Hybrid capacitive deionization using MgAl-LDHs-coated graphite felt electrodes for phosphate removal

• MgAl-LDHs coated on oxidized graphite felt improved electrode capacitance. • Reversible phosphate adsorption of ∼10 mg P/g was maintained over 5 cycles. • Time-dependent uptake behavior explains the initial capacity decline. • The CDI system removed >80 % P from real water with minimal interference from competing anions. • Statistical analysis showed no significant difference between +1.0 V and open circuit operation. Capacitive deionization (CDI) is a promising technology for selective phosphate removal, though its performance is often constrained by electrode materials. This study developed composite electrodes by integrating MgAl-layered double hydroxides (LDHs) onto conductive oxidized graphite felt (OGF) to improve charge storage and ion selectivity. Two types were tested: CLGF prepared with commercial nitrate-intercalated LDHs, and LLGF synthesized with chloride-intercalated LDHs. Phosphate removal performance was evaluated in synthetic mixed ion solutions as well as in real lake sediment dewatering reject water. The novelty of this work lies in both the electrode fabrication method and the integration of MgAl LDHs with oxidized graphite felt. This combination provides dual functionality with enhanced phosphate selectivity and improved charge storage for practical CDI based phosphorus recovery. Kinetic modeling identified chemisorption as the main mechanism, with both LDH-coated electrodes outperforming bare OGF in adsorption and capacitance. LLGF and CLGF showed maximum phosphate removal capacities of ∼60 mg/g, while pristine GF and OGF showed negligible ion adsorption capacity. CDI based steady state adsorption capacities stabilized at ∼10 mg/g over 5 cycles during phosphate removal from 1.0 mM mixed anions solution. Phosphate-to-sulphate selectivity coefficients were highly time dependent, reaching 2.0 (CLGF) and 4.3 (LLGF) under +1.0 V applied voltage. CLGF removed over 80 % of phosphate in reject water at both +1.0 V and open circuit (OC), while LLGF achieved moderate phosphate removal of about 57 % with better selectivity. Energy consumption for the CDI system ranged from 0.03 – 0.25 kWh/m 3 , within reported CDI benchmarks. Statistical analysis revealed that removal performance was significantly influenced by electrode-time and electrode-voltage interactions rather than individual factors. Overall, this study demonstrates MgAl-LDHs-OGF electrodes as a feasible electrode for lake water P removal with high selectivity towards phosphate over other competing anions.