Synergetic effect of a pillared bentonite support on SE(VI) removal by nanoscale zero valent iron

Abstract

A positively-charged pillared bentonite (Al-bent) was used as the carrier of nanoscale zerovalent iron (NZVI) particles for accelerating the reductive removal of anionic Se(VI) from water by NZVI. A synergetic effect on Se(VI) removal by this NZVI/Al-bent composite was observed, with removal efficiency (95.7%) much higher than the sum (72.0%) of NZVI reduction (62.1%) and Al-bent adsorption (9.86%). The multiple roles of Al-bent in the reaction system of Se(VI) with NZVI were comprehensively investigated using XAFS techniques, XRD analysis, kinetic studies and pH monitoring. The results indicate that the positively-charged Al-bent as a good adsorbent to anionic Se(VI) accelerated the reductive transformation of Se(VI) into less soluble Se(-II). Al-bent also acted as a pH buffering agent, and contributed to the enhanced stability and reusability of NZVI. In addition, Al-bent could transfer the insoluble reduction products away from the iron surface according to EXAFS studies. Particularly, Al-bent could activate Fe(II) ions generated in the reaction system through an appropriate bonding at pH 8.0, and provide NZVI with an extra reduction capacity. The results indicate the significance of correct matching of bentonite support to target contaminant for developing novel remediation technique based on this natural mineral.