Effect of Sand Co-Presence on Cr^VI Removal in Fe⁰-H₂O System

The aim of the present study was to provide new knowledge regarding the effect of non-expansive inert material addition on anionic pollutant removal efficiency in Fe⁰-H₂O system. Non-disturbed batch experiments and continuous-flow-through column tests were conducted using Cr^VI as a redox–active contaminant in three different systems: “Fe⁰ + sand”, “Fe⁰ only” and ”sand only”. Both experimental procedures have the advantage that formation of (hydr)oxide layers on Fe⁰ is not altered, which makes them appropriate proxies for real Fe⁰-based filter technologies. Batch experiments carried out at pH 6.5 showed a slight improvement of Cr^VI removal in a 20% Fe⁰ system, compared to 50, 80 and 100% Fe⁰ systems. Column tests conducted at pH 6.5 supported results of batch experiments, revealing highest Cr^VI removal efficiencies for “Fe⁰ + sand” systems with lowest Fe⁰ ratio. However, the positive effect of sand co-presence decreases with increasing pH from 6.5 to 7.1. Scanning electron microscopy—energy dispersive angle X-ray spectrometry and X-ray diffraction spectroscopy employed for the characterization of Fe⁰ before and after experiments indicated that the higher the volumetric ratio of sand in “Fe⁰ + sand” system, the more intense the corrosion processes affecting the Fe⁰ grains. Results presented herein indicate the capacity of sand at sustaining the efficiency of Cr^VI removal in Fe⁰-H₂O system. The outcomes of the present study suggest that a volumetric ratio Fe⁰:sand = 1:3 could assure not only the long-term permeability of Fe⁰-based filters, but also enhanced removal efficiency of Cr^VI from contaminated water.