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The effect of different vacancy defects on the adsorption and solidification of Cd on gypsum was studied. The results show that increasing Ca vacancy defects in gypsum can improve the adsorption and curing properties of gypsum for cadmium.

Phosphogypsum is a by-product of phosphoric acid production by a wet-process and can be used to prepare adsorption materials to treat Cd(II) in sewage. Its main component is calcium sulfate dihydrate (gypsum). In the present study, incorporation and adsorption of cadmium on gypsum in an aqueous environment have been investigated using dispersion-corrected density functional theory, and the effect of different vacancy defects on the adsorption and doping of Cd on gypsum was also studied. The results show that Cd impurity defects can form more easily in gypsum crystals with Ca vacancy defects. The increase in both calcium vacancy defects and sulfate vacancy defects is beneficial to the incorporation of Cd in calcium sulfate dihydrate. In addition, the calcium vacancy defects on the gypsum surface can promote the adsorption of Cd(II), while sulfate vacancy defects on the gypsum surface inhibit the adsorption of Cd(II). Therefore, increasing the Ca vacancy defects of gypsum will help in improving the adsorption and curing properties of phosphogypsum for cadmium.

The adsorption of oxygen molecule (O 2 ) on galena (1 0 0) surfaces containing Ag, Cu, Bi, and Mn impurities was investigated using density functional theory (DFT) method. The calculated results show that the impurities could change the semiconductor electronic structure of galena surface and influence the adsorption of oxygen molecule. The semiconductor type of Cu- and Ag-doped galena surfaces are p -type and that of Mn- and Bi-doped galena surface are n -type. Cu, Ag and Mn impurities greatly influence the Mulliken charge of adjacent S atoms, and the Bi impurity has small influence on galena surface. The adsorption of O 2 on Cu-, Ag- and Bi-doped galena surface are stronger than that of the perfect galena, while the presence of Mn impurity weakens the adsorption of oxygen. For the Bi-doped galena, a strong interaction of Bi with O atom is observed. CV examinations results show that Cu- and Ag-doped galena have stronger oxidation peaks than perfect galena, and the CV curve of Bi-galena slightly enhance the oxidation of galena, while the oxidation peak is completely disappeared for Mn-galena.