Utilization of alkaline silicate wastes for removal of cadmium ions from aqueous solution: Comparative performances and removal mechanisms

Journal of Environmental Chemical Engineering 7 (2019) 103402

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Utilization of alkaline silicate wastes for removal of cadmium ions from aqueous solution: Comparative performances and removal mechanisms

T

Ge Zhang, Pingfeng Fu , Huifen Yang, Peng Fu, Zhaofeng Li ⁎

School of Civil and Resources Engineering, University of Science & Technology Beijing, Beijing 100083, China

ARTICLE INFO

ABSTRACT

Keywords: Silicate waste Cd2+removal Removal capacity Straw ash Chemical precipitation Surface complexation

In this work, alkaline silicate wastes including blast furnace slag (BFS), coal fly ash (CFA), straw ash (SA) and lithium slag (LS) were used to remove Cd2+ from aqueous solution. The removal performances of Cd2+ by silicate wastes were compared in batch experiments, and the removal mechanisms by SA were proposed. All of four silicate wastes showed high acid neutralization capacity with solution pH increasing from 2.0 at initial to 6.27–9.70 at equilibrium. The removal of Cd2+ by four wastes followed the pseudo‒second‒order kinetic model and the equilibrium data were well described by the Langmuir isotherm. The maximum removal capacity of Cd2+ for four wastes had the sequence of SA > BFS > CFA > LS. The SA with the maximum removal capacity of 88.75 mg/g was the best adsorbent for Cd2+ removal. The XRD tests showed that Cd2+ could be chemically precipitated as Cd(OH)2 on the SA surface. The FTIR and XPS analysis exhibited that oxygen functional groups of SA could undergo surface complexation with Cd2+. Hydration products such as calcium silicate hydrate derived from SA could immobilize Cd2+ ions through ion exchange. In summary, the removal of Cd2+ by straw ash had a complicate involvement of chemical precipitation, ion exchange and surface complexation with oxygen functional groups.

1. Introduction The pollution of heavy metals has become a main environmental concern due to their non‒degradable properties. Among various heavy metals, cadmium is one of the most toxic elements with potential risks of acute intoxication, kidney damage, and Itai‒Itai‒disease for human beings [1,2]. Both the International Agency for Research on Cancer and the United States Environmental Protection Agency have classified cadmium as an important carcinogen [3]. Cadmium‒contained wastewaters are mainly discharged from mining, smelting and electroplating activities, etc. [4]. Accordingly, it is necessary and even stringent for the treatment of cadmium‒contained wastewaters. Several technologies have been applied for the removal of cadmium from acid wastewaters such as adsorption, chemical precipitation [5], electrochemical treatment [6], ion exchange [7,8] and membrane process [9]. However, all these techniques have disadvantages and limitations from the perspectives of practical applications. Chemical precipitation produces metallic precipitates which are difficult to further disposal [10]. The regeneration of ion exchange resin causes secondary pollution with high operational cost. Electrochemical and membrane technologies involve high energy consumption [11,12]. Adsorption is considered as a promising technique for the removal of ⁎

heavy metallic ions with multiple virtues of high effectiveness, flexibility and low operational cost [13]. In the past decades, various adsorbents had been developed for the removal of heavy metal from wastewaters [14]. The silicate materials are proved to be promising adsorbents due to abundant reserves, low cost, and high fixation capacity of heavy metals [15]. Numerous silicate minerals, such as kaolinite [16], palygorskite [17], glauconite [18] and zeolite [19], have shown high removal capacity of heavy metals. Some silicate wastes, for instance, steel slag [20,21], fly ash [22], straw ash [23,24] and furnace slag [25], also exhibit excellent removal performances of heavy metals. The utilization of silicate wastes can avoid the exploitation of natural resources, decrease the cost of wastewater treatment and reduce the need of waste disposal. Heavy metallic ions can be immobilized as hydroxides with an alkali‒activation process or encapsulated within structural network of silicates [26–28]. Silicate wastes, containing alkaline oxides such as CaO and MgO, can perform hydration reactions with H2O to release OH−, showing strong acid neutralization and chemical precipitation capacity [29]. Furthermore, silicate wastes have considerable ion‒exchange with metallic ions. Ma et al [30] found that the removal of heavy metallic ions by calcium silicate derived from coal fly ash is achieved mainly via ion‒exchange with the combination of adsorption.

Corresponding author. E-mail address: [email protected] (P. Fu).

https://doi.org/10.1016/j.jece.2019.103402 Received 6 July 2019; Received in revised form 27 August 2019; Accepted 1 September 2019 Available online 06 September 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.

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Elements such as K, Na, Ca and Mg in silicate wastes, can also exchange with metallic ion and reduce their availability in solutions. However, the reported removal capacities of heavy metals by silicate wastes are usually hard to be compared because of different test conditions adopted. Additionally, the removal mechanisms of heavy metals by alkaline silicate wastes are not fully investigated. In this study, blast furnace slag (BFS), coal fly ash (CFA), straw ash (SA) and lithium slag (LS) were chosen as typical alkaline silicate wastes to compare their capacity in removing Cd2+ ions from artificially acid solutions in batch experiments. The kinetic and adsorption isotherm studies were performed to evaluate the characteristics of Cd2+ removal by four wastes. Because the SA had the highest capacity of Cd2+ removal among four wastes, the removal mechanisms by SA were investigated by the X‒ray diffraction (XRD), scanning electron microscope equipped with energy dispersive spectroscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR) and X‒ray photoelectron spectroscopy (XPS) tests.

Freundlich isotherm: qe =

D

2

secound

Intra

order equation: qt = k2 qe2 t /(1 + k2 qe t )

particle diffusion equation: qt = k int t (1/2) +C

(7)

=

n i=1

[(qexp

qcal )2 qcal]

(8)

2.3. Characterization methods The X‒ray fluorescence spectroscopy (XRF, PW2424, Philips) was used to examine main chemical compositions of four silicate wastes. The XRD patterns of samples were recorded using Rigaku‒TTR3 with Cu‒Kα radiation (40 kV, 100 mA) at a scanning rate of 3°/min. The structure and morphology of SA before and after the Cd2+ removal was observed using a scanning electron microscope (SEM, SUPRA 55, Zeiss) equipped with energy dispersive spectroscopy (EDS). Fourier transform infrared (FTIR, FTIR‒8400 s, Shimadzu) analysis was performed and the spectra were recorded in the range of 400–4000 cm−1. The surface properties of SA were investigated by X‒ray photoelectron spectroscopy (XPS, AXIS‒ULTRADLD, Kratos) with an Al anode (Al‒Ka ¼ 1486.7 eV) as the X‒ray source. The spectra were calibrated with the C 1s band at 284.8 eV. The XPS Peak 4.1 software was used to deconvolute the XPS spectra with the Lorentzian‒Gaussian fixed at 25%.

In the batch experiments, a known amount of silicate waste was added to 100 mL Cd2+ solution in a 250 mL conical flask. The flasks were shaken at 25 °C for a given time in an oscillator at 180 rpm. After adsorption, the supernatant was separated by the centrifugation at 5000 rpm for 10 min, and the Cd2+ concentration of the supernatant was determined by ICP‒AES (Varian715‒ES). Meanwhile, the equilibrium solution pH was measured using a HQ30d portable pH meter after adsorption. To investigate the mechanisms of Cd2+ removal by silicate wastes, the adsorption kinetics and adsorption isotherms were examined in this study. The kinetics of Cd2+ adsorption were evaluated using the pseudo‒first‒order (Eq. (1)), pseud‒second‒order (Eq. (2)) and inter‒particle diffusion kinetic models (Eq. (3)), respectively.

exp( k1 t )]

(6)

}

where n was the number of observations in the experimental data, qexp (mg/g) was the experimental removal capacity, and qcal (mg/g) was the removal capacity obtained by calculating from models. The smaller value of χ2 represented the better fitting of the curve.

2.2. Batch experiments of removing Cd2+ ions

Pseudo

R equation: qe = qm exp{ KD [RT ln (1 + 1 Ce

where Ce (mg/L) was the equilibrium concentration of Cd , qe (mg/g) was the removal capacity at equilibrium, qm (mg/g) was the maximum removal capacity, b (L/mg) was the Langmuir adsorption constant related to the free energy of adsorption, Kf ((mg/g)·(mg/L)n) and n were the adsorption constants of Freundlich related to adsorption capacity and intensity, KD (mol2/kJ2) was the D–R model constant related to the free energy of adsorption, R was the gas constant of 8.314 J/(mol·K) and T (K) was the absolute temperature of environmental conditions, bT (J/mol) was the Temkin model constant related to adsorption heat and AT (L/mg) was the equilibrium constant related to binding energy. In this study, all of model parameters were evaluated by nonlinear regression. To optimize the procedure, the Chi-square (χ2) was used to evaluate the fitting quality of the experimental data.

Blast furnace slag (BFS) was obtained from Jintaicheng Building Materials Co., Ltd., Hebei Province, China. Coal fly ash (CFA) was supplied by the Datang power plant, Inner Mongolia, China. Straw ash (SA) was obtained from Huaneng thermal power plant, Jilin Province, China. Lithium slag (LS) was from Shandong lithium industry, Co., Ltd., China. The powder samples were dried at 105 °C for 2 h before used. All chemicals used were in analytical grade. The Cd2+ stock solution with a concentration of 1000 mg/L was prepared by dissolving an accurate quantity of sulfate salt (3CdSO4·8H2O) in deionized water. Tested Cd2+ solutions with different concentrations were prepared by diluting the stock solution. The initial solution pH was adjusted using 0.1 M H2SO4 or 0.1 M NaOH.

order equation: qt = qe [1

(5)

)]2

2+

2.1. Materials and reagents

first

Kf Ce1/ n

Temkin equation: qe = (RT bT )ln(AT Ce)

2. Materials and methods

Pseudo

(4)

Langmuir isotherm:qe = qm bCe/(1+bCe)

3. Results and discussion 3.1. Characterization of silicate wastes The main chemical compositions of four silicate wastes were presented in Table 1. It can be seen that silicate wastes mainly consisted of SiO2, Al2O3, Fe2O3, CaO and MgO, but their contents varied for different samples. Furthermore, alkaline oxides such as K2O and Na2O had the high content in the SA and CFA. As showed in Fig. 1, the crystalline phases of BFS were identified as gehlenite (2CaO·Al2O3·SiO2), merwinite (3CaO·MgO·2SiO2) and calcium silicate (2CaO·SiO2). The CFA was mainly composed of quartz (SiO2),

(1) (2) (3)

2+

where qe and qt (mg/g) were the capacity of Cd removal by silicate wastes at equilibrium and contact time t (min), respectively; k1 (min−1), k2 (g/(mg·min)) and kint (mg/(g·min1/2)) were the removal rate constants of pseudo‒first‒order, pseudo‒second‒order and intra‒particle diffusion models, respectively; C was the intercept proportional to the extent of boundary layer thickness. Four adsorption isotherms, i.e., Langmuir, Freundlich, DubininRaduskevich (D–R) and Temkin models, were selected to investigate Cd2+ removal performances. The corresponding isotherm equations were listed as follows:

Table 1 Main chemical composition of four silicate wastes (wt.%).

2

Samples

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

P2O5

Na2O

K2O

Blast furnace slag Coal fly ash Straw ash Lithium slag

26.15

13.49

2.96

41.41

10.09

0.83

—

—

—

31.8 51.42 65.0

27.0 7.76 21.8

5.49 2.96 0.6

2.95 7.41 3.5

0.78 3.52 0.06

— 1.18 5.15

0.15 2.75 —

0.88 2.96 —

1.30 1.58 0.59

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surface sites of SA and BFS. As shown in Fig. 3(a), the Cd2+ removal capacity of CFA and LS was much lower than that of SA and BFS. As illustrated in Fig. 3(b), the solution pH of all four systems increased rapidly from 2.0 at initial to values close to their equilibrium solution pH within 1 min. For example, the solution pH had increased from 2.0 to 9.28 and 8.26 within 1 min for SA and BFS, respectively. The results indicated that the removal of Cd2+ by four silicate wastes was a very rapid process with the increase of solution pH. The kinetics of Cd2+ removal were fitted by the pseudo‒first‒order, pseud‒second‒order and inter‒particle diffusion models, respectively. The kinetic parameters were listed in Table 2. As can be seen, the pseudo‒second‒order model was the best among three models to describe kinetic experimental data because of the highest correlation coefficients (R2 > 0.990) and lowest χ2 values. In addition, the qe value calculated from the pseudo‒second‒order model was well consistent with its experimental qe for each silicate waste as shown in Fig. 3 and Table 2. The results indicated that the rate‒controlling step in Cd2+ removal was chemisorptive bonds of Cd2+ species and chemisorption involving valence forces through sharing or exchanging electrons between adsorbent and adsorbate [16,17].

Fig. 1. XRD patterns of four silicate wastes: (a) blast furnace slag; (b) coal fly ash; (c) straw ash; (d) lithium slag.

mullite (3Al2O3·2SiO2) and hedenbergite (CaO·FeO·2SiO2). The SA was dominated by quartz, calcium silicate, gehlenite and monticellite (MgO·CaO·SiO2). Quartz and spodumene (Li2O·Al2O3·4SiO2) were two main crystalline phases of LS. Amorphous mineral phases were presented in BFS, CFA and SA, especially in the BFS, which were identified by broad diffraction peaks shown in Fig. 1.

3.4. Adsorption isotherms The adsorption isotherms of Cd2+ removal by four silicate wastes were illustrated in Fig. 4. Table 3 summarized the parameters calculated with four models. For each waste, the equilibrium data fitted by the Langmuir model showed the higher R2 (> 0.990) and lower χ2 than other three models, indicating that the removal of Cd2+ by silicate wastes was a monolayer adsorption process. As shown in Table 3, the qm, exp and qm, cal values of each waste were very close, further indicating that the removal of Cd2+ ions by silicate wastes well obeyed the Langmuir model. For these four silicate wastes, the removal capacity of Cd2+ had a sequence of SA > BFS > CFA > LS, with a qm, cal of 88.75, 67.56, 15.25 and 6.81 mg/g, respectively. The SA was proved to be the best adsorbent for Cd2+ removal among four silicate wastes. Table 4 compared the maximum removal capacity obtained from the Langmuir isotherm in the Cd2+ removal by various adsorbents. It exhibited that the SA and BFS in this work had much higher removal capacity of Cd2+ than some silicate minerals such as kaolinite and bentonite. The SA even achieved the higher Cd2+ removal capacity than biochar and some synthetic porous silicates given in Table 4. Besides, it had various merits of the utilization of solid waste, low cost and avoidance of adsorbent synthesis, etc. The results indicated that the SA was an eco‒friendly, highly effective and low cost material for the removal of Cd2+ from acid wastewaters. Thus, the removal mechanisms of Cd2+ ions by SA were further investigated in this work.

3.2. Effects of silicate waste dosage The effects of wastes dosage on the Cd2+ removal were shown in Fig. 2. With the increase of waste dosage, the removal of Cd2+ increased at first and then leveled off, and the equilibrium solution pH significantly increased at the same time. At the waste dosage of 20 g/L, the Cd2+ removal reached 99.19% and 98.74% for SA and BFS with equilibrium solution pH of 9.70 and 8.99, respectively. However, for CFA and LS, the Cd2+ removal was just 93.65% and 79.15% even at a large dosage of 400 g/L, with the equilibrium solution pH of 7.76 and 6.27, respectively. It suggested that the SA and BFS had higher capacity of the Cd2+ removal compared to CFA and LS. The remarkable increase of solution pH after the adsorption revealed that silicate wastes investigated could release OH− ions with high acid neutralization capacity. 3.3. The kinetics of Cd2+ removal The variations of the Cd2+ removal capacity (qt) and solution pH with contact time were shown in Fig. 3. At contact time of only 1 min, the qt was as high as 1.97 and 1.90 mg/g for SA and BFS, very close to the calculated qe of 1.98 and 1.97 mg/g for SA and BFS, respectively. The results exhibited that Cd2+ ions could be rapidly immobilized on

Fig. 2. Effect of the dosage of silicate wastes on the removal of Cd2+ and equilibrium solution pH. Conditions: initial solution pH of 2.0, initial Cd2+ concentration of 40 mg/L, contact time of 60 min. 3

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Fig. 3. Variations of the Cd2+ removal capacity (qt) (a) and solution pH (b) with contact time (t) by silicate wastes. Conditions: initial solution pH of 2.0, initial Cd2+ concentration of 40 mg/L, wastes dosage of 20 g/L for SA and BFS, and of 400 g/L for CFA and LS, respectively. Table 2 Kinetic parameters for the removal of Cd2+ by four silicate wastes. Model

Pseudo‒first ‒order model Pseudo‒second ‒order model Inter‒particle diffusion model

Parameter

−1

K1 (min ) qe (mg/g) R2 χ2 K2 (g/(mg·min)) qe (mg/g) R2 χ2 Kint (mg/(g·min1/2)) C R2 χ2

Silicate wastes Blast furnace slag

Coal fly ash

Straw ash

Lithium slag

3.390 1.967 0.833 8.64 × 10−5 12.836 1.974 0.995 2.65 × 10−6 0.008 1.928 0.501 2.57 × 10−4

1.773 0.092 0.921 2.30 × 10−6 48.529 0.093 0.997 8.16 × 10−8 0.001 0.084 0.378 1.81 × 10−5

5.446 1.981 0.940 6.56 × 10−7 106.983 1.982 0.995 5.08 × 10−8 0.001 1.975 0.502 5.47 × 10−6

0.668 0.072 0.937 1.01 × 10−5 11.451 0.075 0.994 9.68 × 10−7 0.003 0.048 0.554 7.12 × 10−5

3.5. Mechanisms of Cd2+ removal by straw ash

Fig. 5(e) and (f), some particles with a size of less than 2 μm were presented in the pores and on surface of SA after the Cd2+ removal. These substances might be related to precipitated Cd species as detected by EDS analysis. By comparing the element distribution given in Fig. 5(b) and (d), it can be seen that the contents of Si and Al were reduced significantly, and the elements of Ca, K, Mg and Na disappeared after the Cd2+ removal. It can reasonably infer that these elements were dissolved into acid solution via dissolution and/or ion exchange.

3.5.1. SEM/EDS analysis SEM images and EDS analysis of SA before and after the Cd2+ removal were presented in Fig. 5. The abundant pores and rough surface of SA could provide a large number of active sites for Cd2+ ions (Fig. 5(a)). As showed in Fig. 5(b) and (d), the content of Cd species on SA surface reached 18.81 wt.% by comparing EDS analysis of SA before and after the Cd2+ removal. The EDS surficial scanning test in Fig. 5(c) showed that element Cd was evenly distributed on SA surface. It well demonstrated that Cd species must be immobilized onto SA surface. In

Fig. 4. Adsorption isotherms of Cd2+ removal by four silicate wastes. Conditions: initial solution pH of 2.0, contact time of 120 min, waste dosage of 20 g/L for SA and BFS, and of 400 g/L for CFA and LS, respectively. 4

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Table 3 Parameters of Langmuir, Freundlich, D–R and Temkin models in the removal of Cd2+ ions by four silicate wastes. Isotherm

Langmuir

Freundlich

Temkin

D‒R

Parameter

Silicate wastes

qm, exp (mg/g) qm, cal (mg/g) b (L/mg) R2 χ2 1/n Kf (mg/g)·(mg/L)n R2 χ2 AT (L/mg) bT (kJ/mol) R2 χ2 qm (mg/g) KD (mol2/kJ2) R2 χ2

Blast furnace slag

Coal fly ash

Straw ash

Lithium slag

53.11 67.56 0.005 0.992 2.632 0.562 1.492 0.950 17.749 0.079 198 0.877 43.682 55.308 4.09 × 10−3 0.859 50.312

12.62 15.25 0.008 0.996 0.077 0.439 0.794 0.959 0.733 0.086 780 0.988 0.208 10.82 2.83 × 10−4 0.824 3.124

76.16 88.75 0.035 0.994 3.717 0.382 12.003 0.827 125.556 0.688 153 0.862 100.399 69.91 3.06 × 10−5 0.870 94.258

5.59 6.81 0.010 0.988 0.030 0.396 0.517 0.914 0.210 0.094 1631 0.976 0.059 4.89 2.61 × 10−4 0.839 0.394

to characterize the SA before and after the Cd2+ removal. The XPS wide scan and C 1s, Cd 3d and S 2p spectra of SA were shown in Fig. 8. As shown in Fig. 8(a), the significant decrease of peak intensity of Ca, Mg and Si elements was observed after the Cd2+ removal, indicating these elements were dissolved into acid solution. In contrast, the peaks assigned to Cd species appeared on the SA surface after the Cd2+ removal, well consistent with the presence of Cd species detected by XRD and EDS analysis. In Fig. 8(b), the C 1s peak of SA before the Cd2+ removal was deconvoluted to CeC (284.8 eV), CeOH (286.23 eV) and COOH (289.26 eV). After the Cd2+ removal, the peaks of CeOH and COOH shifted to 286.32 and 289.02 eV, respectively, with a decrease of peak area. It suggested that surface complexation reactions may undergo between Cd species and functional groups (CeOH and COOH) during the removal process. The peaks at 412.41 and 405.65 eV corresponded to Cd 3d3/2 and Cd 3d5/2 in Fig. 8(c), respectively. The binding energies had a shift of about 0.7 eV compared to their general location (Cd 3d3/2 at 411.7 eV and Cd 3d5/2 at 405.0 eV), indicating that a strong bond linkage occurred between Cd species and SA surface [39]. Fig. 8(d) showed that the peak area of S 2p increased remarkably after the Cd2+ removal. This spectrum can be divided into two peaks located at 168.80 and 169.82 eV, which were assigned to metallic sulphates. It was well in accordance with the formation of CaSO4 as detected by XRD.

3.5.2. XRD and FTIR analysis The XRD patterns of SA before and after Cd2+ removal were depicted in Fig. 6. As can be seen, the diffraction intensity of peaks assigned to 2CaO·SiO2, 2CaO·Al2O3·SiO2 and 3CaO·MgO·2SiO2 decreased remarkably. In acid cadmium solution with initial solution pH 2.0, the dissolution of these oxides would occur with the release of Ca2+, Mg2+ and Al3+ ions, which was well consistent with the decrease of element contents by EDS analysis. The emerging peaks at 2θ of 18.90°, 29.45° and 35.36° were in accordance with crystal planes of (001), (100) and (011) of Cd(OH)2 (PDF Card: 73‒0969), respectively. It well demonstrated that Cd2+ ions were removed by generating hydroxide precipitation on the SA surface. As illustrated in Fig. 7, the CeO (1425.30 cm−1) and eOH (3732.00 and 891.05 cm−1) vibrations were observed in received SA. After the Cd2+ removal, the bands assigned to SieOeSi stretching vibration at 1097.42 and 784.97 cm−1 were shifted to 1099.35 and 783.05 cm−1, respectively, with simultaneous change of the transmittance. Hence, it suggested that the SieOeSi might react with Cd2+ species during the removal process. Besides, the new sharp peak at 3618.21 cm−1 could be assigned to the stretching vibration of AleOHeAl [17]. The emerging bands at 3473.56 and 1623.95 cm−1 could be ascribed to HeOeH of adsorbed H2O [37]. The band at 601.75 cm−1 was attributed to sulfate anion vibration modes, confirming the presence of CaSO4 in SA after the Cd2+ removal [38].

3.5.4. Removal mechanisms of Cd2+ The Ca‒silicate, CaO, Al2O3 and MgO presented in SA could undergo hydrolysis and ionization reactions (Eq. (9)). The release of OH−

3.5.3. XPS analysis To reveal the valence state of immobilized Cd species, XPS was used

Table 4 Comparison of removal capacities calculated with Langmuir isotherm model in the removal of Cd2+ ions by different adsorbents. Adsorbent

Straw ash Blast furnace slag Coal fly ash Lithium slag Kaolinite Bentonite Modified sewage sludge wheat straw biochar Ti‒modified ultrasonic biochar Mesoporous silica Microporous layered silicate

Experimental conditions Initial solution pH

Temperature (°C)

2 2 2 2 7 6 5 5 ± 0.03 5 7 5‒6

25 25 25 25 30 22 25.0 ± 1.0 25 20 25 25

5

qm, cal (mg/g)

Refs.

88.75 67.56 15.25 6.81 0.88 9.30 14.7 38.4‒69.8 72.62 85 93.37

This work This work This work This work [16] [31] [32] [33] [34] [35] [36]

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Fig. 5. SEM micrographs and EDS analysis of straw ash before ((a): SEM image; (b): elemental contents in Figure (a)) and after ((c): map distribution of Cd element; (d): elemental contents in Figure (c); (e): SEM image at 10KX magnification; (f): SEM image at 5KX magnification) the removal of Cd2+ ions.

ions resulted in the increase of the solution pH as shown in Fig. 2. This result was well proved by the decreased diffraction intensity of peaks assigned to 2CaO·SiO2, 2CaO·Al2O3·SiO2 and MgO·CaO·SiO2 in XRD test and the significant reduction of element (Si, Al, Ca and Mg) contents in EDS and XPS analysis. MxOy + yH2O ↔ xM(2y/x) +2yOH− (M = Ca, Mg, Al, etc.)

surface alkalinity was a significant factor favoring sorption/precipitation of heavy metal ions on the surface of adsorbents [40]. Higher equilibrium solution pH may mean a growing number of negatively charged sites on SA surface. Thus, the electrostatic attraction between Cd2+ and negatively charged SA surface was enhanced [20]. Furthermore, Cd2+ is the dominated specie in the solution for solution pH < 6, and the main species are Cd(OH)2 at solution pH > 8, and Cd2+ and CdOH+ at 6 < solution pH < 8. Therefore, the chemical precipitation plays a dominant role in the removal of Cd2+ when the solution pH is in alkaline range [29]. As shown in Fig. 2, the equilibrium solution pH was as high as 9.7 after the Cd2+ removal at the SA dosage of 20 g/L.

(9)

2+

The results in Fig. 2 exhibited that the removal of Cd was closely related to the equilibrium solution pH. The higher equilibrium solution pH usually resulted in higher removal efficiency of Cd2+. Similar results were reported for the systems of other inorganic wastes, in which 6

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of SA. Hence, Cd2+ ions could be readily precipitated in form of Cd (OH)2 (Eq. (10)). The generation of Cd(OH)2 on SA surface was also detected by XRD analysis. Cd2+ + 2OH−→Cd(OH)2

(10) −

The hydration of 2CaO·SiO2 in SA can not only release OH but also generate calcium silicate hydrate (CeSeH) (Eq. (11)). The CeSeH can effectively encapsulate and adsorb heavy metals due to extremely high specific surface energy and ion exchange capacity [41,42]. 2CaO·SiO2 + 2H2O→Ca(OH)2 + CaO·SiO2·H2O 2+

(11) 2+

The electronegativity of Cd ions is similar to that of Ca , and the difference between their ionic radii is less than 15%. Therefore, Ca atoms in CeSeH structure can be replaced by Cd2+ via ion‒exchange with the formation of crystalline or gel phases containing Cd silicates [43]. It has been reported that Cd was immobilized by amorphous states of geopolymer in the main form of Cd2+ linked as SieOe‒Cd and AleOeCd [44]. In this study, the crystalline cadmium silicate is not detected by XRD, so it can be inferred that Cd2+ ions were immobilized into amorphous states of CeSeH [45]. This was well confirmed by the change of SieOeSi stretching vibration in FTIR spectrum. As shown in Fig. 2, the removal of Cd2+ ions could reach 92.84% with the equilibrium solution pH of 2.13 when the SA dosage was 1 g/L. The chemical precipitation of Cd2+ species can be neglected at low solution pH of 2.13. Thus, the adsorption of Cd2+ on SA surface may play a key role due to the abundant pores and rough surface of SA. In addition, the type and concentration of surface functional groups have been reported to be an important role in adsorption process [46]. The oxygen functional groups (eOH, CeOH, and eCOOH) of SA might have strong interactions with Cd via surface complexation (Eqs. (12)‒(14)) [47]. When Cd2+ ions were adsorbed onto SA, the CeO and eOH vibrations disappeared according to the FTIR analysis, and the peaks of CeOH and COOH had shifted by XPS analysis, demonstrating that surface complexation occurred between Cd species and SA surface [48].

Fig. 6. XRD patterns of straw ash before (a) and after (b) the Cd2+ removal.

Fig. 7. FTIR spectra of straw ash before (a) and after (b) the Cd2+ removal.

Furthermore, Fig. 3 showed that the Cd2+ removal capacity even at 1 min was very close to the equilibrium data (qe), and the solution pH increased from 2.0 to 9.68 due to the high acid neutralization capacity

Sur−OH + Cd2+ +H2O → Sur−OCd+ +H3O+

Fig. 8. XPS wide scan (a) and C 1s (b), Cd 3d (c) and S 2p (d) high resolution spectra of straw ash before (1) and after (2) the Cd2+ removal. 7

(12)

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C–OH + Cd2+ +H2O → C–OCd+ +H3O+ −COOH + Cd

2+

+H2O → −COOCd

+

+H3O

(13) +

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4. Conclusions In this study, four silicate wastes showed high acid neutralization capacity in the removal of Cd2+ from acid solutions, and the adsorption process followed the pseudo‒second‒order kinetic model and Langmuir isotherm. The maximum removal capacity of Cd2+ for four wastes had the sequence of SA > BFS > CFA > LS. The SA with the maximum removal capacity of 88.75 mg/g was the best adsorbent for Cd2+ removal among four silicate wastes. The Ca‒silicate and alkaline oxides presented in SA could undergo the hydrolysis and ionization reactions, thus increasing solution pH to form Cd(OH)2 precipitate. The hydration products such as CeSeH gels could immobilize Cd2+ ions into CeSeH structure through ion exchange. FTIR and XPS analysis showed oxygen functional groups (eOH, CeOH, and eCOOH) of SA could undergo surface complexation reactions with Cd species. Therefore, the mechanisms of Cd2+ removal by SA were mainly a combination of chemical precipitation, ion exchange and surface complexation with oxygen functional groups. The eco‒friendly SA with the virtues of low cost and high removal capability was a promising material for the removal of Cd2+ from acid wastewaters. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (grant number: 51674017). Authors would like to thank the analytical and testing center of University of Science and Technology Beijing, which supplied us the facilities to fulfill the measurement. References [1] J. Godt, F. Scheidig, C. Grosse‒Siestrup, V. Esche, P. Brandenburg, A. Reich, D.A. Groneberg, The toxicity of cadmium and resulting hazards for human health, J. Occup. Med. Toxicol. 1 (2006) 22. [2] A.H.S. Rebelo, J.M.F. Ferreira, Comparison of the cadmium removal efficiency by two calcium phosphate powders, J. Environ. Chem. Eng. 5 (2017) 1475–1483. [3] N. Ataei, M. Aghaei, M. Panjehpour, Evidences for involvement of estrogen receptor induced ERK1/2 activation in ovarian cancer cell proliferation by cadmium chloride, Toxicol. In Vitro 56 (2019) 184–193. [4] M. Dirbaz, A. Roosta, Adsorption, kinetic and thermodynamic studies for the biosorption of cadmium onto microalgae Parachlorella sp, J. Environ. Chem. Eng. 6 (2018) 2302–2309. [5] Q.Y. Chen, Y. Yao, X.Y. Li, J. Lu, J. Zhou, Z.L. Huang, Comparison of heavy metal removals from aqueous solutions by chemical precipitation and characteristics of precipitates, J. Water Process Eng. 26 (2018) 289–300. [6] T.K. Tran, K.F. Chiu, C.Y. Lin, H.J. Leu, Electrochemical treatment of wastewater: selectivity of the heavy metals removal process, Int. J. Hydrogen Energy 42 (2017) 27741–27748. [7] H. Khanmohammadi, B. Bayati, J. Rahbar‒Shahrouzi, A.A. Babaluo, A. Ghorbani, Molecular simulation of the ion exchange behavior of Cu2+, Cd2+ and Pb2+ ions on different zeolites exchanged with sodium, J. Environ. Chem. Eng. 7 (2019) 103040. [8] C.W. Wong, J.P. Barford, G.H. Chen, G. McKay, Kinetics and equilibrium studies for the removal of cadmium ions by ion exchange resin, J. Environ. Chem. Eng. 2 (2014) 698–707. [9] A. Giwa, A. Dindi, J. Kujawa, Membrane bioreactors and electrochemical processes for treatment of wastewaters containing heavy metal ions, organics, micropollutants and dyes: recent developments, J. Hazard. Mater. 370 (2019) 172–195. [10] F.L. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manage. 92 (2011) 407–418. [11] C.F. Carolin, P.S. Kumar, A. Saravanan, G.J. Joshiba, M. Naushad, Efficient techniques for the removal of toxic heavy metals from aquatic environment: a review, J. Environ. Chem. Eng. 5 (2017) 2782–2799. [12] F. Noli, E. Kapashi, M. Kapnisti, Biosorption of uranium and cadmium using

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