Study On The Preparation Of Expanded Graphite Loaded With ZnO And Its Decolorizing Performance For Dyes

In recent years, dyes, which are frequently used in textile industries, are often discharged into the environment owing to incomplete use and washing operations. Such activities cause health hazards to flora and fauna as well as to humans. Typical techniques such as physical methods, biodegradation, chemical methods are used to decrease the pollution. The physical absorption and the advanced oxidation technology（AOPs） are the most commonly used methods. While the results are not ideal when these methods are used individually. It is necessary to improve the processing efficiency.Expanded graphite has excellent absorption properties, ZnO is an excellent photocatalyst, and ultrasonic method has a certain efficiency for wastewater treatment. In this paper, the photocatalysis of ZnO and ultrasonic techniques were combined with adsorption of expanded graphite to improve treatment efficiency.The research work includes the following four parts:1. The composites of expanded graphite loaded with ZnO（ZnO/EG） was prepared using flake graphite with a particle diameter of 0.3 mm as raw material, KMnO₄ as oxidant, sulfuric acid（H₂SO₄） as inserting agent and Zn（OAc）₂ as auxiliary intercalating agent. The influence of the amount of KMnO₄、H₂SO₄、Zn（OAc）₂, reaction time and temperature on expanded volume（EV） were studied. The feasible conditions were detected as follows: mass ratio of m（C）: m（KMnO₄）: m（H₂SO₄）: m（Zn（OAc）₂） was 1:0.28:5.0:0.4, the reaction lasted 40 min at 40℃, and ZnO/EG with a EV of 617 m L/g was obtained at 800℃. Meanwhile, the flake graphite and the prepared specimens were characterized by means of Fourier transform infrared, X-ray diffraction and scanning electron microscope. The results testified that Zn（OAc）₂ inserted into layers of flake graphite, and after expansion at 800℃, Zn（OAc）₂ in the graphite intercalation compound turned into ZnO particles with a structure of hexagonal crystal system, and they widely distributed in the cavities and the network pores of EG.2. The decoloration properties of ZnO/EG and normal EG1, which was obtained with only H₂SO₄ as intercalating agent, were studied with methylene blue（MB） as the adsorbate. The decoloration capability of ZnO/EG for MB was examined in aqueous solution with respect to the calcination temperature, the concentration of adsorbate, ionic strength and p H. Compared with EG1, the adsorption decolorization and total decolorization capability under UV irradiation of ZnO/EG for MB were significantly enhanced. With the increase of adsorbate concentration and ionic strength, the decoloring amount for MB also increased. When the calcination temperature was 800℃, the obtained ZnO/EG shows a largest decolorization capacity. The adsorption capacity has a little changed with the increasing of p H. The photocatalytic decolorization rate increases with the increasing of adsorbate concentration, but decreases with calcination temperature and ionic strength. With the increasing of p H, the photocatalytic degradation increases first, and then it decreases. Result of dynamic experiments testified that: the decoloration data of ZnO/EG for MB under UV light is accord well with Langmuir-Hinshelwood kinetic model. The adsorption type on EG is typeⅠ. The thermodynamic experimental data of MB on ZnO/EG also can be well accorded with Langmuir equation. Adsorption process of MB on ZnO/EG follows the pseudo-second-order kinetic model.3. The expandable graphite was firstly prepared with single sulfuric acid（H₂SO₄） as inserting agent, then a series of ZnO/EG were prepared by dipping expandable graphite in Zn（OAc）₂ solution and then roasting at different temperature. The dipping sequence and roasting temperature were optimized. It has been confirmed by SEM, FTIR and XRD that Zn（OAc）₂ converted to ZnO after heat treatment, which belongs to a hexagonal system structure. The（ZnO/EG）₅, which was prepared using freshly prepared expandable graphite dipping in Zn（OAc）₂ solution, and then filtration, drying, calcination at 800 ℃ for 30 s, calcination under 450℃ for 3h, shows the best decolorization capacity for MB. Compared the decolorization capacity of ordinary EG₁ and（ZnO/EG）₅ with ZnO/EG（prepared by chemical oxidation method）, the later shows the best decolorization efficiency.4. The adsorption properties of EG and ultrasound technology were combined together, and the decolorization efficiency for MB was investigated. Results show that: 25℃ and 200 W are the optimal sonicatio condition. Ultrasound/EG joint technology has synergistic effect, and it can improve color removal. The treatment results for tannery dye wastewater show that: under UV light/adsorption conditions, ZnO/EG presents the most obvious effect. Through one processing, the chemical oxygen demand（COD） of the wastewater solution can reduce to 3100 mg/L from the initial 5078.32mg/L.