| When the concentration of water-soluble pollutants such as Cr(Ⅵ)andphosphorus exceeds the water standard, there will be a serious danger to theenvironment and human health. Currently, most of the research of Cr(Ⅵ)removal wasfocused on Cr(Ⅵ)removing in industrial effluent, while relatively fewer literatureswere specially involved in Cr(Ⅵ) uptake in drinking water. Phosphorus exists in theform of phosphate, polyphosphate and organic phosphorus in the sewage effluent,while phosphate is the main form. Advanced treatment for phosphate of the effluent ofsecondary sedimentation tank has an extremely important significance for the controlof total phosphorus discharge to receiving waters. The aim of this study is to developean ion-exchange membrane chemoreactor, which will be used for Cr(Ⅵ) andphosphate removal from aqueous solution.The results of our experiments with Cr(Ⅵ) and phosphate separation byanion-exchange membrane showed that the separation efficiency of Cr(Ⅵ) fromfeeding chamber reached86.4%under the conditons of influent Cr(Ⅵ) concentration1.0mg/L, pH6.95, NaCl concentration0.1mol/L in counterion solution, flow rate offeed and counterion solutions2.5mL/min, stirring speed500r/min, and phasetemperature25℃. In addition, the separation efficiency of phosphate byanion-exchange membrane could achieve84.3%under the identical experimentalcondition. Using Na2SO4as counterion solution, the separation efficiency of Cr(Ⅵ)and phosphate was reduced, and the latter got a comparatively greater decrease. Withthe different NaCl concentrations in counterion solution, no change was likely to befound in the separation efficiency of Cr(Ⅵ) and phosphate by anion-exchangemembrane; however the ion flux rised dramatically with the increase of NaClconcentration in counterion solution. When initial pH of feed solution was11.0and3.0, the separation efficiency of Cr(Ⅵ) and phosphate dropped. It was also found thatthe separation efficiency of Cr(Ⅵ) and phosphatewas greatly improved with theincrease of stirring speed and phase temperature in feed and counterion solutions. Theseparation efficiency of Cr(Ⅵ) and phosphate decreased significantly with increasingthe flow rate of feed solution. The competitive removal experiments indicated that thedivalent ion (SO42-) had a profound interfering effect compared to monovalent ions(NO3-and Cl-), and the higher the concentration of coexisting ions, the more stronger of the competitive effect. Orthogonal experiments showed that the flow rate of feedsolution and counterion solution species mostly affected the separation of Cr(Ⅵ) andphosphate, respectively, which had a significant impact on the results of separationexperiments of Cr(Ⅵ) and phosphate by anion-exchange membrane.In the optimum operating parameters of separation, the enrichment content ofboth Cr(Ⅵ) and phosphate in chemoreactor per unit time were increased with theincrease of influent concentration in feed solution. The optimum dosage ofFeSO4·7H2O: Cr(Ⅵ) was found to be20:1with reductant removal of Cr(Ⅵ), andappropriately excessive dosage of FeSO4could be added without adjustment of pH inraw water. Increasing dosage coefficient with the ratio of PFS to P, the removalefficiency of phosphate by chemical precipitation was enhanced. Effluentconcentrations of Cr(Ⅵ) and phosphate treated by ion-exchange membranechemoreactor were less than or close to the corresponding water quality standardsunder three different operation conditions.Ion exchange kinetics of Cr(Ⅵ)and phosphate were also specially investigated.The experimental results showed that the saturated exchange capacity of Cr(Ⅵ) andphosphate with anion-exchange membrane were1.59mmol/g(dry membrance) and0.51mmol/g(dry membrance), respectively. The ion exchange process of both Cr(Ⅵ)and phosphate could be described by the Particle Diffusion Control(PDC) kineticmodel. With the increase of initial concentration and phase temperature of Cr(Ⅵ) andphosphate, the apparent rate constant and particle diffusion coefficient of the two ionsincreased. The transport process of Cr(Ⅵ) and phosphate ions transferred fromfeeding chamber to chemoreactor through the anion-exchange membrane could bedivided into three steps on the whole. Using pretreated anion-exchange membraneimmersing in NaCl solution and increasing Cl-concentration in counterion solutioncould efficiently promote the first and third transport process of Cr(Ⅵ) and phosphateions, respectively. However, the second transport process of Cr(Ⅵ) and phosphateions was mainly depended on the basic characteristic of exchange ion andanion-exchange membrane. When NaCl concentration in counterion solution wasincreased, the content of Cr(Ⅵ)and phosphate ions in anion-exchange membranedecreased significantly. In the experiments with two overlapping membranes, Cr(Ⅵ)ions mainly distributed in membrane1, and fewer were found in membrance2. Withlower and higher NaCl concentration in counterion solution, the content of phosphateions distribution in two overlapping membranes presented increasing and descendingtrend from side of feeding chamber to side of chemoreactor respectively. The charged colloidal particles existed in raw water were the main reason for membrane fouling,and the membrane could be restored by acid-alkali chemical cleaning.In consequence, separation and removal technology using ion-exchangemembrane chemoreactor might be a promise potential process for emergencytreatment of raw water polluted by Cr(Ⅵ)and advanced treatment of phosphatecontaining wastewater, especially in the area where high salted nature water can beutilized. |
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