Synthesis of nitrogen-doped carbon nanotubes-FePO composite from phosphate residue and its application as effective Fenton-like catalyst for dye degradation.

Phosphate residue is regarded as a hazardous waste, which could potentially create significant environmental and health problems if it is not properly treated and disposed of. In this study, nitrogen-doped carbon nanotubes-FePO (NCNTs-FePO) composite was successfully synthesized from phosphate residue, and its application as an effective catalyst was explored. Firstly, an effective method was developed to recover FePO from phosphate residue, achieving an impressive FePO mass recovery rate of 98.14%. Then, the NCNTs-FePO catalyst was synthesized from the recovered FePO by two main reactions, including surface modification and chemical vapor deposition. Finally, the synthesized NCNTs-FePO was applied to photo-degrade 15 mg/L Rhodamine B (RhB) in a Fenton-like system. The results showed that 98.9% of RhB could be degraded in 60 min, closely following the pseudo-first-order kinetics model. It was found that even after six consecutive cycles, NCNTs-FePO still retained a high catalytic capacity (>50%). Moreover, •OH radicals participating in the RhB degradation process were evidenced using quenching experiments and electron paramagnetic resonance analysis, and a rational mechanism was proposed. It was demonstrated that the materials synthesized from hazardous phosphate residue can be used as an effective catalyst for dye removal.