Article pubs.acs.org/est
Immobilizing Water into Crystal Lattice of Calcium Sulfate for its Separation from Water-in-Oil Emulsion Guangming Jiang,† Junxi Li,† Yunliang Nie,† Sen Zhang,§ Fan Dong,† Baohong Guan,*,‡ and Xiaoshu Lv*,† †
Engineering Research Center for Waste Oil Recovery Technology and Equipment, Ministry of Education, Chongqing Technology and Business University, Chongqing 400067, China ‡ Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China § Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: This work report a facile approach to efficiently separate surfactant-stabilized water (droplet diameter of around 2.0 μm) from water-in-oil emulsion via converting liquid water into solid crystal water followed by removal with centrifugation. The liquid− solid conversion is achieved through the solid-to-solid phase transition of calcium sulfate hemihydrate (CaSO4. 0.5H2O, HH) to dihydrate (CaSO4·2H2O, DH), which could immobilize the water into crystal lattice of DH. For emulsion of 10 mg mL−1 water, the immobilizationseparation process using polycrystalline HH nanoellipsoids could remove 95.87 wt % water at room temperature. The separation efficiency can be further improved to 99.85 wt % by optimizing the HH dosage, temperature, HH size and crystalline structure. Property examination of the recycled oil confirms that our method has neglectable side-effect on oil quality. The byproduct DH was recycled to alpha-HH (a valuable cemetitious material widely used in construction and binding field), which minimizes the risk of secondary pollution and promotes the practicality of our method. With the high separation efficiency, the “green” feature and the recyclability of DH byproduct, the HH-based immobilizationseparation approach is highly promising in purifying oil with undesired water contamination.
■
recycled. Typically, Lead et al.15,16 developed a hydrophobic and magnetically- response PVP-coated Fe3O4 nanoparticle (NP) adsorbent, which performed a quantitative (near 100%) oil removal in separating oil−water mixture and could be easily collected by external magnetic field. However, most of current adsorbents and membranes are limited to treat layed water/oil mixtures17,18 or oil-in-water emulsions,19 whereas their separation efficiency in surfactant-stabilized water-in-oil emulsions is still unsatisfactory due to the large oil kinematic viscosity, the small droplet size and the high stability of the emulsion, which prevent the encapsulated water phase to be adsorbed or sequestrated. To our knowledge, no adsorbent has been reported to be efficient for water-in-oil emulsion separation, while for membrane filtration technology, Zhang et al. ever fabricated a superhydrophobic and superoleophilic polyvinylidene fluoride (PVDF) membrane, which could efficiently separate types of micro- and nanosize water-in-oil emulsions with a maximum flux of 100 000 L m−2 h−1
INTRODUCTION Separation of water or oil from their liquid mixture is crucial for many chemical and environmental processes, especially for the recycling of oil that contains undesired water1 and the remediation of water system polluted by oil dumping, spill and leakage.2,3 Current strategies to achieve the separation include the use of adsorbent that can selectively anchor one phase but exclude the other when exposed to the water−oil mixture (adsorption technology), or membrane that only allow one phase to pass through (membrane filtration technology).4 Usually, the adsorbent and the membrane have to be tuned with proper structure and surface chemical properties (e.g., porosity, wettability, and affinity to a specific phase) to enhance the selectivity and separation efficiency.5,6 For example, the adsorbents can be made with the porous structure (foam,7 sponges,8 aerogels,9 and particle/polymer composites10) to increase the surface area and the adsorption capacity. The membrane can be engineered with desirable mesh,11,12 textile,13 and fabric14 to balance its performances in separation, mechanical strength and flow rate (reduced pressure drop). Besides, the absorbent or membrane can be synthetically tailored with other functionalities, enabling absorbent or membrane itself to be conveniently analyzed, collected, and © XXXX American Chemical Society
Received: March 7, 2016 Revised: May 15, 2016 Accepted: June 20, 2016
A
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology bar−1.20,21 However, the membrane stability, antifouling property, the filtration flux and the handling capacity are still a headache issue for the real application of the membrane filtration technology.22 Here, we develop a novel approach to efficiently separate the emulsified water from oil phase (transformer oil was used as a proof-of-concept example with the room-temperature kinematic viscosity of 9.909 mm2 s−1, which is about 10 times larger than 1.006 mm2 s−1 of water) with the aid of the calcium sulfate hemihydrate particles (CaSO4·0.5H2O, HH). HH is a wellknown cementitious material and could be easily prepared by mixing of the Ca2+ and SO42− precursor solutions,23 or the dehydration of gypsum mineral.24 When mixing HH with the emulsion, HH will react with H2O and phase transit into calcium sulfate dihydrate (CaSO.2H2O, DH) through the reaction:25 CaSO4 ·0.5H 2O + 1.5H 2O → CaSO4 ·2H 2O
to remove glycerol and Na2EDTA. After 3 times’ wash, the products were dried at 60 °C for 3.0 h to remove water and acetone for further use. The size of HH NPs was controlled by varying the water volume in Solution I: 50 mL water generates ∼2.0 μm HH NPs. Single crystalline HH prisms with a length/width of 3 μm/ 0.5 μm were prepared in reverse microemulsions of water/nhexanol/CTAB by Kong’s method.26 Typically, two precursor micro -emulsions of Ca2+ and SO42− were prepared in two flasks before synthesis. One microemulsion was composed of 10 mL water with 0.1 mol kg−1 CaCl2, 35 g CTAB and 40 mL n-hexanol (Microemulsion I), while the other was composed of 10 mL water with 0.1 mol kg−1 H2SO4, 35 g CTAB and 40 mL n-hexanol (Microemulsion II). When heating the both microemulsions to 95.0 °C, microemulsion II was quickly mixed with microemulsion I, and the mixture was then maintained at 95.0 °C for 1.0 h to form HH particles. The solid product was collected, washed with boiling water four times, and rinsed with acetone before 2.0 h of drying at 60.0 °C. Preparation of Water-in-oil Emulsions. The surfactantstabilized water-in-transformer oil emulsion was obtained following a reported method.21 0.02 g of Span80 surfactant and 0.2 g of water were added into 20 mL transformer oil in sequence. The mixture was then intensively stirred for 3 h followed by intensive sonication for 3.0 h to form a stable milky emulsion (See Supporting Information (SI) Figure S1). This emulsion could maintain stable over 3 weeks, during which no de-emulsification and precipitation were observed. Separation of the Water-in-Oil Emulsion. The separation process was carried out as follows: controllable amount of dried HH powders were added into 20 mL surfactant-stabilized water-in-oil emulsion in a vial of 25 mL. The suspension was magnetically stirred for 30 min and then kept still for the solid phase settling to the bottom. The oil was collected after the centrifugation treatment of the superatant. The separated DH was washed by hexane ro remove the oil and dried at 60 °C for further treatment. Characterization. Solid sample was subjected to powder Xray diffraction analysis (XRD, D/Max-2550 pc, Tokyo, Japan) and thermogravimetry/differential scanning calorimetry analysis (TG/DSC, NET- ZSCH STA 409 Luxx, Selb/Bavaria, Germany) for phase identification. XRD analysis was performed with CuKα radiation at a scanning rate of 5 o min−1 in 2θ range of 10−60°. For TG/DSC analysis, 20 mg of dry sample was sealed in an Al2O3 crucible with a lid and scanned at 10 °C min−1 under N2 flow. The particle morphology was examined by scanning electron microscopy (SEM, HITACHES-570, Hitachi, Tokyo, Japan), and the particle size distribution and its specific surface area were determined by a laser particle size analyzer (Mastersizer 2000, Malvern, England) after dispersing them in anhydrous ethanol with an ultrasonic bath. The water content (mg mL−1) and Ca2+ concentration (ug g−1) in emulsion was determined by Carr’s moisture titrator (899 coulometer+860 KF Thermoprep., Metrohm, Herisau, Swiss) and multielement oil analyzer (MOA II plus, MOA Instrumentation, Inc. Levittown, PA), respectively. Acid values of the uncontaminated and recycled oil were tested by potentiometric titrator (905 titrando, Metrohm, Herisau, Swiss), whereas their surface tension and kinematic viscosity were measured by an automatic surface/interface tensiometer (JWY-200, Chengde, China) and viscometer (HDSYP 1003-IA, Qingdao, China) according to China National Standard GB/T6541-86 (Petroleum products-mineral oils:
(1)
Such a phase-transition process can immobilize the water into the crystal lattice of DH, and the water can then be separated from the oil along with the removal of DH. Compared with the membrane filtration technology, this immobilization-separation process is capable to continuously separate the emulsion without the concern of the durability, handling capacity and fouling. In addition, the inherent cementitious property of HH will potentially render the DH crystals interlock together to form a large solid paste, which makes it easy to remove them from oil phase. Our results show that the HH-based immobilization-separation process could facilely and cost-effectively separate water from surfactantstabilized water-in-oil emulsion with neglectable side-effect on oil quality. Systematical tuning HH dosage, size and crystalline structure could further optimize the water removal efficiency up to 100 wt %. Overall, this work presents a new and efficient approach to sepatate water-in-oil emulsion, which could potentially find application in the cleanup and recycling of many petroleum products (especially lubricating oil and transformer oil) involving some undesired water, which are easy to emulsify, leading to a dentrimental effect on oil workability.
■
EXPERIMENTAL SECTION Materials. Analytical reagent grade CaCl2, Na2EDTA, (NH 4 ) 2 SO 4 , H 2 SO 4 , Cetyltrimethylammonium bromide (CTAB), hexane, n-hexanol and glycerol were all purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The surfactant Span80 was obtained from Alfa Aesar. All chemicals were used without further purification. Synthesis of Calcium Sulfate Hemihydrate (HH) Particles. Monodisperse polycrystalline HH NPs were synthesized through an EDTA-mediated crystallization process following our previous work.23 Typically, two homogeneous precursor solutions of Ca2+ and SO42− were prepared in two flasks before synthesis. The solution of SO42− was composed of 10 mL H2O, 200 mL glycerol, 3.5 mM Na2EDTA and 38 mM (NH4)2SO4 (solution I), while that of Ca2+ was prepared by dissolving CaCl2 in 50 mL glycerol to ensure the Ca/S molar ratio to be 1:1 (solution II). After heating the both solutions to 90 °C, solution II was immediately injected into solution I, which turns into turbidity quickly. After 30 min’ reaction, the formed solid products were separated by centrifugation (6000 rpm/3 min), purified by dispersing them into a mixture of 5 mL water (5 mL) and acetone (40 mL), and centrifuged once more B
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Determination of interfacial tension of oil against water-ring method, or referring to the ISO Standard 6295-1983) and GB/ T265-88 (Petroleum products: Determination of kinematic viscosity and calculation of dynamic viscosity, or referring to the ISO Standard 3104-1994), respectively.
■
RESULTS AND DISCUSSION Separation of Water from the Emulsion by HH. As reported, an EDTA-mediated crystallization of HH through the mixing of precursor solutions of Ca2+ and SO42− in glycerol will generate monodisperse polycrystalline HH NPs.23 In Figure 1a,
Figure 2. (a−d) Digital images of the whole separation process; (e−f) Optical microscope image of the water-in-oil emusion before and after water separation; (g) SEM images of the collected solid phase after water separation; (h) XRD patterns of the as-synthesized and used HH.
Figure 1. SEM image (a) and XRD pattern (b) of the as-synthesized HH NPs.
the SEM image shows that the as-synthesized NPs present a uniform elliposoid shape with a length of ∼600 nm and a width of ∼300 nm. The particle size distribution analysis in SI Figure S2 further confirms the monodispersity of the NPs with an average size of 538 nm. The surface is quite rough (inset of Figure 1a) since the ellipsoid is formed through an EDTAmediated assembly of smaller HH nanocrystals.20 The XRD pattern in Figure 1b confirms these ellipsoid NPs to be HH phase by the characteristic diffraction peaks at 2θ = 14.70, 25.62, 29.69, 31.89, 42.25, and 49.36 (JSPDS card No. 0410244). The HH elliposoid NPs in the form of dry powders were then added into the water-in-oil emulsion. To remove 200 mg H2O in 20 mL emulsion (10 mg mL−1), the theoretical dosage of HH should be 1.07 g according to immobilization reaction in Equation 1. After HH addition, the mixture was extensively stirred for 30 min at room temperature to form a suspension (Figure 2a), and then kept still. After 1 h, most of the solid phase gradually settled into the bottom of the vial under gravity force (Figure 2b). The solid became more compact after 2 h’ precipitation, while the solution turned to be transparent during this process (Figure 2c). The supernatant was further centrifuged at 4000 rpm for 2 min, which finally yielded a highly transparent liquid (Figure 2d). Figure 2e and 2f compare the optical microscope images of the water-in-oil emulsion before and after the water removal, where the bright spot denotes the water droplet. It is clearly seen that the water droplet has a mean size of ∼2.0 μm, and its number decreases significantly after the water/oil separation, suggesting most of the water has been removed. SEM examination of the collected solid phase in Figure 2g shows that the elliposoid NPs had been totally tranformed into monoclinic cuboid microparticles with a large size of ∼3 μm × 3 μm × 1 μm. The XRD pattern in Figure 2h indicates these particles are mainly DH with a small proportion of HH left in them. It should be noted that the DH crystals are 200 times larger than the as-synthesized HH elliposoid NPs, and simultaneously the DH crystals interlock with each other due to the inherent cementitious property of
HH, both of which help the collection of the DH from the viscous oil by free precipitation or centrifugation. To determine the separation efficiency and the side-effect of separation process on oil quality, the water content, Ca2+ concentration in oil as well as some basic property of the recycled oil, such as acid number, kinematic viscosity, surface tension and oxidation resistance, were analyzed and compared with those of the uncontaminated oil. The water content in recycled oil was measured to be 0.413 mg mL−1, indicating the high efficiency of HH-based water separation process (∼95.87 wt % of water removed ((10−0.413)/10 × 100 wt % = 95.87 wt %)). It is also indicated that 1 g of HH particles can clean up 20 mL emulsion with a water content of 10 mg mL−1, suggesting the high separating capacity of our HH particles. The concentration of Ca2+ in recycled oil was measured to be 6.54 μg g−1. Considering the inorganic HH and DH could not be dissolved into the organic oil, the Ca content should originate from the residue HH/DH particles. The low Ca2+ concentration suggested that the solid phase has been totally removed. The basic property of the uncontaminated oil and the recycled oil are compared in Table 1. Kinematic viscosity of the recycled oil is measured to be 9.657 mm2 s−1, which is slightly lower than that of the uncontaminated oil (9.909 mm2 s−1). Acid number and surface tension of the recycled oil are 0.299 mg KOH g−1 and 25.086 mN m−1, respectively, which are higher than those of the uncontaminated oil. The resulted differences in above properties should be ascribed to the residual water (its lower kinematic viscosity and higher surface tension at 1.006 mm2 s−1 and 71.9 mN m−1 will decrease the kinematic viscosity and raise the surface tension of the oil) and the involved impurity from HH powders (the Ca2+ from the HH NPs may increase the acid number through consuming KOH during acid number test by forming Ca(OH)2). The oxidation resistance of oil is evaluated by its acid number and surface tension change when it undergoes a 72 h’s Cu wirecatalyzed oxidation by pure oxygen.27 Both the uncontaminated C
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Table 1. Basic Property of the Uncontaminted (Original) And Recycled Oil oxidation resistance sample
water content/mg mL−1
kinematic viscosity/mm2 s−1
acid number/mg KOH g−1
surface tension/mN m−1
acid number/mg KOH g−1
surface tension/mN m−1
original recycled
0.005 0.413
9.909 9.657
0.2833 0.2989
24.233 25.086
0.3996 0.4565
26.122 28.393
77.58 mol % DH in collected solid means 77.58 mol % HH has been utilized to immobilize water, i.e. 77.58 wt % water could be theoretically removed by the immobilization-separation process. Considering the original value of 10 mg mL−1, the water content in recycled oil should be 2.242 mg mL−1 (10−10 × 77.58 wt %), which is, however, 5 times higher than the actual value (0.413 mg mL−1). We speculate that the water adsoption by DH solid and the centrifugation process also contribute to the water separation from the emulsion. The temperature-dependent separation efficiency and HH utilization rate should be resulted from the balance between the temperature-dependent thermodynamics and kinetics for the phase transition from HH to DH. From the phase-transition diagram of CaSO4 phase (including DH, HH, and calcium sulfate anhydrite, i.e. AH) in pure water,33 it is found that at a temperature lower than the DH-HH phase-equilibrium temperature of 104.7 °C, DH is more stable than HH, and HH inclines to transform to DH when exposed to free water. A high temperature can usually facilitate the transformation by activating the reaction and promoting the ion and water diffusion, leading to a shortened operating period, enlarged DH content in solid and a high separating efficiency. However, when the temperature gets close to 104.7 °C (for example at 100 °C), DH and HH nearly reach phase equilibrium, where HH tends to retain its phase,25 and thus a longer time is required to trigger the phase transition, resulting in a lower separation efficiency. To pick up the turnover temperature between 75 and 100 °C, four more temperatures of 80, 85, 90, and 95 °C with a smaller interval were chosen to examine the separation efficiency and the utilization rate of HH. Plots of the separation efficiency and the utilization rate of HH versus the temperature were presented in SI Figure S4. With the temperature increasing from 75 to 100 °C, both the separation efficiency and the utilization rate of HH show the similar trend to increase first, reach the peak values of 99.85 wt % and 98.12 mol % at 86.7 °C, and then decrease significantly to 25.42 wt % and 13.59 mol %, respectivle. Accordingly, 86.7 °C is the optimal operating temperature, where the thermodynamics and kinetics of the phase-transition of HH to DH could be ideally balanced. Effect of the HH Dosage. According to the results in Table 2, only 77.58 mol % of HH can be utilized during the room- temperature immobilization-separation process. To achieve a higher separation efficiency at room temperature, more HH than the theoretical value is actually required. Here, HH with the dosage of 1.25 (1.07 × 1.25 = 1.34 g), 1.50 (1.07 × 1.50 = 1.61 g) and 1.75 (1.07 × 1.75 = 1.87 g) times of the theoretical one (1.07 g) were added into the emulsion, respectively. Table 3 shows that with increasing HH dosage from 1.07 to 1.87 g, the water content in recycled oil declines from 0.413 to 0.109 mg mL−1, and the water separation efficiency rises from 95.87 to 98.91 wt %, both of which indicate that an increase in HH dosage will promote the water separation efficiency.
oil and the recycled oil turned to light yellow after the oxidation (see SI Figure S3) due to the formation of carbonyl/carboxyl group in oil molecule.28 The results in Table 1 show that after the oxidation, acid number and surface tension of the recycled oil increase to 0.4565 mg KOH g−1 and 28.393 mN m−1, while those of the uncontaminated oil turn to be 0.3996 mg KOH g−1 and 26.122 mN m−1, respectively. The increase in acid number and the surface tension should be arisen from the formation of free water and carbonyl/carboxyl group in oil molecule during oxidation.29,30 The more significant change observed in the recycled oil suggests that its oxidation resistant is slightly worse than the uncontaminated oil, possibly due to the residual water and the involved impurity (such as HH NP and Ca2+) from HH, which will serve as the catalyst/active site to accelerate the oxidation of oil molecule.31,32 Despite the fact that the quality of recycled oil through our “immobilization-separation” approach is still worse than the uncontaminated one, the result in Table 1 shows that the difference in their basic properties are very small, and the recycled oil could actually be considered as almost the same with the uncontaminated oil. Effect of the Reaction Temperature. Considering the temperature-dependent phase-transition kinetics of HH to DH, four temperatures of 25, 50, 75, and 100 °C were first investigated respectively to illustrate the relationship between the temperature and the separation efficiency as well as the utilization rate of HH. The separation efficiency was evaluated by the elapsed period to achieve the separation as well as the water content in recycled oil. The utilization rate of HH was evaluated by the DH molar ratio in collected soild phase, which was calculated from the overall crystal water content in solid (See the calculation process in SI). A larger DH content means more HH has been utilized to immobilize water, that is, a higher HH utilization rate. Table 2 shows that with an Table 2. Temperature-Dependent Water Separation Efficiency and Utilization rate of HH temperature/ °C
operating period/min
water content/mg mL−1
separation efficiency/ wt %
DH content in solid/mol %
25 50 75 100
30 10 1 8.0 h
0.413 0.128 0.042 7.458
95.87 98.71 99.58 25.42
77.58 85.73 95.92 13.59
increased tempearature from 25 to 75 °C, the required operating period is reduced from 30 to 1 min, the water content in recycled oil drops from 0.413 to 0.042 mg mL−1, while DH content in final solid phase increases from 77.58 to 95.92 mol %, indicating the promoting effect of temperature on both the immobilizing and separating efficiency. However, when further rise in temperature to 100 °C, the water content in recycled oil maintains at a high value of 7.458 mg mL−1 and only 13.59 mol % of HH are utilized. It should be noted in Table 2 that the actual separation efficiency is higher than that achieved by immobilization process. For example, at 25 °C, D
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Table 3. Effect of HH Dosage on the Water Removal Efficiency HH dosage/g
water content/ mg mL−1
separation efficiency/ wt %
DH content in collected solid/ mol %
water separated by immobilization/ wt %
1.07 1.34 1.61 1.87
0.413 0.228 0.122 0.109
95.87 97.72 98.78 98.91
77.58 69.44 61.72 54.24
77.58 86.80 92.58 94.92
Table 3 also shows that the water removed by immobilization-separation process increases significantly from 77.58 to 94.92 wt % when more HH is used. The increase in water immobilization efficiency suggests that a larger HH dosage could immobilize more water into the crystal lattice, which should be responsible for a near 100% removal of water from emulsion even at a room temperature. However, the decreasing DH content in solid suggests that the utilization rate of HH is decreased, which means that large amount of HH materials are actually wasted (near 100−54.24 = 45.76 mol % are wasted when 1.87 g HH is used). To balance the HH utilization rate and the separation efficiency, appropriate HH dosage should be employed in practical application. Effect of the Particle Size and Stucture of HH. The particle size and crystalline structure of the HH material are also found to be important in determining the separation efficiency. To clarify the size effect, monodisperse polystalline HH microspheres with a larger diameter of ∼2.4 μm were prepared (See the SEM image in Figure 3a and the particle size distribution in SI Figure S2) and used to separate the water from emulsion (10 mg mL−1 water). With the same dosage of 1.07 g and processing condition at room temperature, the ∼2.4 μm HH microspheres show a separation efficiency of 85.75 wt %, lower than that of the smaller HH nanoellipsoids (95.87 mol %). The recycled oil appears still a little opaque (Figure 3b), and its water content is 1.425 mg mL−1. The XRD and TG analyses of the final DH hydrate in Figure 3e and 3f find that there is still a large portion of HH unreacted. Considering the crystal water content of 15.84 wt % in collected solid phase (obtained from TG pattern in Figure 3f), it could be calculated that about 34.58 mol % of HH microspheres are unreacted, which is 1.5 times higher than that of 22.42 mol % for the HH nanoellipsoid demonstrated above. The reduced separation efficiency and the lower HH utilization rate should be related to its larger size of 2.4 μm and smaller surface area of 2.82 m2 g−1 (see SI Figure S2), compared to the HH nanoellipsoid with an average size of 518 nm and a specific surface area of 12.51 m2 g−1 (sSee SI Figure S2). With a larger size and a smaller specific surface area, the HH particles will expose less active sites to immobilize the water, and meanwhile the core region of the HH particles will not be so easy to react with the water (the HH in the surface tends to form a compact shell to hinder water’s in-diffusion), both of which will decrease the separation efficiency. To clarify the crystalline structure effect, single crystalline HH prisms with a length/width of 3 μm/0.5 μm were prepared according to Kong’s method26 (See the SEM image in Figure 3c and the particle size distribution with an average size of 2.8 μm in SI Figure S2). Figure 3d shows the treated emulsions after the water immobilization by 1.07 g single crystalline HH prisms at room temperature. The more turbid solution indicates the lower separation efficiency of the single crystalline HH prisms, which is even worse than the ∼2.4 μm
Figure 3. (a) SEM image of the polycrystalline HH microspheres; (b) Digital image of the recyled oil by HH microspheres; (c) SEM image of the single crystalline HH prisms; (d) Digital image of the recyled oil by HH prisms; (e) XRD patterns of the as-synthesized polycrystalline HH microsphere and single crystalline HH prisms, as well as their hydrates formed during the water separation; (f) TG patterns of the hydrate from the polycrystalline HH microsphere and the single crystalline HH prism, respectively.
polycrystalline HH. The XRD analysis shows that obvious characteristic diffraction peaks from HH are still detectable in the hydrate, as seen in Figure 3e. The TG analysis in Figure 3f determines the crystal water content in the collected solid to be 12.98 wt %, suggesting about 54.01 mol % of the HH are unused. Compared with the polycrystalline HH particles, the single crystalline HH has a more perfect crystallinity, which could be roughly reflected by the higher intensity of characteristic diffraction peaks for the single crystalline HH than that for the polycrystalline HH with a nearly the same size, as seen the XRD patterns in Figure 3e. HH with a high crystallinity usually has a stable crystal structure, which thus requires a larger energy for the water to transfer into the crystal lattice, resulting in the lower immobilization capability and separation efficiency. Economic Evaluation. Economy is a crucial factor that should be considered when a method for oil/water separation is under development, which is here evaluated by calculating the total cost for the chemical and energy consumptions of HH systhesis and for the following disposal of byproduct DH, and then compared to the recently developed Fe3O4 adsorption techniques. SI Table S1 gives the total cost for synthesis of every 1.0 ton HH particles, and 1.0 ton Fe3O4 particles according to Duan’s,10 Palchoudhyury’s15 and Mirshahghassemi’s work.16 The results show that the systhesis of every 1.0 ton HH NPs will take $16,895.3 USD, which is much lower than those for Fe 3 O 4 in Duan’s work of 437,764.7 USD, Palchoudhury’s work of 4,023,094.2 USD, and Mirshahghassemi’s work of $4,953,538 USD. When considering the removal E
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Figure 4. (a) Schematic illustration of the immobilization process by HH for the water separation from the emulsion. (b) Crystal structural transition from HH to DH with a view in c-direction.
industrial practicality of our method for real water/oil separation. Mechanism. Based on the above experimental analysis, the “immobilization-separation” process could be schemed as Figure 4a. When HH particles are dropped into the emulsion, they permeate into or adsorb the water droplets. Then, the phase-transition of HH to DH proceeds spontaneously, immobilizing the water molecules into the crystal lattice of DH. Along with the immobilization process, DH crystals grow larger and interlock together, and settle to the bottom of the container. The immobilized water is then separated from oil along with the removal of DH solid. Figure 4b schemes the crystal structural transition of HH to DH along with the water immobilization with a view in cdirection. Interestingly, DH and HH have the same structural motif: (−Ca−SO4−Ca−SO4−) chain, but differentiate in how these chains are assembled and where the water molecules lie. In HH, the chain aligns along the c-direction, and every six chains form a hexagonal channel to accommodate the water molecules. In DH, the chains align to form perfect layers parallel to (010) and the water molecules lie between every two layers, forming a sandwich-like structure. It is clear in the pink region of Figure 4b that every six sulfate ions could fix 12 water molecules in DH, which is four times more than that in HH (only three). The alignment transition of the (−Ca−SO4−Ca− SO4−) chains in crystals from HH to DH provides a larger volume capacity to hold more water molecules, which should be responsible for the capability of HH-DH phase-transition to immobilize the water droplet. Compared with the common adsorption technology, the water immobilization into the crystal lattice could fix the water molecules more firmly and separate them without the concern of desorption. The reactivity of HH particles with the water determines the water immobilization efficiency, which could be tuned by the temperature as well as the size and crystalline structure of HH particles. An appropriate temperature will provide a large driving force to trigger the phase transition of HH to DH and simultaneously accelerate the ion/water diffusion, which could improve the immobilization effiency. HH with a small size usually has a large surface area and a high surface reactivity, which is more possible to adsorb and immobilize a larger number of water molecules. Polycrsytalline HH particles with a
capacity, 1.0 g HH NPs could clean up 20 mL emusion with 10 mg mL−1 water (∼1 wt %), which is nearly equivalent to that of the Fe3O4 adsorbents, 1.0 g of which could handle 10.5 g (2 wt %) or 5.5 g (5 wt %) emulsions.10 In other words, to separate the same amount of emulsion, the dosage of HH is comparable to that of Fe3O4. Overall in the aspects of material synthesis and usage, our approach was more cost-effective compared to the magnetic Fe3O4 adsorbent-based separation technologies. Besides for the material synthesis, the cost for the following disposal of the considerable amount of byproduct DH should also be included to evaluate the economy of our approach. According to eq 1, the theoretical yield of DH is close to 63.7 g when treating 1 L emulsion (10 mg mL−1). To deal with these DH, one alternative route was proposed to recycle the DH to α-HH through a hydrothermal method,34 which is processed in an aqueous salt solution of CaCl2+MgCl2+KCl at 90 °C. It should be noted that the as-collected DH solid usually contains some attached oil (see SI Figure S7a), which should be washed by hexane before recycling (See the cleaning efficiency in SI). Figure S5a shows the SEM image of the as-synthesized HH particles, which shows a typical prism shape of HH with a length of ∼15 μm and a width of ∼1 μm. The XRD analysis in SI Figure S5b confirms the prisms to be HH phase by its characteristic diffraction pattern. TG analysis in SI Figure S5c further determines the crystal water content in the assynthesized HH to be 6.53 wt %, which is very close to the theoretical value of 6.21 wt % for pure HH. The DSC pattern in SI Figure S5d exhibits an endothermic peak at 153 °C followed by an exothermal peak at 177 °C, confirming the HH to be the α-form. The successful transition of the collected DH into αHH confirms the feasibility of DH recycling. Since α-HH is an important and valuable cementitious material widely used in construction and binding field and could be sold at a least price of ∼308 USD ton−1, one may even make some profits from this recycling. SI Table S2 gives a preliminary estimation of the total cost to recycle 1.0 ton of DH to α-HH, including the cost for the chemical and the energy consumption. The total value is calculated to be 78.65 USD ton−1, which indicates there will be a net profit of about 229.35 USD (308−78.65 USD ton−1) to recycle every one ton of DH. The recyclability of DH and the profitability of DH recycling will siginificantly improve the F
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
composites for oil-water separation. Angew. Chem., Int. Ed. 2016, 55 (3), 1178−1182. (9) Wu, Z. Y.; Li, C.; Liang, H. W.; Chen, J. F.; Yu, S. H. Ultralight, flexible, and fire-resistant carbon nanofiber aerogels from bacterial cellulose. Angew. Chem., Int. Ed. 2013, 52 (10), 2925−2929. (10) Duan, C.; Zhu, T.; Guo, J.; Wang, Z.; Liu, X.; Wang, H.; Xu, X.; Jin, Y.; Zhao, N.; Xu, J. Smart enrichment and facile separation of oil from emulsions and mixtures by superhydrophobic /superoleophilic particles. ACS Appl. Mater. Interfaces 2015, 7 (19), 10475−10481. (11) Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D. A super-hydrophobic and super-oleophilic coating mesh film for the separation of oil and water. Angew. Chem., Int. Ed. 2004, 43 (15), 2012−2014. (12) Wang, B.; Guo, Z. pH-responsive bidirectional oil-water separation material. Chem. Commun. 2013, 49 (82), 9416−8. (13) Zhang, J.; Seeger, S. Polyester materials with superwetting silicone nanofilaments for oil/water separation and selective oil absorption. Adv. Funct. Mater. 2011, 21 (24), 4699−4704. (14) Zhou, X.; Zhang, Z.; Xu, X.; Guo, F.; Zhu, X.; Men, X.; Ge, B. Robust and durable superhydrophobic cotton fabrics for oil/water separation. ACS Appl. Mater. Interfaces 2013, 5 (15), 7208−7214. (15) Palchoudhury, S.; Lead, J. R. A facile and cost-effective method for separation of oil-water mixtures using polymer-coated iron oxide nanoparticles. Environ. Sci. Technol. 2014, 48 (24), 14558−14563. (16) Mirshahghassemi, S.; Lead, J. R. Oil recovery from water under environmentally relevant conditions using magnetic nanoparticles. Environ. Sci. Technol. 2015, 49 (19), 11729−11736. (17) Wang, J.; Shang, L.; Cheng, Y.; Ding, H.; Zhao, Y.; Gu, Z. Microfluidic generation of porous particles encapsulating spongy graphene for oil absorption. Small 2015, 11 (32), 3890−3895. (18) Gao, X.; Zhou, J. Y.; Du, R.; Xie, Z. Q.; Deng, S. B.; Liu, R.; Liu, Z. F.; Zhang, J. Robust superhydrophobic foam: a graphdiyne-based hierarchical architecture for oil/water separation. Adv. Mater. 2016, 28 (1), 168−173. (19) Wang, Z. J.; Wang, Y.; Liu, G. J. Rapid and efficient separation of oil from oil-in-Water emulsions using a Janus cotton fabric. Angew. Chem. 2016, 128 (4), 1313−1316. (20) Zhang, W.; Shi, Z.; Zhang, F.; Liu, X.; Jin, J.; Jiang, L. Superhydrophobic and superoleophilic PVDF membranes for effective separation of water-in-oil emulsions with high flux. Adv. Mater. 2013, 25 (14), 2071−2076. (21) Shi, Z.; Zhang, W.; Zhang, F.; Liu, X.; Wang, D.; Jin, J.; Jiang, L. Ultrafast separation of emulsified oil/water mixtures by ultrathin freestanding single-walled carbon nanotube network films. Adv. Mater. 2013, 25, 2422−2427. (22) Ma, Q. L.; Cheng, H. F.; Fane, A. G.; Wang, R.; Zhang, H. Recent development of advanced materials with special wettability for selective oil/water separation. Small 2016, 12 (16), 2186−2202. (23) Jiang, G. M.; Chen, Q. S.; Jia, C. Y.; Zhang, S.; Wu, Z. B.; Guan, B. H. Controlled synthesis of monodisperse alpha-calcium sulfate hemihydrate nanoellipsoids with a porous structure. Phys. Chem. Chem. Phys. 2015, 17 (17), 11509−11515. (24) Jiang, G. M.; Wang, H.; Chen, Q. S.; Zhang, X. M.; Wu, Z. B.; Guan, B. H. Preparation of alpha-calcium sulfate hemihydrate from FGD gypsum in chloride-free Ca (NO3)2 solution under mild conditions. Fuel 2016, 174 (15), 235−241. (25) Li, Z. B.; Demopoulos, G. P. Development of an improved chemical model for the estimation of CaSO4 Solubilities in the HClCaCl2-H2O system up to 100 degrees C. Ind. Eng. Chem. Res. 2006, 45 (9), 2914−2922. (26) Kong, B.; Guan, B. H.; Yates, M. Z.; Wu, Z. B. Control of alphacalcium sulfate hemihydrate morphology using reverse microemulsions. Langmuir 2012, 28 (40), 14137−14142. (27) Alias, N. H.; Yunus, R.; Idris, A.; Omar, R. Effects of additives on oxidation characteristics of palm oil-based trimethylolpropane ester in hydraulics applications. Eur. J. Lipid Sci. Technol. 2009, 111 (4), 368−375.
poor crystallinity owns a less stable interior structure, which make it easy to reassemble its crystal structure and transit into the thermodynamically stable DH, resulting in a high immobilization efficiency of water.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b01152. Digital image of the fresh water-in-oil emulsion; Particle size distributions and specific surface area of the HH particles; Digital images of the uncontaminated and recycled oil before and after the oxidation; Temperaturedependent water separation efficiency and utilization rate of HH; Characterization of the alpha-HH recycled from DH hydrate through the hydrothermal method; Determination of the DH content in the hydrate; Cleaning performance of hexane on byproduct DH; The total cost analysis for the material systhesis; Cost determination for recycling of every one ton DH (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*(B.G.) Phone/fax: 86-571-88982026; e-mail: guanbaohong@ zju.edu.cn. *(X.L.) Phone/fax: 86-23-62769787; e-mail: lyuxiaoshu@zju. edu.cn. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The present work is financially supported by National Natural Science Foundation of China (Project 51508055).
■
REFERENCES
(1) Tao, M.; Xue, L.; Liu, F.; Jiang, L. An intelligent superwetting PVDF membrane showing switchable transport performance for oil/ water separation. Adv. Mater. 2014, 26 (18), 2943−2948. (2) Fontenot, B. E.; Hunt, L. R.; Hildenbrand, Z. L.; Carlton, D. D. J.; Oka, H.; Walton, J. L.; Hopkins, D.; Osorio, A.; Bjorndal, B.; Hu, O. H.; Schug, K. A. An evaluation of water quality in private drinking water wells near natural gas extraction sites in the barnett shale formation. Environ. Sci. Technol. 2013, 47 (17), 10032−10040. (3) Atlas, R. M.; Hazen, T. C. Oil biodegradation and bioremediation: A tale of the two worst spills in U.S. history. Environ. Sci. Technol. 2011, 45 (16), 6709−6715. (4) Ge, J.; Ye, Y. D.; Yao, H. B.; Zhu, X.; Wang, X.; Wu, L.; Wang, J. L.; Ding, H.; Yong, N.; He, L. H.; Yu, S. H. Pumping through porous hydrophobic/oleophilic materials: an alternative technology for oil spill remediation. Angew. Chem., Int. Ed. 2014, 53 (14), 3612−3616. (5) Chu, Z.; Feng, Y.; Seeger, S. Oil/water separation with selective superantiwetting/ superwetting surface materials. Angew. Chem., Int. Ed. 2015, 54 (8), 2328−2338. (6) Xu, Z.; Zhao, Y.; Wang, H.; Wang, X.; Lin, T. A superamphiphobic coating with an ammonia-triggered transition to superhydrophilic and superoleophobic for oil-water separation. Angew. Chem., Int. Ed. 2015, 54 (15), 4527−4530. (7) Chen, N.; Pan, Q. Versatile fabrication of ultralight magnetic foams and application for oil-water separation. ACS Nano 2013, 7 (8), 6875−6883. (8) Jayaramulu, K.; Datta, K. K. R.; Rösler, C. C.; Petr, M.; Otyepka, M.; Zboril, R.; Fischer, R. A. Biomimetic superhydrophobic/ superoleophilic highly fluorinated graphene oxide and ZIF-8 G
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology (28) Wu, Y. X.; Li, W. M.; Zhang, M.; Wang, X. B. Improvement of oxidative stability of trimethylolpropane trioleate lubricant. Thermochim. Acta 2013, 569, 112−118. (29) Wang, H.; Liu, F.; Yang, L.; Zu, Y.; Qu, S.; Zhang, Y. Oxidative stability of fish oil supplemented with carnosic acid compared with synthetic antioxidants during long-term storage. Food Chem. 2011, 128 (1), 93−9. (30) Quinchia, L. A.; Delgado, M. A.; Valencia, C.; Franco, J. M.; Gallegos, C. Natural and synthetic antioxidant additives for improving the performance of new biolubricant formulations. J. Agric. Food Chem. 2011, 59 (24), 12917−12924. (31) Sarin, A.; Arora, R.; Singh, N. P.; Sarin, R.; Malhotra, R. K. Oxidation stability of palm methyl ester: effect of metal contaminants and antioxidants. Energy Fuels 2010, 24, 2652−2656. (32) Bouaid, A.; Martinez, M.; Aracil, J. Production of biodiesel from bioethanol and Brassica carinata oil: Oxidation stability study. Bioresour. Technol. 2009, 100 (7), 2234−2239. (33) Freyer, D.; Voigt, W. Crystallization and phase stability of CaSO4 and CaSO4-based salts. Monatsh. Chem. 2003, 134, 693−719. (34) Guan, B. H.; Yang, L. C.; Wu, Z. B.; Shen, Z. X.; Ma, X. F.; Ye, Q. Q. Preparation of alpha-calcium sulfate hemihydrate from FGD gypsum in K, Mg-containing concentrated CaCl2 solution under mild conditions. Fuel 2009, 88 (7), 1286−1293.
H
DOI: 10.1021/acs.est.6b01152 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
