The Application Of Nano Zeolite Composite For Removal Trace Mtbe From Water Solution And Study Its Mechanism

Methyl tert-butyl ether (MTBE), a widely used gasoline additive, has been identified as potential human carcinogen. The release of MTBE could cause contamination in both ground water and surface water, mainly due to leakage and spillage from underground fuel tanks and pipelines. Recently the United States Environmental Protection Agency (USEPA) has issued a Drinking Water Advisory on MTBE levels not exceeding 20-40μg l-1. Consequently there is an urgent demand for feasible and efficient methods to remove MTBE from water systems. In this study, nano zeolite composites of selective supports have been synthesized and attempted on removing MTBE from water system. All the results given here would contribute the nano zeolite composite to large-scale practice for adsorptive removal of MTBE in aqueous phase system in the coming future.First, in the optimization of nano-sized zeolite crystal seeds including silicalite-1, ZSM-5,β, Y and A, silicalite-1 showed the best performance, reducing the solution concentration from 1000μg l-1 to 120μg l-1, with the percent removal of MTBE from solution ranking as silicalite-1>ZSM-5>β>Y>A. The results showed that the hydrophobicity of the zeolite with suitable size and structure was the dominating factor in the adsorption of MTBE so that silicalite-1, the most hydrophobic zeolite was selected as seeds for the subsequent assembly of the nano zeolite composite.Fly Ash Cenosphere (FAC) and diatomite were selected as supports for the preparation of nano zeolite composites, due to (1) the low cost, abundance and environmental-friendly character; (2) their specific inherent multilevel structures. Through the nanotechnology-seed induced-second hydrothermal treatment, two nano zeolite composites-S/FAC and S/D can be obtained. Having a composite of silicalite-1 and FAC, in which the silicalite-1 is dispersed on the surface of the support FAC/diatomite, can avoid the aggregation and coagulation of nano-sized crystals, and may enable the use of silicalite-1 itself as an adsorbent confined in an isolated and practically usable medium in aquatic solutions. Besides this, the obtained hierarchical composite was found to have macropores linked to the zeolite micropores, which permit easy access to the zeolite micropores owing to the reduction of the intercrystalline diffusion resistance. It is considered that this characteristic of the nano zeolite composite could account for the enhancement of the adsorptive features of silicalite-1.In the batch adsorption, the key experimental conditions, including the ratio of initial MTBE concentration to the amount weight of composites, adsorption time and temperature, have been discussed in detail. Adsorption isotherms of zeolite composites demonstrated that the Langmuir model provided a good fit throughout the concentration rang. The higher MTBE adsorption capacity of S/FAC can be attributed to the higher content of zeolite silicalite-1, which is 21% for S/FAC (q max=92551μg g-1) and 12% for S/D (q max=48465μg g-1), in comparison with the q maxof silicalite-1 64066μg g-1. A faster adsorption rate of the nano zeolite composites S/FAC and S/D were observed than silicalite-1 crystals for the removal of MTBE, which may due to (1) the better diffusion of MTBE to the zeolite (external diffusion) on supports; (2) the hierarchical porous structure facilitate the transport of MTBE.Other factors that may be associated with the practical application of zeolite composites were also compared, and each composite had its own merits. Synthesis conditions of S/FAC are relatively mild and energy-saving, while S/D is more economical when considering the cost of the expensive organic template during the preparation process. The regeneration of the recovered S/FAC and S/D can be realized by heating in air at 500℃for approximately 4 hours. XRD patterns of the composite showed no obvious change in structure after regeneration. Column flow-through adsorption tests were also conducted for S/FAC. The results confirmed the high adsorption capacity of S/FAC for MTBE, which will be useful reference for the practical application of composite.