New insights into soil active substances enhance the biochar/periodate process for remediation of sulfadiazine: The changes of soil properties and toxicity

In recent years, periodate (SPI)-based advanced oxidation processes have been successfully applied in wastewater treatment. However, their application in soil pollution remediation remains limited. To our knowledge, this study represents the first attempt to utilize SPI catalyzed by the Eichhornia crassipes biochar (EBC) system for the remediation of sulfadiazine (SD)-contaminated soil. In the EBC/SPI system, the degradation performance of SD-spiked soils was significantly improved, achieving complete degradation within 60 min, which indicates a clear synergistic effect between SPI and EBC. Notably, our findings highlighted that active soil constituents play crucial roles in SPI activation. Specifically, free Fe-oxides in soil were essential for SPI activation to form reactive species (RS) compared to amorphous Fe-oxides and dissolved Fe, leading to superior SD degradation. Soil organic matter (SOM) also contributed to RS formation and conversion. Adding Fe³⁺, Cl^-, and humic acid accelerated SD elimination, whereas Mn²⁺ and HCO₃^- inhibited it. Quenching experiments and electron paramagnetic resonance spectroscopy confirmed the formation of singlet oxygen, superoxide radicals, and iodate radicals, which actively degraded SD. Analysis of soil properties, including SOM content, total phosphorus, functional groups, crystal structure, and pH value, showed negligible changes after EBC/SPI treatment. Additionally, potential decomposition pathways of SD were proposed based on identified SD intermediates. Ecotoxicity analyses and phytotoxicity tests indicated a marked reduction in the toxicity of these intermediates compared to SD. These findings provide an efficient strategy for soil remediation and offer new insights into the role of inherent substances in the field of contaminated soil remediation.