Enhanced electrochemical performance and boron removal capacity of Mg-Al layered double hydroxide composite for membrane-free capacitive deionization applications

Water purification is significantly inhibited by the ineffective removal of boron from aqueous solutions, particularly because traditional membrane-based purification exhibits poor removal of neutral boric acid. This study investigates the efficiency of a Mg-Al layered double hydroxide electrode supported on carbon paper for boron removal by capacitive deionization (CDI). Cyclic voltammetry was used to confirm the capacitive properties of the electrode, which displayed quasi-rectangular profiles with notable reversibility and rate capability. Chronoamperometric observations revealed a voltage-dependent electrosorption activity; 1.2 V showed optimal charge storage and boron removal performance. At this voltage, the system achieved a mean deionization capacity of 2.58 mg/g, over 94 % boron removal within 20 min, and a peak charge efficiency of 79 %, accompanied by a moderate energy consumption of 0.82 kWh/m3. The system exhibited a notable pH dependence and the optimal performance at pH 11 correlated with the dominance of the negatively charged borate ion (B(OH)⁻). Increasing the pH increased the capacity and rate in the Ragone plot (mean deionization capacity vs. mean deionization rate). Although system efficiency dropped slightly to 88 % after 120 min of operation, the electrode showed remarkable cyclic stability across several charge-discharge cycles. Dynamic variations in real-time pH monitoring during CDI were observed, with an initial increase during charging, followed by a slow decline during discharging. The results demonstrate the effectiveness of Mg-Al layered double hydroxide-based CDI systems as flexible and chemically strong platforms for boron removal in water treatment applications.