Calcium Sulfate Hemihydrate Nanowires: One Robust Material in

Aug 28, 2017 - Here we report a facile and cost-effective wet-chemical approach to the synthesis of calcium sulfate hemihydrate nanowires (HH NWs, CaS...
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Calcium Sulfate Hemihydrate Nanowires: One Robust Material in Separation of Water from Water-in-Oil Emulsion Guangming Jiang, Wenyang Fu, Yuzheng Wang, Xiaoying Liu, Yu Xin Zhang, Fan Dong, Zhiyong Zhang, Xianming Zhang, Yuming Huang, Sen Zhang, and Xiaoshu Lv Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02901 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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Calcium Sulfate Hemihydrate Nanowires: One

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Robust Material in Separation of Water from

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Water-in-Oil Emulsion

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Guangming Jiang,1,* Wenyang Fu,1 Yuzheng Wang,1 Xiaoying Liu,3 Yuxin Zhang,3 Fan Dong,1

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Zhiyong Zhang,2 Xianming Zhang,1 Yuming Huang,4,* Sen Zhang,2 Xiaoshu Lv1,*

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1

Engineering Research Center for Waste Oil Recovery Technology and Equipment, Ministry of Education, Chongqing Technology and Business University, Chongqing 400067, China

7 8

2

Department of Chemistry, University of Virginia, Charlottesville, Virginia, 22904, United States

9 10

3

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

11 12

4

The Key Laboratory of Eco-environments in Three Gorges Reservoir Region, Ministry of

13

Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing

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400715, China

15 16



To whom correspondence should be addressed.

E-mail: [email protected]; [email protected]; [email protected]

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Abstract

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Here we report a facile and cost-effective wet-chemical approach to the synthesis of

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calcium sulfate hemihydrate nanowires (HH NWs, CaSO4.0.5H2O), and their robust

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performance in immobilizing water molecules to the crystal lattice of CaSO4 and then

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separating them from a surfactant-stabilized water-in-oil emulsion (mean droplet size

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of around 1.2 µm). Every gram of HH NWs are capable of treating 20 mL emulsion

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(water content: 10.00 mg mL-1) with a separation efficiency of 99.23% at room

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temperature, and this efficiency can be further improved by tuning the surface charge

25

density of HH. Along with the water immobilization, HH NWs are converted to large

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cubic-like calcium sulfate dihydrate microparticles (DH, CaSO4.2H2O, mean size: 50

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µm), and the accompanied size increment enables efficient collection of the solid

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phase from oil. DH microparticles can be regenerated into HH NWs, which retain the

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high performance of the original NWs. Such a unique renewable feature improves the

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economics of our method and simultaneously prevents the secondary pollution.

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Further economic evaluation finds that purification of every cubic meters of emulsion

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(water content: 10.00 mg mL-1) will cost about $ 34.18 for HH NWs, much lower than

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$ 490.78 for the previously reported HH NPs, and $ 11,052.05-23,420.32 Fe3O4

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NP-based adsorbents, respectively. With the high efficiency, easy collection, low cost

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and renewable feature, HH NWs show highly promising applications in the field of oil

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purification and recycle.

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Graphical Abstract

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 INTRODUCTION

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The ever-increasing consumption of petroleum/synthetic oil products (such as

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lubricating oil, transformer oil and etc.) have led to the generation of million tons of

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wasted oil every year (the wasted oil refers to the one that has lost the function due to

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the inclusion of impurities and the change in composition).[1] The management and

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disposal of them have therefore become a critical issue for governments due to the

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relevant environmental concerns, such as the oil-induced water and soil pollution.[2]

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Oil regeneration via a stepwise purification and refining strategy is one promising

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solution, which can also contribute to saving petroleum resources.[3-4] The purification

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process is intended to remove the undesired impurities in oil, including water,

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particles, polymers and ions.[5] Among them, water droplets are usually stabilized by

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surfactants, emulsified in oil phase, and have a mean size less than ten micrometers,

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making them very difficult to get removed.

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To realize water/oil separation, various techniques, such as magnetic separation,

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oil skimming, electroflotation and selective adsorption, have been developed,[6-7]

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which, however, are limited to separate layered water/oil mixtures[8-9] and oil-in-water

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emulsions.[10-11] Their performance toward wasted oil (kind of a surfactant-stabilized

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water-in-oil emulsion) is still unsatisfactory due to the nano/micro size of droplet, the

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large viscosity of oil phase and the high stability of emulsion.[12-13] Recently, filtration

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techniques, which separate water and oil via selective penetration through one filter

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with superhydrophobic/superoleophilic or superhydrophilic/superoleophobic surfaces,

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have attracted increased research interests.[14,15,16] Till now, various smart membrane, 4

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sponge and mesh-based filters have been developed.[17,18,19,20,21] When armed with

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unique pore structure and surface wettability, these filters can efficiently separate

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water-in-oil emulsions.[22,23,24,25] However during practical applications, the filtration

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techniques are still challenged by the issues associated with the filter fouling,

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processing capacity and flux.[26,27] For example in Zhang’s work[28], the maximum

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flux for the surfactant-stabilized water-in-oil emulsion is reported to be only 1000 L

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m-2 h-1 (oil phase: isooctane; viscosity: 0.53 mPa s-1; droplet size: 5-20 µm), and this

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flux will be further reduced when the droplet size drops to nanoscale, and a more

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viscous oil is used. Very recently, our group reported a novel immobilization-

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separation approach, which can facilely remove 95.87% of water from a

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surfactant-stabilized water-in-oil emulsion (water content: 10.00 mg L-1).[29] In this

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process, EDTA-capped calcium sulfate hemihydrate nanoparticles (HH NPs,

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CaSO4.0.5H2O) were dropped into the emulsion, which can adsorb the water droplets

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and immobilize them into the crystal lattice of HH. The solidified water was then

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separated along with the removal of the solid phase. Compared with the developed

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adsorption separation and filtration technology, [10-11] this immobilization-separation

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approach could fix the water molecules more firmly and remove them without the

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concern of desorption, and can also continuously separate the emulsion without the

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concern of the fouling and processing capacity. However for real use, there is still

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room for improvement in the separation efficiency as well as the cost and yield for

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HH synthesis.

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Herein, we report a more facile large-scale synthesis of well-defined HH 5

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nanowires (NWs) for efficient water separation from a surfactant-stabilized water-in

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-transformer oil emulsion at room temperature. Different from the previously reported

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HH NPs synthesis using (NH4)2SO4 and CaCl2 as precursors,[30] HH NWs were

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prepared from the conversion of gypsum (one earth-abundant mineral, calcium sulfate

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dihydrate, CaSO4.2H2O, DH) in a Mg2+-containing glycerol solution at 95 °C:[31-32] CaSO4·2H2O (DH) →CaSO4·0.5H2O (HH) +1.5H2O

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1)

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With such a simple reaction condition, we were able to produce 10 g of HH NWs in

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one batch, which was around 23 times more than that of the HH NPs (0.44 g)[30]. The

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separation experiments show that every gram of HH NWs are capable of treating 20

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mL emulsion (water content: 10.00 mg mL-1) with a separation efficiency of 99.23%

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at room temperature. This efficiency can be further improved by tuning the surface

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charge density of HH (characterized by Zeta potential). After the water

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immobilization, HH NWs convert into large cubic-like DH microparticles (mean size:

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50 µm), and the accompanied size increment feature is critical to the efficient

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collection of the solid phase from oil. DH could be regenerated into HH NWs with a

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comparable separating performance to the original NWs. Such a unique renewable

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feature improves the economics of our method and also prevents the secondary

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pollution. The economic evaluation finds that the purification of every cubic meter of

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emulsion (water content of 10.00 mg mL-1) will cost about $ 34.18 for HH NWs,

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much lower than $ 490.78 for HH NPs, and $ 11,052.05-23,420.32 Fe3O4 NP-based

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adsorbents, respectively.

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EXPERIMENTAL SECTION 6

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Materials. Analytical reagent grade DH, MgCl2•6H2O, Na2EDTA, hexane, ethanol

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and glycerol were all purchased from Sinopharm Chemical Reagent Co., Ltd,

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Shanghai, China. The surfactant Span80 was obtained from Alfa Aesar. All chemicals

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were used without further purification.

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Synthesis of HH NWs. A typical synthetic approach for HH NWs was as follows: 1.5

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g of MgCl2•6H2O was first dissolved in 5 mL of water, which was added into 100 mL

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glycerol and heated to 95 °C under magnetic stirring (Mg2+ concentration: 12 mM).

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10 g of DH powder was then added into the solution. The mixture was kept at 95 °C

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for 2.0 h to achieve the complete phase transition from DH to HH. After the reaction,

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the hot slurry was filtered, washed with boiling water, and rinsed with ethanol. The

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collected solid was dried at 60 °C to remove any absorbed water or ethanol. To tune

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the surface charge density of HH NWs, more MgCl2.6H2O or controlled amount of

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Na2EDTA were added into the solution right after the 2.0 hour’s reaction, to avoid

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their effects on the shape of HH NWs during synthesis (See the morphologies of HH

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NWs synthesized under 12 and 96 mM MgCl2, respectively, in Figure S1).

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Preparation of Water-in-oil Emulsions and Water Separation. The surfactant-

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stabilized water-in-transformer oil emulsion was prepared following a reported

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work[33]. 2.0 g of water were mixed with 200 mL transformer oil with the assistance of

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0.2 g of Span80 (surfactant). The mixture was then intensively stirred for 3 h and

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sonicated for another 3.0 h (sonication power: 700W; frequency: 40 kHZ; temperature

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25-30 oC) to form a stable milky emulsion (See Figure S2). The surfactant-stabilized

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emulsion is stable over two weeks without any observable precipitation or 7

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de-emulsification.

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To examine the separation efficiency, a certain amount of dried HH NWs was

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mixed with 20 mL of the surfactant-stabilized water-in-oil emulsion in a 25 mL vial.

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The suspension was then magnetically stirred for 30 min and then kept still to allow

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the solid phase settling down to the bottom. The transparent oil could be obtained by

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centrifugation at 3000 rpm for 2.0 minutes. The separated solid phase was washed by

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hexane to remove the adsorbed oil and dried at 60 oC for regeneration or further

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characterization.

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Characterization. The crystal phase in solid samples was investigated by

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thermogravimetry/differential scanning calorimetry analysis (TG/DSC, NET- ZSCH

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STA 409 Luxx) and powder X-ray diffraction analysis (XRD, D/Max-2550 pc). TG

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analysis was also used to determine the crystal water content in solid sample. For

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TG/DSC analysis, 20 mg of the dried sample was sealed in an Al2O3 crucible with a

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lid and scanned at 10 °C min-1 under N2 flow. The XRD analysis was performed with

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CuKα radiation at a scanning rate of 5 ° min-1 in 2θ range of 10 - 60°. Particle

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morphology

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HITACHES-570, Hitachi) and transmission electron microscopy (TEM, JEOL,

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JEM-2010). The particle size distribution of the solid sample was measured by a laser

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particle size analyzer (Mastersizer 2000), and the droplet size in emulsions was

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measured by dynamic light scattering (DLS) (Malvern Zetasizer Nano ZS90). The

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Brunauer–Emmett–Teller (BET) specific surface area of the sample was determined

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using nitrogen adsorption/desorption apparatus (ASAP 2020) with all samples

was examined

by the

scanning

electron

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(SEM,

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degassed at 80 oC for 12 h prior to measurements. The water content (mg mL-1) in

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emulsion before and after the water separation was determined by Carr’s moisture

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titrator (899 coulometer + 860 KF Thermoprep., Metrohm, Herisau, Swiss) and

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multielement oil analyzer (MOA II plus,MOA Instrumentation, Inc. Levittown, USA),

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respectively. The separation efficiency (SE) is calculated by

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ܵ‫= ܧ‬

஼బ ି஼೟ ஼బ

× 100%

2)

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Where C0 and Ct are the initial and final water content in emulsion, respectively.

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 RESULTS AND DISCUSSION

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Synthesis and Characterizations of HH NWs. HH NWs were prepared in a hot

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glycerol solution with gypsum (DH, CaSO4.2H2O) as the precursor and Mg2+ as the

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shape mediator. The volume ratio of glycerol to water and the reaction temperature

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were set to be 10 and 95 oC, respectively, to provide the thermodynamic driving force

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for the phase transition of DH to HH,[30-31] while the presence of Mg2+ contributes to

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the formation of NWs.[34] Figure 1a gives a typical SEM image of the as-synthesized

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HH, which presents a uniform NW structure with an average length of around 20 - 30

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µm. The TEM image in Figure 1b shows the NWs are of an average diameter of 100

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nm, thus their length/width ratio can reach 200-300. Compared with the previous

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reported HH NPs (in the shape of ellipsoid, 600 nm by 300 nm),[29] the present HH

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NWs obviously show a much larger surface area. The N2 adsorption–desorption

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isotherms confirm that HH NWs have a nearly four times larger BET specific surface

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area (4.535 cm2 g-1) than HH NPs (1.351 cm2 g-1, See Figure S3). Since the water

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immobilization into the crystal lattice is an interface-based reaction, a larger surface 9

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area should be beneficial to achieve a higher separation kinetics and efficiency.

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Figure 1. SEM image (a), TEM image and SEAD pattern (b), XRD pattern (c) and

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TG/DSC pattern (d) of the as-synthesized HH NWs.

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The SEAD pattern of the NWs in Figure 1b inset exhibits numbers of spots with

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random orientation as well as several inconspicuous diffraction rings, which suggest a

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polycrystalline nature of these NWs. The XRD pattern of the sample in Figure 1c

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confirms the formation of a pure HH phase (PDF#041-0244) with the characteristic

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diffraction peaks at 2θ=14.70°, 25.62°,29.69°, 31.89°, 42.25° and 49.36°, for HH

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(110), (013), (004), (-411), (503) and (-415), respectively. The TG pattern in Figure

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1d shows a weight loss of 6.25 wt% when HH is heated to 200 oC, which is assigned

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to the escape of the crystal water. It is noted that this value is consistent with the

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crystal water content of 6.22 wt% in the pure HH phase. The DSC pattern (the inset in

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Figure 1d) presents one typical hemihydrate crystal water removal profile of HH, 10

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with the weight loss initiating with an endothermic reaction with the peak temperature

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of 149.2 °C, following by an exothermic reaction with the peak of 180.1 °C.[35-36] All

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these confirm the NWs to be the phase of HH.

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Water Separation from the Water-in-Oil Emulsion. The separation experiment

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proceeds by dropping HH NWs in the form of dried powders into the water-in-oil

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emulsion (0.2 g H2O in 20 mL transformer oil), which was then intensively stirred for

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30 mins at room temperature to drive the immobilization of water into crystal lattice

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of HH. The dosage of HH is set to be 1.07 g, which allows the whole immobilization

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of the water molecules according to the following Equation:

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CaSO4·0.5H2O(HH) +1.5H2O→CaSO4·2H2O(DH)

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As shown in Figure 2a, the system right after the stirring appears to be homogeneous,

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with little difference from the original emulsion (Figure S2). After sitting for 30 min,

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a clear interface is formed as the solids gradually settle down toward the bottom under

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gravity force, leaving the transparent oil phase on top layer (Figure 2b). After settling

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for 120 min, most of the solid was precipitated at the bottom of the vial (Figure 2c),

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indicating the solids are easily separated from the oil phase, avoiding the secondary

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pollution caused by nanostructures. A transparent liquid is facilely achieved by simply

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removing the solid precipitates with a mild centrifugation (Figure 2d). Figure 2e and

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2f compare the optical microscope images of the emulsion before and after the water

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separation, where the bright spot denotes the water droplet. It is clear that a large

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amount of the water droplets with a mean size of 1.2 µm (See the size distribution

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curve in Figure S4) exist in the original emulsion, and the number drops dramatically 11

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after the separation, suggesting most of the water has been removed. The SEM

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examination of the collected solid phase (Figure 2g) shows that the HH NWs have

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transformed into cubic-like microparticles with an average size of 50 µm (See the

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particle size distribution profile in Figure S5). The XRD pattern in Figure 2h

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indicates these microparticles are mainly composed of DH with a small proportion of

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HH left in them, indicating an effective water immobilization in HH NWs. It should

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be noted that the particle size increment along with the water immobilization process

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is important to ensure the efficient removal of the solids from the viscous oil and

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avoid the risk of particle pollution to oil.

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Figure 2. (a-d) Digital images of the whole separation process; (e-f) Optical

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microscope image of the water-in-oil emulsion before and after water separation; (g) 12

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SEM image of the collected solid phase after water separation; (h) Comparison in

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XRD patterns of the as-synthesized HH NWs and the collected solid phase.

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The concentration of water in the purified oil is measured to be 0.077 mg mL-1,

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showing a high separation efficiency of 99.23 wt%. The present HH NWs are even

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more efficient than the previously reported HH NPs, which only show a SE of 95.87

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wt% under the same separation conditions and dosage.[29] The crystal water content in

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the collected solid was determined to be 20.05 wt%. Considering the HH dosage of

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1.07 g and the initial crystal water content of 6.25 wt%, the immobilized water

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reaches 0.185 g, and therefore the immobilization efficiency of the NWs is around

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92.50 % (The immobilization efficiency is defined as the mass of water immobilized

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in HH NWs over the total mass of water in the original emulsion, thus its value here

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equals 0.185 g / 0.200 g = 92.50 %, See the calculation process in Supporting

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Information), much higher than the HH NPs (77.58 %). The significant increase in

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immobilization efficiency should be ascribed to the larger specific surface area and

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the one dimension property of the NW. It should also be mentioned that the

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immobilization efficiency (92.50 %) is a little lower than the separation efficiency

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(99.23 %), which may arise from the adsorption behavior of the solid phase. Overall,

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our HH NWs present an excellent performance toward the water separation from a

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surfactant-stabilized water-in-oil emulsion.

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Influence of the surface charge of HH. The surface charge is usually an important

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descriptor for interface reactions. To understand the role of surface charge during

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water separation, the surface charge on HH NWs is tuned from positive (by inclusion 13

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of Mg2+ during synthesis) to negative (by inclusion of EDTA during synthesis), and is

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characterized by Zeta potential. The dependence of the immobilization and separation

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efficiency on the Zeta potential of HH NWs is presented on Figure 3a. The results

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show that with an increase in Mg2+ dosage from 12 to 24 mM, the Zeta potential of

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the NWs increases from 1.27 to 27.9 mV, and the corresponding immobilization and

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separation efficiencies increase from 68.54 to 93.12 % and 77.56 to 99.51 %,

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respectively. It is suggested that a high zeta potential will promote the immobilization

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process, and this should be induced by the enhanced water affinity of HH NWs with a

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higher surface charge density. When EDTA is introduced during the HH synthesis, the

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Zeta potential of the NWs turns to be negative. With increased dosage of EDTA, the

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Zeta potential moves to more negative, and the corresponding immobilization and

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separation efficiencies increase to the peak of 77.56 % and 97.23 % (at 10 mM

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EDTA). This increment further confirms that a higher surface charge density will

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benefit the water separation. However, at an even higher concentration, the EDTA

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molecule will over-cap the NWs and block the efficient mass transfer between water

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droplets and HH NWs, therefore, lowering the separation efficiency.

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Figure 3. (a) Dependence of the immobilization and separation efficiency on Zeta 14

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potential of the HH NWs; (b) Schematic illustration of the effect of the surface charge

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density of HH material on water immobilization.

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On the basis of the above results, we can conclude that the surface charge density

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should be an important factor that determines the performance of HH NWs in

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water/oil separation. As schemed in Figure 3b, since the water molecule is of higher

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polarity than the oil molecule, it usually shows a stronger affinity to a material with a

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high surface charge density. Such a feature will significantly enhance the possibility

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of interface reactions between water droplets and the surface-charged HH NWs,

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leading to a higher water immobilization efficiency. More importantly, it allows us to

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optimize the separation efficiency via the surface charge engineering of the material.

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Economic Evaluation. Economics is crucial for the commercialization of a new

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rising process. Here, the economy of our method is evaluated by calculating the total

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cost of the chemical and energy consumptions during HH NW synthesis, and then

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comparing it with those of our previous HH NPs-based separation method[29] and the

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developed Fe3O4 NP adsorption techniques.[9,11] Table S2 shows that the synthesis of

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every ton of HH NWs cost $ 638.83, only about 6.96%, 0.14% and 0.31% of that for

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the synthesis of every ton of HH NPs ($ 9173.40), Fe3O4 NP-based adsorbents in

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Duan’s ($ 437,763.70) and Mirshahghassemi’s ($ 206,580.40) work, respectively.

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Considering the similar water removal capacity, we can roughly estimate that the

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purification of every cubic meters of emulsion (water content of 10.00 mg mL-1) will

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take about $ 34.18 for HH NWs, much lower than $ 490.78, $ 23,420.32 and

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$ 11,052.05 for HH NPs and Fe3O4 NP-based adsorbents in Duan’s[11] and 15

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Mirshahghassemi’s[9] work, respectively. All these demonstrate that the HH

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NWs-based method is much more cost-effective.

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As seen in Figure 2g-h, HH NWs are converted into DH after immobilizing the

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water molecules, and become a solid waste. One may thus concern about the disposal

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of these byproducts. Since the collected solid waste is determined by XRD and SEM

288

to be composed of DH with similar particle morphology to the commercial DH

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powders, we propose that the waste may serve as the precursor resources to regenerate

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HH NWs. Figure 4a gives a typical SEM image of the as-synthesized NWs using the

291

collected DH as the precursor, which show nearly the same morphology and size with

292

those synthesized from the commercial DH powders. The XRD pattern in Figure 4b

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confirms that these NWs (as indicated by 1st run) to be the HH phase. Notably, these

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HH NWs maintain a high immobilization efficiency of 94.38 % and a high separation

295

efficiency of 99.29 %, demonstrating that the solid waste could be regenerated into

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HH NWs and used again for water/oil separation.

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Figure 4. (a) SEM image of HH NWs prepared from the collected DH; (b) XRD

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patterns of HH NWs after different runs of separation-regeneration process. 16

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We further extended the runs of the separation-regeneration experiments. Figure

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4b shows that HH NWs could keep their phase with a high purity even after five-run

302

regeneration test. Table 1 summarizes the immobilization and separation efficiencies

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of HH NWs after different runs of separation-regeneration process, which shows that

304

the regenerated HH NWs show comparable performances to the original ones. This

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unique renewable feature is quite appealing, which could reduce the consumption of

306

calcium sulfate resources, and more importantly, avoid the secondary pollution

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induced by the generated solid waste.

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Table 1. Immobilization and separation efficiency of HH NWs after different runs of

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separation-regeneration process Number

Original

1st run

2nd run

3rd run

4th run

5th run

Immobilization efficiency

92.50

94.38

91.56

92.34

93.51

94.51

Separation efficiency

99.23

99.29

99.05

99.17

99.23

99.41

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Environmental Application. In the presented work, we demonstrate that HH NWs

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can immobilize the nano/micro-sized water droplets into their crystal lattice and then

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separate them from the surfactant-stabilized emulsions simply by the removal of the

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solid phase. To check the possibility of real application, we further examined the

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water separation from real wasted emulsified oil. Figure 5 presents the digital images

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of the wasted transformer oil (provided by Chongqing Science and Technology

316

Development Co.) and wasted sealing oil (provided by Chongqing ZhanjiangBaosteel

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Iron & Steel Co., Ltd.) before and after the water removal. It is clear that both of the 17

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transformer oil and sealing oil become much more transparent after the water

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extraction. The corresponding water content decreases from 17.259 to 1.254 mg mL-1

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for sealing oil, while from 15.637 to 0.152 mg mL-1 for transformer oil. The

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separation efficiency and immobilization efficiency are determined to be 92.73 % and

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81.23 % for sealing oil, and 99.02 % and 89.72 % for transformer oil, respectively.

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The lower immobilization efficiency in the case of sealing oil may be attributed to the

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larger kinematic viscosity of sealing oil (47 mm2 s-1) than that of transformer oil (9.9

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mm2 s-1). Despite of these, HH NWs still show promising as robust materials in water

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separation from a water-in-oil emulsion and in the field of oil purification.

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Figure 5. Application of HH NWs in oil extraction from the wasted sealing oil and

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transformer oil.

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SUPPORTING INFORMATION: SEM images of the HH NWs; Digital image of

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the as-synthesized emulsion; N2 adsorption-desorption isotherms of the HH NWs and

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HH NPs; Droplet size distribution in emulsion; Particle size distribution of the

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collected DH microparticles; The total cost for the synthesis of every ton of the 18

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materials. The Supporting Information is available free of charge on the ACS

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Publications websites.

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Corresponding Author:

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*

E-mail: [email protected] (G. M. Jiang);

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*

E-mail:[email protected] (Y. M. Huang);

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*

E-mail: [email protected] (X. S. Lv); Tel./Fax.: 86-023-62769787

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Notes: The authors declare no competing financial interest.

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The present work is financially supported by National Natural Science Foundation of

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China (Project 51608077), China Postdoctoral Science Foundation Funded Project

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(2016M602660), and Innovative Research Team of Chongqing (CXTDG201602014).

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AUTHOR INFORMATION

ACKNOWLEDGES

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