Potential of zero charge regulating highly selective removal of nitrate anions through capacitive deionization

Abstract

Capacitive deionization (CDI) technology is promising for selective removal of target ions from various sewages. However, the low electro-adsorption capacity and poor selectivity in the complex sewage system highly restrict its practical application. Potential of zero charge (Epzc) is the external circuit applied potential as the electrode material exhibiting no ion electro-adsorption from the electrolyte. Although Epzc shows a great prospect for regulating the electro-adsorption performance of the electrode, how to use it to achieve selective electro-sorption remains challenging. Herein, we demonstrate a Sulphur (S) doped ultra-microporous (∼0.7 nm) free-standing wood carbon electrode by one step chemical activation, which exhibits different Epzc values in nitrate or sulphate solution due to the unique S-doped ultra-microporous structure. Accordingly, a fantastic selectivity for NO3– over SO42− (SNO3-/SO42-=32) was achieved as the applied voltage window ranges from 0.12 V (Epzc-NO3–) to 0.43 V (Epzc-SO42−). In addition, the adsorption capacity of the designed material reached up to 637 μmol/g, which is much higher than the maximum value ever reported (∼200 μmol/g). 2000 mL Yellow River water was treated by our tandem CDI cell device through recycling charge and discharge ten times. As expected, the concentrations of NO3– ions in the Yellow River water were dramatically reduced from 152 mg/L to about ∼ 35 mg/L after treated with the tandem CDI cell device, which demonstrates the excellent practical application prospect.