Studies On The Edi Process And Its Applications For The Removal And Concentration Of Cu～(2+) Ions In Water

Electrodeionization(EDI) is a novel separation process combining ion exchangeresins and ion exchange membranes in a stacked unit which is capable of continuousdeionization under the influence of a DC electric field. Electrodeionization isconsistent with a model of a continuous regenerated mixed resin bed and iswell-suited for deep deionization of dilute solutions without chemical regenerations.In recent years, EDI has been widely accepted as a competitive technology to producehigh purity water. In this paper, performances of the EDI process for the removal andconcentration of Cu2+ ions from dilute CuSO4 solutions were investigated. Thepurpose of the study is to evaluate the viability of the recovery of both high puritywater and heavy metals from dilute heavy-metal-ion containing wastewaters by theEDI process. A laboratory two-stage EDI unit with six cell pairs was built for studies on thetreatment of dilute heavy-metal-ion containing wastewaters in this work. Each ionexchange membrane had an effective area of 135cm2.The EDI process was operatedin a flow mode with partial concentrate recirculation. Effects of stack voltage, diluteflowrate, recirculation ratio, Cu2+ concentration and pH of the feed solution on theperformance of the EDI process were investigated. Under certain conditions, for afeed solution with Cu2+ concentration of 50mg/L, EDI was able to produce a purewater product containing non-detectable concentrations of Cu2+. Dilute resistivity ofthe EDI process was in the range of 2.2-5.6 MΩ.cm. Cu2+ removal was greater than99.99%; a concentrate stream with Cu2+ concentrations in the range of 800-1200 mg/Lwas also achieved. The test results suggest that EDI is capable of deep deionizationand concentration of dilute CuSO4 solutions without chemical regenerations and is apotentially viable technology for the recovery of heavy-metal-ion containingwastewaters. Effects of different cell configurations and operating conditions on theperformance of the EDI process for dilute CuSO4 solution treatment were investigated.It was found that the type of the resin, the composition of the mixed resin bed and thestack voltage are crucial to the operation of the process. Gel type resin become blackand ineffective in the electroregeneration regime. A high ratio of anion exchange resinover cation exchange resin leads to the formation of Cu(OH)2 and CuO precipitation IIin the resin bed and on the membrane surface. Copper reduction on the membranesurface take place if the stack voltage is too high. Based on the analyses of thesephenomena, measures were adopted to eliminate those negative effects. Macroporoustype cation/anion exchange resins were mixed with appropriate ratio, and a reductionof stack resistance was fulfilled. With these improvements, a steady and continuousEDI process was achieved. It is demonstrated that EDI works in two different regimes. At high salinity, theresins in the dilute streams remain in the salt forms, and the deionization efficienciesare derived from the resin-enchanced electrical conductivity of the dilutecompartment; at low salinity, the resins are electrochemically converted to thehydrogen and hydroxide forms, and deionization is consistent with a model of acontinuous regenerated mixed resin bed. The characteristics of the two differentregimes are discussed. A three-dimensional diffusion-migration model was established. This model,combining the theory of electroosmosis of the second kind, were used to qualitativelyexplain the ion transport mechanism of the EDI process. The potential gradient of108-109V/m at the resin-solution or membrane-solution interface leads to the secondWien effect which accelerates the water dissociation reaction. Cu2+ ions are involvedin the proton transfer reaction and are catalytically active to the water dissociationreaction. The combination of the second Wien effect with the catalytic effect of Cu2+ions is the origi...