Graphene Oxide–ZnO Nanocomposites for Removal of Aluminum and Copper Ions from Acid Mine Drainage Wastewater

Adsorption technologies are a focus of interest for the removal of pollutants in water treatment systems. These removal methods offer several design, operation and efficiency advantages over other wastewater remediation technologies. Particularly, graphene oxide (GO) has attracted great attention due to its high surface area and its effectiveness in removing heavy metals. In this work, we study the functionalization of GO with zinc oxide nanoparticles (ZnO) to improve the removal capacity of aluminum (Al) and copper (Cu) in acidic waters. Experiments were performed at different pH conditions (with and without pH adjustment). In both cases, decorated GO (GO/ZnO) nanocomposites showed an improvement in the removal capacity compared with non-functionalized GO, even when the pH of zero charge (pH(PZC)) was higher for GO/ZnO (5.57) than for GO (3.98). In adsorption experiments without pH adjustment, the maximum removal capacities for Al and Cu were 29.1 mg/g and 45.5 mg/g, respectively. The maximum removal percentages of the studied cations (Al and Cu) were higher than 88%. Further, under more acidic conditions (pH 4), the maximum sorption capacities using GO/ZnO as adsorbent were 19.9 mg/g and 33.5 mg/g for Al and Cu, respectively. Moreover, the removal percentages reach 95.6% for Al and 92.9% for Cu. This shows that decoration with ZnO nanoparticles is a good option for improving the sorption capacity of GO for Cu removal and to a lesser extent for Al, even when the pH was not favorable in terms of electrostatic affinity for cations. These findings contribute to a better understanding of the potential and effectiveness of GO functionalization with ZnO nanoparticles to treat acidic waters contaminated with heavy metals and its applicability for wastewater remediation.