Flocculation of Both Kaolin and Hematite Suspensions Using the

Nov 24, 2014 - Moreover, the floc properties including the floc size, fractal structure, and regrowth ability of flocs have been systematically studie...
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Flocculation of Both Kaolin and Hematite Suspensions Using the Starch-Based Flocculants and Their Floc Properties Haijiang Li, Tao Cai, Bo Yuan, Ruihua Li, Hu Yang,* and Aimin Li State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China ABSTRACT: In this work, two kinds of starch-based flocculants with the same chemically modified functional groups but far different substitution degrees, (2-hydroxypropyl)trimethylammonium chloride etherified carboxymethyl starch (denoted as CMS-CTA-P and CMS-CTA-N, respectively) were successfully prepared. CMS-CTA-P and CMS-CTA-N bear opposite surface charge properties in water under most of the measured pH range. The flocculation performance of these two flocculants has been systematically studied using kaolin and hematite suspensions as synthetic wastewater under various pH conditions. CMS-CTA-P and CMS-CTA-N exhibited inverse flocculation behavior, but the commonness was that the starch-based flocculants with charge opposite to those of the contaminants in water always had higher flocculation efficiency. On the basis of the apparent flocculation performance and ζ-potential measurement, patching, a special charge neutralization mechanism, was significant here. Moreover, the floc properties including the floc size, fractal structure, and regrowth ability of flocs have been systematically studied by an in situ light-scattering technique in combination with fractal theory for further investigation of the flocculation mechanisms during the flocculation processes. The pH dependence of the floc properties and conformational changes of the polymeric flocculants indicated that bridging also made adequate contributions to efficient flocculation.

1. INTRODUCTION Water pollution in the world has become more severe in the last decades. Various technologies, such as flocculation, adsorption, oxidation, and biotechnology, have been developed and applied in water treatment.1−4 Among them, flocculation is cost-effective and an operation that is easy to perform.5,6 In the flocculation process, the flocculation efficiency highly depends on the selected flocculants because different flocculants bear various properties, such as the ionic nature, charge density, substitution degree of various functional groups, and molecular weight, which are very important for flocculants to achieve the desired flocculation performance aimed at different target pollutants. Small suspended particles in water are aggregated into larger-sized flocs after the addition of flocculants, and then the flocs can be effectively removed via sedimentation. Flocculants are divided into two different classes according to the base of the materials, which determine whether they are inorganic flocculants, such as aluminum sulfate, poly(ferric chloride), and poly(aluminum chloride), or they are organic polymeric flocculants. Among them, inorganic and synthetic polymeric flocculants such as polyacrylamide and its derivatives have been widely used in water treatment,7,8 but residual metal ions or noxious monomers may result in the secondary pollution of water.9 Therefore, natural polymeric flocculants, such as starch,10,11 chitosan,12−14 cellulose15,16 and so on, have drawn more attention recently for the advantages of wide source, low price, low toxicity, and biodegradability. Also, many concerns were paid to starch-based flocculants16−18 because starch is one of the most abundant and popular natural polymers in the world and has been applied in various fields.19,20 As is known, most suspended colloidal particles in water carry charges, and the flocculation efficiency would usually be greatly improved when the flocculants have charges opposite to those of the particles because of the © 2014 American Chemical Society

significant effects of charge interactions. Thus, various ionic moieties have been introduced onto the starch backbone through chemical modification methods including esterification, etherification, and grafting.17,21−24 Nowadays, many kinds of efficient starch-based flocculants have been reported and applied to remove various contaminants from water.17,22−24 The flocculation mechanisms of the natural polymeric flocculants are important and interesting.25,26 However, previous studies investigating the flocculation mechanisms were mainly focused on the apparent flocculation performance, i.e., the removal efficiency of colloidal turbidity and organic pollutants.10,17,27 Little work has been done on the floc properties of starch-based flocculants during the flocculation processes. In fact, it is of great significance to study the floc properties including the floc size, compactness, and strength for understanding the flocculation mechanism and guiding the practical application well. There are strong correlations between the floc properties and the practical efficiencies of several water treatment unit processes such as sedimentation, flotation, and filtration.28,29 In the flocculation process, largersized and denser flocs are favored for higher sedimentation rate and lower sludge volumes. The floc strength would reflect the ability of flocs to resist shear force, and the regrowth ability of broken flocs during further particle−particle contact may improve the settling property. Flocs breakage and regrowth should also be taken into consideration, which has been the subject of in-depth research in recent years.29−32 Received: Revised: Accepted: Published: 59

September 11, 2014 November 24, 2014 November 24, 2014 November 24, 2014 DOI: 10.1021/ie503606y Ind. Eng. Chem. Res. 2015, 54, 59−67

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Industrial & Engineering Chemistry Research

Figure 1. Synthesis route of CMS-CTA flocculants and characteristics by FTIR and 1H NMR.

hydroxypropyl)trimethylammonium chloride (CTA) was obtained from Wuhan Yuancheng Technology Development Co., Ltd. They were both used directly without further purification. Kaolin, with an average particle diameter of 4.18 μm, was purchased from Sinopharm Chemical Reagent Co., Ltd., whereas hematite, with an average particle diameter of 0.18 μm, was obtained from Strem Chemicals Inc. The rest of the chemicals with analytical reagant grade were all purchased from Nanjing Chemical Reagent Co., Ltd. Distilled water was used in all experiments. 2.2. Synthesis and Characterization of Carboxymethyl Starch Etherified CTA (CMS-CTA). According to the synthesis route shown in Figure 1, two kinds of starch-based flocculants with the same chemically modified functional groups but different substitution degrees have been obtained by adjusting the mass feed ratio of CTA and monochloroacetic acid, which were named as CMS-CTA-P and CMS-CTA-N, respectively. The detailed synthesis method was described in previous work.42 Their chemical structures have been confirmed by Fourier transform infrared (FTIR; Bruker model IFS 66/S) and 1H NMR (Bruker AVANCE model DRX-500) spectra. Meanwhile, the ζ potential (ZP) of the starch-based flocculants was measured on a Malvern model Nano-Z zetasizer. ZP is a parameter illuminating electrochemical equilibrium on interfaces43 and plays an important role in the theory of aggregative stability. It brings detailed insight into the causes of dispersion, aggregation, or flocculation and can be applied to improve the formulation of dispersions, emulsions, and suspensions.44 The hydrodynamic radius (Rh) of the polymeric flocculants was also determined by a Brookheaven model BI200SM dynamic light scattering apparatus, and the final results were the average values of three runs. All characterization experiments were performed at room temperature. From FTIR spectra of both CMS-CTA-N and CMS-CTA-P, a strong characteristic peak appeared at 1482 cm−1, which was due to the methyl of the quaternary ammonium groups on

Until now, a number of different techniques, such as image analysis,33,34 laser scattering,35,36 coulter counters,37 and settling rates,38 have already been employed to determine the floc properties. Recently, the laser-scattering technique develops very fast in floc property measurement. The flocculation process could be monitored by the time evolution of the particle size and its distribution. Furthermore, the fractal dimension of flocs, indicating floc compactness, could be calculated from fractal theory39,40 because irregular and porous flocs formed in flocculation are supposed to be geometrically fractal. Three-dimensional fractal dimension (DF) could be obtained by the power law between the mass (M) and characteristic measurement of size (L).30,32 M ∝ LDF

(1)

For Euclidean objects, the value of DF will be 1 for a line and 2 for a two-dimensional planar shape; for a compact threedimensional shape, the DF value will be 3.41 In this present study, corn starch was modified by monochloroacetic acid followed by (3-chloro-2-hydroxypropyl)trimethylammonium chloride with different mass feed ratios. Two different kinds of starch-based flocculants, CMSCTA-P and CMS-CTA-N, have been successfully prepared. The flocculation properties were evaluated systematically using kaolin and hematite suspensions as synthetic wastewater under various pH conditions. Meanwhile, the floc properties, i.e., the floc size, fractal dimension, and floc strength during floc formation, breakage, and subsequent regrowth processes, were monitored in situ by a light-scattering system. The flocculation mechanisms of the starch-based flocculants have been discussed in detail on the basis of the charge properties, flocculation performance, floc properties, and conformational changes of the polymeric flocculants in solutions.

2. MATERIALS AND METHODS 2.1. Materials. Natural corn starch was purchased from Binzhou Jinhui Corn Development Co., Ltd. (3-Chloro-260

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Industrial & Engineering Chemistry Research CTA, and a sharp and narrow band at 1600 cm −1 corresponding to the carboxyl group was also observed.45,46 In 1H NMR spectra, besides the proton signals at 5.3 ppm for H1 and 3.3−3.9 ppm for H2−H6 on the starch backbone, two characteristic peaks at 3.20 and 3.95 ppm were ascribed to the proton signals of the methyl groups of quaternary ammonium groups and methylene of the carboxymethyl groups, respectively.46 The aforementioned results of FTIR and 1H NMR both confirmed that anionic and cationic moieties, i.e., CTA and monochloroacetic acid, have been introduced onto the starch backbone successfully. Moreover, on the basis of the integrated areas of the corresponding characteristic peaks in the 1 H NMR spectra,42 the contents of functional groups on the starch-based flocculants were calculated according to the following equations, respectively. DS(CTA) =

under magnetic stirring at room temperature. Then the flocculation experiments were conducted using 1 L jars and a six-place programmed paddle flocculator model TA6 (Wuhan Hengling Tech. Co., Ltd.) to evaluate the flocculation performance of the starch-based flocculants at room temperature. The procedure of the jar tests included a rapid mixing at 200 rpm for 5 min, a slow mixing at 50 rpm for 15 min, and finally a settling period of 40 min to ensure the flocculation equilibrium. Then the supernatant was collected immediately and analyzed for both transmittance by a UNIC model UV2100 UV−visible spectrophotometer at 550 nm and ZP. The turbidity removal efficiency (TRE) of the flocculants was defined as TRE (%) =

area(methyl on CTA)/9 × 100% area(H1 in the starch backbone)

DS(carboxymethyl group) area(methylene on the carboxymethyl group)/2 area(H1 in the starch backbone) × 100%

(4)

where Ttreated and Traw are transmittances of treated suspensions after reaching flocculation equilibrium and raw wastewater, respectively. 2.3.2. Floc Monitoring. The floc properties were determined in situ by a light-scattering system mode of Malvern Mastersizer 2000. The flocculation process could be easily monitored because the suspension was pumped into the sample cell of the apparatus continuously. The floc formation, breakage, and regrowth in the entire process under various conditions were determined and studied in detail by adjusting the flocculation procedure, which was divided into four stages in this work. After the desired dosage of flocculants was added, the suspension was stirred at 200 rpm for 5 min to ensure sufficient mixing of the flocculants with wastewater (stage 1), followed by slow stirring at 50 rpm for 15 min for floc growth (stage 2). Further, the suspension was subjected to an increased shear rate for 10 min for floc breakage (stage 3) and then finally returned to slow stirring at 50 rpm for 10 min for floc regrowth (stage 4). Separated tests were performed at various increased shear rates (RPM) of 50, 100, 150, and 200 rpm, respectively. The floc size was obtained directly from the data processing software of the Malvern Mastersizer 2000 system. The fractal dimension DF was determined from the relationship between the light intensity I and the scattering vector Q30 as shown in the following:

(2)

=

Ttreated − Traw × 100% 100 − Traw

(3)

The substitution degrees of CTA and the carboxymethyl groups were 43.2% and 20.3% in CMS-CTA-P and 27.4% and 62.3% in CMS-CTA-N, respectively, which were far different with each other. ZPs of CMS-CTA-P and CMS-CTA-N have been performed also and are shown in Figure 2, which further confirms the

I ∝ Q −D F

(5)

The scattering vector Q is given by the following function:

Figure 2. ZPs of CMS-CTA-P, CMS-CTA-N, kaolin and hematite suspensions.

Q=

structural difference between the two aforementioned starchbased flocculants. CMS-CTA-P exhibits obvious charge characteristics of a cationic polyelectrolyte, but CMS-CTA-N bears a negative surface charge under most of the measured pH range, which is due to the far different molar ratios of the substitution degrees of cationic and anionic moieties on respective flocculants. 2.3. Flocculation Performance. 2.3.1. Jar Tests. For the flocculation tests, a 1.0 g/L kaolin or hematite suspension was designed as synthetic wastewater, which was stirred at 200 rpm and ultrasonicated for 3 min to obtain a homogeneous mixture before use. The pH of the suspensions was adjusted by dilute HCl or NaOH aqueous solutions. Stock solutions of CMSCTA flocculants with a weight concentration of 0.1% were prepared freshly with distilled water and fully dissolved in 5 min

4πn sin(θ /2) λ

(6)

In eq 6, n stands for the refractive index of the dispersion medium (1.33 for water in this work), λ is the radiation wavelength (633 nm), and θ is the scattering angle that was previously known in the data processing software. Plotting I against Q on a log−log scale according to eq 6 gives a straight line within a certain range, the absolute value of the slope of which is DF. The coefficient (γ) reflecting the floc strength could be obtained from the aforementioned breakage process. There is a power-law relationship between the floc sizes at the end of the floc breakage period (Dv,breakage) and RPM32 as shown in the following: Dv,breakage ∝ RPM−γ 61

(7) DOI: 10.1021/ie503606y Ind. Eng. Chem. Res. 2015, 54, 59−67

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Industrial & Engineering Chemistry Research Higher γ would indicate lower floc strength, i.e., weaker resistance to breakage.

shown in Figure 2, CMS-CTA-P had a surface charge opposite to that of kaolin particles but the same as that of the hematite ones under the selected pH conditions for respective flocculation experiments, and a similar rule was also found in CMS-CTA-N. It could be concluded that the two starch-based flocculants always bear surface charge characteristics opposite to those of their effective synthetic wastewater but the same as those of their ineffective one, indicating that the charge neutralization flocculation effect played an important role in the current systems. In addition, the transmittance slightly declined with the dosage when the dosage of CMS-CTA-P was more than 0.5 mg/L at pH 4 for the kaolin suspension (Figure 3a), which was due to the restabilization effect at excessive dosage of the flocculants. The more detailed flocculation mechanism will be discussed in the following part. Furthermore, the optimal dosages of the starch-based flocculants at various pH conditions have been observed from Figure 3, which corresponded to the maximal transmittance of the treated water after flocculation equilibrium was reached. The optimal dosages listed in Table 1 also showed distinct pH dependences. The optimal dosages of CMS-CTA-P in the kaolin suspension increased with increasing pH but those of CMS-CTA-N in the hematite suspension decreased. This may be due to the intensity changes of the charge attraction between flocculants and pollutants with the pH. On the basis of the ZP measurement shown in Figure 2, the ZP of kaolin particles would be more negative but that of CMS-CTA-P turned less positive simultaneously with increasing pH, resulting in a reduction of the charge attraction and an increase of the flocculant dosage, whereas for CMS-CTA-N, the aforementioned effects were opposite and the charge attraction was enhanced from pH 4 to 7, causing a decrease of the optimal dosage. To investigate the flocculation mechanism of the starchbased flocculants, which is significant for guiding their practical applications in water treatment plants, the ZP and transmittance of the supernatants as a function of the flocculants’ dosages were determined simultaneously under various pH conditions and are illuminated in Figure 4 because the ZP of the supernatant can be used as an important parameter to judge the mechanism involving in the flocculation process. It was found that the absolute values of the supernatants’ ZPs decreased and went to zero with an increase of the dosage at the beginning not only for CMS-CTA-P in the kaolin suspension (Figure 4a−c) but also for CMS-CTA-N in the hematite suspension (Figure 4d,e) and then increased further across zero as excessive flocculants were dosed. As mentioned earlier, charge neutralization for these two starched-based flocculants had great effects on the flocculation processes. The variation trends of the ZP along with the dosage seemed reasonable. The oppositely charged flocculants neutralized and diminished the surface charge of the suspended particles in water, resulting in destabilization of the pollutants and further floc formation. Then overdosed flocculants would be adsorbed onto the primary flocs, causing regrowth of the ZP. However, there were two abnormal experimental facts found in Figure 4. One was that the ZP of the supernatant did not reach to zero at optimal dosage in each system, as shown in Table 1; the other was that the flocculation efficiency did not obviously decrease for the restabilization effect at excessive dosage of flocculants, which showed a wide “flocculation window”. The aforementioned results indicated that the flocculation processes may not follow the simple charge neutralization mechanism, but

3. RESULTS AND DISCUSSION 3.1. Flocculation Performance. The flocculation performance of these two starch-based flocculants has been studied. As described in the experimental part, two types of synthetic wastewater with fully different characteristics of surface charge, i.e., kaolin and hematite suspensions, were employed for comprehensive investigation of their flocculation behavior. On the basis of the ZP measurement shown in Figure 2, the kaolin suspension bears a full negative surface charge in the whole measured pH range, but the hematite suspension was positive below pH 9.5. Given that the dosage and pH are the two main influencing factors in the flocculation process, the effects of dosage on the flocculation performance of the two starch-based flocculants under various pH conditions were investigated and are shown in Figure 3.

Figure 3. Flocculation performances of (a) CMS-CTA-P and (b) CMS-CTA-N in kaolin and hematite suspension systems.

From Figure 3a, CMS-CTA-P exhibited prominent flocculation efficiency to the kaolin suspension but had almost no effect on the hematite suspension. More interestingly, CMSCTA-N showed a flocculation performance fully inverse to CMS-CTA-P in the two aforementioned types of synthetic wastewater (Figure 3b). On the basis of the ZP measurement 62

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Industrial & Engineering Chemistry Research Table 1. Flocculation Performance and Floc Properties of CMS-CTA Flocculants flocculant

synthetic wastewater

CMS-CTA-P

kaolin suspension (1.0 g/L)

CMS-CTA-N

hematite suspension (1.0 g/L)

pH

optimal dosagea (mg/L)

TREmaxb (%)

floc sizec (μm)

DFc

γ

Rh (nm)

ZPd (mV)

4 7 11 4 7

0.25 0.60 0.80 4.00 3.00

92.8 97.2 97.7 98.0 98.6

157 175 119 45 95

2.40 2.41 1.85 2.48 2.50

0.476 0.353 2.243 1.046 0.272

139.09 118.85 74.39 60.11 176.46

−9.5 −14.0 −15.0 6.6 9.2

The optimal dosage corresponded to the maximal transmittance of the treated water after flocculation equilibrium was reached on the basis of Figure 3. bTREmax was the maximal transmittance of the treated water after flocculation equilibrium was reached at the optimal dosage on the basis of Figure 3. cThe listed floc size and fractal dimension (DF) was at the respective steady state on the basis of Figure 6. dZP at the optimal dosage on the basis of Figure 4. a

Figure 4. Transmittance and the ZP of the supernatants as a function of the dosages of CMS-CTA-P for the kaolin suspension (a−c) and CMSCTA-N for the hematite suspension (d and e).

patching occurred. The polymeric flocculants were attracted and adsorbed onto the surface of the oppositely charged suspended particles easily and rapidly, resulting in heterogeneous coverages. Furthermore, various microregions on the suspended particles bore different charges, and collisions among these particles in water led to further aggregation and formation of larger-sized flocs through electrostatic interactions.

Therefore, the ZP of the supernatant could not reach zero at the optimal dosage and a wide “flocculation window” was observed. Moreover, the bridging mechanism due to the long macromolecular chains of the starch-based flocculants also had some contributions to the aforementioned effects. 3.2. Floc Properties. Besides the apparent flocculation performance as discussed before, the floc properties, such as the 63

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Figure 5. Floc formation, breakage, and regrowth in various flocculation processes at different pH conditions: CMS-CTA-P for the kaolin suspension (a−c) and CMS-CTA-N for the hematite suspension (d and e).

floc size, fractal structure, and strength, greatly affected the efficiency of separation and purification operation in the actual water treatment plants and were also determined to explore the flocculation mechanism. The time courses of floc formation, breakage, and regrowth have been established at corresponding optimal dosages under various pH conditions by an in situ light-scattering technique to understand the flocculation kinetics processes well. The whole flocculation experiments were divided into four stages: a rapid stirring stage for mixing the flocculants with wastewater well, a slow stirring stage for floc formation and growth, then a third stage with an increased shear rate for floc breakage, and finally a slow stirring stage for floc regrowth, which are described in detail in the experimental part. The floc size versus time curves under various conditions in whole flocculation processes are illustrated in Figure 5. 3.2.1. Floc Formation and Fractal Structure. For a better comparison, the time dependences of the floc size and DF in the first two stages are summarized and deduced in Figure 6 according to Figure 5, which contained a rapid mixing at 200 rpm for 5 min and a slow mixing at 50 rpm for 15 min as regular flocculation units for floc formation. The floc size was

provided directly by the data processing software. DF was calculated according to eq 5 by the linear fitting of log Q − log I,30 which is a powerful parameter to reflect the space-filling capacity of irregular and porous flocs. An example for the calculation of DF is shown in Figure 7. From Figure 6a, it was found that the floc size at steady state slightly increased from pH 4 to 7 because of the stronger patching effect at pH 7 since the ZP at the optimal dosage in neutral conditions was farther to zero than that in acidic conditions on the basis of Figure 4a,b. However, the floc size under alkaline conditions decreased to a much smaller one for CMS-CTA-P in the kaolin system, ascribed to the weakened electrostatic interactions between flocculants and kaolin particles as discussed above. In contrast, increasing pH resulted in larger-sized flocs (Figure 6c) for CMS-CTA-N in the hematite system. However, the two aforementioned starchbased flocculants still had something in common; i.e., both CMS-CTA-P and CMS-CTA-N produced relatively larger-sized flocs at lower optimal dosages from Table 1 and had a wide “flocculation window”. It was indicated that other effects 64

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Figure 6. Particle size and fractal dimension of flocs in various flocculation processes at different pH conditions: CMS-CTA-P for the kaolin suspension (a and b) and CMS-CTA-N for the hematite suspension (c and d).

nated, resulting in a reduction of intramolecular electrostatic interactions but an improvement of intramolecular electrostatic repulsion; thus, CMS-CTA-P bore a higher expanded chain conformation at lower pH level,47,48 resulting in improved approachability to contaminants and larger Rh. As for CMSCTA-N, the dominant anionic groups on the starch backbone would also be protonated at acidic conditions; thus, the chain conformation of CMS-CTA-N became compact. As the solution pH rose to higher values, the carboxymethyl groups on CMS-CTA-P were deprotonated, resulting in enhanced intramolecular electrostatic interactions; therefore, the CMSCTA-P chain was transferred into a collapsed state of which Rh was decreased. However, the deprotonation effects on CMSCTA-N would result in a more extended chain conformation and higher Rh. More interestingly, for CMS-CTA-P in the kaolin suspension at pH 4 and 7 from Figure 6a, the floc size showed a downward trend after a sharp increase in the first flocculation stage within 5 min and then increased slowly to a steady-state region at the slow mixing stage (stage 2). At the very beginning, very rapid aggregation of suspended particles in water due to the addition of flocculants took place, resulting in much unoccupied space existing in the interior of the rapidly formed primary flocs. Then, the following structural rearrangements of the flocs engendered the subsequent decline in size. This viewpoint was further confirmed by the sustained growth of DF in Figure 6b, indicating that the floc structure became more and more compact. However, the phenomenon of decreasing the floc size after a rapid increase could not be observed at pH 11 on the basis of Figure 6a. Moreover, DF at acidic and neutral conditions was similar but much higher than that at alkaline conditions from Figure 6b. The aforementioned experimental facts were ascribed to a reduction of the charge attraction and a

Figure 7. Example figure for the calculation of DF by eq 5.

besides charge attraction, such as bridging flocculation, may contribute to the floc growth. Furthermore, dynamic light scattering was employed to determine the pH dependence of the hydrodynamic radius of these two starch-based flocculants, and the results are also summarized in Table 1. Interestingly, Rh of CMS-CTA-P decreased with an increase of the pH, but that of CMS-CTA-N increased on the contrary, indicating that the two ionic flocculants had opposite pH sensitivity and chain conformation bearing different pH dependences in water. Upon combination of the variation trends of the floc size as mentioned above, more extended conformation of flocculants with enhanced bridging effects resulted in larger-sized flocs not only for CMSCTA-P but also for CMS-CTA-N. Although the quaternary ammonium salt groups on the starch-based flocculants are almost independent of the pH, the carboxymethyl groups are sensitive. Under acidic solutions, the small amount of carboxymethyl groups on CMS-CTA-P were mainly proto65

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to the fact that the interactions within the primary flocs were mainly charge attractions and not van der Waals forces. The charge interactions may be strong enough to restructure the flocs.51

weakened bridging effect for a more compact conformation of polymeric flocculants at higher pH conditions. The curve of the floc size at pH 11 could be divided into three steps: lag, growth, and steady state.49 The last step generally reflected a balance between the floc growth and breakage at a given shear rate. As for CMS-CTA-N in the hematite suspension, the decline of the floc size at the first flocculation stage did not appear in Figure 6c. This may be due to the initial smaller-sized hematite particles with less and smaller empty space existing in the primary floc, resulting in insignificant internal structural rearrangement subsequently. This result was also confirmed by the fact that DF in the hematite system was larger than that in the kaolin system at the lag step in Figure 6b,d. Furthermore, from Table 1 and Figure 6c,d, it was shown that increasing pH would cause larger size and DF of flocs but lower optimal dosage. This was ascribed to the fact that the dominant carboxymethyl groups on CMS-CTA-N were deprotonated with an increase of the pH, resulting in a more expanded chain conformation of the polymeric flocculant as well as an improvement of the electrostatic attractions between CMSCTA-N and hematite particles. Accordingly, the bridging and charge neutralization effects were both enhanced at neutral conditions. 3.2.2. Floc Breakage and Regrowth. Subsequently, a higher shear rate was applied for floc breakage and then back to the regular slow stirring for floc regrowth in the last two flocculation stages. From Figure 5, the variation trends of the floc sizes under different pH conditions were quite similar for CMS-CTA-P in the kaolin system. The breakage of kaolin flocs typically included two steps: the sudden decrease of the floc size at the beginning of the third stage was attributed to the fragmentation breakage mechanism, and then a gradual decrease in the floc size was a result of the erosion breakage mechanism.50 The coefficients of floc strength (γ) under various conditions were also derived on the basis of eq 7 and are displayed in Table 1. γ at alkaline conditions in the kaolin suspension was much larger than those at acidic and neutral conditions, indicating that the flocs under pH 11 had the lowest resistance to breakage. Furthermore, the obvious fluctuation of the determined floc size at higher pH (Figure 5c) during the second flocculation stage further confirmed the loose and facilely broken floc structure. At acidic and neutral conditions, lower γ of around 0.4 exhibited more stable floc structures and stronger floc strength. The floc strength highly depended on the interaction force among primary particles in flocs. The higher floc strength at pH 4 and 7 was ascribed to enhanced charge attraction and stronger topological bridging connection for a more extended flocculant conformation. As for CMS-CTA-N in the hematite suspension, the opposite pH dependence of the floc breakage behavior was observed in comparison to that of CMS-CTA-P in the kaolin suspension as mentioned above, but they obeyed similar intrinsic mechanisms. The more obvious decline in the hematite floc size during the floc breakage stage and higher γ at acidic conditions mainly resulted from a reduction of the charge attraction among the primary flocs and weakened bridging effects for a more compact conformation of the polymeric flocculants at lower pH conditions. Interestingly, not only CMS-CTA-P but also CMS-CTA-N mostly returned to or near to their respective level before breakage after sustained exposure to high shear rate, also shown in Figure 5, illuminating good regrowth abilities, which was due

4. CONCLUSION Above all, two kinds of starch-based flocculants with the same chemically modified functional groups but far different substitution degrees were successfully obtained by controlling the mass feed ratio of the two modifiers. CMS-CTA-P showed satisfactory flocculation performance (low optimal dosage and high TRE) and good floc properties (large size, compact structure, high strength, and fine regrowth ability) only in the kaolin system specifically at acidic and neutral conditions, whereas CMS-CTA-N was highly efficient only in the hematite system specifically at neutral conditions. The inverse flocculation performance in the two aforementioned suspensions with opposite surface charges was intrinsically due to the different structural characteristics between those two starchbased flocculants, resulting in various charge properties and pH dependences of the polymeric flocculants’ conformation in water. On the basis of the aforementioned experimental facts, it was found that multiple flocculation mechanisms were involved in the flocculation processes, i.e., patching and bridging effects.

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AUTHOR INFORMATION

Corresponding Author

*Tel and Fax: 86-25-89681272. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by Major Science and Technology Program for Water Pollution Control and Treatment (Grant 2015ZX07204-002), the Natural Science Foundation of China (Grant No. 51378250), and the Fundamental Research Funds for the Central Universities (Grant 20620140494).

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