| Dyes are the organic compounds which can bring bright and firm color to other substances. Today there are more than 10,000 dyes available commercially. Dyes can be classified with two methods according to their chemical structure or application in practice. However, the methods can not be detaching with each other due to the application in industries usually being affected by the dyes'chemical structure.The dyes are more and more important in our society, while they have given enormous interest for human being, they are also resulting in serious environmental problems. Currently, the amount of our country's dyestuff is the first whether the yield or consumption in the world every year. A great deal of dyestuff will run off in the process of producing and using them. In our country, about 1.5×108 tons colored waste water are generated annually, most of them don't be purified and discharged directly into the environmental water body. Some dyes or their metabolites are either toxic or mutagenic and carcinogenic, and this poses a serious hazard to aquatic living organisms and human body health, it has already caused public attention.Dyestuff waste water has large quantity, dark chroma, strong toxicity, high concentration, complicated constituents and usually contains various kinds of dyes, therefore, it is a kind of wastewater difficult to be degraded and hard to be treated. There are many methods for dye removal from dyestuff waste water, such as coagulation, oxidation, adsorption, separation etc., sometimes combination of different processes is often used to achieve the desired water quality due to the most economical way.Amongst the numerous techniques of dye removal, adsorption is the procedure of choice and gives the best results as it can be used to remove different types of coloring materials. Some adsorbents are frequently used in processing wastewater, such as activated carbon, ion exchange resin, natural minerals, coal, coal cinder and some by-products from industry or agriculture etc.. The utilization of activated carbon is limited because of its relatively higher price, higher operating costs and problems from regeneration, though it is the most popular and widely used adsorbent.Some low cost agricultural waste materials had directly been used to remove dyes from wastewater, which included maize stalk, wheat straw, rice husk, coir pith, bark, sawdust. The adsorption capacities of natural wastes from agriculture are mostly lower, pretreatment of them may improve the adsorption abilities and enhance the efficiency of their adsorption. In this article, the rice straw had been modified by citric acid in order to add the amount of carboxyl group onto the cellulose, and then treated by NaOH, the carboxyl group existed with the form of the negative charge. After these pretreated procedures, the adsorption capacity of adsorbent had been greatly enhanced for the basic dyestuff (the cationic dyestuff) removal. The best modified conditions for rice straw: the concentration of citric acid was 0.5 M, the temperature was 120℃for 90min. The physical and chemical properties of the modified rice straw all had been changed due to introducing carboxyl groups comparing with the crude rice straw.The dyes selected as sorbate were two basic dyestuffs: methylene blue (MB), a dye of phenothiazine type; malachite green (MG), a dye of triphenylmethane type. In the study, the adsorption tests of modified materials were compared with natural materials. The effects of various experimental parameters (e.g. initial pH, adsorbent dose, dye concentration, ion strength, contact time) were examined. The initial pH of dye solution did not significantly affect adsorption of two basic dyestuffs on over a wide pH range, the optimal pH for favorable adsorption of dyes on modified rice straw was 4 and above. But the initial pH of dye solution had some effect on the dyes adsorption on crude rice straw. The trend that the ratio of dyes sorbed increased as the adsorbent dose was increased could be seen. For the 250 mg.L-1 of dye solution, the 1.5 g.L-1or up of modified rice straw could almost completely remove the dye from aqueous solution. The trend that the ratio of dyes sorbed decreased on adsorbent as the dyes concentrations were increased.The adsorption isotherms of dyes sorbed on sorbent all followed the Langmuir model, and the isothermal data of malachite green sorbed on modified rice straw also fitted the Freundlich model. The adsorption processes all fitted the pseudo-first-order rate kinetics. After citric acid modification, the intraparticle diffusion rate constant (kid) was obviously increased, and the maximum adsorption capacity had been increased 190.3mg.g-1 and162.1mg﹒g-1 for methylene blue and malachite green, respectively.The adsorption equilibriums were reached at about 10 hour, and the adsorption processes included boundary layer adsorption, intraparticle diffusion and adsorption equilibrium.Rice straw is one of the most abundant by-products from agriculture. As a low-cost material, rice straw is a fine adsorbent of basic dyes after chemical modified with citric acid. It could greatly reduce the operating cost for basic dye removal from wastewater. Therefore, as a kind of new treatment approach for dye wastewater, the method has both economic benefit and environmental significance. |
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