Sorption Mechanisms and Enhancement of Selected Organochlorine Pollutants in Water on Zeolites
Authors: Manda Timothy , Omwoma Solomon , Barasa Godfrey Okumu , Pembere Anthony M. , Sifuna Douglas , Ochilo Livingstone , Lagat Silas , Ngeno Emily , Ssebugere Patrick , Nagawa Christine Betty , Kyarimpa Christine 
Published: January 2024

This study investigates the adsorption capacities of selected organochlorines on zeolites, focusing on hexachlorobenzene (HCB), hexachlorotetradecane (HCTD), hexachlorodecane (HCD), hexachlorocyclohexane (HCH), heptachlorodecane (HPCD), octachlorodecane (OCD), dichlorodiphenyltrichloroethane (DDT), and octachlorotetradecane (OCTD). The structures of the organochlorines were optimized and their Frontier molecular orbitals were calculated. The analysis of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies provided insights into the molecules’ electron-donating and -accepting capabilities. The present research identified the universal force field as suitable for the investigation and used it to evaluate the adsorption capacities of the pollutants on various zeolites. It was found that CLO (a cubic microporous gallophosphate) demonstrated the highest adsorption capacity for HCB among 245 zeolites, with a loading capacity of 65.84 wt%. In terms of molecules adsorbed per cell, CLO remained the highest with 120 molecules per cell for HCB, 113 molecules per cell for HCH, 43 molecules per cell for DDT, 21 molecules per cell for HCTD, 19 molecules per cell for OCTD, 47 molecules per cell for HCD, 30 molecules per cell for HPCD, and 22 molecules per cell for OCD. The analysis revealed correlations between the structural parameters of zeolites (mass, density, HVF, APV, VSA, GSA, DPS, and Di) and their adsorption capacities. The investigation delved into cluster models to understand the interaction of organochlorines with the zeolite framework. The study explored the impact of doping CLO zeolite with different atoms (Al, Si, and Na) on adsorption capacity. The results showed that doping with aluminum improved both loading capacity and adsorption energy and dissociate the chlorinated compounds during adsorption. Quantum chemical calculations show that hydrogen-based bonding of the organochlorides on the CLO is thermodynamically favorable compared to dissociative adsorption. In addition, oxygen atoms in the zeolites provide active adsorption sites. In the present work, laboratory adsorption experiments were performed, treating zeolites with heat at 400°C. Surprisingly, untreated zeolites outperformed treated ones, adsorbing up to 91% of HCB, while treated zeolites reached saturation after the third run. The study attributed the better performance of untreated zeolites to the presence of interstitial water and hydrogen atoms, which are critical for electrostatic interactions with organic compounds. In general, this research provides a comprehensive analysis of the adsorption capacities of organochlorines on zeolites, combining computational simulations and laboratory experiments. This work’s distinctive quality is its methodology that combines molecular simulations, experimental verification, doping, and interstitial water effects. The findings emphasize the importance of zeolite (a high-porosity nanostructured material) structure, composition, and treatment methods in determining their effectiveness as adsorbents for environmental pollutants.