Selenium in soils and sediments

The optimal reaction conditions for the generation of the hydrides can be quite different for the various elements. The type of acid and its concentration in the sample solution often have a marked effect on sensitivity. Additional complications arise because many of the hydrideforming elements exist in two oxidation states which are not equally amenable to borohydride reduction. For example, potassium iodide is often used to pre-reduce AsV and SbV to the 3+ oxidation state for maximum sensitivity, but this can also cause reduction of Se IV to elemental selenium from which no hydride is formed. For this and other reasons Thompson et al. [132] found it necessary to develop a separate procedure for the determination of selenium in soils and sediments although arsenic, antimony and bismuth could be determined simultaneously [133]. A method for simultaneous determination of As III, Sb III and Se IV has been reported in which the problem of reduction of Se IV to Se O by potassium iodide was circumvented by adding the potassium iodide after the addition of sodium borohydride [134], Goulden et al. [123] have reported the simultaneous determination of arsenic, antimony, selenium, tin and bismuth, but it appears that in this case the generation of arsine and stibene occurs from the 5+ oxidation state. 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

In sediments and soils, the chemistry of selenium differs from that of sulfur in that the stability of selenite is similar to that of sulfate. The reduction of selenite to elemental selenium, which tends to immobilise selenium in soils and water, is an important process in the natural environment. Selenates are only stable under alkaline oxidising conditions and have been found, for example, in the Chilean nitrate deposits. 

Fig. 7.2 Generalized chemistry of selenium in soils and weathering sediments.

Which oxidation states can selenium exist in soils and sediment What are the major transformations List the oxidation states which selenium can exist in sediment. 

The use of an organic carbon source to accelerate the reduction of Se(VI) and Se(IV) to elemental Se in soils and sediments has been reported (Tokunaga et ai, 1996 Arbestain and Aros, 2001). The reduction may be part of the selenium detoxification mechanism (Oremland, 1994) or may occur through the following dissimilatory pathway (Tomei et ai, 1992) ... 

Spectrofluorimetric methods are applicable to the determination of aliphatic hydrocarbons, and humic and fulvic acids in soil, aliphatic hydrocarbons polyaromatic hydrocarbons, optical whiteners, and selenium in non-saline sediments, aliphatic aromatic and polyaromatic hydrocarbons and humic and fulvic acids in saline sediments. The only application found in luminescence spectroscopy is the determination of polychlorobiphenyl in soil. Generally speaking, concentrations down to the picogram (pg L 1), level can be determined by this technique with recovery efficiencies near f00%. 

Neutron activation analysis has been used to determine selenium in soil [144-148], Nadkarni and Morrison [149] estimated 47 elements in lake sediments and found 0.3-1.01pg selenium per gram using neutron activation analysis. Dong et al. [162] used mixtures of phosphoric acid, nitric acid and hydrogen peroxide in the digestion of soils prior to the determination of selenium. 

In the case of rocks, soils, and sediments, sufficient material to be representative of the medium to be analyzed should be collected. Soil and sediment samples should be dried at temperatures <35 °C to avoid volatilization losses of arsenic or selenium (Rowell, 1994) and ideally freeze-dried (BGS, 1979-2002). Sampling, analysis, and quality control should be carried out with recognized procedures wherever possible (Darnley et al., 1995 Salminen and Gregorauskiene, 2000). 

The iron oxide and clay content of soils and sediments can affect the bioavailability of selenium markedly. The strong pH dependence of adsorption is an important control. Maximum adsorption occurs between pH 3 and pH 5 and decreases as the pH rises. Organic matter also removes selenium from soil solution, possibly as a result of the formation of organometallic complexes. Addition of PO4 to soils increases selenium uptake by plants, because the PO ion displaces selenite from soil particles making it more bioavailable. Conversely, increasing the concentrations of PO4 in soils can... 

Massehelyn, P. H., and Patrick, W. H., Jr. (1994). Selenium, arsenic, and clu omium redox chemistry in wetland soils and sediments. In Biogeochemistry of Trace Elements, ed. Adriano, D. C., Science and Technology Letters, Northwood, Middlesex, England, 615-625. 

Conditions such as pH, oxidation-reduction potential, and the presence of metal oxides affect the partitioning of the various compounds of selenium in the environment. In general, elemental selenium is stable in soils and is found at low levels in water because of its ability to coprecipitate with sediments. The soluble selenates are readily taken up by plants and converted to organic compounds such as selenomethionine, selenocysteine, dimethyl selenide, and dimethyl diselenide. Selenium is bioaccumulated by aquatic organisms. Very low levels of selenium are found in ambient air. 

The disposal of selenium contaminated waste water has resulted in elevated selenium levels in sediments of Lake Belews, North Carolina. The concentration of selenium in sediments ranged from 4 to 12 pg/g (pre-1986), but has dropped to 1 1 pg/g (1996) due to the discontinued release of selenium laden waste water from a local coal fired power plant (Lemly 1997). Selenium was measured in 445 surface soil samples from Florida with a concentration range of 0.01 1.62 pg/g and an arithmetic mean of 0.25 pg/g (Chen et al. 1999). Selenium was detected in soils and bed sediment from the South Platter River Basin at concentrations of 0.30-3.80 pg/g (Heiny and Tate 1997). The highest levels were observed in areas consisting of a high degree of Precambrian rock formation. 

Much of the literature on heavy-metal-bearing soils and sediments has been devoted to the speciation of anthropogenic metal(oid)s in contaminated matrices, but few papers focus on their crystal chemistry when they are present in trace amounts. This section reviews this topic and supplements and enhances the existing literature by describing the forms of arsenic, selenium, nickel, and zinc in two natural soils. It also attempts to illustrate with one example (Zn) how the novel synergistic use of pSXRF, pSXRD, and pEXAFS provides a quantitative analytical tool to speciate dilute multi-component metals in heterogeneous environmental materials. 

Masscheleyn, P. H., R. D. DeLaune, and W. H. Patrick, Jr. 1991. Biogeochemical behavior of selenium in anoxic soils and sediments an equilibrium thermodynamics approach. Environ. Sci. Health A26(4) 555-573. 

---