Source identification of chromium in the sediments of the Xiaoqing River and Laizhou Bay: A chromium stable isotope perspective

Abstract

Hexavalent chromium, Cr(VI), is a heavy metal contaminant and the reduction of Cr(VI) is accompanied by large isotopic fractionation. In this study, the sources of Cr were explored using the Cr isotopic composition of sediments from the Xiaoqing River, a heavily polluted river located in the Shandong Province of China, which flows into Laizhou Bay. The results show that δ⁵³Cr values of the sediments are the highest upstream near the pollution source, and gradually decrease along the river toward the range for igneous reservoirs observed near the estuary. Based on the calculation of authigenic Cr isotopic composition (δ⁵³Cr_auth) using the detrital index and leaching experiments, we suggest that the authigenic Cr in the sample near the pollution source with the highest δ⁵³Cr_auth value mainly comes from the reduction of Cr(VI) discharged by anthropogenic activity, and authigenic Cr in other samples in the midstream with δ⁵³Cr_auth values slightly higher than the range of igneous reservoirs may come from natural oxidative Cr weathering products. By introducing a Rayleigh model, we calculate that at least 31%-55% of Cr(VI) in the river water had been reduced to Cr(III) near the pollution source. Due to the self-purification ability of the river, Cr(VI) was reduced; thus, there is no record of high δ⁵³Cr_auth values in the downstream of the Xiaoqing River and Laizhou Bay, indicating no obvious Cr pollution in these locations. The limited variation of δ⁵³Cr values for samples from a sediment core in Laizhou Bay is also indicative of no obvious Cr pollution in the history. The Cr isotopic compositions of the river sediments are useful for the identification of Cr sources and can be used to advise environmental remediation on Cr pollution.

Keywords: Chromium contamination; Chromium isotopes; Leaching experiments; River sediments; The Xiaoqing River.