Soil Environmental Chemistry Chapter 15 & 16*

1. Soil Environmental ChemistryChapter 15 & 16* • Why soil environmental chemistry is important? • The place where food and energy source are produced (agriculture/mining) • The dumping ground of municipal refuse, hazardous waste (landfill) and radioactive waste (Section 15.21) • Source of drinking water (groundwater) (Section 15.9) • Soil pollution is related to air pollution and water quality (Section 15.19, 15.20) ***Skip Sections 15.3 -15.5, 15.10 - 15.17 & Sections 16.9-16.11

2. Scope of Soil Chemistry • Geosphere, or solid earth, is that part of earth upon which humans live and from which they extract most of their food, minerals, and fuels • Lithosphere is part of the geosphere that is directly involved with environmental processes through contact with the atmosphere, the hydrosphere, and living things (p. 2-3) • Sediments: dissolved load (1/4) + suspended load (2/3) + bed load (1/12) (Section 15.6) • Soil chemistry, sediment (aquatic) chemistry and geochemistry (Section 15.8) are related

3. Composition of Soil Solid • Inorganics (> 95%): minerals • O, Si, Al, Fe, Ca, Na, K, Mn, trace heavy metals • Organics (< 5%) • Protein, fat, CH2O (10-15% of soil organics) • Humus (85-90% of soil organics) • Pesticide, PAH (trace contaminants) • Water (soil solution) (p. 483) • Cation, anions, ions in hydrolyzed / complexed form • Air (35% of soil volume, =0.35) • 21% O2, 0.03% CO2 Liquid Gas

4. Important Soil Properties • Physical properties • Particle size • Density & porosity • Texture (clay, silt, sand) • Permeability (hydraulic conductivity) • Chemical properties • Total vs. extractable elements • CEC and soil charge (soil is commonly considered to have negative charges) • Soil pH, organic matter • Soil inorganic ions and chelates (functional groups) in soil organics: NH2, -OH, -COO-, -C=O, Cl-, SO42-, HCO3-, OH-

5. Sand Silt Fine Coarse Clay Gravel 2 m 20 m 200 m 2.0 mm Soil Non-soil Soil Particle Size Soil particle size classification according to the International Society of Soil Science

6. Soil Density & Porosity • Soil particle density • Density of individual particles • < 1 g/mL for organic matter, > 5 g/mL for some metals oxides, > 7 g/mL for metal sulfide; average 2.5 ~ 2.8 g/mL • Bulk density • Include the pore spaces between particles • Smaller than particle density; average 1.2 ~1.8 g/mL • Porosity • Pore space (%) = 100 - (bulk density/particle density)*100 • Example: A silt loam soil with particle density = 2.65 and bulk density = 1.5  Pore space = 100- (1.5/2.65)*100 = 43%

7. Soil CEC (Cation Exchange Capacity) • Origins • CEC of clay minerals is due to the presence of negatively charged sites on the minerals • CEC of organic matters is due to the presence of carboxylate group and other functional groups • Typical soil CEC = 10 - 30 meq/100 g soil

8. Soil pH (Section 16.3) • Terminology commonly used to describe the acid-base status of soils: • Strongly acid (pH <4), moderately acid (pH 4-5), slightly acid (5-6), neutral (pH 6-8), slightly alkaline (pH 8-9), moderately alkaline (pH 9-10), strongly alkaline (pH >10) • Origin of soil acidity • FeS2 + 7/2O2 + H2O  Fe2+ + 2H+ + 2SO42- • Adjustment of acidic soil with lime • Soil}(H+)2 + CaCO3  Soil}Ca2+ + CO2 + H2O • Adjustment of alkaline soils by Al or Fe sulfate • 2Fe3+ + 3SO42- + H2O  2H+ + SO42-

9. Total Elements • The composition of major elements (%) and minor elements (mg/kg) of the mineral component in soils Most common elements in soil: O, Si, Al, Fe, Ca, Na, K, Mg

10. Bioavailable Elements • Except for geological time, the “insoluble fraction” of total elements will not play a significant role with respect to plant growth or in terms of most environmental processes • The “bioavailable” or “extractable” elements is the portion of the total element that can take part in a range of chemical and biological reactions • Percentage (%) of total metal extracted from soil using two extractants (DTPA=diethylenetriaminepentaacetic acid)

11. Macronutrients C,H,O  from atmosphere N, P, K  from fertilizer Ca Mg S Micronutrients B Cl Cu Fe Mn Mo Macronutrients vs. Micronutrients (Sections 16.4-16.7)

12. Soil Organic Matter (OM) • Major classes of soil OM (Table 16.1, p.481) • Humus (humic acid, fulvic acid, and humin) (p. 482) • Fats, resin, and waxes • Saccharides • N-containing organics • Phosphorus compounds

13. Soil Minerals (Inorganic Fractions) • Primary minerals (rock-forming minerals)(Table 15.1, p. 434) • Silicates, oxides, carbonates, sulfides, sulfates, halides, native elements • Secondary minerals  Clay (Section 15.7) • Secondary minerals are formed by alteration of parent mineral matter. Clays are silicate minerals, usually containing Al, are one of the most significant classes of secondary minerals

14. Soil Minerals (Inorganic Fractions) • Clays A group of microcrystalline secondary minerals consisting of hydrous aluminum silicates that have sheet-like structure (Si4+-O tetrahedral sheet : Al3+-O octahedral sheet = 1:1 or 2:1) • Kaolinite, Al2Si2O5(OH)4 1:1 • Montmorillonite, Al2(OH)2Si4O10 2:1 • Illite, K0-2Al4(Si8-6Al0-2)O20(OH)4 2:1 • Hydroxides • Fe2O3·nH2O, 2Fe2O3 ·H2O, Fe2O3 ·H2O • Al2O3 ·H2O, Al2O3 ·3H2O • SiO2 ·nH2O

15. Soil Clay (Sections 15.7; 5.5) • Structure (p. 445) • Tetrahedral sheet (Si-4O) • Octahedral sheet (Al-6O) • Importance of clay • Holding water • Protect plant nutrient from leaching (Ca2+, K+, Mg2+) (soil clay is negatively charged due to ion replacement of Si4+ and Al3+ by metal ions of similar size but less charge): [SiO2] + Al3+ [AlO2-] + Si4+ (p. 131)  the reason why soil has cation exchange capacity (CEC) • Can be a pollutant carrier in water (e.g., clay adsorbs metals)

16. Soil Pollution • Major soil pollutants • Heavy metals • Pesticides • Fertilizers (N, P) • Major sources • Pesticides & fertilizers • Solid waste & sludge disposal • Wastewater irrigation

17. Heavy metals Pesticides Redox Hydrolysis Acid-Base reaction Complexation/chelation Precipitation Sorption Biological degradation Physical process (volatilization) Photochemical processes Important Soil Environmental Processes

18. Soil Chemistry of Metals: Mercury (Hg) • Redox • 2Hg+ == Hg2+ + Hg0 • Precipitation • Hg2+ HgS (reduced) • Adsorption • Cationic Hg2+ • Anionic HgCl3-, HgCl42- • Biological • Methylation to form Hg(CH3)2

19. Soil Chemistry of Metals: Cd, Pb, Cr • Cd • Water soluble Cd: pH , concentration  • Adsorbed Cd: pH , adsorption  • Insoluble Cd: CdS cab be formed in reduced environment • Pb (Most Pb in plant from air-borne Pb (gasoline) • Insoluble Pb (PbCO3, Pb3(PO4)2, PbSO4): pH , concentration  (acidic pH will release Pb) • Chelation of Pb with chelates in soil • Cr • Cr3+ can be strongly adsorbed on soil • Anionic Cr (i.e, Cr6+ in the form of Cr2O72- and CrO42-)exist only in weak acid/basic condition

20. Effects of pH on Cu, Cd, Zn, Pb • Reactions • Cu(OH)2 == Cu2+ + 2OH- Ksp = 1.6x10-19 • Cd(OH)2 == Cd2+ + 2OH- Ksp = 2x10-14 • Zn(OH)2 == Zn2+ + 2OH- Ksp = 4.5x10-17 • Pb(OH)2 == Pb2+ + 2OH- Ksp = 4.2x10-15 • Relationship between metal concentration and pH • lg[Cu2+] = 9.2 - 2pH • lg[Cd2+] = 14.3 - 2pH • lg[Zn2+] = 11.65-2pH • lg[Pb2+] = 13.62 -2pH

21. Soil Chemistry of Pesticides • Adsorption • Volatilization • Leaching & solubility • Degradation (p. 496) • Biodegradation • Photochemical degradation • Chemical degradation (hydrolysis)

22. Remediate of Soil Metal Contamination:Use of Lime • In certain pH range, increased pH will reduce soluble metal concentrations • use of limestone to reduce soluble metal concentration and therefore the toxicity to plants • In some cases, further increase in pH will increase metal concentration in soil solution (why?)

23. Remediation of Soil Pollution • Bioremediation • In-situ or Ex-situ • Natural attenuation • Use of self purification capacity • Slow, inexpensive • On-going studies • Phytoremediation • Composting • Slurry reactors

24. Bioremediation • Process by which organic hazardous materials are biologically degraded, usually to innocuous materials such as carbon dioxide, water, inorganic salts and biomass (biotransformation and mineralization)

25. Bioremediation Market Assessment • 100 million tons of hazardous waste generate annually • One third of over 2 million gasoline UST’s are leaking • Over 50,000 historically contaminated sites • All federal installations require extensive remediation action • Estimated cost of $1,700,000,000,000 • EPA consider bioremediation the lowest cost treatment where applicable

26. When Does Biodegradation Occur? • When proper conditions exist • When appropriate metabolic activity is expressed • When there is “contact” between contaminants, nutrients, and organisms • When toxicity or preferential utilization does not occur

27. Natural Attenuation • Natural assimilative capacity • Process by which the indigenous microflora degrades contaminants using ambient levels of nutrients and electron acceptors

28. Phytoremediation • Process by which inorganic and organic contaminants are uptaken by vegetation (plants) from contaminated soils. Plants are then removed by biomass (p.492)

29. Supplemental Water Recycled water (optional) Tumbling Barrel Soil Washing Unit Contaminated Soil Sieve Sievings Washed Soil 1. Nutrients 2. HCl 3. NaOH 4. Compressed Air 5. Air-Lifter 6. Air Diffusers 7. Bottom Rakes 8. Foam Breaker pH 1 2 3 4 Soil Wash Coagulant (CaCl2) 5 8 6 7 Effluent Holding Tank Slurry Tank Emico Slurry Reactors Effluent Solids Soil Slurry Reactor (Zhang et al., 2000)

30. Environmental Chemistry • The study of the sources, reactions, transport, effects, and fates of chemical species in water, soil, air, and living environments, and the effects of technology thereon