Study On The Causes Of Fouling And Selectivity Removal In The Process Of Disopal Hardness And Nitrate By Electrodialysis

Water is the source of life,and one of the indispensable and important material resources for human survival and development.The pollution of the water environment will have a great influence on human health.The current Sri Lankan people are suffered from unknown kidney disease.According to the survey,this phenomenon may link to the excessive concentration of nitrate and hardness of local drinking water.Therefor,According to the national economic level,natural environment conditions and other factors,select the appropriate water is particularly important.Based on electrodialysis’s characteristic of low cost and the high water recovery,Electrodialysis can be used as one of technology for the removal of nitrate and desalination from drinking water in Sri lanka.Considering that the process of electrodialysis disposal solution of hardness is easy to cause ion exchange fouling and to further reduce energy consumption,this study carried out to study the causes of fouling and the selectivity removal of nitrate in the process of electrodialysis.（1）One stage and three cycle electrodialysis equipment was designed,Configureting 400ppm,800ppm and 1600ppm CaCl₂ solution and 400ppm CaCl₂+1000ppm NaHCO₃ solution by electrodialysis.Removal of hardness of CaCl₂and Ca（HCO₃）₂,The formation on ion exchange membrane fouling is studied.The results showed that the A,B,C solution containing only 400ppm,800ppm and1600ppm CaCl₂.Due to the concentration polarization of the A and B solution in dulite,the pH decreased and the Ca（OH）₂ scale was formed on the surface of anion and cation exchange membrane;Treatment of D solutions containing 400ppm CaCl₂and 1000ppm NaHCO₃ by the effect of the electric field force,Ca²⁺and HCO₃^-migrate to the anode and cathode,and accumulate on the surface of anion and cation exchange membrane,The saturation solubility product was formed to form Ca（HCO₃）₂precipitate,The surface of the anion exchange membrane is formed Ca CO₃ by the reaction with OH^-ions.（2）Box–Behnken experimental design combining with response surface modeling（RSM）was employed to study the factors of initial concentration,contact time and operating voltage on selectivity to NO₃^-.Based on the NO₃^-,Cl^-and SO₄^2-desalination rate as the response function,a second-order polynomial regression equation was established.Accorrding to the analysis of variance,the model has a high degree of significance and accuracy.By the result of verification experimental,the model can be used to predict the actual desalting efficiency,the absolute error is not more than 3%（3）By the study the selective removal of NO₃^-in binary systeme(Cl^-,NO₃^-,and NO₃^-,SO₄^2-)and ternary system(Cl^-,NO₃^-and SO₄^2-),the result shows that no matter in ternary system or the binary systeme,NO₃^-always has selectivity to Cl^-,by the same charge,the selectivity of NO₃^-is determined by the ion mobility;The selectivity of NO₃^-/SO₄^2-was mainly affected by the operating voltage,and the desalting efficiency of SO₄^2-was higher in low operating voltage condition,and the NO₃^-/SO₄^2-is lower,The appropriate increase of operating voltage can improve the selectivity of NO₃^-/SO₄^2-.