Catalytic role of soils in the transformation of Cr(VI) to Cr(III) in the presence of organic acids containing α-OH groups

Chromium contamination in soils and natural waters is a significant environmental concern. Remediation efforts often rely on the conversion from the highly toxic Cr(VI) to Cr(III) which has lower toxicity and less environmental mobility. In the present study, the catalytic role of soils in the transformation of Cr(VI) to Cr(III) by two organic acids containing α-OH groups was investigated with or without light. Three types of widely distributed soils in China, Yellow-brown, Chernozem and Red Soils, were examined. The results demonstrated that the presence of soils markedly facilitated the Cr(VI) reduction by organic acids. Without light, the average rate of Cr(VI) reduction by citric acid with 25 g L− 1 soil was increased by 3.9 (Red Soil), 17.5 (Chernozem) and 44.8 times (Yellow-brown Soil), respectively as compared with control (without soil). For tartaric acid, it was 2.6 (Red Soil), 4.2 (Chernozem) and 6.6 times (Yellow-brown Soil) as much as the control. With light and at 1.5 g L− 1 soil loading, the average rates of Cr(VI) reduction increased from 6.2 (citric acid alone) to 27.8 (Red Soil), 44.6 (Yellow-brown Soil), and 57.7 μM h− 1 (Chernozem), an increase of 4.5, 7.2 and 9.3 times, respectively. Tartaric acid showed weaker soil-induced enhancement of photochemical reduction of Cr(VI) than citric acid. The reaction rates in all three soil suspensions added with tartaric acid tended to be approximately the same. Our findings suggest that the catalytic role of soils in the reduction of Cr(VI) by organic acids is mainly from Mn(II) in soil without light and from iron ions in soil with light.

Research Highlights
► Soil catalytic role in Cr(VI) reduction by organic acids in dark results from Mn(II).
► The effect of Fe(III) on Cr(VI) reduction by organic acids with light is greater than Mn(II).
► The complex formation between Mn(II)/Fe(III) and organic acids benefits Cr(VI) reduction.