Removal and simultaneous reduction of Cr(VI) by organo-Fe(III) composites produced during coprecipitation and coagulation processes

Highlights: • Organo-Fe composites (OFC) often formed in natural and waste waters. • OFC precipitated at pH 3 showed superior sorption and reduction for Cr(VI). • As C/(C + Fe) ratio ≤ 0.87, Cr(VI) was mainly sorbed on ferrihydrite. • As C/(C + Fe) ratio ≥ 0.89, major Cr species on OFC was Cr(III). • Adjusting C/(C + Fe) ratios to perform a two-step processing for Cr(VI) removal. -- Abstract: Composites formed during the coprecipitation and/or coagulation of ubiquitous dissolved organic matter (DOM) and Fe in natural and waste water systems might be potential scavengers for Cr(VI) in terms of sorption and reduction. Our objective here was to determine sorption and simultaneous reduction of Cr(VI) on organo-Fe(III) composites (OFC) in relation coprecipitated pH and C/(C + Fe) ratios. Results showed the greatest Cr sorption of 51.8 mg g⁻¹ on the OFC sample that was precipitated at pH 3 and contained the C/(C + Fe) molar ratio of 0.71. Wherein the Cr(VI) removal subsequent to the coprecipitation was dominated by the sorption on Fe hydroxides. Although amounts of total sorbed Cr decreased with increasing C/(C + Fe) molar ratio, the reverse trend on Cr(VI) reducibility compensated the Cr(VI) removal capability of OFC samples. With C/(C + Fe) molar ratios ≥ 0.89, the increasing amounts of coprecipitated organic matter that homogeneously distributed with Fe domains on OFC surfaces could trigger a significantly pronounced Cr reduction. Collectively, our results suggested an alternative method for Cr(VI) remediation by manipulating C/Fe ratios in suspensions. After the sorption of most Cr(VI) on Fe hydroxides, increasing C/Fe ratio in systems could further improve the Cr(VI) removal efficiency by the reduction of remaining Cr(VI) to Cr(III).