Investigations on the fouling characteristic of humic acid and alginate sodium in capacitive deionization

Capacitive deionization (CDI) has been investigated for brackish water desalination, selective removal of ions, and water softening. We used humic acid (HA) and alginate sodium (SA) to simulate different kinds of natural organic matter to investigate the fouling phenomena during CDI operation. Adsorption amount and energy efficiency were studied. Results showed that both SA and HA could decrease the removal of NaCl during CDI operation. There existed a slight decrease of energy consumption in SA solutions which was opposite to that in HA solutions. HA can compete with ions adsorbed by electrodes and attach to electrodes adhesively, resulting in co-ion repulsion. SA is not sensitive to electrical field and its fouling is not obvious. The amount of adsorbed Mg2+ would increase from 0.927 mg/g to 1.508 mg/g in ten cycles' operation and the increment of Ca2+ was from 1.885 mg/g to 2.878 mg/g in SA solutions. This increase of adsorption was due to the complexation between SA and cations. Simultaneously, energy consumption was decreased. In HA solutions, energy consumption of Mg2+ and Ca2+ adsorption increased. In ten cycles' operations, both HA and SA could reduce the efficiency of CDI operation. The types of organic substances are important factors in fouling of CDI electrodes.