Electrochemical Properties Of Polypyrrole And Its Characterization On Heavy Metals Reduction

Heavy metal pollution is becoming an increasing importance because of its serious threatto environment and human health. Most of the conventional methods are inefficient in heavymetal recovery, only transferring the contaminants from one phase to another one. As cleanand safe technology, electrochemical treatment has received considerable attention during thepast two decades. Polypyrrole (Ppy), one kind of many conductive polymers, becomesworldwide research focus because of its good stability and high conductivity. A novelapplication of ppy as electrode material for heavy metals recovery has been proposed in a fewreports. In this paper, adherent polypyrrole films were synthesized by potentiolstatic methodfrom a para-toluenesulfonic sodium solution on stainless steel (SS). On the basis of itsproperties study, Electro-reduction of Cr(â…¥) and Cu(â…¡) in aqueous solutions by ppy-modifiedelectrode was investigated, which is a beneficial trial in terms of ppy application inwastewater treatment.The unique electrochemical properties of ppy distinguish it from the conventionalelectrode materials. Therefore, ppy shows different characterization with conventionalelectrode materials in respect of heavy metals reduction process. Electrochemical propertiesof ppy in different electrolyte solutions have been systematically studied in this thesis withredox characterization, redox stability and cathodic polarization behavior as our main focus.The application of ppy for heavy metals recovery has been conducted on the basis of theelectrochemical properties mentioned above. The influencing parameters and mechanismshave also been discussed.The active redox behavior is one the most remarkable electrochemical properties of ppyfilms. Cyclic voltammetry of ppy-modified electrodes in the wide potential range from -1.6 Vto 0.8 V was conducted in H₂SO₄, Na₂SO₄ and NaOH solutions respectively. The resultsindicate that redox properties depend on electrolytes type and concentration. The cyclicvoltammogram in H₂SO₄ solution is featured by H⁺ insertion during reduction process and H⁺expulsion during oxidation process. A unique peak associated to H⁺ reduction was firstdiscovered. In Na₂SO₄ solution, cyclic voltammogram is featured by Na⁺ insertion duringreduction process and Na⁺ expulsion during oxidation process. In NaOH solution, except thesame pair of oxidation/reduction peaks with that of Na₂SO₄ solution, the cathodic peakassigned to expulsion of para-toluenesulfonate (pTS^-) in the reduction of ppy film was alsoobserved. The results also demonstrate that the oxidation/reduction processes of ppy filmsin all electrolytes are ions diffusion controlled.Redox stability is the redox reversibility of ppy under the applied potential scope inelectrolyte solutions. In this paper, redox stability of ppy-modified electrodes in H₂SO₄, Na₂SO₄ and NaOH solutions is our research emphasis and was studied by means of long-timerecurrent potential step experiments. It was found that when negative potential is lower thanhydrogen evolution potential, hydrogen evolution can make ppy film swelled and lose itsreversibility during oxidation/reduction processes. When more positive potential is exerted,overoxidation will occur, which can destruct conjugated double bonds of ppy chains and alsolose its reversible redox property. The experimental data shows that overoxidation potential isstrongly dependent on pH of solutions. Overoxidation occurs in the lower potential when pHis higher. In the solution of H₂SO₄, the initiatory overoxidation potential is 0.8 V, while inNa₂SO₄, it is 0.5V. In NaOH, irreversible overoxidation arises at whatever potential,indicating that the existence of OH^- in solution is the direct reason of overoxidation.Cathodic polarization behavior is one of the most import electrochemical properties ofppy, especially in the respect of application of ppy as cathodic electrode. Cathodicpolarization in H₂SO₄, Na₂SO₄ and NaOH was also investigated in this paper. The resultsshow that compared with SS electrode, ppy-modified film can restrain hydrogen evolutiondrastically in all the electrolytes mentioned above. And it is more prominent in acidifiedsolutions.Treatments with different electrolyte solutions have important effect on the morphologyand structure of ppy films. SEM micrograph reveals that the as-grown ppy is cauliflower-like.Strong sulfuric acid was proved to erode the film and cause conductivity decay, 18 M sulfuricacid can even solve part of ppy, leading to the appearance of openings and cracks on the filmsurface, but diluted sulfuric acid had little effect on ppy. After Na₂SO₄ treatment, ppy displaysglobe-like or pole-like morphology and its conductivity decreased. By NaOH treatment, ppylose its conductivity and exhibits micro-globular structure. The data of FTIR absorptionspectra demonstrates that NaOH treatment has made extensive structure degradation, butNa₂SO₄ and H₂SO₄ treatments, even high concentration of H₂SO₄ treatment have not madestructure changes on ppy films.On the basis of the study on electrochemical properties mentioned above, reductioncharacterization of ppy-modified films on stainless steel mesh (SSM) through threeexperimental modes of open-circuit reduction, potentiolstatic reduction and potential cyclingreduction was investigated. The results demonstrate that all these three modes are effectiveand the reduction procedure is mass transport limited. But the latter two have higher removalefficiencies than the former. Current efficiency of ppy film is much higher than that of SSMbecause of ppy's inhibition effect on hydrogen evolution, which is the best advantage forheavy metals removal with ppy. The kinetic parameters have been obtained and themechanisms for the reduction processes of Cr (â…¥) and Cu(â…¡) have been proposed. As Cr(â…¥) exists as anion in solution, while Cu(â…¡) exists as cation in solution, the processes of Cr(â…¥)reduction and Cu(â…¡) reduction exhibit different mechanisms.