(197f) A Simple Synthesis Method of Thermoresponsive Polymer Immobilized Magnetite Nanoparticles for of Heavy Metal Ions Recovery

Currently, various adsorbents incorporating ion exchange groups or chelating groups have been utilized to recycle heavy metal ions from wastewater. However, main disadvantage of the method is generation of secondary wastes such as acid solution during recycling process. Therefore, development of new heavy metal ion recycling method in which secondary waste is not generated during the process has been drawing increasing attention. To develop such kind of process, application of thermoresponsive polymer have been tried by various researchers. N-isopropylacrylamide polymer (poly NIPAM) is one of the representative thermoresponsive polymers and is known to have a lower critical solution temperature (LCST) at about 32 °C in an aqueous solution. It exhibits hydrophobicity and hydrophilicity above and below LCST, and the phase transition is reversible. Normally, thermoresponsive “hydrogel”, in which poly NIPAM and chelating ligand were incorporated, were used for heavy metal ion recovery. However, in these cases, slow adsorption rate and very limited ligand group introduction in the gel are the remaining challenges.

On the other hand, in the case of straight chained thermoresponsive polymer, achievement of both high introduction amount of chelating ligand and fast adsorption/desorption can be expected. Therefore, we have been developed straight chained thermoresponsive heavy metal adsorbent incorporating poly NIPAM.

In this study, to enhance the reusability of the adsorbent, we tried to immobilize it onto magnetite nanoparticle (MNP). The conventional copolymer immobilization methods onto nanoparticle such as “grafting to” method, “grafting from” method, and “in situ” method have a disadvantage that the processes are complicated, take long time and in most cases require the usage of organic solvents. On the other hand, Tural et al. reported that when MNP is synthesized by co-precipitation method, if polymer, which have carboxy group, is added in the co-precipitation solution, it is possible to immobilize the polymer on the surface of the synthesized MNP.

The purpose of this study is 1) to simplify the immobilization process by applying Tural’s method to the synthesis of our adsorbent and 2) to evaluate the heavy metal ions recovery ability of the adsorbent. In the experiment, first, straight-chained thermoresponsive copolymers were prepared by free radical polymerization of NIPAM and acrylic acid (AA). Then, immobilization of poly (NIPAM-co-AA) onto MNP was carried out by applying coprecipitation method. Poly (NIPAM-co-AA), iron (II) sulfate heptahydrate, iron chloride (â…¢) hexahydrate were dissolved in deionized water. Then, sodium hydroxide was dropped into the solution to form MNP. The results of X­ray diffractometer (XRD) analysis, Fourier transform infrared spectroscopy (FT­-IR) measurement and thermogravimetric analysis (TGA) indicated that poly(NIPAM­-co­AA) immobilized Fe₃O₄ nanoparticle was successfully prepared and the maximum immobilized amount of the copolymer was 0.760 g/(g­-Fe₃O₄). The results of the Cu (II) adsorption experiment showed that the saturated adsorption amount increased with the increase of immobilized copolymer amount, and the adsorption amount at 60°C was higher than that at 20°C. The recyclable amount of copper (â…¡) ions was 0.13 mmol/(g-adsorbent) at the maximum when temperature swing was carried out between 60°C and 20°C. Despite the simple method, the adsorbent prepared using our method showed the almost the same value as other thermoresponsive adsorbents which were prepared by the complicated and time-consuming conventional methods. In addition, repeated cycle of adsorption and desorption of Cu (II) ions by temperature swing was also tested and no significant performance deterioration was not observed. These results indicated that the reusable thermosensitive adsorbent with high heavy metal recycling ability can be synthesized by our comparatively simple method. Since the adsorbent does not require organic solvent in the preparation step, and also does not require acid solution to recovery heavy metal ions, waste generation throughout the process is very limited. The fact demonstrates the effectiveness of our adsorbent and its preparation method.