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.