| Over the past decade,membrane technology has been increasingly widely used in the field of water treatment,especially reverse osmosis(RO)has become a key technology for water reuse and brine desalination.However,RO technology inevitably has some limitations,,mainly due to the fact that 15% to 30% concentrated brine was produced when fresh water was recovered from source water.The wastewater not only contained various inorganic salts and refractory substances,but also the composition was very complex.In recent years,as a new electrically driven membrane separation process,the development of bipolar membrane electrodialysis(BMED)technology has pointed out the direction for solving the problem of concentrated brine,and its special hydrolysis separation mechanism has also provided a new way for turning waste into treasure for concentrated brine.Although the technical route was very clear,there were still many problems to be solved from its practical application.Based on this,the study intended to treat typical reverse osmosis concentrated brine(ROC),explore its technical feasibility,and conduct a study of the influencing factors and operating conditions during the process of treating concentrated brine using the BMED technology,providing a basic experimental basis for the practical application of this technology.The main conclusions of the paper were as follows:(1)During the BMED process for treating single-component ROC,proper operating voltage and electrolyte concentration can enhance the rate of desalination and acid-alkali output,while increasing the initial acid-alkali concentration and the volume ratio of the salt to product compartment hindered the penetration of water molecules into the interface layer of the bipolar membrane,resulting in the reduction of current efficiency and the increase of energy consumption.Under the optimal operating conditions,after BMED operated for 100 min,the acid and alkali concentrations obtained were 0.70 mol/L and 0.65 mol/L,respectively.The desalination rate was 91.87%,and the energy consumption and current efficiency of acid were 3.14 k W·h/kg and 50.44%,respectively.It showed that it was technically feasible to use BMED to treat ROC.(2)To further investigate the feasibility of BMED treatment of multi-component ROC,the experiments were conducted to investigate the effects of different cations and anions on the electrochemical performance and acid-alkali purity of BMED during the liquid-phase mass transfer process.The results showed that increasing the voltage was beneficial to improve the ion transfer rate and shorten the operation time when the feed concentration was low,but too high voltage caused more water transfer and unnecessary energy loss.When the feed concentration increased,the migration of salt ions was affected by the concentration gradient and potential gradient,which affected the desalination time and the synthesis rate of acid-alkali products.However,the change of flow velocity and initial acid-alkali concentration has not significant impact on BMED,which meant that the kinetic stability of ion diffusion and electromigration was not significantly disturbed.In addition,the water transport phenomenon was attributed to the formation of hydrated ions and the variability of migration rates of different ions was also attributed to the radius and structure of hydrated ions.Considering the production efficiency of acid and alkali,the voltage(24 V),feed concentration ratio(2:1),flow velocity(55 L/h)and initial acid-alkali concentration(0.1mol/L)were selected as the optimal conditions for BMED,and the final mixed acid and mixed alkali concentrations obtained were 0.58 mol/L and 0.53 mol/L,respectively.The desalination effect higher than 99% was achieved,and he alkali energy consumption(2.97 k W·h/kg)was relatively low.Therefore,the study confirmed that BMED method was suitable for treating different types of ROC and could provide a new way for "zero emission" and resource utilization of ROC.(3)The operating voltage,feed concentration ratio and flow velocity were selected as the main influencing factors in the test to optimize the acid concentration and alkali energy consumption in the BMED process with the Box-Behnken design in response surface methodology(RSM).The optimal operating parameters predicted by the RSM model were:voltage(26.56 V),feed concentration ratio(4.34:1)and flow rate(56 L/h),respectively.The optimization results showed that the recovered mixed acid concentration was 0.90 mol/L and the alkali energy consumption was 4.35 k W·h/kg,while the model predicted the mixed acid concentration and alkali energy consumption to be 0.78 mol/L and 5.08 k W·h/kg,respectively.The predicted values matched with the measured values,indicating that the accuracy of the established quadratic polynomial regression model was good.In addition,the economic performance and product purity of BMED treatment ROC were evaluated under optimized conditions,and the estimated production cost for BMED was 7.73 RMB/kg Na OH.It could be seen that the economic benefits of BMED technology were considerable and the future market potential was huge. |
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