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Remediation of oil spill-contaminated sands by chemical-free microbubbles generated in tap and saline water
Journal of Hazardous Materials 366 (2019) 124–129
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Remediation of oil spill-contaminated sands by chemical-free microbubbles generated in tap and saline water Huifang Suna, Hang Liua, Siyu Wanga, Yu Liua,b,
T
⁎
a Advanced Environmental Biotechnology Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, 1 Cleantech Loop, 637141, Singapore b School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Remediation Oil spill-contamination Sands Microbubbles
Oil spill-contaminated sands caused by marine oil spillage could severely impact aquatic and terrestrial ecosystems. In this work, self-collapsing microbubbles (MBs)-based method was explored as a chemical free approach for remediation of oil spill-contaminated sands. Tap water and saline water with 32 g/L of sodium chloride were employed as the media to generate MBs. Results showed that almost all oils were removed from the oil spill-contaminated sands after the 40-min treatment with MBs generated in tap water. The oil removal efficiency of MBs in saline water was slightly lower than that in tap water, while more than 90% of oil removal efficiency was still achieved after the 40-min treatment. The analyses by FTIR and UV spectra further confirmed the oil removal from oil spill-contaminated sands by MBs. Consequently, self-collapsing MBs showed a great potential as an environmentally friendly technology for high-efficiency remediation of oil spill-contaminated sands.
1. Introduction Marine oil spillage has become a global concern due to its high risk for ecosystem [1]. Oil spills caused by the oil production and transportation have been frequently reported in the past decades [2,3]. Oil
⁎
spills associated with release of a wide variety of organic contaminants are posing a great threat to aquatic life, and seriously affect the physical-chemical properties of surrounding soils, such as marine sediments, nearby shoreline sands etc [4,5]. For instance, a collision between two container vessels near Pasir Gudang Port in Singapore
Corresponding author at: School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.jhazmat.2018.11.102 Received 12 July 2018; Received in revised form 21 November 2018; Accepted 27 November 2018 Available online 28 November 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 366 (2019) 124–129
H. Sun et al.
sand samples were collected at the time interval of 10 min for further analyses. All the testes were conducted in duplicate unless otherwise stated.
happened on the 3th Jan 2017, which caused 300 tonnes of oil spillage. After exposed to sunlight, the spilled oils could stick on sands and form more sticky substances (e.g. tar balls). Moreover, the oil burial by the action of beach dynamics could lead to intense oiling at depths of up to several meters, causing serious concerns on their long-term toxicity [6]. So far, several technologies have been explored for remediating oil contaminated sands, including biological, chemical and physical methods [7–13]. However, most of them have limitations in terms of remediation efficiency, high cost, use of hash chemicals, non ecofriendly nature etc. More specifically, bioremediation is a very slow process and unable to tackle bio-refractory organic contaminates [14], while chemical methods are not environmentally friendly due to the use of harsh chemical agents. The physical approaches, such as thermal desorption, ultrasonication and microwave, are highly energy-intensive. Therefore, there is an urgent need for innovative technologies which are eco-friendly, affordable and easily scalable for remediation of oil spill-contaminated sands. Microbubbles (MBs) with a size less than 50 μm have been explored as a chemical-free cleaning approach in many fields. Compared to conventional bubbles with the size of several millimeters, MBs possess unique properties of huge interfacial area, lower rising speed in liquid phase, high bubble density and internal pressure [15,16]. Moreover, MBs have the inherent ability to shrink in size and subsequently collapse under water surface, resulting in strong pressure waves and highspeed water jet, which can help to remove contaminants on solid surfaces [17]. In addition, bursting of MBs could promote the generation of hydroxyl radicals, hence improving the decomposition of organic chemicals [18]. The MBs-based technology has been developed for degreasing of soil surface [19], cleaning of human skin [20] and removal of biofilms from fouled membranes [17,21] by now, while the efficiency of MBs had also been demonstrated in cleaning the artificiallycontaminated sands with diesel and rotary-vane pump oil [22]. However, it should be realized that beach sand contaminated real spilled crude oils with a complex chemical composition under actual environment would be different from those artificially-contaminated sands prepared by using a single type of oil in laboratory. So far, little information is available for the remediation of real oil spill-contaminated sands with MBs. This study thus investigated the potential of MBs for the remediation of oil spill-contaminated beach sands. For this purpose, tap water and saline water were employed as the media for generation of MBs. The operation parameters, such as treatment time and MBs flowrate, were systematically studied and further optimized. It is expected that this study may offer a novel sustainable and chemical-free alternative for remediation of oil spill-contaminated sands.
2.2. TGA analysis Thermogravimetric analyzer (TGA, PerkinElmer, USA) equipped with Pyris™ software was used to determine the remediation efficiency of oil spill-contaminated sands, with nitrogen as purge gas at a constant flowrate of 20 mL/min. About 10 mg of sands was placed in a ceramic crucible and heated at a rate of 10 ℃/min over a range of 30–900 ℃. The percentage of weight loss was calculated according to Eq. (1): Weight loss (%) = ([(mi-mr)/mi] × 100%
(1)
where mi and mr are the initial and remaining weights for oil spillcontaminated sands, respectively. According to the TGA data, the oil removal efficiency can be calculated as follows: Oil removal (%) = [(Wi-Wt)/Wi] × 100%
(2)
where Wi is the weight loss percentage of initial oil spill-contaminated sands, Wt is the weight loss percentage of sands after treatment with MBs. 2.3. TOC and UV analyses After the treatment with MBs, a certain amount of oil spill-contaminated sands was collected and air-dried. 20 g of dried sands were washed in 100 mL of distilled water in a stirring beaker for 30 min. The produced oily water was collected for total organic carbon (TOC) and UV analyses, and the value of which could reflect the amount of oils remaining on sand surfaces. TOC was analyzed by Total Organic Carbon Analyzer (TOC-V CSH, Shimadzu Co. Japan). UV adsorption was measured by UV-1800 (Shimadzu) and the UV spectra were recorded in the region of 200–600 nm. 2.4. FTIR analysis Fourier Transform Infrared Spectroscopy (FTIR, Spectrum 4000, PerkinElmer, USA) was used to characterize the functional groups of oil contaminants on sand surfaces. 1 mg of powered air-dried sands were compacted with 0.1 to 0.15 g of potassium bromide under a hydraulic pressure [23]. The FTIR spectra were recorded in the wavelength of 4000 to 600 cm−1 according to the attenuated total reflection (ATR) method. Each spectrum was the average of 500 scans with a wave number resolution of 2.0 cm−1. A background scan was recorded prior to the measurement and subtracted from the sample spectra.
2. Materials and methods 2.1. Lab-scale cleaning system
3. Results The real oil-contaminated sands were collected from Singapore coast due to an offshore oil spill and treated with MBs in a glass column reactor (h = 25 cm; ID = 3 cm) combined with a mechanical stirring (Fig. 1). A microbubble generator (S-MA III FS, Riverforest Corporation, USA) capable of generating MBs with diameter less than 50 μm was employed in this work. The mechanical mixing was controlled at a constant speed of 60 rpm to suspend the sand matrix and promote the contacts between MBs and oil spill-contaminated sands. Tap water and saline water with 32 g/L of sodium chloride were employed as the media for the generation of MBs. As can be seen in Fig. 1b, the MBs generated in test media appeared milky. In this study, 200 g of oil spill-contaminated sands were put into the column reactor. Self-collapsing MBs were continuously introduced into the column reactor by a peristaltic pump. Meanwhile, the produced oily water was discharged from the reactor outlet. Each experiment lasted for 40 min at different MBs flowrates of 50, 100 and 150 mL/min, respectively. The
3.1. Effect of treatment time Fig. 2 shows the TGA plots of oil spill-contaminated sands at different treatment times of MBs at a constant flowrate of 150 mL/min. It was found that the amount of oils on sands dramatically decreased after 20-min treatment with MBs generated in both tap water and saline water. In addition, nearly no change was observed in the TGA plots of the samples after 40-min treatment, especially for the tap water group, i.e. most of the oils was removed from sands. The remediation efficiencies of MBs generated in tap water and saline water indeed appeared to be comparable. About 99.8% and 92% of oils were removed after 40-min treatment with MBs in tap water and saline water, respectively (Fig. 3a). The TOC data presented in Fig. 3b strongly supported the TGA plots in Fig. 2. For example, after 40-min treatment with MBs in tap water 125
Journal of Hazardous Materials 366 (2019) 124–129
H. Sun et al.
Fig. 1. Schematic illustration of MBs-assisted stirred column reactor (a) and appearance of MBs in test medium (b).
Fig. 2. TGA plots of oil spill-contaminated sands at different treatment times with MBs (a: tap water; b: saline water).
and saline water, the TOC concentrations on the oil spill-contaminated sands decreased from 166.2 mg/L to 1.89 mg/L and 13.11 mg/L, respectively.
Fig. 3. Oil removal efficiency (a) and TOC concentration on oil spill-contaminated sands (b) at different treatment times with MBs.
was determined at different flowrates of 50, 100 and 150 mL/min. TGA analyses showed that the remaining oils on sand surface rapidly decreased with increasing flowrate of MBs generated in both tap water
3.2. Effect of MBs flowrate The remediation efficiency of MBs for oil spill-contaminated sands 126
Journal of Hazardous Materials 366 (2019) 124–129
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Fig. 4. TGA plots of oil spill-contaminated sands after 40-min treatment with MBs at different flowrates (a: tap water; b: saline water).
and saline water (Fig. 4). For MBs in tap water, 75% of oils was removed after 40-min treatment at the flowrate of 50 mL/min, while 80% and 98% of oil removal were achieved at the flowrates of 100 and 150 mL/min, respectively (Fig. 5a). For MBs in saline water, the oil removal efficiency was found to increase from 66.9% to 75.0% and 90.1% with increasing the MBs flowrate from 50 to 100 and 150 mL/ min, respectively. The TOC data presented in Fig. 5b further confirmed the results obtained from the above TGA analyses. These results clearly demonstrated the applicability of MBs for effectively cleaning oil spillcontaminated sands.
Fig. 5. Oil removal efficiency (a) and TOC concentration on oil spill-contaminated sands (b) after 40-min treatment with MBs at different flowrates.
contaminants from sand surfaces. The absorbance of UV spectra at the wavelength of 200 to 400 nm were also found to significantly decrease after 40-min treatment with MBs (Fig. 6b), supported by microscopic observations of oil spill-contaminated sands before and after 40-min treatment with MBs (Fig. 7).
3.3. Characteristics of oil spill-contaminated sands after treatment with MBs
4. Discussion MBs have been known to shrink in size and subsequently to collapse in water [18]. According to the Yong-Laplace equation [29], such decrease in size may result in a significant increase in internal gas pressure inside MBs as shown in Eq (3):
FTIR spectroscopy and UV were employed to determine the changes of oil contaminants on oil spill-contaminated sands after 40-min treatment with MBs at the flowrate of 150 mL/min (Fig. 6). FTIR spectra of the original oil spill-contaminated sands showed strong absorption at the wavelength of 2800-3700 cm−1, 1500-1750 cm−1, 900-1200 cm−1 and 600-800 cm−1, respectively (Fig. 6a). The broad band between 2800 and 3700 cm−1 likely corresponded to CeH stretching modes [24], and the overlapped OeH and NeH stretching vibrations [25,26]. The region between 1500-1750 cm−1 could be assigned to aliphatic and aromatic groups, and the higher wavenumber at 1635 cm-1 should correspond to C]C bonds [27]. The region between 900-1200 cm−1 encompassed stretching vibration of CeO ester group, CeO stretching and bending vibration of CH functional groups, while the peaks at 688 cm−1 should correspond to –(CH2)n groups with n ≥ 4 [24,28]. As shown in Fig. 6a, the intensities of these peaks were all found to significantly decrease after the treatment with MBs generated in tap water and saline water, indicating the removal of the related organic
P = Pl + 2σ/r
(3)
where P is internal gas pressure, Pl is liquid pressure, r is the radius of bubble, and σ is surface tension. Increased intern gas pressure in MBs due to size inevitably triggers quick diffusion of entrapped pressurized gas towards aqueous solution, ultimately resulting in collapsing of MBs with the generation of strong pressure waves and water jets which in turn serve as the physical forces to shear off oils from sand surfaces. In fact, MBs had been applied for cleaning biologically fouled membranes without using any harsh chemicals, such as hypochlorite or ozone [30], while it had also been reported that grease on solid surface could be effectively removed by MBs [19]. In addition, MBs have a large interfacial area and low rising velocity in water phase, which in turn 127
Journal of Hazardous Materials 366 (2019) 124–129
H. Sun et al.
Fig. 6. FTIR (a) and UV spectra (b) of oil spill-contaminated sands before and after 40-min treatment with MBs in tap water and saline water.
improve their contact with contaminants to be removed from a solid surface [19]. Moreover, reattachment of removed oils onto sand surfaces could be effectively prevented through adsorption of oils by MBs due to their hydrophobic nature and good affinity to oils [31]. The possible mechanisms behind the observed oils removal by the self-collapsing MBs are illustrated in Fig. 8 for the remediation of oil spillcontaminated sands. Compared with saline water, the slightly better performance of MBs was observed in tap water (Figs. 3 and 5). In fact, MBs are often negatively charged with a zeta potential of around −35 mV [31]. It is a reasonable consideration that sodium chloride added to saline water reduces the zeta potential of MBs, leading to a decreased electrostatic repulsion among MBs, i.e. the coalescence of MBs is expectable with the formation of coarse bubbles. Therefore, the physical forces generated through the bursting of MBs would be weaken in the presence of sodium chloride. Obviously, further study is still needed to systematically examine the effects of salinity on the characteristics and stability of MBs in the remediation of oil spill-contaminated sands. As discussed above, about 90–99% of oils could be removed from the oil spill-contaminated sands after 40-min treatment by MBs. These clearly demonstrated the
Fig. 8. Illustration of possible remediation mechanisms of oil spill-contaminated sands with MBs.
Fig. 7. Appearances of oil spill-contaminated sands before (a) and after 40-min treatment with MBs in tap water (b) and saline water (c). 128
Journal of Hazardous Materials 366 (2019) 124–129
H. Sun et al.
applicability of MBs for effectively cleaning oil spill-contaminated sands. Moreover, the proposed MBs-based method can be easily scaled up or down according to actual needs, and which should be considered as a sustainable chemical-free approach for the remediation of oilcontaminated sands, which would also be applicable for treating oilcontaminated soils.
[10] J. Robinson, E. Binner, A. Saeid, M. Al-Harahsheh, S. Kingman, Microwave processing of oil sands and contribution of clay minerals, Fuel 135 (2014) 153–161. [11] C. Wang, A. Alpatova, K.N. McPhedran, M.G. El-Din, Coagulation/flocculation process with polyaluminum chloride for the remediation of oil sands process-affected water: performance and mechanism study, J. Environ. Manage. 160 (2015) 254–262. [12] C.-C. Lai, Y.-C. Huang, Y.-H. Wei, J.-S. Chang, Biosurfactant-enhanced removal of total petroleum hydrocarbons from contaminated soil, J. Hazard. Mater. 167 (2009) 609–614. [13] N. Wilton, B.A. Lyon-Marion, R. Kamath, K. McVey, K.D. Pennell, A. Robbat, Remediation of heavy hydrocarbon impacted soil using biopolymer and polystyrene foam beads, J. Hazard. Mater. 349 (2018) 153–159. [14] R.M. Atlas, T.C. Hazen, Oil biodegradation and bioremediation: a tale of the two worst spills in US history, Environ. Sci. Technol. 45 (2011) 6790-6715. [15] A. Serizawa, T. Inui, T. Yahiro, Z. Kawara, Laminarization of micro-bubble containing milky bubbly flow in a pipe, 3rd Europen-Japanese Two-Phase Flow Group Meeting (2003). [16] T. Zheng, Q. Wang, T. Zhang, Z. Shi, Y. Tian, S. Shi, N. Smale, J. Wang, Microbubble enhanced ozonation process for advanced treatment of wastewater produced in acrylic fiber manufacturing industry, J. Hazard. Mater. 287 (2015) 412–420. [17] A. Agarwal, W. Jern Ng, Y. Liu, Removal of biofilms by intermittent low-intensity ultrasonication triggered bursting of microbubbles, Biofouling 30 (2014) 359–365. [18] M. Takahashi, K. Chiba, P. Li, Free-radical generation from collapsing microbubbles in the absence of a dynamic stimulus, J. Phys. Chem. B 111 (2007) 1343–1347. [19] M. Miyamoto, S. Ueyama, N. Hinomoto, T. Saitoh, S. Maekawa, J. Hirotsuji, Degreasing of solid surfaces by microbubble cleaning, J. Appl. Phys. 46 (2007) 1236–1243. [20] J.-W. Lee, H.-W. Kim, J.-I. Sohn, G.-S. Yoon, A study on micro bubbles influence on human skin cleaning, Adv. Sci. Lett. 19 (2013) 2558–2562. [21] A. Agarwal, W.J. Ng, Y. Liu, Cleaning of biologically fouled membranes with selfcollapsing microbubbles, Biofouling 29 (2013) 69–76. [22] A. Agarwal, Y. Zhou, Y. Liu, Remediation of oil-contaminated sand with self-collapsing air microbubbles, Environ. Sci. Pollut. R. 23 (2016) 23876–23883. [23] R.N. Panda, M.F. Hsieh, R.J. Chung, T.S. Chin, FTIR, XRD, SEM and solid state NMR investigations of carbonate-containing hydroxyapatite nano-particles synthesized by hydroxide-gel technique, J. Phys. Chem. Solids 64 (2003) 193–199. [24] A. Boukir, E. Aries, M. Guiliano, L. Asia, P. Doumenq, G. Mille, Subfractionation, characterization and photooxidation of crude oil resins, Chemosphere 43 (2001) 279–286. [25] C. Branca, G. D’Angelo, C. Crupi, K. Khouzami, S. Rifici, G. Ruello, U. Wanderlingh, Role of the OH and NH vibrational groups in polysaccharide-nanocomposite interactions: a FTIR-ATR study on chitosan and chitosan/clay films, Polymer 99 (2016) 614–622. [26] H. Gong, Z. Jin, Q. Wang, J. Zuo, J. Wu, K. Wang, Effects of adsorbent cake layer on membrane fouling during hybrid coagulation/adsorption microfiltration for sewage organic recovery, Chem. Eng. J. 317 (2017) 751–757. [27] A. Boukir, M. Guiliano, L. Asia, A. El Hallaoui, G. Mille, A fraction to fraction study of photo-oxidation of BAL 150 crude oil asphaltenes, Analusis 26 (1998) 358–364. [28] A. Rohman, Y.C. Man, Fourier transform infrared (FTIR) spectroscopy for analysis of extra virgin olive oil adulterated with palm oil, Food Res. Int. 43 (2010) 886–892. [29] V.P. Carey, Liquid–Vapor Phase-Change Phenomena: An Introduction to the Thermodynamics of Vaporization and Condensation Processes in Heat Transfer Equipments, Hemisphere Publishing Corporation, USA, 1992. [30] A. Agarwal, H. Xu, W.J. Ng, Y. Liu, Biofilm detachment by self-collapsing air microbubbles: a potential chemical-free cleaning technology for membrane biofouling, J. Mater. Chem. 22 (2012) 2203–2207. [31] M. Takahashi, ζ potential of microbubbles in aqueous solutions: electrical properties of the gas− water interface, J. Phys. Chem. B 109 (2005) 21858–21864.
5. Conclusions This study clearly demonstrated the feasibility of self-collapsing MBs for the remediation of oil spill-contaminated sands. TGA and TOC analyses revealed that almost all oils were removed from sand surfaces after 40-min treatment with MBs in tap water, while more than 90% of oil removal was achieved with MBs generated in saline water. It is reasonable to consider that seawater instead of fresh water could be used as a medium for generation of MBs in future real application. FTIR and UV spectra further confirmed the high-efficiency of MBs for cleaning oil spill-contaminated sands. Consequently, this study offers a sustainable and chemical-free alternative with self-collapsing MBs for efficient remediation of oil spill-contaminated sands. Acknowledgment This work was supported by the National Environment Agency of Singapore for ETRP (Grant No. 1302 111). References [1] B. Doshi, M. Sillanpää, S. Kalliola, A review of bio-based materials for oil spill treatment, Water Res. 135 (2018) 262–277. [2] G.A. El-Din, A. Amer, G. Malsh, M. Hussein, Study on the use of banana peels for oil spill removal, Alex. Eng. J. (2017), https://doi.org/10.1016/j.aej.2017.05.020. [3] A. Agarwal, Y. Liu, Remediation technologies for oil-contaminated sediments, Mar. Pollut. Bull. 101 (2015) 483–490. [4] N.P. Ventikos, E. Vergetis, H.N. Psaraftis, G. Triantafyllou, A high-level synthesis of oil spill response equipment and countermeasures, J. Hazard. Mater. 107 (2004) 51–58. [5] R.M. Abousnina, A. Manalo, W. Lokuge, J. Shiau, Oil contaminated sand: an emerging and sustainable construction material, Procedia Eng. 118 (2015) 1119–1126. [6] A.M. Bernabeu, D. Rey, Bn. Rubio, F. Vilas, C. Domínguez, J.M. Bayona, J. Albaigés, Assessment of cleanup needs of oiled sandy beaches: lessons from the Prestige oil spill, Environ. Sci. Technol. 43 (2009) 2470–2475. [7] J. Araruna Jr., V. Portes, A. Soares, M. Silva, M. Sthel, D. Schramm, S. Tibana, H. Vargas, Oil spills debris clean up by thermal desorption, J. Hazard. Mater. 110 (2004) 161–171. [8] E. Ceschia, J.R. Harjani, C. Liang, Z. Ghoshouni, T. Andrea, R.S. Brown, P.G. Jessop, Switchable anionic surfactants for the remediation of oil-contaminated sand by soil washing, RSC Adv. 4 (2014) 4638–4645. [9] H.J. Couto, G. Massarani, E.C. Biscaia Jr., G.L. Sant’Anna Jr., Remediation of sandy soils using surfactant solutions and foams, J. Hazard. Mater. 164 (2009) 1325–1334.
129
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Remediation of oil spill-contaminated sands by chemical-free microbubbles generated in tap and saline water Huifang Suna, Hang Liua, Siyu Wanga, Yu Liua,b,
T
⁎
a Advanced Environmental Biotechnology Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, 1 Cleantech Loop, 637141, Singapore b School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Remediation Oil spill-contamination Sands Microbubbles
Oil spill-contaminated sands caused by marine oil spillage could severely impact aquatic and terrestrial ecosystems. In this work, self-collapsing microbubbles (MBs)-based method was explored as a chemical free approach for remediation of oil spill-contaminated sands. Tap water and saline water with 32 g/L of sodium chloride were employed as the media to generate MBs. Results showed that almost all oils were removed from the oil spill-contaminated sands after the 40-min treatment with MBs generated in tap water. The oil removal efficiency of MBs in saline water was slightly lower than that in tap water, while more than 90% of oil removal efficiency was still achieved after the 40-min treatment. The analyses by FTIR and UV spectra further confirmed the oil removal from oil spill-contaminated sands by MBs. Consequently, self-collapsing MBs showed a great potential as an environmentally friendly technology for high-efficiency remediation of oil spill-contaminated sands.
1. Introduction Marine oil spillage has become a global concern due to its high risk for ecosystem [1]. Oil spills caused by the oil production and transportation have been frequently reported in the past decades [2,3]. Oil
⁎
spills associated with release of a wide variety of organic contaminants are posing a great threat to aquatic life, and seriously affect the physical-chemical properties of surrounding soils, such as marine sediments, nearby shoreline sands etc [4,5]. For instance, a collision between two container vessels near Pasir Gudang Port in Singapore
Corresponding author at: School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.jhazmat.2018.11.102 Received 12 July 2018; Received in revised form 21 November 2018; Accepted 27 November 2018 Available online 28 November 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 366 (2019) 124–129
H. Sun et al.
sand samples were collected at the time interval of 10 min for further analyses. All the testes were conducted in duplicate unless otherwise stated.
happened on the 3th Jan 2017, which caused 300 tonnes of oil spillage. After exposed to sunlight, the spilled oils could stick on sands and form more sticky substances (e.g. tar balls). Moreover, the oil burial by the action of beach dynamics could lead to intense oiling at depths of up to several meters, causing serious concerns on their long-term toxicity [6]. So far, several technologies have been explored for remediating oil contaminated sands, including biological, chemical and physical methods [7–13]. However, most of them have limitations in terms of remediation efficiency, high cost, use of hash chemicals, non ecofriendly nature etc. More specifically, bioremediation is a very slow process and unable to tackle bio-refractory organic contaminates [14], while chemical methods are not environmentally friendly due to the use of harsh chemical agents. The physical approaches, such as thermal desorption, ultrasonication and microwave, are highly energy-intensive. Therefore, there is an urgent need for innovative technologies which are eco-friendly, affordable and easily scalable for remediation of oil spill-contaminated sands. Microbubbles (MBs) with a size less than 50 μm have been explored as a chemical-free cleaning approach in many fields. Compared to conventional bubbles with the size of several millimeters, MBs possess unique properties of huge interfacial area, lower rising speed in liquid phase, high bubble density and internal pressure [15,16]. Moreover, MBs have the inherent ability to shrink in size and subsequently collapse under water surface, resulting in strong pressure waves and highspeed water jet, which can help to remove contaminants on solid surfaces [17]. In addition, bursting of MBs could promote the generation of hydroxyl radicals, hence improving the decomposition of organic chemicals [18]. The MBs-based technology has been developed for degreasing of soil surface [19], cleaning of human skin [20] and removal of biofilms from fouled membranes [17,21] by now, while the efficiency of MBs had also been demonstrated in cleaning the artificiallycontaminated sands with diesel and rotary-vane pump oil [22]. However, it should be realized that beach sand contaminated real spilled crude oils with a complex chemical composition under actual environment would be different from those artificially-contaminated sands prepared by using a single type of oil in laboratory. So far, little information is available for the remediation of real oil spill-contaminated sands with MBs. This study thus investigated the potential of MBs for the remediation of oil spill-contaminated beach sands. For this purpose, tap water and saline water were employed as the media for generation of MBs. The operation parameters, such as treatment time and MBs flowrate, were systematically studied and further optimized. It is expected that this study may offer a novel sustainable and chemical-free alternative for remediation of oil spill-contaminated sands.
2.2. TGA analysis Thermogravimetric analyzer (TGA, PerkinElmer, USA) equipped with Pyris™ software was used to determine the remediation efficiency of oil spill-contaminated sands, with nitrogen as purge gas at a constant flowrate of 20 mL/min. About 10 mg of sands was placed in a ceramic crucible and heated at a rate of 10 ℃/min over a range of 30–900 ℃. The percentage of weight loss was calculated according to Eq. (1): Weight loss (%) = ([(mi-mr)/mi] × 100%
(1)
where mi and mr are the initial and remaining weights for oil spillcontaminated sands, respectively. According to the TGA data, the oil removal efficiency can be calculated as follows: Oil removal (%) = [(Wi-Wt)/Wi] × 100%
(2)
where Wi is the weight loss percentage of initial oil spill-contaminated sands, Wt is the weight loss percentage of sands after treatment with MBs. 2.3. TOC and UV analyses After the treatment with MBs, a certain amount of oil spill-contaminated sands was collected and air-dried. 20 g of dried sands were washed in 100 mL of distilled water in a stirring beaker for 30 min. The produced oily water was collected for total organic carbon (TOC) and UV analyses, and the value of which could reflect the amount of oils remaining on sand surfaces. TOC was analyzed by Total Organic Carbon Analyzer (TOC-V CSH, Shimadzu Co. Japan). UV adsorption was measured by UV-1800 (Shimadzu) and the UV spectra were recorded in the region of 200–600 nm. 2.4. FTIR analysis Fourier Transform Infrared Spectroscopy (FTIR, Spectrum 4000, PerkinElmer, USA) was used to characterize the functional groups of oil contaminants on sand surfaces. 1 mg of powered air-dried sands were compacted with 0.1 to 0.15 g of potassium bromide under a hydraulic pressure [23]. The FTIR spectra were recorded in the wavelength of 4000 to 600 cm−1 according to the attenuated total reflection (ATR) method. Each spectrum was the average of 500 scans with a wave number resolution of 2.0 cm−1. A background scan was recorded prior to the measurement and subtracted from the sample spectra.
2. Materials and methods 2.1. Lab-scale cleaning system
3. Results The real oil-contaminated sands were collected from Singapore coast due to an offshore oil spill and treated with MBs in a glass column reactor (h = 25 cm; ID = 3 cm) combined with a mechanical stirring (Fig. 1). A microbubble generator (S-MA III FS, Riverforest Corporation, USA) capable of generating MBs with diameter less than 50 μm was employed in this work. The mechanical mixing was controlled at a constant speed of 60 rpm to suspend the sand matrix and promote the contacts between MBs and oil spill-contaminated sands. Tap water and saline water with 32 g/L of sodium chloride were employed as the media for the generation of MBs. As can be seen in Fig. 1b, the MBs generated in test media appeared milky. In this study, 200 g of oil spill-contaminated sands were put into the column reactor. Self-collapsing MBs were continuously introduced into the column reactor by a peristaltic pump. Meanwhile, the produced oily water was discharged from the reactor outlet. Each experiment lasted for 40 min at different MBs flowrates of 50, 100 and 150 mL/min, respectively. The
3.1. Effect of treatment time Fig. 2 shows the TGA plots of oil spill-contaminated sands at different treatment times of MBs at a constant flowrate of 150 mL/min. It was found that the amount of oils on sands dramatically decreased after 20-min treatment with MBs generated in both tap water and saline water. In addition, nearly no change was observed in the TGA plots of the samples after 40-min treatment, especially for the tap water group, i.e. most of the oils was removed from sands. The remediation efficiencies of MBs generated in tap water and saline water indeed appeared to be comparable. About 99.8% and 92% of oils were removed after 40-min treatment with MBs in tap water and saline water, respectively (Fig. 3a). The TOC data presented in Fig. 3b strongly supported the TGA plots in Fig. 2. For example, after 40-min treatment with MBs in tap water 125
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Fig. 1. Schematic illustration of MBs-assisted stirred column reactor (a) and appearance of MBs in test medium (b).
Fig. 2. TGA plots of oil spill-contaminated sands at different treatment times with MBs (a: tap water; b: saline water).
and saline water, the TOC concentrations on the oil spill-contaminated sands decreased from 166.2 mg/L to 1.89 mg/L and 13.11 mg/L, respectively.
Fig. 3. Oil removal efficiency (a) and TOC concentration on oil spill-contaminated sands (b) at different treatment times with MBs.
was determined at different flowrates of 50, 100 and 150 mL/min. TGA analyses showed that the remaining oils on sand surface rapidly decreased with increasing flowrate of MBs generated in both tap water
3.2. Effect of MBs flowrate The remediation efficiency of MBs for oil spill-contaminated sands 126
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Fig. 4. TGA plots of oil spill-contaminated sands after 40-min treatment with MBs at different flowrates (a: tap water; b: saline water).
and saline water (Fig. 4). For MBs in tap water, 75% of oils was removed after 40-min treatment at the flowrate of 50 mL/min, while 80% and 98% of oil removal were achieved at the flowrates of 100 and 150 mL/min, respectively (Fig. 5a). For MBs in saline water, the oil removal efficiency was found to increase from 66.9% to 75.0% and 90.1% with increasing the MBs flowrate from 50 to 100 and 150 mL/ min, respectively. The TOC data presented in Fig. 5b further confirmed the results obtained from the above TGA analyses. These results clearly demonstrated the applicability of MBs for effectively cleaning oil spillcontaminated sands.
Fig. 5. Oil removal efficiency (a) and TOC concentration on oil spill-contaminated sands (b) after 40-min treatment with MBs at different flowrates.
contaminants from sand surfaces. The absorbance of UV spectra at the wavelength of 200 to 400 nm were also found to significantly decrease after 40-min treatment with MBs (Fig. 6b), supported by microscopic observations of oil spill-contaminated sands before and after 40-min treatment with MBs (Fig. 7).
3.3. Characteristics of oil spill-contaminated sands after treatment with MBs
4. Discussion MBs have been known to shrink in size and subsequently to collapse in water [18]. According to the Yong-Laplace equation [29], such decrease in size may result in a significant increase in internal gas pressure inside MBs as shown in Eq (3):
FTIR spectroscopy and UV were employed to determine the changes of oil contaminants on oil spill-contaminated sands after 40-min treatment with MBs at the flowrate of 150 mL/min (Fig. 6). FTIR spectra of the original oil spill-contaminated sands showed strong absorption at the wavelength of 2800-3700 cm−1, 1500-1750 cm−1, 900-1200 cm−1 and 600-800 cm−1, respectively (Fig. 6a). The broad band between 2800 and 3700 cm−1 likely corresponded to CeH stretching modes [24], and the overlapped OeH and NeH stretching vibrations [25,26]. The region between 1500-1750 cm−1 could be assigned to aliphatic and aromatic groups, and the higher wavenumber at 1635 cm-1 should correspond to C]C bonds [27]. The region between 900-1200 cm−1 encompassed stretching vibration of CeO ester group, CeO stretching and bending vibration of CH functional groups, while the peaks at 688 cm−1 should correspond to –(CH2)n groups with n ≥ 4 [24,28]. As shown in Fig. 6a, the intensities of these peaks were all found to significantly decrease after the treatment with MBs generated in tap water and saline water, indicating the removal of the related organic
P = Pl + 2σ/r
(3)
where P is internal gas pressure, Pl is liquid pressure, r is the radius of bubble, and σ is surface tension. Increased intern gas pressure in MBs due to size inevitably triggers quick diffusion of entrapped pressurized gas towards aqueous solution, ultimately resulting in collapsing of MBs with the generation of strong pressure waves and water jets which in turn serve as the physical forces to shear off oils from sand surfaces. In fact, MBs had been applied for cleaning biologically fouled membranes without using any harsh chemicals, such as hypochlorite or ozone [30], while it had also been reported that grease on solid surface could be effectively removed by MBs [19]. In addition, MBs have a large interfacial area and low rising velocity in water phase, which in turn 127
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Fig. 6. FTIR (a) and UV spectra (b) of oil spill-contaminated sands before and after 40-min treatment with MBs in tap water and saline water.
improve their contact with contaminants to be removed from a solid surface [19]. Moreover, reattachment of removed oils onto sand surfaces could be effectively prevented through adsorption of oils by MBs due to their hydrophobic nature and good affinity to oils [31]. The possible mechanisms behind the observed oils removal by the self-collapsing MBs are illustrated in Fig. 8 for the remediation of oil spillcontaminated sands. Compared with saline water, the slightly better performance of MBs was observed in tap water (Figs. 3 and 5). In fact, MBs are often negatively charged with a zeta potential of around −35 mV [31]. It is a reasonable consideration that sodium chloride added to saline water reduces the zeta potential of MBs, leading to a decreased electrostatic repulsion among MBs, i.e. the coalescence of MBs is expectable with the formation of coarse bubbles. Therefore, the physical forces generated through the bursting of MBs would be weaken in the presence of sodium chloride. Obviously, further study is still needed to systematically examine the effects of salinity on the characteristics and stability of MBs in the remediation of oil spill-contaminated sands. As discussed above, about 90–99% of oils could be removed from the oil spill-contaminated sands after 40-min treatment by MBs. These clearly demonstrated the
Fig. 8. Illustration of possible remediation mechanisms of oil spill-contaminated sands with MBs.
Fig. 7. Appearances of oil spill-contaminated sands before (a) and after 40-min treatment with MBs in tap water (b) and saline water (c). 128
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applicability of MBs for effectively cleaning oil spill-contaminated sands. Moreover, the proposed MBs-based method can be easily scaled up or down according to actual needs, and which should be considered as a sustainable chemical-free approach for the remediation of oilcontaminated sands, which would also be applicable for treating oilcontaminated soils.
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5. Conclusions This study clearly demonstrated the feasibility of self-collapsing MBs for the remediation of oil spill-contaminated sands. TGA and TOC analyses revealed that almost all oils were removed from sand surfaces after 40-min treatment with MBs in tap water, while more than 90% of oil removal was achieved with MBs generated in saline water. It is reasonable to consider that seawater instead of fresh water could be used as a medium for generation of MBs in future real application. FTIR and UV spectra further confirmed the high-efficiency of MBs for cleaning oil spill-contaminated sands. Consequently, this study offers a sustainable and chemical-free alternative with self-collapsing MBs for efficient remediation of oil spill-contaminated sands. Acknowledgment This work was supported by the National Environment Agency of Singapore for ETRP (Grant No. 1302 111). References [1] B. Doshi, M. Sillanpää, S. Kalliola, A review of bio-based materials for oil spill treatment, Water Res. 135 (2018) 262–277. [2] G.A. El-Din, A. Amer, G. Malsh, M. Hussein, Study on the use of banana peels for oil spill removal, Alex. Eng. J. (2017), https://doi.org/10.1016/j.aej.2017.05.020. [3] A. Agarwal, Y. Liu, Remediation technologies for oil-contaminated sediments, Mar. Pollut. Bull. 101 (2015) 483–490. [4] N.P. Ventikos, E. Vergetis, H.N. Psaraftis, G. Triantafyllou, A high-level synthesis of oil spill response equipment and countermeasures, J. Hazard. Mater. 107 (2004) 51–58. [5] R.M. Abousnina, A. Manalo, W. Lokuge, J. Shiau, Oil contaminated sand: an emerging and sustainable construction material, Procedia Eng. 118 (2015) 1119–1126. [6] A.M. Bernabeu, D. Rey, Bn. Rubio, F. Vilas, C. Domínguez, J.M. Bayona, J. Albaigés, Assessment of cleanup needs of oiled sandy beaches: lessons from the Prestige oil spill, Environ. Sci. Technol. 43 (2009) 2470–2475. [7] J. Araruna Jr., V. Portes, A. Soares, M. Silva, M. Sthel, D. Schramm, S. Tibana, H. Vargas, Oil spills debris clean up by thermal desorption, J. Hazard. Mater. 110 (2004) 161–171. [8] E. Ceschia, J.R. Harjani, C. Liang, Z. Ghoshouni, T. Andrea, R.S. Brown, P.G. Jessop, Switchable anionic surfactants for the remediation of oil-contaminated sand by soil washing, RSC Adv. 4 (2014) 4638–4645. [9] H.J. Couto, G. Massarani, E.C. Biscaia Jr., G.L. Sant’Anna Jr., Remediation of sandy soils using surfactant solutions and foams, J. Hazard. Mater. 164 (2009) 1325–1334.
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