Mechanisms of persulfate activation on biochar derived from two different sludges: Dominance of their intrinsic compositions

Highlights: • Organic structures and metals in the SDBCs impacted their PS activation ability. • PS activation by SDBCs was regulated by free radical and nonradical pathways. • Surface nonradical oxidation contributed the same proportions in different SDBCs. • Inherent Fe oxides in SDBC promoted the oxidative efficiency in PS activation. Sludge-derived biochar (SDBC) has been regarded as persulfate (PS) activator during the remediation of organic contamination. However, the complexity of sludge composition makes it difficult to predict the activity of SDBC and the efficacy of PS. To improve the understanding of how the composition of sludge regulated activity of its parent SDBC towards PS activation, we used two SDBCs derived from different sludges with significantly different organic compositions and metals. Results indicated the higher content of organic and nitrogen content in sludge led to higher polymerization and condensation of carbon layer and more moieties in SDBC1, whereas more Fe species (e.g. Fe-O, Fe²⁺ and Fe³⁺) formed in SDBC2. According to the results of phenol (PN) degradation in SDBC/PS, the apparent rate constants (k_obs) of SDBC2-700 (0.0037 min⁻¹) was 2 folds higher than that of SDBC1-700 (0.0016 min⁻¹), whereas the SDBC1-500 (6.0 × 10⁻⁴ min⁻¹) exhibited higher k_obs than that of SDBC2-500 (4.9 × 10⁻⁴ min⁻¹). The difference of PS activation by different SDBCs mainly relied on generated reactive oxygen species (ROS). The persistent free radicals (PFRs) and Fe species acted as redox sites for generated ROS, which were depended on the organic compositions and involved metals in used sludges.