FeCl3/KOH-modified digestate biochar for ciprofloxacin removal: Fe-O coordination-promoted adsorption mechanism and sustainability assessment

Digestate-derived biochar (BC) and an FeCl 3 /KOH-modified biochar (FKBC) were evaluated for ciprofloxacin (CIP) removal from nutrient-rich wastewater. FeCl 3 /KOH modification increased the specific surface area from 76.3 to 176.1 m 2 /g and introduced abundant oxygen-containing functional groups with dispersed Fe O coordination sites. Consequently, FKBC exhibited a Langmuir adsorption capacity of 117.93 mg/g, representing a 91.1% enhancement over pristine BC. Adsorption kinetics of FKBC were well described by the pseudo-second-order model (R 2  ≈ 0.99), with higher rate constants and initial adsorption rates, indicating accelerated chemisorption. FKBC further enabled simultaneous removal of ammonium and phosphate (∼60% at 20–120 mg/L). Adsorption was pH-dependent, favoring acidic conditions. XPS/FTIR analyses combined with density functional theory (DFT) calculations revealed that CIP adsorption is primarily governed by inner-sphere Fe O coordination and carboxylate complexation, with carbonyl anchoring and π-π interactions contributing at higher surface coverage. Stable Fe 2p features before and after adsorption indicate reversible surface coordination without alteration of iron chemical states, supporting low iron-leaching risk. A ranking efficiency product (REP) analysis yielded an overall sustainability score of 85%, highlighting FKBC as an efficient and sustainable adsorbent for antibiotic remediation and resource-oriented digestate valorization.