Challenges and opportunities in the removal of sulphate ions in contaminated mine water: A review

Metal sulphates are one of the major contributors to acid mine drainage (AMD) or acid rock drainage (ARD). AMD occurs by the oxidation of metal sulphides. Sulphides produce sulphates and eventually acidify the medium by converting to sulphuric acid. As AMD contaminates the water sources downstream, the set limit of sulphates for human consumption gets compromised. Stringent standards are imposed on, to comply with limitations set by the regulatory bodies. World Health Organization (WHO) emphasises on a 250 mg/L sulphate concentration in water for human consumption. Therefore, curing AMD of sulphates has become one of the prominent issues in water research. Unregulated disposal of such drainage may cause the increase in salinity and increase in the pH, which can be detrimental to the utility of the water downstream. Corrosion, scaling and health implications are definite results of AMD. Numerous approaches are available to treat sulphates from AMD. Suitability of a specific method depends on the level of removal expected, environmental legislations, available resources, space required, economy and volume of the contaminated water. Several approaches which have demonstrated promising results in the laboratory scale, but their viability at the industrial scale is yet to be established. This paper reviews the remediation methods which are currently in practice. It discusses the approaches in two main topics, both conventional and recent developments. While the conventional methods include lime, limestone and wetlands, the recent developments include filtration, electrocoagulation, adsorption, ion exchange and precipitation with the introduction of a certain level of novelty throughout the last few years. It is well established that lime and limestone treatment of AMD are well suited for pre-treatment processes whereas the rest of the methods can be selected upon the site specific requirements. Even though the novel methods show their potential to reduce sulphate ions greatly, these need to be tested at industrial scale in order to identify the overall effectiveness.