(165f) Characterization of Silica-Coated Fe3O4 Prepared Under Various Conditions Using Electrooxidation Method and Immobilization of Thermoresponsive Polymer

In our laboratory, we have been developing recyclable thermoresponsive heavy metal adsorbent by immobilizing poly(NIPAM-co-AA), which was prepared by copolymerizing N-isopropylacrylamide polymer (poly NIPAM) and acrylic acid (AA), on Fe₃O₄ nanoparticle (magnetite nanoparticle; MNP). Here, poly NIPAM is one of the representative thermoresponsive polymer and acrylic acid (AA) is selected as heavy metal adsorption site since it can form a complex with divalent heavy metal ions.

One disadvantage of the adsorbent in our past study is that heavy metal ion recovery amount is relatively small because the immobilized poly(NIPAM-co-AA) amount on MNP is small. Since the main reason for the small polymer amount is due to the aggregation of MNP prior to the immobilization, it is important to suppress the aggregation. One method for suppressing the MNP aggregation is to coat the surface with silica. Therefore, in our previous study, we synthesized silica coated MNP using the electrooxidation method, and investigated the effect of DC current value during synthesis on the resulting particle properties.

In this study, the purposes of our research are the following 2 points, 1) to investigate the effect of silica source concentration on MNP properties in the synthesis of silica coated MNP using the electrooxidation method, 2) to investigate the effect of the resulting MNP properties on the immobilized amount of the poly(NIPAM-co-AA).

In the experiment, the silica coated MNP was prepared using the electrooxidation method with the constant DC current at 0.60 A and changing the concentration of sodium silicate as a silica source from 0 to 500 ppm. Next, the surface of the resulting samples was modified using a silane coupling agent. Subsequently, poly(NIPAM-co-AA), which was prepared by free radical copolymerization, was immobilized through chemical bonding.

The results of X-ray diffractometer (XRD) analysis and Fourier transform infrared spectroscopy (FT-IR) measurement confirmed that the silica coated MNP was successfully prepared using this method. In addition, the aggregated particle size of the silica coated MNP was examined by a laser diffraction scattering method. The result shows that the aggregated particle size distributions of all the silica coated MNP were shifted to smaller particle size range compared to the bare MNP. This indicates that the aggregation of MNP was suppressed by silica coating.

The results of thermogravimetric analysis (TGA) shows that the maximum immobilized amount of the polymer of 0.314 (g-polymer)/(g-MNP) was obtained when the sample was prepared with the silica source concentration of 450 ppm. The immobilized amount was approximately 4.9 times larger than that of the sample prepared using the MNP without silica coating. To examine the reason for that, the relationship between the polymer immobilized amount and the specific surface area based on the primary particle size of the MNP or that based on the aggregate particle size of the MNP were plotted. The results show that the immobilized polymer amount is essentially proportional to the specific surface area based on the aggregate particle size. This indicates that the successful suppression of MNP aggregation through silica coating using the electrooxidation method contributed to the increase of the available surface area for the polymer immobilization.