Modification of volcanic ash for enhanced removal of malachite green: a complete batch adsorption study

This study investigated the application of HCl and H₃PO₄ as treatment agents for volcanic ash used in the adsorption of malachite green from water. Each acid yielded different results; HCl altered the textural properties of the ash, while H₃PO₄ caused the substitution of PO₄ into the Si and Al groups. The XRD results indicate that both ashes are primarily composed of diopside, lizardite, and anorthite, regardless of the acid used in the treatment. SEM imaging reveals cracked and uneven surfaces for HCl-treated ash, while H₃PO₄-treated ash exhibits discrete and porous surfaces. Malachite green shows the highest adsorption capacity at pH 8, requiring a minimum contact time of 480 min to reach equilibrium. H₃PO₄-treated ash demonstrates the highest adsorption capacity across various water types. Stability tests indicate that there is less than a 1% change in adsorption capacity, even in the presence of NaCl and Na₂CO₃. The adsorption of malachite green follows both the Avrami model and the double-layer model, suggesting fractal adsorption and dual-layer formation. Thermodynamic analysis confirms that the adsorption is spontaneous and endothermic. The proposed adsorption mechanism involves multiple interactions, including hydrogen bonds, electrostatic forces, π–π interactions/stacking, and cation/anion-π interactions.