| 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 into three categories: 1. anionic: direct, acid, and reactive dyes, etc; 2. cationic: basic dyes; and 3. non-ionic: disperse dyes. Dyes usually have a synthetic origin and complex aromatic molecular structures which possibly come from coal-tar based hydrocarbons such as benzene, naphthalene, anthracene, toluene, xylene, etc. The complex aromatic molecular structures of dyes make them more stable and more difficult to biodegrade. Synthetic dyes have been increasingly used in the textile, leather, paper, rubber, plastics, cosmetics, pharmaceutical and food industries. The extensive use of dyes often poses pollution problems in the form of colored wastewater discharged into environmental water bodies. For some dyes, the dye concentration of less than 1 ppm in receiving water bodies is highly visible,so that even small quantities of dyes can color large water bodies. This not only affects aesthetic merit but also reduces light penetration and hydrophyte photosynthesis. In addition, some dyes or their metabolites are either toxic or mutagenic and carcinogenic.The conventional physical-chemical methods for treating dye wastewater are coagulation and flocculation, oxidation or ozonation, membrane separation and activated carbon adsorption. These technologies do not show significant effectiveness or economic advantage. Activated carbon is the most popular and widely used dye sorbent, but its relatively high price, high operating costs and problems with regeneration hamper its largescale application. Activated carbon is expensive and the higher the quality the greater the cost, furthermore, regeneration using solutions produces a small additional effluent, while regeneration by refractory technique results in loss of sorbent and its uptake capacity. Therefore, there is a growing need in finding low cost, renewable, locally available materials as sorbent for the removal of dye colors.Some low cost botanic materials had directly been used as sorbent for dye adsorption from wastewater, which included apple pomace, wheat straw, orange peel, banana peel, maize cob, maize stalk, rice husk, barley husk, wood chip, palm fruit bunch, sawdust, bark, leaf, coir pith, banana pith, bagasse pith and aquatic plants. But in China, yet no any research work about using low cost botanic material as sorbent for removal of dyes from aqueous solution was done. China was the largest producer of synthetic dyes in the world. For a developing nation as China, new, economical and highly effective treatment technologies of dye wastewater were urgently needed.In this paper, the feasibility of granular peanut hull as sorbent for removal of ionic dyes from aqueous solution was investigated. The aim of this study was to search for a locally available, low cost and untried botanic material in China with high dye removal capacity in order to enhance the treatment of dye wastewater. To the best of our knowledge, up to now, there was no any report about peanut hull had been used as dye sorbent. Peanut hull is an agricultural waste residue and is widely available in large quantity in China. The exploitation and utilization of this bioresource must bring obvious economic and social benefit to us. The dyes selected as sorbate were three anionic dyes: amaranth (Am), sunset yellow (SY), fast green FCF (FG) and three cationic dyes: methylene blue (MB), brilliant cresyl blue (BCB), neutral red (NR). The effects of various experimental parameters (e.g. sorbent dosage, particle size, contact time, ion strength, initial dye concentration and pH etc.) were examined and optimal experimental conditions were decided. By chemical modification, action of some fuctional groups in dye adsorption was certified. The ion exchange mechanism of cationic dye adsorption was elucidate by ICP-AES.At the value of initial pH 2.0, the removal efficiencies of three anionic dyes studied were high and above the value of initial pH 4, three cationic dyes studied could be removed effectively. Except BCB, the adsorption percentages of other five dyes decreased with the increase of initial dye concentrations. The percentages of dyes sorbed increased as the sorbent dose was increased for all dyes studied. The smaller the sorbent particle size the higher the ratios of dyes sorbed for six dyes studied. The ion strength of dye solutions obviously affected adsorption of three anionic dyes. To all six ionic dyes, the isothermal data of adsorption followed the Langmuir or Freundlich models. The adsorption processes conformed the pseudo-first-order rate kinetics. By chemical modification of biomass, some conclusions could been drawn. Carboxyl groups inhibited the adsorption of anionic dyes but they were important in the adsorption of cationic dyes. Hydroxyl groups were important functional groups in the adsorption of ionic dyes. The effect of methylation of amino groups on ionic dye adsorption wasn't significant.The results in this study indicated that granular peanut hull was a good ionic dye sorbent and a low cost biomaterial for dye wastewater treatment.
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