Study On Preparation Of Modified Chitosan For Selective Adsorption Of Heavy Metal Ions And Their Application

With the growth of population and the fast development of industry and mining,a large amount of wastewaters with heavy metal ions are discharged into the water without treatment,resulting in serious heavy metal water pollution.Therefore,heavy metal wastewaters must be disposed before they are discharged.Chitosan is a natural polysaccharide whose amino and hydroxyl groups can act as active sites of adsorption for heavy metal ions.Therefore,chitosan can act as an adsorbent for removal of heavy metal ions from solutions.At present,many reseraches on modification of chitosan as common adsorbents have been reported.However,the modified reseraches on chitosan as selective adsorbents for heavy metal ions have been rarely repored.In this paper,three modified chitosan adsorbents with adsorption selectivity for heavy metal ions were prepared.The structures and morphologies of the obtained adsorbents were characterized by FTIR,XRD,SEM and TGA.The effects of pH,adsorption time,temperature and amount of adsorbent on the adsorption of heavy metal ions by the adsorbent were studied.The main results are as follows:?1?Preparation of melamine grafted crosslinked chitosan adsorbent and its adsorption on Pb²⁺and Hg²⁺Using chitosan as a raw material,melamine graft-crosslinked chitosan adsorbent?GMCS?was prepared by crosslinking with glutaraldehyde and grafting with melamine.The structure and morphology of GMCS were characterized by FT-IR,XRD,TGA,¹³C-NMR,SEM,BET,Zeta potential analyses.FT-IR and ¹³C-NMR spectroscopy indicated that the target product was obtained.The results of SEM characterization indicated that the product was formed by the adhesion of irregular particles with a particle size of about 1?m.Adsorption studies showed that GMCS had selectivity and high adsorption capacity for Pb²⁺and Hg²⁺adsorption,and the optimal pH for Pb²⁺and Hg²⁺adsorption were 5 and 6 respectively;the maximum adsorption capacities were 622.5 mg g^-1 and 492.8 mg g^-1,respectively.The adsorption was endothermic and chemical chelated processes.The adsorbent could be regenerated after the adsorption-desorption cycle.?2?Preparation of cinnamic acid grafted crosslinked chitosan adsorbent and its adsorption properties for Hg²⁺Using chitosan as a raw material,cinnamic acid graft-crosslinked chitosan adsorbent?GCCS?was prepared by crosslinking with glutaraldehyde and initiating graft with cinnamic acid.The analysed results of FT-IR and ¹³C NMR spectra indicated that the target product was obtained.The characterization results of SEM indicated that the product was made up of irregular particles with grain size of about 2?m.Adsorption results of GCCS for different heavy metal ions showed that GCCS had selectivity and high uptake for adsorption of Hg²⁺.GCCS for adsorption of Hg²⁺was systematically studied.The results showed that the optimal pH for adsorption was 6,and the maximum adsorption capacity was 714.3 mg g^-1.The adsorption followed the pseudo-second-order kinetics and Langmuir isothermic models.The adsorption was an endothermic and spontaneous chemical process.The adsorption-desorption cycle demonstrated that the adsorbent could be renewed.?3?Preparation and Application of Acrylic Graft Polymerization Crosslinked Chitosan Adsorbent Using Ca²⁺TemplateUsing chitosan as raw material,chitosan was grafted with acrylic acid in the presence of calcium ions,then cross-linked with glutaraldehyde,and finally the calcium ion template was removed.A modified chitosan adsorbent?GACS?of a calcium ion template was prepared.FT-IR spectral analysis showed that the target product was obtained.SEM characterization result indicated that the product consisted of small particles of approximately 700 nm.Adsorption studies showed that GACS had adsorption selectivity for Pb²⁺and Hg²⁺and highest uptake for Hg²⁺.Systematical studies for adsorption of Hg²⁺on GACS indicated that the optimal adsorption pH was 6,and the maximum adsorption capacity was 555.6 mg g^-1.The adsorption conformed to the pseudo second-order kinetics and Langmuir isotherm models.Adsorption was an endothermic process.Adsorption desorption experiments proved that the adsorbent could be recycled.The above results showed that the three chitosan modified adsorbents had high selectivity and uptake for the adsorption of Pb²⁺and Hg²⁺and could be regenerated by EDTA.Therefore,the adsorbents may be used to treat wastewaters containing Pb²⁺or Hg²⁺.