Waste-to-Resource Strategy To Fabricate Highly Porous Whisker

Research Article pubs.acs.org/journal/ascecg

Waste-to-Resource Strategy To Fabricate Highly Porous WhiskerStructured Mullite Ceramic Membrane for Simulated Oil-in-Water Emulsion Wastewater Treatment Mingliang Chen,†,‡ Li Zhu,†,‡ Yingchao Dong,*,†,‡ Lingling Li,†,‡,§ and Jing Liu†,‡,§ †

CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, P. R. China ‡ Ningbo Urban Environment Observation and Research StationNUEORS, Chinese Academy of Sciences, Ningbo 351800, P. R. China § School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: Industrial waste coal fly ash, containing hazardous metal oxides, poses potential threats to the environment and humans. Efficient recycling of such kind of solid state waste is highly desired yet still challenging. This work addressed waste-to-resource fabrication of a highly porous whisker-structured mullite ceramic membrane for separation of simulated oil-in-water emulsion wastewater by recycling of waste fly ash and natural bauxite with addition of WO3. The formation and characterizations of membranes were systematically studied including reaction mechanism, dynamic sintering behavior, open porosity, mechanical property, pore size distribution, microstructure, and pure water flux. The results show mullite formation temperature was decreased about 100 °C with addition of 20 wt % WO3, whereas open porosity significantly increased with WO3 content due to the formation of a highly porous interlocked whisker structure. Even without any pore formers, interestingly, the membrane with addition of 20 wt % WO3 possessed an open porosity as high as 51.9 ± 0.3% after sintering at a high temperature of 1400 °C whereas its mechanical strength (68.7 ± 6.1 MPa) was still improved. An oil-in-water emulsion dead-end microfiltration experiment indicates a significantly improved oil rejection as high as 99% was also obtained for W20 membrane, as compared to that (83%) of the W0 membrane. KEYWORDS: Waste recycling, Coal fly ash, Ceramic membrane, Mullite membrane, Open porosity, Wastewater treatment, Oil-in-water emulsion

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and strong acidic or alkaline media separation.7 Recently, there have been reports using inorganic ceramic microfiltration membranes to separate oily wastewater.8 Compared with some conventional oily wastewater separation techniques such as gravity separation, flotation and centrifugal separation, the major advantages of ceramic membranes rely on their high oil removal efficiency, efficient membrane regeneration, good mechanical/chemical/thermal stability, no chemical additives used, and long operation time.9 When these advantages are considered along with small space requirement, moderate capital cost, and simple operation, ceramic membrane technology provides a very competitive alternative to traditional separation technologies for simultaneous production of oil resource and purified water.10

INTRODUCTION Coal fly ash is a byproduct generated from the combustion of raw coal in thermal power plants that may cause serious environmental pollution problems if no sustainable disposal or treatment is adopted. Thus, it is of great necessity to recycle coal fly ash not only to decrease its environmental impacts but also to produce useful products from it.1 The main components in fly ash are Al2O3 and SiO2, which are very suitable for the preparation of porous mullite-based ceramics with the addition of other Al2O3 sources such as α-alumina, γ-alumina, alumina hydroxide, kaolin, and bauxite.2 Some previous studies have focused on the conversion of fly ash into porous mullite-based ceramic membranes via high-temperature sintering method that show a feasibility of this kind of waste recycling.3 Inorganic ceramic membrane technology, developed in the last 50 years, is becoming a promising technology for environmental applications such as wastewater treatment,4 drinking water production,5 high temperature gas purification,6 © XXXX American Chemical Society

Received: November 17, 2015 Revised: January 24, 2016

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DOI: 10.1021/acssuschemeng.5b01519 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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In practical separation applications, it is highly desired that porous ceramic membranes should have sufficient mechanical strength, high open porosity, and good resistance to acidic and alkaline solutions. As these properties are very important especially when they are used to treat great volumes of liquid/ gas effluents under high pressure. Porous ceramic membrane constructed by mullite whisker crystals is a novel and unique porous membrane, exhibiting a highly porous whisker interlocked microstructure. Such a structure is expected to retain an excellent structural integrity at a higher level of open porosity than traditional porous ceramic membranes with a particulate-packing structure made via particulate sintering method.1 Because of these unique properties, in our previous work efforts have been performed to find the cost-effective ways to grow mullite whiskers through sintering reaction between fly ash and natural bauxite.11 Several approaches have been developed to produce mullite whiskers with controllable morphologies, such as additions of AlF3 and MoO3,12 AlF3 and V2O5,13 and AlF3 and Al(OH)314 to the mixtures of starting materials, which could not only improve mullite phase content at lowered sintering temperatures, but also effectively accelerate the formation-growth of whisker-like mullite crystals to enhance significantly porosity while maintaining high mechanical strength. However, the use of AlF3 should be taken with great care because it becomes hazardous gas species (AlF3, AlOF, and F) during high-temperature sintering.15 because gaseous AlF3-derived species play an important role in causing the growth of mullite whiskers, an airtight sintering environment should be considered to avoid their effusion, which is not suitable for fabrication of large-sized specimens. Therefore, it is necessary to find a green and suitable method for membrane fabrication without use of AlF3. As also well-known, W and Mo are in the same subgroup of the element periodic table; it is therefore expected they may have similar function in reaction formation of mullite whiskers to produce highly porous ceramic membrane. Besides, the effect of WO3 on the growth of mullite whiskers powder from a mixture of pure Al2O3 and SiO2 oxides was studied by Kong et al.16 But little has been done to investigate the effect of pure WO3 as a sintering aid on the preparation of highly porous mullite ceramic membrane. In this work, therefore, with only excess WO3 added and mixed with fly ash and bauxite, a complete reaction conversion of highly porous mullite ceramic membrane was realized at lowered sintering temperature, entirely composed of interlocked well-grown mullite whiskers even after high-temperature sintering at an open atmospheric environment, whereas mechanical strength is still improved. By altering processing conditions, whisker-structured mullite ceramic membranes with various combinations of porosity, pore size, mullite crystal morphology/size and mechanical property were produced. The process parameters including reaction mechanism and dynamical sintering behavior were studied. In addition, the properties of the as-produced membranes were characterized in detail, including mechanical strength, open porosity and pore size, microstructure, and pure water permeation flux. Furthermore, an oil-in-water emulsion separation experiment was conducted to illustrate the advantage in separation performance for WO3doped highly porous whisker-structured mullite ceramic membranes, where a significantly improved oil rejection was achieved for the membrane with 20 wt % WO3, as compared to that of the membrane without addition of WO3.

Research Article

EXPERIMENTAL SECTION

Starting Materials and Membrane Fabrication. Coal fly ash powder was obtained from Beilun thermal power plant (Ningbo, Zhejiang Province, China). Natural bauxite powder was purchased from Yangquan (Shanxi Province, China). Commercially available WO3 (99%+ purity, Sinopharm Chemical Reagent Co., Ltd.) powders was used as sintering aid. On the basis of the composition of 3:2 mullite (3Al2O3·2SiO2), a batch of fly ash and bauxite powders was carefully weighed. A series of WO3 doped and undoped mullite membranes were prepared by adding various weight percents (wt %) of WO3 into the powder mixture of fly ash and bauxite. All samples are labeled as Wx, x represents the mass percentage of WO3 in the samples and x = 0, 5, 10, and 20. The details for preparation method and process of the green membrane compacts can be seen in the Supporting Information. Characterizations. The chemical compositions of fly ash and bauxite were examined by quantitative X-ray fluorescence spectrum analysis (XRF-1800, Shimazduo Corporation, Japan). The particle size distributions of starting materials including fly ash and bauxite were determined by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., UK). Although the phases present in sintered samples were identified by X-ray diffraction (XRD) technique using Cu Kα radiation and the phase contents of mullite, corundum, cristobalite, WO3, and CaWO4 can be quantitatively calculated according to the normalized reference intensity ratio (RIR) method.17 More details about this method are given in the Supporting Information. The morphologies of the fracture surfaces of mullite ceramic membranes were observed by scanning electronic microscope (SEM, S-4800, Hitachi Ltd., Japan). Sintering behaviors of green compacts (20 mm in diameter and 1−2 mm in thickness) were monitored with a horizontal dilatometer (DIL 402C, Netzsch, Germany) at a heating rate of 10 °C·min−1 and an operating temperature from room temperature (25 °C) to 1500 °C. Vernier caliper was used to measure the diametral shrinkages of sintered membranes. Open porosity and bulk density were measured in water medium based on the Archimedes’ principle.18 Pore size distribution and nitrogen permeation flux of membranes sintered at different temperatures were determined by a pore size distribution analyzer (PSDA-20, Nanjing Gaoqian function materials Co. Ltd., China) based on a gas−liquid displacement method. The pore diameter was calculated from the following Washburn’s equation according to bubble-point method:

d=

4γ cos θ Δp

(1)

where d is the pore diameter, γ is the surface tension of liquid, θ is the contact angle of the liquid on the inner pore wall of the membrane and Δp is the applied pressure difference.19 Biaxial flexural strength (BFS) was tested in a universal testing machine (AGS-X, Shimadzu Corporation Ltd., Japan) according to ISO687220 on sintered disc specimens (dimensions: diameter = 18− 21 mm; thickness = 1.3−1.6 mm). The disc specimens were centered and supported on three small zirconia balls (3 mm diameter) positioned 120° apart on a circle with a diameter of 5.0 mm. A crosshead speed of 0.1 mm·min−1 and a preload of 5 N were utilized along with a test jig (lab-designed) with a support radius of 5 mm. BFS was calculated using the following formula for maximum tensile stress given by Timoshenko and Woinowsky-Krieger:21 BFS (MPa) =

P {(1 + υ)[0.485ln(a/t ) + 0.52] + 0.48} t2

(2)

where BFS is the biaxial flexural strength (MPa), P is the maximum load (N), t is the thickness of the specimens, υ is Poisson’s ratio (here υ = 0.257 for mullite-based materials), and a is the radius of the support circle (m). The fracture energy was calculated according to the following equation: B


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Figure 1. XRD patterns of the membranes sintered at various temperatures with different WO3 contents: (a) W0, (b) W5, (c) W10, and (d) W20. “m” represents mullite (3Al2O3·2SiO2; ICSD PDF #83-1881); “c” for corundum (α-Al2O3; ICSD PDF #99-0036); “g” for CaWO4 (ICSD PDF #721624); “s” for cristobalite (SiO2; ICSD PDF #82-0512); “w” for WO3 (ICSD PDF #85-2460). N

Ea =

∑i = 1

Ei 1πa 2ti

N

× 106

J= (3)

V A × Δt

(4)

⎛ Cp ⎞ R = ⎜1 − ⎟ × 100 C⎠ ⎝

where Ea is the fracture energy of unit volume (KJ/m3); Ei is the absorbed energy of sample i, which can be directly obtained from our analysis software (TrapeziumX), which was used for data acquisition and postprocessing. Real-time data were recorded and plotted as loaddeflection graphs using TrapeziumX software. The original data were extracted from the operating software and transferred into Microsoft Office Excel for assessment; a is the circumcircle radius of the three support balls (a = 5 mm in our study); N is the total number of samples we used, and N is ten in our study; ti (mm) is the thickness of sample i. Pure Water Flux and Oil-in-Water Separation Experiment. The pure water flux and oil-in-water emulsion separation experiment of the ceramic membranes were measured by a homemade dead-end flow setup, which is made of stainless-steel. It consists of a top cylindrical water container and a base plate with a provision to keep a support. In the experiments, the trans-membrane pressure provided by nitrogen gas ranges from 0.05 to 0.15 MPa. The oil-in-water emulsions with a concentration of 250 mg·L−1 were prepared by dissolving a certain amount of machine oil (GL-5, Qiangli) into deionized water with addition of sodium dodecyl sulfate, followed by ultrasonication for 12h, finally by vigorous magnetic stirring for 48 h until they appeared approximately turbid and milky white, indicating good stability and homogeneity. For oil-in-water emulsions separation experiment, the oil concentrations were determined by measuring the absorbance using a UV− visible spectrophotometer22 (UV-1800PC, Shanghai MAPADA instrument Corporation Ltd., China). The whole separation process lasted 1 h, and the oil concentrations in the permeate samples varied with time were obtained by measuring the absorbance with an interval of 5 min during the experiment. All the experiments were conducted at room temperature (25 °C), the transmembrane flux (J, μL·m−2·s−1) and the percent oil rejection (R) was evaluated according to the following expressions:

2

(5) 3

where, A (m ) is the effective membrane area, V (m ) is the volume of permeate, Δt (s) is the sampling time, C (mg·L−1), and Cp (mg·L−1) are the concentrations of oil in the feed and permeate, respectively.

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RESULTS AND DISCUSSION Reaction Mechanism. The properties of starting materials such as chemical compositions, phase contents, and particle size distributions are quite important for membrane preparation. The D50 of coal fly ash and bauxite is 11.94 and 5.66 μm, respectively (see Figure S1). The main components of coal fly ash are 56.39 wt % SiO2 and 32.75 wt % Al2O3, whereas those of bauxite are 66.87 wt % Al2O3 and 9.19 wt % SiO2 (Table S1). The major phases for bauxite are diaspore (AlO(OH), PDF #87-0705) and kaolinite (3Al2O3·2SiO2·2H2O, PDF #78-2109), but for fly ash, the major phases are mullite (3Al2O3·2SiO2, PDF #79-1455) and quartz (α-SiO2, PDF #89-8936) (Figure S2). Figure 1 shows the XRD patterns of the ceramic membranes doped with different WO3 levels ranging from 0 to 20 wt %. For the samples without any additives (W0, Figure 1a), the dominant phase in the samples is corundum (72 wt %) at 1100 °C. Mullite phase (25.16 wt %), formed by the reaction of Al2O3 and cristobalite, which was transformed from quartz, can be also detected. Whereas, Chen et al.23 reported that the mullite did not form via the reaction between kaolinite and Al2O3 until reaction temperature increased up to 1200 °C, which is higher 100 °C than that (1100 °C) in this study. From 1200 to 1500 °C, the reflection intensity of corundum, decomposed from the diaspore24 in natural bauxite decreases C


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Figure 2. Variations of contents of detected phases with sintering temperature of the membranes: (a) W0 and (b) W20.

%, PDF# 82-0512) can only be detected at 1100 °C as it reacted with corundum for secondary mullitization, and residual SiO2 also became amorphous at higher temperatures. For W20 membrane, besides mullite, corundum, and cristobalite, two other phases (WO3 and CaWO4) appear and their weight percentages are 0−3.96 wt % and 5.53−8.99 wt % (Figure 2b). The content of main phase mullite increases gradually from 44.7 wt % to 54.39 wt % between 1100 and 1200 °C. With further increasing temperature up to 1300 °C, the reflection intensity of mullite phase dramatically increases with a content as high as 90.58 wt % and at 1450 °C a maximum content of 94.05 wt % was achieved, which is much higher than that (62.17 wt %) of W0 membrane at 1450 °C. A high content of mullite phase is usually accompanied by a low content of corundum phase, as presented in W20 membrane, due to secondary mullitization reaction between corundum and amorphous silica.27 Corundum phase content decreases significantly from 1100 °C, and then reaches almost 0 wt % at 1300 °C. No corundum was detected until sintering temperature increased to 1500 °C when the mullite phase decomposed into corundum and amorphous silica. In summary, the addition of WO3 significantly promoted the formation of more mullite phase at lowered temperatures through secondary mullitization reaction between corundum and silica, derived from starting materials of coal fly ash and bauxite. Similar results also have been reported by Kong et al.16 who added WO3 to the mixture of pure Al2O3 and SiO2 to enhance the formation of mullite phase and its formation temperature could be lowered down to 835 °C with 20 wt % WO3 addition. In this work, the mullite formation temperature of W20 sample is about 100 °C lower than the W0 sample, like other additives such as MoO3 and V2O5 that accelerated the formation-growth rate of mullite crystals through a dissolution−precipitation reaction process.25 From the results discussed above, possible reaction processes27−29 for W20 membrane can be summarized as follows: 500 °C:

gradually. Meanwhile, the reflection intensity of mullite phase increases significantly from 1200 to 1300 °C, and above 1300 °C mullite becomes a dominant phase. While no silica phase is observed in the XRD patterns because it is amorphous at 1300 °C. These results indicate that secondary mullitization reaction between Al2O3 and SiO2 mainly took place above 1300 °C. At higher sintering temperatures (1300−1500 °C), mullite content increases gradually with sintering and then decreases a little at 1500 °C. When comparing the membranes of W0 and W5, it is observed in Figure 1b a new phase of CaWO4 appears in W5 membranes from 1100 to 1500 °C as a result of the reaction between WO3 and CaO. Although the reflection intensity of CaWO4 looks pretty high, actually its content in terms of mass percentage is very low.25 This is confirmed by checking a total weight percentage of CaO (2.39 wt %) in fly ash and bauxite, indicating the highest content of 8.99 wt % CaWO4 by assuming that all CaO completely convert into CaWO4 via reaction with as-added WO3. The reflection intensity of mullite phase in W5 is much higher than W0 sample at 1200 °C, indicating the addition of WO3 is quite effective in promoting secondary mullitization at this temperature. As for the membranes W10 and W20 as shown in Figure 1c,d, it can be found that a trace of residual WO3 (0.68−3.96 wt %) was still left in the W20 membrane. Cristobalite phase was not detected at 1200 °C in both membranes and very weak reflection intensities of corundum are observed only in W10, whereas corundum almost disappears in W20. This implies that the formation temperature of mullite phase in W20 is lowered about 100 °C compared with W0 (without any additives). This positive result of lowering secondary mullitization temperature is similar to V2O5, La2O3, and MoO3.12,20,26 It is also noted that mullite phase content increases with increasing WO3 content, which will be discussed in the following section. Variations of contents of detected phases with sintering temperature in the membranes of W0 and W20, quantified based on RIR analysis, are shown in Figure 2. Figure 2a shows the content variations of three phases (mullite, corundum, and cristobalite) in W0 membrane. It can be clearly observed that the content of mullite (PDF #83-1881) increases gradually from 24.56 to 62.17 wt % with increasing sintering temperature from 1100 to 1450 °C, accompanied by a gradual decrease in corundum (PDF #99-0036) content from 72 to 37.83 wt %. However, the mullite phase is unstable at 1500 °C and decomposes into Al2O3 and amorphous SiO2 as the diffraction peak of corundum appears again at 1500 °C in Figure 1d, which may be due to the coexistence of impurities such as Fe2O3, TiO2, and CaO in the starting materials.20 Cristobalite (2.73 wt

AlO(OH) (diaspore) → Al 2O3 + H 2O

(6)

Al 2Si 2O5(OH)4 (kaolinite) → Al 2Si 2O7 (metakaolin) + H 2O

(7)

1100−1400 °C: WO3 + CaO → CaWO4

(8)

1400−1500 °C: CaWO4 → WO3 + CaO D

(9) DOI: 10.1021/acssuschemeng.5b01519 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. Open porosity and biaxial flexural strength test results of W0 and W20 membranes: (a) open porosity versus sintering temperature, (b) biaxial flexural strength versus sintering temperature, (c) fracture energy versus sintering temperature, (d) biaxial flexural strength versus porosity.

Figure 4. Pore size distributions of the ceramic membranes sintered at different temperatures: (a) W0 and (b) W20.

1100−1450 °C:

property is also improved. More membrane characterizations will be shown and discussed in the following subsections. Characterization of Mullite Membranes. Open Porosity, Mechanical Property and Pore Size Distribution. As can be seen in Figure 3a, the open porosities of W20 are always higher than those of W0 at the respective sintering temperatures. Between 1200 and 1400 °C, the open porosities of W20 and W0 both slightly increase with the sintering temperature, then decrease significantly from 1400 to 1500 °C. The highest open porosity for W20 is 51.9 ± 0.3% even after sintering at a high temperature of 1400 °C. A gradual enhancement in open porosity with the increase of WO3 content is observed at all sintering temperatures. More details about the open porosity of W0, W5, W10, and W20 are displayed in Figure S5 in the Supporting Information. Figure 3b displays the biaxial flexural strength of the membranes of W0 and W20 after sintering from 1100 to 1500 °C for 2 h. The strength of both ceramic membranes gradually increases with sintering temperature. The biaxial flexural strengths increase from 28.4 ± 2.7 MPa to 93.6 ± 5.5

Al 2Si 2O7 (metakaolin) → 3Al 2O3 · 2SiO2 (primary mullite) + SiO2

(10)

Al 2O3 (corundum) + SiO2 → 3Al 2O3 · 2SiO2 (secondary mullite)

(11)

Above 1450 °C: 3Al 2O3 ·2SiO2 (mullite) → Al 2O3 + SiO2 (amorphous) (12)

The dynamic sintering curves of the green rectangular compacts of W0 and W20 membranes are shown in Figure S4. By comparison, it can be also concluded that a much higher volume expansion is achieved for W20 membrane, which is 6.84% greater than that achieved for W0. This unique volume expansion resulted in porosity enhancement while mechanical E


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Figure 5. Fractured surface SEM images of W0 and W20 membranes sintered at various temperatures: (A) W0 at 1200 °C, (B) W0 at 1300 °C, (C) W0 at 1400 °C, (D) W0 at 1500 °C, (a) W20 at 1200 °C, (b) W20 at 1300 °C, (c) W20 at 1400 °C, and (d) W20 at 1500 °C.

MPa for W0 and from 34.5 ± 1.8 MPa to 87.5 ± 12.5 MPa for W20 respectively from 1100 to 1400 °C. Between 1100 and 1400 °C, owing to the growth of acicular mullite crystals promoted by the addition of WO3, the strengths of W20 membranes are always higher than those of W0 membranes. However, at 1500 °C, the strength of W20 (87.5 MPa) becomes slightly lower than that (93.6 MPa) of W0 membrane, which can be ascribed to a much more significant densification (open porosity = 19.5%) for W0 than W20 (open porosity = 44.6%), decreasing open porosity and providing enhanced strength. Figure 3c depicts the fracture energy as a function of sintering temperature for W0 and W20 membranes. As shown, the fracture energy of the samples (W0 and W20) gradually increases with sintering temperature between 1100 and 1300 °C, which is consistent with the variation of biaxial flexural strength with sintering temperature. Whereas the fracture energy of W20 decreases as sintering temperature is higher than 1300 °C. In general, fracture energy of porous ceramic membrane strongly depends on its microstructure details such as grain size and porosity (also as flaws). More nonuniform pores and bigger mullite whisker size would result in an apparent decrease in fracture energy.30,31 As observed from Figure 4b and Figure 5, the pore size distribution for W20 becomes broader and the mullite whisker size grows bigger when sintering temperature is above 1300 °C. As a result, the fracture energy decreases when sintering temperature is higher than 1300 °C. The W20 sample always exhibits higher fracture energy than W0 sample at each temperature, which can be ascribed to that WO3 could promote the growth of more interlocked need-like mullite whiskers with high strength and high modulus in W20, which requires more energy for fracture as compared with partially sintered glassy phase particles in W0. Therefore, the mechanical strength is enhanced significantly.32 This effect is similar to that of the study performed in our group,12 where MoO3 and AlF3 were used to create an acicular microstructure in mullite membranes in order to increase open porosity without degradation of biaxial flexural strength. The relationship between biaxial flexural strength and open porosity of the membranes of W0 and W20 is presented in Figure 3d. It is interesting to note that at the same level of biaxial flexural strength, the W20 membranes always show higher open porosity than the W0 membranes, which again illustrates WO3 could improve both open porosity and biaxial flexural strength. It is found that the open porosity of mullite ceramic membrane prepared from kaolin with additives of

Al(OH)3 and AlF3 could also be above 50%, but its three-point bending mechanical strength is only 60 MPa at 1500 °C.33 The same mechanical strength is obtained at a much lower temperature (only 1200 °C) for W20 membrane in our work. Thus, compared with those ceramic membranes reported in the literature (see Table S2), the W20 membrane in this work has better performances in terms of open porosity and mechanical strength. As illustrated in Figure 4, the pore size distributions of ceramic membranes have a close correlation with sintering temperature. At 1200−1400 °C, the average pore sizes of W0 (Figure 4a) increase from 0.46 to 0.93 μm, due to the unique volume expansion (see Figure S4). Whereas from 1400 to 1500 °C, the average pore sizes decrease from 0.93 to 0.56 μm owning to a sintering redensification. The pore size distributions of W20 membranes sintered from 1300 to 1500 °C are shown in Figure 4b and the mean pore sizes are 0.11, 0.48, and 0.94 μm, respectively. Compared with W0 membranes, it can be inferred that the average pore sizes of W20 become smaller between 1300 and 1400 °C, whereas larger than W0 samples at 1500 °C. This is probably as a result of the formation and growth of stiff skeleton needle-like mullite crystals with increasing temperature, which can be also justified by the SEM results in Figure 5. Microstructural Evolution. The microstructures of the sintered membranes of W0 and W20 sintered at different temperatures were examined by SEM. The results are shown in Figure 5. Below 1300 °C, the surface of W0 is covered by a layer of glass-like amorphous substance, where mullite crystals are not observed, but more acicular mullite crystals embedded in the glassy phase could be clearly identified above 1400 °C. By contrast, well-developed mullite whiskers were formed in W20 membrane even at a low temperature of 1200 °C, similar results are also observed for W5 and W10 membranes (see Figure S7). Mullite whiskers were also obtained by using other additives such as La2O3,34 CeO2,35 B2O3,25 and V2O5.13 The mullite whiskers grew bigger at higher temperatures with an increased aspect ratio (length over width) and the amount of whiskers was enhanced with the increase in doping level from 0 to 20 wt % in this study. The formation mechanism of secondary mullite was controlled by a dissolution−precipitation reaction process, where Al2O3 species dissolved into the coexisting SiO2 liquid phase until its concentration reached a critical value.16,36 As Al2O3 concentration in SiO2-rich bulk phase affected the nucleation and growth of mullite crystals, dissolution velocity of F


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Figure 6. Pure water flux of (a) W0 and (b) W20 mullite ceramic membranes sintered at various temperatures.

Al2O3 into SiO2-rich bulk liquid phase is a key step for mullite whisker formation. It is a known case that mullite phase formation is related not only to the characteristic of Al2O3 and SiO2 as-used, but also to the addition of other compounds.25 In the present work, WO3 was assumed to be as heterogeneous center for mullite nucleation, which lowered the melting temperature and hightemperature viscosity of silica-rich glassy liquid, promoting dissolution of Al2O3 into SiO2 liquid. The formation temperature of mullite phase is lowered in this way. And more mullite nucleation centers were formed with increasing WO3 content. Therefore, formation temperature of secondary mullite was decreased with increasing WO3 content. Because of lower temperature, mullite crystals grew anisotropically in an unconstrained environment before a significant densification process occurred. As a result, more mullite whiskers with higher aspect ratio were formed.37 Pure Water Flux. Figure 6 illustrates the pure water flux of W0 and W20 mullite ceramic membranes at different transmembrane pressures. In all cases, the water flux increases linearly with transmembrane pressure, which means that the permeation of pure water across W0 and W20 membranes in this study follows the viscous flow model.38 For W0 mullite ceramic membrane, the pure water flux increases with increasing temperature from 1200 to 1400 °C, whereas it decreases at 1500 °C. By contrast, the pure water flux of W20 membrane increases gradually with temperature at all temperatures between 1200 and 1500 °C. It is interesting to find that the pure water flux is in a positive correlation with average pore size (Figure 4). Similar phenomenon is also discussed in other literature.39 Separation of Oil-in-Water Emulsion. Ceramic membranes technology is one of the most effective methods for oilin-water emulsion separation.40 To compare further the separation performance of W0 and W20 mullite ceramic membranes, an oil-in-water emulsion dead-end MF separation experiment (see Figure S8) was conducted for membranes sintered at 1400 °C, and the results are shown in Figure 7. In the experiment, the concentration of feed oil (machine oil, GL5, Qiangli) was 250 mg·L−1, the average particle size of the oil droplets was 2 μm and the emulsion could be stable for at least 2 days (Figure S9). Experiments were performed at several transmembrane pressures of 50, 100, and 150 kPa. Figure 7a shows the effect of trans-membrane pressure on the permeate flux. As can be seen, for all the membranes, higher transmembrane pressure results in higher initial permeate flux, which is due to a higher driving force across the membrane. Similar results were also reported by Chang et

Figure 7. Oil-in-water emulsion separation results under operating pressures of 50, 100, and 150 kPa for mullite ceramic membranes of W0 and W20 sintered at 1400 °C: (a) permeate flux with time, (b) oil rejection with time, and (c) the photos of oil-in-water emulsion before and after separation using W20 membrane.

al.41 However, the flux always declines more rapidly when trans-membrane pressure is increased as there is a more frequent and more serious pore blocking in membranes.40 It could also be found in Table 1, as the transmembrane pressure differences exceed some threshold value, the steady state flux is independent of transmembrane pressure. Thus, membrane operation at the transmembrane pressures higher than 100 kPa, could not contribute to flux increase and in that case more energy was consumed. There are two significant problems leading to a decrease in permeate flux with time and limitation of separation efficiency: concentration polarization and membrane fouling.42 At the initial filtration stage of membrane separation, membrane fouling was dominatly controlled by a pore blocking mechanism, where a significant decline of permeate flux was mainly caused by the formation of a layer of oil droplets on the membrane surface. After a certain period of filtration, membrane fouling mechanism was dominated by cake filtration and in this case the permeate flux is stable with increasing operation time. For W0 and W20 membranes, in the first 10 min, the permeate flux of W0 membrane is always higher than that of W20 membrane at different transmembrane pressures as a result of its larger average pore size (0.93 μm for W0, 0.48 μm G


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Table 1. Comparison of Oil-in-Water Emulsion Separation Performance of Mullite Ceramic Membranes of W0 and W20 membrane

transmembrane pressure (kPa)

initial flux (mL·m−2·s−1)

steady-state flux (mL·m−2·s−1)

oil rejection (%)

W0

50 100 150 50 100 150

181.96 188.09 377.73 79.99 106.06 154.87

90 50 49 46 39 41

83−97 83−96 78−92 97−99 97−99 96−98

W20

for W20) at 1400 °C sintering temperature (see Figure 4). While at the first 5 min, the oil concentration for W0 membrane was quite high (even more than 40 mg·L−1 at 50 kPa) and also increases with transmembrane pressure. After a dead-end microfiltration process for half an hour, the oil concentration in the permeate decreased to a stable value and its oil rejection was improved significantly (see Figure 7b). For W0 membrane, an increase in transmembrane pressure enhanced the initial permeate flux but lowered both stable state flux and oil rejection, because larger pore size of W0 membrane could induce passage of oil droplets across the membrane pore and therefore accelerate its fouling on membrane surface. However, for W20 mullite ceramic membrane, the result was quite positive as desired (see Figure 7c), the permeate oil concentration was much lower less than 8 mg·L−1, which complied with the Class B of the Integrated Wastewater Discharge Standard (GB8978-1996). In addition, the oil rejection could be as high as 96−99% without any reduction at various transmembrane pressures. This oil rejection value is a little higher than those reported in other literature. Emani et al.22 used their prepared ceramic membrane to separate the feed oil-in-water emulsion with a concentration of 400 mg·L−1, and its highest oil rejection is 98.52% at a transmembrane pressure of 138 kPa. D. Vasanth et al.43 reported around 85% of oil rejection for a feed oil concentration of 250 mg·L−1 at a pressure of 69 kPa using the ceramic membrane prepared from kaolin, quartz, and calcium carbonate.

could be achieved as high as 99% for W20 membrane. Therefore, this study proves it is feasible to prepare separation membranes for economical treatment of simulated oil-in-water emulsion wastewater form low-cost materials (waste fly ash and natural bauxite) instead of expensive traditional pure oxide materials such as alumina, titania and zirconia. Moreover, these membranes may be afterward used in other applications by surface modification or coating of finer pore membrane layer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01519. Tables and figures on the characterization of raw materials, membrane green compacts preparation and their sintering behaviors. RIR analysis method, open porosity, biaxial flexural strength test, and SEM images of membranes. Oil droplets size distributions and oil-inwater emulsion separation schematic diagram (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Prof. Dr. Yingchao Dong. E-mail: [email protected], [email protected]. Tel.: 0592-6190790. Fax: 05926190977. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS From preparation to application, detailed characterizations, such as reaction mechanism, dynamical sintering, porosity, mechanical property, average pore size, water flux, and oil rejection were compared for the membranes of W0 and W20 and the following conclusions may be drawn: (1) All the membranes prepared from waste coal fly ash and natural bauxite are mainly composed of mullite phase at high sintering temperatures of 1450 °C. Mullite phase content is significantly improved in W20 membrane and its formation temperature is lowered about 100 °C compared with W0 membrane. (2) Because of the addition of WO3, the porosity of the membranes is significantly increased and an open porosity as high as 51.9 ± 0.3% is achieved at 1400 °C. Moreover, even at increased open porosity, the biaxial flexural strength is also improved for W20 membrane, which is higher than W0 membrane between 1100 and 1400 °C. The mechanical strength of W20 membrane sintered at 1400 °C is 68.7 ± 6.1 MPa, which is sufficient for porous membrane application. A highly porous microstructure entirely composed of interlocked mullite whiskers with controllable morphologies is observed from 1200 to 1500 °C by SEM and the size of mullite whiskers increases with sintering temperature. (3) The results of oil-in-water emulsion separation are very positive and the highest oil rejection

ACKNOWLEDGMENTS The authors acknowledge the financial supports from the Natural Science Foundation for Distinguished Young Scholars of Fujian Province (No. 2015J06013), Industry Leading Key Project of Fujian Province, China (No. 2014H0050), the National Natural Science Foundation of China (No. 21301171) and the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. IUEMS201407).

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REFERENCES

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Waste-to-Resource Strategy To Fabricate Highly Porous Whisker

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