Rapid Dewatering and Consolidation of Concentrated Colloidal

May 22, 2019 - ... sands production in north Alberta. Current treatment technologies are limited by slow yet insufficient water release and sludge con...
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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 21610−21618

Rapid Dewatering and Consolidation of Concentrated Colloidal Suspensions: Mature Fine Tailings via Self-Healing Composite Hydrogel Bin Yan,†,§ Linbo Han,‡,§ Hongyan Xiao,† Jiawen Zhang,§ Jun Huang,§ Wenjihao Hu,§ Yingchun Gu,*,† Qi Liu,§ and Hongbo Zeng*,§ †

College of Light Industry, Textile, and Food Engineering, Sichuan University, Chengdu 610065, China College of Health Science and Environmental Engineering, Shenzhen Technology University, Shenzhen 518118, China § Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada

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‡

S Supporting Information *

ABSTRACT: Billions of tonnes of thick waste streams with highly concentrated colloidal suspensions from different origins have accumulated worldwide, exampled as over 220 km2 mature fine tailings (MFT) from oil sands production in north Alberta. Current treatment technologies are limited by slow yet insufficient water release and sludge consolidation. Herein, a self-healing composite hydrogel system is designed to convert concentrated aqueous colloidal suspensions (e.g., MFT with colloidal solid content >30 wt %) into a dynamic double cross-linked network for rapid dewatering and consolidation. The resultant composite hydrogel demonstrates an excellent dewatering performance so that over 50% of water could be rapidly released within 30 min by vacuum filtration. Furthermore, the formed infinite cross-linked network with self-healing ability can effectively trap fine particles of all sizes and capture small flocs during mechanical mixing, thereby enabling a low solid content at the ppm level in the released water. This new strategy outperforms all the previously reported treatment methods; under mechanical compression, over 80% of water is removed from the MFT, thereby generating a stackable material with >70 wt % solids within an hour. These results demonstrate a highly effective approach and provide insight into the development of advanced materials to tackle the challenging environmental slurry issues. KEYWORDS: self-healing hydrogel, concentrated colloidal suspensions, mature fine tailings, filtration, dewatering and consolidation

1. INTRODUCTION

To date, tremendous efforts have been devoted to developing novel technologies to speed up the water release and consolidation of waste slurries over the last few decades.3,17 However, so far only limited success has been reported, and the existing strategies to destabilize dense slurries, such as paste technology with flocculation using watersoluble (hydrophilic) polymers,18−22 composite/consolidated tailings with dilution, adding coarse sands17 or inorganic coagulants,23 and natural drying via evaporation,24 cannot achieve efficient solid−liquid separation for consolidation and inventory reduction as well as timely recycling of valuable process water. These limitations have necessitated the exploration of new strategies that can (1) deal with the original concentrated waste slurries directly without pretreat-

Wastewater with high concentrations of colloidal solids is generated in many industrial activities, for example, from mining and mineral processing operations1−3 to food processing4,5 and leather production.6 With a high colloidal solid concentration, such wastewater often exists in gel-like states and the suspension is extremely stable.7−11 Because it is extremely difficult to dewater, the current practice is to store it on-site using seepage-capture facilities and interceptor ditches. As a result, billions of tonnes of such thick waste streams from different origins have accumulated worldwide.3,12 Impoundment of such an unstable suspension poses severe consequences to public health, economy, and the environment and can be disastrous. It not only considerably increases the operating cost but also creates substantial environmental liabilities. The inability to recycle the trapped process water and the need to occupy large land areas puts great pressure on natural water and land resources.13−16 © 2019 American Chemical Society

Received: April 1, 2019 Accepted: May 20, 2019 Published: May 22, 2019 21610

DOI: 10.1021/acsami.9b05692 ACS Appl. Mater. Interfaces 2019, 11, 21610−21618

Research Article

ACS Applied Materials & Interfaces

dissolution in DCM and precipitation in diethyl ether three times. The final product was dried under vacuum at 50 °C overnight. The characterization of the obtained polymer DFPEO is shown in Figure S1. 2.3. Characterization Methods. Fourier Transform Infrared spectrophotometry (FTIR). The FTIR spectra were obtained on a Thermo Scientific Nicolet iS50 Fourier Transform Infrared Spectrophotometer using attenuated total reflection (ATR) mode. The spectra were collected over a frequency range of 4000−400 cm−1. In a typical ATR measurement, the polymer powder was directly placed on the ATR diamond crystal and then the sample was pressed against the crystal by a pressure tower to ensure the intimate contact between the polymer sample and the crystal. Then the sample spectrum was acquired by allowing the infrared radiation to transmit through the crystal and reflect from the surface of the crystal contacted with the sample with the energy absorbed by the sample. Gel Permeation Chromatography (GPC). Molecular weight measurements on polymers were carried out on a Waters system equipped with a refractive index detector (RI 2414) and a UV/vis 2489 detector at 40 °C. A Waters 510 liquid chromatography pump equipped with one (HR4E) Styragel column was used to determine molecular weight and polydispersibility of polymers. THF was used as the eluent at an elution rate of 1 mL min−1, and polystyrene standards were used for calibration. Nuclear Magnetic Resonance (NMR). 1H NMR measurements were performed on a Varian-Inova (400 MHz) spectrometer at room temperature. Chemical shifts are reported in parts per million (δ) relative to TMS as the internal reference. The polymer samples were dissolved in CCl3D at a concentration of 5 mg/mL. X-ray Diffraction (XRD) Measurement. XRD was performed using a Rigaku X-ray diffraction tool (Cu Kα X-ray source) for crystalline phase analysis. A glass sample holder was used for powder samples. Diffraction data were obtained at 50 kV and 200 mA, scanning from 10° to 110° of 2θ with a scan step of 2°/s. MFT solids were separated by Dean−Stark extraction procedure and ground to a powder using a mortar and pestle.32 Quantification of the mineral phases (Rietveld refinement) was obtained with Siroquant software (Sietronics, Canberra, ACT, Australia). Particle Size Measurement. The particle size distribution of the solids in the MFT was determined using light scattering with a Malvern Mastersizer 3000 Particle Size Analyzer (Malvern Instruments, U.K.). Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) Measurement. Ionic concentration of different cationic ion species in MFT samples was determined using a PerkinElmer Elan 6000 ICP-MS. The MFT water was collected by centrifugation and further filtered by using 0.45 μm filter unit for removal of large particles. Rheology Analysis. Rheological characterization was performed on a TA Instruments AR-G2 stress-controlled rheometer fitted with a Peltier stage using a cone−plate configuration (40 mm, 2° steel cone). The samples were equilibrated at 25 °C for 10 min before measurements were performed. To test self-healing behaviors of the hydrogels and MFT hydrogel composites, dynamic oscillatory strain amplitude sweep measurements were conducted at a frequency of 10 rad/s at different temperatures with the strain changing from 0.1% to 150% to achieve a strain failure, followed by a time-dependent modulus observation at 1% strain. Capillary Suction Time (CST) Measurements. CST measurements were carried out using a Type 319 Multi-Purpose Capillary Suction Timer equipped with five single-radius test heads (Triton Electronics Ltd.). Each of the heads has an inner diameter of 18 mm and a height of 25 mm. CST paper (Triton Electronics Ltd.) with a thickness of 1 mm was used. The sludge sample (5−8 g) of treated MFT or original MFT was taken from the beaker for the CST measurements. Surface Forces Apparatus (SFA). An SFA 2000 (SurForce LLC, Santa Barbara, CA, U.S.A.) system was used to quantitatively study the force interactions between freshly cleaved mica surfaces across polymer solutions. Briefly, two back-silver-coated muscovite mica surface (thickness ∼5 μm, grade #1, S&J Trading, U.S.A.) were glued onto cylindrical glass disks using an UV light (361 nm) cured glue

ment, (2) recover the trapped process water with adequate quality and quantity for recycle/reuse, (3) generate stackable solid materials for rapid reclamation, (4) reduce the inventory volume of these waste streams via effective consolidation and reclamation. Mature fine tailings (MFT), generated from the warm waterbased bitumen extraction from Canadian oil sands, is considered as one of the most stubborn waste slurries in the world. They currently occupy over 220 km2 land area in northern Alberta, Canada.2,12−15,25 Like other concentrated colloidal suspensions, it has a high colloidal solid content of >30 wt % and demonstrates a gel-like property. However, the original MFT gel is extremely stable yet weak, which fails most of the existing technologies to achieve rapid water release and sludge consolidation. To address this challenging issue, we envisioned MFT suspension could be further reinforced with additional polymer networks, and the resultant composite hydrogel of MFT would be compatible with most existing separation technologies for further dewatering and consolidation. Herein, we treated the MFT suspension with a hydrogel system composed of two polymers to form a double crosslinked polymer network, which is one commonly adopted strategy to reinforce the mechanical property of polymer hydrogels.26−28 Different from the previous studies using dual polymer flocculation strategy to destabilize MFT by bridging and charge neutralization mechanisms,21,29−31 our strategy in this work transfers the MFT into a self-healing composite hydrogel with strong mechanical property. This composite hydrogel possessed a dynamic infinite cross-linked network which could effectively trap fine particles with different sizes and capture the formed big flocs and ultrafine particles together during mechanical mixing through its self-healing ability. This new strategy outperformed the existing MFT treatment technologies for rapid water release and consolidation: with mechanical compression, we could easily achieve a net water release of 80% and generate stackable cake containing higher than 70 wt % solids, which is ready for reclamation.

2. MATERIALS AND METHODS 2.1. Materials. Tetrahydrofuran (THF) and dichloromethane (DCM) were distilled over CaH2 and used as prepared. Poly(ethylene oxide) with a molecular weight of 4000 Da (PEO4K) was purchased from Aldrich and purified by precipitating into cold ethyl ether three times and collected by filtration followed by drying under vacuum overnight. All the other chemicals were purchased from Aldrich and used as received. Mature fine tailing samples used in this study were from Syncrude and stored in 18.9 L (5 gallon) plastic pails. The sample in each pail was homogenized with a Makita 6013BR handheld agitator and subsampled to small portions (around 110 or 550 g of MFT) with stirring. 2.2. Synthesis of Dibenzylaldehyde-Functionalized Poly(ethylene oxide). Telechelic dibenzaldehyde-functionalized poly(ethylene oxide) (PEO) (DFPEO) with a molecular weight of 4000 Da was synthesized by esterification of hydroxyl-terminated PEO with 4-formylbenzoic acid. A typical example is shown as follows. A solution of N,N′-dicyclohexylcarbodiimide (0.412 g, 20 mmol) dissolved in 20 mL of freshly distilled THF was added to a solution of PEO4K (2 g, 0.5 mmol), 4-formylbenzoic acid (0.225 g, 1.5 mmol) and 4-(dimethylamino) pyridine (0.015 g) in 20 mL of freshly distilled THF. The whole mixture was stirred under an argon atmosphere at room temperature for 24 h and then the white solid was filtered. The filtrate was concentrated by evaporation and the polymer DFPEO was obtained as a white solid by repeated 21611

DOI: 10.1021/acsami.9b05692 ACS Appl. Mater. Interfaces 2019, 11, 21610−21618

Research Article

ACS Applied Materials & Interfaces (NOA81, Norland Products, Inc., U.S.A.). The thickness of coated silver layer is ∼50 nm. Then the two disks with glued mica surface were mounted into the SFA chamber in a cross-cylinder configuration. The SFA chamber was saturated with water vapor. One hundred microliters of BPEI aqueous solution (0.75 wt %) was directly injected into the two opposing mica surfaces using a pipet, and force measurement was conducted after equilibration for 30 min. Afterward, 70 μL of PEO or DFPEO aqueous solution (0.75%) were injected into the two mica surfaces to study the force interaction change. The film thickness and absolute separation distance were obtained in situ by employing an optical technique named multiple beam interferometry (MBI) using the fringes of equal chromatic order (FECO). The measured force, F, between two curved surfaces, was correlated to the interaction energy per unit area (W(D)) of two planar surfaces using the Derjaguin approximation described as follows, F(D) = 2πRW (D)

Figure 1. Characterization of MFT. (a) Particle size distribution of MFT suspension measured by Mastersizer; (b) FE-SEM image shows the thin plate layers of MFT particles.

suspension is extremely stable and exists in gel-like states without apparent phase separation or dewatering for decades. Gelation of MFT by a Self-Healing Hydrogel. To prepare the aforementioned self-healing composite hydrogel of MFT, two water-soluble polymers were designed. One is a cationic polymer with a hyperbranched architecture, branched poly(ethylenimine) (BPEI), providing physical interactions, mainly attractive electrostatic interactions with negatively charged MFT clay particles to form a primary physical network of large flocs; the other one is a delicately designed cross-linker called difunctional telechelic poly(ethylene oxide) (DFPEO) with two aldehyde end groups33 (see Figure S1 for for details in the Supporting Information (SI) DFPEO synthesis and characterization). Through dynamic Schiff base reactions between aldehyde groups of DFPEO and the amino groups of BPEI on the surfaces of MFT clay particles, the final resultant composite hydrogel of MFT is a double cross-linked hydrogel network reinforced by dynamic chemical bonds and electrostatic interactions. To clearly validate these interactions involved in the formation of the composite hydrogel of MFT, direct force measurements were conducted by employing an SFA. Muscovite mica, a typical aluminosilicate comprising [AlO(OH2)2]m and (Si2O5)n layers, was used as a model surface as it resembles the chemical composition of clay particles.21,33,34 A set of experiments were conducted to evaluate the Schiff base and electrostatic interactions between mica and BPEI, between BPEI and DFPEO. First, BPEI solution was injected between two mica surfaces, and the interactions between mica and BPEI were measured through driving the two surfaces to approach and separate from each other. The normalized force−distance curves are shown as black curves in Figure 2a. A moderate adhesion (3.8 mN/m) was measured between BPEI and mica when the two surfaces were separated from the contact state, evidenced by the abrupt “jump out” behavior. This attractive interaction was mainly ascribed to the electrostatic attraction between positively charged BPEI polymer chains and negatively charged mica surfaces. In order to study the contribution of DFPEO in the formation of gelled MFT, DFPEO solution was then injected between the mica surfaces. After injecting DFPEO solution, an enhanced adhesion (12.3 mN/m) was between DFPEO and BPEI coated mica surfaces (the red line in Figure 2a), which was about 4 times stronger than that between BPEI and mica surfaces. This enhanced adhesion between DFPEO and BPEI is attributed to the formation of Schiff base bonds between benzylaldehyde and amine groups, consistent with our previous study.33 The formation of Schiff base bonds was also confirmed by FTIR spectroscopy (see Figure S2). FTIR spectrum of the freeze-

(1)

Scanning Electron Microscopy (SEM). The microstructure of dried MFT fine particles with or without polymer treatment was examined using a Zeiss EVO LS15 EP-SEM equipped with LaB6 electron source with a resolution of about 100 nm at the highest magnification. It is equipped with a Bruker energy dispersive X-ray spectroscopy (EDS) system with a silicon drift detector with a resolution of 123 eV and a 10 mm2 window area. The samples were collected by freeze-drying to prevent unnecessary contamination or damages during preparation. Filtration Test. The treated or untreated MFT samples were kept still for 30 min prior to filtration tests. Each filtration test for MFT of a small quantity (100 mL) was conducted by a laboratory vacuum filtration set for 40 min under a constant pressure of 40 kPa. The cumulative volume of filtrate water was plotted as a function of filtration time. The formed filter cake was dried in an air-vented oven at 120 °C to determine the final solid content. The performance of filtration was evaluated in terms of initial filtration rate (mL/s), turbidity of filtrate water, and solid content (wt %) of the filter cake. The net water release was defined as the product of the volume of filtrate minus the volume of polymer solution added. Mechanical Measurement. The compressive stress−strain measurements were carried out using an Instron 5967 Testing Machine (Instron Corp., Norwood, MA, U.S.A.). The filtered composite hydrogels of MFT were compressed in air at room temperature at a rate of 0.1 mm/min. The compressive stress versus strain was recorded to determine the compression strength of the filtered composite hydrogel of MFT.

3. RESULTS AND DISCUSSSION Characterization of MFT. The compositions of the indigenous MFT were characterized according to Dean− Stark analysis results and it revealed that MFT is composed of 4.6 wt % bitumen, 31.7 wt % solids, and 63.7 wt % water (Table S1). The salinity of the MFT suspension is quite high and the major cation species in the solution are Na+, K+, Ca2+, and Mg2+ (see details for water chemistry of MFT in Table S2). The size distribution of suspended solids in MFT samples was measured and shows that 93% of tailing particles are smaller than 44 μm with the volume median diameter at 7.4 μm, and 40% of the particles are smaller than 1 μm (Figure 1a). The XRD analysis reveals that MFT solids consist of the major mineral phases of quartz, kaolinite and Illite, and the minor ones of K-feldspar and siderite. Figure 1b shows the FESEM image of dried tailing particles, which clearly demonstrated that the particles of MFT formed stacked thin plate layers and the solids of tailings formed flaky plates in the scale of ∼2 μm in diameter, and aggregated into different sizes of compact and sheetlike masses. Because of existentce of high concentration of platelike colloidal particles, the MFT 21612

DOI: 10.1021/acsami.9b05692 ACS Appl. Mater. Interfaces 2019, 11, 21610−21618

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ACS Applied Materials & Interfaces

Figure 2. Strong composite hydrogel of MFT derived from gelling MFT suspensions with BPEI and DFPEO. (a) Force−distance profiles (F/R vs D) between mica and BPEI, and between BPEI and DFPEO in aqueous solution, showing two distinct interactions involved in the formation of the composite hydrogel of MFT. (b) Strain sweep measurements of the composite hydrogel of MFT (BPEI concentration, 0.5 wt %; DFPEO concentration, 0.5 wt %) and original MFT (solid content, 31.7 wt %) at 25 °C (storage modulus G′ and loss modulus G″ as a function of the strain). Photograph (c) shows an untreated MFT that runs down the beaker when it is inverted, whereas photographs (d,e) show the composite hydrogel of MFT that can hold its own weight in the inverted beaker and is similar to the appearance of the swollen soil.

Figure 3. The dewatering properties of the composite hydrogel of MFT and untreated MFT. (a) Comparison of average CST results of tap water, the composite hydrogel of MFT and untreated MFT. (b) Filtration curves and (c) net water release of the composite hydrogel of MFT and untreated MFT (the same amount of 120 g) as a function of time under 40 kPa. (d,e) Images show the appearance of the corresponding filter cakes after 40 min filtration under 40 kPa.

dried composite hydrogel of MFT clearly shows the characteristic peak at 1640 cm−1 ascribing to the stretching vibration of Schiff base CN bond. Meanwhile, the characteristic peaks of BPEI at 3378 and 1590 cm−1, DFPEO at 1717 and 1268 cm−1, and MFT mineral particles at 3694 cm−1 and strong bands in the 1120−1000 cm−1 regime can be clearly identified. The above results clearly demonstrate that BPEI and DFPEO offer two different interaction mechanisms to form the composite hydrogel with MFT. These two interactions could synergistically enhance the mechanical property of the resultant composite hydrogel by forming a double-cross-linked network

and confer it with self-healing ability (discussed below), which was shown to occur in the following dewatering and consolidating treatment. Rheological measurements further demonstrate that the gelling MFT suspension with BPEI and DFPEO generates a strong composite hydrogel of MFT with a dramatically enhanced mechanical property. As shown in Figure 1b, strain amplitude sweep tests of pure MFT sample demonstrate a typical elastic response of a soft weak hydrogel. It possesses a relatively low storage modulus, G′, of 357 Pa higher than its loss modulus, G″ (see the blue line in Figure 2b), which is 21613

DOI: 10.1021/acsami.9b05692 ACS Appl. Mater. Interfaces 2019, 11, 21610−21618

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ACS Applied Materials & Interfaces

Figure 4. Self-healing properties of the composite hydrogel of MFT ensure low solid content in the released water. (a) Strain sweep measurements of a composite hydrogel of MFT at 25 °C (storage modulus G′ and loss modulus G″ as a function of strain) (left) and an immediate recovery from the 200% strain deformation (right). Photographs (b,c) show transparency of the filtrate of the composite hydrogels collected immediately after mixing (left) or the one after 30 min relaxation after mixing (right); (d) the effect of relaxation time after mixing on the solid content of the aforementioned corresponding filtrates in (b,c). (e) Force−distance profiles (F/R vs D) between mica surfaces and hydrogel of BPEI and DFPEO in aqueous solution as a function of contact time. The contact time was examined by keeping the two surfaces in contact for 0, 5, and 20 min.

contrast, the CST value of the untreated MFT is significantly higher (∼3700 s). To our knowledge, this CST value obtained from our treated MFT hydrogel is lower than all the previously treated MFT samples reported in the literature.3,19 This excellent dewatering performance of obtained composite hydrogel of MFT was further verified by the filtration test using a reported method.41 In brief, the MFT suspension or the composite hydrogel of MFT are loaded in a laboratory filter press and filtered under a constant gauge pressure of 40 kPa. The weight of filtrate is continuously monitored by an electronic balance and recorded via custom-programmed filtration software. The filtration curves shown in Figure 3b are obtained by plotting the cumulative volume of filtrate against the filtration time. The same amount of MFT and composite hydrogel of MFT (i.e., 110 g) was used for the filtration tests. The composite hydrogel of MFT demonstrated an excellent filterability. Within less than 2 min, more than 40 mL of filtrate was collected and then the filtrate volume leveled off after about 20 min. In contrast, the untreated MFT is barely filterable. As shown in Figure 3b, after more than 30 min, only 6 mL of filtrate was collected. This filtration result for untreated MFT is consistent with previous reports30,41 and is mainly attributed to the special three-dimensional card-house structure of MFT suspension. To investigate the role of DFPEO in the dewatering performance, we treated MFT suspension with BPEI only and performed the filtration test on the resultant mixture. As shown in Figure S4, MFT treated with BPEI only shows an inferior filterability when compared with the composite hydrogel of MFT treated with BPEI and DFPEO. It only collected around 20 mL of filtrate in 2 min and less than 40 mL of filtrate in 30 min, whereas the composite hydrogel of MFT could collect more than 40 mL of filtrate within less than 2 min and more than 50 mL in 30 min. Therefore, DFPEO plays an important role in the final dewatering performance of the treated MFT

consistent with the previous study on rheological measurement of MFT.35 Though the untreated MFT suspension displays a gel-like structure, it is too weak to hold its own weight: the suspension behaves like a liquid and starts to flow quickly when the beaker is inverted (Figure 2c). In contrast, after being treated by 0.5 wt % BPEI and 0.5 wt % DFPEO, the soft MFT turns into a cohesive composite hydrogel. Moreover, the obtained composite hydrogel can hold its own weight without apparent breakdown when the container is inverted (see Figure 2d and Figure S3 in the SI). Different from the established destabilization strategies for MFT such as paste technology and composite tailings,20 our strategy did not form big flocs so that no sedimentation for the whole treatment process was observed. Success in preparing this cohesive composite hydrogel of MFT is also verified by strain amplitude sweeps measurement. As shown in Figure 1b, the G′ value of the composite hydrogel of MFT increases dramatically from 357 Pa to around 30 kPa, 2 orders of magnitude higher than the untreated MFT (see the back line). It is also noteworthy that the composite hydrogel of MFT with enhanced mechanical property is very similar to a general soil swelled by water,36 which makes it very suitable for further dewatering and consolidation treatments (Figure 2e). Dewatering Properties of the Composite Hydrogel of MFT. The dewatering performance of the resultant composite hydrogel of MFT was studied by the batch method. We carried out capillary suction time (CST) measurements to assess the dewatering ability of the treated MFT (see Supporting Information). This measurement is a commonly used method to determine the filterability and the easiness of removing water from slurry and sludge in numerous environmental and industrial applications.37−40 A lower CST value indicates better dewaterability. The results in Figure 3a show that the CST value of the resultant composite hydrogel of MFT is very small (75 s), only 5 times higher than that of pure water (15 s). In 21614

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SFA chamber, followed by introducing the BPEI and DFPEO solutions in sequence. The whole system was then equilibrated for 30 min before force measurements. The SFA results clearly demonstrate that the interactions among polymers and clay particles in MFT are dependent on contact/relaxation time (see Figure 4e). When the surfaces were separated immediately after the approaching process, a robust adhesion of ∼11 mN/ m (mJ/m2) was observed, indicating that an instant and strong adhesion can be built up between mica and the two polymers after mixing. This adhesion can be gradually enhanced by increasing the contact time after surface approaching. With 5 min of contact, the adhesion increases quickly to ∼16 mN/m, finally reaching about 17 mN/m and levels off after contacting for over 20 min. These results are consistent with the rheology measurements and explain the time-dependent effect of filtration on the quality of the reclaimed water. With longer contact time (viz., increasing the relaxation time after preparation of the composite hydrogel), more fine particles can be rebounded together to the networked hydrogel via formation of more effective bonds, including electrostatic interactions and Schiff base bonds, between the fine particles or broken flocs, thus capturing more fine particles in the filtered MFT cake and significantly lowering the concentration of suspended solids in the release water. The reclaimed filtrate was used for preparing the two polymer solutions, which were applied to treat MFT sample. Similar results were obtained that a net water release of close to 50% could be achieved within 30 min of filtration (see Figure S6), indicating that combining gelling and filtration technology provides a powerful solution for recovering water for reuse. Consolidation by Mechanical Compression. Interestingly, the filtered cakes produced from the composite hydrogel of MFT possess a high compressive yield stress of above 1.5 MPa (see Figure S7). This strong mechanical property promoted the use of mechanical compression to further consolidate the filtered MFT cakes. Figure 5a shows a

and we attributed this improvement to the strong composite hydrogel with a double-cross-linked network by BPEI and DFPEO. Meanwhile, glutaraldehyde (GA), a commercially available dialdehyde, was used instead of DFPEO as a crosslinker for BPEI to prepare the composite hydrogel of MFT. Because of its low solubility in water, it failed in forming the robust composite hydrogel of MFT and exhibited a similar filterability as that treated by BPEI only. This is attributed to the insufficient cross-linking between BPEI and GA and dilution effect of GA solution on the MFT suspension. Therefore, it is important to design or select suitable watersoluble dialdehyde as cross-linkers for our system to prepare the strong composite hydrogel for rapid dewatering and consolidation of MFT. Interestingly, rapid water release from MFT could be achieved by filtration after the gelling treatment. As shown in Figure 3c, under a constant gauge pressure of 40 kPa, the net water release from the composite hydrogel of MFT dramatically increased to ∼40% in the first 10 min and reached above 50% after ∼30 min. After filtration, the composite hydrogel of MFT turned into a soil cake with a solid content of >50 wt % and some cracks were formed (see Figure 3d). In contrast, for untreated MFT in the first 10 min of filtration hardly any water could be removed. After ∼40 min of filtration, only less than 10% of water was filtered and the state of the remaining MFT was almost the same as the original one (see Figure 3e). Self-Healing Properties of the Composite Hydrogel of MFT. Another intriguing property about this composite hydrogel of MFT is that it possesses an intrinsic self-healing ability, which can form and reform a dynamic infinite crosslinked network to effectively trap fine particles of all kinds and scavenge the formed small flocs and particles under strong mechanical mixing, thus ensuring the low solid content of the released water. We characterized the self-healing property of the composite hydrogel of MFT using strain sweep measurements. As shown in Figure 4a, under a large-amplitude oscillatory force (γ = 100%, frequency ω = 10 rad/s), the G′ value decreases dramatically from around 30 kPa to less than 100 Pa, indicating that the composite hydrogel is turned into a quasi-liquid state. However, once the amplitude is decreased to a lower value (γ = 0.1%) at the same frequency, the G′ can be slowly restored to its initial value in 1 h, suggesting that the asprepared composite hydrogel can automatically heal and recover to a quasi-solid state. We ascribed this self-healing ability to the hydrogel systems we adopted for preparing the composite hydrogel (see Figure S5). This intrinsic self-healing property confers the composite hydrogel with the capacity to collect fine solid particles and reconnect broken flocs of small size after mechanical shear. As a result, the relaxation time after preparation of the composite hydrogel plays a critical role in the final quality of the reclaimed water by filtration. When the filtration was conducted immediately after mixing BPEI and DFPEO with MFT, the released water was translucent due to a high solid content of 0.5 wt % (see Figure 4b,d); in contrast, when 30 min of relaxation time was applied after mixing of BPEI and DFPEO with MFT, a transparent water with a superior low solid content of 5 ppm could be obtained (see Figure 4c,d). To achieve further understanding of this time-dependent behavior, a model system was built to quantify the interactions involved in the formation process of the composite hydrogel of MFT. In brief, two mica surfaces were first mounted in the

Figure 5. Mechanical compression rapidly consolidates filtered MFT cake. (a) A pressing setup composed of a table clamp and two steel plates was used to squeeze out the remaining water from MFT filter cakes. The high pressure of 2 MPa was continuously provided by tuning the gap between two steel plates with the hand wrench. (b) The filtered MFT cake from 120 g of MFT and (c) the pressed MFT soil after 1 h compression. (d) Pressed MFT soil from 500 g of MFT after filtration and compression.

homemade mechanical pressing setup for consolidation, where the filtered MFT cakes were introduced between a table clamp and a high pressure was continuously added on the MFT cakes to squeeze out the remaining water. It is found that this procedure can remove most of the remaining water from the cakes and generate a stackable soil for direct land reclamation. Within 1 h of compression under ∼2 MPa, the cake was continuously dewatered with its volume shrunk more than 3 times as compared to the original filtration cake. The total net water release reached 80%, and the compressed cake 21615

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naturally abundant polysaccharides such as chitosan for BPEI and water-soluble cellulose derivatives with aldehyde groups for DFPEO, because these polymers are relatively cheap and environmentally friendly. Meanwhile, their synthesis procedures are simple and feasible.31,42

contained >70 wt % solids (see Figure S8). Figure 5b,c shows two typical MFT filter cakes before and after compression. The developed strategy can be readily scaled up. As shown in Figure 5d, a similar result was obtained for treating 500 g of MFT in that the resulted soil cake had a solid content of ∼70 wt % with a corresponding total net water release of ∼80%. Therefore, combining gelling MFT suspension via a selfhealing hydrogel with the consecutive filtration and mechanical compression, a facile and feasible strategy has been developed to rapidly dewater and consolidate concentrated colloidal suspensions as schematically illustrated in Figure 6. This

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CONCLUSIONS This work has developed a novel, facile, and feasible strategy to rapidly dewater and consolidate concentrated colloidal suspensions via a self-healing hydrogel. Mature fine tailings from Canadian oil sand production are used a model system for the concentrated colloidal system, which is well-known as an extremely challenging waste slurry that occupies over 220 km2 land area in northern Alberta. After hybridization, the resultant composite hydrogel possesses excellent dewatering and self-healing properties. These characteristics open up the opportunity to rapidly release high quality process water from the concentrated colloidal suspensions by filtration and mechanical compression. Within ∼30 min, more than 50% of water was released from the composite hydrogel by filtration under 40 kPa. In addition, its intrinsic self-healing ability prevents the breakup of the formed flocs and captures the fine particles during mechanical mixing, thereby recovering the process water with a low solid content at ppm level. Moreover, the filter cake possesses a strong mechanical property, which facilitates its further dewatering/consolidation under mechanical compression. A total water release of 80% and stackable soils with a solid content >70 wt % suitable for direct land reclamation can be achieved under mechanical compression at ∼2 MPa. Our results demonstrate a novel and facile strategy to effectively tackle the challenging wastewater slurry issues with concentrated colloidal suspensions commonly encountered in many industrial activities and provide useful insights into the development of advanced materials with environmental applications.

Figure 6. Schematics of rapidly dewatering and consolidation of MFT suspension by combination of gelling with a self-healing hydrogel and the consecutive filtration and mechanical compression.

strategy can rapidly dewater and consolidate the MFT suspension, transferring it into a stackable soil suitable for direct deposit and simultaneously reclaiming most of the processed water from MFT to be ready for recycling. Compared to the existing technologies for MFT, such as paste technology and composite/consolidated tailings, the method in this work can be directly applied to the indigenous MFT without any pretreatment and can generate a depositable/stackable soil with high solid content of >70 wt %. Meanwhile, it outperformed the reported dual polymer flocculation strategy in the following aspects.21,29,30 First, the final dewatering performance of our strategy is insensitive to the applying condition such as mixing or shear strength, whereas that of the dual polymer flocculation strategy in previous studies is highly dependent on the applying condition, viz, applying different shearing strength on mixing MFT with polymers can produce different dewatering results. Second, our strategy produces a strong and cohesive hydrogel of MFT that is suitable for the facile filtration technology at a low pressure of 40 kPa for rapid water release. Nevertheless, the mechanical strength of the treated MFT by the conventional dual polymer flocculation in previous studies is relatively weak and thus the treated MFT sludge can only be dewatered at a higher pressure like 620 kPa by filter press.29 Third, the mechanical strength of the filtered MFT solid in this work is very high (>70 wt %), which can be further consolidated by compression and transferred into a depositable/stackable soil, whereas that of the filtered MFT cake by the dual polymer filtration in previous studies is still too weak for further consolidation. It is noted that the amount of polymers needed in our method is slightly higher than that using other reported methods (see Table S3). We attribute this to the special double cross-linked enhancement mechanism adopted for MFT. As shown in previous research,26−28 the reported strong double cross-linked hydrogels with a high mechanical strength usually possess a high polymer concentration. To reduce the cost and increase environmental sustainability of our method, future study will focus on the possibility to replace these polymers with the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05692. Polymer synthesis routes and more characterization data on the composition of MFT and the self-healing and dewatering performance of the composite hydrogel of MFT (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.G). *E-mail: [email protected]. Phone: +1-780-492-1044. Fax: +1-780-492-2881 (H.Z.). ORCID

Bin Yan: 0000-0001-5426-6179 Hongbo Zeng: 0000-0002-1432-5979 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.Y. acknowledges the financial supports of the National Natural Science Foundation of China (under Grant 21876119), Sichuan Science and Technology Program (under Grant 2018GZ0381), and the Fundamental Research 21616

DOI: 10.1021/acsami.9b05692 ACS Appl. Mater. Interfaces 2019, 11, 21610−21618

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ACS Applied Materials & Interfaces

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Funds for the central Universities (under Grant YJ201732), and H.Z. acknowledges the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chairs program.

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