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Finally, when overlimiting conditions occur, mass transfer characteristics can be assessed by the approach proposed by Nikonenko et al. [192] and Larchet et al. [50] based on the experimental procedure described in [233]. 4.2.3 A critical outlook of ED operations under limiting and overlimiting current conditions In common ED operations, ilim has been assumed as a practical upper threshold for optimal operation [49, 195, 217, 234, 235]. In fact, despite the advantages of enhancing the current thus reducing membrane area requirements, a number of detrimental effects may be encountered in limiting current conditions, such as fouling, scaling and membrane deterioration due to extreme pH values [169, 170, 191, 235, 236]. On the other hand, all complex phenomena occurring in the limiting and overlimiting regions have been misunderstood for long periods in the past and a debate has been on-going in the last 10-20 years on how these can actually affect the operation of an ED stack. Recent publications outline a clear and complete picture of the multifaceted nature and of the complexity characterizing such phenomena [138, 173, 210, 217]. The existence of the overlimiting region was originally attributed to water splitting, i.e. the generation of H+ and OH- ions that contribute as charge carriers [169, 170]. Water splitting has also a secondary effect consisting in the disturbance of the electric field that exalt the transport of salt ions (Kharkats effect of current exaltation [237]). Later, water splitting was found to be promoted by the catalytic activity of some functional groups on the membrane surface in contact with the solution, giving rise to reversible protonation and deprotonation reactions. For example, the splitting reaction occurs intensely in the case of AEMs containing secondary and tertiary amino groups on the surface [238, 239] or CEMs with surface phosphoric acid groups, while it is significantly suppressed in AEMs modified by surface treatment with a strong electrolyte that converts the tertiary and secondary amino groups into quaternary ones [173]. A strong reduction of water splitting was also obtained by AEMs containing crown ethers [240], thus suggesting the possibility to achieve significant effects in fouling prevention. Interestingly, the transport mechanism of charges (i.e., the electrical current) in overlimiting conditions was recently proven to be actually related to the transfer of salt counter-ions by coupled current-induced convection, with water splitting having only a minor effect in generating current. Two types of coupled convection were identified: gravitational convection and electroconvection. Gravitational convection [207, 215, 237, 241, 242] arises from a non-uniform distribution of solution density due to concentration or temperature gradients. Electroconvection [175, 176, 188, 206, 207, 30PDF Image | Electrodialysis for water desalination
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