Unveiling the interaction mechanism between facet-dependent pyrite nanoparticles and Cr(VI) under anaerobic environment: adsorption, redox, and phase transformation

The environmental migration and bioavailability of Hexavalent chromium (Cr(VI)) could be affected by facet-dependent pyrite (FeS 2 ) existing in soils and sediments. Conversely, the occurrence state of facet-dependent FeS 2 might also be influenced by Cr(VI). However, the complex interactive mechanism at the solid–liquid interface between facet-dependent FeS 2 and Cr(VI) still remained ambiguous. Herein, FeS 2 nanobelts, FeS 2 nanosheets, and FeS 2 nanocubes with corresponding dominant facets of (   −1   1   0), (0   0   1), (1   0   0)/(2   1   0) were prepared. The adsorption and reduction experimental results reflected that Cr(VI) species were predominantly adsorbed and a portion of them was reduced to Cr(III) at the FeS 2 interfaces. In particular, the adsorption efficiency of Cr(VI) on (0   0   1) facet was 1.39 and 4.69 times than those on (   −1   1   0) and (1   0   0)+(2   1   0) facets, respectively. The results of in situ Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR) measurements and Density Functional Theory (DFT) calculations suggested that bidentate binuclear (BB) coordinated chromates were generated on the (0   0   1) facet while monodentate mononuclear (MM) coordinated chromates were formed on (   −1   1   0), (1   0   0), and (2   1   0) facets. Due to the stronger adsorption energy and more stable coordination mode of BB for Cr(VI) species on (0   0   1) facet, FeS 2 NS exhibited higher efficiency of Cr(VI) removal and reduction compared with FeS 2 NB and FeS 2 NC. In addition, partial FeS 2 particles were proved to be transformed to goethite under the effect of Cr(VI). Owing to the stronger adsorption of Cr(VI) species on (0   0   1) facet, FeS 2 NS also had a faster phase transformation rate than FeS 2 NB and FeS 2 NC. Above results all reflected the good structure–reactivity relationships between the FeS 2 facets and the Cr(VI) removal efficiency as well as the phase transformation of FeS 2 particles.