Concentration And Removal Of Heavy Metal Ions And Nutrient Salts From Water By EDI

As two kinds of the most primary pollutants in the water, Heavy metals and nutritivesalts are also valuable and useful resources. Discharge of wastewater containing heavy metalions or nutritive salts leads to not only serious environmental pollution, but prodigiousresource waste. Traditional technologies for the correlative wastewater treatment have theirown advantages, however, their disadvantages, such as high cost, low efficiency andsecondary pollution, are outstanding. There are many technological and economic limitationsand it is difficult for them to realize both wastewater purify and resource recycle, especiallyfor dilute solutions.Electrodeionization (EDI) is a clean, efficient and neotype separative technology. It canrealize deep desalination and ions recycle. Though current studies on the treatment ofwastewater containing low concentration of heavy metal ions by EDI prove the processfeasibility, they cannot avoid the precipitation of bivalent metal hydroxide commonlyhappened in the EDI membrane stack, and the long-time stability of EDI has been greatlylimited by this drawback. As for treatment of wastewater containing multiple heavy metalions and wastewater containing nutritive salts, the reports are rare. Therefore, treatment ofdilute wastewater containing heavy metal ions and wastewater containing nutritive salts werestudied respectively in this paper.The electro-regeneration effect of cation exchange resins loaded with nickel ions wasinvestigated in the EDI stack. It was found that Electrode solution, applied voltage and initialmetal ion concentration had remarkable influence on the electro-regeneration of cationexchange resin. Addition of a small amount of Na₂SO₄ into the electrode solution, moderateincrease of applied voltage and initial metal ion concentration could get better regenerationeffect. Na₂SO₄ introduced into the electrode compartments was an initiating reagent of theelectrode reactions, and it could accelerate the anode reaction to generate H⁺ ions. Continuouselectro-regeneration of ion exchange resin were achieved by H⁺ ions produced from the anodereaction and OH^- ions generated form the cathode reaction. Separative fill of anion exchange resins and cation exchange resins and closer arrangement of ion exchange membrane with thesame electric property kept the solution in the concentrate compartment of EDI acid enoughduring the process of electro-regeneration, and the pH value was as low as about 3. Thisacidic environment was very favorable for avoiding Ni(OH)₂ precipitation in the concentratecompartment. It was accordingly hopeful to realize long-time EDI running and continuouswastewater treatment.Effects of different operating conditions on the removal and concentration performanceof nickel ions by the EDI process were investigated. It was found that type of the resin, typeof the membrane, applied voltage and concentration of nickel ion in the wastewater werecrucial to the process. Under the optimized operational condition, simulated wastewatercontaining 50mg/L Ni²⁺ was treated.A steady and continuous process of wastewater treatmentcould be achieved with enrichment factor of 8.5～14.7, removal efficiency of at least 98% andcurrent efficiency of 23.6～37.9%, and concentration of nicked ion in purified water wasbelow 1.0 mg/L. This demonstrated the applicability of deep removal and concentration ofheavy metal ions by EDI without additional chemical regeneration of resins.Behavior difference and selective separation of co-exist nickel, copper and zinc ions inthe EDI membrane stack were discussed, and the ion transport mechanism was analysedcombining Nernat-Planck equation. It showed that the affinity and selectivity sequence wasNi²⁺＞CU²⁺＞Zn²⁺, while transport ability and enrichment factor was Zn²⁺＞Cu²⁺＞Ni²⁺.When the affinity of the ions to resin was different, resins would select and adsorb the ionwith strong affinity. However, the stronger the affinity was, the stronger the desorptionresistance. Improve of selectivity of resins to ions must lead to reduction of transport abilityand drop of enrichment factor.It showed from the description of ion transport mechanism that, the separative coefficientβwas virtually the ratio of ion electromigration rate or ion diffusion coefficient. The larger thevalue ofβwas, the larger the difference of electromigration rate between ions, accordingly thebetter the separative performance. The sequence ofβwasβ_(Zn2+-Ni2+)＞β_(Zn2+-Cu2+)＞β_(Cu2+-Ni2+)·It could be seen that ions were competitive with each other in mixed solutions. Zinc ion wasthe most competitive, thus its electromigration rate was the highest, and due to thecompetition of zinc, electromigration rates of nickel and copper decreased. Nickel ion was influenced by zinc ion more than copper ion, therefore, electromigration rates of copper ionwas lower and that of nickel was the lowest.Emigration and concentration characteristics of nutritive salt anions were explored.Enrichment factor of NO₃^- and PO₄^3- achieved 4.8～6.8 and 2.7～4.0, respectively, after theEDI running of 4 hours. Concentrations of both ions in purified water were below 0. 1mg/L,and the removal rate was more than 97%. This demonstrated the applicability of deep removaland concentration of nutritive salt anions by EDI without additional chemical regeneration ofresins. It was found that the affinity of resin to PO₄^3- ion was larger than that to NO₃^- ion, andthe electromigration rate of NO₃^- ion was about 3.6 times that of PO₄^3- ion. This causeddirectly the decrease of PO₄^3- ion enrichment factor.All the results showed that, it was feasible and applicable to remove deeply andconcentrate both heavy metal cations and nutritive salt anions by the improved EDI process.This was important and significant to to resource recycle and environmental protection.