A new methodology involving stable isotope tracer to compare simultaneously short- and long-term selenium mobility in soils

Abstract

A better understanding of Se fate in soils is required for different environmental issues, such as radioactive waste management or soil fertilization procedures. In these contexts, the mobility and speciation of Se have to be studied at both short and long terms after Se inputs. Here, we present a new methodology to monitor simultaneously the reactivity of added (isotopic enriched tracers) and ambient Se at trace level in soils by high-performance liquid chromatography inductively coupled plasma mass spectrometry (ICP-MS) following specific extractions. To do so, the collision/reaction cell of the ICP-MS instrument and the interference corrections were optimized to measure reliably the four major Se isotopes. To exemplify the method capabilities, the behaviors of added (77)Se(IV) and ambient Se were followed up in two soils submitted to an ageing process during 3 months. The solid/liquid distribution of added Se reached a steady state after 1 month while its speciation and distribution among soil solid phases were still changing after 3 months. The results clearly demonstrate that slow processes are involved in Se retention and transformation in soils. The usual short-term experiments (<1 month) performed after Se addition are thus not suitable for long-term risk assessment. Interestingly, the behavior of added Se tended to that of ambient Se, suggesting that ambient Se would be useful to infer the fate of Se input over long time scales.

Understanding soil selenium accumulation and bioavailability through size resolved and elemental characterization of soil extracts

“…Overall, our study precisely quantifies and characterizes alkalineextractable organic Se, thus shedding insight on the degree and types of Se association to SOM in soils. Previous studies suggested that SOM could mainly play an indirect role on Se cycling through coating of Fe oxides (nano)particles to which Se oxyanions are adsorbed 14,31,55 and/or by favoring (a)biotic Se reduction into Se(0) nanoparticles 9,56,57 , which commonly have sizes >50 nm [50][51][52] . Here, we demonstrate that over a large gradient of SOC content, variable mineral composition and redox conditions, organic Se is a dominant Se pool that consists of different size and chemical fractions.…”

“…However, selectivity of these extractions is questionable [28][29][30] (Supplementary Note 2). Furthermore, speciation analysis of soil extracts using selective reduction of Se(IV) 19,20,22,23 or chromatographic separation of Se oxyanions and amino acids was sometimes performed 13,14,31,32 , but the former only informs on Se oxidation states whereas a large share of extracted Se species (20-100%) remains unrecovered by commonly used liquid chromatographic methods. Currently, the predominant nature (organic vs mineral) and characteristics (e.g., size, charge) of Se species present in soil extracts remain largely unknown.…”