Abstract
Arsenic (As) and chromate (Cr(VI)) contamination of ground and surface waters is a major problem worldwide. Given that a new drinking water limit is anticipated for Cr(VI) and that the limit of arsenic in drinking water is quite low (10 μg/L), there is an urgent need for evaluating technologies that could be efficient for removal of both contaminants simultaneously. In this work, the use of Fe(II) redox assisted coagulation was investigated to simultaneously remove the contaminants of interest. The basic principle of this technology is that Fe(II) could react with Cr(VI) and form Fe(III)-hydroxides and insoluble Cr(III) species, while the freshly formed Fe(III) hydroxides are very efficient adsorbents for As(V). The effect of pH, the water matrix composition, Fe(II) dose, initial contaminant concentrations, NOM presence and phosphate concentration were the examined parameters. The results revealed that with a dose of 2 mg/L Fe(II), residual As(V) and Cr(VI) concentrations were both below 10 μg/L, from initial concentrations of 50 μg/L. Though, this is effective only at circumneutral pH values. This is however not a big obstacle, since most natural waters, especially groundwaters, have near neutral pH values. At these pH values, residual iron concentration was far below 200 μg/L. The presence of phosphate anions inhibited As(V) removal but had no effect on Cr(VI) removal. Increasing Fe(II) concentrations eliminated the effect of phosphate and provided simultaneous phosphate removal. Therefore, Fe(II) coagulation can be applied, with secured results, for simultaneous As(V), Cr(VI) and phosphate removal from waters.
Highlights
IntroductionMore than 750 million people have no access to safe drinking water [1]. More than 250 million inhabitants are regularly exposed to water contaminated with arsenic [2,3], mostly in south east Asia [4] and in Europe [5]
Worldwide, more than 750 million people have no access to safe drinking water [1]
Mitrakas et al, [23] have examined this reaction at the pH range to 8 in artificial groundwater. They showed that at pH 6, from 100 μg/L of Cr(VI) by using Fe(II) concentration of 0.5 mg/L, the removal of total Chromium was only 40%, while Cr(VI) reduction was about 80%. We show that this effect becomes even stronger, as the pH decreases further, and only about 8 to 9% removal of total chromium was accomplished at pH 5, while Cr(VI) removal was higher than 80%
Summary
More than 750 million people have no access to safe drinking water [1]. More than 250 million inhabitants are regularly exposed to water contaminated with arsenic [2,3], mostly in south east Asia [4] and in Europe [5]. By conducting experiments in the presence and absence of chromate and by using Fe(II) or Fe(III) salts, we shed light on the mechanism of arsenic removal by Fe(II) under various scenarios, which are realistic in drinking water treatment. This is the first study to investigate in detail the simultaneous As(V) and Cr(VI) removal from both real surface- and ground- waters focusing on achieving the anticipated new limit for Cr(VI)
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