Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
pubs.acs.org/EF
Synergistic Mechanism of Particulate Matter (PM) from Coal Combustion and Saponin from Camellia Seed Pomace in Stabilizing CO2 Foam Qichao Lv,† Zhaomin Li,*,† Binfei Li,† Maen Husein,*,‡ Songyan Li,† Dashan Shi,† Wei Liu,§ Hao Bai,† and Li Sheng† †
College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, Shandong, China Center for Environmental Engineering Research & Education, University of Calgary, Calgary T2N4 V8, Alberta, Canada § College of Energy, Chengdu University of Technology, Chengdu 610059, Sichuan, China ‡
ABSTRACT: Particulate matter (PM), generated during coal combustion for energy production, is a serious pollution risk to atmospheric environment. Its utilization as an industrial byproduct is an attractive endeavor all over the world. In this work, novel aqueous CO2 foam stabilized by PM from coal combustion in combination with saponin (COSA1) from camellia seed pomace was explored. Addition of COSA1 effectively promoted the dispersion of PM into the aqueous solution through adsorbing onto particle surface and increasing their zeta (ζ) potential. Hence, PM-stabilized foam could be generated much more effectively. In the presence of COSA1, PM migrated to the CO2/liquid interface and formed an “armor” surrounding the bubbles. The surface of the bubbles became solidlike and in the presence of PM, for bubble with 3.2 mM COSA1, the interfacial dilatational viscoelastic modulus dramatically increased from 23 to 65 mN/m. In addition, PM at the interface shielded CO2 gas from the surrounding liquid leading to much slower diffusion of CO2 between the bubbles, despite the small bubble size, leading to more stable foam with an average bubble diameter of ∼110 μm ∼5000 s after generation. Moreover, particle adsorption increased the interfacial viscoelasticity of COSA1/PM foam film leading to higher apparent viscosity, relative to bare COSA1 foam. Nevertheless, high viscosity increases energy dissipation during foam generation and, hence, decreases foaming ability. The mechanical intensity of the adsorbed particle layer overweighed the interfacial tension (IFT) action and resulted in extremely rough bubbles. The high-performance PM-stabilized CO2 foam could potentially be used in enhancing oil recovery, hydraulic fracturing, fire-fighting, mineral flotation, etc.
1. INTRODUCTION Atmospheric particulate matter (PM) is a major urban pollutant.1−4 According to the WHO, over 87% of the world’s population are exposed to higher levels of PM than the safe annual average.5 Fly ash from coal combustion for energy generation contributes to a significant PM pollution, especially PM2.5 (with diameters of 98.0% pure, Sigma−Aldrich, USA) was used to adjust the pH. All glassware was cleaned with a solution of 67 wt % sulfuric acid (H2SO4) and 12 wt % potassium dichromate (K2Cr2 O7) in order to avoid organic contamination. Unless otherwise specified, all experiments were conducted at 25 °C.
preferred, because of their chemical inertness and tenability of their surfaces, as well as availability in different shapes, sizes, and narrow size distributions.28,31,33 Compared to bare surfactant foams, particles irreversibly adsorb at the interface and potentially provide greater long-term stability, even under extreme conditions.34−36 Despite all these attractive features, raw particles fail to favorably participate at interfaces, because of either their unbalanced hydrophobic or hydrophilic properties. Surface modification of particles, on the other hand, may increase the cost of production and, hence, limits industry-scale applications. In particular, for the stabilization of CO2-in-water foams, the requirements for hydrophilic/CO2-philic balance (HCB) of the particles enhances the difficulty of optimization.37−39 The theory of foam stabilized by raw particles from industry still needs to be studied and completed. In this work, synergistic mechanism of PM from coal combustion and saponin from camellia seed pomace in stabilizing CO2 foam was investigated systematically. First, the properties of different aqueous dispersions of PM and saponin were studied in terms of their particle size distribution, saponin adsorption, zeta potential (ζ), and particle wettability. Then, the stability and foaming ability of foams prepared from the aqueous dispersions of the PM and the saponin were elucidated. Potential synergy between PM and saponin toward stabilizing the foam was explored. Moreover, the morphology of the foam was studied from two aspects: the microstructure of the bubbles and the foam structure evolution. Finally, an extrastable and inexpensive PM-stabilized CO2 foams was obtained.
3. METHODS 3.1. Preparation of Dispersions. In this study, the initial PM concentration was fixed at 2.0 wt %, whereas COSA1 concentration was varied to obtain stable dispersion. All mixtures were stirred for 5 h and then subjected to 40 min of ultrasonication using a 2 × 103 W ultrasonic processor at a frequency of 20 kHz (YP-S17, Hangzhou Success Ultrasonic Equipment Co., Ltd., China). In one cycle, the PM dispersion was processed with 10 s of ultrasonication and 30 s of rest, and its temperature was maintained at 25 °C by using a water bath. The initial pH values of the PM/COSA1 dispersions ranged from 3.52 to 4.67, and the ζ potential was adjusted by adding concentrated sodium hydroxide. The stability of the PM/COSA1 dispersion was evaluated by using a settling test. First, After 2 h of standing, 180 mL of solution were collected from the upper 200 mL of the PM/COSA1 dispersion and sonicated for 10 min, then the mass of the particles it contained was determined following centrifugation. Similarly, the mass of the particles in the bottom dispersion (the rejected 20 mL) was also evaluated to close the mass balance. The 180 mL represented the sample with the settling step used in the experiments. 3.2. Particle Size Distribution and Adsorption of COSA1 onto PM Particles. The particle size distribution of PM/COSA 1 dispersions was determined by a laser particle size analyzer (Mastersizer 3000, Malvern, U.K.). The PM/COSA1 dispersion was processed using centrifugation at 8000 rpm (GT10-1, Beijing era Beili Centrifuge, China) for 2 h, then a total organic carbon analyzer (TOC, Shuzhou Ailan Co., Ltd. (China)) was used to measure the COSA1 concentration of supernatant. The PM uptake of COSA1 was then calculated using a mass balance, assuming constant liquid volume, between the initial and the supernatant concentration of COSA1 per unit mass of the PM. 3.3. Zeta (ζ) Potential Measurement and Three-Phase Contact Angle of Particles. The ζ potential of the PM remaining in the dispersion after the 2 h of settling was determined by a Malvern Zetasizer (Nano ZS90, Malvern, U.K.). For each experimental condition, five replicates were used in order to provide reliable error analysis. The particles in the dispersions were separated by centrifugation and were then compressed into circular cakes under 100 MPa for 30 min by a tablet press machine (YPJ, Hongdaboyu Co., China). The three-phase contact angle of the cake was determined by Teclis Tracker-H (France).40 3.4. Interfacial Tension and Dilatational Viscoelasticity Modulus. Interfacial tension (IFT) and dilatational viscoelasticity modulus of the PM/COSA1 dispersions after 2 h of settling were determined by a interfacial rheometer (Tracker-H, Teclis Co., France).41−43 During the measurement, CO2 at ambient pressure was used to fully fill the view cell, then a drop of the PM/COSA1 dispersions was produced in the cell. The apparatus estimates the drop
2. MATERIALS Camellia oleifera saponin A1 (COSA1), provided by Zhongye, Inc. (China), was used as the surfactant. COSA1 is extracted from camellia seed pomace with >97.5% purity. During the manufacturing process, the pomace is first crushed into powder, followed by methanol extraction. The methanol extract is dissolved by deionized water to get a solution/mixture that is filtered through a nanofiltration membrane. The filtrate is passed through a macroporous silica gel column and then is subjected to repeated high-performance liquid chromatography (HPLC) before obtaining the saponin. The molecular structure of COSA1 is shown in Figure 1. The scientific name of COSA1 is 22-O-
Figure 1. Molecular structure of COSA1 surfactant. cis-2-hexenoyl-A1-barrigenol-3-O-[β-D-galactopyranosyl (1 → 2)][β-Dglucopyranosyl (1 → 2)-α-L-arabinopyranosyl (1 → 3)]-β-D-glucopyranosiduronic acid. The as-received COSA1 powder is dried with a temperature of 40 °C for 24 h under vacuum. The dried powder is stored in a desiccator under vacuum before its use. PM samples was kindly donated by a Shengli coal-fired power plant (Dongying, China). The PM sample was obtained from the electrostatic precipitators (ESPs) and was sieved through 800 mesh to obtain sizes that are 3.2 mM, no sedimentation could be observed. This suggests that COSA1 surfactant helps to disperse the PM effectively. Particle size distribution (PSD) in the dispersion was also affected by particle sedimentation. Figure 4a exhibits a PSD ranging between 10 nm and 15 μm for the dispersion prepared as described in Section 3.1 after 2 h of settling, whereas, for the original PM (after sieving), bimodal size distributions with one peak at ∼100 nm and another minor one at ∼1.40 μm were observed. In the absence of COSA1 and after 2 h of settling, the proportion of particles with small size (1 μm) decreased. The bimodal size distributions became unimodal with a peak at ∼500 nm. According to particle mass proportion in Figure 4b, the PM concentration in the upper dispersions was small and most of its particles settled into the bottom. Those results indicated the instability of particles with small size and large size. For the small-sized particles, the specific surface area is relatively large, which promotes flocculation. Large particles, on the other hand, settle under gravity. With 0.4 mM COSA1, the particle proportion in the dispersion increased (Figure 4b), and the PSD peak shifted to the left (Figure 4a). This suggests that, with the addition of COSA1, small-sized particles were the first to disperse by action of entropy force acting against gravity. Upon further increases in COSA1 concentration, larger particles become stabilized in
4. RESULTS AND DISCUSSION 4.1. Properties of the COSA1/PM Dispersions. To get a stable foam, first, it is essential to effectively disperse the particles. Therefore, the stability of the COSA1/PM dispersions was studied. The initial PM concentration was fixed at 2.0 wt %. The addition of COSA1 surfactant introduced a big change in PM dispersion stability. As shown in Figure 3, for the sole PM dispersion, a clear solid sediment appeared after 2 h, mainly C
DOI: 10.1021/acs.energyfuels.8b00245 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 4. (a) Particle size distribution (PSD) in the dispersions after 2 h of settling and (b) PM particle concentration in the dispersions, as a function of initial COSA1.
Figure 5. (a) Adsorption isotherm of COSA1 onto PM in water at 25 °C, (b) ζ potential of COSA1/PM dispersions and the contact angle of PM particles, and (c) schematic of COSA1/PM dispersion and the interaction at the particle surface for different COSA1 concentration. All results were collected after 2 h of settling time.
measured to verify the adsorption behavior, and the result was shown in Figure 5b. At pH 8.0, the ζ potential of the bare PM dispersions is approximately −3 mV. As COSA1 contains negatively charged carboxylate COO− groups, the addition of COSA1 gradually decreases the particle potential to approximately −5 mV at a concentration of 0.4 mM. The ζ potential significantly decreases to below −19 mV at a COSA 1 concentration of 5.5 mM. Beyond this concentration, there is no obvious change in ζ potential. Accordingly, the ζ potential curve can also be divided into three regions. A schematic of COSA1/PM dispersions and the interaction at the surface of the particles at different COSA1 concentration is given in Figure 5c. The adsorption of COSA1 on particle surface was
the dispersion to maximize the entropy of the system, i.e., reduce the overall energy of the system. The mean particle size became larger, and PSD closely matched the original PM. It is likely that the adsorption of COSA1 surfactant on particles changed the dispersion properties of particles.45,46 To study the extent of COSA1 adsorption on the PM, the adsorption isotherm was measured by using depletion from solution method. Figure 5a shows that, as the equilibrium COSA1 concentration increases, the amount of adsorbed COSA1 increased slightly at the beginning and more significantly at a high COSA1 concentration. The adsorption follows an “S”-type isotherm with three regions (A, B, C).46 Meanwhile, the ζ potential of COSA1/PM dispersions was D
DOI: 10.1021/acs.energyfuels.8b00245 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 6. (a) Initial foam volume and (b) half-life time of liquid drainage vs initial COSA1 concentration.
enhanced by a mixture of COSA1 and PM. The COSA1/PM foam without particle settling showed a shorter liquid drainage half-life. This is mainly due to the instability of the particles in COSA1/PM dispersion. The aggregation and sedimentation of a portion of the particles, as stated earlier, pull an amount of liquid out of COSA1/PM foam, which is disadvantageous to foam stability. In Region III, the initial foam volume of the two systems increased again as the COSA1 concentration increased continuously above CM, but the half-life of the liquid drainage decreased sharply. The result is interesting and indicates that the high concentration of surfactant is not essential for foam stability. To explain this trend, the COSA1/PM foam following 2 h particle settling was studied by LSCM at two COSA1 concentrations, namely, 3.2 mM and 6.4 mM. Figures 7a and
mainly controlled by the hydrogen bonds between the hydroxide group from glycone (refer to Figure 1) and the oxygen atom from the particle surface groups (Si−O−Si, Si− O−Al). In region A, particles have a tendency to aggregate, to reduce the surface area, and the sedimentation is easy to form. Although the adsorption of COSA1 increases the ζ potential of the particles, the electrostatic repulsion is too small to resist the aggregation in this zone. In region B, much more COSA1 monomers adsorb onto the particle surface and increase the ζ potential of the particle. Thus, higher electrostatic repulsion decreased particle aggregation. Moreover, the adsorption of COSA1 molecules left the hydrophobic aglycone exposed, thereby changing the wettability of the particles. Figure 5b shows that, first, the contact angle of particles was increased to a maximum above 80° by the addition of COSA1. The hydrophobic change confirms the monolayer adsorption of COSA1 molecules onto the particle surface. As the COSA1 concentration increased continuously, bilayer adsorption, by virtue of hydrophobic associative interactions along the carbon chain, was established. Thus, the particle surface became hydrophobic and the contact angle decreased. In region C, the adsorbed amount of COSA1 molecules reached saturation, thus no noticeable variation in ζ potential with COSA1 concentration could be recorded. 4.2. Foaming Ability and Foam Stability. Experimental results of CO2 foam generated by bare COSA1 solution are shown in Figure 6. As COSA1 concentration increased, the foam volume increased, because the surfactant reduced the IFT of the mixture, leading to less energy being needed for foam formation, thus improving the foaming ability. Beyond a certain COSA1 concentration, the foam volume did not undergo any appreciable change, which is mainly due to saturation of the surface of the surfactant molecules, leading to constant IFT. For COSA1/PM dispersions with or without the 2 h particle settling step, the general trend was common for both samples, as shown in Figure 6a. With COSA1 concentration increasing, the foam volume first increased gradually and then decreased to a relatively low point (red star point) until a certain COSA1 concentration (CM) was reached. Meanwhile, for the two foam systems, the half-life of the liquid drainage increased to a maximum value at the same COSA1 concentration CM. Compared to pure COSA1 foam, the maximum half-life of COSA1/PM foam with and without a particle settling step improved by factors of ∼15 and 7, respectively. This is an important result, indicating that foam stability was dramatically
Figure 7. Confocal fluorescence image of foams prepared with COSA1/PM dispersions: (a,b) COSA1 concentration = 3.2 mM (corresponding to red star point in Figure 6); (c,d) COSA 1 concentration = 6.4 mM (corresponding to the green star point in Figure 6). Time elapsed was 500 s after foam generation.
7b show that, for the foam with a COSA1 concentration of 3.2 mM, the adsorption of particles onto the bubble surface is very obvious. The particles appear to have bridged together and formed a dense particle “armor” surrounding the bubble. The particle “armor” was very stable and contributed to less of a liquid−gas contact surface. Adsorption of a sufficient number of PM shields the CO2 from the surrounding liquid film, which E
DOI: 10.1021/acs.energyfuels.8b00245 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels effectively slows down the CO2 diffusion between the bubbles. Subsequently, Ostwald ripening can be inhibited and the stability of the CO2 foam enhanced. For foam with a relatively high COSA1 concentration of 6.4 mM (Figures 7c and 7d), the adsorption behavior of particles was not clear. The particles in the liquid had a tendency to be drained from the film to a plateau border under the influence of the pressure difference and gravity. Thus, the bubble film became thinner with time, which led to a higher CO2 gas diffusion. The disproportionation of foam in Figure 7c has been obvious at 7 min after foam generation. The interfacial viscoelasticity of a bubble film dictates the bubble resistance to disturbances, due to local fluctuations or deformations, which may jeopardize foam stability. The interfacial dilatational viscoelastic modulus (E) for COSA1/ PM bubbles was studied under different particle adsorption regimes, as in Figure 5a. Figure 8 depicts Ε as a function of
Figure 9. Schematic illustration of the synergistic between COSA1 and PM during COSA1/PM bubble stabilization and its impact on the interfacial viscoelasticity. Yellow arrows suggest a direction for PM migration with increasing COSA1.
COSA1 and COSA1/PM foam in this region. In region II, COSA1 concentration was higher than that in region I, and the surface of the PM was occupied by much more COSA1 molecules with aglycone exposed outside, leading to more hydrophobic particles. The distribution of particles gradually shifted from the liquid to the CO2/liquid interface. Meanwhile, the aggregation and sedimentation of some of the particles consumed COSA1, thus the micellization of COSA1 was hindered. This leads to an apparent increase of the CMC of COSA1, as shown in the inset of Figure 8. When COSA1 concentration increased to near the CMC value, the surfactant molecules bridged the PM together, resulting in the formation of interconnected structure with strong steric integrity, as depicted in Figures 7a and 7b. The bubble surface changed to being solidlike, its intensity was enhanced, and the values of E for COSA1/PM bubble dramatically increased. In this region, COSA1 and PM display a synergistic effect in stabilizing the CO2 foam, as confirmed by region II in Figure 6b. In region III, with the continuing increases of COSA1 concentration, COSA1 molecules contributed to a bilayer coating of the PM, mainly driven by hydrophobic interaction between the aglycone tails, leaving the hydrophilic ends facing outward. This enhanced the hydrophilicity of the PM and the particles gradually migrated away from the CO2/liquid interface back to the bulk aqueous phase. Meanwhile, the solidlike property of the bubble surface was attenuated, and the interfacial dilatational viscoelastic modulus decreased in this region. The synergistic effect between COSA1 and PM weakened rapidly and the foam become unstable, as confirmed by region III of Figure 6b. In region IV, the saturation of the bilayer adsorption of COSA1 make the particle surface extremely hydrophilic. The PM adsorption onto the gas surface vanished, and the foam become unstable in a similar fashion to bare COSA1 foam, as confirmed by region IV of Figure 6b. The apparent viscosity of COSA1/PM foams was studied because it governs energy dissipation during foaming. Figure 10 shows the variation of the apparent viscosity of the foams with
Figure 8. Interfacial dilatational viscoelastic modulus vs COSA1 concentration.
COSA1 concentration. E increased to a maximum at an intermediate COSA1 concentration, then decreased with COSA1 concentration for both bare COSA1 and COSA1/PM systems. For pure COSA1 bubble, when the surfactant concentration was below CMC, i.e.,
