REMOVAL OF SELECTED IONIC AND HALOGENATED ORGANIC POLLUTANTS FROM WATER BY ZEOLITIC ADSORBENTS: COMPUTATIONAL AND EXPERIMENTAL STUDY

Abstract

The presence of contaminants such as toxic anions, heavy metals, halogenated pesticides and pharmaceuticals in various water compartments has caused environmental concern due to the toxicological effects. Conventional water treatment systems are unable to completely remove a large class of these pollutants. It therefore, necessitates the development of more cost effective and environmentally safe methods to remove these pollutants from water. This study explores the use of zeolites as adsorbents for the removal of selected ionic (heavy metals and anions) and organic (halogenated pharmaceuticals and pesticides) pollutants from water. This study was done through computational simulations and batch experiments. In the assessment of zeolite frameworks for the sorption of Pb2+ and Cd2+, 242 zeolites were investigated by employing molecular simulations and machine learning technique. The results showed that only the zeolites with high pore diameter exhibited appreciable loading of the two cations (CLO, FAU, PAU TSC and MWF). It was also observed that the adsorption energy of the cations on the zeolite framework is directly related to the loading capacity, whereby a trend of Pb2+ > Cd2+ is observed. The sorption of Pb2+ and Cd2+ in all the zeolites gave a negative average total energy, showing that the sorption is exothermic. From the machine learning results, pore size and total energy are the most important factor in adsorption of the metal ions onto the zeolites. Therefore, CLO and MWF are recommended for adsorption of Pb2+ and Cd2+, respectively. Furthermore, the capacity of Heulandite (HEU) zeolite to remove Pb2+ and Cd2+ ions from water was investigated using batch experiments and from the results the percentage removal of Pb2+ and Cd2+ ions were 98 and 77 %, respectively. The adsorption of Pb2+ and Cd2+ onto the HEU zeolite follows the Freundlich isotherm model. For the anions (NO3- and PO43-), a screening process involving based on the pore-limiting diameter was done, followed by simulations to identify high-performing zeolites for their removal. CLO, LTN, MWF and TSC zeolites are the best performing. From machine learning the important descriptors that has influence on the removal of the two anions in water were overall cavity diameter, mass and accessible pore volume. From the batch experiments, it was observed that HEU zeolite was able to removal of 59 and 51% of 4 ppm of NO3- and PO43-, respectively after 120 minutes. The adsorption of anions followed the pseudo first order (PFO) kinetics indicating a physisorption-mediated rate-determining step. The adsorption of NO3- and PO43- onto the HEU zeolite follows the Freundlich isotherm model and the adsorption process was multi-mechanistic spontaneous and exothermic. From the computational results, CLO zeolite was the most suitable for the adsorption of diuron, imidacloprid and chlorpyrifos. From the batch experiments, the adsorption of imidacloprid and chlorpyrifos adsorption were best described by the Freundlich isotherm. The percent removal of imidacloprid and chlorpyrifos by HEU zeolite were 50 and 48 %, respectively. The adsorption kinetics followed both pseudo-first order and second order kinetics indicating both physisorption and chemisorption. For the adsorption of selected pharmaceutical products (diclofenac, ciprofloxacin and chloramphenicol, it was observed that CLO was the best zeolite adsorbent. On the HEU zeolite, the percent removal of diclofenac, ciprofloxacin and chloramphenicol were 82, 69 and 51 %, respectively. The adsorption of the pharmaceuticals was best described by Freundlich and Sips isotherm models. The kinetics followed pseudo-first order for diclofenac and ciprofloxacin, while pseudo-second order for chloramphenicol predicting physisorption and chemisorption, respectively. HEU zeolite removed 99, 87 and 70 %, of Ciprofloxacin, imidacloprid and chlorpyrifos, respectively from environmental water samples. The study has established that HEU is a good adsorbent for the removal of pollutants from water. This study forms the background for the innovative use of zeolites adsorbents in development of water purifier gadgets and in wastewater treatment technology.