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Electrodialysis for water desalination

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Electrodialysis for water desalination ( electrodialysis-water-desalination )

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𝜃􏰏􏰒􏰓􏰔 = 𝐶􏰏􏰒􏰓􏰔⁄𝐶􏰏􏰮􏰧􏰯􏰰 (17) 𝜃􏰶􏰒􏰓􏰔 = 𝐶􏰶􏰮􏰧􏰯􏰰⁄𝐶􏰶􏰒􏰓􏰔 (18) where 𝐶􏰮􏰧􏰯􏰰 is the electrolyte concentration at the solution (dilute or concentrate) bulk. In a cell pair, there are four polarization coefficients, one for each IEM-solution interface. Values of 𝜃􏰒􏰓􏰔 can be 􏰕􏰖􏰗 derived from eq. (14), though a more rigorous approach relates them to eq. (11). The membrane potential can then be split into two terms, one accounting for the potential of a membrane ideally in contact with the bulk solutions (∆𝜑􏰷􏰒􏰓􏰔), and the other accounting for the concentration overpotential due to polarization in the DBLs (𝜂􏰒􏰓􏰔 ) [168, 179, 190, 202]: 􏰕􏰖􏰗 􏰶􏰖􏰸 ∆𝜑􏰒􏰓􏰔 = ∆𝜑􏰷􏰒􏰓􏰔 + 𝜂􏰒􏰓􏰔 = 𝛼 􏰘𝑙𝑛 􏰙 􏰶 􏰖 􏰸 􏰒 􏰓 􏰔 𝐹 𝐶 􏰏􏰮 􏰧 􏰯 􏰰 𝛾 􏰏􏰮 􏰧 􏰯 􏰰 where 𝜉􏰏􏰒􏰓􏰔 = 𝛾􏰏􏰒􏰓􏰔⁄𝛾􏰏􏰮􏰧􏰯􏰰 and 𝜉􏰏􏰒􏰓􏰔 = 𝛾􏰶􏰮􏰧􏰯􏰰⁄𝛾􏰶􏰒􏰓􏰔 (often assumed equal to 1). 𝑅𝑇 𝐶􏰶􏰮􏰧􏰯􏰰 𝛾􏰶􏰮􏰧􏰯􏰰 􏰚 − 𝑙𝑛􏰦𝜃􏰒􏰓􏰔𝜃􏰒􏰓􏰔𝜉􏰒􏰓􏰔𝜉􏰒􏰓􏰔􏰪􏰜 􏰶 􏰏 􏰶 􏰏 (19) Interestingly, focusing on hydrodynamics-related features, the DBLs contribute to the total potential drop by means of both Ohmic and non-Ohmic phenomena, which can easily be calculated assuming a linear concentration profile. 𝜂􏰏􏰴􏰗 can be computed by Ohm’s law integrating the resistivity over δ 􏰖􏰲􏰔 and multiplying by the current density (1-dimensional approach, where i = cost). 𝜂􏰏􏰴􏰗 , also called 􏰥􏰒􏰠􏰠 concentration potential or junction potential, is strictly non-Ohmic and originates from the difference in the ionic diffusion coefficients and the need to maintain the local electroneutrality [62]. Note that 𝜂􏰏􏰴􏰗 has a dissipative nature, i.e. it is caused by the irreversible process involving Joule’s effect [62]; 􏰖􏰲􏰔 conversely, non-Ohmic phenomena are (at least ideally) reversible and, ultimately, convert the electrical energy into the chemical energy of a salinity gradient. However, under the hypothesis of a linear variation of concentration within the DBL and of an equivalent conductivity (𝜆) independent of concentration, 𝜂􏰏􏰴􏰗 and 𝜂􏰏􏰴􏰗 can be expressed as [169, 170, 199]: 􏰥􏰒􏰠􏰠 􏰖􏰲􏰔 𝜂􏰏􏰴􏰗 =∓(𝑡 −𝑡 )𝑅𝑇𝑙𝑛􏰦𝜃􏰒􏰓􏰔􏰪 􏰥􏰒􏰠􏰠 􏰹􏰺𝐹􏰕􏰖􏰗 𝜂􏰏􏰴􏰗 = − 𝐹𝐷 𝑙𝑛􏰦𝜃􏰒􏰓􏰔􏰪 􏰖 􏰲 􏰔 􏰦 𝑡 􏰒􏰒 􏰓 􏰔 − 𝑡 􏰒 􏰪 𝜆 􏰕 􏰖 􏰗 (20) (21) where 𝑡􏰹 and 𝑡􏰺 are the transport numbers of anions and cations, respectively. In eq. (20) the sign – is valid when the concentration gradient is negative, as in Figure 9 (thus 𝜂􏰏􏰴􏰗 is positive, i.e. there is a potential drop), while the sign + is valid when the concentration gradient is positive (thus 𝜂􏰏􏰴􏰗 is 􏰥􏰒􏰠􏰠 negative, i.e. there is a potential increase). In very recent years, researchers have shown a wide interest on transport phenomena and DBL effects in IEMs-based systems. The resistance associated with the boundary layer has been measured by 33 􏰥􏰒􏰠􏰠

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