Thermodynamics and Mechanisms of Organochlorine Pollutants Adsorption on Zeolites
Abstract/ Overview
Zeolites, mineral substances composed of aluminum and silicon, have found industrial applications in purification and as a separation technique for compounds. For instance, zeolites can separate isomers, gases and remove volatile organic compounds from some waste air streams. Their porous structures are utilized to “sieve” the molecules/compounds of specific size hence separating them from their mixtures. Technologically, the separation can be enhanced by fine-tuning and adjusting zeolite pore sizes and their surface morphology. In order to achieve this mechanism of fine-tuning zeolites, there is a need to understand zeolite separation mechanisms and their associated thermodynamics. For instance, isosteric heats of an adsorbent can affect its homogeneity and consequently separation efficiency. In the current study, therefore, we explore physical chemical parameters of various types of zeolites adsorption using selected organic pollutant molecules. Effect of doping to adsorption is also investigated. Adsorption parameters including isosteric heats, adsorption sites, adsorption isotherms, activation energies, temperature and loading rates are determined for various types of zeolites using selected organic pollutants. Mulliken charges are also investigated. Thereafter, laboratory adsorption experiments were done to verify loading rates of the zeolites. The average values of isosteric heats for the studied organic pollutants range between 0 to 59 kcal, which is synonymous with physical adsorption mechanisms of the organic compounds and also indicate a homogeneous mixture of the adsorbate and adsorbent. Among the 245 types of zeolites studied, zeolite Cloverite (CLO) gave the best loading rate for all the studied organic pollutants. It is demonstrated that zeolites can separate the various organic chemicals from polluted water environments using different adsorption sites. Furthermore, under low concentrations of adsorbate, the adsorption process on zeolites obeys a pseudo second order kinetic. In addition, sticking probability of adsorbate to adsorbent maximizes at activation energy (Ea) then reduces, indicating the ability of avoiding secondary waste generation through control of Ea. Doping the zeolites with aluminum increases the ability to adsorb organochlorines. It is also noted that charge transfer takes place during adsorption. Laboratory experiments required high silica zeolite in order to pack the columns. Addition of silica gel to the zeolite in the ratio 5;3 was found to be appropriate in order to make slurry. Sorption experiments revealed that clinoptilolite zeolite clogged the columns hence no adsorption. Untreated synthetic zeolite FAU was the best adsorbent with a removal percentage of 95%. Furthermore, untreated zeolite could be reused several times before reaching the saturation point contrary to treated zeolite. The interstitial water within the untreated zeolite provides the active oxygen sites for adsorption thus making it the best adsorbent compared to the treated zeolite with no water in its framework. Experimental results thus agree with our simulation results.