Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 90–99
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Colloids and Surfaces A: Physicochemical andEngineering Aspects
j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa
ource of Unimin kaolin rheological variation–Ca2+ concentration
avanya Avadiara, Yee-Kwong Leonga,∗, Andy Fourieb, Tutun Nugrahab, Peta L. Clodec
School of Mechanical and Chemical Engineering M050, AustraliaSchool of Civil and Resource Engineering M051, AustraliaCentre for Microscopy, Characterisation and Analysis, Australia, The University of Western Australia, 35 Stirling Hwy, Crawley, Perth, WA 6009, Australia
i g h l i g h t s
Rheological (yield stress)-pH vari-ations observed between kaoliniteminerals.Unimin kaolin displays high yieldstress at high pH when highly nega-tively charged.This is due to presence and adsorp-tion of hydrolyzed calcium productsat high pH.These calcium products induceaggregation and increase slurrynetwork strengths.Concentrations and pH of these prod-uct formations vary via acid and basetitration.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 25 April 2014eceived in revised form 27 June 2014ccepted 30 June 2014vailable online 7 July 2014
eywords:a2+ cationsydrolysis-pH
a b s t r a c t
The variations in rheological-pH behaviors between suspensions of low and high calcium (Ca) kaolinwere evaluated via zeta potential, yield stress and sedimentation methods. Elemental analysis via x-ray fluorescence (XRF) showed that Riedel kaolin (0.028%) carries ∼18.0 times lesser Ca concentrationsthan Unimin kaolin where Riedel slurries displayed a maximum yield stress at ∼pH 2.0 when Riedelparticles carried low negative zeta potential magnitudes while Unimin slurries exhibited a maximumyield stress at pH 8.6 when Unimin particles were highly negatively charged. With Ca2+ cations added toRiedel slurries at sufficiently high concentrations, Riedel’s negative zeta potential magnitudes decreasedespecially along high pH, maximum yield stress peaks shifted to high pH and solids content obtained
during consolidation increased by ∼14.0 weight percentages (wt%) at pH 8.0 to be similar to Unimin.These changes in Riedel’s behaviors and the reasons for Unimin’s behaviors is due to the adsorptionof hydrolyzed Ca(OH)+ hydroxy complexes onto these particles, which reduced particle negativity andinduced unlike charge attractions between these particles to improve consolidation and increase slurry
network strengths, and the presence of hydrolyzed Ca(OH)2 precipitates, which increased yield stressesof these kaolin slurries at high pH. The hydrolysis-pH behaviors of Ca2+ cations varied via acid and basetitration paths to correspondingly affect and cause the rheological-pH behaviors of Unimin slurries to
L. Avadiar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 90–99 91
be varied via acid and base titration paths. Overall, this study explains the effect of elemental variationsbetween kaolin particles that affects the rheological-pH behaviors between the respective kaolin slurries.
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. Introduction
Kaolin clay is a valued commodity and it is extensively useds additives and fillers in paper-coating formulations, adhesives,eramics, refractories, pharmaceuticals and cosmetics [1]. This clays also present as dispersed and stable soft sediments clay tailing
aste material that is found in many mineral tailing operations andhat significantly reduces levels of solid loading of waste tailingsnd water recovery for recycling [2]. Due to its continuing applica-ility in various industrial applications and its presence and issuesosed as a waste material, extensive research concerning the prop-rties of kaolin clay such as the surface chemistry, rheology andonsolidation behaviors of kaolin is being widely studied.
Most research on kaolin clay are carried out using commerciallyroduced kaolin samples where kaolin rocks and/or kaolinite-ontaining mineral deposits are mined for kaolinite minerals whereia air flotation, water washing, calcination and/or surface modi-cation, these minerals are produced to be of generally uniformhysiochemical characteristics that allows reproducibility betweenhe similar kaolin minerals analyzed which could be with respecto the origin of the kaolinite mineral.
While analyses on commercially produced kaolin samples,hich generally contain lower concentrations of chemical impu-
ities and accessory minerals, can provide a general trend of thehysiochemical behavior of kaolinite in tailings, caution shoulde exercised concerning the direct relation between kaolinite inhese kaolin samples and kaolinite in tailings due to the higherontents of gangue minerals and process chemicals in the latter.lthough gangue materials and process chemicals could be present
n kaolinite obtained from coal and gold tailings, these materi-ls and chemicals are in negligible quantities in kaolinite obtainedrom mineral sands tailings.
Variations can also be observed between the physiochem-cal characteristics of commercially produced kaolin samplesepending on the origin of the corresponding minerals where con-entrations of elements could vary between these samples to causeariations in the physiochemical such as rheological and consolida-ion behaviors of kaolin slurries analyzed. Most research, however,id not address the effect of the elemental differences that mayxist between kaolin powders and primarily analyzed the x-rayiffraction (XRD) and/or the Brunauer, Emmett, Teller (BET) sur-ace areas of the powders and/or simply stated the origin of theowders, failing to characterize properties, such as the elementalompositions of the kaolin powders used.
Melton and Rand [3] observed that different kaolin slurriestudied by various researchers displayed different rheological-pHesponses with maximum Bingham yield stresses being located atifferent pH levels. Johnson et al. [4] analyzed the effect of Al3+
ons on the zeta potential and shear yield stress behaviors of kaolinuspensions to relate the shifts in isoelectric points (IEPs) and inaximum yield stresses of these suspensions towards the higher
H regions to the adsorption of Al(III)-based hydrolysis products.legmann et al. [5] reported a yield stress maximum at a high pH of.95 for aluminum-kaolinite and related this yield stress-pH max-
mum to the adsorption of polynuclear species, formed from theydrolysis of aluminum ions in solutions, onto kaolin particle sur-
aces.Rheological-pH variations were also observed between Sigma,
luka and Unimin kaolin [6–9]. Maximum yield stress peaks were
observed at pH 2.0 within Sigma and Fluka kaolin slurries wheremagnitudes of zeta potential were low [6] while maximum yieldstress peaks were observed at ∼pH 9.0 within Unimin kaolin slur-ries where magnitudes of zeta potential were high [7–9]. Shankaret al. [8] suggested that Unimin’s high yield stress coupled with itshigh negativity at high pH is due to strong unlike charge attractionsbetween high concentrations of negatively and positively chargedsites on Unimin kaolin particles where the concentrations of neg-atively charged sites are much higher than the positively chargedsites on these particles at high pH. They also suggested the presenceof positively charged sites on Unimin particles at high pH from theincrements in the magnitudes of Unimin’s negative zeta potentialsat high pH from the addition of negatively charged triphosphate,P3O10
5− additives.Increments in negative zeta potential magnitudes and reduc-
tions in yield stresses by P3O105− additives were observed at basic
pH [8]. Adsorption of P3O105− on positively charged sites was
invoked to explain the increase in the magnitude of the negativezeta potential. The removal of the positively charged species ofCa(II) by forming an insoluble phosphate compound is another pos-sibility that has not been considered. The increase of the Uniminparticle negative charge density by P3O10
5− enhanced the strengthof the interparticle repulsion. As a result, the yield stress of Uniminslurries was reduced and progressively with P3O10
5− concentra-tion. At sufficient P3O10
5− concentration, complete dispersion ofthe Unimin slurries was observed at this high pH level.
The IEPs of kaolin particle sites determine the charges carriedby these particles at various pH to relate to the rheological-pHbehaviors of these slurries. While tetrahedral silica sheets in kaolinparticles are understood to carry permanent pH-independent neg-ative charges resulting from the isomorphous substitution of Si4+ byAl3+ groups [10], Gupta and Miller [11] explained that these tetra-hedral silica sheets carry an IEP at pH < 4. The IEP of the Si-OH andAl-OH edge sites is between pH 5.0–7.0 [12–14] and that of octahe-dral alumina sheets is between pH 6.0–8.0 [11]. According to theseIEPs, at high pH, the number of positively charged sites on kaolinsurfaces should be low to exist in negligible quantities. Unimin’svaried rheological-pH behavior could, however, suggest the pres-ence of positively bound sites on Unimin kaolin surfaces most likelycontributed by the adsorption of hydrolyzed products of polyvalentcations such as Al3+, Mg2+ or Ca2+ at high pH.
Hydrolyzed Al(III) products are, however, only present withinkaolin slurries when the corresponding kaolin particles within slur-ries are exposed to low pH of <3–4. This is the case for kaolin slurriesthat do not contain Al3+ cations and is in contrast to slurries ana-lyzed by Flegmann et al. [5] and Johnson et al. [4] that contain Al3+
cations. The reduction in slurry pH to <3–4 results in the dissolutionof kaolin particles that causes the leaching and, as a result, pres-ence of Al3+ cations into slurries where the subsequent hydrolysisof these cations and the presence of hydrolyzed Al(III) products aspH is raised could affect the rheological-pH behaviors of kaolin slur-ries. Flegmann et al. [5] reported that the rheological-pH behavior ofhydrogen-kaolinite was similar to that of aluminum-kaolinite dueto the treatment of kaolinite with acid at pH < 3 during the prepara-tion of hydrogen-kaolinite that most likely caused the dissolutionof kaolin particles that produced Al3+ cations, which hydrolyzed to
alter the rheological-pH behavior of hydrogen-kaolinite to be simi-lar to aluminum-kaolinite. Unless in high concentrations on Uniminkaolin and unless rheological measurements of Unimin slurries
a Multivalent cationic elements in higher concentrations on Unimin than Riedel.b Multivalent cationic elements in higher concentrations on Unimin than Sigma.c Multivalent cationic elements in higher concentrations on Unimin than Fluka.
The particle size distributions (PSDs) of Riedel, Unimin, Sigmaand Fluka kaolin powders measured on a Malvern MastersizerMicroplus Particle Size Analyzer are shown in Fig. 2 (Refer to
Table 2Ex cation compositions (mequiv(H+)/100 g) on Riedel and Unimin kaolin at theirnatural pH in water.
2 L. Avadiar et al. / Colloids and Surfaces A
ere carried out from low (<3–4) to high pH, Al3+ cations wouldot contribute to Unimin’s rheological-pH behavior.
Fuerstenau and Palmer [15] analyzed the hydrolysis trendsf Mg2+ and Ca2+ ions to explain that from pH 10.0, these ionsydrolyze to form positively charged hydroxy complexes, Mg(OH)+
nd Ca(OH)+ respectively. X-ray fluorescence (XRF) results of Flukand Unimin kaolin as shown in Teh et al. [6] and Shankar et al. [8]espectively, however, indicate the similar concentrations of Mg2+
ons on both these kaolin particles where at similar ionic strengths,imilar concentrations and, as a result, adsorptions of Mg(OH)+
omplexes should occur onto these kaolin particle surfaces thatesult in similar yield stress-pH trends between these kaolin slur-ies at high pH; where having not, could eliminate the possibility ofg2+ ions affecting the rheological-pH behavior of Unimin slurries.
his could explain the existence of other polyvalent cations suchs Ca2+ that could have resulted in Unimin’s rheological-pH behav-or as Ca2+ exist in higher concentrations on Unimin than Sigma orluka particles.
In this study, two types of commercially available kaolin pow-ers, Riedel (with low Ca content) and Unimin, were analyzed toetermine if elemental differences contribute to the rheological-pHesponse of Unimin kaolin slurries and if so, how. The rheological-H behaviors of two other commercially available kaolin powders,igma and Fluka that were previously analyzed by Teh et al. [6]ere also compared with behaviors of Riedel and Unimin kaolin.
eta potential, yield stress (rheological) and sedimentation behav-ors of Riedel and Unimin slurries (with and without the addition of
gCl2 or CaCl2 crystals) were evaluated to express the differencesn the rheological-pH and consolidation behaviors between theselurries. Reasons for differences were substantiated using analysesuch as XRF, cation exchange capacity (CEC), hydrolysis (turbidity)-H studies of Mg2+ and Ca2+ elemental solutions and cryogeniccanning electron microscopy (cryo-SEM) imaging of kaolin slur-ies. These analyses examined the effects of elements, such as Mg2+
nd Ca2+, on the rheological-pH and consolidation behaviors ofnimin kaolin slurries to show the source of rheological-pH varia-
ions between the slurries analyzed where this study also examineshe variability between commercially available kaolin powders.
. Materials and methods
.1. Materials and their characterization
The following kaolin powders are mined from kaolinite-ontaining minerals and/or kaolin rocks. Riedel kaolin, which is ofroduct number 18672, is mined from the United Kingdom androcessed and made commercially available from Sigma–Aldrichaborchemikalien GmbH in D-30926 Seelze, Germany as a whiteo yellow or grayish finest kaolin powder. Unimin kaolin, whichs commercially known as PrestigeTM NY Kaolin Forming Clay,s mined from Granville, New South Wales and processed underigid ISO quality programs and made commercially available fromnimin Australia Ltd as a dry powdered speswhite kaolin. Sigmaaolin, which is commercially known as Aldrich kaolin and of prod-ct number 228834, is processed and made commercially availablerom Sigma-Aldrich as a powdered sample. Fluka kaolin, which isommercially known as Fluka Chemika Kaolin 60609 and of gradeh Eur, is processed and made commercially available from Buchs,witzerland, Sigma–Aldrich as a powdered sample.
These kaolin samples are likely to carry impurities as they areined from naturally occurring minerals. The composition and
urity of Riedel and Unimin kaolin powders that were analyzed inhis study were, therefore, studied via XRD. (Refer to Supplemen-ary data for XRD method). The main trace impurities in Riedel were
uscovite and quartz and in Unimin was quartz. These impurities
Concentration differences were empirically selected using a ratio of element com-position (%) on Unimin: element composition (%) on Riedel, Sigma or Fluka of 5:1times or more.
were present in very low concentrations of <5% on these particlesto indicate their minimal effects on the yield stresses of Riedel andUnimin slurries and the low concentrations of elements such asTi and K that would form from these low concentrations of micaminerals to possibly exist as soluble cations in these slurries. Thepresence of Ti and K, if any, in high concentrations on these particleswould be, as a result, elements naturally found on these particles.
Elemental and exchangeable (Ex) cation compositions of thesekaolin clays were determined via XRF spectrometry (Table 1) andCEC analysis (Table 2) respectively and discussed in detail in Section3.2. (Refer to Supplementary data for XRF and CEC methods).
The BET surface areas of Riedel and Unimin kaolin measured ona Micromeritics Gemini using N2 adsorption were 9.9 and 19.9 m2/grespectively (Refer to Supplementary data for reasons concerningdiscrepancies and unreliability of these BET surface areas mea-sured).
SEM images of Riedel and Unimin powders obtained usinga Zeiss 55 field emission SEM are shown in Fig. 1 wherehigh concentrations of micro-islands were observed on boththese poorly-ordered particles and where these similarities inmicrostructures observed between these particles minimize and/oreliminate the possibilities of microstructural variations affectingthe zeta potential-pH and rheological-pH behaviors between Riedeland Unimin slurries (Refer to Supplementary data for SEM samplepreparation and imaging methods).
The natural pH of Riedel and Unimin kaolin in water is 4.90 and8.13 respectively where their corresponding (average) zeta poten-tials measured on a Colloidal Dynamics ZetaProbe were −35.16 and−30.98 mV respectively. Despite the lower natural pH, Riedel kaolincarries a higher negative zeta potential magnitude at its natural pHthan Unimin kaolin at its natural pH. No IEPs were recorded foreither the Riedel or Unimin slurries. (Refer to Supplementary datafor the discussion concerning the absence of IEPs for both Riedeland Unimin particles).
L. Avadiar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 90–99 93
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Fig. 1. SEM images of (a) Riedel and (b) Unimin pa
upplementary data for detailed methods via which PSDs werebtained and a discussion on the effects of particle delaminationuring commercial processing and slurry preparation on theseSDs). While Riedel, Unimin and Fluka kaolin display very simi-ar PSDs to each other with very similar d50 values of 4.00, 4.01 and.98 �m respectively, the PSD of Sigma particles is shifted slightlyo the left with a lower d50 value of 1.50 �m to indicate that thesearticles are slightly smaller. This could affect the yield stress mag-itudes of Sigma slurries to be slightly higher than Riedel, Uniminnd Fluka slurries across all pH levels due to the slightly higherarticle number concentrations within Sigma slurries at constantlurry solids content. These particle size variations are, however,ot expected to affect the yield stress-pH trends across these slur-ies as these trends are affected by the inter-particle forces withinhese slurries at respective pH.
.2. Methods of analysis
All kaolin slurries were prepared by adding appropriatemounts of respective kaolin powders and 1.0–5.0 M NaOH or HClolutions and then 0.1 weight percentages (wt%) CaCl2 or MgCl2tock solutions (when required) to deionized water and then soni-ated using a Branson digital sonifier with a 2.5 cm probe for0–40 s. The solids contents and pH of kaolin slurries prepared aretated under each measurement type explained where slurry pHevels were adjusted by the amounts of NaOH or HCl added. Theolids contents specified are varied and with respect to each mea-urement type undertaken so as to obtain accurate measurements
f the particle properties or slurry behaviors analyzed. Zeta poten-ial and sedimentation measurements require slurries of low solidsontent of 5.0–8.0 wt% while yield stress measurements requireocculated slurries of 20.0–40.0 wt%. These variations in slurry
Fig. 2. PSDs of Riedel, Unimin, Sigma and Fluka kaolin powders.
s at magnifications of 20,000× and scales at 1 �m.
solids contents between the different measurements should notinduce variations in the trends of asymmetric charging of kaolinparticles where pH would affect these trends of kaolin particleasymmetric charging. CaCl2 and MgCl2, provided by Ajax FinechemPty Ltd and Optigen Scientific respectively in their anhydrousforms, were prepared into stock solutions using distilled water.
Zeta potential measurements of 5.0 wt% kaolin slurries ofaverage conductivity between 2.4–2.8 mS/cm at 22.0 ◦C wereundertaken using a ZetaProbe instrument manufactured by Col-loidal Dynamics, USA. Slurry pH was gradually decreased from aninitial pH of 12.0 to 2.0 in a stepwise manner via acid titration using0.5–1.5 M HCl while zeta potential and conductivity measurementswere taken. The ZetaProbe uses an electroacoustic-based techniqueto determine zeta potentials where this technique was developedto measure the zeta potentials of spherical particles with low sur-face conductance. Kaolin particles, on the contrary, have a plateletshape and carry anomalously high surface conductance at very lowelectrolyte concentrations [16]. The zeta potentials calculated bythe ZetaProbe therefore does not represent the true potential valueof the kaolin particles in water where, however, these values canbe used as a representation of the “average” zeta potential value ofthe kaolin particles in water [17]. Discretion was thus maintainedtowards the application of the zeta potential-pH results in relationto the yield stress-pH or sedimentation results where only generalvariations in the zeta potential-pH trends or magnitudes and notspecific zeta potential magnitudes were mentioned in this study.
For yield stress measurements, 20.0–40.0 wt% kaolin slurries ofaverage conductivity between 4.0–4.5 mS/cm at 22.0 ◦C were pre-pared at pH 12.0 or 2.0. The maximum torque, or S values thatrepresent the amounts of torque that the slurries can exert againstthe constant rotational motions created by the vane prior to yield-ing were then measured in a stepwise manner from pH 12.0 to 2.0or pH 2.0 to 12.0 using 0.5–1.5 M HCl or NaOH respectively. S val-ues were measured on the Brookfield LVDV-II and RVDV-II vaneviscometers which measure low to moderate torques of slurriesand have spring viscometer constants, VC (or torque at 100% scalereading) of 0.0673 and 0.7187 mNm respectively. The medium andsmall three-blade vanes of vane constants, K of 3.757 × 10−6 and4.538 × 10−7 m3 respectively were used. Vane rotational speeds onboth viscometers were kept low and constant at 0.6 rpm so as not todisturb the network structures formed within the slurries. From Svalues obtained at each respective pH, the yield stress, �y was calcu-lated with respect to pH using the equation �y = [(S ÷ 100) × Vc] ÷ K.
For sedimentation characterization, 8.0 wt% kaolin slurries withan average conductivity of 0.2 mS/cm at 22.0 ◦C were prepared at
pH 8.0. pH 8.0 corresponds to the pH of many mineral waste tail-ings where variations in consolidation behaviors between Riedeland Unimin slurries in this study could only serve to provide a gen-eral idea concerning similar variations observed in consolidation
Fig. 3. Zeta potential-pH behaviors of 5.0 wt% Riedel and Unimin slurries.
4 L. Avadiar et al. / Colloids and Surfaces A
ehaviors between mineral waste tailings containing kaolin parti-les. As Riedel’s and Unimin’s natural pH in water is 4.90 and 8.13espectively, 1 M NaOH was added into Riedel slurries before son-cation to increase Riedel’s pH to ∼8.0. After sonication, slurries
ere transferred into 250 ml beakers. At regular time intervals,ud-lines, which represent the suspension–solution interfaces,ere measured to calculate solids content with respect to time.
At specific pH levels, Mg2+ and Ca2+ ions hydrolyze to form metalydroxy complexes and/or metal hydroxide precipitates where
nsoluble complexes and precipitates formed render solutions tur-id. Turbidity-pH measurements of MgCl2 and CaCl2 solutions werehus carried out to predict the hydrolysis-pH trends of Mg2+ anda2+ ions respectively where metal hydroxy complexes and metalydroxide precipitates were considered to form at approximatelyimilar pH to each other. MgCl2 and CaCl2 solutions were pre-ared by adding 0.5 dry weight percentages (dwb%) of MgCl2 andaCl2 crystals to deionized water and the initial pH of the solu-ions was adjusted to 12.0 or 2.0 by adding 1.0 M NaOH or HNO3espectively. (The addition of 0.5 dwb% of crystals is with respecto 25 g of kaolin powder, which represents similar kaolin powderoncentrations as to that in yield stress measurements and theydrolysis-pH analyses of these cations via acid titration from pH2.0 correlates to explain the zeta potential-pH and yield stress-pHrends of the slurries analyzed via acid titration in this study.) Theolutions were then left to condition for 15 min on a magnetic stir-er from Industrial Equipment and Control Pty. Ltd., CH2090-001.he turbidity of these solutions was then measured at pH inter-als of 1.0 every 20 min from pH 12.0–2.0 or pH 2.0–12.0 using.5–1.5 M HNO3 or NaOH respectively whilst the solutions wereontinuously conditioned on the magnetic stirrer. Turbidity mea-urements were carried out using a portable turbidimeter, Hannanstruments, HI98703, which functions according to Method 180.1f The Environmental Protection Agency of the United States ofmerica. The temperature of the slurries during measurement wasept constant at 22.0 ◦C.
Cryogenic (or cryo vitrification) techniques were used torepare the kaolin slurry samples for SEM imaging where sam-les were obtained from the middle of the consolidated sedimenteds or turbid supernatants within sedimented slurries. (Refer toupplementary data for cryo-SEM sample preparation and imagingethods).
. Results and discussion
.1. Variations in zeta potential-pH and yield stress-pH behaviorsetween Riedel and Unimin slurries
The Riedel particles had higher negative zeta potential mag-itudes than the Unimin particles especially at high pH (Fig. 3)o suggest the possibility of higher concentrations of positivelyharged sites occurring on the Unimin than the Riedel particlesspecially at high pH.
In accordance to the less negative zeta potential magnitudesf Unimin than Riedel particles, Unimin slurries displayed higherield stresses to Riedel slurries especially at high pH (Fig. 4a).he yield stress-pH trend of 40.0 wt% Riedel slurries was low atigh pH and high at low pH, similar to the yield stress-pH trendsf most other kaolin slurries such as 15.0 wt% Sigma and Flukaaolin slurries studied by Teh et al. [6] via acid titration (Inset inigure 4a). From pH 12.0–8.0, the yield stresses of Riedel slurriesere zero, meaning the slurries were completely dispersed due
o the strong inter-repulsive forces between the highly negativelyharged Riedel particles at this high pH range. As pH decreases to8.0, the yield stresses within Riedel slurries increased to peak at02.5 Pa at pH 2.2. Yield stress increments with decreasing pH is
Fig. 4. (a) Yield stress-pH behaviors of 40.0 wt% Riedel and Unimin slurries and(b) hydrolysis-pH trends: turbidity-pH trends of 0.5 dwb% (with respect to 25.0 gkaolin) MgCl2 and CaCl2 solutions via varied pH titration paths. Inset in Fig. 4(a).Yield stress-pH behaviors of 15.0 wt% Sigma and Fluka slurries [6].
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ttributed to the alumina faces and edge sites on kaolin particlesrotonating at pH 6.0–8.0 [11] and 5.0–7.0 [12–14] respectively toarry positively charged sites. The higher degrees of responsive-ess of edge sites, compared to alumina faces, to interactions isuggested to result in the higher degrees of electrostatic attractionsand/or van der Waals forces of attraction similar to that within sil-ca slurries at low pH) between the negatively charged silica facesnd the positively charged edge sites. The general increments intrength and numbers of particle connections within slurry net-ork structures via these attractive interactions increase the yield
tresses of kaolin slurries such as Riedel, Sigma and Fluka at low pH18].
The yield stress-pH trend of 40 wt% Unimin slurries was, how-ver, very different from Riedel, Sigma and Fluka slurries where theield stresses of Unimin slurries were high at high pH and approx-mately similar to Riedel slurries at low pH (Fig. 4a–Acid titration).nimin’s yield stresses ranged from 53.1 Pa at pH 11.5 to a max-
mum of 143.1 Pa at pH 8.6 and to a minimum of 63.4 Pa at pH.0 where, unlike Riedel slurries, Unimin slurries did not displayny signs of complete dispersion at any pH. At low pH, the naturef the attractive force between the Unimin particles responsibleor the relatively high yield stresses of these slurries would be theame as that for Riedel, Sigma and Fluka particles. However, atigh pH, the nature of the attractive force, which must have sur-assed the strong repulsive forces between the Unimin particles toesult in a yield stress peak that occurred at a pH of high negativearticle zeta potential within the Unimin slurries, is of a differ-nt origin. This attractive force could have been induced from thedsorption of hydrolyzed products of cations [19,20] such as Ca2+
nd Mg2+ within Unimin slurries where the specific adsorptions ofa(II) hydrolyzed products was observed to lead to reductions innegative) zeta potential magnitudes and lower dispersions withinaolin slurries [21]. (Refer to Supplementary data for the expla-ation on the negligible effects of particle sizes and slurry solidsontent on the yield stress-pH trends of kaolin slurries and the dis-ussion on the effects of partial dissolution of Riedel and Uniminaolin particles on the rheological-pH behaviors of these slurries).
.2. Possible cations causing yield stress-pH variations betweeniedel and Unimin slurries
XRF (Table 1) and CEC (Table 2) analyses were carried out toetermine cations in higher concentrations on Unimin than Riedelarticles. XRF analysis showed that Unimin carries lower concen-rations of K2O but higher concentrations of Na2O, TiO2 and CaOhan Riedel. The monovalent, group 1 ions such as K+ and Na+
ould not contribute to Unimin’s high yield stresses as these ionsre usually counterions which adsorb to neutralize particle charges22] and upon hydrolysis exist to be KOH and NaOH precipitateshere despite in high concentrations, K+ did not affect Riedel’s
ield stresses at high pH. TiO2 would also not contribute to Unimin’sigh yield stresses as unlike K2O, Na2O and CaO, which are soluble
n water, TiO2 is insoluble in water and would remain as precip-tates within Unimin slurries where despite in similar or higheroncentrations on Sigma and Fluka kaolin to Unimin kaolin, TiO2id not affect the yield stresses within Sigma and Fluka slurriest high pH. Ca2+, on the other hand, forms highly particle surfacective positively charged complex species at high pH upon hydroly-is that could adsorb onto Unimin particles to affect Unimin’s yieldtress-pH behavior.
Table 1 Elemental compositions (%) of Riedel, Unimin, Sigma andluka kaolin via XRF analysis. CEC analysis showed that Unimin car-
ies lower concentrations of Ex Al3+ but higher concentrations of Exg2+ and Ca2+ than Riedel (Table 2). (Refer to Supplementary data
or reason for the high Ex Al3+ concentration on Riedel kaolin andhe negligible effect of Al3+ on Unimin’s high yield stress). The high
cochem. Eng. Aspects 459 (2014) 90–99 95
Ex Mg2+ concentration on Unimin than Riedel, which is despite XRFanalysis displaying approximately similar or, in fact, marginallyhigher MgO concentrations on Riedel than Unimin, is of signifi-cance and is not within experimental errors. These higher Ex Mg2+
and Ca2+ concentrations on Unimin particles could be attributedto the presence of MgCO3 and CaCO3 soluble salts on these parti-cles where these soluble salts undergo dissolution at generally lowpH to yield Mg2+ or Ca2+ and OH− ions in water. While the pres-ence of OH− forces the natural pH of Unimin kaolin in water up to8.13, the presence of Mg2+ and Ca2+ cations contributes to increasethe Ex Mg2+ and Ca2+ concentrations and the sum of all Ex cations(which includes cations contributed by soluble salts) on Unimin tobe higher at 12.3 mequiv(H+)/100 g compared to the CEC of Unimincalculated at 9.2 mequiv(H+)/100 g. These MgCO3 and CaCO3 con-centrations are inferred to be negligible on Riedel particles of lownatural pH and low Ex Mg2+ and Ca2+ concentrations.
3.3. Hydrolysis (turbidity)-pH trends of Mg2+ and Ca2+ cations inrelation to yield stress-pH trends of Unimin slurries
(Refer to Supplementary data for the reason concerning the neg-ligible adsorptions and yield stress effects of Mg and Ca hydratedions compared to hydrolyzed products on kaolin slurries and forthe hydrolysis-pH analyses of Mg2+ and Ca2+ cations conducted viabatch or base titration analysis by previous studies where the pHof the onset of Mg2+ and Ca2+ hydrolysis was observed to be variedand still vague from these studies).
In this study, the hydrolysis (turbidity)-pH trend of Mg2+ ana-lyzed via acid titration (Fig. 4b) did not coincide to relate to theyield stress-pH trend of Unimin slurries analyzed via acid titration(Fig. 4a). Mg(II) hydrolyzed products namely Mg(OH)+ complexesand Mg(OH)2 precipitates were inferred to form between pH12.0– > 9.0 with high concentrations of insoluble Mg(OH)2 precipi-tates especially forming between pH 12.0–9.5 as inferred from thehigh turbidity of Mg2+ solutions between ∼130–50 Nephelomet-ric Turbidity Units (NTU) respectively. Maximum and high yieldstresses within Unimin slurries, however, occurred between pH9.8–8.6 where negligible Mg(II) hydrolyzed products were inferredto exist at pH 8.6 when Unimin’s yield stress peaked. The pH ofMg2+ hydrolysis via base titration between 10.0–12.0, which wassimilar as to in most literature analyzed, on the other hand, coin-cided with the pH of high yield stresses within Unimin slurries viabase titration from >10.0–12.0 (Fig. 4).
The turbidity-pH trend of Ca2+ via acid titration, which was verydifferent to the trends of Mg2+ via acid or base titration and Ca2+
via base titration (Fig. 4b), on the other hand, coincided to correlateto the yield stress-pH trend of Unimin slurries via acid titration(Fig. 4a). While maximum turbidity of Ca2+ occurred betweenpH 10.5–8.3, maximum and high yield stresses of Unimin slur-ries occurred between pH 9.8–8.6 to fall within the pH range ofCa2+’s maximum turbidity. Beyond the pH values of these maxi-mum points, Ca2+’s turbidity and Unimin’s yield stresses graduallyand correspondingly decreased. The pH of Ca2+ turbidity via basetitration from >10.0–12.0, which was similar to in most litera-ture analyzed, also coincided to the pH of high yield stresses ofUnimin slurries via base titration from >10.0–12.0 (Fig. 4). (In thisstudy, the yield stress of Unimin slurries via base titration wasobserved to increase from pH ∼ 6.5 to display greater incrementsfrom pH > 10.0–12.0. In accordance to this trend, James and Healy[23] observed especially higher degrees of Ca2+ adsorptions frompH > 10.0 onto SiO2 particles and explained that a general qual-
itative relationship exists between the pH of abrupt adsorptionand the pH of metal ion hydrolysis.) These correlations betweenthe hydrolysis-pH trends of Ca2+ and the yield stress-pH trends ofUnimin slurries via both acid and base titrations indicates that Ca2+,
96 L. Avadiar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 90–99
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Fig. 6. Effect of MgCl2 and CaCl2 on yield stress-pH behaviors of 40.0 wt% Riedel
ig. 5. Effect of MgCl2 and CaCl2 on zeta potential-pH behaviors of 5.0 wt% Riedellurries. The plot for Unimin was included for comparison. Riedel (�), 0.2 dwb%gCl2 (�), 0.5 dwb% MgCl2 (♦), 0.2 dwb% CaCl2 (�), 0.5 dwb% CaCl2 (�, Unimin (�).
ompared to Mg2+, exists in higher concentrations on Unimin kaolinnd is the reason for Unimin’s varied rheological-pH behaviors.
The variations in the turbidity-pH trend of Ca2+ via acid titra-ion compared to the other turbidity-pH trends displayed in Fig. 4bomprises of the significantly low turbidity of these Ca2+ solutionsith a maximum of only ∼20 NTU compared to the turbidity ofg2+ solutions analyzed via acid or base titrations at ∼120 NTU.
hese very low turbidity values cannot be primarily attributedo the slightly higher solubility of Ca(OH)2 than Mg(OH)2 precip-tates where this low turbidity indicates the formation of highoncentrations of hydrolyzed soluble complex species and/or nano-ized precipitates where these complex species could comprisef Ca(OH)+ and/or of other possible Ca(II) hydrolyzed complexesnd/or species formed during acid titration. Such species could,owever, not comprise of anionic Ca(OH)3
− complexes as thesepecies would result in higher zeta potential magnitudes and lowerield stresses at high pH within Unimin slurries. Another varia-ion in the turbidity-pH trend of Ca2+ via acid titration involvedhe larger pH range of hydrolysis from 12.0– > 6.0 within these Ca2+
olutions compared to the smaller hydrolysis pH ranges of the otherolutions displayed in Fig. 4b. The maximum turbidity of ∼20 NTUetween pH 10.5–8.3 within these Ca2+ solutions suggest the for-ation of Ca(OH)2 precipitates together with Ca(OH)+ complexes.
he very low turbidity values of these solutions from pH > 10.5nd < 8.3–6.0 suggest the higher concentrations of Ca(OH)+ toa(OH)2 formations. While these hydrolyzed products-pH forma-ions could suggest the greater contribution of Ca(OH)2 precipitateso the yield stress peak of Unimin slurries at pH 8.6, the generallyigh yield stresses of Unimin slurries along high pH is suggested toe due to the greater contribution of the higher concentrations ofydrolyzed soluble and/or nanosized Ca(II) products or Ca(OH)+ tohe yield stresses of Unimin slurries.
.4. Zeta potential-pH and yield stress-pH behaviors of Riedellurries with Mg2+ and Ca2+ cations in relation to behaviors ofnimin slurries
Zeta potential-pH and yield stress-pH behaviors of Riedel slur-ies with MgCl2 and CaCl2 respectively added displayed to confirmhe individual contributions of Mg2+ and Ca2+ cations to theheological-pH behaviors of Unimin slurries via observations oflterations of Riedel’s behaviors to replicate Unimin’s.
Reductions were observed in Riedel’s (negative) zeta potentialagnitudes with Mg2+ and Ca2+ added (Fig. 5), similar to Yukselen
nd Kaya’s [24] observations. These reductions were especially sig-ificant at high pH of >∼6.0 with Ca2+ than Mg2+ added as zeta
potential magnitudes of Riedel with Mg2+ continued increasingwith pH but approached an approximate plateau with Ca2+. Whileincrements in slurry conductivities or ionic strengths with Mg2+ orCa2+ added could cause compressions of the electrical double lay-ers on the particles and reduce the zeta potential magnitudes ofthe particles within the slurries, these average conductivity incre-ments (calculated over pH 12.0 to 2.0) of Riedel slurries with Mg2+
and Ca2+ added were only marginal to without Mg2+ or Ca2+ added.These conductivity increments were also not substantially higherwith 0.2 dwb% CaCl2 (0.01 mS/cm) than MgCl2 (0.01 mS/cm) and0.5 dwb% CaCl2 (0.42 mS/cm) than MgCl2 (0.26 mS/cm) added. Thisexplains that in comparison to the effects of conductivity incre-ments, higher concentrations of formations and adsorptions ofCa(OH)+ than Mg(OH)+ occurred onto Riedel particles from pH > ∼6.0 that resulted in the lower zeta potential magnitudes and thesimilarity in Riedel’s zeta potential-pH behavior to Unimin’s with0.2 dwb% CaCl2 added. Higher Ca(OH)+ to Mg(OH)+ concentrationsare inferred to exist from the lower Ca2+ to Mg2+ turbidity and thelarger Ca2+ to Mg2+ pH range of hydrolysis (Fig. 4b–Acid titration)with either 0.2 or 0.5 dwb% MgCl2 and CaCl2 added.
The continuing slight reductions in Riedel’s zeta potential mag-nitudes at pH below the respective pH of the onset of hydrolysis ofthese ions could indicate the continuing adsorptions of these highlysurface active Mg(OH)+ and Ca(OH)+ complexes formed at higherpH onto these kaolin surfaces and/or to the compressions of elec-trical double layers of kaolin particles from the presence of thesedivalent cations.
Adsorptions of these highly particle surface active complexescan potentially reverse the zeta potentials of particles at the pHvalues where significant concentrations of these complexes arepresent [25]. The absence of surface induced precipitation or chargereversal points within these zeta potential-pH trends of Riedel slur-ries with either Mg2+ or Ca2+ added, despite the formations ofMg(OH)2 and Ca(OH)2 respectively (Fig. 4b–Acid titration), explainsthe low concentrations of these hydroxides formed with the gen-erally low dosages of 0.2 and 0.5 dwb% MgCl2 and CaCl2 addedwhere with higher and sufficient MgCl2 and CaCl2 dosages added,Riedel’s zeta potential magnitudes are expected to progressively beless negative especially along the high pH and to subsequently dis-play surface induced precipitation and charge reversal points thatcoincide to Mg(OH)2 and Ca(OH)2 formations at the high pH.
Higher yield stresses at high pH and shifts in yield stress peaksto high pH were observed within Riedel slurries with Mg2+ andCa2+ added (Fig. 6). With 0.2 dwb% MgCl2, Riedel’s yield stressesincreased especially between pH 11.91–8.77 to 66.5–85.4 Pa and
: Physicochem. Eng. Aspects 459 (2014) 90–99 97
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eaked at pH 10.22. With 0.5 dwb% MgCl2, Riedel’s yield stressesncreased only moderately and did not display a definitive peak atigh pH to remain generally similar between 35.9–58.7 Pa through-ut pH 12.0–2.0. With 0.2 dwb% CaCl2, Riedel’s yield stressesncreased especially between pH 9.77–8.08 to 53.1–82.9 Pa andeaked at pH 8.38. With 0.5 dwb% CaCl2, Riedel’s yield stresses
ncreased moderately between pH 9.71–7.99 to 52.3–68.1 Pa andt greater extents between pH 11.96–10.03 to 73.6–86.2 Pa ando peak at pH 10.72. The yield stress fluctuations observed withinhese slurries with Mg2+ and Ca2+ added could be attributed to theseield stress measurements being carried out via acid titration anal-sis as via base titration analysis, the degrees of such fluctuationsere considerably reduced (the yield stress-pH studies of Riedelith Mg2+ and Ca2+ via base titration analysis were carried out by
he authors for another study). This could explain the possibilities ofotential variations in the adsorption and/or precipitation behav-
ors of Mg(II) and Ca(II) hydrolyzed products onto Riedel particlesia acid titration compared to via base titration, which, however,ill not be a focus in this study.
The respective yield stress peak positions of Riedel slurries with.2 dwb% MgCl2 and CaCl2 added occurred at pH regions whenhe turbidity of the Mg2+ and Ca2+ solutions were high at pH2.0–9.5 and 10.5–8.3 respectively (Fig. 4b- Acid titration) whereiedel’s yield stress-pH trend and peak position was most similaro Unimin’s with 0.2 dwb% CaCl2 added. This high turbidity-yieldtress peak relation explains the contribution of Mg(OH)2 anda(OH)2 to result in these pH-specific increments and yield stresseaks in these Riedel slurries. As inferred from the general reduc-ions in zeta potential magnitudes of these Riedel slurries with.2 dwb% MgCl2 and CaCl2 added, however, Mg(OH)+ and Ca(OH)+
dsorptions also occur onto these Riedel particle surfaces to resultn positively charged sites on these particles that undergo unlikeharge attractions with the negatively charged sites on these par-icles that leads to the general yield stress increments of theselurries. While slurries with 0.5 dwb% MgCl2 displayed no suchorrelation between its insignificant yield stress-pH peak posi-ion and the hydrolyzed Mg(II) products-pH behaviors, the highield stress increments especially between pH 11.96–10.03 withiniedel slurries with 0.5 dwb% CaCl2 added, indicates the pH-specific
ncrements and peaks attributed to Ca(OH)+ complexes. These com-lexes are inferred to exist in greater concentrations than Ca(OH)2rom the low Ca2+ solution turbidity along this high pH rangeFig. 4b–Acid titration) and could be produced in sufficiently higheroncentrations at this higher dosage added to control and causehe higher (negative) zeta potential magnitude reductions and thereater yield stress shifts to higher pH.
Riedel slurries replicated the zeta potential-pH and the yieldtress-pH trend and peak position of Unimin slurries most similarlyith 0.2 dwb% CaCl2 added into Riedel slurries (Figs. 5 and 6) where
his could suggest that concentrations of Ca2+ within Unimin kaolinorrelates to 0.2 dwb% of CaCl2 (which is with respect to 25.0 g ofnimin kaolin). The presence of MgCO3 and CaCO3 soluble salts asrecipitates in Unimin slurries, due to their poor solubility at highH, could explain the higher yield stresses of Unimin slurries at highH that was not attained by Riedel slurries even with the additionf these Ca2+ cations. Mg2+ additions in considerable and similaroncentrations to Ca2+ and the subsequent Mg(OH)+ adsorptionsnd Mg(OH)2 precipitations, on the other hand, was observed tolter Riedel’s behaviors not to replicate Unimin’s behaviors but inccordance to the presence of the Mg(II) hydrolyzed species withespect to pH. This indicates that Mg2+ concentrations are low onnimin kaolin where, as observed from XRF analysis, MgO concen-
rations were, in fact, marginally lower on Unimin than Riedel andluka kaolin (Table 1) and where the high Ex Mg2+ concentrationsn Unimin are, in fact, attributed to the presence of MgCO3 solublealts on Unimin particles. Mg2+ cations are not but Ca2+ cations
Fig. 7. Solids content vs. time behaviors of Riedel slurries at pH 8.0, pH 4.90 and pH8.0 with CaCl2 added and Unimin slurries at pH 8.0.
are the reason for Unimin’s varied rheological-pH behavior athigh pH.
3.5. Particle packing and consolidation behaviors within Riedelslurries with Ca2+ cations in relation to behaviors within Uniminslurries
At pH 8.0, 8.0 wt% Riedel slurries displayed negligible consoli-dation to reach only ∼8.6 wt% after ∼2 days (Fig. 7). This negligibleslurry consolidation is due to the high negative charges and inter-repulsive forces between the Riedel particles at this high pH. Theslurry was completely dispersed where no phase separation ofclear liquid from the suspended solids was observed. These smallRiedel particles, which remained suspended within the super-natants for a prolonged period of time, comprised of large portionsof these slurries and formed small and simple floc microstructures(Fig. 8b). A thin and compact consolidated layer of settled suspen-sion was present at the bottom of the beaker within these slurriesto comprise of the larger sized Riedel particles, which experienced‘incomplete’ face–face (F–F) interactions with the face of one inter-acting particle protruding out and misaligning in relation to theother interacting particle (Fig. 8a).
At the natural pH of Riedel particles in water at 4.90, slurry con-solidation with the appearance of a clear liquid layer was observedwithin the 8.0 wt% Riedel slurries that yielded ∼18.3 wt% after∼2 days (Fig. 7). Consolidation within these slurries at lower pHis facilitated by the lower degrees of negative particle chargesand thus inter-particle-repulsive forces and the higher degrees ofinter-particle interactions via unlike charge attractions, hydrogenbonding and/or van der Waals forces of attraction.
At pH 8.0 and with 0.35 dwb% CaCl2 added, the 8.0 wt% Riedelslurries displayed very similar solids content vs. time behaviorsto that of the 8.0 wt% Unimin slurries at pH 8.0 with both slur-ries reaching ∼22.4 wt% after ∼2 days (Fig. 7). The higher degreesof consolidation within both these slurries are attributed to theadsorption of Ca(OH)+ complexes onto these kaolin particle sur-faces and the presence of Ca(OH)2 precipitates within these slurries.
The Ca(OH)+ adsorptions are proposed to occur at especiallyhigh concentrations onto the pH-independent and negativelycharged silica faces via electrostatic attractive interactions thanonto the pH-dependent edge sites and alumina faces, which gener-ally carry lower concentrations of SiO− and/or AlO− groups along
pH 12.0–2.0. These Ca(OH)+ adsorptions are also suggested to occurat higher concentrations on both these poorly-ordered Riedel andUnimin particles with high concentrations of surface irregulari-ties (Fig. 1) and particle face and edge surface areas than onto
98 L. Avadiar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 90–99
Fig. 8. Floc microstructures at pH 8.0 within (a) consolidated sediment beds of Riedel, (b) turbid supernatants of Riedel, (c) consolidated sediment beds of Riedel with CaCla at 10
wnUsvpwstittC
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nd (d) consolidated sediment beds of Unimin. Magnifications at 5000× and scales
ell-ordered particles. These Ca(OH)+ adsorptions reduced theegativity and inter-particle-repulsive forces within Riedel andnimin slurries and increased the particle aggregations and con-
olidations via unlike charge attractions, hydrogen bonding and/oran der Waals forces of attraction to result in the dense and com-lex microstructures with particles packing closely to one anotherithin the consolidated sediment beds of these Riedel and Unimin
lurries (Fig. 8c and 8d respectively). The presence and sedimen-ation of Ca(OH)2 precipitates at pH 8.0 could also contribute toncrease the solids content of the Riedel and Unimin slurries. (Refero Supplementary data for the possible types of particle interac-ions and floc microstructures formed within kaolin slurries witha2+ cations added).
. Conclusions
Unimin kaolin has been identified to carry sufficiently high con-entrations of Ca2+ cations that have caused its zeta potential-pH,ield stress-pH and consolidation behaviors to be varied to that ofow Ca-Riedel kaolin. While Mg2+ cations could also affect theseehaviors of kaolin slurries, it is, however, not in significant con-
entrations on Unimin kaolin to affect these behaviors of Uniminlurries accordingly. The concentrations of hydrolyzed Ca(II) prod-cts produced varied with pH via acid and base hydrolysis to causeariations in the rheological-pH behaviors of Unimin slurries via
2
�m.
acid and base titration. While the adsorption of Ca(OH)+ com-plexes reduced the (average) negative zeta potential magnitudesof Unimin particles and induced particle aggregation and consol-idation to result in dense and complex floc microstructures andincreased slurry network strengths and yield stresses of Uniminslurries especially at high pH, the presence of Ca(OH)2 precipitateswas observed to contribute to the yield stress peak of Unimin slur-ries at pH 8.6 where the presence of MgCO3 and CaCO3 soluble saltscould have also contributed to increase the yield stress of Uniminslurries at high pH. This study highlights the importance of parti-cle characterization via elemental analysis prior to experimentalanalysis in order to substantiate results obtained.
Acknowledgments
We wish to acknowledge The Australian Research Council (ARC)for funding this project via DP1096528, Lyn Kirilak from the Cen-tre for Microscopy, Characterisation and Analysis (CMCA), UWA forher guidance in preparing samples for cryo-SEM imaging, MichaelSmirk from the School of Earth and Environment, UWA for carryingout and guiding the authors through the XRD, XRF and CEC mea-
surements and Paul McCormick from the School of Mechanical andChemical Engineering, UWA for teaching and guiding the authorsthrough the BET measurement process. We also wish to thank thereviewers for making this a better paper.
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ppendix A. Supplementary data
Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.014.06.048.
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