Nanogravimetric Monitoring of Electrochemically Driven Fluoride Ion Extraction from Water By Aniline-Based Copolymer Films
Authors: Hillman A. Robert , Unal Asuman , Cihangir Salih , Yavuz Abdulcabbar , Ryder Karl 
Published: July 2022

Sustainable access to safe drinking water is a global societal goal [1]. Accordingly, much attention has been given to the development of technologies for remediation of the unintended effects of agricultural practices and industrial processes. The case of fluoride in water is an interesting case in two respects: its desirability up to a point and its natural origins. The beneficial effects of fluoride on dental health are recognized: in some countries drinking water is routinely fluoridated. However, high levels of fluoride can have detrimental effects on teeth, bones and internal organs; the recommended upper limit is 1.5 mg/L. The fluoride concentration in natural waters - the source of most drinking water - is dictated by the presence of calcium ions, through the solubility product of calcium fluoride. Dependent on the local geology, the calcium ion concentration may be relatively low or high, such that either the beneficial effects of fluoride may not be realized or its injurious effects may dominate. Determination of fluoride concentration using ion selective electrodes is well-established. The question then is how to remedy high fluoride levels. Existing methods for removal of excess fluoride from drinking water and wastewater include precipitation (as Ca or Al salts), membrane technologies and adsorption. Each of these has limitations, e.g. high costs, toxic by-products and slow or complex processes.

Here we explore electrochemically controlled fluoride ion extraction by electroactive polymer films. This approach has been studied for water softening (using polypyrrole [2]) and perchlorate removal (using polypyrrole composites [3] and polyaniline-based copolymers [4]). Here we explore the feasibility of using a range of polyaniline-based materials in an electrochemically switched ion exchange system for fluoride removal from water [5]. The concept is based on F- ion uptake as counter ions in the oxidation (p-doping) of the conducting polymer film. Upon oxidation, the film will “capture” F- ions from solution. After separation of the purified water, the F- ions would then be ejected into a concentrated waste stream by reduction (un-doping) of the oxidized polymer.

In this presentation we compare the characteristics and performance of polyaniline, poly(o-aminophenol) and poly(o-toluidine) homopolymer films with each other and with those of their copolymers of various composition. Film deposition is controlled electrochemically and monitored nanogravimetrically using the EQCM. Acoustic impedance enables distinction between gravimetric and viscoelastic interpretation of the response [8]. The extent of redox-driven fluoride uptake is then determined upon exposure to solutions of varying fluoride (and in some cases chloride) concentration. Correlation of EQCM-derived film mass and charge responses is used to assay fluoride and solvent uptake during film oxidation and reduction. Comparison with the total redox site population (from the response in fluoride-free media) yields the efficiency. Observations for these aniline-based homopolymers and copolymers reveal behavior that is quite different to that seen for typical small anionic dopants, such as chloride, nitrate and perchlorate. Further, there are surprisingly diverse responses to fluoride for these relatively similar polymeric materials, notably the extent of film solvation change during fluoride uptake. Use of different electrochemical control functions and timescales reveals differences in fluoride ion uptake and release rates. The relevance of these data to defluoridation will be discussed.

References

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