Design of acid-geopolymers based on clays by-products for methylene blue removal from wastewater

Morocco has significant phosphate reserves, but the extraction process generates a lot of waste rock. To tackle this problem, this study aims to make use of clay, a by-product of phosphate mining, to create acid-activated geopolymers. Four formulations of geopolymers were prepared by combining metakaolin (MK) and calcined clay (CC) in different proportions, which were then activated using phosphoric acid. Different techniques were performed for the characterization of raw and calcined clays as well as the elaborated geopolymers. Based on the XRD, FTIR, and SEM results, it was observed that an increase in the level of CC replacing MK led to the formation of new crystals like Monetite, Newberyite, and Brushite. The quantity of CC influenced the type of crystals formed. Moreover, the specific surface area analysis revealed that the geopolymer (GP2) containing 25% of CC exhibited the highest specific surface area. These materials were then tested for their ability to eliminate methylene blue (MB) from wastewater. The results indicated that GP2, a geopolymer made with 75% MK and 25% CC had the highest efficiency in removing MB with a rate of 98%. The material was highly reactive and achieved adsorption equilibrium in just 15 minutes. It was found to be effective in both acidic and alkaline environments. Furthermore, studies have shown that the Temkin isotherm model best explains how MB (a dye) is absorbed by GP2, with a high correlation coefficient. Additionally, the pseudo-second-order kinetic model was a better fit, suggesting that chemical interactions are more significant than physical interactions. Notably, the use of phosphoric acid to activate GP2 was found to selectively adsorb cationic dyes.