Strengthening Biogas Digestate into Copolymerized, Highly Activated Biochar for Efficient Orange G Removal and Application on Industrial Wastewater

Abstract

As a bridge between waste valorization and water treatment, biogas residue was converted herein into an effective environmental remediation material. In this study, biogas residue, known as digestate, is converted into biochar activated with potassium hydroxide (KOH) and further functionalized by copolymerization with two conducting polymers, polyaniline (PANI) and polypyrrole (PPy). Four different ratios of ABC@PANI@Ppy were synthesized, and their structural and surface properties were thoroughly characterized using XPS, XRD, BET, FTIR, SEM/EDS, TGA/DTA, and zeta potential analysis. Among the synthesized composites, ABC@PANI@Ppy (4:1) exhibited the highest adsorption performance, achieving 90.85% Orange G (OG) removal, which was nearly seven times higher than that of pristine activated biochar (13.15%). A high Langmuir maximum adsorption capacity of 344 mg/g was further obtained, and the adsorption process was found to follow pseudo-second-order kinetics. In addition to electrostatic attraction between OG’s sulfonate groups and adsorbent’s protonated amine and imine groups, bonding and ion exchange respectively are contributing to adsorption. Furthermore, the reusability tests revealed that ABC@PANI@Ppy retained more than 88% of its removal efficiency after five cycles. These materials also demonstrated excellent performance when applied to industrial wastewater treatment, achieving over 95.45% removal efficiency. This study highlights the promising potential of digestate-derived biochar functionalized with conducting polymers for efficient and sustainable wastewater treatment applications.