Efficient removal of Cr(VI) by tannic acid-modified FeS nanoparticles: Performance and mechanisms

Abstract Ferrous sulfide (FeS) nanoparticles constitute an effective hexavalent chromium (Cr(VI)) treatment reagent. However, FeS nanoparticles aggregate easily, significantly limiting their engineering applicability. To overcome this shortcoming and further improve Cr(VI) removal efficiency, this study used tannic acid (TA) to modify FeS nanoparticles. The results demonstrated that TA-modified FeS nanoparticles, TA-nano-FeS, had a significantly reduced tendency to agglomerate, and maintained particle diameters of 10–100 nm, which were much shorter than diameters of FeS nanoparticles. In addition, TA-nano-FeS could combine the surface-active functional groups of TA. The maximum removal capacity of TA-nano-FeS was 381.04 mg/g, which was 2.92 and 1.83 times higher than those of TA and nano-FeS, respectively. Furthermore, the acidic condition was more beneficial for Cr(VI) removal, and the coexisting cations (Ca2+ and Mg2+) slightly decreased the removal efficiency of Cr(VI). Adsorption, reduction, and co-precipitation were the removal mechanisms, and the reaction products included FeCr2O4, Cr2O3, Fe2O3, Cr(OH)3, and S8. The results provided valuable information for the practical application of TA-nano-FeS in Cr(VI) removal.