Ion transmembrane behaviors in selective electrodialysis for acid recovery: Impact of ion categories

Selective electrodialysis (SED) was found to be a promising technology for acid recovery. This study systematically investigated the acid recovery performances using SED for six representative acid systems. The results indicated that SED has superior acid recovery performance for the HCl/AlCl3 system, in which the energy consumption was only 3.81 kWh/t (far lower than 17.46 kWh/t of H2SO4/FeSO4), the acid recovery ratio reached 80.72 %, and the treating cost was as low as 11.39 $/t. Among the various acid systems, the transmembrane rate of the H+ ions presented the order of HCl/AlCl3 > HNO3/Al(NO3)3 > HCl/FeCl2 > H2SO4/Al2(SO4)3 ≈ H2SO4/CuSO4 > H2SO4/FeSO4. Meanwhile, there was an obvious enhancement in the perm-selectivity of the Neosepta@CIMS membrane when treating high-valency species such as Al3+-containing acids. Under the action of the electric field, the large-sized metal ions aggregated near the diffusion boundary layer adjacent to the membrane, and the electrostatic repulsion force caused an increasing rejection for the metal ions compared with hydrogen ions. In addition, the physiochemical properties of the anions affected the purity of the recycled acid. Anions with a large hydration radius and large hydration energy aggravate the leakage of co-ions through the anion-exchange membrane and increase the treatment time.