Ultrasound‒induced facile synthesis of spinel CoFe 2 O 4 ‒PAC magnetic nanocatalyst for remediation of hypersaline petrochemical wastewater: Degradation mechanism, biodegradability enhancement and phytotoxicity mitigation.

In this study, magnetic CoFe2 O4 -PAC nanocatalysts were synthesized through facile hydrothermal and co‒precipitation approaches with ultrasonic irradiation, which were used for the treatment of hypersaline petrochemical wastewater (HPCW). When an ultrasound‒induced synthesis process (US@CoFe2 O4 ‒PAC) was used, a more efficient and stable magnetic spinel CoFe2 O4 ‒PAC nanocatalyst was developed. The application of this nanocatalyst as a PMS activator, not only caused eradication of 90.4% of chemical oxygen demand (COD) of a HPCW after 90 min reaction time under the optimum conditions (pH 5-6, catalyst dose 1.0 g/L and 1.0 mM PMS), but also led to marginal leaching of iron (314 μg/L) and cobalt (95 μg/L) from the nanocatalyst. Recycling experiments over five consecutive runs showed a negligible decrease (7.2%) in COD removal efficiency which proved the stability and reusability of magnetic US@CoFe2 O4 -PAC. Two main mechanisms of adsorption and catalytic oxidation processes (homogeneous and heterogeneous PMS) are involved simultaneously in the PMS/US@CoFe2 O4 -PAC system, which are responsible for the destruction of refractory contaminants of HPCW through the generation of SO4 •‒ and OH• radicals. COD of HPCW was mainly removed through SO4 •- radical attack (73.6%) and the biodegradability of HPCW was enhanced dramatically after 90 min reaction time. The germination index (GI) of raw HPCW was increased 17.1 ± 4.2% and 24.3 ± 8.8% after 15 and 90 min reaction time, respectively, even PMS/US@CoFe2 O4 -PAC system showed less impact on phytotoxicity mitigation. Hence, it can be recommended to dilute the effluent before using for irrigational purpose. The findings of this study present practical significance of spinel US@CoFe2 O4 -PAC, which is an environment‒friendly catalyst, easy to handle and can sustain long‒term operation for the treatment of recalcitrant hypersaline wastewater and the other potential practical applications.