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.