Trace elements mobility in soil

Gerritse RG, Vriesema R, Dalenberg J, et al. 1982. Effect of sewage sludge on trace element mobility in soils. Journal of Environmental Quality 7 359-364. 

Smith and Huyck (1999) described metal mobility under different environmental conditions. Although it is rather difficult to predict trace element mobility in soils and other terrestrial compartments, these authors referred to the capacity of an element to move within fluids after dissolution in surfi-cial environments. The following conditions and behavior of trace elements were distinguished ... 

In general, in heavy neutral soils most trace elements would be less mobile and less phytoavailable than in light acid soils. Trace element mobility in soils is also known to be related to land use. Usually, metals in forest soils are more easily mobile, and therefore easier bioavailable and leached, than are trace elements in agricultural soils. This phenomenon will be an environmental concern in the future due to programs of forestation of poor agricultural quality soils, especially in Central and Eastern Europe. 

Pueyo, M., Sastre, J., Herndndez, E., Vidal, M., Ldpez-Sdnchez, J. F., and Rauret, G. (2003). Prediction of trace element mobility in contaminated soils by sequential extraction. J. Environ. Qual. 32, 2054-2066. 

Van der Sloot HA, de Groot GJ, Eggenkamp HGM, Tielen JALW and Wijkstra J (1987) Versatile Method for the Measurement of the Trace Element Mobilities in Waste Materials, Soils and Bottom-Sediments. Stichting Energie Onderzoek Centrum, Petten, Netherlands, ECN-87-085. 

Leaching and extraction tests are widely used for assessing trace element mobility and phytoavailability. As measurements in these protocols are made in equilibrium conditions, only thermodynamic information is obtained. However, kinetic extraction—desorption studies are a more correct approximation to the distribution of species in natural media. The desorption rate constants of the trace elements in sediments and soils can be related to their mobility and toxicity. Detailed studies are needed in this area. 

Trace metals such as Cr(in), Pu(III), and Co(II) can be oxidized in the disproportionation process of Mn oxides and oxyhydroxides. Oxidation of trace metals can greatly influence their solubility and mobility. When Cr(III) and Pu(III/IV) are oxidized to Cr(VI) and Pu(VI), these elements are quite mobile, because they are not sorbed by soil components to any extent. Manganese oxide can thus enhance the mobility, toxicity, and food chain contamination of Cr and Pu. On the other hand, oxidation of Co(II) to Co(III) by Mn oxides decreases its solubility and mobility in soil and aquatic environments. 

The chemical form and nature of the element. Most trace metal cations have a low mobility in soils because they adsorb strongly on minerals and organic matter, or form insoluble precipitates (e.g., oxides, carbonates, sulfides). Some elements that take the form of anions in soils, such as boron, are relatively mobile. Other elements that form anions, like phosphorus, are considered to be immobile because they form insoluble precipitates and bond strongly with mineral surfaces. In Figure 9.4, many of the elements of interest in soils are classified on the basis of their ionic radii and valence. The elements tend to fall naturally into one of foiu groups ... 

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)... 

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. 

The partition index (IR, which will be discussed in details below) of Cd, Cu, Cr, Ni and Zn in both soils rapidly increased from time zero (calculated value) to one day and further to one year. This was especially true for Cr, Cu and to some extent Ni and Cd (Table 6.5). This result indicates that added trace metals are initially and rapidly transferred from the labile EXC fraction into the more stable fractions. Furthermore, IR of trace metals in native arid soils incubated under the saturated paste regime decreased at the end of year. This indicates mobilization of trace elements in these soils as saturation (Table 6.5). Also, it can be seen that IR decreased, for any given time, with an increase of the loading level (Table 6.5, Fig. 6.5). This means that higher additions of soluble metals result in higher metal content in the labile fractions and lower metal binding intensity in soils. 

Chemical remediation refers to the application of various minerals or chemicals to adsorb, bind, precipitate or co-precipitate trace elements and heavy metals in soils and waters thereby reducing their bioavailability, toxicity, and mobility. In situ immobilization refers to the treatment of contaminants in place without having to excavate the soils or waste, often resulting in substantial cost savings. However, in situ immobilization or extraction by these physicochemical techniques can be expensive and are often only appropriate for small areas where rapid and complete decontamination is required. 

Violante A, Krishnamurti GSR, Pigna M (2008) Mobility of trace elements in soil environments. In Violante A, Huang PM and Gadd G (eds) Wiley-JUPAC series on biophysico-chemical processes of metals and metalloids in soil environments. John Wiley Sons, Hoboken, USA Waltham AC, Eick MJ (2002) Kinetic of arsenic adsorption on goethite in the presence of sorbed silicic acid. Soil Sci Soc Am J 66 818-825 Waychunas GA, Fuller CC, Rea BA, Davis J (1996) Wide angle X-ray scattering (WAXS) study of two-line ferrihydrite structure Effect of arsenate sorption and counterion variation and comparison with EXAFS results. Geochim Cos-mochim Acta 60 1765-1781... 

Bruand A (2005) Toward conditions favourable to mobility of trace elements in soils. C R Geosci 337(6) 549-550... 

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