Batch experiments were conducted to determine whether sub-ppb concentrations of dissolved Cr(VI) could be achieved by redox-assisted coagulation with Fe(II). In this process Fe(II) acts as a reductant for Cr(VI) and, after its oxidation to Fe(III), as a coagulant. With an Fe(II) dose of 30 μM (1.7 mg/l as Fe), dissolved Cr(VI) concentrations in synthetic or natural Colorado River water (CRW) at pH 8.2 could be decreased from an initial value of 1 μM (52 µg/l) to <0.01 μM (<0.5 µg/l) corresponding to a >99% removal efficiency. In contrast, removal of Cr(VI) by coagulation with Fe(III) was poor, even at higher Fe(III) doses (up to 100 μM). The efficiency of Cr(VI) removal by redox-assisted coagulation decreased at lower (but still above stoichiometric) Fe(II) doses due to the incomplete reduction of Cr(VI) to Cr(III), which, under our experimental conditions, can be attributed to the competing reaction of Fe(II) with O2 between pH 7 and 8.2 and to the slow kinetics of reaction of Fe(II) with Cr(VI) between pH 6.5 and 7. Although Cr(VI) was less efficiently removed from natural or synthetic CRW than from the simple electrolyte solution, the desired level of removal could be achieved by increasing the Fe(II) dose.
- water treatment
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