Effect of organic matter on Cr(VI) removal from groundwaters by Fe(II) reductive precipitation for groundwater treatment

Abstract

Due to its toxicity, Cr(VI) is undesirable in groundwater. Its chemical reduction to Cr(III) species, followed by precipitation is the most widely practiced treatment technique for the removal of Cr(VI) from polluted waters. The resulting Cr(III) species present low solubility, is much less toxic, and can be subsequently removed either by precipitation, or by adsorption onto iron oxy-hydroxides and co-precipitation. The effects of several parameters, such as the pH value of water to be treated, the applied Fe(II) dose, and the presence of appropriate mineral surfaces, are well investigated and understood. However, the impact of the presence of humic acids (HAs) in this process has only been considered by rather few studies. The main aim of this study was to determine the effect of humic substances on Fe(II) reductive precipitation of Cr(VI) within a pH range relevant for drinking water treatment. Jar test experiments were performed, using artificial groundwater of defined composition and initial Cr(VI) concentration 100 μg/L, ferrous sulphate dosages 0.25–2 mg Fe(II)/L, and pH values 6.5–8. It was found that Cr(VI) and total chromium (Cr(total)) can be reliably removed in the absence of HAs in the tested pH range with the addition of Fe(II) dosage of 1 mg Fe(II)/L. Further on, the results indicated that the reduction of Cr(VI) is only slightly affected by the presence of HAs. However, increased residual total Cr concentrations were found at lower Fe(II) dosages and/or higher pH values. Additionally, the removal of the Cr(III) species formed during Cr(VI) reduction was strongly inhibited by the presence of HAs under the examined experimental conditions, since residual concentrations higher than 60 μg/L were determined. The results of this study will have implications to the ongoing discussion of a new, stricter, European Union regulation limit, regarding the presence of total chromium in drinking water.

Due to its toxicity, Cr(VI) is undesirable in groundwater. Its chemical reduction to Cr(III) species, followed by precipitation is the most widely practiced treatment technique for the removal of Cr(VI) from polluted waters. The resulting Cr(III) species present low solubility, is much less toxic, and can be subsequently removed either by precipitation, or by adsorption onto iron oxy-hydroxides and co-precipitation. The effects of several parameters, such as the pH value of water to be treated, the applied Fe(II) dose, and the presence of appropriate mineral surfaces, are well investigated and understood. However, the impact of the presence of humic acids (HAs) in this process has only been considered by rather few studies. The main aim of this study was to determine the effect of humic substances on Fe(II) reductive precipitation of Cr(VI) within a pH range relevant for drinking water treatment. Jar test experiments were performed, using artificial groundwater of defined composition and initial Cr(VI) concentration 100 μg/L, ferrous sulphate dosages 0.25–2 mg Fe(II)/L, and pH values 6.5–8. It was found that Cr(VI) and total chromium (Cr(total)) can be reliably removed in the absence of HAs in the tested pH range with the addition of Fe(II) dosage of 1 mg Fe(II)/L. Further on, the results indicated that the reduction of Cr(VI) is only slightly affected by the presence of HAs. However, increased residual total Cr concentrations were found at lower Fe(II) dosages and/or higher pH values. Additionally, the removal of the Cr(III) species formed during Cr(VI) reduction was strongly inhibited by the presence of HAs under the examined experimental conditions, since residual concentrations higher than 60 μg/L were determined. The results of this study will have implications to the ongoing discussion of a new, stricter, European Union regulation limit, regarding the presence of total chromium in drinking water.