Humic acid modified nano-ferrous sulfide enhances the removal efficiency of Cr(VI)

The dissolved hexavalent chromium (Cr(VI)) in natural water system poses a significant risk for public health and the environment. Nanoscale ferrous sulfide (FeS) has been widely used to treat soils, sediments, groundwater, and natural waters contaminated with Cr(VI), but the reactivity of FeS can be significantly reduced due to its easy agglomeration and therefore decreased the remove rate of the dissolved Cr(VI). To overcome this problem, the present study used humic acid (HA) to modify FeS in order to reduce its agglomeration and increase stability. Results showed that the HA modified FeS (HA-FeS) had good stability and reduced FeS agglomeration. The hydrodynamic diameter of HA-FeS was between 235 and 241 nm, which was much smaller than that of the unmodified FeS. The maximum removal quantity of Cr(VI) by HA-FeS was 7.74 and 2.35 times higher that of HA and the unmodified FeS, respectively, and the maximum removal quantity of HA-FeS was 675.03 mg/g. Further, HA-FeS performed better efficiency in removing the dissolved Cr(VI) with high or low concentration. With an increasing concentration of Ca2+ and Mg2+, the efficiency of HA-FeS to remove Cr(VI) showed a decreasing trend. The removal mechanism of Cr(VI) mainly relies on the reducibility of Fe2+ and S2-. XPS and XRD measurements further revealed that 95.03% of Cr(VI) was converted to Cr(III) (FeCr2O4, Cr(OH)(3) and Cr2O3), and another 4.58% of Cr(VI) was adsorbed by HA-FeS. Therefore, this study demonstrated an efficient method to improve the nano-FeS stability and reactivity for the rapid repair of Cr(VI) contamination in natural water.