Low-energy synthesis of iron-phosphate crystalline hydrochar for efficient Fenton-like degradation of bisphenols

The widespread detection of bisphenol S (BPS) in aquatic ecosystems underscores the development of high-performance, sustainable remediation technologies. This study reports the energy-efficient synthesis of a phosphorus-rich heterogeneous Fenton catalyst derived from sewage sludge at low hydrothermal temperature of 125°C. While traditional carbonization typically requires temperatures exceeding 150–200°C, this work demonstrates that this low-temperature hydrothermal synthesis is critical for maintaining a highly active catalytic structure. Integrated spectroscopic and kinetic analyses reveal that the 125°C regime prevents the detrimental segregation of iron-phosphate domains observed at higher temperatures, instead promoting a highly active, surface-enriched Fe(II)/Fe(III) matrix. The resulting hydrochar (HC-125) achieved 90% BPS degradation within 15 minutes, driven by a synergistic mechanism involving surface complexation of iron and H 2 O 2 and accelerated reversible Fe (II)/Fe (III) cycling. Mechanistic probe experiments confirmed that ·OH radicals, generated via a specialized H 2 O 2 activation pathway on iron-phosphate sites, are the primary oxidative species. Although catalytic efficiency is sensitive to pH-induced particle agglomeration and surface passivation, transitioning from 98% at pH 3 to 23.6% at pH 7, the catalyst exhibits good performance in the presence of humic acids and achieves more than 77% removal for five additional halogen-free bisphenol analogues. By elucidating the structural phosphorus-iron interplay, this work provides a sustainable, low-energy framework for the valorization of waste-derived catalysts in advanced oxidation processes. • Inherent phosphorus enables low-temp synthesis of stable sludge Fenton catalyst • Over 90% bisphenol S was removed within 15 min under mild reaction conditions • Phosphorus tunes Fe redox stability and active site accessibility during synthesis • Negligible Fe leaching confirmed a truly heterogeneous Fenton mechanism at pH 3