Mechanism and Efficacy of Phenol Degradation via Photocatalysis Using Pyrolytic Residue from Oilfield Drilling Solids

Addressing the challenge of treating highly toxic and refractory phenolic compounds (e.g., phenol and its derivatives) in wastewater, this study employed phenol as a representative organic pollutant and established a PDWR–H₂O₂–OA photo-Fenton catalytic activation system. Here, pyrolytic drilling waste residue (PDWR) served as the catalyst, H₂O₂ as the oxidant, and oxalic acid (OA) as the chelating agent. The system was designed to investigate the activation mechanism and photocatalytic performance of PDWR toward H₂O₂ under natural sunlight irradiation. Under solar radiation, the PDWR-based system achieved a 7.51-fold increase in H₂O₂ utilization efficiency. The degradation rate of phenol reached 2.2 mg/g/h, with a mineralization rate approaching 100%. Moreover, PDWR exhibited excellent stability: after five consecutive cycles, the cumulative decline in catalytic efficiency was less than 4.00%, while the phenol degradation rate remained above 77.21%. EPR characterization revealed that the primary active species in the PDWR–H₂O₂–OA system was the ·OH radical, accounting for 84.24% of the reactive oxygen species. SEM analysis indicated that the surface morphology of PDWR transformed from a disordered needle-like structure before the photo-Fenton reaction into an ordered block-like structure afterward. BET measurements showed that the pore structure of PDWR was dominated by macropores prior to the reaction, which shifted to a microstructure primarily composed of micropores following the reaction. Furthermore, PL spectroscopy, photocurrent response, UV-Vis diffuse reflectance, and Mott–Schottky measurements demonstrated that the photogenerated charge carriers in PDWR possessed a low recombination rate upon visible-light excitation. These results collectively confirm that PDWR exhibits favorable photocatalytic performance and holds promising potential for practical environmental applications.