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Soil Environmental Chemistry Chapter 15 16

Soil / Groundwater Pollution

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Soil Environmental Chemistry Chapter 15 16

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    1. Soil / Groundwater Pollution & Remediation 1 Soil Environmental Chemistry Chapter 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. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 3 Composition of Soil 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

    4. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 5 Soil Particle Size

    6. Soil / Groundwater Pollution & Remediation 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 / Groundwater Pollution & Remediation 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 / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 9 Total Elements The composition of major elements (%) and minor elements (mg/kg) of the mineral component in soils

    10. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 11 Macronutrients vs. Micronutrients (Sections 16.4-16.7) Macronutrients C,H,O ? from atmosphere N, P, K ? from fertilizer Ca Mg S Micronutrients B Cl Cu Fe Mn Mo

    12. Soil / Groundwater Pollution & Remediation 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 / Groundwater Pollution & Remediation 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 / Groundwater Pollution & Remediation 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 Fe2O3nH2O, 2Fe2O3 H2O, Fe2O3 H2O Al2O3 H2O, Al2O3 3H2O SiO2 nH2O

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

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

    18. Soil / Groundwater Pollution & Remediation 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 / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 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 / Groundwater Pollution & Remediation 21 Soil Chemistry of Pesticides Adsorption Volatilization Leaching & solubility Degradation (p. 496) Biodegradation Photochemical degradation Chemical degradation (hydrolysis)

    22. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 25 Bioremediation Market Assessment 100 million tons of hazardous waste generate annually One third of over 2 million gasoline USTs 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. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 27 Natural Attenuation Natural assimilative capacity Process by which the indigenous microflora degrades contaminants using ambient levels of nutrients and electron acceptors

    28. Soil / Groundwater Pollution & Remediation 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. Soil / Groundwater Pollution & Remediation 29

    30. Soil / Groundwater Pollution & Remediation 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

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