Study On Removal Of Cu²⁺、Mn²⁺ And Zn²⁺ Using Cation Exchange Embranes Based On Donnan Dialysis

Based on Donnan dialysis, the removal of heavy metal ions such as copper,manganese, and zinc has been investigated using a homogeneous cation exchangemembrane. The study includes the effects of experimental operation conditions andphysical and chemical properties of the heavy metal ions and solution on the removalof heavy metal ions, the membrane fouling and cleaning, the dynamic andthermodynamic phenomena of the membrane for the heavy metal ions exchanging. Atthe last, an ion exchange membrane reactor is constructed and tested for the heavymetal removal.It is found that the tested heavy metal ions can be removed by the ion exchangemembrane without applying external electric potential，and when the heavy metalconcentrations is0.0787mmol/L, the removal efficiency of heavy metal ions such ascopper, manganese and zinc, can be up to85%,79%and75%under the experimentalcondition. Increasing any value of the single factor such as temperature, concentrationof compensation ion, hydraulic retention time and stirring speed within a reasonablerange will result in rise of the metal ions removal efficiency.Migration of heavy metal ions through the membrane can be divided into threesteps: the metal ions entering into the membrane from the donor cell （the first step）,mobile of the metal ions in the membrane （the second step）, the metal ions enteringinto the receptor cell from the membrane （the third step）. Preconditioning membranewill improving the ion migration in the first step. Increasing concentration ofcompensation ion can promote the migration in the second and third step, but nosignificant effect on the first step of migration was observed.As for the heavy metal ions of Cu²⁺, Mn²⁺and Zn²⁺with the same charge, thesmaller the size of its hydrated ion, the faster the migration of the ions, and the higherremoval efficiency the membrane can be achieved at the same ion concentrations.When the size of hydrated heavy metal ions is approximately same, the ion with loweratomic number will pose higher removal efficiency to the membrane. Thus, theremoval capacity of membrane for the tested metal ions is ranged as Cu²⁺＞Mn²⁺＞Zn²⁺. When different heavy metal ions coexist, the ions are simultaneously removedwith less efficiency due to interference between each others.Whether pH of the influent solution imposes an effect on the removal of Cu²⁺ depends on the concentration ratio of H+and Cu²⁺. When the H+concentration is10times higher than the Cu²⁺concentration in the influent, H+imposes an opposite effectof Donnan dialysis of K+ions, resulting in and the removal of Cu²⁺decreasedsignificantly. When H+concentration in the compensation solution is high, H+and K+play the same role in of Donnan dialysis, however this additive effect is not soobvious for the film exchange rate and exchange capacity constraints.Adding of compounds such as silicon dioxide, aluminium oxide, calciumcarbonate, humic acid, ammonia, complexon II, cationic surfactant, anionicsurfactant and iron trichloride etc. as pollutants in influent, will decrease the removalof copper ion after long time operation; if there is no occurrence of physical andchemical reaction between copper ion and the adding inorganic substances, such assilica, alumina and non-ionic surfactant, the effect on the exchange membraneseparation of copper ion would be less, and if the complex compounds are formedbetween the Cu²⁺and additives such as complexon II, ammonia, humic acid andanionic surfactant, the serious effect on removal of Cu²⁺is observed; Since appearingcalcium carbonate, ammonia and anionic surfactant will lead to the solution becomingalkaline and then decreasing free Cu²⁺, and humic acid calcium carbonate andcolloidal iron hydroxide are likely attached on the membrane surface and therefore toblock the pore of the membrane, the reduction of exchange capacity of ion membraneis resulted in. Further, it is found that properties of a fouling ion exchange membranecould be well recovery when it is washed by acid and alkali solution.The exchange capacity of ion-exchange membrane for Cu²⁺is determined as0.506mmol/g under the condition of temperature25℃and pH of the solution6.00.Dynamic and thermodynamic study indicates the ion exchange reaction of Cu²⁺follows the first order kinetics, and exchange process is endothermic and entropyincreasing.An ion exchange membrane chemical reactor is constructed by combining ionexchange separation unit with chemical precipitation chamber, and is applied forcopper ion removal. The results shows that the copper ion leasing from receptor poolof the separation unit can be settled down in the chemical precipitation chambersuccessfully. The compensation potassium after copper precipitated is recycled to thereceptor pool for saving the cost of water and chemicals.Ion exchange chemical reactor which explored in the study is a promise newtreatment technology for separation of heavy metal ions in water, with advantages oflow energy consumption, easy and low cost operation, no secondary pollution.