Chromate reduction in wastewater at different pH levels using thin iron wires—A laboratory study

The effectiveness of using thin zero‐valent iron (Fe0) wires in the treatment of wastewater generated from a metal cleaning facility and with a pH in the range of 2 to 10 was examined. It was found that (1) when the sample containing low levels of total chromium (≤14 mg/L) was mixed with iron wires at a pH of 3 to 8, 50 to 90% of the total chromium could be reduced in 4 h; (2) the initial reduction efficiency was pH‐dependent: the lower the pH, the higher the reduction rate; (3) variations of solution pH, redox electrical potential, and electrical conductivity (EC) in samples were also pH‐dependent; (4) the adsorption/reduction efficiency was limited by the diffusion of Cr(VI) from wastewater to the iron surface when the test duration was long; (5) when the initial pH = 3, iron corrosion and redox reaction dominated the reduction process; however, with pH = 8 or 10, corrosion, surface passivation, or metal precipitation could compete with reduction; (6) the used iron wires were still effective in chromium removal in new samples at pH = 3; and (7) some desorption of adsorbed chromium was observed in acidic samples when the test duration was long. Scanning electron microscope images and energy‐dispersive X‐ray spectra collected from iron samples also indicate that the efficiency of chromium adsorption/reduction is pH‐dependent. Our results suggest that using zero‐valent iron to polish acidic wastewater containing low contents of chromium and other heavy metals is feasible.