Activity Coefficient & Cation Adsorption

Soil Chemistry Notes

Fundamentals of Soil Chemistry

• Soil constituents consist of O (46.5%), Si (27.5%), Al (8%), Fe (4.5%), C, Ca (3.5%), Na (3%), K (2.5%), Mg (2%), and Ti (0.4%).
• The solid phase includes rock and mineral fragments, primary minerals (silicates, quartz), secondary minerals (oxides, hydroxides, clay minerals), and organic material (biomass, humus).
• Primary minerals come from the parent material, such as silicate and quartz.
• Secondary minerals result from weathering, including oxides and clay minerals.

Aluminium Oxides

• Examples include Gibbsite (Al(OH)3), Boehmite (γ-AlO(OH)), Diaspore (α-AlO(OH)), Aluminium Oxide (Al2O3), and Pseudoboehmite (AlOOH).

Iron Oxides

• Examples include Goethite (α-FeOOH), Hematite (α-Fe2O3), Magnetite (Fe3O4), and Bernalite (Fe(OH)3).

Carbonates

• Examples include Calcite, Dolomite, and Magnesite.

Silicates (Primary)

• With Al: Feldspars (Orthiklas, Albit, Anothit), Micas (Muscovit, biotit), Pyroxenes (Augite), Amphibole (Hornblende)
• Without Al: Olivin, Quartz, SiO2

Silicates (Secondary)

• Clay Minerals (all with Al): Iliite, Vermiculite, Smectite

Main Processes

• Organic processes that influence components include uptake by plants/microorganisms, efflux from plant roots/microorganisms, and exudates from plant roots/microorganisms.
• Precipitation, dissolution, co-precipitation, co-dissolution, complexation, and chelation are key processes.
• Chelation is the formation of two or more separate bindings between a polydentate ligand and a single central atom, involving organic compounds called chelants.
• Ion exchange is a surface reaction where ions absorbed to the solid phase are replaced by ions of the same charge.
• Exchangeable cations: Soil minerals and soil organic matter (SOM) are predominantly negatively charged.
• The exchange equilibrium with major cations in soil solution is attempted to be reached.
• Major cations include Ca2+, Mg2+, K+, and Na+ (dominate at moderate to high pH).
• Acidic cations include Al3+, Fe3+, Mn2+, and H+ (dominate at low pH).
• Extraction (exchange) occurs by neutral salt solution containing relatively high concentrations of a competitive cation (Ba2+, NH4+).
• In Austria, 0.1 M BaCl2 extraction for 2 hours (end-over-end shaking), filtration and measurement of major cations in the filtrate uses a 100 mL solution to 5g soil ratio.

Cation Exchange Capacity (CEC)

• The process involves washing back the Ba2+ ions by adding 100 mL 0.2 M HCl to the soil remaining in the filter, shaking for 2 hours, filtering, and measuring Ba in the filtrate.
• CEC measures the negative surface charges available for cation sorption in soil.
• CEC is related to the specific surface area and depends on mineral composition and SOM content.
• CEC varies at soil pH ("effective") versus CEC obtained by buffers (pH 8.2) BaCl2 solution ("potential” CEC).
• The buffer solution consists of triethanolamine + 0.2 M HCl (pH > 6.5 to avoid Ca dissolution from carbonates).
• Base saturation is the percentage of "base" cations relative to CEC (%).
  • Significance of CEC is the measure of the buffer capacity of a soil for cations and its role in nutrient and pollutant binding.

Hydrolysis

• Hydrolysis is the cleavage of chemical bonds through the addition of water; the reverse reaction is condensation. Example: X-Y + H-OH <-> X-H + Y-OH (hydrolyses of a molecule X-Y)
• Through hydrolysis, cations (e.g., K, Ca, Mg, Fe) in a rock-framework are replaced by H+ protons, leading to framework instability and collapse.
• This relates to isomorphic substitution.
• Hydrolysis requires the presence of water, which serves as the main transporter for soluble nutrients in soils, subsequently raising the element concentration in solution

Properties

• Chemical speciation refers to the occurrence of different chemical forms (species) of an element within the solid, soil solution, and soil air phases.
• An element in soil solution may exist as a "free" cation, various hydrolysis species, inorganic ion complexes, ion pairs, or organic complexes.
• Chemical speciation affects diffusion coefficients, reactions with charged soil surfaces (sorption), and selective uptake through biological membranes.
• Speciation can affect element transport, leaching, buffering, and uptake by plants and microorganisms (typically "free" ions are taken up most readily).
• Toxicity depends on the form of the element (e.g., "free" Al3+ is more toxic than organic complexes of Al).

Chemical Concentration

• Concentration is the abundance of a constituent divided by the total volume of a mixture.
• The activity of a material measures its effective concentration in a chemical reaction, which may differ from the actual concentration due to interactions (e.g., repulsion/attraction between ions).

Chemical Equilibrium

• It is reasonable to assume equilibrium for fast reactions such as most reactions in soil solution such as hydrolysis and formation of inorganic complexes; nonspecific sorption and cation exchange are typically also fast reactions for which the equilibrium assumption may approximately apply in many cases; however, soil is an open system
• Inputs and outputs can disturb the equilibrium, while precipitation/dissolution reactions are often kinetically limited and may therefore not reflect equilibrium conditions.
• Chemical equilibrium occurs when reactants and products are present at concentrations with no further tendency to change, with the forward reaction proceeding at the same rate as the reverse reaction.
• The most soluble mineral controls the concentration; the solution is supersaturated for the less soluble minerals, which will precipitate.

Mass Action Law

• Concentration-based constants vary with ionic composition/ionic strength; activity constants are real constants.

Chemical Activity

• Chemical activity refers to the "effective concentration" of a chemical species in a mixture, with the activity (a) treated as a dimensionless quantity influenced by its surroundings.
• Activities should be used to define equilibrium constants, but concentrations are often used instead.

Activity Coefficient

• The activity coefficient measures the reactivity at a given concentration.
• Activity coefficient (y) is the activity of substance A divided by the concentration of substance A (aA / cA).
• The coefficient (y) is not a constant; it varies due to the physico-chemical conditions.
  • Ranges between 1 (infinite dilution) and 0 (theoretically).

Debye-Hückel Equation

• Estimates activity coefficient. log γi = -AZi² √μ γi=10-AZi² √μ
• γi is the activity coefficient
• μ is the ionic strength,
• A is the solvent constant (=0.509 for H2O at 25°C),
• Zi is the valence of ion i.

Ionic Strength

• The ionic strength of a solution is a measure of the concentration of ions in that solution.
• Generally, multivalent ions contribute strongly to the ionic strength. μ = 1/2 ΣciZi²
• μ is the ionic strength,
• ci is the ion concentration i (mol L-1),
• Zi is the valence of the ion i.

Base Saturation

• Base saturation is defined as the percentage of "base" cations (Ca2+, Mg2+, K+, or Na+) in relation to CEC (%).
• Alternatively, it is the amount of positively charged ions (excluding H+ and Al3+) that are absorbed on the surface of soil particles.
• Base saturation is positively related to soil pH.

Sorption

• Desorption refers to the transfer of an ion from the solid phase to the soil solution and requires sufficient energy to overcome the bond energy.

Adsorption

• Adsorption is the transfer of an ion from the soil solution to the solid phase, forming a film of the adsorbate on the surface of the adsorbent.
• Cations are preferably adsorbed in temperate climate soils with negatively-charged clay minerals (e.g., illite, vermiculite) and organic matter.

Chemical Reaction vs. Ion Exchange

• Chemical reaction changes one set of chemical substances to another, changing electron positions. Ion exchange is a surface reaction where ions adsorbed to the solid phase are replaced by ions of the same charge.

Non-Specific vs. Specific Adsorption

• Non-specific sorption involves electrostatic (Coulombic) forces.
• Specific sorption involves covalent bonds (e.g., in a clay mineral).
• The equilibrium constant of adsorption reactions depends on the soil because it is not independent of soil minerals.
• Sorption can be determined for each soil through sorption envelopes.

Sorption Isotherm

• Describes the relation between the concentration of an ion/compound in the solid phase to that in the liquid phase (=soil solution).
• Freundlich equation: Q = A * Cn
• Langmuir equation: Q = (b * k * C)/(1 + k * C)

Chemical Reaction vs Sorption

• At equilibrium, chemical reactions can be treated using the mass action law, where the equilibrium constant K is independent of the composition of the system.
• Chemical reactions alter the structure of molecules, while sorption involves transfer of ions from/to the soil solution, creating an adsorbate film.
• Precipitation is the creation of a solid from a solution, with the solid formed called the 'precipitate'.
• Dissolution is the process by which gases, liquids, or solids enter into a liquid.

Species in Soil

• Free ions (anion, cation)
• Ion-pairs
  • Outer sphere complex created by ion impact from a cation and anion.
• Ion Complexes
  • Inner sphere complex and is an aggregate of an ion with one or more molecules

Free Ions

• Free ions are ions whose properties (spectrum and magnetic moment) as they are not significantly affected by other atoms/ions/molecules nearby.
• The ability of a solvent to form free ions dictates the reactivity of the propagating cationic chain.
• Nutrients must be dissolved as ions for plants to absorb them.
• The most readily available to plants and microorganisms

Key Species

• Redox species
• Hydrolosis species
• Organic Complexes

Fate of Elements

• Focus on nitrogen transformation:

1. Mineralization

 - Which converts organic nitrogen to ammonium nitrogen (N: NH; NO2)
      - Organic nitrogen is not directly taken up by plants but gradually transformed by soil microorganisms to ammonium (NH4).
      - Ammonium, a cation attracted to negatively charged clay minerals, is available to plants.

2. Nitrification

 - Which converts ammonium nitrogen to nitrate nitrogen (NH: NO3)
       - Ammonium transforms rapidly to nitrate (NO3 ) in drained, warm soils.
       - Nitrate is the principle nitrogen for plants
      - Nitrate leaches from soil as it is not attracted to clay minerals.

3. Immobilisation

• Nitrate or ammonium to organic nitrogen (NO3;NH: OrganicN)

4. Denitrification

• converts Nitrate nitrogen to gaseous nitrogen (NO3 => N2, N2O).
  • In anaerobic soils, microorganisms convert nitrate to nitrous oxide (N2O) and nitrogen gases (N2).
  • Resulting gases escape to the atmosphere and unavailable to plants
  • Process occurs within days in poorly aerated soil resulting in substantial loss of nitrate type fertilizers

5. Ammonia Volatilisation

• Ammonium nitrogen to ammonia gas (NH=> NH)
  • At high pH 7 5 large amts of NH4 converts to NH3 gas + volatilisation (NH 4 NH 3 H+)
  • Mitigation: to minimize these losses, solid ammonium type fertilizers, urea and anhydrous ammonia should be incorporated below the surface of a moist soil
• Phosphorus exists as phosphate in natural systems (PO4-3).
• Orthophosphate is H2PO4- in acidic and HPO42- in alkaline.
• Phosphorus is most soluble when at pH levels of 6 to 7.
• Soils with high pH (alkaline): Ca- and Mg-phosphates including apatite
• Soils with low pH (acidic): Phosphates adsorbed to clay or Al-/Fe oxyhydroxides e.g. variscite, strengite
• In soil, main phosphorus compounds include phytin (2-50%), phospholipids (1-5%), and nucleic acids (0.2-0.5%).

Arsenic

• Exists as Fe-(oxy-) hydroxides, As-V
• Range from 2 - 50%
• Low pH favors AsO2HO adsorption, higher pH favors AsOOH.
• Increased dissociation with increase of pH
• Exists mostly as arsenate (+V) species dominate (H2AsO4-is in acidic and HAsO42- in alkaline soils).
• In low redox soils (e.g., water logging): As-III (lower oxidation state) exists, as well as As-Fe-Sulfides Under reducing conditions, arsenite (+III) becomes stable, resulting in increased levels of elemental arsenic and arsine (III).
• Exists as Fe-arsenate (Fe3(AsO4)2).

Cadmium

• Solubility controlled by adsorption on clay minerals, hydroxides (alumnium/iron) and organic matter
• Unstable in small quantities, existing mostly as complexes with organic matter in the soil solid phase
• Exists mostly as:
  • Free Cd 2 ++( pH 3-8)
  • Cd(OH) species are not important
• In calcareous soils it exists as CdHCO 3 CdCO 3
• At higher chloride (Cl concentrations in salt affected soils, it exists as significant contribution of CdCI and CdCl 2
• Depending on the concentration and properties of soluble organic matter it forms soluble Cd SOM complexes
• Cadmium Solubility by soil pH: Like any cation you can expect that protons that are more active exchange Cd protons from surfaces,
• Low pH transfer Cd2+ into soil solution, proton is sorbed to negatively charged surfaces also increased by H+ releases Cd2+
• Where Luvisol is higher in Cd2+, lower om content causes less Cd2+ absorbed to the solid phase
• High om enhances higher solubility, this is the case if you have a high proportion of soluble om, mostly effect of solid matter is stronger

At higher chloride (Cl concentrations in salt affected soils it:

• When Cd forms complexes with for example Cl- the solubility gets higher because Cd2+ is removed from the solution, more Cd2+ can get dissolved, same with sulfate SO42-,
• High content of soluble organic matter (SOM) in soil forms also soluble complexes (Podsols)

For Cd normally presents:

• Cd normally present in traces, so normally there there no precipitation because there is never a too high conc.
• Free Cd2+ (hydrated) is the main chemical form- most common between ph 3-8

Soil Analysis

• Includes pH, Electrical conductivity, Exchangeable cations
• Soil minerals and SOM are negatively charged attempting approach to exchange equilibrium in major cations in soil solution Key cations include:
  • Ca2+, Mg2+, K+, Na+ (dominate at moderate to high pH)
  • Al3+, Fe3+, Mn2+, H+ (“acidic"cations, dominate at low pH)
• 0.1 M BaCl2 extraction for 2 hours (end-over-end shaking) to measure major cations in the filtrate, applying soil ratio 100 mL : 5g

Cation Exchange Capacity (CEC)

• CEC [meq/100g] estimates soils attract, retain and exchange cation elements which is calculated in millequivalents per 100 grams of soil.
• CEC measures negative surface charges available for cation sorption.
• High CEC values mean soil holds onto more soil requiring rates of fertilizer or lime to change since High CEC

Carbonate Content

• Soils contain carbonates of Ca and Mg (as calcite, CaCO3; dolomite, CaCO3*MgCO3) method of Scheibler(pressure calcimeter)

Method Of Scheibler

• Add diluted HCI to wetted soil and measure CO2 measuring device; measure volume of CO2 convert to mass equivalent of CaCO3 which = g CaCO3 equivalent kg-1 soil

• Reaction CaCO3 + 2H+ ↔ Ca2+ + CO2 + H2O Relevance

• Pedogenesis / soil formation (loss & redistribution of carbonates)

• Act as pH buffer (carbonate buffer system)

• Influence Dominance of Ca2+ on exchange complex

• Influence Implications for micronutrient availability, low micronutrient availability in calcareous soils

• Liming of required soils in optimal pH range if pH drops (below 5,5 to 6)

Other soil measures

• Measures for Total Carbon & Nitrogen
  • Measure and detect organic carbon in soil humus / organic matter using infrared spectrometry Calculation for total C in g C kg-1 soil
• Organic Carbon calculated from total C minus C in carbonates.
• Total SOM = organic C * 1.724 derived from content of C in SOM the greater of these are a measure of the significance of organic C and SOM

Soil Analysis measures

These focus on: Total nutrient and pollutant content

• Acid digestion of soil on a heating plate or block and measurement of the diluted andfiltered digests for measure of the elements of interest\ through acidic digestion with strong acids (e.g. Aqua regia, mixture of concentrated HCI and HNO3) measure filtrated and nutrients/pollutants using AAS (atomic absorption spectrometry) to measure.

Principles:

• Extract soils with water or a relatively week salt solutionFor micronutrients, use EDTA to account for the action of plant root exudates
• Pollutant concentrate can extract pollutants by relatively weak solutions, and assess soil toxicity.
• EDTA used because it acts as a Chelating agent for plant extraction

This quiz covers activity coefficients in solutions, focusing on their behavior under varying conditions and deviations from ideal behavior. It also explores cation adsorption in soils, including factors influencing preferential adsorption and the role of soil components. Includes cation exchange capacity.