Continuous flow process of Cr(VI) removal from drinking water through reduction onto FeOOH by inorganic sulfur reductants

Abstract

Abstract In this study, the implementation of an iron oxy-hydroxide (FeOOH) as a surface catalyst for Cr(VI) reduction by inorganic sulfur reductants (ISRs) was investigated. Batch Cr(VI) removal tests, performed to evaluate and compare the efficiency of ISRs in the presence of FeOOH, qualified Na2S2O4 as the optimum for drinking water treatment. Application of Na2S2O4 in continuous flow rapid small scale column tests, using a FeOOH adsorbent at pH 7 ± 0.1 and artificial (resembling natural) water matrix, verified the high potential for Cr(VI) removal at sub-ppb level. Indeed, a 15 mg S/L Na2S2O4 dose diminished an initial Cr(VI) concentration of 100 μg/L below the method's detection limit of 1.4 μg/L at least for 105 bed volumes. X-ray absorption fine structure spectroscopy revealed that Cr(VI) forms outer sphere complexes, while Cr(III) is involved in 2E, 2C and 1 V geometries with the surface Fe-oxyhydroxyl groups. It can, therefore, be concluded that FeOOH attracts Cr(VI) to its surface via physisorption, offering a solid surface that promotes the transfer of electrons through bridging ions. Thus, when Na2S2O4 is added in the system, Cr(VI) is reduced to Cr(III), which is subsequently chemisorbed onto the FeOOH surface.