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Sulphate is the specific anion present in the acid rock drainage (ARD). Although sulphate is considered benign or with low toxicity, some countries have set limits for the wastewater discharged to natural receivers, often 500 mg/L or 600 mg/L. By conventional treatment of the ARD with calcium hydroxide, some gypsum can precipitate, depending on the ARD initial sulphate content, but the conformity for sulphate will not be achieved, because the concentration correlated to gypsum solubility exceeds the limit value by...