[Preparation of sulfonic acid-functionalized polymeric ionic liquid-based magnetic adsorbent and its applications to...

In terms of the structure of polar pollutants, the design and development of new materials that can interact with target analytes is vital for effective extraction. Diquat is a broad-spectrum herbicide with strong toxicity. It is sprayed during the growing process of vegetables because of its high efficiency and low cost. Furthermore, diquat is a polar and basic cationic organic compound. Therefore, it is necessary to develop adsorbents that can form strong interaction with diquat to extract it from complex vegetable samples. In this study, sulfonic acid-functionalized ionic liquid, 1-vinyl-3-propyl(3'-sulfonate) imidazolium chloride ([VPImi-SO₃H][Cl]), was synthesized through the reaction of 1-vinylimidazole and 1,3-propanesultone and characterized using nuclear magnetic-resonance spectroscopy. The resulting [VPImi-SO₃H][Cl] was then immobilized on the surface of vinyl-modified magnetic silica to obtain poly-(1-vinyl-3-propyl(3'-sulfonate) imidazolium chloride)-modified magnetic nanoparticles (Poly([VPImi-SO₃H][Cl])-MP) via free radical polymerization between vinyl groups. The structure, morphology and magnetism of the obtained Poly([VPImi-SO₃H][Cl])-MP were characterized using Fourier transform infrared spectroscopy, scanning electron microscope, vibrating sample magnetometer, and thermogravimetric analysis. The characterization results indicated that the Poly([VPImi-SO₃H][Cl])-MP was synthesized with strong magnetic properties. Poly([VPImi-SO₃H][Cl])-MP as magnetic adsorbent exhibited a different pH response to anion and cation. With the increase in solution pH, the adsorption efficiency of Poly([VPImi-SO₃H][Cl])-MP for anionic dye amaranth decreased; whereas, the adsorption efficiency for cationic dye, methylene blue, increased. Under a high pH condition, sulfonic acid groups on the surface of Poly([VPImi-SO₃H][Cl])-MP existed in the form of -SO3-. Thus, the adsorption efficiency of Poly([VPImi-SO₃H][Cl])-MP for methylene blue could reach 95.2% due to the strong electrostatic attraction between adsorbent and positively charged methylene blue; whereas, negatively charged amaranth could hardly be adsorbed due to the electrostatic repulsion between adsorbent and amaranth. The above results showed sulfonic acid modification endowed Poly([VPImi-SO₃H][Cl])-MP with abundant negative charges at certain pH conditions. Thus, the extraction can be achieved by the strong electrostatic attraction between negatively charged Poly([VPImi-SO₃H][Cl])-MP and positively charged diquat. Using green vegetables as the sample matrix and magnetic solid-phase extraction as extraction model, the extraction performance of Poly([VPImi-SO₃H][Cl])-MP for diquat was studied. Some extraction parameters affecting the extraction efficiency, such as solution pH, adsorbent amount, adsorption time, desorption solvent and its volume, and desorption time were optimized using the single factor experiment method. Under the optimized extraction conditions (30 mg of adsorbent, 15 min of adsorption time, 40 μL of ammonia, 600 μL of acetonitrile-formic acid (9∶1, v/v) as desorption solvent, 1 min of desorption time), the performance, and applicability of the proposed method were investigated by combining magnetic solid-phase extraction, high performance liquid chromatography, and UV-visible detector. Good linearity was observed at contents ranging from 0.2 to 20 μg/g with a correlation coefficient (r) of 0.9981. The limit of detection and limit of quantification based on the signal-to-noise ratio of 3∶1 and 10∶1 were 0.09 and 0.2 μg/g, respectively. The spiked recoveries at three levels of 0.5, 1.0, and 2.5 μg/g were obtained in the range of 82.7%-97.5% with the relative standard deviations of 2.8%-5.0% (n=3). The results showed that the sulfonic acid-functionalized Poly([VPImi-SO₃H][Cl])-MP could be employed as a magnetic adsorbent for the quick and effective extraction of diquat, and the proposed method could be used for the determination of diquat from green vegetable samples. It could be anticipated that Poly([VPImi-SO₃H][Cl])-MP could be used as an adsorbent for extracting other cationic polar pollutants by adjusting the solution pH. This work provides a new way to construct adsorbents for extracting polar pollutants.