Single-pass capacitive deionization with a HNO3-modified electrode for fluoride removal

HNO3-modified activated carbon was used to make electrodes for single-pass capacitive deionization for removing F from drinking water. The optimal operating conditions for F- removal were studied, and the F- removal performance, cycle stability, and charge efficiency of the electrode were investigated. Based on these results, an optimization scheme was proposed for practical applications. After HNO3 modification, the proportion of micropores, specific surface area, and number of oxygen-containing functional groups on the activated-carbon surface increased, resulting in a significant increase in the specific capacitance of the electrode. Under optimal operating conditions, the adsorption capacity of the modified electrode was 13% higher than that of the unmodified electrode, while the charge efficiency increased by 25% and reached a peak value after about 1,100 s. The HNO3-modified electrode had good cycle stability, and maintained 83% of the original adsorption capacity after 5 cycles. Optimizing the adsorption time (1,500 s) and desorption time (900 s), 80% of the specific adsorption capacity was maintained after 5 cycles. In addition, the cycle time was reduced by 32%, and the utilization rate of electric-double-layer adsorption sites was optimized, resulting in a reduction in the energy consumption per unit F- removal.