To fully comprehend the efficiency and feasibility of any adsorption process, particularly in wastewater treatment, a thorough thermodynamic analysis is essential. Thermodynamics helps us understand the energy changes involved, the driving forces behind the adsorption, and its inherent spontaneity. This analysis is crucial for predicting how changes in temperature and other conditions might affect the adsorption of Direct Blue 86 (DB86) dye onto cellulose hydrogels (CAH).

The thermodynamic parameters commonly evaluated include the change in Gibbs free energy (ΔG°), the change in enthalpy (ΔH°), and the change in entropy (ΔS°). These parameters provide a comprehensive picture of the adsorption process. The Gibbs free energy change (ΔG°) is a direct indicator of the spontaneity of a process. A negative ΔG° value signifies a spontaneous process, while a positive value indicates a non-spontaneous one.

In the case of DB86 dye adsorption onto CAH, the study revealed that all calculated ΔG° values were positive. Furthermore, these positive values increased with increasing temperature. This indicates that the adsorption process is non-spontaneous under the tested conditions. This finding is important for process design, suggesting that while adsorption does occur, external energy input or specific conditions might be needed to drive it effectively, or that other factors are at play that make the overall process require energy.

The enthalpy change (ΔH°) provides information about the heat absorbed or released during adsorption. A negative ΔH° signifies an exothermic process, where heat is released, typically indicating a favorable adsorption driven by bond formation or molecular attraction. A positive ΔH° suggests an endothermic process, which requires heat input. The research found a negative ΔH° value of -9.36 kJ/mol for the adsorption of DB86 dye on CAH. This negative enthalpy change confirms that the adsorption process is exothermic, meaning that heat is released as the dye molecules bind to the cellulose hydrogel. This exothermic nature is generally considered favorable for adsorption, as it implies that the formation of bonds or interactions between the dye and the adsorbent releases energy.

The entropy change (ΔS°) reflects the change in randomness or disorder at the solid-solution interface during adsorption. A negative ΔS° indicates a decrease in randomness, often observed when adsorbate molecules become more ordered upon binding to the adsorbent surface. Conversely, a positive ΔS° suggests an increase in randomness. The study reported a negative entropy value (ΔS° = -40.84 J/K mol) for the DB86 dye absorption by CAH. This negative entropy change suggests a reduction in disorder at the interface, implying that the dye molecules are attaching to the CAH surface in a relatively ordered manner.

While the positive ΔG° values suggest non-spontaneity, the exothermic nature indicated by the negative ΔH° is a favorable thermodynamic characteristic for adsorption. The negative ΔS° further points to a specific arrangement of the dye molecules on the adsorbent surface. Understanding these thermodynamic aspects is crucial for optimizing the adsorption process. For instance, the exothermic nature suggests that lower temperatures might be preferred for achieving higher adsorption capacities. By carefully considering these thermodynamic principles, researchers and engineers can better design and implement cellulose hydrogel-based systems for efficient and effective removal of Direct Blue 86 dye from contaminated water sources.