The purification of acid mine drainage through the formation of schwertmannite with Fe(0) reduction and alkali-regulated biomineralization prior to lime neutralization

Acid mine drainage (AMD) contains abundant Fe (II), Fe(III), and SO42−, as well as a large amount of dissolved toxic metals and metalloids, posing a serious threat to the environment. In this study, an integrated technique for the treatment of AMD was proposed. The technique started with pre-oxidation followed by Fe(0) reduction and alkali-regulated biomineralization and then ended with lime neutralization. The technique removed toxic metal oxyanions in the pre-oxidation stage and recovered pure schwertmannite during the subsequent alkali-regulated biomineralization. Fe(III), which could not be directly biomineralized, was reduced to Fe(II) by Fe(0). A small amount of alkali was added to regulate the hydrolytic mineralization reaction after Fe(II) oxidation in AMD, which in a single biomineralization could remove in the form of schwertmannite >95 % of soluble Fe in the AMD. In the subsequent lime neutralization process, the amount of lime required and the sludge produced were reduced by 75.4 % and 84.9 %, respectively, compared to the raw AMD. Additionally, the content of non-ferrous metals in the sludge increased 5.6-fold. Compared with non-alkali-regulated biomineralization, the schwertmannite obtained by the alkali-regulated biomineralization had a higher adsorption capacity for oxyanions (e.g., arsenic, chromium, and antimony). The new approach should significantly reduce the treatment cost of AMD and recover Fe and S elements in the form of valuable secondary minerals, such that it is reasonable to expect that it will be widely adopted in practical applications.