Physical and Chemical Parameters Driving the Direct Blue-78 Adsorption from Water Using Chitosan/ZnO Hybrid Sponges Engineered via Atomic Layer Deposition

This study presents hybrid sponges composed of zinc oxide and chitosan, engineered via atomic layer deposition (ALD), as highly efficient adsorbents for the removal of textile dyes from aqueous environments, including complex dye mixtures. ALD is a vapor-phase strategy to conformally deposit photocatalytically active zinc oxide within the porous chitosan matrix, enabling enhanced water stability and improved structural integrity. The main aim is to elucidate the influence of key physicochemical parameters on the adsorption process using Direct Blue-78 as a model dye. The effects of the adsorbent amount, initial dye concentration, solution pH, ionic strength, and temperature were systematically investigated. The process occurred slightly influenced by changing pH values and ionic strength, revealing the contribution of both electrostatic forces and hydrophobic interactions. Interestingly, the increased amount of adsorbent and the reduced concentration of pollutants favored dye removal from water. A very high maximum adsorption capacity of 2000 ± 400 mg/g was observed, denoting the great performance of the proposed material that retained the same behavior if in the presence of azo-dye mixtures composed also by Direct Red 83:1 and Direct Yellow 86. Kinetic modeling was carried out, and several adsorption isotherm models, including Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich, were used to fit to the experimental data. The results suggested that the proposed adsorbent had a heterogeneous adsorption surface and a single mathematical model cannot be able to fit experimental data. Furthermore, thermodynamic parameters were derived, highlighting the spontaneous and endothermic nature of the adsorption process.