Removal Of Nickel And Cobalt Ions From Aqueous Solution Using Complexation-Ultrafiltration Process

Heavy metal pollutants exhibit the following characteristics: wide source, long time with toxicity, easily cumulating and hardly resuming, and so have negative effects on the health of human being and aquatic organisms. Nickel and cobalt are two kinds of heavy metals with very similar chemical properties, they can be found together in deposits, special alloy materials, battery materials and catalysts.The main traditional technologies for the treatment of heavy metal containing waste water are chemical treatment method, physical treatment method and biochemical method. These methods just simply transfer heavy metals from one place to another, and thus cannot thoroughly solve the pollution problems which are caused by heavy metals, much less recycle the heavy metals. The process of complexation-ultrafiltration with polymers, aiming to reuse water and recycle heavy metals, can treat heavy metal containing wastewaters effectively and economically, and hence has a positive significance to slow the lack of resource and reduce environmental pollution.This research focuses on the removal of nickel and cobalt ions from aqueous solutions using complexation-ultrafiltration process. Polyethylene imine (PEI) and sodium polyacrylate (PAANa) are chosen as the polymer to bind with heavy metals and the separation is implemented through a polyether sulfone ultrafiltration membrane. After the declaration of the principle of removing heavy metal ions by complexation ultrafiltration, we discussed about the effect on removal rate of heavy metal ions under different load ratios, ionic strength, pH values and the time of complexation- ultrafiltration process and set the optimized parameter of Complexation–ultrafiltration process. The polymer was recoverd through the acid-hydrolysis-ultrafiltration process. And the removal rate of heavy metal ions is studied when using the recovered polymer.Firstly, PES membrane was chosen,PEI and PAANa were separately used as complexing agents to remove nickel ions by complexation-ultrafiltration. Effects of solution pH, PEI/heavy metal ions ratio and ionic strength on the rejection coefficient of nickel were investigated. The changing of removal rate of nickel ions and membrane flux along the process was also studied. The complex reaction equilibrium constants were calculated according to Langmuir isotherm model. Effects of concentration time on nickel rejection and membrane flux were also studied. When PEI was used as polymer, the removal rate of nickel ions went highest to 93.0% at pH 7.0 with PEI/Ni²⁺ ratio of 5. There is a 35% reduction on removal rate of nickel ions when the ionic strength of the solution was 0.5mol/L. PEI-Ni²⁺ complexation ultrafiltration process was relatively stable, and removal rate of Ni²⁺ and membrane flux do not decreased significantly over time. When the recovered PEI was used as polymer, the removal rate of nickel ions was 85.28%. Although it is lower than the situation when the fresh PEI is used, it remained at high level. Best-fit complexation equilibrium constants at different pH values showed that pH 7.0 was most beneficial to the complex reaction. The number of nickel ions that bind to the PEI monomer increases with the increasing pH.When PAANa was used as polymer, the removal rate of nickel ions went highest to 99.5% at pH 8.0 with PAANa/Ni²⁺ ratio of 5. There was a 80% reduction on removal rate of nickel ions when the ionic strength of the solution was 0.5mol/L. PAANa-Ni²⁺ complexation ultrafiltration process was relatively stable, and removal rate of Ni²⁺ and did not decrease significantly over time. The complexation ultrafiltration process resulted in some membrane fouling. When the recovered PAANa was used as polymer, the removal rate of nickel ions was above 90%. Although it was lower than the situation when the fresh PAANa was used, it remained at high level. Best-fit complexation equilibrium constants at different pH values showed that pH 7.0 was most beneficial to the complex reaction. The number of nickel ions that bind to the PEI monomer increases with the increasing pH. Aftert the comparison of PEI and PAANa, PEI is more suitable to be the polymer for the removal of nickel ions.At last, with PEI as the polymer, the simultaneous removal of nickel and cobalt ions from aqueous solutions using complexation-ultrafiltration process was studied. The removal rate of metal ions went highest at pH 6.0 with PEI/ metal ions ratio of 5. The removal rate of nickel ions was 99.9% while that of cobalt ions was 96.4%. The complexation-ultrafiltration process was a little bit affected by inorganic ions. There was a 20% reduction of metal ions when the ionic strength of Na+ reached 0.5mol/L. During the 12-hour PEI-Ni²⁺/Co²⁺ complexation-ultrafiltration experiment, there was 15%-20% reduction on membrane flux. So we can conclude that nickel and cobalt ions can be simutanously removed from aqueous solutions through PES membrane using complexation-ultrafiltration process with little membrane fouling. Acid solution- ultrafiltration can effectively recover PEI, which has the same function as fresh PEI.Results above showed that the process of complexation-ultrafiltration with PEI and PAANa can effectively remove heavy metal ions from aqueous solutions. Reuse water and polymer can be effectively regenerated.