Sustaining the efficiency of the Fe(0)/H2O system for Cr(VI) removal by MnO2 amendment.

This study aims to provide new knowledge regarding the effect of MnO2 co-presence on efficiency of Cr(VI) removal with Fe(0). Non-disturbed batch experiments (≤40 days) were conducted using two types of Fe(0) (milli- and micro-sized), two Cr(VI) concentrations (5 and 100 mg/L), in three different systems ("Fe(0) only", "MnO2 only", and "Fe(0) + MnO2"), at an initial pH value of 6.9. Compared to "Fe(0) only" system, the efficiency and rate of Cr(VI) removal were highly promoted in "Fe(0) + MnO2" system; moreover, while for the "Fe(0) only" system removal of Cr(VI) was severely hindered by increasing Cr(VI) concentration, in "Fe(0) + MnO2" system comparable high efficacies were noticed both at low and high concentration. Recycling experiments indicated that total Cr(VI) removal capacity of "Fe(0) + MnO2" system was up to 48.1 times greater than of the "Fe(0) only" system. Enhanced removal of Cr(VI) with Fe(0) was achieved at low doses of MnO2, with an optimal mass ratio Fe(0):MnO2 of 4:1. The favorable synergistic effect observed in "Fe(0) + MnO2" system was ascribed to capacity of MnO2 to accelerate Fe(0) oxidative dissolution, and to generate supplementary amounts of secondary adsorbents/reductants with removal ability towards Cr(VI). This study provides compelling evidence that "Fe(0) + MnO2" system could represent a highly efficient and cost-effective alternative for the abatement of Cr(VI) aqueous pollution.