ARTICLE pubs.acs.org/Langmuir
Temperature-Responsive Solid Liquid Separations with Charged Block-Copolymers of Poly(N-isopropyl acryamide) John-Paul O’Shea, Greg G. Qiao, and George V. Franks* Department of Chemical and Biomolecular Engineering, Australian Mineral Science Research Institute, The University of Melbourne, Victoria, 3010 Australia ABSTRACT: Temperature responsive charged block-copolymers of poly(N-isopropylacrylamide) (PNIPAM) have been used in the solid liquid separation of alumina mineral particles from aqueous solution. The effects of temperature, polymer charge-sign and fraction of charged segment have been investigated. Batch settling and adsorption studies showed that rapid sedimentation results for suspensions with polymers of opposite charge-sign to the particle surface-charge (counterionic) at 50 °C. Cooling the suspensions after flocculation at 50 °C was found to increase the final solids volume fraction of the sediment beds formed through a mechanism related to partial desorption of polymer and the reduction of the hydrophobic attraction. Suspension stability results after dosing with polymers of similar charge-sign to the particle surface-charge (co-ionic) at both 25 and 50 °C. Increasing the amount of polymer charge increased the influence of polymer charge-sign on the adsorption and solid liquid separation behavior. The performance of the charged block copolymers are compared to that of the random charged copolymer and neutral homopolymer PNIPAM structures.
1. INTRODUCTION Solid liquid separations are employed at many stages of mineral processing, ranging from product liquor recovery to the tailings processing stages. Increasing the efficiency of these separations can contribute to enormous reductions in running cost and the volume of waste produced. The application of poly(N-isopropylacrylamide) as a temperature-responsive flocculant has been proposed as a novel means of effecting rapid gravity settling in the initial primary separations stage (by creating attraction between particles) while providing a mechanism for increasing the extent to which the sediment bed can densify in the secondary consolidation stage (by switching interparticle forces to repulsion).1 Networked suspensions of particles with attractive interparticle forces aggregate and settle rapidly but resist consolidation under pressure (such as in sediments) due to the strong touching particle network. On the other hand, colloidal particles interacting via repulsive forces will not settle quickly, but do form dense sediment beds because the repulsive interactions allow the particles to rearrange and to consolidate to a greater extent under lower pressures.1 3 PNIPAM can be used to effect a switch between attraction and repulsion, because below a lower critical solution temperature (∼32 °C, LCST) PNIPAM is soluble in aqueous solution. Heating solutions of PNIPAM above the LCST causes an entropically driven phase transition of the polymer molecules and the polymer becomes insoluble.4 6 When the phase change occurs in suspensions of solid particles, polymer molecules deposit on the particle surfaces causing attractive interparticle interactions, aggregation, and rapid settling. By cooling to below r 2011 American Chemical Society
the LCST, a switch to repulsive interparticle interactions occurs and the polymer behaves as a dispersant.7,8 Enhanced consolidation of the sediment follows with very high densities achievable.8 The use of PNIPAM homopolymers as flocculating agents is limited because adsorption of the polymer molecules onto the particle surfaces must occur before the polymer molecules selfaggregate in order for particle aggregation to result. As such PNIPAM homopolymer is ineffective in flocculation when added to a suspension that is already hot. In addition, the polymer-association mechanism responsible means that there is no specific interaction of the PNIPAM homopolymer molecules with mineral particles of different surface charges and hence there is no apparent mineral particle selectivity. To meet these specific challenges while at the same time preserving control of the particle particle interactions to aid enhanced consolidation, our group 9 and others10 14 have previously investigated the use of random, charged copolymers of PNIPAM in order to allow the polymer to interact with mineral particles selectively depending on the charge charge relationship existing. Random polymers with the same charge sign as the mineral surfaces (co-ionic) do not strongly adsorb and produce a repulsive interparticle interaction which results in stability of suspension at both 25 and 50 °C. Random copolymers with the opposite charge to the mineral surfaces (counterionic) adsorb strongly at temperature both above and below the LCST Received: October 3, 2011 Revised: November 8, 2011 Published: November 09, 2011 905
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Table 1. Reaction Conditions for Block Copolymers of PNIPAM samplea
a
monomer 1 (mmol)
RAFT agent (mmol)
initiator (105 mol)
solvent
P(tBA)
tBA, 12.82 g (99)
BPA, 0.29 g (1)
AIBN, (2)
20 mL of DMF
P(DQA)
DQA, 29.33 g (151.5)
BPA, 0.42 g (1.5)
AIBN, (15.2)
70 mL of MeOH
P(tBA01-b-NIPAM99)
NIPAM, 9.2 g (81.3)
P(tBA), 0.1 g
AIBN, (8.1)
20 mL of THF
P(tBA05-b-NIPAM95)
NIPAM, 8.8 g (77.6)
P(tBA), 0.5 g
AIBN, (7.8)
20 mL of THF
P(tBA15-b-NIPAM85)
NIPAM, 5.6 g (49.6)
P(tBA), 1.1 g
AIBN, (5.0)
20 mL of THF
P(tBA30-b-NIPAM70)
NIPAM, 10.05 g (89)
P(tBA), 4.9 g
AIBN, (3.8)
20 mL of THF
P(DQA01-b-NIPAM99)
NIPAM, 9.6 g (84.8)
P(DQA), 0.16 g
APS, (8.5)
20 mL of MeOH
P(DQA05-b-NIPAM95) P(DQA15-b-NIPAM85)
NIPAM, 10.22 g (90) NIPAM, 1.05 g (0.15)
P(DQA), 0.92 g P(DQA), 0.4 g
APS, (0.5) APS, (0.2)
30 mL of MeOH 15 mL of MeOH
P(DQA30-b-NIPAM70)
NIPAM, 10.32 g (91)
P(DQA), 7.6 g
APS, (3.9)
50 mL of MeOH
Subscript numbering refers to mole percentage of the different monomers in the polymerization.
inducing net particle attraction, aggregation and sedimentation. In addition, counterionic copolymers form charge-stabilized polymer colloids in solution at temperatures above the LCST which are able to adsorb on the particle surfaces.15 The adsorption of the counterionic charge stabilized polymer colloids when added to suspension already at 50 °C enables flocculation and rapid settling (not observed with PNIPAM homopolymers).8,9 Although the counterionic random copolymers of PNIPAM afford selective flocculation even when added at a temperature above the LCST, chargepatch and bridging adsorption mechanisms produce strongly and irreversibly flocculated sediments which persist even upon cooling to 25 °C. Unlike the PNIPAM homopolymer, the charged random polymers do not exhibit enhanced consolidation after cooling.9,16,17 Block-copolymers are an alternative architecture for incorporating charge into a polymer molecule which may enable selectivity in adsorption to mineral particle surfaces because the charged segment of the polymer molecule has affinity for counterionically charged surfaces.18 In this paper, we investigate the use of charged block-copolymers of PNIPAM as flocculants. We investigate the settling rates, temperature-responsive sediment consolidation, mineral selectivity and addition at temperature above the LCST using alumina suspensions. This is the first instance of charged and temperature responsive block-copolymers being used in flocculation and solid liquid separations. The influence of charge-sign and fraction of charged segment is investigated by looking at the flocculation performance of cationic and anionic block-copolymers with different charge amounts. Further, we compare the flocculation and consolidation performance of charged block-copolymers of PNIPAM with neutral PNIPAM homopolymer and charged random-copolymers of PNIPAM.
mixture (40:60 by volume). Acrylic acid (AA, >98%, Aldrich) was purified by vacuum distillation and stored in a refrigerator at 4 °C. Dimethylaminoethylacrylate quaternary ammonium (DQA, 80% w/w aqueous solution, CIBA Specialty Chemicals) was precipitated into acetone and the crystalline monomer was refrigerated. Tertiary butyl acrylate (tBA, 97.0%, Aldrich) was distilled under reduced pressure prior to use. Initiators: Azobisisobutyronitrile (AIBN, Aldrich) was recrystallized in methanol and kept refrigerated. Solvents: Toluene, n-hexane, tetrahydrofuran, acetone, 1,4-dioxane, and N,N-dimethylformamide were all AR grade and purchased from Merck. Diethyl ether and methanol were both AR grade and purchased from BDH. All analytical reagent grade organic solvents were used without further purification. RAFT agent: 3-Benzyltrithiocarbonyl propionic acid (BPA) was prepared using the reported procedures of Jesberger et al.19 and Takolpuckdee.20 1H NMR (CDCl3, δ = 7.26 ppm, δ, ppm): 7.39 7.18 (m, 5H, Ph), 4.61 (s, 2H, CH2 Ph), 3.62 (t, 2H, CH2 S, J = 7.2 Hz), 2.84 (t, 2H, J = 7.2 Hz, CH2 CdO). 13C NMR (CDCl3, δ = 77.0 ppm; δ, ppm): 177.5 (q, CdO), 134.7 (q, Ph), 129.2, 128.7, 127.8 (3t, Ph), 41.4 (s, CH2 Ph), 32.8, 30.8 (2s, CH2 CH2S, CH2 CH2S).
2.2. Synthesis of Block Copolymers of NIPAM Using RAFT Polymerization: General Procedure. The block-copolymers used in this study were prepared using standard reversible-addition and fragmentation transfer (RAFT) polymerization techniques.21 The reaction conditions for synthesis are provided in Table 1 and characteristics of the resultant polymers are shown in Figure 1 and Table 2. A schematic representation of the block-copolymers is given in Figure 1. As an example of the procedure for synthesis of block copolymers of NIPAM, the polymerization of poly(N-isopropylacrylamide-b-tertiary butyl acrylate) is described below. Sequential RAFT polymerization was performed for PNIPAM-b-tBA copolymers using poly(tBA) as the macro-chain transfer agent (CTA) for the second block (NIPAM). In the first step, tBA, BPA, AIBN, and THF (20 mL total) were dispensed into 30 mL Schlenk tubes with small stir bars. The tBA/BPA/AIBN ratio was varied between 5000:5:1 and 5000:20:1 (molar ratio) and the monomer concentration was ca. 50% wt/wt. The tubes were subjected to three freeze pump thaw cycles on a vacuum line (10 Pa). The tubes were then backfilled with argon to restore atmospheric pressure and maintain an inert environment. The Schlenk tubes were subsequently immersed in a preheated oil bath maintained at 70 °C with magnetic stirring at 500 rpm and left overnight. The tubes were removed from the oil bath, cooled to room temperature, and diluted with THF (5 mL). Subsequently, the volume of the reaction mixtures was reduced by rotary evaporation, and the polymer was purified by repeated precipitation into diethyl ether and subsequently dried under vacuum at 50 °C. In the second step, the resulting PtBA macro-CTA, NIPAM monomer, AIBN, and THF were added to Schlenk tubes with the NIPAM/macroCTA/AIBN ratio of 1000:4:1 and the monomer concentration of
2. EXPERIMENTAL SECTION 2.1. Materials. AKP-15 Alumina (Sumitomo, Japan, SBET ≈ 7 m2/g,
D50 ≈ 1 μm, F ≈ 3.94 g/cm3) was used as received. 10 wt %/wt stock alumina suspensions were prepared and homogenized by ultrasonication for 10 min at 40 W. Sodium chloride (>99%) was obtained from SigmaAldrich and used as received. Reverse osmosis (RO) water was used in all experiments and stock polymer and salt solution make-ups. PNIPAM stock solutions were prepared several days in advance. All experiments were carried out at pH 5 and 0.01 M NaCl and in temperature controlled environments. Monomers: N-isopropylacrylamide (NIPAM) monomer (Wako Specialty Chemicals, Japan) was recrystallized from a toluene/n-hexane 906
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Figure 1. General structure of the block-copolymers used in this study. Monomeric units are represented as follows: gray circle, N-isopropylacrylamide (NIPAM); b, tertiary-butyl acrylate (tBA); blue circled minus, acrylic acid (AA); red circled plus, dimethylaminoethyl acrylate quaternary chloride (DQA).
Table 2. Characterisation of Block-Copolymers of PNIPAM sample
Mn,total 10‑3 (g/mol)a
DPfirst blockb
DPPNIPAMc
PDtotald
P(tBA)
12
1.01
P(DQA)
8
1.1
LCST (°C)f
ffirst block charge e UNIT (%)
foverall charge f unit (%)
P[(tBA-co-AA)01-b-NIPAM99]
49
34
395
1.2
33.25
45
( )1
P[(tBA-co-AA)05-b-NIPAM95]
45
34
360
1.6
35.05
12
( )4
P[(tBA-co-AA)15-b-NIPAM85]
28
34
299
1.1
35.25
53
( )9
P[(tBA-co-AA)30-b-NIPAM70] P[DQA01-b-NIPAM99]
24 300
34 39
175 2583
1.1 1.5
35.5 33
33 100
( )14 (+)1
P[DQA05-b-NIPAM95]
100
39
826
1.1
33.8
100
(+)4
P[DQA15-b-NIPAM85]
30
39
245
1.1
34
100
(+)15
P[DQA30-b-NIPAM70]
17
39
87
1.3
34.3
100
(+)30
a
DPfirst block: Degree of polymerization of initial polymer block segment. b DPPNIPAM: Degree of polymerization of PNIPAM, the second polymer block segment. c PDtotal: Polydispersity of polymer. d UV vis. e ffirst block charge: Molar percentage of 1st polymer block which is randomly charged via 1H NMR. f foverall charge: Molar percentage of overall polymer that is charge, via 1H NMR. ca. 30 wt %/wt. The degas-reaction-purification routine was performed as outlined above. t-Butyl groups in block copolymer prepared from NIPAM and tBA (i.e., P(NIPAM-b-tBA)) are cleaved upon treatment with HCl. P(NIPAM-b-tBA) (2 g) was dissolved in dichloromethane (50 mL). HCL (1.6 mL, 42 mmol) was added under argon dropwise within 1 h. The reaction was continued for 48 h, after which time the volume of dichloromethane was reduced by rotary evaporation and the polymer repeatedly precipitated in diethyl ether. This resulted in a randomcopolymer structure of tBA with AA as the first block with PNIPAM as the second block. The precipitate was dried at 50 °C under vacuum for 24 h. The RAFT polymerization procedure employed above incorporates charge of a different degree by the sequential polymerization of different lengths of PNIPAM onto initial anionic block length polymers of the same length. Shorter second blocks of PNIPAM meant that overall there was a higher incorporation of tBA but a lower overall block-copolymer molecular weight. Conversely, longer second blocks of PNIPAM meant that there was a lower overall incorporation of tBA but a higher overall block-copolymer molecular weight. There is a similar relationship between the degree of cationic charge incorporation and overall chain length of block-copolymers of PNIPAM and DQA. As described above, the structure of the anionic-block in the anionic block-copolymer was randomly charged resulting in charge-densities in the charged-blocks that are less than 100%. Meanwhile, the chargedensity of the cationic charged block was 100%. However, the length of the charged blocks for a particular charge-sign is the same. Hence the overall molar fraction which is charged in each block-copolymer decreases as the length of the PNIPAM block segment increases. Conversely, as the overall molar fraction which is charged increases,
the length of the PNIPAM block segment decreases. Therefore a reference to increasing overall polymer charge fraction corresponds to decreasing PNIPAM block length, whereas decreasing polymer charge fraction corresponds to increasing PNIPAM block length. The degree of incorporation of the functional groups in the block copolymers was analyzed using 1H NMR (Varian Unity Plus 400 Spectrometer operating at 400 MHz). The changes in molecular weight of the polymers at different reaction stages was monitored using gel permeation chromatography (GPC) carried out on a Shimadzu liquid chromatography system equipped with a Wyatt miniDAWN TREOS and Shimadzu RID-10A refractive index detector using Polymer Laboratories mixed columns (3 PLgel 5 μm MIXED-C) operating at 70 °C. N,N-Dimethylformamide (DMF) containing LiBr (0.05 M) was used as the eluent, at a flow rate 1 mL/min. Astra software (Wyatt Technology Corp., USA) was used to determine the number-average molecular weights (Mn) using injected polymer mass values based on the assumption 100% mass recovery. The lower critical solution temperature (LCST) of polymers was determined by measuring the change in light absorbance with temperature using a Varian-400 UV vis with peltier temperature control. A thermistor probe continuously monitored the temperature inside the cell. In brief, polymer solutions were made up to 10 000 mg/L in RO water and filtered in 0.45 μm filters. A 4 mL sample was added to a 10 mm path length quartz cuvette and placed in the beamline. The transmittance of light at 540 nm was measured with reference to the absorbance of RO water, at 0.2 °C increments between 10 and 50 °C. The LCST was taken as the temperature at which the transmittance had decreased by 5% from the initial baseline value at 10 °C. 907
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Table 3. Conditions for Adsorption/Desorption and Settling Experiments temperature at regime
which polymer is added (°C)
equilibration temperature for adsorption and deposition
settling conditions
abbreviation
1
25
48 h at 25 °C
24 h at 25 °C
25f25 °C
2
25
48 h at 50 °C
24 h at 50 °C
25f50 °C
3
25
24 h at 50 °C followed by 24 h at 25 °C
12 h at 50 °C followed by l2 h at 25 °C
25f50f25 °C
4
50
not applicable
24 h at 50 °C
50f50 °C
Figure 2. (a) Adsorption (mg/m2) of block-PNIPAM onto alumina after equilibrating at 25 °C for 48 h. (b) Deposition (mg/m2) of block-PNIPAM onto alumina after heating to 50 °C and holding for 48 h. (c) Polymer remaining on surface (mg/m2) of block-PNIPAM onto alumina after heating to 50 °C for 24 h and then cooling to 25 °C and holding for 24 h. Block PNIPAM compared: red circle, (+)30%; orange circle, (+)15%; pink circle, (+)4%; green square, ( )1%; teal square, ( )4%; blue square, ( )9%; and dark blue square, ( )14%. Lines are drawn to guide the eye.
2.3. Polymer Aggregate Size Characterization. Polymer aggregates and micelles were sized using a Malvern HTTP dynamic light scattering (DLS) instrument, with volume-averaged diameters reported. The DLS incorporated sample temperature control within the range of 14 55 °C. Polymer solutions were made up to 200 mg/L polymer, 0.01 M NaCl, and pH 5 using stock polymer solutions of 10 g/L. Samples were filtered and transferred to 10 mm path-length polystyrene cuvettes. The clarity of the polystyrene cuvettes at different temperatures was checked using water and no measurable variance in optical quality over the relevant temperature range (14 55 °C) was found. Spectroscopic measurement parameters such as laser position and attenuator were optimized by an instrumental algorithm. After the instrument had reached a desired temperature, the samples were allowed to equilibrate for 5 min before measurements started. Six measurements were then subsequently made and recorded with the average of these reported in this work. 2.4. Adsorption and Deposition Studies. In adsorption experiments suspensions were made up to 15 mL, 0.01 M NaCl, and 5% weight of solid particles per weight of suspension (w/w), at various PNIPAM concentrations, in 24 mL screw-top soda-glass vials at 25 °C. To examine the temperature dependency of PNIPAM adsorption, the samples were securely closed, shaken, and then equilibrated according to one of the first three treatment regimes outlined in Table 3. Subsequently, the samples were centrifuged in a temperature-controlled centrifuge and the clear supernatant drawn off and analyzed for total organic carbon concentration using a Total Organic Carbon Analyzer (TOC, Shimadzu Corp., Japan). The concentration of polymer in the supernatant after equilibration was determined by comparing the supernatant TOC with a calibration plot prepared by measuring the TOC of known concentrations of PNIPAM solutions. These equilibrium
concentrations are reported in milligrams of polymer per liter of supernatant (mg/L). The adsorbed amount is reported as milligrams of polymer per surface area of mineral (mg/m2) and is determined by subtracting the amount remaining in the supernatant from the initial dosage concentration and dividing by the known mineral surface area in the sample. 2.5. Batch Settling Tests. Five weight percent suspensions were made up from alumina, salt, and polymer stock preparations in 100 mL measuring cylinders at 25 °C. Before commencing the experiments, the cylinders were shaken vigorously for 10 s to ensure complete initial dispersion. Settling was observed for four different temperature regimes (Table 3): (1) Polymer was added to suspension at 25 °C and settled at 25 °C for 24 h [25f25 °C]; (2) polymer was added to 25 °C suspension, which was then heated to 50 °C and settled for 24 h at 50 °C [25f50 °C]; (3) polymer was added at 25 °C, suspension settled for 12 h at 50 °C and subsequently for 12 h at 25 °C [25f50f25 °C]; (4) polymer was added to 50 °C suspension which is then settled at 50 °C for 24 h [50f50 °C]. In order to control the temperature of the cylinders (initially at 25 °C with the exception of case (4), they were placed in a temperature controlled bath where the suspensions rapidly approached the temperature of the surrounding water (∼30 s). The time zero was taken as starting as soon as the cylinders were placed in the hot water. The dose of polymer is reported in milligrams of polymer per gram of solid in the suspension (mg polymer/g solid). For clarification, a dose of 1 mg/L is equivalent to a dose of 0.02 mg polymer/g alumina. The initial hindered settling rate was taken as the negative of the initial gradient of an interfacial height versus time plot. Supernatant clarity at the end of a 24 h settling period was found by taking multiple supernatant samples, drying at 100 °C for 24 h and hence calculating the average suspended solids concentration (mg/L). The final solids volume 908
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fraction of the sediment, ϕf has been calculated as ϕf = (mT Vf), mT (total mineral mass, g), ms mS)/(FmVf) with mS = cS(VT (mass of mineral in supernatant, g), VT (total suspension volume, mL), Vf (final sediment bed volume, mL), Fm (mineral density, g/mL), and cs (solids concentration in the supernatant, g/mL).
adsorbed amount, thus demonstrating partial desorption due to reversal of the phase transition. The desorption results in a similar amount of polymer remaining on the substrate surface as for the initial 25 °C adsorption shown in Figure 2a. 3.2. Behavior of Polymer in Solution at Different Temperatures. The volume averaged diameter of polymer in aqueous solution with varying temperature and charge is given in Figure 3. At 25 °C, below the LCST, there is no aggregation, whereas at 50 °C polymer aggregates form. The size of the polymer aggregates appears to decrease with increasing charge density, although this trend is less noticeable for the cationic species. This demonstrates that the formation of micelle structures is facilitated by the charged block segments of the polymers stabilizing the aggregates in solution. 3.3. Settling. No flocculation or sedimentation occurs when polymer is added at 25 °C for either cationic or anionic block copolymers. Figure 4a shows the initial hindered settling rates of alumina suspensions dosed with block-PNIPAM and heated to 50 °C. No destabilization occurs with cationic species resulting in negligible settling rates. In contrast, anionic copolymer flocculates the suspensions even at lower concentrations. The effect of charge density is clearly seen, as there is a strong increase in settling rate with increasing charge density. This shows the important role that counterionic polymer has in adsorbing to the surface by electrostatic interactions, compressing the particle electrical double layer and causing particle aggregation and rapid sedimentation to occur. Figure 4b shows the concentration of alumina remaining suspended in the supernatant of suspensions which have been dosed with block-PNIPAM and settled at 50 °C. The amount of solids remaining suspended in the supernatant after flocculation with anionic block PNIPAM is quite low (
