Authors: Yan Zhang
Source: UWM Digital Commons (University of Wisconsin–Milwaukee) (article, 2018-01-01)
Field: Chemistry — Organic Chemistry

Graphene-Based Materials Coated on Zeolite for the Removal of Persistent Organic Pollutants from Water Adsorption is a fast, low-cost and the most commonly implemented water treatment technology for the removal of multiple contaminations from ground water, drinking water or wastewater. Difficulties in removing persistent organic pollutants (POPs) to improve quality and safety of treated water sources require the exploration of novel and multifunctional materials. Graphene-based materials having unique structures, high specific surface areas and tailorable functional groups are promising candidates as adsorbents.\nThe main goal of this work is to fabricate a novel adsorbent made of GO/rGO attached on natural zeolite substrates for the removal of variably charged organic model compounds and POPs in aqueous solutions. The thin-layered GO/rGO sheets coated on zeolites surface is expected to have a higher removal efficiency for POPs, wider selectivity, lower cost and better recyclability than most of the commercially available adsorbents, such as granular activated carbon (GAC).\nInitially, ex-situ hybridization of GO and natural zeolite are developed and studied. Three different coating methods were evaluated: spin coating, vacuum coating, and dry coating. Surface morphology, elementary, structural and physical properties of the coated zeolites were assessed. Chemical and structural analysis shows that the presence of GO on the surface of natural zeolite for these samples. Results from desorption experiments indicated that the dry-coated samples exhibit strongest bonding between GO and zeolites. The adsorption capability of GO coated zeolites for organic model compounds are strongly dependent on the loading of GO. The removal efficiencies of POPs by DCGZ showed comparable results with granular activated carbon (GAC). It reveals that the dry coating method to attach the GO sheets on the surface of zeolites is an effective and straightforward approach with a higher loading of GO and a better physical stability. In addition, optimization of column test operating conditions was carried out at different pH, temperature, and concentrations of organic model compounds.\nIn the second phase, variable-charge surfactants including cationic hexadecyl trimethyl ammonium bromide (CATB), nonionic polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether (Triton-X 100) and anionic sodium dodecyl sulfate (SDS), are used in conjunction with the GO sheets and zeolites to enhance interaction of GO on zeolites surface. Besides the elemental and morphological characterization, electrochemical analysis is employed to understand the mechanism of interactions between GO, surfactants and zeolites. The electrochemical analysis and desorption experiment studies show that the enhancement of electrostatic interaction and hydrophilicity of GO and natural zeolite can lead to a strong bonding of GO on zeolites surface. The batch adsorption experiment displays that the charged surfactants added DCGZ have strong adsorption capacities for oppositely charged organic model compounds, while they showed the weak adsorption to the same charged ones. Nonionic Triton X-100 is found to effectively enhance the hydrophilicity and\nstability of the adsorbent, and consequently improve the adsorption performance for a wide variety of organic model compounds.\nIn the third phase, higher loading of GO and in-situ reduction of GO attached on the modified zeolites surface are prepared and studied. To understand and control the properties of the adsorbents, a wide range of characterization were used to evaluate the physical, chemical and morphological properties of GO/rGO and zeolites surface. On one side, the experimental results show that zeolites surface modification by APTES enhances the bonding between the GO sheets and zeolites. A step-wise GO coating process increases the loading of GO on modified zeolites due to the positive surface modification. The effect of loading of GO on adsorption capacity for variable-charged organic model compounds was examined by the column test. A better physical stability of four-time graphene-based materials coated on modified zeolites was confirmed by the desorption experiment. On the other side, two methods by microwave and vitamin C for in-situ reduction of GO on the natural zeolite was studied and evaluated. The extent of GO reduction are characterized and well controlled by the factors including reaction time, concentration and adding methods. The thin-layered GO/rGO on modified zeolites substrate has shown a high affinity for organic model compounds exceeding the performance of a reference GAC. Column adsorption studies of a four-time coated rGO on modified zeolites for polychlorinated biphenyl (PCB), perfluorooctanoic acid (PFOA), and bisphenol-A (BPA) show that over 81% adsorbate removal rate is maintained after 24 hours. The thermal stability and recyclability of a four-time coated rGO on modified zeolites were also investigated. The experimental results show that it has a good thermal which maintains over 91% after five cycles after thermal treatment at 500oC for 1 hours.\nFinally, the modeling of adsorption mechanisms of the four-time coated rGO on modified zeolites for organic model compound was studied. The pseudo-second-order kinetics model and Langmuir isothermal model were fitted to the batch experiment results. Zeolite ; Pollutant ; Environmental science ; Water pollutants ; Graphene ; Environmental chemistry ; Waste management ; Chemistry ; Materials science ; Nanotechnology ; Engineering ; Organic chemistry ; Catalysis Nanomaterials for catalytic reactions ; Adsorption and biosorption for pollutant removal ; Graphene and Nanomaterials Applications