Ultrasound-assisted photocatalytic degradation of sulfadiazine using MgO@CNT heterojunction composite: Effective factors, pathway and biodegradability studies

Heterogeneous hybrid systems have emerged as effective approaches for decontamination of aqueous solutions containing non-biodegradable compounds. This is the first study employed hexagonal magnesium oxide (MgO) anchored on carbon nanotubes (CNTs), as heterogeneous catalyst, in coupling with UV and ultrasound (US) waves (marked as MCs/UV/US) for degradation of sulfadiazine (SDZ). MgO/CNT nanocomposites (MCs) were synthesized through an ultrasound-assisted hydrothermal method, and their properties were characterized via BET, FESEM, TEM, XRD, FTIR, EIS, TGA, UV-vis DRS, EDAX, and EDS techniques. The FESEM and TEM results indicated that CNTs were well-incorporated into the hexagonal MgO. The ability of MCs/UV/US system for SDZ decontamination was evaluated as a function of solution pH, catalyst loading, US power, UV intensity, initial SDZ concentration, scavenging agents and co-existing chemicals. As-prepared catalyst showed a significant synergistic effect in the presence of both UV and US irradiation for SDZ mineralization. During sono-photocatalytic process, the complete degradation of SDZ (45 mg/L) was achieved at pH: 11.0; MCs: 0.9 g/L; US power: 200 W; and UV intensity: 150 W within 80 min reaction. MCs exhibited high chemical stability and reusability after six consecutive cycles. However, the addition of co-existing anions, especially Cl-, gradually reduced the efficiency of SDZ degradation over MCs/UV/US system. Trapping studies indicated that hydroxyl radicals and holes were responsible for oxidation of organic chemicals during the reaction period. The plausible pathway and mechanism of sono-photocatalytic decomposition of SDZ were determined in details. Kinetic data confirmed that degradation process was fitted to a pseudo first-order kinetic model with a regression coefficient of 0.98. MCs/UV/US process effectively quenched 89% and 96% of TOC and COD in the pharmaceutical wastewater, respectively. The biodegradability and toxicity of the produced organic chemicals were considerably enhanced after the treatment.