Removal of selenocyanate in water by precipitation: Characterization of copper-selenium precipitate by X-ray diffraction, infrared, and X-ray absorption spectroscopy

During refining of sour crude oils produced from reservoirs containing seleniferous marine shales, selenium is concentrated into process waters and wastewaters. Selenium, present in the waters as selenocyanate, must be removed to meet regulatory discharge limits. Treatment of wastewaters with copper(ll) salts effectively precipitates selenium. An objective of the present study was to determine whether selenium was precipitated as elemental Se or immobilized in the form of cyanate complexes or any other form that remains to be identified. The precipitate had a Se/(Se+S) mole ratio of 0.09. The incorporation of selenium in the copper thiocyanate lattice was proven by X-ray diffraction. The presence of native selenium was disallowed by Se K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Selenium was shown by EXAFS to be present as selenide and surrounded by one nearest C atom and three next-nearest Cu atoms. This local environment is typical of the copper thiocyanate structure, which proves that Se substitutes for S on the atomic scale. Based on results of chemical analyses and structural methods, it is concluded that the sludge consists primarily of an alpha-Cu(So(0.91)Se(0.09))CN solid solution. Some copper is not reduced and is liganded to O, OH, or H2O. These results indicate that, when added to process water, cupric ions are reduced by sodium thiosulfate and sulfites initially present in the solution to yield primarily cuprous thiocyanate/selenocyanate.