EDI device is homemade, vertical structure, specifications for 200mm × 400mm, anion and cation exchange membrane is homogeneous membrane, fresh water chamber filled with gel anion and cation exchange resin (volume ratio of 2:1), anion and cation electrodes are titanium coated with nails electrodes, from 0-200V SCR rectifier provide DC power supply is provided by 0-200V silicon controlled rectifier.
The feed water of EDI for test was prepared by adding Na2SO4 and CaCl2 to the first grade demineralized water.
2
Test results and discussion
2.1
The effect of raw water salinity on the current of the membrane stack
The effect of the salt content of feed water on the operating current, within a certain range of voltage, the current of the membrane stack is linearly related to the voltage when the conductivity of the feed water is 60
μS/cm, and the current of the membrane stack is linearly related to the voltage when the conductivity of the feed water is 20μS/cm. When the conductivity of the feed water is 20μS/cm, the current-voltage relationship curve is roughly bounded by the current equal to 60mA, and the relationship is linear when the current is lower, and the slope increases when it is higher.
Ion migration in the freshwater chamber can be viewed as two parallel processes: one is the migration of anions and cations in water toward the anode and cathode, respectively, and the other is the migration of ions into the resin pores after ion exchange occurs, i.e., in the resin particles [1]. Let the resistance of water and resin in the freshwater chamber be RW and Rr, respectively, R1 is the resistance of the solution phase, and R2 and R3 are the resistance of the diffusion layer of the anion and cation surfaces, i.e.
RW=R1+R2+R3 (1)
The total resistance of the freshwater chamber, R total, is obtained by Ohm's law:
R total=(RrRW)/(Rr+RW)
(2)
In the fresh water chamber, due to the low conductivity of the Shenzhen EDI ultrapure water feed, the resin conductivity is 2-3 orders of magnitude higher than the raw water [2], so the ions in the raw water mainly enter the concentrated water chamber through migration in the resin layer. We can also see from Fig. 1 that the above theory is well explained by the fact that at a current of less than 60 mA, the current of raw water with a conductivity of 60 μS/cm is higher than that of 20 μS/cm under the same voltage, but it is not directly proportional to the ratio of the conductivities, which is due to the fact that, although the resistances of the solution phases are different, the resistance of the freshwater chamber is mainly determined by the series resistance of the resin layer alone, and so the corresponding current is not much different .