# Using Modified Fly Ash for Removal of Heavy Metal Ions from Aqueous Solution

Authors: Thuy Chinh Nguyen; Trang Do Mai Tran; Van Bay Dao; Quoc-Trung Vu; Trinh Duy Nguyen; Hoang Thai
Journal: Journal of Chemistry
 (2020)

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## Abstract

This paper presents the characteristics of fly ash which was modified by 2-mercaptobenzothiazole (MBT) and sodium dodecyl sulfate (SDS) as the surfactants after treating with 1M NaOH solution. The change in morphology, specific surface area, crystal structure, and composition of the unmodified and modified fly ash was evaluated by FTIR, XRD, FESEM, BET, and EDX methods and techniques. The FTIR spectra of modified fly ash showed that there was no chemical reaction between the surfactants and fly ash. The XRD patterns and FESEM images indicated that modified fly ash had zeolite structure with a pore size of about 50 nm. Heavy metal ion adsorption behavior as well as adsorption isotherm models (Langmuir and Freundlich) of Cd2+ and Hg2+ ions of the unmodified and modified fly ash were also investigated and discussed. The amount of adsorbed ions of the modified fly ash was higher than that of the unmodified fly ash. The calculated results from the adsorption data according to the adsorption isotherm models of the above ions displayed that the Langmuir isotherm model was complied for the Cd2+ adsorption process while the Freundlich isotherm model was fitted for the Hg2+ adsorption process.


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## Body

## 1. Introduction

Application of fly ash (FA, waste product of thermal power plants) as additive/filler or adsorption material is opened and focused on study day to day. The main composition of FA includes inorganic oxides such as SiO2, Al2O3, and Fe2O3 and a small fraction of Na2O, MgO, and K2O. High thermal stability and spherical shape are the advantages of FA to broaden its application [1].One of the popular applications of FA is that it is used as a cheap absorbent for adsorption of heavy metal ions, organic substances, anions, dyes in water, and SOx, NOx, and mercury in air [2]. Chemical treatment of FA is an important tool to make FA a more effective absorbent for gas and water cleaning. In some reports, the authors studied on FA as zeolite for removal of heavy metal ions in solution [3–6] and catalytic [7], hydrocarbon contamination [8], lignite mine [9], oxamyl [10], resorcinol [11], phenol [12], and dye in waste water [13]. Based on an optical Fenton system, Visa et al. used FA to adsorb Cd2+ ion, methyl orange, and polyion water [14, 15]. Wang and Wu indicated that unburned carbon content in FA could influence on its adsorption ability [16]. FA is suggested as a promising absorbent for removal of various pollutants [17].In 2002, Sarbak and Kramer-Wachowiak carried out experiments to treat FA by NaOH, NaOH/NH4HCO3, ethylenediamine tetraacetic acid (EDTA), and HCl solutions to change specific surface area and surface and porous structure of FA. As a result, in all cases, the specific surface area and pore volume of treated FA were greater than those of original FA. The SEM images showed that the FA treated by HCl solution had surface smoother than the FA treated by NaOH/NH4HCO3 solution. The porosity of the treated FA was 10 times higher than that of the untreated FA. The treated FA is suitable for application as an adsorption material [1].Agarwal et al. used NaOH-treated FA particles modified by cethyltrimethyl ammonium bromide (CTAB) (FA/NaOH/CTAB) for adsorption of resorcinol in waste water from plastic, dye, and cosmetic factories [11]. The authors showed that the size of FA/NaOH/CTAB was smaller than that of original FA as well as NaOH-treated FA. The specific surface area (calculated according to the BET method), pore volume, and diameter of the FA/NaOH/CTAB reached 51.12 m2/g, 0.176996 cm3/g, and 138.4928 Å, respectively. The resorcinol single layer adsorption efficiency of the FA/NaOH/CTAB was 500 mg/g in the range of pH 5–7 solutions. According to the authors, the resorcinol adsorption mechanism by FA/NaOH/CTAB at pH < 7 solutions was a combination of electrostatic attraction, hydrogen bonding, and organic partitioning. FA-based geopolymer was also used for removal of Li+, Cs+, Co2+, Cs2+, Sr2+, and AsO43 from solution [18, 19].From literature findings, it can be inferred that FA particles has been treated by different agents such as acids or alkalis, EDTA, CTAB, or poly(1, 8-diaminonaphthalene) (PDAN). The treated FA particles have been used for adsorbing organic substances, Cr4+, Ni2+, and Fe3+ ions. In our previous research studies, we investigated modification of FA by vinyltriethoxysilane [20], NaOH and H2SO4 solutions [21]. The treated FA was used for adsorption of Cd2+ and Hg2+ ions. The obtained results confirmed that the FA which was treated by the NaOH solution had a higher adsorption capacity than that by the acidic solution. The NaOH-treated FA modified by organic substances could improve the ions adsorption capacity of FA, leading to the potential application of FA as excellent adsorption materials. In some studies, the authors indicated that organic substances grafted onto the surface of FA could react with metal ions to form the complex [3–5]. As a result, percent removal of heavy metal ion using modified FA was increased significantly. Up to now, FA modified by 2-mercaptobenzothiazole (MBT) or sodium dodecyl sulfate (SDS) and use of the modified FA for Cd2+ and Hg2+ ions adsorption has not been studied. MBT has high affinity for Hg2+ ion; thus, MBT can contribute to the improvement of Hg2+ ion adsorption capacity for FA. SDS is a widely used surfactant which enhances dispersion of FA in water; therefore, it increases contact between Hg2+ ion and FA in aqueous solution. Therefore, in this work, NaOH-treated FA was modified by SDS and MBT, and the modified FA was used for removal of Cd2+ and Hg2+ from aqueous solution. The change in structure, morphology, composition as well as Cd2+ and Hg2+ adsorption capacity of modified FA was presented and discussed. The laboratory investigation results of Cd2+ and Hg2+ ions adsorption uptake from aqueous can be a reliable basic for applying the modified FA in treatment of waste water.


## 2. Experimental

### 2.1. Materials

Fly ash particles (FA) used in this work were provided by Pha Lai Thermal Power Plant (Vietnam). Total content of SiO2 + Fe2O3 + Al2O3 in FA composition is higher than 86 wt.% (F class FA). Its particle size is in the range of 100–10 μm, and its humidity is 0.3%. 2-Mercaptobenzothiazole (MBT), sodium dodecyl sulfate (SDS), HgCl2, and CdCl2 were purchased from Merck Co. Some chemicals such as acetone and ethanol, which were obtained from China, were used without further purification.


### 2.2. Surface Modification of the Fly Ash

FA was treated by 1M NaOH solution (abbreviated as FAN) before being modified by SDS and MBT according to the process reported in [21]. The steps for modifying FAN by SDS and MBT were listed as follows. First, the FAN was added into the solution of 5% SDS (w/v) in water (the suitable content of SDS was chosen based on the data of experimental adsorption). The mixture (solid–liquid system) was stirred for 3 hours at 70°C with a stirring speed of 750 rpm. The mixture was continuously ultrasonic-stirred for 15 minutes before cooling in the air. Then, the SDS-modified FAN was washed by distilled water to remove residual surfactant. Finally, the solid was dried in a vacuum machine at 80°C until a constant weight is achieved, and SDS-modified FAN particles (abbreviated as FASDS) were obtained. The FASDS was reweighted and stored in a sealed PE bag.To modify the FAN by MBT, 8% MBT (w/v) was dissolved in acetone solvent before adding FAN into the MBT-acetone solution (the suitable content of MBT was chosen based on the data of experimental adsorption). The mixture was stirred for 1 hour with a stirring speed of 750 rpm at room temperature. Then, the mixture was allowed for evaporating the solvent at room temperature. After that, filtering and washing the solid by ethanol and distilled water was performed before drying it in a vacuum machine at 80°C until a constant weight is achieved and MBT-modified FAN particles (abbreviated as FAMBT) were obtained. The FAMBT was reweighted and stored in a sealed PE bag.


### 2.3. Characterization

#### 2.3.1. Infrared Spectroscopy (IR)

IR spectra of the FAN and modified FAN particles were recorded by using a Nicolet iS10 spectrometer (USA) at room temperature from 400 to 4000 cm−1 by an average of 8 scans with a resolution of 8 cm−11.


#### 2.3.2. Field Emission Scanning Electron Microscopy (FESEM)

FESEM images of the FAN and modified FAN particles were obtained using an S-4800 FESEM machine (Hitachi, Japan) to observe the change in morphology of the FAN and modified FAN particles.


#### 2.3.3. X-Ray Diffraction (XRD)

XRD analyses of the FAN and modified FAN particles were performed on a Siemens D5000 X-ray diffractometer (XRD) (CuKα radiation source, λ = 0.154 nm) at a generator voltage of 40 kV with a step of 0.03° and a current of 30 mA at a scan speed of 0.043°/s in the 2θ scan range from 5° to 90°.


#### 2.3.4. Brunauer–Emmett–Teller (BET) Isotherm Equation

The specific surface area of the FAN and modified FAN particles was determined by the nitrogen sorption method (BET) on a Micromeritics Tristar 3000 device.


#### 2.3.5. Energy-Dispersive X-Ray Spectroscopy (EDX)

EDX spectra of the FAN and modified FAN particles were carried out on an EDX 6000E device (Japan).


#### 2.3.6. Atomic Absorption Spectroscopy (AAS)

This method was used to evaluate the adsorption effectiveness of the FAN and modified FAN particles on an AAS 3300 Perkin Elmer spectrophotometer (USA) and an AAS ICE 3500 spectrophotometer (USA).


### 2.4. Adsorption of Cd2+ and Hg2+ Ions by FA0, FAN, and Modified FAN Particles

200 mg of FA samples was added into a 50 ml of solution containing Cd2+ or Hg2+ ion with different concentrations (ca. ppm). The mixture was stirred at room temperature for 120 minutes. This is the time for the heavy metal ion adsorption to reach the adsorption equilibrium. The pH value 6 was kept constant during the adsorption process. The amount of Cd2+ and Hg2+ ions adsorbed per gram of adsorbent, Q (mg·g−1), was calculated using the following equation:(1)Q=Co−CeW,where Q is the amount of adsorbed metal ion at equilibrium conditions (metal ion (mg)/adsorbent (g)), V is the solution volume (L), and W is the sorbent mass (g). Co and Ce are the initial and equilibrium concentrations of metal ions in solution (mg·L−1), which are determined by the AAS method.The removal of metal ions was calculated using the following equation:(2)H=Co−Ce⋅100Co,where H is the percent removal of metal ions (%).All experiments were performed in triplicate.


### 2.5. Study on Adsorption Isotherms

In this work, the Langmuir and Freundlich isotherms have been selected for the study on ions adsorption behavior in solid–liquid system.The Langmuir isotherm equation for ion adsorption can be written as follows:(3)qe=qmaxkLCe1+kLCe,where Ce is the concentration of the solute at equilibrium (mol·L−1) and qe is the amount of the adsorbate adsorbed per unit weight of the adsorbent at equilibrium (mol·g−1). The parameters, qmax and kL, are the Langmuir constants. qmax is represented as the maximum adsorption capacity, and kL is related to the binding energy or affinity parameter of the adsorption system.The Freundlich isotherm is expressed as follows:(4)qe=kFCe1/nF,where kF is the constant of the Freundlich isotherm (L1/n mg(1−(1/n))g−1) and 1/nF is the Freundlich exponent.


## 2.1. Materials

Fly ash particles (FA) used in this work were provided by Pha Lai Thermal Power Plant (Vietnam). Total content of SiO2 + Fe2O3 + Al2O3 in FA composition is higher than 86 wt.% (F class FA). Its particle size is in the range of 100–10 μm, and its humidity is 0.3%. 2-Mercaptobenzothiazole (MBT), sodium dodecyl sulfate (SDS), HgCl2, and CdCl2 were purchased from Merck Co. Some chemicals such as acetone and ethanol, which were obtained from China, were used without further purification.


## 2.2. Surface Modification of the Fly Ash

FA was treated by 1M NaOH solution (abbreviated as FAN) before being modified by SDS and MBT according to the process reported in [21]. The steps for modifying FAN by SDS and MBT were listed as follows. First, the FAN was added into the solution of 5% SDS (w/v) in water (the suitable content of SDS was chosen based on the data of experimental adsorption). The mixture (solid–liquid system) was stirred for 3 hours at 70°C with a stirring speed of 750 rpm. The mixture was continuously ultrasonic-stirred for 15 minutes before cooling in the air. Then, the SDS-modified FAN was washed by distilled water to remove residual surfactant. Finally, the solid was dried in a vacuum machine at 80°C until a constant weight is achieved, and SDS-modified FAN particles (abbreviated as FASDS) were obtained. The FASDS was reweighted and stored in a sealed PE bag.To modify the FAN by MBT, 8% MBT (w/v) was dissolved in acetone solvent before adding FAN into the MBT-acetone solution (the suitable content of MBT was chosen based on the data of experimental adsorption). The mixture was stirred for 1 hour with a stirring speed of 750 rpm at room temperature. Then, the mixture was allowed for evaporating the solvent at room temperature. After that, filtering and washing the solid by ethanol and distilled water was performed before drying it in a vacuum machine at 80°C until a constant weight is achieved and MBT-modified FAN particles (abbreviated as FAMBT) were obtained. The FAMBT was reweighted and stored in a sealed PE bag.


## 2.3. Characterization

### 2.3.1. Infrared Spectroscopy (IR)

IR spectra of the FAN and modified FAN particles were recorded by using a Nicolet iS10 spectrometer (USA) at room temperature from 400 to 4000 cm−1 by an average of 8 scans with a resolution of 8 cm−11.


### 2.3.2. Field Emission Scanning Electron Microscopy (FESEM)

FESEM images of the FAN and modified FAN particles were obtained using an S-4800 FESEM machine (Hitachi, Japan) to observe the change in morphology of the FAN and modified FAN particles.


### 2.3.3. X-Ray Diffraction (XRD)

XRD analyses of the FAN and modified FAN particles were performed on a Siemens D5000 X-ray diffractometer (XRD) (CuKα radiation source, λ = 0.154 nm) at a generator voltage of 40 kV with a step of 0.03° and a current of 30 mA at a scan speed of 0.043°/s in the 2θ scan range from 5° to 90°.


### 2.3.4. Brunauer–Emmett–Teller (BET) Isotherm Equation

The specific surface area of the FAN and modified FAN particles was determined by the nitrogen sorption method (BET) on a Micromeritics Tristar 3000 device.


### 2.3.5. Energy-Dispersive X-Ray Spectroscopy (EDX)

EDX spectra of the FAN and modified FAN particles were carried out on an EDX 6000E device (Japan).


### 2.3.6. Atomic Absorption Spectroscopy (AAS)

This method was used to evaluate the adsorption effectiveness of the FAN and modified FAN particles on an AAS 3300 Perkin Elmer spectrophotometer (USA) and an AAS ICE 3500 spectrophotometer (USA).


## 2.3.1. Infrared Spectroscopy (IR)

IR spectra of the FAN and modified FAN particles were recorded by using a Nicolet iS10 spectrometer (USA) at room temperature from 400 to 4000 cm−1 by an average of 8 scans with a resolution of 8 cm−11.


## 2.3.2. Field Emission Scanning Electron Microscopy (FESEM)

FESEM images of the FAN and modified FAN particles were obtained using an S-4800 FESEM machine (Hitachi, Japan) to observe the change in morphology of the FAN and modified FAN particles.


## 2.3.3. X-Ray Diffraction (XRD)

XRD analyses of the FAN and modified FAN particles were performed on a Siemens D5000 X-ray diffractometer (XRD) (CuKα radiation source, λ = 0.154 nm) at a generator voltage of 40 kV with a step of 0.03° and a current of 30 mA at a scan speed of 0.043°/s in the 2θ scan range from 5° to 90°.


## 2.3.4. Brunauer–Emmett–Teller (BET) Isotherm Equation

The specific surface area of the FAN and modified FAN particles was determined by the nitrogen sorption method (BET) on a Micromeritics Tristar 3000 device.


## 2.3.5. Energy-Dispersive X-Ray Spectroscopy (EDX)

EDX spectra of the FAN and modified FAN particles were carried out on an EDX 6000E device (Japan).


## 2.3.6. Atomic Absorption Spectroscopy (AAS)

This method was used to evaluate the adsorption effectiveness of the FAN and modified FAN particles on an AAS 3300 Perkin Elmer spectrophotometer (USA) and an AAS ICE 3500 spectrophotometer (USA).


## 2.4. Adsorption of Cd2+ and Hg2+ Ions by FA0, FAN, and Modified FAN Particles

200 mg of FA samples was added into a 50 ml of solution containing Cd2+ or Hg2+ ion with different concentrations (ca. ppm). The mixture was stirred at room temperature for 120 minutes. This is the time for the heavy metal ion adsorption to reach the adsorption equilibrium. The pH value 6 was kept constant during the adsorption process. The amount of Cd2+ and Hg2+ ions adsorbed per gram of adsorbent, Q (mg·g−1), was calculated using the following equation:(1)Q=Co−CeW,where Q is the amount of adsorbed metal ion at equilibrium conditions (metal ion (mg)/adsorbent (g)), V is the solution volume (L), and W is the sorbent mass (g). Co and Ce are the initial and equilibrium concentrations of metal ions in solution (mg·L−1), which are determined by the AAS method.The removal of metal ions was calculated using the following equation:(2)H=Co−Ce⋅100Co,where H is the percent removal of metal ions (%).All experiments were performed in triplicate.


## 2.5. Study on Adsorption Isotherms

In this work, the Langmuir and Freundlich isotherms have been selected for the study on ions adsorption behavior in solid–liquid system.The Langmuir isotherm equation for ion adsorption can be written as follows:(3)qe=qmaxkLCe1+kLCe,where Ce is the concentration of the solute at equilibrium (mol·L−1) and qe is the amount of the adsorbate adsorbed per unit weight of the adsorbent at equilibrium (mol·g−1). The parameters, qmax and kL, are the Langmuir constants. qmax is represented as the maximum adsorption capacity, and kL is related to the binding energy or affinity parameter of the adsorption system.The Freundlich isotherm is expressed as follows:(4)qe=kFCe1/nF,where kF is the constant of the Freundlich isotherm (L1/n mg(1−(1/n))g−1) and 1/nF is the Freundlich exponent.


## 3. Results and Discussion

### 3.1. Infrared Spectra of the FA0, FAN and Modified FAN Particles

IR spectra of the untreated FA (FA0), FAN, and modified FAN particles in Figure1 show that the IR spectra of the FAN, FASDS, and FAMBT particles exhibit similar characteristic peaks. For example, the peaks at 3250–3700 cm−1 and 1640 cm−1 are for stretching and bending vibrations of hydroxyl group in the FA particles. The asymmetric, symmetric stretching, and bending vibrations of Si-O group in the FA particles correspond at 1070 cm−1, 793 cm−1, and 460 cm−1, respectively [1, 22]. The peak at 555 cm−11 can be attributed for Al-O group in the FA particles. Interestingly, some new peaks in the IR spectrum of the FAMBT particles appeared at 2917 cm−1, 2857 cm−1, and 1465 cm−1 corresponding to asymmetric, symmetric stretching, and bending vibrations of CH group in the heterocyclic structure of MBT. In addition, the peak observed at 1550 cm−1 characterizes for tertiary amine group in MBT. These can confirm that MBT was grafted onto the surface of the FAN particles after modification process [15, 22]. This is explained by the physical interactions (hydrogen bonding or dipolar-dipolar interaction) formed between hydroxyl groups on the surface of the FAN particles and the polar groups or element with a valence electron pair (SH, S) in MBT molecules. As a result, the MBT molecules were kept on the surface of the modified FAN particles.Figure 1
FTIR spectra of FA0, FAN, and modified FAN particles.To observe clearly, the position of characteristic peaks in the IR spectra of the FAN and modified FAN particles are listed in Table1. The IR spectrum of the FA0 is not shown here because it was presented in our previous literature [21], but the main peaks are given in Table 1. The slight shift of some absorption peaks (1–5 cm−1) can be caused by the effect of the surfactants (MBT and SDS). However, there is no chemical reaction between the surfactants and the FAN particles in the treatment process.Table 1
Position of main peaks in FTIR spectra of FA0, FAN, and modified FAN particles.
Sample
Characteristic absorptions (cm−1)
δ
Si-O-Si
δ
Al-O-Al
ν
sym Si-O-Si
ν
asym Si-O-Si
δ
OH
ν
SH
ν
OH
FA0
461
557
793
1070
1636
—
3452
FAN
457
555
793
1057
1642
—
3451
FASDS
453
556
795
1036
1637
—
3452
FAMBT
454
553
790
1037
1642
2927, 2857
3455


### 3.2. Morphology of the FA0, FAN, and Modified FAN Particles

FESEM images of the FA0 and FAN particles before and after modification are demonstrated in Figures2 and 3. It can be observed that FA particles have spherical shape with the particle diameter in the range of 100–10 μm; however, their main particle size is in the range of 3–5 μm. This is the general size of the FA after sieving. The untreated FA (FA0) particles have a smooth surface while the treated FA (FAN) and modified FAN (FASDS and FAMBT particles) have rouge surfaces [23]. In addition, the micropores also appeared on the surface of the FAN, FASDS, and FAMBT particles, as shown higher magnification in Figure 3. These results prove that the surface of the FAN, FASDS, and FAMBT particles can be destroyed and cleaned by the effect of 1M NaOH solution and the surfactants. Besides, the occurrence of reaction between FA0 and NaOH (2NaOH + Al2O3 ⟶ 2NaAlO2 + H2O; NaOH + Al2O3 + SiO2 ⟶ Na2O·Al2O3·nSiO2·mH2O) can lead to formation of micropores on the surface of the FAN, FASDS, and FAMBT particles due to the dissolution of aluminum oxide and silica oxide into the solution in the treatment process. In comparison to the FASDS and FAMBT particles, it can be clearly seen that the surface of the FAMBT particles becomes rougher and jagged than those of the FASDS particles. The size and number of micropores (ca. 50 nm) of the FAMBT particles are smaller but higher than those of the FASDS particles. Paulina Halas also indicated that as for zeolite from fly ash, single crystals with irregular conformation and size 3–6 mm are demonstrated and the edges of the particles are often sharp and jagged [5].Figure 2
FESEM images of FA0 (a), FAN (b), FAMBT (c), and FASDS (d) particles at the magnification of 5.000 times.Figure 3
FESEM images of FA0 (a), FAN (b), FAMBT (c), and FASDS (d) particles at the magnification of 100.000 times.


### 3.3. Crystal Structure of the FAN and Modified FAN Particles

The change in crystal structure of the unmodified and modified FAN particles is evaluated according to the XRD patterns presented in Figure4. As reported in our previous research [21] and some other research studies [3–5, 24], the XRD spectrum of the FA demonstrates that the structure of FA0 particles is composed of mullite, quartz, and hematite while the FAN particles has a zeolite P (Na6Al6Si10O32.12H2O or NaP) structure (at 2θ = 13°, 2θ = 16°, 2θ = 27°, and 2θ = 55°). The appearance of NaP phase could be caused by the effect of NaOH solution on the conversion of alumino-silicate materials by the reaction between NaOH solution with SiO2 and Al2O3 in the FA particles as abovementioned in Section 3.2. Grigorios et al. also mentioned that the polarization of chemical bonds in FAN particles led to the improvement of their active centers in the frame of FAN particles. As a result, the terminal groups (≡Si–OH, ≡Si–ONa, ≡Si–O– and (≡Si–O)3Al–O–) of FA particles are increased as treated by NaOH solution [8]. The crystal structure of FASDS and FAMBT particles is similar to that of FAN particles but the intensity of peaks which characterized for NaP zeolite phase in the XRD pattern of the FASDS and FAMBT samples is three times higher than that of the FAN particles. This result proves that the NaP zeolite phase in the crystal structure of FASDS and FAMBT particles is stronger and clearer than that in FAN particles.Figure 4
XRD patterns of FA0, FAN, and modified FAN particles.


### 3.4. Change in Composition of the FA Samples after Modification

Figure5 and Table 2 display EDX spectra and element weight percentage of the FA0, FAN, and modified FAN particles. Three positions were chosen to record the EDX spectrum for every sample, and the data shown in Table 2 are the average values. It can be observed that O, Al, Si, and Fe are the main elements in all investigated FA samples due to the main composition of the FA particles including main oxides such as SiO2, Al2O3, and Fe2O3. The order of other elements is K, Mg, and Ca. The existence of C and N elements in the FA0 particles could be unburned carbon and organic impurities.Figure 5
EDX spectra of FA0 (a), FAN (b), FAMBT (c), and FASDS (d).
(a)
(b)
(c)
(d)Table 2
Weight percent of elements in FA0, FAN, and modified FAN particles.
Element
Sample
FA0
FAN
FASDS
FAMBT
C
28.10
26.49
26.94
17.89
O
44.35
48.40
47.61
49.62
Al
7.56
8.32
7.87
11.80
Si
10.65
11.42
12.07
14.93
Na
—
1.08
0.99
0.92
K
1.71
1.57
1.56
1.96
Fe
2.48
1.84
2.09
1.97
Mg
1.55
0.31
0.33
0.27
Ca
0.25
0.28
0.27
0.31
N
3.01
—
—
—
Ba
0.33
—
—
—
Ti
—
0.28
0.28
0.34
Total
100
100
100
100The presence of Na element in the FAN, FASDS, and FAMBT particles can be attributed for appearance of NaP zeolite phase as discussed in XRD (Section3.3). The decrease in weight percentage of C element as well as the disappearance of Ba and N elements in the FAN, FASDS, and FAMBT particles is due to the removal of unburned carbon, organic impurities, and BaO from the FAN, FASDS, and FAMBT particles. The weight percentage of Fe, Ca, and K elements in the FAN particles changed insignificantly after modification by SDS and MBT. This shows that the oxides and subtances of Fe, Ca, and K in FASDS and FAMBT particles are relatively stable, unaffected by the NaOH solution or surfactants. Interestingly, the increase in weight percentages of Al and Si elements and the presence of Ti element after the modification process are the evidence for structural damage and surface erosion as displayed in FESEM images. The Ti element does not appear in the FA0 sample due to its deep location in the FA0 structure. On the other side, weight percentages of the Al and Si elements are rised by the formation of NaP zeolite phase. This result is similar to the above XRD result.


### 3.5. Change in Specific Surface Area of the FA0, FAN, and Modified FAN Particles

Figure6 shows the performance of nitrogen adsorption and desorption of the FA0, FAN, and modified FAN particles. According to the IUPAC classification of adsorption isotherms, based on the adsorption and desorption pathways of samples, it can be seen that the adsorption and desorption isotherms are type IV with hysteresis. The type A of hysteresis loop on the adsorption-desorption isotherms corresponded to the cylindrical micropore structure of the FAN, FAMBT, and FASDS particles. Table 3 lists the specific surface area (SSA), pore volume, and pore diameter of the FA0, FAN, and modified FAN particles. The SSA of FAN and FASDS particles is increased while the SSA of the FAMBT particles is decreased in comparison with that of the FA0 particles. In the case of SDS grafted onto the FAN particles, it is difficult to form the physical interaction between SDS and the FAN particles because there is no element with a valence electron pair in SDS molecules; hence, SDS was not kept on the surface of FAN particles. The play of SDS as a detergent is stronger than that as a grafting agent. Therefore, the SSA and pore diameter of the FASDS particles are slightly increased in comparison with the FAN particles. When using MBT for the modification of the FAN particles, the organic layer on the surface of the FAN particles can cover the micropores leading to the reduction of SSA and increase in the pore diameter of the FAMBT particles in comparison with that of the FAN particles [11, 25].Figure 6
Nitrogen adsorption-desorption curves of FA0, FAN, and modified FAN particles.Table 3
Specific surface area, pore volume, and pore diameter of FA0, FAN, and modified FAN particles.
Sample
Specific surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
FA0
2.9489
0.0031
11.3126
FAN
3.5178
0.0043
15.2018
FASDS
4.7190
0.0194
16.4493
FAMBT
1.5275
0.0056
20.0612Interestingly, the pore volume of the FASDS and FAMBT particles (0.0194 and 0.0056 cm3 g−1) are also larger than those of the FA0 and FAN particles. This can be related to the some metal ion adsorption capacity of the FASDS and FAMBT particles as discussed below.


### 3.6. Cd2+ and Hg2+ Ion Adsorption Capacity of the FA0, FAN, and Modified FAN Particles

#### 3.6.1. Choosing the Most Suitable Type of FA for Cd2+ Ion Adsorption

The percent removal of Cd2+ ion (H) and the amount of adsorbed Cd2+ ion at equilibrium conditions (Q) obtained from Cd2+ ion adsorption process by the FA0, FAN, and modified FAN particles at the same initial Cd2+ ion concentration and the same FA weight are determined from data obtained by AAS analysis and are given in Table 4.Table 4
Cd2+ adsorption parameters by FA0, FAN, and modified FAN particles.
Sample
a
b
f
Abs
kq mg/l
H (%)
Q (mg/g)
FAMBT
0.2691
0.0043
5
0.082
1.44
97
12.14
FASDS
0.2691
0.0043
5
0.318
5.83
88
11.04
FAN
0.2691
0.0043
25
0.088
7.78
84
10.55
FA0
0.2691
0.0043
25
0.188
17.07
65
8.23Here,a and b are the parameters obtained from the calibration equation of Cd2+ ion, f is the dilution coefficient, and Abs is the optical absorption of solution.From data in Table4, the FAMBT particles exhibit the highest H and Q values (97% and 12.14 mg·g−1, respectively) while the FA0 particles have the lowest H and Q values (65% and 8.23 mg·g−1, respectively). The increase in percent removal of Cd2+ ion using the FAMBT particles can be explained by the rising its pore diameter in comparison with that of the FAN and FASDS particles (as listed in Table 3) as well as the formation of complex between MBT and Cd2+ ion (Figure 7) [25]. Based on these results, the FAMBT particles can be the most suitable FA particles for further study on Hg2+ and Cd2+ adsorption.Figure 7
General formula of M-MBT complex (M = Hg, Cd).


#### 3.6.2. Cd2+ Adsorption Behavior by FAMBT Particles

Figure8 presents the influence of initial Cd2+ concentration on percent removal of Cd2+ ion (H) and the amount of adsorbed Cd2+ ion at equilibrium conditions (Q) of the FAMBT particles. It is clear that the decrease in percent removal of Cd2+ ion occurs as well as the increase in the amount of adsorbed Cd2+ ion of the FAMBT particles. At Cd2+ concentration lower than 30 mg L−1 (or 30 ppm), the percent removal of Cd2+ ion by the FAMBT particles is quite high, from 98.95 to 99.47%. Therefore, the FAMBT particles are appropriate for Cd2+ adsorption at a concentration lower than 30 ppm in treatment of factory or domestic waste water. The high percent removal of Cd2+ ion of FAMBT particles could be explained by formation of the complex between Cd2+ and MBT grafted onto the surface of the FAN particles as shown in Figure 7. On the other side, high percent removal of Cd 2+ ion of FAMBT particles was also caused by the uniform adsorption layer of Cd2+ ion at adsorption centers. In order to study on the Cd2+ adsorption mechanism of FAMBT particles clearly, the Freundlich and Langmuir isotherms are used for investigation of Cd2+ adsorption behavior by FAMBT particles [10, 12].Figure 8
Percent removal of Cd2+ ion H (-■-) and the amount of adsorbed Cd2+ ion at equilibrium conditions Q (-▲-) of the FAMBT particles at different Cd2+ concentrations.Langmuir and Freundlich isotherms of Cd2+ adsorption by the FAMBT particles are expressed in Figure 9. The Langmuir and Freundlich isotherm parameters are also presented in Table 5. Owing to Figure 9 and data in Table 5, the regression coefficients (R2) obtained from the Langmuir isotherm model is greater than that from the Freundlich isotherm model. Therefore, the Langmuir isotherm model is more suitable for reflecting the Cd2+ adsorption behavior of FAMBT particles. The maximum monolayer adsorption capacity (Qmax) of the FAMBT particles is 12.4 mg/g. The Cd2+ adsorption is a single layer adsorption and active centers on the surface of the FAMBT particles have same adsorption energy. Cd2+ ions adsorbed on the surface of the FAMBT particles do not interact with each other. The existence of a maximum absorbance capacity of FAMBT particles can show the formation of a saturation single layer of heavy metal ions on the surface of FAN particles.Figure 9
Langmuir and Freundlich adsorption isotherms reflecting Cd2+ adsorption by the FAMBT particles.Table 5
Langmuir and Freundlich isotherm parameters reflecting Cd2+ adsorption by the FAMBT particles.
Langmuir
Freundlich
Parameter
Value
Parameter
Value
Q
max (mg/g)
12.40
n
F
3.16
k
L
3.50
k
F
103.16
R
2
0.99
R
2
0.58


#### 3.6.3. Hg2+ Adsorption Behavior by FAMBT Particles

The Hg2+ adsorption behavior of the FAMBT particles is similar to the Cd2+ adsorption behavior with the decrease in the percent removal of Hg2+ ion and increase in the amount of adsorbed Hg2+ ion (Figure 10). However, the percent removal of Hg2+ ion of the FAMBT particles is lower than that of the percent removal of Cd2+ ion. It reaches to 90.80% and 84.10% corresponding to Hg2+ concentration of 0.5 and 1 ppm, respectively, and it falls down fast at Hg2+ concentration in the range of 2–15 ppm. This phenomenon can be explained similarly in the report of Xiaotao et al. Cd2+ ions about higher affinity for FA-modified MBT, which may be attributed to the fact that a longer ionic radius (Cd2+(97 pm) < Hg2+(110 pm)) corresponds to a shorter hydrated ionic radius, resulting in the higher affinity for metal ions of FA-modified MBT [26].Figure 10
Influence of percent removal of Hg2+ ion H (-■-) and amount of adsorbed Hg2+ ion at equilibrium conditions Q (-▲-) on initial Hg2+ concentration (adsorbent of FAMBT).Based on the data of Hg2+ adsorption, it can be suggested that the FAMBT particles is suitable for Hg2+ adsorption at a concentration lower than 1 ppm when application of the FAMBT particles in treatment to factory, urban, and domestic waste water.From the data listed in Table6 and (Figure 11), the Freundlich isotherm model is more appropriate than the Langmuir isotherm model for reflecting the Hg2+ adsorption behavior by the FAMBT particles due to higher regression coefficients (R2 = 0.99). We also compared the capacity of proposed adsorbent in the removal of Hg2+ and Cd2+ ions from aqueous with the results published by other authors as shown in Table 7. From the data in Table 7, it can be seen that the selectivity of modified FA and some other absorbents toward metal ions is found to be in the order Cd2+ ions > Hg2+ ions. This selectivity sequence may be caused by the basis of the first hydrolysis constant values for Hg2+ and Cd2+ ions. The hydrolyzed metal ions are more strongly sorbed than free metal ions. Since Cd2+ ions have smaller hydrated ionic radii than Hg2+ ions corresponding to fewer weakly bonded water molecules, Cd2+ ions tend to move faster to the potential adsorption sites on absorbents. On the other hand, the higher electronegativity of Cd2+ ions can also lead to a greater reaction with potential adsorption sites in absorbents as compared to Hg2+ ions [30].Table 6
Langmuir and Freundlich isotherm parameters reflecting Hg2+ ion adsorption by the FAMBT particles.
Langmuir
Freundlich
Parameter
Value
Parameter
Value
Q
max (mg/g)
1.91
n
1.85
k
L
0.72
k
F
101.85
R
2
0.98
R
2
0.99Figure 11
Langmuir and Freundlich adsorption isotherms reflecting Hg2+ ion adsorption by the FAMBT particles.Table 7
Comparison of maximum adsorption capacity (Qmax) of different adsorbents for Hg2+ and Cd2+ ions.
Adsorbent
Maximum adsorption capacity—Qmax (mg·g−1)
Ref.
Hg2+ ions
Cd2+ ions
Activated carbon fromXanthoceras sorbifolia Bunge hull
235.6
388.7
[26]
Clay mineral montmorillonite
0.86
—
[27]
Graphenes magnetic material
23.03
27.83
[28]
Chitosan
52.63
58.8
[29]
FAMBT
1.91
12.4
This work


## 3.1. Infrared Spectra of the FA0, FAN and Modified FAN Particles

IR spectra of the untreated FA (FA0), FAN, and modified FAN particles in Figure1 show that the IR spectra of the FAN, FASDS, and FAMBT particles exhibit similar characteristic peaks. For example, the peaks at 3250–3700 cm−1 and 1640 cm−1 are for stretching and bending vibrations of hydroxyl group in the FA particles. The asymmetric, symmetric stretching, and bending vibrations of Si-O group in the FA particles correspond at 1070 cm−1, 793 cm−1, and 460 cm−1, respectively [1, 22]. The peak at 555 cm−11 can be attributed for Al-O group in the FA particles. Interestingly, some new peaks in the IR spectrum of the FAMBT particles appeared at 2917 cm−1, 2857 cm−1, and 1465 cm−1 corresponding to asymmetric, symmetric stretching, and bending vibrations of CH group in the heterocyclic structure of MBT. In addition, the peak observed at 1550 cm−1 characterizes for tertiary amine group in MBT. These can confirm that MBT was grafted onto the surface of the FAN particles after modification process [15, 22]. This is explained by the physical interactions (hydrogen bonding or dipolar-dipolar interaction) formed between hydroxyl groups on the surface of the FAN particles and the polar groups or element with a valence electron pair (SH, S) in MBT molecules. As a result, the MBT molecules were kept on the surface of the modified FAN particles.Figure 1
FTIR spectra of FA0, FAN, and modified FAN particles.To observe clearly, the position of characteristic peaks in the IR spectra of the FAN and modified FAN particles are listed in Table1. The IR spectrum of the FA0 is not shown here because it was presented in our previous literature [21], but the main peaks are given in Table 1. The slight shift of some absorption peaks (1–5 cm−1) can be caused by the effect of the surfactants (MBT and SDS). However, there is no chemical reaction between the surfactants and the FAN particles in the treatment process.Table 1
Position of main peaks in FTIR spectra of FA0, FAN, and modified FAN particles.
Sample
Characteristic absorptions (cm−1)
δ
Si-O-Si
δ
Al-O-Al
ν
sym Si-O-Si
ν
asym Si-O-Si
δ
OH
ν
SH
ν
OH
FA0
461
557
793
1070
1636
—
3452
FAN
457
555
793
1057
1642
—
3451
FASDS
453
556
795
1036
1637
—
3452
FAMBT
454
553
790
1037
1642
2927, 2857
3455


## 3.2. Morphology of the FA0, FAN, and Modified FAN Particles

FESEM images of the FA0 and FAN particles before and after modification are demonstrated in Figures2 and 3. It can be observed that FA particles have spherical shape with the particle diameter in the range of 100–10 μm; however, their main particle size is in the range of 3–5 μm. This is the general size of the FA after sieving. The untreated FA (FA0) particles have a smooth surface while the treated FA (FAN) and modified FAN (FASDS and FAMBT particles) have rouge surfaces [23]. In addition, the micropores also appeared on the surface of the FAN, FASDS, and FAMBT particles, as shown higher magnification in Figure 3. These results prove that the surface of the FAN, FASDS, and FAMBT particles can be destroyed and cleaned by the effect of 1M NaOH solution and the surfactants. Besides, the occurrence of reaction between FA0 and NaOH (2NaOH + Al2O3 ⟶ 2NaAlO2 + H2O; NaOH + Al2O3 + SiO2 ⟶ Na2O·Al2O3·nSiO2·mH2O) can lead to formation of micropores on the surface of the FAN, FASDS, and FAMBT particles due to the dissolution of aluminum oxide and silica oxide into the solution in the treatment process. In comparison to the FASDS and FAMBT particles, it can be clearly seen that the surface of the FAMBT particles becomes rougher and jagged than those of the FASDS particles. The size and number of micropores (ca. 50 nm) of the FAMBT particles are smaller but higher than those of the FASDS particles. Paulina Halas also indicated that as for zeolite from fly ash, single crystals with irregular conformation and size 3–6 mm are demonstrated and the edges of the particles are often sharp and jagged [5].Figure 2
FESEM images of FA0 (a), FAN (b), FAMBT (c), and FASDS (d) particles at the magnification of 5.000 times.Figure 3
FESEM images of FA0 (a), FAN (b), FAMBT (c), and FASDS (d) particles at the magnification of 100.000 times.


## 3.3. Crystal Structure of the FAN and Modified FAN Particles

The change in crystal structure of the unmodified and modified FAN particles is evaluated according to the XRD patterns presented in Figure4. As reported in our previous research [21] and some other research studies [3–5, 24], the XRD spectrum of the FA demonstrates that the structure of FA0 particles is composed of mullite, quartz, and hematite while the FAN particles has a zeolite P (Na6Al6Si10O32.12H2O or NaP) structure (at 2θ = 13°, 2θ = 16°, 2θ = 27°, and 2θ = 55°). The appearance of NaP phase could be caused by the effect of NaOH solution on the conversion of alumino-silicate materials by the reaction between NaOH solution with SiO2 and Al2O3 in the FA particles as abovementioned in Section 3.2. Grigorios et al. also mentioned that the polarization of chemical bonds in FAN particles led to the improvement of their active centers in the frame of FAN particles. As a result, the terminal groups (≡Si–OH, ≡Si–ONa, ≡Si–O– and (≡Si–O)3Al–O–) of FA particles are increased as treated by NaOH solution [8]. The crystal structure of FASDS and FAMBT particles is similar to that of FAN particles but the intensity of peaks which characterized for NaP zeolite phase in the XRD pattern of the FASDS and FAMBT samples is three times higher than that of the FAN particles. This result proves that the NaP zeolite phase in the crystal structure of FASDS and FAMBT particles is stronger and clearer than that in FAN particles.Figure 4
XRD patterns of FA0, FAN, and modified FAN particles.


## 3.4. Change in Composition of the FA Samples after Modification

Figure5 and Table 2 display EDX spectra and element weight percentage of the FA0, FAN, and modified FAN particles. Three positions were chosen to record the EDX spectrum for every sample, and the data shown in Table 2 are the average values. It can be observed that O, Al, Si, and Fe are the main elements in all investigated FA samples due to the main composition of the FA particles including main oxides such as SiO2, Al2O3, and Fe2O3. The order of other elements is K, Mg, and Ca. The existence of C and N elements in the FA0 particles could be unburned carbon and organic impurities.Figure 5
EDX spectra of FA0 (a), FAN (b), FAMBT (c), and FASDS (d).
(a)
(b)
(c)
(d)Table 2
Weight percent of elements in FA0, FAN, and modified FAN particles.
Element
Sample
FA0
FAN
FASDS
FAMBT
C
28.10
26.49
26.94
17.89
O
44.35
48.40
47.61
49.62
Al
7.56
8.32
7.87
11.80
Si
10.65
11.42
12.07
14.93
Na
—
1.08
0.99
0.92
K
1.71
1.57
1.56
1.96
Fe
2.48
1.84
2.09
1.97
Mg
1.55
0.31
0.33
0.27
Ca
0.25
0.28
0.27
0.31
N
3.01
—
—
—
Ba
0.33
—
—
—
Ti
—
0.28
0.28
0.34
Total
100
100
100
100The presence of Na element in the FAN, FASDS, and FAMBT particles can be attributed for appearance of NaP zeolite phase as discussed in XRD (Section3.3). The decrease in weight percentage of C element as well as the disappearance of Ba and N elements in the FAN, FASDS, and FAMBT particles is due to the removal of unburned carbon, organic impurities, and BaO from the FAN, FASDS, and FAMBT particles. The weight percentage of Fe, Ca, and K elements in the FAN particles changed insignificantly after modification by SDS and MBT. This shows that the oxides and subtances of Fe, Ca, and K in FASDS and FAMBT particles are relatively stable, unaffected by the NaOH solution or surfactants. Interestingly, the increase in weight percentages of Al and Si elements and the presence of Ti element after the modification process are the evidence for structural damage and surface erosion as displayed in FESEM images. The Ti element does not appear in the FA0 sample due to its deep location in the FA0 structure. On the other side, weight percentages of the Al and Si elements are rised by the formation of NaP zeolite phase. This result is similar to the above XRD result.


## 3.5. Change in Specific Surface Area of the FA0, FAN, and Modified FAN Particles

Figure6 shows the performance of nitrogen adsorption and desorption of the FA0, FAN, and modified FAN particles. According to the IUPAC classification of adsorption isotherms, based on the adsorption and desorption pathways of samples, it can be seen that the adsorption and desorption isotherms are type IV with hysteresis. The type A of hysteresis loop on the adsorption-desorption isotherms corresponded to the cylindrical micropore structure of the FAN, FAMBT, and FASDS particles. Table 3 lists the specific surface area (SSA), pore volume, and pore diameter of the FA0, FAN, and modified FAN particles. The SSA of FAN and FASDS particles is increased while the SSA of the FAMBT particles is decreased in comparison with that of the FA0 particles. In the case of SDS grafted onto the FAN particles, it is difficult to form the physical interaction between SDS and the FAN particles because there is no element with a valence electron pair in SDS molecules; hence, SDS was not kept on the surface of FAN particles. The play of SDS as a detergent is stronger than that as a grafting agent. Therefore, the SSA and pore diameter of the FASDS particles are slightly increased in comparison with the FAN particles. When using MBT for the modification of the FAN particles, the organic layer on the surface of the FAN particles can cover the micropores leading to the reduction of SSA and increase in the pore diameter of the FAMBT particles in comparison with that of the FAN particles [11, 25].Figure 6
Nitrogen adsorption-desorption curves of FA0, FAN, and modified FAN particles.Table 3
Specific surface area, pore volume, and pore diameter of FA0, FAN, and modified FAN particles.
Sample
Specific surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
FA0
2.9489
0.0031
11.3126
FAN
3.5178
0.0043
15.2018
FASDS
4.7190
0.0194
16.4493
FAMBT
1.5275
0.0056
20.0612Interestingly, the pore volume of the FASDS and FAMBT particles (0.0194 and 0.0056 cm3 g−1) are also larger than those of the FA0 and FAN particles. This can be related to the some metal ion adsorption capacity of the FASDS and FAMBT particles as discussed below.


## 3.6. Cd2+ and Hg2+ Ion Adsorption Capacity of the FA0, FAN, and Modified FAN Particles

### 3.6.1. Choosing the Most Suitable Type of FA for Cd2+ Ion Adsorption

The percent removal of Cd2+ ion (H) and the amount of adsorbed Cd2+ ion at equilibrium conditions (Q) obtained from Cd2+ ion adsorption process by the FA0, FAN, and modified FAN particles at the same initial Cd2+ ion concentration and the same FA weight are determined from data obtained by AAS analysis and are given in Table 4.Table 4
Cd2+ adsorption parameters by FA0, FAN, and modified FAN particles.
Sample
a
b
f
Abs
kq mg/l
H (%)
Q (mg/g)
FAMBT
0.2691
0.0043
5
0.082
1.44
97
12.14
FASDS
0.2691
0.0043
5
0.318
5.83
88
11.04
FAN
0.2691
0.0043
25
0.088
7.78
84
10.55
FA0
0.2691
0.0043
25
0.188
17.07
65
8.23Here,a and b are the parameters obtained from the calibration equation of Cd2+ ion, f is the dilution coefficient, and Abs is the optical absorption of solution.From data in Table4, the FAMBT particles exhibit the highest H and Q values (97% and 12.14 mg·g−1, respectively) while the FA0 particles have the lowest H and Q values (65% and 8.23 mg·g−1, respectively). The increase in percent removal of Cd2+ ion using the FAMBT particles can be explained by the rising its pore diameter in comparison with that of the FAN and FASDS particles (as listed in Table 3) as well as the formation of complex between MBT and Cd2+ ion (Figure 7) [25]. Based on these results, the FAMBT particles can be the most suitable FA particles for further study on Hg2+ and Cd2+ adsorption.Figure 7
General formula of M-MBT complex (M = Hg, Cd).


### 3.6.2. Cd2+ Adsorption Behavior by FAMBT Particles

Figure8 presents the influence of initial Cd2+ concentration on percent removal of Cd2+ ion (H) and the amount of adsorbed Cd2+ ion at equilibrium conditions (Q) of the FAMBT particles. It is clear that the decrease in percent removal of Cd2+ ion occurs as well as the increase in the amount of adsorbed Cd2+ ion of the FAMBT particles. At Cd2+ concentration lower than 30 mg L−1 (or 30 ppm), the percent removal of Cd2+ ion by the FAMBT particles is quite high, from 98.95 to 99.47%. Therefore, the FAMBT particles are appropriate for Cd2+ adsorption at a concentration lower than 30 ppm in treatment of factory or domestic waste water. The high percent removal of Cd2+ ion of FAMBT particles could be explained by formation of the complex between Cd2+ and MBT grafted onto the surface of the FAN particles as shown in Figure 7. On the other side, high percent removal of Cd 2+ ion of FAMBT particles was also caused by the uniform adsorption layer of Cd2+ ion at adsorption centers. In order to study on the Cd2+ adsorption mechanism of FAMBT particles clearly, the Freundlich and Langmuir isotherms are used for investigation of Cd2+ adsorption behavior by FAMBT particles [10, 12].Figure 8
Percent removal of Cd2+ ion H (-■-) and the amount of adsorbed Cd2+ ion at equilibrium conditions Q (-▲-) of the FAMBT particles at different Cd2+ concentrations.Langmuir and Freundlich isotherms of Cd2+ adsorption by the FAMBT particles are expressed in Figure 9. The Langmuir and Freundlich isotherm parameters are also presented in Table 5. Owing to Figure 9 and data in Table 5, the regression coefficients (R2) obtained from the Langmuir isotherm model is greater than that from the Freundlich isotherm model. Therefore, the Langmuir isotherm model is more suitable for reflecting the Cd2+ adsorption behavior of FAMBT particles. The maximum monolayer adsorption capacity (Qmax) of the FAMBT particles is 12.4 mg/g. The Cd2+ adsorption is a single layer adsorption and active centers on the surface of the FAMBT particles have same adsorption energy. Cd2+ ions adsorbed on the surface of the FAMBT particles do not interact with each other. The existence of a maximum absorbance capacity of FAMBT particles can show the formation of a saturation single layer of heavy metal ions on the surface of FAN particles.Figure 9
Langmuir and Freundlich adsorption isotherms reflecting Cd2+ adsorption by the FAMBT particles.Table 5
Langmuir and Freundlich isotherm parameters reflecting Cd2+ adsorption by the FAMBT particles.
Langmuir
Freundlich
Parameter
Value
Parameter
Value
Q
max (mg/g)
12.40
n
F
3.16
k
L
3.50
k
F
103.16
R
2
0.99
R
2
0.58


### 3.6.3. Hg2+ Adsorption Behavior by FAMBT Particles

The Hg2+ adsorption behavior of the FAMBT particles is similar to the Cd2+ adsorption behavior with the decrease in the percent removal of Hg2+ ion and increase in the amount of adsorbed Hg2+ ion (Figure 10). However, the percent removal of Hg2+ ion of the FAMBT particles is lower than that of the percent removal of Cd2+ ion. It reaches to 90.80% and 84.10% corresponding to Hg2+ concentration of 0.5 and 1 ppm, respectively, and it falls down fast at Hg2+ concentration in the range of 2–15 ppm. This phenomenon can be explained similarly in the report of Xiaotao et al. Cd2+ ions about higher affinity for FA-modified MBT, which may be attributed to the fact that a longer ionic radius (Cd2+(97 pm) < Hg2+(110 pm)) corresponds to a shorter hydrated ionic radius, resulting in the higher affinity for metal ions of FA-modified MBT [26].Figure 10
Influence of percent removal of Hg2+ ion H (-■-) and amount of adsorbed Hg2+ ion at equilibrium conditions Q (-▲-) on initial Hg2+ concentration (adsorbent of FAMBT).Based on the data of Hg2+ adsorption, it can be suggested that the FAMBT particles is suitable for Hg2+ adsorption at a concentration lower than 1 ppm when application of the FAMBT particles in treatment to factory, urban, and domestic waste water.From the data listed in Table6 and (Figure 11), the Freundlich isotherm model is more appropriate than the Langmuir isotherm model for reflecting the Hg2+ adsorption behavior by the FAMBT particles due to higher regression coefficients (R2 = 0.99). We also compared the capacity of proposed adsorbent in the removal of Hg2+ and Cd2+ ions from aqueous with the results published by other authors as shown in Table 7. From the data in Table 7, it can be seen that the selectivity of modified FA and some other absorbents toward metal ions is found to be in the order Cd2+ ions > Hg2+ ions. This selectivity sequence may be caused by the basis of the first hydrolysis constant values for Hg2+ and Cd2+ ions. The hydrolyzed metal ions are more strongly sorbed than free metal ions. Since Cd2+ ions have smaller hydrated ionic radii than Hg2+ ions corresponding to fewer weakly bonded water molecules, Cd2+ ions tend to move faster to the potential adsorption sites on absorbents. On the other hand, the higher electronegativity of Cd2+ ions can also lead to a greater reaction with potential adsorption sites in absorbents as compared to Hg2+ ions [30].Table 6
Langmuir and Freundlich isotherm parameters reflecting Hg2+ ion adsorption by the FAMBT particles.
Langmuir
Freundlich
Parameter
Value
Parameter
Value
Q
max (mg/g)
1.91
n
1.85
k
L
0.72
k
F
101.85
R
2
0.98
R
2
0.99Figure 11
Langmuir and Freundlich adsorption isotherms reflecting Hg2+ ion adsorption by the FAMBT particles.Table 7
Comparison of maximum adsorption capacity (Qmax) of different adsorbents for Hg2+ and Cd2+ ions.
Adsorbent
Maximum adsorption capacity—Qmax (mg·g−1)
Ref.
Hg2+ ions
Cd2+ ions
Activated carbon fromXanthoceras sorbifolia Bunge hull
235.6
388.7
[26]
Clay mineral montmorillonite
0.86
—
[27]
Graphenes magnetic material
23.03
27.83
[28]
Chitosan
52.63
58.8
[29]
FAMBT
1.91
12.4
This work


## 3.6.1. Choosing the Most Suitable Type of FA for Cd2+ Ion Adsorption

The percent removal of Cd2+ ion (H) and the amount of adsorbed Cd2+ ion at equilibrium conditions (Q) obtained from Cd2+ ion adsorption process by the FA0, FAN, and modified FAN particles at the same initial Cd2+ ion concentration and the same FA weight are determined from data obtained by AAS analysis and are given in Table 4.Table 4
Cd2+ adsorption parameters by FA0, FAN, and modified FAN particles.
Sample
a
b
f
Abs
kq mg/l
H (%)
Q (mg/g)
FAMBT
0.2691
0.0043
5
0.082
1.44
97
12.14
FASDS
0.2691
0.0043
5
0.318
5.83
88
11.04
FAN
0.2691
0.0043
25
0.088
7.78
84
10.55
FA0
0.2691
0.0043
25
0.188
17.07
65
8.23Here,a and b are the parameters obtained from the calibration equation of Cd2+ ion, f is the dilution coefficient, and Abs is the optical absorption of solution.From data in Table4, the FAMBT particles exhibit the highest H and Q values (97% and 12.14 mg·g−1, respectively) while the FA0 particles have the lowest H and Q values (65% and 8.23 mg·g−1, respectively). The increase in percent removal of Cd2+ ion using the FAMBT particles can be explained by the rising its pore diameter in comparison with that of the FAN and FASDS particles (as listed in Table 3) as well as the formation of complex between MBT and Cd2+ ion (Figure 7) [25]. Based on these results, the FAMBT particles can be the most suitable FA particles for further study on Hg2+ and Cd2+ adsorption.Figure 7
General formula of M-MBT complex (M = Hg, Cd).


## 3.6.2. Cd2+ Adsorption Behavior by FAMBT Particles

Figure8 presents the influence of initial Cd2+ concentration on percent removal of Cd2+ ion (H) and the amount of adsorbed Cd2+ ion at equilibrium conditions (Q) of the FAMBT particles. It is clear that the decrease in percent removal of Cd2+ ion occurs as well as the increase in the amount of adsorbed Cd2+ ion of the FAMBT particles. At Cd2+ concentration lower than 30 mg L−1 (or 30 ppm), the percent removal of Cd2+ ion by the FAMBT particles is quite high, from 98.95 to 99.47%. Therefore, the FAMBT particles are appropriate for Cd2+ adsorption at a concentration lower than 30 ppm in treatment of factory or domestic waste water. The high percent removal of Cd2+ ion of FAMBT particles could be explained by formation of the complex between Cd2+ and MBT grafted onto the surface of the FAN particles as shown in Figure 7. On the other side, high percent removal of Cd 2+ ion of FAMBT particles was also caused by the uniform adsorption layer of Cd2+ ion at adsorption centers. In order to study on the Cd2+ adsorption mechanism of FAMBT particles clearly, the Freundlich and Langmuir isotherms are used for investigation of Cd2+ adsorption behavior by FAMBT particles [10, 12].Figure 8
Percent removal of Cd2+ ion H (-■-) and the amount of adsorbed Cd2+ ion at equilibrium conditions Q (-▲-) of the FAMBT particles at different Cd2+ concentrations.Langmuir and Freundlich isotherms of Cd2+ adsorption by the FAMBT particles are expressed in Figure 9. The Langmuir and Freundlich isotherm parameters are also presented in Table 5. Owing to Figure 9 and data in Table 5, the regression coefficients (R2) obtained from the Langmuir isotherm model is greater than that from the Freundlich isotherm model. Therefore, the Langmuir isotherm model is more suitable for reflecting the Cd2+ adsorption behavior of FAMBT particles. The maximum monolayer adsorption capacity (Qmax) of the FAMBT particles is 12.4 mg/g. The Cd2+ adsorption is a single layer adsorption and active centers on the surface of the FAMBT particles have same adsorption energy. Cd2+ ions adsorbed on the surface of the FAMBT particles do not interact with each other. The existence of a maximum absorbance capacity of FAMBT particles can show the formation of a saturation single layer of heavy metal ions on the surface of FAN particles.Figure 9
Langmuir and Freundlich adsorption isotherms reflecting Cd2+ adsorption by the FAMBT particles.Table 5
Langmuir and Freundlich isotherm parameters reflecting Cd2+ adsorption by the FAMBT particles.
Langmuir
Freundlich
Parameter
Value
Parameter
Value
Q
max (mg/g)
12.40
n
F
3.16
k
L
3.50
k
F
103.16
R
2
0.99
R
2
0.58


## 3.6.3. Hg2+ Adsorption Behavior by FAMBT Particles

The Hg2+ adsorption behavior of the FAMBT particles is similar to the Cd2+ adsorption behavior with the decrease in the percent removal of Hg2+ ion and increase in the amount of adsorbed Hg2+ ion (Figure 10). However, the percent removal of Hg2+ ion of the FAMBT particles is lower than that of the percent removal of Cd2+ ion. It reaches to 90.80% and 84.10% corresponding to Hg2+ concentration of 0.5 and 1 ppm, respectively, and it falls down fast at Hg2+ concentration in the range of 2–15 ppm. This phenomenon can be explained similarly in the report of Xiaotao et al. Cd2+ ions about higher affinity for FA-modified MBT, which may be attributed to the fact that a longer ionic radius (Cd2+(97 pm) < Hg2+(110 pm)) corresponds to a shorter hydrated ionic radius, resulting in the higher affinity for metal ions of FA-modified MBT [26].Figure 10
Influence of percent removal of Hg2+ ion H (-■-) and amount of adsorbed Hg2+ ion at equilibrium conditions Q (-▲-) on initial Hg2+ concentration (adsorbent of FAMBT).Based on the data of Hg2+ adsorption, it can be suggested that the FAMBT particles is suitable for Hg2+ adsorption at a concentration lower than 1 ppm when application of the FAMBT particles in treatment to factory, urban, and domestic waste water.From the data listed in Table6 and (Figure 11), the Freundlich isotherm model is more appropriate than the Langmuir isotherm model for reflecting the Hg2+ adsorption behavior by the FAMBT particles due to higher regression coefficients (R2 = 0.99). We also compared the capacity of proposed adsorbent in the removal of Hg2+ and Cd2+ ions from aqueous with the results published by other authors as shown in Table 7. From the data in Table 7, it can be seen that the selectivity of modified FA and some other absorbents toward metal ions is found to be in the order Cd2+ ions > Hg2+ ions. This selectivity sequence may be caused by the basis of the first hydrolysis constant values for Hg2+ and Cd2+ ions. The hydrolyzed metal ions are more strongly sorbed than free metal ions. Since Cd2+ ions have smaller hydrated ionic radii than Hg2+ ions corresponding to fewer weakly bonded water molecules, Cd2+ ions tend to move faster to the potential adsorption sites on absorbents. On the other hand, the higher electronegativity of Cd2+ ions can also lead to a greater reaction with potential adsorption sites in absorbents as compared to Hg2+ ions [30].Table 6
Langmuir and Freundlich isotherm parameters reflecting Hg2+ ion adsorption by the FAMBT particles.
Langmuir
Freundlich
Parameter
Value
Parameter
Value
Q
max (mg/g)
1.91
n
1.85
k
L
0.72
k
F
101.85
R
2
0.98
R
2
0.99Figure 11
Langmuir and Freundlich adsorption isotherms reflecting Hg2+ ion adsorption by the FAMBT particles.Table 7
Comparison of maximum adsorption capacity (Qmax) of different adsorbents for Hg2+ and Cd2+ ions.
Adsorbent
Maximum adsorption capacity—Qmax (mg·g−1)
Ref.
Hg2+ ions
Cd2+ ions
Activated carbon fromXanthoceras sorbifolia Bunge hull
235.6
388.7
[26]
Clay mineral montmorillonite
0.86
—
[27]
Graphenes magnetic material
23.03
27.83
[28]
Chitosan
52.63
58.8
[29]
FAMBT
1.91
12.4
This work


## 4. Conclusions

In conclusion, the structure, morphology, and some other characteristics of FAN particles have been changed significantly after modification by MBT and SDS surfactants. The surface of the FASDS and FAMBT samples is rougher than that of the FA0 and FAN particles. The specific surface area of the samples is arranged in the order FASDS > FAN > FAMBT > FA0. The NaP phase in the structure of the FAMBT and FASDS particles appears clearer and stronger than that of the FAN particles. The FAMBT sample is chosen for study on Cd2+ and Hg2+adsorption because of its highest percent removal of ions among four tested samples. The Cd2+ and Hg2+adsorption behavior of the FAMBT sample is similar with a decrease in the percent removal of ions as increasing the heavy metal ion concentration. The Langmuir isotherm model is complied with Cd2+ adsorption (R2 = 0.99) while the Freundlich is suitable for study on Hg2+adsorption (R2 = 0.99). The modified FA is a promising absorbent for removal of Hg2+ and Cd2+ ions in waste water.



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