Effect of Organic Amendments on the Speciation of Chromium in Contaminated and Uncontaminated Soils

Quantitative speciation of metals and their variability over time is a critical concept in environmental soil chemistry. Chromium is a heavy metal that has negative effects on the environment and human health. There have been several attempts to study the behavior of chromium in soils and its removal from soil. However, there is relatively little knowledge about the immobilization of chromium and its chemical speciation in contaminated and uncontaminated soils when different organic amendments are applied. A laboratory incubation experiment was conducted in a completely randomized design to evaluate the effect of organic amendments on the speciation of chromium in contaminated and uncontaminated soils. Four organic amendments were used: farmyard manure (FYM), rice straw compost (RSC), vermicompost (VC), and biochar (BC). Each of them was applied at 1% to contaminated as well as uncontaminated soils. In both the amended and unamended soils, chromate (CrO42-), bichromate (HCrO4-), and calcium chromate (CaCrO4) in soil-water system followed the order CrO42-> HCrO4-> CaCrO4. The application of FYM, RSC, VC, and BC significantly decreased the CrO42- and CaCrO4 in the soil-water system of both soils. However, only the application of BC affected the HCrO4- contaminated soils. In both soils, HCrO4- exists as the predominant chromium species in the pH range of 2.0 to 6.25. At pH 6.25, CrO42- and HCrO4- each contribute 50% of the total dissolved chromium in soil solution. CrO42- dominated in the system as the pH increased from 6.25 till 8.0. The maximum concentration of CrO42- was at pH 8.0. At pH <3.0, CrO3SO42- contributes less than 10%. Overall, organic amendments decreased the concentration of Cr species in both soils. However, the decrease in concentration was highly dependent on the nature of the amendment applied. Therefore, organic amendments can be exploited to remediate chromium-contaminated soils.