Synthesis and application of novel GO@ZIF-7 nanocomposite for the removal of lead from aqueous solutions

Water pollution is a growing environmental problem with negative impacts on the ecosystem and the public health. Lead is one of the highly toxic water pollutants, requiring effective elimination from contaminated waters. In this study, a novel graphene oxide/zeolitic imidazolate framework-7 (abbreviated as GO@ZIF-7) nanocomposite has been synthesized and utilized for the adsorptive removal of lead from aqueous solutions for the first time, to the best of our knowledge. Despite the low level of GO in the nanocomposite, the nanocomposite extremely outperformed its parental materials (GO and ZIF-7) with a lead adsorption capacity approaching 5,000 mg/g relative to about 500 and 600 mg/g obtained using pristine ZIF-7 and GO, respectively. The adsorptive removal of lead using this nanocomposite was also optimized using the Response Surface Methodology (RSM) and the optimization results revealed a greater role of aqueous pH and the adsorbent dosage in achieving higher lead removal, which reached about 95% at the optimum process conditions (i.e., initial lead concentration of 50 mg/L, adsorbent dosage of 0.6 g/L, and initial solution pH 6). Additionally, even at a lower adsorbent dosage of 0.2 g/L and a higher lead initial concentration of 150 mg/L, more than 83% removal was still obtained at pH 6. The higher lead removal/adsorption at pH 6 is most likely resulting from a greater interaction between the cationic lead species and the adsorbent surface due to comparatively less presence of the cationic H⁺ ions. Furthermore, a greater lead removal at higher adsorbent dosage is also attributed to higher availability of surface sites for lead adsorption. Lead adsorption was also noted to follow the Freundlich adsorption isotherm trend, suggesting a multi-layer adsorption of lead onto the adsorbent surface. This is consistent with the above-mentioned higher lead uptake capacity by the GO@ZIF-7 nanocomposite as compared to the unmodified GO and ZIF-7. The 1/n (n is the heterogeneity factor) obtained from the Freundlich isotherm fitting was found to be 0.574, suggesting that lead adsorption onto GO@ZIF-7 nanocomposite is a favorable process. The outcomes of this study reveal the high commercial potential of the GO@ZIF-7 nanocomposite in successfully treating waters contaminated with lead.