7.5 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electrodialysis | 1/2 | https://en.wikipedia.org/wiki/Electrodialysis | reference | science, encyclopedia | 2026-05-05T10:47:30.386561+00:00 | kb-cron |
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell. The cell consists of a feed (dilute) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. In almost all practical electrodialysis processes, multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation-exchange membranes forming the multiple electrodialysis cells. Electrodialysis processes are different from distillation techniques and other membrane based processes (such as reverse osmosis (RO)) in that dissolved species are moved away from the feed stream, whereas other processes move away the water from the remaining substances. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electrodialysis offers the practical advantage of much higher feed recovery in many applications.
== Method == In an electrodialysis stack, the dilute (D) feed stream, brine or concentrate (C) stream, and electrode (E) stream are allowed to flow through the appropriate cell compartments formed by the ion-exchange membranes. Under the influence of an electrical potential difference, the negatively charged ions (e.g., chloride) in the dilute stream migrate toward the positively charged anode. These ions pass through the positively charged anion-exchange membrane, but are prevented from further migration toward the anode by the negatively charged cation-exchange membrane and therefore stay in the C stream, which becomes concentrated with the anions. The positively charged species (e.g., sodium) in the D stream migrate toward the negatively charged cathode and pass through the negatively charged cation-exchange membrane. These cations also stay in the C stream, prevented from further migration toward the cathode by the positively charged anion-exchange membrane. As a result of the anion and cation migration, electric current flows between the cathode and anode. Only an equal number of anion and cation charge equivalents are transferred from the D stream into the C stream and so the charge balance is maintained in each stream. The overall result of the electrodialysis process is an ion concentration increase in the concentrate stream with a depletion of ions in the dilute solution feed stream. The E stream is the electrode stream that flows past each electrode in the stack. This stream may consist of the same composition as the feed stream (e.g., sodium chloride) or may be a separate solution containing a different species (e.g., sodium sulfate). The E stream is usually employed to prevent the reduction and/or oxidation of salt ions from the feed into the electrode plates. Depending on the stack configuration, anions and cations from the electrode stream may be transported into the C stream, or anions and cations from the D stream may be transported into the E stream. In each case, this transport is necessary to carry current across the stack and maintain electrically neutral stack solutions.
== Anode and cathode reactions == Reactions take place at each electrode. At the cathode,
2e− + 2 H2O → H2 (g) + 2 OH− while at the anode,
H2O → 2 H+ + 1/2 O2 (g) + 2e− or 2 Cl− → Cl2 (g) + 2e− Small amounts of hydrogen gas are generated at the cathode and small amounts of either oxygen or chlorine gas (depending on composition of the E stream and end ion-exchange membrane arrangement) at the anode. These gases are typically subsequently dissipated as the E stream effluent from each electrode compartment is combined to maintain a neutral pH and discharged or re-circulated to a separate E tank. However, some (e.g.,) have proposed collection of hydrogen gas for use in energy production.
== Efficiency == Current efficiency is a measure of how effective ions are transported across the ion-exchange membranes for a given applied current. Typically current efficiencies >80% are desirable in commercial stacks to minimize energy operating costs. Low current efficiencies indicate water splitting in the diluate or concentrate streams, shunt currents between the electrodes, or back-diffusion of ions from the concentrate to the diluate could be occurring. Current efficiency is calculated according to:
ξ
=
z
F
Q
f
(
C
i
n
l
e
t
d
−
C
o
u
t
l
e
t
d
)
N
I
{\displaystyle \xi ={\frac {zFQ_{f}(C_{inlet}^{d}-C_{outlet}^{d})}{NI}}}
where
ξ
{\displaystyle \xi }
= current utilization efficiency
z
{\displaystyle z}
= charge of the ion
F
{\displaystyle F}
= Faraday constant, 96,485 Amp-s/mol
Q
f
{\displaystyle Q_{f}}
= dilute flow rate, L/s
C
i
n
l
e
t
d
{\displaystyle C_{inlet}^{d}}
= dilute ED cell inlet concentration, mol/L
C
o
u
t
l
e
t
d
{\displaystyle C_{outlet}^{d}}
= dilute ED cell outlet concentration, mol/L
N
{\displaystyle N}
= number of cell pairs
I
{\displaystyle I}
= current, Amps. Current efficiency is generally a function of feed concentration. As electrodialysis works by transporting salt ions from the diluted channels to the concentrated channels, energy consumption greatly increases as the feed salt concentration increases. Seawater desalination is usually more energy efficient by reverse osmosis than electrodialysis. However, for water streams with lower salt concentration electrodialysis may be the most energy efficient process. Additionally, water streams with very high salt concentrations, that cannot be separated by reverse osmosis, can be concentrated by electrodialysis up to concentrations near to saturation. This is very useful for Zero Liquid Discharge treatments, providing a reduction in energy consumption compared to evaporation.