The Application Study On The Determination And Transformation Of Cr(Ⅵ) In Crab Shell And Pine Needle

Most of natural polymer materials are microporous structure with reducinggroups, which show the unique adsorption and transformation behaviors for Cr（Ⅵ）.In this thesis, the natural polymer materials, crab shell and pine needle, were used indealing with Cr（Ⅵ） pollutants. The adsorption and transformation of Cr（Ⅵ） with thetwo kinds of natural polymers were studied by electrochemical and UV-visiblespectroscopy. Some interesting results were obtained as the followings:The adsorption and determination of Cr（Ⅵ） on hermit crab shell powder andbismuth modified carbon paste electrode were studied by linear sweep voltammetry.The adsorption of Cr（Ⅵ） on hermit crab shell powder follows the extended Langmuiradsorption isotherm model, Cr₂O₇^2-was electrochemical reduced by the depositedbismuth. The peak current increased with the increase of K₂Cr₂O₇concentrationwith two linear regions of 1.00×10^-8～1.00×10^-6mol/L and 1.00×10^-5～1.00×10^-3mol/L and lowest detection limit was obtained as 1.0×10^-8mol/L, and the recoveries ofK₂Cr₂O₇in waste water were in the range of 91.17%～106.0% with relative standarddeviation of 3.228%. SO₄^2-and Cr（III） ions do not interference the detection. Thecommonly existed CrO₄^2-shows a similar adsorption behavior if the concentration 10times over Cr₂O₇^2-.The adsorption and determination of Cr（Ⅵ） on pine needle powder modifiedcarbon paste electrode were studied by open circuit potential. The adsorption andpreconcentration of Cr₂O₇^2-follows a logarithmic adsorption model. The equilibriumpotential increased linerly with the increase of Cr₂O₇^2-concentration of 10-6mol/L～10-2mol/L with lowest detection limit of 1.0×10^-7mol/L. The experimental resultsshowed that the Cr³⁺,Cu²⁺,SO₄^2-ions did not interference the determination of Cr₂O₇^2-and CrO₄^2-,Pb²⁺interferenced the detection great. The recoveries in waste water werein the range of 94.70%～103.5% with relative standard deviation of 3.443%.The adsorption and transformation processes of K₂Cr₂O₇at natural hermit crabshell powder were studied by UV-visible spectrophotometry.The results indicated thatthe adsorbed K₂Cr₂O₇is transformed into K₂CrO₄in moderate conditions by naturalhermit crab shell powder with the apparent first-order rate constant of 0.002040 min^-1,and equilibrium constant of K=2.27×10¹²M^-1. The adsorption of K₂Cr₂O₇followsthe Freundlich adsorption model with the adsorption constant of KF=888.8 andadsorption free energy change of 16.83 kJ.mol^-1。 The process of the adsorption and transformation of Cr₂O₇^2-into Cr³⁺with pineneedle in strong acid medium were studied by UV-visible spectrophotometry.K₂Cr₂O₇aqueous solution was treated with pine needle for 6 hours, andtransformation rate was upto 87.80%. The apparent first-order reaction constant wasobtained as 0.008767min^-1. The transformation process was controlled by adsorptionand chemical reaction together.