Speciation soil solution

D. Hirsch and A. Banin, Cadmium speciation in. soil solutions. J. Environm. Qual. 19 366 (1990). 

In addition, dissolved organic carbon (DOC) is also an important soil solution solute affecting speciation and bioavailability of many trace elements in soil solution. Many trace elements and heavy metals complex with dissolved organic carbon. This is especially important in arid and semi-arid environments since high soil pH increases the solubility of organic molecules and accordingly increases concentrations of dissolved organic carbon in soil solution. 

In arid and semi-arid soils with a pH range of 6-9, free calcium is the major Ca speciation form in soil solution. When pH > 9.2-9.5, CaP04 becomes a major calcium species in soil solution of neutral and calcareous soils, especially when the activity of H2P04 is > 10 5 M (Lindsay, 1979), such as after phosphate fertilizers are used. Lindsay (1979) further pointed out that CaS04° contributes significantly to the total calcium in solution when S042- is > 10 M. 

Emmerich et al. (1982) reported that in soil solution of arid soils amended with sewage sludge, free Ni2+ ion accounted for 60-70% of the total Ni in soil solution. Sposito and Page (1984) indicated NiHC03+ and NiC03° were also important Ni speciations in alkaline and calcareous soils. 

In arid soil solution, Cd is predominately found in free Cd2+. However, when pH >7.5, Cd(OH)+ and Cd(OH)2° account for only a small percentage of the speciation (Lindsay, 1979). CdHC03+ is significant near pH 8.0. 

Figure 3.2. Changes of Cd speciation in soil solutions of a typical Israeli calcareous soil with pH 4-9 (after Hirsh and Banin, 1990, with permission from Soil Sci. Soc. Am)...

Organic complexed Cd is not important in arid soil solution. Hirsh and Banin (1990) observed 5-10% of Cd bound to organic ligands in Israeli arid soil solution. Emmerich et al. (1982) found that organic-Cd complexes constituted 1-4% of Cd in California arid soil solution. However, Villarroel et al. (1993) reported that in a California sludge-treated soil, Cd was mainly present in both free ion and organic complex forms (each accounted for 32-40% and 30-45% of total Cd in soil solution, respectively), followed by the chloride complexes (8-20%), S04-complex (3-10%), and P04-Cd complex (1.5-7.7%). The nitrate Cd complexes were the lowest. Cadmium activities and speciation is not significantly affected by P and N treatments. 

In addition to soil solution, speciation of trace elements in water of the Nahr-Ibrahim river valley of Lebanon was studied with the AQUACHEM model. The results indicate that a high percentage of Pb and Zn is present as carbonate species, but in low percentages in free hydrated ion species. Cadmium exhibits as a high percentage of a free hydrated Cd2+. 

FACTORS AFFECTING TRACE ELEMENT SPECIATION IN SOIL SOLUTION... 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

Trace element speciation in soil solution is affected by total metal concentrations in soils. Free Cu2+ activity increases with total Cu content in soils from Quebec and New York (Sauve et al., 1997). Total free Cu activity in soils could be predicted from total Cu content and soil pH ... 

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

Hirsh D., Banin A. Cadmium speciation in soil solutions. J Environ Qual 1990 19 366-372. 

The environmental standards based on total heavy metal concentration in the soil solution seem the most important criterion for the exposition of further compartments of the environment. The additional effects connected with metal speciation and complexations were not considered in the study. 

Koopmans GF, Groenenberg JE. Effects of soil oven-drying on concentrations and speciation of trace metals and dissolved organic matter in soil solution extracts of sandy soils. Geoderma 2011 161 147-158. 

Sauve S, Parker DR (2000) Chemical speciation of trace elements in soil solution. In Tabatabai MA and Sparks DL (eds) Chemical processes in soils. Soil Sci Soc Am Book Series no 8, Madison, Wisconsin... 

The mobility of metals in soil solutions is controlled by several processes (1) desorption or dissolution (rate depends on the solubility of metal-mineral form) (2) diffusion (depends on speciation of metal, soil oxidation/reduction potential, and pH) (3) sorption or precipitation (depends on soil solution concentration and rhi-zosphere effects) and (4) translocation in the plants (depends on plant species, soil solution concentration, and competing ions) (McBride... 

Tills and Alloway [113] investigated the speciation of lead in soil solution using a fractionation scheme, ion exchange chromatography and graphite furnace atomic absorption spectrophotometry. Soils from four sites were selected (Snertingdal in Norway, Pen Craig-ddu in Dyfed, Wales, Velvet Bottom... 

David, M. B., and C. T. Driscoll. 1984. Aluminum speciation and equilibria in soil solutions of a haplorthod in the Adirondack Mountains (New York USA). Geoderma 33 297-318. 

Birnessite (8-Mn02) Condensation of glucose and glycine under soil ambient conditions (measurement of optical density) yields of humic substances XANES study of change in speciation of Mn ESR study of Mn speciation in solution 13C CPMAS NMR spectra of FA fraction resembling spectra of natural FAs Jokic et al. (2001b)... 

Matthias, A., Maurer, M., and Parlar, H. (2003). Comparative aluminium speciation and quantification in soil solutions of two different forest ecosystems by 27A1NMR. Fresenius Environ. Bull. 12,1263-1275. 

Alvarez E, Perez A, Calvo R. 1993. Aluminum speciation in surface waters and soil solutions in areas of sulphide mineralization in Galicia (N.W. Spain). Sci Total Environ 133 17-37. 

Boudot J-P, Merlet D, Rouiller J, et al. 1994. Validation of an operational procedure for aluminum speciation in soil solutions and surface waters. Sci Total Environ 158 237-252. 

Gardiner PE, Schierl R, Kreutzer K. 1987. Aluminum speciation in soil solutions as studied by size exclusion chromatography. Plant Soil 103 151-154. 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

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