Removal of Cd2+ from aqueous solution using hydrothermally modified circulating fluidized bed fly ash resulting from coal gangue power plant

Accepted Manuscript Removal of Cd2+ from aqueous solution using hydrothermally modified circulating fluidized bed fly ash resulting from coal gangue power plant

Ruifang Qiu, Fangqin Cheng, Haiming Huang PII:

S0959-6526(17)32909-8

DOI:

10.1016/j.jclepro.2017.11.236

Reference:

JCLP 11375

To appear in:

Journal of Cleaner Production

Received Date:

27 April 2017

Revised Date:

20 October 2017

Accepted Date:

29 November 2017

Please cite this article as: Ruifang Qiu, Fangqin Cheng, Haiming Huang, Removal of Cd2+ from aqueous solution using hydrothermally modified circulating fluidized bed fly ash resulting from coal gangue power plant, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.11.236

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ACCEPTED MANUSCRIPT Highlights 

Circulating fluidized bed fly ash was modified for the removal of Cd2+.



Various parameters had a remarkable effect on the removal of Cd2+ by adsorption.



The maximum Cd2+-adsorption capacity of the adsorbent was 183.7 mg/g.



Sorption and precipitation functionalized on Cd2+ removal by the adsorbent.

ACCEPTED MANUSCRIPT Removal of Cd2+ from aqueous solution using hydrothermally modified circulating fluidized bed fly ash resulting from coal gangue power plant Ruifang Qiu a, b, Fangqin Cheng a *, Haiming Huang c * a

Institute of Resources & Environment Engineering, Shanxi University, State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi Innovation Hub of Resource Recycling and ecological engineering, Taiyuan 030006, China

b

College of Environmental& Resource Sciences, Shanxi University, Taiyuan 030006, China c

Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China

Abstract Circulating fluidized bed (CFB) fly ash is an industrial waste produced from the burning of coal gangue by the CFB technology. We investigated the removal of Cd2+ from wastewater by a hydrothermally modified CFB fly ash (HM-CFB-FA) adsorbent material. The SEM and BET analyses of HM-CFB-FA indicated that the adsorbent was rich in porous structure and had a huge specific surface area. Also, the XRF, XRD, and FTIR characterization of the adsorbent material demonstrated that HM-CFB-FA was rich in zeolite content, which indicates that the adsorbent has excellent adsorption capability. The results of the conditional experiments showed that the adsorbent dosage, solution pH, initial Cd2+ concentration, temperature, and the contact time had a remarkable effect on the adsorption performance of HM-CFB-FA. Under the optimal conditions, the maximum adsorption capacity of Cd2+ by HM-CFB-FA reached 183.7 mg/g. The adsorption isotherm analysis revealed that the adsorption of Cd2+ by HM-CFB-FA was well fitted to the Langmuir model. The kinetic investigation showed that Cd2+adsorption by HM-CFB-FA was controlled by surface reaction and external diffusion processes. The mechanism analysis demonstrated that the adsorption of HM-CFB-FA and the formation of cadmium hydroxide may be responsible for the removal of Cd2+ from aqueous solution. Therefore, it was concluded that HM-CFB-FA is a promising low-cost adsorbent for the removal of Cd2+ from wastewater. Keywords: adsorption, cadmium, circulating fluidized bed fly ash, coal gangue.

*Corresponding Author: Phone: +86 335 8387 741; Fax: +86 335 8061 569; E-mail: [email protected] 1

ACCEPTED MANUSCRIPT 1. Introduction Cadmium (Cd) is one of the most toxic metals, even in low concentrations, that is widely used in the production of pigments, pesticides, fertilizers, and metal plating and in processes, such as mining, smelting, refining, and alloy industries (Rao et al.,2010). Cd2+ poisoning can cause a variety of diseases in the human body such as lung cancer, pulmonary edema, anemia, skin, bone diseases, brain damage, and trachea-bronchitis (Voegelin et al., 2003; Mohan et al., 2005; Deng et al., 2005). Moreover, consumption of any considerable amount of Cd2+ can result in instantaneous intoxication, which can damage the liver and the kidney (Mohan et al., 2005; Deng et al., 2005). Therefore, removal of Cd2+ from wastewater is of critical importance to avoid the harm of Cd2+ to the human body. Presently, various processes, such as chemical precipitation, electrochemical methods, and ion exchange have been applied to the treatment of Cd2+-containing wastewater. Among these processes, adsorption of heavy metals on a solid sorbent is a promising method for the removal of Cd2+, especially for developing countries (Unuabonah et al., 2008; Akar et al., 2009; Šljivić Ivanović et al., 2013). This method has several advantages for the removal of heavy metal ions such as the applicability at low concentrations, suitability for use in batch and continuous processes, ease of operation, minimized production of sludge, the possibility of regeneration and reuse, and lower operating cost (Unuabonah et al., 2008; Akar et al., 2009) . Activated carbon and zeolite are the most commonly used adsorbent materials. Besides these, some new adsorbents, such as the process of mesoporous materials from silicoaluminate mineral, artificial zeolite, and graphene oxide (Anbia et al., 2015; Javadian et al., 2015; Rouholah et al., 2016) can generally achieve high removal efficiency of Cd2+. Unfortunately, all these synthesis materials pose the main drawback of high production cost, which limits the practical application of these adsorption materials. Meanwhile, this conflicted with the subject of upsurge of interest in today’s society to remove heavy metals using low-cost adsorbents derived from industrial waste items, such as steel slag, coal gangue, fly ash, and red mud (Duan et al., 2014; Zhou et al., 2015; Qiu et al., 2016; Agnieszka et al.,2016). Therefore, the development of cost-effective adsorbents using industrial waste would be greatly significant for the removal of Cd2+. Coal gangue is a type of solid waste generated from the production process of coal; and its amount is extremely huge in the world. China is the largest coal gangue producer in the world, the 2

ACCEPTED MANUSCRIPT accumulated amount of coal gangue in which has already reached 3.8 billion tons (Fu et al., 2012); meanwhile, the annual production is estimated to around 300–350 million tons and it will reach over 700 million tons in 2020 (Yu et al., 2016). In addition, it was reported that over 730 million tons of wastes from quarrying and mining activities were generated in the European Union in 2012 (European Commission website database, 2016), and there are 1.09 billion tons coal gauge produced in America in 2011 (Yao et al., 2012). In recent years, since the circulating fluidized bed (CFB) combustion technology was widely disseminated, which is a commercialization clean combustion process with several advantages, such as high combustion efficiency, low combustion temperature, and reduced emissions of NOX and SO2 (Sheng et al., 2007; Stewart et al., 2010; Xia et al., 2013), coal gangue is gradually used as the fuel of CFB boiler to generate electricity in China (Selcuk et al., 2006; Liu et al., 2010; Font et al., 2012). In the process of CFB combustion, a large amount of the byproduct fly ash is generated. Presently, the quantity of generated CFB fly ash (CFB-FA) reached approximately 50 million tons every year in China (Yue et al., 2006). Unfortunately, for a long period of time, the vast majority of CFB-FA was directly discharged as a waste, which added to the economic and environmental burden (Rao et al., 2008; Xia et al., 2013). In fact, CFB-FA generated from coal gangue power plant has significant physical and chemical characteristics, such as irregular shape, high specific surface area, and large contents of dehydrated silicate minerals (quartz and muscovite) and iron-containing minerals (hematite, pyrrhotite) (Lee et al., 2002; Visa et al., 2014) as compared to the fly ash generated from the conventional pulverized coal combustion. Modification of coal fly ash is gaining notice as one of the effective uses for pollutants adsorption. More recently all kinds of modified fly ash, such as acid/FA, alkali/FA, salt/FA, surfactant/FA, acid-salt/FA have been evaluated by many researchers for adsorption of variety of dyes and metals, etc (Gao et al., 2015). Ogata et al. (2015) used hydrothermally modified fly ash with alkaline solution to produce zeolites for tungsten treatment, and found that the modified FA has a good adsorption for tungsten. Javadian et al. (2015) investigated the adsorption of Cd2+ on amorphous aluminosilicate geopolymer adsorbent converted from fly ash with solid NaOH, and found that the Cd2+ adsorption capacity of the adsorbent reached 26.25 mg/g. However, there are few investigations on the removal of heavy metals by the treated CFB-FA, which has the characteristics of low cost and high efficiency. Actually, CFB-FA may be much more easily turned into a potential zeolite-like adsorption 3

ACCEPTED MANUSCRIPT material for the removal of heavy metal ions from wastewater. To achieve the objectives of the reuse of CFB-FA and high efficiency in the removal of Cd2+, in this study, CFB-FA was modified hydrothermally to an adsorbent for the removal of Cd2+ using hexadecyl trimethyl ammonium bromide (HTAB) as the modifier. The adsorption capacity of the adsorbent for the removal of Cd2+ from simulated solution was evaluated under different conditions of adsorbent dosage, solution pH, initial Cd2+ concentration, and contact time. The adsorption kinetic and equilibrium were also investigated. Besides, the mechanism responsible for the removal of Cd2+ was examined in greater detail. 2. Materials and methods 2.1. Experimental materials The CFB-FA used in this study was obtained from the Pingshuo Coal Gangue Power Plant of Shanxi province, China. It was made ultrafine by superheating through the steam jet mill. Its main chemical composition was SiO2 (39.59%), Al2O3 (35.73%), Fe2O3 (4.41%), CaO (9.80%), MgO (2.23%), K2O (0.50%), Na2O (0.15%), and SO3 (4.71%) by X-ray fluorescence. The sum of the major oxides SiO2 and Al2O3 is over 75 wt%. High contents of SiO2 and Al2O3 were advantageous to the restructuring of the materials (Lee et al., 2002). The major particle size of the CFB-FA was approximately 3.0–5.0 μm. The preparation procedures of the adsorbent are described as follows: First, the soluble compounds K2O, Na2O, MgO, and CaO were removed by washing 20 g of the raw CFB-FA thrice with 200 mL of ultra-pure water under the conditions of mechanical stirring at 120 rpm, temperature 25 ± 1 °C for 1 h. Next, the suspension was filtered through a 0.45-μm membrane, and the obtained solid was dried at 105 °C for 24 h. Then, 10 g of the pretreated CFB-FA, 100 mL of NaOH solution (2 M), and 1 g of HTAB (99% purity) were fed to a 1000-mL three-necked, round-bottomed flask, with a reflux condenser. Next, the mixture was continuously stirred at 240 rpm and 100 °C for 24 h. Finally, when the reaction was completed, the colloidal suspension was filtrated in vacuum and washed thrice with ultra-pure water, and the obtained sample (i.e., hydrothermally modified CFB-fly ash or HM-CFB-FA) was dried at 105– 115 °C for 48 h. The wastewater used in this study was the synthetic Cd2+-containing solution. It was prepared by diluting the stock solution (1000 mg/L) into ultra-pure water. The Cd2+ stock solution was prepared by dissolving Cd(NO3)2·4H2O in ultra-pure water. In the experiments, all chemicals 4

ACCEPTED MANUSCRIPT used were of analytical reagent grade, and only ultra-pure water was used. 2.2. Experimental methods Batch experiments were performed to investigate the effects of the adsorbent dosage, the initial concentration of Cd2+, temperature, initial pH, and the contact time on the removal of Cd2+ by HM-CFB-FA. The experimental procedures are described as follows: 100-mL of synthetic Cd2+ solution (10–1000 mg/L) was first added to a 250-ml conical flask with airtight lid, followed by the addition of an appropriate amount of HM-CFB-FA (0.5–10 g/L), and then the flask was placed in a shaker incubator (at 140 rpm) for the desired time period (3h). In the experiments, the pH of the solution was adjusted with 1 M HCl solution or 1 M NaOH solution. To investigate the adsorption kinetics of Cd2+ on HM-CFB-FA, a series of experiments were performed under the following conditions: HM-CFB-FA dosage of 2.5 and 7.5 g/L; 100 mL of initial Cd2+ solution, 50–500 mg/L; initial pHs of 1 and 6; temperature of 25–45 ℃. During the experiments, 2 mL of the slurry samples were removed at time intervals of 360 min and filtered through a 0.45-μm filter membrane for the determination of the Cd2+ concentration of the supernatant. All the experiments were performed in duplicate, and measurements were repeated three times. The adsorption capacity (Q, mg/g) and the removal efficiency (R, %) of Cd2+ were determined according to Eqs. (1) and (2), respectively:

Q

(C 0  C t )V m

(1)

R

(C 0  C t )  100% C0

(2)

where, C0 and Ct represent the initial Cd2+ concentration (mg/L) and the Cd2+ concentration (mg/L) at time (t, min), respectively; V is the volume (L) of the Cd2+ solution; m is the adsorbent mass (g). The experimental data were evaluated according to the pseudo-second-order, Boyd and Kannan models. The thermodynamic isotherm investigation was performed as follows: 1 g/L of HM-CFBFA and 100 mL of Cd2+ solution (500 mg/L) were mixed at the temperature range of 25–45 ℃. After reaching the equilibrium, the maximum adsorption capacity (Qm) of HM-CFB-FA for Cd2+ was predicted through the adsorption isotherms. The Langmuir and Freundlich equations were applied to fit the experimental data. 5

ACCEPTED MANUSCRIPT 2.3. Analysis methods The CFB-FA samples before and after modification were characterized using the Bruker S4-Explorer XRF Spectrometer (PW4400; PANalytical B.V., Netherlands) and a Fouriertransformed infrared spectrometer (FTIR; Perkin Elmer Frontier, USA). The morphology of the samples was examined with a scanning electron microscope (SEM, JSM7001F; JEOL, Japan). The mineral composition of the samples was determined by X-ray diffraction (XRD, D2-Phaser; Bruker, Germany). The specific surface area and porosimetric examination were conducted with the Accelerated Surface Area and Porosimetry System (ASAP 2020; Micrometrics, USA). The specific surface area and the average pore diameter were determined with the Braunauere Emmette Teller (BET) method of multilayered adsorption. The pore volume was determined with Barret, Joyner, and Halenda (BJH) method. The concentration of Cd2+ was determined using an atomic absorption spectrophotometer (AAS, AA-6700; Shimadzu, Japan). 3. Results and discussion 3.1. Characterization of adsorbents The SEM morphologies of CFB-FA and HM-CFB-FA are shown in Figs. 1(a) and (b), respectively. As observed in Fig. 1(a), CFB-FA had various cross-section and size, and their shapes were irregular. Such a morphology is typical of CFB-boilers ash that contains grains of highly irregular shape, mainly elongated with sharp edges as well as isometric, plate-like, and sharp-edged ones

(Szponder et al., 2011); however, it also contains big porous grains of almost

spherical or oval shape that are the remains of unburned coal and biomass (Grammelis et al., 2006). The grains of ash from CFB-boilers do not contain vitreous phase, but they have significant open porosity (Szponder et al., 2011). As seen in Fig. 1(b), the HM-CFB-FA was comprised of twisted and/or crinkled honeycomb particles, resembling the vacuum structure of zeolite. The textural characteristic of CFB-FA and HM-CFB-FA are shown in Table 1. The BET analysis indicated that the specific surface area of CFB-FA increased from 16.2 to 113.2 m2/g after modification. In addition, it was found from the table that the pore diameter of CFB-FA was twice as large as that of HM-CFB-FA, which suggested that the pores in HM-CFB-FA sample may possess greater volume as compared to that in CFB-FA sample. The XRD patterns of CFB-FA and HM-CFB-FA are shown in Fig. 1(c). It can be observed that the hydrothermal modification of CFB-FA with HTAB resulted in a remarkable change of the 6

ACCEPTED MANUSCRIPT mineral structure. The main mineral phases of CFB-FA were quartz, hematite, CaSO4, and CaCO3. However, after hydrothermal modification, the morphology of the generated HM-CFB-FA resembled that of zeolite materials, such as Na6Al6Si10O32 and Na12Al12Si12O48. Considering the intensity of the diffraction peaks, it was found that the diffraction peak intensities of quartz, CaSO4, and hematite in HM-CFB-FA were lesser than that of CFB-FA. This result suggested that the ordered crystal structures of the quartz and CaSO4 phases in CFB-FA were destroyed to a certain extent by hydrothermal modification, which increased the lattice defects and promoted the digestion of Al2O3 and SiO2 in CFB-FA. The active Al2O3 and SiO2 easily reacted with alkali to form zeolite materials, which improved the adsorption performance of HM-CFB-FA (Juan et al., 2002). The FTIR spectra of CFB-FA and HM-CFB-FA are illustrated in Fig. 1(d). As seen in the figure, the absorption band of approximately 1632 cm−1 of less intensity peak can be attributed to deformational vibrations of adsorbed water molecules, and the broad band of around 3438 cm−1 was caused by the surface structural hydroxyl groups and adsorbed water content, which demonstrates the hydrophilic nature of the materials (Manohar et al., 2006). The vibrational bands located at around 1000 cm−1 and 450 cm−1 corresponded to TO4 (T = Si/Al) tetrahedral asymmetric stretch vibrations (Li et al., 2006) and the bending vibrations of the Si-O- groups on the surface (Takeda et al., 2010), respectively. In addition, it was observed that the tetrahedral asymmetric stretch vibrations of TO4 in the samples shifted from 1080 cm−1 to 982 cm−1 after hydrothermal modification and that the bending vibrations peak of Si-O- groups were obviously reduced. Hence, the polymerization degrees of [SiO4] and [AlO6] of CFB-FA were reduced by hydrothermal modification; this observation can be attributed to the fact that Si-O and Al-O bonds of CFB-FA were gradually broken in the hydrothermal process, which resulted in the cracking of a part of network structure constituted by [SiO4] and [AlO6]. This can be confirmed by the significant changes in the bands between 600 cm−1 and 900 cm−1 before and after modification. As a consequence, it can be believed that zeolite and zeolite materials were formed in HM-CFB-FA (Duxson et al., 2007). In addition, a band at 2920 cm−1 and 2850 cm−1 resulting from C–H stretch adsorption can be clearly observed in HM-CFB-FA, which may be derived from HTAB. Fig. 1 here Table 1 here 7

ACCEPTED MANUSCRIPT 3.2. Adsorption behavior of Cd2+ by HM-CFB-FA 3.2.1. Effect of adsorbent dosage Fig. 2 reveals the effect of adsorbent dosage on the adsorption performance of Cd2+ by HMCFB-FA. As shown in the figure, the removal efficiency of Cd2+ by HM-CFB-FA increased with an increase in the dosage from 0.5 to 4 g/L, and thereafter remained stable with a further increase in the adsorbent dosage. Nonetheless, it was observed that the adsorption capacity of HM-CFBFA increased rapidly with increasing adsorbent dosage from 0.5 to 2.5 g/L, with a sharp decline at the adsorbent dosage of 2.5–10 g/L. This result can be attributed to the fact that, at a low adsorbent dosage, the active sites of the adsorbent are fully utilized by the sorbent–solute interaction (Adebowale et al., 2006), whereas, at a high dosage of HM-CFB-FA (≥4 g/L), the active sites available for Cd2+ adsorption were not fully occupied. This may be responsible for the phenomenon that the calculated adsorption capacity decreased steeply with increasing dosage of HM-CFB-FA. When the adsorbent dosage was 2.5 g/L, the sorption capacity of 182 mg/g was obtained. Anbia et al. (2015) developed a magnetic functionalized MCM-48 mesoporous silica and found that the Cd2+ adsorption capacity of the adsorbent was 114.08 mg/g. Javadian et al. (2015) used a specific type of zeolite, synthesized from coal fly ash to adsorb Cd2+ from aqueous solution, and the results showed that the adsorption capacity of the adsorbent was 26.2 mg/g. Zein et al. (2010) reported that, using low-cost mangosteen shell as the adsorbent material for the removal of Cd2+ can achieve a sorption capacity of 3.15 mg/g. Compared to these findings, it can be calculated that the Cd2+-adsorption capacity of the adsorbent material reported in this study was 1.6–58-times that of various low-cost sorbents, such as biomass, clay minerals, and industrial wastes as reported mentioned-above. Fig. 2 here 3.2.2. Effect of initial concentration of Cd2+ The effect of the initial concentration of Cd2+ (5–1000 mg/L) on the adsorption performance of HM-CFB-FA was performed at the adsorbent dosage of 1 g/L (Fig. 3). It was found that the removal efficiency of Cd2+ decreased obviously with an increase in the initial Cd2+ concentration. However, the Cd2+-adsorption capacity of HM-CFB-FA increased at first and remained stable thereafter. This result suggested that the initial Cd2+ concentration had a positive effect on the adsorption of Cd2+ by HM-CFB-FA. At a lower initial concentration (<500 mg/L), 8

ACCEPTED MANUSCRIPT the Cd2+ in the solution would interact with the binding sites, which could facilitate the adsorption of Cd2+. With an increase in the initial concentration of metal ions, the increased driving force of the metal ions toward the active sites on adsorbents could increase the amount of metal ions adsorbed on the adsorbent material (Adebowake et al., 2006). When HM-CFB-FA was added at 1 g/L, the adsorption capacity remained stable at the initial Cd2+ concentration of >500 mg/L, which could be because most of Cd2+ was left unadsorbed in the solution due to saturation of the binding sites. These findings suggest that the number of available sites on the HM-CFB-FA is an important factor for the removal of Cd2+ by adsorption. Fig. 3 here 3.2.3. Effect of initial pH The changes in the adsorption of Cd2+ on HM-CFB-FA with the initial pH are shown in Fig. 4. As seen in Fig. 4a, at initial pH <2, HM-CFB-FA presented a low adsorption capacity due to the competition between H+ and Cd2+ for the active sites of the adsorbent (Sari et al., 2007). At the initial pH >2, the H+ concentration rapidly decreased and the adsorption of Cd2+ rapidly increased. This phenomenon can be attributed to the presence of oxides, such as SiO2 and Al2O3, the charge of which depends on the pH of the medium. The exchange mechanism between hydrogen ions and metal ions is represented by the following equations: -XOH + H3O+ → XOH2+ + H2O

(3)

-XOH + OH– → XO– + H2O

(4)

2(-XO–) + M2+ → (-XO)2M

(5)

where, X represents Si/Al/Fe and M represents the adsorbed metal ions. As the initial pH increased, SiO– derived from the dissociation of hydroxyl group on the surface of HM-CFB-FA increased, thereby resulting in the increase in the electrostatic attraction capacity between the adsorbate and adsorbent (Dawodu et al., 2012). As observed in Fig. 4a, the optimal initial pH for the adsorption of Cd2+ on HM-CFB-FA was 6.2. In addition, the final pH of the solution after reaction was estimated (Fig. 4b). The figure revealed that, at the tested initial pH range, the final pH of the solution significantly increased as compared to the starting pH. When the initial pH was >6.2, the final pH was >8.5. Under this condition, the precipitation of Cd species, such as hydroxides must be considered in the process of Cd2+adsorption on HM-CFB-FA according to the distribution of metal species as a function of pH (Stumm et al., 1996). 9

ACCEPTED MANUSCRIPT Fig. 4 here 3.2.4. Adsorption kinetics The adsorption kinetic results under different reaction conditions are shown in Fig. 5. From the figure, it can be found that, in all cases, the adsorption of Cd2+ undergo a rapid increase in the first period of 60 min and then slowly reached an equilibrium at 120 min. The rapid increase at the initial stage may be owed to the abundance of vacant sites on the surface of HM-CFB-FA and the high concentration gradient of Cd2+ between the solution and the adsorbent (Kavitha et al., 2007). With the gradual occupancy of these sites, the subsequent adsorption became slower, suggesting that the mechanism responsible for the sorption may change. The kinetics data of Cd2+ adsorption were evaluated by the pseudo-second-order model assuming that the rate-control step may be a chemical adsorption involving valence forces through the sharing of electrons between adsorbent and adsorbate. This model can be expressed in Eq. (6) (Ho et al., 1999):

1 1 1 1    2 Q k 1Q e t Qe

(6)

where, k1 is the pseudo-second-order rate constant of adsorption (g/mg · min). The Qe and Q represent the amounts of solute adsorbed at equilibrium and at time t (min), respectively. As shown in Table 2, under all the tested conditions, the high correlation coefficient and the greater degree of closeness between the calculated Qe values and the experimental Qe values indicated that the pseudo-second-order model is greatly suitable for the description of adsorption kinetics of Cd2+ on HM-CFB-FA. These results were consistent with those reported by Duan et al. (2014) and Yan et al. (2014). Therefore, the adsorption of Cd2+ on HM-CFB-FA was not only a diffusion process but also a surface reaction process (Ho et al., 1999). McKay et al. (1983) proposed a Cd2+ adsorption model, which was divided into three steps: i) external diffusion, the diffusion of the Cd2+ in the solution to the surface of adsorbent, ii) intraparticle diffusion, the diffusion of Cd2+ from the surface to the internal sites of adsorbent, and iii) the uptake of Cd2+ by physical–chemical adsorption, ion-exchange, precipitation, or complexation. Since the final adsorption step is extremely rapid, it kinetic process can be neglected. Hence, the overall rate of the adsorption process may be completely affected by either external diffusion or intra-particle diffusion. In the published literature, Boyd equation and Kannan equation have been 10

ACCEPTED MANUSCRIPT widely used to depict the external diffusion and intra-particle diffusion, respectively. Boyd equation:

 ln(1  F)  k 2 t

(7)

Kannan equation:

Q  k 3 t 0.5

(8)

where, F = Q/Qe is the adsorption fraction at time t, and k2 and k3 are the kinetic constants. As shown in Table 2, under the tested conditions, the correlation coefficients for Boyd equation were high, which suggested that the adsorption process of Cd2+ followed the Boyd model. In other words, the external diffusion mechanism controlled the adsorption of Cd2+ on HM-CFB-FA. According to the Kannan model, when the intra-particle diffusion dominated in the adsorption process, the plot of Qt versus t0.5 would be a straight line. In addition, if the plot passed through zero, the intra-particle diffusion would be the sole adsorption rate-controlling step; conversely, if the plot did not pass through zero, it was not the only adsorption rate-controlling step (Chiou et al., 2003). It can be found from Fig. 6 that the intra-particle diffusion was not the only adsorption ratecontrolling step during the uptake of Cd2+ on HM-CFB-FA due to no zero value in the plot. Furthermore, the experimental data can be modeled to multi-linear by Kannan equation as suggested by Wu et al. (2009). In this case, the multi-linear plot reveals that two or more steps control the adsorption process of Ca2+ by HM-CFB-FA. The plot described by experimental data could be divided into three sections: the external diffusion in the initial section, a slow adsorption stage controlled by the intra-particle diffusion in the second linear section, and the final equilibrium stage in the third section. When the initial concentration is <50 mg/L or pH <1, two linear sections could be fitted to be the initial deeper section (external diffusion) and the second section (equilibrium stage) by the Kannan equation. Table 2 here Fig. 5 here Fig. 6 here 3.2.5. Effect of temperature The effect of temperature on the adsorption of Cd2+ on HM-CFB-FA was performed at the temperature range of 25–45 ℃. In Fig. 5(d), it was observed that the adsorption capacity of HMCFB-FA increased from 165.2 mg/g to 191.7 mg/g with the increase in the reaction temperature from 25 ℃ to 35 ℃, and then reached 194.5 mg/g. at 45 ℃. It is well known that the increase of 11

ACCEPTED MANUSCRIPT temperature can decrease in the viscosity of the solution and increase the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle (Dogan et al., 2004). The increase of the adsorption capacity with the increase in the reaction temperature implied that the adsorption process is chemisorption (Dogan et al., 2000). Therefore, the adsorption of Cd2+ on HM-CFB-FA occurs as chemisorption and is endothermic in nature. 3.2.6 Adsorption isotherms The adsorption equilibrium is usually described by an isotherm equation, the parameters of which express the surface properties of the adsorbent. Langmuir and Freundlich models are usually used to describe the adsorption isotherms, which are expressed in Eqs. (9) and (10) (Hall et al., 1966), respectively. Langmuir equation:

Freundlich equation:

Ce C 1   e Q Qmkl Qm lgQ 

(9)

1 lgC e  lgk f n

(10)

where, Qm is the maximum adsorption capacity (mg/g), kl is the Langmuir constant related to the free energy of adsorption, kf is Freundlich constant, and n is the heterogeneity factor. The relative values of Qm, kl, kf, and n obtained as calculated from Langmuir and Freundlich models are listed in Table 3. The results showed that the experimental data were well fitted to the Langmuir isotherm (R2 ≥0.9943) and that the maximal adsorption capacity of Cd2+ increased from 130.2 mg/g to 208.7 mg/g with increasing temperature from 25 °C to 45 °C (Fig.7). This observation suggested that the adsorption process was chemical, monolayer, and endothermic in nature (Teng et al., 1998). Compared with previous literatures shown in Table 4, the HM-CFB-FA adsorbent prepared in this study possessed a higher adsorption capacity for Cd2+. Therefore, it can be considered that HM-CFB-FA is a promising material for Cd2+ removal. The Langmuir constant kl is a characteristic parameter related to the binding energy of solute and adsorbent, which reflects the spontaneity of the adsorption reaction. In other words, the greater the kl, the higher the spontaneity of the adsorption reaction, and, thereby, it has more stable product and better adsorption capacity (Wang et al., 2005). In the experiments, the value of kl value is low. This implied that the adsorption products of the adsorbents and Cd2+ were not stable, which meant that 12

ACCEPTED MANUSCRIPT the Cd2+ adsorbed on the adsorbent could be easily desorbed. Fig. 7 here Table 3 here Table 4 here The favorability and feasibility of the adsorption process can be determined by the equilibrium factor (RL) of Langmuir isotherm, the description equation of which can be given as the following (Hall et al., 1966):

RL 

1 1  k lC0

(11)

where, C0 and RL are the initial Cd2+ concentration (mg/L) and the dimensionless separation factor, respectively. The RL value represents that the shape of the isotherms is to be either unfavorable (RL >1), linear (RL =1), favorable (0 < RL < 1), or irreversible (RL =0). The value of RL calculated lies between 0 and 1 (0.0005–0.0018), implying that the adsorption of Cd2+ on the HM-CFB-FA sample is favorable. 3.3. The adsorption mechanism of Cd2+on HM-CFB-FA The species of Cd in the solution exist in the form of Cd2+, Cd(OH)+, Cd(OH)20, and Cd(OH)2(s) (Snoeyink et al., 1980). At pH < 6, Cd2+ is the only ionic species present in the solution, and at pH 6–8.5, Cd2+ and Cd(OH)+ predominate in the solution, whereas, at pH >8.5, the majority of Cd2+ species are transformed to be the form of Cd(OH)2 (Srivastava et al., 1966). Furthermore, from Fig. 4b, it can be found that, at the initial pH < 6.2, the final pH of the solution was < 7.2. On the other hand, when the initial pH was > 6.2, the final pH of the solution was maintained at 8.5–8.9. When the initial pH was < 6.2, the high concentration of H+ in solution decreased the effect of precipitation, and the adsorption function dominated in the process of the removal of Cd2+. During this adsorption process, the high porosity and surface area of HM-CFBFA would help Cd2+ to move through the pores of the adsorbent. However, when HM-CFB-FA was used as the adsorbent at the initial pH >6.2, part of the oxides of aluminum and calcium may undergo hydrolysis and ionization reaction in the aqueous solution as given below: MxOy + yH2O → xMe2y/x+ + 2yOH–

(M = Al, Ca, Mg, Na, K, etc)

(12)

The release of OH– promotes the solution pH, which results in the chemical precipitation of Cd2+. Hence, based on the above-mentioned analysis, it can be confirmed that the adsorption and 13

ACCEPTED MANUSCRIPT precipitation functions occur simultaneously when the initial pH is > 6.2. In order to elucidate the mechanism of the removal of Cd2+, we proposed a sketch map to describe the removal process of Cd2+ by HM-CFB-FA at the initial pH 6.2 (Fig. 7). At first, in an acidic environment, the surface of HM-CFB-FA was surrounded by a tightly bound shell of H+ and a positive charge developed on the surface of oxides of HM-CFB-FA, according to Eq. (13) (Duan et al., 2014): -MO + H2O → M-OH+ + OH–

(M=Si, Al, etc)

(13)

The positive charge on the HM-CFB-FA tends to weaken the force of holding the protons (–H) to the oxygen, and thus the hydroxyl are easily released. The release of OH– can cause neutralization that increases the pH. The hydroxyl and –OH groups bond to the surface of HM-CFB-FA, leading to the formation of surface hydroxyl group (–M–OH+) (Duan et al., 2014). Next, with an increase in the solution pH, the number of positive charge sites on the surface of HM-CFB-FA decreased and a negative charge developed on the surface of HM-CFB-FA [Eq. (14)]:. -M-OH + OH– → -MO– +H2O

(M = Si, Al, etc)

(14)

Finally, the Cd2+ was adsorbed by new active sites (-SiO–) and (-AlO–) formed on the surface of HM-CFB-FA to form new compounds [Eq. (15)] (Visa et al., 2014): 2-MO– + Cd2+ → - M-O-Cd-O-M

(M = Si, Al, etc)

(15)

In addition, Cd2+ can directly react with OH– to form hydrated heavy metal cations Cd(OH)+, according to Eq. (16), which can further react with the hydroxyl group to form precipitation compounds on the surface of HM-CFB-FA [Eq. (17)] (Visa et al., 2014): Cd2+ + H2O ↔ Cd(OH)+ + H+

(16)

M-OH + Cd(OH)+ ↔ MCd(OH)20

(17) Fig. 8 here

4. Conclusion The process in which CFB-FA was hydrothermally modified for the removal of Cd2+ was feasible, and the generated HM-CFB-FA was an efficient adsorbent for the removal of Cd2+. The zeolite materials formed in HM-CFB-FA was an effective component for the adsorption process, and its high specific surface area (113.2 m2/g) and pore volume (0.143 cm3/g) were beneficial for the adsorption of Cd2+. The adsorbent dosage, initial concentration of Cd2+, solution pH, contact time, and temperature played a significant role in the adsorption behavior. The maximum 14

ACCEPTED MANUSCRIPT adsorption capacity of HM-CFB-FA could reach 183.7 mg/g at 25 °C at an initial pH of 6.0. The adsorption process of Cd2+ on HM-CFB-FA could be well described using pseudo-second-order kinetics and Boyd equation, which indicates that the adsorption process was controlled by surface reaction and external diffusion processes. Isotherm modeling revealed that the adsorption process followed the Langmuir model, which may be due to the heterogeneous distribution of active sites onto the surface of HM-CFB-FA. In view of these results, HM-CFB-FA can be recommended as a highly cost-effective adsorbent material for the removal of Cd2+ from an aqueous solution. Acknowledgments This work was financially supported by coal-based key science and technology project of Shanxi province (Grant No. MC2014-04), the National Natural Science Foundation of China (Grant No. 51408529), the Natural Science Foundation of Hebei Province (Grant Nos. E2014203080), the Outstanding Young Scholars Project of Colleges and Universities of Hebei province (Grant No. BJ2014059), and China Postdoctoral Science Foundation Funded Project (Grant Nos. 2015M580215 and 2016T90215). Reference Adebowale, K.O., Unuabonah, I.E., Olu-Owolabi, B.I., 2006. The effect of some operating variables on the adsorption of lead and cadmium ions on kaolinite clay. J. Hazard. Mater. 134, 130–139. Agnieszka, G.P., Marek, M., Stanisław, P., Dariusz, S., 2012. Simultaneous adsorption of chromium (VI) and phenol on natural red clay modified by HDTMA. Chem. Eng. J. 179, 140– 150. Akar, S.T., Yetimoglu, Y., Gedikbey, T., 2009. Removal of chromium (VI) ions from aqueous solutions by using Turkish montmorillonite clay: effect of activation and modification. Desalination 244, 97–108. Aleksandra, B.,

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20

ACCEPTED MANUSCRIPT Table captions Table 1 Textural characteristic of CFB-FA and HM-CFB-FA Table 2 Kinetics parameters for Cd2+ adsorption by HM-CFB-FA Table 3 The parameters of adsorption isotherm for the removal of Cd2+ by HM-CFB-FA (adsorbent dosage, 1 g/L; initial pH = 6; initial Cd2+ concentration = 500 mg/L; contact time = 3 h). Table 4 Comparison of Cd2+ adsorption capacities of different low-cost adsorbents.

21

ACCEPTED MANUSCRIPT Figure captions Fig. 1. Characterization of HM-CFB-FA and CFB-FA. (a) SEM image of CFB-FA, (b) SEM image of HM-CFB-FA, (c) XRD patterns of HM-CFB-FA and CFB-FA, and (d) FTIR spectrums of HM-CFB-FA and CFB-FA. Fig. 2. Effect of adsorbent dosage on adsorption of Cd2+ by HM-CFB-FA (Reaction conditions: the initial concentration of Cd2+ = 500 mg/L; the initial pH = 6; temperature = 25 °C; reaction time = 3 h). Fig. 3. Effect of initial Cd2+ concentration on the adsorption of Cd2+ by HM-CFB-FA (Reaction conditions: adsorbent dosage = 1 g/L, initial pH = 6; temperature = 25 °C; reaction time = 3 h) Fig. 4. Effect of pH on the adsorption of Cd2+ by HM-CFB-FA (reaction conditions: the initial Cd2+ concentration = 500 mg/L; adsorbent dosage = 2.5 g/L; temperature = 25 °C; reaction time = 3 h). Fig. 5. The adsorption kinetics of Cd2+ on HM-CFB-FA. (a) reaction conditions: initial Cd2+ concentration = 500 mg/L; initial pH = 6; temperature = 25 °C; reaction time = 3 h; (b) adsorbent dosage = 2.5 g/L; initial pH = 6; temperature = 25 °C; reaction time = 3 h; (c) initial Cd2+ concentration = 500 mg/L; adsorbent dosage = 2.5 g/L; temperature = 25 °C; reaction time = 3 h; (d) the initial Cd2+ concentration = 500 mg/L; adsorbent dosage = 2.5 g/L; initial pH = 6; reaction time = 3 h. Fig. 6. Kannan adsorption equation of Cd2+ by HM-CFB-FA on the following conditions: (a) initial Cd2+ concentration = 500 mg/L, initial pH = 6, temperature = 25 °C, contact time = 3 h; (b) adsorbent dosage = 2.5 g/L, initial pH = 6, temperature = 25 °C, contact time = 3 h; (c) initial Cd2+ concentration = 500 mg/L, adsorbent dosage = 2.5 g/L, temperature = 25 °C, contact time = 3 h; (d) initial Cd2+ concentration = 500 mg/L, adsorbent dosage = 7.5 g/L, initial pH = 6, contact time = 3 h. Fig.7. Adsorption isotherms of Cd2+ onto HM-CFB-FA (Adsorbent dosage, 1 g/L; initial pH = 6; contact time = 3 h; initial Cd2+ concentration are 10, 25, 50, 100, 200, 300, 500, 1000 mg/L, respectively). Fig.8. Diagrammatic sketch of the removal of Cd2+ from aqueous solution by HM-CFB-FA.

22

ACCEPTED MANUSCRIPT Table 1

Table 1 Textural characteristic of CFB-FA and HM-CFB-FA. Material

SBET (m2/g) a

Vp (cm3/g) b

Apd (Å) c

CFB-FA HM-CFB-FA

16.2 113.2

0.026 0.143

163.132 84.201

a

SBET: specific surface area

b

Vp: volume of the pores

c

Apd: average pore diameter

23

ACCEPTED MANUSCRIPT Table 2 Table 2 Kinetics parameters for Cd2+ adsorption by HM-CFB-FA. Pseudo-second order model

Adsorbent dosage

T (℃)

Initial

Cd2+ conc.

pH

(mg/L)

(g/L)

k2 (×10–4)

Boyd

Experimental

Calculated

Qe

Qe,

(mg/g)

(mg/g)

R2

ki

R2

7.5

25

6

500

11.2

0.9975

58.5

59.9

0.0416　 0.9865　

2.5

25

6

500

15.4

0.9863

182.3

192.3

0.0399　　0.9901

2.5

25

1

500

172.9

0.9881

8

9.2

0.0385　　0.9932

2.5

25

6

50

108.1

0.9988

19.6

20.3

0.0290　　0.9898

2.5

25

6

200

25.7

0.9954

77.2

83.3

0.0302　　0.9918

7.5

35

6

500

18.3

0.9923

60.1

59.2

0.0743　　0.9889

7.5

45

6

500

22.4

0.9915

63.7

66.9

0.1311　 0.9882

24

ACCEPTED MANUSCRIPT Table 3 Table 3 The parameters of adsorption isotherm for the removal of Cd2+ by HM-CFB-FA (adsorbent dosage, 1 g/L; initial pH = 6; initial Cd2+ concentration = 500 mg/L; contact time = 3 h). Temperature (oC)

Langmuir Experimental Qm

Calculated Qm

( mg/g)

(mg/g)

Freundlich kl

R2

kf

n

R2

25

130.2

134.9

4.2

0.9965

6.1

1.2

0.9638

35

163.5

158.1

6.8

0.9959

8.9

1.4

0.9612

45

208.7

215.1

9.5

0.9943

11.

1.8

0.9587

5

25

ACCEPTED MANUSCRIPT Table 4

Table 4 Comparison of Cd2+ adsorption capacities of different low-cost adsorbents. Adsorbent

Qm (mg/g)

Magnetic chlorapatite nanoparticles

Refs.

1.16 mmol/g

Keochaiyom et al. 2017

biochar from biogas residues

76.34

Aleksandra et al. 2017

alkaline treated algae waste

41.88

Bulgariu et al. 2016

Mesoporous calcium-silicate material from coal fly ash

5.52 mmol/g

Qi et al. 2016

Zeolite-based geopolymer from coal fly ash

26.25

Javadian et al. 2015

Waste FGD gypsum

43.10

Yan et al. 2014

Hydrothermally modified fly ash

87.72

Visa et al. 2014

Modified sewage sludge

14.7

Phuengprasop et al. 2011

Nalco Plant Sand

58.13

Mohapatra et al. 2009

Sodium tetraborate-modified kaolinite clay

44.05

Unuabonahet al. 2008

26

ACCEPTED MANUSCRIPT Figure 1

Fig. 1. Characterization of HM-CFB-FA and CFB-FA. (a) SEM image of CFB-FA, (b) SEM image of HM-CFB-FA, (c) XRD patterns of HM-CFB-FA and CFB-FA, and (d) FTIR spectrums of HM-CFB-FA and CFB-FA.

27

ACCEPTED MANUSCRIPT Figure 2

Fig. 2. Effect of adsorbent dosage on adsorption of Cd2+ by HM-CFB-FA (Reaction conditions: the initial concentration of Cd2+ = 500 mg/L; the initial pH = 6; temperature = 25 °C; reaction time = 3 h).

28

ACCEPTED MANUSCRIPT Figure 3

Fig. 3. Effect of initial Cd2+ concentration on the adsorption of Cd2+ by HM-CFB-FA (Reaction conditions: adsorbent dosage = 1 g/L, initial pH = 6; temperature = 25 °C; reaction time = 3 h)

29

ACCEPTED MANUSCRIPT Figure 4

Fig. 4. Effect of pH on the adsorption of Cd2+ by HM-CFB-FA (reaction conditions: the initial Cd2+ concentration = 500 mg/L; adsorbent dosage = 2.5 g/L; temperature = 25 °C; reaction time = 3 h).

30

ACCEPTED MANUSCRIPT Figure 5

Fig. 5. The adsorption kinetics of Cd2+ on HM-CFB-FA. (a) reaction conditions: initial Cd2+ concentration = 500 mg/L; initial pH = 6; temperature = 25 °C; reaction time = 3 h; (b) adsorbent dosage = 2.5 g/L; initial pH = 6; temperature = 25 °C; reaction time = 3 h; (c) initial Cd2+ concentration = 500 mg/L; adsorbent dosage = 2.5 g/L; temperature = 25 °C; reaction time = 3 h; (d) the initial Cd2+ concentration = 500 mg/L; adsorbent dosage = 2.5 g/L; initial pH = 6; reaction time = 3 h.

31

ACCEPTED MANUSCRIPT Figure 6

Fig. 6. Kannan adsorption equation of Cd2+ by HM-CFB-FA on the following conditions: (a) initial Cd2+ concentration = 500 mg/L, initial pH = 6, temperature = 25 °C, contact time = 3 h; (b) adsorbent dosage = 2.5 g/L, initial pH = 6, temperature = 25 °C, contact time = 3 h; (c) initial Cd2+ concentration = 500 mg/L, adsorbent dosage = 2.5 g/L, temperature = 25 °C, contact time = 3 h; (d) initial Cd2+ concentration = 500 mg/L, adsorbent dosage = 7.5 g/L, initial pH = 6, contact time = 3 h.

32

ACCEPTED MANUSCRIPT Figure 7

Fig.7. Adsorption isotherms of Cd2+ onto HM-CFB-FA (Adsorbent dosage, 1 g/L; initial pH = 6; contact time = 3 h; initial Cd2+ concentration are 10, 25, 50, 100, 200, 300, 500, 1000 mg/L, respectively).

33

ACCEPTED MANUSCRIPT Figure 8

Fig. 8. Diagrammatic sketch of the removal of Cd2+ from aqueous solution by HM-CFB-FA.

34