Selenium fractions in organic matter from Se-rich soils and weathered stone coal in selenosis areas of China

A high degree of association between Selenium (Se) and organic matter has been demonstrated in natural environments, but Se fractions and speciation in organic matter is unclear. In this study, a method for quantifying organic matter associated with Se (OM-Se) was developed to investigate Se fractions in organic matter in Se-rich soils and weathered stone coal from Enshi, China, where Se poisoning of humans and livestock has been documented. Initially, Se was extracted using water and a phosphate buffer. Subsequently, OM-Se was extracted using NaOH, and then speciated into Se associated with fulvic acids (FA-Se) and humic acids (HA-Se). Both FA-Se and HA-Se were further speciated into the weakly bound and strongly bound fractions using a customized hydride generation reactor. The results show that FA-Se (1.91–479 mg kg−1) is the predominant form of Se in all Se-rich soils and the weathered stone coal samples, accounting for more than 62% of OM-Se (3.07–484 mg kg−1). Weakly bound FA-Se (1.33–450 mg kg−1) was prevalent in the total FA-Se, while weakly bound HA-Se (0.62–26.2 mg kg−1) was variable in the total HA-Se (1.15–32.5 mg kg−1). These data indicate that OM-Se could play a significant source and sink role in the biogeochemical cycling of Se in the supergene environment. Weakly bound FA-Se seems to act as a potential source for bioavailable Se, whereas strongly bound HA-Se is a possible OM-Se sink which is not readily transformed into bioavailable Se.

► A reliable method for quantifying OM-Se fractions was developed.
► Fulvic acids Se (FA-Se) was the predominant form in the whole OM-Se.
► OM-Se could play the source and sink role in the biogeochemical cycling of Se.
► Weakly bound FA-Se seems to act as a potential source for bioavailable Se.