Chemically versus thermally reduced graphene oxide: effects of reduction methods and reducing agents on the adsorption of phenolic compounds from wastewater

Graphene has recently emerged as an attractive material for various applications, including water decontamination. However, the pristine graphene oxide (GO) usually provides unsatisfactory adsorptive removal of water pollutants. GO reduction, whether chemically or thermally, has the potential to enhance its adsorption performance, particularly towards organic pollutants. Accordingly, the key aim of this study is to explore how the GO reduction method could alter its textural and chemical properties, and, thus, its capacity in removing phenolic pollutants from synthetic wastewater samples. To achieve this aim, GO was reduced chemically using hydrazine, aluminum foil, and metallic zinc powder, as well as thermally at 300, 500, and 800 oC. The obtained graphene materials showed mixed performance. The chemically reduced GO using hydrazine (i.e., rGO-HD), aluminum foil (i.e., rGO-Al), and metallic zinc powder (labelled as rGO-Zn) showed a better adsorption performance towards bisphenol A (BPA) relative to the pristine GO. Additionally, BPA adsorption on rGO-HD and rGO-Al was comparable while its uptake capacity by rGO-Zn was about 50% lower. Thermal reduction of GO at 300 oC (abbreviated as rGO-300) provided a marginal increase in BPA adsorption relative to the unmodified GO. Contrarily, GO reduction at 800 oC (rGO-800) boosted the BPA saturation uptake capacity (i.e., q(max)) from 53.1 (in the case of the pristine GO) to 193.5 mg/g. In addition to BPA, the adsorption of two other harmful phenolic pollutants (i.e., 2-nitrophenol and 2-chlorophenol) was investigated and the results showed that the respective adsorption of these pollutants on rGO-800 could reach 341.8 and 213.0 mg/g, which are about 8- and 7-fold, respectively, higher than their adsorption capacities on the unmodified GO. The adsorption of these phenolic pollutants does not follow any consistent correlation with the textural properties (i.e., BET surface area, pore volume, and pore size) of the synthesized graphene materials. Additionally, although GO reduction degree (measured by the C/O atomic ratio) plays an important role in the adsorption of phenolic pollutants on graphene materials, no further adsorption enhancement was observed when the C/O atomic ratio increased from about 5 up to 10. Interestingly, higher C/O atomic ratio had detrimental effect on adsorption. The approach adopted in this study revealed that the hazardous hydrazine, which is commonly used to reduce GO, can be replaced with a safer option (i.e., Al foil) without compromising the adsorption performance of the reduced GO. More importantly, the solvent-free (thermal) GO reduction could produce a superior (when conducted at 800 oC) adsorbent than the ones obtained using the harmful chemical reduction methods.