Soil Environmental Chemistry Chapter 15

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

• Inorganics (gt 95) minerals
• O, Si, Al, Fe, Ca, Na, K, Mn, trace heavy metals
• Organics (lt 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

Solid
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-, -CO,
  Cl-, SO42-, HCO3-, OH-

5
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
• lt 1 g/mL for organic matter, gt 5 g/mL for some
  metals oxides, gt 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 lt4), 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 gt10)
• Origin of soil acidity
• FeS2 7/2O2 H2O ? Fe2 2H 2SO42-
• Adjustment of acidic soil with lime
• Soil(H)2 CaCO3 ? SoilCa2 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 (DTPAdiethylenetriaminepent
  aacetic acid)

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 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 11
  or 21)
• Kaolinite, Al2Si2O5(OH)4? 11
• Montmorillonite, Al2(OH)2Si4O10 ? 21
• Illite, K0-2Al4(Si8-6Al0-2)O20(OH)4 ? 21
• Hydroxides
• Fe2O3nH2O, 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
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 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
• lgCu2 9.2 - 2pH
• lgCd2 14.3 - 2pH
• lgZn2 11.65-2pH
• lgPb2 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 ContaminationUse 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 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
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
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