A molecular-design approach for selective sulfate separation from competitive acidic and alkaline aqueous media

Selective and efficient removal of sulfate from aqueous solution having a high concentration of other competing ions is an important aspect of separation science technology and has attracted considerable attention from researchers to develop molecular systems to achieve this challenging goal. Selective sulfate separation from aqueous nuclear waste media with a high nitrate concentration and seawater with a high chloride concentration are the two main objectives to be accomplished along this line. Nuclear power plant-generated radioactive waste disposal and highly effective membrane-based seawater desalination processes require prior removal of corrosion-inducing hydrophilic sulfate ions from the aqueous media to avoid possible environmental risks and membrane blockage, respectively. Further, sulfate removal from highly acidic wastewater discharged from mining and metallurgical industrial operations needs to be seriously addressed to avoid irreversible damage to the aquatic environment. Therefore, to achieve selective sulfate separation from water, several hydrogen bond donor (HBD) macrocyclic and acyclic anion receptors having higher binding affinity for sulfate over other anions have been synthesized. The sulfate removal efficacy of anion receptors has been demonstrated by the industrially applicable liquid–liquid (solvent) extraction method and proof of concept technique involving the selective crystallization (precipitation) of a receptor–sulfate complex from aqueous solution. In this review, we provide the detailed development of sulfate-selective synthetic receptors and their application in effective sulfate separation from simulated wastewater media and seawater. Since the pioneering paper by Sessler and Moyer et al. (2007), significant progress has been made in this field, which needs to be thoroughly assessed and understood to deliver suitable chemical technology for selective sulfate separation.