Structure Modifications of Hydrolytically-Degradable Polymer

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Structure Modifications of Hydrolytically-Degradable Polymer Flocculant for Improved Water Recovery from Mature Fine Tailings Georges R. Younes,† Abbigale R. Proper,† Thomas R. Rooney,† Robin A. Hutchinson,*,† Sarang P. Gumfekar,‡ and João B. P. Soares‡ †

Department of Chemical Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2 V4, Canada

‡

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S Supporting Information *

ABSTRACT: Variants of the partially hydrolytically degradable cationic macromonomer polycaprolactone choline iodide ester methacrylate (PCL2ChMA) have been synthesized to assess the effects of structure on the performance of the resulting polymers in the flocculation of mature fine tailings (MFT) that are a byproduct of bitumen extraction from oil sands. Neither the substitution of PCL with poly(lactic acid) (PLA) units or replacement of the methacrylate functionality with acrylate greatly affected the ability of the resulting cationic flocculants to settle and separate the sediments in diluted MFT suspensions, as the synthesized polymers have similar structures and charge densities. The higher degradation rates of the PLA-based materials, however, led to faster compaction of the MFT sediment, as quantified by the amount of water released from the flocculated materials over time. Over 50% compaction was observed in MFT samples ranging between 2 and 20 wt % held for either 5 days at 50 °C or for 12 weeks at room temperature, whereas no significant amount of water was released from sediment flocculated with a comparable nondegradable cationic polymer or with high molecular-weight nonionic poly(acrylamide). The results demonstrate the potential of these LA-based cationic degradable polymers for dewatering of oil sands MFT or other flocculated sediments.

1. INTRODUCTION Open-pit mining is extensively used to recover the plentiful oil sands ores in Alberta, Canada. Separating the bitumen from this mixture requires a large volume of hot water (40−80 °C), 2.5 m3 of water per barrel of bitumen.1,2 The residue from the extraction, known as oil sands fresh fine tailings (FFT), consists of clays, sands, and what remains of the bitumen suspended in water. The tailings separate into two phases after being transferred to settling ponds: the coarse sand and approximately half of the clay particles which settle quickly, and the bitumen and the rest of the fine clays which form a stable suspension in water. The latter gel-like phase is known as mature fine tailings (MFT) and contains 30−40 wt % of solids dispersed in water.3 Over the past 50 years, MFTs have accumulated in ponds that cover 176 km2 in surface area.1,4 In addition, a large volume of water, around 83 vol % of the MFT, remains unavailable for usage.5 Various processes and technologies have been developed to treat oil sands tailings, with a specific focus on MFT treatment. © XXXX American Chemical Society

Current techniques include consolidated or nonsegregating tailings (CT), inline thickened tailings or paste technology (PT), thin lift drying, centrifugation-drying, and extraction fine tails thickening.6 CT and PT are the only two technologies applied in the oil sands industry on a large scale. During the former process, MFT are mixed with coarse sands from fresh tailings and an inorganic coagulant aid to obtain a nonsegregated mixture whose particles aggregate once deposited in ponds. However, the inorganic coagulant leads to high concentrations of divalent ions in the recovered water, making impossible its reuse for the extraction of bitumen, a significant drawback.7−9 Hence, the alternative PT process was developed to rapidly thicken FFT by the addition of a polymeric flocculant. While settling the fine particles that normally transforms the FFT into Received: Revised: Accepted: Published: A

June 21, 2018 July 23, 2018 July 26, 2018 July 26, 2018 DOI: 10.1021/acs.iecr.8b02783 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

cationic polycaprolactone-based materials that degrade through the hydrolysis of ester bonds.31 In contrast, the PCL2ChMA macromonomer developed for flocculation applications was synthesized via the addition of ε-caprolactone units to 2(dimethylamino)ethanol, with the double bond attached through addition of methacryloyl chloride.27,32 The performance of poly(PCL2ChMA) was compared to that of poly(trimethylaminoethyl methacrylate chloride) (poly(TMAEMC)), synthesized from a methacrylate that has the same cationic end group but does not contain the degradable polyester units. While poly(TMAEMC) also was effective at MFT flocculation, the resulting solids did not show the increased dewaterability over time found with poly(PCL2ChMA).27 In the previous study, a temperature of 85 °C was used to achieve accelerated degradation of the poly(PCL2ChMA). While degradation also occurs at room temperature, the time frame would be significantly longer. Thus, a goal of this work is the modification of these polymeric flocculants to achieve faster degradation (and thus dewatering). Guidance toward achieving this objective is found in the biomedical field, as similar polyester methacrylate macromonomers have been employed to generate degradable nanoparticle (NPs).31 Several studies have shown that degradation rates are accelerated by replacing the CL units on the macromonomer with L-lactide (LA), due to the greater hydrophilicity of the resulting polymers.33−35 Thus, faster degradation is expected using flocculants formed from poly(lactic acid) choline iodide ester methacrylate (PLAnChMA) macromonomers. However, it must be verified that adequate separation performance (settling rates, supernatant turbidity) is achieved when these variants are applied to MFT. Another structural modification considered in this work is to substitute the methacrylate polymerizable group in the macromonomer structure with acrylate functionality, to produce poly(caprolactone) choline iodide ester acrylate (PCLnChA) and poly(lactic acid) choline iodide ester acrylate (PLAnChA). The motivation for this effort arises from radical polymerization literature, as it is known that chain growth of acrylates36 is characterized by higher propagation rate coefficients than methacrylates.37 Thus, it may be possible to produce polymers of higher molecular weight (MW) under similar synthesis conditions; previous work has shown that increased MW leads to better flocculation performance, presumably due to increased bridging between the clay particles to form bigger flocs.10,12−14 In the following, the synthesis and characterization of these new macromonomers, shown in Scheme 1 along with PCL2ChMA and TMAEMC, is described. The resulting polymers are characterized, and their degradation behavior is studied. MFT flocculation experiments are carried out to assess the flocculating performance of the polymers with respect to the initial settling rate (ISR), supernatant turbidity, and capillary suction time (CST). Focused beam reflectance measurements (FBRM) are also conducted to unveil some insights to the interactions between the tailings and the flocculants. Tracking the amount of water released from the sediments over time provides a direct measure of how the partial hydrolytic degradation of these novel materials improves MFT consolidation compared to nondegrading poly(TMAEMC) and nonionic PAM.

MFT, the thickened sediments still contain large volumes of water due to limitations of the flocculants currently available.1,4 As a result, researchers are continuing efforts to develop new polymeric variations to improve the efficiency of this process. As summarized in a recent review on water-soluble polymers for oil sands tailings treatment,10 most studies use commercial or modified polyacrylamide (PAM)-based flocculants in various forms: nonionic, ionic, inorganic−organic hybrid (e.g., Al(OH)3-PAM, FeCl2-PAM), and temperature-responsive. The first three flocculant types work well at low dosages on kaolin, which is often used as a model for the mixture of clays that are the predominant component of oil sands tailings. However, when applied to MFT at similar conditions, the resulting supernatants remain turbid due to incomplete separation of the small highly charged clay particles (98%), stannous octoate (Sn(oct)2, 92.5−100.0%), triethylamine (TEA, ≥99.5%), basic alumina (Brockmann 1), acryloyl chloride (ACl, ≥97%), methyl iodide (ICH3, 99%), 2,2-azobis(2-methylpropionamidine)dihydrochloride (V-50, 97%), [2-(methacryloyloxy)ethyl] trimethylammonium chloride solution (TMAEMC, 80 wt % in H2O), and nonionic polyacrylamide (PAM, 5−6 million Da) were purchased from Sigma-Aldrich and used as received. Tetrahydrofuran (THF, >99%, ACP Chemicals), anhydrous diethyl ether (≥99%, ACP Chemicals), deuterium oxide (D2O, 99.9% D, Cambridge Isotope Laboratories), chloroform-d (CDCl3, 99.8% D, Cambridge Isotope Laboratories), and deionized water (Millipore Synergy water purification system) were used as received. Methacryloyl chloride (MACl, 97%, Sigma-Aldrich) was distilled before use to remove reactive dimers.38 Mature Fine Tailings (MFT, 34% solids content by mass) were provided by Coanda Research and Development Corporation, with a second sample provided from the Muskeg River in Alberta. Characterizations of these materials are reported in the Supporting Information Tables S1 and S2. In the following sections, these MFT will be referred to as Coanda MFT and Muskeg MFT, respectively. Scheme 2. Synthesis Procedure to Produce Poly(PLA4ChMA)

C

DOI: 10.1021/acs.iecr.8b02783 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 3. Synthesis Procedure to Produce Poly(PCL2ChA)

four macromonomers. The samples were prepared by dissolving each macromonomer in water to obtain a 1 wt % solution, followed by a 20-fold dilution in the methanol buffer solution. The MS analysis was conducted under positive-ion mode, and the instrument was externally calibrated with caffeine as a standard (74−1522 Da). The critical micelle concentrations (CMC) for the four macromonomers were determined via surface tension measurements in water using a DuNoüy Ring setup at room temperature; a TensioCAD with a platinum ring was used following EN14370 standards. 2.3. Polymer Synthesis and Characterization. 2.3.1. Polymer Synthesis. The polymers were produced by radical batch homopolymerization of 10 wt % of macromonomer at 70 °C with 0.22 wt % of V-50 as initiator, as previously reported.27 The polymerization kinetics were studied via in situ 1H NMR using a solution of 0.10 g of macromonomer dissolved in 0.90 g of D2O (containing 0.22 wt % of V-50 as initiator). The mixture was bubbled with nitrogen for 1 h while avoiding excessive foaming. The four polymerizations were run at 60 °C in a Bruker Avance instrument operating at 500 MHz using the methodology described elsewhere to maintain near-isothermal operation.39,40 The monomer consumption, and hence the conversion, was tracked by integrating the vinyl peaks with respect to the reference solvent peak. 2.3.2. Polymer Characterization. Images of the polymers were taken using transmission electron microscopy (TEM) on a Hitachi H-7000 instrument operating at 75 kV. Aqueous solutions were prepared with a concentration of 1.0 mg/mL, with the exception of poly(PLA4ChMA) solution, which was diluted to 0.10 mg/mL for better resolution. The TEM images were captured after depositing droplets of the solutions on carbon coated copper grids and removing excess solution after 1 min. The viscosity of polymer solutions diluted to 1 wt % was measured according to ASTM D445 and D2515 standards using viscometer tubes (size 25 F756 and size 200 P449) obtained from the CANNON Instrument Company. Each measurement was repeated three times to check for reproducibility. 2.3.3. Polymer Degradation Study. The polymers were diluted to 1 wt % in water and held at 50 °C in an oil bath. After each day, the samples were cooled to room temperature for analysis before increasing the temperature again. Once the LAbased polymers had degraded (5 days), the temperature of the bath was increased to 85 °C to allow for the CL-based polymers to further degrade. The analyses conducted included pH

TEA (31 mL, 220 mmol) was added and the solution was warmed to 38 °C using an oil bath. The flask was purged with nitrogen before slowly adding 2.44 mL of ACl diluted in 7 mL of THF via syringe over 1 h. The reaction mixture was kept at 38 °C for another 24 h after which the TEA salt was filtered and the residual ACl was removed using a column of basic alumina. THF and the remaining TEA were evaporated under vacuum to yield poly(caprolactone) 2-(N,N-dimethylamino)ethyl ester acrylate (PCL2DeA). The resulting PCL2DeA was dissolved in 232 mL of diethyl ether, cooled to 0 °C in an ice bath, and purged with nitrogen. A volume of 4.33 mL of ICH3 (69.6 mmol) was injected using a syringe and the reaction was progressively warmed to room temperature while proceeding for 48 h. The white orange waxy product, poly(caprolactone) choline iodide ester acrylate (PCL2ChA) was collected after washing several times with cold ether which was evaporated in a last step under vacuum for 4 h. At the end of the synthesis, 7.33 g of PCL2ChA was produced, corresponding to a final yield of 61%. The final product contains less than 10 mol % [2-(acryloyloxyl)ethyl]trimethylammonium iodide (TAEMI), determined by 1H NMR (Figures S4−S6). The synthesis scheme of this macromonomer is shown in Scheme 3. PLA4ChA: In a first step, 7.76 g (54 mmol) of LA was reacted through ROP as in the synthesis of PLA4ChMA, using 21.8 mg (0.054 mmol) of Sn(oct)2 as the catalyst and 2.4 g (27 mmol) of De as the initiator. The ROP reaction lead to PLA4De with n ≈ 4 lactoyl units and 93% conversion. In total, 10.16 g of PLA4De (27 mmol −OH) was then treated following the same acrylation and purification procedures as for PCl2De, with 135 mL of THF, 35 mL of TEA (256.5 mmol), and 2.83 mL of ACl diluted in 7 mL of THF, and purified to produce poly(lactic acid) 2-(N,Ndimethylamino)ethyl ester acrylate (PLA4DeA) with 91% PLA4De conversion, as calculated from 1H NMR (Figures S7−S9). The resulting PLA4DeA was methylated with 5.0 mL of ICH3 (80.7 mmol) in 269 mL of diethyl ether, and the final product purified to obtain 10.9 g of PLA4ChA corresponding to a final yield of 70%, including 10 mol % of unreacted PLA4De remaining after the acrylation reaction. 2.2.3. Macromonomer Characterization. The four macromonomers as well as their intermediate synthesis products were characterized by 1H NMR using a 400 MHz Bruker Avance instrument. Electrospray ionization-mass spectroscopy (ESIMS) was performed using a Thermo Fisher Orbitrap Velos Pro instrument to examine the molecular weight distributions of the D


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Table 1. Molecular Weights (Not Including Counterion) of (Macro)Monomers and Corresponding Charge Densities of Polymers Synthesized in This Study polymers poly(PCL2ChMA) poly(PCL2ChA) poly(PLA4ChMA) poly(PLA4ChA) poly(TMAEMC)

macromonomer CMC (g/L)

macromonomer Mn (g/mol) from NMR

macromonomer Mn (g/mol) from ESI-MS

dispersity (Mw/Mn) from ESI-MS

polymer charge densitya (mmol/g)

0.538 0.510 0.628 0.636

423 409 460 446 172

494 499 548 559

1.08 1.08 1.08 1.08

1.82 1.87 1.70 1.75 4.82

a

As calculated from Mn estimated by NMR.

Figure 1. ESI spectra showing relative intensity (mol basis) as a function of macromonomer molecular weight (Da) of (top) PCL2ChMA and (bottom) PLA4ChMA. Peaks at 400.3 Da for PCL2ChMA and 460.2 Da for PLA4ChMA correspond to the n = 2 and n = 4 structures shown in Scheme 1.

measurement using a Mettler Toledo SevenExcellence pH meter and particle size and solution zeta potential using a Malvern Zetasizer Nano ZS (size range 0.3 nm−10 μm) at 25 °C with backscattering optics (173°) and a 4 mW He−Ne (633 nm) laser. The latter two properties were measured using quartz

cuvettes and universal dip cell (DTS1070), respectively. The reported sizes and zeta potentials represent an intensity average of at least 30 scans each, with every measurement repeated three times. E


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Industrial & Engineering Chemistry Research 2.4. Oil Sands Flocculation Tests. Flocculant dosages were varied between 3000 and 9000 ppm (ppm) at 2000 ppm increment, calculated on a weight basis relative to the weight of solids in the Coanda MFT. For each test, 100 mL of a 2 wt % MFT solution was prepared by diluting the original solid mixture with deionized water. The diluted MFT solutions had pHs between 7.7 and 8.0. Each suspension was transferred to a 250 mL baffled beaker and stirred at 600 rpm for 2 min. The polymer flocculant was added at the desired dosage, and the mixture was kept at 600 rpm for an additional 5 min, followed by 2 min at 300 rpm, following the procedures used previously.27 The final mixture was directly transferred to a 100 mL graduated cylinder, and the mudline (solid−liquid interface) was recorded for 30 min to determine an initial settling rate (ISR). The supernatant was removed after 24 h to measure its turbidity, and the sediments were collected to measure their capillary suction time (CST) following previous procedures.27 The turbidity and the CST were measured three times for each sample, demonstrating excellent reproducibility, as also reported previously.27 FBRM experiments were also conducted in which 100 mL of Coanda MFT diluted with deionized water to 2 wt % solids was placed in a 250 mL unbaffelled beaker for each trial. The FBRM probe and a dual-blade impeller were inserted in the MFT mixture and the speed was set to 400 rpm for 5 min after which the polymer flocculant (with dosage varying between 3000 and 9000 ppm at 2000 ppm increments) was added. The mixing speed was left at 400 rpm for another 2 min and then lowered to 100 rpm for the rest of the 30 min experiment. The Mettler Toledo FBRM instrument has a fast-rotating laser beam inside its probe that provides an estimate of the floc size (in microns) and a count of the number of flocs from reflectance measurements. 2.5. MFT Dewatering Study. Additional flocculation tests were conducted in 100 mL cylinders to monitor the further consolidation of the tailings sediments that occurs with polymer degradation after the initial settling. The same mixing procedure was adopted as before, with the supernatant removed from the sediments in the cylinder after 24 h (2−3 mL of water were left above the mudline). The cylinders were covered and placed in an oil bath held at 50 °C for 5 days, then at 85 °C for another 5 days in order to accelerate dewatering in the presence of the more slowly degrading CL-based materials. As the flocculant degraded, the further compaction of the sediment was monitored by following the change in the sediment height daily, with the extra water released rising to the top of the mixture. An analysis of reproducibility was conducted by preparing six to eight cylinders of 2 wt % solids content Muskeg MFT treated with 10000 ppm of the same polymer. Tests were then conducted using 10000 ppm flocculant with Coanda MFT diluted to 2 and 5 wt %, with poly(TMAEMC) and PAM used as reference flocculants, with additional testing conducted on 10 wt % MFT with 15000 ppm flocculant. Moreover, cylinders with 2 and 20 wt % Coanda MFT mixtures were prepared using 10000 ppm (for 2 wt % MFT) and 25000 ppm (for 20 wt % MFT) poly(PCL2ChMA) and poly(PLA4ChMA) and left at room temperature for 12 weeks, with the mudline levels recorded every week. The solids content of the consolidated sediment was determined by gravimetry.

confirm structures of the intermediates species (Figures S1−S9) formed during the multistep syntheses summarized by Schemes 2 and 3. As shown previously27,32,39 and characterized by ESIMS in this work, there is a distribution of chain lengths that results from the ROP step. MS spectra of the methacrylate macromonomers PCL2ChMA and PLA4ChMA are shown as Figure 1. As found in a previous ESI-MS study of choline-PLA and choline-PCL biodegradable surfactants,41 the mass spectra for the CL-based materials do not show any peak corresponding to chain without CL units (n = 0), despite their detection by 1H NMR. Thus, the first peak corresponds to the macromonomeric cation with one CL or LA unit. For PCL2ChMA, the subsequent peaks are separated by 114 Da (equivalent to one CL unit) with the peaks for n = 2 and 3 of almost equal intensity on a number basis, in agreement with the n = 2.2 average value determined by NMR. Integration over the entire spectra yields a numberaverage MW value of 494 Da, reasonably close to the value of 423 Da estimated from 1H NMR. Moreover, the distribution is relatively narrow, with a dispersity of 1.08. The lactide ring used in the synthesis of PLA4ChMA contains two lactoyl units, each with a MW of 72 Da. Hence, the distribution of macromonomers is dominated by n = 4 and n = 6 structures with small peaks of odd numbers of lactoyl units (n = 1, 3, 5...). Once again, the MW value is in reasonable agreement with that determined by 1H NMR, and the dispersity is 1.08. The findings from analysis of the corresponding acrylate macromonomers (PCL2ChA and PLA4ChA) are similar (see Table 1 and Figure S10). The macromonomer MWs determined by NMR are used to calculate the density of the cationic charges (mmol/g) on the corresponding polymers reported in Table 1. As PCL2ChMA forms micelles in aqueous solution,39 surface tension measurements were conducted to determine and compare the critical micelle concentration (CMC) for the four cationic macromonomers. As shown in Figure 2, the CMC

Figure 2. Surface tension (γ) versus macromonomer concentration (C) for (●) PCL2ChMA, (○) PCL2ChA, (▲) PLA4ChMA, and (△) PLA4ChA in water (γ = 71.3 mN/m) at 26 °C.

values for the three new macromonomers are very similar to that of PCL2ChMA,39 approximately 10−3 mol/L or 0.6 g/L. As PLA is more hydrophilic than PCL, the values for PLA4ChMA and PLA4ChA are slightly above those of PCL2ChMA and PCL2ChA, in agreement with other studies that report higher CMCs for more hydrophilic surfactants.41−44 No consistent difference between the acrylate and methacrylate macro-

3. RESULTS AND DISCUSSION 3.1. Macromonomer Synthesis and Characterization. The MWs and charge densities of the macromonomers synthesized are summarized in Table 1. 1H NMR was used to F


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Figure 3. Visual evidence of polymer aggregation and precipitation during the ex-situ partial degradation of poly(PCL2ChMA) and poly(PLA4ChMA).

morphology as that previously reported for poly(PCL3ChMA).39 The heterogeneous nature of the poly(macromonomer) systems reduces their viscosity relative to water-soluble polymers, with the values for 1 wt % polymer solutions of the four poly(macromonomers) ranging between 1.2 and 2.3 cP (Table S4), significantly lower than the value of 10 cP measured for 1 wt % poly(TMAEMC) of similar MW (weight average values of 6−8 × 105 Da27). 3.3. Polymer Degradation Study. Unlike the cationic polycaprolactone-based materials produced by ROP addition to HEMA that degrade fully to produce water-soluble poly(HEMA),31 it has been shown that poly(PCL2ChMA) synthesized according to the procedures outlined above only partially degrades to yield an insoluble product.27,32 The prime motivation for synthesizing LA-based polyester flocculants is the expected increase in the rate of degradation compared to their CL counterparts under room temperature conditions. As a first comparison of degradation behavior, solutions of 1 wt % polymer in aqueous solution were held at elevated temperatures for an extended period of time, following the procedures used previously to accelerate the partial hydrolysis process.27,32 As detailed elsewhere, choline and PCL (or PLA) oligomers are the principle degradation products,32 although further study is needed, it is likely that toxicity of these byproducts will not be an issue as similar materials are being explored for biomedical applications.31 Instead, we focus on the comparison of the LA and CL-based materials. While measurements of polymer particle average size and dispersity, zeta potential, and pH are all used to track degradation, a simple visual indicator is the precipitation of polymer that occurs as it degrades. The previous study showed that poly(PCL2ChMA) took several days to degrade at 85 °C,27 very similar to the time taken for the degradation of poly(PCL3ChMA).32 When held for 5 days at the lower temperature of 50 °C, however, no visual precipitation of the CL-based polymers occurred, whereas the LA-based materials had degraded to form precipitate, as shown in Figure 3 for the

monomers is observed, with the CMC of PLA4ChA higher than that of PLA4ChMA and the opposite ordering observed for the CL-based macromonomers. Similar complexities are summarized by Joynes and Sherrington in their extensive study on cationic surfactant monomers, known as surfmers.45 The absolute surface tension of the micellar solution, γCMC, provides a measure of surfactant effectiveness, as influenced by various factors including hydrophobicity and chain configuration.41 The values for the CL macromonomers are above those of the LA materials, consistent with their higher hydrophobicity. It has been shown that the (meth)acryloxy groups at the surfmers tails have a loop configuration once dissolved in water;46,47 the hypothesis that the methacryloxy polymerizable group are less flexible, making the γCMC of acrylates macromonomers higher, are consistent with the results in Figure 2. The full CMC and corresponding absolute surface tension results are summarized in Table S3. 3.2. Polymer Synthesis and Characterization. Full conversion was reached in under 30 min by radical polymerization in aqueous solution, as verified by 1H NMR and consistent with previously reported results for the polymerization of PCL2ChMA.27,39 The measurements using in situ NMR showed that the acrylate macromonomers polymerize at a slightly slower rate than the methacrylates (Figure S11). The weight-average molecular weight of poly(PCL2ChMA) previously synthesized under these conditions was 770 000 Da;27 thus, the molecular weights of the two methacrylate polymers synthesized in this study are expected to be in the same 5−10 × 105 Da range. As detailed later, the acrylate polymers provide comparable flocculation performance, and it can thus be hypothesized that they also have similar molecular weights. While TMAEMC polymerization proceeds homogeneously in aqueous solution, PCL2ChMA polymerizes through a complex micellar/surfmer mechanism.39 The resulting solution, although optically clear, is heterogeneous; the TEM images presented in Figure S12 indicate that the polymers formed from the new polyester macromonomers have a similar fibril G


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Figure 4. ISR values for the four polymers as a function of polymer dosage with 2 wt % MFT. Inset photographs compare final mudlines and supernatant clarity 24 h after settling, with dosages of 3000, 5000, 7000, and 9000 ppm polymer (left to right).

Figure 5. Supernatant turbidities after 24 h for 2 wt % MFT samples flocculated using the four polymers applied at the indicated dosages (average error of ±2.0 NTU).

acidity of the polymer increases with its hydrophilicity.48 Since poly(PLA4ChMA) and poly(PLA4ChA) are more hydrophilic than their caprolactone analogues, the pH of the initial 1 wt % polymer solutions were lower, in agreement with other studies on PLA and PCL in which PLA terminal groups degraded faster than the PCL ones (pKa of 3 compared to 5, respectively).35 In addition, poly(PCL2ChA) and poly(PLA4ChA) have lower pH than their methacrylate analogues, in agreement with the increased acidity of poly(acrylic acid) compared to poly(methacrylic acid) determined elsewhere.49

methacrylate polymers. Precipitation of the CL-based materials was only observed when the degradation temperature was raised to 85 °C. No significant difference in degradation behavior between the acrylate (Figure S13) and methacrylate versions of either the CL or the LA based materials was observed. Full results of the degradation study for all four of the polymers are provided in Figures S13−S16 and Table S5, including the variation of polymer particle average size and dispersity, zeta potential, and pH with time. A difference to note between the LA and CL-based polymers, as well as the methacrylate and acrylate versions, is their pH. In general, the H


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Figure 6. CST of the sediments measured 24 h after flocculation by the four polymers applied to 2 wt % MFT at different dosages (average error of ±4.9 s).

Figure 7. FBRM results showing (A) mean size of flocs, (B) number of flocs, and (C) floc size distribution of 2 wt % MFT as flocculated using poly(PCL2ChMA) at different dosages.

Syncrude Canada Ltd.,27 the Coanda-sourced material used in this study proved more difficult to flocculate; it is well-known that the heterogeneous composition of MFTs affects settling behavior.11 Thus, the study was conducted with MFT diluted to 2 wt %, following the procedures outlined in the Materials and Methods. Polymer dosages were varied between 3000 and 9000 ppm at 2000 ppm increments, although it was not possible to determine the initial settling rate (ISR) at a dosage of 3000 ppm, as the mudline could not be clearly tracked in the turbid mixture.

3.4. Oil Sands Flocculation Studies. Although the LAbased polymers clearly degrade more quickly than CL-based materials, it is necessary to compare their relative ability to settle sediments from oil sands MFT. Our previous study compared the flocculation behavior of poly(PCL2ChMA) to control samples of poly(TMAEMC) and PAM.27 Herein, we focus on comparing flocculating behavior of the four degradable polyester compositions. While previously 5000 ppm of poly(PCL2ChMA) was effective in flocculating 5 wt % solids MFT provided from I


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Figure 8. Visual record of dewatering from accelerated in situ degradation of sediments obtained from flocculation of 2 wt % MFT sediments by 10000 ppm poly(TMAEMC), poly(PCL2ChMA), and poly(PLA4ChMA).

tively. The CST values measured for sediments flocculated with the acrylate materials are higher than those flocculated with the methacrylates, consistent with the slightly more hydrophobic character of the latter variations. FBRM was employed to provide additional information regarding MFT flocculation. As shown for poly(PCL2ChMA) in Figure 7, both the size and the number of flocs quickly achieve constant levels after the polymer is mixed with the MFT sample, with the average size increasing and the number of flocs decreasing as the polymer dosage is increased from 3000 to 9000 ppm. At low polymer dosages, the clay particles in the MFT were not as effectively flocculated, as seen in the distributions of the floc sizes in Figure 7C. A significant increase in the floc size is seen as dosage is increased to 7000 ppm, consistent with the ISR and turbidity measurements presented earlier. FBRM results obtained with the other flocculating agents are similar. A comparison of the mean size of the flocs for the four polymers at the different dosages after 30 min is given in Figure S18, with a corresponding comparison of floc counts shown as Figure S19. At a dosage of 3000 ppm, the flocs formed are of similar size (10−11 μm) as the initial MFT, a result consistent with the poor flocculation observed. At higher dosages, the flocs formed in the presence of the poly(acrylates) are smaller than those formed with the methacrylate materials. The poly(PLA4ChMA) applied at 9000 ppm formed flocs 2.5 times larger in diameter on average than the size of the original MFT, hence with more than a 10-fold increase in volume. 3.5. MFT Dewatering Study. Enhanced dewatering was previously demonstrated through an 85% reduction in CST after 1 week accelerated degradation of sediments flocculated with poly(PCL2ChMA) at 85 °C.27 How this improvement translates to solids consolidation in the field, however, is not evident. Thus, we have adopted the decrease of the sediment mudline over time as a direct measure of consolidation. The procedure for this test is straightforward: the sediments are kept in the original graduated cylinder (after removal of the supernatant after 24 h), which is covered and held at the desired temperature in a thermostated water bath. The water released appears as a clear layer on top of the sediment as it consolidates, as shown in

The settling curves are given in Figure S17, with the ISR values summarized in Figure 4, and the corresponding measurements of supernatant turbidity (measured after 24 h) summarized in Figure 5 (see also Table S6). The ISR values for the four polymers were found to be very similar, as were the heights of the final mudline in the samples (inset pictures in Figure 4 taken after 24 h). Although there was little difference in the ISR values, the clarity of the supernatant improved as dosage was increased from 5000 to 9000 ppm (Figure 5). The methacrylates have slightly better supernatant clarity than their acrylate analogues; however, all of the tests conducted at 7000 and 9000 ppm (with the exception of poly(PLA4ChA) at 7000 ppm) result in clear supernatants. Thus, all four compositions of the synthesized flocculants show comparable performances when assessed according to ISR and supernatant turbidity. This finding is perhaps not surprising, as they have similar chain architecture and charge densities. The remainder of the study assesses the relative performance of the materials in promoting dewatering of the sediment before and after degradation. While not greatly affecting separation behavior, the increased hydrophilicity of the LA-based polymers influences the permeability of the sediments, as assessed through the capillary suction test (CST, with lower values indicating better drainage of water from the sediment) conducted after removal of the supernatant from the MFT sediment 24 h after flocculation. It was previously reported that the CST values for sediment flocculated with poly(PCL2ChMA), while slightly higher than those for poly(TMAEMC) 24 h after flocculation, decreased significantly after accelerated degradation conditions were applied;27 the values summarized in Figure 6 are in the same range as that previous work. The sediments flocculated with poly(PLA4ChMA) and poly(PLA4ChA) released water more slowly than those flocculated with the CL materials, as reflected in the higher CST values (see also Table S7). The differences in CST become smaller, however, as polymer dosages are increased to 9000 ppm. At this concentration, the CST value of 22.6 s for poly(PLA4ChMA) compares well with the 14.3 and 28.9 s measured for poly(PCL2ChMA) and poly(PCL2ChA), respecJ


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As summarized in Table S13 and Figure S20, the acrylate version of the flocculating agents showed dewatering behavior similar to the corresponding methacrylates, with the LA material releasing water (50% of the original sediment volume) at 50 °C, while the CL version requiring a temperature increase to 85 °C to achieve the same performance. Control experiments were conducted not only with the cationic poly(TMAEMC), but also with nonionic PAM. While 10000 ppm of PAM flocculated the 2 wt.% solids content MFT, its sediments did not experience any water release (Figure S21). When the same PAM dosage was applied to MFT with 5 wt % solids content, it formed loosely packed sediments with the supernatant still turbid after 24 h, as shown in comparison to poly(PCL2ChMA) and poly(PLA4ChMA) flocculation cylinders after 30 min in Figure 10. Dewatering tests were repeated

Figure 8, with the decrease in mudline position providing a measure of the quantity of water released. Following the same strategy as for the polymer degradation tests in solution, the sediments were held at 50 °C for 5 days, followed by several days at 85 °C. The reliability of the procedure was examined by following the change in mudline in a series of cylinders flocculated under the same conditions. Despite minor differences in the position of the starting mudline, the amount of water released was quite reproducible with an average error of ±0.25 mL (Tables S8−S12). These verification tests were done using MFT from a different source (Muskeg River), which showed significantly better consolidation during the initial flocculation period (mudline of ∼16 mL compared to ∼23 mL for the Coanda-sourced material). While not a focus of the current study, it illustrates the fact that flocculant performance can vary between MFT samples due to their heterogeneity.11 A series of consolidation tests were conducted on sediment collected after flocculation of 2, 5, and 10 wt.% Coanada MFT samples using the four novel degradable polymers as well as the nondegradable poly(TMAEMC). Figure 9 plots the evolution of

Figure 10. Flocculation tests of 5 wt % MFT with 10000 ppm of (from left to right): poly(PCL2ChMA) and poly(PLA4ChMA) after 30 min and PAM after 24 h.

using 5 and 10 wt % MFT solutions, with the same general results achieved (Tables S14 and S15 and Figures S22 and S23): negligible additional water release with poly(TMAEMC) used as the flocculating agent, significant water release only occurring when temperature was increased to 85 °C for CL-based flocculating agents, and significant consolidation (decrease of ∼50% in mudline position) occurring with LA-based flocculants at 50 °C over 5 days. The dewatering rates of flocculated settlements were also determined at room temperature conditions using 2 wt % MFT solutions. As summarized in Table S16, the dewatering achieved with the slower-degrading poly(PCL2ChMA) was relatively low, with the mudline height decreasing from 18 to 13 mL over 12 weeks. Thus, the degradation rate of poly(PCL2ChMA) may be too low under field conditions to achieve significant dewatering. The mudline height, however, decreased from 25 to 8 mL over 12 weeks for the sediment flocculated with poly(PLA4ChMA), an even greater release of water than observed during the accelerated degradation test results at higher temperature. Thus, the flocculants synthesized with LA-based macromonomers are more promising materials for application to MFT tailings remediation, as significant solids consolidation can be achieved at room temperature over a 3 month period, a time scale suitable for field applications. Similar results were achieved with 20 wt % MFT samples (Table S17), where LA-based polymers achieved ∼50% compaction of the flocculated sediment, reaching solids levels

Figure 9. Mudline position of MFT sediments during accelerated in situ degradation tests after flocculation of 2 wt % solids MFT by 10000 ppm (●) poly(PCL2ChMA), (▲) poly(PLA4ChMA), and (□) poly(TMAEMC) at 50 °C for 5 days, followed by an additional 5 days at 85 °C.

the sediment mudlines from 2 wt.% solids MFT treated with poly(PCL2ChMA), poly(PLA4ChMA), and poly(TMAEMC). A small amount of water (∼2 mL) was released from all samples in the first day, likely due to the removal of the supernatant that exerts pressure on top of the sediment. With poly(TMAEMC), no additional water was released at 50 °C, with a total of 4 mL released over the 10 day study. The sediment treated with poly(PCL2ChMA) released 4 mL of water over the 5 days held at 50 °C; however, unlike the behavior with poly(TMAEMC), an additional 4−5 mL was released when temperature was increased to 85 °C to accelerate the degradation of the CL-based flocculant. This behavior is contrasted to that of the poly(PLA4ChMA) sample, for which most of the water (11 out of 13 mL) was released from the sediment during the 5 days at 50 °C. Thus, the faster degradation rate of the LA-based materials observed in the polymer degradation study leads to accelerated consolidation of MFT sediments under in situ conditions. Moreover, as shown in the Figure 8 photographs, the polymer degradation led to a decrease of the sediment volume (mudline position) by approximately 50%. K


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weeks, suggesting that the LA-based materials degrade at a fast enough rate to provide solids compaction under field conditions. Follow-up work is planned to pursue this promising development. Relatively high polymer dosages (10 000 ppm based on sample solids content) were required for the initial separation of the MFT sediments and to achieve good supernatant clarity. This may be due to a combination of factors including wall effects arising from the use of 100 mL graduated cylinders in our small-scale studies and the procedures adopted to mix the flocculant and MFT.51 However, on a comparative basis under identical conditions, the cationic materials synthesized in this study performed significantly better than a high MW PAM homopolymer. Scale-up of both macromonomer synthesis and MFT testing under ambient temperature conditions is under development. In addition, sediment characteristics such as yield stress and the environmental fate52 of the water-soluble degradation products will be studied.

of greater than 40% after 12 weeks at room temperature. Table 2 provides a summary of the volume reduction and final solids Table 2. Volume Reduction and Final Solids Content of Sediments after Polymer Degradation Using Poly(PLA4ChMA) as Flocculant to Treat Diluted MFT MFT solids content/ polymer dosage

degradation conditionsa

volume reduction (%)

final solids content (wt %)

2 wt %/10000 ppm 2 wt %/10000 ppm

accelerated room temperature accelerated accelerated room temperature

57 68

20 25

50 40 50

26 33 43

5 wt %/10000 ppm 10 wt %/15000 ppm 20 wt %/25000 ppm

a 5 days at 50 °C for accelerated degradation or 12 weeks at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

content of the sediment flocculated with poly(PLA4ChMA) from MFT samples of various dilutions. Volume reduction varied between 40 and 70%, with the lower numbers found with higher MFT content likely due to the difficulty in releasing entrapped water from the sediment in the 100 mL cylinders.50 Despite this artifact resulting from the small-scale testing, final solids contents increased to >40% after polymer degradation for the sample with the highest MFT content.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b02783. 1 H NMR spectra for macromonomer and precursors, macromonomer characterization (ESI-MS spectra and critical micelle concentrations), polymer synthesis and characterization (macromonomer conversion profiles and solution viscosity tests), results from ex-situ degradation studies (pH, particle size, and zeta potential), results from MFT flocculation tests (mudline settling curves, supernatant turbidity, CST data, FBRM floc count, average floc size, and floc size distributions), and results from sediments dewatering studies (reproducibility analysis and sediment compaction results after flocculation of 2, 5, 10, and 20 wt % diluted MFT) (PDF)

4. CONCLUSIONS Four variants of polyester-based macromonomers, acrylate and methacrylate versions containing CL or LA units, were synthesized via ring-opening polymerization through simple modifications to the procedure previously developed for PCL2ChMA. The materials were then radically polymerized to produce partially hydrolytically degradable polymers tested as flocculating agents for oil sands MFT. With similar charge densities and chain architectures, the four flocculants showed similar effectiveness in treating the MFT. Changing from a methacrylate polymerizable group to an acrylate and using LA in place of CL units had no effect on the settling of MFT solids, as characterized by ISR and supernatant turbidity. The initial dewaterability of the resulting sediments, as characterized by CST, was slightly lower for the LA-based flocculants due to their increased hydrophilicity compared to the CL-based analogues. The differences between the materials were clearly seen in the polymer degradation studies, in which the LA-based polymers degraded after 5 days at 50 °C, whereas the CL-containing materials showed little change in properties until the temperature was raised to 85 °C. The advantage of the faster degradation rates for application to oil sands tailings was demonstrated by quantifying the amount of further water released by the sediments separated after MFT flocculation. With poly(PLA4ChMA) and poly(PLA4ChA) flocculants, the sediments reduced in volume by greater than 50% within 5 days when held at 50 °C, whereas higher temperature was needed to accelerate the water release from sediments flocculated with the CL-based polymers. The newly developed test, which directly quantifies the increased sediment compaction that occurs in the MFT sample, was also used to demonstrate that only a minimal amount of water is released using a nondegrading cationic polymer as a flocculant. Most significantly, greater than 50% of compaction occurs in MFT sediments flocculated with poly(PLA4ChMA) when held at room temperature over several

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robin A. Hutchinson: 0000-0002-0225-7534 Sarang P. Gumfekar: 0000-0001-7763-5360 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (RAH group) and the Campus Alberta Innovation Program (JBPS group) for the financial support.

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REFERENCES

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Structure Modifications of Hydrolytically-Degradable Polymer

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