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The influence of polymer chemistry on adsorption and flocculation of talc suspensions
Chemical Engineering Journal 220 (2013) 375–382
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The influence of polymer chemistry on adsorption and flocculation of talc suspensions Agnieszka Mierczynska-Vasilev, Mohammad Kor, Jonas Addai-Mensah, David A. Beattie ⇑ Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia
h i g h l i g h t s " The influence of starch chemistry on adsorption and flocculation. " Chemistry influences adsorption, contact angle, and adsorbed layer thickness. " Flocculation not affected by a variation in polymer substitution chemistry. " Flocculation is significantly affected by variation in polymer branching ratio.
a r t i c l e
i n f o
Article history: Received 4 November 2012 Received in revised form 27 December 2012 Accepted 29 December 2012 Available online 9 January 2013 Keywords: Talc Starch Flocculation AFM Adsorption Contact angle
a b s t r a c t The adsorption and action of four starch polymers in talc flocculation has been studied. The polymers include: two chemically modified starch polymers (carboxymethyl (CM) and phenyl succinate (PS) starch), with similar molecular weight but variation in terms of added aromatic functionality; and two unmodified starch polymers, Gelose 80 and Gelose 50, which vary in their amylose:amylopectin ratio. The adsorption behaviour of the polymers indicated that the carboxyl groups of the functionalised polymers discouraged adsorption at low ionic strength, with the non-charged Gelose polymers adsorbing to higher densities in spite of having lower molecular weights. AFM imaging and contact angle measurements indicated that the polymers interact similarly with the talc basal plane in terms of coverage and effect on hydrophobicity, with minor differences in adsorbed layer thickness. CM and PS starch had an equal and best effect on the flocculation behaviour of talc suspension. The settling rate of bare talc was 0.14 m h1; the two functionalised starch polymers increased the settling rate to 1.7 and 2 m h1 for CM and PS starch, respectively (at 8 mg L1 concentration). Whereas different chemical substitution had no pronounced effect on the settling behaviour, the relative content of amylose to amylopectin did produce differences in settling. Gelose 80 (80% amylose; 1.3 m h1 settling rate at 40 mg L1 concentration) performed better when compared Gelose 50 (50% amylose; 0.65 m h1 settling rate at 40 mg L1 concentration). Of the two unmodified starch polymers, Gelose 80 gave the best overall settling performance, highlighting the effect of the linear starch chains in encouraging good bridging flocculation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In mineral processing operations, clay minerals (e.g., talc, kaolinite, smectite) are commonly present within the waste mineral stream (tails). Tailings with high quantities of such layered silicates are difficult to treat to recover the process water (which is recycled back into the mineral processing plant). The use of high molecular weight polymers as flocculants is necessary to increase the efficiency of the solid–liquid separation in gravity-assisted thickeners. Polymeric flocculants have been used in a number of studies to show how the recovery of water from mineral waste slurry can ⇑ Corresponding author. Tel.: +61 8 8302 3676. E-mail address: [email protected] (D.A. Beattie). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.12.080
be improved [1–13]. However, even with the use of polymeric flocculants, low settling rates and voluminous consolidated waste mineral slurries at low solids density are commonly observed in thickeners [14–16]. There is a push to specifically design polymers for enhanced flocculation performance by controlling the molecular weight distribution, chemical structure, and the nature and ratio of functional groups on the polymeric backbone, thus making them effective dewatering aids [8,10–12,17]. The effect of polymer molecular weight on flocculation behaviour has been investigated by some researchers [8,10]. In general, bridging flocculation (the dominant mechanism in polymer-induced flocculation, where a single polymer attaches to more than one particle) is more efficient with higher molecular weight polymers, leading to faster settling rates [18,19], although some
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exceptions exist [20]. Some studies have reported that increased flocculant molecular weight reduces the floc density, and increases the amount of entrained intra-floc water, whilst relatively lower molecular weight flocculants produce denser flocs [18]. Thus, whilst high settling rates result from the increased flocs size, compact consolidation of the resulting sediment can be diminished. The effect of polymer charge density on flocculation and dewatering of tailings is also described in the literature. Treatment of kaolinite dispersions with anionic polyacrylamide flocculants displayed reduced flocculation performance with increasing charge fraction [18]. Non-ionic or weakly charged flocculants are reported to form robust flocs which resist degradation upon shear, whereas highly charged flocculants produce weak and easily ruptured flocs [21]. Enhanced dewatering of sheared kaolinite and smectite flocs produced with high molecular weight polyethylene oxide has been attributed to the non-ionic character of the polymer [6,7]. Less well-studied than issues of molecular weight and charge is the effect of specific variation in the polymer flocculant architecture and functional group chemistry. The work in this paper is concerned with the settling of dilute dispersions containing talc particles, and examines the effect of varying polymer chemistry at a specific molecular weight. The goal of this study is to draw a comparison between: (i) two starch polymers: CM and PS starch, with quite similar molecular weight and carboxymethyl substitution, but PS starch has an additional aromatic functionality; and (ii) between two starch polymers (Gelose 80 and Gelose 50) with similar molecular weight but varying degrees of branching (i.e. amylose:amylopectin ratio). Talc, the chosen mineral for this study, is a hydrophobic layered silicate that displays substantial colloid stability and slow settling, and thus requires flocculation aids to enable such suspensions to be efficiently dewatered. It is also a common waste mineral in flotation, and thus is a major constituent of mineral tailings that require dewatering. Studying the flocculation of talc suspensions is therefore of relevance for many mineral processing operations. In addition to detailed studies of settling of talc suspensions in the absence and presence of the chosen polymers, data were obtained on their adsorption on talc particles, and their ability to alter the wettability of the hydrophobic talc surface (expected to influence the particle–particle interactions). In addition, the morphology of the adsorbed polymer layers on a talc surface was characterized
with in situ atomic force microscopy; such characteristics (coverage, thickness, roughness) are expected to be important for particle bridging.
2. Materials and methods 2.1. Materials and sample preparation Four different starch-based polymers were used as supplied from National Starch. These polymers are: carboxymethyl (CM) and phenyl succinate (PS) starches; Gelose 50 (50–50% combination of linear amylose and branched amylopectin starch) and Gelose 80 (80% linear amylose starch). The chemical structures of linear and branched starches are presented in Fig. 1 – which indicates the major difference between polymers Gelose 50 and Gelose 80. Fig. 2 contains schematic structures of the additional functional groups of the CM and PS starches. These substitutions are connected to the pyranose rings at position C6 (the carbon connected to the a-(1–6) linkage shown in Fig. 1 (the PS diagram does not include the pyranose ring). The anionic polymers (CM and PS) are expected to be negatively charged at pH 7.5. The weight average molecular weight (Mw), the number average molecular weight (Mn), and their quotient, denoted as the polydispersity index (PD), were determined using size exclusion chromatography (SEC) and are listed in Table 1 together with the amylose/ amylopectin ratio. SEC was carried out using a Waters ‘‘Ultrahydrogel Linear’’ aqueous SEC column (300 7.8 mm) fitted with an ‘‘Ultrahydrogel’’ guard column (Waters, Milford, MA, USA). The eluent was a solution of 0.1 M NaNO3 (pH 6.8), pumped at a flow rate of 0.8 ml min1 using a Waters 600E solvent delivery system. The column was heated to 40 °C using a column heating module. The detector used was a Waters Model 410 RI detector. Where UV detection at 190 nm was used, the solvent was 0.1 M NaClO4 due to its transparency at 190 nm. Data acquisition and integration was done using Baseline 815 software from Waters. Polymer samples and dextran were made up as 1 mg ml1 solutions in 0.1 M NaNO3 at pH 6.8. 100 ml of each sample solution was injected using an LS-3200 LC auto-sampler (SGE Australia). All four polymer samples have high polydispersity values, as expected for polymers derived from natural polysaccharide material [22].
Fig. 1. The structure of the (A) linear (amylose) and (B) branched (amylopectin) Gelose starches.
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Fig. 2. The structure of the: (A) CM and (B) PS substituted groups (primarily at the C6 position on the pyranose rings). The structure of CM includes the pyranose ring repeat unit of the starch polymer; the structure of PS simply includes the C6 carbon of the pyranose ring repeat unit.
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103 M NaCl background solution and the appropriate volume of this suspension was placed in individual vials. The required volume of background solution and stock polymer solution at pH 7.5 were added to each vial in order to obtain samples of different initial concentrations. The vials were placed on a rotary tumbler for 2 h and centrifuged. The supernatant was then analysed to determine the concentration of polymer left in solution, via a complexation method between the polysaccharide, phenol and concentrated H2SO4, using UV–vis spectroscopy [24]. In this method, all the amount of polymer depleted from solution is assumed to be adsorbed onto the solid surface. The adsorbed amount (U) was the calculated using the following equation:
C¼
1 ðC f C i ÞV mAs
ð1Þ
where m (g) is mass of the solid substrate, As (m2 g1) the specific area of the solid substrate, ci and cf are the solution polymer concentration(g L1) before and after adsorption, V (L) is the volume of the solution.
Table 1 Characterisation data of starches. Sample
Mn (g/mol)
Mw (g/mol)
PD (Mw/Mn)
Amylose/amylopectin ratioa
CM starch PS starch Gelose 50 Gelose 80
120,000 83,000 25,000 20,000
1,100,000 1,300,000 260,000 250,000
9 16 10 13
– – 50/50 80/20
2.3. AFM imaging
a Starch consists of two types of molecules: amylose, which is linear and amylopectin, which is branched.
All solutions were prepared using high-purity Milli-Q water (Elga UHQ unit; pH 5.5, specific conductivity <0.5 lS cm1 and a surface tension of 72.8 mN m1 at 20 °C). For pH measurement and monitoring, a two-point calibration of the pH meter was performed using 9.22 ± 0.02 and 4.00 ± 0.02 buffer solutions. The solution and suspension pH was adjusted by addition of small quantities of analytical grade 2.0 M HCl and NaOH solutions. In order to maintain a constant ionic strength, polymer solutions and mineral suspensions were prepared in analytical grade 103 M NaCl electrolyte (Merck, Australia). For the preparation of polymer stock solutions a dissolution procedure developed by National Starch was used. A 3.0 g of Milli-Q water was added to 1.25 g of starch, and mixed well to form a slurry. 10 ml of 2% w/w sodium hydroxide with stirring was added and continued to mix until a clear gel was formed. The gel was allowed to sit for 20 min to ensure complete dissolution. Milli-Q water was then added (with mixing) until the desired concentration was reached. Talc rock mineral (Delaware, USA; impurity free [23]) used for atomic force microscopy imaging and captive bubble experiments was obtained from the Mineralogy Department of the South Australian Museum. Freshly cleaved talc samples were prepared by carefully adhering a small piece of sticking tape on a flat section of the mineral and gently peeling the tape to reveal a freshly cleaved mineral basal plane. Fine talc particles (>99% pure; Merck, Germany; XPS analysis indicated that the surface concentration of impurities such as Fe and Al was negligible [23]) were used for the dewatering and polymer adsorption studies. The particles were platy in morphology with a BET surface area of 2.9 m2 g1, the size distribution was in the range 0.5–100 lm, and D10, D50 and D90 of 3.5, 15 and 52 lm, respectively. 2.2. Adsorption isotherms Adsorption studies were performed using the batch depletion method. A 3.5 wt.% solid suspension at pH 7.5 was prepared in
AFM images were obtained using a Nanoscope MultiMode 8 AFM (Bruker) in solution (in situ). Imaging was performed using a commercially available liquid cell and narrow, thin silicon nitride Si3N4 cantilevers (V-shape cantilever configuration) with a typical spring constant of 0.2 N/m and a resonant frequency around 9 Hz. A piezoelectric tube scanner J with a 125 lm2 scan size in the XY direction was used. The fluid cell and cantilevers were cleaned in ethanol, rinsed with high quality Milli-Q water and dried with nitrogen prior to use in imaging. Each mineral talc sample was freshly cleaved and mounted using double-sided tape on a magnetic stainless steel disc. One o-ring from the liquid cell was placed on top of the sample, the liquid cell was then lowered onto the o-ring to form a seal. This arrangement ensures that subsequent solutions only come into contact with the mineral surface and the cleaned interior of the flow cell. The polymer solution was injected into the liquid cell using a 1 ml syringe. The talc surfaces were conditioned for 30 min in a polymer solution of known concentration (at pH 7.5, 1 103 M NaCl). After the designated conditioning time the polymer solution was exchanged for the NaCl background electrolyte solution (pH 7.5) prior to imaging the adsorbed layer. All experiments were conducted in a class-100 clean room at 22 °C. The AFM adsorbed polymer parameters have been determined using 5 5 lm images, subjected to the second-ordered flattening. The Nanoscope software was used to assess the root-meansquared (RMS) roughness and average height of the adsorbed polymer (defined as the difference in average peak-to-valley distances between bare talc and polymer-treated talc). Image J was used to determine the area fraction of polymer visually observed on the images [25]. Height images obtained from the Nanoscope software were converted in 8-bit greyscale images. In the next step the threshold was visually adjusted and the area fraction of polymer calculated. 2.4. Contact angle measurements Advancing contact angle (ha) and receding contact angle (hr) measurements on a freshly cleaved talc sample have been taken using captive bubble technique with the Video Based Contact Angle Meter (OCA 20, Dataphysics – Germany). A flat section of the mineral surface was adhered to a clean glass slide using doublesided tape. The captive bubble method allows one to measure the contact angle formed by an air bubble pushed against a mineral
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surface immersed in a liquid. The freshly cleaved mineral surface was deposited on a holder, and immersed for 30 min in a quartz cell containing polymer solution at desired concentration. An air bubble was pushed below the surface through a needle. Both advancing and receding contact angles were obtained by capturing the silhouette of the bubble and calculating the tangent using Dataphysics software. Experiments were conducted at 22 °C in a class-100 clean room.
Table 2 Freundlich adsorption parameters for four investigated starch polymers on talc at pH 7.5.
2.5. Settling experiments
on molecular weight indicates that the functional groups of the two substituted polymers have an adverse effect on the adsorption. CM and PS Starch both have negatively charged carboxyl groups present in their structure. The surface of talc, both basal and edge faces, will have a negative charge in 103 M solution (the edge due to the presence of ionisable groups at the surface; the face due to the presence of specifically adsorbed hydroxyl ions, in common with many other non-polar hydrophobic surfaces/interfaces, e.g. Teflon [26]). At 103 M NaCl electrolyte concentration, the electrical double layer thickness (Debye length) will be approximately 10 nm, meaning that repulsive electrostatic interactions will not only hinder particle coagulation but may also reduce adsorption for anionic polymers if electrostatic attraction were the dominant mechanism. When a comparison is made between CM and PS starches (similar molecular weight), the adsorbed density on talc was greater for PS compared to CM. It appears that the presence of the aromatic group on PS starch facilitated hydrophobic interactions between the polymer and the talc basal face, and hence enhanced the adsorption process. The comparison of the two smaller polymers, with variation in degree of branching, reveals that the more branched polymer (G50) adsorbs more onto the talc surface than does the more linear polymer (G80). One possible explanation for this behaviour would be the ability of the branched polymer to form a denser conformation on the talc surface. In all cases, the starch polymer isotherms exhibit a continual increase in the adsorbed amount without a clear plateau. High affinity isotherms without a clear plateau may indicate adsorption onto a heterogeneous surface. Such isotherms can be fitted by the Freundlich isotherm. The Freundlich model considers the presence of binding sites with variation in affinity for the adsorbing molecules. Freundlich isotherms are represented by the equation [27]:
Talc suspensions (8 wt.% solid) were prepared by mixing dry talc powder and 103 M NaCl electrolyte solution (at pH 7.5) with an overhead stirrer at an agitation rate of 500 rpm for 1 h. For batch flocculation and settling behaviour investigations, a known volume of starch flocculant solution was added using a syringe to 500 cm3 of talc suspension in a graduated cylinder. The suspension was then mixed for 10 s. The initial settling rate of the flocculated suspension (and also that of the bare talc suspension) was determined by recording the time taken for the ‘mud line’ (interface between the settling solid and the above supernatant) in the 500 cm3 graduated glass cylinder to fall between the 450 and 350 cm3 marks on the glass cylinder. The sediment was then allowed to stand undisturbed for 24 h, after which the consolidated bed volume was recorded (the final position of the mud line). The sediment bed solid content was gravimetrically measured using infrared moisture balance. To determine the supernatant clarity, 20 cm3 of supernatant was withdrawn using a syringe at a fixed distance above the sediment mud line and evaporated/dried in an oven (105 °C). The clarity is calculated as the sold mass of talc divided by the volume of supernatant removed. 3. Results and discussion 3.1. Adsorption isotherms The measured adsorption isotherms for the four starch polymers adsorbed onto the talc mineral at pH 7.5 are shown in Fig. 3, with adsorbed amount plotted as a function of equilibrium polymer concentration. It would be expected that the higher molecular weight polymers should adsorb more on talc, but this is not the case here. The Gelose 50 and Gelose 80 polymers have higher adsorbed density than CM and PS starch. The disconnect between the observed adsorption densities and that expected based
CM starch PS starch Gelose 50 Gelose 80
kF (mg m2)
nF [L mg1]
R2
0.70 ± 0.07 1.76 ± 0.10 2.79 ± 0.14 2.04 ± 0.07
2.44 4.35 6.25 4.76
0.98 0.97 0.97 0.98
1
Cabs ¼ kF C neqF
ð2Þ
where Cabs is the amount of adsorbate adsorbed per unit area of adsorbent (mg m2), Ceq is the equilibrium concentration of the adsorbate (mg L1), kF and nF are empirical constants, with kF as the absorbent capacity, and nF the adsorption affinity constant[28]. The larger the value of nF, the greater the adsorption affinity. Fitting curves (solid lines) determined for the Freundlich isotherms using the non-linear curve fitting tool in OriginPro8.5.1 are also shown on Fig. 3. The values of the empirical constants (kF and nF) are given in Table 2, together with the correlation coefficient for the Freundlich fits. From Fig. 3 it can be seen that Gelose 50 adsorbed with highest affinity on talc particles followed by Gelose 80, PS and CM starch. 3.2. In situ atomic force microscopy imaging
Fig. 3. Adsorption isotherms of CM starch (downward pointing triangles), PS starch (upward pointing triangles), Gelose 80 (squares) and Gelose 50 (circles) on talc at pH 7.5, at 40 mg L1, in NaCl 103 M of background electrolyte. The solid lines correspond to the Freundlich fitting.
Typical tapping mode atomic force microscopy (TMAFM) images of freshly cleaved talc taken in situ are shown in Fig. 4. The untreated talc surface is characterized by an RMS roughness value of 0.1 nm. This low value of roughness indicates that the freshly cleaved mineral surface appear quite smooth at the scale at which the images have been taken with some steps exposed due to the cleavage procedure.
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Fig. 4. AFM height (A) and (B) phase image (2 2 lm) of freshly cleaved bare talc substrate.
Fig. 5 contains in situ height images of four starch polymers after 30 min of polymer adsorption on the cleaved talc surface. The talc surface was conditioned for 30 min in a polymer solution of known concentration (40 mg L1). After the designated conditioning time the polymer solution was exchanged for the background electrolyte solution prior to imaging the adsorbed layer. The solution flushing removes the possibility that any physically deposited polymer is imaged during the experiments; any loosely-bound material will desorb/detach during this step in the procedure. All four investigated polymers adsorb onto the freshly cleaved talc surface, but with different morphologies. The morphology of adsorbed CM starch onto freshly cleaved talc is shown in Fig. 5A. The polymer has chainlike structure – with
highly elongated filaments. The chain-like structures have a width of 45 nm and a length 200–450 nm. The RMS roughness of this surface is 0.5 nm and the calculated apparent layer thickness is 3 nm, with coverage of 80%. It is assumed that the lowest lying regions of the image are the bare talc substrate. Fig. 5B shows the adsorbed structure of PS starch at 40 mg L1 on the freshly cleaved talc surface. The polymer adsorbs as a dense inhomogeneous film. The RMS roughness and PTV distance show an increase compared to the bare talc surface, an apparent layer thickness of 6 nm and 75% coverage (Table 3). The coverage and thickness of the polymer layer allows us to correlate the adsorbed polymer density from the AFM imaging with that obtained from the adsorption isotherms. Based on the
Fig. 5. 5 5 lm AFM height images of adsorbed starches onto talc in situ (A) CM starch; (B) PS starch; (C) Gelose 50; and (D) Gelose 80 (images acquired at pH 7.5, 40 mg L1).
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Table 3 RMS roughness, peak to valley distance (PTV), apparent layer thickness (DPTV) and area fraction of polymer coverage (u) of the 4 starches adsorbed on talc at 40 mg L1, calculated from images depicted in Fig. 5. Sample
RMS roughness (nm) (±0.1)
PTV (nm) (±0.1)
DPTV (nm) (±0.1)
u (%) (±2)
Bare talc Talc/CM starch Talc/PS starch Talc/Gelose 50 Talc/Gelose 80
0.1 0.5 0.6 0.5 0.8
0.2 3.1 6.2 3 4.5
– 2.9 6 2.8 4.3
– 80 75 90 85
adsorption isotherms for polymer concentration of 40 mg L1 we would expect adsorption densities to be approximately 3 and 4 mg m2 for CM and PS starches, respectively. The higher layer thickness similar coverage of the PS Starch AFM image relative to the CM Starch image indicates a similar trend in adsorption density. However, it should be noted that the AFM images represent the hydrated adsorbed layer; difference in hydration water content would influence the amount (coverage and thickness) of the adsorbed polymer layers. In the case of Gelose 50 (Fig. 5C) the morphology is a near-continuous film with small holes. In addition to the near-continuous film there are some larger sections of adsorbed material. The parameters from the analysis of the AFM image are: layer thickness of approximately 3 nm; area coverage of 90%. Gelose 80 adsorbs onto talc as shown in Fig. 5D. Gelose 80 has a similar morphology to the Gelose 50. The layer thickness for the adsorbed Gelose 80 was calculated to be approximately 4 nm. This polymer reached 85% of coverage. The different degree of branching does not seem to have produced a stark contrast in polymer layer morphology. At this concentration, we would also not expect a significant difference in the volume (coverage thickness) of adsorbed material. Based on the adsorption isotherms for a polymer concentration of 40 mg L1 we would expect adsorption densities to be approximately 5 and 5.5 mg m2 for Gelose 80 and Gelose 50, respectively 3.3. Contact angle Contact angles obtained by the captive bubble method for the freshly cleaved talc basal plane in the absence of polymer, as well as the values after immersion for 30 min in starch solutions at 40 mg L1, have been measured. Advancing water contact angles of bare and polymer-coated talc are reported in Table 4. Talc has a very high hydrophobicity for a natural mineral sample, with an advancing water contact angle of over 90°. All four polymers reduced the hydrophobicity of the talc by over 10°, with little contrast between the different polymers. This is not too surprising given the similar degree of coverage of the polymers on the talc basal plane surface (see Table 3). It is noteworthy that in spite of the high coverage values, the hydrophobicity of the talc surface is not dramatically affected. However, this modest reduction in advancing contact angle has been observed for many other polymers on talc.
Table 4 Effect of starch polymers on talc advancing (hA) and receding (hR) contact angle measured with captive bubble technique after 30 min of polymer adsorption at 40 mg L1. Sample
Bare talc CM starch PS starch Gelose 50 Gelose 80
Contact angle ± 2 (deg) hA
hR
93 79 81 83 78
74 44 52 57 53
Contact angle hysteresis (deg)
19 35 29 26 25
Also given in Table 4 are the receding water contact angles of talc, and talc after adsorption of the polymers. The receding water contact angle is lower for bare talc, and this difference between the advancing and receding angles is termed contact angle hysteresis [29,30]. The hysteresis measured for the bare talc surface is likely due to the inherent roughness of the mineral generated by an imperfect cleavage of the basal plane. Adsorption of polymer increases the amount of physical heterogeneities on the surface, in addition to adding chemical heterogeneity. Both types of heterogeneity would be expected to increase the contact angle hysteresis, and this is observed for all starches (Table 4). CM Starch produces the biggest effect on the receding contact angle (and thus on the contact angle hysteresis). Comparison of the AFM images reveals that the morphology of the three other polymers is one of a film with gaps, whereas CM Starch is more structured with the elongated fibrous features. It is this more structured surface formed by the adsorbed CM Starch layer that is most likely responsible for the larger effect on the contact angle hysteresis. 3.4. Settling, consolidation, and supernatant clarity of talc suspensions The settling rates (m h1), sediment bed consolidation (wt.% solid), and clarity of supernatant (in mg L1) following settling were measured as a function of polymer dosage in the range 0– 40 mg L1 for both the unflocculated and flocculated talc suspensions. The data acquired for the two carboxylated starch polymers, PS and CM Starch are shown in Fig. 6, along with data for unflocculated talc suspensions. At pH 7.5 and 103 M KCl, talc particles in a 8 wt.% solid suspension displayed a fairly high colloid stability in the absence of flocculants. The suspension produced very low settling rate (<0.14 m h1) and poor supernatant clarity (>2300 mg dm3) as shown in Fig. 6A and C, respectively. Despite the poor suspension settling behaviour, the sediment bed formed after 24 h displayed relatively high consolidation (44 wt.% solid) (Fig. 6B). A similar high level of consolidation for talc (47 wt.% solid) was also reported in a previous study from this group [31]. It is possible that the hydrophobic character of the basal faces of the talc particles facilitated slow but a more compact packing of the plate-like particles, possibly through the influence of hydrophobic forces due to the presence of nano-bubbles on the talc basal plane surfaces [32]. Upon treatment of the suspension with the two carboxylated starch polymers at the lowest concentration (8 mg L1), the settling rate increased to 1.7 and 2 m h1 for CM and PS, respectively. The presence of the polymers and the resultant bridging flocculation led to a significant increase in settling rate. A further increase in polymer dosages up to 40 mg L1 (equivalent to 500 g polymer t1 solid) had no marked effect on the settling rate (Fig. 6A). However, there was a consistent difference in the ability of the two polymers to increase the settling rate, with PS Starch generally producing a marginally higher settling rate. This small but noticeable flocculation and settling behaviour improvement may be due to the thicker adsorbed layer of PS starch on the talc particles (Table 3), facilitated by its structure and slightly greater molecular weight.
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Fig. 6. Settling rate (A), final consolidation behaviour (B) and supernatant clarity (C) of unflocculated and flocculated talc suspensions at 8 wt.% solid as a function of polymer dosage.
Fig. 7. Settling rate (A), final consolidation behaviour (B) and supernatant clarity (C) of unflocculated and flocculated talc suspensions at 8 wt.% solid as a function of polymer dosage.
The effect of the two carboxylated starch polymers on bed consolidation and supernatant clarity are shown in Fig. 6B and C. Increased addition of polymer causes a modest but a systematic decrease in sediment bed consolidation (Fig. 6B), accompanying the dramatic decrease in supernatant clarity (Fig. 6C). Thus fast settling, but less dense flocs are formed under PS and CM. Furthermore, the polymer-induced flocculation produced a clearer supernatant, with a fivefold decrease in total suspended solids content. Unlike the settling rate behaviour, there is no significant difference between the effect of the two polymers on bed consolidation and supernatant clarity. The data indicate that there is little influence of the varying polymer chemistry in the settling behaviour of the talc suspensions, with only minor changes seen in the settling rate that could be attributed to the differences between the two polymers, or their adsorbed layer characteristics. The results for the two Gelose polymers with variation in degree of branching are shown in Fig. 7. The settling rate data (Fig. 7A) indicate that there is a strong dosage dependence to the settling rate, with the settling rate increasing with increasing concentration of both polymers in the range 8–40 mg L1, equivalent to 100–500 g polymer t1 solid dosage. At the highest dosage settling rate of 1.2 m h1 and 0.65 m h1 were achieved for Gelose 80 and Gelose 50, respectively. Two things are apparent from inspecting the data. First, there is a stark difference in the ability of the two polymers to increase the settling rate of the talc suspension, with Gelose 80, the polymer with more linear amylose starch chains, performing much better than Gelose 50. Second, both polymers perform worse than the two larger carboxylated starches (see Fig. 6A) in terms of the suspension settling behaviour. The latter is due to a molecular weight effect (both Gelose polymers are much smaller than the PS and CM Starch polymers). The consolidation and clarity data are plotted in Fig. 7B and C, respectively. It can be seen that both Gelose polymers, in spite of their small influence on the settling rate, in comparison with the
PS and CM starch polymers, they produce higher sediment consolidation, in fact comparable to that of the bare talc bed. There was no impact of polymer structure type, as the sediments flocculated with both Gelose polymers behaved similarly across the concentration range. The supernatant clarity data (Fig. 7C) displays some polymer structure dependence, with the more linear Gelose 80 sample producing the lowest suspended solid content, and almost equal performance over the concentration range, whereas use of the more branched Gelose 50 led to very poor clarity and a clear dosage dependence. The higher percentage of amylose chains in Gelose 80 produced a much better overall settling performance than Gelose 50. The increased performance can be attributed to a higher particle bridging flocculation efficiency, which produced faster settling, a more compact packing of sedimented flocs and better clarification. The poorer performance of Gelose 50 is in spite of the higher equilibrium adsorption density determined from adsorption isotherms, and the similar polymer layer coverage and reduction in particle hydrophobicity for the two Gelose polymers determined at equilibrium adsorption time. The data from the AFM images on layer thickness does point to more desirable adsorption attributes with Gelose 80 whose adsorption led to a thicker layer (50% thicker than Gelose 50) and greater RMS, both indicative of an expanded interfacial conformation, reflecting longer adsorbed polymer loops and tails, conducive to multi-particle adsorption and bridging.
4. Conclusions The adsorption of four starch polymers on talc has been studied with adsorption isotherms, AFM imaging, and contact angle measurements. Polymer functional group chemistry, molecular weight, and degree of branching are all seen to influence the characteristics of the adsorbed polymer layer at equilibrium (adsorbed amount,
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morphology, and reduction of hydrophobicity). The flocculation and settling behaviour of talc suspensions treated with these polymers has revealed that molecular weight is the primary characteristic that controls settling rate, bed consolidation, and supernatant clarity. The secondary characteristic of the polymers that influences settling behaviour is the degree of branching (amylopectin content) of the starch sample, with more linear starch chains (amylose) being more effective in flocculating the talc suspensions. It was also seen that variations in functional group chemistry (inclusion of an aromatic group alongside carboxyl groups) did not result in settling behaviour modification). Acknowledgments Financial support for this study was received from the Australian Research Council and AMIRA International sponsors of Project P498C (which include National Starch, Anglo Platinum, CP Kelco and Newcrest). Russell Schumann of Levay and Co. Environmental Services is acknowledged for his for assistance with SEC measurements. The authors would also like to thank Associate Professor Allan Pring (South Australian Museum, Adelaide) for the provision of the talc mineral sample used for AFM and contact angle measurements, and Dr. Audrey Beaussart for drawing the polymer structures. References [1] J. Gregory, Rates of flocculation of latex particles by cationic polymers, J. Colloid Interface Sci. 42 (1973) 448–456. [2] Y.D. Yan, S.M. Glover, G.J. Jameson, S. Biggs, The flocculation efficiency of polydisperse polymer flocculants, Int. J. Mineral Process. 73 (2004) 161–175. [3] D.N. Thomas, S.J. Judd, N. Fawcett, Flocculation modelling: a review, Water Res. 33 (1999) 1579–1592. [4] J. Gregory, Effect of cationic polymers on colloidal stability of latex-particles, J. Colloid Interface Sci 55 (1976) 35–44. [5] F. Mabire, R. Audebert, C. Quivoron, Flocculation properties of some watersoluble cationic copolymers toward silica suspensions – a semiquantitative interpretation of the role of molecular-weight and cationicity through a patchwork model, J. Colloid Interface Sci. 97 (1984) 120–136. [6] P. Mpofu, J. Addai-Mensah, J. Ralston, Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions, Int. J. Mineral Process. 71 (2003) 247–268. [7] P. Mpofu, J. Addai-Mensah, J. Ralston, Flocculation and dewatering behaviour of smectite dispersions: effect of polymer structure type, Minerals Eng. 17 (2004) 411–423. [8] H. Li, J. Long, Z. Xu, J.H. Masliyah, Effect of molecular weight and charge density on the performance of polyacrylamide in low-grade oil sand ore processing, Can. J. Chem. Eng. 86 (2008) 177–185. [9] I. Cengiz, E. Sabah, S. Ozgen, H. Akyildiz, Flocculation of fine particles in ceramic wastewater using new types of polymeric flocculants, J. Appl. Polymer Sci. 112 (2009) 1258–1264.
[10] K. Das, S. Raha, P. Somasundaran, Effect of polyacrylic acid molecular weight on the floc stability during prolonged settling, Colloids Surf. A: Physicochem. Eng. Aspects 351 (2009) 1–8. [11] J. Long, H. Li, Z. Xu, J.H. Masliyah, Improving oil sands processability using a temperature-sensitive polymer, Energy Fuels 25 (2011) 701–707. [12] P. Watson, R. Farinato, T. Fenderson, M. Hurd, P. Macy, A. Mahmoudkhani, Novel polymeric additives to improve oil sands tailings consolidation, in, 2011, p. 703–709. [13] P. Ofori, A.V. Nguyen, B. Firth, C. McNally, M.A. Hampton, The role of surface interaction forces and mixing in enhanced dewatering of coal preparation tailings, Fuel 97 (2012) 262–268. [14] Q.D. Nguyen, D.V. Boger, Application of rheology to solving tailings disposal problems, Int. J. Mineral Process. 54 (1998) 217–233. [15] J.B. Farrow, R.R.M. Johnston, K. Simic, J.D. Swift, Consolidation and aggregate densification during gravity thickening, Chem. Eng. J. 80 (2000) 141–148. [16] A. Sworska, J.S. Laskowski, G. Cymerman, Flocculation of the Syncrude fine tailings Part I. Effect of ph, polymer dosage and Mg2+ and Ca2+ cations, Int. J. Mineral Process. 60 (2000) 143–152. [17] S.A. Ravishankar, Pradip, N.K. Khosla, Selective flocculation of iron-oxide from its synthetic mixtures with clays – a comparision of polyacrylic-acid and starch polymers, Int. J. Mineral Process. 43 (1995) 235–247. [18] A.Q. Clark, T.M. Herrington, J.C. Petzold, The flocculation of kaolin suspensions with anionic polyacrylamides of varying molar mass and anionic character, Colloids Surf. 44 (1990) 247–261. [19] M.C. Chang, H.L. Lin, C.L. Huang, Y.Y. Wang, C.C. Wan, Conditioning characteristics of kaolin sludge with different cationic polyelectrolytes, Colloids Surf. A: Physicochem. Eng. Aspects 139 (1998) 75–80. [20] P.F. Richardon, L. Connelly, Industrial Coagulans and Flocculatns, Marcel Dekker, New York, 1988. [21] P. Somasundaran, B.M. Moudgil, Reagents in Mineral Technology, 1988. [22] J. Lefebvre, J.-L. Doublier, Rheological behavior of polysaccharides aqueous systems, in: S. Dumitriu (Ed.), Polysaccharides: Structural Diversity And Functional Versatility, CRC Press, 2004, pp. 357–394. [23] A. Beaussart, A. Mierczynska-Vasilev, D.A. Beattie, Adsorption of dextrin on hydrophobic minerals, Langmuir 25 (2009) 9913–9921. [24] M. Dubois, K.A. Gilles, J.K. Hamilton, P.A. Rebers, S. Fred, Colorimetric method for determination of sugars and related substances, Anal. Chem. 28 (1956) 350–356. [25] M.D. Abramoff, P.J. Magelhaes, S.J. Ram, Image processing with image, J. Biophoton. Int. 11 (2004) 36–42. [26] A. Niecikowska, M. Krasowska, J. Ralston, K. Malysa, Role of surface charge and hydrophobicity in the three-phase contact formation and wetting film stability under dynamic conditions, J. Phys. Chem. C 116 (2012) 3071–3078. [27] R.J. Hunter, Foundations of Colloid Science, Oxford University Press, Oxford, 2001. [28] B.H. Hameed, A.A. Ahmad, N. Aziz, Isotherms, kinetics and thermodynamics of acid dye adsorption on activated palm ash, Chem. Eng. J. 133 (2007) 195–203. [29] R.J. Good, Contact angle wetting, and adhesion – a critical review, J. Adhesion Sci. Technol. 6 (1992) 1269–1302. [30] C.N.C. Lam, R. Wu, D. Li, M.L. Hair, A.W. Neumann, Study of the advancing and receding contact angles: liquid sorption as a cause of contact angle hysteresis, Adv. Colloid Interface Sci. 96 (2002) 169–191. [31] K.Y. Yeap, J. Addai-Mensah, D.A. Beattie, The pivotal role of polymer adsorption and flocculation conditions on dewaterability of talcaceous dispersions, Chem. Eng. J. 162 (2010) 457–465. [32] M. Zbik, R.G. Horn, Hydrophobic attraction may contribute to aqueous flocculation of clays, Colloids Surf. A: Physicochem. Eng. Aspects 222 (2003) 323–328.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
The influence of polymer chemistry on adsorption and flocculation of talc suspensions Agnieszka Mierczynska-Vasilev, Mohammad Kor, Jonas Addai-Mensah, David A. Beattie ⇑ Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia
h i g h l i g h t s " The influence of starch chemistry on adsorption and flocculation. " Chemistry influences adsorption, contact angle, and adsorbed layer thickness. " Flocculation not affected by a variation in polymer substitution chemistry. " Flocculation is significantly affected by variation in polymer branching ratio.
a r t i c l e
i n f o
Article history: Received 4 November 2012 Received in revised form 27 December 2012 Accepted 29 December 2012 Available online 9 January 2013 Keywords: Talc Starch Flocculation AFM Adsorption Contact angle
a b s t r a c t The adsorption and action of four starch polymers in talc flocculation has been studied. The polymers include: two chemically modified starch polymers (carboxymethyl (CM) and phenyl succinate (PS) starch), with similar molecular weight but variation in terms of added aromatic functionality; and two unmodified starch polymers, Gelose 80 and Gelose 50, which vary in their amylose:amylopectin ratio. The adsorption behaviour of the polymers indicated that the carboxyl groups of the functionalised polymers discouraged adsorption at low ionic strength, with the non-charged Gelose polymers adsorbing to higher densities in spite of having lower molecular weights. AFM imaging and contact angle measurements indicated that the polymers interact similarly with the talc basal plane in terms of coverage and effect on hydrophobicity, with minor differences in adsorbed layer thickness. CM and PS starch had an equal and best effect on the flocculation behaviour of talc suspension. The settling rate of bare talc was 0.14 m h1; the two functionalised starch polymers increased the settling rate to 1.7 and 2 m h1 for CM and PS starch, respectively (at 8 mg L1 concentration). Whereas different chemical substitution had no pronounced effect on the settling behaviour, the relative content of amylose to amylopectin did produce differences in settling. Gelose 80 (80% amylose; 1.3 m h1 settling rate at 40 mg L1 concentration) performed better when compared Gelose 50 (50% amylose; 0.65 m h1 settling rate at 40 mg L1 concentration). Of the two unmodified starch polymers, Gelose 80 gave the best overall settling performance, highlighting the effect of the linear starch chains in encouraging good bridging flocculation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In mineral processing operations, clay minerals (e.g., talc, kaolinite, smectite) are commonly present within the waste mineral stream (tails). Tailings with high quantities of such layered silicates are difficult to treat to recover the process water (which is recycled back into the mineral processing plant). The use of high molecular weight polymers as flocculants is necessary to increase the efficiency of the solid–liquid separation in gravity-assisted thickeners. Polymeric flocculants have been used in a number of studies to show how the recovery of water from mineral waste slurry can ⇑ Corresponding author. Tel.: +61 8 8302 3676. E-mail address: [email protected] (D.A. Beattie). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.12.080
be improved [1–13]. However, even with the use of polymeric flocculants, low settling rates and voluminous consolidated waste mineral slurries at low solids density are commonly observed in thickeners [14–16]. There is a push to specifically design polymers for enhanced flocculation performance by controlling the molecular weight distribution, chemical structure, and the nature and ratio of functional groups on the polymeric backbone, thus making them effective dewatering aids [8,10–12,17]. The effect of polymer molecular weight on flocculation behaviour has been investigated by some researchers [8,10]. In general, bridging flocculation (the dominant mechanism in polymer-induced flocculation, where a single polymer attaches to more than one particle) is more efficient with higher molecular weight polymers, leading to faster settling rates [18,19], although some
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exceptions exist [20]. Some studies have reported that increased flocculant molecular weight reduces the floc density, and increases the amount of entrained intra-floc water, whilst relatively lower molecular weight flocculants produce denser flocs [18]. Thus, whilst high settling rates result from the increased flocs size, compact consolidation of the resulting sediment can be diminished. The effect of polymer charge density on flocculation and dewatering of tailings is also described in the literature. Treatment of kaolinite dispersions with anionic polyacrylamide flocculants displayed reduced flocculation performance with increasing charge fraction [18]. Non-ionic or weakly charged flocculants are reported to form robust flocs which resist degradation upon shear, whereas highly charged flocculants produce weak and easily ruptured flocs [21]. Enhanced dewatering of sheared kaolinite and smectite flocs produced with high molecular weight polyethylene oxide has been attributed to the non-ionic character of the polymer [6,7]. Less well-studied than issues of molecular weight and charge is the effect of specific variation in the polymer flocculant architecture and functional group chemistry. The work in this paper is concerned with the settling of dilute dispersions containing talc particles, and examines the effect of varying polymer chemistry at a specific molecular weight. The goal of this study is to draw a comparison between: (i) two starch polymers: CM and PS starch, with quite similar molecular weight and carboxymethyl substitution, but PS starch has an additional aromatic functionality; and (ii) between two starch polymers (Gelose 80 and Gelose 50) with similar molecular weight but varying degrees of branching (i.e. amylose:amylopectin ratio). Talc, the chosen mineral for this study, is a hydrophobic layered silicate that displays substantial colloid stability and slow settling, and thus requires flocculation aids to enable such suspensions to be efficiently dewatered. It is also a common waste mineral in flotation, and thus is a major constituent of mineral tailings that require dewatering. Studying the flocculation of talc suspensions is therefore of relevance for many mineral processing operations. In addition to detailed studies of settling of talc suspensions in the absence and presence of the chosen polymers, data were obtained on their adsorption on talc particles, and their ability to alter the wettability of the hydrophobic talc surface (expected to influence the particle–particle interactions). In addition, the morphology of the adsorbed polymer layers on a talc surface was characterized
with in situ atomic force microscopy; such characteristics (coverage, thickness, roughness) are expected to be important for particle bridging.
2. Materials and methods 2.1. Materials and sample preparation Four different starch-based polymers were used as supplied from National Starch. These polymers are: carboxymethyl (CM) and phenyl succinate (PS) starches; Gelose 50 (50–50% combination of linear amylose and branched amylopectin starch) and Gelose 80 (80% linear amylose starch). The chemical structures of linear and branched starches are presented in Fig. 1 – which indicates the major difference between polymers Gelose 50 and Gelose 80. Fig. 2 contains schematic structures of the additional functional groups of the CM and PS starches. These substitutions are connected to the pyranose rings at position C6 (the carbon connected to the a-(1–6) linkage shown in Fig. 1 (the PS diagram does not include the pyranose ring). The anionic polymers (CM and PS) are expected to be negatively charged at pH 7.5. The weight average molecular weight (Mw), the number average molecular weight (Mn), and their quotient, denoted as the polydispersity index (PD), were determined using size exclusion chromatography (SEC) and are listed in Table 1 together with the amylose/ amylopectin ratio. SEC was carried out using a Waters ‘‘Ultrahydrogel Linear’’ aqueous SEC column (300 7.8 mm) fitted with an ‘‘Ultrahydrogel’’ guard column (Waters, Milford, MA, USA). The eluent was a solution of 0.1 M NaNO3 (pH 6.8), pumped at a flow rate of 0.8 ml min1 using a Waters 600E solvent delivery system. The column was heated to 40 °C using a column heating module. The detector used was a Waters Model 410 RI detector. Where UV detection at 190 nm was used, the solvent was 0.1 M NaClO4 due to its transparency at 190 nm. Data acquisition and integration was done using Baseline 815 software from Waters. Polymer samples and dextran were made up as 1 mg ml1 solutions in 0.1 M NaNO3 at pH 6.8. 100 ml of each sample solution was injected using an LS-3200 LC auto-sampler (SGE Australia). All four polymer samples have high polydispersity values, as expected for polymers derived from natural polysaccharide material [22].
Fig. 1. The structure of the (A) linear (amylose) and (B) branched (amylopectin) Gelose starches.
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Fig. 2. The structure of the: (A) CM and (B) PS substituted groups (primarily at the C6 position on the pyranose rings). The structure of CM includes the pyranose ring repeat unit of the starch polymer; the structure of PS simply includes the C6 carbon of the pyranose ring repeat unit.
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103 M NaCl background solution and the appropriate volume of this suspension was placed in individual vials. The required volume of background solution and stock polymer solution at pH 7.5 were added to each vial in order to obtain samples of different initial concentrations. The vials were placed on a rotary tumbler for 2 h and centrifuged. The supernatant was then analysed to determine the concentration of polymer left in solution, via a complexation method between the polysaccharide, phenol and concentrated H2SO4, using UV–vis spectroscopy [24]. In this method, all the amount of polymer depleted from solution is assumed to be adsorbed onto the solid surface. The adsorbed amount (U) was the calculated using the following equation:
C¼
1 ðC f C i ÞV mAs
ð1Þ
where m (g) is mass of the solid substrate, As (m2 g1) the specific area of the solid substrate, ci and cf are the solution polymer concentration(g L1) before and after adsorption, V (L) is the volume of the solution.
Table 1 Characterisation data of starches. Sample
Mn (g/mol)
Mw (g/mol)
PD (Mw/Mn)
Amylose/amylopectin ratioa
CM starch PS starch Gelose 50 Gelose 80
120,000 83,000 25,000 20,000
1,100,000 1,300,000 260,000 250,000
9 16 10 13
– – 50/50 80/20
2.3. AFM imaging
a Starch consists of two types of molecules: amylose, which is linear and amylopectin, which is branched.
All solutions were prepared using high-purity Milli-Q water (Elga UHQ unit; pH 5.5, specific conductivity <0.5 lS cm1 and a surface tension of 72.8 mN m1 at 20 °C). For pH measurement and monitoring, a two-point calibration of the pH meter was performed using 9.22 ± 0.02 and 4.00 ± 0.02 buffer solutions. The solution and suspension pH was adjusted by addition of small quantities of analytical grade 2.0 M HCl and NaOH solutions. In order to maintain a constant ionic strength, polymer solutions and mineral suspensions were prepared in analytical grade 103 M NaCl electrolyte (Merck, Australia). For the preparation of polymer stock solutions a dissolution procedure developed by National Starch was used. A 3.0 g of Milli-Q water was added to 1.25 g of starch, and mixed well to form a slurry. 10 ml of 2% w/w sodium hydroxide with stirring was added and continued to mix until a clear gel was formed. The gel was allowed to sit for 20 min to ensure complete dissolution. Milli-Q water was then added (with mixing) until the desired concentration was reached. Talc rock mineral (Delaware, USA; impurity free [23]) used for atomic force microscopy imaging and captive bubble experiments was obtained from the Mineralogy Department of the South Australian Museum. Freshly cleaved talc samples were prepared by carefully adhering a small piece of sticking tape on a flat section of the mineral and gently peeling the tape to reveal a freshly cleaved mineral basal plane. Fine talc particles (>99% pure; Merck, Germany; XPS analysis indicated that the surface concentration of impurities such as Fe and Al was negligible [23]) were used for the dewatering and polymer adsorption studies. The particles were platy in morphology with a BET surface area of 2.9 m2 g1, the size distribution was in the range 0.5–100 lm, and D10, D50 and D90 of 3.5, 15 and 52 lm, respectively. 2.2. Adsorption isotherms Adsorption studies were performed using the batch depletion method. A 3.5 wt.% solid suspension at pH 7.5 was prepared in
AFM images were obtained using a Nanoscope MultiMode 8 AFM (Bruker) in solution (in situ). Imaging was performed using a commercially available liquid cell and narrow, thin silicon nitride Si3N4 cantilevers (V-shape cantilever configuration) with a typical spring constant of 0.2 N/m and a resonant frequency around 9 Hz. A piezoelectric tube scanner J with a 125 lm2 scan size in the XY direction was used. The fluid cell and cantilevers were cleaned in ethanol, rinsed with high quality Milli-Q water and dried with nitrogen prior to use in imaging. Each mineral talc sample was freshly cleaved and mounted using double-sided tape on a magnetic stainless steel disc. One o-ring from the liquid cell was placed on top of the sample, the liquid cell was then lowered onto the o-ring to form a seal. This arrangement ensures that subsequent solutions only come into contact with the mineral surface and the cleaned interior of the flow cell. The polymer solution was injected into the liquid cell using a 1 ml syringe. The talc surfaces were conditioned for 30 min in a polymer solution of known concentration (at pH 7.5, 1 103 M NaCl). After the designated conditioning time the polymer solution was exchanged for the NaCl background electrolyte solution (pH 7.5) prior to imaging the adsorbed layer. All experiments were conducted in a class-100 clean room at 22 °C. The AFM adsorbed polymer parameters have been determined using 5 5 lm images, subjected to the second-ordered flattening. The Nanoscope software was used to assess the root-meansquared (RMS) roughness and average height of the adsorbed polymer (defined as the difference in average peak-to-valley distances between bare talc and polymer-treated talc). Image J was used to determine the area fraction of polymer visually observed on the images [25]. Height images obtained from the Nanoscope software were converted in 8-bit greyscale images. In the next step the threshold was visually adjusted and the area fraction of polymer calculated. 2.4. Contact angle measurements Advancing contact angle (ha) and receding contact angle (hr) measurements on a freshly cleaved talc sample have been taken using captive bubble technique with the Video Based Contact Angle Meter (OCA 20, Dataphysics – Germany). A flat section of the mineral surface was adhered to a clean glass slide using doublesided tape. The captive bubble method allows one to measure the contact angle formed by an air bubble pushed against a mineral
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surface immersed in a liquid. The freshly cleaved mineral surface was deposited on a holder, and immersed for 30 min in a quartz cell containing polymer solution at desired concentration. An air bubble was pushed below the surface through a needle. Both advancing and receding contact angles were obtained by capturing the silhouette of the bubble and calculating the tangent using Dataphysics software. Experiments were conducted at 22 °C in a class-100 clean room.
Table 2 Freundlich adsorption parameters for four investigated starch polymers on talc at pH 7.5.
2.5. Settling experiments
on molecular weight indicates that the functional groups of the two substituted polymers have an adverse effect on the adsorption. CM and PS Starch both have negatively charged carboxyl groups present in their structure. The surface of talc, both basal and edge faces, will have a negative charge in 103 M solution (the edge due to the presence of ionisable groups at the surface; the face due to the presence of specifically adsorbed hydroxyl ions, in common with many other non-polar hydrophobic surfaces/interfaces, e.g. Teflon [26]). At 103 M NaCl electrolyte concentration, the electrical double layer thickness (Debye length) will be approximately 10 nm, meaning that repulsive electrostatic interactions will not only hinder particle coagulation but may also reduce adsorption for anionic polymers if electrostatic attraction were the dominant mechanism. When a comparison is made between CM and PS starches (similar molecular weight), the adsorbed density on talc was greater for PS compared to CM. It appears that the presence of the aromatic group on PS starch facilitated hydrophobic interactions between the polymer and the talc basal face, and hence enhanced the adsorption process. The comparison of the two smaller polymers, with variation in degree of branching, reveals that the more branched polymer (G50) adsorbs more onto the talc surface than does the more linear polymer (G80). One possible explanation for this behaviour would be the ability of the branched polymer to form a denser conformation on the talc surface. In all cases, the starch polymer isotherms exhibit a continual increase in the adsorbed amount without a clear plateau. High affinity isotherms without a clear plateau may indicate adsorption onto a heterogeneous surface. Such isotherms can be fitted by the Freundlich isotherm. The Freundlich model considers the presence of binding sites with variation in affinity for the adsorbing molecules. Freundlich isotherms are represented by the equation [27]:
Talc suspensions (8 wt.% solid) were prepared by mixing dry talc powder and 103 M NaCl electrolyte solution (at pH 7.5) with an overhead stirrer at an agitation rate of 500 rpm for 1 h. For batch flocculation and settling behaviour investigations, a known volume of starch flocculant solution was added using a syringe to 500 cm3 of talc suspension in a graduated cylinder. The suspension was then mixed for 10 s. The initial settling rate of the flocculated suspension (and also that of the bare talc suspension) was determined by recording the time taken for the ‘mud line’ (interface between the settling solid and the above supernatant) in the 500 cm3 graduated glass cylinder to fall between the 450 and 350 cm3 marks on the glass cylinder. The sediment was then allowed to stand undisturbed for 24 h, after which the consolidated bed volume was recorded (the final position of the mud line). The sediment bed solid content was gravimetrically measured using infrared moisture balance. To determine the supernatant clarity, 20 cm3 of supernatant was withdrawn using a syringe at a fixed distance above the sediment mud line and evaporated/dried in an oven (105 °C). The clarity is calculated as the sold mass of talc divided by the volume of supernatant removed. 3. Results and discussion 3.1. Adsorption isotherms The measured adsorption isotherms for the four starch polymers adsorbed onto the talc mineral at pH 7.5 are shown in Fig. 3, with adsorbed amount plotted as a function of equilibrium polymer concentration. It would be expected that the higher molecular weight polymers should adsorb more on talc, but this is not the case here. The Gelose 50 and Gelose 80 polymers have higher adsorbed density than CM and PS starch. The disconnect between the observed adsorption densities and that expected based
CM starch PS starch Gelose 50 Gelose 80
kF (mg m2)
nF [L mg1]
R2
0.70 ± 0.07 1.76 ± 0.10 2.79 ± 0.14 2.04 ± 0.07
2.44 4.35 6.25 4.76
0.98 0.97 0.97 0.98
1
Cabs ¼ kF C neqF
ð2Þ
where Cabs is the amount of adsorbate adsorbed per unit area of adsorbent (mg m2), Ceq is the equilibrium concentration of the adsorbate (mg L1), kF and nF are empirical constants, with kF as the absorbent capacity, and nF the adsorption affinity constant[28]. The larger the value of nF, the greater the adsorption affinity. Fitting curves (solid lines) determined for the Freundlich isotherms using the non-linear curve fitting tool in OriginPro8.5.1 are also shown on Fig. 3. The values of the empirical constants (kF and nF) are given in Table 2, together with the correlation coefficient for the Freundlich fits. From Fig. 3 it can be seen that Gelose 50 adsorbed with highest affinity on talc particles followed by Gelose 80, PS and CM starch. 3.2. In situ atomic force microscopy imaging
Fig. 3. Adsorption isotherms of CM starch (downward pointing triangles), PS starch (upward pointing triangles), Gelose 80 (squares) and Gelose 50 (circles) on talc at pH 7.5, at 40 mg L1, in NaCl 103 M of background electrolyte. The solid lines correspond to the Freundlich fitting.
Typical tapping mode atomic force microscopy (TMAFM) images of freshly cleaved talc taken in situ are shown in Fig. 4. The untreated talc surface is characterized by an RMS roughness value of 0.1 nm. This low value of roughness indicates that the freshly cleaved mineral surface appear quite smooth at the scale at which the images have been taken with some steps exposed due to the cleavage procedure.
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Fig. 4. AFM height (A) and (B) phase image (2 2 lm) of freshly cleaved bare talc substrate.
Fig. 5 contains in situ height images of four starch polymers after 30 min of polymer adsorption on the cleaved talc surface. The talc surface was conditioned for 30 min in a polymer solution of known concentration (40 mg L1). After the designated conditioning time the polymer solution was exchanged for the background electrolyte solution prior to imaging the adsorbed layer. The solution flushing removes the possibility that any physically deposited polymer is imaged during the experiments; any loosely-bound material will desorb/detach during this step in the procedure. All four investigated polymers adsorb onto the freshly cleaved talc surface, but with different morphologies. The morphology of adsorbed CM starch onto freshly cleaved talc is shown in Fig. 5A. The polymer has chainlike structure – with
highly elongated filaments. The chain-like structures have a width of 45 nm and a length 200–450 nm. The RMS roughness of this surface is 0.5 nm and the calculated apparent layer thickness is 3 nm, with coverage of 80%. It is assumed that the lowest lying regions of the image are the bare talc substrate. Fig. 5B shows the adsorbed structure of PS starch at 40 mg L1 on the freshly cleaved talc surface. The polymer adsorbs as a dense inhomogeneous film. The RMS roughness and PTV distance show an increase compared to the bare talc surface, an apparent layer thickness of 6 nm and 75% coverage (Table 3). The coverage and thickness of the polymer layer allows us to correlate the adsorbed polymer density from the AFM imaging with that obtained from the adsorption isotherms. Based on the
Fig. 5. 5 5 lm AFM height images of adsorbed starches onto talc in situ (A) CM starch; (B) PS starch; (C) Gelose 50; and (D) Gelose 80 (images acquired at pH 7.5, 40 mg L1).
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Table 3 RMS roughness, peak to valley distance (PTV), apparent layer thickness (DPTV) and area fraction of polymer coverage (u) of the 4 starches adsorbed on talc at 40 mg L1, calculated from images depicted in Fig. 5. Sample
RMS roughness (nm) (±0.1)
PTV (nm) (±0.1)
DPTV (nm) (±0.1)
u (%) (±2)
Bare talc Talc/CM starch Talc/PS starch Talc/Gelose 50 Talc/Gelose 80
0.1 0.5 0.6 0.5 0.8
0.2 3.1 6.2 3 4.5
– 2.9 6 2.8 4.3
– 80 75 90 85
adsorption isotherms for polymer concentration of 40 mg L1 we would expect adsorption densities to be approximately 3 and 4 mg m2 for CM and PS starches, respectively. The higher layer thickness similar coverage of the PS Starch AFM image relative to the CM Starch image indicates a similar trend in adsorption density. However, it should be noted that the AFM images represent the hydrated adsorbed layer; difference in hydration water content would influence the amount (coverage and thickness) of the adsorbed polymer layers. In the case of Gelose 50 (Fig. 5C) the morphology is a near-continuous film with small holes. In addition to the near-continuous film there are some larger sections of adsorbed material. The parameters from the analysis of the AFM image are: layer thickness of approximately 3 nm; area coverage of 90%. Gelose 80 adsorbs onto talc as shown in Fig. 5D. Gelose 80 has a similar morphology to the Gelose 50. The layer thickness for the adsorbed Gelose 80 was calculated to be approximately 4 nm. This polymer reached 85% of coverage. The different degree of branching does not seem to have produced a stark contrast in polymer layer morphology. At this concentration, we would also not expect a significant difference in the volume (coverage thickness) of adsorbed material. Based on the adsorption isotherms for a polymer concentration of 40 mg L1 we would expect adsorption densities to be approximately 5 and 5.5 mg m2 for Gelose 80 and Gelose 50, respectively 3.3. Contact angle Contact angles obtained by the captive bubble method for the freshly cleaved talc basal plane in the absence of polymer, as well as the values after immersion for 30 min in starch solutions at 40 mg L1, have been measured. Advancing water contact angles of bare and polymer-coated talc are reported in Table 4. Talc has a very high hydrophobicity for a natural mineral sample, with an advancing water contact angle of over 90°. All four polymers reduced the hydrophobicity of the talc by over 10°, with little contrast between the different polymers. This is not too surprising given the similar degree of coverage of the polymers on the talc basal plane surface (see Table 3). It is noteworthy that in spite of the high coverage values, the hydrophobicity of the talc surface is not dramatically affected. However, this modest reduction in advancing contact angle has been observed for many other polymers on talc.
Table 4 Effect of starch polymers on talc advancing (hA) and receding (hR) contact angle measured with captive bubble technique after 30 min of polymer adsorption at 40 mg L1. Sample
Bare talc CM starch PS starch Gelose 50 Gelose 80
Contact angle ± 2 (deg) hA
hR
93 79 81 83 78
74 44 52 57 53
Contact angle hysteresis (deg)
19 35 29 26 25
Also given in Table 4 are the receding water contact angles of talc, and talc after adsorption of the polymers. The receding water contact angle is lower for bare talc, and this difference between the advancing and receding angles is termed contact angle hysteresis [29,30]. The hysteresis measured for the bare talc surface is likely due to the inherent roughness of the mineral generated by an imperfect cleavage of the basal plane. Adsorption of polymer increases the amount of physical heterogeneities on the surface, in addition to adding chemical heterogeneity. Both types of heterogeneity would be expected to increase the contact angle hysteresis, and this is observed for all starches (Table 4). CM Starch produces the biggest effect on the receding contact angle (and thus on the contact angle hysteresis). Comparison of the AFM images reveals that the morphology of the three other polymers is one of a film with gaps, whereas CM Starch is more structured with the elongated fibrous features. It is this more structured surface formed by the adsorbed CM Starch layer that is most likely responsible for the larger effect on the contact angle hysteresis. 3.4. Settling, consolidation, and supernatant clarity of talc suspensions The settling rates (m h1), sediment bed consolidation (wt.% solid), and clarity of supernatant (in mg L1) following settling were measured as a function of polymer dosage in the range 0– 40 mg L1 for both the unflocculated and flocculated talc suspensions. The data acquired for the two carboxylated starch polymers, PS and CM Starch are shown in Fig. 6, along with data for unflocculated talc suspensions. At pH 7.5 and 103 M KCl, talc particles in a 8 wt.% solid suspension displayed a fairly high colloid stability in the absence of flocculants. The suspension produced very low settling rate (<0.14 m h1) and poor supernatant clarity (>2300 mg dm3) as shown in Fig. 6A and C, respectively. Despite the poor suspension settling behaviour, the sediment bed formed after 24 h displayed relatively high consolidation (44 wt.% solid) (Fig. 6B). A similar high level of consolidation for talc (47 wt.% solid) was also reported in a previous study from this group [31]. It is possible that the hydrophobic character of the basal faces of the talc particles facilitated slow but a more compact packing of the plate-like particles, possibly through the influence of hydrophobic forces due to the presence of nano-bubbles on the talc basal plane surfaces [32]. Upon treatment of the suspension with the two carboxylated starch polymers at the lowest concentration (8 mg L1), the settling rate increased to 1.7 and 2 m h1 for CM and PS, respectively. The presence of the polymers and the resultant bridging flocculation led to a significant increase in settling rate. A further increase in polymer dosages up to 40 mg L1 (equivalent to 500 g polymer t1 solid) had no marked effect on the settling rate (Fig. 6A). However, there was a consistent difference in the ability of the two polymers to increase the settling rate, with PS Starch generally producing a marginally higher settling rate. This small but noticeable flocculation and settling behaviour improvement may be due to the thicker adsorbed layer of PS starch on the talc particles (Table 3), facilitated by its structure and slightly greater molecular weight.
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Fig. 6. Settling rate (A), final consolidation behaviour (B) and supernatant clarity (C) of unflocculated and flocculated talc suspensions at 8 wt.% solid as a function of polymer dosage.
Fig. 7. Settling rate (A), final consolidation behaviour (B) and supernatant clarity (C) of unflocculated and flocculated talc suspensions at 8 wt.% solid as a function of polymer dosage.
The effect of the two carboxylated starch polymers on bed consolidation and supernatant clarity are shown in Fig. 6B and C. Increased addition of polymer causes a modest but a systematic decrease in sediment bed consolidation (Fig. 6B), accompanying the dramatic decrease in supernatant clarity (Fig. 6C). Thus fast settling, but less dense flocs are formed under PS and CM. Furthermore, the polymer-induced flocculation produced a clearer supernatant, with a fivefold decrease in total suspended solids content. Unlike the settling rate behaviour, there is no significant difference between the effect of the two polymers on bed consolidation and supernatant clarity. The data indicate that there is little influence of the varying polymer chemistry in the settling behaviour of the talc suspensions, with only minor changes seen in the settling rate that could be attributed to the differences between the two polymers, or their adsorbed layer characteristics. The results for the two Gelose polymers with variation in degree of branching are shown in Fig. 7. The settling rate data (Fig. 7A) indicate that there is a strong dosage dependence to the settling rate, with the settling rate increasing with increasing concentration of both polymers in the range 8–40 mg L1, equivalent to 100–500 g polymer t1 solid dosage. At the highest dosage settling rate of 1.2 m h1 and 0.65 m h1 were achieved for Gelose 80 and Gelose 50, respectively. Two things are apparent from inspecting the data. First, there is a stark difference in the ability of the two polymers to increase the settling rate of the talc suspension, with Gelose 80, the polymer with more linear amylose starch chains, performing much better than Gelose 50. Second, both polymers perform worse than the two larger carboxylated starches (see Fig. 6A) in terms of the suspension settling behaviour. The latter is due to a molecular weight effect (both Gelose polymers are much smaller than the PS and CM Starch polymers). The consolidation and clarity data are plotted in Fig. 7B and C, respectively. It can be seen that both Gelose polymers, in spite of their small influence on the settling rate, in comparison with the
PS and CM starch polymers, they produce higher sediment consolidation, in fact comparable to that of the bare talc bed. There was no impact of polymer structure type, as the sediments flocculated with both Gelose polymers behaved similarly across the concentration range. The supernatant clarity data (Fig. 7C) displays some polymer structure dependence, with the more linear Gelose 80 sample producing the lowest suspended solid content, and almost equal performance over the concentration range, whereas use of the more branched Gelose 50 led to very poor clarity and a clear dosage dependence. The higher percentage of amylose chains in Gelose 80 produced a much better overall settling performance than Gelose 50. The increased performance can be attributed to a higher particle bridging flocculation efficiency, which produced faster settling, a more compact packing of sedimented flocs and better clarification. The poorer performance of Gelose 50 is in spite of the higher equilibrium adsorption density determined from adsorption isotherms, and the similar polymer layer coverage and reduction in particle hydrophobicity for the two Gelose polymers determined at equilibrium adsorption time. The data from the AFM images on layer thickness does point to more desirable adsorption attributes with Gelose 80 whose adsorption led to a thicker layer (50% thicker than Gelose 50) and greater RMS, both indicative of an expanded interfacial conformation, reflecting longer adsorbed polymer loops and tails, conducive to multi-particle adsorption and bridging.
4. Conclusions The adsorption of four starch polymers on talc has been studied with adsorption isotherms, AFM imaging, and contact angle measurements. Polymer functional group chemistry, molecular weight, and degree of branching are all seen to influence the characteristics of the adsorbed polymer layer at equilibrium (adsorbed amount,
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morphology, and reduction of hydrophobicity). The flocculation and settling behaviour of talc suspensions treated with these polymers has revealed that molecular weight is the primary characteristic that controls settling rate, bed consolidation, and supernatant clarity. The secondary characteristic of the polymers that influences settling behaviour is the degree of branching (amylopectin content) of the starch sample, with more linear starch chains (amylose) being more effective in flocculating the talc suspensions. It was also seen that variations in functional group chemistry (inclusion of an aromatic group alongside carboxyl groups) did not result in settling behaviour modification). Acknowledgments Financial support for this study was received from the Australian Research Council and AMIRA International sponsors of Project P498C (which include National Starch, Anglo Platinum, CP Kelco and Newcrest). Russell Schumann of Levay and Co. Environmental Services is acknowledged for his for assistance with SEC measurements. The authors would also like to thank Associate Professor Allan Pring (South Australian Museum, Adelaide) for the provision of the talc mineral sample used for AFM and contact angle measurements, and Dr. Audrey Beaussart for drawing the polymer structures. References [1] J. Gregory, Rates of flocculation of latex particles by cationic polymers, J. Colloid Interface Sci. 42 (1973) 448–456. [2] Y.D. Yan, S.M. Glover, G.J. Jameson, S. Biggs, The flocculation efficiency of polydisperse polymer flocculants, Int. J. Mineral Process. 73 (2004) 161–175. [3] D.N. Thomas, S.J. Judd, N. Fawcett, Flocculation modelling: a review, Water Res. 33 (1999) 1579–1592. [4] J. Gregory, Effect of cationic polymers on colloidal stability of latex-particles, J. Colloid Interface Sci 55 (1976) 35–44. [5] F. Mabire, R. Audebert, C. Quivoron, Flocculation properties of some watersoluble cationic copolymers toward silica suspensions – a semiquantitative interpretation of the role of molecular-weight and cationicity through a patchwork model, J. Colloid Interface Sci. 97 (1984) 120–136. [6] P. Mpofu, J. Addai-Mensah, J. Ralston, Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions, Int. J. Mineral Process. 71 (2003) 247–268. [7] P. Mpofu, J. Addai-Mensah, J. Ralston, Flocculation and dewatering behaviour of smectite dispersions: effect of polymer structure type, Minerals Eng. 17 (2004) 411–423. [8] H. Li, J. Long, Z. Xu, J.H. Masliyah, Effect of molecular weight and charge density on the performance of polyacrylamide in low-grade oil sand ore processing, Can. J. Chem. Eng. 86 (2008) 177–185. [9] I. Cengiz, E. Sabah, S. Ozgen, H. Akyildiz, Flocculation of fine particles in ceramic wastewater using new types of polymeric flocculants, J. Appl. Polymer Sci. 112 (2009) 1258–1264.
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