Reduced graphene oxide/geopolymer composite for adsorption of methylene blue

Emerging contaminants, such as methylene blue, pose significant environmental and health risks due to their widespread presence in water sources and their persistence in the environment. This study focuses on developing composite adsorbing materials based on reduced graphene oxide (rGO) and metakaolin-based geopolymer, aiming to enhance the removal of methylene blue from wastewaters. The composites were prepared with rGO amounts of 0.5 % and 1 % w/w and were extensively characterized to evaluate their structural and adsorption properties. Adsorption kinetics and binding isotherms were analyzed, revealing that the process follows a monolayer adsorption model with a physisorption mechanism, described by the Pseudo-First Order model. The kinetic parameters suggest a strong increase in the adsorption rate as the amount of graphene oxide within the composite increases. The Langmuir adsorption isotherms indicate that the composite with 1 wt% rGO achieved a maximum adsorption capacity qmax of 22.7 mg/g, nearly twice that of the unmodified geopolymer (qmax = 13.2 mg/g). Furthermore, desorption and adsorption cycles were conducted to assess the sustainability of the composites’ performance over time. The results demonstrate that all materials maintain approximately 55 % of their initial adsorption capacity up to the fifth cycle, with a maximum of cumulative capture capacity of 60.9 mg/g for the sample with 1 wt% rGO; this shows almost 80 % of capture capacity at the fourth cycle, highlighting the great potential these composites have for industrial wastewater treatment applications.