Removal of hexavalent chromium using biogenic mackinawite (FeS)-deposited kaolinite

The biogenic mackinawite (FeS) is a promising remover for hexavalent chromium. However, FeS is susceptible to aggregation, affecting the removal efficiency of Cr(VI). To address the aggregation of FeS, kaolinite was selected as the stabilizer to disperse biogenic FeS. Results showed that kaolinite could be used as a supporting material for biogenic FeS during the formation process, which efficiently decreased the aggregation of FeS. FeS-kaolinite enabled to increase the rate and capability of Cr(VI) removal, and the maximum removal capability of FeS-kaolinite reached 399 mg/g at pH 7, which was 2.4 times higher than that of FeS only. The optimal concentration of kaolinite was 3 g/L where the percentage of FeS on FeS-kaolinite was 5% (wt). The removal kinetic was well fitted with the pseudo-second-order model and Cr(VI) could be removed quickly and efficiently in the pH range of 3-8. The efficiency of Cr(VI) removal was not dependent on ionic strength (Cl^-, 0.1-1000 mM), HA (5-20 mg/L) and co-existing cations (Ca²⁺, Zn²⁺,Cu²⁺, and Cd²⁺, 30 mM). Cr(VI) could be rapidly reduced to Cr(III) by FeS-kaolinite on the surface and Cr(III) was coprecipitated into Cr(OH)₃ and Cr_xFe_1-x(OH)₃. This study suggests biogenic FeS-kaolinite is a good candidate for remediation of Cr(VI) contaminated environment.