Effects of common groundwater ions on chromate removal by magnetite: importance of chromate adsorption.

Amanda H. Meena,Yuji Arai

doi:10.1186/s12932-016-0033-9

Abstract

BackgroundReductive precipitation of hexavalent chromium (Cr(VI)) with magnetite is a well-known Cr(VI) remediation method to improve water quality. The rapid (<a few hr) reduction of soluble Cr(VI) to insoluble Cr(III) species by Fe(II) in magnetite has been the primary focus of the Cr(VI) removal process in the past. However, the contribution of simultaneous Cr(VI) adsorption processes in aged magnetite has been largely ignored, leaving uncertainties in evaluating the application of in situ Cr remediation technologies for aqueous systems. In this study, effects of common groundwater ions (i.e., nitrate and sulfate) on Cr(VI) sorption to magnetite were investigated using batch geochemical experiments in conjunction with X-ray absorption spectroscopy.ResultsIn both nitrate and sulfate electrolytes, batch sorption experiments showed that Cr(VI) sorption decreases with increasing pH from 4 to 8. In this pH range, Cr(VI) sorption decreased with increasing ionic strength of sulfate from 0.01 to 0.1 M whereas nitrate concentrations did not alter the Cr(VI) sorption behavior. This indicates the background electrolyte specific Cr(VI) sorption process in magnetite. Under the same ionic strength, Cr(VI) removal in sulfate containing solutions was greater than that in nitrate solutions. This is because the oxidation of Fe(II) by nitrate is more thermodynamically favorable than by sulfate, leaving less reduction capacity of magnetite to reduce Cr(VI) in the nitrate media. X-ray absorption spectroscopy analysis supports the macroscopic evidence that more than 75 % of total Cr on the magnetite surfaces was adsorbed Cr(VI) species after 48 h.ConclusionThis experimental geochemical study showed that the adsorption process of Cr(VI) anions was as important as the reductive precipitation of Cr(III) in describing the removal of Cr(VI) by magnetite, and these interfacial adsorption processes could be impacted by common groundwater ions like sulfate and nitrate. The results of this study highlight new information about the large quantity of adsorbed Cr(VI) surface complexes at the magnetite-water interface. It has implications for predicting the long-term stability of Cr at the magnetite-water interface.Graphical abstractEffects of background anions (sulfate and nitrate) on the Cr(VI) surface coverage at the magnetite-water interface at pH 4 and 9

Highlights

Reductive precipitation of hexavalent chromium (Cr(VI)) with magnetite is a well-known Cr(VI) remediation method to improve water quality
point of zero salt effect (PZSE) of magnetite The PZSE of magnetite used in this study was determined using batch titrations with 0.01, 0.05 and 0.1 M NaNO3, and was approximately 5.63, slightly lower than reported PZSE and PZC values of synthetic magnetite, 6.3–6.8 [12, 19, 20]
Comparatively lower PZSE values have been reported with natural magnetite samples [21, 25, 26]

Summary

Introduction

Reductive precipitation of hexavalent chromium (Cr(VI)) with magnetite is a well-known Cr(VI) remediation method to improve water quality. It is well documented that Fe(II) in magnetite facilitates the reduction of Cr(VI) to Cr(III), subsequently forming Cr(III) hydroxide and or a mixed Cr(III)-Fe(III) hydroxide surface precipitate at the surface of an iron oxide [8,9,10,11]. These reductive precipitation reactions often refer to Cr(VI) sorption to magnetite. Based on the following half-reactions 1–3 [13, 14], overall reactions of Cr(VI) reduction by Fe(II) can be written in reactions 4 and 5

Objectives

Results

Conclusion