Extraction Of Chitin From Crab Shells Waste And Its Application In Dye Wastewater Treatment

With increasing attention to environmental protection,concerns are raised about the pollution of marine environment caused by excessive emission of dyestuff effluent.Congo red(CR),as a typical anionic azo dye,has obvious toxic effect on human and marine organism.The wastewater of it has high organic content,high chroma,high salinity and difficult to be biodegraded.Previous studies of CR wastewater treatment by chitin extracted from crab shells waste were less,which made the influencing factors and mechanisms for CR adsorption were not well understood yet.This article extracted chitin(CT)from crab shells waste by the treatment of decalcification and deproteinization,and further prepared chitin with different deacetylation degrees under certain conditions,naming the obtained samples CT-1,CT-2,CT-3,CT-4 and CT-5 respectively,which were all used for the research of adsorptive removal of CR dye.The result of elemental analysis determined the D.D value of these chitin samples and scanning electron microscope showed that CT surface was full of grooves and pores.Thermogravimetric analysis result showed that there were two types of crystal in CT-5 after partial deacetylation,and thermal stability reduced.Fourier transform infrared spectroscopy analysis suggested that the adsorption mainly occur on amino,acetylamino,O-glycosides and-O-on the pyranose ring,but hydroxyl group was not involved in.X-ray diffraction spectra result indicated that the crystallinity of CT-1,CT-2,CT-3,CT-4 and CT-5 decreased compared to CT.A series of adsorption experiments showed that initial dye concentration,contact time,temperature,pH value,salinity and D.D value all have certain effects on CR adsorption onto chitin except for solid-liquid ratio.The optimal condition(t=20 h,pH=5,4,D.D = 17.85%)for CR adsorption under 300 K was determined by orthogonal experiments.Under this condition,the removal rate of 500 mg/L CR can reach up to 99.3%.The isothermal equilibrium adsorption data was best fitted by Double-Langmuir model,theoretical maximum adsorption capacity was 440.7 mg/g.The result of kinetics showed that the pseudo-second-order kinetic model fitted the kinetic data well,and the rate-limiting step for CR adsorption onto CT may be the process of internal surface adsorption-desorption-diffusion and re-adsorption.The thermodynamic parameters?G(?),?H(?)and ?S(?)suggested that CR adsorbed onto CT was a spontaneous,exothermic and entropy reduction process.The small values of ?G(?)and ?H(?)also proved that the adsorption for CR was mainly via physical adsorption.Desorption studies showed that 68.7%of adsorbed CR could be desorbed within 90 minutes under the condition of 0.01 mol/L NaOH and 360 K water bath,indicating that CT has a good potential for reutilization.Compared to other adsorbents reported by previous references,chitin extracted from crab shells waste exhibited an excellent effect for the removal of CR.Combined with its renewable,environmental friendly and low-cost advantages,chitin is a promising adsorption material.