THERMODYNAMICS AND KINETIC MODELING OF THE DISSOLUTION AND ADSORPTIVE APPLICATIONS OF UKPOR, UDI, AND NTEJE CLAYS

AUTHOR: AJEMBA REGINA OBIAGELI 

DEPARTMENT: CHEMICAL ENGINEERING

AFFILIATION: NNAMDI AZIKIWE UNIVERSITY, AWKA

Alumino-silicate materials, enormously abundant in nature, have been considered as a potential source of alumina and adsorbent for removing colour pigments from edible oils. Nonetheless, the effective application of these materials in these areas is limited due to small surface area and presence of net negative surface charge, leading to low dissolution and adsorption capacity. This research was directed towards the modification of clay minerals from Ukpor, Udi and Nteje, and investigation of the thermodynamics and kinetic models of their dissolution and adsorption in the leaching of alumina and bleaching of palm oil. The modification of the raw clay samples was carried out by two physicochemical methods; (1) thermal activation and (2) acid activation. The raw and modified clay samples were characterized by Ethyl Methyl Glycol Ethylene (EMGE) method for surface area, xray fluorescence (XRF) for chemical composition and Fourier transform infrared spectroscopy (FTIR) for the determination of the effect of acid attack. The thermally modified samples were dissolved in acid solutions (H2SO4, HCl and HNO3) to study their dissolution rate, kinetics and thermodynamics at different values of the process parameters- leaching temperature, acid concentration, solid/liquid ratio, particle size, and stirring speed. The dissolution kinetic data were analyzed using the following kinetic models: shrinking core (Yagi and Kunii); Holt, Cutter, and Wadsworth; Ginstling and Brounshtein; and Christian, Avremi and Kabai models. The acid activated samples were used adsorptive bleaching of palm oil to study their adsorptive capacity, and the kinetics and thermodynamics of the adsorption reaction were investigated at 343K, 353K, 363K, 373K, and 393K. To further understand the kinetics, the adsorption data were analyzed by pseudo-first order; pseudo-second order; Elovich; intra-particle diffusion; mass transfer; and Boyd kinetic models. The response surface methodology (RSM) based on the central composite rotatable design (CCRD) was used to optimize both the leaching and adsorption conditions. The characterization results showed that Ukpor clay is Kaolinite; Udi clay is calcite; while Nteje clay is illite and that the activation process resulted in some structural modifications of the clay minerals. Analysis of the dissolution data showed that the process increased with increased calcination and dissolution temperatures; acid concentration, and stirring speed; and decreased with increasing solid/liquid ratio and particle size. Results showed that Ukpor clay dissolution in H2SO4, HCl and HNO3 conformed to the Yagi and Kunii model with activation energies of 19.945, 39.009 and 30.521kJ/mol, respectively. Udi clay dissolution in H2SO4, followed the Holt-Cutter-Wadsworth model with activation energy of 17.833kJ/mol; while in HCl and HNO3 it followed the Yagi and Kunii model, with activation energy of 21.349 and 18.642kJ/mol, respectively. Nteje clay dissolution in H2SO4 followed the Ginstling- Brounshtein model having activation energy of 19.24kJ/mol; in HCl, it followed the first-order homogeneous kinetic model with activation energy of 27.348kJ/mol; while in HNO3 it fitted Yagi and Kunii model with activation energy of 40.652kJ/mol. The dissolution thermodynamic parameters revealed that the process is spontaneous and endothermic. The maximum yield of alumina was 97.8 % and was obtained with kaolinite in H2SO4 at calcination and leaching temperatures of 729 0C and 103 0C, respectively, acid concentration of 2.93M, particle size of 0.045 mm, solid/liquid ratio of 0.027g/ml, and stirring speed of 436rpm. Adsorptive bleaching of palm oil was dependent on the temperature, clay dosage and reaction time. Results from adsorption kinetic studies revealed that the colour pigment adsorption on the activated clays occurs in multi-layers, and did not form a monolayer. The results revealed that the adsorption followed the pseudo-second order kinetics. Analysis of the equilibrium data using Langmuir, Freundlich, and Temkin isotherms showed that Freundlich isotherm provided the best fit. Furthermore, the evaluation of the adsorption thermodynamic parameters revealed that adsorption process was spontaneous and endothermic. A maximum colour reduction of 94.7% was obtained using illite at a bleaching temperature of 120 0C, adsorbent dosage of 80% and a bleaching time of 30 minutes. The results from this study revealed that modification of these alumino-silicates increased their dissolution and adsorptive capacity. Therefore, application of the simple and low cost modification techniques employed in this study has shown that clays are abundant natural adsorbents and source of alumina. 

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