Aqueous Geochemistry PPT PDF

An Overview of Aqueous Geochemistry Abhijit Mukherjee, Ph.D. IIT Kharagpur Chemical definition of water Water (pure) = H2O Natural water (surface/groundwater) = H2O + Solute (dissolved mass) Solute can occur in the subsurface as a solid...

An Overview of Aqueous Geochemistry Abhijit Mukherjee, Ph.D. IIT Kharagpur Chemical definition of water Water (pure) = H2O Natural water (surface/groundwater) = H2O + Solute (dissolved mass) Solute can occur in the subsurface as a solid or amorphous phase gas liquid IIT Kharagpur 2  Dissolved constituents in groundwater Most inorganic solutes are electrolytes, which dissolve to form cations and anions (e.g., NaCl → Na+ + Cl−) Can also have complex ions⎯combinations of simpler cations and anions (e.g., H+ + HCO3− → H2CO3º) Non-electrolytes⎯non-ionic compounds that dissolve to form molecules (e.g., O2 [aq], Trichloroethylene (TCE) ) ✔ Electrolytes are relatively soluble and non-electrolytes are relatively insoluble in water IIT Kharagpur 3  Polar vs. non-polar molecules Polar: charge-balanced molecule, but H+ H+ charge is asymmetrically distributed O2− Non-polar: charge-balanced and charge is H relatively evenly distributed H C H H IIT Kharagpur 4  Dissolved constituents in groundwater Examples of dissolution: minerals, organic liquids Polar -Halite dissolution: NaCl = Na+ + Cl- -Calcite dissolution: CaCO3 + H+ = Ca2+ + HCO3- Non-Polar -Trichloroethylene (TCE) dissolution: TCE = TCEaq IIT Kharagpur 5 TDS and water classification TDS: Total Dissolved Solid Fresh water: sufficiently dilute to potable (TDS < 1,000 mg/L) Brackish water: salty, non potable, but less than sea water (TDS: 1,000 to 20,000 mg/L) Saline water: similar to sea water (TDS ~ 35,000 mg/L) Brines: more saline than sea water (TDS >> 35,000 mg/L) IIT Kharagpur 6 Non-aqueous phase liquids (NAPL) NAPLs are typically non-polar organic compounds that are sparingly soluble in water NAPLs can occur as solutes, pools of liquid, or “blobs” DNAPL: NAPL that’s denser than water (examples: coal tar, creosote, chlorinated solvents) LNAPL: NAPL that’s lighter than water (examples: benzene, toluene, xylenes [gasoline constituents]) IIT Kharagpur 7  Direction of groundwater flow Presence of NAPL in Aquifer 8 Defining Concentration 1. Molar concentration: Number of moles of a species per liter of solution (mol/L) mole: formula weight of a substance in grams 2.Molal concentration Number of moles per kilogram of solvent (mol/Kg) IIT Kharagpur Defining Concentration 3. Equivalent charge: number of equivalent charges of an ion per liter of solution (units: eq/L, meq/L) Equivalent charge = number of moles of an ion multiplied by the absolute value of the charge: Example: 1 M Na+ equals 1 eq/L 1 M Ca2+ equals 2 eq/L 4. Mass per unit mass Mass of a species or element per total mass of the system (ppm, ppb, mg/kg, ug/kg) 5. Mass per unit volume (most common): Mass of a solute dissolved in a unit volume of solution units: mg/L, μg/L) 1 ppm = 1 mg/kg = 1 mg/L IIT Kharagpur  Defining Concentration: conversions Conversion between mg/L to molar concentration: mg/L x 10-3 molarity ___________________________ = formula weight Conversion between mg/L to meq/L: Meq/L = mg/L ___________________________ formula weight/ charge IIT Kharagpur  Use of Stable Isotopes Isotopes are- Unstable: isotopes that continuously and spontaneously break down/decay in other lower atomic weight isotopes (e.g. radio-isotopes) Stable: isotopes that do not naturally decay but can exist in natural materials in differing proportions IIT Kharagpur 12  IIT Kharagpur 13  Stable Isotopic fractionation 14  Ranges of δ2H values in natural systems D = δ2H IIT Kharagpur 15  Ranges of δ13C values in natural systems IIT Kharagpur 16 Fractionation of δ18O along precipitation path 17 Fractionation of δ18O because of recharge from moving monsoon wind: example from West Bengal In West Bengal δ18O shows prominent continental (latitudinal) effect along direction of movement of monsoon wind… Direction of monsoon wind δ18O (‰ VSMOW) N Bay of Bengal S 18 IIT Kharagpur  Fractionation of δ34S in redox reactions: example from West Bengal IIT Kharagpur 19 Water Quality and Analyses IIT Kharagpur Types of Water Analyses Inorganic solutes Organic solids Organic liquids Gases Oxygen carbon dioxide hydrogen sulfide, methane IIT Kharagpur 21  Classification of Relative Abundance of Dissolved Solutes in Groundwater: Inorganic Major Minor Trace (< 0.1 mg/L) (> 5 mg/L) (.01 - 10.0 mg/L) Bicarbonate Boron Aluminum Platinum Copper Silicon Nitrate Arsenic Radium Gold Calcium Carbonate Barium Rubidium Lead Sodium Potassium Beryllium Silver Lithium Chloride Fluoride Bromide Cobalt Manganese Sulfate Strontium Cadmium Selenium Nickel Magnesium Iron Cerium Thorium Phosphate Carbonic Chromium Zinc Platinum Acid Tungsten Vanadium Uranium 22  Classification of Dissolved Solutes in Groundwater by depth: Organic Organic Compounds Organic Compounds (Shallow) (Deep) Humic Acid Acetate Fulvic acid Propionate Carbohydrates Methane Amino acids Tannis Lignins Hydrocarbons 23  Routine Water Analyses Routine: measuring concentration of standard set of most abundant constituents : Major constituents (mg/L) (except Silicon, Carbonic Acid) Minor constituents (mg/L) (except Boron, Strontium) pH Total Dissolved Solid or TDS (mg/L) Eh (oxidation-reduction potential; indicator of oxic conditions) (mV) Specific Conductance (SC): measure of sample’s ability to conduct electricity (μS/cm, μ/cm) IIT Kharagpur 24  Specialized Water Analyses Trace metals ( As, Mn, Cr, Cd, Pb, Zn) Radioisotopes Organic compounds Nitrogen-containing species (NO3-, NH4+) Gases Stable isotopes (δ18O, δ18H, 3H) Specialized analyses done for: Groundwater contamination problems Water quality assessment Research Regulatory issues IIT Kharagpur 25  Water Quality Standards Designed to protect public health by requiring that certain substances in water be less than certain limits Microorganisms Disinfection and disinfection byproducts Inorganic chemicals (e.g. As, Se, Hg) Organic chemicals (e.g. BTEX, TCE) Radionuclides (e.g. Ra, Po) IIT Kharagpur 26 Plotting Hydrochemical Data IIT Kharagpur  Plotting chemical data Presenting results of chemical analyses: Abundance or relative abundance 1. Collins Bar diagram 2. Stiff pattern diagram 3. Pie diagram 4. Piper diagram Abundance and patterns of change Graphical/illustrative type diagrams Statistics IIT Kharagpur 28  Plotting chemical data Presenting results of chemical analyses: Abundance or relative abundance 1. Collins Bar diagram 2. Stiff pattern diagram 3. Pie diagram 4. Piper diagram Abundance and patterns of change Graphical/illustrative type diagrams Statistics IIT Kharagpur 29 Sample 1 Sample 2 STIFF DIAGRAM Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 IIT Kharagpur 30  PIPER DIAGRAM IIT Kharagpur 31  IIT Kharagpur 32 Piper diagram showing the typical groundwater evolution in a carbonate aquifer IIT Kharagpur 33 Influence of dissolved sodium chloride on the Piper diagram 34 IIT Kharagpur  Example of groundwater chemistry from West Bengal 35 IIT Kharagpur Types of reactions of groundwater with aquifer material IIT Kharagpur 36  Definitions of equilibrium Rate of forward reaction (e.g., dissolution) = rate of backward reaction (e.g., precipitation); instantaneously fast For dissolution of a solid like NaCl, equilibrium means solute concentration = aqueous solubility of compound Σ (free energies of products) – Σ (free energies of reactants) = 0 Can have irreversible (non-equilibrium) reactions (e.g., radioactive decay, oxidation-reduction) where reactants are consumed Equilibrium constants Consider generic reaction of the form cC + dD = yY + zZ ▪ c, d, y, z are # of moles ▪ for a dissolution reaction, C and D are compounds and Y and Z are ions ▪ example: NaCl (s) = Na+ + Cl− Can define equilibrium constant as: K = (Y)y(Z)z/((C)c(D)d) = (Na+)(Cl−) for example above ΔGrº (Gibbs free energy of reaction) = −RT ln K ▪ thus K varies with temperature Equilibrium and activity More about equilibrium constant: if solution is dilute, ( ) = concentration; if not, ( ) = activity Activity accounts for non-ideal components in non-dilute solutions (electrostatic interactions in solution) Activity of a solid (and of water) is usually assumed = 1 (thus (NaCl) isn’t shown in the denominator in the previous example) Let (Y) = activity and [Y] = concentration of solute Y: (Y) = γY [Y], where γY is the activity coefficient for dilute solutions, γ = 1 Y  Conceptual models of activity Most commonly used: extended Debye-Hückel equation⎯ for ion i, log γi = −Azi2(I)0.5 1 + Båi2(I)0.5 ▪ A and B are constants that vary with temperature ▪ zi = ionic charge 2 ▪ I = ionic strength of solution = 0.5Σ(Mizi ), where M = molarity ▪ å = hydrated radius of ion (in cm) i Extended Debye-Hückel equation is valid to a maximum I of ~ 0.1 M (total dissolved solids of ~ 5000 mg/L) ▪ need specific-ion interaction model for more saline waters Saturation state Can define ion activity product in or out of equilibrium as IAP = (Y)y(Z)z/((C)c(D)d) Departure of a reaction from equilibrium is indicated by the ratio IAP/K; log (IAP/K) = saturation index (SI) For dissolution of a solid, if SI < 0 (IAP/K < 1), the solution is undersaturated with respect to the solid if reaction is written with reactant(s) on left-hand side and product(s) on right-hand side, then reaction goes to the right Saturation state Departure of a reaction from equilibrium is indicated by the ratio IAP/K; log (IAP/K) = saturation index (SI), where K is equilibrium constant Can define ion activity product in or out of equilibrium as IAP = (Y)y(Z)z/((C)c(D)d) IIT Kharagpur 42 More about saturation state If SI0 (IAP/K > 1), the solution is supersaturated with respect to the solid precipitation of solid or degassing of gas takes place EX: if a groundwater has precipitation of Iron Oxide then SI of Iron Oxide in that water is >0 IIT Kharagpur 43  More about saturation state Because of kinetic inhibitions, precipitation can be slow Precipitation can be induced by change in temperature or by introduction of “seeds” under isothermal conditions SI and γ values can be calculated from chemical analyses of waters by geochemical speciation models Kinetic reactions Consideration of reaction rates is necessary for irreversible reactions (like radioactive decay) for reactions that are slow relative to mass transport First-order kinetic reactions (like radioactive decay) are of the form dC/dt = −kC, where k is the reaction rate constant IIT Kharagpur 45 Acid-base reactions Involve H+ transfer in aqueous phase or between solution and solid Recall pH = −log (H+) pH is acidic if < 7, neutral if = 7, and basic if > 7 pH of groundwater is typically circum-neutral i.e. 6 to 8 pH is generally (but not exclusively) regulated by carbonate equilibria IIT Kharagpur 46 Common acid-base reactions Ionization of carbonic acid: H2CO3 = HCO3− + Η+ and HCO3− = CO32− + Η+ Calcite dissolution/precipitation: CaCO3 + Η+ = Ca2+ + HCO3− Silicate weathering: silicate + H+ = cations + H2SiO3 IIT Kharagpur 47 World-wide occurrence of high fluoride in leachable silica-rich rocks 48 Surface reactions Surface reactions include adsorption, ion exchange, and surface precipitation Partitioning of mass between a solution and a solid surface at a constant temperature is commonly described by an isotherm IIT Kharagpur 49 Surface reactions Surface reactions include adsorption, ion exchange, and surface precipitation Partitioning of mass between a solution and a solid surface at a constant temperature is commonly described by an isotherm of the form: S = (Ci – C) × V/Ms S is commonly (and simplistically) described by a linear isotherm: S = KdC (Domenico and Schwartz)  Sorption of organic compounds Non-polar organic solutes preferentially partition (or sorb) to solid organic matter (SOM), such as humic substances, kerogen, coal, and “black carbon” The partition coefficient is often proportional to the mass fraction of solid organic carbon: K = Kocfoc Koc tends to be specific to classes of organic solutes and to types of SOM  Ion exchange In phyllosilicate (clay) minerals, broken bonds on edges or cation substitution in crystal lattices can result in structural charge deficiencies electrical neutrality maintained by ion sorption (usually cations) Cation exchange involves desorption of one type of cation accompanied by adsorption of another example: Ca2+ + Na-clay = 2Na+ + Ca-clay IIT Kharagpur 53  Ion exchange (continued) Some clay minerals have a fixed structural charge 2:1 layer clays such as smectites (e.g., montmorillonite) and vermiculite have greatest cation exchange capacity sheets of Si-O tetrahedra “sandwich” Al-OH octahedra charge deficiency occurs when lower valence cation substitutes for Al or Si hydrated (exchangeable) cations reside in interlayers (Grim)  Reactions on oxide surfaces Metal (typically Fe, Mn, Al, and Si) oxides and oxyhydroxides have pH-dependent surface charge For metal X, the distribution of surface species is controlled by equilibrium reactions like XOH = H+ + XO− and XOH + H+ = XOH2+ Cations adsorb by reactions like M2+ + XO− = XOM+; oxyanions (e.g., SO42−) can also adsorb Zero point of charge = pH at which surface has no net charge (positively charged below zpc, negatively above) Example from Huhhot Basin, China: No Ion Exchange Example from West Bengal: Evidence of Ion Exchange 57  Oxidation-reduction (redox) reactions Redox reactions involve electron (e−) transfer and are mediated (catalyzed) by microorganisms Elements that have multiple oxidation states act as reductants (e− donors) and oxidants (e− acceptors) e− loss = oxidation (e.g., Fe2+ = Fe3+ + e−) e− gain = reduction (e.g., Fe3+ + e− = Fe2+) IIT Kharagpur 58 More about redox reactions Can define pe = – log (e−); EH (or Eh) = (2.3RT) × pe/F In practice, pe is a calculated quantity, while EH is the measured e− potential for an electrochemical cell Redox reactions are not usually at equilibrium and are essentially irreversible if equilibrium constants are large ▪ e.g., CH O + O → CO + H O 2 2 2 2 Values of pe or EH are constant when one couple dominates the other(s) ▪ because oxidation of organic matter consumes an insignificant amount of O2, EH for the O2/H2O couple remains constant  Effect of Redox Reactions on Iron and Arsenic 60  Model of Arsenic Mobilization in Bengal by Redox Reactions 61  More about redox reactions From thermodynamic considerations, an electrochemical evolution sequence can be defined with dominant oxidants, from most energetically favorable to least: ▪ O2 → H2O ▪ NO −→ N (denitrification) 3 2 ▪ NO3− → NH4+ (dissimilatory NO3− reduction) ▪ Mn+4 → Mn+2 +3 +2 ▪ Fe → Fe ▪ SO42−→ HS− ▪ HCO − → CH (methanogenesis) 3 4 IIT Kharagpur 62 Steps of Redox Reactions: Example from West Bengal 63 Nitrate Pollution by Redox Reactions: Example from Mid-USA 64 Chemical evolution along flow paths Major anion sequence seen by Chebotarev in Australia As travel time and distance increase, dominant anions go from HCO3− → {HCO3− + SO42−} → {SO42− + HCO3−} → {SO42− + Cl−} → {Cl− + SO42−} → Cl− Precipitation (rain or snow): extremely dilute (total dissolved solids of a few mg/L), pH 3 – 6, highly oxidizing Upper zone (local flow): active ground-water flushing; HCO3− produced by oxidation of SOM, root respiration, and carbonate dissolution; low TDS Chemical evolution (continued) Intermediate zone: SO42− may be dominant anion; can have Na-HCO3 hydrochemical facies, as in the Atlantic Coastal Plain Lower (regional) zone: travel times of 103 to 106 years; highly soluble minerals (like evaporites) persist because of minimal flushing; high TDS and Cl− (SO42− may have been reduced anaerobically) Hydrochemical (major ion) facies are controlled by mineral availability and solubility Because of cation exchange, can’t generalize about a cation evolution sequence Can have mixing of chemically distinct waters from different flow paths (e.g., regional and local paths meet in discharge zone)  Groundwater evolution along flow path 67