Preparation Of Polyaniline/Carbon Composites And Their Performances In Heavy Metal Removal From Water

Heavy metal ions, which are toxic and non-biodegradable, exist in various kinds of aquatic systems. They are stable and can be accumulated into the human body through the food chain, causing a serious threat to human health. Thus, how to control and manage heavy metal pollution has become a great issue in the environmental protection field.At present, there are lots of treatment methods for heavy metal removal from water. Among these technologies, adsorption has been widely used because of its high efficiency and simplicity to operate. The excellent adsorbent not only should have fast adsorption rate and high adsorption capacity but also it should be low-cost and easy to regenerate. Polyaniline (PANI) is a new conductive polymer and it contains lots of imine and amine groups, which can react with heavy metal ions to form metal complex. In addition, the imine and amine groups of PANI also have the ability to reduce some heavy metal ions with high oxidation potential. Due to these advantages, PANI has attracted great attention and become a hot topic in the field of waste purification.Aiming at the shortcomings of PANI, such as insoluble in common organic solvents, poor mechanical strength and hard to process, granular activated carbon (GAC) with high specific surface area was used to support PANI in this work, and the performance of PANI/GAC composites in heavy metal ions removal was investigated. In addition, doped PANI modified spectroscopically pure graphite (SPG) electrode was prepared and its electro-catalytic properties for heavy metal ions removal were also examined.The main research works and conclusions are as follows:The PANI/GAC composites were prepared by in situ chemical polymerization. The effect of molar ratio of ammonium persulfate (APS) relative to aniline (AN), sulfuric acid concentration, reaction temperature and reaction time were investigated by orthogonal experiment to optimize the preparation condition. The surface morphology and micro-structure of the PANI/GAC composites were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Results indicated that the optimal preparation condition were1:1of APS/AN molar ratio,1.0M of sulfuric acid concentration,0℃and12h of reaction. Among these conditions, APS/AN molar ratio was the most primary factor. Meanwhile, the yield of PANI/GAC could be enhanced by lowering the reaction temperature and prolonged the reaction time. The cauliflower-like PANI particles were uniformly attached to the surface of GAC, indicative of the successful preparation of the PANI/GAC composites.The adsorption performance of the PANI/GAC composites in Cu(II) ions removal from aqueous solutions was evaluated, and the effects of pH, contact time, dosage of adsorbent, the initial concentration of Cu(II) ions and the reaction temperature on adsorption kinetics and material regeneration performance were investigated. Thermodynamic parameters were also calculated. Various techniques, such as SEM, energy dispersive spectrometer (EDS), FT-IR, XRD and XPS, were used to characterize the PANI/GAC composites before and after adsorption, and the adsorption mechanism was proposed. Experimental results showed that the best pH for Cu(II) ions removal was at around5.5and the dosage of the PANI/GAC composites was2.0g/L corresponding to Cu(II) concentration at400mg/L. The adsorption process followed pseudo second-order kinetics and fitted the Langmuir isotherm excellently. According to the Langmuir equation, the saturated adsorbing capacity of Cu(II) ions on the PANI/GAC composites was calculated as38.97mg/g, which was consistent with experimental data. Thermodynamic results indicated that the adsorption processes was endothermic and could happen spontaneously. Thus, higher temperature favors the adsorption of heavy metal ions. The PANI/GAC composites could be regenerated using0.1M HC1as desorption solution because Cu2+ions were adsorbed on the PANI/GAC composites by chemical reaction. The adsorption capacity of the PANI/GAC composites was with little loss after reused for6times. In summary, the PANI/GAC composites can make full use of the synergistic effect of organic and inorganic materials and have the advantages of high adsorption rate, low cost and good regeneration performance, making it an excellent adsorbent.PANI film was also synthesized on SPG surface by a two-step electrochemical polymerization method from sulfonic acid solution and p-toluenesulfonic acid (pTSA) solution. The SEM characterization showed that the morphology of H2SO4/PANI film had porous morphology and three-dimensional textured structure and was composed of uniform ID nanowires with an average diameter of about100-200nm, while pTSA/PANI film was relatively loose and had short and thick fibers with an average diameter of about300-400nm. Electrochemical experiments indicated that the pTSA/PANI film exhibited a better stability than the H2SO4/PANI film at relatively high electrode potential (0.7-1.0V). After degradation, the FT-IR bands of the pTSA/PANI and H2SO4/PANI film were both blue-shift and with a lower intensity. The results of electrochemical impedance spectroscopy (EIS) illustrated that the ion transfer impedances of two modified electrodes were increased and their conductivities were also significantly reduced. The electro-reduction of Cr(VI) to much less toxic trivalent state (Cr(III)) was subsequently studied on the H2SO4/PANI modified SPG (PANI/SPG) electrode. Compared to the bare SPG, the PANI/SPG electrode exhibited better electro-catalytic performance in Cr(VI) reduction. The partial oxidation state of PANI, EM2+, could act as the catalytic medium for electron transfer between Cr(VI) and the backbone of PANI. It was found that more negative electrode potential, low electrolyte pH, appropriate thickness of PANI film and suitable temperature were beneficial for the removal of Cr(VI). The reduction of Cr(VI) on the PANI/SPG electrode followed pseudo-first-order kinetics (k°=2.57×10-2min-1). The results of cyclic voltammetry and FT-IR illustrated that the H2SO4/PANI film still possessed excellent electrochemical activity and its structure had little changed after electro-catalytic reduction of Cr(VI).