Article pubs.acs.org/IECR
Effect of Template on Structure and Properties of Cationic Polyacrylamide: Characterization and Mechanism Qingqing Guan,†,‡ Huaili Zheng,*,†,‡ Jun Zhai,*,†,‡ Chun Zhao,†,‡ Xiaokai Zheng,†,‡ XiaoMin Tang,†,‡ Wei Chen,†,‡ and Yongjun Sun†,‡ †
Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, and ‡National Centre for International Research of Low-Carbon and Green Buildings, Chongqing University, Chongqing 400045, China ABSTRACT: In this study, a cationic block structure with a strong neutralizing ability was formed through template polymerization. Acryloxyethyltrimethyl ammonium chloride (DAC) and acrylamide (AM) were used as monomers, and the ionic homopolymer sodium polyacrylate (NaPAA) was used as the template. The product containing NaPAA after template polymerization is denoted as NTP, whereas the copolymer obtained after removing the NaPAA is denoted as TP. The common polymer (denoted SP) of AM and DAC was copolymerized through solution polymerization. TP and SP were characterized and compared by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), 1H nuclear magnetic resonance (1H NMR) spectroscopy, and thermogravimetric analysis (TGA). The results of 1H NMR spectroscopy and TGA showed that a cationic block structure was formed in TP. The mechanism of the cationic block polymer used in water treatment was extensively studied through a jar test in which turbidity, zeta potential, and average floc size were used to evaluate the flocculation performance. The results further supported the cationic block structure of TP. TP, with a different structure than SP, resulting in a stronger neutralization ability, showed a better performance in flocculating kaolin suspensions. The dominant mechanisms for TP flocculation behavior at pH 5, 7, and 9 might be patching, charge neutralization, and bridging adsorption. The flocculation performance of NTP was not acceptable, whereas acidic NTP at pH < 2 showed a flocculation effect similar to that of TP.
1. INTRODUCTION The effluents of the paint, rubber, paper filler, coating pigment, and chemical production industries contain large quantities of kaolin clay.1 Kaolin suspensions should be disposed in an environmentally friendly manner. However, settling and consolidation of industrial wastewater with kaolin are difficult because of the colloidal size, anisotropic shape, and repulsive interactions among negatively charged basal faces. Therefore, removing kaolin microparticles from effluents before these effluents are discharged into the environment is a problem for the aforementioned processing industries.2−5 Flocculation, one of the most important industrial processes for water treatment, is widely applied because of its facile operation, high efficiency, and economic advantages.6,7 Synthetic polymers with high molecular weights have been extensively used as flocculants for colloidal suspensions to separate and dewater solid/water systems. Polyacrylamide and its derivatives are among the most commonly used polyelectrolytes for kaolin in aqueous solutions because these polymers can effectively aggregate particles into flocs to promote faster settling.8 Cationic polyacrylamide (CPAM) is one of the most widely applied polymers. Flocculation of fine particles can occur by polymer bridging, charge neutralization (including electrostatic patch effect), polymer−particle surface complex formation, or a combination of these mechanisms.9,10 Bridging flocculation occurs as a result of the simultaneous adsorption of individual polymer chains onto several particles, thus forming a molecular bridge between adjoining particles in the floc. The characteristics of polymers that influence the effects of bridging are the length of the polymer molecular © 2014 American Chemical Society
chain [i.e., the polymer molecular weight (MW)] and the molecular configuration in the solution. As polymer chain length increases, more particles become involved, and thus, flocculation improves. Linear chains can provide more adsorption segments and can form larger flocs than polymers with a coiled configuration. Therefore, the most effective polymers for bridging are linear chains with high molecular weights (up to several million grams per mole).11,12 An electrostatic patch occurs when a polymer with a relatively high charge adsorbs on a weakly charged negative surface and the average distance between surface sites is greater than that between charged segments along the polymer chain. Although a surface might have an overall charge close to neutral, “patches” or “islands” with a positive charge occur among regions of uncoated, negatively charged surfaces. Negative charges on the bare parts of the surface of one particle easily attach to the excess net residual positive charges of the polymer coating on another particle to provide particle attachment and, hence, flocculation.13−15 Charge neutralization becomes the dominant mechanism for polyelectrolytes when suspended particle surface sites with a charge opposite to that of polymer ionic functional groups are present. Repulsion among particles diminishes upon addition of CPAM, thus causing particle charges to decline and flocculation to occur. Studies have demonstrated that the best flocculation performance occurs at Received: Revised: Accepted: Published: 5624
December March 16, March 18, March 18,
5, 2013 2014 2014 2014
dx.doi.org/10.1021/ie404116k | Ind. Eng. Chem. Res. 2014, 53, 5624−5635
Industrial & Engineering Chemistry Research
Article
Scheme 1. Overview of Template Polymerization
flocculant dosages required to neutralize or nearly neutralize the particle charge when the zeta potential is zero or close to zero.16,17 Thus, flocculants with a high charge density (CD) are more effective because they can neutralize more particle surface segments for a particular dosage. However, CD is not the only factor responsible for charge neutralization; rather, the sequence and distribution of cationic groups also play a role in this process. Cationic flocculants synthesized by common methods (e.g., solution copolymerization18) often lead to a random cation distribution along the polymer molecular chain. For such materials, neutralization cannot effectively decrease the particle surface charge because the cations are scattered and the length of the cation group is short. Charge neutralization of cationic flocculants functions effectively if the cationic charges are distributed along the polymer chain in a microblock structure. This structure can provide larger cationic adsorption sites to neutralize particle surface charges and promote flocculation. Inverse microemulsions19 have also been studied for the synthesis of block structure polymers. However, not only does this method require a huge amount of emulsifiers, which influences the purity of the polymer, but the process is also complicated. Thus, a new method should be introduced to synthesize flocculants of cationic block structures. An effective method for synthesizing block polymers is template or matrix polymerization, which has been investigated since 1954. Template polymerization is generally defined as a process in which monomer units are organized by a preformed macromolecule (template) and refers to single-phase systems in which the monomer and the template are soluble in the same solvent or are present in the form of a swollen gel.20 Only a few studies have employed template polymerization to synthesize high-MW flocculants, and most of these synthesized ionic flocculants.21−24 Few studies have focused on the synthesis of cationic polymers, and only Charalambopoulou et al. synthesized cationic polymers through template polymerization when the MW was only at the 105 level.25 In addition, one of the most difficult problems in template polymerization is removing the template. A template is fully mixed in a polymerization reactor, and thus, isolating a daughter polymer is difficult and unacceptable in industry. The flocculation performance of polymers synthesized by template polymerization with and without removal of the template has rarely been studied systematically. Understanding the influence of template on flocculation performance might enable the application of template polymers (TPs). The flocculation mechanism of the involved cationic block structures should be investigated to shed light on similar research and actual applications. In the present study, we report the polymerization of acrylamide (AM) and acryloxyethyltrimethyl ammonium chloride (DAC) prepared in the presence and absence of an ionic homopolymer, namely, sodium polyacrylate (NaPAA), to
understand the effects of the template on the structural control of CPAM. NaPAA was chosen as template for the following reasons: (1) Low-MW NaPAA has a linear configuration in solutions,26 and (2) the price of NaPAA is comparatively low because NaPAA is extensively applied as a dispersant.27 Solution polymerization and template polymerization were both initiated by UV illumination, and the chemical structures of the products (denoted SP and TP, respectively) were characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), 1H nuclear magnetic resonance (1H NMR) spectroscopy, and thermogravimetric analysis (TGA). TP and SP were used to flocculate kaolin at various dosages and pH values. The flocculation mechanisms of these polymers were analyzed in terms of supernatant turbidity, zeta potential, and average floc size. The influence of the template on flocculation performance was explained. Moreover, a method to overcome the drawbacks of the presence of the template was introduced for the polymer without removal of NaPAA.
2. EXPERIMENTAL SECTION 2.1. Materials. The monomer AM (98.5%, w/w) was obtained from Lanjie Tap Water Company (Chongqing, China). The cationic monomer DAC was supplied by Guangchuangjing Company (Shanghai, China). The template was purchased from Jvtao Biological and Scientific Co., Ltd. (Hangzhou, China). The photoinitiator 2,2′-azobis(2methylpropionamide)dihydrochloride was obtained from Ruihong Biological Technology (Shanghai, China). Kaolin was purchased from Guangfu Fine Chemical Institute (Tianjin, China). The other reagents used in the experiments, including ethanol, urea [CO(NH2)2], hydrochloric acid (HCl), and sodium hydroxide (NaOH), were of analytical grade. All aqueous and standard solutions were prepared with deionized water. The purity of nitrogen gas was higher than 99.99%. 2.2. Preparation of Copolymer. The preparation of TP was performed as follows: AM (75.00 wt % of total monomer), DAC (25.00 wt % of total monomer), and NaPAA (mole ratio between a chain unit and DAC of 1.00) were first added to a reaction vessel made of silicate glass. Then, deionized water was poured into the reaction vessel for a total monomer concentration of 30%. Urea (4.0 wt ‰ of total monomer weight), which acted as the cosolvent, and 2,2′-azobis(2methylpropionamide)dihydrochloride (0.65‰ of total monomer weight), which acted as the initiator, were immediately added to the aqueous solution. Meanwhile, the aqueous solution was purged with nitrogen for 30 min prior to UV activation to completely remove oxygen. Through UV-induced polymerization (main radiation wavelength between 300 and 400 nm, 365 nm; average irradiation intensity, 2000 μW/cm2) that continued for 1 h, the copolymer was produced, and the aqueous solution was changed into a semicolorless transparent 5625
dx.doi.org/10.1021/ie404116k | Ind. Eng. Chem. Res. 2014, 53, 5624−5635
Industrial & Engineering Chemistry Research
Article
Scheme 2. Scheme of the Reaction Route for the Preparation of TP and SP
potentials of the kaolin suspension at different pH values are listed in Table 1.
solid. The copolymerization process can be illustrated by Scheme 1 and is discussed below.20 NaPAA was removed after 12 h at room temperature to increase the polymerization degree. The copolymer was dissolved in water in an amount that was 10 times the weight of the copolymer. After the copolymer had been dissolved, the pH was adjusted to be
