Molten salt-assisted synthesis of a nitrogen-doped biochar catalyst at low temperature for enhanced degradation of acetaminophen

Abstract

Nitrogen (N) doping is an efficient modification route to improve the catalytic performance of biochar in peroxymonosulfate (PMS) activation. However, conventional synthetic methods for high-performance N-doped biochar catalysts often require high temperatures (>700 °C), which are energy intensive. To address this issue, a molten salt-assisted method was employed to synthesize a N-doped biochar catalyst from alkaline lignin at a relatively low temperature of 500 °C (named AL@NX). This catalyst was subsequently utilized for the catalytic oxidation of acetaminophen (APAP) through PMS activation. Results demonstrated that the molten salt treatment improved the carbonization of lignin, specific surface area and pyridine N-doping level in the biochar. Notably, pyridine N was identified as the primary active site in AL@NX for PMS activation. Accordingly, AL@NX exhibited a reaction rate constant (K_obs) for APAP oxidation that was 8 times higher than that observed for N-doped biochar synthesized without the use of molten salt. The results of electron paramagnetic resonance (EPR) spectroscopy, quenching experiments and electrochemical measurements revealed that the AL@NX/PMS system operates through an O₂˙⁻-dominated dual activation pathway (radical-based and nonradical). The unique oxidative properties of O₂˙⁻ in the AL@NX/PMS system impart broad pH adaptability, high selectivity for APAP, and high degradation performance in real water bodies. These findings highlight the promising application prospects of the AL@NX/PMS system for water remediation applications. Furthermore, this work proposes a green, energy-efficient approach to the resource utilization of biomass waste, contributing to the achievement of carbon neutrality goal.