Superoxide anion radicals mediated degradation of tetrachloropicolinic acid in biochars-Fe_xP@Fe-Fe_xC/O₂ system with excellent reactivity durability

Activation of oxygen by zero-valent iron (ZVI) to in-situ produce reactive oxidant species (ROS) provides a promising low-carbon and “green” technology for water purification. However, poor ROS yields and easy inactivation limit its engineering application for organic pollutants elimination. Herein, we fabricated a novel Fe-based catalyst with Fe(II)-regenerative surface derived from phosphatized sewage sludge and iron salts. The achieved materials were composed of sludge biochars, Fe_xP, Fe, and Fe_xC (SL-Fe_xP@Fe-Fe_xC) and possessed core/shell structure. SL-Fe_xP@Fe-Fe_xC showed high efficiency in degrading recalcitrant organic pollutants 3,4,5,6-tetrachloropicolinic acid (TCPA) from water at pH 3–10 or in different salts solution without the need of exogenous H₂O₂. When sludge was pretreated with 1.0 M H₃PO₄ and then soaked in 50 mM FeCl₃ solution before carbonization, the obtained SL_1.0M-Fe_xP@Fe-Fe_xC_{50 mM} could degrade TCPA with almost 100 % efficiency in ten consecutive recycle runs. This material demonstrates better activity persistence than most of the reported Fe-based catalysts. The EPR and quenching tests indicated that O₂^•- radicals generated from Fe(II)/O₂ reaction were the main active species for TCPA degradation. The electrochemical experiments revealed that strong affinity of O₂ and fast electron transfer from inner Fe/Fe_xC to SL-Fe_xP shell improved the yields of O₂^•- and regeneration of Fe(II) species.