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

Abstract

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. 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-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.

Keywords

Phosphate recovery / Peanut shell biochar / Anaerobic digestion / Membrane capacitive deionization / Selectivity

CRediT authorship contribution statement

Yuxin Qu: Writing – original draft, Validation, Data curation, Conceptualization. Yin Lu: Visualization, Software. Jiakang Li: Methodology, Formal analysis. Jaehac Ko: Writing – review & editing, Funding acquisition. Qiyong Xu: Writing – review & editing, Supervision, Project administration, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by National Natural Science Foundation of China (22576006) and the Regional Innovation System & Education (RISE) program through the Jeju RISE center funded by the Ministry of Education (MOE) and the Jeju Special Self-Governing Province, Republic of Korea (2025-RISE-17-001).

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