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Demulsification of a new magnetically responsive bacterial demulsifier for water-in-oil emulsions Xiang-Feng Huang, Yongjiao Xiong, Wan Yin, Lijun Lu, Jia Liu, and Kaiming Peng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00687 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 23, 2016
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Demulsification of a New Magnetically Responsive Bacterial Demulsifier for Water-in-Oil Emulsions Xiangfeng Huang†, Yongjiao Xiong†, Wan Yin†, Lijun Lu†, Jia Liu†, and Kaiming Peng*‡ †
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, Tongji University, Shanghai 200092, People’s Republic of China ‡
Tongji University, Shanghai 200092, People’s Republic of China
ABSTRACT A new, magnetically responsive bacterial demulsifier (MRBD) was prepared by grafting magnetite (Fe3O4) nanoparticles onto the surface of demulsifying cells. The demulsification process and performance of the MRBD were investigated using a Turbiscan system. At a mass ratio of magnetite to demulsifying cells of 0.2, the demulsification ratio of MRBD increased from 70% to 80% in the presence of a magnetic field, the demulsification half-life decreased from 3.0 to 2.0 h, and the transmitted intensity increased 4 times compared with the native bacterial demulsifier. Analysis of the demulsification process revealed that an increased mass ratio improved the extent of drop coalescence by adjusting the balance between the surface hydrophobicity and magnetic responsiveness. The magnetic field mainly increased the drop sedimentation rate. The MRBD exhibited good recyclability and could be reused for three cycles, which may minimize demulsification costs. The simple synthesis and highly efficient demulsification performance represents a significant improvement over existing techniques. 1.
INTRODUCTION Large amounts of emulsified wastes are produced in the shipping, cosmetic and, particularly, the 1 / 31
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petrochemical industries.1 Therefore, separating water from oil for the further recovery of saleable oil and purification of water is urgently needed. Treatment methods such as centrifugation, electrochemical techniques, membrane separation, chemical demulsification, and biotechnology, have been used to treat waste emulsion, among which the application of demulsifiers is the most widely used.2, 3 Despite their broad application, chemical demulsifiers would preferentially remain in the separated water phase, inevitably causing environmental contamination when discharged to the environment. Consequently, it is necessary to design more effective, cheap, environment-friendly demulsifiers.4, 5 Magnetic nanoparticles (MNPs) have attracted significant interest during the past decade because they can be recovered quickly and easily from complex multiphase systems by magnetic separation.6 In the demulsification field, these new magnetic materials, which possess demulsification functionality, can be classified into two groups: MNPs decorated with chemical demulsifiers and those modified with active substances. In the former group, Peng et al. grafted demulsifier ethyl cellulose (EC) onto MNPs (M-EC), which led to approximately 4% increase in the demulsification ratio and a separation rate approximately 10 times faster compared to those in the case of EC.7, 8 Li et al. grafted the commonly used chemical demulsifier 5010 onto MNPs (M-5010) and found that the demulsification ratio increased by 5%, with an oil removal rate higher than that of native 5010 used under the same process conditions.9 For another type of magnetic demulsifier, Liang et al. synthesized an oleic acid (OA)-coated magnetite (Fe3O4) nanoparticle and achieved a demulsification efficiency of 98% while oleic acid alone was not able to break the emulsion.10, 11 Moreover, dodecyltrimethoxysilane was used to modify Fe3O4 nanoparticles, achieving a 90% water removal ratio from a water-in-oil (W/O) emulsion.10 These results demonstrate that the demulsification ratio of a chemical demulsifier cannot be improved significantly by MNPs. Rather, the strength of MNPs lies in their acceleration of 2 / 31
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demulsification and the recoverability of the magnetic materials. For active substance-modified MNPs, results have indicated that MNPs endow the active substance with demulsification functionality in a magnetic field. However, to our knowledge, the performance of MNPs decorated with a bacterial demulsifier has not been reported to date. Regarding the demulsification mechanism of magnetic demulsifiers, most researchers have concluded that an external magnetic field enhances the coalescence of magnetically tagged water droplets in an emulsion; consequently, these droplets move downward, leading to separation of the aqueous and oil phases under the magnetic field.11, 12 The enhanced coalescence was confirmed by microscopy images in which the size of the water droplets at the bottom of container increased sharply after treatment with the magnetic demulsifier.8 However, most of the emulsions used in these studies focused on a low internal phase, with volume fractions lower than 20%.10 Without crowding and interaction occurring between drops, sparse droplets can be dragged downward effortlessly. Thus, sedimentation was the main demulsification process involved in low internal phase emulsions, whereas emulsions comprising a high internal phase may behave differently because of the complex interactions between droplets. Accordingly, both coalescence and sedimentation in the demulsification of high internal phase emulsions need further investigation. Biodemulsifiers are environmentally friendly and important in the separation of oil and water emulsions.13, 14 They can be classified into microorganisms with demulsifying ability which refers to demulsifying bacteria, such as Nocardia, Micrococcus, Alcaligenes, Ochrobactrum anthropi, and hybrid strains; and extracellular metabolic, such as fengycin, proteins, or lipopeptides.15, 16 Previous research has revealed demulsifying bacteria (intact cells) as excellent biodemulsifiers. Compared with extracellular demulsifiers and chemical demulsifier, demulsifying bacteria are more 3 / 31
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suitable for the demulsification of complex emulsion systems, owing to their more complicated demulsification
mechanism.
The
strain
of
Alcaligenes
sp.
S-XJ-1
was
isolated
from
petroleum-contaminated soil of the Karamay Oilfield (XinJiang, China) and exhibited excellent demulsifying ability for crude oil in our previous study.17 To investigate the demulsification performance of the strain, a model emulsion with high stability was developed and the bacterial demulsifier possessed demulsification ability for this emulsion.18 It has been tested in our previous study that the demulsification ability of the demulsifying bacteria came from the combination action of cell wall-bound compounds and the intact cell structure.19 Magnetic nanoparticles have been used to enhance the demulsification ability of the chemical demulsifier.8 The combination of MNPs and bacterial demulsifiers might therefore enhance the demulsification performance of bacterial demulsifiers. This combination of MNPs and microorganisms has been used in pathogen detection, water purification, strengthening of microbial desulfurization, and pollutant degradation,20-22 thereby suggesting that combination of MNPs and demulsifying cells might be possible. Here, a new magnetically responsive bacterial demulsifier (MRBD) was synthesized to strengthen the demulsification of a high internal phase emulsion. The macroscopic effects of mass ratio and magnetic field on the bacterial demulsifier performance were investigated using a Turbiscan tower system. In addition, the coalescence and sedimentation behaviors of droplets in the demulsification process were analyzed. This work represents an advance from MNP-enhanced chemical demulsifiers (molecular behavior) to MNP-enhanced demulsifying cells (soft colloidal particles). The results are expected to facilitate the future development of industrial applications of bacterial demulsifiers. 2. MATERIALS AND METHODS 2.1. Preparation of MRBD. The magnetic Fe3O4 nanoparticles were prepared using the chemical 4 / 31
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co-precipitation method.23 Briefly, 10.8 g of FeCl3•6H2O and 3.98 g of FeCl2•4H2O were dissolved in 150 mL of 0.2 mol HCl solution by vigorous mechanical stirring (800 rpm) under nitrogen gas. A total of 180 mL of ammonium hydroxide (25%) was added rapidly, and the resulting suspension stirred for a further 6 h. The black product was collected by a magnet and thoroughly washed with deionized water to remove excess ammonium hydroxide. Alcaligenes sp. S-XJ-1 was cultivated in a modified mineral salts medium (MMSM) containing 4% (v/v) paraffin as the carbon source for 7 d in a fermentation tank. After cultivation, the bacteria were isolated by centrifugation at 18,000 g for 10 min. Three washing steps with n-hexane and separation by centrifugation were performed to remove residual oil. Dry bacterium was obtained using a freeze drier (Scientz-10N, Ningbo, China) at −50 °C for 24 h and utilized for subsequent experiments. The magnetic Fe3O4 nanoparticle suspension was ultrasonically dispersed and then adequately dispersed MNPs were added to tubes containing 800 mg/L of bacterial demulsifier to obtain a mass ratio (M/B) series of 0.1, 0.2, 0.3, 0.4, and 0.5. The tubes were shaken on a vortex mixer at 2000 rpm for 90 s and subsequently put on a shaker at 130 rpm for another hour. The MRBD product was accordingly obtained for subsequent use. 2.2. Characterization of MRBD. The bonding efficiencies of demulsifying cells with MNPs were evaluated by measuring the optical density (OD) (at wavelength 600 nm) of the solutions before and after magnetic trapping.24, 25 The bacterial concentration was adjusted to a desired level (16 g/L), MNPs suspension with different concentration was prepared as well to explore the bonding ability between demulsifying bacteria and MNPs under a series of mass ratios. The OD600 nm of demulsifying bacteria was determined by measure the absorbance of the initial bacteria suspension (usually needed to dilute the suspension to proper concentration range for absorbance test). A 2 mL of demulsifying bacteria 5 / 31
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suspension and MNPs suspension (2 mL) were blended in a container and the container was then placed within a voxtex rotator for 90 s at 2000 r/min. An external magnet was placed near the containers for 60 seconds. The supernatant was then carefully pipetted into a cell to measure its OD600
nm
using a spectrophotometer. The bonding efficiencies of the bacteria and MNPs were calculated from
the decrease of turbidity relative to the original bacteria suspension before magnetic capture. The following relation was used to quantify the bonding ratio: bonding ratio = (ODbefore–ODafter) / ODbefore
(1)
where ODbefore is the optical density of demulsifying cells in the original suspension and ODafter is the optical density of demulsifying cells after magnetic trapping. The morphology of MRBD was characterized by transmission electron microscopy (TEM) (JEM-2100F, Tokyo, Japan). To prepare TEM samples, the MRBD was washed with ethanol three times, and dispersed in ethanol after washing was complete. Two or three drops were placed on a carbon-coated copper TEM grid (200 mesh) and then left to dry in air23.26 The distribution of Fe3O4 nanoparticles on the demulsifying cells was determined directly from the TEM images. Magnetic hysteresis loops of the samples were measured using a Physics Property Measurement System (Quantum Design, California, America) at 35°C. Saturation magnetization was obtained by applying a magnetic field from −60000 to +60000 Oe. The zeta potentials (ZPs) of MRBD and the demulsifying cells were measured using a Zetasizer NanoZ analyzer (Malvern Instruments, Malvern, UK). The demulsifying cells were suspended in distilled water with an initial optical density of 0.8–1.0 (at wavelength 600 nm). The pH was adjusted to approximately neutral. The reported values are based on three samples that were measured in triplicate. 6 / 31
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The surface hydrophobicity of the MRBD was evaluated by its water contact angle (WCA). First, 10 mL of the prepared MRBD or demulsifying cell suspension was filtered under vacuum using a mixed nitrocellulose membrane (0.45 µm) to attain an approximate 0.5mm thick cell layer on the membrane. After 6 h on an agar plate (containing 10 g/L agar and 100 mL/L glycerol) to achieve homogeneous wetting, the membrane was air dried. After placing a 20 µL drop of milli-Q water on the bacterial lawn, the contact angles at both the left and right sides were measured on a SL200B instrument (Shanghai Solon Technology Science Company, Shanghai, China). Each contact angle value was the average of at least five independent measurements. The WCA was obtained by a circular fitting analysis using CAST3.0 software (Solon Technology Company, Shanghai, China). 2.3. Demulsification Test and Analysis Methods. A W/O model emulsion
was prepared as
described in a previous study.19 Aviation kerosene (80 mL containing 1.526 g Tween 80 and 0.074 g Span 80) and distilled water (40 mL) were mixed at 10000 rpm by a high-speed emulsifying machine (WL-500CY; Shanghai Wei Yu Mechanical and Electrical Manufacture, Shanghai, China) for 3.5 min. Distilled water (80 mL) was lowly added at ∼2 mL/s when the mixing started. The blank emulsion had an emulsion breaking ratio of
