Removal Of Cr(Ⅵ) From Drinking Water With Anion Exchange Membrane Based On Donnan Dialysis

The sources of drinking water in some areas of china have been polluted byCr(Ⅵ) to some degree due to the discharge of Cr(Ⅵ)-containing industrialwasterwater and the improper disposal of Cr(Ⅵ)-containing slag. A new approach forthe separation of Cr(Ⅵ) from drinking water with anion exchange membrane based onDonnan dialysis had been studied in this paper. And the best separation condition ofCr(Ⅵ) had been determined by orthogonal experiment. Furthermore, on the basis ofimproving the original separation apparatus, an anion exchange membranechemical-reactor had been constructed to acheieve the long term operation andthorough removal of Cr(Ⅵ).The basic performance parameters of the anion exchange membrane used in theexperiment are as follows: ion-exchange capacity1.65mmol/g (dry membrane), watercontent44.08%and fixed group concentration3.74meq/g water. In order to improvethe separation effect of Cr(Ⅵ), the anion exchange membrane should be immersed in0.1mol/L NaCl solution for24h before used. The separation experimental resultsshowed that the separation effect of Cr(Ⅵ) got a large improvment when the watertempreture increased from10℃to25℃. However, it improved slightly as the watertempreture increased up to40℃. Thus the water tempreture in the subsequentexperiments was set at25℃. When the rotation speed of the stirrer was set at500rpmor above, the resistance to the transfer of ions in the boundary layer was negligiblysmall compared to that in the membrane. Therefore, the rotation speed of the stirrer inthe subsequent experiments was set at500rpm. The chloride which is monovalent andhas a comparable smaller hydration radius was determined as the best driving ionamong those tested. And since the separation effect of Cr(Ⅵ) was scarcely affected bythe kinds of co-ions, NaCl solution which is comparable cheap and common wasrecommended as the the receiver phase. When increasing the concentration of NaClsolution from0to0.100mol/L, there was no change with the removal efficiency ofCr(Ⅵ), though the ion flux rised dramatically. However, both the removal efficiencyand ion flux tended to increase slightly as the concentration of NaCl solutionincreased up to0.500mol/L. When increasing the initial concentration of Cr(Ⅵ),nochange was likely to be found with the removal efficiency, while the ion flux riseddramatically. We have also noticed that the separation effect of Cr(Ⅵ) would come to be the worst under the strong acid condition. However, it shows a satisfactoryseparation effect in the range of conventional pH value (6.5~8.5) of drinking water.With the increase of the flowing rate of feed phase, the removal efficiency came to belower, while the ion flux rised. the adverse effect of the four kinds of accompanyingions in the feed solution on the separation effect of Cr(Ⅵ) followed the order ofHPO42->SO42-> NO3->Cl-. And when its concentration in the feed phase increased, thecompetiton inhibition between accompanying ions and Cr(Ⅵ) enhanced. In addition,further tests demonstrated that the background components of drinking water had onlya slight adverse impact on the separation of Cr(Ⅵ). By orthogonal experiment, thebest separation condition of Cr(Ⅵ) was defined as: the reciver phase was NaClsolution with its concentration of0.1mol/L, the flowing rate of feed phase was1.0mL/min.The original separation apparatus was improved by increasing the area of anionexchange membrane. And further experiments demonstrated that the improvedseparation apparatus showd a better performance in the separation and concentrationof Cr(Ⅵ), even when the initial concentration of Cr(Ⅵ)fluctuated. So the anionexchange membrane chemical-reactor was constracted by the improved separationapparatus and a chemical reactor. The chemical treatment methods employed in thechemical reactor was determined to be Fe2SO4.7H2O reduction-precipitation. The bestdosing mass ratio of Fe2SO4.7H2O/Cr(Ⅵ) wasset as20, and it was suggested to beadded in a continuous manner. This separation process of Cr(Ⅵ)could meet thedrinking water quality standard successfully, and it was especially suitable for smallto medium sized water treatment.