Ion exchange resin – Bipolar membrane electrodialysis hybrid process for reverse osmosis permeate Remineralization: Preparative ion exchange chromatography for Ca2+ and Mg2+ recovery

Reverse osmosis (RO) membrane technology is increasingly applied for the production of potable water on a large scale. However, slightly acidic and unbuffered RO permeate can enhance water piping network deterioration. Therefore, post-treatment i.e., remineralization is commonly applied to protect the water distribution network. An ion exchange resin – bipolar membrane electrodialysis hybrid process has been recently developed for RO permeate remineralization. In the investigated remineralization process, the cation exchange resin regenerant stream – rich with mainly divalent ions – is used partially for RO permeate remineralization. The viability of using preparative ion exchange chromatography principles for monovalent – divalent ion separation was investigated. The elution process was investigated in the case of both single and binary (calcium and potassium as a case study) components. Elution velocity, as well as concentration, was shown to affect the separation process significantly, while elution mode (co/counter current) did not substantially influence the elution process for the conditions studied. Gradient elution profiles (linear and stepwise) were superior to the non-gradient method of ion exchange elution in achieving higher purity and yield of the recovered calcium using the same amount of eluent. Linear gradient elution improved the purity of the calcium recovered to > 99% compared to conventional isocratic elution (85%) at 60% yield. Likewise, linear-gradient elution resulted in higher calcium specific productivity at>95% purity, which is necessary for an efficient remineralization process to avoid adding impurities to the remineralised water. This research showed that preparative ion exchange chromatography principles can be applied in drinking water applications to achieve high purity (>95%) and sufficient specific productivity of the recovered hardness ions (Ca2+ = 0.15–0.20 mol/L.h) required for RO permeate remineralization.