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Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 39–46

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

hitosan based nonstoichiometric polyelectrolyte complexes aspecialized flocculants

arcela Mihai, Ecaterina Stela Dragan ∗

Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41 A, 700487 Iasi, Romania

r t i c l e i n f o

rticle history:eceived 22 April 2009eceived in revised form 20 May 2009ccepted 22 May 2009

a b s t r a c t

The aim of this work was to evaluate the flocculation of kaolin by using positively charged nonstoi-chiometric interpolyelectrolyte complex (N-PEC) nanoparticles formed by the interaction of chitosanwith poly(sodium 2-acrylamido-2-methylpropanesulfonate) (NaPAMPS) and two random copolymers ofAMPS with t-butyl acrylamide, comparative with chitosan. The flocculation efficiency was evaluated by

vailable online 29 May 2009

eywords:locculationaolinhitosan

optical density at 500 nm. The N-PEC nanoparticles were more effective than chitosan in the kaolin sep-aration, especially at low molar ratios between charges, when the flocculation window was more thendouble at an optimum dose lower than that of chitosan. The main advantage of N-PECs is the increaseof critical concentration for kaolin re-stabilization, the N-PEC particles adsorbed on the kaolin surfaceprotecting them more efficient against re-dispersion. The influence of low molar mass electrolytes on the

was a

onstoichiometric polyelectrolyte complexispersions

flocculation with N-PECs

. Introduction

Most particles suspended in aqueous medium have a net sur-ace charge. Interparticle interactions cause repulsion and, in spitef Brownian motion trying to bring them together, the suspen-ion becomes stable, i.e., the particles do not aggregate unless theyre forced. Aggregation of particles is fundamental for improvinghe performance of many solid-liquid separation processes. The

ajor property of polyelectrolytes is their solid-liquid separationfficiency, with extensive application in purification of drinkingater, industrial raw and process water, municipal sewage treat-ent, mineral processing and metallurgy, oil drilling and recovery,

aper and board production, etc. [1–9]. The postulated mechanismsy which polyelectrolytes can bring about flocculation are chargeeutralization, bridging, or electrostatic patch [6,10,11]. Chargeeutralization is caused by the reduction in the electric double

ayer repulsion between particles due to the adsorption of highlyharged polyelectrolytes on oppositely charged particles. It is gen-rally believed that low molar mass polymers tend to adsorb andeutralize the opposite charges on the particles. Bridging floccu-

ation occurs when segments of the same polymer molecule arettached to more than one particle, thereby linking the particlesogether. The electrostatic patch flocculation is thought to be oper-tive for polymers of very high charge density interacting with

∗ Corresponding author. Tel.: +40 2322217454; fax: +40 232211299.E-mail addresses: marcelas@icmpp.ro (M. Mihai), sdragan@icmpp.ro

E.S. Dragan).

927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.05.021

lso investigated.© 2009 Elsevier B.V. All rights reserved.

oppositely charged particles of low charge density. The net residualcharge of the polymer patch on one particle surface can attach tothe bare part of an oppositely charged particle. Comparative withinorganic coagulants, there are some advantages by using polyelec-trolytes [5,6,8,10,11]: lower dose requirements, a smaller volume ofsludge, a smaller increase in the ionic load of the treated water, andcost savings up to 25–30%. The main disadvantage of flocculationwith polymers is the very small flocculation window, at a smallincrease of the dosage the system being re-stabilized. A numberof studies have tried to solve this problem by combination of two(or more) oppositely charged polyelectrolytes that can be addedone after another [12–15] or as nonstoichiometric polyelectrolytecomplexes (N-PEC) [16–23]. In the first case, known as dual floccula-tion, some advantages comparative with single polymer flocculantwere observed: a higher overall level of flocculation, less sensitivityto process variations and polyelectrolyte concentration, improvedpossibilities to control and optimize the flocculation, good sludgedewatering, superior retention, and shear – resistant flocs, etc. Theinvestigations on N-PEC flocculants, as colloidal dispersions bearingpositive or negative charges in excess, which starts with the prelim-inary studies of Suzuki and Kashili [16,17] and developed in the lastyears [18–23], has been concentrated on the use of N-PECs withmolar ratio between charges ranged from 0.4 to 0.8. The authorsobserved that the amount of N-PEC required for reaching optimal

flocculation decreased with the increase of polyelectrolyte molarmasses. The main advantage in flocculation induced by N-PECsis the lower dependence on the concentration of the floccula-tion agents, showing a substantially wider optimum concentrationrange for flocculation. Nevertheless, the method drawback was that

4 s A: Physicochem. Eng. Aspects 346 (2009) 39–46

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tsriTNbtK1tttfcmtfrptm

Table 1Characteristics of oppositely charged polyions.

Sample (�) (dl/g) M̄v (kDa) Mua (g/charge)

Ch I 9.12b 470 197Ch II 12.33b 670 197Ch III 14.07b 780 197NaPAMPS I 0.20c 73 229NaPAMPS II 0.38c 170 229P(AMPS54-co-TBA46) 0.47c 175 337P(AMPS37-co-TBA63) 0.33c – 441

a Mu: mass per charge; Mu = [MIM × fIM + MNM × (1 − fIM)]/fIM, where MIM and MNM

0 M. Mihai, E.S. Dragan / Colloids and Surface

he optimum concentration required for flocculation with N-PECsas much higher than the optimum concentration for flocculationith polycations, further studies on the factors which influence

he complex properties to increase the flocculation window simul-aneously with optimum concentration decrease being stronglyequired.

Last decades, growing demand for environmentally friendlyechnologies promotes the interest in investigation of natural floc-ulants [6–8,24–26]. The most prominent of these is chitosan, aartially deacetylated chitin, its linear charge density dependingn the level of deacetylation. The properties of chitosan, including

ts cationic behavior and molar mass, may be used both for chargeeutralization (coagulating effect for anionic compounds) and forarticle entrapment (flocculating effect) [7–9,27–32].

A constant interest of our group, last years, was the prepara-ion of PECs nanoparticles as stable colloidal dispersions and thetudy of the factors (polyions structure and molar mass, molaratio between charges, addition order, titrant addition rate) whichnfluence particles size, morphology, and storage stability [33–36].he objective of this work is the investigation of the influence of-PECs characteristics, such as polyanion structure, molar ratioetween charges, particles morphology and low molar mass elec-rolyte content, on the flocculation of model kaolin dispersion.aolin, commonly referred to as china clay, is clay that contains0–95% of the mineral kaolinite, formed under acidic conditionshrough weathering or hydrothermal changes of feldspars and -o a lower extent -other aluminosilicates. Kaolin is made up ofiny sheets of triclinic crystals with pseudohexagonal morphology,ormed by rock weathering and possesses some cationic exchangeapacity. Kaolin suspensions were used as a representative colloidalaterial because kaolin, which is a well known and widespread

hickening agent, is a key component in various industrial fluid

ormulations and in consequence has received great interest inecent years [14,25,26,29,37–39]. For this purpose we used someositively charged N-PECs as colloidal dispersions formed byhe interaction of chitosan with poly(sodium 2-acrylamido-2-

ethylpropanesulfonate) (NaPAMPS) and two random copolymers

Scheme 1. Chemical stru

are the molar masses of the ionic and the nonionic monomer and fIM is the molarfraction of the ionic monomer.

b In 0.3 M CH3COOH–0.2 M CH3COONa (1:1, v/v) at 25 ± 0.1 ◦C.c In 1 M NaCl at 25 ± 0.1 ◦C.

of AMPS with t-butyl acrylamide (TBA), the molar ratio betweencharges (n−/n+) ranging between 0.15 and 0.5, comparative withsome chitosan samples with a deacetylation degree of about 80%and different molar masses.

2. Experimental

2.1. Materials

The chemical structures of the polyions used in this study arecollected in Scheme 1.

Chitosan samples were purchased from Heppe GmbH (Biotech-nologische Systeme und Materialien) as flakes, with a deacetylationdegree of 80%, ash content less than 1%, and were used without fur-ther purification. Chitosan was characterized by FT-IR analysis, forthe determination of deacetylation degree (using the method pro-posed in ref. [40]), and by viscometry, for molar mass calculation

(using the method proposed in ref. [41]). NaPAMPS and copolymersof AMPS with TBA were synthesized and purified according to refs.[34] and [42]. Polyions characteristics were summarized in Table 1.

Kaolin powder (from Aldrich) was used as model substrate forfine particles. Average size of kaolin particles was about 600 nm,

cture of polyions.

s A: Physicochem. Eng. Aspects 346 (2009) 39–46 41

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Table 2Characteristics of some N-PEC dispersions.

N-PEC n−/n+ CDa, (mequiv L−1) Dhb (nm) PIc

P(AMPS54-co-TBA46)/Ch II 0.15 0.47 288 0.2540.2 0.44 284 0.2510.3 0.39 267 0.2460.4 0.33 254 0.240.5 0.29 244 0.231

P(AMPS37-co-TBA63)/Ch II 0.15 0.51 326 0.310.2 0.48 321 0.30.3 0.43 311 0.280.4 0.38 306 0.260.5 0.33 302 0.25

M. Mihai, E.S. Dragan / Colloids and Surface

ith a polydispersity index of about 0.58 as determined by dynamicight scattering (DLS) using a Zetasizer 3000 (Malvern Instruments,K). The particle charge density, determined by colloid titrationith Particle Charge Detector PCD 03 (Mütek GmbH, Germany), was

.067 mequiv L−1.

.2. Methods

.2.1. Preparation of polyelectrolyte solutionsThe chitosan solutions, with a concentration of 1 g/L, were

btained by dissolving the flakes in 1 vol.% acetic acid solution andntensive stirring for at least 48 h. For N-PEC preparation, the con-entration of chitosan solution was adjusted to 0.5 mM by dilutionith 1 vol.% acetic acid solution. Aqueous solutions of the anionic

olyelectrolytes with a concentration of 5 mM were prepared bydequate dilution of the stock solutions (10 mM) with distilledater. The molar concentrations of the polyelectrolytes were cal-

ulated taking into account the values of mass per charge (Mu, seeable 1). The concentration of the charged groups in the solutionsas determined by the Particle Charge Detector PCD 03 (MütekmbH, Germany). The polyelectrolyte solutions in the presence ofaCl were prepared by dissolving the polymer in the salt solutionf specific concentration.

.2.2. N-PEC preparationTo ensure a full protonation of chitosan, and to avoid any other

eutralization reactions, the initial solutions of polyelectrolytesere adjusted at pH 4.0. Specific volumes of the aqueous solution

f polyanion, having a constant concentration of 5 mM, were con-inuously added to the aqueous solution of polycation, having aoncentration of 0.5 mM, with a constant addition rate, under mag-etic stirring, at room temperature, to obtain a certain molar ratioetween opposite charges, n−/n+, ranging from 0.15 to 0.5. Afterixing, the formed dispersions were stirred 60 min and were char-

cterized after minimum 24 h by dynamic light scattering using aetasizer 3000 (Malvern Instruments, UK), in order to determinehe particles size and size distribution, and by the Particle Chargeetector PCD 03 (Mütek GmbH, Germany), for charge density deter-ination.

.2.3. Flocculation testsThe kaolin model dispersions, with a concentration of 1 g/L and

H 6, were prepared by sonication for 15 min using an Ultrasonatoronics Vibra-Cell. All the flocculation experiments were conductedt room temperature. Volumes of 50 ml kaolin suspension weretirred at 120–150 rpm in beakers and then different volumes ofocculant were added. Stirring was continued with the same speed

or about 2 min, and then it was decreased to about 50 rpm for5 min. After a settling time of about 20 min, the reading of the opti-al density was performed at a wavelength of 500 nm (OD500) (thebsorption maximum of kaolin) with a SPECORD M42 spectrome-er. Deionized water was used as reference. Residual turbidity (RT)as calculated according to the formula:

T =(

OD500s

OD500i

)× 100, %

here OD500s is OD500 after the addition of flocculant and OD500is OD500 of the model suspension.

. Results and discussion

.1. Characteristics of N-PECs particles

PECs are formed spontaneously in aqueous medium due tohe electrostatic interaction between oppositely charged polyions.epending on the nature of the polyelectrolyte components used

a Particles charge density.b Hydrodynamic diameter.c Polydispersity index.

and the medium conditions during complex formation, a largevariety of structures can be obtained [43,44]: soluble PEC, i.e.macroscopically homogeneous systems containing nanoscopic PECaggregates; turbid colloidal systems with suspended PEC particlesin the transition range to phase separation; two-phase systemsof supernatant liquid and precipitated PEC. Considerable effortshave been lately devoted to the preparation of PECs as colloidaldispersions because of their widespread applications in chemicalengineering, pharmaceutical and biological areas. The N-PEC dis-persions used in this study differ in their structure, charge density,particle size and size distribution (Table 2).

As our previous investigation showed, formation of positivelycharged N-PECs between chitosan and the copolymers of AMPSwith TBA have some peculiarities [47]: at n−/n+ < 0.2, the addedpolyanion react with chitosan chains leading to primary aggregates,which contain more chitosan chains connected by few polyanionchains, with a high density of free positive charges; further additionof polyanion (0.2 < n−/n+ < 0.8) led to the step-by-step neutraliza-tion of the positive charges of chitosan included in the primaryaggregates, accompanied by rearrangements of the chains and theformation of more compact particles with lower sizes. Furthermore,as Table 2 shows, the increase of the TBA content from 46 mol% upto 63 mol% caused distinct differences in the characteristics of thecomplex dispersion. Thus, for the same molar ratio between chargesboth the average hydrodynamic particle sizes and PEC charge den-sity were higher when P(AMPS37-co-TBA63) was used as polyanion(Table 2). The decrease of the charge density and the increase of thenonionic comonomer content had two unfavorable consequenceson the complex formation: the increase of the polyanion amountrequested for the charge compensation, which causes the increaseof the particle sizes, and the formation of higher swollen aggregates[34,35]. The polydispersity index (PI) followed almost the sametrend like the particle sizes, for the same ratio between chargesthe higher values of PI being obtained when P(AMPS37-co-TBA63)was used as polyanion (Table 2).

3.2. Kaolin flocculation with N-PECs based on chitosan

The kaolin flocculation induced by different N-PECs based onChitosan II and NaPAMPS or copolymers of AMPS with TBA, with dif-ferent molar ratios between charges, n−/n+, has been investigatedby following the variation of residual turbidity (RT) as a functionof the ratio between the amount of polymer per substrate (CP/CS,mg/g) (Fig. 1). To obtain a reference on the flocculation ability of the

N-PEC nanoparticles, the flocculation of kaolin by Chitosan II wasalso investigated. In the case of flocculation with N-PECs, the poly-mer mass (CP) is the sum of the mass of polycation and polyanionin the corresponding complex. The ratio CP/CS, considered opti-mum for flocculation, corresponds to the lowest dose of polymer

42 M. Mihai, E.S. Dragan / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 39–46

F kaoli

f0

edtoatb[dmmroscolrNmflosi

ig. 1. Residual turbidity (RT) as a function of polymer per substrate (CP/CS, mg/g) in

or which the turbidity of the supernatant, OD500, was lower than.01 a.u.

The efficiency of flocculation process depends on some param-ters, which characterize the polyelectrolytes, such as the type andensity of charges, polyelectrolyte molar mass, polymer conforma-ion. The polymer conformation is assumed to have a high influencen the flocculation efficiency. As it is already known, chitosan indilute solution (1 g/L in this work) has a worm-like conforma-

ion [45,46], and, having a high molar mass, is able to induceridging by simultaneous interaction with two or more particles7,8,27,29]. Fig. 1 shows that a low dose of chitosan II was needed forestabilization and sedimentation of kaolin suspension. Thus, theaximum removal of the turbidity (100%) was achieved at the opti-um dosage of 2.7 mg flocculant/g kaolin, the flocculation window

anging between 2.7 and 4 mg chitosan/g kaolin. By further increasef chitosan dose, residual turbidity increased, the system being re-tabilized. As Fig. 1 shows, the flocculation of kaolin by N-PECs ofhitosan with synthetic polyanions is very effective in a broad rangef polymer concentration, the values of optimum dose and floccu-

ation window depending on the polyanion nature and the molaratio between charges, n−/n+. When N-PECs were prepared withaPAMPS (Fig. 1a and b), for the same n−/n+, the polyanion molar

ass influenced the amount of N-PEC required to reach optimal

occulation. Thus, the optimum dose decreased with the increasef polyanion molar mass. Buchhammer et al. [19] observed theame influence of the polyelectrolyte molar mass in studies regard-ng the capacity of some N-PEC nanoparticles to remove organic

n flocculation with N-PECs corresponding to different molar ratio between charges.

molecules from aqueous solutions. The presence of hydrophobicnonionic comonomer in the polyanion structure led to the increaseof the N-PEC particles sizes (Table 2). The formed flocs were largerand their sedimentation was faster compared to those formed withChitosan II.

Fig. 2 summarizes the flocculation efficiency (optimum dose andflocculation window broadness) as a function of the molar ratiobetween charges, n−/n+, for all the N-PEC samples investigated,comparative with Chitosan II. The flocculation window broadnesswas calculated as the difference between the re-dispersion con-centration (the highest dose of polymer for which the turbidity ofthe supernatant, OD500, was lower than 0.01 a.u.) and the optimumdose.

Three regions are evident in Fig. 2a: (1) by using N-PECs withn−/n+ = 0.15, the optimum dose abruptly decreased, for all studiedsystems, comparative with optimum dose of Chitosan II (evidencedby a dash line); (2) for N-PECs with molar ratio between 0.15 and0.25 for P(AMPS37-co-TBA63)/Ch II, and 0.15 and 0.35 for NaPAMPSI/Ch II and P(AMPS54-co-TBA46)/Ch II, the optimum dose increasedalmost linearly until the values reached that of chitosan; in thecase of the complexes NaPAMPS II/Ch II, the optimum dose waslower than that corresponding to chitosan, on the whole investi-

gated molar ratios domain; (3) at further increase of molar ratio theamount of flocculants increases over the limit needed for chitosanoptimum flocculation dose.

As concerns the flocculation window broadness, Fig. 2b showsan almost linear increase, for all tested N-PEC, with the increase

M. Mihai, E.S. Dragan / Colloids and Surfaces A: Ph

Fig. 2. The influence of the molar ratio between charges, n−/n+, on the optimumdwfl

oomPnnrpaNtttbw

N

tk

optim (a), and flocculation window broadness (b) as a function ofNaCl concentration using chitosan samples with different molarmasses (470, 670 and 780 kDa) (Table 1).

Generally, the effect of molar mass on the flocculation is mani-fested on the flocculation mechanisms, and less on the flocculation

ose, CP/CS optimum (a), and flocculation window (b) in the flocculation of kaolinith different N-PECs based on chitosan (dashed line marked the CP/CS optimum forocculation with Chitosan II).

f molar ratio between charges, the most important increase beingbserved for P(AMPS37-co-TBA63)/Ch II, on the whole investigatedolar ratio domain. As it was observed by DLS (Table 2), the N-

EC particles size decreased with the increase of the molar ratio−/n+, the complex particles used in this study having the hydrody-amic diameter, Dh, in the range 240–330 nm. Increasing the molaratio between charges the charge density and the size of the com-lex particles decreased, leading thus to the adsorption of a highermount of flocculant on the particle surface. The thicker layer of-PEC particles and the higher hydrophobicity of the flocculant pro-

ected more efficient the particles against re-dispersation, leadingo the increase of the flocculation window. For the same molar ratio,he polyanion nature strongly influenced the flocculation efficiency,oth optimum dose and flocculation window broadness increasingith the polyanion hydrophobicity.

Based on the presented results, a flocculation mechanism with

-PECs was proposed in Scheme 2.

The chitosan characteristics (molar mass, degree of acetyla-ion and chain conformation) suggest a bridging mechanism in theaolin flocculation. The increased flocculation efficiency of N-PECs

ysicochem. Eng. Aspects 346 (2009) 39–46 43

with the molar ratio n−/n+ = 0.15 was most certainly caused by animproved bridging ability of the N-PECs compared to single chi-tosan molecules, mainly due to the larger size and higher densityof free positive charges than chitosan itself (I, Scheme 2). Increas-ing the molar ratio, the particle size and charge density decreased,the N-PEC nanoparticles being adsorbed on the kaolin surface byelectrostatic interactions, neutralizing the kaolin particles surface(II, Scheme 2); the nonattached parts of the N-PECs nanoparticles,may interact with the bare part of another kaolin particles forminghigher aggregates, which are difficult to re-stabilize.

3.3. Effect of low molar mass electrolyte concentration on kaolinflocculation

Strong electrolytes are present in almost all industrial effluentswhich must be processed by flocculation and it is important to knowthe influence of the ionic strength on the efficiency of the kaolinflocculation with chitosan, to establish the relationship betweensalt concentration, polymer molar mass and the performance ofthe flocculation process. As it was already reported, the flocculationefficiency is strongly influenced by the ionic strength of the envi-ronment, the optimum dose decreasing, whereas the flocculationwindow broadened, with increasing ionic strength [26,32]. Accord-ingly, measurement were performed at four different ionic strengthranging between 0.01 and 0.5 M and the flocculation behavior wasobserved and compared for both one component system (chitosan)and N-PEC systems. Fig. 3 shows the influence of ionic strength onthe destabilization of the kaolin dispersions with Chitosan II. Thecorresponding curve of kaolin flocculation with Chitosan II in saltfree aqueous solution was included for comparison.

As Fig. 3 shows, even a small amount of NaCl (0.01 M) sig-nificantly influenced the flocculation behavior of Chitosan II, theoptimum dose sharply decreasing from 2.7 mg flocculant/g kaolinin saltless solution to 1.3 mg flocculant/g kaolin in 0.01 M NaCl. Thefurther increase of NaCl concentration enhanced the flocculationefficiency, the optimum dose decreasing up to 1 mg flocculant/gkaolin when the ionic strength increased to 0.5 M NaCl.

For a better evidence of the flocculation efficiency in the pres-ence of NaCl, Fig. 4 shows the variation of the optimum dose, CP/CS

Fig. 3. Effect of NaCl concentration on the flocculation of kaolin dispersion by Chi-tosan II.

44 M. Mihai, E.S. Dragan / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 39–46

Scheme 2.

Fig. 4. The influence of the NaCl concentration on the optimum dose, CP/CS optimum (a), and flocculation window (b) in the flocculation of kaolin with Chitosan havingdifferent molar masses.

Fig. 5. Residual turbidity (RT) as a function of polymer per substrate (CP/CS, mg/g) in kaolin flocculation with N-PECs based on Chitosan II and copolymers P(AMPS-co-TBA),with a molar ratio n−/n+ = 0.15, prepared in the presence of NaCl with different concentrations.

s A: Ph

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M. Mihai, E.S. Dragan / Colloids and Surface

fficiency [2,3]. This observation is valid for our study, the opti-um dose and the flocculation window broadness being similar

or all chitosan samples regardless the molar mass (Table 1), inaltless solution. A common feature for the investigated samples,bserved in Fig. 4a, is that the optimum dose abruptly decreased

n the presence of small amounts of NaCl (0.01 M), and was almostonstant starting with salt concentration of 0.1 M. Also, optimumose is slightly depending on the chitosan molar mass, increasingith its decrease. At the same time, the flocculation window broad-

ess increased with the increase of chitosan molar mass (Fig. 4b).his behavior may be explained by the higher flexibility of the chi-osan chains, caused by the partial screening on ionic groups onhe polymeric chain by the excess of counterions, which may thusttach to more particles, connecting them in bigger flocs than inbsence of NaCl.

Flocculation of kaolin with N-PECs based on Chitosan II andoth copolymers P(AMPS-co-TBA) used in this study, with a molaratio n−/n+ = 0.15, prepared in the presence of NaCl with differentoncentrations is presented in Fig. 5.

As Fig. 5 shows the optimum dose decreased when the complexarticles were prepared in the presence of NaCl, for both polyan-

ons. The broadness of the flocculation window has been influencedy the polyanion structure, as follows: slowly increased with the

ncrease of ionic strength, in the case of N-PECs prepared with(AMPS54-co-TBA46), and significantly decreased with the increasef ionic strength, in the case of N-PECs prepared with P(AMPS37-o-TBA63). Increasing the concentration of NaCl would have twoain consequences: the increase of the chain flexibility of chitosan

n excess, and the decrease of the complex particle sizes [18,48],oth of them leading to a more effective interaction with kaolinarticles. The prominence of these effects depends on the charac-eristics of the complementary polyelectrolytes. Buchhammer et al.48] observed that the flocculation ability of N-PECs was influencedy the salt concentration and chemical nature of polyelectrolytes,he optimum of flocculation being located in the range of the saltoncentration where the formation of large aggregates of N-PECsook place. The authors supposed that N-PECs act as binders of sus-ended particles and promote the formation of floccs. The presencef NaCl could influence also the particle–particle adhesion, whichecomes weaker increasing the adsorption capacity of the posi-ively charged chitosan (or positively charged N-PECs) on the kaolinegatively charged faces [25].

. Conclusions

The flocculation efficiency of some N-PECs nanoparticles formedy the interaction of chitosan with NaPAMPS and two randomopolymers of AMPS with t-butyl acrylamide, comparative withhitosan, on kaolin suspension was investigated as a function ofhe N-PECs characteristics, such as polyanion structure, molar ratioetween charges, particles morphology and low molar mass elec-rolyte content.

The main conclusions drawn from the study are as follows:

The values of RT decrease with the increase of the chitosan dose,the amount of flocculant required for reaching the maximum clar-ity degree of about 100% (2.7 mg polycation/g kaolin) as well as thebroadness of the optimum flocculation (2.7–4 mg polycation/gkaolin) being almost the same irrespective of the chitosan molarmass;

The N-PEC nanoparticles were more effective than chitosan inthe kaolin separation, especially at low molar ratios betweencharges, when the flocculation window was more then double atan optimum dose lower than that of chitosan. The main advantageof N-PECs is the increase of critical concentration for kaolin re-

[

ysicochem. Eng. Aspects 346 (2009) 39–46 45

stabilization, the N-PEC particles adsorbed on the kaolin surfaceprotecting them more efficient against re-dispersion.

- The flocculation efficiency increased significantly in the presenceof NaCl, for all the chitosan samples investigated, namely increas-ing the ionic strength up to a concentration of 0.1 M NaCl theoptimum dose decreased to lower values than in the absence ofsalts, the flocculation window increasing too, in the same domainof ionic strength.

- The N-PECs performance in kaolin flocculation was enhancedwhen the complex particles were prepared in the presence ofNaCl, the optimum dose decreasing for both polyanions; the ionicstrength influenced in a different way the broadness of the floc-culation window: a little in the case of N-PECs prepared withP(AMPS54-co-TBA46), and much in the case of N-PECs preparedwith P(AMPS37-co-TBA63), which significantly decreased with theincrease of ionic strength.

Acknowledgements

The financial support of this research by the Grant No. 981(Exploratory Research Project) is gratefully acknowledged. Authorsare thankful to Dr. Simona Schwarz (Leibnitz Institute of PolymerResearch, Dresden, Germany) for scientific support.

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