Electrochemical removal and release of perchlorate using poly(aniline-co-o-aminophenol)
Introduction
Since its discovery in groundwater in 1985, perchlorate has attracted much attention because of its adverse impact on public health [1]. It has been reported that perchlorate can displace iodide from the thyroid gland and block the uptake of iodine in the thyroid [2]. Many sources exist that may contribute to discharges of perchlorate into the environment. These include the manufacture and use of ammonium perchlorate as an oxidant in rocket fuel, and munitions [3], in the production of lithium batteries [4], and in electric double layer capacitors [5]. In addition to these anthropogenic sources, chilean nitrate, a common natural fertilizer is considered a potential source of perchlorate contamination [6]. Perchlorate is also produced in the atmosphere, causing its appearance in many rain and snow samples [1].
Perchlorate is difficult to remove from contaminated water because it is extremely water soluble and stable. Many technologies have been investigated for treatments of perchlorate-contaminated water including ion exchange [7], [8] and biological perchlorate reduction [9]. These two technologies, however, are not cost-effective, partially because of the need for the disposal of treatment residuals [8]. Also, very stringent operating conditions must be followed in the biological reduction of perchlorate [10]. New strategies have been investigated to effectively remove perchlorate from water. These include electrochemical and chemical reduction [11], [12], electrically controlled anion exchange [13] and highly selective and regenerable ion-exchange processes [8]. Among these technologies, the conducting polymer polypyrrole-based electrically controlled anion exchange [13] is attractive because this electrically reversible process is simple to operate, generates no secondary contaminant, and thus could be cost-effective.
Conducting polymers have unique properties in conductivity and electrochemical activity suitable for treating contaminated water by ion-exchange processes. When an anodic potential is applied on the conducting polymer, it carries positive charges. The oxidation of the conducting polymer is accompanied with incorporation of anions from the solution into the conducting polymer, i.e. doping, to maintain its electric neutrality. When a cathodic potential is applied, the polymer matrix returns to its reduced state with no or negative charge, causing the doped anions to be released (i.e. de-doping) back into the solution. Therefore, on the basis of doping and de-doping principle, a conducting polymer with higher affinity for the target contaminant (e.g. perchlorate) than other ions could be electrically manipulated to selectively remove the contaminant from aqueous solutions, without using regenerative agents in the ion-exchange process.
The performance of a conducting polymer as an electrically controlled anion exchanger is controlled by its electrochemical activity. Polyaniline is a promising material in acidic systems because it is cheap and has high conductivity, good redox reversibility, and stability [14]. This material, however, has an extremely low conductivity and redox activity in aqueous solution with pH above 5.0 [15], so it is not suitable for perchlorate removal from water in this pH range. Polypyrrole has been reported electrochemically active in exchanging anions of from an acidic solution [16], [17], but not in the alkaline solution because OH− is preferentially doped in the polymer matrix in a basic solution [18]. Hence, it is expected that polypyrrole would not be able to remove perchlorate from alkaline solution. The copolymer synthesized using aniline and aniline derivative with −OH group has demonstrated good redox activity in a wide pH range [19], [20], [21], making it a promising material for removing perchlorate from solutions in a wide range of pHs.
The objective of the current study was to investigate the feasibility of using poly(aniline-co-o-aminophenol) (PANOA) as an electrically controlled ion exchanger to selectively remove perchlorate from aqueous solutions containing background electrolyte NaCl. The associated ion exchange mechanism was studied by cyclic voltammetry, Fourier transformed infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS).
Section snippets
Experimental
Sodium perchlorate (⩾98%) was purchased from Fluka. Other chemicals used were of analytical reagent grade obtained from Shanghai Chemical Company. Aniline was purified by distillation under reduced pressure prior to use. Doubly distilled water was used to prepare all aqueous solutions.
The electrochemical copolymerization of aniline and o-aminophenol was carried out in hydrochloric acid medium in this report. Electrochemical synthesis of PANOA in the sulfuric acid medium has been reported [22],
Redox characteristics of PANOA
Cyclic voltammetry provides a quick and reliable approach to assess the redox properties of a conducting polymer. Fig. 1a and b depict the cyclic voltammograms (CV) of a PANOA film deposited on a glassy carbon (GC) electrode immersed in 0.10 M NaCl solution with pH 5.7 and 9.0, respectively. To test the stability of PANOA, 20 cycles were recorded in Fig. 1a and b, respectively. At both pH values, CVs of the copolymer film reached the stable state after the ninth cycle because the change in the
Conclusions
Results from IR, XPS spectra and determination of perchlorate in the solution demonstrate that PANOA can be used to effectively remove perchlorate ions in the solution. The electrically controlled ion-exchange process on PANOA can be manipulated by modulating the potential applied on this polymer film. At an anodic potential, the polymer film is oxidized causing perchlorate to be selectively incorporated. At a catholic potential, the polymer is reduced, stripping the adsorbed anions. The
Acknowledgment
Funding for this research was provided by the National Science Foundation of China under Grant No. 20377021.
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