Fluoride Electroadsorption on Activated Carbon Electrodes Modified with La(III)
Authors: Martínez-Vargas David Ricardo , Rangel-Mendez José René , Cházaro-Ruiz Luis Felipe 
Published: February 2020

Fluoride pollution is an important problem in San Luis Potosi, México due to its health effects. Conventional water treatment methods do not provide an efficient remotion for this anion, so there is a need in adapt and develop methods for this purpose. ES is an hybrid method that consists in the electrical bias of conducting adsorbents with high specific area; such polarization generates an electric field and promotes anion migration to the anode and cation migration to the cathode. Migration provides a well-oriented drift and should be more effective in diluted systems, in contrast to diffusion-based treatments such as conventional adsorption. Also, the regeneration of the surface is usually achieved through depolarization or inverting te polarization also in contrast with conventional adsorption, which requires saturated material immersion in concentrated solutions. In addition, this makes ES a moe ecological method.

Our research group developed an efficient, economical and selective adsorbent based on activated carbon (AC) impregnated with La(III). Nonetheless, the maximum adsorption capacity of this adsorbent is diminished when evaluated in high ratios of competing anions to a low ratio of fluoride. There are also reports on electroadsorbents modified with metals that increases its remotion capacity towards fluoride when biasing in contrast to its conventional adsorption process. Instead of opting for the optimization of the adsorbent, we plan to understand its electrochemical properties and try to correlate them with its physicochemical and textural properties, so we can induce a selective remotion of fluoride among several ions.

For this, AC was immersed in a La(III) solution 0.1 and 0.15 M for 2 days at 25C and under constant stirring (120-130 rpm). The material was characterized by several physicochemical and electrochemical techniques. The electrode was assemble introduced in stainless steel mesh sacks with a platinum rod for current collection. The counter-electrode was also SS without AC on it; a membrane was placed between electrodes to attain minimum separation and higher repeatability of the results.

From conventional N2 adsorption-desorption isotherms and their corresponding BET and DFT analysis can be appreciated that the total volume of the material comes from the micro and mesopores, which is normal in our bituminous activated carbon (F-400). When impregnated with La(III), there is a minimum reduction in total volume, primarily due to a reduction in micro and mesopores; nonetheless, the area remains practically unchanged among adsorbents.

The La(III) content evaluation by acid digestion followed by ICP analysis demonstrated that the La percentage in samples was also minimal, about 0.5% or less. SEM micrographs and EDS analysis shows the formation of well-distributed La(III) clusters among particles. By XRD analysis it was possible to identify La(OH)3 and La2O3 crystalline phases. Finally, the determination of the point of zero charge shows the acidification of the modified materials and the pKa distribution analysis, an increase in the hydroxyl signal peak due.

Cyclic voltammetry in K2SO4 0.1 M shows that adsorbents behave like double-layer capacitors and that their electrochemical responses are practically indistinguishable. For example, their specific capacitance was calculated and all materials presented a value of approx. 60 F/g. Their integrated capacitance evaluated at different sweep rates were also the same for the three adsorbents. Electrochemical Impedance Spectra (EIS) in same medium (K2SO4 0.1 M) show that modified adsorbents present a minimum but clear higher charge-transfer resistance and also a higher mass-transfer resistance. This resistive behaviors obtained from Nyquist plot do not correlate with La(III) content. Further data was gathered from this these experiments, such as time constants and polarization capacity. From other EIS measurements in 20 ppm NaF it was possible to obtain the potential of zero charge of the electrodes.

Polarization curves show a working potential window that can be applied to the adsorbents where they behave as ideally polarized electrodes. There is a specific potential window where most electrodes develop an increase in the current. Such signals were associated to oxidation reactions of the solvent and the stainless steel mesh.

Preliminary results show a decrease in fluoride concentration when biasing the unmodified AC and the 0.15 M modification at different potential values. Replication of this experiment and new ones will be discussed, such as competition studies.