Effective Extraction of Cr(VI) from Hazardous Gypsum Sludge via Controlling the Phase Transformation and Chromium Species

Through controlling the phase transformation and chromium species under hydrothermal condition, the Cr(VI) was extracted fully from hazardous Cr(VI)-containing gypsum sludge, with a very high efficiency of more than 99.5%. Scanning transmission electron microscopy, X-ray absorption fine structure, and density functional theory calculation results revealed that the dissolution-recrystallization of CaSO4·2H2O into CaSO4 was the key factor to fully release the encapsulated Cr(VI). Moreover, the mineralizer (persulfate salt) provided H+ and SO42- ions, the former made an acidic condition to transform the released CrO42- into the specie (Cr2O72-) with less similarity to SO42-, which further prevented the recombination of the released Cr(VI) with gypsum; and the latter was essential to accelerate crystal growth of calcium sulfate so as to enhance Cr(VI) extraction. This work would provide an instructive guidance to fully extract heavy metals from hazardous solid wastes via the control of crystal transformation and the pollutant species.