Objective The purpose of this work is to explore the adsorption thermodynamics and reaction mechanism of vegetable-aluminum combination tannage.

Methods Using black wattle tannin-tanned leather powder (CF-BWT) as the adsorbent, this study systematically investigated its adsorption behavior toward aluminum sulfate to explore the adsorption thermodynamics of Al(Ⅲ) by CF-BWT, and the effects of hydrogen bonds and hydrophobic bonds on CF-BWT, aluminum tanning leather powder (CF-Al) and vegetable-aluminum combination tannage leather powder (CF-BWT-Al) were investigated.

Results The adsorption kinetics study indicates that the adsorption of Al(Ⅲ) by CF-BWT reached equilibrium within 24 hours. The adsorption kinetic curve followed a pseudo-second-order kinetic model, characterized by a relatively rapid adsorption rate. The equilibrium adsorption capacity increased with rising pH and temperature. Under conditions of pH 4.0 and a temperature of 40 °C, the equilibrium adsorption capacity of CF-BWT reached 23.41 mg/g. The adsorption isotherms were well fitted by the Langmuir model. Thermodynamic calculations showed that under optimal adsorption conditions, the standard Gibbs free energy change (∆_rG^θ), standard enthalpy change (∆_rH^θ), and standard entropy change (∆_rS^θ) for the adsorption of Al(III) onto CF-BWT were -15.67 kJ/mol, 28.00 kJ/mol, and 139.44 J/(mol·K), respectively. Experiments involving the disruption of hydrogen bonds and hydrophobic bonds revealed that when the urea concentration reached 4 mol/L, the thermal shrinkage temperatures (T_d) of CF, CF-BWT, CF-Al and CF-BWT-Al decreased by 7.17 °C, 23.11 °C, 12.20 °C, and 15.01 °C, respectively. When the n-propanol concentration was 20%, the T_d values for CF, CF-BWT, CF-Al and CF-BWT-Al decreased by 3.04 °C, 17.71 °C, 10.40 °C, and 10.99 °C, respectively. Under constant initial Al(Ⅲ) concentration and varying pH conditions, the thermal shrinkage temperature of vegetable-aluminum combination tannage leather powder increased with rising pH. The highest thermal shrinkage temperature, reaching 109.85 °C, was observed at pH 4.0. The Zeta potential of CF-BWT was tested at pH values ranging from 3 to 6, and its isoelectric point was determined to be 4.6. The surface of CF-BWT carried a positive charge when the pH was below 4.6, and it became negatively charged when the pH exceeded 4.6. From the species distribution diagram in this paper, it is known that in a solution with pH＜4.0 and Al(Ⅲ) = 16 mg/L, the content of Al(SO₄)₂⁻ was approximately 30%. This indicates the presence of electrostatic interactions between CF-BWT and Al(Ⅲ). Morphological and specific surface area characterization revealed a higher degree of fiber dispersion in the CF-BWT-Al. Infrared spectral and X-ray photoelectron spectroscopy characterization confirmed the interaction between aluminum and oxygen.

Conclusion Thermodynamic studies on the adsorption of vegetable tanned leather powder onto aluminum sulfate solutions indicate that combination tannage is a spontaneous, endothermic process accompanied by entropy increase. At a fixed pH, the temperature-induced change in ∆_rS^θ is less than approximately 2%; conversely, at a fixed temperature, the pH-induced change in ∆_rS^θ exceeds 15%. The entropy increase in the system drives the spontaneous progression of the binding tanning process, which is closely related to the type and amount of water released from vegetable tanned leather powder due to Al(Ⅲ) cross-linking. Solution pH is a key factor regulating the thermodynamic driving force of this process. In vegetable-aluminum combination tannage, synergistic effects manifest through multiple strong and weak interactions, including coordination, hydrogen bonding, and hydrophobic bonding. Al(Ⅲ) cross-linking significantly stabilizes weak interactions such as hydrogen bonds and hydrophobic bonds between vegetable tannins and collagen fibers, thereby substantially enhancing the wet-heat stability of the product. This work provides a quantitative thermodynamic explanation for the "synergistic effect" in vegetable-aluminum combination tannage, extending the understanding of the mechanism from molecular interactions to the macroscopic levels of energy and changes in system order.