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 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.) 2 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 Figure1. 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 up withinWater 2018 one, 10, week. x FOR PEER The REVIEW stock solution of PAA5K was prepared by dissolving PAA5K in pure3 water of 12 upon 24 h stirring. The prepared polymer solutions were placed in a dark and cold place (5 ◦C) to avoida dark light and exposure-induced cold place (5 °C) degradation. to avoid light The exposure ionic strengths-induced ofdegradation. as-prepared The solutions ionic strengths were adjusted of as- 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 reportedpH at that10 mM the NaCl degree is ca. of, dissociation 0.35. of PAA in this range of pH at 10 mM NaCl is ca., 0.35.
Figure 1. Chemical structures of monomer unit of poly ((2-dimethylamino)ethyl methacrylate) methyl chlorideFigure quaternary 1. Chemical salt structures (PAM) of and monomer polyacrylic unit acidof poly (PAA). ((2-dimethylamino)ethyl Molecular weights methacrylate) of monomer methyl unit for PAMchloride and PAA quaternary are 207.5 salt and (PAM) 71 g/mol, and polyacrylic respectively. acid (PAA). Molecular weights of monomer unit for PAM and PAA are 207.5 and 71 g/mol, respectively. 2.2. Analytical Principle 2.2.The Analytical employed Principle analytical principle was based on the rate of collisions between particles in a mixing flow.The employed Notably, theanalytical frequency principle of collisions was based between on the colloid rate of particles collisions is between equivalent particles to the in rate a of rapidmixing coagulation, flow. Notably, in whichthe frequency case all of collisions collisions resultbetween in successfulcolloid particles coagulation. is equivalen Fort ato suspensionthe rate of composedrapid coagulation, of monodispersed in which spherical case all particles collisions with result a radius in successful of a0, the coagulation. collision rate For can a be suspension expressed as thecomposed temporary of changemonodispersed of the number spherical concentration particles with of colloidal a radius particles, of , theN( t collision), as rate can be expressed as the temporary change of the number concentration of colloidal particles, N(t), as r dN(t) 128πε ( ) 3 2 (1) ==− α−T a 0 N( ()t ), , (1) dt 15 ν 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 is the hydrodynamic interaction-determined capture efficiency of collisions between colloidal colloidalT 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 can be approximately approximately estimated using the approach of Van de Ven et al. [34] as T estimated using the approach of Van de Ven et al. [34] as . = , (2) ( / ) !0.18 A α = , (2) where A and µ denote the HamakerT constantp and fluid 3 viscosity, 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 whereas monomers,A and µ denote Equation the Hamaker 1 can be reduced constant to and a first fluid-order viscosity, expression, respectively. and an approximate If we limit our so discussionlution for to thetemporal initial number stage of concentration flocculation, variation when the canmajority be expressed of clusters as or particles can be regarded as monomers, Equation 1 can be reduced to a( first-order) expression, and an approximate solution(3) for ln = − ( + ) , 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 protruding length of attached polymers N( ont) colloidal particles. Figure 2 shows the typical patterns of ln = −α (a + δ)3Kt, (3) flocculation in the presence and absenceN(0 of) a polymerT flocculant0 under a polymer concentration, Cp, above the optimal dosage. The solid lines represent polymer-induced flocculation, while the dashed wherelineK representsis a constant salt-induced determined rapid bycoagulation. the mechanical setup of the experiment and δ denotes the protruding length of attached polymers on colloidal particles. Figure2 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
aboveWater 2018 the, 10 optimal, x FOR PEER dosage. REVIEW The solid lines represent polymer-induced flocculation, while the dashed4 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 by theAssociation protruding of length colloidal of attached particles polymers with polymeric and therefore coagulants significantly increases increases the effective the collision collision frequency.radius by the Assuming protruding that length the attached of attached polymer polymers structure and is therefore completely significantly permeable, increases i.e., that the correctioncollision frequency. for hydrodynamic Assuming interaction that the attached is negligibly polymer small, structure the flocculation is completely of polymer-coated permeable, particlesi.e., that canthe correction be described for as hydrodynamic interaction is negligibly small, the flocculation of polymer-coated N(t) particles can be described as ln = −α a3Kt. (4) N(0) T 0 ( ) ln = − . (4) Therefore, the enhancement factor ξ, which( ) defines the ratio of the initial polymer-induced flocculationTherefore, rate the and enhancement the rate of salt-induced factor ξ, which rapid coagulation defines the under ratio polymer-freeof the initial conditions, polymer-induced can be expressedflocculation as rate and the rate of salt-induced rapid coagulation under polymer-free conditions, can (a + δ)3 be expressed as = 0 ξ 3 . (5) αT a0 ( + ) (5) = . In our previous studies, the value of δ was confirmed to be correlated with the size of solution-phaseIn our previous polymer studies, molecules the value rather of than was withconfirmed the thickness to be correlated of the adsorbed with the size polymer of solution in the- equilibriumphase polymer state. molecules This finding rather impliesthan with that the a remarkablethickness of behaviorthe adsorbed is expected polymer in in the the extreme equilibrium case whenstate. This the bare finding surfaces implies of colloidalthat a remarkable particles arebehavior exposed is expected to an excess in the dosage extreme of polymercase when flocculant. the bare Thatsurfaces is,the of enhancementcolloidal particles factor are initially exposed increases to an excess with increasingdosage of polymer polymer concentration,flocculant. That whereas is, the flocculationenhancement progress factor initially is abruptly increases terminated with increasing when the polymer particle surfaceconcentration, becomes whereas saturated flocculation with the polymerprogress (Figure is abruptly2). The terminated duration when of the the enhanced particle stage surface is shorter becomes for saturated the case of with higher the polymer polymer concentrations.(Figure 2). The This duration is due to of saturation the enhanced of the stage attached is polymershorter for on colloidal the case surfaces of higher that polymer induces stericconcentrations. stabilization. This is due to saturation of the attached polymer on colloidal surfaces that induces steric stabilization. 2.3. Particle Aggregation Analysis 2.3. ParticleColloid Aggregation mixing was Analysis induced by the turbulent flow generated by the rocking motion of a forked flaskColloid installed mixing on an end-over-endwas induced rotationby the turbulent shaker, as flow illustrated generated in Figure by the3. rocking The detail motion information of a forked can beflask found installed elsewhere on an [35 end]. In-over the- beginning,end rotation 5 mLshaker, of the as PSL illustrated dispersion in Figure was placed 3. The into detail one sideinformation of flask, andcan be the found other endelsewhere was filled [35]. with In the 5 mL beginning, of PAM5M 5 mL solution of the or PSL KCl dispersion solution. Thewas PAM5M placed into solution one side and theof flask, PSL dispersionand the other with/without end was filled PAA5K with were 5 mL mixed of PAM5M using the solution end-over-end or KCl rotationsolution. apparatus The PAM5M at a rotationsolution frequencyand the PSL of 1dispersion Hz until the with/without predetermined PAA5K number were of mixed mixing using times the was end reached.-over-end The rotation degree ofapp coagulation/flocculationaratus at a rotation frequency was monitored of 1 Hz by until determining the predetermined the total number number of of clusters, mixing N times(t), using was areached. Coulter The counter degree (Multisizers of coagulation/flocculation 4e, Beckman Coulter was monitored Inc., Brea, CA,by determining USA). In order the total to evaluate number the of effectsclusters, of N the(t), using polymer a Coulter swelling counter degree (Multisizers in solution, 4e, experimentsBeckman Coult wereer Inc. carried, Brea, out CA, at USA) two. different In order ionicto evaluate strengths. the effects of the polymer swelling degree in solution, experiments were carried out at two different ionic strengths.
Water 2018, 10, 1215 5 of 12 Water 2018, 10, x FOR PEER REVIEW 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 concentrations.The effect Based of the on transient our previous state studieswas analyzed of the same by performing polycation experiments[19], PAM5M at concentrations two different ofPAM5M 2.5 and concentrations. 0.5 mg/L were Based selected. on our The previous concentration studies of PAA5K of the same was set polycation to 0.5 mg/L, [19], and PAM5M the inhibitoryconcentrations effect of was 2.5 and analyzed 0.5 mg/L for were five differentselected. The PAM5M concentration concentrations of PAA5K (2.5, was 1.5, set 0.6, to 0.5 0.5, mg and/L, 0.4and mg/L). the inhibitory The obtained effect resultswas analyzed were compared for five different with those PAM5M of the concentrations corresponding (2.5, blank 1.5, experiments. 0.6, 0.5, and For0.4 experimentsmg/L). The obtained conducted results in the were presence compared of PAA5K, with those 4 mL of of the the corresponding PSL dispersion blank was experiments. mixed with 1For mL experiments of the PAA5K conducted solution in beforehand, the presence whereas of PAA5K, in blank 4 mL experiments,of the PSL dispersion the PAA5K was solutionmixed with was 1 replacedmL of the by PAA5Kan equal solution volume of beforehand, deionized water. whereas All in sets blank of experiments experiments, were the repeated PAA5K at solution least twice was toreplaced confirm by the an reproducibility equal volume of thedeionized obtained water. results. All sets of experiments were repeated at least twice 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 toThe determine viscosities the of size polymer of polymer-only aggregates solutions were employing measured Einstein’s by a capillary viscosity viscometer equation and and nominal used to moleculardetermine weights the size provided of polymer by suppliers, aggregates with employing the results obtained Einstein’s for viscosity each ionic equation strength andsummarized nominal inmolecu Table1lar. When weights two polymers provided were by suppliers, present, viscosity with the measurements results obtained were employed for each to ionic confirm strength the formationsummarized of polyionin Table complexes 1. When two [36 ,polymers37]. Specifically, were present, PAA5K viscosity and PAM5M measurements solutions were employed mixed to affordto confirm the desired the formation concentration of polyion ratios complexes and gently [3 shaken6,37]. Specifically, for 60 times. PAA5K The resulting and PAM5M mixtures solutions were centrifugedwere mixed (Model to afford 3520, the Kubota desired Corp., concentration Osaka, Japan) ratios for and 5 min gently at 150 shaken rpm, andfor 60 the times. supernatants The resulting were sampledmixtures and were used centrifuged for viscosity (Model measurements. 3520, Kubota To Corp reduce., Osaka, the influence Japan) for of 5 human min at error150 rpm, and ensureand the thesupernatants reliability ofwere acquired sampled data, and all used measurements for viscosity weremeasurements. conducted To at reduce high polymer the influence concentrations of human (50error mg/L and PAA5K ensure andthe reliability 50–250 mg/L of acquired PAM5M). 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. ◦ AllTable measurements 1. Hydrodynamic were conducted diameters at (a p controlled) of polymers temperature estimated of by 20 theC Einstein’s and initial viscosity PAA5K andequation. PAM5M All concentrationsmeasurements of were 50 mg/L. conducted at a controlled temperature of 20 °C and initial PAA5K and PAM5M concentrations of 50 mg/L. ap (nm) Sample ap (nm) Sample 10 mM KCl 0.1 mM KCl 10 mM KCl 0.1 mM KCl PAA5K 6.4 7.2
PAM5MPAA5K 2776.4 7.2 341 PAM5M 277 341
3.3. ResultsResults 3.1. Polycation-Induced Flocculation 3.1. Polycation-Induced Flocculation Figure4 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
WaterWater 2018 2018, 10, 10x FOR, x FOR PEER PEER REVIEW REVIEW 6 of6 of12 12 applied method. The slope of the above plot was used in the subsequent analysis of data obtained for method.method. The The slope slope of of the the above above plot plot was was used used in in the the subsequent subsequent analysis analysis of of data data ob obtainedtained for for numerous conditions. numerousnumerous conditions. conditions. MixingMixing TimeTime (s) 0 0 1010 2020 30 4040 5050 0.000.00
-0.05-0.05 0) 0) -0.10-0.10 11 M M KCl KCl t)/N(
(
t)/N( (
N -0.15 N
-0.15ln ln ln -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 the total number of clusters per unit volume N(t) with the number of mixing Figuretime 4. (s) Variation for salt-induced of the totalcoagulation number (N of(0) clusters = 5 × 10 7per cm −unit3). volume N(t) with the number of mixing × 7 −3 time (s)(s) forfor salt-inducedsalt-induced coagulationcoagulation ((NN(0)(0) == 5 × 10107 cmcm−3). ). Figure 5 shows the results obtained for polycation-induced flocculation at different ionic Figure5 shows the results obtained for polycation-induced flocculation at different ionic strengths strengthsFigure 5(0 .1 sh andows 10 the mM), results demonstrating obtained forthat polycationthe enhancement-induced factor flocculation for the PAA5K at di-fffreeerent case ionic is (0.1 and 10 mM), demonstrating that the enhancement factor for the PAA5K-free case is consistent strengthsconsistent (0.1 with and values 10 mM), reported demonstrating elsewhere that[17,19]. the The enhancement calculated enhancement factor for the factors PAA5K are- freeshown case in is with values reported elsewhere [17,19]. The calculated enhancement factors are shown in Table2. consistentTable 2. with values reported elsewhere [17,19]. The calculated enhancement factors are shown in 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. TableFigure 2. Enhancement5. Temporal variation factors obtained of polycation for polymer-induced-induced flocculation flocculation rate underin 0.1 differentand 10 mM conditions. KCl. ξ Sample ξ Sample 10 mM KCl 0.1 mM KCl Table 2. Enhancement factors obtained for polymer10 mM-induced KCl flocculation0.1 mM KCl under different conditions. 0.5 mg/L PAM5M 8.76 10.54 0.5 mg/L PAM5M 8.76 ξ 10.54 2.5 mg/LSample PAM5M 10.64 N/A 2.5 mg/L PAM5M 10 mM10.64 KCl 0.1 mMN/A KCl 0.5 mg/L PAM5M 8.76 10.54 3.2.3.2 Flocculation. Flocculation in in the the Presence Presence of of Two Two Oppositely Oppositely Charged Charged Polymers Polymers 2.5 mg/L PAM5M 10.64 N/A FigureFigure6 shows 6 sho thews influence the influence of PAA5K of PAA5K on the on PAM5M-induced the PAM5M-induced flocculation flocculation of PSL of at two PSL different at two ionic3.2.different Flocculation strengths, ionic in demonstrating strengths,the Presence demonstrating of thatTwo theOppositely PAA5K that Charged the significantly PAA5K Polymers significantly inhibits flocculation. inhibits flocculation. This inhibitory This
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 7 of 12 Water 2018, 10, 1215 7 of 12 Water 2018, 10, x FOR PEER REVIEW 7 of 12 inhibitory effect was most pronounced when the concentration of PAM5M was equal or less than inhibitory effect was most pronounced when the concentration of PAM5M was equal or less than effectthat of was PAA5K, most in pronounced which case when flocculation the concentration was essentially of PAM5M stopped. was Moreover, equal or the less enhancement than that ofof that of PAA5K, in which case flocculation was essentially stopped. Moreover, the enhancement of PAA5K,the initial in floccula whichtion case rate flocculation was disturbed was essentially by the presence stopped. of PAA5K, Moreover, and the this enhancement disturbance became of the the initial flocculation rate was disturbed by the presence of PAA5K, and this disturbance became initialmore flocculationpronounced ratewith wasincreasing disturbed PAM5M/PAA5K by the presence concentration of PAA5K, ratio. and Figure this disturbance 7 shows the became initial more pronounced with increasing PAM5M/PAA5K concentration ratio. Figure 7 shows the initial moreflocculation pronounced rates (0 with to 10 increasing s) observed PAM5M/PAA5K under different concentrationconditions, revealing ratio. Figure that the7 shows rates were the initial clearly flocculation rates (0 to 10 s) observed under different conditions, revealing that the rates were clearly flocculationreduced in rates the presence (0 to 10 s) of observed PAA5K under under different the same conditions, concentration revealing ratio. that Conversely, the rates were at an clearly excess reduced in the presence of PAA5K under the same concentration ratio. Conversely, at an excess reduceddosage of in thePAM5M presence (2.5 ofmg/L), PAA5K a slight under enhancement the same concentration in the rate ratio.of flocculation Conversely, can at be an observed excess dosage in the dosage of PAM5M (2.5 mg/L), a slight enhancement in the rate of flocculation can be observed in the ofpresence PAM5M of (2.5 PAA5K. mg/L), a slight enhancement in the rate of flocculation can be observed in the presence presence of PAA5K. of PAA5K.
FigureFigure6. 6. TemporalTemporal variation variation of of polycation-induced polycation-induced flocculation flocculation rate rate in in 0.1 0.1 mMmM andand 10 10 mM mM KCl KCl Figure 6. Temporal variation of polycation-induced flocculation rate in 0.1 mM and 10 mM KCl determineddetermined in in the the presence presence of of 0.5 0.5 mg/L mg/L PAA5K.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. Figurewith/without 7. Temporal PAA5K. variation of the rate of polycation-induced flocculation in 0.1 mM and 10 mM KCl with/without PAA5K.
Water 2018, 10, 1215 8 of 12
3.3. ReductionWater 2018 of, 10 PAM5M, x FOR PEER Viscosity REVIEW 8 of 12
Figure3.3. Reduction8 shows of PAM5M that the Viscosity viscosity of PAM5M solutions increased with increasing polymer concentrationFigure at all 8 ionic shows strengths. that the viscosity Notably, of the PAM5M viscosities solutions of 50 increased mg/L PAA5K with increasing solutions polymer approximately equaledconcentration that of the atsolvent all ioni sincec strengths. the molecular Notably, weight the viscosities of this polymer of 50 mg/L was PAA5K much less solutions than that of PAM5M.approximately On the other equaled hand, that data of the obtained solvent since for the the PAM5M-PAA5K molecular weight of supernatant this polymer solutionswas much less confirmed that thethan presence that of PAM5M. of PAA5K On the reduces other hand, the data viscosity obtained of for PAM5M the PAM5M solutions.-PAA5K supernatant It should solutio be notedns that the viscosityconfirmed of that PAM5M the presence solutions of PAA5K decreased reduces to the approximately viscosity of PAM5M that solutions. of PAA5K It should solutions be noted 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 PAA5Kby to PAA5K form solid to form complexes solid complexes that were that removed were removed by centrifugation, by centrifugation, in agreement in agreement with with the the results of flocculationresultsrate of flocculation measurements. rate measurements.
1.8 PAM5M only 1.6 (0.1 mM KCl) 1.4 PAM5M with 50 1.2 mg/L PAA5K 1 (0.1 mM KCl) 0.8 PAM5M only (10 mM KCl) 0.6
0.4 PAM5M with 50 Specific viscosity Specific 0.2 mg/L PAA5K 0 (10 mM KCl) 0 50 100 150 200 250
Concentration of PAM5M (mg/L) 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 PAM5MPAM5M concentration concentration at KCl at KCl = 0.1 = 0.1 mM mM and and 1010 mM.mM
4. Discussion4. Discussion
4.1. Enhancement4.1. Enhancement of Initial of Initial Flocculation Flocculation Rate Rate by by PAM5MPAM5M DetailedDetailed analysis analysis of theof the acquired acquired data reveals reveals some some minor minor deviations. deviations. Figure 5 Figureshows that5 shows at low that at low ionicionic strength, strength, the the initialinitial flocculationflocculation rate rate was was significantly significantly enhanced enhanced in the in presence the presence of 2.5 mg/L of 2.5 mg/L PAM5M,PAM5M, with with an even an even more more pronounced pronounced enhancement enhancement observed observed for a PAM5M for a PAM5M concentration concentration of 0.5 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, illustratedwhich in induces Figure saturation2. In high in loCwp dosage.condition, However, adsorption in this case, of polymersthe duration on of colloidalthe initial flocculation particles are fast, which inducesrate enhancement saturation stage in was low too dosage. short in However, view of the in swollen this case, conformation the duration of PAM5M of the chains, initial which flocculation rate enhancementcorresponds to stage the case was of toolow shortCp. Meanwhile, in view in of low the C swollenp, adsorption conformation is continuously of PAM5Mtaking place chains, until which correspondsflocculation to the is case essentially of low C stopped.p. Meanwhile, In contrast, in low theC resultp, adsorption obtained is at continuously high ionic strength taking was place until flocculationconsistent is essentially with that stopped. predictedIn in contrast, Figure 2. the Adachi result et obtained al. [7] reportedat high ionic that strengthboth flocculation was consistent mechanisms; bridging and charge neutralization can be distinguished by the slope (the rate) of with that predicted in Figure2. Adachi et al. [ 7] reported that both flocculation mechanisms; bridging flocculation. Remarkable enhancement accompanied by abrupt termination of flocculation indicates and chargethat flocculation neutralization is taking can place be distinguishedthrough bridging, by which the slope is observed (the rate) in Figure of flocculation. 5 and illustrated Remarkable in enhancementFigure 2. accompanied This behavior is by contrast abrupt with termination the latter mechanism, of flocculation where indicates flocculation that incessantly flocculation occurs is taking place throughwith a rate bridging, similar to salt which-induced is observed rapid coagulation. in Figure 5 and illustrated in Figure2. This behavior is contrastComparison with the latter of flocculation mechanism, rates whereobserved flocculation for both ionic incessantly strengths indicated occurs that with (1) a the rate most similar to salt-inducedpronounced rapid incr coagulation.ease of the initial flocculation rate was observed at low ionic strength and (2) the 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 pronouncedexhibit increase an elongated of the form initial rather flocculation than a coiled rate form was because observed of atthe low strong ionic repulsion strength between and (2) ionic the highest 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 at highWater ionic 2018 strength, 10, x FOR PEER leads REVIEW to the accumulation of counter-ions and results in the adoption of9 of coil-like 12 conformations. This explanation is consistent with the results of viscosity measurements (Figure8), groups in the polymer chain, which increases the collision size of the polyelectrolyte and hence wherepromotes a steeper the slope collisions was observed of the polyelectrolyte at low ionic and strength. particles. Furthermore, On the other Table hand,1 confirms the reduction that theof the size of polyelectrolyteabove repulsion molecules at high decreases ionic strength with increasing leads to the ionic accumulation strength. Finally,of counter the-ions increase and results of the maximumin the flocculationadoption rate of with coil-like increasing conformations. ionic strength This explanation can be explained is consistent by the withconcomitant the results reduction of viscosity of Debye length,measurementsK−1, which is(Figure inversely 8), where proportional a steeper toslope salt was concentration. observed at Atlow low ionic ionic strength. strength, Furthermore, the repulsion betweenTable polyelectrolytes 1 confirms that isthe long size rangedof polyelectrolyte due to the molecules swollen conformationdecreases with and increasing induces ionic saturation strength. of the particleFinally, surface the atincrease lower of adsorbed the maximum amounts. flocculation When rate the with salt increasing level was ionic increased, strength coil-like can be explained conformation −1 of polyelectrolytesby the concomitant owing reduction to screening of Debye of the intramolecular length, K , which electrostatic is inversely repulsion proportional between to charged salt 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.the intramolecular As a result, ele largerctrostatic amounts repulsion of between polyelectrolytes charged segments are needed resulted to inducein local saturationaccumulation of the particlesof positive while particle-to-particlecharge on the negatively collision charged that particle. leads to The flocculation tail of adsorbed is proceeding. polyelectrolytes would then attach to neighboring bare particles due to electrostatic attraction. As a result, larger amounts of 4.2. Shrinkagepolyelectrolytes of PAM5M are needed in the Presenceto induce of saturation PAA5K of the particles while particle-to-particle collision Onethat leads of the to mostflocculation significant is proceeding. 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 (Figure9). The above behavior wasOne explainedof the most bysignificant the regulatory results of effect the present of PAA5K study adsorption is the disturbing on the effect conformation of PAA5K ofon PAM5Mthe 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 considerablyPAM5M chains decreased exhibiting (Figure electrostatic 10). Therefore, repulsion- theinduced charge swelling, density i.e., neutralized of the excess parts PAM5M of the latter dosage (2.5 mg/L)chains wasare no reduced longer involved to the charge in repulsive density interactions, in a lower and dosage the pe condition,rsistence length which of resulted the polymer in a is slight increasetherefore of the considerably flocculation decreased rate. In-depth (Figure analysis 10). Therefore, of Figure the6 chargeprovides density an insight of the excess of the PAM5M flocculation mechanismsdosage (2.5 involved. mg/L) was In reduced the initial to the stage charge of flocculationdensity in a lower (0 to dosage 10 s), condition, the rate of which flocculation resulted in of the 2.5 mg/La slight PAM5M increase was of enhanced the flocculation by the rate. increment In-depth of analysis collision of radius Figure by 6 provides protruding an insight chain of of PAM5M the attachedflocculation to the surfacemechanisms of colloidal involved. particle In the initial and stage gradually of flocculation terminated (0 to 10 in s), the the later rate of stage flocculation (10 to50 s). Flocculationof the 2.5 in mg/L this PAM5M condition was can enhanced be interpreted by the increment as bridging of collision flocculation. radius by protruding With the exceptionchain of of 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, flocculationwhich is rate an effect was of most charge pronounced neutralization at high by patch ionic interaction strength [7] in. viewNotably, of theinhibition counter-ion of the initial screening effectsflocculation and the reduction rate was ofmost PAM5M pronounced charge at density high ionic by PAA5K.strength Thein view subsequently of the counter observed-ion screening flocculation behavioreffects at a and PAM5M:PAA5K the reduction concentration of PAM5M charge ratio ofdensity 3:1 resembled by PAA5K. that The of salt-induced subsequently coagulation observed at high ionicflocculation strength, behavior whereas at a noPAM5M:PAA5K such similarity concentration was observed ratio at of low 3:1 ionicresembled strength. that of Such salt- trendinduced can be explainedcoagulation by Aoki at high and Adachiionic strength, [19], where whereas the no presence such similarity of counter-ions was observed and at smaller low ionic polyelectrolytes strength. prolongsSuch the trend time can required be explained for PSLby Aoki particles and Adachi to reach [19], saturation. where the presence of counter-ions and smaller 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 Water 2018, 10, 1215 10 of 12 PAA5K.
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 attractionThe results between of viscosity two measurements oppositely charged demonstrated polyelectrolytes that at a resulted constant in concentration the formation ratio, of solid the attractioncomplexes, between which decreased two oppositely the content charged of soluble polyelectrolytes polymer molecules resulted in in the the formationsuspension of and solid its viscosity.complexes, The which nature decreased of solid the complexes content of formation soluble polymer can be molecules explained in by the considering suspension the and total its viscosity.charge concentrations The nature of ofsolid the complexes polymers formation in solution. can Izumrudov be explained and by Sybachinconsidering [38 the] reported total charge that concentrationsnon-equimolar mixtureof the polymers of quaternary in solution. polyamines Izumrudov and polycarboxylates and Sybachin in [3 a8 salt] reported solution that makes non an- equimolarinsoluble polyelectrolyte mixture of quaternary complex in polyamines a certain range and ofpolycarboxylates ratio of the molar in concentrations a salt solution of makes charged an insolublegroups of polyelectrolyte polycation and polyanioncomplex in and a certain demonstrates range of a ratio maximum of the turbidity molar concentrations when the ratio of is charged close to groupsunity. The of polycation molar concentrations and polyanion of charged and demonstrates groups of PAM5M a maximum and PAA5K turbidity in solution when the can ratio be estimated is close toby unity. multiplying The molar the molar concentrations concentration of ofcharged monomer groups unit of by PAM5M the degree and of dissociation.PAA5K in solution At 0.5 mg/L can be of polymers,estimated by molar multiplying concentrations the molar of the concentration monomer unit of monomer of PAM5M unit and by PAA5K the degree are 2.36of dissociation.× 10−6 mol/L At 0.5and mg/L 7.04 ×of 10polymers,−6 mol/L, molar respectively. concentrations Therefore, of the the monomer charge concentrationsunit of PAM5M of and PAM5M PAA5K and are PAA5K 2.36 × 10are−6 2.41mol/L× and10− 67.04mol/L × 10 and−6 mol/L, 2.45 respectively.× 10−6 mol/L Therefore, by adopting the charge the values concentrations of 1.0 and of 0.35 PAM5M [33] as and the PAA5Kdegree of are dissociation 2.41 × 10−6 ofmol/L PAM5M and 2.45 and PAA5K,× 10−6 mol/L respectively. by adopting In the the case values of a of concentration 1.0 and 0.35 ratio[33] as of the 1:1, degreethe respective of dissociation distribution of PAM5M of charge and in PAA solution5K, respectively. of polyanion In and the polycation case of a concentration are approximate, ratio whichof 1:1, thegives respective rise to complete distribution charge of charge neutralization in solution through of polyanion electrostatic and polycation interactions. are Hence, approximate, the formation which givesof polyion rise to complexes complete charge was concluded neutralization to terminate through the electrostatic adsorption interactions. of polycations Hence, on PSLthe formation and thus ofinhibit polyion flocculation. complexes was concluded to terminate the adsorption of polycations on PSL and thus inhibit flocculation. 5. Conclusions 5. Conclusions The large enhancement of the initial flocculation rate observed at excess dosages of a cationic polymerThe flocculantlarge enhancement was remarkably of the inhibitedinitial flocculation in the presence rate observed of an oppositely at excess charged dosages small of a polymer.cationic Thispolymer phenomenon flocculant was was attributed remarkably to theinhibited adsorption in the of presence this oppositely of an oppositely charged polymer charged onto small the polymer. cationic Thisflocculant, phenomenon which resulted was attributed in electrostatic to the neutralization adsorption of of this certain oppositely chain parts charged and therefore polymer decreased onto the cationicthe degree flocculant, of swelling. which Subsequently, resulted in electrostatic decreased theneutralization net charge of of certain cationic chain flocculant parts and and therefore effective decreasedcollision radius the degree of particles. of swelling. This effect Subsequently, was more significantdecreased inthe high net ioniccharge strength of cationic due toflocculant the presence and effectiveof counter-ions, collision and radius hence of induced particles. patch This flocculation effect was more at specific significant concentration in high ionic ratio strength of PAM5M:PAA5K. due to the Therefore,presence of it was counter concluded-ions, a thatnd hence the same induce electrostaticd patch attraction flocculation can at occur specific between concentration negatively chargedratio of PAM5M:PAA5K.NOM and the positively Therefore, charged it was polymer concluded flocculant that the backbone, same electrostatic which, at attraction a charge ratiocan occur close between to unity, negativelycaused complete charged neutralization NOM and the topositively occur to charged form solid polymer complexes. flocculant Thus, backbone, flocculation which, of at colloidala charge particlesratio close occurred to unity, only caused when the complete demand neutralization of PAA5K for to the occur cationic to flocculant form solid charge complexes. was satisfied. Thus, flocculationIn the present of study, colloidal we have particles conducted occurred experiments only when under the only demand weakly of acidic PAA5K conditions. for the cationic Similar 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.
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