Effects of liming on Cr(VI) reduction and Cr phytotoxicity in Cr(VI)-contaminated soils

Liming is the most common approach for the amelioration of soil acidity in agriculture, and is widely used to reduce the mobility and bioavailability of heavy metals in soil. The purpose of this study was to investigate the hexavalent chromium [Cr(VI)] reduction and chromium (Cr) phytotoxicity in Cr(VI)-contaminated soils at different pH levels as a result of liming. Calcium carbonate (CaCO3) was added to two acid agricultural soils of Taiwan (Neipu and Pinchen) at their natural pH of about 4, to adjust soil pH to approximately 5 and 6, respectively. The soils were then spiked with six levels of Cr(VI) (0, 150, 300, 500, 1000 and 1500 mg kg−1). X-ray absorption near edge structure spectroscopy (XANES) of Cr was used to determine the extent of the reduction of Cr(VI) at different pH levels. At the same time, extractions of Cr by Dowex M4195 and Chelex 100 resins were carried out to determine the availability of Cr(VI) and trivalent chromium [Cr(III)] in the Cr(VI)-spiked soils, respectively. Also, a pot experiment with wheat (Triticum vulgare) seedlings was carried out to test the phytotoxicity of the Cr(VI)-spiked soils. The results showed that for Cr(VI)-contaminated soils which contain a high amount of organic matter, such as the Neipu soil, the effect of liming on Cr(VI) reduction and Cr phytotoxicity is insignificant. However, for Cr(VI)-contaminated soils which have a low amount of organic matter, such as the Pinchen soils, liming could decrease the extent of Cr(VI) reduction and increase the availability of Cr(VI), thereby enhancing the phytotoxicity of Cr.