Electro-sorption of simple electrolytes onto porous carbon-based composite electrodes from water

Abstract

Capacitive deionization (CDI) is an emerging technology for the removal of charged ionic species from aqueous solution; also it has several merits like low cost and high energy efficiency. Electrodes made of commercial activated carbon with high specific surface area supported on graphite sheet (AC/G) were successfully fabricated and used to remove sodium chloride from dilute aqueous solution. A series of experiments on NaCl electro-sorption by NSA/G electrodes were run at different initial NaCl concentrations, applied potentials, and solution pH values. Direct measurement is found to be more accurate than indirect measurement of conductivity when assessing NaCl removal from aqueous solution. Langmuir and Freundlich isotherm models were used to describe sodium electro-sorption at different time periods (i.e., 80 min and 10 min). The electro-sorption and desorption of sodium on NSA/G electrodes followed the pseudo-first-order model in 1mM NaCl. Results showed applied voltage and pH were both important factors on sodium electro-sorption. The highest sodium adsorption of ca. 0.24 mmol/g in 1 mM NaCl occurred at E-Epzc = -1.0 V (vs. SCE) and pHi = 10. The Gouy-Chapman theory can be adopted to preliminarily estimate the adsorbed sodium charge density in terms of potential and pH effects. The cyclic voltammetry results suggested that adsorption time was an important factor when predicting the sodium charge density in 1 mM NaCl through CV method. Moreover, a ratio around 0.3:1 of the sodium charge density estimated from CV to that measured from 10 min NaCl electro-sorption at the same applied potential was successfully derived, indicating the possibility of using CV for predicting ion removal capacity.