Selenite fixation by soil particle‐size separates

Abstract

The fixation of selenite by clay-( < 2 pm), silt-(2-20 pm) and sand-size (20-2000 pm) separates from two arable soils was examined in solutions of 75Se-labelled sodium selenite using a Se/sample ratio of 1/106. Size separates were isolated by ultrasonic dispersion and gravity sedimentation. Selenite fixation was determined after equilibration periods ranging from 5 min to 26 h. Hydrogen peroxide-treated samples were included to examine the effect of organic matter on selenite fixation capacity.The relative distribution of native Se, C, dithionite/citrate-extractable Fe and A1 between size separates was similar. Concentrations in clay were four to nine times higher than in whole soils, silt showing two to five times higher concentrations and sand being very low in Se, C, Fe and Al.After 1 h, clay, silt and sand fixed 64-65%, 45-61% and < 5 % of the selenite added, respectively. The fixation on whole soils was 1418%. After 1 day, fixation on clay, silt, sand and whole soil increased to 78-87%, 67-79%, 3-1470 and 31-39%, respectively.Hydrogen peroxide-treatment reduced the selenite fixation capacity of whole soil, silt and sand to very low levels. Fixation on peroxide-treated clay was in accord with values for pure clay minerals reported in the literature. Generally, the fixation capacity of peroxidetreated natural clay and pure clay minerals was only half that observed for intact claysize separates, demonstrating the importance of organic matter in soil selenite fixation capacity.

Sorption of selenium(VI) onto anatase: Macroscopic and microscopic characterization

“…Selenium, a non-metallic trace element occurring naturally in the environment, is an essential oligo-element for human beings and animals (Gissel-Nielsen et al, 1984;Burau, 1985;Christensen et al, 1989;Conde and Alaejos, 1997;Lemly, 2004). However, in large amounts, the toxicity of selenium compounds for animals, humans, wildlife and aquatic species has been evidenced (Burau, 1985;Conde and Alaejos, 1997).…”

Selenium Speciation in Soils and Plants

“…While selenite is strongly adsorbed to soil surfaces, selenate is much more weakly adsorbed (11)(12)(13). Studies on Se adsorption onto soils have indicated that the most important soil components are Fe-and Al-oxides and oxyhydroxides, Mn-oxides, clays, organic matter, and carbonates (11,(13)(14)(15). The levels of Se adsorption by Fe-and Mn-oxides are similar, with Al-oxides adsorbing slightly lower levels (16)(17)(18).…”

“…Kaolinite adsorbs greater levels of selenite than montmorillonite below pH 6, even when the minerals have similar surface areas (13,19). Selenite adsorption is sometimes correlated with organic carbon in soils (14,15). However, negatively charged organic substances may also block adsorption sites, preventing Se adsorption (20,21).…”

Proton consumption and release of organic‐cabon, silicate, and sulfate by anion sorption on andosols

“…However, selenite, fluoride, and phosphate sorption mechanisms on real soils would be extremely complex and not fully revealed. Selenite sorption is influenced by the presence of organic matter (Christensen et al, 1989) and other anions (Brown and Carter, 1969), and also phosphate by organic substances (Moshi, 1974;Borggaard et al, 1990). Soil differs from pure systems in that the sites for ligand exchange reactions are not restricted to OH and water molecules (Pardo and Guadalix, 1990).…”