Effect of Cr(VI) concentration on gas and particle production during iron oxidation in aqueous solutions containing Cl⁻ ions

Zero-valent iron (ZVI) is commonly used as a medium in permeable reactive barriers (PRBs) because of its high reducing ability. The generation of H₂gas in PRBs, however, can decrease the permeability of PRBs and reduce the contact area between the PRB and contaminated groundwater. This study investigated the effect of the initial Cr(VI) concentration ([Cr(VI)_init]) in aqueous solutions containing Cl⁻ions on the generation of H₂gas. ZVI chips were reacted in reactors with 0.5-M NaCl solutions with [Cr(VI)_init] ranging between 51 and 303 mg/L. The initial pH was set at 3. The oxidation of ZVI chips by Cr(VI) in aqueous solutions containing Cl⁻ions produced H₂gas and particles (Fe(III)–Cr(III)(oxy)hydroxides). The Cr(VI) removal from aqueous solutions increased as the [Cr(VI)_init] increased, as did H₂gas generation. The positive effect of [Cr(VI)_init] on H₂gas generation might be due to an increase in the redox potential gradient as [Cr(VI)_init] increases. This increased gradient would enhance H⁺ion penetration through the passive film (Fe(III)–Cr(III)(oxy)hydroxides), which formed on the ZVI surface, by diffusion from the solution to pits beneath the passive film.