Study On The Application Of Removal Of Cr(Ⅵ) In Solution By Layered Double Oxide

In this dissertation,a series of NO3--LDHs were prepared by co-precipitation method and the corresponding LDO were prepared by calcination,then the preparation process conditions of LDO were investigated and determined.The simulated waste water containing Cr(VI)was prepared using K2Cr2O7 solution and treated with the as-prepared LDO,and then the adsorption process conditions were investigated and optimized.The saturated LDO after adsorption was regenerated by desorption-calcination method and the regeneration process conditions were investigated and optimized.The mechanism of desorption-calcination was preliminarily discussed.The adsorption thermodynamics and kinetics of the adsorption were studied,and the mechanism of adsorption was preliminarily discussed.The main conclusions about the research are as follows:(1)The optimized process conditions for the preparation of LDO are:the molar ratio of Mg to A1 3:1,crystallization temperature 85℃,crystallization time 12h,calcination temperature 500℃,calcination time 5h.The characterization of LDHs and LDO showed that the layered structure of LDO was partly destroyed after calcination except(2)The optimized process conditions of the removal of Cr(Ⅵ)by Mg3Al-LDO are:the initial concentration of Cr(Ⅵ)200mg·L-1,the initial pH value of the solution 6.0,the addition of LDO 2g·L-1,the adsorption time 120min,the solution temperature 20℃.The adsorption capacity could reach 95.28mg·g-1 and the removal rate of Cr(Ⅵ)could reach 95.28%under this condition,with both high adsorption capacity and high utilization rate.The influence order of coexisting anions on the adsorption effect is as follows:CO32->PO43->SO42->Cl->NO3-,and the effect on the adsorption increases with the increase of the concentration of coexisting ions in the solution.(3)Study on adsorption thermodynamics shows that:the adsorption process of Cr(Ⅵ)ions by Mg3Al-LDO accords with the Langmuir isothermal adsorption model which is monolayer adsorption.Both the △Hθ and △Gθ of the adsorption system are less than 0,indicating that it is a spontaneous and exothermic process.Study on adsorption kinetics shows that:the adsorption process accords with the pseudo-second-order kinetic model and the activation energy(Ea)is 32.26kJ·mol-1(more than 8.4kJ·mol-1)suggesting the adsorption process is dominated by chemical adsorption.The Cr2O72-in the solution is introduced into the interlayer to realize the removal of Cr(Ⅵ)during the reconstruction of the layered structure,depending on the "structural memory" effect of the LDHs.(4)Desorption-calcination method is adopted to regenerate the saturated LDO.The regeneration rate of LDO can maintain 67.85%after 5 times of regeneration,indicating LDO has good reusability.The regeneration process conditions are optimized as follows:the mixed solution of 1.5mol·L-1 NaOH-3mol·L-1 Na2CO3 is used as desorption solution,the desorption time 3h,the calcination temperature 500℃,the calcination time 5h,under which the primary regeneration rate could reach about 93.4%.Study on the desorption kinetics shows that the desorption process accords with the pseudo-second-order kinetic model and the activation energy(Ea)is 21.74kJ·mol-1(more than 8.4kJ·mol-1)and the desorption process is mainly a chemical change.