Exploring the polarization window during fluoride electrosorption in two activated carbons with significant differences in their pore-size distribution

Abstract

This work reports the electrosorption of fluoride by two commercial activated carbons (coconut-shell and bituminous carbons) with significant differences in their pore-size distribution but with a similar surface area, surface chemistry and charging processes. The coconut-shell carbon is 95% microporous, while the bituminous carbon presents micro (75%) and mesoporosity (25%). Fluoride electrosorption was evaluated in sacrifice cells by polarizing at several potentials (Ecell = 0, 1.2, 1.6, 2 and 2.4 V). The fluoride removal capacity and rate without polarizing was really low and increased at different degrees as a function of increasing the applied potential for both carbons. At 2 Vcell, the bituminous and coconut-shell carbons increased their capacity from 0.073 to 0.431 mg g−1 and from 0.047 to 0.256 mg g−1, respectively, compared to conventional adsorption. On the other hand, the removal rate of the bituminous carbon was ∼ 4 times faster than that of the coconut-shell carbon at short times. This was associated to a higher presence of mesopores, as they enhance the mass-transport and the accessibility of the anion to the polarizable surface, and present less overlapping of the electrical double-layer (EDL). In this regard, a first approximation of the decrease of the EDL thickness was obtained from mathematical approximations and the obtained experimental results at 2 Vcell, in which the overlapping decreased by half, from a pore width of 18.67 nm to ∼ 9.5 nm. Parasitic faradaic reactions were induced at all the applied potentials, which decreased the charge efficiency of the process, although these reactions were more significant at 2.4 Vcell.