Seed-mediated growth of schwertmannite for treatment of acid mine drainage: Improved adsorption of As(V) and Cr(VI) due to structure-related mechanisms

The biomineralization-driven formation of schwertmannite is an environmentally friendly strategy for acid mine drainage (AMD) treatment. In this study, a targeted approach using seed-mediated growth of schwertmannite was proposed. The biomineralization time was successfully shortened by one-third compared to biomineralization without seed mediation. In addition, the seed-mediated schwertmannite (S-Sch) exhibited superior adsorption performance towards As(V) and slightly improved adsorption of Cr(VI) than a non-seed-mediated schwertmannite sample (Sch). Furthermore, the whiskers that create the typical pincushion structure grew directly on the surface of the seed during the growth process with S-Sch, whereas biomineralization of Sch needed to nucleate crystals before growing the whiskers. More whiskers were found on S-Sch than on Sch, and the whiskers were arranged loosely on S-Sch while they were packed tightly on Sch. Following adsorption, As(V) is enriched at the whisker sites, where substantial > Fe-OH/OH2 existed, thereby inhibiting subsequent sulfate exchange at inner sites, the open structure of S-Sch decreases this passivation and allows continuous sulfate exchange. Cr(VI) adsorption is mainly due to sulfate exchange with little > Fe-OH/OH2 involved. Cr(VI) was distributed evenly on the surface of S-Sch, but an open mineral structure allows more inner sites to be reached, explaining the slightly better performance of S-Sch. The results suggest that the method proposed in this paper could be an effective way to shorten the biomineralization time and provide schwertmannite with excellent performance for the removal of As(V) and Cr(VI), which could lead to practical applications of biomineralization in the treatment of AMD.