| Chromium and its compounds are widely used in plating, leather tanning, dye,and photography industries producing large quantities of toxic pollutants. Chitinand its deacetylated product chitosan, the world’s second most abundant naturalpolymers after cellulose,has been widely used as absorbents. The purpose of thepresent work was to study the removal of hexavalent chromium from aqueoussolutions by novel modifed chitosan using microwave irradiation.With chitosan as the material, chitosan gel adsorbent and PEI modifiedchitosan were prepared by microwave irradiation through crosslinking reaction.Theoptimum condition for adsorbent production was obtained by single factorexperiment. Sorption parameters includ ing pH, temperature, initia l concentration,contact time and coexistent ions have been investigated. The optimum pH ranges,the equilibrium time for and the optimum temperature the adsorption was obtained.The influence of coexisted negative ion for adsorption were conducted and thesequence were obtained.The optimal modification condition of chitosan gel adsorbent is confirmed: theoptimum concentration of acetic acid is0.5mol/L, the optimum power of microwaveis350W, the dosage of epoxy chloropropane is5mL; The optimal modificationcondition of PEI modified chitosan is confirmed: the grafting reaction time is2h,the grafting reaction tempreture is60℃, the dosage of epoxy chloropropane,1%PEI, Glutaraldehyde are5mL,10.0ml,4mL respectively.The adsorption equilibriumtime was about2h, the results showed that optimum ranges of pH for twoadsorbents were2~7,2~5respective ly. The adsorption capacity decreases withtemperature increasing. Coexist anions have some influence on the adsorption ofCr(VI).The results obtained from equilibrium sorption studies are ftted in varioussorption models such as Langmuir isotherm model, Freundlich isotherm model andthe model parameters have been evaluated. The maximum adsorption capacity areindicated from the Langmuir isotherm and are147mg/g and214mg/grespectively.The adsorption difficult or easy degree are measured by n value offreundlich isotherm.. Kinetic models includ ing Lagergren first-order, second-orderand intragranular transport model were used to test kinetic data. Thermodynamicparameters inc lude free energy, entropy and enthalpy of sorption were alsoinvestigated at different temperatures. The adsorbents were characterized by SEMand FTIR. Through the adsorbent regeneration experiment the effect of different desorption agent has been systematically studied. The results showed that bothLagergren first-order and second-order have good agreement with experimental datafor two adsorbents. Within the whole range of concentration, the disposal quantitiescurve can be fitted well by Langmuir isotherm model and Freundlich isothermmodel. The negative values of Gibbs free energy and enthalpy changes for Cr(VI)indicated the spontaneous and exothermic nature of the sorption process. |
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