Elements in soil

COMPARISON OF MICROWAVE ASSISTED EXTRACTION METHODS FOR THE DETERMINATION OF PLATINUM GROUP ELEMENTS IN SOIL SAMPLES BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY AFTER PHASE SEPARATION-EXTRACTION... 

The procedure followed describes methods for the determination of total levels, and in certain cases, available amounts of trace elements in soils. The determination of arsenic in soil by hydride generation AAS is included. 

Trace elements in soils and sludges (CRMs 141R-146R 597) pp 225-232 (1998k) Chlorobi-phenyls in sewage sludge (CRM 392) pp 240-243 (1998I) Trimethyl-lead in urban dust (CRM 605) pp 274-278 In Production of Certified Reference Materials for Pollutants in Environmental Matrices. European Commission Report EUR 18157 EN, CCF Academic Press, Tarbes. 

The enormous difference in certified values between methods and between analytes illustrates well how much care is needed in matrix/method matching. Further evidence of the importance of matrix matching is provided by an interlaboratory study on trace elements in soil reported by Maier et al. (1983) and the certification of a sewage sludge described by Maaskant et al. (1998). 

The primary sources of trace elements in soils are the parent materials from which soils are derived. These parent materials constitute the reserve for trace elements. Concentrations of trace elements in soils are directly dependent upon their abundance in the earth crust. In general, concentrations of most trace elements in global soils are from one third to three times those in the earth s crust. The logarithm ratios of their concentrations in the global soils over the earth s crust are in the range 0.5 (Fig. 2.2). 

This is especially important for trace elements in soils in undeveloped arid and semi-arid zones. In agricultural ecosystems, two more... 

TRACE ELEMENTS IN SOILS AND ROCKS A GLOBAL VIEW... 

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. 

CONCENTRATIONS OF TRACE ELEMENTS IN SOIL SOLUTION OF ARID ZONES... 

Table 3.2. Concentrations of trace elements in soil solutions of the California soils that received sludge applications. 

Table 3.6. The first hydrolysis constants of selected elements in soil chemistry3...

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. 

Concentrations of trace elements in soil solution may be controlled by the solubility of certain solid phases via dissolution/(co-)precipitation or by other physicochemical and biological processes such as adsorption-desorption, complexation, and redox reactions. 

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... 

Reduction-oxidation is one of the most important processes controlling solubility and speciation of trace elements in soils, especially for those elements with changeable values, such as Cr, As and Se. Within normal ranges of redox potentials and pH commonly found in soils, the two most important oxidation states for Cr are Cr(III) and Cr(VI). Cr(III) is the most stable form of chromium and less soluble and nontoxic, but Cr(VI) is mobile, soluble and toxic. The main aqueous species of Cr(III) are Cr3+, Cr(OH)2+, Cr(OH)3° and Cr(OH)4" and the major aqueous species of Cr(VI)... 

Table 4.1. Some selective sequential dissolution procedures employed to fractionate trace elements in soils... 

The effects of the pH of the NaOAc-HOAc solutions on the subsequent fractions are related to the partitioning patterns of elements in soils. Calcium and Cd in the calcareous soils are predominately present in the CARB fraction (Banin et al., 1990). Cadmium and Ca in the CARB fraction of the soils studied accounted for 40-50% and 75-99%, respectively. Even NaOAc-HOAc solutions at pH 7.0 extracted 3-6% and... 

Figure 4.5. Cumulative sums of extractable elements in soil J3 (CaC03 33.3%) in consecutive steps of the selective sequential dissolution procedure as a function of the cumulative steps. 1 CARB (carbonate) 2 CARB (carbonate) + ERO (easily reducible oxide) 3 CARB + ERO + OM (organic matter) 4 CARB + ERO + OM + RO (reducible oxide) (after Han and Banin, 1995. Reprinted from Cornmun Soil Sci Plant Anal, 26, Han and Banin A., Selective sequential dissolution techniques for trace metals in arid-zone soils The carbonate dissolution step, p 573, Copyright (1995), with permission from Taylor Francis US)...

There are very limited comparative studies on fractionations and distribution of trace elements in soils extracted by various selective sequential dissolution protocols. Sutherland and Tack (2003) compared fractionation of Cu, Pb and Zn in reference soils using three selective... 

The sulfides of trace elements in soils and sediments are also of importance in controlling the availability and mobility of trace elements, especially for land disposal of sulfide-rich sediments or anaerobic digested sludge. Due to the oxic nature in arid soils, most of the sulfur is present as sulfate thus, this problem may not be pressing. In most current SSD schedules, the majority of the sulfide forms are included in the organic bound or residual fractions. 

Carbonates, organic matter, Fe and Mn oxides, and clay minerals play important roles in controlling overall reactivity of trace elements in soils and sediments. This chapter addresses the interaction of trace elements with carbonates, organic matter, Fe and Mn oxides and clay minerals. Analytical techniques for trace element speciation in solid-phase and their distribution among various solid-phase components in arid and semi-arid soils are reviewed. Solubilities of trace elements in solid phases and their mineralogical characteristics in arid and semi-arid soils also are discussed. 

Two parameters, the redistribution index (Uts) and the reduced partitioning parameter (IR), are used to describe the redistribution processes of trace elements in contaminated arid soils (Figs. 6.5-6.6) (Han et al., 2003a). The redistribution index depicts the removal or attainment of element-contaminated soils from or to the fractional distribution pattern characteristic of non-amended soils. However, the reduced partitioning parameter quantifies the relative binding intensity of trace elements in soils. 

Bioavailable trace elements in soil correlate with plant uptake and concentrations in plants. Extractants for bioavailable trace elements include chelating agents, diluted inorganic acid, neutral salt solutions, and water (Table 7.2). The most popular extractant for bioavailable trace elements in arid and semi-arid soils is DTPA-TEA (triethanolamine), which was developed by Lindsay and Norvell (1969, 1978) to extract available Cu, Zn, Fe and Mn from neutral and calcareous soils. Use of this chelating agent, DTPA, is based on the fact that it has the most favorable combination of stability constants for simultaneous complexation of Cu, Fe, Mn and Zn... 

The bioavailability of trace elements in soil-water-plant ecosystems of arid zones is largely determined by their partitioning between solution and solid-phase components. The redistribution and transformation of trace elements among solid-phase components under various biogeochemical conditions strongly adjust their lability and bioavailability. Soluble, exchangeable and organically bound forms are bioavailable to plants. The carbonate bound fraction represents a reservoir of potentially bio-available trace elements to plants. In paddy soils, the amorphous Fe/Al oxide fraction contributes to trace element availability as well. 

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