Fabrication of Hydrophilic and Hydrophobic Sites on Polypropylene

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Fabrication of hydrophilic and hydrophobic sites on polypropylene nonwoven for oil spill cleanup: Two dilemmas affecting oil sorption Xiangyu Zhou, Feifei Wang, Yali Ji, Weiting Chen, and Junfu Wei Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b06007 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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Environmental Science & Technology

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Fabrication of hydrophilic and hydrophobic sites on polypropylene nonwoven

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for oil spill cleanup: Two dilemmas affecting oil sorption

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Xiangyu Zhou1, 2, 3, Feifei Wang1, 3, Yali Ji1, 2, Weiting Chen1, 2, Junfu Wei1, 2, 4 *

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1. State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin

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Polytechnic University, Tianjin 300387, P.R. China;

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2. School of Environmental and Chemical Engineering, Tianjin Polytechnic

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University, Tianjin 300387, P.R. China;

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3. School of Materials Science and Engineering, Tianjin Polytechnic University,

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Tianjin 300387, P.R. China

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4. Tianjin Engineering Center for Safety Evaluation of Water Quality & Safeguards

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Technology, Tianjin 300387, China

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Correspondence to: Junfu Wei *([email protected]).

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School of Environmental and Chemical Engineering, Tianjin Polytechnic University,

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Tianjin 300387, P.R. China.

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Tel.: +86-022-8395-5898

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ACS Paragon Plus Environment


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Abstract

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This article mainly deals with following dilemmas, which affect oil sorption and

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sorbent preparation: (1) hydrophobization could facilitate oil sorption but had adverse

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impacts on emulsion sorption; (2) Micropores of conventional oil sorbent did not

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exhibit effectiveness to emulsion sorption. To solve above contradictions, hydrophilic

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and hydrophobic sites were fabricated onto polypropylene (PP) nonwoven through

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electron beam (EB) radiation and subsequent ring-opening reaction. Further, a similar

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structure without hydrophilic site was constructed as comparison to verify the

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dilemmas. Oil sorption and emulsion adsorption experiment revealed that the PP

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nonwoven with specific hydrophilic and hydrophobic sites is more suitable for oil

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cleanup. The hydrophobic site preserved its hydrophobicity and sorption capacity, and

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the hydrophilic site on PP surface effectively increased the affinity between the

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hydrophilic interface of emulsion and sorbent. The overlapped and intertwined

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structures could provide spaces that large enough to accommodate oil and emulsion.

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In addition, the oil and emulsion sorption behaviors were systematically analyzed.

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The PP nonwoven fabricated in this study may find practical application in the

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cleanup of oil spills and the removal of organic pollutants from water surfaces.

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1. Introduction

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With the rapid growth of offshore oil production and transportation, oil spillage and

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chemical leakage accidents occur frequently worldwide. Such accidents can cause

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severe ecological disasters and the cleanup of these organic pollutants from water

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sources is a major environmental issue that continues to attract a great deal of


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attention.1-4 Current strategies used to tackle these problems can be usually

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categorized into three main types: physical methods (e.g. sorbents skimmers, booms),

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chemical methods (i.e. in situ burning, solidifiers), and biological methods (e.g.

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bioremediation).5,6 Among these methods, sorbents are considered as economical and

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efficient countermeasures for combating oil spills.7-9

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It is generally considered that the excellent oil sorbent should have following

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characteristics: (1) Hydrophobicity. Component analysis reveals that crude oil mainly

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consists of paraffins (alkanes that can be linear or branched), naphthenes

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(non-aromatic multi-ring structures) and aromatic compounds.10 Thus enhancing

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hydrophobicity of sorbent surface will facilitate the Van der Waals force or

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hydrophobic interaction between oil molecules and sorbent and increase the sorption

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capacity. (2) Sufficient porosity and large specific surface area. Sorbents with

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sufficient specific surface area and porosity will provide enough sorption sites or

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spaces for oil entrapment. Guided by above principles, most researches of oil sorbent

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materials are focused on hydrophobic modification and porous structure development.

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In recent years, numerous superhydrophobic sorbents (contact angle > 150o) with

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spectacular specific surface area such as carbon fiber, aerogel, carbon nanotube

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sponge, etc. have been developed as promising candidates for potential

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application.11-15

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However, both principles are on the basis of ideal experiments. It should be noted

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that researchers on fabrication of oil sorbent are excessively focused on sorption

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capacity, but ignore a fundamental fact: the complexity of natural water and crude oil.



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Some chemicals in practical contaminated water e.g., auxiliary agent used in oil

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production, surfactant discharged in ecological environment, metabolites of marine

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creatures, etc. can be regarded as emulsifiers or dispersants. Apart from oil spreading

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over water, plenty of oil is in the form of oil-in-water emulsion and steadily exists in

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natural water.10 Compared with surface spill, the oil emulsion has quite different

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physical nature and morphological structure. Figure 1 reveals the schematic diagram

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of typical oil slicks. Generally, the hydrophobic ends of emulsifiers are embedded in

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the oil droplet and the hydrophilic groups are exposed to water. The stable structure in

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water indicates that the oil emulsion is actually hydrophilic. This can be viewed as the

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first dilemma: Enhancing the affinity between oil molecule and sorbent means

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increasing the hydrophobicity of sorbent surface. However, excessively hydrophobic

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nature weakens the interaction between the hydrophilic emulsion and hydrophobic

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sorbent. This dilemma seriously impedes the sorption efficiency and performance in

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application. We believe that the key to conquer the first challenge is amphiphilicity.

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This notion is originated from biochemistry and widely researched in pollution

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resistance and cell adhesion. Inspired by this notion, the hydrophilic and the

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hydrophobic sites were introduced onto polypropylene (PP) nonwoven, which is the

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most commercialized material for oil sorption.16 The amphiphilic modification will

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perfectly solve the problem.

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Figure 1. Schematic diagram of typical oil slicks.

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Another dilemma pertains to the pore size of sorbent. In recent years, many

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researchers have been attempting to improve the sorption performance of oil sorbent.

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Theoretically, remarkable oil sorbents should have excellent sorption capacity for oil,

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which means the oil sorbent should theoretically provide enough spaces or sites to

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accommodate oil molecules. Accordingly, researchers have developed numerous

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sorbents with huge specific surface area and porosity. However, they seem to ignore

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the diameter of emulsified oil droplet in real application. For many oil sorbents, it

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may be unsuitable for emulsion sorption, because the emulsion can not penetrate into

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the micropores of sorbent.

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Thus in this study, two dilemmas on oil sorption and sorbent preparation were

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proposed: (1) hydrophobization could facilitate oil sorption but had adverse impacts

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on emulsion sorption; (2) Micropores of conventional oil sorbent did not exhibit



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effectiveness to emulsion sorption. Accordingly, hydrophilic and hydrophobic sites

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were fabricated onto PP nonwoven fabric. The hydrophobic site preserved its

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hydrophobicity and sorption capacity. The hydrophilic site on PP surface effectively

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increased the affinity between the hydrophilic interface of emulsion and sorbent. As a

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result, the emulsion could be adsorbed as well. In addition, the overlapped and

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intertwined structures of PP nonwoven provided enough interspaces that large enough

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to accommodate oil emulsion. The PP nonwoven fabricated in this study may find

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practical application in the cleanup of oil spills and the removal of organic pollutants

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from water surface.

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2. Experimental section

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2.1 Sorbent preparation. The polypropylene nonwoven with hydrophilic and

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hydrophobic site was prepared by grafting of glycidyl methacrylate (GMA) onto PP

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nonwoven through electron beam (EB) radiation induced graft polymerization, and

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subsequently converted the epoxy group of GMA to hydrophilic hydroxyl and

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hydrophobic long-chain alkylamine through reaction with n-octylamine.17 The

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schematics of surface modification are shown in Figure 2.


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Figure 2. Synthetic route and chemical structure of PP-g-GMA-OA nonwoven.

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2.2 Characterization. The surface morphologies of PP nonwoven were observed

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using the scanning electron microscopy (SEM) by an S-2500C (Hitachi, Japan)

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microscope (Figure S1). The surface chemical composition of polypropylene

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nonwoven was characterized by X-ray photoelectron spectroscopy (XPS), and the

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analysis was carried out on an AEM PHI 5300X spectrometer with an Al Kα X-ray

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source (1486.71 eV of photons) to determine the C, N and O (Figure S2). The

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software package Thermo Avantage 3.9.3 was used to fit the spectra peaks. To analyze

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the functional groups on the surface of PP samples, the FTIR spectra was recorded on

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Fourier transform infrared spectrometer (Necolet 6700, USA) in the wavenumber

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range of 600-4000 cm-1 under ambient condition (Figure S3). To observe the

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three-dimensional structures of PP substrate, confocal laser scanning microscopy

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(Leica TCS SP8, Germany) was utilized in this study. In addition, the emulsion

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diameters were recorded by dynamic light scattering analyzer (Beckman Coulter LS

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13-320, USA).



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2.3 Oil sorption experiment. The practical use of sorbent for oil cleanup might

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take place in the presence of water. In a typical measurement of this study, 400ml of

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water was poured into a 500 ml glass beaker with appropriate oil to form 6 mm oil

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film. Diesel, gasoline, crude oil (ρ 0.89g/mL, γ 0.032 N/m, η 0.132 Pas), kerosene and

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toluene were employed. The PP nonwoven (3 × 3 cm) was slowly placed on oil

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surface for oil/water sorption. After 30 min of sorption, the samples were weighed and

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the oil sorption capacity was calculated as:

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‫ݍ‬୫ = (݉୤ − ݉଴ )/݉଴

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where qm is the maximum sorption capacity (g/g), mf is the final weight of the of

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samples after sorption and m0 is the initial weight of PP samples. All the sorption

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measurements were performed in triplicate.

(1)

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2.4 Emulsion adsorption experiment. In this section, sodium dodecylbenzene

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sulfonate (SDBS), sodium dodecyl sulfate (SDS) and Tween 80 (C64H124O26) were

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used to stabilize diesel emulsion. Predetermined surfactant (0.05g for SDBS and SDS,

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100 µL for Tween 80) was added to 500mL water with 5mL diesel under the agitation

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of 1000 rpm. After equilibrium, the mixture was stabilized and separated. The

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emulsion phase was selected for emulsion adsorption experiment. The concentration

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of oil in emulsion phase was measured by analyzing the total organic carbon (TOC) in

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the mixture.

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3. Results and discussion

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3.1 Explorations on hydrophobicity. As discussed previously, the weak interaction

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between hydrophobic sorbent surface and hydrophilic interface of emulsion may


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hinder the sorption process. To test this hypothesis, a similar structure without

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hydrophilic site is fabricated. It is noticed that the structure of lauryl methacrylate

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(LMA) is similar to grafted monomer in this study but lacks the hydrophilic part

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(Figure 3a). Therefore, PP-g-LMA nonwoven is prepared as comparison. The contact

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angle (CA) is a parameter of surface wettability and is demonstrated in Figure 3b. It is

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demonstrated that hydrophobic LA monomers significantly raised the contact angle

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(from 111.3o to 130.7o), while the hydrophilic site reduced the contact angle (41.7o).

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However, due to the addition of hydrophobic site and inherent hydrophobicity of PP

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substrate, the water droplet cannot completely penetrate into the nonwoven.

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Figure 3. (a) Structure of PP-g-GMA-OA and PP-g-LMA nonwoven and (b) water



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contact angle of PP substrate, PP-g-GMA-OA and PP-g-LMA nonwoven.

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The results of emulsion adsorption are presented in Figure 4a. Compared with

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initial concentration, the hydrophobic PP-g-LMA nonwoven showed little sorption

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capacity towards emulsion. The tiny decrease of TOC may be ascribed to the

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dissolved and dispersed ingredient of diesel that adsorbed on PP-g-LMA. Meanwhile,

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PP-g-GMA-OA nonwoven presented a considerable effect and approximately 60-70%

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of TOC can be removed. We believe that the most predominant reason for this

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phenomenon is the special structure of emulsion. For oil-in-water emulsion, the

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hydrophilic end of surfactant faces outward, which lower the interaction between the

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hydrophilic interface and hydrophobic surface. In other words, the hydrophilic

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interface impedes the contact of emulsion and sorbent (Figure 4b). However, the

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hydrophilic site on PP-g-GMA-OA reduces these diffusion resistances and increases

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their interaction. Thus emulsion adsorption could take place.

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The results can be explained by capillary theory. Previous studies revealed that the

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capillary action played a dominant role in oil sorption, which can be quantitatively

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described by Young-Laplace equation:18

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∆ܲ =

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where ∆P is capillary pressure between PP filaments. γlν is surface tension that liquid

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oil spreads on solid PP surface. θ is contact angle of oil and PP substrate. r is

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equivalent radius of capillary. In theory, the addition of surfactant will significantly

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decrease the surface tension γlν and accordingly the capillary as the driving force for

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oil sorption is seriously affected. However, the hydrophilic site on PP surface could

ଶఊౢν ୡ୭ୱఏ ௥

(2)


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counteract this decline. Amphiphilic PP-g-GMA-OA nonwoven still had strong

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affinity to oil emulsion.

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Figure 4. (a) Emulsion adsorption experiment and (b) schematic of hydrophobic

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sorbent surface and hydrophilic emulsion interface.

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Many studies mentioned the sorption selectivity, i.e. the sorbent should adsorb oil

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preferentially instead of water.19,20 Figure 5a reveals the pure oil sorption experiment

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on water surface. Compared with hydrophobic PP-g-LMA nonwoven, merely a small

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decrease could be observed and amphiphilic PP-g-GMA-OA nonwoven showed a

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moderate sorption capacity. The optical microscope images of emulsion before and

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after PP-g-GMA-OA sorption are shown in Figure S4. Importantly, no obvious water



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uptake was observed. We considered that this is mainly ascribed to the inherent

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hydrophobicity of polypropylene substrate (hydrophobic carbon backbone) and the

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introduction of hydrophobic site. Another point we believe is that, due to the lower

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densities of oil and PP substrate, they will contact preferentially on the surface of

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water. This initial oil sorption step can be regarded as hydrophobic coating in some

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sense. Thus some hydrophilic natural materials such as woolen, cotton and so forth

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can be utilized in oil sorption as well.21,22 The dynamic sorption process of oil droplet

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(toluene, 0.8µL) on PP-g-GMA-OA nonwoven is presented in Figure 5b. The droplet

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could rapidly penetrate into the nonwoven within 0.36s, which indirectly reveals the

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loose structure and hydrophobic nature of PP substrate.

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Figure 5. (a) Oil sorption experiment and (b) dynamic sorption process of toluene


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droplet.

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3.2 Explorations on porous structure. In recent years, most researchers have been

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attempting to enhance the sorption capacity of sorbent. As mentioned above, an

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excellent sorbent should provide enough spaces to accommodate oil molecules.

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However, most researchers seemed to ignore the diameter of oil emulsion. Generally,

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sorbent with abundant porosity and large specific surface area usually have

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micropores. Figure 6 demonstrates the optical microscope image and diameter

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distribution of emulsified oil (SDBS+diesel in section 2.4) in aqueous solution. It can

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be observed that the light interface wraps the emulsion and the diameter of oil

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emulsion is in the range of 1-100 µm. The median particle diameters (D0.5) calculated

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by dynamic light scattering analyzer is 20.7 µm, which is much larger than typical

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pore size (micropore < 2nm, mesopore 2-50nm, macropore >50nm). Although the

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emulsion diameter could be influenced by factors such as agitation, surfactant and oil

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types, surfactant and oil content, etc., a typical emulsion diameter is mostly in the

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range of micron-scale.23 The differences of oil diameter and pore size may in turn

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impact the emulsion adsorption.

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Figure 6. (a) Optical microscope image and (b) diameter distribution of emulsified oil.



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In fact, due to the complexity of emulsion structure and poor sorption performance,

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seldom studies have focused on emulsion sorption process. It is assumed that the

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process may occur in either of the following ways: (1) Due to the stronger

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hydrophobic interaction between oil molecules and sorbent, the oil is adsorbed

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preferentially, and the surfactant is released. In other words, this process can be seen

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as demulsification. (2) The emulsified oil is adsorbed onto or penetrates into sorbent

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pores as a whole. In this case, both the oil and surfactant molecules are adsorbed.

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It is difficult to directly observe this process at molecular level. However, it is noted

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that the major differences for both processes are the content of oil and surfactant that

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remained in solution. In this part, toluene and SDBS were utilized to simulate

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emulsion and the contents of the compositions before and after sorption were

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quantitatively detected by high performance liquid chromatography (HPLC). The

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result indicated that the content of toluene and SDBS reduced simultaneously,

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revealing that the emulsion was adsorbed as a whole and nonselective sorption

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occurred (Figure7).

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Figure 7. Schematic of emulsion adsorption on PP substrate.


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Theoretically, adsorption is an exothermic and the energy that discharged in

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physical adsorption is usually less that 42 kJ/mol.24 Apparently, the energy discharged

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in sorption can not afford to break emulsion. In addition, the hydrophobic interaction

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is not a selective driving force. Thus the emulsion was adsorbed onto PP-g-GMA-OA

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nonwoven as a whole. This test reveals that appropriate pore size is needed in oil

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sorption. If the pore size is far less than emulsion diameter, the emulsion droplet may

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not penetrate into the tiny pores. In this case, most of specific surface area and pore

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volume of sorbent are wasted (Figure 8).

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Figure 8. Schematic of emulsion diameter and conventional pores.

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Compared with porous materials, the overlapped and intertwined structure of PP

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filament perfectly solves this contradiction. The scanning electron microscope (SEM)

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and three-dimensional confocal laser scanning images of PP nonwoven are shown in

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Figure 9. Even though there are no porous structure that can be observed on the

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smooth surface (Figure 9a), the overlapped and intertwined structure provides enough

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macropores that in the range of micron-scale for oil uptake. The emulsion can be



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stored in the interspaces of PP filaments.

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Figure 9. (a) Scanning electron microscope image and (b) three-dimensional

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confocal laser scanning image of PP nonwoven.

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4. Conclusion

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In summary, two dilemmas affecting oil sorption in practical application were

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proposed. The first is that hydrophobicity could enhance the hydrophobic interaction

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between oil and sorbent, but lower the affinity of hydrophilic emulsion interface. The

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second pertains to the pore size. HPLC analysis revealed that the emulsion was

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adsorbed as a whole. Typical pore size (micropore < 2nm, mesopore 2-50nm,

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macropore >50nm) is much smaller than emulsion diameter (micron-scale), thus


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leading to the fact that the emulsion droplet cannot penetrate into the tiny pores.

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Accordingly, to overcome the challenges inherent in adsorbing both emulsified and

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liquid oil, hydrophilic and hydrophobic sites were fabricated onto polypropylene (PP)

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nonwoven through electron beam (EB) radiation and subsequent ring-opening

278

reaction. On the one hand, the hydrophobic site preserved its hydrophobicity and

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sorption capacity. On the other hand, the hydrophilic site on PP surface effectively

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increased the affinity between the hydrophilic interface of emulsion and sorbent. As a

281

result, the emulsion could be adsorbed as well. In addition, the overlapped and

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intertwined structures of PP nonwoven provided interspaces that large enough to

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accommodate oil emulsion.

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Acknowledgements

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This research was supported by the National Natural Science Foundation of China

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(No. 41301542), National High Technology Research and Development Program of

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China (2013AA065601) and Key Technologies R&D Program of Tianjin

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(15ZCZDSF00880).

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Supporting Information Available

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Experimental details and characterizations of original and modified PP nonwoven are

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presented. This material is available free of charge via the Internet at

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http://pubs.acs.org.

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TOC

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Fabrication of Hydrophilic and Hydrophobic Sites on Polypropylene

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