Covalently-bonded quaternized activated carbon for selective removal of NO3- in capacitive deionization

Nitrate pollution has become an increasingly serious water pollution problem worldwide. Capacitive deionization (CDI) technology is an emerging water treatment technology that can efficiently remove nitrate in water with advantages of simple process, high water recovery rate, low cost and energy consumption. In this work, we engineer a covalently bonded quaternized activated carbon (QRAC) material by silanization and investigate its selective adsorption of nitrate in mixed solution with extended voltage capacitive deionization (eV-CDI) mode. The results show that (1) the QRAC electrodes achieve selective adsorption due to the different adsorption binding energies of the quaternary ammonium group with Cl- and NO3-. The selectivity coefficient of QRAC-5 to NO3- reaches 2.7 at 0.4 V and the electrosorption capacity is 140% higher than that of AC in 5/5 mM KCl/KNO3 solution. (2) The electrosorption of QRAC electrodes are dominated by two aspects: physical adsorption by external electric field which occurs in the incipient stage and ion exchange adsorption by quaternary ammonium groups occurring in the following stage. (3) CDI experiments in simulated municipal wastewater show that SO42- had a negative effect on the adsorption capacity of NO3-, but QRAC electrode can still achieve separation effect in Cl-/NO3- solution system. These results show that covalently bonded quaternized activated carbon can achieve the effective separation and removal of NO3- in polluted water, and our work provides new insights into the selective electrosorption of CDI technology.