Goethite-modified peanut shell biochar for selective phosphate recovery from digestate-derived permeate via membrane capacitive deionization

Selective phosphate recovery from digestate-derived effluents remains a technical bottleneck due to high background salinity and strong multi-anion competition. To address this, waste peanut shells were valorized into activated biochar and functionalized with goethite (α-FeOOH) to form a regenerable anode for membrane capacitive deionization (MCDI). When treating real anaerobic-digestion membrane-bioreactor (AD-MBR) permeate, the optimized 20 wt% α-FeOOH/BC anode achieved a phosphate enrichment factor of 11.46 ± 0.61 and selectivity factors ≥ 5.47 against major competing anions, producing a phosphate-rich concentrate of 695–790 mg L −1 . The process remained stable over 20 adsorption-desorption cycles, maintaining a desorption efficiency of 0.94 ± 0.03 and negligible Fe loss (0.59% of the initial Fe inventory). XPS analysis confirmed the mechanism involved reversible inner-sphere Fe–O–P complexation. The mean recovery-specific energy consumption was 9.07 ± 0.64 kWh kg −1 -P. Overall, this study establishes a sustainable waste-to-resource framework, demonstrating that α-FeOOH-functionalized biochar electrodes can effectively couple agricultural residue valorization with closed-loop phosphate recovery from complex digestate permeates. • Peanut-shell biochar supports goethite anode for phosphate-selective MCDI. • Phosphate selectivity (coefficient > 5.47) was achieved in real AD-MBR permeate. • Stable 20-cycle cycling achieved 94% desorption efficiency and 0.6% Fe loss. • XPS indicates reversible Fe–O–P binding with re-hydroxylation on regeneration • Competitive SEC (∼9.1 kWh kg -1 -P) supported sustainable nutrient looping.