Phase transformation and crystal growth of gypsum via hydrothermal treatment with high pressure CO2 for Cr(VI) extraction

Heavy metal-containing gypsum poses a serious threat to the environment. In this work, the Cr(VI) was extracted fully (>= 99.6%) from hazardous Cr(VI)-containing gypsum sludge via hydrothermal treatment with high pressure CO2, and neutral solutions with a high Cr(VI)-concentration can be recovered due to the instability of carbonic acid. It was found that the phase transformation occurred in hydrothermal stage and crystal growth in standing stage were two essential parts for releasing and separating Cr(VI). The crystal growth can be accelerated by decreasing pH or the solid-liquid ratio. Moreover, the form of Cr(VI) species changed and the size of products increased under the acidic and high-pressure environment generated by passing CO2, which repelled the adsorption of Cr(VI). This work would provide an environment-friendly measure to fully extract heavy metals from hazardous gypsum wastes with just water and CO2.

Automated summary

In this study, Cr(VI) was fully extracted from hazardous gypsum sludge using hydrothermal treatment with high pressure CO2, leading to the recovery of high Cr(VI)-concentration solutions. The crystal growth process was found to be essential for releasing and separating Cr(VI), and could be accelerated by adjusting pH or the solid-liquid ratio. Additionally, changes in the form of Cr(VI) species and product size were observed under the acidic and high-pressure environment generated by passing CO2.