Potential application of inorganic sulfur reductants for Cr(VI) removal at sub-ppb level

Abstract

In this study, the efficiency of inorganic sulfur reductants (ISRs) for the Cr(VI) removal from drinking water at sub-ppb level was evaluated. Particularly, NaHSO₃, Na₂S₂O₃, Na₂S₂O₄, Na₂S₂O₅, and Na₂S were examined at doses up to 50 mg S/L, which is equivalent to an increase in sulfate concentration up to 150 mg/L in the treated water. Experimental results showed that the removal efficiency is strongly influenced by solution pH. The optimum pH lies at the range 2–4 for all ISRs tested, whereas their efficiency gradually decreases due to the competition with dissolved oxygen, as reaction pH value increases from 4 to 9. At the typical for natural water pH 7, only Na₂S₂O₄ was found capable to reduce an initial Cr(VI) concentration of 100 μg/L down to the sub-ppb level. The reaction kinetics for this reagent showed that 85% of total Cr(VI) removal was achieved within 2 h. It is important to note that at the equilibrium, reached after 8 h, sulfate concentration was safely below the drinking water regulation limit. On the other side, the application of Na₂S₂O₅, Na₂S₂O₃, and NaHSO₃ resulted in residual Cr(VI) concentrations not lower than 60 μg/L, while the addition of Na₂S, which proved more efficient (residual Cr(VI) 18 μg/L), was responsible for an unpleasant odor indicating the sulfide’s presence.

Keywords:

• Cr(VI) removal
• Drinking water
• Inorganic sulfur reductants

Acknowledgments

The research Project is co-financed by the European Union–European Social Fund (ESF) and National Sources, in the framework of the program “THALIS” of the “Operational Program Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) 2007-2013.

Notes

Presented at the 1st EWaS-MED International Conference on Improving Efficiency of Water Systems in a Changing Natural and Financial Environment, 11–13 April 2013, Thessaloniki, Greece