Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Recyclable Functional Magnetic Nanoparticles for Fast Demulsification of Waste Metalworking Emulsions Driven by Electrostatic Interactions Kaiming Peng,†,‡ Yongjiao Xiong,‡ Lijun Lu,‡ Jia Liu,‡ and Xiangfeng Huang*,‡ Post-Doctoral Research Station, and ‡College of Environmental Science and Engineering, State Key Laboratory of Pollution Control and Resource Reuse, Ministry of Education Key Laboratory of Yangtze River Water Environment, Shanghai Institute of Pollution Control and Ecological Security, Tongji University, Shanghai 200092, People’s Republic of China
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S Supporting Information *
ABSTRACT: Complex multiphase waste metalworking emulsions, which contain large amounts of surfactants and mineral oil, are difficult to treat efficiently by traditional molecular demulsifiers. We synthesized one type of functionalized magnetic nanoparticles grafted with amino groups (M@ NH2) to treat waste metalworking emulsions from different mechanical processing factories and investigated the demulsification mechanism. The M@NH2 showed an excellent demulsification performance, achieving 85−97% chemical oxygen demand (COD) removal for most of the waste metalworking emulsions. The results indicated the advantage of the three-step demulsification process (adsorption of M@ NH2 on droplets, droplet coalescence, and the magnetic transfer of droplets in the magnetic field) over traditional two-step demulsification (adsorption of MNPs on droplets and the transfer of droplets in the magnetic field). In addition, electrostatic interactions between M@NH2 and surfactants were confirmed as the driving force of demulsification. Isothermal titration calorimeter quantified the interactions at the molecular level; the enthalpy was 1.83 kJ/mol, affinity coefficient between M@NH2 and the surfactant was 1.5 × 103, and the stoichiometric number of the surfactant and M@NH2 was 11.5. This research provides a new perspective for the treatment of waste metalworking emulsions. KEYWORDS: Magnetic nanoparticles, Metalworking wasted emulsion, Demulsification, Drop migration, Drop coalescence, Magnetic field
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secondary pollutants.8,9 Therefore, the development of effective demulsifiers is of great importance. As compared to molecular chemical agents, particles have attracted significant attention recently,10−12 especially in waste emulsions treatment due to their fast oil−water separation and recycling abilities.13,14 Particles used in demulsification can be classified into carbon base materials (graphene,15 carbon nanotubes16) and functional magnetic nanoparticles (MNP decorated with OA,17 EC,18 demulsifier 501019). The carbon base materials are especially effective for crude oil emulsion because they can form π−π interactions or n−π interactions with active substances (asphaltenes or resins) in crude oil.15,20 The demulsification ratio of more commonly used magnetic nanoparticles could reach as high as 90% within a few minutes. In addition, the MNPs can be reused 5−10 times without any decrease in the demulsification performance.13,14 However,
INTRODUCTION Large volumes of waste emulsions are produced by lubrication, cooling, surface cleaning, and corrosion prevention in metalworking industry.1 The effluents usually contain an emulsified oil concentration of 1500−90 000 mg/L, chemical oxygen demand (COD) of 60 000−200 000 mg/L, and large amounts of surfactants.2,3 Because anionic surfactants are the most commonly used surfactants, the emulsion droplets are negatively charged. The layer of surfactant film on the oil− water interface and the electrostatic repulsion between oil droplets make the emulsion extremely stable.4 The treatment of these complex waste emulsions generally includes demulsification pretreatment and subsequent biological treatment.5 The demulsification pretreatment is a key stage that determines the effectiveness of postbiological treatment.6,7 Flocculating agents, acid, alkali, salt, and chemical demulsifiers are the most commonly used demulsification methods in industrial applications. However, most of these agents have poor universality and are consumed at extremely high dosages. They produce large amounts of flocs, which are considered © XXXX American Chemical Society
Received: January 31, 2018 Revised: May 17, 2018 Published: June 15, 2018 A
DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
were measured using a Physics Property Measurement System (Quantum Design, CA) at 25 °C. Saturation magnetization was obtained by applying a magnetic field from −70 000 to +70 000 Oe. The zeta potentials of M@NH2 and M@SiO2 were measured using a Zetasizer NanoZ analyzer (Malvern Instruments, Malvern, UK). The water contact angle of M@NH2 was used to characterize its surface hydrophobicity. The prepared M@NH2 was pressed to form a slice (by means of a vacuum compressor). Contact angles were then measured with water on an SL200B instrument (Shanghai Solon Technology Science Company, Shanghai, China). Emulsion Preparation and Characterization. Six practical waste metalworking emulsions were acquired from different mechanical processing factories including a metalworking emulsions production factory, a precision electromechanical factory, a photoelectric production factory, an injection molding factory, and an aluminum cutting plant. Oil/water (O/W) model emulsions were formulated using paraffin as the base oil. Cetyltrimethylammonium bromide (CTAB), sodium dodecyl benzenesulfonate (SDBS), and polyoxyethylene sorbitan monooleate (Tween-80) were used as cationic, anionic, and nonionic surfactants, respectively, to prepare three types of O/W emulsions. The emulsions were prepared with 0.5% (w/w) base oil in deionized water with an emulsifying machine. A series of model emulsions with, 0, 0.25, 0.50, 1, 2, and 10 times the critical micelle concentration (CMC) of SDBS were prepared by the same method. The practical emulsions were characterized by COD, droplet size distribution, and zeta potential. The COD of the emulsions was measured by an ultraviolet spectrophotometer after digestion treatment with a Hach COD device. The size distribution (by volume percent) and zeta potential of the emulsions were measured using a Malvern 3000 particle sizer and zeta potential analyzer (Malvern Instruments, UK). The model emulsions were characterized by the same methods as practical emulsions. Emulsion stability of model emulsions was characterized with a Turbiscan TOWER (Formulaction, L’Union, France). Twenty milliliters of the emulsion was transferred to a flat-bottomed glass cylindrical cell (6 cm in height and 2.5 cm in diameter) for measurement within 24 h at 35 °C. The stability described as backscatter and transmitted light is shown in Figures S1 and S2. Demulsification Performance and Recyclability Evaluation. The demulsification tests were performed with the addition of M@ NH2 or M@SiO2 to the practical or model emulsions (3.2 g L−1) using a vortex equipment at 2000 rpm, 25 °C for 60 s. Practical emulsions were diluted 10 times before being used for demulsification, while model emulsions were used directly. The mixture was allowed to settle for 5 min. A magnet then was placed near the bottle for 10 min. The supernatant was collected for the COD test. The COD removal ratio was calculated by dividing the original COD of the emulsions by the decrease in COD relative to the original emulsion before the magnetic capture. M@NH2 was collected using a magnet for redemulsification, where two groups of experiments were conducted. One group of recovered M@NH2 was thoroughly washed with methanol and deionized water, respectively, while the other group did not undergo any treatment. Recovered M@NH2 was used for demulsification tests for another cycle. Micro-observation of Emulsion Structure. Fluorescence microscopy was used to observe the aggregation and coalescence of droplets under the effects of M@NH2 and the responsiveness of emulsion droplets in a magnetic field. To hold the emulsion samples, a specially made grooved glass with a circular hole (diameter = 10 mm) was prepared. The paraffin oil was dyed with Nile red (3 mg/L) before being emulsified. The mixture of emulsion and M@NH2 was placed in the circular groove, and a coverslip was placed over the sample. A magnet was placed near the sample during the observation to trace the magnetic droplets using a fluorescence microscope (Nikon Eclipse 80i). Distribution characteristics of M@NH2 in the emulsion during the demulsification process were evaluated using cryo-scanning electron microscopy (Cryo-SEM, Phenom ProX). Emulsions and the M@NH2 suspension were mixed with a vortex equipment at 2000 rpm, 25 °C
they are limited to the treatment of simple model emulsions characterized by low internal phase (lower than 0.5%21,22) and low surfactant content (milligrams per liter or even surfactantfree23). Furthermore, the oil phase contains mostly short-chain hydrocarbons with low viscosity, such as hexane, toluene, and kerosene.19,22 The application of particles, especially MNPs, to complex waste metalworking emulsions has been scarcely reported. The surface wettability, surface charge, and particle size of MNPs are key factors that affect the demulsification performance. Especially the wettability, which can be adjusted by grafting active molecules on MNPs, is the main method used to achieve efficient demulsification. The most efficient demulsification performance can be attained when the threephase contact angles of MNPs are around 90°.17,23 The proper wettability of MNPs facilitates their distribution on the oil− water interface or on dispersed droplets to change the property of the interface. A magnetic field can expedite the transfer of MNPs; the droplets with MNPs can be magnetically isolated from the continuous phase rapidly. However, these studies do not consider the key role of the emulsifier in stabilizing the emulsion process. Because the droplets of metalworking emulsions are negatively charged, we aimed to design magnetic nanoparticles with positive surface charges for treatment of waste metalworking emulsions. With the aid of electrostatic interactions, MNPs can combine surfactants efficiently, reducing the stabilizing effect of the surfactant on the emulsion droplet and the electrostatic repulsion between the droplets and promoting the coalescence of the droplets. With the external magnetic field, MNPs accelerate the transfer of the emulsifier and the droplet. The directional migration and aggregation of droplets during the transfer process further promote demulsification. In this study, functional magnetic nanoparticles with positive charge were synthesized to investigate their demulsification performance for waste metalworking emulsions. The role of magnetic nanoparticles on demulsification was investigated by observing the droplet behavior and characterizing the interactions between MNPs and surfactants to provide a new perspective for treating waste metalworking emulsions.
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MATERIALS AND METHODS
Synthesis and Characterization of Functional Magnetic Nanoparticles. Fe3O4 MNPs were prepared by chemical coprecipitation. Briefly, 10.8 g of FeCl3·6H2O and 3.98 g of FeCl2·4H2O were dissolved in deionized water under nitrogen gas. A total of 180 mL of ammonium hydroxide (25%) was added rapidly, and the resulting suspension was stirred for 6 h. SiO2 MNPs (M@SiO2) were synthesized by hydrolysis with silicon alkylation reagents.24 The amino group was grafted on M@SiO2 with triethylenetetramine, named M@NH2. Two grams of M@SiO2 powder was dispersed in 100 mL of toluene solution, which contained 2 mL of silane coupling agent (KH-560) solution, for 6 h. The powder collected by a magnet was washed with toluene three times and redispersed in 100 mL of toluene containing 1.0 mL of triethylenetetramine for another 6 h. The powder collected by a magnet was washed with ethanol and dried for subsequent use. The morphology and element composition of the M@NH2 were characterized by transmission electron microscopy (TEM-EDX, JEM2100F, Tokyo, Japan). X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer with a copper target at 40 kV and 40 mA with 2θ value ranging from 20° to 80°. Thermal analysis experiments were conducted by Discovery TGA apparatus (New Castle, DE) operated at a heating rate of 10 °C/min under nitrogen atmosphere. Magnetic hysteresis loops of the samples B
DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 1. Characterizations of M@NH2. (A) TEM image of M@NH2, (B) XRD pattern of M@NH2, (C) EDX spectra of M@NH2, and (D) magnetization curve of M@NH2.
Table 1. Characterization of Waste Emulsions from Different Sources source of waste emulsions
COD (mg/L)
pH
size (nm)
zeta potential (mV)
surface tension (mN/m)
critical micelle dilution
COD removal ratio
Francool metalworking fluid Meipei precision electromechanical Engle injection molding process aluminum plant cutting fluid photoelectric production photoelectric production
161083 45083 182916 178833 99750 195916
7.8 6.2 9.2 9.5 6.5 6.4
242 352 221 121 1126 844
−54.8 −42.2 −59.6 −70.4 −41.2 −41.8
30.7 33.4 29.4 32.8 26.3 26.2
109 75 110 63 92 90
85.1% 95.6% 66.7% 41.2% 97.9% 89.2%
for 60 s. The mixed emulsions were frozen with liquid nitrogen rapidly, and one piece of a frozen sample was subjected to imaging in a frozen chamber at −25 °C. Characterization of Interactions between Surfactant and M@NH2. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to confirm the interactions between M@NH2 and the surfactant. Surfactant solutions with concentrations 0, 0.25, 0.5, 1, 2, and 10 times the CMC of SDBS were prepared. M@NH2 was dispersed in the surfactant solution using a vortex equipment at 2000 rpm, 25 °C for 60 s. M@NH2 then was collected by a magnet and vacuum-dried at −40 °C for FTIR (Nicolet 5700; Thermo Electron Corporation, Beverly, MA) and TGA (Discovery TGA, New Castle, DE). An isothermal titration calorimeter (Affinity ITC-LV, TA) was used to characterize the interaction between SDBS and M@NH2. The SDBS solution (7.2 mM) was titrated into the M@NH2 suspension (0.025 mM) at 25 °C at the stirring rate of 125 rpm. The injection interval was 200 s to allow the system to equilibrate. The results of a blank experiment with the SDBS solution (7.2 mM) titrated into the deionized water were subtracted from the experimental data before further analysis. NanoAnalyze (TA Instruments) was applied to analyze and fit the ITC data. All solutions were degassed for 10 min prior to the experiments to ensure that no bubbles were present.
(Figure 1A). The crystal structure of the M@NH2 was characterized by XRD (Figure 1B). The diffraction peaks of the M@NH2 are similar to that of the Fe3O4 magnetic naniparticle, which can be indexed to the cubic spinel structure.25 The XRD results indicated that modification of Fe3O4 magnetic naniparticle did not change its crystal structure. The existence of Si and N element was confirmed by element analysis (EDX) of the M@NH2, indicating the successful grafting of SiO2 and triethylenetetramine on the surface of Fe 3 O 4 (Figure 1C). The mass ratio of triethylenetetramine that grafted on M@SiO2 was 3.05% based on TGA results. Also, the magnetic saturation value of M@NH2 was 36 emu/g (Figure 1D). On the basis of a successful grafting of amino groups characterized by FTIR and TGA in our previous study, the zeta potential of M@NH2 was +40 mV (pH = 7.0) with a water contact angle of 20°.26 Thus, the M@NH2 used in this study was a hydrophilic nanoparticle with a positive charge. Characteristics of these emulsions are shown in Table 1. The COD of the emulsions ranged from 45 000 to 190 000 mg L−1, representing many concentrated oil droplets. The zeta potential of the emulsion droplets varied from −70 to −40 mV, indicating that the emulsions were extremely stable under electrostatic repulsion. Critical micelle dilution (CMD), which was obtained by diluting the emulsions until the surface tension remained unchanged, could be used to characterize the surface activity of complex emulsions; a larger CMD means a stronger surface activity of the surfactant in the emulsion. The
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RESULTS AND DISCUSSION Application of M@NH2 for Waste Metalworking Emulsions. The demulsification performance of M@NH2 for six waste emulsions from different sources in the metalworking industry was investigated. The M@NH 2 magnetic nanoparticle tended to aggregate, and the size of the single M@NH2 was 15−20 nm based on a TEM image C
DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 2. Demulsification performance and recycling ability of M@NH2 for waste metalworking emulsions. (A) Demulsification process, (B) redemulsification results, and (C) relationship of demulsification performance and surface charge of emulsion droplets.
demulsification performance for these complex and recalcitrant emulsions. For chemical demulsifier (take 7520 as an example), which is more commonly used in the treatment of wasted emulsions that were produced in the practical industrial process, the COD removal ratio of the six wasted emulsions was 10−65%, and it was difficult to realize fast water−oil separation. The addition dosage of chemical demulsifier was high, and it was not able to be reused, producing wasted flocs, which was considered secondary pollutant. To our knowledge, in the particle category, only the mineral particles30 and blast furnace dust (BFD)31 have been reported for the treatment of metalworking emulsions. The artificial zeolite was used to treat one type of steel emulsion with 38 700 mg/L COD and achieved 90% COD removal within 2 h at pH = 1.0 and 60 °C.30 At pH = 3.0 and 5.0, the BFD removed 90− 98.5% of the TOC in 3 h for a metalworking emulsion (COD = 28 300 mg/L).31 The demulsification and COD removal were strongly affected by pH. Some emulsions can also be demulsified under acidic conditions as a result of combined demulsification with acids.32 However, acid addition has strict equipment requirements, produces a large amount of salt, and has a negative impact on subsequent processes. As compared to these two studies,30,31 our M@NH2 can achieve efficient demulsification without adjusting pH, showing a greater application potential. Because of its magnetic responsiveness, M@NH2 showed a faster separation rate and more convenient recovery than traditional particles. On the basis of the characterization of emulsions, the demulsification performance of M@NH2 showed positive correlations with COD, droplet size, the surface tension of the emulsion, and the surface charge of emulsion droplets. The relationship between the performance of M@NH2 and the surface charge of emulsion droplets was the most significant (Figure 2C). When the zeta potential of emulsion droplets ranged from −70 to −40 mV, more negative charge of emulsion droplets led to a lower demulsification performance. From −50 to −40 mV, M@NH2 could achieve 80−95% COD removal. The strong repulsion force between the droplets would make the emulsion more stable if the droplets were
CMD of these emulsions was in the range of 90−100, demonstrating that the surface activity of the surfactants was extremely high. Except for one emulsion from photoelectric production process, the size of the rest emulsion droplets ranged from 100 to 844 nm, implying they were nanoemulsions. The strong activity of the surfactant and the nanosized emulsion droplets made the emulsion extremely stable and difficult to be treated efficiently. M@NH2 was subject to the blending, sedimentation, and magnetic separation processes, showing different demulsification performance for emulsions with various sources (Figure 2A). A 40−97% demulsification ratio was achieved after 10 min of magnetic separation, where at least 85% COD removal could be achieved for most of the emulsions (Table 1). Redemulsification experiments conducted with the emulsion from Francool metalworking fluid showed that the COD removal ratio decreased quickly if the M@NH2 was not washed after demulsification (Figure 2B). For the other group, the COD removal ratio decreased more slowly (from the fourth round) with a wash procedure for the recovered M@ NH2. The demulsification efficiency decrease is likely caused by the loss of M@NH2 during the washing and the recovery process. We aim to eliminate the M@NH2 loss and increase the recovery frequency in a future study. MNPs with special wettability or interface activity were used to treat various types of emulsions, where high efficiency (more than 90% demulsification ratio), fast separation, and good recoverability were achieved. 27−29 However, the emulsions used in these studies were mostly simple model emulsions, with low internal phase (0.01−0.20%), low surfactant content (0.01−0.50 g/L, even surfactant-free), and short-chain hydrocarbons (toluene, hexane, and kerosene) used as the oil phase.19,22 For metalworking emulsions, mineral oil was the most commonly used oil phase, which was extremely viscous. The oil droplets were nanosized, and the total oil volume was in the range of 1−5%. Moreover, the emulsions contain large amounts of surfactants and additives. The properties of waste metalworking emulsions make them challenging to treat. M@NH2 still exhibited a comparable D
DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering extremely negatively charged.33 The bridge effect of M@NH2 for droplets was limited, decreasing the aggregation and coalescence rates of the droplets. Decreasing the surface potential of oil microdroplets is essential to reducing the electrostatic repulsion and enhancing droplet flocculation.31 Thus, the positively charged particles are suitable for most metalworking emulsions, the surface of which is negatively charged. Effects of Electrostatic Attraction Force on Demulsification Performance. To investigate the effects of electrostatic attraction force on the demulsification process, positively charged M@NH2 and negatively charged M@SiO2 were used to treat the model paraffin−water emulsions stabilized with cationic (CTAB), anionic (SDBS), and nonionic (Tween 80) surfactants. The surface charges of M@NH2 and M@SiO2 were +40 and−28 mV, respectively. Demulsification results demonstrated that M@NH2 showed a higher demulsification performance for the emulsion stabilized by SDBS (−53.8 mV) followed by the emulsion stabilized by Tween 80 (−31.8 mV), while the effect on the emulsion stabilized by CTAB (+50.1 mV) was poor (Figure 3). The M@SiO2 exhibited the opposite
decreased from 92% to 10% when the surfactants in the emulsion increased from 0.125 to 5.0 g/L. These results suggest that the surfactant concentration has a significant impact on demulsification, where emulsions with high surfactant concentrations are more difficult to be treated. O/ W emulsions are stabilized by steric barriers of surfactants and electrostatic repulsion between oil droplets.1 An increase in surfactant concentrations leads to a significant increase in the stability of small droplets.36 Emulsion droplets were more negatively charged (from −52.0 to −72.3 mV) as surfactant concentrations in the emulsions increased from 0.125 to 5.0 g/ L (Table 2). There was a positive correlation between the demulsification performance of M@NH2 and the surface charge of emulsion droplets. This relation was in accordance with the results of practical waste metalworking emulsions, which further confirmed the effects of opposite surface charges on demulsification. Analysis of Demulsification Process Characteristics. The distribution of M@NH2 in the emulsion was observed using cryo-SEM to reveal its role during the demulsification process. Because the surfactants self-assembled on the interface with their hydrophilic head in the water and hydrophobic tail in oil droplets, the oil droplets were coated with a layer of hydrophilic film. We found that the M@NH2 was mainly distributed in the water phase and at the water−oil interface as small aggregates, indicating that M@NH2 was transferred from the continuous phase to the interface (Figure 4A). The aggregates were scattered in the water phase, while they were compact at the water−oil interface. The M@NH2 aggregation was located at the interfaces (white points in the SEM images in Figure 4B) and even covered the whole droplet surface (Figure 4C). Cryo-SEM is a suitable method to characterize the emulsion and is mainly used to track the microstructures of emulsions under different conditions.37,38 After M@NH2 was transferred from the continuous phase to the oil−water interface, it attached on the surface of the droplet. The attachment induced the wrinkle and bump of the interface or the droplet surface, which could result in changes in the droplet behavior. The particle adsorption at the oil−water interface is an important step in demulsification. The adsorption is driven by the interaction energy (hydrophobic interaction), calculated from the particle surface free energy based on the acid−base theory presented in previous studies.39,40 In this study, the electrostatic interaction was the main reason for the attachment. To investigate the changes in the emulsion microstructure caused by M@NH2 attachment, the coalescence and transfer of droplets dragged by a magnet were observed using a fluorescence microscope. The original emulsion is shown in Figure S4A. After the addition of M@NH2, three different coalescence modes were detected: drop−drop coalescence, Ostwald ripening, and drop−interface coalescence (Figure S3B−D). The three coalescence modes cooperated to change the emulsion microstructure, inducing an apparent reduction in the number of droplets and an increase in droplet size. The
Figure 3. Demulsification performance of M@NH2 and M@SiO2 for emulsions stabilized with different types of surfactants.
results: a higher demulsification capacity for emulsions stabilized with CTAB than the emulsions stabilized by Tween 80 or SDBS. These results confirmed that M@NH2 shows a better performance for emulsion droplets with negative charges. Previous research used electrostatic force in the design of a three-dimensional positively charged sponge and a ceramic membrane to effectively separate oil-in-water emulsions stabilized by anionic, nonionic, and cationic surfactants.34,35 In this study, electrostatic interactions between M@NH2 and surfactants or oil droplets were the reason for demulsification. Because anionic surfactants are the most frequently used types in metalworking emulsions, the demulsification performance of M@NH2 focused on SDBS-stabilized emulsions with various surfactant concentrations (0−5 g/L). The COD removal ratio was near 100% after the addition of M@NH2 to the emulsion, which was free of surfactants, while it
Table 2. Effects of SDBS Concentration on Demulsification Performance of M@NH2 concentration of SDBS (g/L) zeta potential (mV) COD removal ratio (%)
0
0.125
0.25
0.5
1
5
24.2 98.1%
−52.0 92.9%
−52.7 62.5%
−54.7 36.8%
−55.1 32.4%
−72.3 3.6%
E
DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 4. Distributions of M@NH2 in emulsions by cryo-SEM. (A) Distribution of M@NH2 in the continuous phase, (B) M@NH2 at the oil− water interface, and (C) M@NH2 on the droplet surface.
Figure 5. Fluorescence microscope images of droplet behavior under effects of magnetic field from spatial and temporal perspectives. (A−C) Behavior of droplets that locate different distances from the magnet, where S1, S2, and S3 represent the distances between the magnet and droplets, S1 < S2 < S3; (D−F) behavior of droplets under effects of magnetic separation with different periods of time, (D) T = 0 s, (E) T = 2 s, and (F) T = 4 s.
Figure 6. Characterization of combination between M@NH2 and SDBS with concentrations ranging from 0 to 10 CMC of SDBS. (A) FTIR of M@NH2 that adsorbed SDBS, and (B) TGA of M@NH2 that adsorbed SDBS.
droplets deform completely and coalesce clearly, forming a large oil phase (Figure 5A−C). Regarding the temporal differences, large numbers of droplets traveled from the right to the left in only 4 s (Figure 5D−F). Eventually, oil droplets were separated from the water quickly. The demulsification process experienced the attachment of M@NH2 on oil droplets and the coalescence and migration of the droplets. Most researchers agree on a two-step separation mode and attached great importance to the magnetic
attachment of M@NH2 neutralized the electrostatic repulsion between the droplets and disturbed the surfactant film on the droplet, resulting in droplet coalescence. Under the effects of a magnet, the droplets with M@NH2 on the surface dragged by the magnetic force were transferred quickly. Because of the attraction force from the magnet, droplets pushed and pressed each other, causing further coalescence. Droplets with different distances from the magnet displayed a different extent of deformation, where the closest F
DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering separation process.22,41 In the first step, functional magnetic nanoparticles attach to the droplets because of proper hydrophobicity and impart their magnetic properties on dispersed droplets. In the second step, under an applied magnetic field, the magnetically dragged droplets are isolated from the continuous phase rapidly.18,42 This two-step process is suitable for simple emulsions with a low internal phase; without any crowding or interaction occurring between the droplets, sparse droplets can be dragged downward effortlessly. In our previous study, we investigated the performance of a magnetically responsive bacterium for emulsions comprising a higher internal phase.8,26 Here, for complex emulsions that have a stable surfactant network and microstructure, the coalescence was confirmed as an important stage of demulsification. Thus, a certain extent of droplet coalescence is necessary to increase the magnetic responsiveness of the droplets and create more room for droplet transfer. Interactions between M@NH2 and Surfactant. The interactions between the surfactants and M@NH2 played an important role in emulsion destabilization. To confirm the interactions, we designed a series of experiments, and the results were characterized by FTIR, TGA, and ITC. Another study reported that the characteristic peaks of SDBS at 1007, 1034, 2850, and 2925 cm−1 indicated the presence of the −SO3 group and aliphatic C−H stretching mode at the long alkyl tail of SDBS.43 As compared to the spectrum of SDBS, the characteristic peaks were found at 1460, 2853, and 2928 cm−1, indicated as the C−C stretch and C−H stretch in the spectrum of M@NH2 (Figure 6A). FTIR confirmed the combination of M@NH2 and surfactant at the functional group level and showed that M@NH2 could adsorb SDBS in a wide range of concentrations. TGA characterized the combined mass of SDBS on M@NH2 further (Figure 6B). The adsorption mass of SDBS on M@NH2 increased along with the increasing of surfactant concentration from 0 to 1.0 CMC and then decreased when it exceeded 1.0 CMC, implying that the adsorption might reach saturation at 1.0 CMC. The saturation value might provide an evaluation of the capacity of M@NH2 for treating emulsions. All of these results confirmed the existence of interactions between M@NH2 and surfactants with different concentrations. ITC was used to investigate the M@NH2−surfactant interactions at the molecular level by analyzing the binding constant (K), the number of binding sites (N), and thermodynamic binding parameters. This technique is mainly used to characterize the interactions between molecules in solution, for example, proteins, polysaccharides, DNA, lipids, surfactants, and minerals.44−46 It is also used to measure the enthalpy changes (ΔH) associated with the interactions in colloidal systems, for example, the dissociation of surfactant micelles47 and binding of surfactants to polymers.48 In this study, the SDBS solution was titrated into the M@NH2 suspension, and the titration reached a plateau after approximately 10 titration steps (Figure 7). ΔH of the process was determined to be 1.83 kJ/mol surfactant by a fit, indicating an endothermic effect of the electrostatic interaction between SDBS and M@NH2. The K and N values between the surfactant and M@NH2 were 1.5 × 103 and 11.5, respectively. As compared to the reported interaction strengths that ranged from 1.0 × 103 to 1.0 × 106, the K value showed a moderate binding strength between SDBS and M@NH2. As for N, 11.5 was a rather low combination ratio when considering the size of M@NH2 and the molecule.
Figure 7. Titration of SDBS solution to the M@NH2 suspension by ITC.
Among all of the parameters, ΔH was the most commonly used index to analyze the interaction process and the mechanism. ΔH values of the typical hydrogen bonding between asphaltenes and inhibitors or resins were reported to be from −10 to −40 kJ mol−1, and permanent dipole interactions were from −4 to −20 kJ mol−1.49 The ΔH of the physical bonds such as the van der Waals forces and hydrogen bonds between asphaltenes and C5PeC11 was from −1.5 to −0.3 kJ/mol.50 The ΔH values of hydrophobic binding and electrostatic interactions between SDS and polyelectrolyte were 2.9 kJ mol−1 (pH = 9) and 4.9 kJ mol−1 (pH = 4), respectively.51 Only a few studies have investigated the electrostatic or hydrophobic interactions between particles and molecules. Here, we quantified the electrostatic interactions between M@NH2 and surfactants at the molecular level using ITC. In this study, M@NH2 was synthesized for successful demulsification of waste metalworking emulsions. The M@ NH2 showed an excellent demulsification performance, achieving 85−97% COD removal for most waste metalworking emulsions. The results confirmed the advantage of the threestep demulsification process (adsorption of M@NH2 on droplets, droplet coalescence, and the magnetic transfer of droplets in the magnetic field) over traditional demulsification. Electrostatic interactions between M@NH2 and surfactants were the driving forces for demulsification, and ITC was introduced as a new method to quantify the interactions at the molecular level.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00499. Backscattered light and transmitted light of an emulsion with different types of surfactant; backscattered light and transmitted light of an emulsion with different concentrations of surfactant; and fluorescence microscope images of three different coalescence modes of emulsion droplet (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone/fax: +86 21 65982399. E-mail:
[email protected]. ORCID
Kaiming Peng: 0000-0003-0633-1364 Xiangfeng Huang: 0000-0003-4898-7951 G
DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant numbers 51608375, 51478325), the China Postdoctoral Science Foundation (grant numbers 2016M591711, 2017T100311), the Shanghai Institute of Pollution Control and Ecological Security, and the China Major Science and Technology Project of Water Pollution Control and Management, China (grant number 2017ZX07202-003). We gratefully acknowledge the support of TA Instruments for use of the isothermal titration calorimeter in Shanghai.
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DOI: 10.1021/acssuschemeng.8b00499 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
