Sono-catalytic activation of persulfate by nZVI-reduced graphene oxide for degradation of nonylphenol in aqueous solution: Process optimization, synergistic effect and degradation pathway

A hybrid persulfate (PS) activation system based on sonication (US) and nZVI-reduced graphene oxide catalyst (PS/US/nZVI-rGO) was investigated to degrade nonylphenol (NP). Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy mapping (EDX-mapping), X-ray diffraction analysis (XRD) and vibrating sample magnetometer (VSM) analyses confirmed the successful synthesize of nZVI-rGO catalyst. The effect of five independent variables on NP degradation efficiency in the PS/US/nZVI-rGO oxidation system was optimized by a five-level full central composite design (CCD). The Adequacy and validity of the proposed model for predicting NP degradation were confirmed with P-value < 0.0001 and R-2 = 0.9996. The optimum values for solution pH, reaction time, PS concentration, nZVI-rGO dosage, and US power were obtained to be 4.2, 50 min, 6.5 mM, 0.6 g L-1, and 300w, respectively. Under these conditions, the maximum NP removal efficiency in PS/US/nZVI-rGO, PS/nZVI-rGO and PS/US oxidation systems was 98.2, 64 and 9 %, respectively, which is indicative of significant synergy in the performance of the hybrid activation system. The nZVI-rGO had good reusability. However, XRD, VSM, and FESEM techniques confirmed the formation of Fe3O4 in the spent catalyst structure. Radical scavenging experiments showed that radical sulfate (SO4 center dot-) plays the major role of NP degradation in the PS/US/nZVI-rGO oxidation system. The most abundant reaction intermediates caused by the degradation of NP (m/z = 220) were compounds with m/z < 79 (butane, benzene and but-1-ene), which confirms the degradation of the NP in the oxidation system.