Assessment of Isotope Exchange Methodology to Determine the Sorption Coefficient and Isotopically Exchangeable Concentration of Selenium in Soils and Sediments

Abstract

Isotope exchange methodology is invaluable to determine the solution-solid-phase distribution (Kd) and isotopically exchangeable concentration (Evalue) of elements in soils and sediments. This work examined the use of species-specific stable isotope exchange techniques to determine the Kd and E value of selenium (Se), as selenite (SeO3) and selenate (SeO4), in nine soils and sediments varying in concentration and source of Se. High-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) was used to quantify the isotope (e.g., 76Se, 78Se, 80Se, and 82Se) concentrations of the soluble Se oxyanions. The two Se oxyanions were detected in the solution phase of all of the soils and sediments. However, upon spiking the suspensions with stable isotope-labeled 78SeO3 and 76SeO4, it was observed that isotope self-exchange was insignificant to the derivation of Se oxyanion Kd and E values during 24 h (and up to 120 h in four of the samples). These results demonstrate that valid determinations of the Evalue of Se necessitate that the Se oxyanions are speciated in solution. This is clearly evident for these soils and sediments where it was observed that the Evalues of SeO3 and SeO4 represented, respectively, 5-97% and 3-95% of the total Se E value.

Assessing chromate availability in tropical ultramafic soils using isotopic exchange kinetics

“…Finally, the compartment into which the tracer is added must behave homogenously in terms of exchange properties. In this case, the assumption made is that all chromate ions from the soil solution have the same behavior in terms of isotopic exchange properties (Collins et al 2006;Hamon et al 2004). According to the Cr Eh-pH diagram and the pH range of soils, HCrO 4 − represents 70 to 95% and CrO 4 2− 5 to 30% of the stable forms of Cr(VI) which can be present (Kotas and Stasicka 2000) and were defined hereafter as Cr(VI).…”

Labile Cd and Pb in vegetable-growing soils estimated with isotope dilution and chemical extractants

“…But labile pool of HMs is now widely measured using stable isotope technique because stable isotope techniques have several advantages over those employing radioactive tracers, such as a non-radioactive environment for the operator, less disposal and waste restrictions (Sterckeman et al, 2009). In addition, the species-specific E values such as selenite and selenate in soils can be also measured with the stable isotope exchange technique (Collins et al, 2006).…”

Influence of submergence and subsequent drainage on the partitioning and lability of added selenium fertilizers in a sulphur‐containing Fluvisol

“…In flooded soils, reduction of added soluble Se(IV) or Se(VI) occurs within weeks with >80% of the added Se being fixed into non‐labile pools (Premarathna et al , 2010). Oxidation of reduced Se species is slow in soils with <20% of Se(0) being oxidized to Se(VI) within 60 days (Collins et al , 2006; Premarathna et al , 2010). Tokunaga et al (1991) found that more than 55 days were required to oxidize half of the newly formed (biologically) colloidal Se(0) to Se(IV) in a contaminated sediment.…”

“…In addition, these sequential fractionation methods do not provide information about the strength of binding of Se with soil surfaces which may change with changing redox conditions. Isotope dilution methods using enriched stable or radioactive Se isotopes have been used successfully to examine the partitioning and lability of both Se(IV) and Se(VI) species simultaneously in aerobic and anaerobic soils (Collins et al , 2006; Premarathna et al , 2010). The study explained in the present paper was undertaken with a closed reaction system to examine in a controlled environment the influence of redox processes on solution speciation, partitioning and lability of added Se(IV) and Se(VI).…”