Stabilization of Chromium (VI) in the Presence of Iron (II): Method Development and Validation

Daniel Mahringer,Fatima El-Athman,Chantal Polenz

doi:10.3390/w12040924

Abstract

The presence of Cr (VI) in drinking water is mainly caused by leaching of chromium-containing aquifer material into groundwater. In contrast to Cr (III), it has been classified as highly toxic. For this reason, the WHO recommends the implementation of separate guideline values, instead of the so far used limit value of total chromium. The separate evaluation of Cr (VI) in raw water and during removal processes requires the Cr (VI) concentration to remain stable after sampling. In the presence of Fe (II), a stabilization of the samples is necessary to inhibit further reduction of Cr (VI) by Fe (II). In this study, two methods of Cr (VI) stabilization in Fe-(II)-containing water samples are investigated: Fe (II) oxidation by oxygen at high pH values in the presence of buffers and Fe (II) complexation by chelating agents. When adding hydrogen carbonate buffer, Cr (VI) recovery reached 100% at pH values of 10 to 12 in the presence of up to 3 mg L–1 Fe (II). Using hydrogen phosphate buffer, Cr (VI) recovery reached 100% only at pH 12 but for a Fe (II) concentration up to 6 mg L–1. Ammonium buffer was found to be less suitable for Cr (VI) stabilization. The addition of EDTA and citrate resulted in low recovery of Cr (VI), whereas citrate was found to accelerate the Cr (VI) reduction.

Highlights

Chromium is a transition metal and the twenty-first most abundant element in the earth’s crust [1].Due to its high corrosion resistance, it is often used in alloys, mainly for the production of stainless steel [2]
The dominant naturally occurring form of chromium is the trivalent state (Cr (III)), while hexavalent chromium (Cr (VI)) compounds are only found in small quantities [1]
The oxidation state and complexation of chromium is determined by the redox potential, the pH, and the total chromium concentration of the medium [7]

Summary

Introduction

Chromium is a transition metal and the twenty-first most abundant element in the earth’s crust [1].Due to its high corrosion resistance, it is often used in alloys, mainly for the production of stainless steel [2]. The dominant naturally occurring form of chromium is the trivalent state (Cr (III)), while hexavalent chromium (Cr (VI)) compounds are only found in small quantities [1]. Anthropogenic sources, such as effluent discharges of chromium-working industries, make up the largest part of Cr (VI) occurrence in the environment [3]. The oxidation state and complexation of chromium is determined by the redox potential, the pH, and the total chromium concentration of the medium [7]. The hexavalent soluble species hydrogen chromate, chromate, and dichromate are formed depending on the pH and chromium concentration [3]

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Conclusion