Green Graphene Development for Removal of Bisphenol-S from Water

The bisphenol (BPs) family of organic chemicals emerged in the 20th century as a versatile monomer in the evolving polymer industry. The most notable of the bisphenol family of compounds, bisphenol A (BPA), is used widely to make polycarbonate. Unfortunately, it was eventually linked to health problems including a negative impact on human reproductivity. Therefore, there has been an increased need for a safer and more environmentally friendly BPA replacement. Bisphenol-S (BPS) is one common replacement of BPA, but unfortunately BPS was also linked to the same health problems. Thus, there is a growing interest in decontaminating water containing BPS. Adsorption is currently the preferred method for water treatment for the removal of anthropogenic chemicals such as BPs. The efficiency of adsorption processes is inherently dependent on the existence of effective adsorbents for various chemical types. In the current study, methods to develop graphene oxide (GO) materials in environmentally friendly ways for the purpose of absorbing BPS from water were investigated. First GO was synthesized by modifying different existing methods reported in the literature to be more environmentally friendly. The newly synthesized GO (EF-GO) performance was comparable to as-received commercially available GO (AR-GO) when used in solutions of BPS of concentration 10 mg/L at room temperature. EF-GO achieved 63 mg/g adsorption capacity compared to 67 mg/g for AR-GO, and 32% percentage BPS removal compared to 34% for AR-GO. Next, ascorbic acid (AA) was used to reduce the GO, referred to as AA-rGO, to improve its adsorption performance and be classified as green material due to the safety of ascorbic acid. AA was used in two different procedures to produce reduced graphene oxide (AA-rGO) with different defect structures. The performance of these two AA-rGO samples (A and B) was investigated. AA-rGOB was sonicated for 9 minutes during reduction while AA-rGOA was not. Both samples were heated and stirred for 2 hours at 60°C. Reduction of GO by ascorbic acid successfully eliminated most of the oxygen containing functional groups (OCFGs) and preserved the graphene sheets\u27 sp2 domain for both samples. Reduction also boosted the adsorption efficiency of the rGO to 80% removal and 159 mg BPS/g rGO adsorption capacity. Sonication during the reduction process was found beneficial to restoring the sp2 domain of the graphene sheets which was attributed to better adsorption performance. The low level of OCFGs as part of the defect population was directly correlated with the performance of AA-rGO in BPS removal, as well as the structural defects. The AA-rGOB material, which was found to have fewer defects, performed better than AArGOA at higher adsorbent dose (e.g., \u3e 1.0 mg/L) mainly because AA-rGOB has higher ?? interaction affinity. This was attributed to AA-rGOB having higher purity of sp2 carbon domain than AA-rGOA (i.e., small ID/IG ratio in Raman spectrum). Fourier transform infrared (FTIR) spectra suggested the adsorption is mainly a surface interaction phenomenon via ?-? interaction and hydrogen bonding between the hexagonal lattice on the AA-rGO surface and the aromatic ring on the BPS core, rather than its dependence on the adsorbent\u27s structural defects. Nevertheless, the role of structural defects warranted further investigation. The adsorption data was fitted to both Freundlich and Langmuir isotherms. Both models were an excellent fit to the data, as the adsorbent concentrations used in the experiments were low and within the linear regime of the isotherms. This suggests that the bulk concentration of the adsorbents was not high enough to distinguish between adsorption mechanisms directly from the adsorption data, therefore the adsorption mechanism elucidation relied on the FTIR spectra findings