Abstract
A facile method for fabricating superhydrophobic and superoleophilic powder with 5A zeolite and stearic acid (SA) is reported in this study. The effect of different contents of SA on contact angle (CA) was investigated. The maximum water CA was 156.2°, corresponding to the optimum SA content of 1.5 wt%. The effects of SA and the mechanism of modified 5A zeolite powder by SA were analyzed by sedimentation analysis experiment, FTIR analysis, particle size analysis, and SEM characterization. The SA-modified 5A zeolite was used as an oil sorbent to separate oil–water mixture with potential use in floating oil. The separation efficiency was above 98%.
1 Introduction
The discharge of oily wastewater in oil spill accidents and in the progress of oil progressing, oil transportation, etc., has caused destructive pollution of water resources. Oily wastewater has caused great harm to the environment and human health and also waste of oil and water resources. Therefore, the separation of the oil–water mixture should be carried out from the perspective of water recycling or environmental management. Traditional methods for separating oil–water mixtures in the application are mainly classified into four categories: physical methods, chemical methods, physical-chemical methods, and biological methods, such as centrifugal separation method [1,2], coagulation method [3], and flotation method [4]. However, all these methods have problems such as poor recyclability, low separation efficiency, complex separation instrument, and even environmental pollution. It can be seen that the traditional separation methods have failed to meet the actual needs and environmental requirements. Therefore, how to effectively separate oil spills and oily wastewater is a worldwide problem, especially for large-scale floating oil. And, how to fabricate high efficient and robust separation materials for oil–water separation is also an insurmountable problem. In recent years, super wetting materials for oil–water separation, including superhydrophobic and superoleophilic materials [5,6,7,8] and superhydrophilic and underwater superoleophobic materials [9,10,11], are receiving great attention owing to their high efficiency and high flux. The essence of oil–water separation is the interface problem. Through a reasonable design, the different wettability of water and oil droplets on the solid surface is realized, so that the oil–water mixture can be effectively separated.
Separation of oil–water mixtures by absorbing solid materials, such as zeolite, activated carbon [12,13], calcium carbonate, etc., has become a trend in the oil industry due to their environmentally friendly and porous adsorption properties. However, these materials can absorb both water and oil simultaneously, resulting in the decrease of separation efficiency. Therefore, it is necessary to modify these materials so that they only absorb oil while absorbing water. Hu and Deng [14] prepared superhydrophobic and oleophilic calcium carbonate powder by fatty acid as a selective oil sorbent to separate oil–water mixture. Wang et al. [15] used dodecanoic acid modifying calcium carbonate to separate oil–water mixture. However, among the adsorption materials, zeolite is a class of adsorbents with uniform micropores, mainly composed of silicon, aluminum, oxygen, and some other metal cations. Zeolite A type has excellent chemical stability (acid, salt, corrosion-resistant), thermal stability (up to 1,000°C), and a large specific surface area. The application prospect of zeolite is increasingly broad, such as zeolite membranes [16,17] for separating gas [18], catalytic microreactors [19], and ion-exchange electrodes [20]. Recently, due to the high selectivity of the zeolite, zeolite as adsorption materials has attracted lots of researchers’ interest. And zeolite, as an ideal adsorbent material, is non-toxic, inexpensive, and recoverable for reuse. Unfortunately, most absorbent materials, including zeolite, can absorb both water and oil, resulting in the decrease of separation efficiency. The preparation progress of zeolite membranes is complex, such as seeding and the removal of structure-direct agents [21,22]. Poor reproducibility of zeolite membranes is still a big problem for practical application.
Herein, we report the superhydrophobic and superoleophilic 5A zeolite modified by stearic acid [23], an excellent surface modifier, via a facile, economical, and environmentally friendly one-step approach. The prepared 5A zeolite was characterized in detail while its wettability was determined by CA. The oil–water separation experiments and the mechanism of modified 5A zeolite were also investigated.
2 Experimental section
2.1 Materials
Stearic acid (SA), sodium chloride, and ethanol were purchased from Aladdin (Shanghai China). 5A zeolite was purchased from Jiangxi Xintao Technology Co., Ltd. Distilled water was prepared in our laboratory. Methyl red was used as an oil dyeing agent. All reagents were analytical grade and used as received without further purification or modification.
2.2 Preparation of superhydrophobic and superoleophilic 5A zeolite
The superhydrophobic and superoleophilic 5A zeolite was prepared by the surface modification with SA and designated as SSMAZ in the text. For preparation, 1.5 g of SA was dissolved in 50 mL ethanol at 25°C. Then, 10.0 g of 5A zeolite was dispersed in 1.5% SA solution under magnetic stirring at a speed of 1,000 rpm for 1 h. Then, it was washed three times with ethanol to remove unreacted and unadsorbed SA. The mixed solution was dried at 80°C for 24 h.
2.3 Oil–water separation experiment
The adsorption capacity of SSMAZ for kerosene was measured by the following equation (1):
where k is the oil absorption capacity, m II is the weight of recovered water weight, and m I is the initial water weight.
2.4 Characterizations
The Fourier transform infrared spectroscopy (FTIR) was performed using Thermo Nicolet with a scanning wavenumber range of 4,500–400 cm−1.
Particle size analysis was carried out with Mastersizer 3000 (UK).
X-ray diffraction (XRD) analysis of the samples was performed using a Bruker D8 with Cu-Kα radiation. The voltage was 40 kV, and the current was 40 mA. The scanning range (2θ) was 10–80° at a scanning rate of 2° min−1. Phase identification was conducted by comparison with the Joint Committee of Powder Diffraction Standards (JCPDSs) database.
The micromorphology was observed by a field emission scanning electron microscopy (FESEM, QUANTA FEG450).
Water contact angle (WCA) was measured by a contact angle analyzer (JY-PHB, China) with 5 μL of water droplets. The SSMAZ was flattened on a glass slide. The WCA was measured at three different positions, and then, the average value was taken.
3 Result and discussion
3.1 FTIR analysis
The state of chemical groups and the interaction can be studied by FTIR. For 5A zeolite, due to the high specific surface area and high surface energy, the powder can easily adsorb moisture, forming a hydroxyl layer (–OH) on the surface and multiple layers of physically adsorbed water. As shown in Figure 1(a), 5A zeolite has a wide and large hydroxyl absorption peak around 3,300 cm−1. As shown in Figure 1(b), the characteristic absorption peak at 2,921 and 2,858 cm−1 can be found in SSMAZ, which was consistent with the fatty long carbon chain group in SA which is shown in Figure 1(c). This indicated the presence of SA on the surface of the 5A zeolite. Compared with the unmodified 5A zeolite, the hydroxyl absorption peak in the SSMAZ almost disappeared. It confirmed that the carboxyl group in the SA was chemically or physically bonded to the hydroxyl group on the surface of the 5A zeolite.
FTIR spectrum of (a) 5A zeolite, (b) SSMAZ, and (c) SA.
3.2 XRD analysis
Figure 2(a) and (b) shows the XRD pattern of 5A zeolite and SSMAZ. 5A zeolite owned obvious diffraction peaks, belonging to SiO2, Al2O3, Na2O, and CaO [24,25]. The XRD pattern of 5A zeolite as shown in Figure 2(a) was consistent with the values in the standard card (JCPDS No.75-1151). The diffraction peaks observed at 10.4°, 12.4°, 16.4°, 21.7°, 23.8°, 26.4°, 27.2°, 29.5°, 33.9°, 34.1°, and 52.3° can be assigned to the (110), (111), (210), (221), (331), (320), (321), (410), (411), (332), and (550) crystal planes of aluminosilicate, respectively. The XRD pattern of SSMAZ was similar to that of 5A zeolite without any peaks attributing to the SA molecules, indicating that the crystalline phase of 5A zeolite was not changed after the treatment with SA, and there were no residues of the SA.
XRD pattern of (a) 5A zeolite and (b) SA-modified 5A zeolite.
3.3 Particle size analysis
As shown in Table 1, the particle size distribution of unmodified 5A zeolite ranged from 106 to 5,560 nm. The particle size of 5A zeolite was mainly distributed at 4,800 and 5,550 nm. After the modification of SA, the particle size was mainly distributed at 1,110 and 1,280 nm. Due to the formation of the hydroxyl layer on the surface of the 5A zeolite, the interaction force and surface activity between the powder were increased and promoted the agglomeration. SA as a powder modifier, the carboxyl group (–COOH) in the SA reacted with the surface hydroxyl group (–OH) of the 5A zeolite, and the surface of the 5A zeolite was coated with an organic molecular chain. The surface of 5A zeolite changed from the strongly polar polyhydroxy group which caused agglomeration became a non-polar surface structure coated by the organic molecular chain, which eliminated the agglomeration of the 5A zeolite powder and made the 5A zeolite particles to disperse better. It may be also the surface of SA was negatively charged when ethanol was used as the medium and can be adsorbed on the surface of the gaps of 5A zeolite by van der Waals force, and the osmotic pressure acted together to make the agglomeration strength lower, causing the clusters to be broken into smaller crystals.
Particle size distribution of 5A zeolite and SSMAZ
| Sample | The intensity of different particle sizes (%) | ||||||
|---|---|---|---|---|---|---|---|
| 106 nm | 122 nm | 1,110 nm | 1,280 nm | 4,150 nm | 4,800 nm | 5,560 nm | |
| 5A zeolite | 2.89 | 7.57 | — | — | 6.16 | 29.2 | 54.2 |
| SSMAZ | 24.5 | 14.8 | 29.1 | 31.5 | — | — | — |
3.4 Morphology analysis
As shown in Figure 3(a) and (b), the unmodified 5A zeolite particles were stacked on each other and were agglomerated severely. However, the agglomeration had been improved and the particle size became smaller after SA modification. This was consistent with the results obtained from the particle size analysis. It indicated that the dispersibility of the 5A zeolite was increased and the degree of agglomeration was decreased by SA. The addition of SA played two roles, one was the dispersion and fragmentation of 5A zeolite, and the other was to prevent the particles from re-aggregating. Gaps exist in the particles clusters, and fragmentation of the clusters occurred in these places. These micro-slits were seen as capillaries, so that penetration can occur in these capillaries. SA can not only automatically adsorb on the surface of the particles but also automatically penetrate the micro-cavities.
SEM image of (a) unmodified 5A zeolite and (b) SSMAZ.
3.5 CA analysis
Wettability is determined by measuring the CA of water and kerosene droplets on the surface of unmodified 5A zeolite and SSMAZ as shown in Figure 4(b). The unmodified 5A zeolite was quickly wetted by water and kerosene droplets as shown in Figure 4(a). The WCA of the SSMAZ was 156.2°, and the kerosene droplets spread quickly on the surface of SSMAZ as shown in Figure 5(b).
Image of water and kerosene droplets on (a) unmodified 5A zeolite and (b) SSMAZ fixed on a glass slide.
WCA on SSMAZ fixed on a glass slide of different SA contents.
For porous powder, the liquid is required to penetrate between particles. Wetting as a capillary action phenomenon is related to capillary rise. The rate of entry of a liquid into a capillary radius r is given by Washburn [26,27]. The model is originally proposed to simulate the penetration of liquids into a single cylindrical capillary and can be used to describe the absorption behavior of oil and water in a porous matrix of porous materials. The Washburn equation is shown as follows:
where r is the radius of the capillary, γ is the liquid surface tension, η is the liquid viscosity, and θ is the CA for the liquid on the solid surface.
Superhydrophobic and superoleophilic 5A zeolite powder can be seen as a capillary matrix. If too much liquid like kerosene is left in the capillary, the observed times of flow will be too short, the value of θ of the oil is small, while if the wall is not completely wet, the value of θ will be greater than zero.
Wettability behavior can also be explained by the Cassie–Baxter equation [28,29] (equation (3)),
where θ 1 and θ 2 are the WCAs on the surface of the unmodified 5A zeolite and SSMAZ, respectively. f 1 and f 2 (f 1 + f 2 = 1) are the area frictions of water connecting with SSMAZ and air, respectively. According to equation (3), the value of f 2 can be calculated to be 0.957. The WCA value is consistent with the interpretation of the Washburn equation that water cannot penetrate the capillary matrix of the SSMAZ. However, the CA of kerosene droplets cannot be measured due to the rapid penetration on the surface of the SSMAZ.
WCA of the modified 5A zeolite is given in Figure 5, with the increase of SA content, the WCA showed a maximum value. The maximum WCA was 156.2°, corresponding to the optimum SA content of 1.5 wt%. Before the WCA reached the maximum, only part of the surface of the 5A zeolite reacted with SA, and the hydrophobic group of the SA taking part in the reaction was all outward and voids existed between hydrophobic chains. After the WCA reached the maximum, the voids were filled by the SA molecules. Interaction between SA hydrophobic chains, SA was arranged in a disorderly manner as shown in Figure 6, and some polar groups were facing outward causing the WCA decreased. Therefore, the best content of SA was 1.5 wt%.
Illustration of the orientation of SA on the surface of modified 5A zeolite. (a) Before the best SA content and (b) after the best SA content.
3.6 Settlement experiment analysis
According to Stokes’ law [30,31,32], the particles will settle due to gravity in the dispersion medium. The sedimentation velocity is related to the size and quality of the particles. The larger the particle, the faster the sedimentation rate, and the smaller the particle, the slower the sedimentation rate.
According to the principle of similar compatibility, liquid paraffin was used as a dispersing solvent to characterize the dispersion properties of 5A zeolite before and after the surface modification of SA.
About 0.3 g of 5A zeolite before and after modification was weighed, placed in a reagent bottle containing 30 mL liquid paraffin, ultrasonically dispersed for 30 min, and then observed for sedimentation. It was found that the unmodified 5A zeolite settled in the bottom of the reagent bottle, and the SSMAZ was uniformly dispersed in the liquid paraffin after 12 h. The unmodified 5A zeolite precipitated to the bottom, and the upper layer solution became clear after standing for 12 h as shown in Figure 7(a). The SSMAZ solution was milky white, and there was no significant precipitation and delamination after standing for 12 h as shown in Figure 7(b). It indicated that the surface polarity of the modified 5A zeolite particle was similar to liquid paraffin. Due to the high surface polarity, large surface tension, and hydrophilicity of unmodified 5A zeolite particles, the surface of the 5A particle was coated with a layer of a fatty chain after modification; so, the polarity was reduced, the surface tension was reduced, and it was hydrophobic. The surface tension of the liquid paraffin was low. Therefore, the 5A zeolite particles were well dispersed in the liquid paraffin.
Settlement analysis of (a) unmodified 5A zeolite and (b) SSMAZ after 12 h.
Generally, the inorganic powder is usually modified by physical adsorption, chemical adsorption, or chemical reaction. From the viewpoint of the modification effect, chemical reactions have stronger interface properties than physical adsorption and chemical adsorption. It can be seen from the WCA that the surface of 5A zeolite changed from hydrophilic (0°) to hydrophobic (156.2°), from polar to nonpolar (the powder can be well suspended in a non-polar solution such as paraffin, but cannot be suspended in water) after SA modification. The reaction mechanism is that the –COOH group in SA is first ionized to –COO− and H+, and then H+ interacts with OH in the surface of 5A zeolite. A hydrophobic layer composes of a fatty chain of SA formed on the surface of the 5A zeolite to obtain the purpose of surface modification.
3.7 Oil–water separation experiment
In oil–water separation experiment, different proportions of distilled water and kerosene, which dyed with methyl red to simulate floating oil. Then, 5A zeolite powder was gradually sprinkled into the oil–water mixture. The SSMAZ powder floated at the oil–water interface, where it only absorbed the floating oil while repelling the water due to the modified 5A zeolite that was superhydrophobic and superlipophilic. The more 5A zeolite powder, the more the amount of oil was absorbed, and it settled to the bottom of the beaker by gravity and the separation process is shown in Figure 8.
Oil–water mixture (a) before 5A zeolite addition, (b)–(d) after 5A zeolite addition, and (e) after separation.
The kerosene was completely removed from the oil–water mixture by 5A zeolite. As shown in Table 2, the oil absorption capacity was above 98%. The water was carried when the 5A zeolite absorbed oil causing a little bit of water loss.
The oil absorption efficiency of kerosene
| Oil:water (weight ratio) | Absorption efficiency (%) |
|---|---|
| 1:200 | 99.3 |
| 1:100 | 99.0 |
| 1:50 | 98.6 |
| 1:10 | 98.1 |
These results showed that SSMAZ provided a huge space for oil storage and high adsorption efficiency. The 5A zeolite and SA are low-cost and environmentally friendly. More importantly, SA-modified zeolite is demonstrated with excellent recyclability as shown in Figure 9 and the absorption efficiency can still reach 90% after four separations. The residual oil in the pores of 5A zeolite can be removed by calcination. SSMAZ can be reused at least twice.
Absorption efficiency under different separation times.
4 Conclusion
In this communication, SSMAZ was used as an oil sorbent to separate oil–water mixture, and the separation efficiency was above 98%. The SSMAZ can be a candidate for floating oil clean-ups. The fabrication method of SSMAZ is simple to make mass production possible. The modification mechanism is to modify the surface of 5A zeolite powder, and the surface of 5A zeolite powder changes from hydrophilic (0°) to hydrophobic (156.2°), from polar to non-polar.
Acknowledgements
The project is funded by the National Natural Science Foundation of Study on Mechanism of Magnetic Nanometer Fe3O4@SiO2 Composite Hyperbranched Macromolecule for Treating the Sewage Water of Oilfield. Fund No. 51774089.
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Funding information: The project is funded by the National Natural Science Foundation of Study on Mechanism of Magnetic Nanometer Fe3O4@SiO2 Composite Hyperbranched Macromolecule for Treating the Sewage Water of Oilfield. Fund No. 51774089.
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Author contributions: Ting Liang and Biao Wang contributed equally to this work. Ting Liang and Zhenzhong Fan initiated the project. Ting Liang and Biao Wang were supervised by Zhenzhong Fan and Qingwang Liu. Ting Liang prepared the Superhydrophobic Surfaces. Biao Wang did the oil-water separation experiment.
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Conflict of interest: Authors state no conflict of interest.
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Data availability statement: The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Reference
[1] Moosai R, Dawe RA. Gas attachment of oil droplets for gas flotation for oily wastewater cleanup. Sep Purif Technol. 2003;33(3):303–14.10.1016/S1383-5866(03)00091-1Search in Google Scholar
[2] Hami ML, Al-Hashimi MA, Al-Doori MM. Effect of activated carbon on BOD and COD removal in a dissolved air flotation unit treating refinery wastewater. Desalination. 2007;216(1–3):116–22.10.1016/j.desal.2007.01.003Search in Google Scholar
[3] Hassan I, Nirdosh I, Sedahmed GH. Separation of oil from oil–water emulsions by electrocoagulation in an electrochemical reactor with a fixed-bed anode. Water Air Soil Pollut. 2015;226(8):1–12.10.1007/s11270-015-2521-4Search in Google Scholar
[4] Xu H, Liu J, Wang Y, Cheng G, Deng X, Li X. Oil removing efficiency in oil–water separation flotation column. Desalin Water Treat. 2015;53(9):2456–63.10.1080/19443994.2014.908413Search in Google Scholar
[5] Su C, Xu Y, Zhang W, Liu Y, Li J. Porous ceramic membrane with superhydrophobic and superoleophilic surface for reclaiming oil from oily water. Appl Surf Sci. 2012;258(7):2319–23.10.1016/j.apsusc.2011.10.005Search in Google Scholar
[6] Khosravi M, Azizian S. Preparation of superhydrophobic and superoleophilic nanostructured layer on steel mesh for oil–water separation. Sep Purif Technol. 2017;172:366–73.10.1016/j.seppur.2016.08.035Search in Google Scholar
[7] Qing W, Shi X, Deng Y, Zhang W, Wang J, Tang CY. Robust superhydrophobic-superoleophilic polytetrafluoroethylene nanofibrous membrane for oil/water separation. J Membr Sci. 2017;540:354–61.10.1016/j.memsci.2017.06.060Search in Google Scholar
[8] Ma W, Zhao J, Oderinde O, Han J, Liu Z, Gao B, et al. Durable superhydrophobic and superoleophilic electrospun nanofibrous membrane for oil–water emulsion separation. J Colloid Interface Sci. 2018;532:12–23.10.1016/j.jcis.2018.06.067Search in Google Scholar PubMed
[9] Xue Z, Wang S, Lin L, Chen L, Liu M, Feng L, et al. A novel superhydrophilic and underwater superoleophobic hydrogel-coated mesh for oil/water separation. Adv Mater. 2011;23(37):4270–3.10.1002/adma.201102616Search in Google Scholar PubMed
[10] Feng Z, Gao S, Zhu Y, Jin J. Alkaline-induced superhydrophilic/underwater superoleophobic polyacrylonitrile membranes with ultralow oil-adhesion for high-efficient oil/water separation. J Membr Sci. 2016;513:63–7.10.1016/j.memsci.2016.04.020Search in Google Scholar
[11] Liu L, Chen C, Yang S, Xie H, Gong M, Xu X. Fabrication of superhydrophilic-underwater superoleophobic inorganic anti-corrosive membranes for high-efficiency oil/water separation. Phys Chem Chem Phys. 2015;18(2):1317–25.10.1039/C5CP06305ASearch in Google Scholar PubMed
[12] Yang Y, Chen R, Xing W. Integration of ceramic membrane microfiltration with powdered activated carbon for advanced treatment of oil-in-water emulsion. Sep Purif Technol. 2011;76(3):373–7.10.1016/j.seppur.2010.11.008Search in Google Scholar
[13] Wang Z, Ji S, Zhang J, Liu Q, He F, Peng S, et al. Tannic acid encountering ovalbumin: a green and mild strategy for superhydrophilic and underwater superoleophobic modification of various hydrophobic membranes for oil/water separation. J Mater Chem A. 2018;6(10):13959–67.10.1039/C8TA03794ASearch in Google Scholar
[14] Hu Z, Deng Y. Superhydrophobic surface fabricated from fatty acid-modified precipitated calcium carbonate. Ind Eng Chem Res. 2010;49(12):5625–30.10.1021/ie901944nSearch in Google Scholar
[15] Wang C, Piao C, Zhai X, Hickman FN, Li J. Synthesis and characterization of hydrophobic calcium carbonate particles via a dodecanoic acid inducing process. Powder Technol. 2010;198(1):131–4.10.1016/j.powtec.2009.10.026Search in Google Scholar
[16] Zeng J, Guo Z. Superhydrophilic and underwater superoleophobic MFI zeolite-coated film for oil/water separation. Colloids Surf A Physicochem Eng Asp. 2014;444(Complete):283–8.10.1016/j.colsurfa.2013.12.071Search in Google Scholar
[17] Li Y, Shang X, Zhang B. One-step fabrication of the pure-silica zeolite beta coating on stainless steel mesh for efficient oil/water separation. Ind Eng Chem Res. 2018;57(51):17409–16.10.1021/acs.iecr.8b04172Search in Google Scholar
[18] Zhu X, Wang H, Lin YS. Effect of the membrane quality on gas permeation and chemical vapor deposition modification of MFI-type zeolite membranes. Ind Eng Chem Res. 2015;49(20):10026–33.10.1021/ie101101zSearch in Google Scholar
[19] Stefanidis GD, Kaisare NS, Vlachos DG. Modeling ignition in catalytic microreactors. Chem Eng Technol. 2010;31(8):1170–5.10.1002/ceat.200800238Search in Google Scholar
[20] And HLC, Shih WH. Synthesis of zeolites A and X from fly ashes and their ion-exchange behavior with cobalt ions. Ind Eng Chem Res. 2013;39(11):4185–91.10.1021/ie990860sSearch in Google Scholar
[21] Zhong T, Kim SJ, Gu X, Dong J. Microwave synthesis of MFI-type zeolite membranes by seeded secondary growth without the use of organic structure directing agents. Microp Mesop Mater. 2009;118(1):224–31.10.1016/j.micromeso.2008.08.029Search in Google Scholar
[22] Yoo WC, Stoeger JA, Lee PS, Tsapatsis M, Stein A. High-performance randomly oriented zeolite membranes using brittle seeds and rapid thermal processing. Angew Chem. 2010;49(46):8699–703.10.1002/anie.201004029Search in Google Scholar PubMed
[23] Li Y, Weng W. Surface modification of hydroxyapatite by stearic acid: characterization and in vitro behaviors. J Mater Sci Mater Med. 2008;19(1):19–25.10.1007/s10856-007-3123-5Search in Google Scholar
[24] Liu H, Peng S, Shu L, Chen T, Bao T, Frost RL. Magnetic zeolite NaA: Synthesis, characterization based on metakaolin and its application for the removal of Cu2+, Pb2+. Chemosphere. 2013;91(11):1539–46.10.1016/j.chemosphere.2012.12.038Search in Google Scholar
[25] Ghadiri M, Kang AK, Gorji NE. X-ray diffraction of graphene contacted perovskite solar cells for moisture degradation and recovery at dark rest. Superlattices Microstruct. 2020;146:106677.10.1016/j.spmi.2020.106677Search in Google Scholar
[26] Washburn EW. The dynamics of capillary flow. Phys Rev J Archive. 1921;17(3):273–83.10.1103/PhysRev.17.273Search in Google Scholar
[27] Adamson AW. The physical chemistry of surfaces. Phys Chem Surf. 1977;124:192C.10.1149/1.2133374Search in Google Scholar
[28] Cassie A, Baxter X. Wettability of porous surfaces. Transactions of the Faraday. Society. 1944;40(1):546–51.10.1039/tf9444000546Search in Google Scholar
[29] Arbatan T, Fang X, Shen W. Superhydrophobic and oleophilic calcium carbonate powder as a selective oil sorbent with potential use in oil spill clean-ups. Chem Eng J. 2011;166(2):787–91.10.1016/j.cej.2010.11.015Search in Google Scholar
[30] Clifton J, Mcdonald P, Plater A, Oldfield F. An investigation into the efficiency of particle size separation using Stokes’ Law. Earth Surf Process Landf. 1999;24(8):725–30.10.1002/(SICI)1096-9837(199908)24:8<725::AID-ESP5>3.0.CO;2-WSearch in Google Scholar
[31] Schappert GT. Correction to Stokes Law for a charged sphere moving in a polarizable, compressible liquid. Phys Fluids. 1967;10(12):2702.10.1063/1.1762095Search in Google Scholar
[32] Stout JE, Arya SP, Genikhovich EL. The effect of nonlinear drag on the motion and settling velocity of heavy particles. J Atmos Sci. 2010;52(22):3836–48.10.1175/1520-0469(1995)052<3836:TEONDO>2.0.CO;2Search in Google Scholar
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