ABSTRACT

Increased utilization of mechanically stable synthetic matrices specifically silica gel as a solid support for adsorption purposes has proven to be economical and eco-friendly. In this current work, a Schiff base (E)-2-(3-Methylimino) Methylphenol was synthesized. The Schiff base was used for the impregnation and immobilization of the silica gel. The chemical modification was done by functionalizing the activated silica with aminopropyltrimethoxysilane (APTS) covalently grafting by the Schiff base. Four solid surfaces which include: (pure silica (PS), physically modified silica (PMS) and chemically modified silica (CMS) were used for the spectrophotometric adsorption studies of Cr (VI), Cd (II), Ni (II) and Co (II) ion from their respective aqueous solutions. The choice for Co(II) and Ni(II) ions in this study was as a result of the harmful nature of these metals to human health and lives of other living organisms within our environment. Elemental analysis of the solid supports was done using scanning electron microscope and differential scanning calorimeter. The results revealed that the adsorption capacities of the adsorbents was of the order PS<PMS<APTS<CMS. It was discovered that the CMS, APTS, PMS and PS had degradation temperatures of -108^oC, -57^oC and -27^oC respectively. Thus, the thermal stability trend, PS< PMS< CMS was observed. The adsorption of Cr (VI), Cd (II), Ni (II) and Co (II) ion were done in batches. It was discovered that highest amount of the adsorbates was adsorbed at optimum temperature, pH, time and concentration of 50^oC, 5, 60 mins and 2.5 g/mol respectively. Upon comparison, the adsorbents had higher preference for C_o(II) ion than for other metal ions. The high selectivity of the Co (II) ion could be attributed to the high ionic potential of C_o (II) ion than those of the other metals, thus more of the C_o (II) ion was adsorbed electrostatically by the adsorbents. The study showed that the Gibb’s free energy (∆G) of the process was negative suggesting the process was feasible and a spontaneous process. The enthalpy (∆H) and entropy (∆S) of the process were both positive with optimum Gibb’s free energy, enthalpy and entropy values at the temperature of 30 ^oC. The negative values of ∆G indicates spontaneity, while positive values of ∆H and ∆S implies endothermic and a degree of disorderliness respectively. The The sorption kinetics of the process was studied using pseudo first order of the Lagergen kinetic model, pseudo second order kinetic model, Elovich kinetic model and Weber-Morris intra-particle diffusion model. The result revealed that the adsorption process followed second order kinetics-with average rate constant K₂ equals 0.0112954. The sorption isotherm was equally investigated using Langmuir, Freundlich, and Temkin isotherm models. These mathematical isotherm models were used to describe the distribution of the adsorbate species among the solid and liquid phases. The fit of the experimentally determined data to the models were evaluated in terms of linear regression analysis where R² value of each was approximately one, an indication of best fit of a model.