Removal of Mercury From Aqueous Solution Using Polyaniline/ Poly Vinyl Alcohol composite

Mercury is one of the most important heavy metals which, owing to its peculiar physical properties, is used in a wide variety of applications. Due to its toxicity, mercury released to the environment brings detrimental effects; therefore, the removal of which before releasing into the environment has drawn much attention. In the face of mercury’s characteristic, a variety of methods have been used to remove mercury ions from aqueous solutions, including precipitation, ion exchange, adsorption and reverse osmosis. Of these methods, adsorption has been extensively investigated by researchers. In this study, polyaniline/ poly vinyl alcohol composite was used to remove mercury from aqueous solutions. The chemicals used in this study, due to the high purity, were used without further purification, except for aniline monomer which was distilled and stored in the fridge before use. Aniline monomer, sulfuric acid, potassium iodate and poly vinyl alcohol were purchased from Merck. A digital scale model FR200, magnetic mixer model MK20, Arian's Spectra AA series atomic absorption spectrometry and LEO 440i scanning electron microscope were employed. To prepare the polyaniline/ poly vinyl alcohol composite, 1.6 g potassium iodate was initially dissolved in 200 cc of 1 M sulfuric acid using the magnetic mixture, to obtain a uniform homogeneous mixture. Then 0.2 g polyvinyl alcohol was added to the solution under constant stirring for about 15 to 20 minutes, to which 2cc of pure aniline monomer was added and stirred for about 5 h at room temperature. After only a few minutes the solution turned green and darker, which suggested progress in polymerization reaction and the formation of polyaniline/ poly vinyl alcohol composite. After 5 h, the polymer was recovered using centrifugation. To remove oligomers as well as other possible impurities, the resulting polymer was washed several times with distilled water and acetone and then dried in oven at 50°c for 48 hours. The product was finally pulverized in a porcelain mortar prior to use. This experiment was repeated with the addition of 0.4 to 2 g poly vinyl alcohol to the solution, to examine the effects of poly vinyl alcohol dosage present in the composite. Mercury adsorption by the synthesized composites was carried out in batch mode at room temperature. In each experiment, 0.5 g of the synthesized polymer was added to the 50 mg solution containing 35 ppm mercury at a pH of 6. The mixture was stirred using a shaker at 120 rpm for 30 minutes. After reaching equilibrium, polymer was filtered from the solution using filter paper. Then, by determining initial and final mercury concentrations, mercury removal ratio was calculated.
The results of this study indicate that polyaniline and its composites with poly vinyl alcohol have a good performance in the removal of mercury from aqueous solutions and the percentage of mercury removal by polyaniline and its composites is higher than that of activated carbon. The reported ideal chemical structure of polyaniline consists of 1000 or more repetitive units (-ph-n-) containing alternating reduced (benzenoid amine) and oxidized (quinoide imine) states. Mercury adsorption by, Polyaniline/ poly vinyl alcohol occurs due to the presence of nitrogen-containing sites (-NH) in this polymer. Owing to the presence of electron in its s2p3 orbit, this nitrogen could form weak bonds with Hg2 ions, thereby absorbing them. Yet another plausible mechanism concerns the formation of surface complexes between Polyaniline/ poly vinyl alcohol and mercury. Poly vinyl alcohol as an additive surfactant, was able to have a positive effect on the efficiency of synthesized polyaniline and to increase mercury removal capacity of composite. As a surfactant, poly vinyl alcohol is absorbed by the growing polymer particle through hydrogen bonding during the polymerization process, exerting an effect on shape, morphology, uniformity, lateral surface, size and properties of polymer.
Dosage of poly vinyl alcohol used in the synthesis of composite is highly significant, so the dosage needs to be optimized as underuse results in insignificant effect on polyaniline and overuse leads to gelatinized Polyaniline. To estimate the effect of poly vinyl alcohol concentration, varying doses of the compound, as the additive substance, at 1-10 g/l concentrations were investigated in the synthesis of Polyaniline. Optimal mercury removal was observed at 2 g/l concentration of poly vinyl alcohol with 96% removal. Several studies suggest that the single most important parameter in removal of heavy metals by adsorbents is the initial pH value of the solution. This parameter has an impact on metal ion species in the solution as well as on adsorption sites. To evaluate the impact of solution pH on mercury removal, the experiments were conducted in a range of pH from 2 to 6. As the results indicate, the pH approaches more to the neutral point, the higher mercury removal efficiency is achieved. Due to mercury sedimentation, the use of alkaline solutions was avoided. Optimal removal was observed around pH 6 at which polyaniline/poly vinyl alcohol demonstrates the greatest potential for removal. At the neutral pH value, polyaniline/poly vinyl alcohol turns into an oxidizing agent (taking electrons onto itself), then free –N groups in the polymer will be available to form a ligand (binding of large molecules to a central metal atom), thereby increasing the sorption of Hg2 ion. At acidic pH values, –N groups absorb proton, turning into –NH whose positive charge repel Hg2 ions across polyaniline/poly vinyl alcohol. This results in an inhibitory effect on metal uptake. To evaluate the impact of exposure time, adsorption experiments were carried out at pH 6 and initial concentration of 35 mg/l mercury for different periods of time. The results showed that mercury uptake ratio increased as exposure time increased. Optimal exposure time is 30 minutes, after which no significant change in mercury removal is observed.