Green synthesis of zeolite 4A using fly ash fused with synergism of NaOH and Na2CO3

Accepted Manuscript Green synthesis of zeolite 4A using fly ash fused with synergism of NaOH and Na2CO3 Liyun Yang, Xiaoming Qian, Peng Yuan, Hao Bai, Takahiro Miki, Fanxu Men, Hong Li, Tetsuya Nagasaka PII:

S0959-6526(18)33662-X

DOI:

https://doi.org/10.1016/j.jclepro.2018.11.259

Reference:

JCLP 15018

To appear in:

Journal of Cleaner Production

Received Date: 15 June 2018 Revised Date:

17 September 2018

Accepted Date: 27 November 2018

Please cite this article as: Yang L, Qian X, Yuan P, Bai H, Miki T, Men F, Li H, Nagasaka T, Green synthesis of zeolite 4A using fly ash fused with synergism of NaOH and Na2CO3, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2018.11.259. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Green synthesis of zeolite 4A using fly ash fused with synergism of

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NaOH and Na2CO3

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Liyun Yang1,2 *, Xiaoming Qian1, Peng Yuan1, Hao Bai1, Takahiro Miki2, Fanxu Men1, Hong Li1and Tetsuya Nagasaka2

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1 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 100083, P.R. China 2 Department of Metallurgy, Graduate School of Engineering, Tohoku University, 02 Aoba-yama, Sendai 980-8579, Japan.

Abstract

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A high-purity zeolite 4A was synthesized by the hydrothermal method using fly ash as the

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raw material. The effects of sodium hydroxide (NaOH) or/and sodium carbonate (Na2CO3) on

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the activation of fly ash were studied, and the removal efficiency of Cu2+ in aqueous solution

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was also investigated for the synthesized zeolite. The formation process of the zeolite from fly

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ash was surveyed by ex situ techniques such as X-ray diffraction (XRD), thermogravimetric

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analysis and differential scanning calorimetry (TG-DCS), scanning electron microscopy and

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energy dispersive X-ray spectroscopy (SEM-EDX) and Fourier transform infrared

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spectroscopy (FTIR). The important influential factors of energy and water consumption were

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analysed by orthogonal tests, and the best conditions for green synthesis were accurately

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determined through batch tests. The factors affecting the green synthesis of zeolite are the

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alkali mixture ratio, alkali melting temperatures, solid-to-liquid ratios, crystallization times

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and crystallization temperatures. The alkali mixture ratio was indicated to be an important

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factor for green synthesis according to the results of the orthogonal test. Compared with the

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use of alkali alone, when NaOH and Na2CO3 were mixed at a mass ratio of 1:2.8, the alkali

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melting temperature (760°C) and solid-to-liquid ratios (1:5) were both lower, the

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crystallization time (4 h) was shorter in the zeolite synthesis process, and the relative

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crystallinity was the highest at 75.8%. The removal rate of 100 mg/L Cu2+ solution from pH 3

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to pH 7 by 0.18 g of zeolite synthesized for 60 min was close to 100%, and the adsorption

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capacity was 55.5 mg/g. After the zeolite was desorbed and reused 4 times, the removal

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efficiency of Cu2+ was maintained at 73% at a pH of 3.

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Keywords: Coal fly ash; zeolite 4A; NaOH and Na2CO3; the hydrothermal method.

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1 Introduction

Fly ash is a by-product of coal combustion for power generation, and each ton of coal

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produces approximately 0.15 tons of fly ash depending on its composition (Mei et al., 2016; R,

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2016). Coal is one of the world’s most important fossil fuels. With the development of the

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world economy and the rapidly increasing energy demand, large amounts of fly ash are

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produced by power generation or industrial coal combustion. According to statistics,

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approximately 500 Mt fly ash is produced by coal-fired power generation in China each year

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(Xiaotong et al., 2015). Due to the lack of adequate use, approximately 50% of fly ash is

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expelled through waste stacks or deposited in landfills (Izidoro et al., 2012), which not only

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wastes resources but also causes environmental pollution. It has been found that fly ash

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contains a large amount of Si and Al, which can be used as raw materials for the synthesis of

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high-value-added inorganic materials such as zeolites (Aldahri et al., 2016; Fukasawa et al.,

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2017).

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Fly ash has a half-empty or solid spherical structure composed of a glass substrate with

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inner and outer shells, where Si and Al exist in the amorphous phase and α-quartz and mullite

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comprise the crystal phase (Tanaka et al., 2006). Si and Al are presence in high proportions in

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the α-quartz and mullite crystals phases and are difficult to dissolve in water, which prevents

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the release of active Si and Al in solution (Marion et al., 2010). On the other hand, dissolution

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of the Si and Al components plays a crucial role in zeolite nucleation and crystallization

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(Ojumu et al., 2016). Therefore, the activation of Si and Al in fly ash is one of the important

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links in the synthesis of zeolite from fly ash. The melting-hydrothermal synthesis is a

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traditional and conventional method which has potential for the scale-up production for the

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development of zeolite from fly ash (Hong et al., 2017). This traditional method mainly uses

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alkaline activators to convert SiO2 and Al2O3 into zeolite crystalline phases under

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hydrothermal conditions (high temperature and water saturation pressure) (Holler and

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Wirsching, 1985). Alkaline activators can weaken the binding between SiO2 and Al2O3 in the

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α-quartz and mullite crystal phases of fly ash, damage the SiO2 and Al2O3 bond structure,

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ACCEPTED MANUSCRIPT produce a large number of faults and free ends at the break point, cause particle movement,

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induce a thermodynamically unstable state, and form soluble amorphous Si, Al and

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hydroxysodalite (Cheng, 2006; Ke Ming et al., 2007; Wang et al., 2013). It is known that Na+

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has a good promotion effect on the nucleation and crystallization of the zeolite synthesis

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process (Murayama et al., 2002). Therefore, researchers generally use molten NaOH or

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Na2CO3 to treat fly ash, in which the alkaline component activates Si and Al to produce a

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high-activity component (And and Shih, 1998) and Na+ participates in nucleation and

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crystallization.

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And and Shih (1998) found that the quartz and mullite crystalline phases of fly ash are

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gradually converted to water-soluble sodium silicate and sodium aluminate by melting with

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NaOH at 550°C. El-Naggar et al. (2008) observed that when fly ash mixed with NaOH at the

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mass ratio of 1:1 and melted at 550°C, a rough porosity gradually developed on the originally

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smooth spheres. In additional, deeper pores were created with an increase in the alkali fusion

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time. Their observations indicate that the quartz in the original fly ash gradually transformed

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into sodium silicate. In previous studies, it was noted that OH- in NaOH has a high alkalinity.

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Thus, it activates the inert Si and Al in fly ash. Na2CO3 is not as basic as NaOH, but, under the

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same alkalinity conditions, Na2CO3 provides a high content of Na+. Murayama et al. (2002)

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successfully synthesized zeolite P, which has a relatively high crystallinity, from fly ash

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treated with Na2CO3. For the pretreatment and nucleation of fly ash, Na2CO3 has an optimal

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alkalinity and a high content of Na+, which should have beneficial effects on the zeolite

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nucleation process (Murayama et al., 2002). Comparing NaOH to Na2CO3 at the same

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alkalinity, NaOH provides less Na+, while Na2CO3 provides excess Na+. Therefore, NaOH and

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Na2CO3 mixed at a certain percentage to treat fly ash can provide the appropriate amount of

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OH- and Na+ and may lead to the highest activation of Si and Al in the fly ash to achieve the

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highest degree of crystallization, as well as low energy and water consumption, which is

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probably useful in realizing a green synthesis. However, related studies have not been yet

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conducted in this area.

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Due to this background, our study activated fly ash using NaOH, Na2CO3, and a

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combination of NaOH and Na2CO3 to synthesize zeolite 4A with orthogonal and batch tests.

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The important influential factors of energy and water consumption were analysed by the 3

ACCEPTED MANUSCRIPT orthogonal tests and the optimal conditions for green synthesis were determined by batch

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experiments. The efficiency and reuse times of synthetic zeolite 4A for the adsorption of Cu2+

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in aqueous solution were also investigated.

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2 Materials and methods

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2.1 Materials and equipment

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The fly ash used in the experiments was obtained from the Datang International

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coal-fired power plant of Erdos City in China. To reduce the cost of actual production in the

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future, diatomite, obtained from Fengsheng mining in Chin, was used as a silicon source

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instead of a pure chemical reagent. Analytical-grade NaOH, Na2CO3 and Cu(NO3)2 were

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obtained from the Beijing chemical plant of China. The purity levels of the analytical grade

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NaOH, Na2CO3 and Cu(NO3)2 are 96%, 99.8% and 99.5% respectively. Among them, NaOH

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and Na2CO3 were used to synthesize zeolite, and Cu(NO3)2 was used for the adsorption

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experiment. Deionized water with a conductivity of 1.5 µs/cm and total organic carbon of

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0.48 mg/L was used in the experiments. The samples were oven-dried at 105°C for 12 h to

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remove any adsorbed moisture prior to analysis. The identification and quantification of the

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mineral phases present in the raw materials and zeolite products were carried out using

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qualitative X-ray diffraction (XRD, Japan Science Company). The samples were step-scanned

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over a range of 5°< 2θ < 90° at intervals of 0.07° and measured for 0.5 s per step. Crystalline

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mineral phases present in the samples were identified with the help of Jade 5.0 software by

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comparing the spectra with the standard line patterns from the powder diffraction file

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database supplied by the International Centre for Diffraction Data. The high-temperature

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reaction product from the fly ash mixture was examined by means of thermal gravity analysis

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using a distributed control system (TG-DCS, Japan Electronics Corporation). The

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morphology and elemental content of both the raw materials and solid products were

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examined by scanning electron microscopy and energy dispersive X-ray spectroscopy

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(SEM-EDX, Japan Electronics Corporation). Compositional analysis of both the raw

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materials and products was carried out using X-ray fluorescence spectrometry (XRF,

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ACCEPTED MANUSCRIPT Shimadzu Corporation), and the functional groups present in the fly ash and the solid products

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were identified using Fourier transform infrared spectroscopy (FTIR, Nicolet Company).

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Physical and chemical adsorption analysis of the products was carried out using the

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Brunauer-Emmett and Teller equation (BET) and temperature programmed desorption (TPD,

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Micromeritics Instrument Corporation). The metal ion concentration in the liquid samples was

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determined using a flame atomic absorption spectrophotometer (FAAS, Shimadzu

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Corporation).

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2.2 Methods

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2.2.1 Synthesis of zeolite 4A

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The hydrothermal synthesis experimental procedure of fly ash alkali melted separately

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with NaOH, Na2CO3, NaOH and Na2CO3 was performed as follows. The fly ash was ground

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through a ball mill and passed through a 325-mesh sieve to obtain a particle size of less than

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45 µm; then, it was mixed with diatomite and NaOH/Na2CO3/NaOH and Na2CO3. After

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mixing, the fly ash was packed in the crucible and placed in a box sintering furnace for

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high-temperature calcination, and a constant temperature was maintained for a certain period

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of time. After cooling in the furnace, the calcined product was removed, stirred with

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deionized water at room temperature for 2 h and aged for 4 h. Then, the product was

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transferred into a PTFE-lined stainless-steel reactor and placed in a drying oven for a certain

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period of time. Then, after filtration, washing, and drying at 80°C, the zeolite 4A product was

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obtained. The detailed experimental process is shown in Fig. 1.

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Fig. 1. Process for the synthesis of zeolite 4A by the hydrothermal method. A: Alkali fusion

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hydrothermal synthesis using only NaOH, B: Alkali fusion hydrothermal synthesis using only

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Na2CO3, C: Alkali fusion hydrothermal synthesis using NaOH and Na2CO3. The characteristic diffraction peaks, Im of the zeolite 4A were determined from the X-ray

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diffraction pattern of standard commercial 4A molecular sieves with known crystallinity

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(Rayalu et al., 2005). The relative crystallinity of the sample is

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Pi = Ii / Im × Pm

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where Pi and Pm are the relative crystallinity of the sample zeolite and the crystallinity of the

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commercial-grade zeolite, respectively. Ii and Im are the peak intensities of the characteristic

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diffraction peaks of the zeolite and the commercial-grade zeolite 4A at 7.2°, respectively.

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2.2.2 Orthogonal tests and batch experiments

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According to the synthetic method described above (2.2.1), the orthogonal test was

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carried out to determine the important influence factors for green synthesis. In the process of

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zeolite 4A synthesis, alkali melting temperature, alkali mixture ratio, solid-liquid ratio,

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crystallization time and crystallization temperature are the main parameters, in which, alkali

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melting temperature, crystallization time and crystallization temperature determine energy 6

ACCEPTED MANUSCRIPT consumption and solid-liquid ratio influences water consumption. According to the former

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literature (Cardoso et al., 2015; Hu et al., 2017; K. S. Hui and C. Y. Chao et al., 2005; Hui

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and Chao, 2006; Ke-Ming and Zhu, 2007; Kim and Lee, 2009), the optimal crystallization

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temperature is 90°C whether choosing NaOH or Na2CO3 alone as the alkali source, thus the

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crystallization temperature was preliminarily set at 90°C in these orthogonal tests. Alkali

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melting temperature, alkali mixture ratio, solid-liquid ratio and crystallization time were the

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four main factors and five levels were designed in orthogonal tests (Table 1). A total of groups

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of tests were carried out.

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Alkali mixture ratio Factors Levels

melting temperature

Solid-liquid Crystallization

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Alkali

(A)

ratio

time

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(D)

700°C(A1)

18g Na2CO3 (B1)

1:3(C1)

2h(D1)

2

760°C(A2)

13.5g Na2CO3, 3.75g NaOH (B2)

1:5(C2)

4h(D2)

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800°C(A3)

9g Na2CO3, 7.5g NaOH (B3)

1:6(C3)

5h(D3)

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840°C(A4)

4.5g Na2CO3, 11.25g NaOH (B4)

1:8(C4)

6h(D4)

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900°C(A5)

1:10(C5)

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Table 1 Orthogonal tests for the importance influential factors of green synthesis of zeolite

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4A

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After the orthogonal tests, the values Kjm, kjm and Rj need to be calculated to determine

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the important influential factors (Gao et al., 2016a). Kjm is the sum of the test index valus

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corresponding to the m (1, 2, 3, 4, 5) level of the j (A, B, C, D) factor. For example, KA1

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means the sum of the test index values of relative crystallinity while using the alkali melting

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temperature of 700°C. kjm is the mean value of Kjm. Rj is the range of the j factor. For example,

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RA corresponds the difference of the maximum and minimum of kAm. In the results of the

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orthogonal tests, the larger the Rj, the greater the impact of this factor on the experimental

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index (Jiaqiang et al., 2018). According to Rj, the order of the primary and secondary factors

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can be judged as the important influential factors (E et al., 2018). These parameters are shown

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in Table 4.

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above processes (2.2.1 and Fig. 1).

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2.2.3 Cu2+ adsorption To confirm the removal efficiency and reuse times of the synthesized 4A zeolite for

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heavy metal ions, the Cu2+ adsorption experiment was carried out using the single-factor

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method. The Cu2+ solution was prepared by dissolving copper nitrate in deionized water to

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reach a concentration of 100 mg/L. Then, a 150 mL Erlenmeyer flask was filled with 100 mL

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of Cu2+. 180 mg of the synthesized 4A zeolite was added to the flask and shaken in a

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temperature-controlled shaker at 150 rpm and at 25°C constant temperature. After the reaction,

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10 mL of the solution was collected by centrifugation at 4000 rpm for 10 minutes. The filtrate

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was collected in a polyethylene tube and the concentration of Cu2+ was determined by FAAS

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after dilution. All experiments were performed in duplicate.

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The effect of the initial solution pH on the synthesized 4A zeolite was determined by the

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process described above for pH gradients at 1, 2, 3, 4, 5, 6, and 7. The reaction time was 60

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min. The pH was adjusted with 0.1 M HNO3 and 0.1 M NaOH to prepare acidic and neutral

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solutions, respectively.

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The adsorbent efficiency (ŋ, %) and the adsorption amount at time t (qt, mg/g) of Cu2+

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per unit of 4A zeolite synthesized were calculated by the following equations.

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ŋ=

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 =

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where C0, Ce and Ct represent the initial Cu2+ concentration, the equilibrium Cu2+

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concentration and the Cu2+ concentration (mg/L) at time t (min), V is the solution volume (L),

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and m is the mass of the 4A zeolite synthesized (g).

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(  ) × 100% 

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An adsorption kinetic study of the synthesized 4A zeolite was carried out at the time

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gradients for 5 min, 10 min, 30 min, 60 min and 120 min and the initial solution pH was 4.

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The pseudo-first-order (Vernadakis, 1907) and the pseudo-second-order kinetic models (Ho

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and Mckay, 1998) were applied to analyse the adsorption of the synthesized 4A zeolite. The 8

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pseudo-first order equation is as follows:

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ln( −  ) = ln  −  

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where qe (mg/g) is the adsorption capacity at the reaction equilibrium and k1 (g/mg·min–1) is

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the rate constant of the pseudo-first-order process. The pseudo-second order kinetics model is

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based on the adsorption capacity of a solid surface, which depends on the number of surface

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reaction sites. Compared with other models, this model assumes that the chemical adsorption

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is the predominant mechanism (Ho, 2006). Its linear equation is

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where k2 is the pseudo-second order rate constant (g/mg·min-1).

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=

(3)

For determining the reuse possibility of the synthesized zeolite, 180mg of the

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synthesized 4A zeolite was used to treat 100 mg/L Cu2+ solution at pH 3 by the process

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described above. After testing the Cu2+ adsorption, desorption of zeolite 4A was carried out

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with 5 mol/L NaCl for 60 min, followed by filtering and drying at 105°C for 24 h. The dried

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and recovered zeolite 4A was used to adsorb 100 mg/L Cu2+4 times under the same conditions

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as the previous adsorption experiments.

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3 Results and discussion

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3.1 Physicochemical properties of raw materials

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The chemical composition of fly ash is shown in Table 2. The fly ash used in this work is

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a type of high-alumina fly ash in which the contents of SiO2, Al2O3 and Fe2O3 are over 80%.

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Aluminium and silicon in fly ash are the main raw materials for the zeolite synthesis. The

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main components in diatomite are shown in Table 3. The SiO2 content in diatomite is

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approximately 90%, which can provide silicon for zeolite synthesis.

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Component

Al2O3

CaO

Fe2O3

SiO2

LOI

Other

Content (%，ω)

45.66

4.00

2.22

43.80

0.10

4.22

LOI

Other

Table 2 Chemical compositions of coal fly ash by XRF (%, ω) LOI: loss on ignition Component

Al2O3

CaO

Fe2O3 9

SiO2

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2.49

0.28

1.15

89.16

2.76

4.16

Table 3 Chemical compositions of diatomite by XRF (%, ω) LOI: loss on ignition Fig. 2 (a) shows the XRD pattern of fly ash, which indicates that the mineral phase of fly

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ash mainly consists of mullite (JCPDS card 15-0776) and quartz (JCPDS card 46-1045), and

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the main crystal phase is mullite. The surface of the fly ash is smooth and clean, and the

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contents of Al and Si were high (Fig. 2 (b)). According to the XRD pattern of diatomite in Fig.

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2(c), the mineral phase of diatomite mainly consists quartz. Diatomite forms strips and the

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surface is rough and concave. The content of silica was particularly high (Fig. 2 (d)).

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Fig. 2. (a) XRD pattern of coal fly ash, (b) SEM image and EDX analysis of coal fly ash, (c) XRD pattern of diatomite, (d) SEM image and EDX analysis of diatomite

3.2 Theoretical basis for the synthesis of zeolite 4A

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Accurate control of zeolite synthesis requires many factors, mainly the type and ratio of

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reactants, liquid-to-solid ratio, alkalinity, and crystallization temperature and time. Synthetic

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4A molecular sieves usually require a raw material ratio in the range of n(SiO2)/n(Al2O3) =

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1.3~2.4,n(Na2O)/n(SiO2)

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n(SiO2)/n(Al2O3) = 2:1 is the most suitable for the formation of pure 4A-type molecular

=

0.8~3.0,

and

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n(H2O)/n(Na2O)

=

35~100.

The

ratio

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but a silicon source (diatomite) can be added. According to the Si-Al ratio of pure 4A-type

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molecular sieves and the contents of Si and Al in fly ash and diatomite (Table 2 and Table 3),

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fly ash and diatomite are mixed in an 8.5:1.5 ratio. The main reaction equation for the

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synthesis of zeolite 4A is as follows (Yao, 2010): 96Na2SiO3 +96NaAlO2 +312H2O → Na96Al96Si96O384·216H2O +192NaOH

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3.3 The important influential factors Alkali mixture ratio (B)

Solid-liquid ratio (C)

Crystallization time (D)

Relative crystallinity

1

1(A1)

1(B1)

1(C1)

1(D1)

1.7%

2 3 4 5

1 1 1 1

2(B2) 3(B3) 4(B4) 5(B5)

2(C2) 3(C3) 4(C4) 5(C5)

2(D2) 3(D3) 4(D4) 5(D5)

2.5% 38.7% 34.5% 55.2%

6

2(A2)

1

2

3

4.0%

7 8 9 10

2 2 2 2

2 3 4 5

3 4 5 1

4 5 1 2

2.2% 47.8% 3.4% 3.4%

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3(A3)

1

3

5

1.0%

12 13 14 15

3 3 3 3

2 3 4 5

4 5 1 2

1 2 3 4

1.7% 30.8% 46.9% 52.1%

4(A4)

1

4

2

37.3%

17 18 19 20

4 4 4 4

2 3 4 5

5 1 2 3

3 4 5 1

36.5% 36.2% 49.6% 8.6%

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5(A5)

1

5

4

11.1%

22 23 24 25 K1 K2

5 5 5 5 132.6% 60.8%

2 3 4 5 55.1% 43.6%

1 2 3 4 88.9% 111.9%

5 1 2 3 19.1% 90.1%

0.7% 3.7% 16.1% 31.7%

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Alkali melting temperature (A)

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132.5% 168.2% 63.3% 26.5% 12.2% 26.5% 33.6% 12.7% 21.4%

157.2% 150.5% 151.0% 11.0% 8.7% 31.4% 30.1% 30.2% 22.7%

66.6% 153.0% 137.0% 17.8% 22.4% 13.3% 30.6% 27.4% 17.3%

157.8% 136.1% 154.3% 3.8% 18.0% 31.6% 27.2% 30.9% 27.8%

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K3 K4 K5 k1 k2 k3 k4 k5 Range R

Table 4 The results of orthogonal tests

By comparing the R values in Table 4 (Gao et al., 2016b), the sequence of the important

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factors that affect the test index is DBAC. Thus, the crystallization time has the greatest

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influence, followed by the alkali mixture ratio. The alkali mixture ratio is the second key

249

factor, which indicates this parameter has an important effect on the synthesis of zeolite 4A,

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especially for energy and water consumption. Therefore, it is indicated that the alkali mixture

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ratio does have an important effect on the green synthesis according to the results of

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orthogonal test.

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3.4 Determination of the green synthetic parameters

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3.4.1 Crystallization time

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The crystallization time has an important effect on the crystal morphology, size, crystal

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number and crystallinity during zeolite crystallization. The crystallization period involves the

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dissolution of the silicon aluminium source and the growth of the zeolite crystal (Xu, 2004).

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Fig. 3(a) shows that the product was crystallized with Na2CO3 for 4 h to obtain the zeolite 4A

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(JCPDS card 43-0142) with a relative crystallinity of 44.5%. As the crystallization time was

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extended, the characteristic diffraction peak at 7.2° gradually increased and reached a

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maximum at 6 h, and the maximum relative crystallinity was 72.4%. Fig. 3(b) shows that the

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optimum crystallinity was 68.2% for 4 h when NaOH was chosen to treat fly ash; above 4 h,

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the sodalite crystal phase appears.

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The above analysis shows that suitable crystallization times of Na2CO3 or NaOH-treated

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fly ash were 6 or 4 h, respectively. The crystallization time of the zeolite 4A synthesized by 12

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NaOH-treated fly ash was short compared with Na2CO3-treated fly ash, thus, saving energy,

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but the relative crystallinity decreased by 4.2%.






1200

(b)






-Zeolite 4A







Intensity

Intensity

6h



















1200

6h

800











400




0 30

40

50







4h

0 20













4h

10


-Zeolite 4A







0







800

400




















1600

RI PT

(a)

0

60

5

10

15

20

25

30

35

40

45

2-Theta(°)

SC

2-Theta(°)

(b) at different crystallization times

268

3.4.2 Alkali melting temperature

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Fig. 3. XRD patterns of the hydrothermal crystallization products of Na2CO3 (a) and NaOH

Fig. 4 shows the TG-DCS curve of fly ash treated with by NaOH or Na2CO3. The weight

270

of the Na2CO3-treated fly ash sample decreased from 82°C to 150°C due to the loss of

271

unbound water on the surface of the sample. An exothermic peak occurs at 300°C to 400°C

272

due to the exothermic reaction of fly ash and diatomite mixed with Na2CO3. At approximately

273

850°C, a violent exothermic reaction occurs due to mullite and quartz in the fly ash and

274

diatomite reacting with Na2CO3 to produce sodium aluminosilicate and sodium silicate. This

275

outcome released a large amount of carbon dioxide, resulting in a sharp decrease in the weight

276

of the sample (Li and Ma, 2004). Compared with the Na2CO3 alkali melting calcination, the

277

NaOH alkali melting reaction temperature decreased by approximately 90°C. The highest

278

exothermic reaction occurred at approximately 760°C, indicating that the most insoluble

279

mullite phase began to react.

AC C

EP

TE D

269

280

Compared to the Na2CO3 alkali flux, the NaOH alkali melting temperature is low, and the

281

heat flow in the reaction is rapid and intense, which can save energy. However, the reaction

282

product easily bonds with the crucible during the experiment, making scraping necessary.

283

Although the Na2CO3 alkali melting temperature is high, the CO2 produced during the 13

ACCEPTED MANUSCRIPT reaction causes agitation, preventing the alkali melting product from caking on the crucible.

285

Therefore, according to the TG-DCS curve analysis, when NaOH and Na2CO3 were used to

286

treat fly ash, 760°C and 850°C, respectively, were selected as the ideal temperatures.

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SC

RI PT

284

287

289

Fig. 4. TG-DCS curves of fly ash, diatomite and Na2CO3/NaOH

3.4.3 Alkalinity ratio

TE D

288

Na2CO3 calcination can cause glassy silica and aluminium oxide to dissolve to produce

291

soluble sodium aluminosilicate salt, which is a precursor for synthetic zeolite; however,

292

different alkalinity ratios during zeolite synthesis will create different synthetic zeolite species

293

(Liu et al., 2008; Ming-Yan et al., 2006; Molina and Poole, 2004). Fig. 5(a) shows that when

294

the Na2CO3 alkalinity ratio is 0.8, compared with the target product zeolite 4A (JCPDS card

295

43-0142), the crystallization product is nepheline (NaAlSiO4) (JCPDS card 35-0424), in

296

which the proportion of each element is the same, except for the number of water molecules.

297

The molecular weight of nepheline is very small, which may be due to the lower amount of

298

Na2CO3 transforming mullite and quartz minerals into small molecular weight aluminosilicate.

299

When the alkalinity is increased to 1.8, the characteristic peak intensity at 7.2° was the

300

highest, and the relative crystallinity was 48.8%, indicating that Na+ could promote the

301

product morphology and crystallization reaction (Murayama et al., 2002). However, when the

AC C

EP

290

14

ACCEPTED MANUSCRIPT alkalinity ratio is higher than 1.8, the 4A molecular sieve is still present in the product, but the

303

relative crystallinity is greatly reduced. A proper amount of Na2CO3 can dissolve the inert

304

silica aluminium in fly ash and diatomite into the initial silicon aluminium gel. Fig. 5(b)

305

shows that the crystallinity of the zeolite 4A is the best when NaOH-treated fly ash is used

306

with an alkalinity ratio of 1.5.

RI PT

302

Compared with the amount of Na2CO3, the amount of NaOH used was lower, but the

308

relative crystallinity was only 42.6%. It is clear that the use of Na2CO3 alone produces zeolite

309

4A with a slightly higher relative degree of crystallinity, but the amount of alkali required is

310

higher. （ a） 







 

1000

















3000

  2000



Intensity

2000



1.8

■■

■

■

■



■

0.8

-Zeolite 4A ■-Nepheline      1.8





 

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Intensity



（ b）

-Zeolite 4A ■-Nepheline  2.2

SC

307









 

1.5

1000

■ ■

■ ■■

■

0.8

0

0 10

20

30

40

10

20

30

40

2-Theta (°)

TE D

2-Theta(°)

Fig. 5. XRD patterns of the hydrothermal crystallization products of Na2CO3 (a) and NaOH (b) with different basicity ratios

3.4.4 Solid-to-liquid ratio

EP

311

At a solid-to-liquid ratio of 1:5, the synthetized zeolite 4A (JCPDS card 43-0142) had the

313

highest purity and a relative crystallinity of 72.4% (Fig. 6(a)), with a CO 3 concentration of

314

1.2 mol/L. Compared with Na2CO3 alkali melting, it can be concluded that, for NaOH alkali

315

melting, the solid-to-liquid ratio of 1:8 produces the most crystalline products (Fig. 6(b)),

316

with a relative crystallinity of 64.8%. When the solid-to-liquid ratio is 1:3, 5 mol/L OH- is

317

present in solution, and the sodalite phase is completely synthesized. The ring skeleton of the

318

zeolite 4A beta cage has a metastable structure, and the structural energy is higher than that of

319

sodalite (Xiaoqiang et al., 2016). Thus, a high-alkalinity environment will cause the zeolite

320

4A skeleton to crack, accompanied by sodalite formation.

AC C

312

2-

15

ACCEPTED MANUSCRIPT 321

Therefore, the best soild-to-liquid ratios of Na2CO3-treated or NaOH-treated fly ash were

322

1:5 or 1:8, respectively. When Na2CO3-treated fly ash is used, the water consumption is low,

323

resulting in a corresponding decrease in the waste liquid, and the relative crystallinity of the

324

product is 72.4%.





800 400
































10

1:8











▲

30

0

40

▲

▲

▲


▲ ▲-Sodalite ▲

1:8

▲

▲

▲

▲

1:5




10

2-Theta (°)

▲

▲

▲

500

1:3

20

▲

1000

1:5


■-Zeolite 4A

▲

▲ ▲ ▲





●

0

1500














（ b）

■-Zeolite 4A ●-Sodalite

SC

Intensity

1200

Intensity

（ a）

RI PT

325

20







30




1:3


40

M AN U

2-Theta (°)

Fig. 6. XRD patterns of the hydrothermal crystallization products of Na2CO3 (a) and NaOH (b) with different solid-to-liquid ratios

326

3.4.5 Crystallization temperature

Crystallization temperature can affect the reactor pressure, crystallization rate, crystal

328

type and size (Ke-Ming and Zhu, 2007). At the crystallization temperature of 40°C, the degree

329

of crystallization is minimal, and the product is mainly amorphous aluminosilicate (Fig. 7(a)).

330

When the temperature is increased to 60°C, a small amount of nepheline phase appears.

331

Because the hydrothermal reaction is carried out in a closed autoclave, when the temperature

332

increases, the gas expands and increases the pressure on the PTFE liner, increasing thermal

333

motion and effective collisions and improving the dissolution and crystallization rates of the

334

hydrothermal reaction. Thus, when the crystallization temperature reaches 90°C, zeolite 4A

335

(JCPDS card 43-0142) with a relative crystallinity of 72.4% is synthesized. The optimum

336

crystallization temperature of zeolite 4A synthesized by alkali fusion with NaOH is also 90°C

337

(Fig. 7(b)).

338 339

AC C

EP

TE D

327

Therefore, the crystallization temperature has a large influence on the synthesized zeolite, and the optimum crystallization temperature of the two alkalis is 90°C.

16

ACCEPTED MANUSCRIPT




















（ b）


-Zeolite 4A
-Nepheline

▲

1400

90℃
























80℃

700

0




10

20

▲ ▲

▲

▲

▲-Zeolite 4A ■-Nepheline

▲

▲

▲

90℃ 60℃

■

400

60℃











▲

800




Intensity

Intensity

2100




■

■ 40℃

30

40℃

0

40

10

2-Theta(°)

RI PT

（ a）

20

30

40

2-Theta (°)

Fig. 7. XRD patterns of the hydrothermal crystallization products of Na2CO3 (a) and NaOH

3.4.6 Green synthetic parameters

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340

SC

(b) at different crystallization temperatures

The advantages and disadvantages of the synthetic parameters are shown in Table 5 for

342

the synthesized zeolite 4A from fly ash treated by NaOH or Na2CO3. To achieve the green

343

synthesis of zeolite 4A, the advantages of two kinds of alkali fusion were combined, and the

344

synthetic parameters of low energy and water consumption (alkali melting temperature of

345

760°C, solid-to-liquid ratio of 1:5, and crystallization time of 4 h) were selected as the green

346

synthetic condition when the mixture of NaOH and Na2CO3 is used to treat fly ash. The

347

selection of these parameters effectively reduced the energy and water consumption in the

348

synthesis process and realized a green synthesis.

EP

TE D

341

Single experiment

AC C

Alkali melting temperature (°C) Solid-to-liquid ratio (g/mL) Crystallization time (h) Crystallization temperature (°C) Advantages

Disadvantages 349

Synthesis of zeolite 4A by Na2CO3 850

Synthesis of zeolite 4A by NaOH 760

1:5

1:8

6 90

4 90

Relatively high degree of crystallinity (72.4%), less waste High energy consumption, long synthesis cycle

Low energy consumption, short synthesis cycle Relative crystallinity is low (68.6%), more wastewater

Table 5 Parameters for the synthesis of zeolite 4A using NaOH or Na2CO3 17

ACCEPTED MANUSCRIPT 350

3.5 Synthesis of zeolite 4A from a mixture of Na2CO3 and NaOH

Group

Fly ash (g)

Na2CO3 (g)

NaOH (g)

352

1

10

16.2

1.5

2

10

12.6

4.5

3

10

9

7.5

4

10

5.4

10.5

353

RI PT

351

Table 6 The mixing amount of Na2CO3 and NaOH

354 355

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Intensity

        

2000

  

1000

 

  



10

15

20

25

30

35

1

40

2-Theta (°)

Component

TE D

Fig. 8. XRD patterns of the hydrothermal crystallization products produced from mixing NaOH with Na2CO3

359

SiO2

Al2O3

Na2O

CaO

MgO

Other

EP

39.36 30.43 22.78 3.04 0.19 4.2 Content (%,ω) Table 7. XRF patterns of the hydrothermal crystallization products produced from mixing NaOH with Na2CO3 Under the conditions of the green synthetic parameters, the mixing amount of Na2CO3

AC C

362





5

360 361



 

0 356 357 358

SC

3000

-Zeolite 4A -Sodalite      4      3      2

363

and NaOH is shown in Table 6. Hydrothermal crystallization using Na2CO3 mixed with

364

NaOH at a mass ratio of 2.8:1 produced zeolite 4A (JCPDS card 43-0142) with the highest

365

relative crystallinity of 75.8% in group 2, and the product had almost no sodalite (JCPDS card

366

37-0476) (Fig. 8). When the two bases exist together, the equations concerning water

367

solubility are Na2CO3⇌2Na+ + CO 3 , CO 3 + H2O⇌HCO 3 + OH- and NaOH⇌Na++OH-.

368

Comparing the same mass of Na2CO3 with NaOH, Na2CO3 provides an appropriate amount of

369

Na+ but little OH-. Upon OH- consumption, CO 3 is continuously hydrolysed to produce OH-.

2-

2-

2-

2-

18

ACCEPTED MANUSCRIPT In addition, OH- can only be stably kept in solution at a relatively low concentration, which is

371

not beneficial to activation and crystallization. The NaOH hydrothermal synthesis is

372

characterized by the ability to provide enough OH- for the hydrothermal reaction but little Na+.

373

OH- and Na+ are indispensable in the process of hydrothermal crystallization, and the contents

374

of the two ions need appropriate proportions. Mixing Na2CO3 with NaOH at a mass ratio of

375

2.8 to 1 gives the best crystallinity, which may be because the most appropriate OH- and Na+

376

concentrations are obtained and the OH- concentration is relatively stable during the reaction.

RI PT

370

According to the XRF component analysis (Table 7), the silica alumina ratio (the mole

378

ratio of SiO2 and Al2O3) in the synthesized 4A zeolite was 1, which was consistent with the

379

silica alumina ratio in the chemical formula of 4A zeolite. However, when XRF was used to

380

analyse chemical compositions, the atomic number of the detected elements was higher, with

381

higher accuracy. Because the atomic numbers of Si and Al are in the front, this may cause

382

some errors in the analysis results. Therefore, FTIR and SEM-EDX were used to further

383

investigate the synthesized 4A zeolite.

M AN U

SC

377

Fig. 9 shows the FTIR characterization of the fly ash and synthesized 4A zeolite under

385

the green synthetic conditions. The main infrared absorption peaks are located near 459, 873,

386

1103 and 3413 cm-1 in Fig. 9 (a), which belong to the symmetrical stretching vibration peaks

387

of Si-O, Si-O-Si, Si-O-Si and Al-O-Si bonds, and the bending vibration peaks of H2O,

388

respectively. These functional groups were in conformity with mullite and silica’s, and the

389

FTIR results were consistent with Table 1 and Fig. 1(a). Fig. 9 (b) is the FTIR of the

390

synthesized zeolite 4A. The peak at approximately 3467 cm-1 is associated with the OH-

391

hydrogen-bond stretching vibration. The peak at 1640 cm-1 is associated with the OH-

392

hydrogen-bond bending vibration of zeolite 4A, indicating that zeolite 4A is bound to water.

393

The peak at 993 cm-1 corresponds to the characteristic peak of the T-O-T (where T is Al or Si)

394

tetragonal stretching vibration, and the peak at 558 cm-1 is the characteristic peak for the

395

bicyclic vibrations in a tetrahedron. The peak at 465 cm-1 corresponds to T-O (where T is Al

396

or Si) flexural stretching vibrations (Purnomo et al., 2012). These results are consistent with

397

the FTIR spectrum of the synthesized zeolite 4A by Tanaka et al. (2008). The functional

398

groups of FTIR are shown in Table 8.

AC C

EP

TE D

384

19

ACCEPTED MANUSCRIPT

Transmittance

Trasmittance

3413

465 558 1640

993

459

3467

1103

0

500

RI PT

873

1000 1500 2000 2500 3000 3500 4000

0

500

1000 1500 2000 2500 3000 3500 4000

Wave number/cm-1

Wave number/cm-1

(a)

(b)

SC

Fig. 9. FTIR characterization of the (a) fly ash and (b) synthesized zeolite 4A 399

400

Functional groups Si-O bicyclic vibrations in a tetrahedron Si-O-Si and Si-O-Al OH- hydrogen-bond

M AN U

Wave number/cm-1 459, 465 558 873,993,1103 1640,3413,3467

Table 8 The functional groups of samples by FTIR Fig. 10(a)-(c) shows SEM images of the zeolite 4A produced under the optimum

402

conditions from fused fly ash via hydrothermal crystallization. From Fig. 10(a), it can be seen

403

that the baked surface of fused fly ash has irregular holes with different sizes, and is rough

404

and wrinkled. The crystalline products in Fig. 10(b) are well dispersed and exhibit a regular

405

cubic structure. Fig. 10(c) represents a low-power microscope image of the synthesized

406

zeolite 4A under the optimum conditions, which mostly comprise of regular cubic grains, and

407

Si, Al and Na are the main components of the zeolite by EDX analysis.

AC C

EP

TE D

401

20

RI PT

ACCEPTED MANUSCRIPT

Fig. 10. SEM image of baked fly ash (a), high-magnification (b) and low-magnification of the

SC

synthesized zeolite 4A and the element content analysis by EDX (c), image and the element content of the synthesized zeolite 4A after being reused 4 times (d).

M AN U

408

XRD, XRF, FTIR and SEM-EDX analyses showed that the synthesized product

410

conformed to the characteristics of 4A zeolite in term of the crystal phase, composition,

411

functional groups present and morphology. In addition, the zeolite and reached the highest

412

relative crystallinity of 75.8%. Therefore, when Na2CO3 and NaOH are synergistic, the 4A

413

zeolite with high crystallinity can be successfully synthesized under low energy and water

414

consumption conditions.

3.6 Removal efficiency by the synthesized zeolite 4A

EP

415

TE D

409

The removal rate of the zeolite 4A was only approximately 22% when the initial solution

417

pH was 1 or 2. However, when the initial solution pH ≥ 3, the Cu2+ removal rate by the zeolite

418

4A was close to 100%, and the highest Cu2+ removal amount was 55.5 mg/g (Fig. 11(a)).

419

Under strong acid conditions (pH was 1 or 2), the H+ ions concentration was high, because the

420

zeolite was highly selective for H+ ions (Hui et al., 2005), the Cu2+ removal efficiency by

421

zeolite was relatively lower. At an initial solution pH≥3, because the surface hydroxyl groups

422

generated by the zeolite surface with water would neutralize the H+ (Hui et al., 2005) and the

423

final solution pH reached neutral (Fig. 11(b)), the Cu2+ removal efficiency by the zeolite 4A

424

was increased and approached 100%. The TPD of zeolite 4A showed that there was a weak

425

acid site centre near 159°C and a strong acid site centre near 338°C (Fig. 12), which indicated

AC C

416

21

ACCEPTED MANUSCRIPT that the zeolites synthesized had good adsorption properties. The pseudo-second order

427

kinetics model well described the Cu2+ adsorption by the zeolite (Table 9), which indicates

428

that chemical adsorption is the predominant mechanism. This result is consistent with other

429

research findings in which the removal of Cu2+ by zeolite 4A mainly occurred through

430

adsorption and ion exchange between Na+ and Cu2+ (Hui et al., 2005; Jha et al., 2008).

RI PT

426

431

12

60

100

60

30

40 20

10

1

2

3

4 pH

5

6

(a)

7

8

Removal rate (%) pH after reaction

60 40

M AN U

20

80

6 4

pH after reaction

40

SC

80

10

Removal rate (%)

50

Removal rate (%) adsorption capacity

adsorption capacity(mg/g)

Removal rate (%)

100

2

20

1

2

3

4

5

6

7

0

pH

(b)

Fig. 11. (a) Ability of the synthesized zeolite 4A to remove Cu2+ in solution at different initial

433

pH values and (b) Cu2+ removal rate and the final pH of solution (180 mg of zeolite, C0 = 100

434

mg/L for Cu2+, 60 min)

435

0.375

TE D

432

0.365

AC C

TCD Signal (a.u.)

EP

0.370

0.360 0.355 0.350 0.345 0.340

436 437

338

159

100

200

300

400

Temperature(℃ ) Fig. 12. TPD of the synthesized zeolite 4A

438

22

500

ACCEPTED MANUSCRIPT Metal

Pseudo first order

Pseudo two order

kinetic equation

kinetic equation

ion qc(mg/g)

k1 × 10-2

R2

19.70

4.80

0.9116

Cu2+ 439

qc(mg/g)

k2 × 10-2

57.14

0.035

R2 0.9982

Table 9 Parameters and regression coefficients (R2) of the adsorption kinetic models The removal efficiency of Cu2+ by the synthesized zeolite 4A is superior to that of natural

441

zeolite or modified natural zeolite (Table 10). The average pore size of the synthesized

442

zeolites is smaller than natural zeolites, which may increase the pore per unit volume and

443

enhance the removal ability for Cu2 +. However, due to the difference in removal conditions,

444

the removal efficiency of Cu2+ by the zeolite will be greatly affected (Sprynskyy et al., 2006;

445

Hui et al., 2005).

M AN U

Maximum BET specific Average Removal conditions adsorption surface area pore size capacity (mg/g) (m2/g) (nm)

Zeolite 4A

55.5

Chabazite zeolite

11.5

18.33

159

AC C

EP

25.8 Clinoptilolite (Theoretical zeolite value) Zeolite composite fibre 446

Reference

Cu2+ 100 mg/L, 0.18 g of zeolite, the initial pH This work 11.34 3, contact time 1 h and temperature 298 K Cu2+ 115 mg/L, 1.0 g of zeolite, the initial pH (Egashira et al., 2.5-5, contact time 0-24 2012) h and temperature 300 K Cu2+ 800 mg/L, 0.5 g of (Sprynskyy et 95.62 zeolite al., 2006)

TE D

Adsorbent

SC

RI PT

440

28.6 (Theoretical value)

13.2

16.88

24.6

Cu2+ 200 mL/L, 4 g/L adsorbent, the initial pH 5.5, contact time 48 (Ji et al., 2012) h and temperature 298 K

Table 10 Adsorption capacities of Cu2+ with various adsorbents.

447

After Cu2+ was absorbed, it was desorbed from zeolite 4A in 5 mol/L NaCl and then

448

filtered, and the zeolite 4A after for 4 recycling processes was able to remove Cu2+ in an

449

acidic aqueous solution with a pH of 3. The adsorption rate was stable at approximately 73%

450

(Fig. 13), which was lower than the adsorption the first time (close to 100%). Ji et al. (2012) 23

ACCEPTED MANUSCRIPT measured the desorption and reuse of the composite fibre with zeolite and showed that

452

desorption ratio gradually decreased after being used 5 times. However, the synthesized

453

materials still maintain a certain removal efficiency for Cu2+. SEM images showed the regular

454

cubic structure of the reused zeolite 4A remained, but Cu was found in the zeolite by EDX

455

after the fourth desorption (Fig. 10 (d)). Complete desorption was not possible because

456

electrostatic and complexation reactions occurred between the sorbent and the metal ion

457

(Mishra and Sharma, 2011) and some of the adsorption sites were irreversibly occupied by

458

Cu2+ (Zhang et al., 2017).

RI PT

451

SC

70

60

50

40

1

M AN U

Removal rate(%)

80

2

3

4

Repeated times of zeolite 4A

459

Fig. 13. Recycling of the synthesized zeolite 4A to absorb Cu2+ (pH 3, 180 mg of zeolite, 100

461

mg/L Cu2+, 60 min)

462

4 Conclusion

EP

TE D

460

When zeolite 4A was synthesized by the hydrothermal method using coal fly ash as the

464

raw material, the important factors influencing energy and water consumption were the alkali

465

mixture ratio, alkali melting temperatures, solid-to-liquid ratios, crystallization times and

466

crystallization temperatures. According to the results of the orthogonal tests, the ratio of the

467

NaOH and Na2CO3 mixture was proven to be an important influential factor for the green

468

synthesis. Through batch experiments, green synthetic parameters were determined: an alkali

469

melting temperature of 760°C, a solid-to-liquid ratio of 1:5, a crystallization time of 4 h and a

470

crystallization temperature of 90°C. Because the synergism of Na2CO3 and NaOH provided

471

an appropriate amount of OH- and Na+ as well as a stable OH- concentration during the

AC C

463

24

ACCEPTED MANUSCRIPT reaction process, the mixture of these two bases can lead to the highest activation of Si and Al

473

in fly ash to achieve the highest degree of crystallization as well as low energy and water

474

consumption. Therefore, when Na2CO3 and NaOH were mixed in a mass ratio of 2.8 to 1, the

475

synthesized zeolite 4A by fly ash had the highest crystallinity and reached 75.8% under low

476

energy and water consumption conditions.

RI PT

472

The synthesized zeolite 4A can effectively remove Cu2+ from acidic aqueous solution

478

(3≤pH<7), with a removal rate close to 100% and a removal capacity of 55.5 mg/g. When

479

Cu2+ was desorbed from zeolite 4A and when the zeolite was reused 4 times, the Cu2+ removal

480

efficiency by the reused zeolite 4A was stable at 73% in an aqueous solution with a pH of 3.

481

Acknowledgments

482

Support from the program of China Scholarships Council is gratefully acknowledged. We are

483

very grateful to the referees and the editors for their helpful suggestions.

M AN U

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477

References:

484

Aldahri, T., Behin, J., Kazemian, H., & Rohani, S., 2016. Synthesis of zeolite Na-P from coal fly ash

TE D

by thermo-sonochemical treatment. Fuel 182, 494-501. And, H. L. C., & Shih, W. H., 1998. A General Method for the Conversion of Fly Ash into Zeolites as Ion Exchangers for Cesium. Ind Eng Chem Res 37(37), S188-S189. Cardoso, A. M., Horn, M. B., Ferret, L. S., Azevedo, C. M., & Pires, M., 2015. Integrated synthesis of zeolites 4A and Na-P1 using coal fly ash for application in the formulation of detergents and swine

EP

wastewater treatment. J. Hazard Mater 287, 69-77.

Cheng, Q., 2006. Study on determining mineral composition of treated flyash by X-ray diffractometer. Modern Instruments (1), 39-41.

Egashira, R., Tanabe, S., & Habaki, H., 2012. Adsorption of heavy metals in mine wastewater by

AC C

485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506

Mongolian natural zeolite. Procedia Engineering 42, 49-57. Elnaggar, M. R., Elkamash, A. M., Eldessouky, M. I., & Ghonaim, A. K., 2008. Two-step method for preparation of NaA-X zeolite blend from fly ash for removal of cesium ions. J. Hazard Mater 154(1), 963-972.

Fukasawa, T., Karisma, A. D., Shibata, D., Huang, A. N., & Fukui, K., 2017. Synthesis of zeolite from coal fly ash by microwave hydrothermal treatment with pulverization process. Adv Powder Technol 28(3), 798-804. Gao, J., Yin, J., Zhu, F., Chen, X., Tong, M., & Kang, W., et al., 2016a. Orthogonal test design to optimize the operating parameters of CO 2

desorption from a hybrid solvent MEA-Methanol in a

packing stripper. J. Taiwan Inst. Chem. E. 64, 196-202. Gao, J., Yin, J., Zhu, F., Chen, X., Tong, M., & Kang, W., et al., 2016b. Orthogonal test design to optimize the operating parameters of a hybrid solvent MEA–Methanol in an absorber column packed 25

ACCEPTED MANUSCRIPT with three different packing: Sulzer BX500, Mellapale Y500 and Pall rings 16 ×   16 for post-combustion CO2 capture. J. Taiwan Inst. Chem. E. 68, 218-223. Ho, Y. S., 2006. Review of second-order models for adsorption systems. J. Hazard Mater 136(3), 681-689. Ho, Y. S., & Mckay, G., 1998. Sorption of dye from aqueous solution by peat. Chem Eng J. 70(2), 115-124. Holler, H., & Wirsching, G. U., 1985. Zeolite formation from fly ash. Fortshritte Minerals 63(21).

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ACCEPTED MANUSCRIPT Alkali Alkali mixture ratio Factors Levels

Solid-liquid Crystallization

melting ratio

time

(C)

(D) 2h(D1)

temperature (B) (A) 700°C(A1)

18g Na2CO3 (B1)

1:3(C1)

2

760°C(A2)

13.5g Na2CO3, 3.75g NaOH (B2)

1:5(C2)

4h(D2)

3

800°C(A3)

9g Na2CO3, 7.5g NaOH (B3)

1:6(C3)

5h(D3)

4

840°C(A4)

4.5g Na2CO3, 11.25g NaOH (B4)

1:8(C4)

6h(D4)

5

900°C(A5)

15g NaOH (B5)

1:10(C5)

8h(D5)

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1

Table 1 Orthogonal tests for the importance influential factors of green synthesis of zeolite

Component

Al2O3

CaO

Content (%，ω)

45.66

4.00

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4A Fe2O3

SiO2

LOI

Other

2.22

43.80

0.10

4.22

Table 2 Chemical compositions of coal fly ash by XRF (%, ω) LOI: loss on ignition Al2O3

CaO

Content（%，ω）

2.49

0.28

Fe2O3

SiO2

LOI

Other

1.15

89.16

2.76

4.16

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Component

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Table 3 Chemical compositions of diatomite by XRF (%, ω) LOI: loss on ignition

ACCEPTED MANUSCRIPT Alkali mixture ratio (B)

Solid-liquid ratio (C)

Crystallization time (D)

Relative crystallinity

1

1(A1)

1(B1)

1(C1)

1(D1)

1.7%

2 3 4 5

1 1 1 1

2(B2) 3(B3) 4(B4) 5(B5)

2(C2) 3(C3) 4(C4) 5(C5)

2(D2) 3(D3) 4(D4) 5(D5)

2.5% 38.7% 34.5% 55.2%

6

2(A2)

1

2

3

4.0%

7 8 9 10

2 2 2 2

2 3 4 5

3 4 5 1

4 5 1 2

2.2% 47.8% 3.4% 3.4%

11

3(A3)

1

3

5

1.0%

12 13 14 15

3 3 3 3

2 3 4 5

4 5 1 2

1 2 3 4

1.7% 30.8% 46.9% 52.1%

16

4(A4)

1

4

2

37.3%

17 18 19 20

4 4 4 4

2 3 4 5

5 1 2 3

3 4 5 1

36.5% 36.2% 49.6% 8.6%

21

5(A5)

1

5

4

11.1%

2 3 4 5 55.1% 43.6% 157.2% 150.5% 151.0% 11.0% 8.7% 31.4% 30.1% 30.2% 22.7%

1 2 3 4 88.9% 111.9% 66.6% 153.0% 137.0% 17.8% 22.4% 13.3% 30.6% 27.4% 17.3%

5 1 2 3 19.1% 90.1% 157.8% 136.1% 154.3% 3.8% 18.0% 31.6% 27.2% 30.9% 27.8%

0.7% 3.7% 16.1% 31.7%

Table 4 The results of orthogonal tests

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5 5 5 5 132.6% 60.8% 132.5% 168.2% 63.3% 26.5% 12.2% 26.5% 33.6% 12.7% 21.4%

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22 23 24 25 K1 K2 K3 K4 K5 k1 k2 k3 k4 k5 Range R

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Tests

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Alkali melting temperature (A)

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Alkali melting temperature (°C) Solid-to-liquid ratio (g/mL) Crystallization time (h) Crystallization temperature (°C) Advantages

Synthesis of zeolite 4A by Na2CO3 850

Synthesis of zeolite 4A by NaOH 760

1:5

1:8

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Single experiment

6 90

4 90

Disadvantages

Low energy consumption, short synthesis cycle

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Relatively high degree of crystallinity (72.4%), less waste High energy consumption, long synthesis cycle

Relative crystallinity is low (68.6%), more wastewater

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Table 5 Parameters for the synthesis of zeolite 4A using NaOH or Na2CO3 Group

Fly ash (g)

Na2CO3 (g)

NaOH (g)

1

10

16.2

1.5

10

12.6

4.5

10

9

7.5

10

5.4

10.5

2 3 4

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Table 6 The mixing amount of Na2CO3 and NaOH Component

SiO2

Al2O3

Na2O

CaO

MgO

Other

39.36 30.43 22.78 3.04 0.19 4.2 Content (%,ω) Table 7. XRF patterns of the hydrothermal crystallization products produced from mixing NaOH with Na2CO3

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Wave number/cm-1 459, 465 558 873,993,1103 1640,3413,3467

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Functional groups Si-O bicyclic vibrations in a tetrahedron Si-O-Si and Si-O-Al OH- hydrogen-bond

Table 8 The functional groups of samples by FTIR

Metal

Pseudo first order

Pseudo two order

kinetic equation

kinetic equation

ion Cu2+

qc(mg/g)

k1 × 10-2

R2

19.70

4.80

0.9116

qc(mg/g) 57.14

k2 × 10-2 0.035

R2 0.9982

Table 9 Parameters and regression coefficients (R2) of the adsorption kinetic models

ACCEPTED MANUSCRIPT Adsorbent

Maximum BET specific Average Removal conditions adsorption surface area pore size capacity (mg/g) (m2/g) (nm)

55.5

18.33

Chabazite zeolite

11.5

159

28.6 (Theoretical value)

13.2

Cu2+ 200 mL/L, 4 g/L adsorbent, the initial pH 5.5, contact time 48 (Ji et al., 2012) h and temperature 298 K

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Zeolite composite fibre

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25.8 Clinoptilolite (Theoretical zeolite value)

Cu2+ 100 mg/L, 0.18 g of zeolite, the initial pH 11.34 This work 3, contact time 1 h and temperature 298 K Cu2+ 115 mg/L, 1.0 g of zeolite, the initial pH (Egashira et al., 2.5-5, contact time 0-24 2012) h and temperature 300 K Cu2+ 800 mg/L, 0.5 g of (Sprynskyy et 95.62 zeolite al., 2006)

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Zeolite 4A

16.88

24.6

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Table 10 Adsorption capacities of Cu2+ with various adsorbents.

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Reference

ACCEPTED MANUSCRIPT 

The synergistic reaction of NaOH and Na2CO3 has an important effect on the zeolite green synthesis.



The optimal parameters gained by batch test are as green synthesis condition of

The synthetic zeolite can efficiently adsorb Cu2+ from acid solution and be

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reused.

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

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two alkali mixture.