Assessing the phytotoxicity of chromium in Cr(VI)-spiked soils by Cr speciation using XANES and resin extractable Cr(III) and Cr(VI)

Abstract

The phytotoxicity of soil Cr usually depends on the plant availability of Cr(VI) in Cr-contaminated soils. However, Cr(VI) is favorably reduced to Cr(III) under acidic conditions, and increased availability of Cr(III) in acid soils can also cause phytotoxicity. The objective of this study was to determine the Cr phytotoxicity in acid soils in relation to their oxidation state and availability. Chromium X-ray absorption near edge structure spectroscopy (XANES), Dowex-M4195 and Chelex-100 resins, and wheat seedling growth experiments were used to determine the extent of Cr(VI) reduction, extractable Cr(VI) and Cr(III), and the phytotoxicity in two Cr(VI)-spiked acid soils. The results of the XANES spectra showed that Cr(VI) added into the Neipu soil, which had a high content of organic matter, was completely reduced to Cr(III). In addition, both resin extractable Cr(VI) and Cr(III) were very low. Meanwhile, no toxic effect of Cr on the wheat seedlings was observed and the wheat seedling growth increased with the increase in pH as a result of Cr addition. However, for the Pinchen soil which has a low content of organic matter, the XANES spectra showed that Cr(VI) could not be reduced completely and that both resin-extractable Cr(VI) and Cr(III) increased with the addition of Cr. The growth of the wheat seedlings also decreased with the addition of Cr(VI) >500 mg kg−1soil. The significant retardation of the wheat seedlings grown in the Pinchen soil was the result of both Cr(III) and Cr(VI) simultaneously. The speciation of total Cr by XANES and using resin extraction for determination of available Cr(III) and Cr(VI), as demonstrated using Cr(VI)-spiked acid soils in this study, can be used to assess the phytotoxicity of Cr in Cr-contaminated soils.