Evaluation of the Flocculation Characteristics of Polyacrylamide

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Evaluation of the Flocculation Characteristics of Polyacrylamide Grafted Xanthan Gum/Silica Hybrid Nanocomposite Soumitra Ghorai,† Asish Sarkar,† Asit Baran Panda,‡ and Sagar Pal†,* †

Polymer Chemistry Laboratory, Department of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India Nano-structure Materials Division, Central Glass & Ceramic Research Institute (CSIR-CGCRI), Kolkata 700032, India

‡

S Supporting Information *

ABSTRACT: This manuscript illustrates the feasibility of polyacrylamide grafted xanthan gum/silica based nanocomposite toward its potential application as high performance flocculant for treatment of synthetic effluents and mine wastewater. The flocculation performance of the nanocomposite was systematically evaluated by floc size measurement as well as by traditional turbidity and settling velocity measurement. The flocculation kinetics is in good agreement with the aggregation of particle and particle collision model simultaneously. Furthermore, the effect of flocculant dosage on the synthetic effluent (kaolin and iron-ore suspensions) suggests that the bridging flocculation mechanism is predominating here. The graft copolymer-based nanocomposite also possesses a comprehensive color removal ability from mine wastewater. functional properties.8 In the graft copolymer-based flocculants, the approachability of the synthetic polymer chains toward metallic and nonmetallic contaminants increased significantly and thus they are bestowed with highly efficient attributes.7a However, modified polysaccharides (such as polyacrylamide grafted polysaccharides) have certain limitations such as lower hydrodynamic volume, lower hydrodynamic radius, and poor thermal stability as well as low surface area. Because of these limitations, inorganic nanoparticle-modified polysaccharidebased nanocomposites have been developed, which showed effective enhancement in fine solid flocculation.9 Owing to the synergistic effect between the interaction of inorganic nanofiller and organic polymer matrix, the hybrid nanocomposites show a crucial change in mechanical and thermal properties as well as high surface area and hydrodynamic radius in comparison with a pure organic polymer, which makes nanocomposites to be an efficient flocculant/adsorbent for the treatment of wastewater.9 Recently, our laboratory developed a novel biodegradable hybrid nanocomposite (XG-g-PAM/SiO2), which showed excellent efficiency as an adsorbent for removal of Pb (II) ion from aqueous solution.10 This article focuses on the application of the developed nanocomposite as a high performance flocculant for treatment of synthetic effluents as well as mine wastewater. In addition, to investigate the effect of flocculant dosage on the flocculation performances, we have designed the kinetics of flocculation process based on the aggregation of particle and collision frequency model. Furthermore, the flocculation properties of the XG-g-PAM/SiO2 nanocomposite have been studied in detail based on the polymer bridging mechanism. To our knowledge, this is the first report on modified xanthan gum and silica-based nanocomposite, which finds potential application in the field of flocculation.

1. INTRODUCTION In recent years, polymer-based nanocomposites have been extensively used as adsorbents as well as flocculants for the treatment of wastewater.1 The hybrid nanocomposites have been developed using inorganic nanofiller and organic polymer.1 They have remarkable properties that depend not only on their individual components but also on their morphological and interfacial characteristics.2 The most common procedure for the synthesis of hybrid materials is based on the sol−gel process for the formation of an inorganic network onto the backbone of a polymer matrix.3 Recently, work has been carried out on the synthesis of monodispersed silica nanoparticles because of their uniform size, shape, composition, and functional properties as well as their efficiency as a surface-modified substrate.4 Also, the high surface area of silica nanoparticles would enhance the hydrodynamic volume as well as hydrodynamic radius of the nanocomposite in solution, which affects the flocculation efficiency. Wastewater and industrial effluent treatment require removal of a wide range of toxic derivatives, in particular heavy metals, suspended particles, pesticides, surfactant, dyes, and aromatic molecules for purification and possible reuse.5 Flocculation is an efficient technique for primary treatment of wastewater and industrial effluent.6 It is promoted by the addition of a minute quantity of chemicals, known as flocculants. Both natural and synthetic flocculants are extensively used for wastewater treatment, although they have some advantages as well as limitations. In the past, several attempts have been made to combine the best properties of both by grafting synthetic polymers onto the backbone of natural polysaccharides.7 One of the main advantages obtained is the reduced biodegradability, due to the drastic change in the original regular structure of the polysaccharide as well as the increased synthetic polymer content within the graft copolymer.7a It has been reported that by grafting flexible polyacrylamide chains onto natural polysaccharide backbone, it is possible to develop efficient flocculants having controlled biodegradability, shear stability, and improved © 2013 American Chemical Society

Received: Revised: Accepted: Published: 9731

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Table 1. Composition of Various Flocculants and Results of Weight Average Molecular Weight (Mw), Hydrodynamic Radius (RH), Settling Velocity, and Average Floc Size of Various Flocculantsa settling velocity (cm/sec) of suspensions

polymer without flocculant XG XG-g-PAM-i XG-g-PAM-ii XG-g-PAM-iii XG-g-PAM-iv XG-g-PAM-v XG-g-PAM-vi XG-g-PAM/SiO2-1 XG-g-PAM/SiO2-2 XG-g-PAM/SiO2-3 XG-g-PAM/SiO2-4 a

initiator conc. (mole ×10−5)

0.925 1.85 3.70 5.55 1.85 1.85 1.85 1.85 1.85 1.85

AM conc. (mole)

0.14 0.14 0.14 0.14 0.17 0.21 0.17 0.17 0.17 0.17

SiO2 concn (wt %)

0.53 0.70 0.80 0.95

Mw (g·mol−1) 4.06 × 105 1.81 × 106 8.71 × 106 6.03 × 106 4.68 × 106 9.39 × 106 5.45 × 106 1.94 × 107 4.09 × 107 3.65 × 107 1.07 × 107

average floc size (nm) of suspensions

RH (nm)10

kaolin

Fe ore

kaolin

Fe ore

7.85 7.95 14.2 12.15 10.70 15.70 11.95 18.85 21.90 18.95 17.75

0.035 ± 0.042 0.332 ± 0.063 0.423 ± 0.012 0.641 ± 0.031 0.572 ± 0.042 0.504 ± 0.034 0.721 ± 0.063 0.471 ± 0.052 0.936 ± 0.086 1.194 ± 0.138 1.061 ± 0.096 0.808 ± 0.074

0.029 ± 0.012 0.282 ± 0.023 0.407 ± 0.062 0.563 ± 0.044 0.528 ± 0.023 0.501 ± 0.046 0.595 ± 0.068 0.454 ± 0.019 0.736 ± 0.037 0.945 ± 0.082 0.838 ± 0.122 0.671 ± 0.091

123.3 ± 2.4 223.9 ± 4.2 245.2 ± 4.8 572.4 ± 3.9 257.1 ± 2.7 712.3 ± 6.8 1464.1 ± 9.2 702.1 ± 6.4 2171.2 ± 10.6 3055.4 ± 11.8 2321.3 ± 11.2 2004.4 ± 9.7

397.1 ± 4.6 635.3 ± 7.7 811.4 ± 11.4 1028.2 ± 12.8 947.5 ± 11.2 930.7 ± 10.6 1093.2 ± 10.7 901.2 ± 8.9 1146.1 ± 13.2 1375.2 ± 12.6 1151.1 ± 11.3 1110.4 ± 10.2

Results (for settling velocity and average floc size) represented here are ± SD (n = 3).

2. EXPERIMENTAL SECTION Materials. Xanthan gum and tetraethylorthosilicate (98% TEOS) were used as received from Sigma-Aldrich, Germany. Acrylamide was procured from E. Merck, Mumbai, India. Potassium persulfate was purchased from Qualigens Fine Chemicals, Mumbai, India. Kaolin (suspension zeta potential = −16.9 mV at pH = 7; particle size, 123.3 nm) was obtained from B. D. Pharmaceutical Works Pvt. Ltd., Howrah, West Bengal, India, and iron ore (suspension zeta potential = −13.3 mV at pH = 7; particle size, 397 nm) was obtained as a gift sample from Tata steel, Jamshedpur, India. Mine wastewater (suspension zeta potential = −20.1 mV at pH = 7) was collected from a mine near Dhanbad, India. Synthetic polyacrylamide was purchased from Otto Chemicals, India. Three different commercial flocculants, namely, FL 920 (nonionic in nature, Mw = 1.80 × 107 g·mol−1), FL 424 (cationic in nature, Mw = 4.46 × 10 6 g·mol−1) and FL 934 (anionic in nature, Mw = 3.79 × 106 g·mol−1) were collected from Tata Steel, Jamshedpur, India. All the chemicals used were as received without further purification. For all the experiments, double distilled water was used. Preparation of Polyacrylamide Grafted Xanthan Gum/ Silica Nanocomposite (XG-g-PAM/SiO2). The graft copolymerization of polyacrylamide onto xanthan gum was carried out by a free-radical polymerization technique in nitrogen atmosphere using potassium persulfate (KPS) as initiator. By varying the reaction parameters, various grades of graft copolymers have been synthesized (Table 1) and the best one was selected with respect to its higher % GE, hydrodynamic radius, and intrinsic viscosity. Afterward, the nanoscale Stöber silica (sol−gel process using Stöber method) was incorporated in situ on the surface of the graft copolymer by hydrolysis and condensation of TEOS in a mixture of alcohol, water, and ammonia.10 Characterization. The hydrodynamic volume of polymers and nanocomposites in aqueous solution was measured by measurement of intrinsic viscosity using the Ubbelohde viscometer in 1 M sodium nitrate solution. FTIR spectra of the various samples were recorded using KBr pellet method (model IR-Perkin-Elmer, Spectrum 2000). The scan range was 400 and 4000 cm−1. The weight average molecular weights (MW) of xanthan gum, graft copolymers, and various nanocomposites was determined using a Debye Plot by static light scattering (SLS)

analysis using a light scattering spectrophotometer (Nano ZS, Malvern, UK). The surface morphology and microstructure of XG, XG-g-PAM, and the nanocomposite was performed using field emission scanning electron microscopy (Zeiss Ultra 55cv FESEM) operating at 25 °C. Thermal analysis (TGA/DTG) of xanthan gum, graft copolymer, and the nanocomposite was carried out by thermogravimetric analysis (TGA Q 500) with a heating rate of 5 °C/min in nitrogen atmosphere. Investigation of Flocculation Characteristics. Flocculation Characteristics Using Synthetic Effluents. The flocculation performance of xanthan gum, graft copolymers, nanocomposites, synthetic PAM, and commercial flocculants was carried out using the standard jar test and settling test method. To explicate the flocculation mechanism, we have used two types of suspensions (iron ore and kaolin). The floc size was measured using DLS analysis (Zeta Sizer Nano ZS, Malvern, UK). The zeta potential of various suspensions (before and after flocculation) was measured using the Zeta Sizer Nano ZS (Malvern, UK). Jar Test Method. A conventional jar test apparatus consists of a flocculator (Make: Gon Engineering Works, Dhanbad, India) and a turbidity meter (Digital Nephelo-Turbidity Meter 132, Systronics, India). Suspensions (0.25 wt % ) of iron ore and kaolin (prepared by mixing 1 g in 400 mL of distilled water) were used for the flocculation study. The suspensions were taken in a 1 L beaker and the flocculant dosages were added in solution form. The following procedure was uniformly applied: immediately after the addition of flocculant, the suspension was stirred at a constant speed of 75 rpm for 2 min, followed by low stirring at 25 rpm for 3 min. The flocs were then allowed to settle for 15 min. At the end of the settling period, clean supernatant liquid was drawn from a depth of 1 cm, and its turbidity was measured using a turbidity meter. Distilled water was used as reference. The flocculant dosage was varied from 0.25 ppm to 3 ppm. Settling/Column Test Method. The column test employs a 100 mL stoppered graduated cylinder (height, 40 cm; inner diameter, 2 cm) and stopwatch. First the suspension was taken in the cylinder, and then the polymer solution was added into it. The cylinder was inverted 10 times for thorough mixing. After that, the cylinder was set upright and the height of interface between supernatant water and settling flocs was measured at regular interval. 9732

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Kinetics Study. A concentration of 1 wt % suspension of iron ore and kaolin was prepared by mixing 4 g in 400 mL of distilled water with vigorous stirring using the jar test apparatus for 30 min. Afterward, the desired dose of flocculant was added and the suspension was stirred dynamically at a constant speed of 100 rpm for 5 min. The flocs were then allowed to settle down for predetermined time periods and then a fixed volume was withdrawn by suction, filtered, dried, and weighed. The ionic strength of the suspension was maintained using 0.01 M NaCl. Treatment of Mine Wastewater. The flocculation efficacy of different polymers and nanocomposites was investigated using mine wastewater with the jar test procedure but with 200 mL of wastewater using an optimized dosage (9 ppm) of flocculant. The supernatant liquids were drawn from the different sets of experiments and were subjected to the following chemical/ environmental analyses: (i) Turbidity was measured using calibrated nephelo-turbidity meter. (ii) Total solids (TS), total dissolved solids (TDS), and Total suspended solids (TSS) were measured by gravimetric method.11 (iii) COD was determined by the mixing of each 50 mL sample with 25 mL of digestion solution (sulfuric acid/mercuric sulfate/potassium dichromate/ water) and 10 mL of acid reagent (sulfuric acid + silver sulfate) followed by heating in a COD digestion apparatus (make: REMCO, Kolkata, India) at 150 °C for 2 h. The mixture was cooled and the COD value was measured using a UV−visible spectrophotometer (Shimadzu, model: UV 1800). (iv) Color measurement of the treated and untreated mine wastewater has been determined and represented graphically (absorbance vs wavelength) using a UV−vis spectrophotometer (Shimadzu, model: UV 1800)

with varying amount of silica (Figure S1, Supporting Information), XG-g-PAM/SiO2-2 nanocomposite showed maximum intrinsic viscosity due to better polymer−silica interaction. In the FTIR spectrum of XG (Figure S2.a, Supporting Information), the broad peak at 3412 cm−1 and a small peak at 2928 cm−1 are attributed to the stretching vibrations of O−H and C−H, respectively. The peak at 1737 cm−1 is due to CO stretching of alkyl esters of XG. The peaks at 1060 cm−1 and 904 cm−1 are assigned to CH2−O−CH2 stretching vibrations. In the FTIR spectrum of XG-g-PAM-v (Figure S2.b, Supporting Information), it has been observed that there are few additional peaks present in comparison with the polysaccharide. These peaks are assigned to amide-I (1668 cm−1), amide-II (1601 cm−1), and C−N (1445 cm−1) stretching vibrations, which confirms the grafting of polyacrylamide chains onto the XG backbone. However, the spectrum of XG-g-PAM/SiO2-2 (Figure S2.c, Supporting Information) also has few additional peaks compared to graft copolymer. A broad peak at 3776 cm−1 is assigned to the silanol OH group. Peaks at 1112 and 1010 cm−1 are attributed to Si−O−Si and Si−OH linkages, respectively. The presence of these extra peaks indicates the presence of SiO2 particles on the nanocomposite structure. The weight average molecular weight (Mw) of the xanthan gum, various graft copolymers, and all the nanocomposites (Figures S3, S4, and S5, respectively, Supporting Information) were determined from Debye plot using SLS analysis, and Mw values have been reported in Table 1. It is evident that all graft copolymers have higher molecular weight in comparison with xanthan gum because of the presence of grafted PAM chains. However, the graft copolymer having a higher percentage GE, i.e., XG-g-PAM-v,10 has the maximum Mw, which indicates the presence of longer PAM chains in that composition (i.e., XG-gPAM-v). Further, all the nanocomposites have relatively high Mw, which is because of the presence of nano filler onto the surface of the matrix. Again, the nanocomposite XG-g-PAM/ SiO2-2 has the highest molecular weight, which may be because of the better interaction between the nanofiller and polymer matrix. The field emission scanning electron microscopy (FE-SEM) of XG, XG-g-PAM-v and XG-g-PAM/SiO2-2 is shown in Figure S6, Supporting Information. The grafting introduces the change in the surface morphology and also its physical and chemical characteristics. After grafting with PAM, the granular morphology of xanthan gum (Figure S6a) was distorted and the graft copolymer (i.e., XG-g-PAM-v) showed a unique fibrillar structure (Figure S6b). This is because the grafted PAM chains were agglomerated randomly onto the xanthan gum backbone, which makes the morphology fibrillar. The FE-SEM micrograph of XG-g-PAM/SiO2-2 (Figure S6c) shows that silica particles (marked by a blue arrow) are in the form of white round beads in the clustered irregular morphological surface of the graft copolymer. This observation indicates that the silica particles have been uniformly dispersed into the clustered irregular morphological surface of the graft copolymer matrix without agglomeration and formed a homogeneous composition. This explains the interfacial interaction within the polymer matrix and silica nanoparticle. Figure S7 (Supporting Information) explains the TGA and DTG analysis of XG, XG-g-PAM-v, and XG-g-PAM/SiO2-2 nanocomposite. TGA curve of xanthan gum (Figure S7a, Supporting Information) indicates two distinct zones of weight loss. The initial weight loss occurred in the temperature region of 0−100 °C, which may be due to the loss of absorbed water. The

3. RESULTS AND DISCUSSIONS Synthesis. The probable mechanism for the formation of the graft copolymer is based on the fact that initially KPS generates free radical sites on the XG backbone, and these active free radicals react with monomer (acrylamide) to produce the graft copolymer. The hybrid nanocomposite was prepared by in situ incorporation of nanosilica (prepared through hydroxylation of TEOS followed by condensation of the formed monomers silanols) onto the surface of optimized grade XG-g-PAM matrix.10 The nanocomposite formation mechanism is based on the assumption that there is interaction between the hydroxyl and amide groups of modified polysaccharides and the silanol groups of silica particles through hydrogen bonding.10 Various grades of nanocomposites have been prepared by varying the nano filler concentration, and it has been observed that the composite having 0.7 wt % of SiO2 content (i.e., XG-g-PAM/ SiO2-2) shows better matrix−filler interaction in comparison with other nanocomposites, which has been confirmed through rheological properties (XG-g-PAM/SiO2-2 shows maximum shear viscosity) and 13C NMR spectral data (XG-g-PAM/SiO2-2 shows greater shift toward higher δ values)10 as well as TEM analysis.10 Characterization. It is well-known that intrinsic viscosity is a measure of hydrodynamic volume of polymer in solution. From Figure S1 (Supporting Information), it is evident that the hydrodynamic volume of the graft copolymer was enhanced through the grafting of PAM chains on the XG backbone, which was increased drastically upon in situ incorporation of silica nanoparticles on the graft copolymer matrix. Since the nanocomposite is in the heterophase, the increase in viscosity clearly implies an excellent interaction between the copolymer matrix and silica. Out of synthesized various nanocomposites 9733

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Figure 1. Jar test result of (a) XG and various graft copolymers in kaolin suspension, (b) XG, XG-g-PAM-v, and various nanocomposites in kaolin suspension, (c) XG and various graft copolymers in iron ore suspension, (d) XG, XG-g-PAM-v, and various nanocomposites in iron ore suspension. Results represented here are ± SD (n = 3).

second weight loss occurred in the region of 225−330 °C, which might be because of decomposition of the polysaccharide backbone. The TGA-DTA thermogram of XG-g-PAM-v (Figure S7b, Supporting Information) illustrates three zones of weight loss. The additional weight loss zone in the region of 360−480 °C probably due to the elimination of NH3 from the grafted polyacrylamide chains.12 This extra zone of weight loss confirms that polyacrylamide chains have been grafted onto XG backbone. On comparing the thermograms of XG-g-PAM-v and XG-gPAM/SiO2-2 (Figure S7c, Supporting Information), it is obvious that the nanocomposite has an extra zone of weight loss in the region of 525−700 °C. This additional zone of weight loss may be due to the decomposition of incorporated nanosilica present on the surface of the graft copolymer matrix. On the basis of the above findings, it can be concluded that functionalization of inorganic silica nanoparticle improves the thermal stability of the nanocomposite, which might be because of the H-bonding interaction between the polymer matrix and nanofiller. The hydrodynamic radius (RH) of XG, graft copolymer, and all nanocomposites was measured by DLS analysis.10 From Table 1, it is obvious that by grafting the synthetic PAM chains on the XG backbone enhances the hydrodynamic radius of the graft copolymer. The further rise of RH values in the case of the nanocomposite indicates the presence of nanosilica onto the surface of graft copolymer matrix.

Flocculation Properties in Synthetic Effluent. The flocculation characteristics of XG, graft copolymers, and nanocomposites were investigated in kaolin (Figure 1a,b; Figure 2a,b) and iron ore suspensions (Figure 1c,d; Figure 2c,d) using the standard jar test and settling test method. In the conventional jar test experiment (Figure 1), the relationship between polymer concentration and residual turbidity of the supernatant liquid was plotted. From Figure 1, it is evident that at low flocculant doses, the turbidity of the supernatant liquid is significantly high because of insufficient flocculant molecules for bridging the particles to form flocs. With an increase in flocculant dose, more particles bridge together to form flocs, and hence the turbidity of the supernatant liquid is significantly reduced.13 However, in the over dosage region, residual turbidity of the supernatant liquid again increases due to the steric stabilization and electrostatic repulsion.14 The flocculation performance of a polymer is also linearly correlated with the settling velocity. From the settling curves (Figure 2), it has been observed that the fall of the interface is linear for a considerable height before it becomes retarded. The initial settling rate was calculated from the slope of the linear portion of the settling curves. In the present study it has been observed that satisfactory linearity was maintained for about a 20 cm fall of the interface in both suspensions. The settling rate of 9734

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Figure 2. Settling characteristics of (a) XG and various graft copolymers in kaolin suspension, (b) XG, XG-g-PAM-v, and various nanocomposites in kaolin suspension, (c) XG and various graft copolymers in iron ore suspension, (d) XG, XG-g-PAM-v, and various nanocomposites in iron ore suspension. Results represented here are ± SD (n = 3).

Table 2. Settling Velocity and Average Floc Size of XG-g-PAM/SiO2-2 Nanocomposite and Various Commercial Flocculants. Results Represented Here Are ± SD (n = 3) settling velocity (cm/sec) of suspensions

average floc size (nm) of suspensions

polymer

kaolin

Fe ore

kaolin

Fe ore

XG-g-PAM/SiO2-2 synthetic PAM FL 920 FL 424 FL 934

1.194 ± 0.138 1.019 ± 0.121 1.172 ± 0.206 1.124 ± 0.173 0.869 ± 0.144

0.945 ± 0.082 0.668 ± 0.047 0.914 ± 0.097 0.785 ± 0.104 0.627 ± 0.088

3055.4 ± 11.8 2284 ± 10.4 2876.4 ± 13.2 2543.6 ± 9.7 2192 ± 12.9

1375.2 ± 12.6 1089.2 ± 14.7 1308.7 ± 11.5 1278.6 ± 10.7 974.6 ± 9.3

molecular weight, and hydrodynamic volume, as well as radius of gyration, the approachability of the contaminants toward the modified polysaccharide are enhanced and thereby increase the flocculation efficacy.15,16 The better flocculation property of XGg-PAM/SiO2-2 is also reflected in its higher settling velocity as well as the corresponding higher average floc size (Table 1). It is apparent that flocs obtained from XG-g-PAM/SiO2-2 are much larger in size (3.1 μm in case of kaolin suspension and 1.375 μm in case of iron ore suspensions) as compared to other flocculants, which confirms that XG-g-PAM/SiO2-2 is the best flocculant out of various graft copolymers and other nanocomposites developed in this study (details of results of average floc size measurement have been given in the Supporting Information Figures S8−S13). Comparative Study of the Flocculation Performances of XG-g-PAM/SiO2-2 Nanocomposite with Commercial Flocculants. The flocculation efficiency of XG, XG-g-PAM-v, and XGg-PAM/SiO2-2 nanocomposite was compared with synthetic

the kaolin and iron ore suspensions with the addition of various flocculants is reported in Table 1. It is clear from the Figure 1a,c and Figure 2a,c that in both the suspensions graft copolymers show better flocculation characteristics than XG. Xanthan gum has not been found to be efficient as a flocculant as it is a hydrophilic polymer which does not interact with colloidal particles and hence cannot give rise to bridging. Instead, xanthan gum is also known to induce depletion flocculation which is typical for noninteracting polymers. However, because of the presence of dangling branches of PAM chains on the rigid polysaccharide backbone in graft copolymers; they are better able to approach the contaminants, resulting in better flocculation efficiency.15 It has also been observed that the graft copolymer (XG-g-PAM-v) having higher molecular weight, hydrodynamic volume, and hydrodynamic radius (Table 1), shows higher turbidity removal efficiency as well as better settling characteristics out of all graft copolymers. Further, it may be concluded from a perusal of the figures (Figure 1 and 2) that in both suspensions, XG-g-PAM/SiO2-2 shows better flocculation efficiency. This may be explained by the fact that XG-g-PAM/SiO2-2, because of the better matrix−filler interaction, shows higher hydrodynamic volume and hydrodynamic radius (Table 1), which results in a better flocculation property. It has been reported that with an increase in branching,

−

PAM (nonionic in nature, Mw = 9.96 × 106 g·mol 1) and three commercially available flocculants namely FL920 (nonionic in −

nature, Mw = 1.80 × 107 g·mol 1), FL424 (cationic in nature, Mw = 4.46 × 106 g·mol−1), and FL934 (anionic in nature, Mw: 3.79 × 106 g·mol−1) in kaolin and iron suspensions. The comparative 9735

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Figure 3. Kinetics curves of flocculation of (a) kaolin and (b) iron ore particles by XG-g-PAM/SiO2-2 nanocomposite at various initial concentrations.

Table 3. Parameters of Flocculation Kinetics Induced by XG-g-PAM/SiO2-2 Nanocomposite at Different Dosages kinetics of aggregation of particles k1 (count−1 s−1)

frequency of collisions of particles

k2 (s−1)

K (s−1)

R2

flocculant dose (ppm)

Kaolin (×10−17)

Fe ore (×10−15)

Kaolin (×10−3)

Fe ore (×10−2)

Kaolin (×10−17)

Fe ore (×10−15)

Kaolin

Fe ore

0.75 1.50 2.25 3.00 3.50

8.12 12.65 23.32 8.22 6.97

3.5 6.58 10.79 9.68 8.73

4.06 1.68 1.01 1.37 1.49

4.24 3.33 1.11 1.39 1.48

1.76 6.94 22.46 20.11 17.25

1.01 4.69 15.43 13.05 11.54

0.9932 0.9966 0.9983 0.9975 0.9923

0.9449 0.9830 0.9967 0.9941 0.9932

flocculation efficiency of the nanocomposite and commercial flocculants has been represented graphically in Figure S14 in the Supporting Information. The results demonstrate that in both suspensions, XG-g-PAM/SiO2-2 nanocomposite shows comparably better flocculation properties. The settling velocity as well as the floc size of both Fe-ore and kaolin particles using XGg-PAM/SiO2-2 nanocomposite and different commercial flocculants have been reported in Table 2. It is obvious that in both suspensions, the nanocomposite shows markedly better flocculation characteristics than those of commercial flocculants. This may be because of the higher molecular weight, as well as branching nature of the nanocomposite. It is also to be mentioned that most of the commercial flocculants are mainly linear in nature. It has been reported that in case of linear polymers, the polymer segments attach to the surface in trains, project into the solution as tails, or form a part of loops, which link trail together and they can form bridges between the colloidal particles to form flocs.17 However, in case of graft copolymer based nanocomposite, because of the branching nature and presence of grafted polyacrylamide chains as well as the presence of silica particles, they can easily bind the colloidal particles through bridging and form the flocs. This type of intense bridging is not possible in the case of linear flocculants. Flocculation Kinetics. Investigation of the flocculation kinetics of colloidal suspensions is important for understanding the mechanism and for process control of solid−liquid system separation.18 It is well established that the flocculation kinetics with polymers depends on several factors like mixing condition,

adsorption on the particles, reconformation of the adsorbed polymers, collision efficiency, and breakage of flocs.19 Kinetics of Aggregation of Particles. The kinetics of flocculation, deflocculation, and reflocculation processes were investigated following Smoluchowski’s classical model, based on the existence of two simultaneous processes: the aggregation of particles, with second order kinetics, and the aggregate breakage, with first order kinetics:20 ⎛ N ⎞2 d(Nt /N0) N = −N0k1⎜ t ⎟ + k 2 t dt N0 ⎝ N0 ⎠

(1)

where N0 is the number concentration of kaolin and iron ore particles at t = 0, Nt is the number concentration at time t; k1 and k2 are kinetic constants for aggregation of particles and kinetic constants for the aggregate breakage, respectively. This relationship is used to compare the kinetics of aggregation of particles and aggregate breakage at different initial concentration. The relationship between both types of kinetics explains the equilibrium situation. Figure 3a,b demonstrates that after the addition of flocculant, maximum particle aggregation took place within the first 120 s. This indicates that the XG-g-PAM/SiO2 nanocomposite-based flocculant works by a complex aggregation mechanism. The mechanism elucidates a fast aggregation of small particles in the first step, followed by bridge flocculation in the second step, which results in larger flocs. From eq 1, the rate constants k1 and k2 were determined and have been reported in Table 3. For 9736

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Figure 4. (N0/Nt)1/2 as a function of settling time at different initial concentration of XG-g-PAM/SiO2-2 using (a) kaolin and (b) iron ore suspension.

Figure 5. Effect of (a) flocculant dosage and (b) pH on zeta potential values.

kaolin and iron ore suspensions, with an increase in flocculant dose from 0.75 ppm to 2.25 ppm, the aggregation rate constant k1 increases and the aggregate breakage rate constant k2 decreases. This suggests that the flocs formed with a high dosage of flocculant were harder than the flocs formed with a low dosage of flocculant.14a Beyond 2.25 ppm flocculant dose, flocculation rate decreases as the kinetics constant k1 for the aggregation of particles reduced and the aggregate breakage constant k2 increased. This may be because of the fact that higher concentration of flocculant (i.e., XG-g-PAM/SiO2-2 nanocomposite) covers most of the available sites on each particle, and the bridging mechanism becomes negligible,20b which makes the flocs destabilized in the suspension due to steric and electrostatic repulsion. Therefore, an excess of flocculant (here it is XG-g-PAM/SiO2-2 nanocomposite) dosage affects the flocculation kinetics as well as floc properties.21

Frequency of Collisions of Particles. Further, to analyze the XG-g-PAM/SiO2 dosage effect, a flocculation kinetics model of the particle collisions was employed. It was well established that the order of flocculation process was mostly bimolecular:20b,22 N0 1 = 1 + kN0t Nt 2

(2)

where N0 is the initial number concentration of the kaolin and iron ore particles, Nt is the number concentration of these particles at time t and k is the rate constant for collisions between the singlets. N0 values for a known weight of kaolin (1 g) was calculated to be 3.918 × 1014 by considering the particle diameter (0.123 μm) and density of kaolin (2.6 g cm−3), and for 1 g of iron ore sample, the N0 value was 8.476 × 1012 (particle diameter of 0.397 μm and density of 3.6 g cm−3). Similarly, knowing the weight of particles flocculated in a specified time period, it is 9737

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possible to determine the value of Nt (here the hypothesis is that the portion of the particles which did not flocculate is singlets). A plot of (N0/Nt)1/2 vs t provides a straight line (Figure 4a,b) with an intercept 1 for a bimolecular process, and the rate constant (k) obtained from the slope of the curve has been represented in Table 3. From Table 3, it has been found that for kaolin and iron ore suspensions, the flocculation process follows the kinetics model of particle collisions (based on the linear correlation coefficient R2 > 0.99 under optimum concentration). Experimental results indicate that the rate constant k increases with an increase in flocculant concentration. At very low dosages (i.e., 0.75 ppm), the rate constant is very small (1.76 × 10−17 for kaolin and 1.01 × 10−15 for iron ore suspension) but shows a 13-fold increase for kaolin suspension and 15-fold increase for the iron ore suspension at an optimum polymer concentration of 2.25 ppm. This trend suggests that the most efficient molecular collision between polymer nanocomposite and suspended particles occurs at the optimal dosage. This phenomenon may be explained as under the optimized flocculant concentration, the flocculation process was jointly controlled by both charge neutralization and bridging mechanism, though the latter plays a key role for producing larger and stronger floc size.23 However, a lower dosage of flocculant is not enough for both charge neutralization and bridging mechanism, and results in a lower k value. On the other hand, in the overdose region (3 ppm to 3.50 ppm), because of the presence of steric and repulsive forces between different polymeric chains,24 the flocculation rate constant (k) value gradually decreases. Effect of Flocculant Dosage on Zeta Potential. It is wellknown that flocculant dosage is one of the most crucial factors for the flocculation characteristics. Figure 5a shows the zeta potential of suspended particles as a function of XG-g-PAM/SiO2-2 concentration. It has been observed that with an increase in flocculant dosage to optimized concentration (2.25 ppm), the magnitude of the zeta potential decreases. For XG-g-PAM/SiO22 nanocomposite (which is anionic in nature) the zeta potential decreases with an increase in flocculant dosage. This may be explained by the fact that there might be a shift in the position of plane shear due to the adsorbed layer, which is being offset by an increase in charge around the particles of negatively charged SiO2 of the polymer.25 Also, the number of active binding sites, which are available for effective bridging on particle surfaces is higher at lower polymer concentration in comparison to higher concentration. However in the overdosage region, the magnitude of the zeta potential of suspension increases. At higher polymer concentrations, because of the higher surface coverage and the strong electrostatic repulsion between the similarly charged polymer and particles, the number of tails and loops available for bridging decreases, which retards the flocculation performance.26 Effect of Zeta Potential As a Function of pH on the Flocculation Properties of Nanocomposite. The flocculation efficiency of flocculants depends on various parameters, of which pH is one of the most important parameters especially for ionic flocculants. Figure 5b represents the zeta potential vs pH curve for XG-g-PAM/SiO2-2 nanocomposite, before and after treated kaolin and iron ore suspensions. It is obvious from the figure that both kaolin and iron ore particles are negatively charged over the entire pH range, whereas XG-g-PAM/SiO2-2 nanocomposite has an isoelectric point at around pH 4. Interestingly, it has been observed that after the addition of XG-g-PAM/SiO2-2 nanocomposite as a flocculant onto kaolin and iron ore suspensions, the magnitude of zeta potential decreases and shifts the

isoelectric point (iep) toward lower pH value. However, at pH > 4 (pHzpc) the magnitude of the zeta potential of the treated suspension gradually increases. In the pH range below the iep (4), the negative charges on the XG-g-PAM/SiO2-2 nanocomposite increases. Therefore strong electrostatic repulsion between the flocculant and the suspended particles leads to the higher negative zeta potential value. It is also to be mentioned that under neutral or alkaline conditions, the negative charges on XG-g-PAM/SiO2-2 nanocomposite creates a more extended conformation of the polymer chain resulting in intramolecular electrostatic repulsion.27 As a result, the hydrodynamic radius as well as hydrodynamic volume of the polymer solution increases, which improves the efficiency of bridging flocculation. Thus it can be concluded that in acidic pH, the charge neutralization effect predominates in the flocculation mechanism, whereas at neutral or alkaline pH, the bridging effect plays a significant role. Flocculation Mechanism. The major mechanistic pathway of flocculation by modified polysaccharides may be explained by either the charge neutralization mechanism or polymer bridging mechanism. Charge neutralization occurs if the charge of the flocculant is opposite in sign to that of the suspended particles. For high molecular weight, neutral flocculants/slightly ionic flocculants, polymer bridging is the leading flocculation mechanism.28 The better flocculating power of graft copolymer-based nanocomposites over other polymers is probably due to the polymer bridging, because segments of a polymer chain adsorb onto different particles surfaces and form bridges between adjacent particles. Adsorption may occur by electrostatic forces, van der Waals forces or through H-bonding.7a This indicates that a particle−polymer−particle aggregate is formed in which the polymer served as bridge. For effective bridging to occur, the length of the polymer/nanocomposite chains should be long as well as the radius of gyration should be high, so that the adsorbed polymer molecules would tend to adopt a more extended configuration for interaction with more than one particle. Another crucial factor to control the probability of polymer bridging is the availability of unoccupied space on the particle surface in the solution. This phenomenon is observed up to optimum flocculant dosage (2.25 ppm), beyond which flocculation diminishes; this process is known as steric stabilization. The larger floc sizes (3.1 μm for kaolin suspension and 1.375 μm for iron ore suspension) are obtained in the settling test using the optimal dose of XG-g-PAM/SiO2-2 flocculant. This result indicates that bridging is the major mechanism in this case; as the flocs formed by charge neutralization are always smaller than those formed by the bridging mechanism.29 Treatment of Mine Wastewater. It is well-known that industrial wastewater constitutes an important part of the total wastewater from various sources. Proper treatment of mine wastewater has become more important in recent years. Because of a dwindling supply and increasing demand for quality water resources in the agricultural and industrial sectors, a better alternative to direct discharge of treated industrial wastewater is to elevate the water quality further to an appropriate level for possible agricultural and industrial reuse. Flocculation is an important phenomenon for purification of mine wastewater for 9738

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Table 4. Flocculating Effect for Treatment of Industrial Wastewater. Results Represented Here Are ± SD (n = 3) flocculant

turbidity (NTU)

TS (ppm)

TDS (ppm)

TSS (ppm)

COD (ppm)

without flocculant XG XG-g-PAM-i XG-g-PAM-ii XG-g-PAM-iii XG-g-PAM-iv XG-g-PAM-v XG-g-PAM-vi XG-g-PAM/SiO2-1 XG-g-PAM/SiO2-2 XG-g-PAM/SiO2-3 XG-g-PAM/SiO2-4

480 ± 12 240 ± 8 105 ± 5 87 ± 5 72 ± 4 86 ± 4 54 ± 3 90 ± 4 41 ± 3 29 ± 2 37 ± 3 44 ± 4

760 ± 15 510 ± 12 320 ± 9 276 ± 4 290 ± 4 298 ± 3 264 ± 3 305 ± 2 202 ± 2 175 ± 2 190 ± 2 210 ± 2

280 ± 8 150 ± 3 210 ± 3 186 ± 2 195 ± 2 198 ± 3 184 ± 3 200 ± 2 162 ± 2 145 ± 2 155 ± 3 165 ± 2

480 ± 8 360 ± 6 110 ± 4 90 ± 4 95 ± 6 100 ± 5 80 ± 2 105 ± 1 40 ± 1 30 ± 2 35 ± 1 45 ± 2

375 ± 6 260 ± 5 190 ± 4 149 ± 3 158 ± 3 165 ± 3 140 ± 2 176 ± 2 117 ± 1 102 ± 1 109 ± 1 123 ± 2

available flocculants. The flocculation kinetics follows satisfactorily with the aggregation of particle and the particle collision model simultaneously. The flocculation mechanism explains that at optimized concentration, the bridging effect predominates, producing a rapid flocculation rate, and requires less time to reach equilibrium as compare to a higher flocculant dose.

its reuse, as it can decrease the total pollutant content from the wastewater. Table 4 represents the results of analysis of treatment of mine wastewater using various flocculants. It clearly shows that XG-g-PAM/SiO2-2 considerably reduces the overall pollutant load (i.e., turbidity, TS, TDS, TSS, COD). Further, Figure 6

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ASSOCIATED CONTENT

S Supporting Information *

Intrinsic viscosity measurement, FTIR spectra, Debye plot for molecular weight determination, FESEM analysis, TGA analysis, results of measurement of average floc size, comparison of flocculation efficacy of the nanocomposite with commercial flocculants. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 0091-326-2235769. Fax: 0091-326-2296615. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support from the Department of Science and Technology, New Delhi, India, in the form of a research grant [No. DST/TSG/WP/69/2009] to carry out the reported investigation.

Figure 6. Comparative study of efficiency of various flocculants in color removal from mine wastewater.

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REFERENCES

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