The use of carboxylated graphene oxides and related materials for the adsorption of metals and dyes: A review

Environmental pollution caused by the release of effluents contaminated by metals and dyes puts human safety and biodiversity at risk. Adsorption is considered the most suitable technique for treating these wastes. Graphene oxide (GO) appears as one of the adsorbents with the greatest potential for large-scale use. GO’s performance can be further improved through carboxylation, a simple and straightforward approach that involves the use of a strong base and a carboxylic acid source, mostly sodium hydroxide and chloroacetic acid, respectively, as the only reagents. The present review addresses the use of carboxylated graphene oxides and related materials to adsorb metals and dyes, mainly methylene blue in the later case. It also describes the differences in reported approaches to carry out carboxylation, the properties of the adsorbent evidenced by the main characterization techniques, the adsorption mechanisms, and the factors that influence the performance of the adsorbents, such as contact time, initial amount of adsorbate, pH, and ionic strength. In general, increases in adsorption capacity of the order of almost 90% for methylene blue and more than 500 % for mercury, for example, have been reported after the carboxylation of graphene oxides. The incorporation of the adsorbent into composites was also found to improve other properties beyond the adsorption capacity, such as increasing the pure water flux (LMH) and hydrophilicity of membranes. Finally, further studies to optimize carboxylation are suggested, through which the optimal experimental conditions will be determined, such as reaction time and temperature and concentration of reagents. The importance and necessity of this review stems from the growing interest in carboxylation as a surface modification technique for graphene oxides, which is justified by its simplicity and high efficiency, in addition to a great potential for future large-scale use in adsorption processes. Thus, this paper will contribute to disseminating knowledge on this topic and will serve as a reference for future work.