Development of a novel pyrite/biochar composite (BM-FeS@BC) by ball milling for aqueous Cr(VI) removal and its mechanisms.

As a natural reduction mineral, pyrite (FeS), was combined with biochar by simply ball milling technology to synthesize FeS@biochar composite (BM-FeS@BC) and applied for the removal of hexavalent chromium (Cr(VI)) in aqueous solution. SEM, XRD, FTIR, and XPS characterization results showed that the FeS and biochar were successfully combined and biochar suppressed the agglomeration of FeS. Batch sorption experiments showed that the BM-FeS@BC700 composite (mass ratio of FeS-to-biochar = 3:1) had enhanced Cr(VI) removal capacity of 134 mg·g, which were 3-25 times higher than those of the corresponding pristine and ball-milled biochar and FeS. The removal of Cr(VI) by BM-FeS@BC700 was dosage and pH dependent. The addition of oxalic acid (OA) exhibited a promotion effect on the removal of Cr(VI) by increasing the removal rate of Cr(VI) from 56% to 100%. Reduction, adsorption, and surface complexation were the dominate mechanisms for Cr(VI) removal by BM-FeS@BC700. At the equilibrium Cr(VI) concentration of 15.7 mg·L, 92.25% of Cr(VI) was removed through reduction/precipitation and 8.75% was removed by adsorption/surface complexation. The fitting results of the Langmuir model proved that the removal of Cr(VI) by BM-FeS@BC700 composite was chemical surface monolayer adsorption. This work demonstrates the potential of ball milling for the preparation of FeS@BC composite to remove Cr(VI) from water and wastewater.