Synthesis of co-pyrolyzed biochar using red mud and peanut shell for removing phosphate from pickling wastewater:...

Iron (Fe)/iron oxide-modified biochar has practicable adsorption capability for phosphorus (P), but it is expensive. In this study, we synthesized novel low-cost and eco-friendly adsorbents co-pyrolyzed biochars using Fe-rich red mud (RM) and peanut shell (PS) wastes via a one-step pyrolysis process for removing P from pickling wastewater. The preparation conditions (heating rate, pyrolysis temperature, and feedstock ratio) and P adsorption behaviors were systematically investigated. In addition, a series of characterization and approximate site energy distribution (ASED) analyses were conducted to understand the P adsorption mechanisms. The magnetic biochar (BR7P3) with m (RM):m (PS) of 7:3 prepared at 900°C and 10 °C/min had a high surface area (164.43 m²/g) and different abundant ions (including Fe³⁺, and Al³⁺). In addition, BR7P3 exhibited the best P removal capability (142.6 mg/g). The Fe₂O₃ from RM was successfully reduced to Fe⁰, which was easily oxidized as Fe³⁺ to precipitate with H₂PO₄^-. The electrostatic effect, Fe-O-P bonding, and surface precipitation were the main mechanisms of P removal. ASED analyses revealed that high distribution frequency and solution temperature led to a high P adsorption rate of the adsorbent. Therefore, this study provides new insight into the waste-to-wealth strategy by transforming PS and RM into mineral-biomass biochar with excellent P adsorption capability and environmental adaptability.