Low-cost and facile synthesis of geopolymer-zeolite composite membrane for chromium(VI) separation from aqueous solution

Journal Pre-proof Low-cost and facile synthesis of geopolymer-zeolite composite membrane for chromium(VI) separation from aqueous solution Pan Yang He (Methodology) (Investigation) (Writing - original draft), Yao Jun ZhangReview) (Supervision), Hao Chen (Software) (Visualization), Zhi Chao Han (Resources)Writing – review and editing), Li Cai LiuWriting – review and editing)

PII:

S0304-3894(20)30347-2

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122359

Reference:

HAZMAT 122359

To appear in:

Journal of Hazardous Materials

Received Date:

28 October 2019

Revised Date:

17 February 2020

Accepted Date:

18 February 2020

Please cite this article as: He PY, Zhang YJ, Chen H, Han ZC, Liu LC, Low-cost and facile synthesis of geopolymer-zeolite composite membrane for chromium(VI) separation from aqueous solution, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122359

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Low-cost and facile synthesis of geopolymer-zeolite composite membrane for chromium(VI) separation from aqueous solution

Pan Yang He, Yao Jun Zhang*, Hao Chen, Zhi Chao Han, Li Cai Liu

College of Materials Science and Engineering, Xi’an University of Architecture and

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Technology, Xi’an 710055, China

*Corresponding author. Tel.: 86 29 82202467, Fax: 86 29 85535724

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E-mail: [email protected] (Y.J. Zhang); [email protected]

Postal address of corresponding authors:

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College of Materials Science and Engineering,

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Xi’an University of Architecture and Technology, No.13 Yan Ta Road, Xi’an 710055,

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People’s Republic of China

Graphical abstract

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Highlights

A low-cost and facile geopolymerization-hydrothermal treatment was proposed.

A novel geopolymer-Li-ABW zeolite composite membrane was synthesized.

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The membrane had a compact zeolite layer with thickness about 1.5 μm.

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The membrane showed high rejection for Cr(VI) under small TMP.

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The mechanism separation included size exclusion and electrostatic interaction

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ABSTRACT

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Inorganic membranes in wastewater treatment have captured increasing attention

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due to their numerous advantages. However, high cost and complicated producing process restricted their benign developments. This study proposed an novel inorganic geopolymer-zeolite composite membrane which was synthesized by using circulating fluidized bed fly ash (CFBFA) solid waste as initial material and via a low-cost and facile geopolymerization-hydrothermal treatment processes, further, the membrane 2

was employed to separate Cr(VI) ion from aqueous solutions. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectra results indicated that geopolymer-zeolite (Li-ABW) composite membrane was obtained successfully. Field emission scanning electron microscopy (FESEM) results demonstrated that the membrane had a compact zeolite layer with thickness about 1.5 μm. The effects of transmembrane pressures (TMP), Cr(VI) concentration, pH, ionic strength, and

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co-existing ions on Cr(VI) rejection were investigated, and the results revealed that the Cr(VI) rejection reached 85.45% under 10 kPa of TMP, 1000 mg∙L-1 of Cr(VI),

and pH 7. The separation mechanism of Cr(VI) on the geopolymer-zeolite composite

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membrane was considered to be size exclusion and electrostatic interaction. These

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results suggested that the geopolymer-zeolite composite membrane had a potential application for the effective removal of Cr(VI) contaminants from wastewater.

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Keywords: Inorganic membrane; Geopolymer-zeolite; Hexavalent chromium;

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Separation 1. Introduction

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Hexavalent chromium (Cr(VI)) with pervasive applications in leather, electroplating, pigment, and wood preservation industries is an extremely toxic and

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hazardous pollutant in industrial wastewater [1,2]. It is notorious for flora and fauna health due to its acute toxicity, mutagenicity, and carcinogenicity [3]. For this reason, the Cr(VI) level must be below 0.2 mg∙L-1 in industrial and civil wastewaters, and it should be below 0.005 mg∙L-1 in superficial and underground water according to the environmental regulations issued by European Union [4]. The World Health 3

Organization strictly regulated the highest level of Cr(VI) for potable water is 0.05 mg∙L-1 [5]. Therefore, Cr(VI) removal from wastewaters is imperative and becomes increasing significant. Up to now, various methods have been developed to remove Cr(VI) from industrial wastewater, such as ion exchange [6], adsorption [7,8], membrane filtration [9], and electrolysis [10]. These methods possessed their respective advantages and

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disadvantages. For examples, ion exchange shows merits including large capacity and high efficiency but exhibits weak selectivity; adsorption offers advantages like low cost, availability, and easy operation but difficult recollection of the adsorbent;

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electrolysis has high efficiency and selectivity but high operation cost [11,12]. The

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membrane separation process captured increasing attention in recent years because of its competitive edge over all the other methods, for instance, lesser energy

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consumption, without any addition of chemicals, selective recovery, as well as room

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temperature operation [13,14]. The core component of membrane separation process is the membrane that can be classified into two categories according to the material of

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the membranes: organic polymeric membranes and inorganic membranes. The membrane technology based on organic polymeric membranes has been widely

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recognized for Cr(VI) wastewater treatment. Recently, as an important member of membrane materials, the inorganic membranes offered many advantages over polymeric membranes, such as outstanding mechanical strength, high temperature resistance, and excellent chemical stability [15,16]. Therefore, increasing attention and efforts were payed to prepared inorganic ceramic membrane to remove Cr(VI) 4

from industrial wastewater. Shukla et al. [17] prepared a NOx modified zeolite–clay composite membranes by sintered, in situ crystallization, and calcination processes, and discovered that the NOx modification could enhance the intrinsic rejection coefficients of the membranes. For the purpose of decrease cost, Vasanth et al. [18] fabricated a ceramic membrane using budget initial materials (kaolin, quartz, and calcium carbonate) through dry compaction followed by sintering process, and

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obtained the maximum Cr(VI) rejection of 94% with the flux of 2.07×10-5 (m3∙m-2∙s-1). Basumatary et al. [16,19] synthesized ceramic-supported MCM-41, MCM-48 and faujasite zeolite membrane via uniaxial compaction, sintering, and hydrothermal

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sedimentation of the zeolites on surface of ceramic matrix. The highest Cr(VI)

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rejection achieved 82%, 75%, and 77% for FAU, MCM-41 and MCM-48 membrane, respectively in cross flow mode under the condition: Cr(VI) concentration of 1000

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mg∙L-1, pH of 2.35, and TMP of 345 kPa. Kumar et al. [20] reported an analcime-C

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zeolite membrane supported by ceramic through in situ hydrothermal crystallization method, and the maximum Cr(VI) rejection of 84% was obtained under TMP of 207

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kPa. Choudhury et al. [21] fabricated a CuO/hydroxyethyl cellulose composite ceramic membrane, when this membrane was used to remove Cr(VI) and Pb(II) from

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wastewater, the Cr(VI) rejection reached 91.44% at 2 bar transmembrane pressure. However, as mentioned above, ceramic membranes were known disadvantages involving complex preparation process, indispensable sintering at high temperatures, and high production cost. Hence, it is significant to develop low cost inorganic membranes with simple synthesis process. 5

Geopolymer, also known as alkali-bounded-ceramic [22], is a novel amorphous aluminosilicate inorganic polymer, which has analogous property with ceramic but more environmentally friendly (utilization of solid waste and without CO2 emission in producing) and low cost (without sintering) [23]. Recently, geopolymer was developed to inorganic membrane material, and it showed promising application in removing heavy metal ion from wastewater [24]. Further, several latest studies

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indicated that hydrothermal process of geopolymer could obtain geopolymer-zeolite composites [25-27], where the geopolymers on the surface and in the pores were

preferentially converted to zeolites which could alter the surface and pore properties

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[28]. To the best of our knowledge, geopolymer-zeolite composite membrane for

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Cr(VI) separation from wastewater has not been reported to date.

In this study, a new and low-cost geopolymer-Li-ABW zeolite composite

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membrane was fabricated via a facile geopolymerization followed by hydrothermal

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process using circulating fluidized bed fly ash (CFBFA) as raw materials. The inorganic membrane was further used to remove Cr(VI) from aqueous solutions,

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because the microporous Li-ABW zeolite membrane layer supported by geopolymer could provide excellent separation efficiency for Cr(VI) contaminant. It is significant

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for decreasing the environmental hazards due to recycling of CFBFA industrial solid wastes as well as removal of virulent Cr(VI). In addition, the effects of transmembrane pressures, Cr(VI) concentration, pH, ionic strength, and co-existing ions on Cr(VI) rejection were investigated. This study aims to achieve a triple-objective simultaneously: high-value-added utilization of CFBFA industrial 6

solid wastes, low-cost production of a novel inorganic membrane, and efficient separation of Cr(VI) pollutants from aqueous solution. 2. Experimental 2.1. Materials CFBFA was provided by the Shenhua Junggar Energy Corporation in Junggar, Inner Mongolia, China, and it was used as initial material to prepared geopolymer

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after milling for 30 s in a vibromill. Precipitated white carbon black (SiO2 ≥ 90%)

purchased from Beimo Industrial Corporation, Shanghai, China was used to adjust

Si/Al mole ratio of geopolymer. The lithium hydroxide (LiOH), potassium dichromate

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(K2Cr2O7), and various inorganic salts were purchased from Sinopharm Chemical

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Reagent Co. Ltd. in Xi’an, China.

2.2. Fabrication of geopolymer-zeolite composite membrane

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The typical fabrication of geopolymer-zeolite composite membrane included

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preparation of geopolymer membrane and the partial conversion of geopolymer membrane into geopolymer-zeolite composite membrane through a hydrothermal

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treatment. An alkali-activator solution was fabricated by dissolving 5.04 g of LiOH in 50 mL deionized water, 100 g of CFBFA and 18 g of precipitated white carbon black

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were homogenized and then sufficiently stirred with the alkali-activator solution in a net stirrer to uniform paste, which was poured into a disk-like stainless steel mold with the size of 40 mm in diameter and 6 mm in thickness. The mould was packed in a plastic film bag to curing at 80°C for 24 h to obtain CFBFA based geopolymer membrane. Hydrothermal process was performed in a 200 mL autoclave with 7

Teflon-liner containing a geopolymer membrane and 50 mL of LiOH solution under different conditions as shown in Table 1. A geopolymer-Li-ABW zeolite composite membrane could be obtained after sufficient washing and drying at 80°C for 12 h. Further, geopolymer-zeolite composite blocks with size of 20 mm × 20 mm × 20 mm were fabricated to detect compressive strength of the composite. 2.3. Characterization of geopolymer-zeolite composite membrane

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The chemical composition of CFBFA was characterized using a Bruker S4 Pioneer analyser. Particle dimension of CFBFA was detected by a laser particle

analyzer (Sympatec, Helos-Rodos, Germany). XRD patterns of specimens were

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recorded using an X-ray diffractometer (Rigaku D/MAX-2400, Japan) in the 2θ range

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of 5–70° at a scanning rate of 10°∙s-1 under working current of 40 mA and working voltage of 40 kV. FT-IR spectra of specimens were detected in the range from 400 to

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4000 cm-1 using an infrared spectrometer (Bruker Tensor 27, Germany). The

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morphologies of samples were captured using a Hitachi SU8000 field emission scanning electron microscope. The nitrogen adsorption experiments were carried out

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using a porosimetry analyser (Micromeritics ASAP 2020, America) at 77 K. The surface area and pore volume of samples were obtained by the

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Brunauer-Emmett-Teller (BET) model and Barrett-Joyner-Halenda (BJH) method respectively. The thermogravimetric analysis (TGA) was performed on a simultaneous thermal analyzer (Mettler Toledo TGA/DSC 1, Switzerland) in the range of 50–800°C under air atmosphere. Zeta potential of the geopolymer-zeolite composite membrane was carried out on a solid surface Zeta potential analyzer 8

(Anton Paar SurPASS 3, Austria) at various pH values. 2.4. Pure water flux Pure water flux test was conducted in a home-made dead-end filtration device immobilized a geopolymer-zeolite composite membrane. Prior to pure water flux measurement, 200 mL of deionized water was permeated through the membrane at 100 kPa to obtain a clean membrane. Then, pure water was injected into the device by

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a peristaltic pump and the penetrant was collected every ten minutes at various

transmembrane pressures (10–90 kPa). Mass of penetrant (m (kg)) was conducted by

membrane is determined by Eq. (1): m

(1)

A×t

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J=

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an electronic balance and the pure water flux of geopolymer-zeolite composite

where J (kg∙m-2∙h-1) refers to the pure water flux, A (m2) is to the effective filtration

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penetration time.

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area of the geopolymer-zeolite composite membrane, and t (h) refers to the

2.5. Membrane separation of Cr(VI)

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Cr(VI) solution with concentration of 1000 mg∙L-1 was prepared by dissolving K2Cr2O7 into deionized water. A typical membrane separation of Cr(VI) experiment

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includes permeation flux test as well as rejection test of Cr(VI). The test method of permeation flux is similar to that of pure water flux. The feed (K2Cr2O7 solution) was injected into the device by a peristaltic pump, and the penetrant was collected every ten minutes, and permeation flux was calculated by Eq. (1). The rejection Cr(VI) in the penetrant of the experimentally simulated Cr(VI) solution was detected on an 9

inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer Optima 8000, America, limit of detection: 2×10-4 mg∙L-1), and the rejection rate (R (%)) of Cr(VI) was determined by Eq. (2): R(%) =

Cf - Cp Cf

× 100

(2)

where Cf and Cp refer to Cr(VI) concentration (mg∙L-1) in the feed and the penetrant, separately.

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In order to study the effect of pH value of the feed, the pH values of Cr(VI)

solution (1000 mg∙L-1) were adjusted in ranges from 3 to 11 with 0.1 M hydrochloric acid or 0.1 M sodium hydroxide. The Cr(VI) solution with different pH values were

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filtrated on the geopolymer-zeolite composite membrane under the TMP of 10 kPa,

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and the penetrants were collected to obtain the Cr(VI) rejection. To investigate the influence of Cr(VI) concentration, the Cr(VI) solutions with different concentration

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(10~1800 mg∙L-1) were prepared by dissolving K2Cr2O7 into deionized water, and

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were filtrated under the TMP of 10 kPa and pH value of 7. The effect of ionic strength was studied by a series of Cr(VI) samples (100 mg∙L-1, pH=7) with different

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concentrations of NaCl (concentration = 0, 1, 3, 5, 7, and 9 mM). For the influence of coexisting anions, the coexisting anions solutions were prepared by dissolving sodium

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salts of chlorides (Cl-), sulfates (SO42-), nitrate (NO3-), hydrogen phosphate (HPO42-), and mixture of them in the Cr(VI) solution with concentration of 100 mg∙L-1 respectively. In order to study the Cr(VI) separation performance for the real environmental water on the as-prepared geopolymer-zeolite composite membrane, the river water samples were collected from Ba River and Wei River in Xi’an, Shaanxi 10

province. The samples were filtrated by a Poly(vinylidene fluoride) (PVDF) membrane with diameter of 5 cm and pore size of 45 μm to remove the effects of silt and organic pollutants. There might be Cr(III) and Cr(VI) in real water samples, but the ICP-OES technology was used to detect the total of chromium ions in different valence states of Cr(III) and Cr(VI). Therefore, the concentration of Cr(VI) in river water sample was measured with the 1,5-diphenylcarbohydrazide spectrophotography

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using a Visible Spectrophotometer (Jinghua 752N, China, limit of detection: 4×10-3 mg∙L-1), and the Cr(VI) rejection rate was calculated by Eq. (2). To reduce

experimental error, the average values of three times repetitive tests of Cr(VI)

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rejection and flux were reported.

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3. Results and discussion

3.1. The physicochemical properties of the CFBFA

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The CFBFA used in this study is same as that in our previous work [29]. The

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chemical composition of CFBFA is determined by X-ray Fluorescence (XRF), in which the contents of SiO2, Al2O3, CaO, Fe2O3, and TiO2 are 35.14wt%, 45.35wt%,

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2.87wt%, 2.61wt%, and 1.82wt% respectively. Fig. 1 (a) shows the particle size distribution of the CFBFA. It is discovered that the particle size distribution of

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CFBFA is wide. The particle size of CFBFA in the ranges of < 20 μm, 20~40 μm, 40~60 μm, 60~80 μm, 80~100 μm, and >100 μm are 62.03%, 17.29%, 9.13%, 6.55%, 3.46%, and 1.54% respectively, and the median particle size of the CFBFA is 12.46 μm. The pore size distribution of CFBFA measured by N2 adsorption-desorption method is displayed in Fig. 1 (b). It can be seen that CFBFA contains mainly 11

mesoporous and macropores, further, the CFBFA has a BET surface area of 9.8304 m2∙g-1 and an average pore diameter of 11.5941 nm. 3.2. Effects of variables on synthesis of geopolymer-zeolite composite membrane It is considered that the hydrothermal condition is the key procedure for synthesis of geopolymer-zeolite composite membrane [25, 30]. Therefore, various variables including hydrothermal temperature, time, and LiOH concentration were

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investigated to obtain geopolymer-zeolite composite membrane having Li-ABW

zeolites as much as possible (the highest diffraction peak intensity). The XRD pattern of the geopolymer membrane is shown in Fig. 2(a). There is a typically broad diffuse

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hump in ranges of 20–40°, which are derived from amorphous CFBFA geopolymer.

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Fig. 2(b) displays the XRD patterns of geopolymer-zeolite composite membrane under different hydrothermal temperature (120–200°C) for 24 h in 2 M LiOH solution.

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It is found that there are a series of diffraction peaks at 2θ of 13.78°, 17.24°, 20.51°,

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20.96°, 28.29°, 29.65°, and 36.08°, which are assigned to Li-ABW zeolite (JPCDS card No. 80-0463). Further, the intensities of diffraction peak gradually increases with

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the hydrothermal temperature increasing from 120 to 180°C; however, the intensities rapidly decreases as the hydrothermal temperature further rising to 200°C, which is

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caused by the collapse of zeolite crystals framework and transformation into the amorphous matter under high temperature [31]. Therefore, the optimal temperature of is determined to 180°C for further synthesis of geopolymer-zeolite composite membrane. Some previous literatures also reported that the optimal synthesis temperature of Li-ABW zeolite is 180°C [32]. 12

The influence of hydrothermal times of 18–36 h on synthesis of the geopolymer-zeolite composite membrane at 180°C in 2M LiOH solution is investigated (Fig. 2(c)). The Li-ABW zeolite phase appears after hydrothermal treatment for 18 h, and a well-crystallized Li-ABW zeolite is obtained after 24 h. However, the XRD peaks do not further intensify but decrease slightly when the hydrothermal time is further prolonged to 30 and 36 h, implying that the optimal

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hydrothermal time is 24 h.

It is known that the nucleation and growth of zeolites significantly depend on the kind and concentration of alkali metal cations [33]. Fig. 2(d) shows XRD patterns of

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the geopolymer-zeolite composite membrane synthesized in LiOH solutions with

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different concentration (0–4M) at 180°C for 24 h. None crystalline was detected when deionized water was used as the hydrothermal mother liquor. A

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well-crystallized Li-ABW zeolite is obtained when the LiOH concentration increases

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to 1 M, however, the diffraction peak intensity generally decreases and a peak at 33.35° assigned to LiOH (JPCDS card No. 76-1074) as the LiOH concentration

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further rises to 2 M, 3 M, and 4 M, suggesting too high LiOH concentration can dissolve the zeolite framework [34]. Therefore, the optimal synthesis condition of

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geopolymer-zeolite composite membrane is the hydrothermal temperature at 180°C, time for 24 h, and LiOH concentration of 1M. The compressive strength of geopolymer-zeolite composite block synthesized under the optimal condition is 12.4 MPa. In addition, the content of Li-ABW zeolite phase in the composite membrane synthesized under the optimal condition is determined to 51.86% by an external 13

standard method (corundum standard sample) using the Rietveld refinement method in PANalytical X’Pert HighScore Plus Software [25]. To verify the repeatability of the hydrothermal process, the XRD patterns of two repeat samples prepared under the optimal condition are detected, and the results are shown in Fig. 2(e). The XRD patterns of two contrast samples are almost same, suggesting the hydrothermal treatment is a reliable method for preparation of the geopolymer-Li-ABW zeolite

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composite membrane.

3.3. Characterization of geopolymer-zeolite composite membrane

Fig. 3 shows FESEM and HRTEM images of the geopolymer membrane and the

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geopolymer-zeolite composite membrane. As shown in Fig. 3(a), the surface of

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geopolymer membrane consists of flat gel and some obvious porous structure. The morphology of cross section shows that the geopolymer membrane is comprised of

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amorphous geopolymer gel particles and intergranular pore (Fig. 3(b) and (c)). The

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surface of the geopolymer-zeolite composite membrane shows a compact zeolite layer in Fig. 3(d), which has thickness about 1.5 μm as shown in Fig. 3(e). The zeolite layer

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plays an important role in membrane separation process, because the ABW framework has one-dimensional 8-membered ring channel system (0.34 × 0.38 nm)

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which is smaller than sizes of Cr(VI) contaminants [35]. Under the zeolite layer, the amorphous geopolymer as support and rod-like Li-ABW zeolite can be observed coexisting inside the membrane in Fig. 3(f). It is discovered in Fig. 3 (g and h) that there are amorphous geopolymers and crystal zeolite in the composite membrane, which is in accordance with the results of FESEM. In addition, the d-spacing with 14

0.3170 nm and 0.3028 nm are ascribed to the (310) and (121) planes of Li-ABW zeolite respectively as shown in Fig. 3(i). FT-IR spectra of the geopolymer membrane and the geopolymer-zeolite composite membrane are displayed in Fig. 4(a). In the spectrum of the geopolymer membrane, the presences of the bands in the ranges of 3700–3100 cm-1 and at 1650 cm-1 derive from the stretching and bending vibrations of hydroxyl groups, separately

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[36]. Two weak adsorption peaks at 1445 and 1500 cm-1 are attributed to the stretching vibrations of carbonate which is from the reaction of LiOH with

atmospheric CO2 [37]. The band appeared at 465 cm-1 refers to Si-O-Si bending

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vibration, while the peak around 1020 cm-1 is assigned to the asymmetric stretching

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vibration of Si-O-Si (Al) [38]. There are remarkable difference in ranges of 1100–400 cm-1 between the geopolymer membrane and the geopolymer-zeolite composite

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membrane. A series of adsorption bands at 440 cm-1, 540 cm-1, 609 cm-1, 700 cm-1,

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930 cm-1, 1004 cm-1, and 1085 cm-1 are characteristic bands of Li-ABW zeolite [32]. As shown in the XPS spectra of the geopolymer membrane and the

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geopolymer-zeolite composite membrane (Fig. 4 (b)), both the membranes are comprised of Si, Al, and O elements. There are usually four types chemical states of

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oxygen including Si-O-Si, Si-O-Al, Si-O-H, and Si-O-Li (Na or K) in the geopolymer and zeolite according to the previous literature [39]. From the fitted parameters of the O1s XPS spectra in Fig. 4 (c and d), there are much more Si-O-Si and Si-O-Li in the geopolymer-zeolite composite membrane, but much more Si-O-Al and Si-O-H in the geopolymer membrane. The above results indicate that the geopolymer-zeolite 15

composite membrane is obtained successfully. Fig. 5 shows the N2 adsorption-desorption isotherms (Fig. 5(a)) and pore dimension distribution curves (Fig. 5(b)) of the geopolymer membrane and the geopolymer-zeolite composite membrane. Both the geopolymer membrane and the geopolymer-zeolite composite membrane show type IV isotherm, indicating there are analogous mesoporous structures in both membranes. The pore dimension distribution

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curves display that both membranes have a broad pore size distribution in ranges of

4–100 nm. These mesoporous and macroporous are formed by random accumulation of geopolymer gel and zeolite particles within the membrane as shown in Fig. 3(c)

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and (f), the effect of these porous in the geopolymer-zeolite composite membrane can

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be eliminated when the thickness of the membrane approaches a certain value [30]. The BET specific surface area, average pore diameter size, and pore volume of the

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geopolymer membrane and the geopolymer-zeolite composite membrane are listed in

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Table 2. The geopolymer-zeolite composite membrane has bigger BET surface area and pore volume but smaller average pore size than that of geopolymer membrane. In

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addition, the porosities of the geopolymer membrane and the geopolymer-zeolite composite membrane are determined to 43.61% and 46.86% according to the

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Archimedes drainage method, suggesting the pore structure of geopolymer membrane modified by the hydrothermal process. Fig. 6 shows TG and DSC curves of the geopolymer-zeolite composite membrane from room temperature to 800°C. There are two weightlessness stages in the thermogravimetric curve of the geopolymer-zeolite composite membrane. The 16

first stage exhibits 10.92% of weight loss from 50°C to 295°C, and presents an endothermal peak in DSC (390.38 J∙g-1). This stage is contributed by the loss of adsorbed and occluded water in geopolymer-zeolite composite membrane. The second weight loss stage is 2.78% in ranges of 295–800°C without any obvious exothermal or endothermal peak. These results suggest the excellent thermal stability of the geopolymer-zeolite composite membrane [32].

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The surface charge of membrane plays a very important role in membrane

separation processes, and it is closely related to the pH value of the contact solution [19]. Zeta potentials of the geopolymer-zeolite composite membrane at various pH

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values are illustrated in Fig. 7. The isoelectric point (IEP) of the composite membrane

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is discovered to be pH 4.6. The surface charge of composite membrane is negative at pH values less than the IEP, while the surface charge of membrane is positive at pH

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values bigger than the IEP. The result derives from the contribution of calcium species

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in the circulating fluidized bed fly ash, and similar phenomenon is discovered in a calcium-containing system in previous literature [40].

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3.4. Membrane performance and Cr(VI) separation Pure water flux of the geopolymer-zeolite composite membrane under different

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transmembrane pressures (TMP) is shown in Fig. 8(a). The result clearly demonstrates that the pure water flux of the geopolymer-zeolite composite membrane increases linearly with enhancement of TMP, which follows Darcy’s law [17]. 3.4.1. Effect of TMP The effects of TMP on the rejection and permeate flux of Cr(VI) are researched 17

for a solution with 1000 mg∙L-1 of Cr(VI) at natural pH and the results are presented in Fig. 8(b). The permeate flux of Cr(VI) solution also increases linearly and is a little higher than the pure water flux, which is considered that ions adsorbed on the membrane surface weaken the electrostatic interaction of membrane and polar water so as to increase the flux [41]. Remarkably, the Cr(VI) rejection decreases from 95.38% to 54.90% with increasing of TMP from 5 kPa to 25 kPa, which is associated with

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larger pore diameter of the geopolymer-zeolite composite membrane. Similar reduced rejection with increasing of TMP is also reported in previous literature [42]. 3.4.2. Effect of Cr(VI) concentration

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The influences of various concentration of Cr(VI) (10–1800 mg∙L-1) in feed

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solution on the rejection and permeate flux are studied at a constant TMP of 10 kPa as well as natural pH and the result is displayed in Fig. 8(c). It is clear that the permeate

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flux decrease slightly from 9.82 to 8.21 kg∙m-2∙h-1 with increasing of Cr(VI)

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concentration from 10 mg∙L-1 to 1800 mg∙L-1. In contrary, the rejection of Cr(VI) increases from 70.92% to 90.65% with increasing of Cr(VI) concentration from 10

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mg∙L-1 to 1800 mg∙L-1. The interaction between Cr(VI) ions and membrane can be enhanced with increasing Cr(VI) concentration, resulting in a increasing rejection [43].

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Ashim Kumar Basumatary et al. [19] also reported that Cr(VI) rejection on the MCM-41-ceramic composite increased with Cr(VI) concentration from 250 mg∙L-1 to 2000 mg∙L-1 and the Cr(VI) rejection was constant after 2000 mg∙L-1. 3.4.3. Effect of pH value Previous studies proved that separation performance of Cr(VI) heavily depended 18

on the chemical forms of Cr(VI) [19, 44]. In aqueous solution containing Cr(VI), the Cr(VI) species exist in different anionic forms, such as HCrO4−, CrO42−, and Cr2O72−. The chemical forms of Cr(VI) strongly depend on both the solution pH and total Cr(VI) concentration [17]. The main equilibrium reactions among different Cr(VI) species are displayed in equations (3–5). HCrO4- dominates at pH range of 1.0–6.5, while HCrO4- coexists with Cr2O72- under a higher Cr(VI) concentration (>10-3 M).

H2CrO4 (aq)

HCrO4− (aq) + H+ (aq)

HCrO4− (aq)

CrO42− (aq) + H+ (aq)

(3)

(4)

Cr2O72− (aq) + H2O (l)

(5)

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2HCrO4− (aq)

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When the pH value is higher than 8, CrO42- is the dominant Cr(VI) species [45].

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Hence, the influences of various pH in feed solution containing 1000 mg∙L-1 of Cr(VI) on the rejection and permeate flux are researched at a constant TMP of 10 kPa

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(Fig. 8 (d)). The pH value of the solution in range of 3–11 is adjusted by adding 0.1 M

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of hydrochloric acid or sodium hydroxide solution. According to Donnan Effect, when the charged membrane is in contact with chromium solution, the co-ions (ions

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have the same charge as the membrane) near the surface of the membrane have lower concentration than that in the solution while the concentration of counter-ions (ions

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with opposite charge in the membrane) is higher than that concentration in the solution, thus a potential difference is formed in the interface between the membrane and the solution. Due to the potential difference, the membrane rejects diffusion of co-ions, and the counter-ions are also repelled to keep the electrochemical equilibrium of the solution and the membrane [19, 20]. As shown in Fig. 7, the membrane surface 19

has negative charge under pH value of 3 and has positive charge at pH value from 5 to 9. As a result, the geopolymer-zeolite composite membrane shows high rejection of Cr(VI) (>75.86%) under the pH of the solution from 3 to 11 in Fig. 8(d). According to the above results, the mechanism separation of Cr(VI) on the geopolymer-zeolite composite membrane should be based on size exclusion and electrostatic interaction

3.4.4. Effects of ionic strength and coexisting anions

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[46].

In the practical wastewater, there are usually a certain concentration salts which may affect the membrane separation efficiency. The Cr(VI) solutions with different

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concentrations of NaCl (ranging from 0 to 9 mM) are filtrated by the as-prepared

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geopolymer-zeolite composite membrane to investigate the effect of ionic strength, and the results are shown in Fig. 8(e). It can be seen that the rejection of Cr(VI)

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increases from 78.06% to 94.99% with the enhancement of NaCl concentration from

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0 to 9 mM. Further, the practical wastewater generally contains more than one kind of co-existing anions. Thus, the Cr(VI) solutions with different co-existing anions

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including Cl-, SO42-, HPO42-, NO3-, and the mixture of these anions (concentration of each anions is 5 mM) are filtrated by the as-prepared geopolymer-zeolite composite

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membrane to study the influence of coexisting anions. It is clear that the Cr(VI) rejection for solution with Cl-, HPO42-, NO3-, and mixture of anions are 85.56%, 84.71%, 85.43%, and 89.28% which are higher than the blank sample (78.06%), indicating the above three anions can improve the separation efficiency. However, SO42- shows negative influence on the rejection of Cr(VI), which may be due to the 20

SO42- ions’ high affinity with the membrane [47]. To verify the practical application value, the as-prepared geopolymer-zeolite composite membrane is used to separate Cr(VI) in the real environmental water samples collected from Ba River and Wei River in Xi’an, Shaanxi province. As shown in Table 3, the pH value of water from Ba River and Wei River are 8.0 and 8.1, and the initial Cr(VI) concentration are 0.014 mg∙L-1 and 0.015 mg∙L-1. After filtration on

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the geopolymer-zeolite composite membrane under TMP of 10 kPa, the residual

Cr(VI) concentration in both Ba River and Wei River water decrease by 0.004 mg∙L-1 which reaches the allowance of the environmental regulations defined by European

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Union. The results indicate that the as-prepared geopolymer-zeolite composite

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membrane has promise potential in practical applications.

3.4.5. The durability of the geopolymer-zeolite composite membrane

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After utilization in practice, the pollutants on the membrane are usually washed

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by strong acid or alkali, thus, the durability of the membrane is an important parameter. In this study, the as-prepared geopolymer-zeolite composite membrane was

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soaked into the solution of 1 M NaOH or 1 M HCl for 1 h, and then the rejection as well as permeate flux for Cr(VI) solution (1000 mg∙L-1) were detected to evaluate the

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durability of the composite membrane. As shown in Fig. 9, the membrane soaked in 1 M HCl shows slightly decrease Cr(VI) rejection and the increasing flux due to the dissolving out of some aluminum in the membrane. In addition, the Cr(VI) rejection and the flux on the membrane soaked in NaOH are consistent with the blank sample, suggesting the geopolymer-zeolite composite membrane has excellent alkali 21

resistance and less acid resistance. 3.4.6. Comparison of the geopolymer-zeolite composite membrane with other membranes Table 3 summarizes the previously published results for preparation of various inorganic and organic membranes and their removal of Cr(VI) from solution. For the preparation of inorganic membranes, almost all reported inorganic membranes need

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ceramic support (sintering at high temperature (800–1000°C)) and complex

preparation process, causing high energy consumption and production cost. The

geopolymer-zeolite composite membrane uses CFBFA solid wastes as raw materials

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and only needs facile geopolymerization followed by hydrothermal treatment, which

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greatly reduce the production cost. For separation properties of Cr(VI), the geopolymer-zeolite composite membrane has excellent rejection (85.45%) with the

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highest permeation flux (0.851 kg∙m-2∙h-1∙kPa-1) under smaller TMP (10 kPa). The

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organic polymeric membranes show high rejection of Cr(VI). The Cr(VI) rejection on the as-prepared geopolymer-zeolite composite membrane is 10% less than that on

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organic membranes, but the permeation flux is a tenfold increase. Both preparation cost and separation performance suggest that the geopolymer-zeolite composite

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membrane has a bright prospect in treatment of wastewater containing Cr(VI). 4. Conclusions A novel inorganic geopolymer-zeolite composite membrane was synthesized using CFBFA solid waste as initial material by facile geopolymerization-hydrothermal treatment process, and was employed for removal of Cr(VI) from aqueous solutions. 22

The production cost of the geopolymer-zeolite composite membrane was much less than traditional inorganic ceramic membrane. It was discovered that the as-prepared geopolymer-zeolite composite membranes consisted of amorphous geopolymer accompanying with a compact Li-ABW zeolite layer and had excellent thermal stability. The hydrothermal process could effectively modify the pore structure of geopolymer by conversion of the geopolymer into the geopolymer-zeolite composite

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membrane with bigger BET specific surface area, higher pore volume, and smaller

average pore size. This geopolymer-zeolite composite membrane not only provided a new solution for high-value-added utilization of CFBFA industrial solid wastes, but

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also showed a promising prospect in removal of hypertoxic Cr(VI) from wastewater.

Credit Author Statement

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Author’s contributions: Pan Yang He: Methodology, investigation, writing - original Draft. Yao Jun Zhang*: Review, and supervision. Hao Chen: Software and visualization. Zhi Chao Han: Resources and writing. Li Cai Liu: Editing.

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments This work was supported by the National Natural Science Foundation of China [grant number 21676209], the Key Research and Development Project of Shaanxi Province [grant number 2019GY-137], and the Cultivating Fund of Excellent 23

Doctorate Thesis of Xi’an University of Architecture and Technology [grant number

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6040318008].

24

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Figure captions

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Fig. 1. (a) Particle size distribution and (b) N2 adsorption-desorption isotherm and pore size distribution of the CFBFA.

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Fig. 2. XRD patterns of (a) geopolymer membrane, effects of (b) temperature, (c) time, and (d) LiOH concentrations on geopolymer-zeolite composite membranes, (e) repeat samples under the optimal condition. Fig. 3. FESEM images of (a) surface of geopolymer membrane, (b) and (c) cross section of geopolymer membrane, (d) surface of geopolymer-zeolite composite membrane, (e) and (f) cross section of geopolymer-zeolite composite membrane, and HRTEM images (g, h, and i) of geopolymer-zeolite composite membrane. Fig. 4. (a) FT-IR spectra of the geopolymer membrane and the geopolymer-zeolite composite membrane, (b) XPS spectra of the geopolymer membrane and the 32

geopolymer-zeolite composite membrane, and the O 1s high resolution XPS spectra of (c) the geopolymer membrane, and (d) the geopolymer-zeolite composite membrane. Fig. 5. (a) N2 adsorption-desorption isotherm and (b) pore size distribution of geopolymer membrane and geopolymer-zeolite composite membrane. Fig. 6. TG-DSC curves of the geopolymer-zeolite composite membrane. Fig. 7. Zeta potential of the geopolymer-zeolite composite membrane at various pH.

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Fig. 8. (a) Variation of water flux with TMP for the geopolymer-zeolite composite membrane; Effect of (b) TMP, (c) Cr(VI) concentration, (d) pH, (e) ionic strength, and (f) co-existing anions on rejection of Cr(VI).

60

0.3

40 20 0

10 Particle Size (m)

0.1 0.0

100

-1 3

15 10

-1

3

0.05 0.04 0.03 0.02 0.01 0.00

10

100

Pore Diameter (nm)

5 0 0.0

0.2

0.4

0.6

Relative Pressure (P/Po)

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1

0.2

Quantity Adsorbed (cm g )

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dV/dlog(D) Pore Volume (cm g

Particle Size Distribution (%)

0.5

80

Fig. 1.

(b)

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0.6

)

25

(a)

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Cumulative Proportion (%)

100

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Fig. 9. The durability of the as-prepared geopolymer-zeolite composite membrane.

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0.8

1.0

(b)

(a)

Intensity (a.u.)

Intensity (a.u.)

200C

180C

150C 120C Li-ABW zeolite

10

20

30

40

50

60

70

10

20

2 Theta ()

30

40

50

60

70

2 Theta () (d)

(c)

4M

36 h

3M

24 h

2M

1M

18 h

0M

Li-ABW zeolite 10

20

30

40

50

60

Li-ABW zeolite

70

10

20

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60

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Intensity (a.u.)

30

70

2 Theta ()

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Fig. 2.

40

2 Theta ()

(e)

20

30

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2 Theta ()

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Intensity (a.u.)

Intensity (a.u.)

30 h

34

50

60

70

(a)

(b)

(c)

(d)

(e)

(f)

(g)

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1.5 μm

(i)

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(h)

Si 2s Al 2s Si 2p Al 2p

Si 2s Al 2s Si 2p Al 2p

O 1s

C 1s

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Intensity (a.u.)

O 1s

C 1s

1020

1650 1500 1445

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4000 3500 3000 2500 2000 1500 1000

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1085 1004 930 700 540 440

1600 1500 1445

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3613 3476 3450

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Transmittance (a.u.)

Geopolymer membrane

Geopolymer-zeolite composite membrane

(b) O KLL

(a)

Geopolymer-zeolite composite membrane

O KLL

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Fig. 3.

Geopolymer membrane

500

1200

1000

800

600

400

Binding energy (eV)

-1

Wave Number (cm )

35

200

0

Experiment Fit Baseline Si-O-Li Si-O-Si Si-O-Al Si-O-H

(c)

Intensity (CPS)

120.0k Name Position(eV) Area(%) 4.10 26.72 52.46 16.72

530.51 531.42 532.15 533.05

Si-O-Li Si-O-Si 90.0k Si-O-Al Si-O-H

60.0k

150.0k 120.0k Intensity (CPS)

150.0k

90.0k

Experiment Fit Baseline Si-O-Li Si-O-Si Si-O-Al Si-O-H

(d) Name Position(eV) Area(%) 12.61 Si-O-Li 530.53 60.35 Si-O-Si 531.46 19.39 Si-O-Al 532.08 7.65 Si-O-H 533.10

60.0k 30.0k

30.0k 0.0 546

543

540

534

537

531

0.0

525

528

540

545

535

530

525

Binding Energy (eV)

Binding Energy (eV)

Geopolymer-zeolite composite membrane Geopolymer membrane

80 60 40

0 0.0

0.2

0.4

0.6

0.8

Geopolymer-zeolite composite membrane Geopolymer membrane

0.12 0.10 0.08 0.06 0.04

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20

(b)

0.14

-p

-1

dV/dlog(D) Pore Volume(cm3g )

-1

0.00

1.0

1

10 Pore Diameter (nm)

lP

Relative Pressure (P/Po)

0.02

na

Fig. 5.

100

3

TG DSC

Weight loss: 10.92%

1 Weight loss: 2.78% 0

-390.38 Jg

80

-1

-1

295C

-2

Jo

Weight (%)

ur

90

-3

70

-4 -5

60 100

2

200

300

400

500

600

Temperature (C)

Fig. 6. 36

700

-6 800

Heat Flow (mw)

Quantity Adsorbed (cm3g )

100 (a)

ro of

Fig. 4.

100

8

0

IEP 4.60

-4

ro of

Zeta Potential (mV)

4

-8 -12 3

4

5

6

7

8

9

10

-p

pH

re

Fig. 7.

100

20

(b)

lP

80 60

12

10

na

-2

-1

Flux (kgm h )

-1

Flux (kgm-2h )

16

15

5

0

0

5

10

15

20

40

8

Permeate Flux Rejection

4 0

25

0

5 10 15 20 25 Transmembrane Pressure (kPa)

ur

Transmembrane Pressure (kPa)

40

4

20

800

1200

1600

80

-1

8

0

Flux (kgm-2h )

60

400

100

16

80

12

0

0 30

(d) Peremate flux Rejection

-1

Flux (kgm-2h )

Jo

16

20

20

100

(c)

Rejection (%)

20

Rejection (%)

(a)

0 2000

Permeate flux Rejection

12 8

40

4

20

0

0 2

-1

Cr(VI) Concentration (mgL )

4

6

8 pH

37

60

10

12

Rejection (%)

20

100

100

(f)

(e) Cr(VI) Rejection (%)

Rejection (%)

90 80 70 60

80 60 40 20 0

50 0

2

4

6

8

10

Blank

Cl

-

-

SO4

-

PO4

3-

Co-existing Anions

Cl concentration (mM)

NO3

-

Mixing Anions

90

ro of

Fig. 8.

10.0 85.45

85.43 82.56

70

-2

8.51

8.51

8.5

-p

Rejection (%)

9.0 8.75

8.0

re

60

7.0

NaOH

ur

Table captions

HCl

7.5

na

Fig. 9.

lP

50 Blank Sample

-1

80

Flux (kgm h )

9.5

Jo

Table 1 Hydrothermal condition to synthesize geopolymer-zeolite composite membranes. Table 2 BET surface area, average pore size, and pore volume of the geopolymer membrane and the geopolymer-zeolite composite membrane. Table 3 Removal of Cr(VI) in the real environmental water samples. Table 4 Comparison of the as-prepared membrane with other membranes.

38

Table 1 Temperature (°C)

Time (h)

LiOH concentration (M)

1 2 3 4 5 6 7 8 9 10 11 12

120 150 180 200 180 180 180 180 180 180 180 180

24 24 24 24 12 18 30 36 24 24 24 24

2 2 2 2 2 2 2 2 0 1 3 4

BET surface area (m²∙g-1) 13.36

Pore volume (cm³∙g-1)

18.89

0.0827

30.19

15.99

0.1582

Table 3

pH

Initial concentration (mg∙L-1)

Residual concentration (mg∙L-1)

Rejection (%)

8.0 8.1

0.014 0.015

0.004 0.004

71.42 73.33

ur

Samples

na

lP

Geopolymer membrane Geopolymer-zeolite composite membrane

Average pore size (nm)

re

Sample

-p

Table 2

ro of

Number

Jo

Water from Ba River Water from Wei River

39

40

ro of

-p

re

lP

na

ur

Jo

f oo

Table 4

Separation condition Inorganic membrane

Preparation process

MCM-41-ceramic membrane

Uni-axial compaction, support sintering

MCM-48-ceramic membrane

(800–1000°C), hydrothermal crystallization, and calcination (550°C)

Ceramic microfiltration membrane Analcime-C zeolite-ceramic composite membrane

pr

Cr(VI) Concentration (mg∙L-1)

345

1000

e-

Jo ur

CuO/hydroxyethyl cellulose composite ceramic membrane Hydrophilic and surface functional polysulfone membrane M-phenylene isophthalamide membrane Geopolymer-zeolite composite membrane

Casting clay mixture in gypsum surface, sintering (900°C), hydrothermal crystallization, and calcination (450°C) Uniaxial dry compaction, dried at 100°C followed 200°C, and sintered (900°C) Uni-axial pressing and sintering (950°C) to ceramic support, in situ hydrothermal crystallization, and calcination (400°C) Dispersing CuO in binder, adding PEG solution into CuO suspension, coating slurry on support, and dried

Pr

Zeolite–clay membrane modified with NOx

na l

Faujasite-ceramic membrane

TMP (kPa)

Continuous contacting of polysulfone with dopamine solution, and washed Preparation of casting dopes, casting, and formation of asymmetric structure Stirring alkali-activator and CFBFA, casting, curing, and hydrothermal treatment

pH value 2.35

Rejection (%)

Flux (kg∙m-2∙h-1∙kPa-1)

Reference

75

0.217

[16]

77

0.150

[16]

82

0.219

[16]

483

1000

∼2

66

0.0054

[17]

207

100

1

94

0.0036

[18]

207

1000

2.3

84

0.104

[20]

200

5

7.1

91.44

0.350

[21]

200

5

3

94

0.065

[44]

800

5

7

98

/

[14]

10

1000

7

85.45

0.851

This work

41