Simultaneous Removal of Arsenate and Chromate from Ground- and Surface- Waters by Iron-Based Redox Assisted Coagulation

Abstract

<jats:p>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.</jats:p>

Innovative Approaches for Drinking- and Waste-Water Treatment: An Editorial Review Summarizing and Assessing the Findings of the Special Issue

“…The required ratio of [Fe]/[Cr] for an efficient Cr(VI) removal, at pH 7.2 was slightly above 10, much above the stoichiometry of the reaction between Fe(II) and Cr(VI). Previous relevant studies accomplished in batch mode [16] have demonstrated that the main reason for this difference is the competition of this reaction by oxygen. Oxygen reacts also with Fe(II) and the kinetics of Fe(II) oxidation by oxygen increase as pH increases above pH 7 [17].…”

Adsorption and Reduction of Aqueous Cr by FeS-Modified Fe-Al Layered Double Hydroxide

“…This indicated that the main form of Cr in the reacted material is Cr(III) [40]. Obviously, the main Cr removal from the solution mechanism of FeS/LDH involves (1) anion exchange allowing Cr(VI) to enter the interlayered structure of LDH, (2) reduction of Cr(VI) to Cr(III) utilizing reductive properties of Fe 2+ and S 2− , and (3) Fe 3+ and Cr 3+ co-precipitation to remove Cr(T) [34,49]. The schematic diagram of the removal mechanism is shown in Figure 8.…”

“…precipitation to remove Cr(T) [34,49]. The schematic diagram of the removal mechanism is shown in Figure 8.…”

“…Meanwhile, the introduction of FeS on LDH layers can not only assist the better dispersion of FeS nanoparticles to avoid agglomeration, but also improve the reduction properties of the material. It was expected that the FeS-modified Fe-Al LDH shows improvement in ion exchange capacity and removal efficiency from the dual reduction capacity of Fe 2+ and FeS [30,34].…”

Phosphate Removal Using Polyethylenimine Functionalized Silica-Based Materials

“…Although a variety of materials have been applied to remove phosphorus from aqueous solutions, such as zeolite [12], hydrotalcite [13], bentonite [14] and metal oxides [15], it is imperative to develop materials, which are effective in phosphate removal but also can offer multiple contaminant removal, since phosphate is usually present in waters with other pollutants [16]. In our study, we applied materials made up of silica, which we modified with the use of polyethylenimine, to provide plenty of cationic functional groups, which could enhance phosphate removal from waters.…”