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Article

Treatment of Oil Sands’ Mature Fine Tailings Using Advanced Wet Air Oxidation (WAO) and Wet Air Peroxide Oxidation (WAPO)

by
Muhammad Faizan Khan
1,2 and
Haitham Elnakar
3,4,*
1
Department of Civil & Environmental Engineering, University of Alberta, 9211 116 St. NW, Edmonton, AB T6G 1H9, Canada
2
Environment and Protected Areas, Regulatory Assurance Division, Government of Alberta, 2938 11 St. NE, Calgary, AB T2E 7L7, Canada
3
Department of Civil and Environmental Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
4
Interdisciplinary Research Center for Construction and Building Materials, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1518; https://doi.org/10.3390/catal12121518
Submission received: 23 October 2022 / Revised: 16 November 2022 / Accepted: 20 November 2022 / Published: 25 November 2022
(This article belongs to the Section Environmental Catalysis)
Browse Figures
Versions Notes

Abstract

:
Mature Fine Tailings (MFT) generated from oil sands processing represent a growing environmental issue, as settling of these tailings’ emulsion can take decades, increasing the risk of the toxic material’s leaching if left untreated. This study uses advanced wet air oxidation (WAO) and wet air peroxide oxidation (WAPO) to break down the MFT emulsions for faster settling. Three oxidation time intervals (5, 15, and 30 min) were investigated using compressed air and hydrogen peroxide in a pressurized vessel of 3.1–3.4 MPa internal pressure and at 200 °C temperature. The results showed that the WAO process was able to break the MFT emulsion, release trapped water, and recover residual bitumen. The WAPO process was much faster in breaking the emulsion; however, the presence of extra oxidants also resulted in the degradation of the residual bitumen. The 5 min oxidation time interval was found to be sufficient in breaking emulsions, separating water from soil particles, and recovering residual bitumen under the tested conditions. The oxidation process proved to be efficient by degrading all inorganic carbon, whereas 70% of the dissolved organic carbon in the recovered water after oxidation comprised only low molecular weight biodegradable hydrocarbons. Therefore, the WAO process was capable of breaking the MFT emulsions and allowing a faster settling of these tailings, with the added benefit of recovering residual bitumen.

1. Introduction

Oil sands or bituminous sands are unconventional petroleum deposits that contain bitumen mixed with sand, clay, and water [1]. It has been reported that for every barrel of oil extracted from the sands, the oil industries use around three barrels of fresh water [2]. The resulting process water contains sand, silt, and clay from the oil sands and is considered to be unsafe for discharge to the receiving environment; consequently, it is dumped into tailing ponds along with the underflow tailings from the separation vessels [3]. The oil sands tailings ponds consist of four distinct layers: oil sands process-affected water (OSPW), fluid or thin fine tailings (FFT or TFT), mature fine tailings (MFT), and sand [4]. When fresh, MFT are typically comprised of 30–38% solids (including fine clay particles), 3–4% of bitumen, and the rest is water; as it ages, the solids increase to 60% [5]. In MFT, clay minerals are made up primarily of kaolinite, muscovite, and others in minute amounts [6]. The OSPW is initially recycled in the extraction process. However, constant recycling increases its organic and inorganic content [7], until reaching a point where it is deemed hazardous to reuse [8]; then it is dumped back into the tailings pond, thereby making the MFT rich in toxic organic compounds. Within MFT, the soil particles are in the form of a gel-like substance, where residual bitumen appears as blobs forming inter-particle bridges and trapping pore water in between. This leads to the problem of settling these fine clay particles, which can take decades to settle down, thus forming large tailings ponds. Polymers, dual polymers, and flocculation processes have been used to accelerate the dewatering of MFT to consolidate the particles into a solid material [9,10,11], but increased bitumen content in MFT can lead to decreased polymer flocculation [12]. Furthermore, the consolidated MFT still contains ~5% of residual bitumen [13], which can leach off, releasing toxic organics to the environment if applied untreated. Despite all the research efforts, there is still no optimal way to treat MFT.
Over the last two decades, advanced oxidation processes (AOPs) have gained increasing attention for their ability to handle several recalcitrant pollutants [14,15]. Among the various AOPs available, wet air oxidation (WAO) appears to have more advantages over others to degrade refractory effluents [11,16]. In this oxidation process, toxic organic contaminants can easily be reduced to carbon dioxide and water or to a more biodegradable state. Acute conditions of 150–320 °C and pressures ranging from 0.5–20 MPa are used in WAO in order to keep the waste in the aqueous phase [17,18]. High pressure keeps the reactions in the aqueous state by keeping the applied pressure higher than the vapor pressure [19]. WAO can treat a variety of contaminants at the same time, and the destruction of pollutants takes place in the liquid phase, thus reducing any air pollutants. This technology is well-suited for wastewaters with suspended solids that are too diluted for incineration and not biodegradable, such as sludge wastes [19] including oily sludges [20,21,22]. Additionally, WAO has also previously been applied to the treatment of a variety of industrial wastewaters including those of pulp and paper [23], dyes [24], pharmaceuticals [25], olive oil mills [26], and the petrochemical industry [27] as well as phenol and its derivatives [28]. In wet air peroxide oxidation (WAPO), hydrogen peroxide (H2O2) is added to enhance the oxidation of organics [29,30]. This is most commonly applied in combination with a catalyst in catalytic wet air oxidation (CWAO) to lower the operative costs by reducing the harsh conditions of WAO [16,31].
The presence of approximately 38% solids and some bitumen in MFT gives it a sludge-like characteristic that would be difficult to treat biologically. Consequently, the objective of this study is to investigate the feasibility of WAO to break the MFT from the tailings pond in order to consolidate the solid particles, maximize the recovery of the water, minimize the process time, and produce an environmentally safe residue. This study will also investigate the possibility of natural minerals in MFT acting as potential catalysts during the oxidation process, by making the conditions of the process milder. To the best of our knowledge, WAO has never been applied before for the treatment of MFT.

2. Results

2.1. Effect of Temperature, Pressure, and Oxidant

Preliminary investigations on the oxidation of synthetic clay slurries at 120 °C and internal pressure of 1.9–2.1 MPa resulted in a rapid consolidation of the clay particles (Figure S1). Higher temperature and pressure (200 °C and 3.1–3.4 MPa) showed significant breakage for a 25% (v/v) slurry of an MFT emulsion with a layer of froth recovered on the top after WAO (Figure 1a). Although less froth was visible after WAPO, the addition of H2O2 showed approximately 50% faster particle consolidation (Figure 1b). The internal pressure increased faster with increasing temperature in the presence of H2O2.
Separation of water from particles showed about 48% water was recovered after 210 min of WAO, as shown in Figure 2a. However, more than 50% of the recovered sample was water after WAPO for 30, 60, and 90 min (Figure 2b). A layer of foam was also produced above the extracted water.

2.2. Bitumen Recovery

Bitumen recovery in terms of froth collected from the surface during WAO showed gradual froth reduction after each cycle, with the exception of the second cycle of 5 min intervals, as shown in Figure 3a. Thirty-minute intervals produced approximately 93% froth after 5 cycles, whereas almost 92% was recovered during five-minute oxidation cycles. A WAOextended cycle resulted in the collection of most of the residual bitumen as froth after 180 min (Figure 3b). The recovered froth was completely soluble in Dichloromethane (DCM) as opposed to water and methanol.

2.3. Water Chemistry

The released water after oxidation showed high transmissivity. Water analysis showed that the dissolved organic carbon (DOC) concentration increased with residence time during WAO, whereas the inorganic carbon decreased (Figure 4a). An 80% higher DOC was recorded with WAPO (Figure 4b) during 90 min of oxidation, which was similar to the DOC at 180 min during WAOextended (Figure 4c). The total inorganic carbon (TIC) was reduced by 66.4% using WAO, while a 92.6% reduction was achieved after WAPO (Figure 4a,b). TIC was eliminated with 99% removal after WAOextended (Figure 4c). Acetic and formic acid concentrations increased along with DOC; however, oxalic acid concentrations were low during WAO (Figure 4a). WAPO resulted in 66% (acetic acid) and 48% (formic acid) higher concentrations (Figure 4b). Even higher concentrations were recorded with WAOextended (Figure 4c). Possible separation layers of treated MFT samples into froth, water, and sediment are shown in Figure 5.
High concentrations of sulfate and calcium ions were detected in the extracted water after WAO. Concentrations of sulfate (100%) and calcium (>75%) increased during WAPO, whereas sodium increased briefly and then declined (Figure 6a,b). WAOextended showed increased sulfate (126%) and calcium concentrations compared to WAO, while sodium was reduced by 30% (Figure 6c). During WAO, chloride and nitrite ions were reduced by 5.8% and 34.6%, respectively (Figure 6c). Similarly, WAPO caused chloride and nitrite ions to reduce by 43.7% and 89.8%, respectively (Figure 6d). Increased concentrations of potassium and magnesium ions were recorded by both the WAO and WAPO processes, whereas magnesium ions started precipitating after 30 min of WAPO. The WAOextended process reduced chloride and nitrite ion concentrations by 51% and 100%, respectively, while potassium and magnesium ions increased (Figure 6f).

2.4. Particle Characterization

After oxidation, the percentage of hydrocarbons remaining in the settled solid particles was determined by ignition at 900 °C. The hydrocarbon fractions decreased with the oxidation time interval. Less than 30% of the hydrocarbons were left in the residue particles after 210 min of WAOextended (Figure 7).

2.5. Consolidated Particle Surface

SEM analysis recorded the surface area of consolidated particles before and after oxidation. The surface area was found to increase after WAO and WAPO (Figure 8). X-ray diffraction (XRD) analysis showed sharper peaks with reduced intensity for the samples treated with WAPO as compared to WAO (Figure S3). The majority of the high peaks were identified as quartz, while the shorter peaks belonged to kaolinite, muscovite, and halite.

3. Discussion

The oxidation conditions of 1.0 MPa compressed air and 120 °C temperature with an internal pressure of 1.9–2.1 MPa were enough to cause the fast sedimentation of the particles in the synthetic clay slurry, as shown in Figure S1. These conditions allowed particle sedimentation after 5 min of oxidation, which indicates that oxidation had changed the surface chemistry of particles. However, the MFT slurry did not show the same consolidation results under similar conditions. Increasing the temperature to 200 °C and internal pressure to 3.1–3.4 MPa, along with compressed air of 0.7 MPa, disrupted the MFT emulsion (Figure 1a). Temperature is an important factor in the oxidation of organic compounds. Increasing temperature not only yields and produces higher COD removal ratios but also achieves faster degradation of organic compounds. This is because higher temperature increases the rate constant of the radicals, enhancing their reaction with organic compounds towards degradation [20,21,22]. At 200 °C and 3.4 MPa, the oxidation caused desorption of the residual bitumen that was stuck to soil particles, releasing trapped water and a layer of bitumen froth on top of the water layer (Figure S2). Due to its non-polar nature, bitumen is insoluble in water; once released from particles, it moves upward to the surface, while the clay and sand particles consolidate under the influence of gravity.
Higher temperature causes the instantaneous decomposition of hydrogen peroxide [32], generating large amounts of hydroxyl free radicals that can degrade residual organic bitumen faster and release the trapped water. This was observed during WAPO in Figure 1b, where oxidative degradation of bitumen by surplus hydroxyl radicals might have resulted in degrading the residual bitumen into dissolved organic carbon (DOC) in the released water; hence, there was less visible froth on the surface during WAPO (Figure 1b). A rapid increase in internal pressure was noticed, which perhaps was due to the formation of oxygen from the decomposition of hydrogen peroxide. The presence of a foamy layer (Figure S2) on top was due to the reaction of hydrogen peroxide with the naturally available surfactants, such as naphthenic acids, in MFT. They promote the desorption of bitumen from the particles with the formation of foamy bubbles [33].
WAPO resulted in less froth, but the extraction of trapped water was much faster in these samples. Figure 2b shows that as oxidation proceeds with time, the percentage of released water from the emulsion increases. This is due to the attack by hydroxyl radicals on the residual bitumen. The more hydroxyl radicals formed at high temperatures, the faster the separation is observed. The presence of organic bitumen on particle surfaces results in higher viscosity and constant suspension [34]; however, oxidation by hydroxyl radicals removes the attached bitumen, causing faster separation. High temperature and pressure also lower the density of water, causing it to move upwards. Hydroxyl radicals in the upward-moving water degrade and detach residual bitumen, while the particles, free from residual bitumen, fall downwards. WAO showed a slower release of water, indicating there were fewer radicals produced by WAO, at a much slower pace compared to WAPO.
The oxidation process degrades the bitumen attached to soil particles through charge imbalances. The non-polar detached bitumen floats to the surface as a thick layer of froth. Figure 3a reflects the percentage of froth recovery after each process time cycle, and the difference between the three process times appears quite minimal. A difference of approximately 1% between five cycles of the 5 and 30 min process times means that 5 min is enough to detach and degrade the residual bitumen during WAO. Bioresistant organics in wastewaters have been degraded to eliminate toxicity using WAO before [35,36,37,38]. In this study, the control sample was estimated to contain about 17.88% of volatile organic hydrocarbons (dry weight of MFT). After 210 min of WAOextended, 5% (wt.) froth was collected. A 30 min cycle with no oxidant produced around 1.5% (dry wt. MFT) less froth compared to WAO. This means that WAO conditions can produce radicals without the supply of any oxidant, through the dissociation of water molecules as well as the presence of oxygen from atmospheric air already present in the reactor. Oxidation without adding any oxidant occurs at a very slow rate, which indicates a longer process time is required to achieve complete oxidation. The solubility of recovered froth in DCM but not in water proved that it is composed of hydrocarbons and had an odor similar to gasoline.
Increasing DOC in extracted water indicates the degradation of organic bitumen from the breaking of emulsion by oxidation. As oxidation/residence time and temperature increase, the concentration of dissolved carbon increases as more organic bitumen is degraded and dissolved in water [39,40]. Molecular oxygen (in the air) absorbs energy from heat and attacks the weakest C–H bond to give two radicals: an organic radical (R•) and a hydroperoxyl radical (HO2•) (Equation (1)) [35,41]. This reaction has a high rate constant and is most likely to be the initial reaction for the degradation pathway.
RH + O2 → R• + HO2•
A series of propagation reactions ensues, resulting in the formation of hydroxyl radicals that attack the organic compounds through hydrogen abstraction reactions (Equation (2)) [41,42].
RH + OH• → R• + H2O
Due to the low activation energy of this reaction, the reaction rate is rapid and degrades some of the organic bitumen all the way to dissolved organic carbon. The decomposition of hydrogen peroxide provides access to more hydroxyl radicals; consequently, a faster disintegration of organic compounds results in the sudden increase in DOC during WAPO. However, it is noted during WAOextended that after 180 min, the gradual increase in DOC is starting to level off (Figure 4c). Perhaps all the bitumen that had initially oxidized and become dissolved in the water is now being further oxidized and reaching its limit, while all the organic bitumen from the MFT emulsion had been extracted out either as froth or dissolved in water. From this point onwards, it is possible that the level of DOC would start to decline, as further oxidation will reduce DOC concentrations in water. The concentration of DOC after 180 min of WAOextended was similar to the DOC 90 min after WAPO. This shows that hydrogen peroxide resulted in fast and efficient oxidation.
In contrast, the inorganic carbon was reduced during oxidation. The reduction of inorganic carbon by WAO causes the conversion of carbonates to carbon dioxide. While 180 min of WAOextended provided >90% TIC reduction, this was achieved in 90 min with WAPO, indicating the oxidative power of hydroxyl radicals. Hydroxyl radicals degrade the carbonates and bicarbonates to carbonate radicals (Equations (3) and (4)), which are later mineralized to end products (Equation (5)) [43].
OH• + HCO3− → •CO3− + H2O
OH• + CO32− → •CO3− + OH−
OH• + •CO3− → Products
Further analysis showed the presence of acetic and formic acid in the extracted water, which indicated that the oxidation mechanism was rapid under the given conditions and led straight to low molecular weight carboxylic acids. The organic compounds could be ring structures with long carbon chains, and a double bond is easier to break into a single bond. Therefore, it is possible that due to the oxidation conditions provided, all the ring structures and double bond intermediates must have been broken down instantly due to the oxidation conditions. Another explanation could be that the ring structures were removed during froth removal, and the carbon chains were degraded and dissolved in the water in the form of acetic acid and formic acid, as shown in Figure 5. This leads to the accumulation of acetic acid and formic acid in the extracted water as the last stage of oxidation for the allotted residence time. Less-visible froth with the addition of H2O2 indicates the oxidation power of hydroxyl radicals to degrade the ring compounds as well.
The increasing concentration of these acids with the increase in residence time indicates the degradation of the organic bitumen attached to the clay particles. The reason behind having very low concentrations of oxalic acid could be because of the strong oxidation conditions, along with the naturally occurring catalysts in the MFT sample. These conditions were able to oxidize the organic compounds faster through the degradation pathway towards mineralization; therefore, fewer oxalic acids were formed [44].
The higher the temperature of oxidation, the greater number of organic acids will be produced [45,46,47]. Acetic acid is the last stage of degradation before carbon dioxide and water. Figure 4 revealed the concentration of acetic acid to be higher in each process, which means that most of the organic compounds have been degraded by WAO and WAPO to acetic acid. The reason why the acetic acid concentration was increasing faster than the formic acid concentration could be the difference in the rate of their oxidation pathways. At temperatures, 300 °C or below, acetic acid is quite resistant to WAO and is found as a refractory end product [18,38,48,49,50]. It was estimated that about 70–80% of DOC in extracted water was low molecular weight carboxylic acids.
The difference between total DOC and total carboxylic acids is roughly around 23% after WAOextended (Figure 4c). This 23% of the dissolved compounds could be natural organic matter (NOM). These NOMs are part of oil sand ores, are not easily separated from the soil particles after the extraction process [51], and could be still present in the MFT sticking to the clay particles. They are present as insoluble organic matter (IOM) (insoluble in organic solvents) and are associated with inorganic minerals in the oil sands. It was reported that this organic matter could be the reason for the incompressibility of oil sands tailings [52]. WAO oxidizes and detaches these organic materials from the minerals and is later dissolved in water as low molecular weight end products with the yellowish-brown color of the extracted water.
Increasing oxidation time intervals released sulfate, calcium, potassium, and magnesium ions into the extracted water from the emulsified MFT (Figure 6). Higher concentrations were detected with WAPO, which shows the faster breakdown of emulsion. Nitrites were 100% reduced after WAOextended (210 min) and >90% with WAPO (90 min). Decreasing sodium and chloride concentrations proves that the salinity in the treated sample water is being reduced by oxidation. An increase in magnesium ions by H2O2 addition is due to the release of ions from the degradation of clay particles, but sedimentation is observed later as the concentration declines. Potassium is quite similar in all trials with very small increments. The presence of Sodium is from the Clark hot water extraction process, where NaOH is added during froth recovery, while calcium, magnesium, and ferrous sulfate were also reported in bituminous sands from Northern Alberta [53]. Bitumen is composed of hydrocarbons containing sulfur compounds and other inorganic ions such as chloride, nitrate, calcium, etc. [54,55,56], whereas potassium and magnesium can be found in the clay particles. Therefore, the presence of these ions in the extracted water after oxidation is an indication of organics degradation.
Hydrophobic surfaces on the clay particles from the tailings have the tendency to form inter-particle bridging through residual bitumen, giving the tailings a gel-like consistency that resists consolidation. The presence of humic and fulvic substances on the particle surfaces contributes to hydrophobicity [57,58]. Figure 7 shows a reduction in hydrocarbon percentage from the consolidated particles as oxidation proceeds. This indicates that the residual bitumen bridging these particles together has been either extracted out as froth or dissolved in water as DOC by oxidation, allowing particle sedimentation.
Kaolinite was reported as the dominant clay mineral in MFT [59,60]. Similarly, hydrometer analysis showed around 70% clay, 29% silt, and 1% sand in the MFT used in this study. The majority of the clay particles in the settled solids were kaolinite, as observed under XRD analysis. Sharper peaks after WAPO indicate the liberation of clay minerals from the emulsion as the surface becomes bitumen-free. As more clay particles become released from the bitumen-based emulsion, the peak becomes sharper. Nevertheless, WAPO also causes the degradation of clay particles, as some peaks had reduced intensity.
Figure 8 reveals the effect of oxidation on the particle surfaces during the WAO and WAPO processes. Under oxidation, hydroxyl radicals attack the residual bitumen on the particle surface to desorb and degrade it. In this process, the radical’s scavenging effect on the surface causes the degradation of particles as well, resulting in increased surface area. WAPO produces higher concentrations of hydroxyl radicals; hence, the effect is much more rapid, as depicted by 5 min of oxidation compared to 30 min of WAO. As more surface area of the particles is exposed, improvement in the natural catalytic properties of the minerals can also take place, which can further enhance the oxidation mechanism.
Further evidence of changes in surface chemistry was supported by XPS analysis. The intensity of alkane bonds (bitumen) increased as oxidation proceeded but reduced considerably after 210 min of WAOextended, as shown in Figure S4. The increase in the intensity of alkane bonds (C-C, C-H) during the early stage of oxidation was due to the breakage of MFT emulsion. As the emulsion breaks, more bitumen was exposed; hence, the intensity increased. The appearance of a carbonyl bond (C=O) was assumed to be from the unsaturated hydrocarbons formed during oxidation, which suggests that the alcohol intermediates had been converted to aldehydes and later to carboxylic acids during oxidation. The ester bonds (O-C=O) after 90 min could indicate the precipitation of the carboxylate salts of the carboxylic acids, which shows that some carboxylic acids are being converted to their respective salts during oxidation. On the contrary, Fischer esterification can also take place in the presence of hydrogen ions, leading to the formation of esters. After 210 min, the reduction in the intensity of alkane bonds indicates the degradation of residual bitumen. As more of the surface is exposed to hydroxyl radicals, more bitumen is degraded and desorbed; after 210 min of oxidation, most of the residual bitumen is either extracted out as froth or dissolved in water as DOC, resulting in bitumen-free particles.

4. Materials and Methods

4.1. Materials

The MFT used in this study were obtained from the tailings pond from Northern Alberta, Canada, and were stored at 4 °C. The WAO system fittings, connectors, and regulator were purchased from Swagelok Edmonton Valve & Fitting Inc (Edmonton, AB, Canada). A ferromagnetic steel reactor (230 mL, 1/8″ wall thickness) was manufactured with a pressure-holding capacity of 20.7 MPa. OMEGA’s type-K Thermocouple (Norwalk, CT, USA) and Pressure Gauge from WIKA Instruments Ltd. (Edmonton, AB, Canada) were used to observe the conditions. The reagents, including H2O2 (50%), and glassware were purchased from Fisher Scientific (Ottawa, ON, Canada). An induction cooktop was used as the heating device for the experiment. Compressed air (extra dry, O2 19.5–23.5%) was obtained from Praxair Canada Inc. (Edmonton, AB, Canada).

4.2. Experimental Method

A preliminary investigation of WAO conditions was conducted using synthetic emulsions of commercial kaolin, bentonite clay, and sand 30% (w/w) in deionized water to mimic the heterogeneous characteristics of MFT, apart from residual bitumen. The silt was commercially unavailable and was not used in these emulsions. Based on the preliminary results, a 25% MFT slurry solution was prepared with a total volume of 150 mL v/v (75% water). Two sets of batch scale experiments were conducted on 25% MFT slurries: wet air oxidation and wet air peroxide oxidation with the addition of 5 mL hydrogen peroxide (50%). The reactor was filled with the slurry sample manually and then connected to the system.
Three oxidation time intervals were investigated (5, 15, and 30 min), and each time three samples were collected for three individual run cycles in sequence, as shown in Table 1. The 5 min third cycle (III) means the sample was initially treated for 5 min; the effluent was collected to remove the floating froth on top; this effluent was placed back into the reactor for the second run cycle, which lasted another 5 min; and, subsequently, a third run cycle lasted for 5 min. Similar samples were collected at 15 min and 30 min oxidation time intervals. In addition, 0.7 MPa of compressed air was introduced as the primary oxidant before every cycle. Each sample was treated in the reactor under pressure between 3.1–3.4 MPa and heated to about 180–220 °C by means of induction heating. A similar protocol was followed for WAPO batch-scale studies. For each oxidation time interval, 5 mL of hydrogen peroxide (50%) was added at the start of the first run cycle. Based on observation and analyzed data, a WAOextended batch-scale study for 30 min of retention time was carried out. Subsequent run cycles were carried out up to 7 cycles for a maximum of 210 min retention time.

4.3. Analytical Methods

Oxidized MFT samples had separate liquid and solid layers. Some of the treated samples needed centrifugation at 10,000 rpm for 2 min in Sorvall LYNX 4000 Superspeed Centrifuge (Fisher Scientific, Canada). OrionTM 136S and Accumet® 15 were used for conductivity and pH measurements (Fisher Scientific, Canada). An Isotemp Furnace Oven (Model 550-126; Fisher Scientific, Canada) was used for overnight drying at 110 °C for dry weight analysis and later ignited in a furnace (Fisher Scientific) at 900 °C for 30 min to measure volatile organic compounds according to standard methods. For total organic carbon (TOC) measurements, the supernatant was filtered against 0.45µm filters (PTFE, Fisher chemical, Canada) and diluted 5x with Milli-Q water (18.2 MΩ). Non-purgeable organic carbon (NPOC) and total inorganic carbon (TIC) of each sample were measured using TOC-LCPH/CPN Analyzer equipped with ASI-L Autosampler from Shimadzu Corporation (Kyoto, Japan).
Dionex ICS-2100 equipped with an AS-AP autosampler from Thermo Fisher Scientific was used to analyze the anions and organic acid concentrations in the oxidized 25% MFT samples. All samples were filtered using 0.2 µm pore size syringe filters (PTFE, Fisher chemical, Canada) and diluted 10× using Milli-Q water (18.2 MΩ). Glassware was acid-washed prior to the storage of samples. An inductively coupled plasma mass spectrometry (ICP-MS, Perkin Elmer SCIEX ELAN 9000, Waltham, MA, USA) instrument was used for the analysis of cations and trace metal elements in the extracted liquid effluent. The samples were diluted 100x using a 1% nitric acid solution (HNO3; Trace Metal Grade, Fisher Scientific, Canada).
The X-ray diffraction (XRD) instrument used for the mineral composition of consolidated solids was the Rigaku Ultima IV unit (Tokyo, Japan) with a cobalt tube of 38kV and 38mA. Samples were run using a top-pack mount at a speed of 2-degrees 2-theta per minute with a step size of 0.02 degrees from 5 to 90 degrees. Data interpretation was done using JADE 9.1. The samples were dried in an oven overnight at 110 °C and crushed to a fine powder before placing them into the instrument. Surface analysis for the consolidated sediments was analyzed using a Zeiss scanning electron microscope (SEM, Oberkochen, Germany). Before placing the samples in SEM, the powdered samples were stuck on double-sided carbon tape and coated with carbon. X-ray photoelectron spectrometer (XPS) measurements were performed by AXIS 165 spectrometer (Kratos Analytical, Stretford, UK) to identify the chemical bonds in the sediments after treatment with WAOextended. Powdered samples were used for XPS measurement as well.

5. Conclusions

In summary, the following conclusions can be drawn from this study:
  • A temperature of 200 °C and internal pressure of 3.4 MPa can sufficiently break emulsions in MFT and separate bitumen, water, and particles using 0.7 MPa of air as the oxidant. This is in addition to the natural catalyst properties of clay minerals.
  • The addition of hydrogen peroxide provides faster separation of particles and water, although most of the bitumen is degraded and dissolved in water due to fast oxidation reactions by access of hydroxyl radicals.
  • Compared with 30 min, the 5 min oxidation time interval was found to be the optimum interval to recover froth. In addition, 5% (wt.) of the froth can be recovered by WAO in 210 min.
  • DOC increases with the oxidation time interval due to the degradation of organics, while inorganic carbon is completely oxidized. More than 70% of the DOC in the recovered water is composed of low molecular weight carboxylic acids, which are environmentally safe and easily biodegradable. Faster release of dissolved anions and cations can be accomplished with the aid of hydrogen peroxide to reduce the process time.
  • Consolidated particles show a reduced amount of bitumen, calcium, and sulfate ions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121518/s1. Figure S1: Effect of temperature and pressure on (a) 30% (w/w) kaolinite clay slurry at 120 °C, 1.0 MPa compressed air, and 1.9–2.1 MPa internal pressure; Figure S2: Effect of oxidant on emulsified MFT; Figure S3: XRD analysis of control (1), 5 min (2), 5 min with H2O2 (3), 15 min (4), 15 min with H2O2 (5), 30 min (6), and 30 min with H2O2 (7) oxidized sediment after overnight drying at 110 °C; Figure S4: X-ray photoelectron spectrometer analysis of sediment of control, 30 min, 60 min, and 90 min samples displaying chemical bonding of carbon bonds.

Author Contributions

M.F.K.: experimentation and writing—original draft preparation; H.E.: writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

The author at KFUPM acknowledges the support of King Fahd University of Petroleum & Minerals and the Interdisciplinary Research Center for Construction and Building Materials to this publication.

Data Availability Statement

Not applicable.

Acknowledgments

M.F.K. would like to express their sincere gratitude to Leonidas A. Pérez-Estrada for their research support during their time at the University of Alberta. Special thanks to Roland Jaikaran for their assistance in acquiring materials for this research and to Chen Liang for their valuable help and assistance during the experimental work.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01518 g001 550
Figure 1. Effect of temperature, pressure, and oxidant on (a) 25% (v/v) MFT slurry at 200 °C and 3.4 MPa for 15 min and (b) 25% (v/v) MFT slurry at 200 °C and 3.4 MPa for 5 min with the addition of H2O2.
Figure 1. Effect of temperature, pressure, and oxidant on (a) 25% (v/v) MFT slurry at 200 °C and 3.4 MPa for 15 min and (b) 25% (v/v) MFT slurry at 200 °C and 3.4 MPa for 5 min with the addition of H2O2.
Catalysts 12 01518 g001
Catalysts 12 01518 g002 550
Figure 2. Effect of oxidant on separation of water and solid particles with 200 °C and 3.4 MPa internal pressure using (a) 0.7 MPa air and (b) 0.7 MPa air + 5 mL H2O2.
Figure 2. Effect of oxidant on separation of water and solid particles with 200 °C and 3.4 MPa internal pressure using (a) 0.7 MPa air and (b) 0.7 MPa air + 5 mL H2O2.
Catalysts 12 01518 g002
Catalysts 12 01518 g003 550
Figure 3. Bitumen recovery (a) percentage of froth removal after 5, 15, and 30 min interval cycles of WAO and (b) extraction of froth by weight during WAO.
Figure 3. Bitumen recovery (a) percentage of froth removal after 5, 15, and 30 min interval cycles of WAO and (b) extraction of froth by weight during WAO.
Catalysts 12 01518 g003
Catalysts 12 01518 g004 550
Figure 4. Concentrations of dissolved organic, inorganic carbon, and organic acids in extracted water after oxidation with (a) WAO (90 min), (b) WAPO (90 min), and (c) WAOextended (210 min); 90 min = 3 × 30 min cycles, and 210 min = 7 × 30 min cycles.
Figure 4. Concentrations of dissolved organic, inorganic carbon, and organic acids in extracted water after oxidation with (a) WAO (90 min), (b) WAPO (90 min), and (c) WAOextended (210 min); 90 min = 3 × 30 min cycles, and 210 min = 7 × 30 min cycles.
Catalysts 12 01518 g004
Catalysts 12 01518 g005 550
Figure 5. Possible separation layers of treated MFT samples into froth, water, and sediment. Enhancement is of possible assumption of froth layer showing nonpolar organic rings and water layer showing dissolved acetic and formic acids.
Figure 5. Possible separation layers of treated MFT samples into froth, water, and sediment. Enhancement is of possible assumption of froth layer showing nonpolar organic rings and water layer showing dissolved acetic and formic acids.
Catalysts 12 01518 g005
Catalysts 12 01518 g006 550
Figure 6. Concentrations of dissolved ions after oxidation with (a,d) WAO (90 min), (b,e) WAPO (90 min), and (c,f) WAOextended (210 min).
Figure 6. Concentrations of dissolved ions after oxidation with (a,d) WAO (90 min), (b,e) WAPO (90 min), and (c,f) WAOextended (210 min).
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Catalysts 12 01518 g007 550
Figure 7. Percentage of hydrocarbons remaining in the consolidated particles after (a) WAO, (b) WAPO, and (c) WAOextended.
Figure 7. Percentage of hydrocarbons remaining in the consolidated particles after (a) WAO, (b) WAPO, and (c) WAOextended.
Catalysts 12 01518 g007
Catalysts 12 01518 g008 550
Figure 8. Surface area of particles: (a) control, (b) 30 min WAO, and (c) 5 min WAPO.
Figure 8. Surface area of particles: (a) control, (b) 30 min WAO, and (c) 5 min WAPO.
Catalysts 12 01518 g008
Table
Table 1. Process times for WAO and WAPO.
Table 1. Process times for WAO and WAPO.
Interval Time
(min)		Cycle Time
(min)		Batch Cycles
55I
10III
15IIIIII
1515I
30III
45IIIIII
3030I
60III
90IIIIII
3030I
60III
90IIIIII
120IIIIIIIV
150IIIIIIIVV
180IIIIIIIVVVI
210IIIIIIIVVVIVII
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Khan, M.F.; Elnakar, H. Treatment of Oil Sands’ Mature Fine Tailings Using Advanced Wet Air Oxidation (WAO) and Wet Air Peroxide Oxidation (WAPO). Catalysts 2022, 12, 1518. https://doi.org/10.3390/catal12121518

AMA Style

Khan MF, Elnakar H. Treatment of Oil Sands’ Mature Fine Tailings Using Advanced Wet Air Oxidation (WAO) and Wet Air Peroxide Oxidation (WAPO). Catalysts. 2022; 12(12):1518. https://doi.org/10.3390/catal12121518

Chicago/Turabian Style

Khan, Muhammad Faizan, and Haitham Elnakar. 2022. "Treatment of Oil Sands’ Mature Fine Tailings Using Advanced Wet Air Oxidation (WAO) and Wet Air Peroxide Oxidation (WAPO)" Catalysts 12, no. 12: 1518. https://doi.org/10.3390/catal12121518

APA Style

Khan, M. F., & Elnakar, H. (2022). Treatment of Oil Sands’ Mature Fine Tailings Using Advanced Wet Air Oxidation (WAO) and Wet Air Peroxide Oxidation (WAPO). Catalysts, 12(12), 1518. https://doi.org/10.3390/catal12121518

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