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

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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.

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