Catalytic Removal of Aqueous Contaminants on N-Doped Graphitic Biochars: Inherent Roles of Adsorption and Nonradical Mechanisms

TLDR

Environmentally friendly, low‑cost catalysts are essential for rapid mineralization of organic contaminants in advanced oxidation processes. This study reports N‑doped graphitic biochars as inexpensive, efficient catalysts for peroxydisulfate activation and degradation of diverse organic pollutants in water treatment. The biochars, produced at >700 °C, possess highly graphitic nanosheets, large surface areas, and rich nitrogen, enabling peroxydisulfate activation via both radical and nonradical pathways, with singlet oxygen and surface‑confined activated persulfate–carbon complexes dominating in the 900 °C material. N‑BC900 achieved a 39‑fold higher degradation rate than N‑BC400 and 6.5‑fold higher than pristine biochar, surpassing most metal or nanocarbon catalysts, and the study demonstrated that pre‑adsorption of organics accelerates nonradical oxidation, offering robust, cheap materials and the first insight into graphitic biochar‑based nonradical catalysis.

Abstract

Environmentally friendly and low-cost catalysts are important for the rapid mineralization of organic contaminants in powerful advanced oxidation processes (AOPs). In this study, we reported N-doped graphitic biochars (N-BCs) as low-cost and efficient catalysts for peroxydisulfate (PDS) activation and the degradation of diverse organic pollutants in water treatment, including Orange G, phenol, sulfamethoxazole, and bisphenol A. The biochars at high annealing temperatures (>700 °C) presented highly graphitic nanosheets, large specific surface areas (SSAs), and rich doped nitrogen. In particular, N-BC derived at 900 °C (N-BC900) exhibited the highest degradation rate, which was 39-fold and 6.5-fold of that on N-BC400 and pristine biochar, respectively, and the N-BC900 surpassed most popular metal or nanocarbon catalysts. Different from the radical-based oxidation in N-BC400/PDS via the persistent free radicals (PFRs), singlet oxygen and nonradical pathways (surface-confined activated persulfate–carbon complexes) were discovered to dominate the oxidation processes in N-BC900/PDS. Moreover, the adsorption of organics was determined to be the key step determining reaction rate, revealing that the pre-adsorption of reactants significantly accelerated the nonradical oxidation pathway. This study not only provides robust and cheap carbonaceous materials for environmental remediation but also enables the first insight into the graphitic biochar-based nonradical catalysis.