Green ceramsite production via calcination of chromium contaminated soil and the toxic Cr(VI) immobilization mechanisms

Chromium contaminated soil (CCS) is challenging to be disposed because of the high toxicity and mobility of Cr(VI). This paper proposed a way to dispose CCS through calcining ceramsite. Three CCSes and an ordinary soil were selected, and a suitable calcination temperature was determined according to the properties of ceramsite. The stability of Cr(VI) immobilization in ceramsite was investigated and the Cr(VI) immobilization mechanism was analyzed through XRF, XPS, XRD, and BSE. Results showed that all three CCSes could be used to manufacture ceramsite when the calcination temperature was higher than 1100 °C, and the optimal calcination temperature was 1180 °C. The Cr(VI) leaching concentration of ceramsite was much lower than that specified in standard. It was also observed that the calcination temperature should be higher than 1000 °C to ensure the immobilization of Cr(VI). Otherwise, the Cr(VI) leaching concentration would be much higher than that of the original CCS. The Cr(VI) immobilization mechanism of ceramsite was contributed to the transformation of Cr(VI) to Cr(III) and the formation of chromium-bearing mineral, (Mg,Fe)(Cr,Al)2O4, as long as the calcining temperature was higher than 1100 °C. This work found that use of CCS in manufacturing ceramsite is a safe way of CCS utilization, but the calcination temperature should be carefully determined. All the results could be potentially referred for the safe and valuable utilization of waste soil.