Citric acid as an alternative lixiviant for zinc oxide dissolution

The reactivity of ZnO during its dissolution by citric acid in different electrolytic environments was studied. It was observed that at low concentration (0.05 mol/L) citric acid was able to dissolve 90.4% of ZnO after 1 h at 50 °C. The addition of chlorides, nitrates and sulfates enhanced the dissolution to 98%, 84% and 67% respectively attained after 15 min at 40 °C. In the absence of citric acid, Cl−, NO3− and SO42 − also reacted with the solid surface leading to dissolve 4.6%, 4.9% and 13.7% of ZnO respectively at 50 °C after 60 min of reaction. The dissolution was controlled by surface chemical reaction in case of zinc dissolved by citric acid mixed with chloride and nitrate ions. When the dissolution was performed in the presence of citric acid alone and citric acid mixed with sulfates, the chemical reaction in an initial stage and transport through the boundary layer in a second stage were the rate controlling steps.