Highly efficient water softening by mordenite modified cathode in asymmetric capacitive deionization

Hardness ions mainly including Ca2+ and Mg2+ can cause water quality issue. Softening of hard water is still challenging nowadays. Capacitive deionization (CDI) has attracted much attention in recent years because of its high efficiency, environmental-friendliness and low energy consumption. Mordenite is a porous ion exchange material. Herein, the porous mordenite modified activated carbon (MOR-AC) was used as cathode to selectively remove Ca2+ and Mg2+ by CDI technique. At the optimal MOR doping amount, the MOR-AC electrode shows good wettability and high specific capacitance of 162 F/g. The removal amount of Ca2+ ions on the MOR-AC cathode reached 248 mu mol/g, much higher than that on the AC electrode (164 mu mol/g). Distinct from AC electrode, the MOR-AC electrode delivers a much enhanced selectivity to hardness ions in multi-salt mixture solution. In the synthetic hard water, the MOR-AC electrode displays a highly selective adsorption to Ca2+ and Mg2+ with the selectivity coefficient alpha Ca2+ Na+ and alpha Mg2+ Na+ of 8.0 and 7.5, respectively. This CDI device is very stable and regenerable, and the adsorption capacity of Ca2+ retains 87% after 50 cycles in hard water. In addition, the scaling of electrode has been efficiently inhibited.