Trace Elements and Compounds in Soil

Carbon dioxide-water systems play an important role in controlling the pH of alkaline and calcareous soils as well as adjusting solubility of most trace elements and their compounds. Carbon dioxide dissolves in water to form dissolved C02 and dissociated carbonic acid, H2CO30 ... 

Since most trace elements in soils are at parts per million levels, a separate compound may be not formed. Most likely, trace amounts of these trace elements and their compounds are adsorbed on the surfaces of clay minerals and various crystalline and amorphous Fe/Mn/Al oxides and hydroxides. Curtin and Smillie (1983) reported that the solubilities of Mn2+ and Zn2+ in limed soils were not consistent with the solubilities of any... 

Dissolved organic molecules have many acidic functions (hydroxol and carbonic groups) to complex trace elements and their compounds to form soluble chelates. This is one of the reasons why solubility and bioavailability of trace elements in the rhizosphere are higher than bulk soils. At the same time, many organic acids also directly dissolve trace elements and their compounds in soils. Plant-produced phytosiderophores facilitate elements, such as Fe and Zn, uptake by plants (Zhang et al., 1991 Romheld, 1991 Hopkins et al., 1998). However, Shenker et al. (2001) did not find significant uptake of the Cd-phytosiderophores complex by plant roots. 

Nickel is a silver-white, lustrous, hard, malleable, ductile, ferromagnetic metal that is relatively resistant to corrosion and is a fair conductor of heat and electricity. Nickel is a ubiquitous trace metal that occurs in soil, water, air, and in the biosphere. The average content in the earth s crust is about 0.008%. Nickel ore deposits are accumulations of nickel sulfide minerals (mostly pentlandite) and laterites. Nickel exists in five major forms elemental nickel and its alloys inorganic, water-soluble compounds (e.g., nickel chloride, nickel sulfate, and nickel nitrate) inorganic, water-insoluble compounds (e.g., nickel carbonate, nickel sulfide, and nickel oxide) organic, water-insoluble compounds and nickel carbonyl Ni(CO). ... 

Trace element speciation analysis is a base for the description of forms and compounds in which they occur in soils. Other definitions are also used to define properties of trace elements in soils. In fact, the general term availability has been recently defined as geoavailability, geochemical availability, biogeochemical availability and bioavailability (Wolt 1994, Smith and Huyck 1999) ... 

The biomethylation of metals and metalloids in soils and sediments appears to be a widespread phenomenon. Owing to their altered physical properties like volatility and lipid solubility, the methylated compounds are important in the mobilization, transport, and bioaccumulation of trace elements in the environment. The methylation of Cr(II) by methyl-cobalamine appears to proceed by a homolytic pathway involving the transfer of a methyl radical. CH3Cr(H203) is the product of this reaction, but is rapidly cleaved under acidic conditions to give methane and Cr(III). 

Compounds of boron, such as boric acid (H3BO3) and borax (Na2B407, IOH2O), have also been used as insecticides or herbicides, but since borates are relatively non-toxic to animals and tend to be readily leached from the soil, their use is unlikely to produce any serious long-term problems. Since boron is a trace element which is in limited supply in uncontaminated soils, the use of its compounds as pesticides may even have beneficial nutritional effects, particularly where root crops are grown. 

Over 98% of the soil is made up of only eight major chemical elements, listed in Table 51, in order of decreasing abundance. The 90-odd others make up the remaining 2% many occur in the soil as secondary or minor elements, while a large number of still others are present in only very low, often trace, concentrations. Thus all soils contain main, minor, and trace elements combined into chemical compounds and aggregated into complex particles of varying shape, size, and chemical composition (see Textbox 8). 

Most primary and secondary minerals found in soil systems are barely soluble in the soil solution. The amount of mass from the bulk phase to hydrated ions in soil solution is negligible compared to the total mass of the solid phase. In arid and semi-arid soils, concentrations of most trace metals in soil solution may be controlled by their carbonates and to some extent by their hydroxides. Other than carbonates, trace elements in arid and semi-arid soils may also occur as sulfate, phosphate or siliceous compounds, or as a minor component adsorbed on the surface of various solid phase components. The solubility of carbonates, sulfates and other common minerals of trace elements in arid and semi-arid soils will be discussed in Chapter 5. Badawy et al. (2002) reported that in near neutral and alkaline soils representative of alluvial, desertic and calcareous soils of Egypt, the measured Pb2+ activities were undersaturated with regard to the solubility of... 

Soil redox also strongly affects solubility of the compounds of other trace elements in arid soils. Amrhein et al. (1993) found that Fe, Mn, Ni and V in an evaporation pond soil were more soluble under reducing conditions. Han and Banin (2000) reported that after one year of saturated incubation, the solubility of Fe, Mn, Co, V, Ni, Cu and Zn in two Israeli arid soils with 0.5-23% CaC03 increased, while the solubility of Cd decreased with time. During saturated incubation, soil pH in highly calcareous arid soil containing high content of carbonates decreased. In a loessial soil from Israel, Han and Banin (1996) reported that soil pH decreased from 8.0 to 7.0-7.4 over saturated incubation. With the decrease in Eh over incubation, the parameter pe+pH also decreased from initial values of 12-13.6 to 4 after initial 7-9 days of saturation incubation. 

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