Influence of dissolved oxygen on aqueous Cr(VI) removal by ferrous ion

Hexavalent chromium [Cr(VI)] is a highly toxic and carcnogenic pollutant, introduced to the environment due to several industrial activities. Ferrous iron (Fe(II)], an important natural reductant of the Cr(VI), and its release from iron-bearing materials is now becoming a viable alternative for transforming Cr(VI) to the relatively non toxic trivalent form of chromium [Cr(III)]. Cr(VI) removal by ferrous ion at different pH is widely discussed in the literature but the influence of dissolved oxygen on the Cr(VI) removal with respect to pH, is still a contradictory subject. In the present study a systematic approach has been made to estimate the Cr(VI) removal by ferrous ion in oxygen-free and oxygen-containing aqueous solutions of pH 3 to 7. Quantitative analysis results suggest that the influence of dissolved oxygen on the Cr(VI) reduction starts at pH 4. Though, its effect appears to be in significant up to pH 6, becomes quite pronounced at pH 7 because nearly 10 to 12% less removal of the Cr(VI) was observed in the presence of the stoichiometric requirement of three equivalents of Fe(II) in oxygenated solution of pH 7. This was evidenced by carrying detailed spectrophotometric absorption measurements of the Cr(VI) in the presence of Fe(II) in its different equivalent ratios in oxygenated and deoxygenated solutions.