Soil and Water Chemistry: An Integrative Approach PDF

SOIL and WATER CHEMISTRY An Integrative Approach SOIL and WATER CHEMISTRY An Integrative Approach MICHAEL E. ESSINGTON CRC PR E S S Boca Raton London New York Washington, D.C. This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” /LEUDU\RI&RQJUHVV&DWDORJLQJLQ3XEOLFDWLRQ'DWD (VVLQJWRQ0LFKDHO( 6RLODQGZDWHUFKHPLVWU\DQLQWHJUDWLYHDSSURDFKE\0LFKDHO((VVLQJWRQ SFP ,QFOXGHVELEOLRJUDSKLFDOUHIHUHQFHVDQGLQGH[ ,6%1 DONSDSHU 6RLOFKHPLVWU\:DWHUFKHPLVWU\,7LWOH 6( d³GF 7KLV ERRN FRQWDLQV LQIRUPDWLRQ REWDLQHG IURP DXWKHQWLF DQG KLJKO\ UHJDUGHG VRXUFHV 5HSULQWHG PDWHULDO LV TXRWHG ZLWK SHUPLVVLRQDQGVRXUFHVDUHLQGLFDWHG$ZLGHYDULHW\RIUHIHUHQFHVDUHOLVWHG5HDVRQDEOHHIIRUWVKDYHEHHQPDGHWRSXEOLVK UHOLDEOHGDWDDQGLQIRUPDWLRQEXWWKHDXWKRUDQGWKHSXEOLVKHUFDQQRWDVVXPHUHVSRQVLELOLW\IRUWKHYDOLGLW\RIDOOPDWHULDOV RUIRUWKHFRQVHTXHQFHVRIWKHLUXVH 1HLWKHUWKLVERRNQRUDQ\SDUWPD\EHUHSURGXFHGRUWUDQVPLWWHGLQDQ\IRUPRUE\DQ\PHDQVHOHFWURQLFRUPHFKDQLFDO LQFOXGLQJ SKRWRFRS\LQJ PLFURÀOPLQJ DQG UHFRUGLQJ RU E\ DQ\ LQIRUPDWLRQ VWRUDJH RU UHWULHYDO V\VWHP ZLWKRXW SULRU SHUPLVVLRQLQZULWLQJIURPWKHSXEOLVKHU 7KHFRQVHQWRI&5&3UHVV//&GRHVQRWH[WHQGWRFRS\LQJIRUJHQHUDOGLVWULEXWLRQIRUSURPRWLRQIRUFUHDWLQJQHZZRUNV RUIRUUHVDOH6SHFLÀFSHUPLVVLRQPXVWEHREWDLQHGLQZULWLQJIURP&5&3UHVV//&IRUVXFKFRS\LQJ 'LUHFWDOOLQTXLULHVWR&5&3UHVV//&1:&RUSRUDWH%OYG%RFD5DWRQ)ORULGD 7UDGHPDUN 1RWLFH 3URGXFW RU FRUSRUDWH QDPHV PD\ EH WUDGHPDUNV RU UHJLVWHUHG WUDGHPDUNV DQG DUH XVHG RQO\ IRU LGHQWLÀFDWLRQDQGH[SODQDWLRQZLWKRXWLQWHQWWRLQIULQJH 9LVLWWKH&5&3UHVV:HEVLWHDWZZZFUFSUHVVFRP E\&5&3UHVV//& 1RFODLPWRRULJLQDO86*RYHUQPHQWZRUNV ,QWHUQDWLRQDO6WDQGDUG%RRN1XPEHU /LEUDU\RI&RQJUHVV&DUG1XPEHU ISBN 0-203-49614-0 Master e-book ISBN ISBN 0-203-58557-7 (Adobe eReader Format) Preface Soil and Water Chemistry: An Integrative Approach was written to meet the needs of undergraduate and ﬁrst-year master’s students in soil and environmental chemistry courses. The book may also serve as a reference for professionals in the soil sciences and allied disciplines. The discipline of soil chemistry, or its contemporary counterpart, environmental soil chemistry, examines the chem- ical and mineralogical characteristics of the soil environment and the chemical processes that distribute matter between the soil solid, solution, and gaseous phases. Essentially, a soil chemistry course and this book offer a basic understanding of the complexity of the natural system that occupies an exceedingly thin layer at the Earth’s surface. Traditionally, the application of chemical principles to the study of soils has been limited to agronomic systems, and primarily to the behavior of agrichemicals. However, it is well established that as a discipline soil chemistry is not limited to describing the processes that control the availability of nutrients to plants. Indeed, the chemical properties and processes that control the behavior of nutrients and pesticides in soils are the same as those that operate on a vast array of inorganic and organic substances that are outside the purview of production agriculture. Recent texts in soil chemistry, those published in the last decade, have attempted to embrace the environmental aspects of the discipline, as evidenced by their various titles (e.g., Environmental Soil Chemistry, Environmental Chemistry of Soils, and Environmental Soil and Water Chemistry: Principles and Applications). Topically, Soil and Water Chemistry: An Integrative Approach con- tinues the “environmental” trend established by its predecessors. However, my intent is to focus on the needs of undergraduate students in soil chemistry and allied disciplines and offer a balanced presentation of the chemical processes operating in soils. This book contains more information and topic coverage than an instructor might cover in a single semester, and introduces some topics that may be too advanced for an undergraduate course. This extensive coverage is by design. I envision that it will allow instructors the latitude to choose their own “essential” topics, while providing additional information or a more advanced treatment for others. This book also contains more than 300 original ﬁgures and approximately 90 tables to help make the material more accessible. I have also reviewed some of the more common method- ologies and analytical techniques used to characterize soil chemical properties. In addition, each chapter contains several sample examples that illustrate problem solving techniques. Each chapter concludes with a section containing numerous exercises. The problems are not esoteric, nor do they require advanced training in soil chemistry to begin to formulate a solution. Each problem has at one time or another appeared in one of my problem sets. They are tested, relevant, and doable, but ask more of students than to simply (or not so simply) generate a number. It may be inevitable that the path to complete a computation or series of computations is tortuous, but this tortuosity should not blind the student to the purpose for determining a numerical answer. It is important for students to understand concepts, and to recognize that the answers to their computations have physical meaning. I have attempted to include exercises that embody both traits; compute an answer and discuss its signiﬁcance. This book has its roots in the undergraduate course I offer in environmental soil chemistry. This is a required course for the environmental and soil sciences major, and is perhaps the last technically oriented course these students will take during their undergraduate experience. I have high expectations of students, particularly with respect to the amount of information retained from prerequisite courses. However, I am also a pragmatic person and recognize that materials introduced in an organic chemistry course taken as a sophomore may have long since been relegated to the “recycle bin” of the mind. The information is there, it just needs to be restored. As with my course in environmental soil chemistry, this textbook begins with an overview of the soil environment and the chemical processes that operate to distribute matter between the soil solid, solution, and atmosphere. Students are then introduced to the concept of speciation, and they are presented with a list of common oxidation states and species for nearly every element as it might exist in the soil solution (with the exception of aqueous complexes). I do not speciﬁcally discuss units, unit conversions, or mass transfer computations in lecture (e.g., if an X g mass of soil is extracted with Y mL of water and the water contains Z mg L–1 of element A, what was the concentration of A in the soil, in mg kg–1?). Instead, I rely on problem sets to refresh and restore this information. However, I am annually queried, “Where was I supposed to have learned how to do this?” I now recognize that 3 years or more of college has not prepared students for this important rudimentary capability. Therefore, these topics are also covered in Chapter 1. Also introduced in Chapter 1 is the concept of spatial variability and spatial statistics. We often discuss the elemental composition of soils in soil chemistry courses, indicating that for every element there is a mean and median value and a range of concentrations observed in soils of the world. Soil chemical properties are spatially variable on a local scale as well; they change with location on a landscape and depth in a proﬁle. Chapters 2, 3, and 4 are devoted to the soil solids. Chapter 2, “Soil Minerals,” begins by discussing the “glue” that bonds atoms together in mineral structures and the rules that describe how these atoms are arranged in three-dimensional space (Pauling’s rules). The remainder of the chapter describes the silicates, emphasizing the phyllosilicates, and the hydrous metal oxides. Finally, x-ray diffraction and its application to identifying clay minerals are discussed. Chapter 3, “Chemical Weathering,” focuses on clay mineral transformations. This chapter also (re)introduces a very important capability that must be mastered by any individual in a chemistry-based course or discipline: balancing chemical reactions. Chapter 4, “Organic Matter in Soil,” examines the organic component of the soil solid phase. The reader is (re)introduced to the organic functional groups and structural components that occur in soil organic matter. The distinction between non- humic and humic substances is drawn, as well as the mechanisms for isolating humic substances. The nonhumic substances are described, as are their transformations from biomolecules to humic substances. The chemical and (pseudo)structural characteristics of the humic substances are also discussed. One of the largest chapters in this book is Chapter 5, “Soil Water Chemistry.” The chapter begins by discussing chemical characteristics of water, the universal solvent, and ends by examining some important analytical methods used to determine the concentrations of dissolved substances in soil solutions. These topics, and those in between, constitute a course in water chemistry and reﬂect my belief that the aqueous chemistry of a substance dictates its fate and behavior. The nonideality of soil solutions, hydration-hydrolysis, Lowry-Brønsted and Lewis acidity and basicity, aqueous complexation, geochemical modeling, and soil solution sampling methods are topics that are addressed with detail and rigor. Chapters 6, 7, and 8 examine the processes that distribute matter between the soil solid and solution phases. In Chapter 6, “Mineral Solubility,” the soil solid and solution characteristics that control the precipitation and dissolution of soil minerals are examined. This chapter also examines the inﬂuences of temperature and impurities in soil minerals on mineral stability and the compositions of soil solutions. Chapter 7, “Surface Chemistry and Adsorption Reactions,” rivals Chapter 5 in size and scope. Adsorption and partitioning reactions are the principal mechanisms by which all organic solutes and many inorganic substances are retained in soils (the other mechanism is precipitation). The chapter describes the soil surfaces and identiﬁes the inorganic and organic functional groups that react with solutes to form surface species. The chapter also examines those factors that inﬂuence the reactivity of soil surface functional groups and applies surface- and solute-speciﬁc information to predict adsorption behavior (surface complexation mod- eling). The descriptive models that are commonly employed to provide an empirical characterization of adsorption are also examined (e.g., Langmuir and Freundlich isotherm models). Although ion exchange is also an adsorption process (or is it—all adsorption processes are ion exchange pro- cesses?), it is standard practice to discuss exchange phenomena separately from adsorption. This is done in Chapter 8, “Cation Exchange.” This chapter focuses on the history, methods of charac- terizing the soil’s capacity to exchange cations, the qualitative characteristics of cation exchange, and the techniques to quantify exchange behavior. Oxidation-reduction processes in soils are examined in Chapter 9, “Oxidation-Reduction Reactions in Soils.” Although this topic is introduced in a later chapter, this should not be taken to imply that the redox behavior of an element is of minor importance. Quite the contrary; the redox status of an environment is a master chemical variable (along with pH) that directly dictates the fate and behavior of redox-sensitive elements, which in turn may inﬂuence the chemistry of other soil constituents. Methods for determining soil redox status, reduction-oxidation sequences in soils, and the redox chemistry of chromium, selenium, and arsenic are discussed. The ﬁnal two chapters (Chapters 10 and 11) are devoted to topics of regional interest: “Acidity in Soil Materials” and “Soil Salinity and Sodicity.” The genesis, characterization, management, and chemical prop- erties of these differing soil systems are discussed. I have also included case studies that examine the reclamation of pyritic acid mine spoils and sodic mine spoils. I am deeply appreciative to the many individuals who donated their time and expertise to the preparation of this book. John Sulzycki at CRC Press planted the “textbook bug” and gave me the opportunity to bring this project forward. He also provided encouragement and continued to demonstrate conﬁdence that I would complete this project, even after I missed several deadlines. Julia Nelson critically reviewed every chapter with a keen eye. Her critiques were thorough and immeasurably improved the clarity of the manuscript and caught my many typos, misspellings, and grammatical errors. I am deeply indebted to her for her efforts. I am also indebted to Gary Pierzynski, George Vance, April Ulery, Dean Hesterberg, and Malcolm Sumner for their highly constructive reviews and suggestions. I applaud their selﬂess contribution to the discipline of soil and water chemistry and to the education of future “Earth” scientists and technicians by giving of their time and expertise. I was buoyed by their positive feedback and by their desire to see a book that has utility for students and professionals. Finally, I have made every effort to produce a text that is complete for the intended audience and conceptually sound. I also recognize that no book is without errors. For the errors that remain, I hope they are few in number and minor in magnitude. If errors are discovered, or if you as the reader have comments and suggestions that would improve future editions of this book, please being them to my attention. I welcome your input. In addition to the individuals cited above who helped shepherd this book from manuscript to reality, there are many people who have mentored me and provided me with the tools, insight, and drive necessary to complete this book. First and foremost is a fellow soil scientist who steered me away from a major in biology and toward the soil science curriculum at New Mexico State University. The late Edward Essington uttered these words during a job fair while I was still a senior in high school: “Get a degree that will allow you to do more that wait tables when you graduate.” I took his advice. I should thank George O’Connor and Al Page for their continual encouragement and support throughout my career, and Shas Mattigod and Garrison Sposito, who set the bar for me many years ago. Finally, without the students whom I have directed or who have taken my soil chemistry courses, this book would not have come to fruition. Their research has provided an abundance of material for this text, and their response to the materials in the lecture notes has been instrumental in the production of this book. You all have my gratitude. Michael E. Essington Knoxville, Tennessee The Author Michael E. Essington is professor of soil and water chemistry in the Institute of Agriculture at The University of Tennessee in Knoxville. In addition to teaching courses in soil chemistry and clay mineralogy, Dr. Essington’s special research interests center on the role of aqueous speciation in environmental chemistry, with particular emphasis on trace element adsorption and precipitation phenomena. These interests have resulted in more than 120 publications and technical reports. Dr. Essington received his B.S. in agriculture from New Mexico State University in 1980 and his Ph.D. in soil science from the University of California, Riverside, in 1985. He was a research scientist at the Western Research Institute in Laramie, Wyoming from 1985 until 1990 and has been at The University of Tennessee since then. He is a member of the Soil Science Society of America, the American Society of Agronomy, Sigma Xi, and Gamma Sigma Delta. Dr. Essington’s professional activities include serving as an associate editor for the Soil Science Society of America Journal; soil chemistry division chair for the Soil Science Society of America; and USDA-NRI panel member for the Soils and Soil Biology Program. Table of Contents Chapter 1 The Soil Chemical Environment: An Overview..........................................................1 1.1 Phases and Chemical Processes in Soil...................................................................................1 1.1.1 Intraphase Soil Processes.............................................................................................3 1.1.2 Interphase Soil Processes.............................................................................................4 1.2 Elements in the Soil Environment: Their Concentrations and Important Species.................6 1.3 Units and Conversions............................................................................................................14 1.4 Heterogeneity of Soil Chemical Characteristics....................................................................19 1.4.1 Descriptive Statistical Properties................................................................................21 1.4.2 Geostatistics................................................................................................................23 1.4.2.1 Variograms...................................................................................................24 1.4.2.2 Interpolation by Kriging.............................................................................28 1.5 Exercises.................................................................................................................................31 References........................................................................................................................................34 Chapter 2 Soil Minerals.............................................................................................................35 2.1 Chemical Bonds.....................................................................................................................35 2.2 Pauling’s Rules.......................................................................................................................38 2.3 Crystal Structure.....................................................................................................................43 2.4 Silicate Classes.......................................................................................................................45 2.4.1 Nesosilicates...............................................................................................................46 2.4.2 Sorosilicates................................................................................................................48 2.4.3 Cyclosilicates..............................................................................................................48 2.4.4 Inosilicates..................................................................................................................48 2.4.5 Phyllosilicates.............................................................................................................49 2.4.6 Tectosilicates...............................................................................................................53 2.5 Clay Mineralogy.....................................................................................................................56 2.5.1 Division 1:1 Phyllosilicate Minerals..........................................................................61 2.5.2 Division 2:1 Phyllosilicate Minerals..........................................................................65 2.5.2.1 Pyrophyllite Group......................................................................................65 2.5.2.2 Smectite Group...........................................................................................68 2.5.2.3 Vermiculite Group.......................................................................................74 2.5.2.4 Mica and Illite Groups................................................................................75 2.5.2.5 Chlorite Group............................................................................................76 2.5.2.6 Interstratiﬁed Layer Silicates......................................................................78 2.6 Hydrous Metal Oxides...........................................................................................................80 2.6.1 Hydrous Aluminum Oxides........................................................................................80 2.6.2 Hydrous Iron Oxides..................................................................................................83 2.6.3 Hydrous Manganese Oxides.......................................................................................86 2.6.4 Allophane and Imogolite............................................................................................88 2.7 X-Ray Diffraction Analysis....................................................................................................90 2.7.1 Principles....................................................................................................................90 2.7.2 Clay Mineral Characterization...................................................................................93 2.8 Exercises................................................................................................................................98 References......................................................................................................................................100 Chapter 3 Chemical Weathering................................................................................................101 3.1 Hydrolysis and Oxidation....................................................................................................101 3.2 Balancing Chemical Reactions............................................................................................105 3.3 Mineral Stability: Primary Silicates in the Sand- and Silt-Sized Fractions........................109 3.4 Mineral Stability: Clay-Sized Fraction................................................................................111 3.5 Weathering and Formation Characteristics of the Phyllosilicates.......................................113 3.5.1 Mica..........................................................................................................................113 3.5.2 Chlorite.....................................................................................................................120 3.5.3 Vermiculite................................................................................................................122 3.5.4 Smectite....................................................................................................................123 3.6 General Weathering Scheme for the Phyllosilicates............................................................124 3.7 Exercises...............................................................................................................................125 References......................................................................................................................................127 Chapter 4 Organic Matter in Soil..............................................................................................129 4.1 Determination of Soil Organic Carbon Concentrations......................................................131 4.2 Organic Functional Groups: A Review................................................................................133 4.3 Nonhumic Substances..........................................................................................................140 4.3.1 Carbohydrates...........................................................................................................140 4.3.2 Nitrogen, Sulfur, and Phosphorus Compounds.......................................................143 4.3.3 Lipids........................................................................................................................150 4.3.4 Lignins......................................................................................................................152 4.4 Humic Substances................................................................................................................155 4.4.1 Genesis of Humic Substances..................................................................................157 4.4.2 Chemical and Structural Characteristics of Humic Substances..............................163 4.4.2.1 Elemental Content.....................................................................................163 4.4.2.2 Functional Groups and Structural Components.......................................167 4.4.2.3 Molecular Mass and Conﬁguration..........................................................173 4.5 Exercises...............................................................................................................................178 References......................................................................................................................................180 Chapter 5 Soil Water Chemistry...............................................................................................183 5.1 Nature of Water....................................................................................................................183 5.2 Ion Hydration........................................................................................................................185 5.3 Electrolyte Solutions............................................................................................................188 5.4 Hydrolysis of Cations...........................................................................................................195 5.5 Lowry-Brønsted Acids and Bases........................................................................................198 5.6 Complex Ions and Ion Pairs.................................................................................................207 5.6.1 Stability of Soluble Complexes................................................................................208 5.6.2 The Irving and Williams Series...............................................................................211 5.6.3 Metal–Fulvate Interactions.......................................................................................212 5.7 The Ion Association Model..................................................................................................216 5.8 Ion Speciation in Soil Solution.......................................................................................... 223 5.8.1 Acid Soil Solution.................................................................................................. 224 5.8.2 Alkaline Soil Solution..............................................................................................225 5.9 Qualitative Aspects of Ion Speciation..................................................................................227 5.10 Soil Water Sampling Methodologies...................................................................................232 5.10.1 In Situ Soil Water Sampling.....................................................................................232 5.10.2 Ex Situ Soil Water Sampling....................................................................................236 5.11 Methods of Chemical Analysis: Elemental Analysis...........................................................238 5.11.1 Atomic Spectrometry................................................................................................238 5.11.1.1 Flame Atomic Absorption Spectrophotometry.........................................238 5.11.1.2 Atomic Emission Spectrometry................................................................243 5.11.1.3 Graphite Furnace Atomic Absorption Spectrophotometry.......................243 5.11.1.4 Hydride Generation Atomic Absorption Spectrophotometry...................245 5.11.2 Inductively Coupled (Argon) Plasma Spectrometry................................................246 5.12 Exercises...............................................................................................................................248 References......................................................................................................................................251 Chapter 6 Mineral Solubility.....................................................................................................255 6.1 Mineral Solubility: Basic Principles....................................................................................255 6.1.1 Mineralogical Controls on Ion Activities in Soil Solutions....................................257 6.1.2 The Ion Activity Product and Relative Saturation...................................................260 6.2 Application of Mineral Solubility Principles: Impediments...............................................263 6.2.1 The Estimation of Standard Free Energies of Formation.......................................264 6.2.2 Metastability and the Step Rule...............................................................................268 6.3 The Deviation of Ksp from Kdis.............................................................................................269 6.3.1 Poorly Crystalline and Microcrystalline Solids.......................................................269 6.3.2 Solid Solutions.........................................................................................................271 6.4 Mineral Solubility and Solution Composition.....................................................................274 6.5 Stability Diagrams................................................................................................................276 6.5.1 Activity Diagrams....................................................................................................277 6.5.2 Activity Ratio Diagrams...........................................................................................280 6.5.3 The Temperature Dependence of the Equilibrium Constant...................................283 6.5.4 In Situ Stabilization: Observed and Predicted Transformations of Metallic Lead in Alkaline Soil............................................................................290 6.5.5 Predominance Diagrams...........................................................................................297 6.6 Predicting Solution Composition.........................................................................................302 6.7 Exercises...............................................................................................................................305 References......................................................................................................................................308 Chapter 7 Surface Chemistry and Adsorption Reactions.........................................................311 7.1 Surface Functional Groups and Complexes.........................................................................311 7.1.1 Inorganic Soil Particle Surfaces and the Source of Charge....................................313 7.1.2 Surface Complexes...................................................................................................321 7.2 The Solid-Solution Interface: A Microscopic View............................................................322 7.2.1 Surface Charge Density............................................................................................322 7.2.2 Points of Zero Charge..............................................................................................327 7.2.3 The Electric Double-Layer.......................................................................................330 7.3 Quantitative Description of Adsorption...............................................................................334 7.3.1 The Adsorption Isotherm..........................................................................................335 7.3.2 Linear Partition Theory............................................................................................348 7.3.3 Adsorption Kinetics and Desorption Hysteresis......................................................358 7.4 Speciﬁc Retention of Metals and Ligands...........................................................................364 7.5 Ligand Effects on Metal Adsorption....................................................................................370 7.6 Organic Surface Functional Groups and Organic Molecular Retention Mechanisms........373 7.6.1 Organic Surface Functional Groups.........................................................................373 7.6.2 Molecular Retention Mechanisms............................................................................373 7.7 Surface Complexation Models.............................................................................................377 7.7.1 The Nonelectrostatic Model.....................................................................................380 7.7.2 Electrostatic Effects on Adsorption..........................................................................386 7.7.2.1 The Constant Capacitance Model.............................................................387 7.7.2.2 The Modiﬁed Triple Layer Model............................................................391 7.8 Exercises...............................................................................................................................394 References......................................................................................................................................397 Chapter 8 Cation Exchange.......................................................................................................399 8.1 Cation Exchange: A Beginning for Soil Chemistry............................................................399 8.2 Qualitative Aspects of Cation Exchange.............................................................................401 8.3 Cation Exchange Capacity and Exchange Phase Composition...........................................404 8.3.1 Exchange Phase Composition..................................................................................404 8.3.2 Methods of Measuring the Cation Exchange Capacity...........................................407 8.3.2.1 Ammonium Acetate (pH 7 or 8.2)...........................................................407 8.3.2.2 Cation Exchange Capacity of Arid Land Soils........................................408 8.3.2.3 Cation Exchange Capacity of Acid Soils.................................................409 8.3.2.4 Unbuffered Salt Extraction.......................................................................409 8.4 Quantitative Description of Cation Exchange.....................................................................409 8.4.1 Cation Exchange Selectivity....................................................................................410 8.4.1.1 Kerr Selectivity Coefﬁcient.......................................................................410 8.3.1.2 Vanselow Selectivity Coefﬁcient..............................................................412 8.4.1.3 Rothmund and Kornfeld Selectivity Coefﬁcient......................................416 8.4.1.4 Gapon Selectivity Coefﬁcient...................................................................418 8.4.1.5 Gaines and Thomas Selectivity Coefﬁcient..............................................420 8.4.1.6 Davies Selectivity Coefﬁcient...................................................................421 8.4.1.7 Regular Solution Model............................................................................422 8.4.2 Exchange Isotherms and Preference........................................................................424 8.4.3 Exchange Equilibrium Constant..............................................................................430 8.5 Exercises...............................................................................................................................440 References......................................................................................................................................442 Chapter 9 Oxidation-Reduction Reactions in Soils..................................................................445 9.1 The Electron Activity...........................................................................................................446 9.2 Redox Potential Measurements............................................................................................449 9.3 Redox Status in Soils...........................................................................................................454 9.3.1 Reduction-Oxidation Sequences in Soils.................................................................455 9.3.2 Redox Zones.............................................................................................................458 9.4 pe–pH Predominance Diagrams...........................................................................................459 9.4.1 Construction of pe–pH Diagrams: The Cr–H2O System.........................................460 9.4.2 Examples of pe–pH Diagrams for Redox-Sensitive Elements................................464 9.4.2.1 Iron and Manganese..................................................................................465 9.4.2.2 Selenium....................................................................................................467 9.4.2.3 Arsenic.......................................................................................................469 9.5 Exercises...............................................................................................................................471 References......................................................................................................................................472 Chapter 10 Acidity in Soil Materials..........................................................................................473 10.1 The Measurement of Soil Solution pH................................................................................474 10.1.1 The pH Electrode System........................................................................................474 10.1.2 Soil Solution pH.......................................................................................................476 10.2 Chemical and Biochemical Processes that Inﬂuence Soil Solution pH..............................477 10.3 Acid-Neutralizing Capacity and the Quantiﬁcation of Soil Acidity...................................483 10.4 Neutralization of Soil Acidity..............................................................................................484 10.5 Acid Generation and Management in Mine Spoils: The Oxidation of Pyrite....................488 10.5.1 Acid Generation and Neutralization in Pyritic Wastes............................................488 10.5.2 Pyritic Mine Spoil Reclamation: Case Study..........................................................492 10.6 Exercises...............................................................................................................................495 References......................................................................................................................................497 Chapter 11 Soil Salinity and Sodicity.........................................................................................499 11.1 Sources of Salts....................................................................................................................500 11.2 Diagnostic Characteristics of Saline and Sodic Soils.........................................................501 11.3 Irrigation Water Quality Parameters and Relationships......................................................504 11.4 Genesis, Management, and Reclamation of Salt-Affected Soils........................................506 11.4.1 Saline Soils............................................................................................................. 507 11.4.2 Sodic Soils.............................................................................................................. 511 11.4.2.1 Classiﬁcation of Sodic Soils.....................................................................511 11.4.2.2 Flocculation and Dispersion in Colloidal Suspensions............................513 11.4.2.3 Genesis and Management of Sodic Soils.................................................516 11.4.3 Sodic Mine Spoil Reclamation................................................................................518 11.5 Exercises..............................................................................................................................519 References......................................................................................................................................521 Index..............................................................................................................................................523 Dedicated to my girls Nina Erin Meghan Chelsea and Deanna The Soil Chemical Environment: 1 An Overview Soil, the thin layer of unconsolidated material that covers the Earth’s surface, is a natural resource that is perhaps uniquely responsible for human development and continued existence on this planet. Humans require food, water, shelter (including clothing and housing), and a means to separate themselves from their own waste products. Soil is the main reservoir that supplies water and essential nutrients for the growth of plants for food, ﬁber for clothing, and forests for building materials. At the same time, soil is the principal receptacle for waste material, including human, animal, and industrial wastes. Soil is expected to ﬁlter and process human and animal waste products, and to tie-up or degrade industrial waste substances to prevent toxic materials from being transferred back into the food chain and drinking water supplies. However, land development and population growth continue to consume and overtax soil resources such that productivity requirements and environ- mental demands require an increasingly precise level of management to optimize the performance of soils. Soil chemistry is central to providing the understanding needed to predict the fate and behavior of substances in the terrestrial environment, whether involving the controlled release of plant nutrients required for sustainable crop production or the strong binding and immobilization of potentially toxic elements to protect water quality, human health, and the environment. Soil chem- istry aims to understand fundamental processes that regulate the transfer of substances between various phases in soils (solid, aqueous, and gaseous), while integrating these individual processes into a comprehensive set of rules that can be used to predict and manage how substances are retained in or removed from soils. This chapter provides a general overview of the soil environment, the chemical processes that occur in soil, and the chemical nature (concentration and speciation) of elements in soil. 1.1 PHASES AND CHEMICAL PROCESSES IN SOIL If soils did not have the inherent ability to bind chemical elements, the plant nutrients and waste products applied to soils would be transported vertically (leached) by percolating rainwater deep into the earth where they would be unavailable for plant growth or degradation by soil microor- ganisms and they would accumulate in groundwater. Alternatively, chemical elements could be transported laterally through subsoil and deeper geologic materials and ultimately discharged to surface waters. The resulting pollution problems would make groundwater unusable for drinking water, and rivers and streams unﬁt for human consumption and recreation. Before the mid-1800s the ability of soils to actually react with chemicals was unknown. Indeed, soil was simply considered to be a support medium for plants, only retarding the ﬂow of water and plant nutrients by physical ﬁltration. It was during the 1850s that an agricultural scientist named J.T. Way, spurred on by the preliminary ﬁndings of an agriculturalist named H.S. Thompson, conducted a series of studies that illustrated the innate reactivity of the soil solid phase. His results were not easily accepted by the scientiﬁc establishment of the time; indeed, some of the most respected agricultural scientists (such as J. von Liebig) urged the scientiﬁc community to “oppose” the experiments of J.T. Way. There is no doubt that mobility of chemical substances in soils may be retarded by the solid phase. It is also known that the ability of soil solids to regulate the movement of chemicals is 1 2 Soil and Water Chemistry: An Integrative Approach Inputs Precipitation, H2O Losses Evapotranspiration, H2O Fluvian and eolian deposition Erosion Gases, CO2 and O2 Plant uptake Anthropogenic materials Gases, CO2 and O2 Volatilization Soil Atmosphere Synthesis [gaseous phase] Soil Solids Decay [solid phase] Soil organic matter: Synthesis Functional group formation Soil Solution Decay Synthesis/decay [aqueous phase] Precipitation Structural rearrangement Hydration: Soil minerals: Al3+ + 6H2O Al(H2O)63+ Weathering Crystallization: Hydrolysis: Ion am-Fe(OH)3 Al(H2O)63+ Al(H2O)5OH2+ + H+ Exchange FeOOH (geothite) Acid-base: Adsorption Functional group formation H2PO4- HPO42- + H+ Oxidation/reduction: Partitioning Oxidation-reduction: FeII FeIII Fe2+ + 0.25O2 + H+ Fe3+ + 0.5H2O Desorption Complexation: Cd2+ + Cl- CdCl+ Inputs Capillary action, H2O Outputs Water Water table fluctuation (losses) Weathering products Salts Degradation products Anthropogenic compounds Anthropogenic compounds FIGURE 1.1 This diagram presents an overview of the components of and the processes that occur in a soil environment. dictated by the processes that distribute chemicals between the immobile soil solids and the mobile soil water and gaseous phases. In chemistry, a phase is a part of a system that is uniform (homo- geneous) throughout in chemical composition and physical properties, and that is separated from other uniform parts by a boundary. The soil environment may be described as consisting of three phases: the soil atmosphere or gaseous phase, the soil solution or aqueous phase, and the soil solids or solid phase (Figure 1.1). The soil gaseous and aqueous phases are true phases; however, the soil solid phase is not a single phase, but rather a composite of several phases. This distinction will be discussed in later chapters; however, for the purpose of this general discussion, the soil solid phases may be collectively divided into compartments. These solid phase compartments are mineral (inorganic compounds), humus (soil organic matter), and biotic (living organisms). Distinctly different from the soil solid, aqueous, and gaseous phases are the boundary phases, which are comprised of substances that accumulate at the interface between soil minerals or organic matter and the soil solution. For example, ions that are exchangeable may be identiﬁed as residing in the soil exchanger phase. Ions and molecules that are retained by processes other than exchange or precipitation (the formation of a true solid phase) may be identiﬁed as residing in the adsorbed phase. These boundary phases may also be included in the soil solid phase. The soil environment is bounded by the soil surface (at the top of the soil proﬁle) and the soil parent material (beneath the soil proﬁle). In addition to these boundaries, arbitrary lateral boundaries may also be imposed. Soils are open systems, so energy (thermal) and matter can enter and exit The Soil Chemical Environment: An Overview 3 the soil environment at these boundaries. At the soil surface, matter can enter the soil environment via rainfall (H2O), atmospheric particulates, fertilizers and other anthropogenic inputs (waste disposal and utilization), the diffusion of gases, such as carbon dioxide (CO2), oxygen (O2), and nitrogen (N2), and deposition (ﬂuvial and eolian). Matter may also exit the soil environment via evaporation and transpiration, diffusion of gases (CO2, O2, NH3), plant uptake of essential nutrients and nonessential elements followed by biomass removal, direct ingestion by grazing animals, volatilization (organic compounds), and erosion. At the lower boundary, matter may be leached out of the soil proﬁle, ultimately appearing in ground and surface waters. Substances may also enter the soil by lateral ﬂow or at the lower boundary by ﬂuctuating water tables or in water drawn up by capillary action. 1.1.1 INTRAPHASE SOIL PROCESSES Within each of the three soil phases, a single element may exist in several different chemical forms. Each form of an element has unique characteristics that impact the fate and behavior of the element. We are surrounded by soils that contain very high levels of aluminum (71,000 mg Al kg–1, on average), a nonessential element that can be toxic to plants, aquatic life, and humans. Yet, humans show no concern for the fact that they are surrounded by large quantities of this potentially toxic substance; lakes, rivers, and streams teem with aquatic life, even though a portion of the water they hold may have percolated through soil; and soils naturally sustain a diverse and plentiful array of microbial and plant life. However, not all environments are unaffected by Al. Aluminum toxicity in plants can occur in humid regions when poor crop management practices are employed. Aquatic organisms in streams that are impacted by highly acidic runoff and leachates from pyritic mine spoils will show the effects of Al toxicity. Research by soil scientists has shown that when properly managed the toxic effects of Al can be reversed by returning Al to solid phases that are sparingly soluble. For example, Al phytotoxicity and high mine spoil leachate concentrations can be easily remedied by raising the pH of the soil or the mine spoil by liming with calcium carbonate (calcite, CaCO3), altering the solid form of Al and returning the element to mineral phases that are sparingly soluble and unavailable to plants. Potentially toxic metals such as lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), and chromium (Cr) are ever-present constituents of soils, albeit in much lower concentrations than Al. As with Al, these metals are of little concern in a natural setting. However, land disposal of industrial wastes and the utilization of municipal wastes (e.g., sewage sludge) often elevate the soil concentrations of these potentially toxic metals to levels that could have a detrimental environmental impact. Again, research has illustrated that the solid phases in which these elements reside dictate their availability and toxicity, and that soils play a key role in the transformation of potentially toxic elements from soluble and available forms to those that are relatively innocuous. As the discussion above suggests, elements that reside in the soil solid phase may change form. Indeed, within each of the three soil phases, substances may change chemical form by participating in chemical reactions (Figure 1.1). The soil solid phase is comprised of organic substances (biotic and abiotic matter) and soil minerals (inorganic compounds). Within the soil organic fraction, organic compounds are synthesized and degraded, the structure of the organic compounds may rearrange into different conﬁgurations, and functional groups (carboxyl, phenolic-hydroxyl, carbo- nyl, and amino) may form, transform, or ionize (develop charge). In the soil mineral phase, amorphous solids (minerals that have no long-range structure) crystallize, surface functional groups form and ionize ( AlOH0 → AlO– + H+; or AlOH0 + H+ → AlO H 2+, where Al represents an aluminum atom in a mineral structure), and redox-sensitive elements in mineral structures undergo oxidation (FeII → FeIII) or reduction (MnIV → MnII). A soil’s reactivity is mostly regulated by the aqueous phase. In the soil solution, oxidation- reduction, hydration-hydrolysis, acid-base, and complexation reactions occur. These reactions impact the speciation (chemical form) of substances in the soil solution, which in turn impacts 4 Soil and Water Chemistry: An Integrative Approach bioavailability, toxicity, the interactions that occur between the soil phases, and mobility. For example, chromate (Cr O2− 4 ), a highly toxic, mobile, anionic form of Cr, may be reduced to Cr , 3+ 2+ a less toxic, relatively immobile, cationic form of Cr. Metal cations, such as Pb , are surrounded by a sphere of water molecules (they are hydrated, as are anions). Depending on the metal and the pH of the soil solution, a water of hydration can decompose (deprotonate or ionize) and produce a proton and a hydrolysis product. For example, when soil solution pH is greater than 7.7, Pb predominantly exists as the hydrolysis product PbOH+, formed by the reaction: Pb2+ + H2O → PbOH+ + H+. The PbOH+ is a chemically unique aqueous species that reacts quite differently in soil than Pb2+. Hydrolysis detoxiﬁes soluble Al by removing the element from its toxic form, Al3+, and placing it into AlOH2+, Al(OH) 2+ , Al(OH) 3− , and Al(OH) −4 forms. Nearly all dissolved substances in the soil solution are capable of accepting or donating protons. Species that accept protons are called Lowry-Brønsted bases, and those that donate protons are Lowry-Brønsted acids. For example, an organic amino group is a base because it may accept a proton: R N H 20 + H+ → R N H3+. Not only does this process help buffer (control) the pH of the soil solution, the ionization of the organic moiety alters its reactivity and behavior in the soil environment. Acidic species, those that donate protons to the soil solution, also aid in the buffering of the soil solution, and the change in chemical form alters their environmental behavior as well. The hydrolysis of Pb2+ described above is an example of an acid-producing reaction that alters the + chemical form of Pb. Similarly, the dissociation reaction, H2P O −4 → HP O2− 4 + H , is also an acid- producing reaction that alters the chemical form of phosphorus. Substances that are dissolved in the soil solution may interact to form soluble species that display chemically different behavior than they did before the interaction. For example, the free form of Cd in soil solution is the divalent cation, Cd2+. In the presence of the chloride ion, Cl–, Cd will exist as the free cation and in association with chloride, as CdCl+. The CdCl+ is a unique soluble species (it is not a solid) that behaves quite differently from the divalent ion, Cd2+. The process of forming the ion association, or adduct for an addition product, is called aqueous complexation or ion pair formation: Cd2+ + Cl– → CdCl+. Aqueous complexation is not limited to inorganic substances. Naturally occurring and synthetic organic compounds, particularly those than contain carboxyl and amino groups, form strong complexes with metal cations, which drastically increase their mobility and bioavailability. Inorganic and organic species that participate in these types of reactions—and nearly all soluble substances in the soil solution do participate in these reactions—are called Lewis acids and bases. A Lewis acid is a cation, often a metal cation, such as Cd2+, that initiates a chemical reaction by employing an unoccupied electronic orbital. A Lewis base is a ligand, a substance that initiates a chemical reaction by employing a doubly occupied electronic orbital. Ligands may be charge neutral (such as H2O or an organic amine, R N H 20 ) or an anion (such as Cl–). Lowry-Brønsted acidity is a special case of Lewis acidity, where H+ is the cation. 1.1.2 INTERPHASE SOIL PROCESSES Industrial activities in modern society are supported by energy derived from the burning of fossil fuels such as petroleum and coal. As a result of this combustion, the concentration of carbon dioxide (CO2) (and other gases) in the Earth’s atmosphere has steadily increased since the mid- to late 1800s. At present, the atmospheric content of CO2 is considered to be increasing at a rate of 7.2 Pg C yr–1 (1 Pg = 1012 kg), principally from fossil fuel combustion (5.5 Pg C yr–1) and land use changes (1.6 Pg C yr–1) (Swift, 2001). This increase in global atmospheric CO2 has triggered substantial international concerns over its potential effects on global warming and global weather patterns. Soil plays an important role in the regulation of atmospheric CO2 levels and in carbon sequestration. Current estimates indicate that the global reservoir of soil-bound carbon is about 3300 Pg (found in carbonates and organic carbon), which is greater than four times the amount of carbon in the atmosphere (720 Pg). Current estimates also indicate that CO2 emissions from soils The Soil Chemical Environment: An Overview 5 by microbial respiration, 60 Pg C yr–1, are more than 10 times greater than that from the combustion of fossil fuels (5.5 Pg C yr–1). Additionally, carbon emission by respiration from plants (60 Pg C yr–1) and carbon lost through the net destruction of vegetation (2 Pg C yr–1) indicate that there is a net loss of carbon (2 Pg C yr–1) from soils and associated ecosystems. Capturing more CO2 into plant biomass and transferring this carbon into soil organic matter is an important mechanism that might be employed through the manipulation of land management practices to offset fossil fuel emissions. On a smaller and more general scale, the transfer of matter between phases in the soil environment directly inﬂuences the chemical processes that impact the fate and behavior of soil components (Figure 1.1). In the soil atmosphere, the chemically signiﬁcant components are CO2 (as it impacts soil solution pH) and O2 (as it impacts soil redox status). These soil atmosphere gases readily diffuse through the soil atmosphere under water-unsaturated condi- tions, and they can dissolve into the soil solution. However, gases have differing water solubilities. For example, CO2 has a water solubility of 33.8 mmol L–1 at 25°C and 1 atm CO2 pressure; whereas, O2 has a water solubility of 1.28 mmol L–1 at 25°C and 1 atm O2 pressure. Carbonic acid (H2C O30) is created when CO2 dissolves and is hydrated (CO2 + H2O → CO2 H2O). Carbonic acid makes rainfall naturally acidic and hastens the weathering of soil minerals. Oxygen gas is an oxidizing agent (electron acceptor) and the presence of oxygen results in an oxidizing (aerobic) environment, while the absence of O2 leads to reducing conditions. The low water solubility of O2 can cause the development of reduced conditions in water-saturated soil. Whether the environment is oxi- dizing or reducing greatly impacts both biotic (biological) and abiotic (nonbiological) processes in soil. During the degradation of biopolymers in soil, organic matter such as lignin, proteins, and carbohydrates, CO2, inorganic substances, and low-molecular-mass organic compounds such as acetic acid and citric acid are released to the soil solution. Inorganic elements are also released during the dissolution, or weathering, of soil minerals. Many of the released elements reprecipitate to form new and relatively stable mineral phases, such as the hydrous metal oxides. However, some substances may remain soluble and quite mobile, as if the soil had no retention capacity. The ecological disaster that befell the Kesterson National Wildlife Refuge located on the west side of the San Joaquin Valley in California was two decades in the making. During the 1950s and 1960s large tracts of land were brought under irrigated agriculture. However, facilities to remove drainage from the region were not available. Without adequate drainage, salinity levels in the soils of the region and in the shallow groundwater gradually increased during the 1960s and 1970s. Increasing soil salinity prompted even more intense irrigation practices to maintain productivity, resulting in the additional build-up of salinity, water-logging, and ﬁnally the abandonment and loss of arable land. Beginning in 1981, the San Luis Drain was opened to discharge the subsurface drainage to a reservoir at the Refuge. In 1983 it was discovered that high levels of selenium (Se) in the reservoir caused a high incidence of deformity and mortality in waterfowl hatchlings at the Wildlife Refuge. The introduction of irrigated agriculture into the valley led to the solubilization of Se from the seleniferous soils that formed on alluvium derived from the sedimentary rocks that border the valley. Selenium concentrations in the shallow groundwater continued to build during the two decades prior to the opening of the San Luis Drain. When the drain was ﬁnally opened, the Se- rich water was transported to the Wildlife Refuge. In this case, Se in the soil was solubilized because the Se-bearing minerals were easily dissolved by the irrigation waters. The Se remained soluble and mobile because the chemical form of this element in the soil solution was not effectively retained by the reactivity of the soil (the soil had no ability to regulate the particular solution form of Se). The Kesterson example above illustrates that not all substances are effectively retained in soil. In reality, all substances are mobile to some degree in soils (no substance is truly immobile). In addition to precipitation (the formation of a solid phase), substances may be retained in soils by mechanisms that are generally described as sorption processes. Organic substances and numerous clay-sized ( 9.8) will silicic acid deprotonate. Aluminum exists in only the AlIII valence state. Depending on solution pH, Al may be present in soil solutions as the free Al3+ species (note that AlIII denotes the oxidation state of aluminum, while Al3+ is a species that contains aluminum as AlIII), or as a hydrolysis product: AlOH2+, Al(OH)2+ , Al(OH)30 , or Al(OH)−4. Iron is found in two oxidation states, FeII and FeIII. In moderately reduced (anaerobic) systems and in primary minerals, Fe2+ is the dominant form of iron. In oxidized systems and in secondary minerals, Fe3+ dominates. In soil solutions, ferric iron occurs in several hydrolysis products: FeOH2+, Fe(OH)2+ , Fe(OH)30 , or Fe(OH)−4 (depending on pH). Both aluminum and iron(III) are amphoteric because they may exist as a cationic or an anionic species in solution. All alkali metals (Periodic group IA elements), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs), exist only in the +I oxidation state, and occur as monovalent cations: Li+, Na+, K+, Rb+, and Cs+. Similarly, the alkaline earth metals (Periodic group IIA elements), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba), exist only in the +II oxidation state, and occur as divalent cations: Be2+, Mg2+, Ca2+, Sr2+, and Ba2+. Another group of elements that are found in only one oxidation state are the halides (Periodic group VIIB elements). These elements are found in the –I state in soil as the monovalent anions: ﬂuoride (F–), chloride (Cl–), bromide (Br–), and iodide (I–). Elements in each of these Periodic groups tend to display very similar environmental behavior, that is, the behavior of Sr2+ in the soil is similar to that of Ca2+. Titanium (Ti) and zircon (Zr) exist in the +IV valence state; however, their occurrence in the soil environment is restricted to the solid phase where they are surrounded by oxygen atoms. Many of the environmentally problematic elements occur in uncontaminated soils in very low concentrations. These elements are commonly referred to as trace elements or heavy metals. The use of these terms, however, relates very little information concerning environmental behavior or biological toxicity. Indeed, elemental classiﬁcation according to geochemical abundance or speciﬁc gravity does not imply biological hazard, even though trace elements and heavy metals are terms that are used to denote potentially toxic elements. The least descriptive term is heavy metals. Elements from beryllium (9.01 g mol–1) to uranium (238.03 g mol–1) have been identiﬁed as heavy metals, although the ﬁrst row transition elements (vanadium, 50.94 g mol–1, to zinc, 65.39g mol–1), as well as arsenic (74.92 g mol–1), selenium (78.96 g mol–1), molybdenum (95.94 g mol–1), cadmium (112.41 g mol–1), mercury (200.59g mol–1), and lead (207.2 g mol–1), are typically implied by this ambiguous moniker. Unlike a heavy metal, which is a vaguely deﬁned term, a trace element may be deﬁned as an element whose soil concentration is less than 100 mg kg–1 or whose soil solution concentration is signiﬁcantly below 10–4 mol L–1 (and typically below 10–6 mol L–1). Because of their low concentrations, trace elements are not found in soils in discrete mineral phases. Instead, trace elements occur as minor substituents in silicates and aluminosilicates (olivines, pyroxenes, amphiboles, micas, and feldspars); hydrous metal (iron, aluminum, and manganese) oxides; iron sulﬁdes; calcium and magnesium carbonates; and calcium, iron, and aluminum phosphates. A signiﬁcant percentage of the total soil content of a trace element may also reside in the adsorbed phase. 14 Soil and Water Chemistry: An Integrative Approach Minor and trace elements that are typically found in only one oxidation state in soils include boron (BIII in boric acid, B(OH)30 , and borate, B(OH)−4 ), cobalt(II) (Co2+), nickel(II) (Ni2+), cop- per(II) (Cu2+), zinc(II) (Zn2+), molybdenum (MoVI in molybdate, MoO2− 2+ 4 ), cadmium(II) (Cd ), and 2+ lead(II) (Pb ). However, many minor and trace elements are redox sensitive within the range of redox conditions observed in the environment. The two oxidation states of chromium found in the soil environment are CrIII (in the relatively inert Cr3+ form) and CrVI (principally in the toxic chromate forms, HCrO −4 and CrO2− II 4 ). Manganese is found in three valence states: Mn , Mn , and III IV 2+ Mn. In soil solutions, the Mn species dominates; whereas, all three Mn oxidation states are found in soil minerals. Both arsenic and selenium exist as oxyanions in the environment. Arsenic can occur in the AsIII and AsV oxidation states as the arsenite ( HAsO32− ) and arsenate (H2 AsO −4 and HAsO2− 4 ) species (arsenate speciation is similar to that of phosphate). Selenium can occur in the Se–II, SeIV, and SeVI oxidation states as the selenide (H2Se0, HSe–, and Se2–), selenite ( HSeO3− and SeO32− ), and selenate ( SeO2−4 ) species (selenate speciation is similar to that of sulfate). Several oxidation states are possible for vanadium; however, VIV (as VO2+) and VV (as VO2+ and the hydrolysis products, VO2 (OH)2− and VO3(OH)2–) are stable throughout the range of soil solution pH and redox conditions. In primary minerals, V3+ (VIII) is common; whereas, vanadate (VV as VO3−4 ) is common in secondary V-bearing minerals. Mercury, which is one of the least abundant elements in soils, occurs in the HgII oxidation state in aerated solutions. The Hg(OH)20 species dominates throughout a wide range of soil pH conditions. In anaerobic conditions, HgI (in Hg2+ 2 ) and Hg0 (elemental vapor or liquid Hg) may occur. 1.3 UNITS AND CONVERSIONS Launched December 11, 1998, the mission of the Mars Climate Orbiter was to provide information on the Martian climate. As the orbiter neared the Martian atmosphere on September 23, 1999, after spending 286 days (9 1 2 months) in transit, controllers lost contact. Loss of the orbiter was not the result of a collision with a cosmic particle or the failure of a critical component at a crucial time. Instead, the failure of the $655.2 million project was the result of a miscommunication between the spacecraft team in Colorado and the mission navigation team in California. Speciﬁcally, one team was using English units (yards), while the other was using metric (meters) as they worked to guide the orbiter to Mars. The problem was not that each team was using different units, but that each team assumed the other was using comparable units. The 214.6 billion yard distance (196.2 billion meters) to Mars is signiﬁcantly less than the 214.6 billion meter distance assumed by one of the teams. Thus, the Orbiter rather abruptly intersected with Mars at a speed of 12,300 miles per hour (5.5 km sec–1), still programmed to travel an additional 18.4 billion meters. While several lessons were learned from the Mars Climate Orbiter debacle, there are two points to be gleaned from this illustration. First, there are often several ways in which to express a physical quantity. For example, distance can be expressed in an English unit (foot), a centimeter-gram-second (cgs) unit (centimeter), or a meter-kilogram-second (mks) unit (meter). Second, if the system used to express a physical quantity is not agreed upon, or standardized, the consequences of the miscom- munication may be expensive and embarrassing, and potentially detrimental to human health and the environment. In order to remove ambiguities in reported measurements and to bring uniformity of style and terminology to communicating measurements, the SI system (Système International d’Unités, or International System of Units) of reporting measurements has been adopted by scientiﬁc societies and countries the world over. It is also the system employed in this text. There are two classes of SI units: base units and derived units. Base units are dimensionally independent (e.g., meters, kilograms, and seconds) (Table 1.2); whereas derived units are expressed as algebraic terms of base units (e.g., area, m2, and volume, m–3). The derived units can also be given special names (e.g., hectares and liters) which themselves may be used in expressing other derived units. For example, the SI unit for pressure (force per unit area) is the pascal (Pa). It is a derived unit expressed as N m–2 The Soil Chemical Environment: An Overview 15 TABLE 1.2 Base Units of the International System of Units Quantity Unit Symbol Length meter m Mass kilogram kg Time second s Electric current ampere A Thermodynamic temperature kelvin K Amount of substance mole mol Luminous intensity candela cd TABLE 1.3 Examples of Derived Units in the International System of Units (SI) Expression in Terms of Quantity Name Symbol SI Base or Other Derived Units Acceleration meter per second squared — m s–2 Area square meter — m2 hectare ha m2 Capacitance farad F C V–1, m–2 kg−1 s4 A2 Celsius temperature degree Celsius ºC K Concentration mole per cubic meter — mol m–3 Density kilogram per cubic meter — kg m–3 Electric charge, coulomb C sA quantity of electricity Electrical conductance siemen S A V–1, m–2 kg–1 s3 A2 Electric potential, volt V W A–1, m2 kg s–3 A–1 potential difference, electromotive force Electric resistance ohm ω V A–1, m2 kg s–3 A–2 Energy, work, joule J N m, m2 kg s–2 quantity of heat Force newton N m kg s–2 Frequency hertz Hz s–1 Heat capacity, entropy — J K–1 m2 kg s–2 K–1 Pressure pascal Pa N m–2, kg s–2 Power watt W J s–1, m2 kg s–3 Speciﬁc energy J kg–1 m2 s–2 Speciﬁc heat capacity, — J kg–1 K–1 m2 s–2 K–1 speciﬁc entropy Speciﬁc surface area — — m2 kg–1 Velocity meter per second — m s–1 Volume liter L m3 (newtons per square meter). The SI unit for force is the newton, which is derived from the base units, m kg s–2. The derived units having special interest to environmental soil chemistry are identiﬁed in Table 1.3. The SI system also employs preﬁxes to indicate orders of magnitude of SI units (Table 1.4). The objective in employing preﬁxes is to reduce the use of nonsigniﬁcant digits or leading zeros in decimal fractions. Preferably, the preﬁx should be selected so that the numerical 16 Soil and Water Chemistry: An Integrative Approach TABLE 1.4 Preﬁxes Employed to Indicate Orders of Magnitude Order of Magnitude Preﬁx Symbol Order of Magnitude Preﬁx Symbol 1018 exa E 10–1 deci d 1015 peta P 10–2 centi c 1012 tera T 10–3 milli m 109 giga G 10–6 micro µ 106 mega M 10–9 nano n 103 kilo k 10–12 pico p 102 hecto h 10–15 femto f 101 deka da 10–18 atto a TABLE 1.5 Selected Units Used in Environmental Soil Chemistry Quantity Application Unit Symbol Angle of diffraction X-ray diffraction degrees two-theta º2Θ Bulk density soil bulk density megagram per cubic meter Mg m–3 Cation exchange capacity ion retention centimole of charge per kilogram cmolc kg–1 Concentration mass basis mole per kilogram mol kg–1 (m)a milligram per kilogram mg kg–1 gram per kilogram g kg–1 volume basis moles per cubic meter mol m–3 mole per liter mol L–1 (M)a millimole per liter mmol L–1 milligram per liter mg L–1 microgram per liter µg L–1 Electrical conductivity soil salinity decisiemen per meter dS m–1 Interatomic spacing crystallography, nanometer nm clay mineralogy Angstrom Å a m, molal or mole per unit mass; M, molar or mole per unit volume. value of the measurement lies between 0.1 and 1000. In addition to the units presented in Tables 1.2 and 1.3, there are preferred units for the expression of select soil properties (Table 1.5). One of the more vexing of problems encountered in the presentation and assimilation of information is unit conversion. The mechanics of performing unit conversions are not dependent on the particular property that is measured, although the appropriate application and manipulation of units is best examined in measurement-speciﬁc discussions. The concentration of a substance in a system is a recognizable characteristic. Yet, there are many ways in which to express compo- sition. X-ray ﬂuorescence spectrometry (XRF) is an analytical technique employed in the geological sciences to determine the elemental composition of geologic materials. The results of an XRF analysis of a mine spoil material are presented in Table 1.6. Note that the instrument is calibrated to generate compositional data on an oxide basis for some substances and on an elemental basis for others, in percentage (%) units (also known as parts per hundred). For example, 22.3% of the mine spoil is composed of Al2O3, and 0.49% is composed of S. It is important to recognize that the spoil material probably does not contain the compound Al2O3 or elemental S, but that the total concentrations of Al and S are expressed as if they were present in these forms. As is indicated in Table 1.5, the acceptable units for expressing composition data on a mass basis are mol kg–1, g kg–1, The Soil Chemical Environment: An Overview 17 TABLE 1.6 The Composition of a Mine Spoil Material as Determined by X-Ray Fluorescence Spectrometry (XRF) and Presented using the International System of Units Oxide Presentation Elemental Presentation Compound % g kg–1 Element g kg–1 mol kg–1 Al2O3 22.3 223 Al 118 4.37 Cl 0.40 4.0 Cl 4.0 0.11 Fe2O3 7.78 77.8 Fe 54.4 0.974 K2O 3.46 34.6 K 28.7 0.734 MgO 1.6 16 Mg 9.6 0.39 P2O5 0.133 13.3 P 7.49 0.242 S 0.49 4.9 S 4.9 0.15 SiO2 55.7 557 Si 260 9.26 TiO2 1.01 10.1 Ti 6.06 0.127 or mg kg–1. The conversion of percentage units to SI units is necessitated because percentages are unacceptable units (they are ambiguous). The ﬁrst step in performing the conversion of units is to recognize that a percentage indeed represents parts per hundred. Another way to express parts per hundred is in the units of gram per hectogram (g hg–1). In order to convert g hg–1 (a non-SI unit) to grams per kilogram (g kg–1, an SI unit), the conversion factor must have units of hectograms per kilogram. Since there are 10 hectograms in a kilogram, the conversion factor is 10 hg kg–1. The conversion process is illustrated for Al2O3: g Al 2 O 3 22.3 o o Al 2 O 3 = 22.3 (1.6) hg spoil g Al 2 O 3 10 hg g Al 2 O 3 22.3 × = 223 (1.7) hg spoil kg kg spoil Note that the units are simply treated as fractions. In Equation 1.7, the hg unit in the denominator of the Al2O3 concentration units is cancelled by the hg unit in numerator of the conversion factor, leaving the units of g kg–1. Expressing compositional data on an oxide basis is appropriate; however, it is more common to express the chemical composition of a material on an elemental basis. Again using Al2O3 as an example, the oxide composition of Al in the mine spoil is 223 g kg–1. To convert the units of g of Al2O3 per kg of spoil to the units of g of Al per kg of spoil, a conversion factor that has units of g Al per g Al2O3 must be employed. In order to determine the mass of Al in a given mass of Al2O3, we will use the molecular masses of each substance (which can be determined using the molecular masses of Al and O in Figure 1.2). The molecular mass of Al is 26.98 g mol–1, and of Al2O3 is 101.96 g mol–1 (2 × 26.98 g Al mol–1 + 3 × 16 g O mol–1). There are two moles of Al in every mole of Al2O3, or: g Al 2 mol Al g Al 26.98 × = 53.96 (1.8) mol Al mol Al 2O3 mol Al 2O3 Multiplying this value by the reciprocal of the molecular weight of Al2O3: g Al mol Al 2O3 g Al 53.96 × = 0.529 (1.9) mol Al 2O3 101.96 g Al 2O3 g Al 2O3 18 Soil and Water Chemistry: An Integrative Approach This is the conversion factor needed to convert from the g Al2O3 kg–1 basis to g Al kg–1. The elemental composition of Al in the spoil material is computed as: g Al 2O3 0.529 g Al g Al 223 × = 118 (1.10) kg spoil g Al 2O3 kg spoil The expression of the elemental composition data in Table 1.6 in the units of g kg–1 is acceptable, as the values lie between 0.1 and 1000. The elemental composition may also be expressed on a mole basis. To convert from g kg–1 to mol kg–1 requires a conversion factor that is the reciprocal of the molecular mass of Al: g Al mol Al mol Al 118 × = 4.37 (1.11) kg spoil 26.98 g Al kg spoil or 4.37 mol kg–1. Another common unit transformation is the conversion of an extractable concentration, in volume-based units, back to a soil mass–based concentration. Consider the nitric acid digestion of a soil sample that has potentially received waste materials containing cadmium. A 2.5-g sample of soil (dry weight basis) is digested in 25 mL of 4 M HNO3. After digestion, the solution is separated from the solid by ﬁltration. A 1-mL aliquot of the extract is placed in a 100-mL volumetric ﬂask, which is brought to volume with metal-free, deionized-distilled water. The diluted aliquot is then analyzed for Cd and found to contain 0.53 mg Cd L–1. A question that is commonly asked is: based upon this analysis, has this soil received cadmium wastes? As mentioned previously, the knowledge of elemental concentrations provides little, if any, information about risk. There must be some basis for establishing contamination, such as the Cd content of the same soil that is known to have not received waste (background levels). Again, reality does not always conform to the optimal, and in this case the question must be answered without knowledge of background levels. The ﬁrst step in addressing the question is to compute the concentration of Cd on a soil basis. There are two conversion factors that must be employed in order to accomplish this. Dilutions are often required to bring the solution concentration of a substance into the analytical range of an instrument. In this case, a 100-fold dilution was used (1 mL of soil extract into 100 mL total volume), resulting in a measured Cd concentration of 0.53 mg L–1. To convert back to the Cd concentration in the original soil digestate, the following expression is used: caliquot × Valiquot = cdilution × Vtotal (1.12) In this expression, caliquot is the concentration of the analyte in the undiluted extract, Valiquot is the volume of the undiluted sample used to prepare the dilution, cdilution is the concentration of the analyte in the diluted sample, and Vtotal is the total volume of dilution. In our example, caliquot is the quantity we wish to compute, Valiquot is 0.001 L (1 mL), Vtotal is 0.1 L (100 mL), and cdilution is 0.53 mg L–1. The concentration of Cd in the original digestate may then be computed: mg Cd 0.1 L caliquot = 0.53 × = 53 mg Cd L−1 (1.13) L 0.001 L The second conversion will result in the expression of Cd concentration on a soil basis. For this conversion we note that 2.5 g of soil was extracted with 25 mL of nitric acid; or 0.0025 kg of soil was extracted with 0.025 L of nitric acid. Because we want to convert mg L–1 to mg kg–1, the The Soil Chemical Environment: An Overview 19 conversion factor must have units of L kg–1. Therefore, our conversion to obtain the concentration of Cd extracted from the soil on a mass basis is: mg Cd 0.025 L Cd soil = 53 × = 530 mg Cd kg −1 (1.14) L 0.0025 kg According to the information presented in Table 1.1, the median Cd content of uncontaminated soil is 0.35 mg kg–1. Further, the upper limit of uncontaminated soil Cd is 2 mg kg–1. The concentration of Cd extracted from the potentially contaminated soil was 530 mg kg–1. Although the 4 M HNO3 digestion is a rather harsh extractant, the soil Cd content computed in Equation 1.14 is not a total concentration, it is an extractable concentration (total Cd would be greater than extractable). Irrespective, the extractable Cd level in the soil far exceeds the median and upper limit values of total Cd in uncontaminated soil. Thus, the probability that this soil has received Cd- bearing waste is quite high. 1.4 HETEROGENEITY OF SOIL CHEMICAL CHARACTERISTICS It was indicated in Section 1.2 that the elemental content of soils is highly variable relative to location on the landscape and depth within the proﬁle. A compilation of the elemental compositions of surface soil samples collected from around the world indicates a composition range of several orders of magnitude for any given element (Table 1.1). However, one need not venture far to observe the spatially heterogeneity of soil chemical and physical properties. It is a basic tenet of soil science that soil properties differ with depth due to the process of horizonation. This process, which is under the inﬂuence of the soil forming factors (parent material, vegetation, climate, topography, and time), is responsible for the elution (mobilization) and illution (accumulation) of soil compo- nents within a pedon. For example, the elution and illution of soil organic carbon (SOC) is a distinguishing feature of a Spodosol (relative to other soil orders, such as that illustrated for an Alﬁsol) (Figure 1.4a), just as the illution and neoformation of clay-sized material in the subsoil is (a) (b) 0 0 Clay CEC 20 20 40 40 Depth, cm Depth, cm Spodosol Alfisol 60 60 80 80 100 100 0 40 80 120 0 200 400 600 SOC, g kg-1 Clay (g kg-1) or CEC (mmolc kg-1) FIGURE 1.4 Soil properties as a function of depth in the soil proﬁle: (a) soil organic carbon (SOC) in a Spodosol and an Alﬁsol; (b) clay content and cation exchange capacity (CEC) in an Aridisol. 20 Soil and Water Chemistry: An Integrative Approach Grenada Calloway N Henry 100 m Loring Henry Routon Henry Grenada Calloway Memphis FIGURE 1.5 Soil map of a 10.5 ha production cotton ﬁeld located at the Milan Agricultural Experiment Station in Milan, TN. Each rectangle represents a 27.4 m × 16.2 m monitoring unit. The classiﬁcations of the soil series at the subgroup level are: Calloway, Aquic Fraglossudalf; Grenada, Oxyaquic Fraglossudalf; Henry, Typic Fragiaqualf; Loring, Oxyaquic Fragiudalf; Memphis, Typic Hapludalf; and Routon, Typic Epiaqualf. an indicator of an argillic horizon as seen in the Aridisol (Figure 1.4b). However, and aside from soil classiﬁcation criteria, the clay and organic matter content of soil are properties that greatly inﬂuence compound fate and behavior. The observed behavior of an ionic substance in a 0- to 15- cm surface soil sample of the Aridisol described in Figure 1.4b (73 g kg–1 clay with a cation exchange capacity of 6.0 cmolc kg–1) may be quite different from that observed in a 30- to 50-cm subsoil sample (587 g kg–1 clay with a cation exchange capacity of 32.6 cmolc kg–1). Similarly, the behavior of a nonpolar and hydrophobic organic compound in a 0- to 8-cm surface sample of an Alﬁsol (10.0 g kg–1 SOC) will differ from that in a 45- to 60-cm subsoil sample (1.8 g kg–1 SOC) (Figure 1.4a). Thus, it is important to recognize that the behavior of a substance in the soil environment, which is often inferred from ex situ studies, is dependent upon the location in the soil proﬁle from which the soil sample originated. The variability of surface soil characteristics on the landscape may also be extensive, even within soil mapping units. Consider the 10.5-ha production cotton ﬁeld located at the Milan Agricultural Experiment Station in Milan, TN. The ﬁeld consists of several soil series (Figure 1.5); however, the soils are loessal and generally differ from one another only with respect to the depth to fragipan (ranging from 60 to 150+ cm) and the degree to which the fragipans have developed. Thus, the chemical characteristics of surface 15-cm samples are not expected to be inﬂuenced by slight changes in the soil classiﬁcation (the chemical properties in the ﬁeld soil are expected to be relatively homogeneous). Discrete (grid cell) and intensive sampling of this ﬁeld has resulted in 235 composite surface (0 to 15 cm) soil samples, each composite sample representing a grid cell deﬁned by an imaginary grid (Figure 1.5). The samples were then subjected to standard chemical characterization, including pH, Mehlich-3-extractable elements (Mehlich-3 is a pH 2 extracting solution that contains ammonium ﬂuoride, the synthetic chelate EDTA, acetic acid, and nitric acid and is commonly employed to predict plant response to applied fertilizers and plant available trace elements), SOC, and effective cation exchange capacity (ECEC = sum of exchangeable Ca2+, Mg2+, K+, Na+, Mn2+, and Al3+). These data will be examined in the following sections. The Soil Chemical Environment: An Overview

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