Preparation of a fly ash-based geopolymer for removal of a cationic dye: Isothermal, kinetic and thermodynamic studies

Journal Pre-proof Preparation of a fly ash-based geopolymer for removal of a cationic dye: Isothermal, kinetic and thermodynamic studies Ozkan Acisli, Ilker Acar, Alireza Khataee

PII:

S1226-086X(19)30603-3

DOI:

https://doi.org/10.1016/j.jiec.2019.11.012

Reference:

JIEC 4854

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

4 October 2019

Revised Date:

7 November 2019

Accepted Date:

10 November 2019

Please cite this article as: Acisli O, Acar I, Khataee A, Preparation of a fly ash-based geopolymer for removal of a cationic dye: Isothermal, kinetic and thermodynamic studies, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.11.012

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Preparation of a fly ash-based geopolymer for removal of a cationic dye: Isothermal, kinetic and thermodynamic studies

Ozkan Acisli,a,* Ilker Acar, a Alireza Khataee, b,c,*

a

Department of Petroleum and Natural Gas Engineering, Faculty of Earth Sciences, Ataturk

b

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University, 25400, Oltu, Erzurum, Turkey Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department

of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran c

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Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam

*

[email protected] (O. Acisli)

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Corresponding authors:

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Graphical Abstract

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[email protected], [email protected] (A. Khataee)

Highliths 

Fly ash-based geopolymer was prepared for the removal of Basic Yellow 2. 1



The removal efficiency increased with the geopolymer dosage.



An increase in the initial dye concentration caused a decrease in the efficiency.



The Langmuir and Temkin models were well correlated with the obtained results.



The adsorption process occurs spontaneously and shows endothermic character.

Abstract A class F fly ash-based geopolymer was prepared for the removal of Basic Yellow 2 (BY2)

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from aqueous solutions. The geopolymerization process transformed the spherical fly ash particles into a porous and amorphous polymeric structure, sodium-alumina-silicate hydrate gel. The removal efficiency increased with the geopolymer dosage while it decreased with BY2

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concentration. Specifically, the efficiency of 94.47% was achieved with the geopolymer dosage

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of 2.0 g L-1. However, further increases resulted in a substantial reduction, down to 57.76% for 3.0 g L-1. At 293 K for 300 min, the efficiency decreased from 84.69 to 64.19% for BY2

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concentrations of 10 and 50 mg L-1, respectively. Based on the isothermal, kinetic and thermodynamic investigations of the adsorption process, the Langmuir and Temkin models well

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correlated with the obtained results. The adsorption occurs spontaneously and shows endothermic character. The pseudo-second order and the intra-particle diffusion models are

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valid for 293 K while the adsorption results well suited with the pseudo-second order for 313 and 323 K. In conclusion, the prepared fly ash-based geopolymer provided favorable results for

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the removal of Basic Yellow 2 from aqueous solutions.

Keywords: Fly ash; Geopolymer; Alkaline activation; Degradation; Cationic dye.

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1. Introduction Clean water resources have been contaminated day to day by a variety of pollutants like heavy

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metals, dyes, pharmaceuticals, pesticides and others. From these pollutants, especially organic dyes even in trace amounts can be very hazardous due to their toxic, carcinogenic and mutagenic effects. Though a large number of conventional methods attempted, adsorption is by

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far the most widely used process for the removal of dyes owing to its simplicity, low cost and high efficiency. Specifically, very high surface area, good stability and long durability

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properties make activated carbon the most feasible adsorbent. However, its high synthesis cost

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and difficulties in the regeneration process have forced researchers to look for lower-cost alternatives [1-6].

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The term, “Geopolymer”, first used by Davidovits in 1976, can be defined as a negatively charged three dimensions alumina-silicate framework in which Na+, K+ and Ca2+ ions function

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as charge-balancing cations [3, 7]. Geopolymers can generally be prepared by activating various silica and alumina rich materials with strongly alkaline solutions at ambient or slightly elevated

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temperatures [8, 9]. In literature, various raw materials and waste products such as metakaolin [4, 7], coal fly ash [2, 10], blast furnace slag [11, 12], natural alumina-silicates [13, 14], red mud [8] and biomass fly ash [6] have been studied for this purpose. Depending on the activator and processing conditions used, the resulting structure is mostly amorphous alumina-silicates or semi-crystalline zeolites [15-18].

3

Coal fly ash is the major by-product generated in thermal power plants. Its annual worldwide production is in the range of 500-600 million tons, almost half of which is mainly used in cement and concrete industry. The remaining part is being disposed in especially landfills, and also ash ponds and lagoons [19-23]. The resulted environmental pollution can be reduced by turning this waste into a beneficial material. Since coal fly ash is mainly constituted by amorphous alumina-silicate framework, it can be considered as cheap and readily available raw material for the synthesis of geopolymers [10].

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Fly ash-based geopolymers have some significant advantageous properties like high mechanical strength and temperature resistance alongside low permeability and shrinkage [15, 19]. In addition, less energy requirement and low CO2 emission in the manufacturing process

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make fly ash-based geopolymer a promising replacement material for ordinary Portland cement [8, 9, 24]. The extensive utilization of fly ash-based geopolymer seems to take place in

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construction industry [16]. The other application areas involve adsorption of heavy metals [3,

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7, 10, 19] and dyes [2, 4, 6, 15], and immobilization of toxic metals [16, 25]. In this study, a class F fly ash-based geopolymer prepared by a combination of alkaline activation and thermal treatment was used for the removal of Basic Yellow 2 from aqueous

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solutions. In terms of fly ash-based geopolymer usage, a considerable body of literature has been concentrated on cement replacement, and there are also several studies on heavy metal

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adsorption. However, the existing literature on dye removal is very limited to only a few studies.

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In addition, there is no report in the literature as much detailed as this study which covers a combination of the detailed geopolymer characterization, the isothermal, kinetic and thermodynamic investigations as well as the effect of operational parameters.

2. Experimental 2.1. Materials 4

The coal fly ash sample from Çatalağzı Power Plant, Turkey was used as the feedstock. In the experimental stage, the as-received sample (CFA) was first subjected to a specific hydraulic classification process to recover its ultra-fine fraction (CUFA). The obtained CUFA was then used as the alumina-silicate source for the geopolymer (GEO) synthesis in which analytical grade NaOH pellets were used as the alkaline activator. Chemically pure grade Basic Yellow 2 (BY2) was supplied from Haining Deer Chemical Co.. BY2, which has a commercial name as Auramine-O, is a cationic dye. Its chemical formula, molecular weight and maximum

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adsorption wavelength (λmax) are C17H22ClN3, 303.834 g mol-1 and 432 nm, respectively. The main chemical structure can be seen in the study conducted by Öztürk and Malkoc, 2014 [26].

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Distilled water was used throughout the experiments.

2.2. Methods

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2.2.1. Hydraulic classification

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The main framework of the classification used is based on a lamella hydraulic classifier. The particle separation is accomplished by the differences in particle settling rates in water. In this

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system, ultrafine particles (CUFA) are recovered from the top far end, overflow discharge area. Detailed information about the classifier can be obtained from Robl and Groppo, 2011 [27] and

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McCarthy et al., 2013 [28].

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2.2.2. Preparation of the geopolymer A certain amount of CUFA was mixed with 10 M NaOH solution using a solid mass ratio of 1:1.2. After thoroughly stirred with a glass rod for 10 min, a homogenized geopolymer paste started to form. The top of the beaker was closed with a round-bottom glass plate and then treated at 350 oC for 1 h to obtain a fused form. After cooling to room temperature, it was mixed with distilled water using a mass ratio of 1:4, followed by curing for 24 h in a thermostatic 5

shaker at room temperature and 100 rpm. The slurry was then filtrated and washed with distilled water many times to remove excess NaOH. The obtained geopolymer paste (GEO) was finally dried at 105 oC overnight prior to its utilization.

2.2.3. Characterization of materials A Spectro IQ X-ray fluorescence (XRF) device was used to analyze the major chemical

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inclusions. Loss on ignition (LOI) was determined based on ASTM C311. Particle size measurements were done by a Malvern Hydro 2000S instrument. The crystalline structures were examined by a PANalytical Empyrean X-ray diffractometer (XRD) operating with Cu-Kα

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radiation (λ = 1.54051 Å) at 40 kV and 30 mA over 10-80°. Microstructural characterization and point elemental content were determined using high-resolution Zeiss Sigma 300 and

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Hitachi S-4800 scanning electron microscopes (SEM) equipped with an energy dispersive X-

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ray (EDX) analyzer. Fourier transform infrared (FT-IR) spectra of the samples were recorded by a Tensor 27, Bruker instrument using the KBr pellet technique in 4000-400 cm-1. The surface

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area, total pore volume and pore size distribution were measured through N2 adsorption method at approximately -197 oC using a Micromeritics 3 Flex instrument. The BET surface areas of CUFA and its geopolymer were determined for the relative pressure range of 0.0-1.0. The total

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pore volume was determined for a single point P/P0 value of 0.950. The pore size distributions

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of the samples were inferred from the isotherms by means of the Barrett-Joyner-Halenda (BJH) method. The measured samples were degassed for 15 h at 100 oC prior to the adsorption tests. The natural pH values of the samples were determined based on the method suggested by Li et al., 2006 [2].

2.2.4. Batch adsorption experiments 6

Batch adsorption tests were conducted in 100 mL glass flasks which were shaken at 100 rpm and a certain temperature in a thermostatic shaker. The volume of BY2 solution was kept constant as 100 mL throughout the experiments. At predetermined time intervals, the approximately 5-mL sample was withdrawn from the treated solution and then centrifuged two times for 3 min at 5000 rpm by a Hettich EBA 20 instrument. BY2 concentration in the solution was determined using an Optizen pop UV-Vis spectrophotometer at 432 nm (λmax for BY2). The adsorbed amount of BY2 (mg g-1) at equilibrium (qe) and a specific time (qt), and its

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removal efficiency (RE, %) were calculated from the commonly-used well-known equations [6, 10, 17, 29].

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3. Results and discussion

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3.1. Characterization of materials

3.1.1. Chemical composition and basic physical properties

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The main chemical compositions and basic physical properties of CFA, CUFA and CUFAbased geopolymer (GEO) are shown in Table 1. As seen from Table 1, Çatalağzı fly ash has a

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typical chemical inclusion of a regular class F type. According to the ASTM C618 standard, both CFA and CUFA meet the chemical specifications of a class F fly ash owing to their high

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cumulative amounts of silica, alumina and iron oxide (SiO2+Al2O3+Fe2O3 ≥ 70) with low SO3 (≤ 5) and LOI (≤ 6) values [20, 21]. The mean sizes (d50) by volume were determined as 39.29,

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5.25 and 0.70 µm for the respective CFA, CUFA and GEO. Accordingly, the BET surface area values of 0.83, 6.09 and 47.38 m2 g-1 were obtained for the sequential CFA, CUFA and GEO.

Table 1

7

The elemental inclusions in CUFA and GEO were also examined using EDX analysis, and the results were indicated with superscript a. According to the EDX analysis, O, Si and Al constitute the main structure of CUFA. The amount of these elements decreased and Na participated in the structure due to the alkaline activation with NaOH, resulting in the formation of sodiumalumina-silicate hydrate gel [30]. Fig. 1 illustrates the particle size distributions of CFA, CUFA and GEO. As seen from Fig. 1, CFA with a coarse mean size of 39.29 µm has a very large size distribution while its overflow

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product, CUFA, with a very fine mean size of 5.25 µm has a relatively close distribution. After the geopolymerization, a very narrowly-sized distributed GEO product varying in the range of

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400-1100 nm was obtained with a mean size of approximately 700 nm.

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Fig. 1

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3.1.2. Mineralogical analysis

The XRD patterns of CUFA and GEO are shown in Fig. 2. ICSD database was used to analyze

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the diffraction peaks via a software program. As seen from Fig. 2, CUFA consists of mullite (Al4.68Si1.32O9.66, ICSD 98-008-0498) and quartz (SiO2, ICSD 98-005-7052) as the major

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crystalline inclusions, and also includes a considerable amount of hematite (Fe2O3, ICSD 98001-2749). In addition, the XRD analysis indicates that a vast majority of CUFA is constituted

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by amorphous glass phase which substantially affects source material’s reactivity for the synthesis of geopolymers. In other words, higher amorphous content means better reactivity [10, 31].

Fig. 2

8

After the geopolymerization process, all the diffraction peaks extensively decreased, and most of the mullite peaks disappeared. The relative mullite content in the total crystalline inclusions diminished from 61.4 to 29.0% probably due to its reactive alumina-silicate structure. The sharp peaks converted into the broadband peaks with much lower intensities, which are characteristics for amorphous materials. All of these results indicate that the crystalline alumina-silicate phase altered to the amorphous structure as a result of the geopolymerization, which is consistent with

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the work of Al-Zboon et al., 2011 [10].

3.1.3. Microstructural analysis

SEM images provide detailed information in terms of a material’s morphology and surface

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texture. Fig. 3 exhibits the SEM micrographs of CUFA and GEO. Fig. 3-(a-d) show various CUFA images in different magnifications. As seen clearly from these images, CUFA mainly

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consists of solid and hollow spherical particles with mostly smooth surfaces, and also low

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amounts of irregularly shaped particles [20, 21]. These SEM images also verify the results obtained from the laser size analysis. Fig. 3-(e) clearly exhibits the transformation of spherical

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CUFA particles into amorphous structure after the geopolymerization process [32]. Fig. 3-(f) shows the geopolymeric structure in conjunction with the unreacted CUFA particles, indicating

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incomplete geopolymerization. These unreacted particles are the refractory parts which are unaffected by the process mainly because of their strong crystalline nature [33]. Considering

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Fig. 3-(f) together with the XRD analyses, it can be said that most of these unreacted particles are constituted by the crystalline quartz phase. The negative impact of higher Si/Al ratio of starting material on degree of geopolymerization was also reported by Siyal et al., 2016 [33]. According to the researchers, this can be resulted from the rapid setting of the paste prior to the completion of a major portion of dissolution. This explanation clarifies the incomplete geopolymerization in this study in which dissolution of mostly mullite and partly quartz phases 9

takes place. In other words, quartz particles are mostly solidified before the complete dissolution due to the chemical inclusions (SiO2). As far as is known, there is no specific method available for determining the quantity of this refractory part in geopolymers [33]. Fig. 3 (g-h) clearly indicates the alumina-silicate gel structure which is also observed from the literature [15, 19]. Fig. 3

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The elemental inclusions in CUFA and its geopolymer were examined using EDX analysis. According to the EDX spectra shown in Fig. 4, O, Si, Al and Fe constitute the main structure of CUFA. After geopolymerization, Na accompanied with these elements due to the formation

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of sodium-alumina-silicate hydrate gel [30].

3.1.4. FT-IR analysis

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Fig. 4

FT-IR analysis is a commonly used method to examine the changes and modifications in the

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structure of a substance due to any chemical or physico-chemical treatment processes [34]. The FT-IR spectra of CUFA and its geopolymer, GEO, can be seen from Fig. 5. The main

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characteristic bands, their identified species and the changes with the geopolymerization can be

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clarified as follows.

Fig. 5

The strong asymmetric band at 3433 cm-1 is the stretching vibration of the H-O-H groups due to adsorbed molecular water in the fly ash. The weak bands placed at 1636 cm-1 and 2300 cm-1 could be assigned to the respective stretching and deformation vibrations of the OH [32, 35] 10

and simultaneous stretching vibrations of structural OH and Si-O bonds [6]. The band located at ~980 cm-1 can be related to the stretching of asymmetric Al-O-Al/Si-O-Si bonds [6, 36]. Two bands at 720 and 590 cm-1 could be assigned to the stretching vibrations of the symmetric SiO-Si (Al) bridges, corresponding to cyclo-silicates vibrations [31]. The band located at ~1440 cm-1 can be attributed to sodium carbonate inclusion, resulted from the reaction of residual sodium with atmospheric carbon dioxide, indicating the geopolymer formation [6, 32]. The bands at around 2500 cm-1 could be associated with the

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stretching and bending vibrations of the respective –OH and H–O–H bonds in the water, which is adsorbed on the surface of the geopolymer and in the large cavities of the polymeric network [6, 37]. The band at 472 cm-1 are due to symmetric bending vibrations of Si-O-Si or Al-O-Al

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bonds in the geopolymer while, similarly, the one located at 452 cm-1 could be assigned to SiO or Al-O bonds [33]. The band located at 880 cm-1 could be assigned to the symmetric

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stretching in Al-O tetrahedral framework which also specifies the geopolymeric inclusion [32, 38]. The peak at 2160 cm-1 can be attributed to stretching bending vibrations of Si-H. Moreover,

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the weak peaks observed in 1660 and 1974 cm-1 can be caused by C≡C and C=O stretching

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bending of unburned carbons in the fly ash.

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3.1.5. Surface area and pore properties Fig. 6-(a) displays the nitrogen adsorption/desorption isotherms of CUFA and GEO which were

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analyzed with the BET model. Based on the IUPAC classification, GEO follows the H3 type of the hysteresis loops. There are two main characteristic features of this loop. Firstly, the adsorption branch looks like a Type II isotherm. The other one is that the desorption branch has a lower limit normally located at the P/P0 caused cavitation. The H3 type loop resulted from the aggregation of non-rigid stratified particles like certain clays if macro-pores, which are not fully

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filled with pore condensate, constitute the main pore network [39]. On the other hand, CUFA did not exhibit this loop.

Fig. 6

Fig. 6-(b) shows the BJH pore size distributions of the materials. Pore volume of GEO was mainly distributed in the size range of 2-20 nm. However, much lower pore volume values were

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observed for CUFA. In this case, the pore volume decreased linearly with pore width and their distribution mainly varied from 2 to 10 nm. As also seen from the table in Fig. 6, CUFA has a total pore volume of 0.008 cm3 g-1 and an average pore width of 4.970 nm. Surface area values

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of 6.090 and 5.717 m2 g-1 were obtained based on the BET and BJH methods, respectively. After the geopolymerization, total pore volume and average pore width increased to the

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3.2. Isothermal investigations

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were also observed from this table.

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respective 0.156 cm3 g-1 and 9.792 nm. Accordingly, extensive increases in the surface areas

Isotherm studies were carried out with the constant adsorbent dosage of 2.0 g, the equilibrium

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time of 300 min and the natural pH of about 9.85 for the three different temperature values, 293, 313 and 323 K. A total of six adsorption isotherms, namely the Langmuir, Freundlich,

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Temkin, BET, Dubinin-Radukevisch and Harkins-Jura were used to determine the adsorption mechanism of BY2 on the produced geopolymer. Together with the equations used, Table 2 summarizes the parameters of these isotherms calculated from the obtained experimental results.

Table 2 12

As seen clearly from Table 2, the experimental results were better correlated with the Langmuir and Temkin isotherms due to their much higher R2 values than those of the other ones. As it is well known, the Langmuir model assumes the adsorption as monolayer and homogenous. This type of adsorption can be resulted from not only physical interactions but also very strong electrostatic attractions. The first situation occurs when the pore radius is large enough for attaching adsorbate in a single layer, while the second one happens when there is an electrostatic

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attraction between adsorbent and adsorbate [29, 40]. The favorability of the Langmuir isotherm can be expressed by a dimensionless constant, RL, defined by Eq. 1 [41, 42]:

1

𝑅𝐿 = (1+𝐾𝐶 )

(1)

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0

in which K and Co are the Langmuir constant and the initial dye concentration, respectively.

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According to this approach, the Langmuir isotherm is irreversible for the dimensionless constant equal to 0 (RL=0), favorable for the values between 0 and 1 (0< RL<1), linear for 1 and

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not favorable for the values higher than 1 (RL>1) [41, 42]. Since the obtained RL values in Table 2 vary between 0.091 and 0.151, it can be said that the Langmuir isotherm is favorable for the

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adsorption of BY2 on the geopolymer in all the tested temperatures. The Freundlich model is used to examine the heterogeneity of an adsorption process, which is

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determined by the heterogeneity factor, n. In this model, if n>1, adsorption occurs via physical interaction while the condition of n<1 points out chemical interaction between adsorbent and adsorbate. In addition, adsorption is a linear process if n equals to 1.0 [40]. As seen from Table 2, the low R2 values show that the adsorption process is unsuited with the Freundlich isotherm so the calculated n values have no meaning in this case. On the other hand, the Temkin model is better correlated with the obtained results compared to the other models tested especially for 13

the trial performed at 293 K. The R2 value of 0.987, the highest one obtained from the whole isotherm studies, was attained at this point. These results indicated that the Temkin model wellcharacterized the interactions between the geopolymer and the dye [43]. The heat of adsorption of the all molecules in the layer reduced linearly with coverage layer, and the energy is uniformly distributed [44]. In the Temkin equation, bT and aT are the constants for the respective heat of adsorption and maximum binding energy. The parameters calculated from the Temkin equation emphasized that the adsorption of BY2 at 293 K was characterized by a homogeneous

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distribution binding energies up to the maximum point [40]. The low regression coefficients obtained from the BET, Dubinin-Radukevisch and Harkins-Jura isotherms clearly pointed out that the adsorption process was not compatible with these isotherms.

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The determination of the appropriate isotherm type has vital importance in the design of adsorption systems. Since adsorption isotherm clarifies mechanism between adsorbent and

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adsorbate, it is critical in optimizing the appropriate adsorbent dosage [45]. Fig. 7 illustrates the

equilibrium time of 300 min.

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Fig. 7

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adsorption isotherm patterns at three different temperatures, 293, 313 and 323 K, for the

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As seen from Fig. 7, the efficiency increased with temperature, indicating endothermic

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character of the adsorption process. Similarly, the amount of adsorbed BY2 also increased with temperature. The equilibrium adsorption isotherm resembles the Type-I. In particular, the isotherm curves obtained at 313 and 323 K are also similar to the Type-I isotherm. This type of isotherm can be characterized with the mono-molecular chemical adsorption occurring as monolayer. On the other hand, the sharp increases take place close to the isotherm curves of micro-porous solids. When the surfaces of micro-pores with high adsorption potential are

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coated mono-molecularly, the adsorption proceeds until the pores are completely filled. Adsorbed amount in the Type-I isotherm converges a limiting value mainly ruled by the volume of effective micro pore instead of the internal surface area [46].

3.3. Kinetic study The three kinetic models, namely pseudo-first-order, pseudo-second-order and intra-particle

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diffusion were used to evaluate the kinetic investigation of BY2 adsorption on the geopolymer surface. Kinetic studies were conducted at 293, 313 and 323 K with the varying dye concentrations (10-50 mg L-1) for the constant adsorbent dosage of 2.0 g and the natural pH of

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9.85. The equations of these models are sequentially given by Eq. 2-4 [45, 47-51].

=

1 𝑘2 𝑞𝑒2

+

𝑡 𝑞𝑒

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𝑡 𝑞𝑡

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𝑙𝑛(𝑞𝑒 − 𝑞𝑡 ) = 𝑙𝑛𝑞𝑒 − 𝑘1 𝑡

(3) (4)

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𝑞𝑡 = 𝑘𝑖 √𝑡 + 𝐶

(2)

where qe and qt are the adsorbed BY2 amounts at the equilibrium and a specific time, k1 (s-1)

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and k2 (g mg-1 s-1) express the rate constants for the respective pseudo-first and pseudo-secondorder models, ki (mg g-1 s-1/2) can be defined as the rate constant of the intra-particle diffusion

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model and C (mg g-1) represents the boundary layer thickness [47-49]. The obtained values of the kinetic constants are summarized in Table 3.

Table 3

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According to Table 3, the pseudo-first order model did not fit to the experimental results. The highest correlation efficiency obtained based on this model was only 0.7173. On the other hand, the pseudo-second order model extremely well correlated with the results especially for the temperatures of 313 and 323 K. It was also understood that the obtained R2 values were independent of the initial dye concentrations. However, the rate constants (k2) generally decreased with the increasing dye concentrations except for 293 K. This result can be explained by more competition for the surface active sites at higher concentrations [29, 40]. The Weber

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intra-particle diffusion model was also used to describe the adsorption process. Unlike the pseudo-second order model, this time the intra-particle diffusion was compatible with the results especially for 293 K (R2>0.9326) and did not fit to the ones obtained at 323 K. The

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growing C values with the increasing dye concentrations showed that thickness of the boundary layer has an effective role in the adsorption process for 293 K, indicating a larger boundary

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layer diffusion effect [29, 41, 52]. However, the boundary layer most probably begins to

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disappear with increasing temperature, inhibiting the diffusion effect. Overall results suggest that the pseudo-second order and the intra-particle diffusion models both occur at the same time

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for the low temperature.

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3.4. Thermodynamic investigation

The adsorption process was also applied to thermodynamic investigation in which the changes

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of enthalpy (∆Hads), entropy (∆Sads) and Gibbs free energy (ΔGo) were calculated by Eq. 5-7 [53]:

𝑑(𝑙𝑛𝐶)

= 𝑑(1/𝑇)

∆𝐻𝑎𝑑𝑠

𝑑(𝑙𝑛𝐶)

∆𝑆𝑎𝑑𝑠

𝑑(𝑙𝑛𝑇)

=

(5)

𝑅

(6)

𝑅

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∆𝐺 𝑜 = −𝑅𝑇𝑙𝑛𝐾

(7)

where T is temperature (K) and R is the gas constant (8.314x10-3 kJ mol-1 K-1). According to Eq. 5, when (ln C) is plotted against (1/T), the slope of the straight line will yield a value of (∆H/R). Similarly, based on Eq. 6, when (ln C) is plotted against (ln T), this time the slope will be (∆S/R). Thus, ∆H and ∆S values can be determined from simple calculations using the above

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approximations. The obtained results are summarized in Table 4.

Table 4

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As seen from Table 4, ∆Hads has a positive value while ∆Sads takes a negative one. The growing amount of the adsorbed dye (q) with increasing temperature implies the endothermic character,

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also verifying the positive enthalpy change. On the other hand, the negative entropy indicates

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an increased order at the interface. Mobility of the dye ions increases with increasing temperature, causing the ions to escape from the solid surface to the liquid phase. This results in a decrease in the amount of dyes adsorbed. However, negative ∆G values specify the

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spontaneous nature of the adsorption for all the temperatures and dye concentrations tested. After all, it can be said that these results can be used to characterize adsorption processes with

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similar adsorbents and adsorbates.

3.5. The effects of operational parameters on the adsorption 3.5.1. The adsorbent dosage Determination of appropriate adsorbent dosage is vital in terms of the capacity for a given initial concentration of the adsorbed material [54]. In this stage, the effect of GEO dosage varying from 0.5 to 3.0 g L-1 was examined as a function of processing time for the constant initial BY2 17

concentration of 30 mg L-1, the temperature of 293 K and natural pH of 9.85. The obtained results were illustrated in Fig. 8.

Fig. 8

As seen in Fig. 8-(a), the removal efficiency of BY2 increased with processing time independent of the dosages tested. However, much higher increasing ratios were obtained with 2.5 and

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especially 2.0 g L-1. When the time was kept constant at 900 min, from Fig. 8-(b), the efficiency substantially increased from 13.28 up to 94.47% for the adsorbent dosages of 0.5 and 2.0 g L1

, respectively. The observed improvement can be attributed to the increase in surface area and

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availability of more active adsorption sites due to the increasing dosage [55]. On the other hand, further increase reduced the efficiency down to 57.76%, which was obtained for 3.0 g L-1. Since

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the highest efficiency value, 94.47%, was achieved with the geopolymer dosage of 2.0 g L-1,

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this value was used to examine the effect of initial dye concentration.

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3.5.2. The effect of initial dye concentration

The removal of pollutants in high concentrations is one of the success parameters used to evaluate the efficiency of a treatment process. Therefore, in this stage, BY2 concentrations

ur

varying from 10 to 50 mg L-1 were studied at three different temperatures, 293, 313 and 323 K,

Jo

for the constant GEO dosage of 2.0 g L-1 and the natural pH of 9.85. Fig. 9 exhibits the obtained efficiency values as a function of reaction time.

Fig. 9

18

According to Fig. 9, the efficiency decreased with the increasing dye concentrations for all of the three-temperature-set tested. This reduction can be attributed to the insufficient surfaceactive cites as a result of the increasing BY2 concentrations. However, for a specific point of the residence time, the differences between the maximum and minimum efficiency values decreased with the increasing temperature. For the constant reaction time of 300 min, the differences in the efficiencies are 20.50, 16.22 and 5.38% for the respective temperatures of 293, 313 and 323 K. Fig. 9 also showed that the equilibrium time of the adsorption reduced

ro of

with the increasing temperature. The reaction equilibrium was reached after about 700 min for 293 K and 300 min for both 313 and 323 K. In particular, as seen explicitly from Fig. 9-(c), the reaction almost attained to its equilibrium in 60 min at 323 K for most of the dye concentrations

-p

tested.

The obtained results were figured for the fixed BY2 concentration of 50 mg L-1, which can be

re

seen from Fig. 10. Similarly, the efficiency increased with temperature, and the differences in

lP

the efficiencies for a certain time decreased with the increasing reaction time. According to Fig. 10, for the reaction time of 300 min, the efficiency values of 64.19, 76.71 and 88.10% were obtained at the respective temperatures of 293, 313 and 323 K. The differences between the

na

maximum and minimum efficiency values were 46.06, 23.91 and 6.11% for the constant

Jo

Fig. 10

ur

residence time values of 120, 300 and 720 min, respectively.

4. Conclusion

In this study, a class F fly ash was used as alumina-silicate source for the preparation of a geopolymeric material which was then tested as an adsorbent for the removal of Basic Yellow 2 from aqueous solutions. The characterization results showed that the fly ash particles with 19

mostly smooth spherical shape turned into a porous and amorphous polymeric structure, sodium-alumina-silicate hydrate gel, as a result of the geopolymerization process. Specifically, the BET surface area and total pore volume values increased from 6.090 m 2 g-1 and 0.008 cm3 g-1 to the respective 47.380 m2 g-1 and 0.156 cm3 g-1. The experimental results indicated that the removal efficiency of BY2 extensively increased from 13.28 to 94.47% for the geopolymer dosages of 0.5 and 2.0 g L-1, respectively. However, the efficiency dramatically decreased down to 57.76% for 3.0 g L-1. On the other hand, efficiency reduced with increasing BY2

ro of

concentrations. At 293 K for 300 min, the efficiency values of 84.69 and 64.19% was obtained for the respective BY2 concentrations of 10 and 50 mg L-1. Based on the isothermal and thermodynamic investigations, the experimental results well suited with the Langmuir and

-p

Temkin models, and the adsorption can be considered as a spontaneous and endothermic process. In addition, the equilibrium adsorption isotherms resemble the Type-I, which is

re

characterized with the monolayer chemical adsorption. Kinetic studies exhibited that the

lP

pseudo-second order and the intra-particle diffusion models both occur simultaneously for 293 K while the pseudo-second order model is valid for 313 and 323 K. In conclusion, the prepared geopolymer gave promising results for the removal of cationic Basic Yellow 2 from aqueous

na

solutions. However, further research is required to achieve a shorter and more applicable

ur

equilibrium time for the adsorption process.

Jo

Declaration of Interest Statement We would like to confirm that there is no known conflict of interest associated with this publication.

Acknowledgement The technical support by East Anatolia High Technology Application and Research Center (DAYTAM) of Atatürk University is thankfully appreciated. 20

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Jo

ur

na

lP

re

-p

ro of

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22

Figures captions: Fig. 1. Particle size distribution of the materials. Fig. 2. Mineralogical analysis of the materials. Fig. 3. SEM images of CUFA (a-d) and GEO (e-h). Fig. 4. EDX spectra of the materials. Fig. 5. FT-IR spectra of CUFA and GEO. Fig. 6. Surface area and pore properties of the materials.

ro of

Fig. 7. The obtained isotherm patterns at three different temperatures for 300 min.

Fig. 8. The effect of adsorbent dosage on the removal efficiency of BY2 as a function of time (a) for the processing time of 900 min (b). Experimental conditions: Natural pH = 9.85,

-p

[BY2]0 = 30 mg L-1 and temperature = 293 K.

Fig. 9. The effect of initial BY2 concentration on the removal efficiency at 293 K (a), 313 K

re

(b) and 323 K (c). Experimental conditions: Natural pH = 9.85 and GEO dosage = 2 g L-1.

lP

Fig. 10. The effect of temperature on the removal efficiency. Experimental conditions: Natural

na

pH = 9.85, GEO dosage = 2 g L-1 and [BY2]0 = 50 mg L-1.

Jo

ur

Figures

23

30 GEO

CUFA

CFA

Intensity (%)

25 20

15 10

0 0.1

1

10

100

Jo

ur

na

lP

re

-p

Particle size (μm)

Fig. 1

24

ro of

5

1000

10 20 30 40

Position

Fig. 2

25

(o2Theta) 50

48.10-M 49.40-M-H 50.15-Q L

45.64-Q L

42.54-M-QL

40.83-M-H

39.23-M-QL -H

35.22-M 36.53-Q L 37.00-M

33.20-M-H

60

70

81.10-Q L

74.19-M 74.99-M-H

70.36-M

68.06-Q L

64.47-M

59.88-Q L 60.59-M

57.52-M-H

53.90-M-H

16.45-M

25.00-M 26.25-M

20.86-Q L

16.45-M

30.96-M

ro of

-p

re

lP

na

ur

Jo

68.12-Q L

59.99-Q L-M

50.15-Q L

36.55-Q L

26.63-Q L

20.89-Q L

Intensity (count)

26.67-Q L

QL: Quartz M: Mullite H: Hematite

GEO

Mullite; 29 Quartz; 71

Hematite; 8.9

CUFA

Quartz; 29.7

Mullite; 61.4

80

(a)

(b)

(c)

(d)

Solid sphere

ro of

Hollow sphere (Cenosphere)

-p

Unburned carbon

Irregularly-shaped particles

re

Irregularly-shaped particles

(f)

Unreacted CUFA

na

lP

(e)

ur

(h)

Jo

(g)

GEO

26

Fig. 3 CUFA

Element Weight % O 47.44 Mg 0.57 Al 16.35 Si 26.84 K 3.65 Fe 5.16

Si O

K

K

Mg Fe

Fe

K

0

2

4

Fe

6

8

Energy (keV) GEO

Element Weight % O 41.79 Na 11.90 Mg 0.30 Al 14.08 Si 24.08 K 1.17 Fe 6.66

Intensity

-p

Si

O

re

Al

Na KK

Mg

0

Fe

lP

K Fe

10

ro of

Intensity

Al

2

4

6

Jo

ur

na

Energy (keV) Fig. 4

27

Fe

8

10

4000 3600 3200

2800

2400

Fig. 5

28 2000

CUFA

1600

Wavenumber (cm-1)

1200

1200

720 590

1600

Si-O-Si/Al stre tching

2000

ro of

2400

Si-O-Si/Al stretching Si-O/Al stretching

880 Al-O stretching

1440 Na 2CO 3 / O-C-O stretching

1660 C=O stretching

1974 C≡C stretching

2160 Si-H Stretching

472 452

Transmittance (%)

~2500 H-O-H Vibration

980 Al-O-Al/Si-O-Si asym stretching

2800

1636 -OH Stretching

3200

-p

re

3600

2351 -OH /Si-H Stretching

3433 H-O-H Vibration

Transmittance (%) 4000

lP

na

ur

Jo GEO

800

800

400

400

8

CUFA 7

Parameter

CUFA

GEO

6

BET surface area (m2/g)

6.09

47.38

BJH cumulative surface area (m2/g)

5.717

51.63

Total pore volume (cm3/g) Average pore width (nm)

0.008 4.97

0.156 9.792

5 4

2

0.006

GEO

1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

GEO

100 80

60 40 20

0 0.0

(a)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CUFA 0.005

0.004

0.003

0.002

0.001

0.000

1.0

1

Relative pressure (p/p0)

(b)

Jo

ur

na

lP

re

Fig. 6

29

10

Pore width (nm)

-p

0

ro of

0

Pore volume, dV/dlog(w) (cm³/g·nm)

Quantity adsorbed (cm³/g STP)

3

100

50

q (mg/g)

40

30

20 323 K 10

313 K

0 0

5

10

ro of

293 K 15

Ce (mg/L)

Jo

ur

na

lP

re

-p

Fig. 7

30

20

Removal efficiency (%)

100

(a)

80

60

40

0 0

200

400

600

Time (min) 1.0 g/L

1.5 g/L

Removal efficiency (%) 100

(b)

94.47

re

57.76

lP

29.60

40 13.28

ur

0.5

1.0

12.21

7.52

4.44

1.99

Jo

3.0 g/L

50.11

60

na

Uptake (%)

80

0

2.5 g/L

83.26

q (mg/g)

20

2.0 g/L

800

-p

0.5 g/L

ro of

20

1.5

2.0

12.49

2.5

Adsorbent dosage (g/L) Fig. 8

31

8.66

3.0

100

(a)

Removal efficiency (%)

84.69

80

60

10 mg/L

64.19

20 mg/L 40 30 mg/L

40 mg/L

20

0

0

100

200

300

400

500

600

Time (min) 100

re

60

800

900

10 mg/L

20 mg/L 30 mg/L

lP

40

20

0

700

-p

80

na

Removal efficiency (%)

(b)

ro of

50 mg/L

100

200

300

400

50 mg/L 500

Time (min)

Jo

ur

0

40 mg/L

32

600

700

800

900

100

Removal efficiency (%)

(c) 80

60 10 mg/L

40

20 mg/L 30 mg/L 20

40 mg/L 50 mg/L

0 100

200

300

400

500

600

Time (min) Fig. 9

100

60

lP

64.19

40

20

0

100

Jo

ur

0

200

300

900

-p

76.71

800

re

80

na

Removal efficiency (%)

88.10

700

ro of

0

323 K 313 K

293 K 400

500

Time (min) Fig. 10

33

600

700

800

900

Table 1. Chemical composition and basic physical properties of the materials. Chemical composition (%)

CFA

Sia

57.09

55.34

26.84a

24.08a

Al2O3

Ala

27.46

29.39

16.35a

14.08a

Fe2O3

Fea

6.56

5.97

5.16a

6.66a

1.97

1.20

NIb

Mga

2.38

2.22

0.57a

0.30a

Na2O

Naa

0.31

0.26

NIb

11.90a

K2O

Ka

3.89

4.21

3.65a

1.17a

ro of

MgO

TiO2

1.13

1.23

NIb

SO3

<0.01

<0.01

NIb

Oa

NIb

LOI

1.68

Physical properties 39.29

BET surface area (m2 g-1)

0.83

re

Mean size (d50) by volume (μm)

NIb

47.44a

NIb

5.25

≈ 0.70

6.09

47.38

Jo

ur

na

lP

Determined by EDX elemental analysis (The average values of 10 measurements) Not included

Table 2. Parameters of the adsorption isotherms used.

34

41.79a

2.07

-p

-

b

GEO

SiO2

CaO

a

CUFA

Equation

Langmuir

C/q = 1/kqm+(1/qm)C

Freundlich

lnq = lnk+nlnC

Temkin

Parameters

293 K

313 K

323 K

qm k R2 RL n k R2 bT aT R2 qm k R2 K qDR R2 B A R2

36.364 0.282 0.917 0.151 1.006 2.496 0.788 0.293 1.161 0.987 -7.424 0.951 0.146 0.000 12.485 0.007 1.500 227.273 0.177

41.667 0.490 0.924 0.093 0.913 4.792 0.586 0.334 1.826 0.903 21.739 0.810 0.146 0.000 13.924 0.079 1.037 185.185 0.371

53.476 0.501 0.730 0.091 1.200 6.043 0.522 0.513 1.690 0.893 53.191 0.533 0.047 0.000 14.421 0.001 0.699 136.986 0.367

qe = (RT/bT)lnaT+(RT/bT)lnCe

BET

C/q(1-C) = 1/(qmk)+[(k-1/qmk)]C lnq = Kε2 + lnqDR

Harkins-Jura

1/q2 = (B/A)-(1/A)logC

Jo

ur

na

lP

re

-p

DubininRadukevisch

ro of

Isotherm

Table 3. Parameters of the kinetic models. Initial Temp. conc. (K) (mg L-1)

Pseudo-first order R2

Pseudo-second order k2

qe,exp 35

qe,cal

Intra-particle diffusion R2

ki1

C

R2

0.7390 0.9893 0.9931 0.9908 0.9920 0.9991 0.9966 0.9974 0.9970 0.9974 0.9951 0.9999 0.9998 0.9995 0.9997

re lP na ur 36

(mg g-1 s-1/2) 0.0301 0.0465 0.0697 0.0763 0.0789 0.0191 0.0379 0.0523 0.0634 0.0669 0.0144 0.0220 0.0310 0.0395 0.0445

0.3685 1.4366 2.0443 5.2287 8.9575 1.5660 2.4567 4.3200 6.9164 10.7960 2.3588 5.7297 8.8397 12.0790 16.0710

ro of

0.6259 0.6422 0.6628 0.6343 0.7173 0.2085 0.6447 0.5930 0.6718 0.6640 0.0113 0.1714 0.2878 0.3446 0.4028

(mg g-1) 6.0790 10.9649 16.1031 20.1613 24.4499 5.3996 10.3413 15.1286 20.0803 24.6305 5.1706 9.7847 14.6199 19.6078 24.5098

-p

293 293 293 293 293 313 313 313 313 313 323 323 323 323 323

(mg g-1) 3.9405 7.6216 11.4836 15.3974 19.4578 4.7940 8.9319 12.6819 17.1216 21.5181 4.6216 9.4491 14.0181 18.4233 23.3371

Jo

10 20 30 40 50 10 20 30 40 50 10 20 30 40 50

(g mg-1 s-1) 0.00001 0.00001 0.00001 0.00001 0.00001 0.00008 0.00003 0.00002 0.00002 0.00002 0.00010 0.00014 0.00010 0.00006 0.00006

0.9326 0.9572 0.9372 0.9456 0.9574 0.8507 0.9068 0.9102 0.9168 0.9199 0.5898 0.6053 0.6532 0.6768 0.6781

Table 4. Thermodynamic parameters of the geopolymer-BY2 adsorption system. T Ca (K) (mg L-1) 293 4.757 Geopolymer 313 2.136 (Co:20 mg/L) 323 1.102

q 1/T lnT lnC (mg g-1) 7.622 0.003 5.680 1.438 8.932 0.003 5.746 0.792 9.449 0.003 5.778 0.265

ΔGo (kJ mol-1) -27.672 -30.994 -32.047

ΔHads (kJ mol-1)

ΔSads (kJ mol-1)

29.752

-0.097

Jo

ur

na

lP

re

-p

ro of

Adsorbent

37