Interpretation of the adsorption of metals on quartz crystal based-macromolecule via advanced modeling of equilibrium isotherms

Abstract

In this article, new insights about the metals-porphyrin complexes are proved by analyzing the zinc, nickel and chromium adsorption process over the well-known porphyrin macromolecule. The use of the quartz crystal microbalance (QCM) apparatus allows the control of the complexation systems’ experimental adsorption data operating at four temperatures. The experimental results and the physical models reveal that the zinc and nickel complexation processes are to be examined using the mono layer adsorption model. While, the double layer model describes the interaction between the chromium compound and the porphyrin. Actually, the three metals are shown to be adsorbed by a multi-docking process in the physicochemical description. The endothermic character of the investigated processes is shown through the appropriate data of the principal parameter adsorbent sites’ density. Hence, several porphyrin sites are exclusively stimulated at high temperature. The parameters of van del Waals, depicting the influences of the lateral interactions, explain the nickel isotherms down trend. The chemical bonds are shown to be carried out between the zinc and the porphyrin through the calculated adsorption energies. Considering the thermodynamic study, and referring to the configurational entropy and the free enthalpy, it is to be noted that the disorder peak of the three mechanisms is reached when the equilibrium concentration is equal to the energetic parameters’ values for each system. The nickel enthalpy revealed for high concentration that the adsorbates’ lateral interactions disapproved the nickel chloride adsorption. The free enthalpy trends, that observed two stability states of the chromium compound, confirmed the chromium double layer mechanism.