Development of novel ZnZr-COOH/CNT composite electrode for selectively removing phosphate by capacitive deionization

• The terephthalic acid intercalated carbon nanotube composite was developed for phosphate removal by CDI. • ZnZr-COOH/CNT electrode showed greatest phosphate adsorption capacity. • Phosphate adsorption occurred via the electrostatic attraction, inner sphere complexation and coordination exchange. • The energy consumption of CDI dephosphorization was sixteenth of that of chemical phosphate removal process. The discharge of phosphate from sewage water can lead to water eutrophication and endanger the aquatic ecosystem. Capacitive deionization (CDI) has displayed great potential for salt removal and nutrient recovery due to its excellent performance such as environmental friendliness and energy efficient. In this study, the terephthalic acid intercalated carbon nanotube composite material (ZnZr-COOH/CNT) was developed as an anode and its selective removal of phosphate in CDI was investigated. At a applied voltage of 1.2 V, ZnZr-COOH/CNT electrode exhibited great adsorption capacity of phosphate with low concentration (∼10 mg/L) at a wide range of pH (3 – 10), with the equilibrium concentration as low as 0.3 mg/L which was less than the national first-class emission standard (0.5 mg/L). Moreover, the adsorption of phosphate by ZnZr-COOH/CNT composite electrode was mildly disturbed by the coexisting ions (Cl − , NO 3 – , SO 4 2− and HCO 3 – ). X-ray Photoelectron Spectroscopy (XPS) analysis indicated that the zirconium and zinc were coordinated with phosphate to form inner sphere complex, and the hydrogen bond was formed between the hydroxyl group of phosphate and the carboxyl group of electrode material, which enhanced the selective adsorption of phosphate. Furthermore, the energy consumption of CDI dephosphorization was calculated as low as 0.0075 kWh/g P and 0.043 kWh/m 3 water at 1.2 V. This study was expected to provide the potential electrode materials and theoretical basis for the application of CDI dephosphorization.