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Journal Pre-proofs 2D→3D conversion of superwetting mesh: A simple but powerful strategy for effective and efficient oil/water separation Tingping Lei, Dahai Lu, Zhenjin Xu, Weikang Xu, Jing Liu, Xudong Deng, Junjie Huang, Lei Xu, Xiaomei Cai, Liwei Lin PII: DOI: Reference:
S1383-5866(19)34322-9 https://doi.org/10.1016/j.seppur.2019.116244 SEPPUR 116244
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
22 September 2019 18 October 2019 22 October 2019
Please cite this article as: T. Lei, D. Lu, Z. Xu, W. Xu, J. Liu, X. Deng, J. Huang, L. Xu, X. Cai, L. Lin, 2D→3D conversion of superwetting mesh: A simple but powerful strategy for effective and efficient oil/water separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116244
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2D → 3D conversion of superwetting mesh: A simple but powerful strategy for effective and efficient oil/water separation Tingping Leia,b,1, Dahai Lub,1, Zhenjin Xub,1, Weikang Xub, Jing Liub, Xudong Dengb, Junjie Huangc, Lei Xud, Xiaomei Caic,*, and Liwei Line aFujian
Provincial Key Laboratory of Special Energy Manufacturing，Huaqiao University, Xiamen
361021, China bCollege cSchool
dSchool
of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China
of Science, Jimei University, Xiamen 361021, China of Mechanical and Electric Engineering, Jingdezhen Ceramic Institute, Jingdezhen
333403, China eDepartment
of Mechanical Engineering, University of California, Berkeley, California 94720,
USA
* Corresponding author: [email protected] (X. Cai) 1
These authors contributed equally to this work.
Keywords: 2D-to-3D conversion; superwetting mesh; pumping; oil/water separation; “Taylor cone” container Abstract: Previously unattainable success in using a single superwetting mesh to separate oil/water mixtures of various densities was achieved via 2D→3D conversion of superwetting mesh. Unlike 2D superwetting mesh, 3D mesh container allows both oil and water to touch the container wall, ensuring the passage of wetting phase through the mesh. By properly pumping out the non-wetting phase accumulated in the container, continuous oil/water separation can be realized. Experimental results demonstrate that the use of a single metal mesh only coated with the conveniently available superoleophilic-superhydrophobic materials not only realizes continuous separation of large-volume oil/water mixtures of any density ratios with high efficiency and high purity, but also shows good capacity in consecutive collection of oil from the water with the promising potential in separation of oil/water emulsions. The idea of 2D→3D conversion of the membrane together with external pumping strategy is a novel concept for oil/water separation, which will bring technologies based on special wettability a step closer to practical applications.
1. Introduction The endless discharge of oily wastewater and frequent oil leakages/spillages not only induce sever water pollution but also impose great threats to human health and the ecosystem.[1-3] Conventional countermeasures to massive oily water include in situ burning, gravity separation, centrifugation, air flotation, and so on. However, a majority of traditional methods are relatively inefficient and energy-consumptive and some even produce secondary pollution,[3-4] encouraging people to develop more effective strategies. In recent years, membrane separation technology based on special wettability has been intensively studied as a promising and cost-effective method for addressing oily water. [1-3, 5-9] By tuning the surface topography and chemical composition of highly-porous materials, researchers have turned metal meshes, filter paper, fabrics, etc. that are naturally amphiphilic (oil-wet and water-wet) to become “oil-removing” superoleophilic
and
superhydrophobic
(SOPhi-SHPho)
or
“water-removing”
superhydrophilic and superoleophobic (SHPhi-SOPho) structures for selective oil/water separation.[5, 10-15] However, continuous separation of both light (ρoil < ρwater) and heavy (ρoil > ρwater) oil/water mixtures with high efficiency and high purity has never been achieved via current single superwetting (SOPhi-SHPho or SHPhi-SOPho) membrane. There are two main reasons. First, the separation membranes documented in a large number of research publications are in two-dimensional (2D) planar structure; such a structure cannot avoid the problem of liquid accumulation (except when it is used as a “run-off” channel where the
separation capacity and efficiency are very unsatisfactory[16-17]).[4, 18-20] Specifically, in the case of the separation of light (ρoil<ρwater) oil/water mixtures with “oil-removing” membranes, water tends to accumulate to block the permeation of light oils; thus “oil-removing” membranes are not suitable for separating light oil/water mixtures. Similarly, for the separation of heavy (ρoil>ρwater) oil/water mixtures with “water-removing” membranes, heavy oil tends to accumulate to block the permeation of water, so membranes of this type are not suitable for separating heavy oil/water mixtures. Second, the existing studies based on single superwetting membrane focus on the removal of wetting phase but pay little or no attention to the removal of the non-wetting phase simultaneously.[21-24] As a result, the accumulation of the non-wetting phase will restrict the separation capacity of the separation device to a very limited amount for light or heavy oil/water mixture. To the best of our knowledge, currently huge effort is still devoted to the preparation of high-performance superwetting materials,[25-30] and too little attention is given to investigations of the optimal design of the separation membrane and the separation system. In this work, we convert the conventional 2D planar superwetting (e.g. SOPhi-SHPho) mesh into three-dimensional (3D) mesh container, such that the two phases (oil and water) can touch the container wall and thus guarantees easy passage of one phase (wetting phase) through the mesh. By properly pumping out the accumulated phase (non-wetting phase) in the container, we can easily realize continuous separation of large amounts of oil/water mixtures irrespective of light or heavy oils. The idea of 2D→3D conversion of superwetting
mesh together with external pumping strategy also shows good capacity in consecutive collection of oil (water) from the water (oil) surface and promising potential in separation of oil/water emulsions. 2. Experimental 2.1. Fabrication of Superwetting Mesh Container PVDF-candle soot (PVDF-CS), PVDF-SiO2 and PVDF-TiO2 were prepared as typical coating materials. Briefly, 0.4 g PVDF powder (a raw material that we have intensively studied previously[31-32]) and 0.6 g home-made CS were added to the mixed solvents of 12 mL N, N-dimethylformamide (DMF) and 8 mL acetone, with continuous stirring at room temperature for 8 h to form 36.2% cocktail of PVDF-CS. The cocktails of another two materials were obtained following exactly the same procedure and conditions except for replacing CS with commercially available oleophilic SiO2 and TiO2 nanopowders, respectively. The electrospray technique and dip coating method were respectively employed to fabricate superwetting PVDF-CS mesh and superwetting PVDF-SiO2 and PVDF-TiO2 meshes. Specifically, PVDF-CS cocktail was electrosprayed onto acetone- and ethanol-treated Cu mesh at a fixed voltage of 7 kV, a feed rate of 500 μL/h and a working distance (tip-to-mesh distance) of 10 cm, whereas superwetting PVDF-SiO2 and PVDF-TiO2 meshes were obtained by dipping the as-treated Cu meshes into the corresponding cocktails. All coating experiments were performed under an air atmosphere with relative humidity between 55 and 60%. The as-prepared superwetting mesh was transferred onto a stainless perforated colander or strainer (used as a support) to construct a built-in “Taylor
cone” container (Figure 2B-C), where a large gap was naturally formed between the strainer and the mesh. If not stated otherwise, the mesh number for Cu mesh is 120. 2.2. Preparation of Oil/Water Mixtures and Water-in-Oil Emulsion Three representative mixtures of n-hexadecane (0.77 g/cm3), p-chlorotoluene (1.06 g/cm3) and carbon tetrachloride (1.59 g/cm3) with water were simply prepared by pouring some amounts of oils into red dyed water with or without magnetically stirring for specific purposes. Water-in-oil (O/W) emulsion was prepared by mixing water and n-hexadecane at a volume ratio of 1:100 with addition of 2.0 g/L of Span 80 under violent stirring for 6 h to produce a white and milky solution.[33] Commonly, the obtained emulsion could be stable for 12 h when stored under ambient conditions. 2.3. Mixtures and Emulsion Separation Experiments (1) Continuous filtration of various oil/water mixtures was conducted using the proposed system (Figure 2A), where gravity-driven separation was realized for both light and heavy oil/water mixtures. By switching on the left pump (pump 1), the mixture was continuously pumped out into the superwetting “Taylor cone” container, and then the right pump (pump 2) was turned on for pumping out the accumulated water in the “Taylor cone” container. During the filtration process, the pumping rate was adjusted as expected and an electronic balance was included for on-line measurement of the mass of filtrated oil. (2) Oil collection was performed by using a pump and two pipes to collect n-hexadecane, an example light oil, from its oil/water mixture that was continuously stirred to simulate oil spills at sea. Similarly, a balance
was used to measure net weight (N.W.) of oil during the whole process. (3) Separation of O/W emulsion was carried out by pouring the as-prepared emulsified oil/water mixture into the superwetting “Taylor cone” container for gravity driven separation. Unlike Cu mesh (120 mesh) used in separation of immiscible mixtures, a stainless steel mesh with higher mesh density (600 mesh) was employed for coating. 2.4. Instruments and Characterization The surface morphologies and the elemental distribution of the as-prepared samples were investigated using field-emission scanning electron microscopy (FE-SEM, Zeiss, Sigma-HD-01-36, Germany) combined with energy-dispersive X-ray spectroscopy (EDS, 51-XMX1003, Oxford Instruments, UK). Optical microscopy images were taken on a scanning confocal microscope connected with Raman spectroscope (WITec alpha300 Raman, Germany). The wettability characterization was conducted on a contact angle analyzer (JC2000D3, Shanghai, China) using a droplet (9 μl) of water or hexane as an indicator at room temperature. The contact angle data were figured out based on ellipse fitting method and the final result was averaged from five measurements for each sample. The spreading and permeating behavior of n-hexadecane droplets on the as-prepared superwetting meshes was recorded using a high speed camera (MIRO M110, Vision Research, USA). The water content in the filtered oils was tested by a Karl Fischer Titrator (SN-WS200A).
3. Results and discussion
Figure 1A schematically illustrates a universal configuration for the selective passage of oil or water from oil/water mixtures by using a typical cone-shaped container made of the wetting-treatment mesh. The surface of the mesh is coated with SOPhi-SHPho and SHPhi-SOPho materials for the synergistic capillary-driven and gravity-driven removal of oil (top) and water (bottom) from various oil/water mixtures, respectively. According to Jurin’s law,[34] the pressure difference across the gas–liquid interface (known as “Laplace pressure”) for an idealized pore with a circular cross-section is expressed as
p =
2 cos = gh r0
(1)
where θ is the contact angle of the liquid drop on the capillary wall, r0 is the pore radius, γ is the surface tension of liquid, h is the rising (or falling) height of liquid, ρ is the liquid mass density (mass per unit volume), and g is the gravitational acceleration. When the surface is wettable (θ<90°), the liquid is spontaneously drawn into the pore because of the positive capillary effect (h>0). But it should be noted that in the case that the liquid is static, this effect is not enough to propel the liquid out of the pore, since a new equilibrium will be reached through the self-adjusting of the contact angle (Part I in supporting information). Nevertheless, in the case that the liquid is loaded into the wettable container, the additional pressure from the upper liquid will make the liquid within the pore grow up to a weight that is unbearable by the resultant forces such that the liquid of the lower position can pass through the pore. If the surface is non-wettable (θ>90°), a negative capillary effect (h<0) will result in the liquid to stay on the surface without penetration. Therefore, with the loading of the
oil/water mixture (of any density ratios) into a superwetting mesh container, the wetting phase will freely penetrate through the mesh and the non-wetting phase will accumulate in the container (Figure 1A). By further sucking out the non-wetting phase, a continuous oil/water separation can be realized. The container made of superwetting meshes can be configured into many different shapes. An inverted cone, a cylinder, and a frustum of a cone have been designed and compared in this work, especially for the case that the lower density phase is separated from the oil/water mixtures. In this case, the droplets on the sidewall will have different force conditions. As shown in Figure 1B, the gravity force (Fg) in the inverted cone (left) has a y-directional component (Fg,y), such that the adsorption of the droplet to the sidewall is weakened to assist the passage of the lower density phase. In contrast, Fg in the frustum of a cone (right), has a opposite y-directional component (−F′g,y) to slow down the passage of the lower density phase, whereas Fg in the cylinder (middle) remains “neutral” to the passage of the lower density phase. The above theoretical analysis is also supported by the experimental results from the separation of light oil/water mixture (Figure 1C). Details about the experiment are available from supporting information (Part II-a). In addition, the container in inverted cone shape also allows an easier control of the separation process as compared with the cylinder (Part II-b in supporting information). However, the experiments in the following study we adopted a mesh container in the shape of quasi-inverted cone (“Taylor cone”), because of problems encountered for the fabrication process and the practical usages (Part II-a in supporting information). Such
a mesh container can be simply made by coating superwetting materials on the mesh and transferring the as-coated meshes onto a perforated colander or strainer (used as a support) to construct a built-in “Taylor cone” container. Figure 1D shows several other structures from previously published works for comparison. For convenience of demonstration, below a SOPhi-SHPho mesh is used for the structures with a single mesh. The most common one is to sandwich a SHPho-SOPhi mesh between two containers (Figure 1D(d1)).[18, 20, 24, 35-36] However, the mesh in such a configuration (usually horizontally fixed) is not suitable for gravity-driven removal of light oils (ρoil<ρwater), because water naturally settles below oil to form a barrier layer to block oil permeation. Figure 1D(d2) shows that by tilting the mesh, the oil/mesh contact area can be increased to help the removal of light oils.[37-38] However, this tilt strategy has limited separation capacity and it is only suitable for the mixture with small amounts of water. The way to include a “run-off” channel to selectively allow one liquid to permeate through and the other to overflow the mesh (Figure 1D(d3&d4)),[16-17] have helped addressing these problems. But the separation capacity and efficiency of the “run-off” design are very unsatisfactory and it also easily has the oil/water mixture in the run-off channel as shown. A solution to the above problems is to use two antagonistic (SOPhi-SHPho and SHPhi-SOPho) meshes simultaneously as shown in Figure 1D(d5),[19-20, 39] where oil and water can continuously pass through SOPhi-SHPho and SHPhi-SOPho meshes, respectively. However, SHPhi-SOPho materials are very rare (in theory, oil-repellent surfaces often also water-repellent for water having a significantly higher
surface tension than oils).[29, 40-41]. An alternative is to use smart materials with switchable superwettability (controlled through external stimuli),[2, 42-44] but these triggered surfaces also suffer from tedious modification procedures with limited stability in complex oil/water environment. Unlike these five typical structures (Figure 1D), our proposed “Taylor cone” container based on 2D→3D conversion of single superwetting mesh is free of the above troubles when combined with the pumping operation. By properly pumping out the non-wetting phase at the bottom or top of the container (depending on the density of the oil), continuous separation of large amounts of oil/water mixtures (irrespective of either light or heavy oils) can be easily realized. For example, when the “Taylor cone” mesh container is coated with SOPhi-SHPho materials (Figure 2), light oils will penetrate from the side wall of the container, whereas heavy oils tend to penetrate from the bottom portion of the container. To realize the continuous separation of large amounts of light oil/water mixture, the outlet pipe #3 should be inserted near the bottom of the container to pump out the accumulated water (Figure 2, top left). For the case of heavy oil/water mixture, this pipe can be inserted close to the top of the container (Figure 2, top right). Figures 3A and 3B show representative snapshots of SOPhi-SHPho mesh based experimental system that was utilized to separate light (n-hexadecane, 0.77 g/cm3) and heavy (carbon tetrachloride, 1.59 g/cm3) oil/water mixtures, respectively. Details about the experiment are available from supporting information (Part III) and movies S1-S2. Briefly, the stratified light oil/water mixture with 105 mL n-hexadecane and
300 mL water was automatically separated by running the system, whereas continuous stirring was another added for the heavy oil/water mixture with 69 mL carbon tetrachloride and 2.3 L water during the whole separation process. For the sake of distinguishing the two phases, water was dyed in red color while oil remains transparent. After a period of pumping and filtration, we successfully realized the continuous separation of small amounts (~400 mL) of light oil/water mixture and large amounts (~2.4 L) of heavy oil/water mixture, as shown in Figures 3A and 3B, respectively. This SOPhi-SHPho mesh based system is also capable of continuously separating large-volume oil/water mixture with the oil density very close to that of water (e. g. p-chlorotoluene, 1.06 g/cm3) and light oil/water/heavy oil ternary mixtures (e. g. A separation efficiency over 95% was obtained for both light and heavy oils when n-hexadecane/water/carbon tetrachloride mixture with each weight about 100 g were used, Figure S5 and Table S1). These achievements are much better than the results reported previously; Table 1 lists some representative publications for comparison. The above results indicate that when SOPhi-SHPho mesh is used, our designed system can be applied to continuously separate oils of any density from industrial wastewaters, oil-spill mixtures, and polluted oceanic waters (Figure 3C). Here mesh containers based on “oil-removing” (SOPhi-SHPho) materials, such as PVDF-candle soot (PVDF-CS), PVDF-SiO2 and PVDF-TiO2 coatings were demonstrated for the separation system. The chemical compositions and wetting behavior of these coatings are confirmed by energy dispersive spectroscopy (EDS, Figure S6) and measured water and oil contact angles (insets, Figure 4E),
respectively. Based on the system (Figure 2A) and PVDF-CS, PVDF-SiO2 and PVDF-TiO2 coated mesh containers, the separation of three representative mixtures of n-hexadecane (0.77 g/cm3), p-chlorotoluene (1.06 g/cm3) and carbon tetrachloride (1.59 g/cm3) with water was studied. Typical separation processes of the above three different mixtures are available from movies S3-S5. The separation efficiency, η, was calculated as follows:
mp 100% m0
(2)
where m0 and mp are the mass of the oil before and after the separation process, respectively. Figure 4A shows the separation efficiency calculated for all mixtures (oil amount less than 90 mL with the exception of carbon tetrachloride ~ 120 mL). Because some oil can be absorbed on the SOPhi-SHPho coatings and some oil remains may be stuck on the pipe and container walls, the value of η is generally lower than those reported in other literatures (where no pipes and other supports were included).[10-11, 45] The average efficiency of over 90% is obtained for the separation of both light and heavy oil/water mixtures as well as the mixture with the oil density very close to that of water. The water content in the filtered n-hexadecane, p-chlorotoluene and carbon tetrachloride is 51, 43, and 47 ppm, respectively. Increasing the oil amount in the mixture will result in a higher efficiency and our experiments demonstrate that η of 99% or higher can be realized when the oil amount exceeds 390 mL (Figure 4B). The separation efficiency (η) versus the recycle number was
also
investigated
by
successively
separating
the
same
volume
of
n-hexdecane/water mixture (c.a. 325 mL (250 g) n-hexadecane and 300 mL water). A
high efficiency over 97.5% is calculated for all types of coatings in the 1st cycle. As the cycle number increases, the efficiency fluctuates between 98% and 102.5% (Figure 4C). This fluctuation comes from different amounts of the oil adsorbed and stuck to the coated meshes (as indicated by SEM in Figure 4(E)) and pipes during the cycling tests such that some of the results would show efficiency of over 100%. Furthermore, the PVDF-CS coating fails in the 15th cycle (Figure 4C) with lowest efficiency in the first separation cycle as CS is fragile and is difficult to be coated on the mesh, while it has the highest oil absorption amounts among the three coatings (Figure S7). It is noted that when the mixture is with toxic oils (e.g. carbon tetrachloride), the separation cycle is normally reduced (the separation cycle for PVDF-SiO2 and PVDF-TiO2 coated mesh is dramatically reduced to less than 8 during the separation of 100-mL carbon tetrachloride/100-mL water mixture). We also studied the effect of mesh sizes on the filtration by comparing PVDF-SiO2 coated meshes (100, 120, 150, 200) for the same amount of light oil/water mixture (c.a. 260 mL (200 g) n-hexdecane and 50 mL water). Figure 4D shows that the mass of filtrated oil during the same interval normally increases as the mesh number is reduced. However, it should be noted that if the mesh number is below 100 (e.g. 80 mesh), the mesh is not suitable for the separation function. Interestingly, we found that PVDF-SiO2 mesh with large mesh number of 200 could continuously separate large-volume mixture of n-hexadecane and water for at least 50 hours, which is meaningful for practical use. Apart from filtration, we can employ SOPhi-SHPho mesh container to collect oil
from the oil/water mixture surface. Figure 5A shows the combined use of PVDF-TiO2 coated mesh container with pump and pipe to continuously collect oil from the oil/water mixture. Since oil can penetrate the mesh, the as-prepared mesh was wrapped outside the strainer (pore size: 1 mm×1 mm) to form a “Taylor cone” container. The oil/water mixture with 61.25 g n-hexadecane (Figure 5A-a1) was continuously stirred to simulate oil spills at sea (Figure 5A-a2). In the pumping process, 53.38 g n-hexadecane was collected from the water surface within 30 s through a pipe (I.D. 5 mm & O.D. 7 mm) (Figure 5A(a3&a4), also Movie S6) with a calculated collection efficiency of about 87%. This relatively low efficiency is due to the aforementioned possible loss as well as the considerable idle oil between the gap of the strainer and the mesh (due to noncontact between the strainer bottom and the mesh, see Experimental Section) as the pipe fails to reach the gaps. By digging a big hole on the strainer bottom to extend the suction pipe deep into the gap, the collection efficiency over 90% was obtained (Figure S8). Such a mesh container can also be used for gravity driven separation of emulsified oil/water mixtures. Figure 5B-b1 shows the separation of water-in-oil (W/O) emulsion using the PVDF-TiO2 coated mesh (600 mesh) container with SEM micrographs of raw stainless steel with PVDF-TiO2 coating (left) and without coating (right) shown in Figure 5B-b3. The emulsion droplets demulsified once they touched the coated mesh, as evidenced by oil permeating through the mesh container within a few seconds (Movie S7). The optical photos from the microscope showed that before and after the filtration process with no droplets left in the collected filtrate (Figure 5B-b2).
Hence, by properly pumping out the accumulated water, continuous separation of large-volume W/O emulsions could be achieved. Our separation system based on a simple combination of superwetting mesh container with the external pumping operation is advantageous over the state-of-art systems: i) allowing the mesh only coated with the conveniently available SOPhi-SHPho materials to continuously filtrate immiscible oil/water mixtures of different densities with high efficiency and high purity; ii) universal for other wettable material systems (e.g. SHPhi-SOPho materials) to achieve the same or similar filtration results; iii) capable of collecting oils ( or water) from the water/oil surface and separating W/O emulsions (or O/W emulsions); iv) very long service life for the coated mesh. With the progress made in superwetting coatings,[28-29, 45-52] Janus (unidirectional) membranes,[53-55] and “double-layer” meshes (that integrate separation of oil/water mixtures and degradation or absorption of soluble pollutants in water),[56-58] our proposed engineering system can receive great interests for tackling the worldwide problems in oily wastewater, oil spills, and water scarcity, etc. 4. Conclusions In summary, a powerful strategy for the effective and efficient separation of oil/water mixtures with any density ratios has been demonstrated via 2D→3D conversion of superwetting mesh simply combined with the external pumping operation. Results show that the use of a single mesh only coated with the conveniently available superwetting (e.g. SOPhi-SHPho) materials, not only realizes continuous separation of large-volume oil/water mixtures (irrespective of the oil
density) with high efficiency and high purity, but also shows good capacity in consecutive collection of oil (water) from the water (oil) surface with the promising potential in separation of oil/water emulsions. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 61404059 and 11762007). References [1]
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Tables & Figures Table 1 Comparison of the representative oil/water separation based on a single SHPhi-SOPho mesh Device configuration
3D mesh + pump
2D mesh sandwiched (Figure 1D-d1) 2D mesh tilted (Figure 1D-d2) 2D mesh in “run-off” (Figure 1D-d3&d4)
Oil/water mixture ρoil < ρwater
Water amount
Separation mode
Practicality
Reference
Unlimited
Continuous
Good
Our work
ρoil < ρwater
Extremely limited
Batch
Poor
40
ρoil > ρwater
Limited
Batch
Good
40,41
ρoil < ρwater
Very limited
Batch
Poor
32
ρoil > ρwater
Limited
Batch
Good
33
Unlimited
Continuous
Very poor
15,16
ρoil > ρwater
ρoil < ρwater ρoil > ρwater
A
B Superoleophilic-superhydrophobic
y
y x
x
-F' g,y
F g,y Oil
Fg
C
ρOil < ρWater
Pump 1
Pump 1
ρOil > ρWater Pump 2
Wate r
Oil/water mixture
Fg
Inverted cone
ρOil > ρWater Cylinder
150 100
Pump 2
50 0
Superhydrophilic-superoleophobic
D
200 Mass of filtrated oil (g)
ρOil < ρWater
Fg
d1
d2
d3 Oil
Water
0
20
d4
40
60 80 Time (s)
100
120
d5 Oil
Water
Figure 1 (A) Schematic illustration of the selective removal of oil (top) and water (bottom) from both light and heavy oil/water mixtures with SOPhi-SHPho and SHPhi-SOPho mesh containers, respectively. (B) Effect of droplet gravity on its passage through the sidewall of inverted cone, cylinder and frustum of a cone. (C) Filtrated oil from n-hexadecane/water mixture (c.a. 325 ml (250 g) n-hexadecane and 50 ml water) during the filtration for 120 s (mixture pumping rate: 180 ml/min) using both SOPhi-SHPho inverted cone and cylinder of the same volume (Part II-a in supporting information). (D) Five typical device configurations commonly used for the filtration of immiscible oil/water mixtures (black fine arrow indicates the location of SOPhi-SHPho mesh while red coarse arrow indicates the location of SHPhi-SOPho mesh) in prior publications.
Figure 2 The proposed system for the continuous separation of large-volume oil/water mixtures: A pipe (#1) connected to the inlet of a peristaltic pump (pump 1) is inserted into the bottom of leftmost container that is filled with oil/water mixture, and the outlet of pump 1 is connected to a SOPhi-SHPho “Taylor cone” mesh container by another pipe (#2); oil will penetrate through the “Taylor cone” container, while water is accumulated; to pump out the accumulated water, another pipe (#3) connected to pump 2 is inserted into the mesh container at a bottom location for light oil/water mixtures and at the top location for heavy oil/water mixtures.
A
Light oil/water mixture
Before
B
Heavy oil/water mixture Before
SOPhi-SHPho mesh
SOPhi-SHPho mesh Pump 1
Pump 2
NIL
H2O
NIL NIL
Oil
Pump 2 Oil
a1
Pump 1
NIL
H2O
Magnetic stirrer
b1 After
After
H2O Oil
Oil
H2O
a2
b2
C
Water
Mixture
SOPhi-SHPho mesh container Pump
Irrespective of oil density Oil
Pump
Oil collector Water outlet
Inlet
Oil/water mixture
Figure 3 Snapshots of the use of experimental system to separate light oil/water mixture (A) and heavy oil/water mixture with continuous stirring (B). (C) Illustration of the SOPhi-SHPho mesh-based system for continuous separation of oily wastewater irrespective of the oil density. NIL in Figures indicates nothing in the beaker.
100
Separation efficiency (%)
95 90 85
Separation efficiency (%)
()
()
()
80 75
C
PVDF-TiO2
n-Hexadecane
p-Chlorotoluene
B 100 Separation efficiency (%)
PVDF-SiO2
PVDF-CS
~ 99.09% (394.5 ml)
98
~ 99.53% (987 ml)
97 96 95 94 0
100 200 300 700 Mass of n-Hexadecane (g)
800
D
103
Pumping rate: 200 ml/min Mesh 100 Mesh 120
180
102 101 100 99 98 97
PVDF-CS PVDF-SiO2
96
PVDF-TiO2
95
99
93
Carbon tetrachloride
Mass of filtrated oil (g)
A
0
10
E
20 30 Number of cycles
40
150 120 90
Mesh 100 Mesh 120 Mesh 150 Mesh 200 Pumping rate: 150 ml/min
60 30 0
50
0
10 20 30 40 50 60 70 80 90 Time (s) WCA~ 149o
WCA~ 144o
WCA~ 152o
500 μm
PVDF-CS
PVDF-SiO2
500 μm
PVDF-TiO2
500 μm
Cu
OCA~ 0
100 μm
OCA~ 0
100 μm
OCA~ 0
100 μm
Figure 4 (A) The separation efficiency of three representative mixtures of n-hexadecane, p-chlorotoluene and carbon tetrachloride with water using the proposed system with PVDF-CS, PVDF-SiO2 and PVDF-TiO2 coated mesh containers. (B) The separation efficiency of light oil/water mixture with different amounts of n-hexadecane and 300 mL water using the as-prepared PVDF-SiO2 mesh container. (C) The cycling number of the prototype meshes for the oil/water separation process by using the n-hexadecane/water mixture (c.a. 325 mL (250 g) n-hexadecane and 300 mL water) for different mesh containers. (D) The effect of mesh number on the filtration of the same amount of light oil/water mixture (c.a. 260 mL (200 g) n-hexadecane and 50 mL water) recorded during the filtration for 90 s using PVDF-SiO2 coated meshes (100, 120, 150, 200) under different pumping rates (150 or 200 ml/min). (E) FE-SEM images of the as-prepared PVDF-CS, PVDF-SiO2 and PVDF-TiO2 coated meshes; insets are water contact angle (left) and oil contact angle (right) of the corresponding meshes.
B
A a1
b1
Before
b2
Water
Oil 61.25 g (N.W.)
a4
After
a2
Oil
10 μm
Oil
Mixture
53.38 g (N.W.)
Before a3 Oil collection in process
pipe pump
After 10 μm
b3 100 μm
500 μm
100 μm
500 μm
Figure 5 (A) Snapshots of continuously collecting n-hexadecane from water surface using the PVDF-TiO2 coated mesh container with a pump and a pipe to extract oil: (a1) pure oil (61.25 g, left) and pure water in red (right); (a2) oil/water mixture with continuously stirring to simulate oil spills at sea; (a3) oil collection in process by putting the mesh container into the oil/water mixture; (a4) pure oil (53.38 g) collected within 30 s. (B) The solely gravity-driven separation of water-in-oil (W/O) emulsion using the stainless steel mesh (600 mesh) dip-coated with PVDF-TiO2: Photographs (b1) and optical micrographs (b2) of the as-prepared W/O emulsion before and after separation; (b3) FE-SEM images of raw stainless steel with PVDF-TiO2 coating (left) and without coating (right).
Highlights: The importance of 2D→3D conversion of superwetting mesh is demonstrated. A simple, versatile and universal oil/water separation system has been developed. External pumping operation was introduced for the removal of non-wetting phase. The use of single SOPhi-SHPho mesh can separate oil/water mixtures effectively. A mechanism of synergistic capillary-driven and gravity-driven separation is proposed.
2D → 3D Conversion of Superwetting Mesh: A Simple but Powerful Strategy for Effective and Efficient Oil/Water Separation
Tingping Leia,1, Dahai Lua,1, Zhenjin Xua,1, Weikang Xua, Jing Liua, Xudong Denga, Junjie Huangb, Lei Xuc, Xiaomei Caib,*, and Liwei Lind aCollege
of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021,
China bSchool
of Science, Jimei University, Xiamen 361021, China
cSchool
of Mechanical and Electric Engineering, Jingdezhen Ceramic Institute, Jingdezhen
333403, China dDepartment
of Mechanical Engineering, University of California, Berkeley, California
94720, USA *Corresponding 1These
author: [email protected] (X. Cai)
authors contributed equally to this work.
Previously unattainable success in using a single superwetting mesh to separate oil/water mixtures of various densities was achieved via 2D→3D conversion of superwetting mesh. Unlike 2D superwetting mesh, 3D mesh container allows both oil and water to touch the mesh, ensuring the passage of wetting phase through the mesh. By pumping out the accumulated non-wetting phase, continuous separation was realized. Keywords: 2D-to-3D conversion; superwetting mesh; pumping; oil/water separation; “Taylor
cone” container
The authors declare no competing financial interest.
S1383-5866(19)34322-9 https://doi.org/10.1016/j.seppur.2019.116244 SEPPUR 116244
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
22 September 2019 18 October 2019 22 October 2019
Please cite this article as: T. Lei, D. Lu, Z. Xu, W. Xu, J. Liu, X. Deng, J. Huang, L. Xu, X. Cai, L. Lin, 2D→3D conversion of superwetting mesh: A simple but powerful strategy for effective and efficient oil/water separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116244
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© 2019 Published by Elsevier B.V.
2D → 3D conversion of superwetting mesh: A simple but powerful strategy for effective and efficient oil/water separation Tingping Leia,b,1, Dahai Lub,1, Zhenjin Xub,1, Weikang Xub, Jing Liub, Xudong Dengb, Junjie Huangc, Lei Xud, Xiaomei Caic,*, and Liwei Line aFujian
Provincial Key Laboratory of Special Energy Manufacturing，Huaqiao University, Xiamen
361021, China bCollege cSchool
dSchool
of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China
of Science, Jimei University, Xiamen 361021, China of Mechanical and Electric Engineering, Jingdezhen Ceramic Institute, Jingdezhen
333403, China eDepartment
of Mechanical Engineering, University of California, Berkeley, California 94720,
USA
* Corresponding author: [email protected] (X. Cai) 1
These authors contributed equally to this work.
Keywords: 2D-to-3D conversion; superwetting mesh; pumping; oil/water separation; “Taylor cone” container Abstract: Previously unattainable success in using a single superwetting mesh to separate oil/water mixtures of various densities was achieved via 2D→3D conversion of superwetting mesh. Unlike 2D superwetting mesh, 3D mesh container allows both oil and water to touch the container wall, ensuring the passage of wetting phase through the mesh. By properly pumping out the non-wetting phase accumulated in the container, continuous oil/water separation can be realized. Experimental results demonstrate that the use of a single metal mesh only coated with the conveniently available superoleophilic-superhydrophobic materials not only realizes continuous separation of large-volume oil/water mixtures of any density ratios with high efficiency and high purity, but also shows good capacity in consecutive collection of oil from the water with the promising potential in separation of oil/water emulsions. The idea of 2D→3D conversion of the membrane together with external pumping strategy is a novel concept for oil/water separation, which will bring technologies based on special wettability a step closer to practical applications.
1. Introduction The endless discharge of oily wastewater and frequent oil leakages/spillages not only induce sever water pollution but also impose great threats to human health and the ecosystem.[1-3] Conventional countermeasures to massive oily water include in situ burning, gravity separation, centrifugation, air flotation, and so on. However, a majority of traditional methods are relatively inefficient and energy-consumptive and some even produce secondary pollution,[3-4] encouraging people to develop more effective strategies. In recent years, membrane separation technology based on special wettability has been intensively studied as a promising and cost-effective method for addressing oily water. [1-3, 5-9] By tuning the surface topography and chemical composition of highly-porous materials, researchers have turned metal meshes, filter paper, fabrics, etc. that are naturally amphiphilic (oil-wet and water-wet) to become “oil-removing” superoleophilic
and
superhydrophobic
(SOPhi-SHPho)
or
“water-removing”
superhydrophilic and superoleophobic (SHPhi-SOPho) structures for selective oil/water separation.[5, 10-15] However, continuous separation of both light (ρoil < ρwater) and heavy (ρoil > ρwater) oil/water mixtures with high efficiency and high purity has never been achieved via current single superwetting (SOPhi-SHPho or SHPhi-SOPho) membrane. There are two main reasons. First, the separation membranes documented in a large number of research publications are in two-dimensional (2D) planar structure; such a structure cannot avoid the problem of liquid accumulation (except when it is used as a “run-off” channel where the
separation capacity and efficiency are very unsatisfactory[16-17]).[4, 18-20] Specifically, in the case of the separation of light (ρoil<ρwater) oil/water mixtures with “oil-removing” membranes, water tends to accumulate to block the permeation of light oils; thus “oil-removing” membranes are not suitable for separating light oil/water mixtures. Similarly, for the separation of heavy (ρoil>ρwater) oil/water mixtures with “water-removing” membranes, heavy oil tends to accumulate to block the permeation of water, so membranes of this type are not suitable for separating heavy oil/water mixtures. Second, the existing studies based on single superwetting membrane focus on the removal of wetting phase but pay little or no attention to the removal of the non-wetting phase simultaneously.[21-24] As a result, the accumulation of the non-wetting phase will restrict the separation capacity of the separation device to a very limited amount for light or heavy oil/water mixture. To the best of our knowledge, currently huge effort is still devoted to the preparation of high-performance superwetting materials,[25-30] and too little attention is given to investigations of the optimal design of the separation membrane and the separation system. In this work, we convert the conventional 2D planar superwetting (e.g. SOPhi-SHPho) mesh into three-dimensional (3D) mesh container, such that the two phases (oil and water) can touch the container wall and thus guarantees easy passage of one phase (wetting phase) through the mesh. By properly pumping out the accumulated phase (non-wetting phase) in the container, we can easily realize continuous separation of large amounts of oil/water mixtures irrespective of light or heavy oils. The idea of 2D→3D conversion of superwetting
mesh together with external pumping strategy also shows good capacity in consecutive collection of oil (water) from the water (oil) surface and promising potential in separation of oil/water emulsions. 2. Experimental 2.1. Fabrication of Superwetting Mesh Container PVDF-candle soot (PVDF-CS), PVDF-SiO2 and PVDF-TiO2 were prepared as typical coating materials. Briefly, 0.4 g PVDF powder (a raw material that we have intensively studied previously[31-32]) and 0.6 g home-made CS were added to the mixed solvents of 12 mL N, N-dimethylformamide (DMF) and 8 mL acetone, with continuous stirring at room temperature for 8 h to form 36.2% cocktail of PVDF-CS. The cocktails of another two materials were obtained following exactly the same procedure and conditions except for replacing CS with commercially available oleophilic SiO2 and TiO2 nanopowders, respectively. The electrospray technique and dip coating method were respectively employed to fabricate superwetting PVDF-CS mesh and superwetting PVDF-SiO2 and PVDF-TiO2 meshes. Specifically, PVDF-CS cocktail was electrosprayed onto acetone- and ethanol-treated Cu mesh at a fixed voltage of 7 kV, a feed rate of 500 μL/h and a working distance (tip-to-mesh distance) of 10 cm, whereas superwetting PVDF-SiO2 and PVDF-TiO2 meshes were obtained by dipping the as-treated Cu meshes into the corresponding cocktails. All coating experiments were performed under an air atmosphere with relative humidity between 55 and 60%. The as-prepared superwetting mesh was transferred onto a stainless perforated colander or strainer (used as a support) to construct a built-in “Taylor
cone” container (Figure 2B-C), where a large gap was naturally formed between the strainer and the mesh. If not stated otherwise, the mesh number for Cu mesh is 120. 2.2. Preparation of Oil/Water Mixtures and Water-in-Oil Emulsion Three representative mixtures of n-hexadecane (0.77 g/cm3), p-chlorotoluene (1.06 g/cm3) and carbon tetrachloride (1.59 g/cm3) with water were simply prepared by pouring some amounts of oils into red dyed water with or without magnetically stirring for specific purposes. Water-in-oil (O/W) emulsion was prepared by mixing water and n-hexadecane at a volume ratio of 1:100 with addition of 2.0 g/L of Span 80 under violent stirring for 6 h to produce a white and milky solution.[33] Commonly, the obtained emulsion could be stable for 12 h when stored under ambient conditions. 2.3. Mixtures and Emulsion Separation Experiments (1) Continuous filtration of various oil/water mixtures was conducted using the proposed system (Figure 2A), where gravity-driven separation was realized for both light and heavy oil/water mixtures. By switching on the left pump (pump 1), the mixture was continuously pumped out into the superwetting “Taylor cone” container, and then the right pump (pump 2) was turned on for pumping out the accumulated water in the “Taylor cone” container. During the filtration process, the pumping rate was adjusted as expected and an electronic balance was included for on-line measurement of the mass of filtrated oil. (2) Oil collection was performed by using a pump and two pipes to collect n-hexadecane, an example light oil, from its oil/water mixture that was continuously stirred to simulate oil spills at sea. Similarly, a balance
was used to measure net weight (N.W.) of oil during the whole process. (3) Separation of O/W emulsion was carried out by pouring the as-prepared emulsified oil/water mixture into the superwetting “Taylor cone” container for gravity driven separation. Unlike Cu mesh (120 mesh) used in separation of immiscible mixtures, a stainless steel mesh with higher mesh density (600 mesh) was employed for coating. 2.4. Instruments and Characterization The surface morphologies and the elemental distribution of the as-prepared samples were investigated using field-emission scanning electron microscopy (FE-SEM, Zeiss, Sigma-HD-01-36, Germany) combined with energy-dispersive X-ray spectroscopy (EDS, 51-XMX1003, Oxford Instruments, UK). Optical microscopy images were taken on a scanning confocal microscope connected with Raman spectroscope (WITec alpha300 Raman, Germany). The wettability characterization was conducted on a contact angle analyzer (JC2000D3, Shanghai, China) using a droplet (9 μl) of water or hexane as an indicator at room temperature. The contact angle data were figured out based on ellipse fitting method and the final result was averaged from five measurements for each sample. The spreading and permeating behavior of n-hexadecane droplets on the as-prepared superwetting meshes was recorded using a high speed camera (MIRO M110, Vision Research, USA). The water content in the filtered oils was tested by a Karl Fischer Titrator (SN-WS200A).
3. Results and discussion
Figure 1A schematically illustrates a universal configuration for the selective passage of oil or water from oil/water mixtures by using a typical cone-shaped container made of the wetting-treatment mesh. The surface of the mesh is coated with SOPhi-SHPho and SHPhi-SOPho materials for the synergistic capillary-driven and gravity-driven removal of oil (top) and water (bottom) from various oil/water mixtures, respectively. According to Jurin’s law,[34] the pressure difference across the gas–liquid interface (known as “Laplace pressure”) for an idealized pore with a circular cross-section is expressed as
p =
2 cos = gh r0
(1)
where θ is the contact angle of the liquid drop on the capillary wall, r0 is the pore radius, γ is the surface tension of liquid, h is the rising (or falling) height of liquid, ρ is the liquid mass density (mass per unit volume), and g is the gravitational acceleration. When the surface is wettable (θ<90°), the liquid is spontaneously drawn into the pore because of the positive capillary effect (h>0). But it should be noted that in the case that the liquid is static, this effect is not enough to propel the liquid out of the pore, since a new equilibrium will be reached through the self-adjusting of the contact angle (Part I in supporting information). Nevertheless, in the case that the liquid is loaded into the wettable container, the additional pressure from the upper liquid will make the liquid within the pore grow up to a weight that is unbearable by the resultant forces such that the liquid of the lower position can pass through the pore. If the surface is non-wettable (θ>90°), a negative capillary effect (h<0) will result in the liquid to stay on the surface without penetration. Therefore, with the loading of the
oil/water mixture (of any density ratios) into a superwetting mesh container, the wetting phase will freely penetrate through the mesh and the non-wetting phase will accumulate in the container (Figure 1A). By further sucking out the non-wetting phase, a continuous oil/water separation can be realized. The container made of superwetting meshes can be configured into many different shapes. An inverted cone, a cylinder, and a frustum of a cone have been designed and compared in this work, especially for the case that the lower density phase is separated from the oil/water mixtures. In this case, the droplets on the sidewall will have different force conditions. As shown in Figure 1B, the gravity force (Fg) in the inverted cone (left) has a y-directional component (Fg,y), such that the adsorption of the droplet to the sidewall is weakened to assist the passage of the lower density phase. In contrast, Fg in the frustum of a cone (right), has a opposite y-directional component (−F′g,y) to slow down the passage of the lower density phase, whereas Fg in the cylinder (middle) remains “neutral” to the passage of the lower density phase. The above theoretical analysis is also supported by the experimental results from the separation of light oil/water mixture (Figure 1C). Details about the experiment are available from supporting information (Part II-a). In addition, the container in inverted cone shape also allows an easier control of the separation process as compared with the cylinder (Part II-b in supporting information). However, the experiments in the following study we adopted a mesh container in the shape of quasi-inverted cone (“Taylor cone”), because of problems encountered for the fabrication process and the practical usages (Part II-a in supporting information). Such
a mesh container can be simply made by coating superwetting materials on the mesh and transferring the as-coated meshes onto a perforated colander or strainer (used as a support) to construct a built-in “Taylor cone” container. Figure 1D shows several other structures from previously published works for comparison. For convenience of demonstration, below a SOPhi-SHPho mesh is used for the structures with a single mesh. The most common one is to sandwich a SHPho-SOPhi mesh between two containers (Figure 1D(d1)).[18, 20, 24, 35-36] However, the mesh in such a configuration (usually horizontally fixed) is not suitable for gravity-driven removal of light oils (ρoil<ρwater), because water naturally settles below oil to form a barrier layer to block oil permeation. Figure 1D(d2) shows that by tilting the mesh, the oil/mesh contact area can be increased to help the removal of light oils.[37-38] However, this tilt strategy has limited separation capacity and it is only suitable for the mixture with small amounts of water. The way to include a “run-off” channel to selectively allow one liquid to permeate through and the other to overflow the mesh (Figure 1D(d3&d4)),[16-17] have helped addressing these problems. But the separation capacity and efficiency of the “run-off” design are very unsatisfactory and it also easily has the oil/water mixture in the run-off channel as shown. A solution to the above problems is to use two antagonistic (SOPhi-SHPho and SHPhi-SOPho) meshes simultaneously as shown in Figure 1D(d5),[19-20, 39] where oil and water can continuously pass through SOPhi-SHPho and SHPhi-SOPho meshes, respectively. However, SHPhi-SOPho materials are very rare (in theory, oil-repellent surfaces often also water-repellent for water having a significantly higher
surface tension than oils).[29, 40-41]. An alternative is to use smart materials with switchable superwettability (controlled through external stimuli),[2, 42-44] but these triggered surfaces also suffer from tedious modification procedures with limited stability in complex oil/water environment. Unlike these five typical structures (Figure 1D), our proposed “Taylor cone” container based on 2D→3D conversion of single superwetting mesh is free of the above troubles when combined with the pumping operation. By properly pumping out the non-wetting phase at the bottom or top of the container (depending on the density of the oil), continuous separation of large amounts of oil/water mixtures (irrespective of either light or heavy oils) can be easily realized. For example, when the “Taylor cone” mesh container is coated with SOPhi-SHPho materials (Figure 2), light oils will penetrate from the side wall of the container, whereas heavy oils tend to penetrate from the bottom portion of the container. To realize the continuous separation of large amounts of light oil/water mixture, the outlet pipe #3 should be inserted near the bottom of the container to pump out the accumulated water (Figure 2, top left). For the case of heavy oil/water mixture, this pipe can be inserted close to the top of the container (Figure 2, top right). Figures 3A and 3B show representative snapshots of SOPhi-SHPho mesh based experimental system that was utilized to separate light (n-hexadecane, 0.77 g/cm3) and heavy (carbon tetrachloride, 1.59 g/cm3) oil/water mixtures, respectively. Details about the experiment are available from supporting information (Part III) and movies S1-S2. Briefly, the stratified light oil/water mixture with 105 mL n-hexadecane and
300 mL water was automatically separated by running the system, whereas continuous stirring was another added for the heavy oil/water mixture with 69 mL carbon tetrachloride and 2.3 L water during the whole separation process. For the sake of distinguishing the two phases, water was dyed in red color while oil remains transparent. After a period of pumping and filtration, we successfully realized the continuous separation of small amounts (~400 mL) of light oil/water mixture and large amounts (~2.4 L) of heavy oil/water mixture, as shown in Figures 3A and 3B, respectively. This SOPhi-SHPho mesh based system is also capable of continuously separating large-volume oil/water mixture with the oil density very close to that of water (e. g. p-chlorotoluene, 1.06 g/cm3) and light oil/water/heavy oil ternary mixtures (e. g. A separation efficiency over 95% was obtained for both light and heavy oils when n-hexadecane/water/carbon tetrachloride mixture with each weight about 100 g were used, Figure S5 and Table S1). These achievements are much better than the results reported previously; Table 1 lists some representative publications for comparison. The above results indicate that when SOPhi-SHPho mesh is used, our designed system can be applied to continuously separate oils of any density from industrial wastewaters, oil-spill mixtures, and polluted oceanic waters (Figure 3C). Here mesh containers based on “oil-removing” (SOPhi-SHPho) materials, such as PVDF-candle soot (PVDF-CS), PVDF-SiO2 and PVDF-TiO2 coatings were demonstrated for the separation system. The chemical compositions and wetting behavior of these coatings are confirmed by energy dispersive spectroscopy (EDS, Figure S6) and measured water and oil contact angles (insets, Figure 4E),
respectively. Based on the system (Figure 2A) and PVDF-CS, PVDF-SiO2 and PVDF-TiO2 coated mesh containers, the separation of three representative mixtures of n-hexadecane (0.77 g/cm3), p-chlorotoluene (1.06 g/cm3) and carbon tetrachloride (1.59 g/cm3) with water was studied. Typical separation processes of the above three different mixtures are available from movies S3-S5. The separation efficiency, η, was calculated as follows:
mp 100% m0
(2)
where m0 and mp are the mass of the oil before and after the separation process, respectively. Figure 4A shows the separation efficiency calculated for all mixtures (oil amount less than 90 mL with the exception of carbon tetrachloride ~ 120 mL). Because some oil can be absorbed on the SOPhi-SHPho coatings and some oil remains may be stuck on the pipe and container walls, the value of η is generally lower than those reported in other literatures (where no pipes and other supports were included).[10-11, 45] The average efficiency of over 90% is obtained for the separation of both light and heavy oil/water mixtures as well as the mixture with the oil density very close to that of water. The water content in the filtered n-hexadecane, p-chlorotoluene and carbon tetrachloride is 51, 43, and 47 ppm, respectively. Increasing the oil amount in the mixture will result in a higher efficiency and our experiments demonstrate that η of 99% or higher can be realized when the oil amount exceeds 390 mL (Figure 4B). The separation efficiency (η) versus the recycle number was
also
investigated
by
successively
separating
the
same
volume
of
n-hexdecane/water mixture (c.a. 325 mL (250 g) n-hexadecane and 300 mL water). A
high efficiency over 97.5% is calculated for all types of coatings in the 1st cycle. As the cycle number increases, the efficiency fluctuates between 98% and 102.5% (Figure 4C). This fluctuation comes from different amounts of the oil adsorbed and stuck to the coated meshes (as indicated by SEM in Figure 4(E)) and pipes during the cycling tests such that some of the results would show efficiency of over 100%. Furthermore, the PVDF-CS coating fails in the 15th cycle (Figure 4C) with lowest efficiency in the first separation cycle as CS is fragile and is difficult to be coated on the mesh, while it has the highest oil absorption amounts among the three coatings (Figure S7). It is noted that when the mixture is with toxic oils (e.g. carbon tetrachloride), the separation cycle is normally reduced (the separation cycle for PVDF-SiO2 and PVDF-TiO2 coated mesh is dramatically reduced to less than 8 during the separation of 100-mL carbon tetrachloride/100-mL water mixture). We also studied the effect of mesh sizes on the filtration by comparing PVDF-SiO2 coated meshes (100, 120, 150, 200) for the same amount of light oil/water mixture (c.a. 260 mL (200 g) n-hexdecane and 50 mL water). Figure 4D shows that the mass of filtrated oil during the same interval normally increases as the mesh number is reduced. However, it should be noted that if the mesh number is below 100 (e.g. 80 mesh), the mesh is not suitable for the separation function. Interestingly, we found that PVDF-SiO2 mesh with large mesh number of 200 could continuously separate large-volume mixture of n-hexadecane and water for at least 50 hours, which is meaningful for practical use. Apart from filtration, we can employ SOPhi-SHPho mesh container to collect oil
from the oil/water mixture surface. Figure 5A shows the combined use of PVDF-TiO2 coated mesh container with pump and pipe to continuously collect oil from the oil/water mixture. Since oil can penetrate the mesh, the as-prepared mesh was wrapped outside the strainer (pore size: 1 mm×1 mm) to form a “Taylor cone” container. The oil/water mixture with 61.25 g n-hexadecane (Figure 5A-a1) was continuously stirred to simulate oil spills at sea (Figure 5A-a2). In the pumping process, 53.38 g n-hexadecane was collected from the water surface within 30 s through a pipe (I.D. 5 mm & O.D. 7 mm) (Figure 5A(a3&a4), also Movie S6) with a calculated collection efficiency of about 87%. This relatively low efficiency is due to the aforementioned possible loss as well as the considerable idle oil between the gap of the strainer and the mesh (due to noncontact between the strainer bottom and the mesh, see Experimental Section) as the pipe fails to reach the gaps. By digging a big hole on the strainer bottom to extend the suction pipe deep into the gap, the collection efficiency over 90% was obtained (Figure S8). Such a mesh container can also be used for gravity driven separation of emulsified oil/water mixtures. Figure 5B-b1 shows the separation of water-in-oil (W/O) emulsion using the PVDF-TiO2 coated mesh (600 mesh) container with SEM micrographs of raw stainless steel with PVDF-TiO2 coating (left) and without coating (right) shown in Figure 5B-b3. The emulsion droplets demulsified once they touched the coated mesh, as evidenced by oil permeating through the mesh container within a few seconds (Movie S7). The optical photos from the microscope showed that before and after the filtration process with no droplets left in the collected filtrate (Figure 5B-b2).
Hence, by properly pumping out the accumulated water, continuous separation of large-volume W/O emulsions could be achieved. Our separation system based on a simple combination of superwetting mesh container with the external pumping operation is advantageous over the state-of-art systems: i) allowing the mesh only coated with the conveniently available SOPhi-SHPho materials to continuously filtrate immiscible oil/water mixtures of different densities with high efficiency and high purity; ii) universal for other wettable material systems (e.g. SHPhi-SOPho materials) to achieve the same or similar filtration results; iii) capable of collecting oils ( or water) from the water/oil surface and separating W/O emulsions (or O/W emulsions); iv) very long service life for the coated mesh. With the progress made in superwetting coatings,[28-29, 45-52] Janus (unidirectional) membranes,[53-55] and “double-layer” meshes (that integrate separation of oil/water mixtures and degradation or absorption of soluble pollutants in water),[56-58] our proposed engineering system can receive great interests for tackling the worldwide problems in oily wastewater, oil spills, and water scarcity, etc. 4. Conclusions In summary, a powerful strategy for the effective and efficient separation of oil/water mixtures with any density ratios has been demonstrated via 2D→3D conversion of superwetting mesh simply combined with the external pumping operation. Results show that the use of a single mesh only coated with the conveniently available superwetting (e.g. SOPhi-SHPho) materials, not only realizes continuous separation of large-volume oil/water mixtures (irrespective of the oil
density) with high efficiency and high purity, but also shows good capacity in consecutive collection of oil (water) from the water (oil) surface with the promising potential in separation of oil/water emulsions. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 61404059 and 11762007). References [1]
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Tables & Figures Table 1 Comparison of the representative oil/water separation based on a single SHPhi-SOPho mesh Device configuration
3D mesh + pump
2D mesh sandwiched (Figure 1D-d1) 2D mesh tilted (Figure 1D-d2) 2D mesh in “run-off” (Figure 1D-d3&d4)
Oil/water mixture ρoil < ρwater
Water amount
Separation mode
Practicality
Reference
Unlimited
Continuous
Good
Our work
ρoil < ρwater
Extremely limited
Batch
Poor
40
ρoil > ρwater
Limited
Batch
Good
40,41
ρoil < ρwater
Very limited
Batch
Poor
32
ρoil > ρwater
Limited
Batch
Good
33
Unlimited
Continuous
Very poor
15,16
ρoil > ρwater
ρoil < ρwater ρoil > ρwater
A
B Superoleophilic-superhydrophobic
y
y x
x
-F' g,y
F g,y Oil
Fg
C
ρOil < ρWater
Pump 1
Pump 1
ρOil > ρWater Pump 2
Wate r
Oil/water mixture
Fg
Inverted cone
ρOil > ρWater Cylinder
150 100
Pump 2
50 0
Superhydrophilic-superoleophobic
D
200 Mass of filtrated oil (g)
ρOil < ρWater
Fg
d1
d2
d3 Oil
Water
0
20
d4
40
60 80 Time (s)
100
120
d5 Oil
Water
Figure 1 (A) Schematic illustration of the selective removal of oil (top) and water (bottom) from both light and heavy oil/water mixtures with SOPhi-SHPho and SHPhi-SOPho mesh containers, respectively. (B) Effect of droplet gravity on its passage through the sidewall of inverted cone, cylinder and frustum of a cone. (C) Filtrated oil from n-hexadecane/water mixture (c.a. 325 ml (250 g) n-hexadecane and 50 ml water) during the filtration for 120 s (mixture pumping rate: 180 ml/min) using both SOPhi-SHPho inverted cone and cylinder of the same volume (Part II-a in supporting information). (D) Five typical device configurations commonly used for the filtration of immiscible oil/water mixtures (black fine arrow indicates the location of SOPhi-SHPho mesh while red coarse arrow indicates the location of SHPhi-SOPho mesh) in prior publications.
Figure 2 The proposed system for the continuous separation of large-volume oil/water mixtures: A pipe (#1) connected to the inlet of a peristaltic pump (pump 1) is inserted into the bottom of leftmost container that is filled with oil/water mixture, and the outlet of pump 1 is connected to a SOPhi-SHPho “Taylor cone” mesh container by another pipe (#2); oil will penetrate through the “Taylor cone” container, while water is accumulated; to pump out the accumulated water, another pipe (#3) connected to pump 2 is inserted into the mesh container at a bottom location for light oil/water mixtures and at the top location for heavy oil/water mixtures.
A
Light oil/water mixture
Before
B
Heavy oil/water mixture Before
SOPhi-SHPho mesh
SOPhi-SHPho mesh Pump 1
Pump 2
NIL
H2O
NIL NIL
Oil
Pump 2 Oil
a1
Pump 1
NIL
H2O
Magnetic stirrer
b1 After
After
H2O Oil
Oil
H2O
a2
b2
C
Water
Mixture
SOPhi-SHPho mesh container Pump
Irrespective of oil density Oil
Pump
Oil collector Water outlet
Inlet
Oil/water mixture
Figure 3 Snapshots of the use of experimental system to separate light oil/water mixture (A) and heavy oil/water mixture with continuous stirring (B). (C) Illustration of the SOPhi-SHPho mesh-based system for continuous separation of oily wastewater irrespective of the oil density. NIL in Figures indicates nothing in the beaker.
100
Separation efficiency (%)
95 90 85
Separation efficiency (%)
()
()
()
80 75
C
PVDF-TiO2
n-Hexadecane
p-Chlorotoluene
B 100 Separation efficiency (%)
PVDF-SiO2
PVDF-CS
~ 99.09% (394.5 ml)
98
~ 99.53% (987 ml)
97 96 95 94 0
100 200 300 700 Mass of n-Hexadecane (g)
800
D
103
Pumping rate: 200 ml/min Mesh 100 Mesh 120
180
102 101 100 99 98 97
PVDF-CS PVDF-SiO2
96
PVDF-TiO2
95
99
93
Carbon tetrachloride
Mass of filtrated oil (g)
A
0
10
E
20 30 Number of cycles
40
150 120 90
Mesh 100 Mesh 120 Mesh 150 Mesh 200 Pumping rate: 150 ml/min
60 30 0
50
0
10 20 30 40 50 60 70 80 90 Time (s) WCA~ 149o
WCA~ 144o
WCA~ 152o
500 μm
PVDF-CS
PVDF-SiO2
500 μm
PVDF-TiO2
500 μm
Cu
OCA~ 0
100 μm
OCA~ 0
100 μm
OCA~ 0
100 μm
Figure 4 (A) The separation efficiency of three representative mixtures of n-hexadecane, p-chlorotoluene and carbon tetrachloride with water using the proposed system with PVDF-CS, PVDF-SiO2 and PVDF-TiO2 coated mesh containers. (B) The separation efficiency of light oil/water mixture with different amounts of n-hexadecane and 300 mL water using the as-prepared PVDF-SiO2 mesh container. (C) The cycling number of the prototype meshes for the oil/water separation process by using the n-hexadecane/water mixture (c.a. 325 mL (250 g) n-hexadecane and 300 mL water) for different mesh containers. (D) The effect of mesh number on the filtration of the same amount of light oil/water mixture (c.a. 260 mL (200 g) n-hexadecane and 50 mL water) recorded during the filtration for 90 s using PVDF-SiO2 coated meshes (100, 120, 150, 200) under different pumping rates (150 or 200 ml/min). (E) FE-SEM images of the as-prepared PVDF-CS, PVDF-SiO2 and PVDF-TiO2 coated meshes; insets are water contact angle (left) and oil contact angle (right) of the corresponding meshes.
B
A a1
b1
Before
b2
Water
Oil 61.25 g (N.W.)
a4
After
a2
Oil
10 μm
Oil
Mixture
53.38 g (N.W.)
Before a3 Oil collection in process
pipe pump
After 10 μm
b3 100 μm
500 μm
100 μm
500 μm
Figure 5 (A) Snapshots of continuously collecting n-hexadecane from water surface using the PVDF-TiO2 coated mesh container with a pump and a pipe to extract oil: (a1) pure oil (61.25 g, left) and pure water in red (right); (a2) oil/water mixture with continuously stirring to simulate oil spills at sea; (a3) oil collection in process by putting the mesh container into the oil/water mixture; (a4) pure oil (53.38 g) collected within 30 s. (B) The solely gravity-driven separation of water-in-oil (W/O) emulsion using the stainless steel mesh (600 mesh) dip-coated with PVDF-TiO2: Photographs (b1) and optical micrographs (b2) of the as-prepared W/O emulsion before and after separation; (b3) FE-SEM images of raw stainless steel with PVDF-TiO2 coating (left) and without coating (right).
Highlights: The importance of 2D→3D conversion of superwetting mesh is demonstrated. A simple, versatile and universal oil/water separation system has been developed. External pumping operation was introduced for the removal of non-wetting phase. The use of single SOPhi-SHPho mesh can separate oil/water mixtures effectively. A mechanism of synergistic capillary-driven and gravity-driven separation is proposed.
2D → 3D Conversion of Superwetting Mesh: A Simple but Powerful Strategy for Effective and Efficient Oil/Water Separation
Tingping Leia,1, Dahai Lua,1, Zhenjin Xua,1, Weikang Xua, Jing Liua, Xudong Denga, Junjie Huangb, Lei Xuc, Xiaomei Caib,*, and Liwei Lind aCollege
of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021,
China bSchool
of Science, Jimei University, Xiamen 361021, China
cSchool
of Mechanical and Electric Engineering, Jingdezhen Ceramic Institute, Jingdezhen
333403, China dDepartment
of Mechanical Engineering, University of California, Berkeley, California
94720, USA *Corresponding 1These
author: [email protected] (X. Cai)
authors contributed equally to this work.
Previously unattainable success in using a single superwetting mesh to separate oil/water mixtures of various densities was achieved via 2D→3D conversion of superwetting mesh. Unlike 2D superwetting mesh, 3D mesh container allows both oil and water to touch the mesh, ensuring the passage of wetting phase through the mesh. By pumping out the accumulated non-wetting phase, continuous separation was realized. Keywords: 2D-to-3D conversion; superwetting mesh; pumping; oil/water separation; “Taylor
cone” container
The authors declare no competing financial interest.