Sustainable Valorization of Anaerobic Digestate Into PEG-Assisted Fe-C Adsorbents for Efficient Congo Red Removal From Wastewater

Anaerobic digestate, a typical by-product of large-scale biogas plants, faces mounting disposal pressure due to the rapid increase in organic solid waste. In this study, an iron-carbon composite was produced from digestate through polyethylene glycol (PEG)-assisted iron modification followed by low-temperature carbonization. The resulting material exhibited a mesoporous texture, oxygen-containing surface functionalities, and Fe 3 O 4 /Fe 3 C domains, enabling efficient removal of Congo red in batch systems. Removal was favored under mildly acidic conditions. At pH 4 and 298 K, the maximum experimentally observed equilibrium uptake reached 1267.35 mg·g −1 at high initial concentration, and removal efficiencies approached 99% under the tested conditions. The adsorbent retained more than 95% of its initial removal performance after five regeneration cycles. Kinetic and equilibrium analyses, together with FTIR/XPS and post-adsorption characterization, support that uptake involves Fe-associated interactions, electrostatic effects, and hydrogen bonding, with additional high-coverage accumulation contributing at elevated concentrations. Overall, this work demonstrates a practical route to valorize anaerobic digestate into a reusable adsorbent for removing azo dyes from wastewater. Graphical Valorizing anaerobic digestate into a magnetically recoverable Fe-C adsorbent enables efficient Congo red removal: near-complete decolorization at pH 4, 298 K, maximum observed q e 1267.35 mg·g −1 , and > 95% performance after 5 cycles. A single-step, low-temperature process offers a scalable, low-cost wastewater solution.