Adsorption of Anionic Dyes on the Biopolymer Chitin

Journal of the Brazilian Chemical Society

Sanchez, Maria de Nazaré de M.¹  Furlan, Lígia¹  Pozza, Fabíola¹  Laranjeira, Mauro C.M.¹  Longhinotti, Elisane¹  Universidade Federal de Santa Catarina, Florianópolis, Brazil¹  Klug, Marilene¹  Fávere, Valfredo T.¹ 

关键词: anionic dyes;    Langmuir adsorption isotherm;    chitin;   

DOI  :  10.1590/S0103-50531998000500005

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Chitin is a very common natural polysaccharide, found in marine animals, insects and fungi. This biopolymer consists predominantly of repeated units of N-acetyl-b-D-glucosamine (Fig. 1a) and will normally contain a small number of b-D-glucosamine units (Fig. 1b) in a chain1-3.   Chitin has been utilized as an adsorbent for a variety of substrates. Yang and Zall4 have studied the adsorption of various metallic ions. Giles and Hassan5 have investigated the adsorption of ionic species of sulfonated azo dyes and inorganic and organic acids. McKay et al.6 have studied in detail the adsorption of blue acid dye 25, blue acid 158, mordant yellow 5 and direct red 24. The study of adsorption is important for various physico-chemical processes and for an understanding of phenomena such as textile dyeing, clarification and depollution of industrial liquid effluents7,8. For industrial liquid effluents, color is the first indication of water pollution. The industrial dumping of effluents containing dyes not only mars the natural beauty of the rivers, but is extremely toxic to aquatic life, interfering in the transmission of sunlight and thus reducing the action of photosynthesis9. The removal of dyes in the effluents is one of the major problems requiring solution by the textile industries. Some processes have been employed in order to solve this problem, such as adsorption9,10, chemical floculation11, chemical oxidation12,13 and biological techniques14,15. Adsorption appears to be a good alternative for the treatment of effluents. The aim of the present study is to investigate the adsorption of the dyes orange G (OG), orange IV (OIV) and xylenol orange (XO) by the biopolymer chitin, utilizing Langmuir isotherm to determine the adsorption parameters.   ExperimentalChitin extractionChitin was extracted from the exoskeleton of shrimp with NaOH 1 mol L-1 for 24 h. This process was repeated three times with alkaline solution. The material was washed with water until the pH became neutral and demineralization was then carried out with HCl 2 mol L-1 at room temperature. The pigments were oxidized with NaClO 1% w/v and the material was washed until its pH was neutral. The material was then dried at 100 °C for two hours16,17 and pulverized at 100 mesh. The degree of deacetylation of the chitin determined by elemental analysis (Perkin Elmer 2400) was 10% (w/w).DyesThe dyes utilized in the adsorption experiments are illustrated below and were used as commercial salts. Orange G as supplied by Synth (95 % purity) (Fig. 2).   Orange IV as supplied by Sigma (98 % purity) (Fig. 3).   Xylenol Orange as supplied by Vetec ( 90% purity) (Fig. 4).  Dependence on pHSamples of 100 mg of chitin were placed in a conical flask, with 25 mL of dye solution. The pH was adjusted to the desired value by the addition of acid or alkaline solution and control of the medium ionic strength was achieved by addition of NaCl 0.10 mol L-1. The flask was then closed and placed in a bath controlled by a thermostat at 25.0 ± 0.5 °C, for two hours. After the contact period, supernatants were separated from the solid phase by centrifugation, and the concentrations of dyes in solution were determined from the respective analytical curves. The quantity of the dyes adsorbed was determined by the following equation:where {QD} is the quantity of dye adsorbed (mg g-1), Co is the initial concentration of dye in the solution (mg mL-1), Cf is the final concentration of dye in the solution (mg mL-1), V is the volume of the solution in milliliters (mL) and W is the mass of chitin in grams.Adsorption isothermsThe adsorption isotherms were obtained by the batch method, employing 100 mg of chitin and 25 mL of the dye solution at different concentrations. These solutions were buffered at a pH optimum (pH 4.0) for adsorption. These solutions were stirred in mechanical shakers until they reached adsorption equilibrium (60 min). The polymer was separated by centrifugation and the quantity of dye adsorbed was determined by employing a spectrophotometer UV-Vis, Hitachi model U-3000, in the respective lmax. of each dye.   Results and Discussion The effect of the different values of pH on the adsorption of dyes by chitin is represented in Fig. 5. The data indicates that the adsorption capacity of dyes by chitin is dependent on pH. The biopolymer has functional groups OH, NHCOCH3 and NH2. According to Giles and Hassan5, the hydroxyl groups of the polymer are strongly hydrated and are virtually incapable of forming hydrogen bonds with dyes; nevertheless they can adsorb, by the formation of a hydrogen bond, by van der Waals interactions and also by ion exchange with other groups.   Depending on the pH in the water, the polymers containing the amino groups are neutral (-NH2) or cationic (-NH3+). At a pH of higher acidity, these groups are protonated and to maintain neutrality in an aqueous environment, negative counter-ions are adsorbed. These ions are movable and are exchanged by ions from the dyes at appropriate pH. In this

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