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Effects of various fillers on anionic polyacrylamide systems for treating kaolin suspensions
Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 306–311
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Effects of various fillers on anionic polyacrylamide systems for treating kaolin suspensions A. Ariffin a,∗ , M.S. Musa a , M.B.H. Othman a , M.A.A. Razali a , F. Yunus b a School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Penang, Malaysia b Acme Chemicals (Malaysia) Sdn. Bhd. No. 38, Jalan Selat Selatan 21, Sobena Jaya Industrial Park, Pandamaran, 42000 Port Klang, Selangor, Malaysia
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• 0, 10, 20 and 30 wt% loading of CO (NH2 )2 , KCl, NaCl, and Na2 SO4 utilized. • Concentration of A-PAM system fixed (0.5% w/v) according to industry requirement. • Ion dissociation affected dissolution rate, viscosity and turbidity reduction.
a r t i c l e
i n f o
Article history: Received 31 May 2013 Received in revised form 18 September 2013 Accepted 19 September 2013 Available online xxx Keywords: Anionic polyacrylamide (A-PAM) Dissolution rate Viscosity Electrolytes Flocculation
a b s t r a c t This study examines the effects of different types of salt fillers and salt filler loadings on anionic polyacrylamide (A-PAM). Urea, potassium chloride, sodium chloride, and sodium sulfate were incorporated into the A-PAM solution with 10, 20, and 30 wt.% loadings. The characteristics of the A-PAM solution were measured based on the dissolution rate, viscosity, potential, turbidity reduction and floc size. The introduction of salt fillers increased the dissolution rate and reduced the viscosity based on the electrolyte strength of the salt fillers. Stronger electrolytes showed an increased dissolution rate, decreased viscosity, increased turbidity reduction, and decreased floc size. Stronger electrolytes also showed fluctuating and uneven values in the (zeta) potential due to the uneven placement of ions in the A-PAM solution system. The urea-filled A-PAM showed a potential trend similar to that of the unfilled A-PAM. It can be concluded that the strength of ion dissociation plays an important role in the dissolution rate, viscosity, and flocculation characteristics of a salt-filled A-PAM solution. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, synthetic polyelectrolytes, particularly anionic polyacrylamide (A-PAM), have been widely used as flocculants in wastewater treatment due to their efficiency, the reduced cost of chemical usage, and the smaller quantity required [1–3]. Common commercial anionic polyacrylamide (A-PAM) is normally a long-chain synthetic polymer with a high molecular weight, high
∗ Corresponding author. Tel.: +60 4 5946176; fax: +60 4 5941011. E-mail addresses: [email protected], [email protected] (A. Ariffin). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.09.021
viscosity, and low dissolution rate in solutions. This is due to the size, charge, coiled shape, and attraction forces between the polymer chains (cohesive and attractive both intra and intermolecular forces hold these coils together). Thus, the stronger the molecules are bound together, the more viscous the solution or system will be; thus, larger flocs will be produced. Brown et al. [4] found that polymers in papermaking processes produce large or macro flocs, which result in poor drainage and formation. Ahmad et al. [5], Mpofu et al. [6] and Clark et al. [7] found the same flocculation properties. Lee et al. [8] proposed salt filler-PAM hybrid polymers as an attractive material in wastewater treatment due to their ability to destabilize colloids. A combination of a salt filler with a
A. Ariffin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 306–311
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Table 2 Summary of formulations used to prepare the A-PAM solution series.
Fig. 1. The chemical structures of A-PAM.
Salt loadinga (% w/w)
Weight A-PAM: salt (g):(g)
Net Weight A-PAM +salt (g)
Distilled water (mL)
A-PAM concentrationb (% w/v)
0 10 20 30
1.0:0 0.9:0.1 0.8:0.2 0.7:0.3
1 1 1 1
200 200 200 200
0.5 0.5 0.5 0.5
a
Salt loading (w/w) = weight salt (g)/(weight salt (g) + weight A-PAM (g)). A-PAM concentration measured is Net weight A-PAM + Salt (g)/distilled water (mL). b
synthetic flocculating polymer strengthens the aggregating power of a system [9]. Recently, various combinations of fillers have been introduced to improve the treatment performance [10]. The presence of salt fillers in certain solutions will affect the dissolution rate, viscosity, and flocculation. This is because the overall volume fraction of the dispersed phase in the mixture is altered, and the colloidal equilibrium may be disturbed. This study aims to determine the effects of various salt fillers (CO (NH2 )2 , KCl, NaCl and Na2 SO4 ) in an A-PAM solution. Several A-PAM solution systems were prepared by utilizing 10%, 20% and 30% (all in wt.%) of salt filler loadings. The characteristics of the APAM solutions were determined by the dissolution rate and viscosity measurements and flocculation tests. 2. Methodology 2.1. Materials
2.4. Viscosity The viscosity was measured using an HB Brookfield viscometer with a 30-rpm spindle at ambient temperature. The reading was taken for 5 days continuously. 2.5. Kaolin flocculation The flocculation was performed according to jar test procedures. First, 1.5 g of kaolin was suspended in 500 mL of distilled water and introduced with 10 ppm of A-PAM solution (200 rpm for 3 min and 60 rpm for 6 min). Flocs formed and were allowed to settle for 30 min. Then, the floc size was measured. 2.6. Analytical techniques
Anionic polyacrylamide (A-PAM: Mw = ∼16,000,000 g/mol: pH 7; potential = −53.60 mV; conductivity = 0.0222 mS/cm) was provided by ACME Chemicals Sdn. Bhd. Sodium chloride (NaCl), sodium sulphate (Na2 SO4 ), potassium chloride (KCl), and urea (CO (NH2 )2 were obtained from Sigma Aldrich (Malaysia) Sdn. Bhd. Fig. 1 shows the chemical structure of A-PAM, and Table 1 summarizes the details of the salt fillers.
The particle sizes of the various fillers were examined using the Sympatec-Mastersizer Analyzer. The potential and floc size resulting from the flocculation of each A-PAM series were determined using a Malvern-Mastersizer 2000. The turbidity was measured using the HACH 2100P portable Turbidity Meter. 3. Results and discussions
2.2. Sample preparation
3.1. Effects on dissolution rate
First, 0.5% (w/v) of the A-PAM solution was prepared by dissolving 1 g of A-PAM/salt filler in 200 mL of distilled water. The A-PAM solution was stirred at 200 rpm at ambient temperature. Although the salt load was increased up to 30%, the concentration of the A-PAM was fixed according to industry requirements. The formulation is summarized in Table 2.
The dissolution rate is an important characteristic for polymeric electrolytes. The highest or fastest dissolution rate determines the efficiency of the A-PAM. Fig. 2 shows the dissolution rates of the A-PAM solution series corresponding to the salt filler loadings. Obviously, the incorporation of salt fillers into the A-PAM solution significantly increased the dissolution rate. This suggests that
2.3. Dissolution rate The dissolution rates were calculated according to the following equation: Dissolution rate =
dm dt
(1)
where m is the amount of dissolved material in mg, t is the time in s to complete the dissolution of the solid (A-PAM) in the solvent, and the dissolution rate value is expressed in mg/s. Table 1 Physical properties and particle sizes of various fillers. Filler type
Molecular weight (g/mol)
Density (g/mL)
Solubility in water at 20 ◦ C (g/L)
Particle size (m)
NaCl Na2 SO4 KCl (CO (NH2 )2
58.44 142.04 74.55 60.06
2.165 2.664 1.984 1.320
359.0 85.4 344.0 1079.0
30.72 29.49 31.58 33.67
Fig. 2. The effect of different types of fillers and filler loadings on the dissolution rate of A-PAM.
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A. Ariffin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 306–311 Table 3 Viscosity region of fillers and A-PAM.
Fig. 3. The hydration energy of each salt filler utilized.
(a) Cationic series: Li+> Na+> K+ > Rb+> Cs+ (b) Anionic series OH- > CNS- > I- > Br- > Cl- >F- > NO3- > SO42Fig. 4. Relative hydrated ionic radii for cationic and anionic radii [26,27].
the salt filler ions penetrated the A-PAM chain and attached to the counter ion [11] of the A-PAM, thus disturbing and reducing the van der Waals forces between the A-PAM chains. This produced an aperture that allowed water molecules to pass through the matrix and loosen the A-PAM chains from their coiled shape. Therefore, segments of the A-PAM chains became “solvated;” an this continued until the whole loosened coil diffused to form the aqueous A-PAM. These phenomena have been consistently reported by Isemura and Imanishi [12]. In addition, increasing the filler loading consistently showed an increasing dissolution rate for all types of salt fillers. This suggests that the presence of excess salt filler ions enhances the efficiency of the A-PAM chain to dissolve. The high dissolution rate in Fig. 2 followed the order of NaCl > Na2 SO4 > KCl > CO (NH2 )2 > for the unfilled A-PAM. This is related to the type of electrolyte, which corresponds to the capability of the salt fillers to ionize in water. Ganguly (2012) noted that the solubility of an ionic salt depends on the hydration energy. The hydration energy ( hydration) of each salt filler is shown in Fig. 3. The greatest change suggests that the salt filler ions have a great tendency to hydrate; thus, solubility should be better. NaCl, which is known as a strong electrolyte, completely dissociates in water, unlike Na2 SO4 . NaCl has a lower electron configuration than KCl and thus has a better dissociation ability to form ions. Since the cations from salt fillers will initially be attracted to the counter ions in the A-PAM, this suggests that the Na+ ion is better than the K+ ion in terms of the group 1 characteristics in the periodic table (Fig. 4a). Furthermore, the Cl− ions are considered better than the SO4 2− ions in terms of the halogen group (Fig. 4b). The smaller size increased the hydration energy of the ions, which increased the ionization process of the ionic compound in the APAM system. NaCl has higher hydration energy than KCl, because Na is smaller than K, which contributes to the greater solubility of most alkali metals and their high ionic characteristics. Thus, NaCl has higher hydration energy as a strong electrolyte and is more effective in the dissolution rate process than KCl.
Filler/A-PAM
Viscosity (mPa s)
A-PAM Urea, CO (NH2 )2 Potassium chloride, KCl Sodium chloride, NaCl Sodium sulfate, Na2 SO4
1067–1280 746–990 497–913 320–853 300–533
with regard to better flow ability brought about the reduction in viscosity. Fig. 5 presents the 5-day viscosity values for each salt filler type in the A-PAM solution corresponding to 0–30% (w/w) salt fillers loading. Generally, the increasing filler loadings decreased the viscosity in the filled A-PAM system. Increasing the filler loadings for all samples resulted in an increase in the ion concentration, which eventually decreased the viscosity of the A-PAM. In addition, the amount of A-PAM was also reduced in the solution, which increased the hydrodynamic volume of the polymer system and resulted in a lower viscosity. The unfilled (0%) A-PAM had an uninterrupted, high summation of intermolecular forces in the backbone, which restricted the chain mobility and showed the highest viscosity. Except for urea, other fillers with stronger electrolytes produced a much lower viscosity. The viscosity of urea is in the range of 746–990 mPa s, which is a little bit lower compared to the unfilled A-PAM solution. With the lowest ionic strength, urea had less effect on the viscosity compared to the other fillers. No significant changes in viscosity were observed in the A-PAM system after increasing the urea loading. This might be due to the nature of urea as a weak electrolyte that ionizes partially in an aqueous solution. These effects are consistently similar to the results reported by Cohen and Priel [13], Pelton and Allen [14] and Wyatt et al. [15]. The Na2 SO4 filled polymer solution had the lowest range of viscosity, as it is a multivalent salt with a higher concentration of ions in the A-PAM system. Shielding or screening, complex formation, and the spacing effect of the high ion concentration further detached the polymer backbone chain, thus increasing the polymer free volume. This increased the mobility of the A-PAM and further reduced the viscosity. Salts having high solubility and surface area were preferentially selected to reduce the viscosity. 3.3. Effects on potential The potential is the potential at the surface of an electrokinetic unit moving through a solution. The electrokinetic entity may include ions specifically adsorbed from the solution; this will be reflected in the value of the potential, which indicates the degree of repulsion between adjacent, similarly charged particles in the dispersion. The greater the potential, the more likely the suspension is to remain in a stable form [16]. Thus, colloids with high potential (negative or positive) are electrically stabilized, while colloids with low potential tend to coagulate or flocculate as shown in Table 4. The potential values for the salt-filled polymers for five continuous days are illustrated in Fig. 6. Urea follows a similar pattern to A-PAM, whereas the other fillers give fluctuating results.
3.2. Effects on viscosity The range of viscosities for different filler systems is shown in Table 3. The viscosity decreased with the incorporation of fillers. A similar effect of fillers on the A-PAM system as observed by the dissolution rate was also found in the study of viscosity. The ions produced by the fillers reduced the polymer–polymer intermolecular forces, thereby increasing the mobility of the polymer chains. The changes in the rheological properties of the polymer chains
Table 4 Degree of stability as a function of potential [28]. potential [mV]
Stability behavior (colloid)
0 to ±5 ±10 to ±30 ±30 to ±40 ±40 to ±60 ±60 to ±100
Rapid flocculation Incipient instability Moderate stability Good stability Excellent stability
A. Ariffin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 306–311
Fig. 5. The 5-day viscosity values corresponding to filler loadings.
Fig. 6. The potential values of salt loadings corresponding to 5 days of measurement.
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Zingg et al. [17] state that the potential value is highly sensitive to the chemical composition of the solvent (i.e. ionic species, pH, and conductivity). In this case, urea has less ionic strength and less interference with the ionic content of A-PAM. A lower dissociation of the ions in the solution did not yield a significant effect on the potential values. The other fillers showed a random pattern and fluctuated with an increase in filler loadings. The addition of salt resulted in a disruption of the negative ions of the A-PAM to give fluctuating results. This is because increasing the filler loading increases the amount or concentration of ions in the solution. These ions disrupted the negative ions in the A-PAM. The presence of cations and anions of salts disturbed the negatively charged of A-PAM and gave inconsistent readings by day evaluation. 3.4. Effect of fillers on turbidity reduction According to Wong et al. [18], turbidity, an indicator of flocculation efficiency, is a measure of the light-transmitting properties of water with respect to colloidal and residual suspended matter. The effect of various fillers and filler loadings on the reduction of turbidity in kaolin suspensions is shown in Fig. 7. The A-PAM with 10 wt% of fillers showed a decrease in the turbidity reduction compared to the unfilled A-PAM. This is probably due to the decrease in the amount of A-PAM in the solution. Generally, A-PAM causes bridging of the loops and tails between suspended particles. Decreasing the contents reduced the likelihood of the bridging effect between the suspended particles, thus decreasing the turbidity reduction. In this case, the effect of polymers is dominant compared to the effect of ions by the fillers [19]. Except for urea, increasing the salt loading to 20 and 30 wt.% increased the turbidity reduction. This contributed to the difference in the adsorption and flocculation of the negatively charged kaolin particles. The addition of urea decreased the likelihood of the A-PAM adsorption with kaolin. As mentioned earlier, urea is an organic compound and a weak electrolyte. It has relatively less effect than the other fillers but does decrease the A-PAM contents due to the decrease in the kaolin suspension. The addition of KCl increased the turbidity reduction due to its effect on ionic strength in the A-PAM system. Several factors influence this efficiency, including the pH, the type of salt, and the water alkalinity [20]. In this experiment, the addition of KCl contributed to the decrease in
Fig. 7. The effect of various fillers and filler loadings on turbidity reduction.
the double layer of particles due to the shielding of the A-PAM with K+ monovalent ions, which reduced the repulsive forces between the negatively charged polymers and negatively charged kaolin surface. Decreasing the double layer of particles allowed the A-PAM molecules to extend and produce loops and tails to contribute to the bridging mechanism effectively. This led to the increase in turbidity reduction. The turbidity reduction increased with the addition of 20–30 wt% of NaCl and Na2 SO4 due to the concentration of the added salt ions. This reduced the repulsive forces between the anionic segments of the polymeric chains and the negatively charged surfaces of the particles. Consequently, reducing the repulsion forces promoted better adsorption [21]. Although NaCl and Na2 SO4 are both strong electrolyte salts, the effect of Na2 SO4 was much better since Na2 SO4 is a multivalent salt that can very effectively neutralize the negatively charged kaolin. According to Chukwudi et al. [22], a decrease in the compression of the electrical double layer means that the particles can get close enough for the attractive van der Waals forces to dominate the electrostatic repulsions, therefore rendering the suspension unstable. As a result, the particle charge may be partially or completely neutralized,
Fig. 8. Decreased floc size with decreasing polymer concentration for various fillers.
A. Ariffin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 306–311
thereby reducing the components of the electrostatic repulsive forces; hence, flocculation is enhanced [22]. 3.5. Effect of filler on floc size Fig. 8 presents the floc size for different types of fillers and filler loadings on A-PAM. Clearly, increasing the filler contents reduced the floc size. Clark et al. [7] reported that floc size is dependent on the polymer charge content. The addition of filler content from 10 to 30 wt% decreased the concentration of the A-PAM charge content. Decreasing the amount of anionic surface charge of the A-PAM chains reduced the electrostatic repulsive effect between the negatively charged kaolin surface and the A-PAM to increase the adsorption and the bridging mechanism. The adsorption mechanism is also related to the electrostatic charge interaction. Kaolin, with a negative potential, prevents strong adsorption of anionic polymers, leaving long polymer chains to form bridges with kaolin particles and resulting in large flocs [23]. The decreased electrostatic repulsion between the negatively charged A-PAM chains and the negative kaolin surface was related to the addition of fillers, which increased the adsorption and reduced the bridging mechanism (and thus the floc size). Consequently, the flocs produced by the filled samples had smaller dimensions than those produced by the unfilled samples [24]. Increasing the filler loadings of urea, KCl, NaCl, and Na2 SO4 from 10 to 30 wt% reduced the floc size. The decrease in the floc size can be explained by the reduced charge repulsion in producing loops and tails, leading to the small size [25]. Subsequently, A-PAM with 30 wt% of all types of fillers presented small floc size and consistent floc distribution. The effectiveness of the flocculation mechanism was measured not only by the floc size but also by the turbidity reduction. Despite the small size, the results showed good turbidity reduction. In addition, the turbidity reduction of the small floc size was better than for the large floc size. According to Jarvis et al. [25], smaller flocs tend to have greater strength than larger flocs. 4. Conclusion An anionic polyacrylamide (A-PAM)/salt filler solution for wastewater treatment utilizing 10 wt%, 20 wt% and 30 wt% loadings of CO (NH2 )2 , KCl, NaCl, and Na2 SO4 , was prepared. The effects of different types of salt fillers and salt filler loadings on anionic polyacrylamide (A-PAM) were experimentally studied on kaolin. The results showed that the dissolution rate and turbidity reduction improved compared to the unfilled A-PAM solution, while viscosity and flocs decreased. The strong electrolyte salt fillers were more efficient following the order NaCl > Na2 SO4 > KCl > urea. The potential showed a fluctuating value due to disruption of the negative ions of the A-PAM. Acknowledgements We wish to acknowledge the technical support provided by the School of Materials and Mineral Resources Engineering, Engineering Campus Universiti Sains Malaysia, and Acme Chemicals (Malaysia) Sdn. Bhd. The authors also wish to acknowledge the financial support provided by the MOSTI science fund (1001/ pbahan/814131).
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Effects of various fillers on anionic polyacrylamide systems for treating kaolin suspensions A. Ariffin a,∗ , M.S. Musa a , M.B.H. Othman a , M.A.A. Razali a , F. Yunus b a School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Penang, Malaysia b Acme Chemicals (Malaysia) Sdn. Bhd. No. 38, Jalan Selat Selatan 21, Sobena Jaya Industrial Park, Pandamaran, 42000 Port Klang, Selangor, Malaysia
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• 0, 10, 20 and 30 wt% loading of CO (NH2 )2 , KCl, NaCl, and Na2 SO4 utilized. • Concentration of A-PAM system fixed (0.5% w/v) according to industry requirement. • Ion dissociation affected dissolution rate, viscosity and turbidity reduction.
a r t i c l e
i n f o
Article history: Received 31 May 2013 Received in revised form 18 September 2013 Accepted 19 September 2013 Available online xxx Keywords: Anionic polyacrylamide (A-PAM) Dissolution rate Viscosity Electrolytes Flocculation
a b s t r a c t This study examines the effects of different types of salt fillers and salt filler loadings on anionic polyacrylamide (A-PAM). Urea, potassium chloride, sodium chloride, and sodium sulfate were incorporated into the A-PAM solution with 10, 20, and 30 wt.% loadings. The characteristics of the A-PAM solution were measured based on the dissolution rate, viscosity, potential, turbidity reduction and floc size. The introduction of salt fillers increased the dissolution rate and reduced the viscosity based on the electrolyte strength of the salt fillers. Stronger electrolytes showed an increased dissolution rate, decreased viscosity, increased turbidity reduction, and decreased floc size. Stronger electrolytes also showed fluctuating and uneven values in the (zeta) potential due to the uneven placement of ions in the A-PAM solution system. The urea-filled A-PAM showed a potential trend similar to that of the unfilled A-PAM. It can be concluded that the strength of ion dissociation plays an important role in the dissolution rate, viscosity, and flocculation characteristics of a salt-filled A-PAM solution. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, synthetic polyelectrolytes, particularly anionic polyacrylamide (A-PAM), have been widely used as flocculants in wastewater treatment due to their efficiency, the reduced cost of chemical usage, and the smaller quantity required [1–3]. Common commercial anionic polyacrylamide (A-PAM) is normally a long-chain synthetic polymer with a high molecular weight, high
∗ Corresponding author. Tel.: +60 4 5946176; fax: +60 4 5941011. E-mail addresses: [email protected], [email protected] (A. Ariffin). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.09.021
viscosity, and low dissolution rate in solutions. This is due to the size, charge, coiled shape, and attraction forces between the polymer chains (cohesive and attractive both intra and intermolecular forces hold these coils together). Thus, the stronger the molecules are bound together, the more viscous the solution or system will be; thus, larger flocs will be produced. Brown et al. [4] found that polymers in papermaking processes produce large or macro flocs, which result in poor drainage and formation. Ahmad et al. [5], Mpofu et al. [6] and Clark et al. [7] found the same flocculation properties. Lee et al. [8] proposed salt filler-PAM hybrid polymers as an attractive material in wastewater treatment due to their ability to destabilize colloids. A combination of a salt filler with a
A. Ariffin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 306–311
307
Table 2 Summary of formulations used to prepare the A-PAM solution series.
Fig. 1. The chemical structures of A-PAM.
Salt loadinga (% w/w)
Weight A-PAM: salt (g):(g)
Net Weight A-PAM +salt (g)
Distilled water (mL)
A-PAM concentrationb (% w/v)
0 10 20 30
1.0:0 0.9:0.1 0.8:0.2 0.7:0.3
1 1 1 1
200 200 200 200
0.5 0.5 0.5 0.5
a
Salt loading (w/w) = weight salt (g)/(weight salt (g) + weight A-PAM (g)). A-PAM concentration measured is Net weight A-PAM + Salt (g)/distilled water (mL). b
synthetic flocculating polymer strengthens the aggregating power of a system [9]. Recently, various combinations of fillers have been introduced to improve the treatment performance [10]. The presence of salt fillers in certain solutions will affect the dissolution rate, viscosity, and flocculation. This is because the overall volume fraction of the dispersed phase in the mixture is altered, and the colloidal equilibrium may be disturbed. This study aims to determine the effects of various salt fillers (CO (NH2 )2 , KCl, NaCl and Na2 SO4 ) in an A-PAM solution. Several A-PAM solution systems were prepared by utilizing 10%, 20% and 30% (all in wt.%) of salt filler loadings. The characteristics of the APAM solutions were determined by the dissolution rate and viscosity measurements and flocculation tests. 2. Methodology 2.1. Materials
2.4. Viscosity The viscosity was measured using an HB Brookfield viscometer with a 30-rpm spindle at ambient temperature. The reading was taken for 5 days continuously. 2.5. Kaolin flocculation The flocculation was performed according to jar test procedures. First, 1.5 g of kaolin was suspended in 500 mL of distilled water and introduced with 10 ppm of A-PAM solution (200 rpm for 3 min and 60 rpm for 6 min). Flocs formed and were allowed to settle for 30 min. Then, the floc size was measured. 2.6. Analytical techniques
Anionic polyacrylamide (A-PAM: Mw = ∼16,000,000 g/mol: pH 7; potential = −53.60 mV; conductivity = 0.0222 mS/cm) was provided by ACME Chemicals Sdn. Bhd. Sodium chloride (NaCl), sodium sulphate (Na2 SO4 ), potassium chloride (KCl), and urea (CO (NH2 )2 were obtained from Sigma Aldrich (Malaysia) Sdn. Bhd. Fig. 1 shows the chemical structure of A-PAM, and Table 1 summarizes the details of the salt fillers.
The particle sizes of the various fillers were examined using the Sympatec-Mastersizer Analyzer. The potential and floc size resulting from the flocculation of each A-PAM series were determined using a Malvern-Mastersizer 2000. The turbidity was measured using the HACH 2100P portable Turbidity Meter. 3. Results and discussions
2.2. Sample preparation
3.1. Effects on dissolution rate
First, 0.5% (w/v) of the A-PAM solution was prepared by dissolving 1 g of A-PAM/salt filler in 200 mL of distilled water. The A-PAM solution was stirred at 200 rpm at ambient temperature. Although the salt load was increased up to 30%, the concentration of the A-PAM was fixed according to industry requirements. The formulation is summarized in Table 2.
The dissolution rate is an important characteristic for polymeric electrolytes. The highest or fastest dissolution rate determines the efficiency of the A-PAM. Fig. 2 shows the dissolution rates of the A-PAM solution series corresponding to the salt filler loadings. Obviously, the incorporation of salt fillers into the A-PAM solution significantly increased the dissolution rate. This suggests that
2.3. Dissolution rate The dissolution rates were calculated according to the following equation: Dissolution rate =
dm dt
(1)
where m is the amount of dissolved material in mg, t is the time in s to complete the dissolution of the solid (A-PAM) in the solvent, and the dissolution rate value is expressed in mg/s. Table 1 Physical properties and particle sizes of various fillers. Filler type
Molecular weight (g/mol)
Density (g/mL)
Solubility in water at 20 ◦ C (g/L)
Particle size (m)
NaCl Na2 SO4 KCl (CO (NH2 )2
58.44 142.04 74.55 60.06
2.165 2.664 1.984 1.320
359.0 85.4 344.0 1079.0
30.72 29.49 31.58 33.67
Fig. 2. The effect of different types of fillers and filler loadings on the dissolution rate of A-PAM.
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Fig. 3. The hydration energy of each salt filler utilized.
(a) Cationic series: Li+> Na+> K+ > Rb+> Cs+ (b) Anionic series OH- > CNS- > I- > Br- > Cl- >F- > NO3- > SO42Fig. 4. Relative hydrated ionic radii for cationic and anionic radii [26,27].
the salt filler ions penetrated the A-PAM chain and attached to the counter ion [11] of the A-PAM, thus disturbing and reducing the van der Waals forces between the A-PAM chains. This produced an aperture that allowed water molecules to pass through the matrix and loosen the A-PAM chains from their coiled shape. Therefore, segments of the A-PAM chains became “solvated;” an this continued until the whole loosened coil diffused to form the aqueous A-PAM. These phenomena have been consistently reported by Isemura and Imanishi [12]. In addition, increasing the filler loading consistently showed an increasing dissolution rate for all types of salt fillers. This suggests that the presence of excess salt filler ions enhances the efficiency of the A-PAM chain to dissolve. The high dissolution rate in Fig. 2 followed the order of NaCl > Na2 SO4 > KCl > CO (NH2 )2 > for the unfilled A-PAM. This is related to the type of electrolyte, which corresponds to the capability of the salt fillers to ionize in water. Ganguly (2012) noted that the solubility of an ionic salt depends on the hydration energy. The hydration energy ( hydration) of each salt filler is shown in Fig. 3. The greatest change suggests that the salt filler ions have a great tendency to hydrate; thus, solubility should be better. NaCl, which is known as a strong electrolyte, completely dissociates in water, unlike Na2 SO4 . NaCl has a lower electron configuration than KCl and thus has a better dissociation ability to form ions. Since the cations from salt fillers will initially be attracted to the counter ions in the A-PAM, this suggests that the Na+ ion is better than the K+ ion in terms of the group 1 characteristics in the periodic table (Fig. 4a). Furthermore, the Cl− ions are considered better than the SO4 2− ions in terms of the halogen group (Fig. 4b). The smaller size increased the hydration energy of the ions, which increased the ionization process of the ionic compound in the APAM system. NaCl has higher hydration energy than KCl, because Na is smaller than K, which contributes to the greater solubility of most alkali metals and their high ionic characteristics. Thus, NaCl has higher hydration energy as a strong electrolyte and is more effective in the dissolution rate process than KCl.
Filler/A-PAM
Viscosity (mPa s)
A-PAM Urea, CO (NH2 )2 Potassium chloride, KCl Sodium chloride, NaCl Sodium sulfate, Na2 SO4
1067–1280 746–990 497–913 320–853 300–533
with regard to better flow ability brought about the reduction in viscosity. Fig. 5 presents the 5-day viscosity values for each salt filler type in the A-PAM solution corresponding to 0–30% (w/w) salt fillers loading. Generally, the increasing filler loadings decreased the viscosity in the filled A-PAM system. Increasing the filler loadings for all samples resulted in an increase in the ion concentration, which eventually decreased the viscosity of the A-PAM. In addition, the amount of A-PAM was also reduced in the solution, which increased the hydrodynamic volume of the polymer system and resulted in a lower viscosity. The unfilled (0%) A-PAM had an uninterrupted, high summation of intermolecular forces in the backbone, which restricted the chain mobility and showed the highest viscosity. Except for urea, other fillers with stronger electrolytes produced a much lower viscosity. The viscosity of urea is in the range of 746–990 mPa s, which is a little bit lower compared to the unfilled A-PAM solution. With the lowest ionic strength, urea had less effect on the viscosity compared to the other fillers. No significant changes in viscosity were observed in the A-PAM system after increasing the urea loading. This might be due to the nature of urea as a weak electrolyte that ionizes partially in an aqueous solution. These effects are consistently similar to the results reported by Cohen and Priel [13], Pelton and Allen [14] and Wyatt et al. [15]. The Na2 SO4 filled polymer solution had the lowest range of viscosity, as it is a multivalent salt with a higher concentration of ions in the A-PAM system. Shielding or screening, complex formation, and the spacing effect of the high ion concentration further detached the polymer backbone chain, thus increasing the polymer free volume. This increased the mobility of the A-PAM and further reduced the viscosity. Salts having high solubility and surface area were preferentially selected to reduce the viscosity. 3.3. Effects on potential The potential is the potential at the surface of an electrokinetic unit moving through a solution. The electrokinetic entity may include ions specifically adsorbed from the solution; this will be reflected in the value of the potential, which indicates the degree of repulsion between adjacent, similarly charged particles in the dispersion. The greater the potential, the more likely the suspension is to remain in a stable form [16]. Thus, colloids with high potential (negative or positive) are electrically stabilized, while colloids with low potential tend to coagulate or flocculate as shown in Table 4. The potential values for the salt-filled polymers for five continuous days are illustrated in Fig. 6. Urea follows a similar pattern to A-PAM, whereas the other fillers give fluctuating results.
3.2. Effects on viscosity The range of viscosities for different filler systems is shown in Table 3. The viscosity decreased with the incorporation of fillers. A similar effect of fillers on the A-PAM system as observed by the dissolution rate was also found in the study of viscosity. The ions produced by the fillers reduced the polymer–polymer intermolecular forces, thereby increasing the mobility of the polymer chains. The changes in the rheological properties of the polymer chains
Table 4 Degree of stability as a function of potential [28]. potential [mV]
Stability behavior (colloid)
0 to ±5 ±10 to ±30 ±30 to ±40 ±40 to ±60 ±60 to ±100
Rapid flocculation Incipient instability Moderate stability Good stability Excellent stability
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Fig. 5. The 5-day viscosity values corresponding to filler loadings.
Fig. 6. The potential values of salt loadings corresponding to 5 days of measurement.
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Zingg et al. [17] state that the potential value is highly sensitive to the chemical composition of the solvent (i.e. ionic species, pH, and conductivity). In this case, urea has less ionic strength and less interference with the ionic content of A-PAM. A lower dissociation of the ions in the solution did not yield a significant effect on the potential values. The other fillers showed a random pattern and fluctuated with an increase in filler loadings. The addition of salt resulted in a disruption of the negative ions of the A-PAM to give fluctuating results. This is because increasing the filler loading increases the amount or concentration of ions in the solution. These ions disrupted the negative ions in the A-PAM. The presence of cations and anions of salts disturbed the negatively charged of A-PAM and gave inconsistent readings by day evaluation. 3.4. Effect of fillers on turbidity reduction According to Wong et al. [18], turbidity, an indicator of flocculation efficiency, is a measure of the light-transmitting properties of water with respect to colloidal and residual suspended matter. The effect of various fillers and filler loadings on the reduction of turbidity in kaolin suspensions is shown in Fig. 7. The A-PAM with 10 wt% of fillers showed a decrease in the turbidity reduction compared to the unfilled A-PAM. This is probably due to the decrease in the amount of A-PAM in the solution. Generally, A-PAM causes bridging of the loops and tails between suspended particles. Decreasing the contents reduced the likelihood of the bridging effect between the suspended particles, thus decreasing the turbidity reduction. In this case, the effect of polymers is dominant compared to the effect of ions by the fillers [19]. Except for urea, increasing the salt loading to 20 and 30 wt.% increased the turbidity reduction. This contributed to the difference in the adsorption and flocculation of the negatively charged kaolin particles. The addition of urea decreased the likelihood of the A-PAM adsorption with kaolin. As mentioned earlier, urea is an organic compound and a weak electrolyte. It has relatively less effect than the other fillers but does decrease the A-PAM contents due to the decrease in the kaolin suspension. The addition of KCl increased the turbidity reduction due to its effect on ionic strength in the A-PAM system. Several factors influence this efficiency, including the pH, the type of salt, and the water alkalinity [20]. In this experiment, the addition of KCl contributed to the decrease in
Fig. 7. The effect of various fillers and filler loadings on turbidity reduction.
the double layer of particles due to the shielding of the A-PAM with K+ monovalent ions, which reduced the repulsive forces between the negatively charged polymers and negatively charged kaolin surface. Decreasing the double layer of particles allowed the A-PAM molecules to extend and produce loops and tails to contribute to the bridging mechanism effectively. This led to the increase in turbidity reduction. The turbidity reduction increased with the addition of 20–30 wt% of NaCl and Na2 SO4 due to the concentration of the added salt ions. This reduced the repulsive forces between the anionic segments of the polymeric chains and the negatively charged surfaces of the particles. Consequently, reducing the repulsion forces promoted better adsorption [21]. Although NaCl and Na2 SO4 are both strong electrolyte salts, the effect of Na2 SO4 was much better since Na2 SO4 is a multivalent salt that can very effectively neutralize the negatively charged kaolin. According to Chukwudi et al. [22], a decrease in the compression of the electrical double layer means that the particles can get close enough for the attractive van der Waals forces to dominate the electrostatic repulsions, therefore rendering the suspension unstable. As a result, the particle charge may be partially or completely neutralized,
Fig. 8. Decreased floc size with decreasing polymer concentration for various fillers.
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thereby reducing the components of the electrostatic repulsive forces; hence, flocculation is enhanced [22]. 3.5. Effect of filler on floc size Fig. 8 presents the floc size for different types of fillers and filler loadings on A-PAM. Clearly, increasing the filler contents reduced the floc size. Clark et al. [7] reported that floc size is dependent on the polymer charge content. The addition of filler content from 10 to 30 wt% decreased the concentration of the A-PAM charge content. Decreasing the amount of anionic surface charge of the A-PAM chains reduced the electrostatic repulsive effect between the negatively charged kaolin surface and the A-PAM to increase the adsorption and the bridging mechanism. The adsorption mechanism is also related to the electrostatic charge interaction. Kaolin, with a negative potential, prevents strong adsorption of anionic polymers, leaving long polymer chains to form bridges with kaolin particles and resulting in large flocs [23]. The decreased electrostatic repulsion between the negatively charged A-PAM chains and the negative kaolin surface was related to the addition of fillers, which increased the adsorption and reduced the bridging mechanism (and thus the floc size). Consequently, the flocs produced by the filled samples had smaller dimensions than those produced by the unfilled samples [24]. Increasing the filler loadings of urea, KCl, NaCl, and Na2 SO4 from 10 to 30 wt% reduced the floc size. The decrease in the floc size can be explained by the reduced charge repulsion in producing loops and tails, leading to the small size [25]. Subsequently, A-PAM with 30 wt% of all types of fillers presented small floc size and consistent floc distribution. The effectiveness of the flocculation mechanism was measured not only by the floc size but also by the turbidity reduction. Despite the small size, the results showed good turbidity reduction. In addition, the turbidity reduction of the small floc size was better than for the large floc size. According to Jarvis et al. [25], smaller flocs tend to have greater strength than larger flocs. 4. Conclusion An anionic polyacrylamide (A-PAM)/salt filler solution for wastewater treatment utilizing 10 wt%, 20 wt% and 30 wt% loadings of CO (NH2 )2 , KCl, NaCl, and Na2 SO4 , was prepared. The effects of different types of salt fillers and salt filler loadings on anionic polyacrylamide (A-PAM) were experimentally studied on kaolin. The results showed that the dissolution rate and turbidity reduction improved compared to the unfilled A-PAM solution, while viscosity and flocs decreased. The strong electrolyte salt fillers were more efficient following the order NaCl > Na2 SO4 > KCl > urea. The potential showed a fluctuating value due to disruption of the negative ions of the A-PAM. Acknowledgements We wish to acknowledge the technical support provided by the School of Materials and Mineral Resources Engineering, Engineering Campus Universiti Sains Malaysia, and Acme Chemicals (Malaysia) Sdn. Bhd. The authors also wish to acknowledge the financial support provided by the MOSTI science fund (1001/ pbahan/814131).
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