Speciation in soils

The existence of an element in different chemical forms in the gaseous, solid or aqueous solution phase provides the conceptual basis for speciation in soils. More particularly, a chemical species in soil refers either to a specific molecular arrangement of the atoms of an element or, quite often, to the result of an operational process of detection and quantitation aimed at elucidating chemical forms (Bernhard et at., 1986, pp. 7-14). In principle, the former definition should be the outcome of the latter, methodological definition. In practice, this connection is difficult to achieve in natural systems (Bernhard et al., 1986) (see Chapter 1 for a definition of speciation). Understanding speciation is important in assessing the availability of plant nutrients, plant uptake of potentially toxic elements (e.g. Al, Cd), and the movement of both nutrient and toxic substances into waterways or other parts of an ecosystem (Da Silva et al., 1991). 

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

A method for estimating the TSCF for equation 14.24 is given in Table 14.10. The root concentration factor is also defined in Table 14.10 as the ratio of the contaminant in the roots to the concentration dissolved in the soil water (pg/kg root per pg/L). This is important in estimating the mass of contaminant sorbed to roots in phytoremediation systems. The values of TSCF and RCF for metals depend on the metals redox states and chemical speciation in soil and groundwater. 

This paper discusses (1) soil and groundwater and (2) aquatic equilibrium and ranking models. The second category deals with the chemical speciation in soil and groundwater, and with the environmental rating of waste sites, in cases where detailed modeling is not desirable. 

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)...

FACTORS AFFECTING TRACE ELEMENT SPECIATION IN SOIL SOLUTION... 

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. 

Haswell, S.J., P. O Neill, and K. C Bancroft. 1985. Arsenic speciation in soil-pore waters from mineralized and unmineralized areas of south-west England. Talanta 32 69-72. 

Marin A, Lopez-Gonzalvez A, Barbas C. Development and validation of extraction methods for determination of zinc and arsenic speciation in soils using focused ultrasound application to heavy metal study in mud and soils. Anal. Chim. Acta 2001 442 305-318. 

Ure AM, Davidson CM. Chemical speciation in soils and related materials by selective chemical extraction. In Ure AM, Davidson CM (eds.), Chemical Speciation in the Environment, 2nd ed. Malden, MA Wiley-Blackwell Science, Inc. 2002, pp. 265-299. 

Scheinost AC, Kretzschmar R, Pflster S, Roberts DR. Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil. Environ. Sci. Technol. 2002 36 5021-5028. 

Bowell, R.J., Morley, N.H. and Din, V.K. (1994) Arsenic speciation in soil porewaters from the Ashanti mine, Ghana. Applied Geochemistry, 9(1), 15-22. 

Fodor and Fischer [84] have investigated problems of chromium speciation in soils. When employing spectrophotometric detection, only a method based on the diphenylcarbazide reaction was found suitable for chromium speciation analysis. 

Prokisch et al. [85] described a simple method for determining chromium speciation in soils. Separation of different chromium species was accomplished by the use of acidic activated aluminium oxide. Polarographic methods have been applied in speciation studies on chromium(VI) in soil extracts [86]. Mi-lacic et al. [88] have reviewed methods for the determination of chromium(VI) in soils. 

Marques [94] has reviewed literature on chromium speciation in soils. The determination of chromium is also discussed under Multi-Metal Analysis of Soils in Sect. 2.55. 

Solomon, D., Lehmann, J., Kinyangi, J., Liang, B., and Schafer, T. (2005a). Carbon K-edge NEXAFS and FTIR-ATR spectroscopic investigation of organic carbon speciation in soils. Soil Sci. Soc. Am. I. 69,107-119. 

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. 

Keizer, M.G. (1991) ECO SAT A Computer Program for the Calculation of Speciation in Soil-Water Systems. Department of Soil Science and Plant Nutrition, Agricultural University, Wageningen, The Netherlands. 

Sposito, G. and Coves, J. (1988) SOILCHEM A Computer Program for the Calculation of Chemical Speciation in Soils. The Kearny Foundation of Soil Science, University of California, CA. 

In this chapter, we shall introduce soil speciation concepts by consideration of inorganic trace elements in dissolved and adsorbed forms, with reference to both their molecular speciation and their operationally defined soil component speciation. We shall then consider the implications of chemical speciation in soils for agriculture and soil pollution. 

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