| In 2011, the 12 th five-year planning for the prevention of heavy metal pollution pulled open the prelude of heavy metal pollution control. Severe pollution status also makes the heavy metal pollution treatment is imminent. Lots of work is about to carried out, but traditional methods seek breakthrough is difficult. Recently, liquid membrane technique derived from solvent extraction has been paid wide attention, because it can realize simultaneous removal and recovery of heavy metals. In this dissertation, solvent extraction and strip dispersion supported liquid membrane(SLMSD) have been used for Cr(VI) remoal. The mass transfer kinetics have been investigated, and the overall mass transfer coefficient model was obtained. Besides, the mass transfer mechanism has been analyzed with the model. The aim of the dissertation is to establish a theoretical basis of liquid membrane technologies for Cr(VI) separation.In the process of complexation extraction, the effects of extraction equilibrium thermodynamic parameters including temperature, p H, acidic medium, equilibrium time on Cr(VI) separation were discussed. Besides, interferences of coexisting species were also investigated. According to the results, N235 is a low energy consumption reagent, Cr(VI) removal is a fast chemical reaction process. Under the optimal conditions of 25℃, [HCl]=0.1 mol L-1, 0.004 mol L-1 N235 as extractant, 100% removal rate of Cr(VI) was obtained. Afterwards, a 96% recovery rate was observed when 0.1 mol·L-1 Na OH was used as stripping agent. The enthalpy change(ΔH) of Cr(VI) separation using N235 as the extractant, and kerosene or n-heptane as the diluent were calculated as-35.95 k J·mol-1 and-45.80 k J·mol-1, respectively. The gibbs free energy(ΔG) were calculated as-7.58 k J·mol-1,-9.67 k J·mol-1, and entropy change(ΔS) were-95.18 J·mol-1·K-1 and-121.24 J·mol-1·K-1, respectively. The reaction was proved to be a spontaneous exothermic process.Because the Cr(VI) separation is a mass transfer process, hollow fiber membrane module was used as reactor owing to the large surface area per volume. SLMSD technique was used to discuss the influence of types of membrane contractor, liquid flow state, liquid flow, physical and chemical properties of liquid on the removal and recovery of Cr(VI). From the point of reducing occupied area, vertical membrane contractor is considered as the optimal reactor. When membrane contact area was 0.05 m2, Qaq was 100 m L·min-1, Qsd was 80 m L·min-1, transmembrane pressure was 20 k Pa, N235 concentration was 0.02 mol·L-1, the removal rate of 0.01 g·L-1 Cr(VI) was 96%, the recovery rate of Cr(VI) was kept above 90% with 1.0 mol·L-1 Na OH as stripping agent.On the basis of thermodynamic equilibrium theory, the kinetics of Cr(VI) transport were discussed. Mole method was introduced to investigate the reaction mechanism of Cr(VI) with N235. According to the analysis results of the mole method, 0.01 g·L-1 of Cr(VI) species in 0.1 mol L-1 hydrochloride solution was HCr O4-, then HCr O4- complex with NR3H+ with a ratio of 1:1 forming(R3NH+)HCr O4-. Kinetics analysis proved that the Cr(VI) separation with N235 was dominated by diffusion, and the chemical reaction occured on the liquid-liquid interface. The apparent activation energys of Cr(VI) separation using N235 as extractant, and n-heptane or kerosene as diluent were calculated with Arrhenius equation as 3.76 k J·mol-1 and 3.96 k J·mol-1, respectively. On the basis of the relation between ln[N235], ln[Cr(VI)] and ln R0. The rate equations can be obtained.Besides, the significance of kinetic parameters on extraction rate were also evaluated with the grey relational grade analysis and response surface methodology. The significance of influence factors(stirring speed, temperature, interface area) is stirring speed > temperature> interface area. Grey relatonal degree(GRD) were 0.810, 0.679 and 0.617 with n-heptane as the diluent, respectively. GRD were 0.955, 0.712 and 0.650 with kerosene as the diluent, respectively. According to the analysis results of response surface, there is interactivity between temperature and interface area, stirring speed.On the basis of kinetic characteristics and Fick’s law, an overall mass transfer coefficient model was proposed to discuss the permeability of Cr(VI) in liquid membrane. Mixed flow model and two-dimensional flow model(2D model) proved the feasibility of analyzing mass transfer mechanism with overall mass transfer coefficient model. The modeling results showed that the mass transfer resistance mainly originated from the diffusion of Cr(VI) in tube side and inside the liquid membrane. According to mixed flow model, the mass transfer resistance of diffusion in tube side is 840 s·cm-1, the mass transfer resistance of diffusion in liquid membrane is in the range 2.63×102~5.67×102 s·cm-1. According to 2D flow model, the mass transfer resistance of diffusion in tube side is 746 s·cm-1, the mass transfer resistance in liquid membrane is in the range 3.04×102~6.27×102 s·cm-1. When SLMSD technique was used for Cr(VI) recovery, the on-line renewal and regeneration was realized, the outlet meets the industrial wastewater discharge standard and the reaction device possess long-term stability. |
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