Capacitive deionization exploiting La-based LDH composite electrode toward energy efficient and selective removal of phosphate

Abstract

Capacitive deionization (CDI) is a promising technology for removing phosphate from wastewater. Its practical implementation is however hindered by the constraints on the electrode materials. To boost the adsorption capacity, phosphate selectivity, and cost-effectiveness of the electrode, this study proposed a composite electrode blending Lanthanum-based layered double hydroxide (Ca-La LDH) and activated carbon (AC). It capitalizes on the synergistic effects of electric double layer capacitance (EDLC) of AC and the diffusion-controlled charge storage (pseudocapacitive behavior) of Ca-La LDH. By optimizing the mass ratios of the constituents and the electrode material loading capacities, the composite electrode AC/Ca-La LDH-50₂₀ was developed, which contains 20 mg of 50 wt% Ca-La LDH. This composition achieved a remarkable phosphate adsorption capacity of 34.8 mg P /g and a low energy consumption of 0.0051 kWh/g P in constant voltage (CV) mode. It represented a 241 % increase in adsorption capacity (mg P/g) and 71 % decrease in specific energy consumption (kWh/g P) compared to the electrode made solely of AC. Particularly the moderate inclusion of Lanthanum contributes to its cost-effectiveness. Moreover, further studies extensively examined the impacts of electrical driving force, including applied voltage in constant voltage (CV) mode and applied current in constant current (CC) mode, on the phosphate removal efficiency. The composite electrode remained stable performance with the presence of the high content of coexisting anions (e.g.，Cl⁻，SO₄²⁻, HCO₃⁻, NO₃⁻), obtaining high selectivity coefficient of phosphate over other anions. This study highlighted the practical potential of AC/Ca-La LDH composite electrode for advancing CDI technology for phosphate removal in an efficient, energy-saving and cost-effective manner.