Article pubs.acs.org/IECR
UV-Initiated Polymerization of Hydrophobically Associating Cationic Polyacrylamide Modified by a Surface-Active Monomer: A Comparative Study of Synthesis, Characterization, and Sludge Dewatering Performance Yi Liao,†,‡ Huaili Zheng,*,†,§ Li Qian,∥ Yongjun Sun,† Li Dai,† and Wenwen Xue† †
Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, §National Centre for International Research of Low-carbon and Green Buildings, and ∥College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400045, China ‡ Department of Architectural Engineering, Sichuan University of Science and Engineering, Zigong, Sichuan 643000, China S Supporting Information *
ABSTRACT: Two cationic polyacrylamides, PAA and PAD, were synthesized for sludge dewatering. The advanced instruments such as 1H NMR, FTIR, and SEM were used to characterize the two copolymers. Their hydrophobic association properties in water were investigated by viscosimetry as well as the dewatering performance studied by the sludge dewatering experiment. The results showed that the optimum conditions for preparation of PAA were that the initiator concentration, urea concentration, molar ratio of AODBAC to AM, and irradiation time were 0.3‰, 1.0%, 10:90, and 60 min, respectively. Also, it was found that PAA had a stronger hydrophobic interaction with a lower intrinsic viscosity and longer dissolution time as well as a better dewatering performance. Furthermore, the charge neutralization and bridging effects were found to contribute much to the sludge dewatering by PAA in which the dewatering performance was able to be enhanced further by the hydrophobic interaction.
1. INTRODUCTION Treatment of industrial and domestic wastewater produces large and the annual increase of excess activated sludge.1,2 In general, the water content remaining in the sludge is usually up to 95−99%.3 It is well documented that the sludge dewatering is one of the most challenging technical tasks in the field of wastewater engineering.4 The colloidal and compressible nature of sludge resulting from organic components, primarily consisting of bacterial cells and EPS (extracellular polymeric substances), makes it very difficult to dewater even at high pressure.4,5 The problem can be overcome in part through chemical conditioning of sludge prior to the mechanical dewatering.5,6 In the chemical conditioning, the flocculants have been extensively used among which the cationic polyacrylamides (CPAMs) have a superior performance, because they have many advantages such as less dosage, higher efficiency, and better solubility. Despite the fact that the various types of commercial CPAMs are available, there is still a need to improve their efficiency and to develop new types of polymers for special applications. The hydrophobic modification of the CPAMs has been suggested as an effective way to enhance the polymer performance.7 For hydrophobically associating cationic polyacrylamides (HACPAMs), the electroneutralization effect, adsorption bridging effect, and hydrophobic association effect could interact with each other in the sludge dewatering process,8 and the three effects primarily are dependent on the cationic degree, molecular weight, and hydrophobic group, respectively.9 Therefore, the high performance HACPAMs are in general characterized by higher molecular weight and cationic degree as well as stronger hydrophobic group. It has been long found that © 2014 American Chemical Society
the hydrophobic group possibly increasing the hydrophobic association property would lead to a decrease in the solubility of HACPAMs, thus affecting the quality of product.10,11 Therefore, during the performance improvement of the HACPAMs by stronger hydrophobic group, the solubility of the product needs to be highly concerned. The most commonly used method to prepare the HACPAMs is micellar copolymerization in which the external conventional surfactant as the micelles to solubilize the hydrophobic comonomer thus overcoming the problems of insolubility of hydrophobic comonomers in water is usually employed.8,12 However, the external surfactants could cause some negative impacts, such as chain transfer effects, complex post-treatment processes, and undesirable toxicity.13,14 These problems can be alleviated by replacing the commonly used hydrophobic monomer with surface-active monomer (surfmer)14,15 because the surfmer containing amphiphilic structure and polymerizable vinyl double bonds not only has the surface activity but also can be used as a monomer or comonomer for polymerization.13,15 Thus, the absence of the surfactant also promotes the reaction to occur through the free radical polymerization in water. In this study, the surfmer, acryloyloxyethyl dimethylbenzyl ammonium chloride (AODBAC), used to prepare the P (AM-AODBAC) (PAA) was adopted in that it has been reported that the benzyl group in cationic polymers could greatly increase the hydrophobic Received: Revised: Accepted: Published: 11193
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association behavior of these polymers in aqueous solution.16−18 Another monomer, acryloyloxyethyltrimethylammonium chloride (DAC) in this study used to prepare the copolymer (P (AM-DAC) (PAD)), was employed because the DAC is similar to AODBAC in structure but has less phenyl groups. A comparison between PAA and PAD in this study was made. Free radical copolymerization in aqueous solution can be initiated by heat,8 rays,19 microwave,20,21 and ultraviolet (UV).22 Compared with other methods, the UV-initiation technique has been recognized as an easily controlled and environmentally-friendly process. In addition, it is believed to benefit the surface modification.3,23 Thus, this initiation method in this work was explored to prepare the HACPAM, which was applied to the sludge dewatering process. At present, the copolymerization reaction between AODBAC and AM has been reported,24,25 but as a surfmer, the AODBAC used for the reaction is not well-known, especially for those reactions initiated by the UV light. Furthermore, a comparison of hydrophobic association property and sludge dewatering performance between PAA and PAD in the same conditions are not clearly present. PAA that were modified by AODBAC in this study was therefore synthesized through UV-initiated polymerization with 2,2′-azobis (2-methylpropionamide) dihydrochloride (V-50) as initiator, which was applied to sludge dewatering. In order to make a comparison, PAD was synthesized under the same synthetic conditions, and its sludge dewater performance was also investigated. The copolymers were characterized by Fourier transform-infrared spectroscopy (FTIR), magnetic resonance spectrometry (1H NMR), and scanning electron microscopy (SEM). During sludge dewatering, parameters affecting sludge dewatering performance, such as dosage, intrinsic viscosity, cationic monomer feed ratio, and pH were investigated, and the dewatering mechanisms were studied.
Figure 1. Photoinitiated reaction device.
sealed immediately and transferred to the reaction device. The irradiation time under the UV lamp was set to 60 min in a series of experiments. After irradiation, the product was purified with acetone and ethanol for several times and then dried in a vacuum oven at 40 °C until a constant weight was obtained. The final product was obtained through granulation and packing. The proposed reaction mechanism for the polymerization of PAA is outlined in Scheme 1. PAD was prepared by following the same synthesis process. Scheme 1. Proposed Reaction Scheme of the Synthesis of PAA
2. EXPERIMENTAL SECTION 2.1. Materials. AM, AODBAC, and DAC used in this experiment were of technical grade, whereas the remaining reagents were of analytical grade. AM was sourced from Chongqing Lanjie Tap Water Company (Chongqing, China); AODBAC (80 wt % in water) was obtained from Wuhan Yihuacheng Science and Technology Development Co., Ltd. (Wuhan, China); DAC (80 wt % in water) was provided by Guangchuangjing Import and Export Co., Ltd. (Shanghai, China); V-50 was purchased from Ruihong Biological Technology Co., Ltd. (Shanghai, China); and urea was obtained from Chongqing Boyi Chemical Reagent Co., Ltd. (Chongqing, China). Deionized water was used throughout this work. All these reagents were used without further purification. 2.2. Synthesis of Copolymers. The synthesis of the copolymer of AM and AODBAC was carried out in a Pyrex glass vessel at ambient temperature. The photoinitiated reaction device used in the experiment is shown in Figure 1. A 500 W high-pressure mercury lamp with main radiation wavelength of 365 nm and average light intensity of 1200 μW·cm−2 (Shanghai Jiguang Special Lighting Electric Factory, China) was used as a UV light source to initiate the polymerization. Desired amounts of AM, AODBAC, and urea were dissolved in a certain amount of deionized water to maintain the monomer concentration at 40%. The solutions were deoxygenated through nitrogen bubbling for half an hour. Then, predetermined initiators (V50) were added into the solution. The reaction vessel was
2.3. Characterization of Copolymers. The intrinsic viscosity of the copolymers was determined with an Ubbelohde viscometer (Shanghai Shenyi Glass Instrument Co., Ltd., China) at a constant temperature of 30 ± 0.1 °C in 2 M NaCl aqueous solution. The intrinsic viscosity is related to the molecular weight of the copolymer22,26 and can thus be used to evaluate the absorption and bridge effect of the prepared copolymers.7 Dissolution time was measured using the conductance method.23 Apparent viscosity measurements were conducted on a rotational viscometer (NDJ-79/7, Shanghai Changji Geological Co., Ltd., China) at 25 °C and a shear rate of 850 s−1 according to the method reported in a previous work.13 The FTIR spectra of the copolymers were obtained using KBr pellets on the 550Series II infrared spectrometer (Mettler Toledo Instruments Co., Ltd., Switzerland); the 1H NMR spectra of the dried copolymer samples were recorded in deuterium oxide (D2O) with an AVANCE 500 NMR spectrometer (BRUKER Company, Germany). The morphol11194
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with monomer molecules.28 As the initiator increased to 0.3‰, more primary radicals were generated and escaped from their “cages” to react with the monomers, thereby increasing the intrinsic viscosity. However, further increase in the initiator concentration above 0.3‰ might increase the termination and chain transfer rate, thereby decreasing the intrinsic viscosity. This trend was in complete agreement with the classical kinetic theory, as indicated by previous works.27,29 However, unlike the heat-initiated polymerization with a uniform active center distribution and reaction speed, the UV-initiation system had a nonuniform distribution of free radicals because the light intensity decreased along the radiation path. The gradual decrease in light intensity resulted from the light absorption of the initiator and the scattering effect in the reaction medium, which was conducive to the production of polymers with ultrahigh intrinsic viscosity.30 In addition, Figure 2 showed that the change in dissolution time resembled that of intrinsic viscosity, indicating that dissolution time relied mainly on the intrinsic viscosity. Therefore, the favorable value of the initiator concentration was 0.30‰. 3.1.2. Effect of Irradiation Time on Copolymerization. As shown in Figure 3, the intrinsic viscosity of the obtained
ogy of the copolymers was observed with a VEGA II LMU SEM (TES-CAN Company, Czech) after the pretreatment with spray gold. Based on the SEM image, the fractal dimensions of the flocculants were calculated with Image-pro Plus 6.0 software. 2.4. Dewatering Tests. The sludge samples were obtained from Dadukou Wastewater Treatment Plant in Chongqing, China. The sludge was characterized by a moisture content of 97.2%, a mass density of 1.002 kg·L−1, and a pH value of 7.3. A program-controlled jar test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co., Ltd., China) was used in the sludge dewatering experiments. In the dewatering experiment, 500 mL of sludge was transferred into beakers and then added with different dosages of flocculants. The pH of the sewage sludge was adjusted with 1.0 M NaOH and HCl and then rapidly mixed by stirring at 100 rpm for 30 s to attain the complete mix of the flocculant with sludge, followed by 30 s of slow stirring at 50 rpm to promote floc growth. Then, the conditioned sludge was poured into a Buchner funnel for filtering under a vacuum pressure of 0.09 MPa for 30 min or until the vacuum could not be maintained (in PAA-10-11.6 > PAM-10-9.0. This ranking was consistent with that of FCMC. 3.4.3. Effect of Different Cationic Monomers on Dewatering Performance. The flocculation performance of PAA-1014.0 and PAD-10-14.9 is illustrated in Figure 13 to investigate the effect of the hydrophobic benzyl group in the PAA series on sludge dewatering performance. The two polymers had basically identical intrinsic viscosities and the same cationic monomer feed ratio. Therefore, we could reasonably assume
Figure 14. Zeta-pH profiles of raw sludge, sludge conditioned with PAA-10-14.0 (0.8 g·kg−1) and PAD-10-14.9 (0.8 g·kg−1). 11200
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ratio of AODBAC and AM, and irradiation time were 0.3‰, 1.0%, 10:90, and 60 min, respectively. Under these conditions, the maximum intrinsic viscosity and the corresponding dissolution time were 11.0 dL·g−1 and 53 min, respectively. Compared with the PAD, PAA had a slightly lower intrinsic viscosity and slightly higher dissolution time under the same synthetic conditions. 2) The viscosity measurement indicated that PAA had a stronger hydrophobic association than PAD because of the introduction of the benzyl group to the PAA. 3) FTIR and 1H NMR confirmed the structure of the two synthesized polymers. Besides, the surface morphology of the polymer characterized by SEM showed that the structure of PAA was more porous, which was favorable for adsorbing and bridging behavior between polymer molecules and sludge particles. 4) The dewatering results demonstrated the superiority of PAA over PAD at a pH ranging from 4.0 to 9.0. The minimum FCMC and RTS of 68.0% and 5.7 NTU, respectively, were achieved with PAA at the optimum coagulant dosage of 0.8 g· kg−1 and a sludge initial pH of 6.0. By contrast, the minimum FCMC and RTS of 73.5% and 8.3 NTU, respectively, were achieved with PAD under the same flocculant dosage and pH. In addition, the sludge dewatering performance and zeta potential tested in this work indicated that charge neutralization and bridging had important roles for PAA in the sludge dewatering process in which the hydrophobic interaction could enhance the dewatering performance significantly.
zeta potentials of the supernatant of the sludge significantly changed after conditioning with PAA and PAD. This result confirmed that the charge neutralization mechanism had an important role during the sludge dewatering process at various pHs.51 Besides, the zeta potential of the supernatant conditioned with PAA-10-14.0 was less negative than that conditioned with PAD-10-14.9, possibly because PAA could compress the double electric layer more effectively because of its stronger hydrophobic interaction compared with PAD. The effect of pH on the FCMC of the two copolymers is shown in Figure 15, where PAA and PAD display similar
Figure 15. Effect of pH on the FCMC for PAA-10-14.0 and PAD-1014.9 at dosage of 0.8 g·kg−1.
profiles. For the supernatant of the sludge conditioned with the PAA series, its FCMC slightly decreased when the pH changed from 4.0 to 6.0 and reached minimum at a pH of 6.0. Subsequently, the FCMC increased when the pH increased from 6.0 to 9.0. A large increase in FCMC occurred at a pH of 9.0, which could be ascribed to the significant decrease in the zeta potential (δ = −19.0 V). However, even at such zeta potential, the FCMC could reach 72.4%, indicating that the PAA series was not sensitive to the change in the pH from 4.0 to 9.0. A similar phenomenon was observed in the PAD series. The pH insensitivity of the two samples should be ascribed to the pH-independent behavior of the quaternary ammonium salts.34 Additionally, at the same pH, the difference in the FCMC conditioned with the two copolymers was probably due to the differences in their zeta potentials and hydrophobic interactions.2 Moreover, the optimum pH corresponding to the minimum RTS of PAA-10-14.0 and PAD-10-14.9 is shown in the Supporting Information as Table S4. As shown in Table S4, both PAA-10-14.0 and PAD-10-14.9 achieved minimum RTS at a pH level of 6.0. This result was consistent with that obtained by FCMC. The above experiment results indicated that at a pH range of 4.0 to 9.0, PAA showed better sludge dewatering performance than PAD.
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ASSOCIATED CONTENT
* Supporting Information S
The minimum residual turbidity of the supernatant and the corresponding optimum polymer dosage of PAA and PAD at different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86 23 65120827. Fax: +86 23 65120827. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support provided by the National Natural Science Foundation of China (Project No. 21177164), the Major Projects on Control and Rectification of Water Body Pollution (Project No. 2013ZX07312-001-03-03) and the 111 Project (Project No. B13041).
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4. CONCLUSION A novel flocculant, PAA, was synthesized in this study through the copolymerization of AM and AODBAC in an aqueous solution via UV-initiated polymerization technology for sludge dewatering. Meanwhile, another commonly used flocculant PAD copolymerized with AM and DAC was prepared and compared with PAA. Their structures were characterized with some advanced instruments, such as FTIR, 1H NMR, and SEM. Finally, the sludge dewatering performance of PAA as well as its dewatering mechanisms was investigated. The main conclusions of this study were as follows: 1) The optimum conditions for the preparation of the PAA was that the initiator concentration, urea concentration, molar 11201
ABBREVIATIONS EPS = extracellular polymeric substances CPAMs = cationic polyacrylamides HACPAMs = hydrophobically associating cationic polyacrylamides surfmer = surface-active monomer AODBAC = acryloyloxyethyl dimethylbenzyl ammonium chloride DAC = acryloyloxyethyltrimethylammonium chloride UV = ultraviolet V-50 = 2,2′-azobis (2-methylpropionamide) dihydrochloride FTIR = Fourier transform-infrared spectroscopy dx.doi.org/10.1021/ie5016987 | Ind. Eng. Chem. Res. 2014, 53, 11193−11203
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H NMR = magnetic resonance spectrometry SEM = scanning electron microscopy D2O = deuterium oxide FCMC = filter cake moisture content RTS = residual turbidity of the supernatant HAPAM = hydrophobically modified associating polyacrylamide A = projected area L = characteristic length
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