Electrochemically activated layered manganese oxide for selective removal of calcium and magnesium ions in hybrid capacitive deionization

Selective removal of ions is a promising new venture with tremendous upsides for resource recovery and sustainability. Traditional methods such as membrane filtration, electrodialysis or ion-exchange resins are either non-selective or too expensive. To fulfil the gap in technology, a hybrid capacitive deionization (HCDI) device utilizing an electrochemically activated layered MnO2 cathode was proposed. The cathode was infused with structural water and can effectively screen strong electrostatic repulsions from Mn atoms within the MnO2 layers, enabling preferential intercalation of divalent cations Ca2+ and Mg2+. Comparative ion removal experiments showed a high selectivity of Ca2+ over K+ (S-D/M = 18.0) and Mg2+ over K+ (S-D/M = 17.5) at a discharge current of 10 mA g(-1) in equimolar binary salt solutions. Preferential divalent cation removal was even maintained in solutions where the ionic strength of monovalent cations was equivalent to divalent cations. In-situ Raman spectroscopy furthered showed that the intercalation of cations resulted in the replacement of water molecules which consequently altered the intensities of v(1) and v(2) bands of MnO2. The extent of water replacement was characterized by a change in the ratio of v(2) to v(1) intensity and higher divalent cation selectivities were observed to follow smaller changes in intensity ratio.