| Dye wastewater without proper treatment would lead to serious water pollution. It can affect on the organisms in the water because of the toxicity of dyes, abnormal coloration and light absorbance which can cause the reduction of photosynthesis. Therefore it is significant to treat dye wastewater and remove the dyes. However, dye wastewater usually is hard to decolor. It is acid-resistant, alkali-resistant and antioxidable. And sometimes it even contains some dis-biodegradable members. It would have a big advantage to treat dye wastewater by adsorption. Although active carbon is a common adsorbent and effective to most dyes, it is limited due to its high price. In recent years, it becomes a research point to treat dye wastewater with low-cost adsorbents which can replace active carbon.Diatomite is abundant in Jilin province. It is not only light and poriferous, but also has high specific surface area, high melting point and stability in chemistry. Thus it has potential to replace active carbon as a cheap adsorbent. However, at present using three-class diatomite as adsorbent is short of reports.In this research, the adsorption of three types of dyes (cationic dye Rhodamine B (RB), direct dye Direct Green B (DG) and reactive dye Reactive Red M-8B (RR)) onto raw three-class diatomite was investigated firstly, and the raw diatomite was characterized. From characterization, the content of SiO2 in the raw diatomite used in this study is 65.76%. And it is poriferous and has a high specific surface area of 19.875 m2?g-1, so it has potential adsorption ability for dyes. The result of adsorption experiment shows that different dyes have different adsorption capabilities onto raw diatomite. RB has a high adsorption capability onto raw diatomite and its calculated saturated adsorbance (Qmax) is 104.1mg?g-1. Meanwhile, its equilibrium adsorption data were best fitted with Freundlich model. However, DG has a relative low adsorption capability on raw diatomite which needs to be improved. Its Qmax is only 14.98mg?g-1 and Langmuir adsorption model can describe this adsorption best. Little RR can be adsorbed onto raw diatomite.Because of the high quantity of—OH groups on the surface of raw diatomite, the adsorption capability of some anionic dyes, such as RR and DG, would be very low. In order to widen the using of diatomite in the treatment of dye wastewater, the raw diatomite needs to be modified. These days, there are only few method s can modify diatomite effectively. Thus it is necessary to search for a new effective method of modification.Recently, researchers pay a lot of attention on chitosan as an adsorbent. It has strong adsorption ability for many dyes due to its plenty of—NH2 and—OH groups. However, normal chitosan power does not have large specific surface area, which limits its adsorption ability. Considering the special structure of diatomite, chitosan was chosen to modify the cheap raw diatomite into a new chitosan-modified diatomite. And this method of modifying diatomite is quite new.In this research, modified diatomites with different mixture ratio were prepared and compared. 50mgCTS/gRD has the best adsorption ability to both RR and DG whose adsorbance can be 90mg?g-1 and 137.1mg?g-1. The adsorption of RB on chitosan-modified diatomite is worse than that on raw diatomite. The result of isotherm experiment of DG on 50mgCTS/gRD can be fitted by Freundlich model best. Comparing the FTIR, SEM and specific surface area of 50mgCTS/gRD and raw diatomite, it infers that chitosan is bonding or surface adsorption on the surface of diatomite.On the basis of the former experiments, the adsorption of the three different dyes onto their best adsorbents needs to be studied. Therefore in this study different diatomite-adsorbents were used to adsorb different dyes (raw diatomite adsorbed RB, 50mgCTS/gRD adsorbed RR and DG). The influence factors of dye adsorption were discussed, and the kinetics, thermodynamics and adsorption mechanisms were studied. The conclusions are showing as below:1. Adsorption time, temperature and pH all have influence on the adsorption. Longer adsorption time can heighten the adsorbance of three dyes; higher adsorption temperature will improve the adsorption of dyes on the diatomite-adsorbents; the adsorption ability under lower pH would be enhanced, but high pH (pH>10) would lower and even inhibit the adsorption.2. To the kinetics, pseudo-second-order kinetics can describe the processes of dyes adsorption onto diatomite-adsorbents very well.3. To the thermodynamics, the positive values of△H show that the adsorption of dyes on diatomite-adsorbents is an endothermic process, therefore rising temperature would be favorable to the adsorption; the positive values of△S suggest that randomness increases during adsorption; the values of△G become more negative with increasing temperature, which indicates the spontaneity of these dyes on diatomite-adsorbents.4. Chemisorption is the main adsorption mechanism of these three dyes. The adsorption mechanism of cationic dye RB on raw diatomite is mainly about cationic exchange and hydrogen bond interaction; the adsorption of RR and DG onto the chitosan-modified diatomite is the result of the electrostatic force between the protonated amino groups and anion groups.Furthermore, the applicability of diatomite-adsorbents in real condition was discussed in the research. It shows that reusing diatomite-adsorbents would decrease the adsorbance of these dyes; the mixed dyes can be well adsorbed onto the mixed diatomite-adsorbent and the adsorption rate can be 83% after 2 hours'adsorption; the diatomite-adsorbents with bigger size (65mesh |
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