| Abstract:Silver ion-containing water is mainly discharged by silver-gilt industry, photographic industry and chemical analysis in laboratory and so on. The discharged wastewater without any treatment will not only pollute the environment but also result in the waste of resources. Adsorption method is an effective and environmental-friendly way to remove and recycle the noble metal ions. The exploitation of new-type adsorbents for noble metal ion has always been the research hotspot. In this paper, poly(m-phenylenediamine) nanorods with high surface area was synthesized by chemical oxidative polymerization when using methanol as solvent, m-phenylenediamine as monomer and sodium persulfate as oxidant. The introduction of organic amines into the reaction system can promote the polymerization process and synchronously control the morphology and molecular structure of the final product. The adsorption ability of the product for silver ions was investigated. The polymerization and adsorption mechanisms were explored with the aid of TEM, SEM, N2adsorption-desorption isotherm measurements, TG, FTIR, XRD and so on. The results are shown as follow:(1) The introduction of methanol is benefinal to the formation of one-dimensional nanostructures of poly(m-phenylenediamine). The resulting product own high surface area and good adsorption ability for silver ion. Besides, the yield is enhaced by nearly10%.(2) The possible formatin mechanism of poly(m-phenylenediamine) is proposed:The polymerization rate is slow in methanol, therefore at the initial stage only protonated poly(m-phenylenediamine) nanoparticles are obtained. The affinity between these protonated nanoparticles and methanol is accordingly impaired due to the high protonation extent of the nanoparticles. This solvophobic effect inevitably makes the nanoparticles show a strong tendency of aggregation to decrease their contact area with the solvent. Then, the nanoparticles connect with each other and finally grow into intact fibre through filling the gap among the connected nanoparticles.(3) In methanol solvent, the addition of organic amines can promotet the polymerization efficiency of poly(m-phenylenediamine). The presence of propylamine, dipropylamine, tripropylamine, ethylenediamine and tetramethylenediamine can effectively enhance the yield and lower the oxidation state of the product. Despite that, these organic amines have little influence on its morphology. However, adopting diethanolamine to assist the polymerization can not only promote the yield and lower the oxidation state of the polymerization but also induce the formation of one-dimensional hollow nanostructures of poly(m-phenylenediamine). The concentration of diethanolamine has a significant influence on the morphology of the product. The optimal concentration is～1.0M in this study.(4) The function of DEA was discussed:as adding DEA into the reaction system, the protonation extent of the monomer and polymer will be restrained because DEA can neutralize the generated hydrogen ions. Meanwhile, the non-protonated amino groups that have lone pairs of electrons on polymer chains is assumed to form hydrogen bonds with the hydroxyl groups on DEA molecules, thereby making DEA clung tightly to PmPD particles. It should be noted that DEA is highly soluble in methanol. As a consequence, the DEA-attached nanoparticles are more inclined to be dispersive in methanol thus their aggregation trend might be lessened, leading to the appearance of hollow morphology.(5) The adsorption behavior for silver ions was investigated systematically. The maximum adsorbance of poly(m-phenylenediamine) is2318.5mg/g. Weak acid condition favors the adsorption process. The adsorption ability of poly(m-phenylenediamine) will improve through increasing its surface area and declining its oxidation state. The adsorption process can be described by Langmuir and pseudo-second-order models, indicating that the adsorption is monolayer chemisorption. In the five times of adorption-desorption experiment, the accumulated adsorbance was4162mg/g. (6) The adsorption mechanism includes physical adsorption, chelation adsorption and redox adsorption, among which the redox adsorption is proved to be the main mechanism. |
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