Improved selenium bioavailability of selenium-enriched slate via calcination with a Ca-based sorbent

• A calcination/heating procedure to improve Se bioavailability was proposed.
• Bioavailable Se in calcinated slate reached 10 times of those in the original slate.
• Se(IV) was the predominant form of bioavailable Se lower than 500 °C.
• The dominant form for bioavailable Se converted to Se(VI) when exceeded 500 °C.
• High-Se potato was obtained when grown on soil amended with the activated slate.

Fertilizing soil with selenium (Se)-enriched slate could increase Se levels in crops; however, this process is limited by the low Se bioavailability of Se-enriched slate. In this work, a calcination procedure for Se-enriched slate using CaO as a sorbent was developed, and the process significantly improved the bioavailability of Se. The calcination experiments were conducted with the calcium (Ca)-based sorbents CaO or CaCO3 at 300–800 °C. Most of the Se in the organic matter and sulfides were emitted between 400 and 650 °C, and CaO exhibited a higher adsorption efficiency of Se compared with CaCO3. The optimum temperature was 600 °C as determined by the high contents of bioavailable Se (10 times that of the original slate) and Se(VI) in the calcinated samples with the sorbent CaO. Both the physical adsorption of CaO and the formation of CaSeO3 contributed to the conversion of Se associated with organic matter and sulfide minerals to bioavailable Se species. The transformation of Se(IV) to Se(VI) might have been caused by the co-oxidation of O2 and the generation of HCl during the calcination process. The Se concentrations in potatoes amended with activated slate reached 650 ± 23 μg/kg, which verified the high Se bioavailability of the calcinated product. The proposed calcination procedure could support the exploitation of Se-enriched slate and the preparation of Se soil amendments.