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Synthesis and characterization of a novel terpolymer and the effect of its amphoteric property on the sludge flocculation.

INTRODUCTION

In recent years, with the increasing pace of urbanization and industrialization, huge amounts of sewage sludge have been produced in municipal sewage and industrial effluent treatment. The sewage sludge has a high content of water and the nature of a hydrogel with enhanced fluidity, which makes it quite difficult to be treated and reused. Sludge flocculation can greatly impact the mechanical dewatering, and thus should be the first task for sludge treatment [1-5]. Conventionally, cationic polyacrylamides have been used for flocculation treatment of sludge [6-12]; however, these conventional flocculants for sludge treatment are no longer sufficient because of the increases in the volume of generated sludge and the deterioration of characteristics of the sludge. Therefore, the development of a sludge flocculant with high flocculation efficiency, low dosing quantity and the ability to be widely applied has become an urgent task.

The concept of amphoteric sludge flocculants has been brought up after further understanding of sludge flocculation [4, 13, 14]. Sludge formed in the biological processing of sewage is commonly referred to as organic sludge and generally contains large quantities of negatively charged colloidal matters. However, the composition of organic matters formed in this biological processing is not necessarily uniform and changes according to the treatment conditions and water quality [4]. The soluble organics (polysaccharides and proteins) as a major portion of sludge system can damage the destabilization of colloidal particles, meanwhile too much organics that increase the chemical oxygen demand (COD) of the filtrate have adverse effects on the filtrate recycling. Metal residuals generated from primary clarification include metal hydroxides bound to the sludge particle surfaces and metal ions in the solution [4, 15], they affect the flocculation by changing the zeta potential of sludge particles. Additionally, the formation of sludge floes with porous structure has a great impact on improving dewatering performance, which is relatively close to the hydrophilic/hydrophobic regions of flocculant [14]. Therefore, it is necessary to reduce the water content of treated sludge by flocculating these compounds. Watanabe [4] has demonstrated that a amphoteric system gave a two times higher dewatering rate, producing a dewatered sludge cake having a moisture content 2-5% lower than the conventional methods. Therefore, a sludge flocculant with controlled molecular weight and monomer composition is promising.

Conventional polymer modification which modifies the neutral homopolymer through chemical reactions, such as Hoffmann, Mannich, hydrolyzation, methylolation, and sulfomethylation reaction, has previously been employed in the synthesis of flocculant [10, 11, 16-19]. However, the practical applications of these flocculants are limited due to their instability and the residual of chemical agents from the processing. By contrast, free radical copolymerization has been proven to be a convenient and effective approach, which integrates specific functional groups of monomers while simultaneously generating some new features for improvement of flocculation ability [20-22]. Additionally, redox/azo composite initiation has exhibited a significant effect on copolymerization. Compared with conventional initiators, azo components of composite initiation decompose slowly and without chain transfer effects, thus the redox/azo initiation system can maintain the concentration of free radicals to improve the molecular weight in the late stage of reaction [23].

In addition to its simpler chemical structure and better solubility, a new cationic monomer acryloyloxyethyl trimethyl ammonium chloride (DAC) has a high polymerization activity due to its lack of stereo-hindrance effects, which is beneficial to improving the polymerization degree of polymer [24, 25]. The nonionic monomer acrylamide (AM) that serve as the main ingredient of a molecular chain facilitates the formation of high molecular-weight polymer, taking positive effects on floes coagulation. The anionic monomer 2-acrylamido-2-methyl-propane sulfonate (AMPS) with sulfonic groups can adjust the solubility and permeation, as well as the hydrophilic/hydrophobic regions of polymer, which greatly impact the structure of treated sludge floes [26, 27]. Thus DAC, AM, and AMPS is considered to be ideal monomers in the synthesis of flocculant.

Taking into account all of the factors mentioned above, the amphoteric copolymer poly (acrylamide-acryloyloxyethyl trimethyl ammonium chloride--2-acrylamido-2-methyl-propane sulfonate) (PADA) has been synthesized in aqueous solution by ternary free radical copolymerization. The composition of [(N[H.sub.3]).sub.2][S.sub.2][O.sub.8]-NaHS[O.sub.3] and 2,2'azobis(2-methylpropionamidine) dihydrochloride ([V.sub.50]) was adopted as the initiator. Subsequently, single factor experiments and response surface methodology (RSM) were conducted to study preparation variables. Fourier-transform infrared spectroscopy (FTIR), [sup.1]H nuclear magnetic resonance ([sup.1]H NMR) and thermogravimetric analysis (TGA) were used in polymer characterization [28, 29]. Additionally, a possible sludge flocculation process for PADA was proposed to explain the effects of amphoteric properties of PADA on the sludge flocculation. Finally, sludge flocculation performance was tested by comparing effects of PADA and control samples on content of water (CW) and specific resistance to filtration (SRF) under different flocculant concentrations and pH of sludge.

EXPERIMENTAL

Material

DAC (70 wt%) was supplied by Mitsubishi Chemical Holdings (Japan). AMPS (60 wt%) was purchased from the Lubrizol (America), and AM from Jiujiang Chemical Reagent (Jiangsu, China). [V.sub.50] was supplied by Chuangyuan Reagent (Guangzhou, China). Toluidine blue indicator, potassium polyvinyl sulfate (PVSK) were sourced from the Xi'an Chemical Reagent Factory (Xi'an, China).All other chemicals are of analytical grade.

The experimental sludge was obtained from the sludge thickener of the Datansha Wastewater Treatment Plant (Guangzhou, China). The important parameters of the sludge are as follows: density of 1.034 g/[cm.sup.3], pH of 6.37, SRF of 10.87 X [10.sup.13] m/kg, CW of 97.5% and organics of 43.2% (dry basis).

Nonionic polymer polyacrylamide (PAM) and cationic polymer poly(acrylamide-acryloyloxyethyl trimethyl ammonium chloride) (PAD) were used as control samples. PAM was prepared by polymerization of an aqueous solution of AM with [(N[H.sub.3]).sub.2][S.sub.2][O.sub.8]-NaHS[O.sub.3] initiation. The synthesis of PAD was the same as that of PADA except that the copolymer only consisted of two monomer (AM and DAC). To facilitate the contrastive analysis, PAM and PAD with characters similar to PADA (monomer mass fraction of 90:8:2) were obtained, where intrinsic viscosity of PAM is 967.2 mL/g, intrinsic viscosity and cationic degree of prepared PAD were 971.3 mL/g and 7.73%, respectively.

Synthesis of PADA

A 500 mL three-necked flask supplied with a reflux condenser and a water bath stirrer equipped with a thermostat were set up for the copolymerization. A given amount of DAC, AMPS, AM and boiled water were added into the flask to achieve the solution with a certain mass fraction. The solution was stirred at a rate of 150 rpm and purged with nitrogen gas for 20 min to deoxygenate the reaction medium. After that, a given amount of initiators [[(N[H.sub.3]).sub.2][S.sub.2][O.sub.8], NaHS[O.sub.3] and [V.sub.50]] were added drop wise into the solution to start the copolymerization. Afterwards, the reaction mixture was stirred at a certain temperature for 5 h until homogenous under nitrogen atmosphere. The hybrid polymer was obtained in gel form and precipitated with excess acetone to remove unreacted acrylamide. Finally, the purified hybrid polymer was then vacuum-dried and grounded into powder. The copolymerization reaction is illustrated in Fig. 1.

Response Surface Methodology

RSM was devoted to the evaluation of relations existing between a cluster of controlled experimental factors and measured responses [30-32]. The range and center point value of three independent variables are summarized in Supporting Information Table S1. To apply Box-behnken central composite design, 17 experimental runs were carried out and the zero experiment was repeated five times. The Design Expert 8.0 software package was used to establish the mathematical model and obtained the optimum conditions of technological progress. In developing the regression equation, the test factors are coded according to the equation:

[x.sub.i] = [X.sub.i] - [X.sup.X.sub.i]/[DELTA]Xi (1)

where x, is the coded value of ith independent variable, [X.sub.i] is the natural value of ith independent variable, [X.sup.X.sub.i] is the nature value of the i th independent variable at the center point and [DELTA][X.sub.i] is the step change value.

Y = [b.sub.0] + [summation over (i)] [b.sub.i][x.sub.i] + [summation over (i)][summation over (j)] [b.sub.ij][x.sub.i][x.sub.j] + [summation over (i)][b.sub.ii][x.sub.i.sup.2] (2)

where Y is the measured response, [b.sub.0] is the intercept term, [b.sub.i] is the first-order model coefficient, [b.sub.ii] is the quadratic coefficient for the factor i, [b.sub.ij] is the linear model coefficient for the interaction between factors i and j. The variable [x.sub.i][x.sub.j] represents the first-order between [x.sub.i], and [x.sub.j] (i < j)

Characterization

Reaction System Viscosity. 1 g solution (or hydrogel) obtained at certain time in reaction was put into 100 mL distilled water for 1 h to reach dissolution equilibrium at room temperature. The reaction system viscosity was measured by NDJ Series Digital Viscometer.

Intrinsic Viscosity ([H]). 0.05 g sample was dissolved in 50 mL deionized water, before adding 50 mL of 2.0 mol/L NaCl solution to make aqueous solution samples with different concentration. The flow time of the samples were measured by Ubbelohde viscosity meter and digital stopwatch. The intrinsic viscosity was evaluated from Huggins equation (Eq. 3) [33].

[[eta].sub.sp]/c = [eta] + [k.sub.H][[[eta]].sup.2]c (3)

Anionic Degree (AD) and Cationic Degree (CD). The AD of copolymer was measured by colloidal titration. The copolymer was diluted by distilled water (concentration of 0.001 g/L) before adding an excess amount of polybrene (5 mL at 0.001 N),then the solution was agitated for 3 min at pH 10. The polybrene which had not reacted was measured by the back titration with potassium polyvinyl sulfate (PVSK). Toluidine blue was used as an indicator. The end of the titration was observed by the change of color from blue to purple. The titration of a blank was carried out with distilled water for a similar volume of polybrene (5 mL).The value of anionic degree (AD) is expressed as follows:

AD = [C.sub.PVSK]([V.sub.PVSK,blank] - [V.sub.PVSK,sample]) x 207.24/[m.sub.PADA] x 100% (4)

where [C.sub.PVSK] is the molar concentration of PVSK (mol/L), [V.sub.PVSK,blank] and [V.sub.PVSK,sample] are the titration volume of PVSK in the blank and sample (L), 207.24 is the molecular weight of AMPS (g/mol) and [m.sub.PADA] is the mass of PADA involved by the titration (g)

The CD measurement of copolymer was the same as AD except that the solution was titrated at pH 3. The value of cationic degree (CD) is expressed as follows:

CD = [C.sub.PVSK]([V.sub.PVSK,sample] - [V.sub.PVSK,blank]) x 193.67/[m.sub.PADA] x 100% (5)

where [C.sub.PVSK] is the molar concentration of PVSK (mol/L), [F.sub.PVSK,blank] and [V.sub.PVSK,sample] are the titration volume of PVSK in the blank and sample (L), 193.67 is the molecular weight of DAC (g/mol) and [m.sub.PADA] is the mass of PADA involved by the titration (g)

For each sample the titration was repeated three times. Finally, the standard deviation was 2-4%.

Fourier-Transform Infrared Spectroscopy (FTIR). Samples and KBr were mixed by triturating with mass ratio of 1:99, and pressed to sheet samples, which were scanned at 4000-400 [cm.sup.-1] with a Thermo Nicolet 6700 Infrared spectrometer (instrument resolution of 0.4 [cm.sup.-1]). The data was analyzed by Omnic analysis software to determine the functional groups.

[sup.1]H Nuclear Magnetic Resonance ([sup.1]HNMR). The [sup.1]H NMR analysis was obtained with an INOVA 400 MHz spectrometer (AVANCE III, Switzerland). The samples were purified, dried, and then dissolved in deuteroxide ([D.sub.2]O) before the analysis.

Thermogravimetric (TGA) Analysis. Sample powders were characterized by thermogravimetric analysis for the determination of thermal stability, using a thermal analyzer (TGA-50, Shimadzu, Japan) under nitrogen atmosphere with a heating rate of 10[degrees]C/min to 600[degrees]C.

Transmission Electron Microscopy. The sample aqueous solution (0.05 wt%) was analyzed by Philips CM 12 transmission electron microscopy to investigate the polymer structure after dissolution. One drop of copolymer solution was carefully placed on the copper grid and dried with filter paper. The solution-coated copper grid was then put under transmission electron microscopy (TEM) for image viewing.

Zeta Potential. Original sewage sludge (10 g), 50 g of deionized water, and flocculant were mixed, and then 3 ml of the mixture was placed into an electrophoresis pool to measure the zeta potential using a JS94H microscopic electrophoresis instrument (Shanghai, China). Each sample was tested five times and the results averaged.

Scanning Electron Microscope. Scanning electron microscope (SEM) images of the samples were obtained using a Hitachi S-4800 field emission scanning electron microscope (FESEM, Hitachi, Japan). The sample was adhered on an aluminum stub and sputter-coated with gold for conductivity.

Flocculation Capability Experiment

Four mg of flocculant was added into 100 mL sludge while rapidly stirring at 200 rpm for 0.5 min, followed by slow stirring at 40 rpm for 3min and quiescent settling. Then, the treated sludge was poured into a Buchner funnel to filter under a vacuum pressure of 0.06 MPa for 20 min or until the vacuum could not be maintained (whichever was shorter).

The flocculation ability of flocculant is evaluated in terms of SRF and CW of treated sludge [34], which are defined as follows:

[W.sub.C] = [m.sub.0] - [m.sub.1]/[m.sub.0] x 100% (6)

Where [m.sub.0] and [m.sub.1] are the weights of the initial sludge and the sludge dried at 105[degrees]C, respectively.

SRF = 2P[A.sup.2]b/[mu][omega] (7)

where P is the filtration pressure (N/[m.sup.2]), A is the filter area ([m.sup.2]), [mu] is the viscosity of the filtrate (Ns/[m.sup.2]), b is the slope of filtrate discharge curve (s/[m.sup.6]), and [omega] is the weight of cake solids per unit volume of filtrate (kg/[m.sup.3]).

RESULTS AND DISCUSSION

Single Factor Experiment

As seen in Fig. 2a, increasing the mass fraction of monomer will lead the high speed growth of system viscosity in the early stage and eventually improve the system viscosity in the end. However, it is obvious that the increase of monomer mass fraction is not exactly in direct proportion to that of final system viscosity, which is in agreement with the correlation of monomer mass fraction and intrinsic viscosity of polymer (Fig. 2b).In the range of 0% - 17 wt%, the values of cationic and anionic degree are close to the theoretical values, meanwhile polymer shows good flocculation capacity. However, as monomer mass fraction continues to go up, the irregular crosslinkings and excessive electrostatic neutralization occurs due to excessively fast copolymerization. These tendencies have adverse effects on sludge flocculation, leading to an increase of SRF and CW of sludge accordingly. As a result, the adopted optimum monomer mass fraction is 15-20 wt%.

From Table 1, it is clear to find that copolymer with higher intrinsic viscosity is obtained at high mass ratio of AM, meanwhile the ionic degree of copolymer with high mass ratio of DAC and AMPS differ from its theoretical value due to the excessive neutralization between the functional groups. Thereby, the copolymer with high mass ratio of AM (90%) fits for the improvement of flocculation capacity. Anionic groups existing in AMPS can produce static repulsion, which is useful to chain stretch in the copolymerization [12]. Besides, since AMPS has a larger molecular weight and more similar monomer reactivity ratio to AM, the intrinsic viscosity of copolymer increases with the mass ratio of AMPS increasing. However, too high content of anionic degree undoubtedly weaken the electrostatic neutralization effects, the resistance influence of side base in AMPS may have adverse effects on growth of molecular chain at the same time [35]. Thus, the flocculation capacity will not be ideal with high or low mass ratio of AMPS, the adopted optimum mass ratio of m (AM):m (DAC):m (AMPS) is 90:8:2.

As shown in Fig. 3a, the reaction system viscosity sustains at the relatively low value at below 40[degrees]C, indicating that this temperature is too low for monomers to be copolymerized adequately. In the range of 40-50[degrees]C, high temperature facilitates the rate of initiator decomposition, so the system viscosity steadily rises over time until reach a high value. In this case, final polymer shows a good flocculation capacity by significantly reducing the SRF and CW of sludge (Fig. 3b). However, excessive heat leading to high speed copolymerization at early stage and acceleration for inducing chain-terminating, it hinders the continuous growth of molecular chain, result in a limited rise in the system viscosity at late stage of reaction. This is observed as the temperature is above 50[degrees]C. Under the circumstances, SRF and CW of sludge rise again due to the decrease of copolymerization degree. However, the cationic and anionic degree stay steady instead of going down, which implies that molecules integrated with functional groups exist in the form of short chains mostly.

As shown in Fig. 3c, as low pH in system facilitates the rate of initiator decomposition and easily causes intramolecular and intermolecular imide reactions, the system viscosity rises greatly at early stage because of the formation of high-viscosity copolymers with crosslinkings and branches, which is proved by the fact that polymer with high intrinsic viscosity has low ionic degree in low pH (Fig. 3d). Consequently, lacking of adequate charge neutralization leads to undoubted ineffective sludge flocculation. While, in the pH range of 4-6, the copolymerization proceeds smoothly and the intrinsic viscosity of final polymer increases generally, as well as the cationic and anionic degree are close to the theoretical values, which means monomers are well copolymerized in this condition. However, rising pH above 7 makes reaction hardly proceed, which is attributed to alkaline system resulting in hydrolysis and carboxylation reactions during the copolymerization, thus the polymer has little effect on sludge flocculation. In sum, temperature 40-50[degrees]C and pH 4-6 are adopted as the optimal condition.

Figure 4 indicates that redox and azo initiators greatly impact the copolymerization in different ways. As seen in Fig. 4a, when few redox initiator exists in system (0.03 wt% or lower), the system viscosity sustains in low. Nevertheless, the low content of azo initiator, or even none at all, makes less differences to copolymerization at early stage (Fig. 4b). it shows that the initial reaction is mainly induced by redox initiator rather than azo initiator. Higher mass fraction of both initiators improve the generation of free radicals and a stronger copolymerization, thus obtaining the polymer with better flocculation capacity (Fig. 4c and d). Whereas if the initiator mass fraction is excessive (0.1 wt% for redox initiator and 0.05 wt% for azo initiator), the flocculation capacity of PADA is inclined to decrease instead. This is because chain-terminating step caused by excessive free radicals accelerate the termination of the reaction. In sum, 0.05-0.1 wt% for redox initiator and 0.01 0.03 wt% for azo initiator are adopted as the optimum condition, respectively.

Response Surface Methodology

From previous discussion in single experiments, temperature, pH, redox initiator mass fraction are main variables significantly impacting the properties of PADA and its flocculation capacity. To know the interaction among them, further studies were needed with Box-Behnken central composite design (CCD). It was selected with three-level-three-factor. Coefficients of a full model were evaluated by regression analysis and tested for their significance (p-values < 0.05 indicate model terms are significant). The insignificant coefficients were eliminated stepwise on the basis of p-value after testing the coefficients. Finally, the best-fitting model was determined by regression and stepwise elimination. It is represented that three linear coefficients ([X.sub.1], [X.sub.2], [X.sub.3]), two cross-product coefficients ([X.sub.1], [X.sub.2], [X.sub.2], [X.sub.3]) and one quadratic coefficients ([X.sup.2.sub.1]) are significant for SRF (Table 2). While three linear coefficients ([X.sub.1], [X.sub.2], [X.sub.3]), two cross-product coefficients ([X.sub.1], [X.sub.2], [X.sub.2], [X.sub.3]) and one quadratic coefficients ([X.sup.2.sub.1]) are significant for CW (Supporting Information Table S2).

The ANOVA for response surface reduced quadratic model of SRF and CW are shown in Table 2 and Supporting Information Table S2, respectively. The coefficients of the response surface model as provided by Eq. 2 were evaluated. To minimize error, all the coefficients were considered in the design. According to the ANOVA analysis of factors, there is low lack of fit (0.0366 for SRF and 0.0257 for CW). This indicates that the model represents the actual relationships of reaction variables well within the ranges selected [31].

The final estimative response model equations, eliminating the insignificant variables to estimate PADA synthesis, are as follows:

[Y.sub.1] = 2.33 -0.22[X.sub.1] +0.15[X.sub.2] -0.088[X.sub.3] +0.49[X.sub.1] [X.sub.2]+0.67[X.sub.2] [X.sub.3] +0.30[X.sub.1.sup.2] (8)

[Y.sub.2] = 77.50-0.80[X.sub.1]-0.94[X.sub.2]-0.31[X.sub.3] + 2.36[X.sub.1] [X.sub.2]+1.89[X.sub.1] [X.sub.3] + 1.84[X.sub.1.sup.2] (9)

where [Y.sub.1] and [Y.sub.2] are the response factors, SRF and CW. [X.sub.1], [X.sub.2], [X.sub.3] are the values of the independent factors, temperature, pH, redox initiator mass fraction, respectively.

The goodness of the fit model is checked by determination coefficient ([R.sup.2]). In this case, correlation coefficients for SRF ([R.sup.2] = 0.9643) and for CW ([R.sup.2] = 0.9888) indicate that the models are suitable to adequately represent the real relationship among the factors selected.

The graph of RSM was made to reflect the interaction between the independent variables on the SRF and CW of sludge treated with PADA (Fig. 5). In all the presented figures, the other factors were kept at level zero. The slope of response surface plot represents the magnitude of response value [30]. The minimum predicted value is indicated by the surface confined by the smallest ellipse in the contour diagram. As shown in Fig. 5, by the center of the three influence factors (zero level), and the minimum SRF and CW of sludge can be obtained being located inside the experimental region. The slope of response surface plot and p-value indicates that it has extremely significant interaction effect on SRF between temperature and pH, pH and redox initiator mass fraction, respectively. While it has extremely significant interaction effect on CW between temperature and pH, temperature and redox initiator mass fraction, respectively.

Contour plot representing the response value is the same within an ellipse-shaped region. Ellipse shows a significant interaction effect between factors whereas circular shows no significant impacts [31]. As shown in Fig. 5d and f, the minimum response value is obtained in the center of ellipse-shaped region and gradually increasing from center to edge, the ellipses in contour plot are arranged more densely, indicating that the response values have more effect on CW than SRF with the change of reaction conditions.

The optimal values of the variables were obtained using Design Expert software. The optimum reaction conditions for PADA (temperature 46[degrees]C, pH 4.85, redox initiator mass fraction 0.08 wt%) were obtained from the regression equation (Eq. 2). The minimum predicted response values of SRF and CW under the above conditions are 2.29 X [10.sup.13] m/kg and 77.65%, respectively. The adequacy of the predicted model here is examined by an additional independent experiment at the suggested optimal synthesis conditions. The actual experimental values are SRF 2.27 X [10.sup.13] m/kg and CW 77.3%, indicating that experimental values are significantly the same as the predicted values. Thus, modeling and optimization of the synthesis of PADA for best flocculation capacity to sludge are successfully developed by RSM.

Characterization of PADA

The FTIR spectrum of PAM is seen in Fig. 6a. The peaks and their corresponding absorptions are given as follows: 3435 [cm.sup.-1] (stretching vibration absorption N-H of -N[H.sub.2]), 2992 [cm.sup.-1], 2763 [cm.sup.-1], 1454 [cm.sup.-1] (stretching and bending vibration absorption peak of--C[H.sub.2]), 1414 [cm.sup.-1], 1647 [cm.sup.-1] (stretching vibration absorption C=O). Compared with PAM, the FTIR spectrum of PADA exhibits spectral absorption peaks corresponding to functional monomers: 1163 [cm.sup.-1] (asymmetric stretching vibration absorption C--O--C of--COOC[H.sub.2]-), 1120 [cm.sup.-1] (C--N[H.sub.2]), 1041 [cm.sup.-1] (--HS[O.sub.3]), 952 [cm.sup.-1] (characteristic absorption peaks of quaternary ammonium salt). Thus, it can be seen that PADA is an amphoteric copolymer consisting of AM, DAC, and AMPS.

The [sup.1]H NMR spectroscopy of the PADA solution in [D.sub.2]O and the corresponding assignment of the signals are shown in Fig. 6b. The most significant signal at 4.8 ppm is due to the spectrum of protons in the solvent [D.sub.2]O. The sharp signal at 1.1 ppm (g) is attributed to the spectrum of protons of the methyl groups on the quaternary ammonium salt. The [sup.1]H signals from the backbone methine and methylene protons appeared in the ranges of 2.02-2.25 ppm (e) and 1.25-1.75 ppm (f), respectively. The two sharp peaks at 3.58 ppm (c) and 3.17 ppm (d) are both attributed to the proton signals of methylene in the grafting chain. Fig. 6b also shows that the -NH- and the -N[H.sub.2] groups give rise to weak signals at 7.22 ppm (a) and 6.63 ppm (b), respectively. The analytical results of the [sup.1]HNMR spectrum provide further support for the formation of the PADA.

In Fig. 6c, it shows that the thermal behavior of PADA is distinct from that of PAM. PAM has weight loss in three main stages as shown in its TGA profile. The first weight loss occurs in the range 30-100[degrees]C, which corresponds to the loss of absorbed and bound water. The second stage of weight loss starts at 250[degrees]C and continues up to 330[degrees]C because of the breakage of chemical bonds. The third stage of weight loss occurs between 350 and 450[degrees]C as a result of the decomposition of the carbon backbone. PADA contains a large number of strong hydrophilic groups which induce the dry sample to combine with water molecules leading to moisture absorption. However, unlike PAM, PADA exhibits two main quick-weight-loss stages, which is attributed to the introduction of new chemical bonds result in modification of the chemical structure. One quick-weight-loss stage occurs in the range 18Q-320[degrees]C with weight loss about 24.59% due to some processes involving the imine reaction of the amide group and the thermal decomposition of hydrophobic side chain. The other occurs in the range of beyond 320[degrees]C with weight loss about 39.57%, which assigns to the complete thermal degradation of PADA. At about 480[degrees]C, the copolymer decomposes completely. Results of TGA indicate that the integrating of new functional groups significantly changes the thermal stability of the polymer.

Flocculation Characteristics of PADA

To find out the effects of amphoteric structure in PADA flocculation, its flocculation characteristics and flocculation process was studied based on the contrastive analysis of polymer morphology, charge effects and floe characterization with control samples.

The SEM images of PAM, PAD, and PADA are exhibited in Fig. 7a-c to further investigate the effects of composition of monomers on the morphology of copolymer. In Fig. 7a, as the high polymerization activity, the monomer AM can be polymerized at a great extent, which results in its homopolymer being a dense and compact structure with smooth surface. The introduction of cationic monomer DAC increases the hydrophobicity of polymer, making the molecular chains of PAD hard to be evenly distributed in the solution during the preparation. Besides, monomer AM and DAC are hardly copolymerized well due to their different reactivity ratio, thereby PAD is the irregular-shaped structure with lots of big pores on its surface (Fig. 7b). From TEM images of dissolved polymer, lots of colloidal particles without sufficient dissolution are found on chains of PAM and PAD, meanwhile branch chains can be seen on every long chain.

As can be seen in Fig. 7c, it is obvious that anionic monomer AMPS with hydrophilic groups greatly modifies the structure of polymer, it exhibits a slightly looser structure with small pores and gaps. In this case, polymer has a better solubility and stretching, which is proved by Fig. 7f that PADA has a long chain with better extensibility and fewer colloidal particles. It is better for PADA in sludge flocculation to trap more sludge particles, which significantly enhances its charge neutralization and bridging effects.

The zeta potentials of sludge treated with the different flocculants are shown in Fig. 8. Assuming that charge neutralization accompanies successful destabilization, a zeta potential near zero should indicate good flocculation. Without charge neutralization, PAM barely changes the zeta potential of sludge, its little effect on the zeta potential is through relocation of the shear plane outward from the sludge surface [36]. After being treated with PAD, the zeta potential of the sludge increases from -25 to 10 mV, which means the negative charges of sludge particles are neutralized by excess positive charges generated from cationic groups of PAD. For PADA, the negative zeta potential of sludge gently closes to a neutral value. This is because, on one hand, PADA traps more sludge particles due to its better extension in solution. On the other hand, metal hydroxides bound to the sludge surface or metal ions dispersed in the solution which can be neutralized by anionic groups of PADA partly weaken the rise of zeta potential. With the flocculant concentration increasing, zeta potential of sludge still retains the neutral constant due to repulsive forces between the anionic groups and sludge charges. They expand molecular chains of PADA and provide more adsorption sites for opposite charges to be neutralized.

Figure 9a indicates that the sludge floes treated with PAM are scattered as a result of the attachment of extracellular polymers. Due to the lack of charge neutralization, the sludge particles cannot be destabilized and treated well, thus resulting in small-sized floes. After being treated with PAD, the size of the sludge floes greatly increases and the structure becomes slightly tight (Fig. 9b). Small pores can be seen on the surface of the floes, which serve as drainage channels caused by the hydrophobic regions of flocculant [14]. By contrast, PADA produces even larger floes with more pores and more obvious gaps (Fig. 9c), which benefits from balanced hydrophobic/hydrophilic regions after the introduction of AMPS [14], where the hydrophobic groups are the alkane structure of DAC and AM, the hydrophilic groups are the amide and quaternary ammonium groups, as well as the sulfonic groups of AMPS. Additionally sludge particles with neutral zeta potential is beneficial for the formation of floes for the solid/liquid separation [14].

Based on the discussion above, the flocculation characteristics of PADA are proposed in its flocculation process (Supporting Information Fig. S1):

1. Due to the hydrophilic effects of sulfonic groups of AMPS, PADA has good solubility, leading to the sufficiently stretch of molecular chains in the sludge system.

2. Amphoteric structure of PADA enhances charge neutralization which accompanies successful destabilization, resulting in good permeation and dispersion in the sludge system. The repulsive forces between the anionic groups and sludge charges in dense regions further increase the extension of the molecular chains.

3. The cationic groups of the PADA react with and bind to the negative charges of sludge through the neutralization reaction. Through the neutralization reaction, charged anionic portions react with and bind to the positive charges of metal hydroxides or metal ions. Meanwhile, soluble organics are absorbed and the sedimentation rate of inorganics is accelerated (step 3).

4. The functional groups of the PADA that adsorbed onto the surface of a sludge particle bind to the functional groups of another polymer, and the effective molecular weight of the polymer becomes gigantic, then surrounding sludge particles are bridged and bound tightly, thus the size of the polymers is shrank, resulting in the large floes with porous and clearly demarcated structures (step 4).

Flocculation Experiment of PADA

To testify the better flocculation capacity of PADA, the SRF and CW of sludge treated with PADA, PAD and PAM at various concentrations and pH were measured, which are shown in Fig. 10. It can be seen that the amphoteric flocculant PADA exhibits a better flocculation capacity at all concentrations, the minimum SRF and CW of sludge treated with PADA are 2.4 X [10.sup.13] m/kg and 77.3%, respectively. Additionally, PADA maintains a high efficiency even at a concentration of 60 to 100 mg x [L.sup.-1], thus it can be proved to be used in a wider range of flocculant concentration. Additionally, in the pH range of 7-10, PADA maintains the high flocculation efficiency, implying that it has a broader pH window. Although polymer can be hydrolyzed accompanied with sludge floes being deteriorated under alkali conditions, sulfonic groups of AMPS can inhibit the hydrolysis so that the destabilization of the sludge floes caused by charge reversion can be attenuated. Meanwhile, the repulsive forces between sulfonic groups enhance the flexibility of the polymer under alkali conditions [37], which is beneficial for the bridging action.

CONCLUSIONS

The amphoteric terpolymer PADA was synthesized by free radical copolymerization in a redox/azo initiation system. The range of optimum reaction conditions were obtained by single factor experiments: total monomer mass fraction of 15-20 wt%, monomer mass ratio of 90:8:2, reaction temperature of 40-50[degrees]C, pH of 4-6, redox initiator mass fraction of 0.05-0.1 wt% and azo initiator mass fraction of 0.01-0.03%. With analysis of RSM, there were significant interaction effects on SRF and CW of sludge among main variables, the optimal SRF and CW of flocculated sludge predicted by the statistical model were 2.29 X [10.sup.13] m/kg and 77.65%, respectively. Subsequently, PADA was characterized by FTIR, [sup.1]HNMR, confirming it was the integrate of three different monomers, while results of TGA indicated that the integrating of new functional groups significantly changed its thermal stability. From SEM and TEM images, it showed porous structure in PADA was good for its extension and absorption to particles, meanwhile zeta potential tests confirmed PADA provided more adsorption sites for opposite charges to be neutralized, which both exhibited the amphoteric properties took positive effects on the sludge flocculation. Finally, treatment of sludge indicate PADA was a high efficient flocculant even in a concentration range of 50-100 mg-[L.sup.-1] and sludge pH of 5-10.

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Liang Qi, (1) Jian-Hua Cheng, (1) Xiao-Yan Liang, (1) Yong-You Hu (1,2)

(1) Ministry of Education Key Laboratory of Pollution Control and Ecological Remediation for Industrial Agglomeration Area, College of Environment and Energy, South China University of Technology, Guangzhou, 510006, China

(2) State Key Lab of Pulp and Paper Engineering, College of Light Industry and Food Science, South China University of Technology, Guangzhou, 510640, China

Correspondence to: J.-H. Cheng; E-mail: jhcheng@scut.edu.cn

Contract grant sponsor: National Natural Science Fund of China (Foundation of Guangdong Province of China); contract grant number: U1401235; contract grant sponsor: Fundamental Research Funds for the National Central Universities; contract grant number: 2014ZZ0052.

Additional Supporting Information may be found in the online version of this article.

DOI 10.1002/pen.24238

TABLE 1. The influences of monomer mass ratio on the
properties of PADA and its flocculation ability.

      Mass ratio
      of monomer
                   [[eta]]                      SRF (m/kg,
AM    DAC   AMPS   (mL/g)    CD (%)   AD (%)   [10.sup.-13])   CW (%)

90    10     0      868.3     9.87      0          3.33         80.5
90     9     1       914      8.85     0.91        2.76         79.9
90     8     2      973.3     7.83     1.82        2.33         77.5
90     7     3     1040.4     6.86     2.84        2.44         78.3
90     6     4     1079.3     5.78     3.81        2.87         79.4
90     5     5     1193.2     4.56     4.47        3.37         83.1
80    18     2      642.6    12.38     1.02        4.43         82.3
80    16     4      662.3    11.36     2.42        4.35         81.9
80    14     6      677.3     8.58     3.13        5.12         84.1
80    10     10     702.2     4.96     3.21        6.21         86.3
70    27     3      375.3     16.8     0.82        7.82         88.6
70    15     15     393.6     6.23     4.12        8.13         89.3

Other conditions: total monomer 17 wt%, 45[degrees]C, pH 5,
redox initiator 0.075 wt% and azo initiator 0.02 wt%, SRF
and CW were measured at 50 mg/L concentration.

TABLE 2. ANOVA for response surface quadratic model of SRF.

                          Sum of    Degree of     Mean
Source                   squares     freedom     square    F-Value

Model                    4.289369       6       0.476597   21.73051
[X.sub.1]-Temperature    0.391613       1       0.391613   17.85564
[X.sub.2]-pH             0.183013       1       0.183013   8.344488
[X.sub.3]-Redox          0.06125        1       0.06125    2.792705
  initiator mass
  fraction
[X.sub.1][X.sub.2]       1.031684       1       1.031684   47.03983
[X.sub.2][X.sub.3]       1.890105       1       1.890105   86.17969
[X.sup.2.sub.1]          0.385289       1       0.385289   17.56734
Residual                 0.153525      10       0.016932
Lack of fit              0.078525       6       0.014175     0.74
Pure error                0.075         4       0.01875
Cor total                4.442894      16

Source                   p (Prob > F)   Significant a

Model                       0.0003       Significant
[X.sub.1]-Temperature       0.0039            *
[X.sub.2]-pH                0.0234            *
[X.sub.3]-Redox             0.0386            *
  initiator mass
  fraction
[X.sub.1][X.sub.2]          0.0002           **
[X.sub.2][X.sub.3]         < 0.0001          **
[X.sup.2.sub.1]             0.0041           **
Residual
Lack of fit                 0.0366            *
Pure error
Cor total

* Significant (p < 0.05); ** Extremely significant (p < 0.01).
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Author:Qi, Liang; Cheng, Jian-Hua; Liang, Xiao-Yan; Hu, Yong-You
Publication:Polymer Engineering and Science
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Geographic Code:9CHIN
Date:Feb 1, 2016
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