Fabrication of Unidirectional Diffusion Layer onto Polypropylene Mat

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Fabrication of Unidirectional Diffusion Layer onto Polypropylene Mat for Oil Spill Cleanup Xiangyu Zhou,†,‡,§ Feifei Wang,†,§ Yali Ji,†,‡ and Junfu Wei*,†,‡,∥ †

State Key Laboratory of Hollow Fiber Membrane Materials and Processes, ‡School of Environmental and Chemical Engineering, and School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, P.R. China ∥ Tianjin Engineering Center for Safety Evaluation of Water Quality & Safeguards Technology, Tianjin 300387, China §

ABSTRACT: In this study, a cross-linking layer which performed as a unidirectional diffusion layer was fabricated onto a polypropylene (PP) mat through UV radiation graft polymerization. On the one hand, the introduction of acrylate monomers enhanced the hydrophobility of the sorbent, and the swelling of PP sorbent provided more spaces, which increased the sorption capacity. On the other hand, the diffusion channels were locked after saturation adsorption. Thus, the retention capacity was dramatically intensified. A reusability test revealed that the cross-linking layer also performed as a scaffold to sustain the inner fibrous structure, which improved the reusability. Also, the oil sorption and retention mechanism were systematically elucidated. The study proposed a potential method to fabricate fibrous sorbent and increase the oil sorption and retention capacity.

1. INTRODUCTION

Generally, most studies on oil sorbent are generally focused on hydrophobic modification.16,17 However, due to the inherent limitations of capillarity and hydrophobic interaction, the improvement of sorption and retention capacity is limited. We believed the microstructure fabrication may be the key to solve this problem.18 Inspired by this notion, we attempted to propose another solution and introduce the three-dimensional network structure onto polypropylene mat surface through ultraviolet (UV) radiation graft polymerization. Accordingly, the PP mat could be distinctively divided into cross-linking layer and storage layer. After saturation adsorption, the crosslinking layer swelled and the grafted chains stretched. As a consequence, the cross-linking layer performed as a unidirectional diffusion layer and the adsorbed oil was trapped into the PP mats. Hydrophobic acrylate monomers were introduced onto sorbent, which also increased its hydrophobicity and adsorption capacity. Further, the oil adsorption and retention mechanism were systematically discussed. The approach proposed in this study could be a facile and efficient way to fabricate fibrous sorbent (e.g., cotton, kapok, wool, polypropylene nonwoven, etc.) and effectively increase its oil adsorption and retention capacity.

With the growth of offshore oil production and transportation, oil spills and chemical leaks have become the most significant threat to the coastal environment and ecosystem. The cleanup of these organic pollutants from water sources is a major environmental issue that continues to attract a great deal of attention.1−3 Accordingly, many processes such as mechanical extraction, in situ burning, and bioremediation have been developed to clean oil from contaminated areas.4 Among these methods, one of the most economical and efficient countermeasures for combating oil spills is sorption by sorbent.5−7 The preferable sorbent materials are those which, besides being inexpensive and readily available, demonstrate fast oil sorption capacity, high oil retention capacity during transfer and excellent reusability.8 Different materials such as inorganic minerals (silica, carbon, zeolite),9 organic natural resources (straw, wood, cellulose and cotton),10−12 and organic synthetic products (rubber, foam, and polypropylene fibrous mats)5,13,14 have been utilized as potential sorbents for oil spill cleanup. However, there is no doubt that the most common and commercialized materials for oil removal in practical application are polypropylene (PP) mats/nonwovens. As a synthetic material, PP mats have excellent features such as strong hydrophobicity, low cost, good mechanical strength, highly overlapped fibrous structure, low density, etc., and these characteristics made PP mats suitable and efficient for oil− water separation.13−15 However, PP-based materials also face enormous challenges: (1) Low adsorption capacity. Although PP material is of nonpolarity, the sorption capacity needs to be improved compared with other oil sorbent. (2) Poor retention capacity. The adsorbed oil is mainly retained in the interspaces of PP filaments through capillary action. Previous studies indicated that fibrous materials could be desorbed by a simple squeeze, which means external disturbances such as ocean waves and towing during use and collection may cause the leakage of oil from sorbents as well.13 © 2015 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Polypropylene mat (approximately 120 g/m2) used in this work was supplied by Haidexin Chemical Factory (Jiangsu, China). Lauryl acrylate (LA) was purchased from Guangfu Chemical Factory (Tianjin, China). Ethylene glycol dimethacrylate (EGDMA) and benzophenone (BP) were purchased from J&K Scientific Ltd. Sodium diethyldithiocarbamatre (iniferter) was obtained from Tianyi Chemical Reagent Institute Co., Ltd. (Tianjin, China). Received: Revised: Accepted: Published: 11772

October 9, 2015 November 8, 2015 November 16, 2015 November 16, 2015 DOI: 10.1021/acs.iecr.5b03778 Ind. Eng. Chem. Res. 2015, 54, 11772−11778

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematics of preparation route and structure of unidirectional diffusion layer.

min of sorption, the wet sample was weighed and the maximum oil sorption capacity was calculated as

All the reagents were of analytical grade (A.R.) and used without further purification. Gasoline, diesel, crude oil, toluene, and kerosene were purchased from local suppliers. 2.2. Adsorbent Preparation. The PP-g-LA-co-EGDMA mat was fabricated through UV radiation graft polymerization. Preparation details are presented below: (1) The PP mat was pretreated with acetone to elute residual solvent during the manufacturing process and then dried in the oven for 2 h. Then lauryl acrylate (LA, monomer, 6 wt %), EGDMA (cross-linker, 1 wt %), BP (photo initiator, 1%), and sodium diethyldithiocarbamatre (iniferter, 0.5%) were dissolved in an isopropyl alcohol/water (1:4, v/v) solution under stirring. The mixed solution was poured into a polyethylene bag with the PP mat of 10 cm × 10 cm. After that, the bag was aerated by bubbling nitrogen gas for 20 min and then sealed. (2) UV radiation with an energy of 500 W was applied to initiate the graft polymerization. A cooling infrastructure was used to keep a constant temperature. The UV irradiation graft polymerization reaction then occurred on the surface of the PP mat. Afterward, the grafted PP mat was extracted by ethanol for about 4 h to remove the unreacted monomers and homopolymers and then dried in a vacuum oven. The schematics of preparation are shown in Figure 1. 2.3. Characterization. To analyze the functional groups on the surface of PP samples, the FTIR spectra was recorded on Fourier transform infrared spectrometer (Necolet 6700, USA) in the wavenumber range of 600−4000 cm−1 under ambient condition. The surface chemical composition was characterized by X-ray photoelectron spectroscopy (XPS), and the analysis was carried out on an AEM PHI 5300X spectrometer with an Al Kα X-ray source (1486.71 eV of photons) to determine the C and O elements. The software package Thermo Avantage 3.9.3 was used to fit the spectra peaks. The PP samples were coated with gold−palladium alloy prior to observation, and the surface morphologies of PP samples were observed using scanning electron microscopy (SEM) by an S-2500C microscope (Hitachi, Japan). Contact angle measurements of the pristine PP mat and PP-g-LA-co-EGDMA mat were made on a contact angle analyzer (KRUSS DSA100, Germany). 2.4. Oil Sorption and Mechanical Retention Experiment. The maximum oil capacity was evaluated at room temperature in pure oil medium. In a typical measurement, PP mat (10 × 10) was placed on top of 250 mL of the abovementioned oil samples in a tray without any stirring. After 30

qm = (mf − m0)/m0

(1)

where qm is the maximum sorption capacity (goil/gsorbent), mf is the final weight of the of samples after sorption, and m0 is the initial weight of PP samples. All the sorption measurements were performed in triplicate. Also, sorption experiments were conducted at various time intervals to determine the kinetics. As mentioned above, for fibrous materials, the adsorbed oil could be readily removed by simple squeeze which may cause secondary pollution during use. To quantify this process and ascertain the retention capacity of the PP mat, a mechanical retention experiment was designed. The PP mat after saturation adsorption was placed on a sieve and mechanical forces were employed to press the saturated samples. The mechanical oil retention capacity was calculated as

η=

(mf − m0 − mi) (mf − m0)

(2)

where η is the mechanical oil retention capacity (goil/gsorbent) and mi is the weight of oil drained from the PP mat under different mechanical forces. 2.5. Desorption and Repeatability. In this study, the three-dimensional network structure was introduced onto a polypropylene surface and consequently the samples could not be fully desorbed by squeezing. Thus, in this study, the PP mat was desorbed by using centrifugation under 1000 rpm. The adsorption−desorption process was repeated for 20 cycles to evaluate the reusability of original PP mat and the PP-g-LA-coEGDMA mat.

3. RESULTS AND DISCUSSION 3.1. Characterizations. A unidirectional diffusion layer was fabricated onto the PP mat and the SEM images of (a) pristine PP mat, (b) PP-g-LA-co-EGDMA mat, and (c) cross section of PP-g-LA-co-EGDMA mat are shown in Figure 2. It could be observed that the surface of the pristine PP mat (Figure 2a) was relatively smooth and had a typically overlapped fibrous structure. After modification, the fibrous surface of PP-g-LAco-EGDMA (Figure 2b) was covered with a compact polymer layer which could be verified by its image of the cross section (Figure 2c) as well. The PP-g-LA-co-EGDMA can be distinctively divided into two layers from the cross section 11773

DOI: 10.1021/acs.iecr.5b03778 Ind. Eng. Chem. Res. 2015, 54, 11772−11778

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to C−C/C−H species. This also confirmed the chemical composition of PP mat. After modification, a comparison of wide-scan spectrum between the original PP nonwoven (Figure 4a) and PP-g-LA-co-EGDMA (Figure 4c) indicated that the O 1s signal appears. The C 1s spectrum of PP-g-GMA (Figure 4d) nonwoven can be curve fitted into three peak components with bond energyies (BEs) of 284.6, 287.7, and 288.7 eV, which is attributed to the C−C/C−H, O−C−O, and O−CO species, respectively. The presence of these peaks confirmed the chemical modification of the PP mat on the surface. Contact angle measurements were carried out to investigate the wettability of the PP mat. The images of water contact of the (a) original PP mat, (b) PP-g-LA-co-EGDMA mat, and (c) dynamic sorption process of oil droplet on PP-g-LA-coEGDMA mat are presented in Figure 5. The water contact angle of the PP-g-LA-co-EGDMA mat was as high as 132°, which was much higher than that of the pristine PP mat (117.7°). This is because hydrophobic lauryl acrylate (LA) monomers were introduced onto the surface of the PP mat, obviously increasing its hydrophobicity. Although it is observed from SEM images that the PP surface was covered with a compact cross-linking layer, the oil droplet could rapidly penetrate into the PP-g-LA-co-EGDMA mat within 0.36 s, which revealed that the diffusion channel still existed in the cross-linking layer. 3.2. Oil Sorption Capacity. Figure 6a presents the maximum organic medium sorption capacities of the pristine PP and the PP-g-LA-co-EGDMA mats. It is obvious that the sorption capacities of the PP-g-LA-co-EGDMA mat were much higher than that of the pristine PP mat. The higher sorption capacity can be ascribed to the enhanced hydrophobicity and swelling of cross-linking layer compared to that of the original PP mat. It is generally considered that, for fibrous sorbents, the predominant mechanisms for sorption are adsorption and interfiber capillarity (Figure 6b).10,19 In pure organic medium, capillary action will play a dominant role in a sorption system, which can be described by the Young−Laplace equation:14

Figure 2. SEM images of (a) pristine PP mat, (b) PP-g-LA-coEGDMA mat, and (c) cross section of PP-g-LA-co-EGDMA mat.

morphology, namely cross-linking layer and storage layer. The cross-linking layer is approximately 500 μm thick and this thickness is ascribing to the weak penetrability of UV radiation. Thus, the graft polymerization only occurred on the exterior surface of the PP mat. The FTIR spectra of (a) original PP mat, (b) PP-g-LA-coEGDMA are depicted in Figure 3. The bands at the range of

ΔP =

2γlν cos θ r

(3)

where ΔP is capillary pressure between PP filaments. γlν is surface tension and θ is contact angle of organic medium and PP substrate. r is equivalent radius of capillary in this system. After modification, the surface of the PP-g-LA-co-EGDMA mat became more compact and hydrophobic, which dramatically increased the cos θ between the oil droplet and the PP substrate and reduced the equivalent radius r. Consequently, the capillary action and affinity between oil and PP substrate were enhanced. It is concluded from Figure 6a that PP sorbent had a relatively larger sorption capacity for organic media with high viscosity (crude oil and kerosene). A previous study indicated that the viscosity of liquid oil, as defined by the relevant internal friction when a fluid flows, had a critical effect on the spread of oil into sorbent.8 A lower viscosity will facilitate the penetration of oil, while a higher viscosity can be expected to slow the leaching of sorbed oil and is beneficial for oil adherence. Compared with common porous sorbent, it is not difficult for liquid oil to diffuse through the interspaces between filaments of PP mat. Accordingly, retention capacity and adherence became the key factors for sorption capacity. Thus, the larger sorption capacity for oils with higher viscosity could be observed.

Figure 3. FTIR spectra of (a) original PP mat, (b) PP-g-LA-coEGDMA.

3000−2800 cm−1 could be attributed to the typical stretching vibration of C−H. Similarly, the bands observed at 1460 and 1380 cm−1 were assigned to the deformation vibration of CH2 and CH3, respectively. The above bands confirmed the composition of polypropylene mat. After modification, the peaks at 1750 and 1180 cm−1 appeared, which corresponded to the stretching vibration of CO and C−O−C. The new peaks indicated that lauryl acrylate and ethylene glycol dimethacrylate monomers were introduced onto the PP mat. To further confirm the surface modification of the PP mat, XPS analysis was conducted to explore the chemical components of the original PP and PP-g-LA-co-EGDMA mat. The respective XPS wide scan and C 1s core-level spectra of (a and b) pristine PP and (c and d) PP-g-LA-co-EGDMA mat are presented in Figure 4. The C 1s core-level spectrum (Figure 4b) of the original PP mat can only be curve fitted into peaks with binding energies (BEs) of 284.6 eV, which is attributable 11774

DOI: 10.1021/acs.iecr.5b03778 Ind. Eng. Chem. Res. 2015, 54, 11772−11778

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Figure 4. XPS wide scan and C 1s core-level spectra of (a and b) pristine PP and the (c and d) PP-g-LA-co-EGDMA mat.

Figure 5. Water contact angle of (a) original PP mat, (b) PP-g-LA-co-EGDMA mat, and (c−f) dynamic sorption process of oil droplet on the PP-gLA-co-EGDMA mat.

from chemical reaction, which are based on the whole process of adsorption without considering the diffusion steps. Considering the different morphologies of the cross-linking layer and storage layer, a double-exponential model is used herein and presented as follows:20,21

To further explore the sorption mechanism of this system, sorption kinetics was systematically studied. Various kinetics models have been suggested for adsorption analysis. The pseudo-first-order and pseudo-second-order models are commonly used for oil sorption which is expressed as pseudo-first-order

qt = qe(1 − e

−k1t

)

qt = qe − qD1 e−KD1t − qD2 e−KD2t (4)

where qD1 and qD2 are the adsorption capacities (g/g) of the rapid and slow step, respectively. In this study, they can be defined as adsorption capacity of storage layer and cross-linking layer, respectively. KD1 and KD2 are diffusion parameters (min−1) which control the adsorption process of the rapid and slow step, respectively. All the kinetic parameters and nonlinear regression correlation coefficients are listed in Tables 1 and 2. The pseudo-second-order model (R2 > 0.99) and double-exponential model (R2 > 0.98) were fitted better than the pseudo-firstorder model (R2 > 0.89). In addition, the experimental qe,exp values were quite close to adsorption capacity calculated from the pseudo-second-order kinetic model (q2e,cal) and double-

pseudo-second-order qt =

qe2k 2t 1 + qek 2t

(6)

(5) −1

where k1 and k2 (min ) are rate constants of pseudo-first-order and pseudo-second-order, respectively, qe is the oil absorbency at equilibrium, and qt is oil absorbency at time t. It can be observed from Figure 6c that the kinetics curve was distinctly divided into three steps, namely rapid step (0−1 min), gradual saturation step (1−12 min), and equilibrium step (>12 min). However, above adsorption reaction kinetics are originated 11775

DOI: 10.1021/acs.iecr.5b03778 Ind. Eng. Chem. Res. 2015, 54, 11772−11778

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Figure 6. (a) Sorption capacity of PP-g-LA-co-EGDMA and pristine PP for different kinds of oils, (b) schematic of oil sorption, and (c) sorption kinetics of PP-g-LA-co-EGDMA.

Table 1. Parameters for Pseudo-first-order and Pseudo-second-order Kinetic Model pseudo-first-order kinetic model

pseudo-second-order kinetic model

adsorbate

qe,exp (g/g)

k1 (min−1)

q1e,cal (g/g)

R2

k2 (g/(g min))

q2e,cal (g/g)

R2

crude oil diesel gasoline kerosene toluene

17.63 12.72 13.24 16.40 15.82

2.14 1.12 1.31 1.48 1.41

16.92 12.07 12.64 15.84 15.13

0.9322 0.9042 0.9225 0.9504 0.9366

0.1149 0.0854 0.1066 0.1153 0.1044

17.95 13.12 13.56 16.73 15.99

0.9963 0.9944 0.9921 0.9972 0.9996

diffusion layer, and the diffusion channels in the cross-linking layer were “shut off”. As a result, the oil was reserved in the storage layer. (3) Dynamic equilibrium step. The sorption rate of different steps could also be quantitatively explained using Washburn equation:22

Table 2. Parameters for Double-Exponential Model rapid step

slow step

adsorbate

qDe,cal (g/g)

qD1 (g/g)

KD1 (min−1)

qD2 (g/g)

KD2 (min−1)

R2

crude oil diesel gasoline kerosene toluene

17.02 12.86 13.33 16.44 15.64

8.50 7.86 8.77 11.53 9.02

6.78 3.58 3.30 4.20 6.34

8.41 5.00 4.57 4.91 6.61

0.921 0.153 0.166 0.302 0.288

0.9978 0.9865 0.9898 0.9865 0.9977

⎛ γ cos θ ⎞ L2 = ⎜ ⎟rt ⎝ 2η ⎠

(7)

which could be transformed to t=

exponential model (qDe,cal). Adsorption reaction models (pseudo-first-order models and pseudo-second-order models) assumed the adsorption process as chemical reactions. It could be evaluated that the pseudo-second-order model was more suitable compared with pseudo-first-order model. This is because both the sorbent and adsorbate dosage had tremendous influence on sorption rate. According to morphology of cross section and doubleexponential kinetic parameters, the sorption process could be distinctly divided into three steps and described as follows: (1) Rapid sorption step (storage layer sorption). In this stage, oil droplet rapidly penetrated through channels of cross-linking layer into storage layer. (2) Gradual sorption step (cross-linking layer sorption). In this stage, the cross-linking layer gradually swelled and its grafted chains stretched. The cross-linking layer after saturation adsorption performed as a unidirectional

2ηL2 γr cos θ

(8)

where t is time for liquid of viscosity η and surface tension γ to penetrate a distance (penetration depth) L into a wettable, porous material whose average radius is r. Although PP-g-LAco-EGDMA had stronger hydrophobicity (larger cos θ), the equivalent radius of the cross-linking layer was much smaller than that of the storage layer. Thus, the gradual sorption step (cross-linking layer sorption) needed more time for equilibrium. The adsorption capacity of double-exponential model (qD1 and qD2) also revealed that the storage layer played a dominant role in sorption capacity for all kinds of oil (qD1 > qD2), which is consistent with the thickness of both layers. To dynamically observe the swelling process of the crosslinking layer on the polypropylene mat, a microparticle grafting experiment was designed based on the determination of 11776

DOI: 10.1021/acs.iecr.5b03778 Ind. Eng. Chem. Res. 2015, 54, 11772−11778

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Industrial & Engineering Chemistry Research stimuli-responsive materials. First, polypropylene granules were milled into microparticles in a high speed ball mill. Afterward, the grafted layer was introduced through the method mentioned in section 2.2. The median particle diameters (D0.5) of PP-g-LA-co-EGDMA microparticles in toluene were recorded by dynamic light scattering (Beckman Coulter LS 13320) and shown in Figure 7. The median particle diameters

Figure 8. Mechanical retention capacity of PP-g-LA-co-EGDMA and the original PP mat.

modification, namely enhancing the hydrophobicity of sorbent and increasing the ΔP of capillarity. In this study, we proposed a new solution, and a cross-linking layer was fabricated onto the PP sorbent through ultraviolet radiation graft polymerization. The oil could penetrate through channels of the cross-linking layer into the storage layer rapidly. After saturation, the crosslinking layer swelled and the channels were locked. The oil molecules were actually trapped into the three-dimensional network structure of the cross-linking layer, and this layer performed as a unidirectional diffusion layer. Thus, the retention capacity of PP-g-LA-co-EGDMA was dramatically enhanced. 3.4. Repeatability of Oil Sorption. Reusability is an important parameter for an oil sorbent. In real applications, the saturated sorbent could be mechanically centrifuged and used repeatedly. Figure 9 indicates the repeatability of the PP-g-LAco-EGDMA and pristine PP mats in a kerosene sorption test. It was noted that PP-g-LA-EGDMA showed an excellent reusability. After 20 circles, the sorption capacity of PP-g-LAEGDMA was retained at 70% compared with that of the first

Figure 7. Median particle diameters (D0.5) of PP-g-LA-co-EGDMA microparticles in toluene.

increased rapidly and attained equilibrium within 20 min, which corresponded to the sorption kinetics as well. This experiment confirmed the swelling behaviors of the cross-linking layer. In addition, the swelling provided more spaces for oil, thus dramatically increasing the sorption capacity. 3.3. Mechanical Retention Experiment. The oil retention capacity during field application, transfer, and handling operation is an important factor for sorbent evaluation. Actually, in most studies, the adsorbed oil can be easily squeezed by mechanical force for fibrous sorbents, which means that the external interferences such as ocean waves and towing during use and collection may cause the leakage of oil from sorbents. In this study, the mechanical retention experiment was designed and shown in Figure 8. The results indicated that compared with the original PP mat, PP-g-LA-coEGDMA could sustain considerable mechanical forces and the retention capacity under mechanical forces was dramatically intensified. The oil retention capacity of the original PP mat dropped by 81% under 100 kPa mechanical pressure. In contrast, there was merely a 32% decrease for the PP-g-LA-coEGDMA mat under the condition of identical pressure. In theory, the sustainable forces for fibrous sorbents are limited, which is closely related to capillarity (ΔP in Young−Laplace equation). When the exterior forces (e.g., ocean waves, towing) exceeded the threshold, the oil stored in the sorbent would inevitably leak out and cause second pollution. To overcome this defect, most researches are focused on hydrophobic

Figure 9. Repeatability of the PP-g-LA-co-EGDMA and pristine PP mats. 11777

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(7) Wang, D.; McLaughlin, E.; Pfeffer, R.; Lin, Y. S. Adsorption of oils from pure liquid and oil−water emulsion on hydrophobic silica aerogels. Sep. Purif. Technol. 2012, 99, 28−35. (8) Hubbe, M. A.; Rojas, O. J.; Fingas, M.; Gupta, B. S. Cellulosic substrates for removal of pollutants from aqueous systems: A Review. 3. Spilled oil and emulsified organic liquids. BioResources 2013, 8, 3038−3097. (9) Adebajo, M. O.; Frost, R. L.; Kloprogge, J. T.; Carmody, O.; Kokot, S. Porous materials for oil spill cleanup: a review of synthesis and absorbing properties. J. Porous Mater. 2003, 10, 159−170. (10) Singh, V.; Kendall, R. J.; Hake, K.; Ramkumar, S. Crude oil sorption by raw cotton. Ind. Eng. Chem. Res. 2013, 52, 6277−6281. (11) Deschamps, G.; Caruel, H.; Borredon, M. E.; Albasi, C.; Riba, J. P.; Bonnin, C.; Vignoles, C. Oil removal from water by sorption on hydrophobic cotton fibers. 2. Study of sorption properties in dynamic mode. Environ. Sci. Technol. 2003, 37, 5034−5039. (12) Deschamps, G.; Caruel, H.; Borredon, M. E.; Bonnin, C.; Vignoles, C. Oil removal from water by selective sorption on hydrophobic cotton fibers. 1. Study of sorption properties and comparison with other cotton fiber-based sorbents. Environ. Sci. Technol. 2003, 37, 1013−1015. (13) Wei, Q. F.; Mather, R. R.; Fotheringham, A. F.; Yang, R. D. Evaluation of nonwoven polypropylene oil sorbents in marine oil-spill recovery. Mar. Pollut. Bull. 2003, 46, 80−783. (14) Rengasamy, R. S.; Das, D.; Karan, C. P. Study of oil sorption behavior of filled and structured fiber assemblies made from polypropylene, kapok and milkweed fibers. J. Hazard. Mater. 2011, 186, 526−532. (15) Choi, H. M.; Moreau, J. P. Oil sorption behavior of various sorbents studied by sorption capacity measurement and environmental scanning electron microscopy. Microsc. Res. Tech. 1993, 25, 447−455. (16) Duan, B.; Gao, H.; He, M.; Zhang, L. Hydrophobic Modification on Surface of Chitin Sponges for Highly Effective Separation of Oil. ACS Appl. Mater. Interfaces 2014, 6, 19933−19942. (17) Wang, C.; Yao, T.; Wu, J.; Ma, C.; Fan, Z.; Wang, Z.; Yang, B. Facile approach in fabricating superhydrophobic and superoleophilic surface for water and oil mixture separation. ACS Appl. Mater. Interfaces 2014, 1, 2613−2617. (18) Zheng, X.; Guo, Z.; Tian, D.; Zhang, X.; Li, W.; Jiang, L. Underwater self-cleaning scaly fabric membrane for oily water separation. ACS Appl. Mater. Interfaces 2015, 7, 4336−4343. (19) Singh, V.; Jinka, S.; Hake, K.; Parameswaran, S.; Kendall, R. J.; Ramkumar, S. Novel Natural Sorbent for Oil Spill Cleanup. Ind. Eng. Chem. Res. 2014, 53, 11954−11961. (20) Chiron, N.; Guilet, R.; Deydier, E. Adsorption of Cu (II) and Pb (II) onto grafted silica: isotherms and kinetic models. Water Res. 2003, 37, 3079−3086. (21) Wilczak, A.; Keinath, T. M. Kinetics of sorption and desorption of copper (II) and lead (II) on activated carbon. Water Environ. Res. 1993, 65, 238−244. (22) Washburn, E. W. The dynamics of capillary flow. Phys. Rev. 1921, 17, 273.

circle. However, the pristine PP mat only retained 22% of its original capacity. The decrease of fibrous sorbents was mainly ascribed to the deformation of the inner structure. Generally, a fibrous sorbent with excellent reusability should have good flexibility and an elastic nature. After modification, the crosslinking layer performed as a scaffold to sustain the fibrous structure, which intensified the mechanical strength and reduced the inner collapse.

4. CONCLUSIONS In summary, cross-linking layer was fabricated onto a PP mat and performed as unidirectional diffusion layer. The oil could penetrate rapidly through the cross-linking layer into the storage layer. After saturation, the cross-linking layer swelled, and its grafted chains stretched. The swelling of the crosslinking layer offered more spaces for oil molecules. The diffusion channels in the cross-linking layer were closed, and the oil was mostly reserved in the storage layer. Besides, with the addition of lauryl acrylate monomers, the hydrophobility and sorption capacity of PP-g-LA-co-EGDMA were dramatically improved compared with that of pristine PP. The reusability test revealed that the cross-linking layer also performed as a scaffold to sustain the inner fibrous structure, which improved the reusability. The approach proposed in this study could be a facile and efficient method to fabricate fibrous sorbent (e.g., cotton, kapok, wool, polypropylene nonwoven, etc.) and increase its oil adsorption and retention capacity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-022-8395-5898. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (No. 41301542), National High Technology Research and Development Program of China (2013AA065601), and Key Technologies R&D Program of Tianjin (13ZDSF00100).

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REFERENCES

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DOI: 10.1021/acs.iecr.5b03778 Ind. Eng. Chem. Res. 2015, 54, 11772−11778