Application of polyaniline for the reduction of toxic Cr(VI) in water
Introduction
Conducting polymers have become one of the most attractive subjects of investigation in last decades. Their unique properties such as electrical conductivity and electrochemical properties (like metals), mechanical strength and ease of processing (like polymers) and possibility of both chemical and electrochemical synthesis, make them useful in wide area of applications. These materials have been used in many applications, including rechargeable batteries [1], [2], chemical and electrochemical sensors [3], [4], electro-chromic devices [5], corrosion protection [6], [7] and switchable membranes [8].
The electrochemical properties and reactions of conducting polymers have been studied extensively [9], [10], [11]. In this regard some studies have focused on electrochemical interaction of conducting polymers with toxic metals [12], [13], [14]. Results of these studies have lead to the application of conducting polymers in analysis [4], [14], [15], [16], [17] and abatement [18], [19], [20] of some toxic metals from the environment.
One of the most important and very toxic metals studied in this regard is chromium hexavalent [Cr(VI)]. The two more significant oxidation states of chromium are Cr(VI) and Cr(III) [21]. Chromium as Cr(VI) and Cr(III) enters to air, water and soil as a result of natural processes and anthropogenic sources such as chromium plating, leather tanning, wood preserving and making of steel and other alloys, bricks, dyes and pigments [22]. Cr(VI) is extremely mobile in the environment and very toxic, carcinogenic and mutagenic to living organisms, whereas Cr(III) is less toxic and can be easily isolated and precipitated using organic and inorganic reagents [21], [23]. Thus, most of the methods for removal or abatement of Cr(VI) from environmental samples are based on the reduction of Cr(VI) to Cr(III) followed by precipitation and removal of Cr(III). Conventional reduction methods are based on the treatment of Cr(VI) with reducing agents such as sulfur dioxide or sodium metabisulfite which aren’t entirely satisfactory and are accompanied with secondary wastes and high volume of sludge products [13].
The use of conducting polymers for the reduction of very toxic Cr(VI) to less toxic Cr(III) has been reported by Rajeshwar and co-workers first time in 1993 [24]. They have reported the complete reduction (100% efficiency) of Cr(VI) by electrosynthesized polypyrrole films. The advantages of conducting polymers for Cr(VI) reduction with respect to usual reducing agents, is the reversibility of reduction process and possibility of recycling of conducting polymers for further treatments. Rajeshwar and co-workers have also reported the catalytic reduction of Cr(VI) by polypyrrole and its composites with carbon black [25], [26], [27]. They showed that the content of carbon black in polypyrrole-carbon black composite affects on the reduction process so that the increasing of carbon black content of the composite, caused to an increase in the reduction process performance.
More recently the use of polyaniline films for the reduction of toxic Cr(VI) species in solution has been reported [28], [29]. Breslin and co-worker [28] have studied the mechanism and kinetics of Cr(VI) reduction by various oxidation states of electrochemically synthesized polyaniline films. They have also reported the effect of polyaniline film thickness on the efficiency of reduction process.
In all previous works, conducting polymer films have been used for the reduction of Cr(VI) or other toxic metals. However polyaniline can be prepared chemically or electrochemically in various forms such as powder, free standing film or film on electrodes. Also polyaniline, as a reducing agent, have different oxidation states of leucoemeraldine and emeraldine. The aim of this work is to study the efficiency and kinetics of Cr(VI) reduction by various forms of polyaniline. The effect of synthesis method, film or powder form, oxidation state and some other parameters such as polymer film thickness, initial Cr(VI) concentration and solution pH, on the kinetics and performance of Cr(VI) reduction was also studied.
Section snippets
Reagents and materials
Hydrochloric acid, sulfuric acid, potassium dichromate, ammonium persulfate, ammonia, acetone and aniline were all purchased from Merck chemicals (Germany) and were used as received without any further purification, except aniline which was distilled under vacuum prior to use.
Instrumentation
A galvanostat/potentiostat WENKING TG 97 from Bank (Germany) and a pH meter 654 from Metrohm Co. (CH 9101-Herisau, Switzerland) were used for the electrosynthesis of polyaniline and its reduction under potentiostatic
Reduction of Cr(VI) by chemically synthesized polyaniline
Polyaniline powder as emeraldine salt (ES) form was obtained via chemical synthesis route. The ES was treated with ammonia solution (1 M) for 6 h to obtain the reductant emeraldine base (EB) form of polyaniline. Different amounts of the chemically synthesized polyaniline powder, in the emeraldine base form, were added to 10 ml of Cr(VI) solutions with different concentrations. The exposure was performed in different times followed by filtration and separation of polyaniline particles.
The
Conclusion
Chemically and electrochemically synthesized polyaniline as powder and film forms can be used to reduce very toxic Cr(VI) to less toxic Cr(III) in aqueous solutions. With increasing the thickness of the polyaniline films prepared electrochemically in contrast to polymer films synthesized chemically, the reduction performance of the polymer for Cr(VI) is increased. High concentrations of Cr(VI) causes to the overoxidation and degradation of all forms of polyaniline. Increasing the acidity of
Acknowledgments
This work has been supported by the Research Institute for Fundamental Sciences, Tabriz, Iran. The authors would like to thank this support.
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