Development and characterisation of electropolymerised polypyrrole and polypyrrole-reduced graphene oxide composite films as a potential treatment method for azo dyes

In this thesis, we report the development and application of conducting polypyrrole films as a potential green technology for electrochemical treatment of the model azo dye, Acid Red 1. Synthetic dyes, for example azo dyes, are extensively used as colouring agents in various industries including textile, paper, printing, and pharmaceutical industries. As a result, discharge of dye-containing industrial effluents into the aquatic ecosystems will generate undesirable colours in the water, reducing sunlight and oxygen penetration, and resisting photochemical and biological attack. In some cases, their degradation products can also be toxic, carcinogenic or even mutagenic. In addition, the majority of these dyes are chemically stable and resistant to microbiological attack that often exhibits a low degradation efficiency due to their complex structure. Therefore, discharge of dye-containing effluent in the hydrosphere without proper treatment is a major environmental concern. Several treatment methods for azo dyes have hitherto been reported. However, there are severe limitations associated with these methods. Notably, many treatment methods often produce toxic by-products and hazardous residues during operation. Therefore, there is a need for developing alternative treatments that are effective and environmentally friendly in removing dyes from textile effluents. Electropolymerised conducting polypyrrole films have been chosen to avoid such a problem in this study. The synthesis of polypyrrole usually involves electrochemical oxidation of its monomer, pyrrole, to yield a polymeric chain with a positive backbone. In order to neutralise this charge, a counter anion, for example that of Acid Red 1, is entrapped in the backbone structure. Many studies have shown that this entrapment process is electrochemically reversible, enabling polypyrrole to act as an anion exchanger, depending upon the mobility of the entrapped counter anion. This has then been exploited as the basis for developing an eco-friendly treatment for dye in textile effluents. The azo dye, Acid Red 1, was used as a model dye in this study. This thesis will begin with Chapter 1 presenting a brief introduction of different dyes, risks associated with dye effluents and their degradation products in the environment, conventional dye effluent treatment techniques and limitations. This section will also cover the fundamental chemistry of such conducting polymers as polyaniline and polypyrrole and justification for their applications to dye removal from dye containing effluents. The scope of this study will also be outlined in Chapter 1. Chapter 2 will provide details of experimental techniques used in this research. Here, we will describe the synthesis and characterisation techniques of polypyrrole films, and method for the evaluation of Acid Red 1 entrapment-liberation in polypyrrole films. This chapter will also describe the methods used for experimental data analysis. In Chapter 3, we report the optimised synthesis parameters of polypyrrole-Acid Red 1 films and characterisation of polypyrrole films. Polypyrrole films were synthesised by anodically polymerising pyrrole in the presence of Acid Red 1 as a supporting electrolyte. These Acid Red 1-entrapped polypyrrole films were characterised by electrochemistry, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction analysis. Based on a two-level factorial design, we have identified the solution pH, the Acid Red 1 concentration and the polymerisation duration as the significant experimental parameters affecting the entrapment efficiency. The entrapment process will potentially aid in decolourising an Acid Red 1-containing solution. Similarly, in a cathodic process, electrons are supplied to neutralise the polypyrrole backbone, liberating Acid Red 1 into the solution. This allows the recovery of Acid Red 1 for recycling purposes. In Chapter 4, we focus on the kinetic models, isotherms and thermodynamics of the electrochemical entrapment of Acid Red 1, at conducting polypyrrole films. The Acid Red 1 entrapment kinetic data were found to follow a pseudosecond order model involving an intraparticle diffusion. However, the equilibrium data obtained for Acid Red 1 entrapment in polypyrrole film failed to obey any common adsorption models such as the Langmuir and Freundlich isotherms. Therefore, enhanced quantity of dye may then be achievable by entrapment, making it a more effective and efficient technique than those involving only adsorption. Similarly, dye leakage from polypyrrole film surface to a sample matrix will be easily prevented. Thermodynamically, a negative standard Gibbs free energy of entrapment range between -1.46±0.78 and -2.94±0.24 kJ mol⁻¹ at the corresponding temperature range of 298 K – 318 K, and a standard enthalpy change of 20.5±2.5 kJ mol⁻¹ indicate a spontaneous and endothermic entrapment process. Also, a positive entropy change (73.6±8.2 J mol⁻¹ K⁻¹) reveals increased randomness of the interface and an affinity of Acid Red 1 towards polypyrrole films. A low activation energy (7.67±0.80 kJ mol⁻¹) confirms a physical process for Acid Red 1 entrapment in polypyrrole films. Unfortunately, problems were uncounted when the same polypyrrole films were used in repeated entrapment-liberation process of Acid Red 1 due to their poor stability during cycling. This is because of swelling and shrinkage of polypyrrole films that take place during the entrapment-liberation process of Acid Red 1. This leads to mechanical degradation of the polypyrrole films and weakening of their electrochemical performance. To minimise this effect, polypyrrole-reduced graphene oxide composite films were considered. Accordingly, Chapter 5 is devoted to synthesis, characterisation and evaluation of Acid Red 1 entrapment and liberation at mechanically stable polypyrrole-reduced graphene oxide composite films. Initially, we anodically synthesised polypyrrole-graphene oxide films by in situ electropolymerisation of pyrrole and graphene oxide. Next, a reduction potential was applied to obtain a polypyrrole-reduced graphene oxide film from a polypyrrole-graphene oxide composite film. The synthesised composite films were then characterised by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, surface analysis, thermogravimetric analysis and scanning electron microscopy. Brunauer, Emmett and Teller surface area analysisshowed a 7.4-fold increase in surface area of a polypyrrole-reduced-graphene oxide film compared to that of a polypyrrole film. Also, mechanical testing results revealed that the tensile strength of polypyrrole-reduced graphene oxide films was enhanced by 12.7 folds compared to that of polypyrrole films. We evaluated the entrapment-liberation efficiency of Acid Red 1 entrapped polypyrrole composite films and estimated a 95 % entrapment of Acid Red 1 in polypyrrole-reduced graphene oxide films, which is significantly higher than 58% in polypyrrole films. Similarly, a 73% liberation efficiency at polypyrrole-reduced graphene oxide composite films was found to be higher than 36% at polypyrrole films. Finally, a preliminary study of Acid Red 1 entrapment in polypyrrole-reduced graphene oxide films in the presence of Indigo Carmine was also investigated to evaluate the selectivity towards Acid Red 1 of polypyrrole-reduced graphene oxide film. We observed that electropolymerised polypyrrole-reduced graphene oxide film showed excellent memory effect for selective entrapment of Acid Red 1. Finally, in Chapter 6, some concluding remarks on the development and applicability of entrapment and liberation of Acid Red 1 will be presented. To this end, limitations of this treatment will also discussed. Further, several suggestions will be proposed as feasible future polypyrrole and polypyrrole-reduced graphene oxide composite films to electrochemical work for this project.