Synergetic effect of pyrite on Cr(VI) removal by zero valent iron in column experiments: An investigation of mechanisms

In this work, a novel combination of pyrite with zero valent iron (ZVI) was used to enhance the efficiency of Cr (VI) removal in column experiments, simulating the operation of permeable reactive barriers (PRBs).The influence of pyrite dosage, initial pH, and the mixing manner on the removal efficiency were evaluated. The synergetic effect between pyrite and ZVI was found to contribute to enhanced Cr(VI) removal in column experiments using the ZVI/pyrite mixture, by which the Cr(VI) removal could be kept at > 90% for 50-100 PVs in comparison with similar to 30 PVs by using ZVI alone.The Cr(VI) removal efficiency by the ZVI/pyrite mixture was also much higher than the sum of those by ZVI and pyrite separately in the batch experiments. Mechanism investigations indicated that the synergetic effect was principally attributed to the more reactive Fe2+ regenerated in the columns using the ZVI/pyrite mixture, due to reaction of pyrite with Fe3+ produced during Cr(VI) reduction. In addition, pyrite could suppress the pH increase and retard the surface passivation of active materials. The by-product SO42- from pyrite oxidation is also beneficial for enhanced removal efficiency. X-ray photoelectron spectroscopy (XPS) and Scanning electron microscopy-Energy dispersive spectrometry (SEM-EDS) studies confirmed the reduction of Cr(VI) into Cr(III), and that more pyrite was consumed in the Cr(VI) reduction by the ZVI/pyrite mixture. These findings are useful for developing cost-efficient methods for enhancing the removal of pollutants by ZVI-PRBs.