Fluoride electrosorption by hybrid La(III)-activated carbon electrodes under the influence of the La(III) content and the polarization profile

Chronic and excessive fluoride consumption has important implications for human health. Several methods have been implemented to decrease its concentration to acceptable levels. This work studies fluoride electrosorption by an activated carbon impregnated with 3 La(III) percentages: La-0.5%, La-1.5% and La-2.0%. Lanthanum clusters decreased the surface area and pore volume of the adsorbents, which also decreased their polarizable surface. The deprotonation of lanthanum hydroxyls at the working conditions increased the input of negative surface charges, reflected in the acidification and anodization of their points (pHPZC < 5) and potentials (EPZC > 0.1 V) of zero charge, respectively, compared to the pristine carbon (pHPZC= 8.5 and EPZC= 0 V). Electrosorption was evaluated by applying 0.8 V (vs. Ag/AgCl/3 M NaCl) in two profiles: (i) polarizing from the beginning and (ii) polarizing after the adsorption equilibrium. For La-0.5%, the same removal (~3 mg g-1) was reached independently of the profile, although the first profile (i) promoted a faster rate. For La-1.5%, the second profile (ii) increased by 50% its removal (~9 mg g-1), compared to the first (i) profile (~6 mg g-1). The lanthanum hydroxyls are exchanged by fluoride during conventional adsorption, which decreased their input of negative charges and thus their competition when polarized at anodic potentials after adsorption. This was demonstrated by obtaining more alkaline pHPZC, less anodic EPZC and better charging processes after fluoride adsorption. Then, polarizing after adsorption could benefit the performance of electrodes, especially those that undergo synergistic changes in their surface charges during adsorption.