Revisiting Selenium Interactions with Pyrite: From Adsorption to Coprecipitation

Interactions of selenium (Se), a trace element bioessential at low concentrations but highly toxic at high concentrations, with the most abundant sulfide mineral in the earth's crust, namely, pyrite, were investigated over a wide range of time scales from nanoseconds to days. At the nanosecond scale, selenate Se(VI)O-4(2-) adsorption onto the neat pyrite surface is shown by ab initio computations to proceed via the formation of a chemical bond between an oxyanion oxygen atom and a surface Fe atom, weakening the other Se-O bonds and reducing the Se atom oxidation state. At the hour-to-day scale, the adsorption and coprecipitation of selenate Se(VI)O-4(2-) and selenite, Se(IV)O-3(2-), were investigated through wet chemical batch experiments at various pH values at different sulfide concentrations. Selenium removal from solution is slower and weaker for selenate than for selenite. After 24 h, only 10% of selenate, against 60% of selenite (up to 100% in the presence of sulfide), is removed by the pyrite surface. Independently of its original oxidation state, adsorbed Se is completely reduced to elemental trigonal selenium via adsorption, precipitation, or coprecipitation, as shown by XANES spectroscopy. Our EXAFS results, compared to published data on Se-rich pyrite, show a Se to S substitution within the pyrite structure. The reductive coprecipitation mechanism of selenium with pyrite represents valuable new insights for improving our understanding of modern and ancient biogeochemical cycles involving Se. In addition, several industries can benefit from direct applications of our findings, such as water treatment, green technologies, and sustainable mining.