Characteristic and model of phosphate adsorption by activated carbon electrodes in capacitive deionization

The advanced treatment for phosphate (P) has been focused on in the wastewater treatment field in recent years. Developing an innovative P removal technology with high efficiency, low cost, and low energy consumption has become an imperative requirement. Capacitive deionization (CDI) is an emerging environmentally friendly technology for removing charged ions from aqueous solutions. In this study, we investigated the performances of P removal by CDI with homemade activated carbon electrodes. Furthermore, kinetics, thermodynamics and the equilibrium Gouy-Chapman-Stern (GCS) double-layer model were studied to reveal the mechanism of P adsorption. It was observed that the adsorption capacity was positively correlated with the voltage. The pH first increased and then decreased during the adsorption because of water electrolysis and forms transformation of phosphate. The experimental results were validated by pseudo-first-order adsorption kinetics and Freundlich isotherms. The adsorption rate increased with increasing temperature, voltage, and concentration. The adsorption capacity decreased with increasing temperature, and the maximum adsorption capacity was 8.53 mg/g. Thermodynamic analysis confirmed that the electrosorption of P on the activated carbon electrode was an endothermic process, and the high temperature was conducive to reducing the adsorption potential of P on the activated carbon electrode. The theoretical results of the double layer model were in agreement with the experimental data, which showed that the ion adsorption can be reflected by surface charge. This study can help to understand the adsorption mechanism of P on activated carbon electrodes and provide a theoretical basis for the application of P removal by CDI.