Synergistic immobilization of Cr from real tannery sludge by formation of spinel phases with TiO2 and ZnO

With the rapid development of tanning industry, a noticeable number of chromium-containing tannery sludge (Cr-ITS) has been produced, which poses an impending threat to ecosystem and human health, and needs special disposal ahead of landfilling. In this study, an alternative method for thermally stabilizing Cr-ITS via employing ZnO and TiO2 as dopants was developed. Oxidation of Cr(III), competitive formation of Cr-spinel, immobilization of Cr, and leaching toxicity of sintered products were investigated during Cr-ITS thermal treatment at the temperature range of 800-1200 degrees C. Cr in Cr-ITS could be immobilized effectively through transforming Cr into spinel structure, as demonstrated by XRD analysis. Thus, the toxic Cr-ITS would be remedied and detoxified. Co -doping of ZnO and TiO2 not only noticeable inhibited oxidation of Cr(III), but also accelerated conversion of Cr (III) to spinel dominated by ZnCr2O4. Increasing temperature was conducive to the formation of spinel. Most of Cr were incorporated into spinel phase when thermal treatment temperature was over 1000 degrees C, where Cr(VI) leached was not detected, and leaching concentrations of Cr and Zn after prolonged leaching time of 25 d were 13.32 mg/L and 23.07 mg/L respectively, which all meet regulatory limits. When increasing temperature to 1100 degrees C, their leaching risk was further reduced to 4.39 mg/L and 5.24 mg/L respectively, successfully achieving simultaneous stabilization of heavy metals. The overall results indicate that Cr-ITS immobilized with ZnO and TiO2 is a promising and feasible strategy by incorporating Cr into spinel phases using sintering temperature reached in ceramic industry.

Automated summary

This study developed an alternative method for thermally stabilizing chromium-containing tannery sludge (Cr-ITS) by using ZnO and TiO2 as dopants. The results showed that Cr could be immobilized effectively by transforming it into a spinel structure. The co-doping of ZnO and TiO2 inhibited the oxidation of Cr(III) and accelerated the conversion of Cr(III) to spinel. Increasing temperature promoted the formation of spinel. The overall results indicate that immobilizing Cr-ITS with ZnO and TiO2 using sintering temperature reached in the ceramic industry is a promising and feasible strategy.