Fabrication of Ecofriendly Recycled Marimo-like Hierarchical

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Separations

Fabrication of eco-friendly recycled marimo-like hierarchical micro-nanostructure superhydrophobic materials for effective and selective separation of oily pollutants from water Shan Xue, Xu Xu, and Lei Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06411 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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Industrial & Engineering Chemistry Research

Fabrication of eco-friendly recycled marimo-like hierarchical micro-nanostructure superhydrophobic materials for effective and selective separation of oily pollutants from water

Shan Xue, Xu Xu, Lei Zhang College of Chemistry, Liaoning University, 66 Chongshan Middle Road, Shenyang 110036, P.R. China

Shan Xue, [email protected], College of Chemistry, Liaoning University, 66 Chongshan Middle Road, Shenyang 110036, P.R. China Xu Xu, [email protected], College of Chemistry, Liaoning University, 66 Chongshan Middle Road, Shenyang 110036, P.R. China Lei Zhang, [email protected], College of Chemistry, Liaoning University, 66 Chongshan Middle Road, Shenyang 110036, P.R. China

* Corresponding

author. Tel.: +86 24 62207809; Fax: +86 24 62202380. E-mail address: [email protected] (X. Xu). E-mail address: [email protected] (L. Zhang). 1

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ABSTRACT: Marimo-like hierarchical micro-nanostructure superhydrophobic β-FeOOH (MHMSFe) firmly anchored onto the skeleton of sponge/cotton fabric (MHMSFe@sponge/MHMSFe@cotton fabric) as superhydrophobic materials were successfully designed and fabricated via a one-step surfactant and urea assisted hydrothermal method. By the combination of micro-nanostructure design, three-dimensional (3D) hierarchical architecture was built from spherical assembly of high integration and density superhydrophobic β-FeOOH nanorods which tended to stack up to each other in an orderly manner to decrease their surface energy. The obtained superhydrophobic materials with a static contact angle (CA) of 168° could not only undergo rapid oil/water separation but also display excellent absorption capacity up to 23 times of its own weight and great separation efficiency up to 99.6%. Besides, the superhydrophobic materials performed satisfactory reusability. This study not only provides a new strategy to design superhydrophobic materials, but also demonstrates its prospective application in recovery of oil and hydrophobic organic pollutants on a large scale. KEYWORDS:

Marimo-like

hierarchical

micro-nanostructure

Superhydrophobicity; Oil/water separation

2


β-FeOOH;

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Introduction In recent years, frequent occurrences of water pollution caused by the increased oil-leakage/spillage accidents have led to environmental pollution and fatal damage 1. The danger of oil and organic solvents pollutants has seriously affected the quality of human life 2. Consequently, the development of simple, efficient and economical methods to deal with the oil-leakage/spillages for environmental and water source protection has become urgent 3. Generally, some main technologies 4, such vacuum suction 5, controllable burn

6

and chemical decomposition

7

have been adopted in

removal of oils/organic solvents from water. Unfortunately, these conventional methods have been proved to be time-consuming, environment unfriendly and expensive 8. Recently, novel superhydrophobic and superoleophilic filter membrane and absorbent materials have become a hot topic because of their low cost, chemical durability and high performance polyurethane sponge

16-17

8-14.

For instance, modified cotton fabric

and melamine sponge

18

15,

etc. have demonstrated great

efficiency for removal of oil/organic solvents from water and oil/water separation. Zhou et al. fabricated superhydrophobic and superoleophilic cotton fabrics by functionalization of pristine fabrics with polyaniline and fluorinated alkylsilane

19.

Xue et al. prepared superhydrophobic cotton fabrics by immersing the ordinary fabrics into a TiO2 solution followed by modification with stearic acid to form the surface roughness 20. Wu et al. prepared superhydrophobic, magnetic and durable PU sponges with high oil/water separation capacity via chemical vapor deposition and 3



modification with fluoropolymer

21.

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Ruan et al. fabricated a superhydrophobic and

superoleophilic material via functionalization of commercial melamine sponge with polydopamine and 1H,1H,2H,2H-perfluorodecanethoil reported

strategies

required

toxic

fluorinated

non-biodegradable and environmentally undesirable

22.

However, most of the

compounds,

10.

which

were

Besides, these strategies still

required complicated processes or expensive reagents, which hindered the large scale production

23.

Thus, the further development of designing of multifunctional

superhydrophobic and superoleophilic materials which can remove oil/organic solvents from water and separate oil/water mixture in a selective, efficient and eco-friendly way are in high demand. Nowadays, different structures of FeOOH have received much attention in preparation of superhydrophobic material because of its abundant hydroxyl groups on the surface, which are favorable for the subsequent hydrophobic modification. Special-structured

FeOOH,

such

as

γ-FeOOH

nanosheets

modified

with

polydimethylsiloxane 24 and spherical α-FeOOH deposited by octadecyltrichlorosilane 25

have been used to prepare superhydrophobic materials. However, a large proportion

of relevant reports on superhydrophobic FeOOH required tedious post-treatment steps and expensive reagents, further increasing the cost of production. Therefore, a timesaving one-step method for preparing superhydrophobic FeOOH materials should be developed. In this work, MHMSFe@sponge/MHMSFe@cotton fabric was designed and prepared by a green one-pot synthetic method with the aid of surfactant and urea for 4


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efficient

oil/water

separation

and

oil

removal.

The

MHMSFe@sponge/MHMSFe@cotton fabric demonstrated superhydrophobicity, brilliant mechanical stability, excellent selectivity in oil-absorption and satisfactory reusability. It had high absorptive capacity in oil-removal and great separation efficiency both in emulsified oil/water mixture and continuous oil/water separation. Importantly, through this one-step method in water-based reaction system, complex pre-treatment or post-treatment process, expensive or toxic reagents such as fluoride was not required, which was satisfied the environmental chemistry requirements.

Experimental Section Chemicals Iron chloride hexahydrate (FeCl3·6H2O, ≥99.0 wt%), sodium dodecyl sulfonate (SDS, ≥97.0 wt%), urea (CO(NH2)2, ≥99.0 wt%) and organic solvents including hexane, methylbenzene and dichloromethane were obtained from Sinopharm Chemical Reagent Co Ltd. Sudan III was obtained from J&K Scientific Ltd. Cotton fabric and PU sponge were obtained from local market in Shenyang, China. All these chemicals were used without any further purification. Oils used in this research included soybean oil, gasoline and lubricating oil were all obtained from local suppliers. Synthesis of superhydrophobic materials The

schematic

description

of

the

fabrication

of

MHMSFe@sponge/MHMSFe@cotton fabric and MHMSFe microspheres was 5



demonstrated in Scheme 1. The MHMSFe@sponge was synthesized by a simple hydrothermal method. 10.8 g FeCl3·6H2O, 2.4 g CO(NH2)2, and 0.05 g SDS were dissolved in 60 mL of deionized water and stirred constantly for 20 minutes. Then the homogeneous suspension was poured into a 100 mL Teflon-lined autoclave. A proper amount (3cm*3cm*3cm) of sponge was immersed in the homogeneous suspension and sealed at 100 ℃ in an oven for 12 hours. After the autoclave was cooled down to room temperature naturally, the obtained yellowish brown sponge was washed with deionized water several times and dried at 60 ℃ for 12 hours. The MHMSFe@cotton fabric was also synthesized by the same experiment conditions as above for oil/water separation. To further investigate the influence of SDS, the MHMSFe microspheres and β-FeOOH nanorods were synthesized under the same reaction conditions as above with and without SDS, respectively.

Scheme 1. Schematic illustration of preparation process of MHMSFe@sponge/MHMSFe@cotton fabric and MHMSFe microspheres. 6


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Characterization The morphologies and microstructures of MHMSFe microspheres and MHMSFe@sponge/MHMSFe@cotton fabric were characterized by Scanning electron microscopy (SEM) (HITACHI SU8000, Japan) under an acceleration voltage of 10.0 kV. X-ray powder diffraction (XRD) pattern of superhydrophobic β-FeOOH microspheres was recorded by Cu Kα radiation (Diffraktometer D8, Bruker AXS, Germany) at the range 2θ of 10-80˚. FTIR spectrum was obtained by Avatar 330 FT-IR spectrometer (Nicolet Co., USA) with KBr method. Thermogravimetric analysis (TGA) was performed using a SDT Q600 thermoanalyzer (TA, USA). The static contact angles (CAs) were measured by a DSA100 optical contact angle measuring instrument (Kruss Co. German) using 5 μL water droplet at an ambient temperature. Abrasion test Low stability is the main issue limiting the widespread application of superhydrophobic

materials.

Herein,

the

stability

of

the

MHMSFe@sponge/MHMSFe@cotton fabric was assessed by abrasion tests, including finger press, hands twisting, water jetting and water-sand jetting. The wetting property was investigated after each test. Absorptive capacity experiment A piece of MHMSFe@sponge/MHMSFe@cotton fabric was immersed in oil at room temperature. Then the MHMSFe@sponge/MHMSFe@cotton fabric was taken out of the oil after 10 s, held for a few seconds and wiped with filer paper to remove 7



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extra liquid. The obtained MHMSFe@sponge/MHMSFe@cotton fabric was weighed for the calculation of absorption capacity (Q) by the mass change of the MHMSFe@sponge/MHMSFe@cotton fabric before and after absorption according to the following equation : Q = (mt – m0)/m0

(1)

where m0 and mt are the mass of the MHMSFe@sponge/MHMSFe@cotton fabric before and after absorption, respectively 26. The absorption kinetic of the MHMSFe@sponge/MHMSFe@cotton fabric was tested by placing it into the oils or organic solvents and then measuring the mass as a function of absorption time. The absorption kinetic can be described by the following pseudo-first-order kinetics model 27-28: ln(Q − Qt) = lnQ – Kt

(2)

where t is the absorption time, Qt is the absorption capacity at time t, Q is the saturation absorption capacity, K is the absorption constant. Selective absorption and oil removal test Soybean oil (light oil) and dichloromethane (heavy oil) were chosen to determine the selective absorptive property of MHMSFe@sponge/MHMSFe@cotton fabric. They were both dyed with Sudan III (Oil-soluble dye 29) and then mixed with water to generate oil/water mixture. A piece of MHMSFe@sponge/MHMSFe@cotton fabric was contacted with soybean oil/dichloromethane and the selective absorption processes was recorded with a smart phone. Oil/water separation test 8


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The continuous oil/water separation was performed using MHMSFe@sponge with a home-made equipment. One end of the tube was stuffed into the MHMSFe@sponge, which was held at the oil/water interface. Meanwhile the other end of the tube was connected to a peristaltic pump as the input of the pump. The output of the pump was collected by a beaker. Once the switch of peristaltic pump was turned on, the continuous oil/water separation test began. The separation efficiency was calculated according to the following equation: η = (m1/m0) × 100%

(3)

where m0 and m1 are the mass of the water before and after absorption process, respectively. The

continuous

oil/water

separation

was

also

performed

by

using

MHMSFe@cotton fabric, which was put in a self design funnel separation device as a filter for gravity-driven. Then it could be used directly for heavy oil/water separation without other auxiliary tools. Heavy oil was able to pass through the water layer driven by gravity, passed the MHMSFe@cotton fabric, and was collected by a beaker. The oil/water separation test began as a Sudan III-dyed dichloromethane/water mixture was poured into the funnel. The separation efficiency was calculated according to the equation (3) mentioned above. The

separation

of

emulsified

oil/water

mixture

was

explored

using

MHMSFe@sponge/MHMSFe@cotton fabric. A Span 60 stabilized water-in-toluene emulsion (1:99, v/v) with a droplet size at the micrometer scale was prepared for separation of emulsified oil/water mixture. MHMSFe@sponge was used to absorb 9



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toluene directly. MHMSFe@cotton fabric was put in funnel separation device as a filter for gravity-driven to separate toluene. The remaining water in toluene were determined by using a UV-Vis spectrophotometer. The separation efficiency was calculated by the following equation: η = (C0 - Cw) / C0 × 100%

(4)

where C0 is the original water concentration in the emulsion (ppm), Cw is the remaining water concentration (ppm). Mechanical stability Mechanical stability of MHMSFe@sponge/MHMSFe@cotton fabric was explored with the help of a homemade equipment. The balance weight (200 g) was used to perform compression and stretching experiments. The superhydrophobicity, oil absorbency and mechanical property of MHMSFe@sponge after repeated compression (50 times) and MHMSFe@cotton fabric after repeated stretching (50 times) were investigated. Recyclability The

recyclability

of

MHMSFe@sponge/MHMSFe@cotton

fabric

was

determined by simple squeezing, wiping and evaporation process. The oil-absorbed MHMSFe@sponge/MHMSFe@cotton fabric was regenerated and reused by squeezing manually followed by wiping with filter paper for removing absorbed oils and evaporating in a common oven at 60 ℃ for 1 hour. After each cycle, the absorptive capacity of the MHMSFe@sponge/MHMSFe@cotton fabric was calculated for comparison. 10


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Results and Discussion Characterization The morphology and microstructure of the MHMSFe microspheres was studied by SEM images. As shown in Figure 1a, the sample with the addition of SDS exhibited obvious 3D marimo-like micro-nanostructure with a diameter of ~5 μm. These marimo-like microspheres which constructed by a large number of needle-like branches with the width size of 100 nm (Figure 1b) were tended to stack up to each other to decrease their surface energy. In order to investigate the effect of SDS on the synthesized MHMSFe microspheres, the morphology of β-FeOOH nanorods synthesized without SDS was also investigated. Inversely, the sample synthesized without SDS was composed of numerous nanorods which dispersed disorderly (Figure 1c-d). As a typical anionic surfactant, SDS containing hydrophobic alkane chains inside and hydrophilic-SO3- ions outside in aqueous solution was effective to emulsify immiscible mixture and control the growth of microspheres. At the same time, SDS could graft to MHMSFe microspheres through dehydration reaction by reacting with NH4+ provided by urea to obtain dodecyl sulfonic acid. Thus, the addition of SDS could assemble the dispersed nanorods. Combining these morphological differences with relevant reports 30, SDS could be considered to act as a structure-directing agent in the formation process of marimo-like microspheres. In order to confirm the successful attachment of the MHMSFe microspheres onto sponge/cotton fabric and observe the rough surface structure of the composite 11



sponge/cotton fabric, the microstructures of pristine sponge/cotton fabric and composite sponge/cotton fabric were observed by SEM. For pristine sponge, it was clearly seen that the sponge was consisted of numerous interconnected micropores with a diameter of ~200 μm (Figure 1e). The skeleton of the sponge with a thickness of ~50 μm was very smooth. As presented in Figure 1f, the framework became very rough after modification with MHMSFe microspheres, which were deposited uniformly on the surface and interior of the sponge. The surface morphologies of the pristine and composite cotton fabrics were also investigated by SEM. The pristine fabrics were consisted of numerous microscale fibers, which exhibited quite smooth surfaces under high magnification (Figure 1g). In contrast, the composite cotton fabric were modified with MHMSFe microspheres. Figure 1h showed that the MHMSFe microspheres were uniformly dispersed and widely enriched on the microscale fabric surface. Therefore, with introduction of the MHMSFe microspheres, the modified sponge/cotton fabric formed a rough surface structure leading a great hydrophobicity and oil absorption capacity, which further enhanced its oil/water separation performance.

Figure 1. SEM images of MHMSFe microspheres (a,b), β-FeOOH nanorods (c,d), 12


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original PU sponge (e), MHMSFe@sponge (f), original cotton fabric (g) and MHMSFe@cotton fabric (h). The XRD patterns of synthesized β-FeOOH with and without SDS were presented in Figure 2a. In these XRD patterns, the diffraction peaks at 2θ = 11.9°, 16.9°, 26.8°, 34.1°, 35.3°, 39.3° and 55.9°, matched well with the (110), (200), (310), (400), (211), (301) and (521) crystal planes of β-FeOOH synthesized with and without SDS in the standard data (JCPDS Card No.34-1266). Furthermore, several major sharp diffraction peaks at 2θ = 11.9°, 16.9°, 26.8° and 55.9° became weaker after the superhydrophobic modification by SDS. The β-FeOOH synthesized with and without SDS were also characterized by FTIR spectroscopy in the range of 4000-450 cm-1 (Figure 2b). Absorption peaks at wavelength of 3414 and 1620 cm-1 could be attributed to O-H vibrations of the absorbed water. Absorption peaks at wavelength ranges of 839, 665, 638 and 486 cm-1 could be attributed to the typical vibration modes of the FeO6 coordination octahedron of β-FeOOH

31.

Compared with

β-FeOOH (curve a), the peaks at 2924 cm-1 and 2859 cm-1 which belonged to the asymmetric and symmetric stretching vibrations of C-H groups and a new peak at 1046 cm-1 (curve b) ascribed to the symmetric stretching vibration of -SO3 also indicated that the MHMSFe superhydrophobic microspheres were grafted by alkane chains originating from SDS. Additionally, TGA experiments of β-FeOOH synthesized with and without SDS were also studied to investigate the thermal stability from 50 ℃ to 750 ℃ (Figure 2c). The slight mass loss about 5% before 200 ℃ was associated with absorbed water, 13



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whereas the quick decrease at 200-400 ℃ was due to the decomposition of grafted alkane chains. The results from TGA investigated that the prepared MHMSFe microspheres was thermally stable below 200 ℃.

Figure 2. (a) XRD pattern of β-FeOOH and superhydrophobic β-FeOOH; (b) FTIR spectroscopy of β-FeOOH and superhydrophobic β-FeOOH; (c) TGA curve of β-FeOOH and superhydrophobic β-FeOOH from 50 ℃ to 750 ℃ Wetting properties To determine the hydrophobicity of the materials, the wetting properties were investigated by state contact angle (CA) measurements. Several water droplets stably rested

and

remained

spherical

on

the

surface

of

the

MHMSFe@sponge/MHMSFe@cotton fabric (CA=150°/168°), which indicated the excellent superhydrophobicity (Figure 3a-d)

32.

This could be explained that the

magical 3D marimo-like micro-nanostructure not only enhanced surface roughness, but also provided many microscopicpores as barriers to reduce the contact area between

material

and

water

33.

As

shown

in

Figure

3e-f,

the

MHMSFe@sponge/MHMSFe@cotton fabric was completely immersed into water with external forces. It could be seen that the surface of material was covered by several air bubbles and looked like a silver mirror. The results could be explained that a

lot

of

air

existed

on

the

hydrophobic

14


surface

of

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MHMSFe@sponge/MHMSFe@cotton fabric and the light in the interface between water and air occurred refraction phenomenon.

Figure 3. Photograph of water droplets on the cross section of MHMSFe@cotton fabric (a) and MHMSFe@sponge (b). Water contact angle of MHMSFe@cotton fabric (c) and MHMSFe@sponge (d). Photographs of MHMSFe@cotton fabric (e) and MHMSFe@sponge (f) totally immersed in water by an external force. Abrasion test The

stability

of

the


fabric

was

investigated by several abrasion tests. It was found that water droplet still maintained a spherical shape on the surface as shown in the insets images after abrasion tests including finger press, hands twisting, water jetting and water-sand jetting. Therefore, the MHMSFe@sponge retained its water repellent property after each test, as shown 15



in Figure 4a-d, and so did MHMSFe@cotton fabric (Figure 4e-h).

Figure 4. Abrasion tests for MHMSFe@sponge: (a) finger press; (b) hands twisting; (c) water jetting; (d) water-sand jetting. Abrasion tests for MHMSFe@cotton fabric: (e) finger press; (f) hands twisting; (g) water jetting; (h) water-sand jetting. The insets show the images of water droplets on the superhydrophobic materials after tests. Absorption capacity Figure 5a depicted the change in the absorption capacities of MHMSFe@sponge as a function of absorption time towards various kinds of oils (soybean oil, lubricating oil, gasoline) and organic solvents (hexane, methylbenzene, dichloromethane). The absorption capacities increased with absorption time until reaching saturation state at 10 s, which indicated that the MHMSFe@sponge had a fast absorption rate. The adsorption constant K and R2 were shown in Table 1. The absorption constant K was calculated by the slope of linear regression plot of ln(Q-Qt) versus t (Figure 5b). The absorption followed pseudo-first-order absorption kinetics (R2 > 0.99), indicating that it should be a physisorption process. In order to further study the practical application of the MHMSFe@sponge, various kinds of oils (soybean oil, lubricating oil, gasoline) and organic solvents 16


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(hexane, methylbenzene, dichloromethane) were used to determine the mass-based absorptive capacity. The absorptive capacities of the MHMSFe@sponge for different test liquids were in the range of 8.63-22.82 g/g (Figure 5c). It indicated that the MHMSFe@sponge had high absorptive capacity and could be a promising absorbent material for oil removal applications. The comparison of absorption capacities for various 3D porous absorbent materials was shown in Table S1. The absorption capacities, absorption kinetics and saturated absorption capacities of MHMSFe@cotton fabric to six oils (soybean oil, lubricating oil, gasoline, hexane, methylbenzene and dichloromethane) were also investigated and shown in Figure S1. The absorption capacities increased with absorption time until reaching saturation state at 5 s, which indicated that the MHMSFe@cotton fabric had a fast absorption rate. The adsorption constant K and R2 were shown in Table S2. The absorption followed pseudo-first-order absorption kinetics (R2 > 0.97), indicating that it should be a physisorption process.

Figure 5. (a) Absorption capacities of MHMSFe@sponge in oils and organic solvents; (b) Pseudo-first-order absorption kinetics of MHMSFe@sponge for oils and organic solvents; (c) The saturated absorption capacities of MHMSFe@sponge for oils and organic solvents. 17



Page 18 of 35

Samples K (s-1)

R2

Soybean Oil

-0.45446

0.99096

Lubricating Oil

-0.54876

0.99141

Gasoline

-0.47838

0.99111

Hexane

-0.49774

0.99101

Methylbenzene

-0.48318

0.99033

Dichloromethane

-0.4414

0.99003

(Oils/Organic solvents)

Table 1. Fitting parameters for the adsorption kinetics of MHMSFe@sponge in oils and organic solvents. Selective absorption Considering the interconnected porous structure, the superhydrophobic structure and high oil absorptive capacity, the selective oil absorption of MHMSFe@sponge from water was also explored. The absorption process of Sudan III-dyed soybean oil represented for light oil was shown in Figure 6a1-a4. When the oil/water surface was contacted with the MHMSFe@sponge, soybean oil could be selectively absorbed and totally absorbed from water by the MHMSFe@sponge in a few seconds. After absorption of the soybean oil, the MHMSFe@sponge was floated on the water surface owing to its low density and superhydrophobicity, indicating that it could applied in selective removal of oil spills. Furthermore, the MHMSFe@sponge could also absorb Sudan III-dyed dichloromethane effectively, which represented for heavy oil (Figure 6b1-b4). When 18


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the MHMSFe@sponge contacted with dichloromethane under the water, it could totally

absorb

dichloromethane

in

a

few

seconds,

indicating

that

the

MHMSFe@sponge could be a suitable absorption material to remove heavy organic pollutants under water. After absorption, a simple squeezing process could collect the soybean oil or dichloromethane absorbed in the MHMSFe@sponge. There were no red pollutants left in the water, indicating that the MHMSFe@sponge have high separation efficiency with no secondary pollution. It was proposed that the critical role in the oil removal process of the MHMSFe@sponge was physisorption. The oil/organic solvents could be absorb into the inner part under the hydrophobic interactions force when they were contacted with the MHMSFe@sponge. The same selective absorption process of the MHMSFe@cotton fabric was shown in Figure S2.

Figure 6. The absorption process of soybean oil (dyed with Sudan III) on water surface with MHMSFe@sponge (a1-a4); the absorption process of dichloromethane (dyed with Sudan III) under water with the MHMSFe@sponge (b1-b4). Oil/water separation Continuous oil/water separation of the MHMSFe@sponge The continuous oil/water separation device was set up to study the continuous 19



separation of light and heavy oils from water by using the MHMSFe@sponge as illustrated in Figure 7. As shown in Figure 7a1-a3, the MHMSFe@sponge was placed at the interface of hexane (dyed with Sudan III) and water mixture. Owing to the superhydrophobic and superoleophilic property the MHMSFe@sponge, it could absorb hexane quickly and repel water completely. The hexane was absorbed and removed from water surface through the MHMSFe@sponge after the peristaltic pump was turned on. The continuous hexane stream was observed in the tube and the hexane layer gradually reduced. At last, the hexane layer was completely removed and collected by the MHMSFe@sponge with the aid of the peristaltic pump. The remaining water without hexane and collected hexane without water droplets could be observed (Figure 7a4). Besides, the continuous oil/water separation processes of dichloromethane (heavy oil) under water using the MHMSFe@sponge were similar to the above processes (Figure 7b1-b4). The results illustrated that the heavy oil could also be continuously separated from the water with the device. In addition, the separation efficiency of the MHMSFe@sponge was also studied. The separation efficiency was obtained by the ratio between the mass of water after absorption and that initially added to the mixture. The separation efficiency of the MHMSFe@sponge was calculated up to 99.6% for the hexane-water mixture and above 97.5% for other oils and organic solvents (Figure 8). The results illustrated that the MHMSFe@sponge had high oil/water separation efficiency and could be successfully applied in continuous oil/water separation.

20


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Figure 7. Continuous oil/water separation processes with the MHMSFe@sponge: continuous removal of hexane (light oil) (a1-a4); continuous removal of dichloromethane (heavy oil) (b1-b4).

Figure 8. The separation efficiency of the oil/water mixtures. Continuous Oil/water separation of the MHMSFe@cotton fabric The oil/water separation of the MHMSFe@cotton fabric was also investigated. For gravity-driven continuous oil/water separation, The MHMSFe@cotton fabric was placed in a funnel separation equipment as a filter (Figure 9), which can be directly used for effective oil/water separation without other auxiliary accessories. Oil could pass though the water layer driven by gravity, pass the MHMSFe@cotton fabric, and be collected in a beaker. When the sudan III-dyed dichloromethane-water mixture was 21



poured into the funnel, it could be seen that dichloromethane got through the cotton fabric immediately, while the water remained upside. After the last drop of dichloromethane was dropped into the beaker and held for ten seconds, no droplets continued to drip. The remaining liquid in the funnel was collected into a tube and it can be seen that there was no red droplet. The separation efficiency of the MHMSFe@cotton fabric was calculated up to 99.2%. This high separation efficiency was attributed to the magical micro-nanostructure of MHMSFe@cotton fabric and excellent superhydrophobicity.

Figure 9. Separation and removal of Sudan-dyed dichloromethane from water using the MHMSFe@cotton fabric: (a) before separation; (b) during separation; (c) after separation; (d) clean water after separation. Separation of emulsified oil/water mixture The separation of oil from emulsified oil/water mixtures is also a challenge in practical oil/water separation application. MHMSFe@sponge and MHMSFe@cotton fabric were both used to investigate emulsified oil/water separation. Figure 10a-b and Figure 10c-d demonstrated the optical microscopic images of the emulsified oil/water mixture before and after absorption and separation. No water droplet was observed in 22


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the Figure 10b and Figure 10d. The separation efficiencies of MHMSFe@sponge and MHMSFe@cotton fabric were calculated up to 97.7% and 95.1%, respectively.

Figure 10. Optical micrographs of the emulsified oil/water mixture before (a) and after (b) MHMSFe@sponge absorption; Optical micrographs of the emulsified oil/water mixture before (c) and after (d) separation by MHMSFe@cotton fabric. Mechanical stability As shown in Figure 11b, the balance weight (200 g) was put onto the MHMSFe@sponge to simulate the compression condition. The superhydrophobicity and mechanical property of MHMSFe@sponge after repeated compression (50 times) were shown in Figure 11c. The oil absorption capacity maintained 96.2% of original capacity for dichloromethane after 50 times compression. Figure 11d performed the simulation of repeated stretching condition (50 times) and the superhydrophobicity for MHMSFe@cotton fabric. MHMSFe@cotton fabric remained efficient (97.7%) for dichloromethane-water mixture after 50 times stretching.

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Figure 11. (a)-(c) The compression of MHMSFe@sponge and the inset show the superhydrophobicity of MHMSFe@sponge after repeated compression. (d) The stretching of MHMSFe@cotton fabric and the inset show the superhydrophobicity of MHMSFe@cotton fabric after repeated stretching. Wetting mechanism The mechanism of wetting property is revealed and the synergistic effect of both chemical

compositions

and

physical

structures

of

the

MHMSFe@sponge/MHMSFe@cotton fabric may be the possible explanation. On one hand, the 3D MHMSFe microspheres composed of hydrophobic alkyl chains have strong resistance ability for water molecules, which is chemically beneficial to repel water droplets. On the other hand, as explained in Cassie

34

and Wenzel

35

equations,

the unique 3D hierarchical micro-nanostructure constructed via densely build-up layer by layer assembled from hydrophobic nanoleaves can greatly enhance the surface roughness, which further improve hydrophobicity and oleophilicity. Each hydrophobic nanoleaf emanated from the marimo sphere center can form a large amount of open-free space in the marimo sphere and greatly enlarge the contact area 24


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between the oil and hydrophobic nanoleaves, which promote the oil diffusion. To further investigate the mechanism of the selectively separation ability of MHMSFe@sponge/MHMSFe@cotton fabric for oil/water mixtures, the schematic diagram of the wetting model is set up and shown in Figure 12. Generally, the intrusion pressure (ΔP) can be described by the following equation: ΔP = -lγ(cosθA)/A

(5)

where γ is the interfacial tension, l is the pore perimeter, A is the cross-sectional area of the pore and θA is the advancing contact angle of liquid on the material surface. As shown in Figure 12a, unless external pressure is applied, water cannot permeate the material spontaneously when θ > 90° because ΔP > 0. On the contrary, when θ < 90°, oil can spontaneously pass through the material without any external force because ΔP < 0 (Figure 12b). As a result, the unique hierarchical micro-nanostructure of MHMSFe microspheres on the surface is favorable to increase the advancing CA of

water

and

decrease


the fabric

advancing is

CA

of

superhydrophobicity

oil. and

superoleophilicity, which can be well used in selectively oil/water separation 36-40.

Figure 12. Schematic diagrams of the wetting model of MHMSFe@sponge/MHMSFe@cotton fabric. (a) Water cannot permeate the material 25



because Δp > 0. (b) Oil can spontaneously permeate the material because Δp < 0. Recyclability It is important to investigate the recyclability of adsorption and filter membrane materials in oil/water separation application. Thus, the recyclable use of MHMSFe@sponge/MHMSFe@cotton fabric was evaluated. As illustrated in Figure 13a, the saturated absorptive capacities of MHMSFe@sponge still maintained more than 86% of original capacities for six different oil/water mixtures after 10 cycles, indicating excellent recyclability of MHMSFe@sponge. As shown in Figure 13b, MHMSFe@cotton fabric remained efficient (>98.7%) for dichloromethane-water mixture after ten cycles, indicating the good recyclability of MHMSFe@cotton fabric. Besides, the MHMSFe@sponge/MHMSFe@cotton fabric remained hydrophobic (CA>144°) after 10 cycles. The results demonstrated that the MHMSFe@sponge and MHMSFe@cotton fabric possessed excellent recyclability, making them ideal materials in practical applications of oil/water separation.

Figure 13. (a) The recyclability of MHMSFe@sponge for absorption of different oils and organic solvents. (b) The recyclability of MHMSFe@cotton fabric for separation of dichloromethane. 26


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Summary and Conclusions In conclusion, we developed a simple method to prepare low cost MHMSFe@sponge/MHMSFe@cotton fabric with excellent superhydrophobicity via a

one-pot

surfactant

and

urea

assisted

hydrothermal

method.

The

MHMSFe@sponge/MHMSFe@cotton fabric exhibited superhydrophobicity, brilliant mechanical stability, excellent selectivity in oil-absorption and satisfactory reusability. It had high absorptive capacity in oil-removal and great separation efficiency both in emulsified oil/water mixture and continuous oil/water separation. Furthermore, mechanisms were revealed and fully elucidated in detail. The results indicated that the MHMSFe@sponge/MHMSFe@cotton fabric was promising in oil/water separation and the selective removal of oils from water.

27



Supporting Information Absorption capacities for various absorbent materials. Absorption capacity and selective absorption of the MHMSFe@cotton fabric. (PDF)

Acknowledgements This work was supported by the Science and Technology Foundation of Ocean and Fisheries of Liaoning Province (No. 201408, No. 201406), Liaoning Provincial Doctor Startup Fund Program (No. 201601092), the General project of scientific research of the Education Department of Liaoning Province (No. LQN201707), the Foundation for Young Scholars of Liaoning University (No. 2013LDQN13) and the Foundation for National Advance declaration of Liaoning University (No. LDGY201406).

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Table of Contents

3D marimo-like micro-nanoscale β-FeOOH superhydrophobic materials were prepared by an eco-friendly one-step method and exhibited excellent oil absorptive capacity and high separation efficiency in oil/water separation.

35


Fabrication of Ecofriendly Recycled Marimo-like Hierarchical

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