Study Of Bipolar Membrane Electrodialysis Treating Ammonium Chloride Wastewater To Genrate Acid And Base

The bipolar membrane electrodialysis?BMED?treating ammonium chloride wastewater to generate HCl and NH₃?H₂O was studied.The influences of initial concentration of NH₄Cl,current density,salt solution volume,initial concentration of acid and base,membrane stack structure,ions competitive migration and membrane fouling on the concentration of HCl and NH₃?H₂O,current efficiency and unit energy consumption were investigated.The feasibility of BMED treating NH₄Cl wasterwater was conformed and some new conclusions were obtained.?1?Higher initial concentration of NH₄Cl was preferred to promote the yield of acid and base,reduce unit energy consumption and increase current efficiency of the BMED system.At the current density of 48mA?cm^-2,the highest concentration of HCl and NH₃?H₂O with initial concentration of 110 g?L^-1?NH₄Cl was 57.67 g?L^-11 and 45.85 g?L^-1,respectively.The concentration of NH₄Cl below 8000 mg?L^-1?inflection point concentration?was no longer suitable for BMED because of higher energy consumption.The HCl and NH₃?H₂O concentration increased more quickly with the increase of current density.Increasing the volume of NH₄Cl could improve the yield of acid and base.The high initial concentration of acid and base could improve the final concentration of them,while the growth rate was decreased.Compared with the BMED system with three compartments,the growth rate of HCl concentration with the two compartments?anion exchange membrane and bipolar membrane?was higher and its unit energy consumption was lower.?2?The influence of membranes on the BMED system was investigated.The results showed that the bipolar membranes BP-1E and FBM had good performances on the voltage of membrane stack,yield of acid and base,current efficiency and energy consumption.Compared with the other two combinations,the membrane combination FAB-FBM-FKB could obtain a higher concentration of acid and base in a same time period.The membrane combination AHA-?BP-1E?-CMB had lower energy consumption and higher current efficiency than the other two membrane combinations.When the stability of anion and cation exchange membrane in strong acid and base solution were investigated,the anion exchange membrane AHA changed least on the membrane resistance and the contact angle after soaked in strong acid for a week,and the cation membrane FKB changed least on the surface morphology,contact angle and membrane resistance after soaked in base solution for a week,respectively.Thus AHA and FKB had better stability in strong acid and base solutions,respectively.?3?Whatever the concentration ratio of NH₄Cl and?NH₄?₂SO₄ in salt solution was,the Cl^-always transferred preferentially at the beginning of experiment.With the increase of?NH₄?₂SO₄ concentration ratio in the salt solution,the migration rate of Cl^-decreased,while the migration rate of SO₄^2-increased.With the increase of NH₄Cl concentration ratio in salt solution,the current efficiency of HCl increased and its energy consumption reduced.In conclusion,the ion competitive migration was closely related with ion ratio and ionic valence.?4?When the membrane fouling of BMED was investigated,it was found that the membrane resistance increased and there was obvious pollutant on the membrane surface from the SEM pictures.The content of S increased by energy spectrum analysis,which further confirmed that the anion exchange membrane was seriously polluted by SDBS.There was no obvious pollution on the surface of cation membrane when the concentration of CaCl₂ in the salt solution was relatively low.While when the concentration of CaCl₂ was 400 mg?L^-1,the membrane resistance of cation exchange membrane increased.The content of Ca on the surface of cation membrane increased obviously by spectrum analysis.Therefore,the NH₄Cl wastewater need pretreatment to remove the impurities such as organic and Ca²⁺.