Sustainable and efficient persulfate activation by pristine pumpkin seed pomace biochar: a low-energy regeneration strategy and singlet oxygen-dominated pathways

Water contamination by persistent organic pollutants like Rhodamine B (RhB) necessitates sustainable and efficient remediation strategies. This study explores the potential of pristine biochar derived from waste pumpkin seed pomace, pyrolyzed at 600 °C (PS600) and 800 °C (PS800), as a catalyst for peroxydisulfate (PDS) activation to degrade and mineralize RhB in aqueous solution. PS800 exhibited superior catalytic performance, achieving over 99.38% RhB degradation under the optimum conditions of 20 mg L^-1 RhB, 2 g L^-1 catalyst, pH 5.18, and 10 mM PDS within 30 minutes, with a pseudo-first-order rate constant of 0.1644 min^-1 and significant mineralization (81.4% TOC removal in 60 min). Characterization (SEM, EDS, XRD, FTIR, XPS, N₂ sorption) revealed that PS800 possessed a relatively developed porous structure, graphitic carbon, and abundant surface functional groups (C-OH, pyridinic N) crucial for PDS activation. Remarkably, the catalyst demonstrated excellent reusability over five cycles with a simple and effective grinding regeneration method, restoring its activity. Quenching experiments identified singlet oxygen (¹O₂) and superoxide (O₂ ^-˙) as the dominant reactive oxygen species, with minor contributions of sulfate (SO₄ ^-˙) and hydroxyl (˙OH) radicals. The PS800/PDS system showed robustness across a wide pH range (3-11) and in the presence of common water matrix components, except for carbonate. This work highlights PS800 as a cost-effective, environmentally friendly, simple, and low-energy regeneration method and highly efficient catalyst for advanced oxidation processes in water treatment.