Evaluation of Graphene Oxide and Reduced Graphene Oxide for the Removal of Selected Halogenated Phenols from Water

Abstract

Carbon-based materials especially graphene nanocomposites (GNS) have attracted wide attention in recent years. In this study, graphene oxide (GO) and reduced graphene oxide (rGO) were prepared by Improved Hummers method having high suspension stability in water. Both GO and rGO were investigated for the adsorption of halogenated compounds from water, its stability at the GNS-water interface and its effective application in the debromination of brominated flame retardant. Emerging contaminants (ECs) are compounds of emerging concern that are of raising concern in the past 20 years. ECs such as bisphenol A (BPA), 4-nonylphenol (4-NP) and tetrabromobisphenol A (TBBPA) pose threat to both humans and the ecosystem. GNS including GO and rGO are also considered as EC due to its potential hazard. The adsorption of organic contaminants such as the phenolic ECs on GNS affects the stability at the GNS-water interface and the fate of organic contaminants, thus causing further environmental risk. Various spectroscopic tools such as SEM, TEM, XRD, Raman, FTIR, and XPS were used to characterize the nanomaterial synthesized. The obtained results confirmed that the size of GO and rGO were with a surface area of 2.02 and 227.32 m2/g. The XRD analysis shows that the values of diffraction peak 2θ were 10.01 and 26.09 confining to the synthesized GO and rGO. Later both GO and rGO were used to study the adsorption behaviour of some ECs and common phenolic compounds that include 4-chlorophenol, 2,4-dichlorophenol, 2,4,6– trichlorophenol and phenol considering its stability in water interface were studied. The adsorption capacity of GNS with phenol, TBBPA, and BPA was examined for its thermodynamic equilibrium at different temperatures. The adsorption equilibrium was reached less than 10 h and was fitted using both Langmuir and Freundlich isotherms. The kinetics and isotherms models of the sorption of aromatic compounds on GNS were investigated at ambient conditions. It was also demonstrated that GO and rGO that varied in C/O ratio is identified as an efficient approach for debromination of TBBPA. A pathway of TBBPA, tri-BBPA, di-BBPA, mono-BBPA, and BPA was thus proposed for TBBPA degradation. Debromination was observed by using metal-free carbon-based nanomaterial. The structural defects of GBMs, act as active sites responsible for catalytic performance. Furthermore, ESR analysis provided insights into the evolution of reactive oxygen species (ROS) such as superoxide radical (O⁻ ₂•) and singlet oxygen (1O2) during the debrominationprocess. Therefore, these active species were identified to be the primary radicles generated onto the surface of GBMs, which results in the formation of less brominated BPA. Finally, reuse of the adsorbents for all the pollutants were investigated, and we observed that adsorbent reusability was >93% of its activity up to 5 cycles. These novel findings unveil the crucial role of oxygen functional groups on GBMs surface for the catalytic degradation of TBBPA. These findings emphasize that when carbon-based materials are used for sorption studies of halogenated compounds more attention should be considered on estimating the adsorption capacity in addition to the degradation.