Photocatalytic degradation of oilfield produced water ...

Chemosphere 204 (2018) 79e86

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Photocatalytic degradation of oilfield produced water using graphitic carbon nitride embedded in electrospun polyacrylonitrile nanofibers Nur Hashimah Alias a, b, Juhana Jaafar a, *, Sadaki Samitsu c, Norhaniza Yusof a, Mohd Hafiz Dzarfan Othman a, Mukhlis A Rahman a, Ahmad Fauzi Ismail a, Farhana Aziz a, Wan Norharyati Wan Salleh a, Nur Hidayati Othman b a

Advanced Membrane Technology Research Center (AMTEC), Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia Department of Oil and Gas Engineering, Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia Data driven Polymer Design Group, Center for Materials Research by Information Integration, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Absorption and in-situ degradation of oil droplets by photocatalytic nanofiber. Purification of oilfield produced water by GCN incorporated photocatalytic nanofiber. Purification by polymeric nanofiber incorporated GCN photocatalyst.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 November 2017 Received in revised form 25 February 2018 Accepted 5 April 2018 Available online 6 April 2018

Separation and purification of oilfield produced water (OPW) is a major environmental challenge due to the co-production of the OPW during petroleum exploration and production operations. Effective capture of oil contaminant and its in-situ photodegradation is one of the promising methods to purify the OPW. Based on the photocatalytic capability of graphitic carbon nitride (GCN) which was recently rediscovered, photodegradation capability of GCN for OPW was investigated in this study. GCN was synthesized by calcination of urea and further exfoliated into nanosheets. The GCNs were incorporated into polyacrylonitrile nanofibers using electrospinning, which gave a liquid-permeable self-supporting photocatalytic nanofiber mat that can be handled by hand. The photocatalytic nanofiber demonstrated 85.4% degradation of OPW under visible light irradiation, and improved the degradation to 96.6% under UV light. Effective photodegradation of the photocatalytic nanofiber for OPW originates from synergetic effects of oil adsorption by PAN nanofibers and oil photodegradation by GCNs. This study provides an insight for industrial application on purification of OPW through photocatalytic degradation under solar irradiation. © 2018 Elsevier Ltd. All rights reserved.

Handling Editor: Jun Huang Keywords: Oilfield produced water Purification Photodegradation Photocatalytic nanofiber Graphitic carbon nitride Electrospinning

1. Introduction * Corresponding author. Advanced Membrane Technology (AMTEC) Research Centre, Universiti Teknologi Malaysia, 81310 UTM Johor Bharu, Johor. Malaysia. E-mail address: [email protected] (J. Jaafar). https://doi.org/10.1016/j.chemosphere.2018.04.033 0045-6535/© 2018 Elsevier Ltd. All rights reserved.

Securing water resources is one of the important targets for sustainability in our society. Separation and purification of oilfield

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N.H. Alias et al. / Chemosphere 204 (2018) 79e86

produced water (OPW) is a major environmental challenge due to massive amount of the OPW generated during petroleum exploration and production activities (Campos et al., 2002; Pardue et al., 2014). Physical separation technologies have been traditionally used to collect oil component from recovered oil-water fluid. Although, methods such as floatation, centrifugation, evaporation and extraction are effective to separate most of oil components, small traces of oil contaminant still present in the OPW (Lu et al., 2006). Furthermore, under recent stringent environmental regulation, the contaminant is not negligible for discharging (Dong et al., 2011). Since physical separation process is difficult to remove the oil contaminant in a cost-effective manner, attention has been diverted to material-based separation technology. Numerous technologies including adsorption, aggregation precipitation and membrane separation have been examined as the best available technology for produced water management (Fakhru'lRazi et al., 2009; Saththasivam et al., 2016). Owing to its unique characteristics, most of these methods can collect the contaminants, but recovery or removal of the collected contaminants usually become a severe problem. Therefore, effective capture of oil contaminant and its in-situ photodegradation may be a promising way to purify the OPW. Recent emerging rediscovery of graphitic carbon nitride (GCN) as the visible-light-driven photocatalyst for effective utilization of the solar spectrum, spurred the interest in remarkable applications (Liu et al., 2016; Zhang et al., 2013). The GCN has an appealing electronic band structure, facile synthesis, high physicochemical stability and ‘earth-abundant’ nature (Ong et al., 2016). Previous literature only reported on photodegradation performance of GCN for Rhodamine B (RhB) and methylene blue (MB) but none has reported on treatment of OPW. Here, we study the photodegradation performance of GCN to purify OPW. As-synthesized bulk graphitic carbon nitride (bGCN) was obtained as agglomerated powder with reduced effective surface area (Molinari et al., 2016). In addition, to achieve high photoactivity to irradiation, we prepared nanosheets graphitic carbon nitride (nsGCN) by liquid exfoliation method (Yang et al., 2013). It is essential to make the separation material easy to handle as it is a significant requirement to design and operate separation process. Thus, to make specific-shaped GCN with high surface area, we made polymer nanofiber composite of GCN using electrospinning. Electrospinning is a technique that can effectively disperse GCN into polymer based nanofibers (Xu et al., 2015). We dispersed GCN into polyacrylonitrile (PAN) solution and made nonwoven mat of PAN nanofibers with embedded GCN. The PAN matrix adsorbs and accumulates oil contaminant near to GCN and effectively photodegrade them even at low-concentration solution. The synergetic effects of photodegradation and adsorption of oil in polymer matrix help to enhance the photocatalytic performance. 2. Materials and methods 2.1. Synthesis, fabrication and characterization Details of the materials are described in Supplementary Materials. The GCN was simply synthesized from urea by a facile template-free method. The as-synthesized pale-yellow powder was ground in a mortar and used as bulk graphitic carbon nitride (bGCN). The bGCN was further treated by sonication-assisted liquid exfoliation method to form nanosheets graphitic carbon nitride (nsGCN). A spinning dope was prepared by a method previously reported by previous study (Xu et al., 2015) with a few modifications. Nanofibers Electrospinning Unit (Progene Link Sdn. Bhd., NF1000) was employed to fabricate PAN nanofibers containing GCN. The nanofibers containing bGCN and nsGCN were referred as NFbGCN and NF-nsGCN hereafter. Details of the synthesis and the

fabrication of GCN nanofibers are provided in Supplementary Materials. Surface morphology of samples was observed by field-emission scanning electron microscopy was performed using FESEM (Hitachi High-Tech. Co., Hitachi SU8020). Transmission electron microscopy was performed using HR-TEM (Hitachi High-Tech. Co., Hitachi HT7700) under the acceleration voltage of 120 kV. Nitrogen adsorption/desorption measurement was performed at 196 C with an automatic gas adsorption instrument (BEL Japan, Belsorpmax). Chemical structure was characterized using Fourier transform infrared spectroscopy (JASCO Inc., FT/IR-6100) with transmission configuration. Details of the materials characterizations are provided in Supplementary Materials.

2.2. Photocatalytic activity measurement The formulation of synthetic oilfield produced water (OPW) was freshly prepared by a method previously reported by previous study (Ong et al., 2014). Crude oil (66 API Grade), obtained from PETRONAS Refinery Malacca, Malaysia, was added to RO water containing sodium dodecylbenzenesulfonate (SDS). Total concentration of OPW was prepared at 1000 ppm, which contains 9 to 1 mixing ratio of oil to SDS. The solution was blended by a high-speed blender (Khind-Mistral Sdn. Bhd., BL 310AW) for 2 min with an agitation speed of 50 Hz at room temperature. The prepared OPW solution was stabilized for 15 min before it was ready for photocatalytic measurements. All photocatalytic measurements were conducted at room temperature in a self-fabricated suspension mode photocatalytic reactor as shown in Fig. S1. The suspension solution was added in a 450 ml beaker and placed 15 cm distance from the source of light. Two light source of UV light (UV lamp, wavelength region of 312 nm, 30 W) and visible light (white light-emitting diode lamp, wavelength region of >420 nm, 30 W) were used for excitation on photocatalytic activity. Air bubble was supplied by an air diffuser to supply sufficient oxygen to the reaction. 0.20 g of photocatalyst was added to suspension solution of 200 ml and stirred in darkness conditions for 60 min to ensure adsorption equilibrium prior to irradiation. Suspensions were sampled at 30 min interval during irradiation and the absorbance of the sampled suspension was measured with UVevis spectrophotometer (Hach, DR5000). The absorbance value of OPW was recorded at 238 nm. The photocatalytic activity was expressed in the percentages of the degradation according to the following Eq. (1),

% of Degradation ¼

Ao A 100 Ao

(1)

where Ao is the initial absorbance of OPW solution and At is the absorbance at time of 30e480 min. In order to ensure the degradation of organic compounds in OPW, TOC measurement was carried out using TOC analyzer (Shimadzu Co., TOC-LPCN). TOC degradation (%) of the OPW solution was determined according to Eq. (2).

TOC degradation ð%Þ ¼

I0 It 100 I0

(2)

where, I0 (ppm) is the TOC intensity of initial OPW solution and It (ppm) is the TOC intensity of OPW solution at reaction time of 60 mine180 min. The organic species in a crude oil and OPW solution after 480 min of photodegradation was analyzed by gas chromatograph mass spectroscopy (Agilent Tech., 6890N Network Gas Chromatograph).


3. Results and discussion 3.1. Fabrication and characterization of GCN nanofibers Characterization of bGCN and nsGCN are discussed in Supplementary Materials. It is essential to ensure that the separation material is easy to handle as it is a significant requirement to design and operate separation process. Thus, to make a specific-shaped GCN with high surface area, we made polymer nanofiber composite of GCN using electrospinning. PAN polymer was used as matrix for nanofibers because it has no optical absorbance in visible light region. The PAN consists of similar molecular structure with GCN, which is favorable for miscibility and dispersion of GCN. FESEM images shown in Fig. 1 (a) and 1 (b) reveals that electrospinning technique had successfully generated nanofibers containing GCNs, NF-bGCN and NF-nsGCN. The electrospun fibers were found straight with an average diameter in nanometer size. Meanwhile, the nanofibers mat formed ‘nodule-less’ structure indicating that GCN were uniformly distributed along the PAN nanofibers surface. Furthermore, the nanofiber mats have a network of highly well interconnected open pore structure with the size of several micrometers. Fig. 1 (c) and 1 (d) showed the histograms of nanofiber diameters assessed by digital imaging analysis from SEM images. In comparison with NF-bGCN (mean nanofiber diameter, 207 ± 2 nm), the mean nanofibers diameter of NF-nsGCN were 262 ± 6 nm with smooth and straight in infinite length structure, as shown in TEM images in Fig. 1(d and e). As shown in Table S1, results from nitrogen adsorption measurement determine SBET of NF-bGCN and NF-nsGCN to be 15.4 m2/ g and 13.3 m2/g respectively. Since the SBET values of nanofibers were much smaller than that of GCNs, we suppose the SBET were only contributed by outer surface of PAN nanofibers, not by GCN incorporated into PAN matrix. To further verify these hypotheses, we calculate the geometric surface area of nanofibers using Eq. (3) where S, specific surface area: A, area of the nanofibers: r, density of PAN: V, volume: L, length and d, diameter of the nanofibers.

S¼

A

rV

pdL 4 ¼ pd2 ¼ r d r 4 L

(3)

Using SBET of 15.4 m2/g for bGCN and 13.3 m2/g for nsGCN, and r of 1.18 g/cm3 (Gulgunje et al., 2015), average diameter of nanofiber was calculated to be 220 nm for bGCN and 255 nm for nsGCN. This calculated diameter is similar with mean diameter of the nanofibers obtained by digital image analysis of the SEM images (Fig. 1a and b), directly proving the SBET value only represents the surface area on the nanofibers surface not from porous structure of GCN. This is probably because the porous GCN was encapsulated in the PAN polymer matrix. In addition, the total pore volumes for both NF-bGCN and NF-nsGCN were much lower than GCNs because of sparse stacking structure of nanofibers which consists of macropores much larger than 200 nm, and therefore the macropores are not accessible by gas adsorption measurement. Fig. S6 shows the FTIR spectra of NF-bGCN, NF-nsGCN and PAN nanofibers. Both NF-bGCN and NF-nsGCN exhibits characteristic peaks of PAN such as stretching vibration of nitrile group (eCNe) at 2242 cm1 and the stretching vibration and bending vibration of methylene (eCH2e) at 2936 cm1 and 1453 cm1 (Pan et al., 2010) because the nanofibers were mostly made by PAN polymer matrix (weight ratio of PAN to GCN is 10:1). In addition to the peaks of PAN, the characteristic peaks of GCN were evidently found at 810 cm1 (aromatic CN heterocycles); 1243 cm1, 1320 cm1 (CeN stretching); 1563 cm1, 1621 cm1 (C^N stretching); 3000e3380 cm1 (amine groups, -NH2 or ¼ NH), which reveals that GCNs were

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successfully embedded into the PAN polymer matrix. These nanofiber structure of NF-bGCN and NF-nsGCN photocatalysts offer large surface area and defined open pores which could facilitate mass diffusion and rapid adsorption of targeted molecules. Moreover, well dispersion of GCN into PAN polymer matrix promotes vast numbers of active sites for effective interactions between the reactant and photocatalyst, thus enhancing the photocatalytic activity. 3.2. Photocatalytic activity 3.2.1. Photodegradation of oilfield produced water (OPW) We then investigated the purification of oilfield produced water (OPW) using photodegradation with GCN. Fig. S7 (a) shows the chromatogram of crude oil sample measured by GC-MS chromatography. The chromatograph showed a large number of sharp peaks, indicating that the crude oil contained numerous organic species consisting of cyclic alkanes, aromatic compounds, branched alkanes, linear alkanes as well as alkenes. Even though most of the peaks was significantly reduced for OPW solution after 480 min of photodegradation under visible light by NF-nsGCN as shown in Fig. S7 (b), it was difficult to quantitatively analyze each component due to the numerous peaks. As prepared OPW solution exhibited pale colorization due to organic components and had clearly decreased the colorization after photodegradation as shown in Fig. S7 (c). Therefore, we quantitatively assessed the photodegradation of OPW from the variation of UV absorbance. Visible light (l > 420 nm) was irradiated to OPW containing either bGCN, nsGCN or without GCNs (direct photolysis) and the photodegradation of OPW were recorded as shown in Fig. 2 (a). The summary of the percentage of degradation for OPW under visible light irradiations is presented in Table 1. Direct photolysis caused no variation on the degradation of OPW when turning on visible light illumination and presented only 4.2% degradation even after 480 min, indicating high stability of OPW under visible light irradiation. We therefore need the assistance of GCNs to enhance the oil degradation and purify the OPW. Incorporation of bGCN and nsGCN into the OPW solutions led to small degradation of 5.6% and 9.5% in dark condition after 60 min, respectively. The increase in the percentage of degradation is actually not due to photodegradation but the adsorption of oil species on porous structure of GCNs. Even though the SBET of bGCN is higher than nsGCN, the adsorption of OPW was found to be slightly higher for nsGCN, which is in agreement with previous study (Kamarudin and Alias, 2013), that adsorbent with high surface area would not necessarily result in higher adsorption capacities of organic molecules. In contrast to the dark conditions, the bGCN obviously accelerated degradation of OPW just after light illumination and exhibited 62.0% degradation after 480 min. The nsGCN gave higher degradation than bGCN in all of the time recorded and exhibited 11.6% higher degradation after 480 min. The rapid increased in degradation just after visible light illumination indicates the effective photodegradation by GCNs. Compared with bGCN, the higher adsorption capacity and photocatalytic degradation efficiency of nsGCN may originated from a well-dispersed nsGCN in aqueous solution by electrostatic repulsive interaction (Ma et al., 2014). The nsGCN allows to rapidly generate highly reactive species such as hydroxyl radicals (OH) and oxy radicals (O), both of which quickly react and effectively degrade the organic species in OPW. The two-dimensional structure of nsGCN may provide much shorter distance than the bGCN for photogenerated electron to travel to the photocatalyst surface and further reduce superoxo before being released from the surface of the nsGCN (Zhang et al., 2015). Since both bGCN and nsGCN are fine powders dispersed well in the OPW solutions, we needed to separate them using 10 min of

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Fig. 1. FESEM images and nanofibers diameter distribution approximated with Gauss function: (aec) NF-bGCN, (bed) NF-nsGCN and TEM images: (e and f) NF-nsGCN.

centrifugation at 3000 rpm before measuring the absorbance. Since the requirement for centrifugation is crucially inconvenient for the practical application of OPW purification, we incorporated the GCNs into thin nanofibers made of PAN using electrospinning and obtained a liquid-permeable self-supporting nanofiber mat that can be handled by hand. The mats allowed us to readily separate purified OPW just by pipetting the solution. The method is therefore an effective way to make a new shape of materials that is easy to handle for separation purpose as compared with a powdered sample. Fig. 2 (b) plots the percent of OPW degradation for nanofibers samples. PAN nanofibers gradually increased the percent of degradation even without the inclusion of GCNs. Since the rate of the degradation was almost similar before and after visible light illumination, the increase is probably not due to photodegradation but due to absorption of oil species into PAN nanofibers. Furthermore, no photodegradation capability of the PAN polymer is

expected because PAN polymers have no optical absorption at visible wavelength range. Assuming uniform absorption of oil into the nanofibers and 48.1% of degradation after 480 min, weight ratio of oil in the nanofibers is calculated to be 46.5 wt% with respect to PAN. Compared to very low concentration of oil in initial suspension (about 0.1 wt%), the concentration of oil in PAN nanofibers increased more than 300 times, indicating excellent oil adsorption capability of the PAN nanofibers. With respect to the opening size of the nanofiber mesh, the oil droplets produced in this study are likely to be small enough to readily permeate in the nanofiber mat. The droplets sometimes collide with the PAN nanofibers and the oil species are eventually absorbed in the nanofiber due to favorable affinity of oil species to PAN polymers. It is well known that diffusion rates of small molecules increase with excess free volume of a polymer matrix (Freeman and Yampollskii, 2010). PAN nanofibers produced by electrospinning possibly include large free volume between polymer chains of PAN because rapid evaporation


Fig. 2. Percentage of degradation for OPW under visible light irradiation of (a) photolysis, bGCN and nsGCN: (b) PAN nanofibers, NF-bGCN and NF-nsGCN. Solid lines are drawn to guide the eyes.

Table 1 Summary for percentage of degradation for OPW using as-prepared samples. Photocatalyst

bGCN nsGCN NF-bGCN NF-nsGCN PAN nanofibers Photolysis a b

OPW UVa

Visb

76.5 83.1 90.3 96.6 53.7 5.2

62.0 73.6 77.9 85.4 48.1 4.2

Degradation (%) of OPW under UV irradiation after 480 min. Degradation (%) of OPW under visible light after 480min.

of solvent during electrospinning causes polymer conformation to be frozen in non-equilibrium state (Makaremi et al., 2015; Sperling, 2005). The excess volume with small nanofiber diameter therefore facilitates rapid molecular diffusion of oil species in overall nanofibers. The excess volume also enhanced absorption amount due to

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Langmuir-type adsorption site of oil species (Samitsu et al., 2013). Both NF-bGCN and NF-nsGCN present remarkable increased in the percent of degradation just after visible light irradiation, indicating the significant contribution of photodegradation capability of GCNs. NF-bGCN and NF-nsGCN respectively gave 77.9% and 85.4% of OPW degradation after 480 min, which correspond to 29.8% and 37.3% increase compared with that of PAN nanofiber. The results confirmed that GCNs embedded in PAN nanofibers showed sufficient photocatalytic activity similarly with GCNs directly dispersed in a suspension solution. Very interestingly, both NF-GCNs were associated with 10e20% higher percent of degradation than the corresponding GCNs even though they contain less than one-tenths of GCNs. The result is probably due to oil concentrate capability of PAN nanofibers, suggesting that PAN nanofibers itself is one of the key parameters that influenced the photocatalytic activity of NFGCNs by the synergetic effects of adsorption and degradation of oil species. Although NF-nsGCN gave a maximum degradation of 85.4% under visible light irradiation, we investigated the effect of UV light irradiation to obtain OPW degradation higher than 95%. Similar with visible light irradiation, high stability of OPW under UV light irradiation was found, which was associated with only 5.2% degradation after 480 min by the direct photolysis as shown in Fig. 3 (a). The bGCN and nsGCN had increased the photodegradation by 14% and 10%, respectively, with respect to visible light irradiation, which resulted from higher absorption efficiency of UV light as expected from the UVeVis spectra shown in Fig. S5 (a). The nsGCN presented 5e10% higher degradation compared to bGCN, which in agreement with the result under visible light irradiation. PAN nanofiber exhibited the similar percent of degradation curve under UV light irradiation with that under visible light, again confirming that the percent of degradation for PAN nanofibers was originated from absorption of oil in the nanofibers not due to photodegradation of oil species by PAN nanofibers. UV light irradiation had increased the degradation of NF-bGCN and NFnsGCN by 10e15% with respect to visible light irradiation, which is in agreement with the results for bGCN and nsGCN. The agreements validated the reproducible characterization of the photocatalytic activity using this test system. Both NF-bGCN and NF-nsGCN plateaued within 180 min and exhibited high degradation of 90.3% and 96.6% after 480 min respectively. NF-nsGCN under UV light irradiation was found to offer highest percent of degradation investigated in this study, achieving degradation of more than 95% and associating with less than 50 ppm of residual oils in purified water. Under UV light illumination, the NF-nsGCN itself was still found robust and tough even after 480 min. Therefore, this work provides a comprehensive study for purification of OPW under UV and visible lights through synergetic effects of adsorption on oil contaminants and effective photodegradation by NF-nsGCN nanofibers. Furthermore, this study may be an insight for future industrial application on purification of OPW through photocatalytic degradation under solar irradiation. Fig. 4 illustrates the comparison on percentage of degradation under UV irradiation evaluated by TOC and UV absorbance for NFnsGCN, PAN nanofibers and nsGCN. TOC measurement gave 86.6% of degradation for NF-nsGCN after 180 min, which is almost similar with the 91.5% of photodegradation assessed by UV absorbance. The result proved high photodegradation capacity of NF-nsGCN for OPW regardless of type of oil molecules. In addition, the trend evaluated by UV absorbance was almost the same with the photodegradation experiment separately performed as shown in Fig. 3, confirming the reproducibility of the data obtained. TOC measurement gave 83.9% of degradation for PAN nanofibers after 180 min, which is significantly different from 35.7% degradation obtained from UV absorbance measurement. In general, TOC

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Fig. 4. Comparison on percentage of degradation under UV irradiation measured by TOC (open symbol) and UV absorbance measurement (closed symbol) for NF-nsGCN (red), PAN nanofibers (green) and nsGCN (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3. Percentage of degradation for OPW under UV light irradiation of (a) photolysis, bGCN and nsGCN: (b) PAN nanofibers, NF-bGCN and NF-nsGCN. Solid lines are drawn to guide the eyes.

measurement measures the total concentration of organic compounds, while UV absorbance measures the present of decolorization of OPW solution. As confirmed by GC-MS shown in Fig. S7, oil droplets in OPW solutions contain various types of organic molecules, for example, colored and colorless molecules. The difference in percentage by UV absorbance and TOC measurements is therefore possibly due to selective adsorption of colorless molecules by PAN nanofibers. The low efficiency for removal of colored molecules seems to be their serious drawback for practical applications because we always need to obtain clear water for usage of treated water as well as for safety drainage for environmental remediation. TOC measurement for nsGCN showed 97.7% of degradation, which is higher than 73.8% degradation recorded by UV absorbance. The nsGCN was loaded in large amount in OPW solution as compared to the amount of nsGCN incorporated in NFnsGCN, because we must obey the condition that total weight of photocatalyst is equal for all tests, not equal total weight of GCN. The high loading amount possibly results in higher percentage of degradation. Moreover, the low degradation percentage estimated

by UV absorbance may result from small GCN particles floating in the solution because they can strongly absorb UV light, which considerably decrease degradation percentage when UV absorbance at 238 nm is used for evaluation. Since the working principles (i.e. sensitivity to types of molecules) of UV and TOC measurement are different, use of both TOC and UV enabled us to partly elucidate the removal mechanism by NF-nsGCN. In order to evaluate the photocatalytic activity for OPW degradation in repeating cycles, we performed a 180-min photocatalytic degradation test for OPW under UV light irradiation three times using NF-nsGCN, PAN nanofiber and nsGCN (Fig. 5). At the end of each cycle, the photocatalyst was recovered by draining out the OPW solution from a beaker, and then a fresh OPW solution with a concentration of 1000 ppm was filled into the beaker and continued the next cycles. The NF-nsGCN, which had highest percent of degradation for 480 min UV light irradiation evaluated by UV absorbance (Fig. 3), retained 66.0% of degradation even after three 180-min cycles. The value is highest compared to PAN nanofibers and nsGCN although a 24.9% decrease was observed with respect to the first cycle. While PAN nanofibers gradually increased the percent of degradation in a first cycle, they exhibited the saturation of percent of degradation at the low value of 16.2% at the end of a third cycle. The saturation behavior of PAN nanofiber corresponds to slow absorption kinetics found in long time photocatalytic test in Fig. 3. The result indicates that PAN nanofiber can absorb oil, not degrade it and finally saturate its absorption capacity which is behaves similarly with conventional oil absorbents. The nsGCN exhibited 75.2% of OPW degradation in the first cycle, while the percentage was significantly reduced down to 41.2% at the end of third cycle. Based on the photodegradation capacity of nsGCN, the nsGCN is, in principle, expected to be reusable for repeating photodegradation tests as similar with NF-nsGCN. However, large amount of nsGCN powders was unfortunately lost during draining the OPW solution due to their small size and welldispersed state in an aqueous solution. In fact, we found considerable numbers of nsGCN particles in the drained OPW solution even providing 30 min sedimentation time for all drainage. Despite high capability of photodegradation of OPW, the difficulty to recover nsGCN from OPW solution for every cycle of operation


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photoluminescence measurements and Dr. Masanobu Naito at NIMS for his kind support. N.H.A, J.J and N.Y. would like to express their sincere gratitude towards Malaysia Ministry of Higher Education for the research funding provided under UTM-HiCOE Research Grants (R.J090301.7846.4J184) and (R.J090301.7846.4J185) and UTM for the financial support under Research University Grant (GUP) Tier 1 (Q.J130000.254616H43). N.H.A. would like to thank Universiti Teknologi Malaysia (UTM)e National Institute for Materials Science (NIMS) Cooperative Graduate School Program (ICGP) 2017/18 for the graduate fellowship awarded. A part of this work was supported by "Nanotechnology Platform" (project No. A-17-NM-0208) of the Ministry of Education, Culture, Sports, Science and Technology(MEXT), Japan. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.chemosphere.2018.04.033. Fig. 5. Photocatalytic cycle tests for NF-nsGCN (red), nsGCN (blue) and PAN nanofibers (green) monitored by UV absorbance. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

seems to be a serious problem causing considerable decrease in OPW photodegradation efficiency, thus making this photocatalyst not feasible for practical applications. In contrast, NF-nsGCN could be retained with immediate quick drainage just after photodegradation tests. As a result, NF-nsGCN is the most promising candidate to perform OPW purification with high reusability and degradation performance. The results also proved that the NFnsGCN is stable under prolonged time of operation and can potentially be used for industrial application of OPW treatment. 4. Conclusions A powder of graphitic carbon nitride (GCN) was obtained from facile synthesis of urea and successfully transformed to nanosheet form (nsGCN) by sonication-assisted liquid exfoliation method without any chemical reaction involved. The GCNs were embedded into PAN nanofibers (NFs) using electrospinning, which resulted in nonwoven NF mats that have excellent features such as high specific surface area, large opening enough to permeate oil droplets, well dispersed GCNs incorporated in nanofibers, and selfsupported sheet for easy handling by hand. Both GCNs and NFs had photodegradation capability for oil produced water under visible light irradiation as well as UV light. The PAN nanofiber was found to effectively absorb oil species inside the nanofibers and significantly concentrate the low-concentration oil contaminants due to favorable affinity with oil. The nsGCN incorporated into NFs exhibited OPW degradation of 85.4% under visible light irradiation and 96.6% under UV light, which is the highest value recorded in this study. The synergetic effects of concentrate and degradation of oil species are the important factors for high photodegradation efficiency for OPW. Declarations of interest None. Acknowledgments The authors gratefully acknowledge PETRONAS Penapisan (Melaka) Sdn. Bhd. for the supply of crude oil sample. N.H.A. and S.S. would like to thank Dr. Takeshi Yasuda for his guidance on the

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