Characterization of adsorbent composition in co-removal of hexavalent chromium with copper precipitation.

Mechanisms of hexavalent chromium co-removal with copper precipitation by dosing Na2CO3 were studied with a series of well-designed batch tests using solutions containing 150 mg l-1 Cu(II) and 60 mg l-1 Cr(VI). It was found that direct precipitation of chromium through formation of copper-chromium bearing precipitates (in the form of CuCrO4) was one of the main mechanisms contributing to chromium co-removal at pH close to 5.0, and adsorption of chromium at a higher pH by freshly formed copper-carbonate precipitates (adsorbent) contributed to further chromium co-removal. Since, according to solubility products, neither copper-carbonate nor copper-hydroxide precipitates can be produced at pH around 5.0 for a pure 150 mg l-1 copper precipitation, characterization of copper-carbonate precipitates (adsorbent) was carried out through developing pC-pH curves of the systems by both equilibrium calculations and MINEQL+ 4.5 (a chemical equilibrium modeling software), and also through laboratory determination of the precipitate composition, such as gravimetric analyses, inorganic carbon percentage and EDAX spectrum analyses. CuCO3.Cu(OH)2, or a combination of CuCO3.Cu(OH)2 (in majority) and Cu(OH)2 (in minority) were suggested to be the major constituent of the precipitates obtained from the copper solution with Na2CO3 dosing.