Structural analysis of polymer flocculants

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Structural Analysis of Polymer Flocculants
Abstract: The initiation systems and relative molecular masses of two polymer flocculants produced by a foreign company were analyzed using techniques such as infrared spectroscopy, nuclear magnetic resonance spectroscopy, and X-ray energy spectroscopy. The results showed that one of them was polyacrylamide with a relative molecular mass of approximately 3.5 × 10^6; its initiation system might be Na2S2O8 or Na2S2O8-Na2S2O4. The other compound was an acrylamide-acrylic acid copolymer, with a molar ratio of the copolymerized units of about 1:1. The initiator for this compound was the same as that used in the first case.
Keywords: Polymer flocculants; analysis; infrared spectroscopy; nuclear magnetic resonance spectroscopy; X-ray energy spectroscopy
With the development of modern industry, water consumption has increased sharply, leading to more severe water pollution and a shortage of water resources. Wastewater treatment has thus become one of the important ways to address this problem. Flocculants play a crucial role in water and wastewater treatment. Currently, flocculants are mainly divided into two categories: inorganic and organic. Compared to inorganic flocculants, organic polymer flocculants offer advantages such as lower usage amounts, wider application ranges, less susceptibility to the effects of salts and environmental conditions, reduced sludge production, and better treatment effects. This has attracted increasing attention. Synthetic organic polymer flocculants hold a dominant position in the market due to their large relative molecular masses and numerous functional groups on their molecular chains. Among these, polyacrylamide-based products are the most widely used. The flocculation mechanism of flocculants is quite complex, and the relevant theories are still not fully developed. It is generally believed that the flocculation process involves two mechanisms: flocculation and coagulation. Coagulation refers to the process by which colloidal particles lose stability and form small aggregates, while flocculation is the process by which these small aggregates combine under the action of the flocculant to form larger flocculates. A representative definition of flocculation was given by Gruchy: it is the process by which colloidal and suspended particles are linked together by polymer flocculants to form larger flocculates. During the flocculation process, charge neutralization also occurs alongside the formation of larger flocculates. For example, some organic polymer flocculants possess both charge-neutralizing capabilities and the ability to link particles together. Two samples of polymer flocculants from a foreign company were analyzed using available analytical methods to determine their initiation systems and relative molecular masses. Sample 1 was a white solid particle, while Sample 2 was a beige-colored gel-like liquid. 1 Experimental Section 1.1 Instruments and Reagents BRUKER V-70 Fourier transform infrared spectrometer, manufactured by BRUKER GmbH in Germany; BRUKER AV500 nuclear magnetic resonance spectrometer, also from BRUKER GmbH in Germany; KQ3200B ultrasonic oscillator, produced by Jiangsu Kunshan Ultrasonic Instrument Co., Ltd.; JSM-5610LV scanning electron microscope; EX-FALCON X-ray energy dispersive spectrometer, made by JEOL Corporation in Japan; Ubbelohde viscometer; constant temperature water bath. Deuterated chloroform (CDCl3), Beijing Chemical Factory; Heavy water (D2O), Beijing Chemical Factory; Pure water (H2O), Liming Chemical Research Institute. NaCl, analytically pure, Tianjin Chemical Reagents Factory. 1.2 Experimental conditions and methods 1.2.1 Dissolution experiment: A small amount of the sample was taken for this experiment. Approximately 0.03 g of the sample was weighed and placed in a beaker, to which 50 mL of pure water was added. The mixture was stirred thoroughly, then placed in an ultrasonic oscillator for 30 minutes. Finally, it was left to stand for 24 hours to determine whether the sample dissolved. 1.2.2 Infrared spectroscopy (FT-IR): Testing was carried out using a BRUKER V-70 Fourier-transform infrared spectrometer, with a scanning range of 400–4,000 cm-1. Sample 1 was prepared by coating it on a substrate using a solution method, while Sample 2 was coated directly. 1.2.3 Nuclear magnetic resonance spectroscopy (1H-NMR): Measurements were performed using a BRUKER AV500 nuclear magnetic resonance spectrometer. Sample 1 was placed in a 5 mm quartz sample tube, and D2O was added to dissolve it. Once completely dissolved, the sample tube was inserted into the NMR instrument for 1H-NMR analysis. The operating frequency was 50.0 MHz, with 65,536 data points per acquisition, 16 acquisitions in total, 2 dummy acquisitions, and a spectral width of 7,002.801 Hz. Sample 2 was similarly processed: it was placed in a 5 mm quartz sample tube, CDCl3 was added to dissolve it, and after complete dissolution, the sample tube was used for 1H-NMR analysis. The parameters here were: operating frequency of 50.0 MHz, 65,536 data points per acquisition, 32 acquisitions in total, 2 dummy acquisitions, and a spectral width of 7,002.801 Hz. 1.2.4 Electron Spectroscopy (XPS): The polymer was prepared as a sample on a polyester surface, and its elemental composition was analyzed using an X-ray spectrometer after vacuum gold spraying. The acceleration voltage used was 20.0 V, with a magnification of 200; the sample was placed at an inclination of 0 degrees. 1.2.5 Viscosity Tests: The measurement was carried out at a temperature of (30 ± 0.1) °C. The samples were dissolved in a 1 mol/L NaCl aqueous solution, and their relative viscosity was determined using an Ubbelohde viscometer. The intrinsic viscosity was calculated by extrapolation.

2 Results and Discussion 2.1 Solubility Test: Sample 1 could be dissolved in water after ultrasonic treatment followed by 2–4 hours of standing time; however, Sample 2 did not dissolve, with an off-white gel-like substance remaining suspended in the water. Therefore, for the 1H-NMR analysis, Sample 1 was dissolved in D2O, while Sample 2 was dissolved in CDCl3. 2.2 Sample Analysis 2.2.1 Sample 1: FT-IR analysis showed that the characteristic absorption peaks of Sample 1 matched those of polyacrylamide (Figure 1), and they were almost identical to the infrared spectrum of polyacrylamide in BRUKER V-70’s standard infrared spectrum database. The structure was confirmed using 1H-NMR spectra (Figure 2): the protons at chemical shifts of 1.55 and 1.66 corresponded to the two protons of methylene groups, while those at 2.11 and 2.23 corresponded to the proton of an imine group. Due to spin coupling between adjacent hydrogen nuclei in the acrylamide molecule, both sets of absorption peaks showed varying degrees of splitting, forming double peaks. The peak at 4.67 corresponds to the characteristic peak of D2O. XPS analysis (Figure 3) revealed that the sample contained 1.45% Na and 0.28% S by mass, values that are higher than those found in polyacrylamide. Based on existing studies on polyacrylamide, it is inferred that the initiator used was either Na2S2O8 or a Na2S2O8-Na2S2O4 system. The results of elemental analysis are shown in Table 1. The intrinsic viscosity, determined from the limiting differential viscosity test, was 781 mL/g. Using the relationship between the intrinsic viscosity of polyacrylamide and its molecular weight (i.e., 3.73 × 10^-4 to 0.66, where Mw is the mean molecular weight), the molecular weight of Sample 1 was estimated to be approximately 3.50 × 10^6. 2.2.2 Sample 2: Preliminary FT-IR analysis showed that the characteristic absorption peaks of Sample 2 were similar to those of acrylamide-acrylic acid copolymers; however, there was a hydroxyl group absorption peak at 2,925 cm^-1, which did not match the standard spectrum (Figure 4). This peak corresponds to the -CH3 group, and it may be related to the organic solvent present. Therefore, the sample was re-analyzed after being treated with infrared light, and the results were almost identical to those of the acrylamide-acrylic acid copolymer in BRUKER V-70’s standard infrared spectrum database (Figure 4). The structure was further confirmed using 1H-NMR spectra. Based on these spectra (Figure 5), it can be determined that the organic solvent involved was a C8 long-chain aliphatic hydrocarbon. The double peaks at shifts of 0.88 and 1.26 corresponded to the solvent protons; there were 16 protons in total. The peak at 1.67 corresponded to the methylene group, while the peak at 2.22 corresponded to the imine group attached to the amide group. The peak at 3.43 corresponded to the imine group attached to the carboxylic acid group. The integral areas of these two peaks were approximately equal, indicating that the molar ratio of acrylamide to acrylic acid units was about 1:1. From the ratio of the integral areas of the imine group peaks, it can be inferred that there were approximately 4 protons at shift 1.67, meaning 2 methylene groups. Additionally, the peak at 4.75 corresponded to the protons in water, indicating that the sample also contained some water. The peak at 7.61 corresponded to CDCl3. XPS analysis (Figure 6) revealed that the sample contained 7.41% Na and O by mass, again indicating a high content of these elements. It is likely that the initiator used was either Na2S2O8 or a Na2S2O8-Na2S2O4 system. The results of elemental analysis are shown in Table 2.

3 Conclusions Sample 1 is polyacrylamide, with a molecular weight of approximately 3.50 × 10^6. The initiator may be Na2S2O8 or a Na2S2O8-Na2S2O4 system. Sample 2 is an acrylamide-acrylic acid copolymer, with a molar ratio of acrylamide to acrylic acid units of approximately 1:1. The initiator may also be Na2S2O8 or a Na2S2O8-Na2S2O4 system. The sample contains some organic solvent, likely a C8 long-chain aliphatic hydrocarbon.