Effect of the Oxidation Degree of Graphene Oxides on their Adsorption, Flocculation, and Antibacterial Behavior

The effect of the oxidation degree of two graphene oxides (GOs) on the adsorption of cationic methylene blue (MB) and anionic methyl orange (MO) dyes and on the coagulation of cationic goethite and anionic kaolin minerals was studied with respect to water treatment in the textile and mining industries. To this purpose, two GOs with high (GO-H) and low (GO-L) oxidation degrees were synthesized by the Hummer method and characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and zeta potential. The antibacterial behavior against Gram-negative Escherichia coli bacterium was further tested under continuous agitation. GO-H and GO-L presented a high capacity for removing MB with maximum monolayer adsorption capacities (q(m)) of 476.2 and 312.5 mg/g, respectively. Despite its negative charge, GO-L adsorbed MO with a q(m), of 263.2 mg/g that was higher than the values from GO-H (105.3 mg/g). Regarding the coagulation process, GO-H interacted with goethite decreasing the turbidity of the respective clay suspension, while GO-L presented a much lower effect. In kaolin suspensions the flocculation was less pronounced confirming the relevance of the minerals on the GO behavior. Both types of GO were able to kill Escherichia coli species at 0.5 mg/mL of particles with GO-L presenting the highest effectivity. All these results allow the generation of a general mechanism explaining the interactions of GO with the different contaminants based on its functional groups. Our results show that GO nanoparticles can be designed as a universal agent for water treatment removing ionic dissolved organic dyes and cationic particles as well as bacteria such as Escherichia coli.