Optimizing direct acid leaching of Egyptian kaolin for aluminum extraction and value-added adsorbent production for water treatment applications

Abstract Aranda Mastin technology (AMT) is an emerging industry focused on the extracting aluminum from secondary sources, amid the decline of traditional bauxite resources. However, the cost-effectiveness of AMT operations remains challenged by the limited added value of its by-products. This study investigates the potential of utilizing acid-leached residues from Egyptian kaolin as low-cost adsorbents for removing methylene blue (MB) from aqueous solutions. Four mineral acids—HCl, HNO 3 , H 2 SO 4 , and H 3 PO 4 —were evaluated for their effectiveness in direct kaolin dealumination under varying experimental conditions, including concentration, temperature, stirring time, and solid/liquid ratio. Optimal conditions were found to be: 7 mol L −1 acid concentration, 100 °C, > 240 min of reaction time, and a 1/20 solid/liquid ratio. The leaching efficiency followed the order: H 3 PO 4 > H 2 SO 4 > HCl > HNO 3 . X-ray diffraction (XRD) analysis confirmed amorphization due to dealumination in the residues from H 3 PO 4 and H 2 SO 4 . The specific surface area ( S BET ) of the residues increased in the order: H 2 SO 4 > HCl > H 3 PO 4 residues. Despite this, the highest dye removal efficiency (84.1%) was observed with residue treated using H 3 PO 4 , followed by HCl then with H 2 SO 4 was the lowest efficiency. Adsorption kinetic studies followed the pseudo-second-order model for both H 3 PO 4 and HCl residues, with high correlation coefficients (0.997–0.999), suggesting a heterogeneous adsorption mechanism. Additionally, the adsorption data for H 3 PO 4 -treated residue fit both the Langmuir and Freundlich isotherm models, indicating the coexistence of monolayer and multilayer adsorptions. When the removal efficiency was tested in MB-spiked seawater samples across various salinities and pH values (7.84–8.5), it remained consistently high (89.4–91.8%). These results highlight that while both H 3 PO 4 and H 2 SO 4 leaching effectively promote dealumination, only H 3 PO 4 -derived residues exhibit significant adsorption potential—making them suitable for cost-effective water treatment applications in addition to their role in aluminum recovery.