Preparation of novel heterogeneous catalysts by adsorption of a cationic tetrapyrrole on to bentonite: Equilibrium, kinetics, and thermodynamics

Adsorption of an octacationic tetrapyrrole, octakis(2- trimethylammoniumethylsulfanyl)porphyrazinatocobalt octaiodide (QCoPz), from aqueous solutions on to negatively charged bentonite was investigated. Effects of temperature, dye concentration, solid concentration, and contact time on adsorption were determined. Zeta potential and ion-release measurements were also used as supporting experiments. Experimental data were analyzed using four adsorption kinetic models; a pseudo second-order kinetic model resulted in better correlation with experimental results than the others. Experimental equilibrium data were analyzed by non-linear regression using five adsorption isotherm models with two, three, or four terms. Free energies, enthalpies, and entropies for the adsorption process were determined. The results indicated that adsorption of QCoPz on bentonite was exothermic and spontaneous in nature. FT-IR spectroscopy of the composite and its desorptive behavior were also investigated to identify the mechanism of adsorption. The novel QCoPz-bentonite composites obtained are likely to be used in "green chemistry" and in a wide range of optical and/or catalytic applications, especially those crucially important in the petroleum and pulp/paper industries for waste water cleaning (destruction of mercaptans, sulfides, phenol, and halogenated aromatics, etc.) and removal of bad odor.