ABSTRACT 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.
Zhang et al. (Sun,) studied this question.