Activation of peroxydisulfate by alkali-activated algal biochar for the enhancement of enrofloxacin degradation in water: Role of singlet oxygen and electron transfer pathway

The biochar has garnered significant attention for its potential in carbon sink and versatile application, such as catalyst. However, the catalytic mechanism for peroxydisufate (PDS) activation by alkali-activated biochar is not yet clear. In this work, algal biochar (BC) was synthesized using a simple one-step pyrolysis method and used as a metal-free catalyst to activate PDS. The efficacy of the BC/PDS system for the degradation of enrofloxacin (ENR) was systematically studied. The optimal conditions for BC preparation were identified as a pyrolysis temperature of 650 °C and a ratio of NaOH: algae biomass ratio as 2:1 (650NBC), resulting in a biochar with a large surface area, high defect density, more carbonyl group (C[dbnd]O) and excellent electron transfer capabilities. Mechanistic studies using electron paramagnetic resonance (EPR), quenching experiments, and electrochemical analysis showed that both radical and non-radical pathways were involved in ENR degradation. Notably, singlet oxygen (¹O₂) and electron transfer pathway (ETP) play crucial roles in the degradation of ENR molecules. The alkali-activated biochar accelerated the electron transfer between PDS and ENR by increasing the electrochemical specific surface area and facilitating the formation of a metastable 650NBC-PDS* complex. Through characterization analysis, the significantly increased C[dbnd]O groups and defect sites provide more active sites for ¹O₂ generation. Moreover, the intermediate degradation products of ENR from mass spectrometry indicated a possible pathway through the density functional theory (DFT) method. Overall, our study enhances the understanding of the mechanism in PDS activation by alkali-activated biochar, and proposes a novel approach for the recycling of solid waste.