Soil / Groundwater Pollution & Remediation 1 Soil Environmental Chemistry Chapter 15 & 16* Why soil environmental chemistry is important? The place where.
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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 16.9-16.11Scope of Soil ChemistryGeosphere, or solid earth, is that part of earth upon which humans live and from which they extract most of their food, minerals, and fuelsLithosphere 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 relatedComposition of SoilInorganics (> 95%): mineralsO, Si, Al, Fe, Ca, Na, K, Mn, trace heavy metalsOrganics (< 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 formAir (35% of soil volume, =0.35)21% O2, 0.03% CO2SolidLiquidGasImportant Soil PropertiesPhysical propertiesParticle sizeDensity & porosityTexture (clay, silt, sand)Permeability (hydraulic conductivity)Chemical propertiesTotal vs. extractable elementsCEC and soil charge (soil is commonly considered to have negative charges)Soil pH, organic matterSoil inorganic ions and chelates (functional groups) in soil organics: NH2, -OH, -COO-, -C=O, Cl-, SO42-, HCO3-, OH- Soil Particle SizeSoil particle size classification according to the International Society of Soil ScienceSoil Density & PorositySoil particle densityDensity 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/mLBulk densityInclude the pore spaces between particlesSmaller than particle density; average 1.2 ~1.8 g/mLPorosityPore space (%) = 100 - (bulk density/particle density)*100Example: A silt loam soil with particle density = 2.65 and bulk density = 1.5 Pore space = 100- (1.5/2.65)*100 = 43%Soil CEC (Cation Exchange Capacity)OriginsCEC of clay minerals is due to the presence of negatively charged sites on the mineralsCEC of organic matters is due to the presence of carboxylate group and other functional groupsTypical soil CEC = 10 - 30 meq/100 g soilSoil pH (Section 16.3)Terminology commonly used to describe the acid-base status of soils:Strongly acid (pH 10)Origin of soil acidityFeS2 + 7/2O2 + H2O Fe2+ + 2H+ + 2SO42-Adjustment of acidic soil with limeSoil}(H+)2 + CaCO3 Soil}Ca2+ + CO2 + H2OAdjustment of alkaline soils by Al or Fe sulfate2Fe3+ + 3SO42- + H2O 2H+ + SO42-Total ElementsThe composition of major elements (%) and minor elements (mg/kg) of the mineral component in soilsMost common elements in soil: O, Si, Al, Fe, Ca, Na, K, MgMajor elements%Minor elementsmg/kgSiAlFeTiCaMgKNa30-452.4-7.41.2-4.30.3-0.70.01-3.90.01-1.60.2-2.5trace-1.5ZnCuNiMnCoCrPbAs10-2505-1520-30~4001-2010-501-501-20Bioavailable ElementsExcept 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 processesThe bioavailable or extractable elements is the portion of the total element that can take part in a range of chemical and biological reactionsPercentage (%) of total metal extracted from soil using two extractants (DTPA=diethylenetriaminepentaacetic acid)ExtractantCoNiCuPbCrNH4OAc (pH 7)DTPA0.361.30.862.71.15.00.518.70.370.11McLeod and vanLoon (1981), Ontario Geography, 17, 91-104Macronutrients vs. Micronutrients (Sections 16.4-16.7)MacronutrientsC,H,O from atmosphereN, P, K from fertilizerCaMgSMicronutrientsBClCuFeMnMoSoil 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 waxesSaccharidesN-containing organicsPhosphorus compoundsSoil TypeOrganic Matter ContentTemperature agricultural soilsTropical agricultural soilsForest soils (surface horizons)Peat soils1 5%0.1 2%>10%>20%Soil Minerals (Inorganic Fractions)Primary minerals (rock-forming minerals)(Table 15.1, p. 434)Silicates, oxides, carbonates, sulfides, sulfates, halides, native elementsSecondary 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 mineralsSoil Minerals (Inorganic Fractions)ClaysA 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 1:1Montmorillonite, Al2(OH)2Si4O10 2:1Illite, K0-2Al4(Si8-6Al0-2)O20(OH)4 2:1HydroxidesFe2O3nH2O, 2Fe2O3 H2O, Fe2O3 H2OAl2O3 H2O, Al2O3 3H2OSiO2 nH2OSoil Clay (Sections 15.7; 5.5)Structure (p. 445)Tetrahedral sheet (Si-4O)Octahedral sheet (Al-6O)Importance of clayHolding 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)Soil PollutionMajor soil pollutantsHeavy metalsPesticidesFertilizers (N, P)Major sourcesPesticides & fertilizersSolid waste & sludge disposalWastewater irrigationImportant Soil Environmental ProcessesHeavy metals PesticidesRedoxHydrolysisAcid-Base reactionComplexation/chelationPrecipitationSorptionBiological degradationPhysical process (volatilization)Photochemical processesSoil Chemistry of Metals: Mercury (Hg)Redox2Hg+ == Hg2+ + Hg0PrecipitationHg2+ HgS (reduced) AdsorptionCationic Hg2+Anionic HgCl3-, HgCl42-BiologicalMethylation to form Hg(CH3)2 Soil Chemistry of Metals: Cd, Pb, CrCdWater soluble Cd: pH , concentration Adsorbed Cd: pH , adsorption Insoluble Cd: CdS cab be formed in reduced environmentPb (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 soilCrCr3+ can be strongly adsorbed on soilAnionic Cr (i.e, Cr6+ in the form of Cr2O72- and CrO42-) exist only in weak acid/basic conditionEffects of pH on Cu, Cd, Zn, PbReactionsCu(OH)2 == Cu2+ + 2OH-Ksp = 1.6x10-19Cd(OH)2 == Cd2+ + 2OH-Ksp = 2x10-14Zn(OH)2 == Zn2+ + 2OH-Ksp = 4.5x10-17Pb(OH)2 == Pb2+ + 2OH-Ksp = 4.2x10-15Relationship between metal concentration and pHlg[Cu2+] = 9.2 - 2pHlg[Cd2+] = 14.3 - 2pHlg[Zn2+] = 11.65-2pHlg[Pb2+] = 13.62 -2pHSoil Chemistry of PesticidesAdsorption VolatilizationLeaching & solubilityDegradation (p. 496)BiodegradationPhotochemical degradationChemical degradation (hydrolysis)Remediate of Soil Metal Contamination:Use of Lime In certain pH range, increased pH will reduce soluble metal concentrationsuse of limestone to reduce soluble metal concentration and therefore the toxicity to plantsIn some cases, further increase in pH will increase metal concentration in soil solution (why?)Remediation of Soil PollutionBioremediationIn-situ or Ex-situNatural attenuationUse of self purification capacitySlow, inexpensiveOn-going studiesPhytoremediationCompostingSlurry reactorsBioremediationProcess by which organic hazardous materials are biologically degraded, usually to innocuous materials such as carbon dioxide, water, inorganic salts and biomass (biotransformation and mineralization)Bioremediation Market Assessment100 million tons of hazardous waste generate annuallyOne third of over 2 million gasoline USTs are leakingOver 50,000 historically contaminated sitesAll federal installations require extensive remediation actionEstimated cost of $1,700,000,000,000EPA consider bioremediation the lowest cost treatment where applicableWhen Does Biodegradation Occur?When proper conditions existWhen appropriate metabolic activity is expressedWhen there is contact between contaminants, nutrients, and organismsWhen toxicity or preferential utilization does not occurNatural AttenuationNatural assimilative capacityProcess by which the indigenous microflora degrades contaminants using ambient levels of nutrients and electron acceptorsPhytoremediationProcess by which inorganic and organic contaminants are uptaken by vegetation (plants) from contaminated soils. Plants are then removed by biomass (p.492)Soil Slurry Reactor (Zhang et al., 2000)Environmental ChemistryThe study of the sources, reactions, transport, effects, and fates of chemical species in water, soil, air, and living environments, and the effects of technology thereonGeneralSoil / Groundwater Pollution & RemediationGeneralSoil / Groundwater Pollution & Remediation
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