water Article
Inhibition of Cationic Polymer-Induced Colloid Flocculation by Polyacrylic Acid Voon Huey Lim 1 , Yuji Yamashita 2, * , Yen Thi Hai Doan 1 and Yasuhisa Adachi 2 1 2
*
Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8572, Japan;
[email protected] (V.H.L.);
[email protected] (Y.T.H.D.) Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8572, Japan;
[email protected] Correspondence:
[email protected]; Tel.: +81-29-853-6638
Received: 31 July 2018; Accepted: 6 September 2018; Published: 8 September 2018
Abstract: Although the dosage of cationic flocculants used for water treatment is well known to increase in the presence of natural organic matter (NOM), the underlying reasons for this increase are not properly understood. Herein, we studied the flocculation behavior of polystyrene latex (PSL) particles in the presence and absence of polyacrylic acid (PAA5K) as an NOM analogue using an end-over-end rotation apparatus for standardized flow mixing. In the absence of PAA5K, the initial rate of cationic flocculant (PAM5M)-induced flocculation was enhanced, as reflected by the size of flocculant in solution. Additionally, flocculation experiments were performed in the presence of 0.5 mg/L PAA5K for five concentrations of PAM5M and two ionic strengths, and the increase of the initial rate of PAM5M-induced flocculation was suppressed by PAA5K immediately after mixing, with the most pronounced suppression obtained at a PAM5M concentration similar to PAA5K. Based on the above insights and the results of viscosity measurements, the inhibitory effect of PAA on flocculation was ascribed to (1) the reduction of PAM charge and the concomitant shrinkage via electrostatic association with PAA and (2) the termination of polycation adsorption on PSL caused by polyion complex formation when the charge ratio of PAM:PAA is close to unity. Keywords: polymer flocculation; polystyrene latex; initial flocculation rate; cationic polyelectrolyte; inhibition of flocculation by natural organic matter
1. Introduction The affinity of polyelectrolytes to oppositely charged surfaces is used to promote flocculation in numerous practical applications such as water purification [1], soil conditioning [2], mineral processing [3], and papermaking [4]. In the case of water treatment, the utilization of polyelectrolytes in the coagulation-flocculation process facilitates the control of floc structure and enhances the rate of solid/liquid separation. In addition, polymeric coagulants exhibit a range of advantages (e.g., lower dosage, decreased sludge production, cost-effectiveness, and decreased increment of ionic load and amount of Al in treated water) over the commonly used alum [5]. In general, polymer-induced flocculation is thought to proceed via two mechanisms involving (1) the neutralization of colloidal particle charge by patches of adsorbed polyelectrolyte [6,7] or/and (2) the formation of bridges between colloidal particles by simultaneous adsorption of polyelectrolyte chains onto more than two particles [8,9]. The improvement of flocculation efficiency requires a deep understanding of polyelectrolyte adsorption onto the surface of colloidal particles, and considerable efforts have therefore been directed at deepening our theoretical and experimental understanding of polymer behavior at the interface of thermodynamically well defined surfaces [10–13]. However, despite the large number of corresponding Water 2018, 10, 1215; doi:10.3390/w10091215
www.mdpi.com/journal/water
Water 2018, 10, 1215
2 of 12
studies, the exact mechanism of polymer-induced flocculation has not been clarified yet. For instance, in numerous engineering flocculation practices, it can be easily imagined that systems experience far-from-equilibrium conditions just after the onset of flocculation. That is, the progress and results of flocculation are strongly influenced by non-equilibrium aspects such as the transportation of polymer flocculants from bulk solution to surface of colloidal particles that is followed by a transition from a coil-like solution-phase conformation to a more or less flattened adsorbed-phase conformation [13–15]. To evaluate the transient states of polymer coagulants, we have previously developed a procedure of fluid mixing normalization in terms of colloidal particle collision frequency equivalent to the rate of rapid coagulation [16] and applied the developed technique to the analysis of polymer adsorption-induced flocculation of colloidal particles. One of the most remarkable results obtained from polymer-induced flocculation is that the maximum rate of flocculation in the beginning stage reflects the size of the polymer in solution [17–19]. Most of the recent flocculation investigations were performed under single-flocculant conditions [20–22]. However, real-life water treatment is commonly performed in the presence of naturally-occurring polymers, i.e., natural organic matter (NOM), which are opposite in charge to the flocculants employed [23,24]. The presence of such polymers complicates water treatment by deteriorating water quality and promoting the formation of strongly toxic disinfection by-products such as trihalomethanes. Among the numerous components of NOM, the most abundant ones belong to the class of humic substances and can be further categorized into fulvic and humic acids [25,26], which account for the anionic charge of humic substances and are considered to play an important role in flocculation processes because of their tendency to interact with cationic flocculants. Similar effects are found in the addition of anionic surfactant in the latex flocculation with polycation, where cationic flocculant appeared to be less competent and induces bridging or patch flocculation in low concentration of anionic surfactant [27]. Although the above interaction explains why higher flocculant dosages are required to treat NOM-containing water in the coagulation-flocculation process [28,29], only few studies have investigated the underlying reasons of this behavior. Herein, well-characterized polyacrylic acid (PAA) [30] is used as an NOM analogue with a similar polymer size and effective charge in solution [31,32] to study the inhibitory effect of NOM on the cationic polyelectrolyte-induced flocculation of colloidal particles. Notably, the flocculation rate is shown to be strongly enhanced by the attachment of swollen-chain cationic polyelectrolytes (formed during the transition from the solution-phase coil-like conformation to the adsorbed flattened conformation at the particle interface) to colloidal particles. The behavior of the cationic polyelectrolytes in solution is considered to be most clearly influenced by the presence of PAA at this transient stage. 2. Materials and Methods 2.1. Materials A suspension of monodispersed negatively charged polystyrene latex (PSL; Thermo Fisher Scientific Inc., Waltham, MA, USA) was employed as a model colloid. According to the supplier, PSL was synthesized by standard polymerization of styrene in the absence of surfactants, and the diameter of latex particles was determined as 1.2 ± 0.01 µm by electron microscopy. The initial number concentration of particles, N(0), was set to 5.0 × 107 cm−3 . Prior to flocculation experiments, the PSL suspension was sonicated for 30 min to eliminate aggregates. A linear cationic polyelectrolyte, poly ((2-dimethylamino) ethyl methacrylate) methyl chloride quaternary salt (PAM5M; Kaya Floc Co. Ltd., Tokyo, Japan; nominal molecular weight = 4.9 × 106 g/mol), was used as a flocculant. The charge density of PAM5M, i.e., the ratio of the number of charged segments to the total number of chain segments, equaled 100%. Negatively charged PAA (PAA5K; Wako Pure Chemical Industries Ltd., Osaka, Japan; nominal molecular weight = 5 × 103 g/mol) was employed as a humic substance model. The chemical structures of both polymers are indicated in Figure 1. The stock solution of PAM5M was prepared by dissolving PAM5M in dilute aqueous KCl to the required concentration and was used
Water 2018, 10, 1215
3 of 12
Water 2018, 10,week. x FOR PEER of 12 up within one The REVIEW stock solution of PAA5K was prepared by dissolving PAA5K in pure3water upon 24 h stirring. The prepared polymer solutions were placed in a dark and cold place (5 ◦ C) to a dark cold place (5 °C)degradation. to avoid lightThe exposure-induced degradation. The ionic strengths of asavoid lightand exposure-induced ionic strengths of as-prepared solutions were adjusted prepared solutions were adjusted with 0.1 and 10 mM KCl solutions. KCl solutions and deionized with 0.1 and 10 mM KCl solutions. KCl solutions and deionized water were filtered through a 0.2 µm water were filtered through a 0.2 µm cellulose acetate (CA) membrane filter prior to usage. The initial cellulose acetate (CA) membrane filter prior to usage. The initial pH of PAA5K was determined as pH of PAA5K was determined as 5.3. Although pH was not strictly controlled, the final pH of 5.3. Although pH was not strictly controlled, the final pH of mixtures for each ionic strength and mixtures for each ionic strength and polyelectrolyte concentration was measured as 5.2–5.6 by using polyelectrolyte concentration was measured as 5.2–5.6 by using a glass electrode. Kodama et al. [33] a glass electrode. Kodama et al. [33] reported that the degree of dissociation of PAA in this range of reported theNaCl degree of dissociation of PAA in this range of pH at 10 mM NaCl is ca., 0.35. pH atthat 10 mM is ca., 0.35.
Figure 1. Chemical structures of monomer unit of poly ((2-dimethylamino)ethyl methacrylate) methyl Figure 1. Chemicalsalt structures monomer unit acid of poly ((2-dimethylamino)ethyl chloride quaternary (PAM) of and polyacrylic (PAA). Molecular weightsmethacrylate) of monomer methyl unit for chloride quaternary and polyacrylic acid (PAA). Molecular weights of monomer unit for PAM and PAA are 207.5salt and(PAM) 71 g/mol, respectively. PAM and PAA are 207.5 and 71 g/mol, respectively.
2.2. Analytical Principle
2.2. Analytical Principle The employed analytical principle was based on the rate of collisions between particles in a mixing flow. Notably, the frequency of collisions between equivalent to theinrate The employed analytical principle was based on thecolloid rate of particles collisionsisbetween particles a mixing flow. Notably, the frequency collisionsresult between colloid particles is equivalent the rate of of rapid coagulation, in which case allofcollisions in successful coagulation. For atosuspension rapid coagulation, in which case allparticles collisions result in successful suspension composed of monodispersed spherical with a radius of a0 , thecoagulation. collision rateFor cana be expressed composed of monodispersed spherical particles with a radius of , the collision rate can be as the temporary change of the number concentration of colloidal particles, N(t), as expressed as the temporary change of the number r concentration of colloidal particles, N(t), as 128πε 3 dN (t) (1)(1) (= ) −α a N ( t )2 , = −T dt 15ν 0 ( ) , where ε denotes the rate of energy dissipation per unit mass, ν is the kinematic viscosity of the fluid, where ε denotes the rate of energy dissipation per unit mass, ν is the kinematic viscosity of the fluid, and is the hydrodynamic interaction-determined capture efficiency of collisions between and α T is the hydrodynamic interaction-determined capture efficiency of collisions between colloidal colloidal particles. Assuming that the concept of effective mixing shear rate is valid, can be particles. Assuming that the concept of effective mixing shear rate is valid, α T can be approximately approximately estimated using the approach of Van de Ven et al. [34] as estimated using the approach of Van de Ven et al. [34] as .
(2) ( / ) A p and fluid 3viscosity, αT = , (2) where A and µ denote the Hamaker constant respectively. If we limit our 36µπ (4ε/15πν) a0 discussion to the initial stage of flocculation, when the majority of clusters or particles can be regarded as monomers, Equation 1 can be reduced a first-order expression, and an approximate solution for where A and µ denote the Hamaker constanttoand fluid viscosity, respectively. If we limit our discussion temporal number concentration variation can be expressed as to the initial stage of flocculation, when the majority of clusters or particles can be regarded as =
, !0.18
) (3) monomers, Equation 1 can be reducedlnto a( first-order = − ( expression, + ) , and an approximate solution for ( ) temporal number concentration variation can be expressed as where K is a constant determined by the mechanical setup of the experiment and δ denotes the oncolloidal particles. Figure 2 shows the typical patterns of protruding length of attached polymers N (t) 3 ln = −α T ( aflocculant 0 + δ ) Kt, under a polymer concentration, C(3) flocculation in the presence and absence p, N (0of ) a polymer above the optimal dosage. The solid lines represent polymer-induced flocculation, while the dashed lineKrepresents salt-induced rapid by coagulation. where is a constant determined the mechanical setup of the experiment and δ denotes the protruding length of attached polymers on colloidal particles. Figure 2 shows the typical patterns of flocculation in the presence and absence of a polymer flocculant under a polymer concentration, Cp ,
Water 2018, 10, 1215
4 of 12
above the optimal dosage. The solid lines represent polymer-induced flocculation, while the dashed Water 2018, 10, x FOR PEER REVIEW 4 of 12 line represents salt-induced rapid coagulation.
Figure 2. Schematic diagram of progressive polymer-induced flocculation. Figure 2. Schematic diagram of progressive polymer-induced flocculation.
Association of colloidal particles with polymeric coagulants increases the effective collision radius Association colloidal particles polymers with polymeric coagulants increases increases the effective collision by the protrudingoflength of attached and therefore significantly the collision radius by the protruding length of attached polymers and therefore significantly increases frequency. Assuming that the attached polymer structure is completely permeable, i.e., that the collision frequency. Assuming that the attached polymer is completely permeable,particles i.e., that correction for hydrodynamic interaction is negligibly small,structure the flocculation of polymer-coated the correction forashydrodynamic interaction is negligibly small, the flocculation of polymer-coated can be described N (t) particles can be described as ln = −α T a30 Kt. (4) N ((0)) ln ( ) = − . (4) Therefore, the enhancement factor ξ, which defines the ratio of the initial polymer-induced flocculation rate the andenhancement the rate of salt-induced rapid coagulation can be Therefore, factor ξ, which defines theunder ratio polymer-free of the initial conditions, polymer-induced expressed as flocculation rate and the rate of salt-induced rapid coagulation under polymer-free conditions, can ( a0 + δ )3 be expressed as ξ= . (5) α T a30 ( + ) (5) . In our previous studies, the value of=δ was confirmed to be correlated with the size of solution-phase polymer molecules rather the thickness of the adsorbed polymer in the In our previous studies, the value of than was with confirmed to be correlated with the size of solutionequilibrium state. This finding thatthe a remarkable is expected case phase polymer molecules ratherimplies than with thickness of behavior the adsorbed polymerininthe theextreme equilibrium when the bare surfaces of colloidal particles are exposed to an excess dosage of polymer flocculant. state. This finding implies that a remarkable behavior is expected in the extreme case when the bare That is, the factor initially increases with increasing whereas surfaces of enhancement colloidal particles are exposed to an excess dosage of polymer polymerconcentration, flocculant. That is, the flocculation is abruptly terminated when the polymer particle surface becomes saturated with the enhancementprogress factor initially increases with increasing concentration, whereas flocculation polymer 2). The durationwhen of thethe enhanced is shorter forsaturated the case of higher progress (Figure is abruptly terminated particlestage surface becomes with the polymer polymer concentrations. This is due to saturation of the attached polymer on colloidal surfaces that polymer induces (Figure 2). The duration of the enhanced stage is shorter for the case of higher steric stabilization. concentrations. This is due to saturation of the attached polymer on colloidal surfaces that induces
steric stabilization. 2.3. Particle Aggregation Analysis 2.3. Particle ColloidAggregation mixing wasAnalysis induced by the turbulent flow generated by the rocking motion of a forked flask Colloid installedmixing on an end-over-end shaker, asflow illustrated in Figure The detail information can was induced rotation by the turbulent generated by the3.rocking motion of a forked be found elsewhere [35]. In the beginning, 5 mL of the PSL dispersion was placed into one side of flask, flask installed on an end-over-end rotation shaker, as illustrated in Figure 3. The detail information and thefound other end was filled 5 mL of PAM5M solution KCldispersion solution. The solution and can be elsewhere [35].with In the beginning, 5 mL of theorPSL wasPAM5M placed into one side the PSL dispersion with/without PAA5K were mixed using the end-over-end rotation apparatus at a of flask, and the other end was filled with 5 mL of PAM5M solution or KCl solution. The PAM5M rotation frequency of 1 Hz until the predetermined number of mixing times was reached. The degree solution and the PSL dispersion with/without PAA5K were mixed using the end-over-end rotation of coagulation/flocculation was monitored determining the total number apparatus at a rotation frequency of 1 Hz by until the predetermined numberofofclusters, mixing N(t), timesusing was areached. CoulterThe counter (Multisizers 4e, Beckman Coulter Inc., Brea, CA, USA). In order to evaluate degree of coagulation/flocculation was monitored by determining the total numberthe of effects of the polymer swelling degree in solution, experiments were carried out at two different clusters, N(t), using a Coulter counter (Multisizers 4e, Beckman Coulter Inc., Brea, CA, USA). In order ionic strengths. to evaluate the effects of the polymer swelling degree in solution, experiments were carried out at
two different ionic strengths.
Water 2018, 10, 1215 Water 2018, 10, x FOR PEER REVIEW
5 of 12 5 of 12
Figure 3. Schematic diagram of the employed end-over-end rotation shaker. Figure 3. Schematic diagram of the employed end-over-end rotation shaker.
The effect of the transient state was analyzed by performing experiments at two different PAM5M The effect Based of theon transient state studies was analyzed by performing two different concentrations. our previous of the same polycation experiments [19], PAM5Matconcentrations PAM5M concentrations. Based on our previous studies of the same polycation [19], PAM5M of 2.5 and 0.5 mg/L were selected. The concentration of PAA5K was set to 0.5 mg/L, and the concentrations 2.5 and 0.5 mg/L selected. The concentration of PAA5K to 0.5 inhibitory effectofwas analyzed for were five different PAM5M concentrations (2.5,was 1.5,set 0.6, 0.5,mg/L, and and the inhibitory effect results was analyzed for five different PAM5M 1.5, 0.6, 0.5, and 0.4 mg/L). The obtained were compared with those of theconcentrations corresponding(2.5, blank experiments. 0.4 mg/L). The obtained results were compared with those of the corresponding blank experiments. For experiments conducted in the presence of PAA5K, 4 mL of the PSL dispersion was mixed with conducted the presencewhereas of PAA5K, 4 mL experiments, of the PSL dispersion wassolution mixed with 1For mLexperiments of the PAA5K solutioninbeforehand, in blank the PAA5K was1 mL of the PAA5K solution beforehand, whereas in blank experiments, the PAA5K solution was replaced by an equal volume of deionized water. All sets of experiments were repeated at least twice equal volume of the deionized water. All sets of experiments were repeated at least twice toreplaced confirmby theanreproducibility obtained results. to confirm the reproducibility of the obtained results. 2.4. Viscosity Measurements 2.4. Viscosity Measurements The viscosities of polymer-only solutions were measured by a capillary viscometer and used to determine theof size of polymer aggregates employing Einstein’s viscosityviscometer equation and The viscosities polymer-only solutions were measured by a capillary andnominal used to molecular weights provided by suppliers, with the results obtained for each ionic strength summarized determine the size of polymer aggregates employing Einstein’s viscosity equation and nominal inmolecular Table 1. When two provided polymers were present, viscosity measurements were employed confirm the weights by suppliers, with the results obtained for each to ionic strength formation of polyion [36,37]. Specifically, PAA5Kviscosity and PAM5M solutions were employed mixed to summarized in Tablecomplexes 1. When two polymers were present, measurements afford the desired concentration ratioscomplexes and gently[36,37]. shakenSpecifically, for 60 times. The resulting mixtures were to confirm the formation of polyion PAA5K and PAM5M solutions centrifuged 3520, Corp., Osaka, Japan) forand 5 min at 150 rpm, and thetimes. supernatants were were mixed(Model to afford theKubota desired concentration ratios gently shaken for 60 The resulting sampled used for viscosity measurements. reduce the influence of 5human and ensure mixturesand were centrifuged (Model 3520, KubotaToCorp., Osaka, Japan) for min aterror 150 rpm, and the the reliability of acquired data, measurements conductedTo at reduce high polymer concentrations supernatants were sampled andall used for viscositywere measurements. the influence of human (50 mg/L 50–250 mg/L PAM5M). error andPAA5K ensure and the reliability of acquired data, all measurements were conducted at high polymer concentrations (50 mg/L PAA5K and 50–250 mg/L PAM5M). Table 1. Hydrodynamic diameters (ap ) of polymers estimated by the Einstein’s viscosity equation. ◦ and initial PAA5K and PAM5M All measurements were conducted temperature Table 1. Hydrodynamic diametersat(aapcontrolled ) of polymers estimatedofby20theCEinstein’s viscosity equation. All concentrations 50 mg/L. measurementsofwere conducted at a controlled temperature of 20 °C and initial PAA5K and PAM5M
concentrations of 50 mg/L.
ap (nm) Sample a p (nm) 0.1 mM KCl Sample 10 mM KCl 10 mM KCl 0.1 mM KCl PAA5K 6.4 7.2 6.4 7.2 341 PAM5MPAA5K 277 PAM5M 277 341
3.3.Results Results 3.1. Polycation-Induced Flocculation 3.1. Polycation-Induced Flocculation Figure 4 depicts the results of salt-induced rapid coagulation experiments, showing that Figure 4 depicts the results of salt-induced rapid coagulation experiments, showing that ln(N(t)/N(0)) was linearly dependent on the mixing time (s), which verified the validity of the ln(N(t)/N(0)) was linearly dependent on the mixing time (s), which verified the validity of the applied
Water 2018, 10, 1215
6 of 12
Water 2018, 10, 10, x FOR PEER REVIEW Water 2018, x FOR PEER REVIEW
6 of 6 of 12 12
applied method. The slope of the above plot was used in the subsequent analysis of data obtained for method. TheThe slope of ofthe analysis ofof data dataobtained obtainedforfor method. slope theabove aboveplot plotwas wasused used in in the the subsequent subsequent analysis numerous conditions. numerous conditions. numerous conditions.
00 0.00 0.00
1010
Mixing Time Time (s) Mixing 20 20 30
40 40
50 50
ln N(t)/N(0)
ln N(t)/N(0)
-0.05 -0.05 -0.10 -0.10
1 M KCl
1 M KCl
-0.15 -0.15 -0.20 -0.20 -0.25 -0.25
Figure 4. Variation of the total number of clusters per unit volume N(t) with the number of mixing Figure 4. Variation of number of per −3unit volume Figure of the the total total number of clusters clusters volume N(t) N(t) with with the the number number of of mixing mixing time4.(s)Variation for salt-induced coagulation (N(0) = 5 × 1077per cm unit −).3 ). time (s) for salt-induced coagulation (N(0) = 5 × 10 cm 7 −3 time (s) for salt-induced coagulation (N(0) = 5 × 10 cm ).
Figure 5 shows the results obtained for polycation-induced flocculation at different ionic Figure 5 shows the results obtained for polycation-induced flocculation at different ionic strengths Figure 5(0.1 shows results obtained for flocculation at different strengths and 10the mM), demonstrating that polycation-induced the enhancement factor for the PAA5K-free caseionic is (0.1 and 10 mM), demonstrating that the enhancement factor for the PAA5K-free case is consistent consistent reported elsewhere that [17,19]. calculated enhancement are shown in is strengths (0.1with andvalues 10 mM), demonstrating theThe enhancement factor for thefactors PAA5K-free case with values reported elsewhere [17,19]. The calculated enhancement factors are shown in Table 2. Table 2. with values reported elsewhere [17,19]. The calculated enhancement factors are shown in consistent
Table 2.
Figure 5. Temporal variation of polycation-induced flocculation rate in 0.1 and 10 mM KCl. Figure 5. Temporal variation of polycation-induced flocculation rate in 0.1 and 10 mM KCl. Table 2. Enhancement factors obtained for polymer-induced flocculation under different conditions. Table 2. Enhancement factors obtained for polymer-induced flocculation conditions. Figure 5. Temporal variation of polycation-induced flocculation rate under in 0.1 different and 10 mM KCl. ξ Sample ξ 10 mM KClflocculation 0.1 under mM KCl Samplefor polymer-induced Table 2. Enhancement factors obtained different conditions. 10 mM KCl 0.1 mM KCl 0.5 mg/L PAM5M 8.76 0.5 mg/L PAM5M 8.76 10.54 10.54 2.5 mg/L Sample PAM5M 10.64 ξ N/A
2.5 mg/L PAM5M 10 mM 10.64 KCl
N/A KCl 0.1 mM 0.5 of mg/L PAM5M 8.76 10.54 3.2.3.2. Flocculation in in thethe Presence Two Oppositely Flocculation Presence of Two OppositelyCharged ChargedPolymers Polymers 2.5 mg/L PAM5M 10.64 N/A Figure 6 shows the influence of PAA5K on theon PAM5M-induced flocculation of PSLof at PSL two different Figure 6 shows the influence of PAA5K the PAM5M-induced flocculation at two ionic strengths, demonstrating that the PAA5K significantly inhibits flocculation. This inhibitory different ionic strengths, demonstrating that the PAA5K significantly inhibits flocculation. This 3.2. Flocculation in the Presence of Two Oppositely Charged Polymers
Figure 6 shows the influence of PAA5K on the PAM5M-induced flocculation of PSL at two different ionic strengths, demonstrating that the PAA5K significantly inhibits flocculation. This
Water 2018, 10, x FOR PEER REVIEW Water 2018, 10, 1215 Water 2018, 10, x FOR PEER REVIEW
7 of 12 7 of 12 7 of 12
inhibitory effect was most pronounced when the concentration of PAM5M was equal or less than inhibitory effect in was mostcase pronounced when concentration of PAM5M wasthe equal or less than that ofwas PAA5K, which flocculation wasthe essentially stopped. Moreover, enhancement of effect most pronounced when the concentration of PAM5M was equal or less than that of that of PAA5K, in which case flocculation was essentially stopped. Moreover, the enhancement of the initial was disturbed by the presence of PAA5K, andthe thisenhancement disturbance became PAA5K, in flocculation which case rate flocculation was essentially stopped. Moreover, of the the initial flocculation was disturbed by the presence of PAA5K, andFigure this disturbance became more pronounced withrate increasing PAM5M/PAA5K concentration 7 shows the initial initial flocculation rate was disturbed by the presence of PAA5K,ratio. and this disturbance became more pronounced with increasing PAM5M/PAA5K concentration ratio. Figure 7 rates shows the clearly initial flocculation rates (0 to 10 s) observed under differentconcentration conditions, revealing that the were more pronounced with increasing PAM5M/PAA5K ratio. Figure 7 shows the initial flocculation ratespresence (0 to 10 s) observed under different conditions, revealing that the rates were clearly reduced in rates the PAA5K under underdifferent the sameconditions, concentration ratio.that Conversely, at anclearly excess flocculation (0 to 10 s)ofobserved revealing the rates were reduced in the presence of PAA5K under the same concentration ratio. Conversely, at an excess dosage of PAM5M (2.5of mg/L), a slight in the rate ratio. of flocculation canatbe in the reduced in the presence PAA5K underenhancement the same concentration Conversely, anobserved excess dosage dosage of of PAM5M (2.5 mg/L), a slight enhancement in the rate of flocculation can be observed in the PAA5K. ofpresence PAM5M (2.5 mg/L), a slight enhancement in the rate of flocculation can be observed in the presence presence of PAA5K. of PAA5K.
Figure6.6. Temporal Temporal variation variation of of polycation-induced polycation-induced flocculation flocculation rate rate in in 0.1 0.1 mM mM and and10 10mM mMKCl KCl Figure Figure 6. Temporal variation of polycation-induced flocculation rate in 0.1 mM and 10 mM KCl determined in the presence of 0.5 mg/L PAA5K. determined in the presence of 0.5 mg/L PAA5K. determined in the presence of 0.5 mg/L PAA5K.
Figure 7. Temporal variation of the rate of polycation-induced flocculation in 0.1 mM and 10 mM KCl Figure 7. Temporal variation of the rate of polycation-induced flocculation in 0.1 mM and 10 mM KCl with/without PAA5K. Figure 7. Temporal variation of the rate of polycation-induced flocculation in 0.1 mM and 10 mM KCl with/without PAA5K. with/without PAA5K.
Water 2018, 10, 1215
8 of 12
Water 2018,of10,PAM5M x FOR PEER REVIEW 3.3. Reduction Viscosity
8 of 12
3.3. Reduction of PAM5M Viscosity Figure 8 shows that the viscosity of PAM5M solutions increased with increasing polymer concentration at all ionic strengths. Notably,ofthe viscosities of 50increased mg/L PAA5K solutionspolymer approximately Figure 8 shows that the viscosity PAM5M solutions with increasing equaled that of the at solvent since the molecular of this polymer was PAA5K much less than that of concentration all ionic strengths. Notably,weight the viscosities of 50 mg/L solutions approximately equaled that of the solvent since thePAM5M-PAA5K molecular weight ofsupernatant this polymer solutions was much less PAM5M. On the other hand, data obtained for the confirmed than that of PAM5M. On thereduces other hand, obtainedof forPAM5M the PAM5M-PAA5K that the presence of PAA5K thedata viscosity solutions.supernatant It shouldsolutions be noted that confirmed that the presence of PAA5K reduces the viscosity of PAM5M solutions. It should be noted the viscosity of PAM5M solutions decreased to approximately that of PAA5K solutions when the that the viscosity of PAM5M solutions decreased to approximately that of PAA5K solutions when concentrations of both polymers were equal, at which point PAM5M was completely neutralized by the concentrations of both polymers were equal, at which point PAM5M was completely neutralized PAA5KbytoPAA5K form solid complexes that were by centrifugation, in agreement with thethe results of to form solid complexes thatremoved were removed by centrifugation, in agreement with flocculation measurements. resultsrate of flocculation rate measurements. 1.8
PAM5M only (0.1 mM KCl)
1.6
Specific viscosity
1.4
PAM5M with 50 mg/L PAA5K (0.1 mM KCl) PAM5M only (10 mM KCl)
1.2 1 0.8 0.6 0.4
PAM5M with 50 mg/L PAA5K (10 mM KCl)
0.2 0 0
50 100 150 200 Concentration of PAM5M (mg/L)
250
Figure 8. Specific viscosities of PAM5M and PAM5M-PAA5K supernatant solutions as functions of Figure 8. Specific viscosities of PAM5M and PAM5M-PAA5K supernatant solutions as functions of PAM5M concentration at KCl = 0.1 mM PAM5M concentration at KCl = 0.1 mMand and10 10 mM. mM
4. Discussion 4. Discussion 4.1. Enhancement of Initial Flocculation Rate 4.1. Enhancement of Initial Flocculation Rateby byPAM5M PAM5M Detailed analysis of the acquired data some minor deviations. Figure 5Figure shows that at low that at Detailed analysis of the acquired datareveals reveals some minor deviations. 5 shows ionic strength, the initial flocculation rate was significantly enhanced in the presence of 2.5 low ionic strength, the initial flocculation rate was significantly enhanced in the presence mg/L of 2.5 mg/L PAM5M, an even morepronounced pronounced enhancement observed for a PAM5M concentration of 0.5 PAM5M, with with an even more enhancement observed for a PAM5M concentration of mg/L. This scenario can be explained by enhancement factor and polymer adsorption rates as 0.5 mg/L. This scenario can be explained by enhancement factor and polymer adsorption rates as illustrated in Figure 2. In high Cp condition, adsorption of polymers on colloidal particles are fast, illustrated Figuresaturation 2. In high Cp dosage. condition, adsorption of polymers onofcolloidal particles are fast, whichininduces in low However, in this case, the duration the initial flocculation which induces saturation in low dosage. However, in this case, the duration of the initial flocculation rate enhancement stage was too short in view of the swollen conformation of PAM5M chains, which corresponds to the case oftoo low short Cp. Meanwhile, p, adsorption is continuously taking place until which rate enhancement stage was in view in oflow theCswollen conformation of PAM5M chains, flocculation essentially stopped. In contrast, theCresult obtained isatcontinuously high ionic strength corresponds to theiscase of low C in low taking was place until p . Meanwhile, p , adsorption consistent with thatstopped. predictedInincontrast, Figure 2.the Adachi al. [7] reported that strength both flocculation flocculation is essentially result et obtained at high ionic was consistent mechanisms; bridging and charge neutralization can be distinguished by the slope (the rate) of with that predicted in Figure 2. Adachi et al. [7] reported that both flocculation mechanisms; bridging flocculation. Remarkable enhancement accompanied by abrupt termination of flocculation indicates and charge neutralization canplace be distinguished bywhich the slope (the rate) of flocculation. Remarkable that flocculation is taking through bridging, is observed in Figure 5 and illustrated in enhancement accompanied by abrupt termination of flocculation indicates that flocculation is taking Figure 2. This behavior is contrast with the latter mechanism, where flocculation incessantly occurs place through which is observed in Figure 5 and illustrated in Figure 2. This behavior with a ratebridging, similar to salt-induced rapid coagulation. of flocculation rates where observed for both ionic strengths indicated is contrast Comparison with the latter mechanism, flocculation incessantly occurs that with(1)a the ratemost similar to pronounced increase of the initial flocculation rate was observed at low ionic strength and (2) the salt-induced rapid coagulation. highest maximum flocculation rate was observed at high ionic strength. The first finding can be Comparison of flocculation rates observed for both ionic strengths indicated that (1) the most explained by the incubation of electrolyte concentrations. At low ionic strength, PAM5M molecules pronounced of theform initial flocculation rateform was because observed ionic strength and (2)ionic the highest exhibitincrease an elongated rather than a coiled of at thelow strong repulsion between maximum flocculation rate was observed at high ionic strength. The first finding can be explained by the incubation of electrolyte concentrations. At low ionic strength, PAM5M molecules exhibit an elongated form rather than a coiled form because of the strong repulsion between ionic groups in the polymer chain, which increases the collision size of the polyelectrolyte and hence promotes the
Water 2018, 10, 1215
9 of 12
collisions of the polyelectrolyte and particles. On the other hand, the reduction of the above repulsion Water 2018, 10, x FOR PEER REVIEW 12 at high ionic strength leads to the accumulation of counter-ions and results in the adoption of9 ofcoil-like conformations. This explanation is consistent with the results of viscosity measurements (Figure 8), groups in the polymer chain, which increases the collision size of the polyelectrolyte and hence wherepromotes a steeperthe slope was observed at low ionic and strength. Furthermore, Table 1 confirms that of thethe size of collisions of the polyelectrolyte particles. On the other hand, the reduction polyelectrolyte molecules decreases with increasing ionic strength. Finally, the increase of the maximum above repulsion at high ionic strength leads to the accumulation of counter-ions and results in the flocculation rate increasing ionic strength can be explained by the with concomitant reduction of Debye adoption ofwith coil-like conformations. This explanation is consistent the results of viscosity measurements (Figure 8), where a steeper to slope observed at At lowlow ionic strength. Furthermore, length, K−1 , which is inversely proportional salt was concentration. ionic strength, the repulsion Table 1 confirms that is thelong sizeranged of polyelectrolyte decreases withand increasing strength.of the between polyelectrolytes due to themolecules swollen conformation inducesionic saturation Finally, the increase of the maximum flocculation rate with increasing ionic strength can be explained particle surface at lower adsorbed amounts. When the salt level was increased, coil-like conformation by the concomitant of Debye length, K−1, which is inversely proportional to charged salt of polyelectrolytes owingreduction to screening of the intramolecular electrostatic repulsion between concentration. At low ionic strength, the repulsion between polyelectrolytes is long ranged due to the segments resulted in local accumulation of positive charge on the negatively charged particle. The tail swollen conformation and induces saturation of the particle surface at lower adsorbed amounts. of adsorbed polyelectrolytes would then attach to neighboring bare particles due to electrostatic When the salt level was increased, coil-like conformation of polyelectrolytes owing to screening of attraction. As a result,electrostatic larger amounts of between polyelectrolytes are needed to induce of the the intramolecular repulsion charged segments resulted in local saturation accumulation particles while particle-to-particle collision that particle. leads toThe flocculation is proceeding. of positive charge on the negatively charged tail of adsorbed polyelectrolytes would then attach to neighboring bare particles due to electrostatic attraction. As a result, larger amounts of
4.2. Shrinkage of PAM5M in the Presence PAA5K of the particles while particle-to-particle collision polyelectrolytes are needed to induceofsaturation
that leads to most flocculation is proceeding. One of the significant results of the present study is the disturbing effect of PAA5K on the large flocculation rate increase observed in the presence of PAM5M. Notably, this disturbance 4.2. Shrinkage of PAM5M in the Presence of PAA5K becomes more pronounced with decreasing PAM5M:PAA5K concentration ratio (Figure 9). The above One explained of the mostby significant results of the present study is the disturbing of PAA5K of onPAM5M the behavior was the regulatory effect of PAA5K adsorption on theeffect conformation large flocculation rate increase observed in the presence of PAM5M. Notably, this disturbance chains exhibiting electrostatic repulsion-induced swelling, i.e., neutralized parts of the latter chains are becomes more pronounced with decreasing PAM5M:PAA5K concentration ratio (Figure 9). The no longer involved in repulsive interactions, and the persistence length of the polymer is therefore above behavior was explained by the regulatory effect of PAA5K adsorption on the conformation of considerably decreased (Figure 10). Therefore, the charge density of the excess PAM5M dosage PAM5M chains exhibiting electrostatic repulsion-induced swelling, i.e., neutralized parts of the latter (2.5 mg/L) was reduced to the charge density in a lower dosage condition, which resulted in a chains are no longer involved in repulsive interactions, and the persistence length of the polymer isslight increase of theconsiderably flocculationdecreased rate. In-depth of Figure provides an insight of thePAM5M flocculation therefore (Figureanalysis 10). Therefore, the6charge density of the excess dosage (2.5 mg/L) was to the charge density in a lower which resulted in mechanisms involved. Inreduced the initial stage of flocculation (0 todosage 10 s), condition, the rate of flocculation of the a slight increase of enhanced the flocculation In-depthofanalysis ofradius Figure by 6 provides an insight of PAM5M the 2.5 mg/L PAM5M was by therate. increment collision protruding chain of flocculation mechanisms involved.particle In the initial of flocculation (0 to 10 thelater rate of flocculation attached to the surface of colloidal andstage gradually terminated ins), the stage (10 to 50 s). of the 2.5 mg/L PAM5M was enhanced by the increment of collision radius by protruding chain of of Flocculation in this condition can be interpreted as bridging flocculation. With the exception PAM5M attached to the surface of colloidal particle and gradually terminated in the later stage (10 to PAM5M:PAA5K concentration ratio of 3:1, flocculation proceeds persistently at high ionic strength, 50 s). Flocculation in this condition can be interpreted as bridging flocculation. With the exception of which is an effect of charge neutralization by patch interaction [7]. Notably, inhibition of the initial PAM5M:PAA5K concentration ratio of 3:1, flocculation proceeds persistently at high ionic strength, flocculation was of most pronounced at high ionicinteraction strength [7]. in view of the counter-ion screening which israte an effect charge neutralization by patch Notably, inhibition of the initial effectsflocculation and the reduction PAM5M chargeatdensity by PAA5K. subsequently observed flocculation rate was of most pronounced high ionic strength The in view of the counter-ion screening behavior at aand PAM5M:PAA5K ratio of 3:1 resembled thatThe of salt-induced effects the reduction concentration of PAM5M charge density by PAA5K. subsequently coagulation observed at flocculation behavior at a no PAM5M:PAA5K concentration ratioatoflow 3:1 ionic resembled that of salt-induced high ionic strength, whereas such similarity was observed strength. Such trend can be coagulation at high ionic strength, whereas no such similarity was observed at low ionic strength. explained by Aoki and Adachi [19], where the presence of counter-ions and smaller polyelectrolytes Suchthe trend canrequired be explained by Aoki and Adachi [19],saturation. where the presence of counter-ions and smaller prolongs time for PSL particles to reach polyelectrolytes prolongs the time required for PSL particles to reach saturation.
Figure 9. Schematic diagram of progressive polycation-induced flocculation in the presence of PAA5K.
Water 2018, 10, x FOR PEER REVIEW
10 of 12
Figure 9. Schematic diagram of progressive polycation-induced flocculation in the presence of 10 of 12 PAA5K.
Water 2018, 10, 1215
Figure 10. Schematic representation of conformational changes of PAM5M upon the adsorption Figure 10. Schematic representation of conformational changes of PAM5M upon the adsorption of of PAA5K. PAA5K.
4.3. Polyion Complex Formation 4.3. Polyion Complex Formation The results of viscosity measurements demonstrated that at a constant concentration ratio, The resultsbetween of viscosity demonstrated that at aresulted constantinconcentration the the attraction two measurements oppositely charged polyelectrolytes the formationratio, of solid attraction two oppositely charged polyelectrolytes resulted in solid complexes,between which decreased the content of soluble polymer molecules in the the formation suspensionofand its complexes, which decreased the content of soluble polymer molecules in the suspension and its viscosity. The nature of solid complexes formation can be explained by considering the total viscosity. The nature ofof solid canIzumrudov be explained by Sybachin considering thereported total charge charge concentrations the complexes polymers formation in solution. and [38] that concentrations of the polymers in solution. Izumrudov and Sybachin [38] reported that nonnon-equimolar mixture of quaternary polyamines and polycarboxylates in a salt solution makes an equimolar mixture of quaternary polyamines and of polycarboxylates in concentrations a salt solution of makes an insoluble polyelectrolyte complex in a certain range ratio of the molar charged insoluble complex inand a certain range ofa ratio of theturbidity molar concentrations groups of polyelectrolyte polycation and polyanion demonstrates maximum when the ratioofischarged close to groups of polycation and polyanion and demonstrates a maximum turbidity when the is close unity. The molar concentrations of charged groups of PAM5M and PAA5K in solution can ratio be estimated to The molar concentrations of of charged groups PAA5K in solution can be by unity. multiplying the molar concentration monomer unit of by PAM5M the degreeand of dissociation. At 0.5 mg/L of estimated by multiplying the molar concentration of monomer unit byPAA5K the degree of dissociation. At polymers, molar concentrations of the monomer unit of PAM5M and are 2.36 × 10−6 mol/L −6 mol/L, 0.5 of 10 polymers, molar concentrations of thethe monomer unit of PAM5Mofand PAA5K arePAA5K 2.36 × andmg/L 7.04 × respectively. Therefore, charge concentrations PAM5M and −6 −6 − 6 − 6 10 mol/L × 10and mol/L, Therefore, the charge concentrations PAM5M are 2.41 × and 10 7.04 mol/L 2.45 respectively. × 10 mol/L by adopting the values of 1.0 and of 0.35 [33] asand the −6 mol/L and 2.45 × 10−6 mol/L by adopting the values of 1.0 and 0.35 [33] as the PAA5K are 2.41 × 10 degree of dissociation of PAM5M and PAA5K, respectively. In the case of a concentration ratio of 1:1, degree of dissociation of PAM5M andinPAA5K, thepolycation case of a concentration ratio which of 1:1, the respective distribution of charge solutionrespectively. of polyanionInand are approximate, the respective distribution of charge in solution of polyanion and polycation areHence, approximate, which gives rise to complete charge neutralization through electrostatic interactions. the formation gives rise to complete charge neutralization through electrostatic interactions. Hence, the formation of polyion complexes was concluded to terminate the adsorption of polycations on PSL and thus of polyion complexes was concluded to terminate the adsorption of polycations on PSL and thus inhibit flocculation. inhibit flocculation. 5. Conclusions 5. Conclusions The large enhancement of the initial flocculation rate observed at excess dosages of a cationic polymer was remarkably in the presence of an oppositely small The flocculant large enhancement of the inhibited initial flocculation rate observed at excesscharged dosages of a polymer. cationic This phenomenon was attributed to the adsorption of this oppositely charged polymer onto the cationic polymer flocculant was remarkably inhibited in the presence of an oppositely charged small polymer. flocculant, which resulted in electrostatic certain chain parts and therefore This phenomenon was attributed to theneutralization adsorption ofofthis oppositely charged polymerdecreased onto the the degree of swelling. Subsequently, decreased the net charge of of certain cationicchain flocculant and therefore effective cationic flocculant, which resulted in electrostatic neutralization parts and collision radius of particles. This effect was more significant strength due to the presence decreased the degree of swelling. Subsequently, decreased in thehigh net ionic charge of cationic flocculant and of counter-ions, and henceofinduced patch at specific concentration ratio strength of PAM5M:PAA5K. effective collision radius particles. Thisflocculation effect was more significant in high ionic due to the Therefore, it was concludedand that hence the same electrostatic can at occur between negatively charged presence of counter-ions, induced patch attraction flocculation specific concentration ratio of NOM and the positively charged flocculant which, atattraction a charge ratio closebetween to unity, PAM5M:PAA5K. Therefore, it waspolymer concluded that thebackbone, same electrostatic can occur caused complete occur tocharged form solid complexes. Thus, flocculation negatively chargedneutralization NOM and theto positively polymer flocculant backbone, which,ofatcolloidal a charge particles occurred onlycaused when the demandneutralization of PAA5K for to theoccur cationic charge was satisfied. ratio close to unity, complete to flocculant form solid complexes. Thus, In the presentofstudy, we have conducted experiments under weakly conditions. Similar flocculation colloidal particles occurred only when the only demand of acidic PAA5K for the cationic experiments over a wider range of pH conditions will lead to more comprehensive knowledge.
Water 2018, 10, 1215
11 of 12
Author Contributions: Conceptualization, Y.Y. and Y.A.; Methodology, Y.Y. and Y.A.; Validation, V.H.L. and Y.T.H.D.; Formal Analysis, Y.Y. and Y.A.; Investigation, V.H.L. and Y.T.H.D.; Resources, Y.Y. and Y.A.; Data Curation, Y.Y.; Writing-Original Draft Preparation, V.H.L.; Writing-Review & Editing, Y.Y., Y.T.H.D. and Y.A.; Visualization, V.H.L.; Supervision, Y.A.; Project Administration, Y.Y.; Funding Acquisition, Y.Y. and Y.A. Funding: This research was funded by the Japan Society for the Promotion of Science [grant numbers 16H06382, 15K12236]; and the Kurita Water and Environment Foundation [grant number 14A073]. The APC was funded by the Japan Society for the Promotion of Science [grant number 16H06382]. Conflicts of Interest: The authors declare no conflict of interest.
References 1.
2.
3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
20.
Kadooka, H.; Kiso, Y.; Goto, S.; Tanaka, T.; Jami, M.S.; Iwata, M. Flocculation behavior of colloidal suspension by use of inorganic and polymer flocculants in powder form. J. Water Process. Eng. 2017, 18, 169–175. [CrossRef] Zezin, A.; Mikheikin, S.; Rogacheva, V.; Zansokhova, M.; Sybachin, A.; Yaroslavov, A. Polymeric stabilizers for protection of soil and ground against wind and water erosion. Adv. Colloid Interface Sci. 2015, 226, 17–23. [CrossRef] [PubMed] Farrow, J.B.; Swift, J.D. A new procedure for assessing the performance of flocculants. Int. J. Miner. Process. 1996, 46, 263–275. [CrossRef] Rojas, J.; Hubbe, M.A. The dispersion science of papermaking. J. Dispers. Sci. Technol. 2005, 25, 713–732. [CrossRef] Bolto, B.; Gregory, J. Organic polyelectrolytes in water treatment. Water Res. 2007, 41, 2301–2324. [CrossRef] [PubMed] Gregory, J. Rates of flocculation of latex particles by cationic polymers. J. Colloid Interface Sci. 1973, 42, 448–456. [CrossRef] Adachi, Y.; Feng, L.; Kobayashi, M. Kinetics of flocculation of polystyrene latex particles in the mixing flow induced with high charge density polycation near the isoelectric point. Colloids Surf. A Physicochem. Eng. Asp. 2015, 471, 38–44. [CrossRef] Wang, T.K.; Audebert, R. Flocculation mechanisms of a silica suspension by some weakly cationic polyelectrolytes. J. Colloid Interface Sci. 1987, 119, 459–465. [CrossRef] Zhou, Y.; Franks, G.V. Flocculation mechanism induced by cationic polymers investigated by light scattering. Langmuir 2006, 22, 6775–6786. [CrossRef] [PubMed] Fleer, G.; Stuart, M.C.; Scheutjens, J.; Cosgrove, T.; Vincent, B. Polymers at Interfaces, 1st ed.; Chapman Hall: London, UK, 1993. De Gennes, P.G. Polymers at an interface; a simplified view. Adv. Colloid Interface Sci. 1987, 27, 189–209. [CrossRef] Netz, R.R.; Andelman, D. Neutral and charged polymers at interfaces. Phys. Rep. 2003, 380, 1–95. [CrossRef] Gregory, J. Polymer adsorption and flocculation in sheared suspensions. Colloids Surf. 1988, 31, 231–253. [CrossRef] Peng, P.; Garnier, G. Effect of cationic polyacrylamide adsorption kinetics and ionic strength on precipitated calcium carbonate flocculation. Langmuir 2010, 26, 16949–16957. [CrossRef] [PubMed] Adachi, Y. Dynamic aspects of coagulation and flocculation. Adv. Colloid Interface Sci. 1996, 56, 1–31. [CrossRef] Adachi, Y.; Stuart, M.A.C.; Fokkink, R. Kinetics of turbulent coagulation studied by means of end-over-end rotation. J. Colloid Interface Sci. 1994, 165, 310–317. [CrossRef] Matsumoto, T.; Adachi, Y. Effect of ionic strength on the initial dynamics of flocculation of polystyrene latex with polyelectrolyte. J. Colloid Interface Sci. 1998, 204, 328–335. [CrossRef] [PubMed] Adachi, Y.; Wada, T. Initial stage dynamics of bridging flocculation of polystyrene latex spheres with polyethylene oxide. J. Colloid Interface Sci. 2000, 229, 148–154. [CrossRef] [PubMed] Aoki, K.; Adachi, Y. Kinetics of polyelectrolyte adsorption onto polystyrene latex particle studied using electrophoresis: Effects of molecular weight and ionic strength. J. Colloid Interface Sci. 2006, 300, 69–77. [CrossRef] [PubMed] Divakaran, R.; Sivasankara Pillai, V.N. Flocculation of river silt using chitosan. Water Res. 2002, 36, 2414–2418. [CrossRef]
Water 2018, 10, 1215
21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
32. 33.
34. 35. 36. 37.
38.
12 of 12
Ashmore, M.; Hearn, J.; Karpowicz, F. Flocculation of latex particles of varying surface charge densities by chitosans. Langmuir 2001, 17, 1069–1073. [CrossRef] Szilagyi, I.; Trefalt, G.; Tiraferri, A.; Maroni, P.; Borkovec, M. Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation. Soft Matter 2014, 10, 2479–2502. [CrossRef] [PubMed] Kam, S.; Gregory, J. The interaction of humic substances with cationic polyelectrolytes. Water Res 2001, 35, 3557–3566. [CrossRef] Wang, H.; Adeleye, A.S.; Huang, Y.; Li, F.; Keller, A.A. Heteroaggregation of nanoparticles with biocolloids and geocolloids. Adv. Colloid Interface Sci. 2015, 226, 24–36. [CrossRef] [PubMed] Nikolaou, A.D.; Golfinopoulos, S.K.; Lekkas, T.D.; Kostopoulou, M.N. DBP Levels in Chlorinated Drinking Water: Effect of Humic Substances. Environ. Monit. Assess. 2004, 93, 301–319. [CrossRef] [PubMed] Matilainen, A.; Vepsäläinen, M.; Sillanpää, M. Natural organic matter removal by coagulation during drinking water treatment: A review. Adv. Colloid Interface Sci. 2010, 159, 189–197. [CrossRef] [PubMed] Pelton, R.; Lawrence, D. The influence of surfactants on latex flocculation with poly (diallyldimethyl ammonium chloride). Colloid Polym. Sci. 1989, 267, 907–914. [CrossRef] Narkis, N.; Rebhun, M. Flocculation in presence of organic macromolecules of natural water and secondary effluents. Water Sci. Technol. 1997, 36, 85–91. [CrossRef] Lurie, M.; Rebhun, M. Effect of properties of polyelectrolytes on their interaction with particulates and soluble organics. Water Sci. Technol. 1997, 36, 93–101. [CrossRef] Miyajima, T. Metal complexation in polyelectrolyte solutions. In Physical Chemistry of Polyelectrolytes, 1st ed.; Radeva, T., Ed.; CRC Press: Boca Raton, FL, USA, 2001; Volume 99, pp. 829–874. Roger, G.M.; Durand-Vidal, S.; Bernard, O.; Mériguet, G.; Altmann, S.; Turq, P. Characterization of humic substances and polyacrylic acid: A high precision conductimetry study. Colloids Surf. A Physicochem. Eng. Asp. 2010, 356, 51–57. [CrossRef] Avena, M.J.; Vermeer, A.W.P.; Koopal, L.K. Volume and structure of humic acids studied by viscometry pH and electrolyte concentration effects. Colloids Surf. A Physicochem. Eng. Asp. 1999, 151, 213–224. [CrossRef] Kodama, H.; Miyajima, T.; Mori, M.; Takahashi, M.; Nishimura, H.; Ishiguro, S. A unified analytical treatment of the acid-dissociation equilibria of weakly acidic linear polyelectrolytes and the conjugate acids of weakly basic linear polyelectrolytes. Colloid Polym. Sci. 1997, 275, 938–945. [CrossRef] Van de Ven, T.G.M.; Mason, S.G. The microrheology of colloidal dispersions. Colloid Polym. Sci. 1977, 255, 794–804. [CrossRef] Adachi, Y.; Matsumoto, T. Dynamics of initial stage flocculation of polystyrene latex spheres with polyelectrolytes. Colloids Surf. A Physicochem. Eng. Asp. 1996, 113, 229–236. [CrossRef] Takahashi, T.; Takayama, K.; Machida, Y.; Nagai, T. Characteristics of polyion complexes of chitosan with sodium alginate and sodium polyacrylate. Int. J. Pharm. 1990, 61, 35–41. [CrossRef] Chen, S.; Zhang, Q.; Nakamoto, T.; Kawazoe, N.; Chen, G. Highly active porous scaffolds of collagen and hyaluronic acid prepared by suppression of polyion complex formation. J. Mater. Chem. B 2014, 2, 5612–5619. [CrossRef] Izumrudov, V.A.; Sybachin, A.V. Phase separation in solutions of polyelectrolyte complexes: The decisive effect of a host polyion. Polym. Sci. Ser. A 2006, 48, 1098–1104. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
