Fluoride Electrosorption By an Activated Carbon Doped with Lanthanum

Abstract

Fluorine is an essential element for human health at low doses, but its excessive and chronic consumption at higher doses promotes undesired health conditions. This research reports the electrosorption of fluoride with a commercial bituminous activated carbon impregnated with Lanthanum, as Lanthanum is an excellent fluoride sorbent and presents interesting electrochemical properties. The impregnation was performed in a doping concentration to avoid significant changes in the textural (surface area and pore volume) and capacitive properties (by cyclic voltammetry) of the modified carbon. SEM-EDS mappings were used to evaluate La(III) impregnation and dispersion on the carbon surface, and ICP-OES, to determine its content (0.45% w/w). The anchoring of La(III) modified abruptly the physicochemical properties of the carbon surface, decreasing its point of zero charge (pHPZC) from 8.5 to 4.9, and increasing its potential of zero charge (EPZC) from 0.0 V to 0.3 V (vs. Ag/AgCl/3M NaCl). This was attributed to the exposed hydroxyls from La(III) clusters, which present a Bronsted-acid character and can deprotonate as function of the solution pH. The carbon performance was evaluated by two polarization profiles: stepped-potential increases and single potentials applied from the beginning (0.4, 0.8 and 1.2 V vs. Ag/AgCl/3M NaCl). The anodic polarization of the carbons increased their removal capacity and velocity towards fluoride and a similar capacity was reached independently of the polarization profile, for most applied potentials. At 1.2 V (C0 = 20 ppm F-, pH0 = 7) the removal of the modified carbon increased 2.55 times, compared to its adsorption capacity without polarization (5.93 vs. 2.33 mg g-1, respectively), while the pristine carbon reached only 4.26 mg g-1 . Desorption by depolarization was inefficient for the modified carbon but was attained by reversing the polarization to -0.5 V (94% and 51% desorption for the pristine and modified carbon, respectively). This was due to fluoride chemisorbed to the modifed carbon surface by a ligand-exchange mechanism, in which the exposed hydroxyls provided by La(III) clusters were exchanged by fluorides in solution. Surface charges (pHPZC and EPZC) played a major role in explaining the obtained capacities and velocities, as has been widely studied and reported in desalination using the amphoteric Donnan model.