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Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology
Journal of Cleaner Production xxx (xxxx) xxx
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Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology Peng Chen a, Di Yin a, Pengfei Song b, Yiyang Liu a, Lankun Cai a, Hualin Wang a, Lehua Zhang a, c, * a
State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai, 200237, China Division of Science Engineering and Technology, Thomas Nelson Community College, 99 Thomas Nelson Drive, Hampton, VA, 23666, USA c Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2019 Received in revised form 25 August 2019 Accepted 2 October 2019 Available online xxx
A novel electrochemical micromembrane technology coupling membrane and electric method was developed to demulsify oily wastewater and recover oil. The diameter of the oil droplets increased to 55.0 mm from the initial 7.0 mm after treating the emulsion with the electrochemical micromembrane technology. Chemical oxygen demand removal rate and oil recovery reached 87.89% and 5173 mg/L, respectively, after cycling for 90 min at 10.0 V voltage with a 10.0 mm electrode distance, a 5.0 mm micromembrane pore size and a 0.5 L/min flow rate. Micromembrane pore sizes of 1.0 and 5.0 mm were both effective for decreasing chemical oxygen demand and recovering oil. The recovered oil was the mixture of mineral oil and ester according to the Raman infrared spectroscopy analysis. Surface charge redistribution and electrogenerated destruction for hydrophilic groups of surfactant molecules are responsible for oil coalescence. The electrochemical micromembrane technology used in this study offer an attractive treatment option to demulsify oily wastewater. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Prof. S Alwi Keywords: Demulsification Oil recovery Electrochemistry Membrane separation Stainless steel membrane
1. Introduction Large amounts of oily water are produced as liquid waste in a variety of industrial processes, including metalworking industry (Kobya et al., 2008), petroleum refining and food processing (Kumar et al., 2017). Untreated oily water causes severe environmental pollution (Tran et al., 2018) since the small oil droplets are not only abundant but also stable. Conventional methods to remove oil droplets include biological treatment (Cai et al., 2018), chemical treatment (Su et al., 2018), adsorption (Zhang et al., 2016), centrifugation, demulsifier addition (Rajak et al., 2016), microwave treatment or heating (Assenheimer et al., 2017). However, drawbacks, such as high costs, complex operations, secondary pollution and low efficiency, limit their application in demulsifying oil emulsions.
* Corresponding author. State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resources and Environmental Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China. E-mail address: [email protected] (L. Zhang).
Electrodemulsification technology, which is simple to operate and does not require additional chemicals, has been extensively studied and become increasingly accepted by industry. Electrostatic coalescence of the droplets under an external electric field is considered as the major force that causes demulsification (Lu et al., 1997). The stabilized small oil droplets are hard to fuse due to electric double layer repulsion between adjacent oil droplets (Zolfaghari et al., 2016). Nevertheless, the surface charge density of oil droplets is changed under an external electric field, and the energy barrier that prevents oil droplets from approaching each other is overcome, and then promotes eventual fusion of the droplets (Ichikawa, 2007). Although demulsification of W/O emulsions has been extensively researched, similar attention has not been given to O/W emulsions owing to the unique challenge: the continuous aqueous phase of the O/W emulsion is conductive, which leads to a higher current density (Hosseini et al., 2012) and formation of oil droplet chains (Zhang et al., 2018). A few studies have proved the concept of demulsifying an O/W emulsion using an electrostatic coalescence method, but the requirement for platetype electrodes and high electric voltage would generate secondary fine droplets, cause high voltage breakdown effect, and
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Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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consume high energy (Zolfaghari et al., 2016). Membrane method is a useful technology to separate oil/water emulsion. Researchers found the modified membrane, which own super-hydrophilicity (Chen et al., 2019) or lipophilicity (Lu et al., 2016a,b), have the great potential in oily wastewater separation. Although the membrane technology can separate the oil with high efficiency, the fouling and low permeate flux would restrict its wide-ranging applications (Zolfaghari et al., 2016). Some researchers have applied electrically conducting membrane to treat oily wastwater, including carbon nanotube-based ultrafifiltration membrane (Zhu et al., 2018), carbon nanotube-polyvinyl alcohol composite ultrafiltration membranes (Dudchenko et al., 2014), and hydrophilic Fe2O3 dynamic membrane (Lu et al., 2016a,b). In these cases, the electrostatic repulsion or attraction between oil droplet and electrically conducting membrane play the key role for oil/ water separation under low voltage. The interactive force can change surface charge distribution on oil droplet surface (Zhu et al., 2018) and compel oil droplets transform to unstable state, but these electrically conducting membranes under low voltage rarely involve electrochemical reaction, which would decline the removal efficiency for chemical oxygen demand. The investigation regarding electrochemical reaction of oily wastewater has been reported nek lately (Dick et al., 2014; Kim et al., 2014; Park et al., 2016; Troja et al., 2018). These studies revealed that, when oil droplet collided with electrode, reductive or oxidative molecules of the oil emulsion droplet would start to be electrolyzed. Electrogenerated oxidation or reduction reaction might result in phase transformation (Zhang et al., 2018) and deformation (Kim et al., 2015). The previous study found the 94.93% COD was removed using electro-oxidation process to treat cutting fluid, but oil can not be recovered owing to electrogenerated decomposition reaction (Ramanpreet et al., 2017). Here, an electrochemical micromembrane (EM) technology, which coupling membrane method, was developed to retrieve oil from the oil-in-water cutting fluid, which was usually generated in mechanical processing and metal manufacturing due to its desirable cooling and lubricating effects (Kobya et al., 2008). The working electrode was designed into membrane-electrode but not plate-style electrode. Electrochemical reactions occurred and the surface charge distribution was simultaneously changed when oil droplets passed through the micromembrane electrode. The effects of voltage, cycling time, flow rate, electrode distance, and pore size of the micromembrane on separation efficiency were examined. This work lays the foundation for a novel approach to demulsify O/ W emulsions and recovers oil simultaneously. 2. Materials and methods 2.1. Characterization of the wastewater The wastewater was taken from a local machinery factory (ASSA ABLOY Brand–Baodean Security Products Co., LTD, Taizhou, China). Various water quality indices were measured at the beginning of study and are listed in Table 1. 2.2. Design of the demulsification apparatus As shown in Fig. 1, the device is comprised of a stainless steel cylindrical micromembrane as the anode, a copper sheet as the
cathode, and a plexiglass container. The micromembrane measured 28.0 mm (i.d.), 60.0 mm (o.d.), and 254.0 mm (L). It was positioned in the center of the container. Two membranes with different pore sizes of 1.0 mm and 5.0 mm were used in this study. The copper sheet measured 2.0 mm in thickness and 254.0 mm in length. It was folded into a cylinder and was adhered to the container inner wall. The emulsion was fed into the micromembrane column. Liquid then flowed into the annular region between the two electrodes after passing through the micromembrane electrode. Finally, the emulsion flowed out of the device through the outlet on the top of the plastic container. Fig. 2 illustrates the experimental setup for the demulsification test. The system included the demulsification device (EM), a DC power supply, a circulating pump, a storage tank, and a suction filtration funnel. A fiber filter paper (diameter is 90.0 mm) with the pore size of 30.0 mm was placed in the funnel to separate oil and water. 2.3. Demulsification test by EM technology As the demulsification tests of EM technology, 4 L of cutting fluid were fed into the demulsification apparatus from the storage tank at constant pumping speeds of 0.5, 1.0, 2.0, 5.0, and 10.0 L/min. The demulsification device (EM) was provided 1.0, 2.0, 5.0, 10.0, and 15.0 V of voltage by the DC voltage supply. The output emulsion returned to the storage tank for a second cycle until the reaction time reached 30, 60, 90, 120, 150 min, respectively. The liquid and oil phases were separated by suction filtration after electrochemical mcromembrane treatment. The filter cake was weighed after drying at 105 C in a drying oven. COD of the filtrate was measured using the potassium dichromate method (Netherlands Standardization Institute, 2006). Oil content of raw cutting fluid and filtrate were determined by means of gravimetric method (Feng et al., 2018). Table 2 indicates the COD and oil content of the samples used in experiments. 2.4. Cyclic voltammetry (CV) test O/W emulsion and deionized water were electrochemically characterized by applying a potential scan in the oxidation direction. The arrangement setup consisted in a typical cell (volume ¼ 50 mL) with three-electrodes which consisted of a saturated calomel reference electrode (SCE), platinum plate as counter-electrode and a stainless steel plate as working electrode. The cell was connected to a CHI model 760E electrochemical workstation (CH Instruments, Shanghai, China), and voltammograms were obtained by scanning at 50 mV/s, from 1.6 to þ1.6 V. Prior to each experiment, the surface of the working electrode was polished using Buehler alumina powder (final grain size 0.05 mm) to a mirror finish. The electrode was then rinsed with deionized water and was placed in an ultrasonic bath for 5 min. 2.5. Sample analysis To assess performance of the EM technology for treating the cutting fluid, COD removal efficiency (%) and oil recovery (mg/L) was calculated based on the equation below. Data presented in the figures was reported as mean value (three repeated
Table 1 Water quality indices of the raw waste cutting fluid. Chemical oxygen demand (COD) (mg/L)
Electrical conductivity (mS/cm)
Oil content (mg/L)
pH
50,300e57,100
5.27e5.39
4639e5590
8.1e8.5
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 1. Structure of the electrochemical micromembrane (EM) apparatus.
where m0 is COD of the raw fluid (mg/L) and m1 is the COD of the fluid treated by the demulsification device (mg/L).
oil recovery ¼ Q 0 Q 1
Fig. 2. Schematic diagram of the experimental setup for treating oil-in-water cutting fluid with the electrochemical micromembrane technology.
Table 2 COD and oil content of the feed samples under different conditions.
Voltage (V)
Flow rate (L/min)
Electrode distance (mm) Membrane pore size (mm)
1.0 2.0 5.0 10.0 15.0 0.5 1.0 2.0 5.0 10.0 10.0 20.0 1.0 5.0
COD (mg/L)
Oil content (mg/L)
52,600 53,700 54,350 50,300 54,700 57,100 57,100 57,100 57,100 57,100 50,300 51,100 53,170 50,300
5424 5424 5470 5465 5590 4639 4639 4639 4639 4639 5465 5571 5547 5465
m0 m1 100 % m0
where Q0 is the oil content of the initial cutting fluid (mg/L), Q1 is the oil content of the treated cutting fluid by the demulsification device (mg/L). The crude and recovered oil were analyzed between 500 and 4000 cm1 via Raman infrared spectroscopy (Nicolet 6700, America Thermo Fisher Scientific Company) using the coating method. A microscope with an attached digital camera (XSP-BM-12CAC, Shanghai BM Optical Instruments Manufacture Co., LTD.) was used to observe the emulsion and the diameter of oil droplet was measured at 640 magnification before and after the demulsification treatment. As described in previous study (Feng et al., 2017), emulsion were taken from the outlet of the EM apparatus before the suction filtration. A certain amount of solution was absorbed using a dropper and then was dropped in the middle of the slide. More than 100 droplets were measured under each condition. 3. Results and discussion 3.1. Effects of voltage on performance of the EM technology
experiments) ± standard deviation. Statistical analysis was processed using the Origin software (version 8.0).
CODremoval efficiency ¼
(2)
(1)
As shown in Fig. 3, when voltage increased from 1.0 to 15.0 V, COD removal and oil recovery efficiencies both increased. The improvement was significant and gradual when the voltage rose from 1.0 to 10.0 V but diminished when it further increased to 15.0 V. The results indicated that the supplied voltage affected the demulsifying performance of the EM technology. The increased voltage promotes the generation of a strong electric field between the anode and the cathode, which encourages oil coalescence. Consequently, the number of large oil droplets inside the EM device will increase. Additionally, high voltage enhances the anode potential, which facilitates demulsification as oil droplets move through the micromembrane. For example, the anode potential at the low voltage of 1.0 V was 0.52 V, but its value gradually climbed
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 3. Effects of voltage on COD removal (A) and oil recovery (B) at various amounts of cycling. Experimental conditions: flow rate ¼ 0.5 L/min; electrode distance ¼ 10.0 mm; membrane pore size ¼ 5.0 mm.
to 1.65, 2.66, 5.40 and 8.24 V when the supplied voltage increased to 2.0, 5.0, 10.0 and 15.0 V, respectively. The increment of anode potential produces electric potential differences for oxidation of reductive species. In addition, the electric attractive force between electrode and the charged oil droplets is strengthened. The increased electric attractive force causes the redistribution of surface charge distribution and leads to the formation of unstable oil droplets. Consequently, oil droplets break and fuse together, which results in the improvement of COD removal and oil recovery. When the voltage increased beyond the optimum point, however, it generated hydrogen and oxygen bubbles at the electrode surface (Bande et al., 2008). These gas bubbles would block oil droplets collide with electrode and carrier small oil droplets out, which disturbed the coalescence of oil droplets. Since unbroken oil droplets will continue to grow under an electric field, these droplets could subject to demulsification at the next cycle. As shown in Fig. 3, under low voltage (1.0, 2.0, or 3.0 V), COD removal and oil recovery gradually increased as the cycling time increased. At a higher voltage (10.0 or 15.0 V), the rates of COD removal and oil recovery rose rapidly during the first 60 min of cutting fluid cycling, and then they both plateaued as cycling continued to 150 min. Results revealed that prolonging the cycling time can enhance demulsification at low voltage. The reason would be that the longer cycling time could increase the frequency of collision between oil droplets and electrode, thus providing enough time for the oxidation of emulsion. Taken together, these results suggested that voltage and cycling time both positively affect COD removal and oil recovery. At 10.0 V and 90 min of cycling, 89.6% COD removal and 5085 mg/L oil recovery were achieved. These conditions therefore were chosen to demulsify and recover oil from cutting fluid using the novel EM technology. The rough comparison with the traditional demulsification technology reported in recent years indicated that, the demulsification effect of EM technology is higher than the electrostatic coalescence method (Hosseini and Shahavi, 2012), is equivalent to membrane method (Chen et al., 2019) and lower than electro-oxidation technology (Ramanpreet et al., 2017). Noted that different results would be generated when the compared wastewater with different characteristics. 3.2. Effect of flow rate on performance of the EM technology The effect of flow rate on COD removal and oil recovery is shown in Fig. 4. As the flow rate increased, the demulsification and oil recovery decreased. COD removal dropped from the mean value of 85.99% to 30.65% when the flow rate increased from 0.5 to 10.0 L/ min. Oil recovery decreased from the average of 4379 to 1429 mg/L. Better performance was observed at a lower flow rate. This result
Fig. 4. Effect of flow rate on COD removal and oil recovery. Experimental conditions: voltage ¼ 10.0 V; cycling time ¼ 90 min; electrode distance ¼ 10.0 mm; membrane pore size ¼ 5.0 mm.
would be relevant to the prolongation of residence time of the cutting fluid in the EM apparatus. Lower flow rate means longer residence time for oil droplet in EM apparatus. The increased residence time provide longer time for electrochemical reaction and surface charge redistribution than high flow rate. The higher flow rate reduces residence time of oil droplets in EM apparatus resulting to the low separation efficiency. This result is consistent with a study from Roques-Carmes et al. (2014), who reported lower oil separation efficiency at a higher flow rate. 3.3. Effect of electrode distance on the performance of the EM technology When the distance between the two electrodes was changed, COD removal and oil recovery efficiencies also changed (Fig. 5). During the first 30 min, this change was minimal, but as the cycling time increased, the difference became apparent at the two different distances. The results showed that at 90 min, the average of COD removal and oil recovery were at 65.36% and 3224 mg/L, respectively, when the anode-cathode distance was 20.0 mm. These values increased to 89.60% and 5085 mg/L, respectively, when the distance was reduced to 10.0 mm. The electric current at the 10.0 mm distance was maintained at 1.47 A, which was much higher than the value of 0.94 A produced at 20.0 mm electrode distance. This increment of the current could provide higher energy for surface charge redistribution and collision of oil droplets (Fouad et al., 2009), which ultimately led to improved performance in oil recovery and COD removal.
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 5. Effect of electrode distance on COD removal (A) and oil recovery (B) at various cycling times. Experimental conditions: voltage ¼ 10.0 V; flow rate ¼ 0.5 L/min; membrane pore size ¼ 5.0 mm.
3.4. Effect of membrane pore size on the performance of the EM technology Micromembranes with two different pore sizes (1.0 mm and 5.0 mm) were used in this experiment. As shown in Fig. 6, the 1.0 mm membrane performed slightly better in recovering oil during the first 60 min of cycling, but the difference became negligible thereafter. At 90 min of cycling, the amounts of COD removed and oil recovered were almost the same with these two methods (COD removal (mean value): 87.89% vs. 89.60%; oil recovery (mean value): 5085 vs. 5173 mg/L). This result likely occurred due to the majority of the oil droplets in the emulsion being larger than 5.0 mm. These parts droplets can easily collide with the electrode and take reaction when they passed through the micromembrane electrode with small pore size of 1.0 mm and 5.0 mm. This behavior creates the similar demulsification effect under the two micromembrane pore sizes of 1.0 and 5.0 mm. 3.5. Characterization of cutting fluid and the recovered substance by EM technology Fig. 7 shows microscopic images of the cutting fluid before and after demulsification with the electrochemical microfiltration technology. Floating oil droplets uniformly dispersed in the cutting fluid before the treatment with a diameter of 7.0 mm. After 90 min of demulsification in the designed device, small oil droplets coalesce to large oil droplet with the maximum diameter of 55.0 mm, which demonstrated the effectiveness of the electrochemical micromembrane action in demulsifying oil-in-water emulsion. These large oil drops were easy to recover in the suction filtration step. Small fractions of the oil droplets were so fine (smaller than 1.0 mm) that they can smoothly pass through the micromembrane
pore. This result can explain the minor difference in COD removal and oil recovery when using membranes with different pore sizes (1.0 mm and 5.0 mm). 3.6. Results of IR and CV IR spectrum results of crude oil and recovered oil were presented in Fig. 8. The functional groups that were identified include eOH characteristic absorption peak (3384.5 cm1), -CHn-stretching of the saturated bonds (2930.0e2853.2 cm1), the peak of the ester C¼O group (1740.0 cm1), eCOOe characteristic absorption peak (1559.6 cm1), -CHn-bending of the saturated bonds (1460.0 cm1), and -CHn-symmetrical bending of the saturated bonds (1377.1 cm1). The peak at 722.7 cm1 indicates the presence of long chain alkyl groups (-(CH2)n-, n 4). Thus, the crude oil was the mixture of fatty acid salts, ester and mineral oil. However, the recovered material was a mixture of ester and mineral oil. Fatty acid salts is often added to emulsion as anionic surfactant, aiming to strengthen emulsification effect (Guan et al., 2019). Hydroxyl is a hydrophilic group, which is inserted into the molecular structure of surfactant to improve its hydrophilicity (Zhou et al., 2017). There is an approximate conclusion that the anionic surfactant is a kind of fatty acid salt with the hydroxyl group. The electrochemical reaction is considered as the main reason resulting in dispersion of fatty acid salts and hydroxyl in recovered oil. As depicted in Fig. 9, an oxidation peak around 0.2e0.3 V (vs SCE) occurred in CV of the O/W cutting fluid. Trace oil droplets floated out of the emulsion surface only when this oxidation peak emerges in the cyclic voltammetry curve. Accordingly, collision of oil droplet with electrode would cause the separation of oil from oily wastewater (Zhang et al., 2017). When oil droplet collide electrode the hydrophilic group, eOH and eCOOe, is partially or
Fig. 6. Effect of membrane pore size on COD removal (A) and oil recovery (B) at various cycling times. Experimental conditions: voltage ¼ 10.0 V; flow rate ¼ 0.5 L/min; electrode distance ¼ 10.0 mm.
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 7. Morphology of the oil droplets in the cutting fluid before (a) and after (b, c, d) treatment with the electrochemical micromembrane technology. Experimental conditions: voltage ¼ 10.0 V; flow rate ¼ 0.5 L/min; electrode distance ¼ 10.0 mm; membrane pore size ¼ 5.0 mm; magnification ¼ 640X.
3.7. Mechanism of demulsification in the electrochemical micromembrane technology
Fig. 8. IR spectrum of the crude oil and recovered oil.
Fig. 9. Voltammogram of the O/W emulsion and the deionized water over the potential range from 1.6 to þ1.6 V at a scan rate of 50 mV/s.
absolutely oxidized, destroying the stability of oil droplet or changing the surfactant to hydrophobic material (Kim et al., 2015). This would be the reason why some absorption peaks of hydrophilic group (-OH and eCOOe) disappeared in the IR spectrum of recovered oil.
Based on the aforementioned results for the batch experiments, a possible pathway for separating oil from O/W cutting fluid wastewater via the EM technology is summarized here and is illustrated in Fig. 10. In a stable oil-water emulsion, the electrostatic repulsive force and hindrance from the interfacial film are the main coalescence barriers among the oil droplets (Ren and Kang, 2018). When two spherical droplets approach each other, the electric double layers of neighboring oil droplets overlap (Zhang et al., 2010), and a thin liquid film is formed between two approaching droplets (Yang, 2007). This behavior can be attributed to the barring effect of the electrostatic repulsive force (Fer), which is caused by some charges adsorbed on the oil surface. Followed the DLVO theory, this is one of the main coalescence barriers among the oil drops (Ichikawa, 2007). Moreover, the action of hindrance works by the surfactants that stop oil-droplets from coalescing by forming a tight and rigid interfacial film on an oil drop surface (Miao et al., 2015). However, the resistance in oil droplet coalescence by the electrostatic repulsive force and the interfacial film can be shrunken in the EM apparatus. As can be seen in Fig. 10(a), when the oil droplet adsorbed negative charged surfactant enters the anodic membrane pore, the electrostatic attractive force (Fea) between the membrane electrode and the oil droplet leads to the redistribution of surface charge adsorbed on oil droplet surface (Wu et al., 2018). The negative charged surfactant that is perpendicular to the direction of flow will migrate to the area parallel to the flow, and cluster around of electrode, which form the non-equivalent surface charge density distribution. These unstable oil droplets will coalesce, because the barriering energy (Fer) that hinders oil droplets approaching each other is reduced (Liu et al., 2015). Additionally, there is another possible mechanism (Fig. 10(b)) functioned for the separation of oil droplet in EM apparatus. When the oil droplet collides with anode, some hydrophilic groups of the surfactant, such as eOH and eCOOe, are destroyed by electrochemical reaction. Destruction of group change the amphiphilic molecular (R(M)) to the hydrophobic molecular (R’). Change in molecular hydrophilicity result in the disappearance of the interfacial film coating on oil droplet surface, which is beneficial to reduce the obstruction of coalescence among these oil droplets (Srivastava et al., 2017). Some finely unbroken droplets also coalesce and enlarge to large oil droplet with increasing cycle time until to breaking in EM system, under the function of the surface charge redistribution and surfactant molecular oxidation. 4. Conclusions Design of the novel electrochemical micromembrane technology was effective to demulsify oil-in-water emulsion. Results
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 10. Mechanism of electrochemical demulsification of O/W emulsions by the electrochemical micromembrane technology.
suggested that 87.89% COD was removed and 5173 mg/L oil was recovered at the optimum conditions of 10.0 V voltage, 90 min cycling time, 0.5 L/min flow rate, 10.0 mm electrode distance and 5.0 mm micromembrane pore size. Under these conditions, the diameter of oil droplets enlarged to 55.0 mm from an initial size of 7.0 mm. Raman infrared spectroscopy indicated the recovered substance was mostly mineral oil. Surface charge redistribution and destruction of hydrophilic group are likely the main demulsifying mechanisms for this electrochemical micromembrane technology. Declaration of competing interest None. Acknowledgement This work was supported by the National Natural Science Foundation of China (NSFC) (21876050), the National key research and development plans of special project for site soils (2018YFC1800600) and the Special Fund from State Key Joint Laboratory of Environment Simulation and Pollution Control (18K10ESPCT). References Assenheimer, T., Barros, A., Kashefi, K., Pinto, J.C., Tavares, F.W., Nele, M., 2017. Evaluation of microwave and conventional heating for electrostatic treatment of a water-in-oil model emulsion in a pilot plant. Energy Fuel. 31, 6587e6597. Bande, R.M., Prasad, B., Mishra, I.M., Wasewar, K.L., 2008. Oil field effluent water treatment for safe disposal by electroflotation. Chem. Eng. J. 137, 503e509. Cai, Q., Zhu, Z., Chen, B., Zhang, B., 2018. Oil-in-water emulsion breaking marine bacteria for demulsifying oily wastewater. Water Res. 149, 292e301. Chen, X., Huang, G., An, C., Feng, R., Yao, Y., Zhao, S., Huang, C., W, Y., 2019. Plasmainduced poly(acrylic acid)-TiO2 coated polyvinylidene fluoride membrane for produced water treatment: synchrotron X-Ray, optimization, and insight studies. J. Clean. Prod. 227, 772e783. Dick, J.E., Renault, C., Kim, B.K., Bard, A.J., 2014. Electrogenerated chemiluminescence of common organic luminophores in water using an emulsion system. J. Am. Chem. Soc. 136, 13546e13549. Dudchenko, A.V., Rolf, J., Russell, K., Duan, W., Jassby, D., 2014. Organic fouling inhibition on electrically conducting carbon nanotube-polyvinyl alcohol composite ultrafifiltration membranes. J. Membr. Sci. 468, 1e10. Feng, W., Yin, Y., Mendoza, M.D.L., Wang, L., Chen, P., Liu, Y., Cai, L., Zhang, L., 2018. Oil recovery from waste cutting fluid via the combination of suspension crystallization and freeze-thaw processes. J. Clean. Prod. 172, 481e487. Feng, W., Yin, Y., Mendoza, M.D.L., Wang, L., Chen, X., Liu, Y., Cai, L., Zhang, L., 2017. Freeze-thaw method for oil recovery from waste cutting fluid without chemical additions. J. Clean. Prod. 148, 84e89. Fouad, Y.O.A., Konsowa, A.H., Farag, H.A., Sedahmed, G.H., 2009. Performance of an electrocoagulation cell with horizontally oriented electrodes in oil separation compared to a cell with vertical electrodes. Chem. Eng. J. 145, 436e440.
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology Peng Chen a, Di Yin a, Pengfei Song b, Yiyang Liu a, Lankun Cai a, Hualin Wang a, Lehua Zhang a, c, * a
State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai, 200237, China Division of Science Engineering and Technology, Thomas Nelson Community College, 99 Thomas Nelson Drive, Hampton, VA, 23666, USA c Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 July 2019 Received in revised form 25 August 2019 Accepted 2 October 2019 Available online xxx
A novel electrochemical micromembrane technology coupling membrane and electric method was developed to demulsify oily wastewater and recover oil. The diameter of the oil droplets increased to 55.0 mm from the initial 7.0 mm after treating the emulsion with the electrochemical micromembrane technology. Chemical oxygen demand removal rate and oil recovery reached 87.89% and 5173 mg/L, respectively, after cycling for 90 min at 10.0 V voltage with a 10.0 mm electrode distance, a 5.0 mm micromembrane pore size and a 0.5 L/min flow rate. Micromembrane pore sizes of 1.0 and 5.0 mm were both effective for decreasing chemical oxygen demand and recovering oil. The recovered oil was the mixture of mineral oil and ester according to the Raman infrared spectroscopy analysis. Surface charge redistribution and electrogenerated destruction for hydrophilic groups of surfactant molecules are responsible for oil coalescence. The electrochemical micromembrane technology used in this study offer an attractive treatment option to demulsify oily wastewater. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Prof. S Alwi Keywords: Demulsification Oil recovery Electrochemistry Membrane separation Stainless steel membrane
1. Introduction Large amounts of oily water are produced as liquid waste in a variety of industrial processes, including metalworking industry (Kobya et al., 2008), petroleum refining and food processing (Kumar et al., 2017). Untreated oily water causes severe environmental pollution (Tran et al., 2018) since the small oil droplets are not only abundant but also stable. Conventional methods to remove oil droplets include biological treatment (Cai et al., 2018), chemical treatment (Su et al., 2018), adsorption (Zhang et al., 2016), centrifugation, demulsifier addition (Rajak et al., 2016), microwave treatment or heating (Assenheimer et al., 2017). However, drawbacks, such as high costs, complex operations, secondary pollution and low efficiency, limit their application in demulsifying oil emulsions.
* Corresponding author. State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resources and Environmental Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China. E-mail address: [email protected] (L. Zhang).
Electrodemulsification technology, which is simple to operate and does not require additional chemicals, has been extensively studied and become increasingly accepted by industry. Electrostatic coalescence of the droplets under an external electric field is considered as the major force that causes demulsification (Lu et al., 1997). The stabilized small oil droplets are hard to fuse due to electric double layer repulsion between adjacent oil droplets (Zolfaghari et al., 2016). Nevertheless, the surface charge density of oil droplets is changed under an external electric field, and the energy barrier that prevents oil droplets from approaching each other is overcome, and then promotes eventual fusion of the droplets (Ichikawa, 2007). Although demulsification of W/O emulsions has been extensively researched, similar attention has not been given to O/W emulsions owing to the unique challenge: the continuous aqueous phase of the O/W emulsion is conductive, which leads to a higher current density (Hosseini et al., 2012) and formation of oil droplet chains (Zhang et al., 2018). A few studies have proved the concept of demulsifying an O/W emulsion using an electrostatic coalescence method, but the requirement for platetype electrodes and high electric voltage would generate secondary fine droplets, cause high voltage breakdown effect, and
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consume high energy (Zolfaghari et al., 2016). Membrane method is a useful technology to separate oil/water emulsion. Researchers found the modified membrane, which own super-hydrophilicity (Chen et al., 2019) or lipophilicity (Lu et al., 2016a,b), have the great potential in oily wastewater separation. Although the membrane technology can separate the oil with high efficiency, the fouling and low permeate flux would restrict its wide-ranging applications (Zolfaghari et al., 2016). Some researchers have applied electrically conducting membrane to treat oily wastwater, including carbon nanotube-based ultrafifiltration membrane (Zhu et al., 2018), carbon nanotube-polyvinyl alcohol composite ultrafiltration membranes (Dudchenko et al., 2014), and hydrophilic Fe2O3 dynamic membrane (Lu et al., 2016a,b). In these cases, the electrostatic repulsion or attraction between oil droplet and electrically conducting membrane play the key role for oil/ water separation under low voltage. The interactive force can change surface charge distribution on oil droplet surface (Zhu et al., 2018) and compel oil droplets transform to unstable state, but these electrically conducting membranes under low voltage rarely involve electrochemical reaction, which would decline the removal efficiency for chemical oxygen demand. The investigation regarding electrochemical reaction of oily wastewater has been reported nek lately (Dick et al., 2014; Kim et al., 2014; Park et al., 2016; Troja et al., 2018). These studies revealed that, when oil droplet collided with electrode, reductive or oxidative molecules of the oil emulsion droplet would start to be electrolyzed. Electrogenerated oxidation or reduction reaction might result in phase transformation (Zhang et al., 2018) and deformation (Kim et al., 2015). The previous study found the 94.93% COD was removed using electro-oxidation process to treat cutting fluid, but oil can not be recovered owing to electrogenerated decomposition reaction (Ramanpreet et al., 2017). Here, an electrochemical micromembrane (EM) technology, which coupling membrane method, was developed to retrieve oil from the oil-in-water cutting fluid, which was usually generated in mechanical processing and metal manufacturing due to its desirable cooling and lubricating effects (Kobya et al., 2008). The working electrode was designed into membrane-electrode but not plate-style electrode. Electrochemical reactions occurred and the surface charge distribution was simultaneously changed when oil droplets passed through the micromembrane electrode. The effects of voltage, cycling time, flow rate, electrode distance, and pore size of the micromembrane on separation efficiency were examined. This work lays the foundation for a novel approach to demulsify O/ W emulsions and recovers oil simultaneously. 2. Materials and methods 2.1. Characterization of the wastewater The wastewater was taken from a local machinery factory (ASSA ABLOY Brand–Baodean Security Products Co., LTD, Taizhou, China). Various water quality indices were measured at the beginning of study and are listed in Table 1. 2.2. Design of the demulsification apparatus As shown in Fig. 1, the device is comprised of a stainless steel cylindrical micromembrane as the anode, a copper sheet as the
cathode, and a plexiglass container. The micromembrane measured 28.0 mm (i.d.), 60.0 mm (o.d.), and 254.0 mm (L). It was positioned in the center of the container. Two membranes with different pore sizes of 1.0 mm and 5.0 mm were used in this study. The copper sheet measured 2.0 mm in thickness and 254.0 mm in length. It was folded into a cylinder and was adhered to the container inner wall. The emulsion was fed into the micromembrane column. Liquid then flowed into the annular region between the two electrodes after passing through the micromembrane electrode. Finally, the emulsion flowed out of the device through the outlet on the top of the plastic container. Fig. 2 illustrates the experimental setup for the demulsification test. The system included the demulsification device (EM), a DC power supply, a circulating pump, a storage tank, and a suction filtration funnel. A fiber filter paper (diameter is 90.0 mm) with the pore size of 30.0 mm was placed in the funnel to separate oil and water. 2.3. Demulsification test by EM technology As the demulsification tests of EM technology, 4 L of cutting fluid were fed into the demulsification apparatus from the storage tank at constant pumping speeds of 0.5, 1.0, 2.0, 5.0, and 10.0 L/min. The demulsification device (EM) was provided 1.0, 2.0, 5.0, 10.0, and 15.0 V of voltage by the DC voltage supply. The output emulsion returned to the storage tank for a second cycle until the reaction time reached 30, 60, 90, 120, 150 min, respectively. The liquid and oil phases were separated by suction filtration after electrochemical mcromembrane treatment. The filter cake was weighed after drying at 105 C in a drying oven. COD of the filtrate was measured using the potassium dichromate method (Netherlands Standardization Institute, 2006). Oil content of raw cutting fluid and filtrate were determined by means of gravimetric method (Feng et al., 2018). Table 2 indicates the COD and oil content of the samples used in experiments. 2.4. Cyclic voltammetry (CV) test O/W emulsion and deionized water were electrochemically characterized by applying a potential scan in the oxidation direction. The arrangement setup consisted in a typical cell (volume ¼ 50 mL) with three-electrodes which consisted of a saturated calomel reference electrode (SCE), platinum plate as counter-electrode and a stainless steel plate as working electrode. The cell was connected to a CHI model 760E electrochemical workstation (CH Instruments, Shanghai, China), and voltammograms were obtained by scanning at 50 mV/s, from 1.6 to þ1.6 V. Prior to each experiment, the surface of the working electrode was polished using Buehler alumina powder (final grain size 0.05 mm) to a mirror finish. The electrode was then rinsed with deionized water and was placed in an ultrasonic bath for 5 min. 2.5. Sample analysis To assess performance of the EM technology for treating the cutting fluid, COD removal efficiency (%) and oil recovery (mg/L) was calculated based on the equation below. Data presented in the figures was reported as mean value (three repeated
Table 1 Water quality indices of the raw waste cutting fluid. Chemical oxygen demand (COD) (mg/L)
Electrical conductivity (mS/cm)
Oil content (mg/L)
pH
50,300e57,100
5.27e5.39
4639e5590
8.1e8.5
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 1. Structure of the electrochemical micromembrane (EM) apparatus.
where m0 is COD of the raw fluid (mg/L) and m1 is the COD of the fluid treated by the demulsification device (mg/L).
oil recovery ¼ Q 0 Q 1
Fig. 2. Schematic diagram of the experimental setup for treating oil-in-water cutting fluid with the electrochemical micromembrane technology.
Table 2 COD and oil content of the feed samples under different conditions.
Voltage (V)
Flow rate (L/min)
Electrode distance (mm) Membrane pore size (mm)
1.0 2.0 5.0 10.0 15.0 0.5 1.0 2.0 5.0 10.0 10.0 20.0 1.0 5.0
COD (mg/L)
Oil content (mg/L)
52,600 53,700 54,350 50,300 54,700 57,100 57,100 57,100 57,100 57,100 50,300 51,100 53,170 50,300
5424 5424 5470 5465 5590 4639 4639 4639 4639 4639 5465 5571 5547 5465
m0 m1 100 % m0
where Q0 is the oil content of the initial cutting fluid (mg/L), Q1 is the oil content of the treated cutting fluid by the demulsification device (mg/L). The crude and recovered oil were analyzed between 500 and 4000 cm1 via Raman infrared spectroscopy (Nicolet 6700, America Thermo Fisher Scientific Company) using the coating method. A microscope with an attached digital camera (XSP-BM-12CAC, Shanghai BM Optical Instruments Manufacture Co., LTD.) was used to observe the emulsion and the diameter of oil droplet was measured at 640 magnification before and after the demulsification treatment. As described in previous study (Feng et al., 2017), emulsion were taken from the outlet of the EM apparatus before the suction filtration. A certain amount of solution was absorbed using a dropper and then was dropped in the middle of the slide. More than 100 droplets were measured under each condition. 3. Results and discussion 3.1. Effects of voltage on performance of the EM technology
experiments) ± standard deviation. Statistical analysis was processed using the Origin software (version 8.0).
CODremoval efficiency ¼
(2)
(1)
As shown in Fig. 3, when voltage increased from 1.0 to 15.0 V, COD removal and oil recovery efficiencies both increased. The improvement was significant and gradual when the voltage rose from 1.0 to 10.0 V but diminished when it further increased to 15.0 V. The results indicated that the supplied voltage affected the demulsifying performance of the EM technology. The increased voltage promotes the generation of a strong electric field between the anode and the cathode, which encourages oil coalescence. Consequently, the number of large oil droplets inside the EM device will increase. Additionally, high voltage enhances the anode potential, which facilitates demulsification as oil droplets move through the micromembrane. For example, the anode potential at the low voltage of 1.0 V was 0.52 V, but its value gradually climbed
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 3. Effects of voltage on COD removal (A) and oil recovery (B) at various amounts of cycling. Experimental conditions: flow rate ¼ 0.5 L/min; electrode distance ¼ 10.0 mm; membrane pore size ¼ 5.0 mm.
to 1.65, 2.66, 5.40 and 8.24 V when the supplied voltage increased to 2.0, 5.0, 10.0 and 15.0 V, respectively. The increment of anode potential produces electric potential differences for oxidation of reductive species. In addition, the electric attractive force between electrode and the charged oil droplets is strengthened. The increased electric attractive force causes the redistribution of surface charge distribution and leads to the formation of unstable oil droplets. Consequently, oil droplets break and fuse together, which results in the improvement of COD removal and oil recovery. When the voltage increased beyond the optimum point, however, it generated hydrogen and oxygen bubbles at the electrode surface (Bande et al., 2008). These gas bubbles would block oil droplets collide with electrode and carrier small oil droplets out, which disturbed the coalescence of oil droplets. Since unbroken oil droplets will continue to grow under an electric field, these droplets could subject to demulsification at the next cycle. As shown in Fig. 3, under low voltage (1.0, 2.0, or 3.0 V), COD removal and oil recovery gradually increased as the cycling time increased. At a higher voltage (10.0 or 15.0 V), the rates of COD removal and oil recovery rose rapidly during the first 60 min of cutting fluid cycling, and then they both plateaued as cycling continued to 150 min. Results revealed that prolonging the cycling time can enhance demulsification at low voltage. The reason would be that the longer cycling time could increase the frequency of collision between oil droplets and electrode, thus providing enough time for the oxidation of emulsion. Taken together, these results suggested that voltage and cycling time both positively affect COD removal and oil recovery. At 10.0 V and 90 min of cycling, 89.6% COD removal and 5085 mg/L oil recovery were achieved. These conditions therefore were chosen to demulsify and recover oil from cutting fluid using the novel EM technology. The rough comparison with the traditional demulsification technology reported in recent years indicated that, the demulsification effect of EM technology is higher than the electrostatic coalescence method (Hosseini and Shahavi, 2012), is equivalent to membrane method (Chen et al., 2019) and lower than electro-oxidation technology (Ramanpreet et al., 2017). Noted that different results would be generated when the compared wastewater with different characteristics. 3.2. Effect of flow rate on performance of the EM technology The effect of flow rate on COD removal and oil recovery is shown in Fig. 4. As the flow rate increased, the demulsification and oil recovery decreased. COD removal dropped from the mean value of 85.99% to 30.65% when the flow rate increased from 0.5 to 10.0 L/ min. Oil recovery decreased from the average of 4379 to 1429 mg/L. Better performance was observed at a lower flow rate. This result
Fig. 4. Effect of flow rate on COD removal and oil recovery. Experimental conditions: voltage ¼ 10.0 V; cycling time ¼ 90 min; electrode distance ¼ 10.0 mm; membrane pore size ¼ 5.0 mm.
would be relevant to the prolongation of residence time of the cutting fluid in the EM apparatus. Lower flow rate means longer residence time for oil droplet in EM apparatus. The increased residence time provide longer time for electrochemical reaction and surface charge redistribution than high flow rate. The higher flow rate reduces residence time of oil droplets in EM apparatus resulting to the low separation efficiency. This result is consistent with a study from Roques-Carmes et al. (2014), who reported lower oil separation efficiency at a higher flow rate. 3.3. Effect of electrode distance on the performance of the EM technology When the distance between the two electrodes was changed, COD removal and oil recovery efficiencies also changed (Fig. 5). During the first 30 min, this change was minimal, but as the cycling time increased, the difference became apparent at the two different distances. The results showed that at 90 min, the average of COD removal and oil recovery were at 65.36% and 3224 mg/L, respectively, when the anode-cathode distance was 20.0 mm. These values increased to 89.60% and 5085 mg/L, respectively, when the distance was reduced to 10.0 mm. The electric current at the 10.0 mm distance was maintained at 1.47 A, which was much higher than the value of 0.94 A produced at 20.0 mm electrode distance. This increment of the current could provide higher energy for surface charge redistribution and collision of oil droplets (Fouad et al., 2009), which ultimately led to improved performance in oil recovery and COD removal.
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 5. Effect of electrode distance on COD removal (A) and oil recovery (B) at various cycling times. Experimental conditions: voltage ¼ 10.0 V; flow rate ¼ 0.5 L/min; membrane pore size ¼ 5.0 mm.
3.4. Effect of membrane pore size on the performance of the EM technology Micromembranes with two different pore sizes (1.0 mm and 5.0 mm) were used in this experiment. As shown in Fig. 6, the 1.0 mm membrane performed slightly better in recovering oil during the first 60 min of cycling, but the difference became negligible thereafter. At 90 min of cycling, the amounts of COD removed and oil recovered were almost the same with these two methods (COD removal (mean value): 87.89% vs. 89.60%; oil recovery (mean value): 5085 vs. 5173 mg/L). This result likely occurred due to the majority of the oil droplets in the emulsion being larger than 5.0 mm. These parts droplets can easily collide with the electrode and take reaction when they passed through the micromembrane electrode with small pore size of 1.0 mm and 5.0 mm. This behavior creates the similar demulsification effect under the two micromembrane pore sizes of 1.0 and 5.0 mm. 3.5. Characterization of cutting fluid and the recovered substance by EM technology Fig. 7 shows microscopic images of the cutting fluid before and after demulsification with the electrochemical microfiltration technology. Floating oil droplets uniformly dispersed in the cutting fluid before the treatment with a diameter of 7.0 mm. After 90 min of demulsification in the designed device, small oil droplets coalesce to large oil droplet with the maximum diameter of 55.0 mm, which demonstrated the effectiveness of the electrochemical micromembrane action in demulsifying oil-in-water emulsion. These large oil drops were easy to recover in the suction filtration step. Small fractions of the oil droplets were so fine (smaller than 1.0 mm) that they can smoothly pass through the micromembrane
pore. This result can explain the minor difference in COD removal and oil recovery when using membranes with different pore sizes (1.0 mm and 5.0 mm). 3.6. Results of IR and CV IR spectrum results of crude oil and recovered oil were presented in Fig. 8. The functional groups that were identified include eOH characteristic absorption peak (3384.5 cm1), -CHn-stretching of the saturated bonds (2930.0e2853.2 cm1), the peak of the ester C¼O group (1740.0 cm1), eCOOe characteristic absorption peak (1559.6 cm1), -CHn-bending of the saturated bonds (1460.0 cm1), and -CHn-symmetrical bending of the saturated bonds (1377.1 cm1). The peak at 722.7 cm1 indicates the presence of long chain alkyl groups (-(CH2)n-, n 4). Thus, the crude oil was the mixture of fatty acid salts, ester and mineral oil. However, the recovered material was a mixture of ester and mineral oil. Fatty acid salts is often added to emulsion as anionic surfactant, aiming to strengthen emulsification effect (Guan et al., 2019). Hydroxyl is a hydrophilic group, which is inserted into the molecular structure of surfactant to improve its hydrophilicity (Zhou et al., 2017). There is an approximate conclusion that the anionic surfactant is a kind of fatty acid salt with the hydroxyl group. The electrochemical reaction is considered as the main reason resulting in dispersion of fatty acid salts and hydroxyl in recovered oil. As depicted in Fig. 9, an oxidation peak around 0.2e0.3 V (vs SCE) occurred in CV of the O/W cutting fluid. Trace oil droplets floated out of the emulsion surface only when this oxidation peak emerges in the cyclic voltammetry curve. Accordingly, collision of oil droplet with electrode would cause the separation of oil from oily wastewater (Zhang et al., 2017). When oil droplet collide electrode the hydrophilic group, eOH and eCOOe, is partially or
Fig. 6. Effect of membrane pore size on COD removal (A) and oil recovery (B) at various cycling times. Experimental conditions: voltage ¼ 10.0 V; flow rate ¼ 0.5 L/min; electrode distance ¼ 10.0 mm.
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 7. Morphology of the oil droplets in the cutting fluid before (a) and after (b, c, d) treatment with the electrochemical micromembrane technology. Experimental conditions: voltage ¼ 10.0 V; flow rate ¼ 0.5 L/min; electrode distance ¼ 10.0 mm; membrane pore size ¼ 5.0 mm; magnification ¼ 640X.
3.7. Mechanism of demulsification in the electrochemical micromembrane technology
Fig. 8. IR spectrum of the crude oil and recovered oil.
Fig. 9. Voltammogram of the O/W emulsion and the deionized water over the potential range from 1.6 to þ1.6 V at a scan rate of 50 mV/s.
absolutely oxidized, destroying the stability of oil droplet or changing the surfactant to hydrophobic material (Kim et al., 2015). This would be the reason why some absorption peaks of hydrophilic group (-OH and eCOOe) disappeared in the IR spectrum of recovered oil.
Based on the aforementioned results for the batch experiments, a possible pathway for separating oil from O/W cutting fluid wastewater via the EM technology is summarized here and is illustrated in Fig. 10. In a stable oil-water emulsion, the electrostatic repulsive force and hindrance from the interfacial film are the main coalescence barriers among the oil droplets (Ren and Kang, 2018). When two spherical droplets approach each other, the electric double layers of neighboring oil droplets overlap (Zhang et al., 2010), and a thin liquid film is formed between two approaching droplets (Yang, 2007). This behavior can be attributed to the barring effect of the electrostatic repulsive force (Fer), which is caused by some charges adsorbed on the oil surface. Followed the DLVO theory, this is one of the main coalescence barriers among the oil drops (Ichikawa, 2007). Moreover, the action of hindrance works by the surfactants that stop oil-droplets from coalescing by forming a tight and rigid interfacial film on an oil drop surface (Miao et al., 2015). However, the resistance in oil droplet coalescence by the electrostatic repulsive force and the interfacial film can be shrunken in the EM apparatus. As can be seen in Fig. 10(a), when the oil droplet adsorbed negative charged surfactant enters the anodic membrane pore, the electrostatic attractive force (Fea) between the membrane electrode and the oil droplet leads to the redistribution of surface charge adsorbed on oil droplet surface (Wu et al., 2018). The negative charged surfactant that is perpendicular to the direction of flow will migrate to the area parallel to the flow, and cluster around of electrode, which form the non-equivalent surface charge density distribution. These unstable oil droplets will coalesce, because the barriering energy (Fer) that hinders oil droplets approaching each other is reduced (Liu et al., 2015). Additionally, there is another possible mechanism (Fig. 10(b)) functioned for the separation of oil droplet in EM apparatus. When the oil droplet collides with anode, some hydrophilic groups of the surfactant, such as eOH and eCOOe, are destroyed by electrochemical reaction. Destruction of group change the amphiphilic molecular (R(M)) to the hydrophobic molecular (R’). Change in molecular hydrophilicity result in the disappearance of the interfacial film coating on oil droplet surface, which is beneficial to reduce the obstruction of coalescence among these oil droplets (Srivastava et al., 2017). Some finely unbroken droplets also coalesce and enlarge to large oil droplet with increasing cycle time until to breaking in EM system, under the function of the surface charge redistribution and surfactant molecular oxidation. 4. Conclusions Design of the novel electrochemical micromembrane technology was effective to demulsify oil-in-water emulsion. Results
Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698
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Fig. 10. Mechanism of electrochemical demulsification of O/W emulsions by the electrochemical micromembrane technology.
suggested that 87.89% COD was removed and 5173 mg/L oil was recovered at the optimum conditions of 10.0 V voltage, 90 min cycling time, 0.5 L/min flow rate, 10.0 mm electrode distance and 5.0 mm micromembrane pore size. Under these conditions, the diameter of oil droplets enlarged to 55.0 mm from an initial size of 7.0 mm. Raman infrared spectroscopy indicated the recovered substance was mostly mineral oil. Surface charge redistribution and destruction of hydrophilic group are likely the main demulsifying mechanisms for this electrochemical micromembrane technology. Declaration of competing interest None. Acknowledgement This work was supported by the National Natural Science Foundation of China (NSFC) (21876050), the National key research and development plans of special project for site soils (2018YFC1800600) and the Special Fund from State Key Joint Laboratory of Environment Simulation and Pollution Control (18K10ESPCT). References Assenheimer, T., Barros, A., Kashefi, K., Pinto, J.C., Tavares, F.W., Nele, M., 2017. Evaluation of microwave and conventional heating for electrostatic treatment of a water-in-oil model emulsion in a pilot plant. Energy Fuel. 31, 6587e6597. Bande, R.M., Prasad, B., Mishra, I.M., Wasewar, K.L., 2008. Oil field effluent water treatment for safe disposal by electroflotation. Chem. Eng. J. 137, 503e509. Cai, Q., Zhu, Z., Chen, B., Zhang, B., 2018. Oil-in-water emulsion breaking marine bacteria for demulsifying oily wastewater. Water Res. 149, 292e301. Chen, X., Huang, G., An, C., Feng, R., Yao, Y., Zhao, S., Huang, C., W, Y., 2019. Plasmainduced poly(acrylic acid)-TiO2 coated polyvinylidene fluoride membrane for produced water treatment: synchrotron X-Ray, optimization, and insight studies. J. Clean. Prod. 227, 772e783. Dick, J.E., Renault, C., Kim, B.K., Bard, A.J., 2014. Electrogenerated chemiluminescence of common organic luminophores in water using an emulsion system. J. Am. Chem. Soc. 136, 13546e13549. Dudchenko, A.V., Rolf, J., Russell, K., Duan, W., Jassby, D., 2014. Organic fouling inhibition on electrically conducting carbon nanotube-polyvinyl alcohol composite ultrafifiltration membranes. J. Membr. Sci. 468, 1e10. Feng, W., Yin, Y., Mendoza, M.D.L., Wang, L., Chen, P., Liu, Y., Cai, L., Zhang, L., 2018. Oil recovery from waste cutting fluid via the combination of suspension crystallization and freeze-thaw processes. J. Clean. Prod. 172, 481e487. Feng, W., Yin, Y., Mendoza, M.D.L., Wang, L., Chen, X., Liu, Y., Cai, L., Zhang, L., 2017. Freeze-thaw method for oil recovery from waste cutting fluid without chemical additions. J. Clean. Prod. 148, 84e89. Fouad, Y.O.A., Konsowa, A.H., Farag, H.A., Sedahmed, G.H., 2009. Performance of an electrocoagulation cell with horizontally oriented electrodes in oil separation compared to a cell with vertical electrodes. Chem. Eng. J. 145, 436e440.
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Please cite this article as: Chen, P et al., Demulsification and oil recovery from oil-in-water cutting fluid wastewater using electrochemical micromembrane technology, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118698