Superhydrophobic coating on quartz sand filter media for oily wastewater filtration

Accepted Manuscript Title: Superhydrophobic coating on quartz sand filter media for oily wastewater filtration Authors: Liu Jianlin, Zhu Xiaofei, Zhang Hongwei, Wu Fuping, Wei Bigui, Chang Qing PII: DOI: Reference:

S0927-7757(18)30485-0 https://doi.org/10.1016/j.colsurfa.2018.06.007 COLSUA 22576

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

11-4-2018 1-6-2018 2-6-2018

Please cite this article as: Liu J, Zhu X, Zhang H, Wu F, Wei B, Chang Q, Superhydrophobic coating on quartz sand filter media for oily wastewater filtration, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2018), https://doi.org/10.1016/j.colsurfa.2018.06.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Superhydrophobic coating on quartz sand filter media for oily wastewater filtration

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Liu Jianlin, Zhu Xiaofei, Zhang Hongwei, Wu Fuping, Wei Bigui*, Chang Qing

(School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou,

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Gansu, 730070, PR China)

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* Corresponding author.

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Graphical abstract

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E-mail: [email protected] (Wei B G)

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Abstract:

Deep bed filtration is commonly used to remove oil from oily wastewater. The wettability of the filter media greatly influences the oil removal efficiency. A filter with a strong hydrophobicity and oleophilicity results in a higher oil removal efficiency. To improve the hydrophobicity and

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oleophilicity, ZnO nanoparticles and octadecyltrichlorosilane (OTS) were coated on quartz sand filter media. SEM, FTIR and XPS were used to study the morphology and chemical composition of the filters, and adsorption capacity and oil removal efficiency experiments were performed to assess

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the oil removal performance. The results show that the OTS molecules are grafted onto the surface of the quartz sand filter through chemical bonding, and the OTS/ZnO-coated quartz sand filter is superhydrophobic and oleophilic, with a water contact angle of 154.1° and an oil contact angle of

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0°. The OTS/ZnO-coated quartz sand filter has very high mechanical stability and acid resistance.

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The oily wastewater treatment performance is greatly improved after application of the OTS/ZnO

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coating, and the superhydrophobic quartz sand filter media can potentially be used to filter oily

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wastewater and achieve high oil removal efficiency.

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Keywords:

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oily wastewater; filtration; superhydrophobic; quartz sand filter; superoleophilic

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1. Introduction

Oily wastewater is commonly found in the petroleum, chemical, metallurgical, mechanical processing and textile industries. Every year, hundreds of billions of tons of oily wastewater are discharged into rivers and lakes around the world, resulting in a serious waste of resources and environmental pollution that poses a serious threat to the environment [1]. Deep bed filtration is a tertiary treatment technology for low-concentration oily wastewater, especially for oil particles with 2

sizes less than 10 μm [2]. The wettability of the filter media seriously influences the oil removal efficiency [3, 4]. A filter with a stronger hydrophobicity and oleophilicity results in a higher oil removal efficiency [5]. Therefore, the oil removal efficiency of the filtration process can be improved by improving the hydrophobicity of the surface of the filter media. There are two aspects to improving the hydrophobicity. One is grafting a low surface energy material to the filter material surface, making the surface hydrophobic, and the other is the addition of a

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rough structure on the surface[6] to create a superhydrophobic surface. Superhydrophobic surfaces are usually superoleophilic. Superhydrophobic and superoleophilic surfaces are defined as having a

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water contact angle greater than 150° and an oil contact angle less than 5° [7]. Superhydrophobic

surfaces have broad application prospects in oil/water separation [5, 8-13], ice-over delay [14, 15], self-cleaning surfaces [16], anticorrosive technologies [17, 18], medicine [19] and other fields.

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Superhydrophobic materials have received extensive attention from researchers since 1977 when

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Barthlott and Neinhuis [20], inspired by the lotus leaf, first proposed them.

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At present, the low surface energy materials commonly used to fabricate superhydrophobic and

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superoleophilic surfaces include alkyl mercaptan [21, 22], hydrophobic silica gel [23] and longchain fatty acids [16]. The matrix materials used for oil/water separation include fibres [8, 9],

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sponges [10-12], cotton [13], and filter paper [5]. The oil removal process includes immersing one of these materials in an oil/water mixture and then removing it, using the material’s adsorption

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selectivity for oil/water separation. The other main matrix material is metal mesh [24, 25]. A superhydrophobic and superoleophilic membrane can be formed on the metal mesh by physical and

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chemical methods. Gravity cross-flow filtration can be used to separate oil and water because oil has a super affinity to pass through the membrane pores, while the water is intercepted by the membrane.

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Although superhydrophobic and superoleophilic surfaces are widely used in the field of oil/water separation, there have been few reports of superhydrophobic and superoleophilic filter media for deep bed filtration processes. Therefore, in this paper, the surface of a quartz sand filter was coated by octadecyltrichlorosilane (OTS) and ZnO nanoparticles to form an OTS/ZnO-coated quartz sand filter. The OTS/ZnO-coated quartz sand filter is superhydrophobic and superoleophilic and can be used to filter oily wastewater with a high oil removal efficiency. 3

2. Experimental section 2.1 Materials and agents The quartz sand filter media was produced by the Gongyi Hongda Filter Plant in the Henan Province of China. OTS, ZnO nanoparticles, dopamine hydrochloride, tris(hydroxymethyl)aminomethane (Tris, chemically pure), cyclohexane, sodium chloride, and concentrated sulfuric acid (analytically pure) were purchased from Lanzhou Honghao Instrument and Equipment Co., Ltd., China. Gasoline

oil (Jinlongyu) was produced by Jiali Grain and Oil (China) Co., Ltd.

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2.2 Pretreatment of quartz sand filter

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engine oil (Kunlun Tiange SD40) was produced by China Petroleum Lube Oil Company. Rapeseed

To obtain a filter medium with size fractions of +0.55 to 0.83 mm, the quartz sand filter was dry sieved through stainless steel sieves. The filter medium was washed several times with deionized

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water, followed by washing with ethanol until the surface of the quartz sand was free of impurities.

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Finally, the purified filter medium was dried at 110 ℃ for 12 h. The effect of any impurities on the

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filter medium surface was thereby substantially eliminated.

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2.3 Fabrication of OTS/ZnO-coated quartz sand filter

A Tris-buffer solution was prepared by adding 25 g solid Tris particles into a 500 mL ethanol

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solution. Then, 1.7 g ZnO nanoparticles, 1.0 mg dopamine hydrochloride and 40 g pretreated quartz sand filter were successively added to the 500 mL Tris-buffer solution. The solution was

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ultrasonically dispersed at 40 kHz for 5 min using a KQ5200DB ultrasonic cleaner (Kunshan Ultrasonic Instruments Company, China), followed by stirring at 650 rpm for 16 h using a JB-2

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constant-temperature magnetic stirrer (Shanghai Leici, China). Then, the quartz sand filter was removed and dried to a constant weight in an oven at 80 °C. Then, 10 mL of OTS and the quartz sand filter were successively added into 100 mL toluene, followed by stirring at 650 rpm for 2 h

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using the JB-2 constant-temperature magnetic stirrer. Again, the quartz sand filter was leached and dried to a constant weight in the oven at 80 °C. By this procedure, the OTS/ZnO-coated quartz sand filter was fabricated. 2.4 Characterization The superhydrophobicity of the quartz sand filter media was assessed by the Krüss DSA100 4

Drop Shape Analysis System (Krüss, Germany). The contact angle values of water or oil drops (7 μL) on the quartz sand filter media surface were measured after 15 s under ambient conditions at a temperature of 20 °C. The surface morphology of pristine and OTS/ZnO-coated quartz sand filters was characterized by a JEOL JSM-5600LV scanning electron microscope (SEM, Kevex, USA). The XPS spectra of the pristine and OTS/ZnO-coated quartz sand filters were recorded using a Perkin Elmer PHI 5702 X-ray photoelectron spectrometer (XPS, American Physical Electron

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Company, USA) to further study the elemental composition and interfacial interactions. The chemical structures were measured by an IFS66V/S infrared spectrometer (FTIR, BRUKER

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Corporate, Germany) within the wavenumber range of 4000 cm-1~400 cm-1 with a resolution of 2 cm-1. 2.5 Oil absorption capacity and oil removal efficiency tests

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To estimate the absorption performance of the quartz sand filter, the water and oil absorption

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capacities of the pristine and OTS/ZnO-coated quartz sand filter media were measured. First, 20 g

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of the filter media was added into 100 mL pure water or oil for 1 h and then taken out and weighed. The absorption capacity is calculated, in mg/g, by 𝑚𝑤 − 𝑚𝑑 𝑚𝑑

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𝑄=

(1)

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where 𝑚𝑤 and 𝑚𝑑 are the mass (g) of the filter media before and after absorption, respectively. Using engine oil as the raw material, artificial emulsified oily wastewater was prepared by ultrasonic

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emulsification and mechanical agitation [26, 27]. After 24 h, the oil concentration changed by <10%, the stability was good [28], and the diameters of the oil drops were between 1 and 10 μm. Then, 10

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g pristine or OTS/ZnO-coated quartz sand filter media was added to 5 mL oily wastewater. The oil concentration of the supernatant was determined after oscillating for 10 h at 160 rpm with the temperature kept at 25±0.5 °C and standing for 30 min. The oil removal efficiency was calculated

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by

𝜂=

𝐶0 − 𝐶1 𝐶0

(2)

where 𝜂 is the oil removal efficiency (%), and 𝐶0 and 𝐶1 are the initial concentration and adsorption equilibrium concentration of oily wastewater, respectively (mg/L). 5

All the experimental data were averaged from three sets of parallel experiments, and the error bars corresponds to one standard deviation. 2.6 Durability of the OTS/ZnO-coated quartz sand filter The OTS/ZnO-coated quartz sand filter was ultrasonically treated in ethanol at a frequency of 40 kHz for 12 h, 24 h and 48 h. The filter was then taken out and washed five times to remove the

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absorbed ethanol, followed by drying to a constant weight in an oven at 80 °C. The OTS/ZnO-coated quartz sand filter was soaked in water for 48 h at pH values of 1, 5, 10

and 14. The filter was then taken out and washed five times, followed by drying to a constant weight

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in an oven at 80 °C. The pH values of the solutions were adjusted by HCl and NaOH. 3. Results and discussion 3.1 Coating mechanism

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Dopamine hydrochloride can react with the Tris-buffer solution to form polydopamine (PDA)

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particles. PDA is rich in active groups (–OH, –NH2), making it a versatile platform for secondary

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reactions [29], especially the formation of ligands with metal ions that can generate a nuclear/shell

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structure of PDA/metal oxide [30]. PDA particles react with ZnO and with the hydroxyl groups on the surface by a condensation reaction. As a result, the ZnO particles are coated on the surface of

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the quartz sand filter, and the roughness increases.

Silanization occurs when the OTS comes in contact with the ZnO-coated quartz sand filter [31].

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The main reaction mechanism is shown in Fig. 1. First, the active head of the OTS molecule physisorbs onto the surface of the quartz sand filter. Then, a hydrolysis reaction occurs to form a

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silanol group —Si(OH)3 between the active head group (—Cl) of the OTS and trace water on the surface of the quartz sand filter. Next, one Si—OH of the —Si(OH)3 condenses with the hydroxyl groups of the quartz sand surface to form a covalent bond, thus grafting the OTS molecule on the

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surface of the quartz sand filter. Finally, a condensation reaction occurs between two adjacent OTS molecules, forming a dense long-chain alkyl group —C18H37 arranged in a layer, which decreases the surface energy of the quartz sand filter surface [32].

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C 18H 37 Si

Cl

Cl

C 18H 37 Si

Cl

Cl H

OH

Cl

w a te r la y e r

H O OH OH OH

C 18H 37

C 18H 37

Si

Si

a d so rb e d

H O

H 2O

HCl

Cl

HO

OH HO

OH

H

OH

OH

quartz sand filter

HO

C 18H 37 Si

OH HO O

OH

OH

O

OH

quartz sand filter

( a ) P h y s is o r p tio n

C 18H 37 Si

C 18H 37

C 18H 37

Si

O

Si

O

OH

O

OH

OH OH OH OH

H 2O

OH

OH

OH

quartz sand filter

( b ) H y d r o ly s is

HO

OH

OH

quartz sand filter

( c ) C o v a le n t g r a f tin g to

( d ) C o n d e n s a tio n

th e q u a r tz s a n d s u r f a c e

b e tw e e n O T S m o le c u le s

Fig. 1 Schematic illustration of the synthesis of OTS on the quartz sand filter.

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3.2 Characterization

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b y c o n d e n s a tio n

Fig. 2 shows photographs of water and oil droplets on the surface of the pristine and OTS/ZnOcoated quartz sand filter media, where the water is dyed with methylene blue and the oil is dyed

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with oil red O. The oil and water can be quickly wetted on the surface of the pristine quartz sand

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filter, indicating the oleophilic and hydrophilic properties of the filter. However, the water forms a

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spherical shape on the OTS/ZnO-coated quartz sand filter surface, while the oil still presents an

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oleophilic property. As measured by the Drop Shape Analysis System, apparent water contact angles of 0° for pristine quartz sand filter and 154.1° for superhydrophobic quartz sand filter are observed.

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Therefore, the surface wettability of the OTS/ZnO-coated quartz sand filter is successfully

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(a)

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converted from hydrophilic to superhydrophobic.

(d)

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(c)

(b)

Fig. 2 Photographs of liquid drops on the surface of the quartz sand filter media. Water (dyed with methylene blue) and oil (dyed with oil red O) droplets on the (a) pristine and (b) OTS/ZnO-coated quartz sand filter media surface, captured by digital camera; water droplets on the (c) pristine and (d) superhydrophobic quartz sand filter media surface, captured by DSA100.

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To investigate the superhydrophobicity of the quartz sand filter, the two main factors governing the surface wettability, the surface morphology and the chemical composition, are considered. SEM images at ~50× and ~1000× of the pristine and OTS/ZnO-coated quartz sand filters are shown in Fig. 3. It can be observed that the surface of the pristine quartz sand filter media has an uneven and heterogeneous appearance. The OTS/ZnO-coated quartz sand filter media surface is extremely rough with white submicron-sized projecting structures, which should be attributed to the ZnO and

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OTS. The FTIR spectra of the pristine and OTS/ZnO-coated quartz sand filters are shown in Fig. 4.

For the pristine quartz sand filter, the two absorption bands observed at 3415 cm-1 and 1627 cm-1

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correspond to the stretching vibration and deformation vibration of the O—H of the absorbed water,

respectively [33]. For the OTS/ZnO-coated quartz sand filter, the absorption band that appears at 2960 cm-1 can be attributed to the C—H stretching vibration of the CH3 group in OTS; the

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absorption peaks at 809 cm-1 and 1028 cm-1 should be attributed to the asymmetric and symmetric

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stretching vibrations of Si—O—Si between the OTS and quartz sand filter as well as the OTS

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molecules with each other, respectively [34], as illustrated in Fig. 1. The absorption peaks at 2850

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cm -1 and 2922 cm-1 can be attributed to the —CH3 and —CH2 long-chain alkyl groups of OTS [35]. An XPS analysis (Fig. 5) demonstrates that the main elements of the pristine quartz sand filter are

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oxygen (532.8 eV) and silicon (103.5 eV), while the OTS/ZnO-coated quartz sand filter contains two additional elements that are nitrogen at 399 eV and chlorine at 200 eV. The contents of oxygen

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and silicon decrease, and that of carbon (288.8 eV) increases sharply. Combined with the FTIR and XPS results, it can be confirmed that the long-chain alkyl —C18H37 moiety of OTS was mainly

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grafted onto the quartz sand surface by chemical bonding rather than physical adsorption. As a result, the surface of the OTS/ZnO-coated quartz sand filter became superhydrophobic with little absorption of water, which can also be confirmed by the disappearance of the absorption bands at

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3415 cm-1 and 1627 cm-1.

(a)

(b)

(c)

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(d)

Fig. 3 SEM images of (a, b) pristine and (c, d) OTS/ZnO-coated quartz sand filter media at different magnifications.

3900 3400 2900 2400 1900 1400 900

400

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Wavenumber(cm-1)

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809

1028

1627

2850

2960 2922

Transmittance(%)

b

3415

a

Fig. 4 FTIR spectra of (a) pristine and (b) OTS/ZnO-coated quartz sand filter media.

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a

b

1000

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C1s Si2s Si2p

N1s

800

Cl2p

600 400 200 binding energy (eV)

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intensity (a.u.)

O1s

0

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Fig. 5 XPS survey spectra of (a) pristine and (b) OTS/ZnO-coated quartz sand filter media.

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3.3 Oil removal performance

The adsorption capacity of the pristine and OTS/ZnO-coated quartz sand filters was studied

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using water and different oils. As shown in Fig. 6, the amount of water absorbed by the OTS/ZnOcoated quartz sand filter is 1 mg/g, which is only 1/50 the weight absorbed by the pristine quartz sand filter. The absorption capacities of the OTS/ZnO-coated quartz sand filter for engine oil and

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rapeseed oil are increased by 54% and 45%, respectively, over that of the pristine quartz sand filter; for n-hexane, this value is five times higher, and for ethylene glycol, methyl alcohol, isopropyl alcohol, acetone and methylbenzene, the value is 11~20 times higher than that of the pristine quartz sand filter. Therefore, the OTS/ZnO-coated quartz sand filter shows very good oil absorption and hydrophobicity. 9

pristine quartz sand OTS/ZnO coated quartz sand

300 200 100 0

Type of oil

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Fig. 6 Absorption capacity of pristine and OTS/ZnO-coated quartz sand filter media.

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Absorption capacity (mg/g)

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Static adsorption experiments were conducted to assess the oily wastewater treatment

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performance, as shown in Fig. 7. When the initial oil concentrations are 9.08 mg/L and 11.02 mg/L

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for engine oil and rapeseed oil, respectively, the supernatant oil concentrations of the pristine and OTS/ZnO-coated quartz sand filter media are 3.07 mg/L and 0.03 mg/L for engine oil and 3.45 mg/L

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and 0.39 mg/L for rapeseed oil, respectively. The oil removal efficiencies of the OTS/ZnO-coated

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quartz sand filter media are 99.65% and 96.46%, respectively, which are 33.48% and 27.77% higher

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than those of the pristine coated quartz sand filter media, respectively. Wei et al. [4] modified quartz sand filter media using a 3-aminopropyltriethoxy silane coupling agent to improve the

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hydrophobility, and the oil removal efficiency increased to 40.96%. Liu et al. [36] grafted the titanate coupling agent DN101 onto the quartz sand filter surface and achieved oil removal

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efficiencies of 54% and 57% for a modified dry process and a modified wet process, respectively. Zhou et al. [28] modified polystyrene resin with quaternary ammonium surfactant and achieved an oil removal efficiency of 78.4% by filtration with an initial oil concentration of 500 mg/L. It can be

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seen from the adsorption capacity and the oil removal efficiency that the oily wastewater treatment performance is greatly improved after application of the OTS/ZnO coating and is higher than that of other modified granule filter media.

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Oil removal efficiency (mg/g)

pristine quartz sand OTS/ZnO coated quartz sand 100

80

60

40

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engine oil rapeseed oil Type of oil

Fig. 7 Oil removal efficiencies of pristine and OTS/ZnO-coated quartz sand filter media for engine oil and rapeseed

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oil.

3.4 Durability of OTS/ZnO-coated quartz sand fifer

Ultrasonic processing (ultrasonication) is usually used in colloidal chemistry and for cleaning

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materials because it can physically damage a fragile surface where the chemical bonding with the

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substrate is not strong [37]. For this reason, ultrasonication has been used as a test to evaluate the

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mechanical stability of the wettability of surfaces [38]. The water contact angle of an ultrasonically

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treated OTS/ZnO-coated quartz sand filter is shown in Fig. 8. Ultrasonication can decrease the water contact angle only slightly, and after 48 h of ultrasonication, the water contact angle is still greater

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than 150°, confirming that the OTS/ZnO-coated quartz sand filter has good mechanical stability. The durability of the OTS/ZnO-coated quartz sand filter in corrosive solutions with pH values

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ranging from 1 to 14 was investigated [39]. As shown in Fig. 9, the water contact angle is still greater than 150° for the acid solution, while it decreased to 143.8° and 139.0° for pH values of 10 and 14,

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respectively. It can be concluded that the OTS/ZnO-coated quartz sand filter has very high

Water contact angle (︒)

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mechanical stability and acid resistance.

170 160 150 140

154.1

152.1

150.7

150.2

130 120 110 100 0

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24 time (h)

36

48

11

170 160

150.1

151.6

150

143.8

139.0

140 130 120 110

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Water contact angle (︒)

Fig. 8 Variation of the water contact angle of OTS/ZnO-coated quartz sand filter under ultrasonication.

100 1

5

10

14

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pH

Fig. 9 Variation of the water contact angle of OTS/ZnO-coated quartz sand filter exposed to acidic and alkaline solutions.

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4. Conclusions

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The quartz sand filter medium is inexpensive, non-toxic, environmentally friendly, and stable, and

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it is commonly used to treat oily wastewater. By coating with OTS and ZnO nanoparticles through

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hydrogen bonding, the quartz sand filter medium exhibits superhydrophobic and superoleophilic properties. The OTS/ZnO-coated quartz sand filter has a very high mechanical stability and acid

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resistance, and compared with the pristine filter, the coated filter exhibits a dramatically improved adsorption capacity and oil removal efficiency. As a result, the OTS/ZnO-coated quartz sand filter

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can potentially be used to filter oily wastewater.

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Acknowledgments

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This work was supported by the National Natural Science Foundation of China (No. 51668032)

and the Foundation of A Hundred Youth Talents Training Program of Lanzhou Jiaotong University (No. 152022).

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Figure captions Fig. 1 Schematic illustration of the synthesis of OTS on the quartz sand filter.

Fig. 2 Photographs of liquid drops on the surface of the quartz sand filter media. Water (dyed with methylene blue) and oil (dyed with oil red O) droplets on the (a) pristine and (b) OTS/ZnO-coated

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superhydrophobic quartz sand filter media surface, captured by DSA100.

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quartz sand filter media surface, captured by digital camera; (c) oil and (d) water droplets on the

Fig. 3 SEM images of (a, b) pristine and (c, d) OTS/ZnO-coated quartz sand filter media at

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different magnifications.

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Fig. 4 FTIR spectra of (a) pristine and (b) OTS/ZnO-coated quartz sand filter media.

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Fig. 5 XPS survey spectra of (a) pristine and (b) OTS/ZnO-coated quartz sand filter media.

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Fig. 6 Absorption capacity of pristine and OTS/ZnO-coated quartz sand filter media.

Fig. 7 Oil removal efficiencies of pristine and OTS/ZnO-coated quartz sand filter media for engine

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oil and rapeseed oil.

Fig. 8 Variation of the water contact angle of OTS/ZnO-coated quartz sand filter under

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ultrasonication.

Fig. 9 Variation of the water contact angle of OTS/ZnO-coated quartz sand filter exposed to acidic and alkaline solutions.

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