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Conversion of waste FGD gypsum into hydroxyapatite for removal of Pb2+ and Cd2+ from wastewater
Journal of Colloid and Interface Science 429 (2014) 68–76
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Conversion of waste FGD gypsum into hydroxyapatite for removal of Pb2+ and Cd2+ from wastewater Yubo Yan, Xiaoli Dong, Xiaolei Sun, Xiuyun Sun ⇑, Jiansheng Li, Jinyou Shen, Weiqing Han, Xiaodong Liu, Lianjun Wang ⇑ Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu Province, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 15 January 2014 Accepted 3 May 2014 Available online 19 May 2014
Flue gas desulfurization (FGD) gypsum, a familiar waste generated from coal-fired power plants, was successfully transformed to hydroxyapatite (FGD-HAP) by hydrothermal method. The obtained FGD-HAP was characterized by XRD, FTIR, TEM and BET methods and investigated as adsorbent for removal of Pb2+ and Cd2+ from wastewater. Batch experiments were performed by varying the pH values, contact time and initial metal concentration. The result of pH impact showed that the adsorption of two ions was pH dependent process, and the pH 5.0–6.0 was found to be the optimum condition. The achieved experimental data were analyzed with various kinetic and isotherm models. The kinetic studies displayed that the pseudo-second order kinetic model could describe adsorption processes well with high correlation coefficient, and the Langmuir isotherm model provided the best fit to the equilibrium experimental data. The maximum adsorption capacities calculated from Langmuir equation were 277.8 and 43.10 mg/g for Pb2+ and Cd2+, respectively, which can compete with other adsorbents. The thermodynamic parameters revealed the adsorption processes were endothermic and spontaneous in nature. In binary adsorption, the amount of Cd2+ adsorbed on FGD-HAP decreased by 46.0% with increasing concentration of Pb2+, which was higher than that of Pb2+(21.7%), demonstrating the stronger affinity between FGDHAP and Pb2+. The highest amount of Pb2+ and Cd2+ desorbed from saturated FGD-HAP by EDTA solution confirmed the FGD-HAP was a promising alternative adsorbent in treatment of toxic Pb2+ and Cd2+ wastewater. Ó 2014 Elsevier Inc. All rights reserved.
Keywords: FGD gypsum Adsorption Lead Cadmium Desorption
1. Introduction With the rapid industrial development, the increasing levels of heavy metals are discharged into surface waters. Pollution by heavy metals, such as Pb2+ and Cd2+, has drawn much international attention due to their non-biodegradability, persistence in nature, accumulation in the food chain and toxicity even at low concentration [1]. Pb2+ and Cd2+ are commonly presented in wastewater through their intensive industrial application, for example, mining operations, tanneries, metal plating, ceramics, metal finishing, electroplating, battery manufacture and painting [2]. Consequently, it is necessary to reduce such poisonous heavy metals before wastewater discharge. Conventional technologies for heavy metals removal from wastewater include chemical precipitation, reverse osmosis, electrocoagulation, ion exchange, evaporation ⇑ Corresponding authors. Fax: +86 25 84315941. E-mail (L. Wang).
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http://dx.doi.org/10.1016/j.jcis.2014.05.010 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
Sun),
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and membrane filtration [3,4], etc. Many of them, however, either not effective enough or are too expensive [5]. Adsorption, compared with above approaches, is considered as a most promising technique for removal of heavy metals from wastewater because of its high removal efficiency, easy operation and less residue production. During the past decade, hydroxyapatite (Ca10(PO4)6(OH)2, HAP) has attracted particular interest in treating heavy metal wastewater due to its high sorption capacity, low water solubility and high stability under oxidizing and reducing conditions [6–10]. Currently, utilization of hydroxyapatite or its nanocomposites to immobilize metal ions from aqueous solution is still a research hotspot. Zhao et al. [11] synthesized three-dimensional HAP nanosheet-assembled microspheres as adsorbent for removal of Pb2+, Cd2+ and Cu2+. Yang et al. [12] prepared Fe3O4@HAP nanocomposite for adsorbing Pb2+, Y3+, Eu3+ and Sb3+. Aliabadi et al. [13] reported that Chitosan/HAp composite nanofiber had high sorption capacity for Pb2+ (296.7 mg/g), Ni2+ (213.8 mg/g) and Co2+ (180.2 mg/g). They also confirmed that Chitosan/HAp composite
Y. Yan et al. / Journal of Colloid and Interface Science 429 (2014) 68–76
nanofiber possessed an effective regenerability. On the other hand, Minh et al. [14] used calcium carbonate as low cost Ca source to synthesize HAP for adsorbing Pb2+. Kongsri et al. [15] converted waste fish scale to nanocrystalline HAP for selenium adsorption. Generally, Ca-contained reagent or Ca-contained biowaste is primarily selected as Ca sources for preparation of HAP. However, little research focused on the Ca-contained industrial waste. Flue gas desulfurization (FGD) gypsum, a common waste from the wet flue gas desulfurization process in coal power plants, is mainly composed of CaSO42H2O, which can be an ideal Ca source. China Association of Environmental Protection Industry (CAEPI) reported over 43 million tons FGD gypsum was produced in 2010 [16]. A considerable portion of FGD gypsum is dumped directly, such disposal not only occupy a large amount of land resources but also cause serious dust pollution and groundwater pollution. Thus, finding an appropriate method to transform waste FGD gypsum into useful materials may be an economically valuable solution to this problem. Up to now, no report has ever been published regarding conversion of waste FGD gypsum into hydroxyapatite and then application for adsorbing heavy metals. Therefore, the objectives of this work were (1) to assess the feasibility of preparing HAP (FGDHAP) using waste FGD gypsum as Ca source, (2) to thoroughly characterize the FGD-HAP by XRD, TEM, FTIR and BET methods, (3) to firstly evaluate the FGD-HAP’s adsorption properties (pH effects, adsorption kinetics, isotherms, thermodynamics and competitive adsorption) and regenerability toward Pb2+ and Cd2+, (4) to deeply investigate the mechanisms involved in adsorption process.
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transferred to Teflon-lined stainless steel autoclave and kept in an oven at 150 °C for 24 h. (iv) The solid obtained from the cooled autoclave was rinsed several times with Milli-Q water and ethanol, then dried at 80 °C for overnight. (v) The dried sample was milled to pass through a 200 mesh sieve and stored in a desiccator for further analysis. The crystalline phases of the FGD-HAP powder were analyzed by X-ray diffraction (XRD) (D8 Advance, Bruker, Germany) with Cu Ka radiation. The surface area and pore volume of material were determined by BET apparatus (ASAP 2020, Micromeritics Instrument, USA), using nitrogen gas as the adsorbate at 77 K. The morphology of sample was analyzed through transmission electron microscopy (TEM). Infrared spectra were obtained using Fourier transform infrared (FTIR) (IR Prestige-21, Shimadzu, Japan) spectroscopy. The zeta potential of powder was measured by a Zeta meter (ZetaPALS, Brookhaven Instruments, USA), using 0.01 M KCl solution as a background electrolyte. 2.3. Batch adsorption experiments
2. Materials and methods
The adsorption experiments of Pb2+ and Cd2+ were carried out according to batch method. The conical flasks containing 0.05 g FGD-HAP and 50 mL single-component solution at desired concentration and pH were placed in an air bath oscillator with a constant speed of 200 rpm and constant temperature of 20 °C. After adsorption, the mixtures were filtered through 0.22 lm membrane filter and the filtrates were analyzed for residual metal concentration via an inductive coupled plasma atomic emission spectroscopy (ICP-AES) (Optima 7000DV, PerkinElmer, USA). The Pb2+ and Cd2+ adsorption capacities at time t, qt (mg/g), are calculated according to:
2.1. Materials
qt ¼
The FGD gypsum was obtained from Huarun power plant, Nanjing, China. The sample was washed thoroughly with Milli-Q water and then dried at 100 °C for 24 h. The 200 mesh sieve was used to collect FGD gypsum with particle size smaller than 0.04 mm. The components of screened sample were analyzed by energy dispersive X-ray fluorescence (XRF) (LAB CENTER XRF-1800, Shimadzu, Japan) and the results were listed in Table 1. Pb(NO3)2, Cd(NO3)2, (NH4)2HPO4, NH3H2O, NaOH and HNO3, purchased from Nanjing Chemical Reagent Company, are all analytical grade. Simulated stock wastewaters with 1000 mg/L were prepared by respectively dissolving appropriate amounts of Pb(NO3)2 and Cd(NO3)2 in Milli-Q water. The desired concentrations in experiments were prepared by diluting stock wastewaters.
where C0 and Ct (mg/L) are the single-metal concentration in the initial solution and at time t, respectively; V (L) is the volume of solution, and m (g) is the weight of the sample added to the solution. For determination of pH impact on adsorption, the pH of was adjusted using 1 M NaOH or 1 M HNO3 ranging from 2.0 to 6.0 in 200 mg/L Pb2+ or 100 mg/L Cd2+ wastewater. Adsorption kinetics of Pb2+ and Cd2+ were both studied in the range of 10–300 min at optimum pH. Adsorption isotherms were investigated in the range of 50–500 mg/L for Pb2+ and 20–200 mg/L for Cd2+. Adsorption thermodynamic studies were carried out with 250 mg/L Pb2+ and 50 mg/L Cd2+ at 20, 30 and 40 °C, respectively.
ðC 0 C t ÞV m
ð1Þ
2.4. Binary adsorption
2.2. FGD-HAP synthesis and characterization The FGD-HAP powder was synthesized by the following procedures: (i) 2 g FGD gypsum and 100 mL (NH4)2HPO4 solution (0.086 mol P/L) was mixed at room temperature. (ii) The mixture was maintained at a pH value 10–11 by adding NH3H2O and vigorously stirred for 4 h. (iii) After stirring, the mixture was
Table 1 Chemical composition of FGD gypsum by XRF. Composition
Value (wt.%)
CaO SiO2 Al2O3 Fe2O3 MgO SO3
40.1 2.12 1.23 0.29 0.18 55.9
2.4.1. Preferential adsorption Preferential adsorption systems were prepared by solubilizing a combination of Pb–Cd in presence of each metal with other metal present in equal concentrations (20, 50, 100, 150 and 200 mg/L). 2.4.2. Competitive adsorption The competitive adsorption studies of Pb2+ and Cd2+ in binary system consists of two parts: (i) Effect on the adsorption capacity of Pb2+ with Cd2+ present in system. In this part, the initial Pb2+ concentration remained at 50, 100, 150, 200, 250, 300, 400 and 500 mg/L respectively, while the concentration of Cd2+ varied from 50 to 200 mg/L respectively. (ii) Effect on the adsorption capacity of Cd2+ with Pb2+ present in solution. In this section, the initial Cd2+ concentration fixed at 20, 40, 60, 80, 100, 120, 150 and 200 mg/L respectively, whereas the other one ranged from 50 to 200 mg/L respectively.
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2.5. Desorption experiments Desorption experiments were carried out to estimate the effects of different eluants on desorption of Pb2+ or Cd2+ from FGD-HAP. Initially, FGD-HAP was centrifuged after equilibrium experiments, then FGD-HAP loaded Pb2+ or Cd2+ was rinsed and dried at 80 °C for overnight. Subsequently, 0.05 g collected FGD-HAP was added into 25 mL EDTA (0.01 M), HNO3 (0.01 M), MgCl2 (0.1 M) and NaCl (0.1 M) solution, respectively, shaking at 22 ± 2 °C for overnight. After experiments, the concentration of Pb2+ or Cd2+ in each eluant was measured by ICP-AES. 3. Results and discussion 3.1. Characterization of FGD-HAP Fig. 1 shows wide-angle XRD pattern of FGD-HAP. Eight characteristic peaks of HAP were observed at 2h = 25.9°, 31.8°, 32.1°, 32.9°, 34.1°, 39.8°, 46.7° and 49.5°, which could be indexed as (0 0 2), (2 1 1), (1 1 2), (3 0 0), (2 0 2), (3 1 0), (2 2 2) and (2 1 3) reflections, respectively. These typical peaks are matched well with JCPDS file No. 09-432, indicating the structure of FGD-HAP belongs to the hexagonal P63/m space group with lattice constant of a = b = 9.418 Å, c = 6.884 Å. FTIR spectra of FGD-HAP are shown in Fig. 2. The adsorption bands at 1093, 1033, 962, 601 and 565 cm1 represent phosphate groups [17,18]. The peaks observed at 3570 and 634 cm1 are indicative of the hydroxyl groups [15,17,18], which supported adsorbing metal ions on FGD-HAP through surface complexation. A bi-modal peak at 1460 and 1420 cm1 as well as single peak at 873 cm1 correspond to carbonate groups, indicating carbonate partly substituted for the phosphate while exposed FGD-HAP to atmosphere [14,19]. In addition, two weak broad bands were observed at 3450 and 1637 cm1 can be attributed to the adsorbed water [15]. Fig. 3a and b show the TEM images of FGD-HAP. The FGD-HAP particles (15–25 nm in diameter and 100–250 nm in length) present uniform rod-like shape and most particles seem to aggregate together by melting their surfaces. Fig. 4 shows the textural properties of FGD-HAP. As can be seen from figure, the FGD-HAP possessed the mesoporous structure, high specific surface area and large pore volume, which were beneficial to ion-exchange and diffusion [20].
Fig. 2. FTIR spectra of FGD-HAP.
3.2. Effect of pH The solution pH is one of the most important factors for the adsorption of heavy metals, which would significantly influence
Fig. 3. TEM images of FGD-HAP.
Fig. 1. XRD pattern of FGD-HAP.
both the hydrolytic species of metal ions and the surface charge of adsorbent. In order to prevent precipitation of Pb2+ and Cd2+ in the form of metal hydroxides, the pH ranges in this study were both chosen as 2.0–6.0. The effect of pH was shown in Fig. 5. It was noticed that the plots of Pb2+ and Cd2+ had similar variation trend. The adsorption amount of two metals increased obviously with increasing pH from 2.0 to 4.0, which could be reasonably explained by: (i) the activated sites reduce with the partial dissolution of FGD-HAP in strongly acidic media, (ii) the competition between protons and metal ions for the available adsorption sites
Y. Yan et al. / Journal of Colloid and Interface Science 429 (2014) 68–76
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Fig. 4. Textural properties of the FGD-HAP.
Fig. 5. Effect of pH on the uptake of Pb2+ and Cd2+ and the zeta potential of FGDHAP at various pH values.
in lower pH value [21], (iii) the negative charge on the FGD-HAP surface increases and thus attracts the positively charged ions more strongly [22]. At pH above 4.0, the adsorption capacity of Pb2+ had a slight change while the Cd2+ exhibited a slow increase initially and then remained constant. Therefore, the optimum pH for adsorption of Pb2+ and Cd2+ was determined as 5.0–6.0 and was selected for the subsequent experiments. 3.3. Sorption kinetics The effect of contact time on the adsorption of Pb2+ or Cd2+ was illustrated in Fig. 6a. It was evident that the adsorption amount of Pb2+ and Cd2+ increased steeply within the first 60 min. This can be interpreted by the fact that a large number of active sites on the FGD-HAP surface are available at the beginning of the adsorption. Afterward the uptake of metal ions increased slightly and finally reached equilibrium. This appearance is attributed to the availability of binding sites are exhausted gradually. The equilibrium time for both metals was 150 min, which was chosen for the further experiments. In order to explicate in depth the controlling mechanism of the adsorption process such as mass transfer and chemical reaction, the pseudo-first order and pseudo-second order [23,24] models were employed to analyze the experimental data, and their linear forms can be expressed as:
Pseudo-first-order : logðqe qt Þ ¼ log qe
k1 t 2:303
ð2Þ
Fig. 6. (a) Effect of time on the uptake of Pb2+ and Cd2+ onto FGD-HAP at 20 °C, (b) pseudo-first order kinetic plots for adsorption of Pb2+ and Cd2+ and (c) pseudosecond order kinetic plots for adsorption of Pb2+ and Cd2+.
Pseudo-second-order :
t 1 t ¼ þ qt k2 q2e qe
ð3Þ
where qe and qt are the adsorption capacity (mg/g) at equilibrium and at time t (min), k1 (min1) and k2 (g/(mg min)) are the rate constants for the pseudo-first order and pseudo-second order, respectively. Furthermore, when t ? 0, the initial sorption rate (h mg/(g min)) of Pb2+ or Cd2+ can also be defined by the following equation:
h ¼ k2 q2e
ð4Þ
The kinetic parameters (Table 2) of pseudo-first order model and pseudo-second order model were respectively calculated from the linear plot of log(qe qt) versus t (Fig. 6b) and the plot of t/qt versus
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t (Fig. 6c). Based on Table 2 and Fig. 6b–c, it can be found that the correlation coefficients of the pseudo-second order model (0.9984 for Pb2+ and 0.9995 for Cd2+) are higher than the results obtained from the pseudo-first order model (0.9753 for Pb2+ and 0.9361 for Cd2+). The theoretical qe calculated from pseudo-second order equation were 212.8 and 38.31 mg/g for Pb2+ and Cd2+ respectively, which were extremely close to the experimental data (199.5 and 36.70 mg/g). However, the calculated qe from the pseudo-first order equation (226.5 mg/g for Pb2+ and 14.98 mg/g for Cd2+) were obviously different from the experimental data. Therefore, the sorption behavior of Pb2+ and Cd2+ can be well described by pseudo-second order model, suggesting that the rate-limiting step for adsorption process was chemical sorption [25]. Additionally, the initial sorption rate of Pb2+ was 13.13 mg/(g min) while the Cd2+ was 3.23 mg/(g min), indicating that the adsorption of Pb2+ at the beginning was much faster than Cd2+. Fig. 7. Adsorption isotherms of Pb2+ and Cd2+ on FGD-HAP.
3.4. Sorption isotherms Adsorption equilibrium data, expressed by the mass of adsorbate adsorbed per unit weight of adsorbent and liquid phase equilibrium concentration of adsorbate, were usually represented by adsorption isotherms, which is essential for the practical design and critical for predicting the maximum adsorption capacity of adsorbent. In this study, the adsorption isotherms of Pb2+ and Cd2+ on FGD-HAP are plotted in Fig. 7. It can be seen that the adsorption capacity of two metals increased initially with equilibrium concentration and then reached saturation. The uptake of Pb2+ is throughout higher than Cd2+, indicating that the interaction between FGD-HAP and Pb2+ is stronger than that of Cd2+. For a better interpretation of adsorption behaviors, the Langmuir, and Freundlich isotherm models were firstly applied to fit the experimental equilibrium data. The Langmuir model [26] assumes adsorption occurs on a homogenous surface and no interaction between the adsorbed in the plane of the surface. The equation of the Langmuir isotherm is as follows:
Ce Ce 1 ¼ þ qe qmax bqmax
ð5Þ
where Ce is the equilibrium concentration of Pb2+ or Cd2+ (mg/L), qe is the amount of Pb2+ or Cd2+ adsorbed under equilibrium (mg/g), qmax is the maximum adsorption capacity (mg/g), b is a Langmuir constant related to the affinity of the binding sites and energy of adsorption (L/g). The Freundlich model [27] is commonly used to describe adsorption characteristics for heterogeneous surface. The equation of the Freundlich isotherm can be written as:
log qe ¼ log K F þ
1 log C e n
ð6Þ
where KF is the Freundlich constant and gives the capacity of adsorbent (mg/g (1/mg1/n)) and n is Freundlich exponent that provides an indication of favorability of the adsorption process [28]. The calculated Langmuir and Freundlich constants are summarized in Table 3. By comparing the R2 values, it can be discovered that the adsorption processes of both Pb2+ and Cd2+ were accurately described by the Langmuir model. The maximum adsorption
capacity (qmax) deduced from Langmuir equation were 277.8 and 43.10 mg/g for Pb2+ and Cd2+, respectively, which are similar to or even greater than those of other adsorbents (Table 4), indicating the FGD-HAP is a promising adsorbent for removal of Pb2+ or Cd2+ from the real wastewater. The n value obtained from Freundlich equation is an indication of the favorability of adsorption. Values of n in this study were both in the range 2–10 (6.105 for Pb2+ and 3.880 for Cd2+), representing favorable adsorption and high affinity between FGD-HAP and metals [38]. The essential characteristic of Langmuir isotherm can be expressed in terms of a dimensionless separation factor, RL:
RL ¼
1 1 þ bC 0
ð7Þ
where C0 is denoted to the highest initial Pb2+ or Cd2+ concentration. The values of RL reflect the adsorption process to be either irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1). The plots of RL values against initial concentrations are presented in Fig. 8. All RL values were between 0 and 1, implying Pb2+ and Cd2+ are both favorably adsorbed by FGD-HAP, which was in good agreement with the results regarding the n values. For each metal, the RL values decreased with initial concentration increasing, indicating the adsorption was more beneficial at higher initial concentration. Moreover, the much lower RL values for Pb2+ suggested that the FGD-HAP was more effective in removing Pb2+ than Cd2+. In order to deeply evaluate the mechanism of Pb2+ and Cd2+ adsorption on FGD-HAP, as well as distinguish between physisorption and chemisorption, the Dubinin–Radushkevich (D–R) model [39] was chosen to apply on sorption study. The linear expressions of this model can be represented by:
ln qe ¼ ln qm be2
ð8Þ
where qm is the theoretical saturation adsorption capacity (mg/g), b is a constant correlated with the mean free energy of adsorption (mol2/J2), e is the Polanyi potential, which is equal to:
e ¼ RT ln 1 þ
1 Ce
ð9Þ
Table 2 Pseudo-first order and pseudo-second order kinetic parameters for the adsorption of Pb2+ and Cd2+ on FGD-HAP. Adsorbate
Pb2+ Cd2+
qe,exp (mg/g)
199.5 36.70
Pseudo-first order kinetic model
Pseudo-second order kinetic model
k1 (min1)
qe,cal (mg/g)
R2
k2 (g/mg min)
qe,cal (mg/g)
R2
4.0 102 1.9 102
226.5 14.98
0.9753 0.9361
3.3 104 2.4 103
212.8 38.31
0.9984 0.9995
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Y. Yan et al. / Journal of Colloid and Interface Science 429 (2014) 68–76 Table 3 Langmuir, Freundlich and Dubinin–Radushkevich parameters for the adsorption of Pb2+ and Cd2+onto FGD-HAP at 20 °C. Adsorbate
2+
Pb Cd2+
Langmuir model
Freundlich model 2
qmax (mg/g)
b (L/mg)
R
277.8 43.10
0.4865 0.0825
0.9992 0.9953
KF (mg/g(1/mg
))
127.8 11.76
Table 4 Comparison of adsorption capacity (qmax) for Pb2+ and Cd2+ with various adsorbents. Metals
Adsorbent
qmax (mg/g)
Reference
Pb2+
FGD-HAP Alkali treated tea residue HAp/PU composite foams Activated carbon CHAP US-MWCNT
277.8 64.10 150 31.2 94.3 227.3
This study [29] [30] [31] [32] [33]
FGD-HAP Orange peel Peat Hazel nut shells Bamboo charcoal Commercial hydroxyapatite
43.10 35.71 22.50 5.42 12.08 290.0
This study [34] [35] [36] [37] [7]
Cd2+
Dubinin–Radushkevich model
1/n
R
qm (mg/g)
E (kJ/mol)
R2
6.105 3.880
0.6079 0.9609
436.6 80.99
18.90 13.87
0.6260 0.9631
3.5. Thermodynamic study Thermodynamic parameters, such as the Gibbs free energy change (4G°, kJ/mol), the enthalpy change (4H°, kJ/mol) and the entropy change (4S°, kJ/mol/K) are critical for understanding whether the adsorption is endothermic or exothermic, and the spontaneity of the adsorption process, which are determined by following equations [42]:
Cs Ce
ð11Þ
DG ¼ RT ln K ln K ¼
Fig. 8. Separation factor, RL, for the adsorption of Pb2+ and Cd2+ at different initial concentrations on FGD-HAP.
where R is the universal gas constant (8.3145 J/mol/K), and T is the absolute temperature in Kelvin (K). The constant b gives information about the mean free energy (E, mol2/kJ2) of sorption per molecule of the sorbate when it is transferred to the surface of the solid from infinity in the solution and can be computed by [40]:
1 E ¼ pffiffiffiffiffiffi 2b
n
FGD-HAP by the Pb2+ or Cd2+ occurring in the wastewater. In addition to ion exchange, some other chemical reactions may be occurred on the surface of FGD-HAP for Pb2+ removal.
K¼
Note: Hap/PU = hydroxyapatite/polyurethane. CHAP = synthetic carbonate hydroxyapatite. US-MWCNT = ultrasonic bath – multiwalled carbon nanotubes.
2
ð12Þ
DS DH R RT
ð13Þ
where K is the adsorption equilibrium constant, Cs is the amount of Pb2+ or Cd2+ adsorbed on per weight unit of FGD-HAP after equilibrium (mg/g), Ce is the Pb2+ or Cd2+ concentration in solution at equilibrium (mg/L), R is the universal gas constant (8.3145 J/mol/K), and T is the temperature (K). The parameters of thermodynamics are listed in Table 5. The values of 4G° were below zero at all temperatures, suggesting that the adsorption of Pb2+ and Cd2+ on FGD-HAP are spontaneous and thermodynamically favorable. The decrease in 4G° values with the rise in temperature indicates more efficient adsorption at higher temperature. In addition, the Gibbs free energy of Pb2+ was greater than that of Cd2+ illustrating the stronger affinity for Pb2+ adsorption on FGD-HAP. The positive value of 4H° confirmed that the adsorption processes of Pb2+ and Cd2+ are endothermic. Besides, according to the values of 4H°, the procedure can be classified as physical (2.1 kJ/mol < 4H° < 20.9 kJ/mol) and chemical (20.9 kJ/ mol < 4H° < 418.4 kJ/mol) process [43]. The values of 4H° for Pb2+ and Cd2+ were 107.3 and 34.69 kJ/mol respectively, concluding that the nature of Pb2+ and Cd2+ adsorption are chemisorption, which is consistent with the results of kinetics study and D–R model. The positive 4S° values reflected the increase of randomness at the solid-solute interface during the adsorption of metal ions onto FGD-HAP. Similar result was found from the literature [44].
ð10Þ
The value of E is very useful in predicting the type of adsorption. If the value is below 8 kJ/mol then the adsorption is physical in nature, between 8 and 16 kJ/mol the adsorption is due to ion exchange and over 16 kJ/mol the adsorption type can be explained by a stronger chemisorption than ion exchange [41]. The values of E found in the present work were 18.90 and 13.87 kJ/mol for Pb2+ and Cd2+, respectively. So, it seems that the ion exchange mechanism is dominating in both adsorption processes. The probable metals adsorption is due to the exchange of Ca2+ existing in
Table 5 Thermodynamic parameters for the adsorption of Pb2 + and Cd2 + on FGD-HAP. Metals
T (°C)
4G° (kJ/mol)
4H° (kJ/mol)
4S° (kJ/mol/K)
Pb2+
20 30 40
5.97 7.46 13.75
107.3
0.38
20 30 40
0.29 0.85 2.70
34.69
0.12
Cd2+
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3.6. Binary adsorption study In practice, the wastewater normally contains Pb2+ and Cd2+ simultaneously. Hence, it is very significant to investigate the binary adsorption of metals and to know if there is competition between Pb2+ and Cd2+ for the available binding sites. 3.6.1. Preferential adsorption The separation factor, aPb Cd , was employed to assess the preference of FGD-HAP for one of the two ions in binary systems, which was defined by [45]:
aPb Cd ¼
qPb C Cd qCd C Pb
ð14Þ
If the Pb2+ is preferred, the factor is greater than unity, while if the Cd2+ is preferred, the factor is smaller than unity. The calcuPb lated aPb Cd varied in the range of 36.74 < aCd < 133.8, highlighting 2+ that the preference of FGD-HAP for Pb was higher than that of Cd2+, i.e., the greater affinity of Pb2+ than Cd2+, in accord with the results of binding energy (Langmuir constant, bPb > bCd). This could be attributed to (i) the smaller hydrated radii of Pb2+ (0.261 nm) than Cd2+ (0.275 nm), resulting in the Pb2+ is more accessibility to the FGD-HAP’s surface and pores, (ii) the higher electronegativity of Pb2+ (2.33) than Cd2+ (1.69), leading to more strongly attraction between Pb2+ and FGD-HAP’s surface, (iii) the ionic radii of Pb2+ (0.118 nm) is larger than Ca2+ (0.099 nm) while that of Cd2+ (0.097 nm) is smaller than Ca2+, causing the Cd2+ has less chance to be incorporated into the FGD-HAP structure [46–48]. 3.6.2. Competitive adsorption The adsorption isotherm of Pb2+ or Cd2+ with the presence of another metal ion was shown in Fig. 9a and b, respectively. From the figures, it can be observed that for each ion, the equilibrium adsorption capacity notably decreased with increasing the other ion concentration, suggesting the metals competition for the adsorption sites. For Pb2+, when the initial concentration of Cd2+ varied from 0 to 200 mg/L, the qmax calculated from Langmuir isotherm model reduced from 277.8 to 217.4 mg/g (reduction by 21.7%). While for Cd2+, the qmax reduced from 43.10 to 23.26 mg/ g (reduction by 46.0%) with raising the initial concentration of Pb2+ from 0 to 200 mg/L. In addition, we could found that the interference of Pb2+ with the Cd2+ uptake was much more pronounced, since a distinct reduction of the Cd2+ uptake was observed even at a relatively low Pb2+ concentration, which also confirmed that the FGD-HAP had greater affinity for Pb2+ than for Cd2+. In short, the Pb2+ exhibited stronger antagonism to Cd2+ during the competitive adsorption.
Fig. 9. (a) Pb2+ adsorption in binary system with the presence of Cd2+ and (b) Cd2+ adsorption in binary system with the presence of Pb2+.
3.7. Desorption Considering the economic value, desorption of Pb2+ and Cd2+ from FGD-HAP with different eluants were studied, and the results are shown in Fig. 10. It is clearly found that the desorption efficiency of both metals were maximum (74.3% for Pb2+ and 81.4% for Cd2+) with EDTA solution as the eluant. This phenomenon may be explained by the formation of complex between EDTA and metal ions [49]. When using HNO3 solution, the desorption efficiency was decreased to 10.7% and 36.0% for Pb2+ and Cd2+ respectively. Furthermore, the metals could also be desorbed slightly from FGD-HAP by NaCl or MgCl2 solution. The possible reason is that abundance of Na+ or Mg2+ in solution can exchange with adsorbed Pb2+ or Cd2+. Another interesting finding is that the desorption efficiency of Cd2+ was higher than that of Pb2+ no matter what eluants were used, which once again confirmed the stronger affinity between FGD-HAP and Pb2+.
Fig. 10. Desorption of Pb2+ and Cd2+ from loaded FGD-HAP with various eluants.
3.8. Sorption mechanism To better investigate the mechanism involved in Pb2+ and Cd2+ adsorption, the composition, formation and morphology of final solid (FGD-HAP after adsorption) were measured. Fig. 11 illustrates the XRD patterns of final solid. It can be seen that after Cd2+ adsorption, the final solid still consists of a single phase of HAP. This is due to that the ion-exchange and complexation were the two main mechanisms in the process of Cd2+ adsorption, which
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process of Pb2+ adsorption was the dissolution of HAP followed by re-precipitation of hydroxypyromorphite, which was more stable than the HAP phase [50]. Fig. 12 showed the TEM images and EDX spectrum of final solid. From Fig. 12a, it could be found that the rod-like FGD-HAP dissolved and the needle-shaped product generated. Moreover, a strong peak of Pb was observed in related EDX spectrum, implying the new product was mostly composed of hydroxypyromorphite. On the contrary, after Cd2+ adsorption, the final solid still maintained the original morphology (Fig. 12b), indicating no FGD-HAP dissolved and no Cd5(PO4)3OH formed. The trace of Cd was also observed in related EDX spectrum, confirming the Cd2+ was adsorbed through ion-exchange or complexation reaction. The following equations exhibit the possible mechanisms of adsorption between FGD-HAP and metal ions.
Ca10 ðPO4 Þ6 ðOHÞ2 þ 14Hþ ! 10Ca2þ þ 6H2 PO4 þ 2H2 O 2þ
5Pb Fig. 11. XRD patterns of FGD-HAP after Pb2+ and Cd2+ adsorption. (Initial concentration 500 mg/L).
þ 3H2 PO4 þ H2 O ! 7Hþ þ Pb5 ðPO4 Þ3 OH
Ca10 ðPO4 Þ6 ðOHÞ2 þ xPb 2þ
2½BPOH þ Pb
2þ
! ½BPO 2 Pb þ 2Hþ 2þ
Ca10 ðPO4 Þ6 ðOHÞ2 þ xCd 2þ
2½BPOH þ Cd
! Ca10x Pbx ðPO4 Þ6 ðOHÞ2 þ xCa2þ
! Ca10x Cdx ðPO4 Þ6 ðOHÞ2 þ xCa2þ
! ½BPO 2 Cd þ 2Hþ
ð15Þ ð16Þ ð17Þ ð18Þ ð19Þ ð20Þ
4. Conclusion This work not only reached the goal of reducing waste FGD gypsum but also developed an efficient adsorbent for removal of heavy metals from wastewater. Based on the experiments, the following conclusions can be drawn: (1) The feasibility of using FGD gypsum as Ca source to produce hydroxyapatite was demonstrated according to the characterization of FGD-HAP. (2) The adsorption of two metals was both controlled by chemical process due to the kinetic data were better described by the pseudo-second-order kinetic model. (3) The equilibrium data of both metals correlated well with the Langmuir isotherm model. The maximum adsorption capacity of Pb2+ obtained from Langmuir equation was 277.8 mg/g and 43.10 mg/g for Cd2+. The values of 4G°, 4H° and 4S° revealed adsorption process of Pb2+ or Cd2+ onto FGD-HAP was spontaneous and endothermic. (4) The competitive adsorption studies signified the uptake of Cd2+ was more adversely affected by the simultaneous presence of Pb2+ than that of Pb2+, which demonstrated the FGDHAP had a stronger interaction with Pb2+ than Cd2+. (5) The desorption experiments showed that the FGD-HAP immobilized Pb2+ or Cd2+ could be satisfactorily desorbed by EDTA solution, exhibiting a great potential for FGD-HAP to separate and enrich Pb2+ and Cd2+ from industrial wastewater. Fig. 12. TEM images and EDX spectrum (inner images) of FGD-HAP after Pb2+ (a) and Cd2+ (b) adsorption. (Initial concentration 500 mg/L).
Acknowledgment is consistent with that discussed by Zhao et al. [11]. However, after Pb2+ adsorption, the final solid was mixture of HAP and hydroxypyromorphite (Pb5(PO4)3OH). The dominated mechanism in
The authors wish to thank the Jiangsu Technological Support Plan of China for financial support (No. BE2011834).
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Conversion of waste FGD gypsum into hydroxyapatite for removal of Pb2+ and Cd2+ from wastewater Yubo Yan, Xiaoli Dong, Xiaolei Sun, Xiuyun Sun ⇑, Jiansheng Li, Jinyou Shen, Weiqing Han, Xiaodong Liu, Lianjun Wang ⇑ Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu Province, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 15 January 2014 Accepted 3 May 2014 Available online 19 May 2014
Flue gas desulfurization (FGD) gypsum, a familiar waste generated from coal-fired power plants, was successfully transformed to hydroxyapatite (FGD-HAP) by hydrothermal method. The obtained FGD-HAP was characterized by XRD, FTIR, TEM and BET methods and investigated as adsorbent for removal of Pb2+ and Cd2+ from wastewater. Batch experiments were performed by varying the pH values, contact time and initial metal concentration. The result of pH impact showed that the adsorption of two ions was pH dependent process, and the pH 5.0–6.0 was found to be the optimum condition. The achieved experimental data were analyzed with various kinetic and isotherm models. The kinetic studies displayed that the pseudo-second order kinetic model could describe adsorption processes well with high correlation coefficient, and the Langmuir isotherm model provided the best fit to the equilibrium experimental data. The maximum adsorption capacities calculated from Langmuir equation were 277.8 and 43.10 mg/g for Pb2+ and Cd2+, respectively, which can compete with other adsorbents. The thermodynamic parameters revealed the adsorption processes were endothermic and spontaneous in nature. In binary adsorption, the amount of Cd2+ adsorbed on FGD-HAP decreased by 46.0% with increasing concentration of Pb2+, which was higher than that of Pb2+(21.7%), demonstrating the stronger affinity between FGDHAP and Pb2+. The highest amount of Pb2+ and Cd2+ desorbed from saturated FGD-HAP by EDTA solution confirmed the FGD-HAP was a promising alternative adsorbent in treatment of toxic Pb2+ and Cd2+ wastewater. Ó 2014 Elsevier Inc. All rights reserved.
Keywords: FGD gypsum Adsorption Lead Cadmium Desorption
1. Introduction With the rapid industrial development, the increasing levels of heavy metals are discharged into surface waters. Pollution by heavy metals, such as Pb2+ and Cd2+, has drawn much international attention due to their non-biodegradability, persistence in nature, accumulation in the food chain and toxicity even at low concentration [1]. Pb2+ and Cd2+ are commonly presented in wastewater through their intensive industrial application, for example, mining operations, tanneries, metal plating, ceramics, metal finishing, electroplating, battery manufacture and painting [2]. Consequently, it is necessary to reduce such poisonous heavy metals before wastewater discharge. Conventional technologies for heavy metals removal from wastewater include chemical precipitation, reverse osmosis, electrocoagulation, ion exchange, evaporation ⇑ Corresponding authors. Fax: +86 25 84315941. E-mail (L. Wang).
addresses:
[email protected]
(X.
http://dx.doi.org/10.1016/j.jcis.2014.05.010 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
Sun),
[email protected]
and membrane filtration [3,4], etc. Many of them, however, either not effective enough or are too expensive [5]. Adsorption, compared with above approaches, is considered as a most promising technique for removal of heavy metals from wastewater because of its high removal efficiency, easy operation and less residue production. During the past decade, hydroxyapatite (Ca10(PO4)6(OH)2, HAP) has attracted particular interest in treating heavy metal wastewater due to its high sorption capacity, low water solubility and high stability under oxidizing and reducing conditions [6–10]. Currently, utilization of hydroxyapatite or its nanocomposites to immobilize metal ions from aqueous solution is still a research hotspot. Zhao et al. [11] synthesized three-dimensional HAP nanosheet-assembled microspheres as adsorbent for removal of Pb2+, Cd2+ and Cu2+. Yang et al. [12] prepared Fe3O4@HAP nanocomposite for adsorbing Pb2+, Y3+, Eu3+ and Sb3+. Aliabadi et al. [13] reported that Chitosan/HAp composite nanofiber had high sorption capacity for Pb2+ (296.7 mg/g), Ni2+ (213.8 mg/g) and Co2+ (180.2 mg/g). They also confirmed that Chitosan/HAp composite
Y. Yan et al. / Journal of Colloid and Interface Science 429 (2014) 68–76
nanofiber possessed an effective regenerability. On the other hand, Minh et al. [14] used calcium carbonate as low cost Ca source to synthesize HAP for adsorbing Pb2+. Kongsri et al. [15] converted waste fish scale to nanocrystalline HAP for selenium adsorption. Generally, Ca-contained reagent or Ca-contained biowaste is primarily selected as Ca sources for preparation of HAP. However, little research focused on the Ca-contained industrial waste. Flue gas desulfurization (FGD) gypsum, a common waste from the wet flue gas desulfurization process in coal power plants, is mainly composed of CaSO42H2O, which can be an ideal Ca source. China Association of Environmental Protection Industry (CAEPI) reported over 43 million tons FGD gypsum was produced in 2010 [16]. A considerable portion of FGD gypsum is dumped directly, such disposal not only occupy a large amount of land resources but also cause serious dust pollution and groundwater pollution. Thus, finding an appropriate method to transform waste FGD gypsum into useful materials may be an economically valuable solution to this problem. Up to now, no report has ever been published regarding conversion of waste FGD gypsum into hydroxyapatite and then application for adsorbing heavy metals. Therefore, the objectives of this work were (1) to assess the feasibility of preparing HAP (FGDHAP) using waste FGD gypsum as Ca source, (2) to thoroughly characterize the FGD-HAP by XRD, TEM, FTIR and BET methods, (3) to firstly evaluate the FGD-HAP’s adsorption properties (pH effects, adsorption kinetics, isotherms, thermodynamics and competitive adsorption) and regenerability toward Pb2+ and Cd2+, (4) to deeply investigate the mechanisms involved in adsorption process.
69
transferred to Teflon-lined stainless steel autoclave and kept in an oven at 150 °C for 24 h. (iv) The solid obtained from the cooled autoclave was rinsed several times with Milli-Q water and ethanol, then dried at 80 °C for overnight. (v) The dried sample was milled to pass through a 200 mesh sieve and stored in a desiccator for further analysis. The crystalline phases of the FGD-HAP powder were analyzed by X-ray diffraction (XRD) (D8 Advance, Bruker, Germany) with Cu Ka radiation. The surface area and pore volume of material were determined by BET apparatus (ASAP 2020, Micromeritics Instrument, USA), using nitrogen gas as the adsorbate at 77 K. The morphology of sample was analyzed through transmission electron microscopy (TEM). Infrared spectra were obtained using Fourier transform infrared (FTIR) (IR Prestige-21, Shimadzu, Japan) spectroscopy. The zeta potential of powder was measured by a Zeta meter (ZetaPALS, Brookhaven Instruments, USA), using 0.01 M KCl solution as a background electrolyte. 2.3. Batch adsorption experiments
2. Materials and methods
The adsorption experiments of Pb2+ and Cd2+ were carried out according to batch method. The conical flasks containing 0.05 g FGD-HAP and 50 mL single-component solution at desired concentration and pH were placed in an air bath oscillator with a constant speed of 200 rpm and constant temperature of 20 °C. After adsorption, the mixtures were filtered through 0.22 lm membrane filter and the filtrates were analyzed for residual metal concentration via an inductive coupled plasma atomic emission spectroscopy (ICP-AES) (Optima 7000DV, PerkinElmer, USA). The Pb2+ and Cd2+ adsorption capacities at time t, qt (mg/g), are calculated according to:
2.1. Materials
qt ¼
The FGD gypsum was obtained from Huarun power plant, Nanjing, China. The sample was washed thoroughly with Milli-Q water and then dried at 100 °C for 24 h. The 200 mesh sieve was used to collect FGD gypsum with particle size smaller than 0.04 mm. The components of screened sample were analyzed by energy dispersive X-ray fluorescence (XRF) (LAB CENTER XRF-1800, Shimadzu, Japan) and the results were listed in Table 1. Pb(NO3)2, Cd(NO3)2, (NH4)2HPO4, NH3H2O, NaOH and HNO3, purchased from Nanjing Chemical Reagent Company, are all analytical grade. Simulated stock wastewaters with 1000 mg/L were prepared by respectively dissolving appropriate amounts of Pb(NO3)2 and Cd(NO3)2 in Milli-Q water. The desired concentrations in experiments were prepared by diluting stock wastewaters.
where C0 and Ct (mg/L) are the single-metal concentration in the initial solution and at time t, respectively; V (L) is the volume of solution, and m (g) is the weight of the sample added to the solution. For determination of pH impact on adsorption, the pH of was adjusted using 1 M NaOH or 1 M HNO3 ranging from 2.0 to 6.0 in 200 mg/L Pb2+ or 100 mg/L Cd2+ wastewater. Adsorption kinetics of Pb2+ and Cd2+ were both studied in the range of 10–300 min at optimum pH. Adsorption isotherms were investigated in the range of 50–500 mg/L for Pb2+ and 20–200 mg/L for Cd2+. Adsorption thermodynamic studies were carried out with 250 mg/L Pb2+ and 50 mg/L Cd2+ at 20, 30 and 40 °C, respectively.
ðC 0 C t ÞV m
ð1Þ
2.4. Binary adsorption
2.2. FGD-HAP synthesis and characterization The FGD-HAP powder was synthesized by the following procedures: (i) 2 g FGD gypsum and 100 mL (NH4)2HPO4 solution (0.086 mol P/L) was mixed at room temperature. (ii) The mixture was maintained at a pH value 10–11 by adding NH3H2O and vigorously stirred for 4 h. (iii) After stirring, the mixture was
Table 1 Chemical composition of FGD gypsum by XRF. Composition
Value (wt.%)
CaO SiO2 Al2O3 Fe2O3 MgO SO3
40.1 2.12 1.23 0.29 0.18 55.9
2.4.1. Preferential adsorption Preferential adsorption systems were prepared by solubilizing a combination of Pb–Cd in presence of each metal with other metal present in equal concentrations (20, 50, 100, 150 and 200 mg/L). 2.4.2. Competitive adsorption The competitive adsorption studies of Pb2+ and Cd2+ in binary system consists of two parts: (i) Effect on the adsorption capacity of Pb2+ with Cd2+ present in system. In this part, the initial Pb2+ concentration remained at 50, 100, 150, 200, 250, 300, 400 and 500 mg/L respectively, while the concentration of Cd2+ varied from 50 to 200 mg/L respectively. (ii) Effect on the adsorption capacity of Cd2+ with Pb2+ present in solution. In this section, the initial Cd2+ concentration fixed at 20, 40, 60, 80, 100, 120, 150 and 200 mg/L respectively, whereas the other one ranged from 50 to 200 mg/L respectively.
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2.5. Desorption experiments Desorption experiments were carried out to estimate the effects of different eluants on desorption of Pb2+ or Cd2+ from FGD-HAP. Initially, FGD-HAP was centrifuged after equilibrium experiments, then FGD-HAP loaded Pb2+ or Cd2+ was rinsed and dried at 80 °C for overnight. Subsequently, 0.05 g collected FGD-HAP was added into 25 mL EDTA (0.01 M), HNO3 (0.01 M), MgCl2 (0.1 M) and NaCl (0.1 M) solution, respectively, shaking at 22 ± 2 °C for overnight. After experiments, the concentration of Pb2+ or Cd2+ in each eluant was measured by ICP-AES. 3. Results and discussion 3.1. Characterization of FGD-HAP Fig. 1 shows wide-angle XRD pattern of FGD-HAP. Eight characteristic peaks of HAP were observed at 2h = 25.9°, 31.8°, 32.1°, 32.9°, 34.1°, 39.8°, 46.7° and 49.5°, which could be indexed as (0 0 2), (2 1 1), (1 1 2), (3 0 0), (2 0 2), (3 1 0), (2 2 2) and (2 1 3) reflections, respectively. These typical peaks are matched well with JCPDS file No. 09-432, indicating the structure of FGD-HAP belongs to the hexagonal P63/m space group with lattice constant of a = b = 9.418 Å, c = 6.884 Å. FTIR spectra of FGD-HAP are shown in Fig. 2. The adsorption bands at 1093, 1033, 962, 601 and 565 cm1 represent phosphate groups [17,18]. The peaks observed at 3570 and 634 cm1 are indicative of the hydroxyl groups [15,17,18], which supported adsorbing metal ions on FGD-HAP through surface complexation. A bi-modal peak at 1460 and 1420 cm1 as well as single peak at 873 cm1 correspond to carbonate groups, indicating carbonate partly substituted for the phosphate while exposed FGD-HAP to atmosphere [14,19]. In addition, two weak broad bands were observed at 3450 and 1637 cm1 can be attributed to the adsorbed water [15]. Fig. 3a and b show the TEM images of FGD-HAP. The FGD-HAP particles (15–25 nm in diameter and 100–250 nm in length) present uniform rod-like shape and most particles seem to aggregate together by melting their surfaces. Fig. 4 shows the textural properties of FGD-HAP. As can be seen from figure, the FGD-HAP possessed the mesoporous structure, high specific surface area and large pore volume, which were beneficial to ion-exchange and diffusion [20].
Fig. 2. FTIR spectra of FGD-HAP.
3.2. Effect of pH The solution pH is one of the most important factors for the adsorption of heavy metals, which would significantly influence
Fig. 3. TEM images of FGD-HAP.
Fig. 1. XRD pattern of FGD-HAP.
both the hydrolytic species of metal ions and the surface charge of adsorbent. In order to prevent precipitation of Pb2+ and Cd2+ in the form of metal hydroxides, the pH ranges in this study were both chosen as 2.0–6.0. The effect of pH was shown in Fig. 5. It was noticed that the plots of Pb2+ and Cd2+ had similar variation trend. The adsorption amount of two metals increased obviously with increasing pH from 2.0 to 4.0, which could be reasonably explained by: (i) the activated sites reduce with the partial dissolution of FGD-HAP in strongly acidic media, (ii) the competition between protons and metal ions for the available adsorption sites
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Fig. 4. Textural properties of the FGD-HAP.
Fig. 5. Effect of pH on the uptake of Pb2+ and Cd2+ and the zeta potential of FGDHAP at various pH values.
in lower pH value [21], (iii) the negative charge on the FGD-HAP surface increases and thus attracts the positively charged ions more strongly [22]. At pH above 4.0, the adsorption capacity of Pb2+ had a slight change while the Cd2+ exhibited a slow increase initially and then remained constant. Therefore, the optimum pH for adsorption of Pb2+ and Cd2+ was determined as 5.0–6.0 and was selected for the subsequent experiments. 3.3. Sorption kinetics The effect of contact time on the adsorption of Pb2+ or Cd2+ was illustrated in Fig. 6a. It was evident that the adsorption amount of Pb2+ and Cd2+ increased steeply within the first 60 min. This can be interpreted by the fact that a large number of active sites on the FGD-HAP surface are available at the beginning of the adsorption. Afterward the uptake of metal ions increased slightly and finally reached equilibrium. This appearance is attributed to the availability of binding sites are exhausted gradually. The equilibrium time for both metals was 150 min, which was chosen for the further experiments. In order to explicate in depth the controlling mechanism of the adsorption process such as mass transfer and chemical reaction, the pseudo-first order and pseudo-second order [23,24] models were employed to analyze the experimental data, and their linear forms can be expressed as:
Pseudo-first-order : logðqe qt Þ ¼ log qe
k1 t 2:303
ð2Þ
Fig. 6. (a) Effect of time on the uptake of Pb2+ and Cd2+ onto FGD-HAP at 20 °C, (b) pseudo-first order kinetic plots for adsorption of Pb2+ and Cd2+ and (c) pseudosecond order kinetic plots for adsorption of Pb2+ and Cd2+.
Pseudo-second-order :
t 1 t ¼ þ qt k2 q2e qe
ð3Þ
where qe and qt are the adsorption capacity (mg/g) at equilibrium and at time t (min), k1 (min1) and k2 (g/(mg min)) are the rate constants for the pseudo-first order and pseudo-second order, respectively. Furthermore, when t ? 0, the initial sorption rate (h mg/(g min)) of Pb2+ or Cd2+ can also be defined by the following equation:
h ¼ k2 q2e
ð4Þ
The kinetic parameters (Table 2) of pseudo-first order model and pseudo-second order model were respectively calculated from the linear plot of log(qe qt) versus t (Fig. 6b) and the plot of t/qt versus
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Y. Yan et al. / Journal of Colloid and Interface Science 429 (2014) 68–76
t (Fig. 6c). Based on Table 2 and Fig. 6b–c, it can be found that the correlation coefficients of the pseudo-second order model (0.9984 for Pb2+ and 0.9995 for Cd2+) are higher than the results obtained from the pseudo-first order model (0.9753 for Pb2+ and 0.9361 for Cd2+). The theoretical qe calculated from pseudo-second order equation were 212.8 and 38.31 mg/g for Pb2+ and Cd2+ respectively, which were extremely close to the experimental data (199.5 and 36.70 mg/g). However, the calculated qe from the pseudo-first order equation (226.5 mg/g for Pb2+ and 14.98 mg/g for Cd2+) were obviously different from the experimental data. Therefore, the sorption behavior of Pb2+ and Cd2+ can be well described by pseudo-second order model, suggesting that the rate-limiting step for adsorption process was chemical sorption [25]. Additionally, the initial sorption rate of Pb2+ was 13.13 mg/(g min) while the Cd2+ was 3.23 mg/(g min), indicating that the adsorption of Pb2+ at the beginning was much faster than Cd2+. Fig. 7. Adsorption isotherms of Pb2+ and Cd2+ on FGD-HAP.
3.4. Sorption isotherms Adsorption equilibrium data, expressed by the mass of adsorbate adsorbed per unit weight of adsorbent and liquid phase equilibrium concentration of adsorbate, were usually represented by adsorption isotherms, which is essential for the practical design and critical for predicting the maximum adsorption capacity of adsorbent. In this study, the adsorption isotherms of Pb2+ and Cd2+ on FGD-HAP are plotted in Fig. 7. It can be seen that the adsorption capacity of two metals increased initially with equilibrium concentration and then reached saturation. The uptake of Pb2+ is throughout higher than Cd2+, indicating that the interaction between FGD-HAP and Pb2+ is stronger than that of Cd2+. For a better interpretation of adsorption behaviors, the Langmuir, and Freundlich isotherm models were firstly applied to fit the experimental equilibrium data. The Langmuir model [26] assumes adsorption occurs on a homogenous surface and no interaction between the adsorbed in the plane of the surface. The equation of the Langmuir isotherm is as follows:
Ce Ce 1 ¼ þ qe qmax bqmax
ð5Þ
where Ce is the equilibrium concentration of Pb2+ or Cd2+ (mg/L), qe is the amount of Pb2+ or Cd2+ adsorbed under equilibrium (mg/g), qmax is the maximum adsorption capacity (mg/g), b is a Langmuir constant related to the affinity of the binding sites and energy of adsorption (L/g). The Freundlich model [27] is commonly used to describe adsorption characteristics for heterogeneous surface. The equation of the Freundlich isotherm can be written as:
log qe ¼ log K F þ
1 log C e n
ð6Þ
where KF is the Freundlich constant and gives the capacity of adsorbent (mg/g (1/mg1/n)) and n is Freundlich exponent that provides an indication of favorability of the adsorption process [28]. The calculated Langmuir and Freundlich constants are summarized in Table 3. By comparing the R2 values, it can be discovered that the adsorption processes of both Pb2+ and Cd2+ were accurately described by the Langmuir model. The maximum adsorption
capacity (qmax) deduced from Langmuir equation were 277.8 and 43.10 mg/g for Pb2+ and Cd2+, respectively, which are similar to or even greater than those of other adsorbents (Table 4), indicating the FGD-HAP is a promising adsorbent for removal of Pb2+ or Cd2+ from the real wastewater. The n value obtained from Freundlich equation is an indication of the favorability of adsorption. Values of n in this study were both in the range 2–10 (6.105 for Pb2+ and 3.880 for Cd2+), representing favorable adsorption and high affinity between FGD-HAP and metals [38]. The essential characteristic of Langmuir isotherm can be expressed in terms of a dimensionless separation factor, RL:
RL ¼
1 1 þ bC 0
ð7Þ
where C0 is denoted to the highest initial Pb2+ or Cd2+ concentration. The values of RL reflect the adsorption process to be either irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1). The plots of RL values against initial concentrations are presented in Fig. 8. All RL values were between 0 and 1, implying Pb2+ and Cd2+ are both favorably adsorbed by FGD-HAP, which was in good agreement with the results regarding the n values. For each metal, the RL values decreased with initial concentration increasing, indicating the adsorption was more beneficial at higher initial concentration. Moreover, the much lower RL values for Pb2+ suggested that the FGD-HAP was more effective in removing Pb2+ than Cd2+. In order to deeply evaluate the mechanism of Pb2+ and Cd2+ adsorption on FGD-HAP, as well as distinguish between physisorption and chemisorption, the Dubinin–Radushkevich (D–R) model [39] was chosen to apply on sorption study. The linear expressions of this model can be represented by:
ln qe ¼ ln qm be2
ð8Þ
where qm is the theoretical saturation adsorption capacity (mg/g), b is a constant correlated with the mean free energy of adsorption (mol2/J2), e is the Polanyi potential, which is equal to:
e ¼ RT ln 1 þ
1 Ce
ð9Þ
Table 2 Pseudo-first order and pseudo-second order kinetic parameters for the adsorption of Pb2+ and Cd2+ on FGD-HAP. Adsorbate
Pb2+ Cd2+
qe,exp (mg/g)
199.5 36.70
Pseudo-first order kinetic model
Pseudo-second order kinetic model
k1 (min1)
qe,cal (mg/g)
R2
k2 (g/mg min)
qe,cal (mg/g)
R2
4.0 102 1.9 102
226.5 14.98
0.9753 0.9361
3.3 104 2.4 103
212.8 38.31
0.9984 0.9995
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Y. Yan et al. / Journal of Colloid and Interface Science 429 (2014) 68–76 Table 3 Langmuir, Freundlich and Dubinin–Radushkevich parameters for the adsorption of Pb2+ and Cd2+onto FGD-HAP at 20 °C. Adsorbate
2+
Pb Cd2+
Langmuir model
Freundlich model 2
qmax (mg/g)
b (L/mg)
R
277.8 43.10
0.4865 0.0825
0.9992 0.9953
KF (mg/g(1/mg
))
127.8 11.76
Table 4 Comparison of adsorption capacity (qmax) for Pb2+ and Cd2+ with various adsorbents. Metals
Adsorbent
qmax (mg/g)
Reference
Pb2+
FGD-HAP Alkali treated tea residue HAp/PU composite foams Activated carbon CHAP US-MWCNT
277.8 64.10 150 31.2 94.3 227.3
This study [29] [30] [31] [32] [33]
FGD-HAP Orange peel Peat Hazel nut shells Bamboo charcoal Commercial hydroxyapatite
43.10 35.71 22.50 5.42 12.08 290.0
This study [34] [35] [36] [37] [7]
Cd2+
Dubinin–Radushkevich model
1/n
R
qm (mg/g)
E (kJ/mol)
R2
6.105 3.880
0.6079 0.9609
436.6 80.99
18.90 13.87
0.6260 0.9631
3.5. Thermodynamic study Thermodynamic parameters, such as the Gibbs free energy change (4G°, kJ/mol), the enthalpy change (4H°, kJ/mol) and the entropy change (4S°, kJ/mol/K) are critical for understanding whether the adsorption is endothermic or exothermic, and the spontaneity of the adsorption process, which are determined by following equations [42]:
Cs Ce
ð11Þ
DG ¼ RT ln K ln K ¼
Fig. 8. Separation factor, RL, for the adsorption of Pb2+ and Cd2+ at different initial concentrations on FGD-HAP.
where R is the universal gas constant (8.3145 J/mol/K), and T is the absolute temperature in Kelvin (K). The constant b gives information about the mean free energy (E, mol2/kJ2) of sorption per molecule of the sorbate when it is transferred to the surface of the solid from infinity in the solution and can be computed by [40]:
1 E ¼ pffiffiffiffiffiffi 2b
n
FGD-HAP by the Pb2+ or Cd2+ occurring in the wastewater. In addition to ion exchange, some other chemical reactions may be occurred on the surface of FGD-HAP for Pb2+ removal.
K¼
Note: Hap/PU = hydroxyapatite/polyurethane. CHAP = synthetic carbonate hydroxyapatite. US-MWCNT = ultrasonic bath – multiwalled carbon nanotubes.
2
ð12Þ
DS DH R RT
ð13Þ
where K is the adsorption equilibrium constant, Cs is the amount of Pb2+ or Cd2+ adsorbed on per weight unit of FGD-HAP after equilibrium (mg/g), Ce is the Pb2+ or Cd2+ concentration in solution at equilibrium (mg/L), R is the universal gas constant (8.3145 J/mol/K), and T is the temperature (K). The parameters of thermodynamics are listed in Table 5. The values of 4G° were below zero at all temperatures, suggesting that the adsorption of Pb2+ and Cd2+ on FGD-HAP are spontaneous and thermodynamically favorable. The decrease in 4G° values with the rise in temperature indicates more efficient adsorption at higher temperature. In addition, the Gibbs free energy of Pb2+ was greater than that of Cd2+ illustrating the stronger affinity for Pb2+ adsorption on FGD-HAP. The positive value of 4H° confirmed that the adsorption processes of Pb2+ and Cd2+ are endothermic. Besides, according to the values of 4H°, the procedure can be classified as physical (2.1 kJ/mol < 4H° < 20.9 kJ/mol) and chemical (20.9 kJ/ mol < 4H° < 418.4 kJ/mol) process [43]. The values of 4H° for Pb2+ and Cd2+ were 107.3 and 34.69 kJ/mol respectively, concluding that the nature of Pb2+ and Cd2+ adsorption are chemisorption, which is consistent with the results of kinetics study and D–R model. The positive 4S° values reflected the increase of randomness at the solid-solute interface during the adsorption of metal ions onto FGD-HAP. Similar result was found from the literature [44].
ð10Þ
The value of E is very useful in predicting the type of adsorption. If the value is below 8 kJ/mol then the adsorption is physical in nature, between 8 and 16 kJ/mol the adsorption is due to ion exchange and over 16 kJ/mol the adsorption type can be explained by a stronger chemisorption than ion exchange [41]. The values of E found in the present work were 18.90 and 13.87 kJ/mol for Pb2+ and Cd2+, respectively. So, it seems that the ion exchange mechanism is dominating in both adsorption processes. The probable metals adsorption is due to the exchange of Ca2+ existing in
Table 5 Thermodynamic parameters for the adsorption of Pb2 + and Cd2 + on FGD-HAP. Metals
T (°C)
4G° (kJ/mol)
4H° (kJ/mol)
4S° (kJ/mol/K)
Pb2+
20 30 40
5.97 7.46 13.75
107.3
0.38
20 30 40
0.29 0.85 2.70
34.69
0.12
Cd2+
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3.6. Binary adsorption study In practice, the wastewater normally contains Pb2+ and Cd2+ simultaneously. Hence, it is very significant to investigate the binary adsorption of metals and to know if there is competition between Pb2+ and Cd2+ for the available binding sites. 3.6.1. Preferential adsorption The separation factor, aPb Cd , was employed to assess the preference of FGD-HAP for one of the two ions in binary systems, which was defined by [45]:
aPb Cd ¼
qPb C Cd qCd C Pb
ð14Þ
If the Pb2+ is preferred, the factor is greater than unity, while if the Cd2+ is preferred, the factor is smaller than unity. The calcuPb lated aPb Cd varied in the range of 36.74 < aCd < 133.8, highlighting 2+ that the preference of FGD-HAP for Pb was higher than that of Cd2+, i.e., the greater affinity of Pb2+ than Cd2+, in accord with the results of binding energy (Langmuir constant, bPb > bCd). This could be attributed to (i) the smaller hydrated radii of Pb2+ (0.261 nm) than Cd2+ (0.275 nm), resulting in the Pb2+ is more accessibility to the FGD-HAP’s surface and pores, (ii) the higher electronegativity of Pb2+ (2.33) than Cd2+ (1.69), leading to more strongly attraction between Pb2+ and FGD-HAP’s surface, (iii) the ionic radii of Pb2+ (0.118 nm) is larger than Ca2+ (0.099 nm) while that of Cd2+ (0.097 nm) is smaller than Ca2+, causing the Cd2+ has less chance to be incorporated into the FGD-HAP structure [46–48]. 3.6.2. Competitive adsorption The adsorption isotherm of Pb2+ or Cd2+ with the presence of another metal ion was shown in Fig. 9a and b, respectively. From the figures, it can be observed that for each ion, the equilibrium adsorption capacity notably decreased with increasing the other ion concentration, suggesting the metals competition for the adsorption sites. For Pb2+, when the initial concentration of Cd2+ varied from 0 to 200 mg/L, the qmax calculated from Langmuir isotherm model reduced from 277.8 to 217.4 mg/g (reduction by 21.7%). While for Cd2+, the qmax reduced from 43.10 to 23.26 mg/ g (reduction by 46.0%) with raising the initial concentration of Pb2+ from 0 to 200 mg/L. In addition, we could found that the interference of Pb2+ with the Cd2+ uptake was much more pronounced, since a distinct reduction of the Cd2+ uptake was observed even at a relatively low Pb2+ concentration, which also confirmed that the FGD-HAP had greater affinity for Pb2+ than for Cd2+. In short, the Pb2+ exhibited stronger antagonism to Cd2+ during the competitive adsorption.
Fig. 9. (a) Pb2+ adsorption in binary system with the presence of Cd2+ and (b) Cd2+ adsorption in binary system with the presence of Pb2+.
3.7. Desorption Considering the economic value, desorption of Pb2+ and Cd2+ from FGD-HAP with different eluants were studied, and the results are shown in Fig. 10. It is clearly found that the desorption efficiency of both metals were maximum (74.3% for Pb2+ and 81.4% for Cd2+) with EDTA solution as the eluant. This phenomenon may be explained by the formation of complex between EDTA and metal ions [49]. When using HNO3 solution, the desorption efficiency was decreased to 10.7% and 36.0% for Pb2+ and Cd2+ respectively. Furthermore, the metals could also be desorbed slightly from FGD-HAP by NaCl or MgCl2 solution. The possible reason is that abundance of Na+ or Mg2+ in solution can exchange with adsorbed Pb2+ or Cd2+. Another interesting finding is that the desorption efficiency of Cd2+ was higher than that of Pb2+ no matter what eluants were used, which once again confirmed the stronger affinity between FGD-HAP and Pb2+.
Fig. 10. Desorption of Pb2+ and Cd2+ from loaded FGD-HAP with various eluants.
3.8. Sorption mechanism To better investigate the mechanism involved in Pb2+ and Cd2+ adsorption, the composition, formation and morphology of final solid (FGD-HAP after adsorption) were measured. Fig. 11 illustrates the XRD patterns of final solid. It can be seen that after Cd2+ adsorption, the final solid still consists of a single phase of HAP. This is due to that the ion-exchange and complexation were the two main mechanisms in the process of Cd2+ adsorption, which
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process of Pb2+ adsorption was the dissolution of HAP followed by re-precipitation of hydroxypyromorphite, which was more stable than the HAP phase [50]. Fig. 12 showed the TEM images and EDX spectrum of final solid. From Fig. 12a, it could be found that the rod-like FGD-HAP dissolved and the needle-shaped product generated. Moreover, a strong peak of Pb was observed in related EDX spectrum, implying the new product was mostly composed of hydroxypyromorphite. On the contrary, after Cd2+ adsorption, the final solid still maintained the original morphology (Fig. 12b), indicating no FGD-HAP dissolved and no Cd5(PO4)3OH formed. The trace of Cd was also observed in related EDX spectrum, confirming the Cd2+ was adsorbed through ion-exchange or complexation reaction. The following equations exhibit the possible mechanisms of adsorption between FGD-HAP and metal ions.
Ca10 ðPO4 Þ6 ðOHÞ2 þ 14Hþ ! 10Ca2þ þ 6H2 PO4 þ 2H2 O 2þ
5Pb Fig. 11. XRD patterns of FGD-HAP after Pb2+ and Cd2+ adsorption. (Initial concentration 500 mg/L).
þ 3H2 PO4 þ H2 O ! 7Hþ þ Pb5 ðPO4 Þ3 OH
Ca10 ðPO4 Þ6 ðOHÞ2 þ xPb 2þ
2½BPOH þ Pb
2þ
! ½BPO 2 Pb þ 2Hþ 2þ
Ca10 ðPO4 Þ6 ðOHÞ2 þ xCd 2þ
2½BPOH þ Cd
! Ca10x Pbx ðPO4 Þ6 ðOHÞ2 þ xCa2þ
! Ca10x Cdx ðPO4 Þ6 ðOHÞ2 þ xCa2þ
! ½BPO 2 Cd þ 2Hþ
ð15Þ ð16Þ ð17Þ ð18Þ ð19Þ ð20Þ
4. Conclusion This work not only reached the goal of reducing waste FGD gypsum but also developed an efficient adsorbent for removal of heavy metals from wastewater. Based on the experiments, the following conclusions can be drawn: (1) The feasibility of using FGD gypsum as Ca source to produce hydroxyapatite was demonstrated according to the characterization of FGD-HAP. (2) The adsorption of two metals was both controlled by chemical process due to the kinetic data were better described by the pseudo-second-order kinetic model. (3) The equilibrium data of both metals correlated well with the Langmuir isotherm model. The maximum adsorption capacity of Pb2+ obtained from Langmuir equation was 277.8 mg/g and 43.10 mg/g for Cd2+. The values of 4G°, 4H° and 4S° revealed adsorption process of Pb2+ or Cd2+ onto FGD-HAP was spontaneous and endothermic. (4) The competitive adsorption studies signified the uptake of Cd2+ was more adversely affected by the simultaneous presence of Pb2+ than that of Pb2+, which demonstrated the FGDHAP had a stronger interaction with Pb2+ than Cd2+. (5) The desorption experiments showed that the FGD-HAP immobilized Pb2+ or Cd2+ could be satisfactorily desorbed by EDTA solution, exhibiting a great potential for FGD-HAP to separate and enrich Pb2+ and Cd2+ from industrial wastewater. Fig. 12. TEM images and EDX spectrum (inner images) of FGD-HAP after Pb2+ (a) and Cd2+ (b) adsorption. (Initial concentration 500 mg/L).
Acknowledgment is consistent with that discussed by Zhao et al. [11]. However, after Pb2+ adsorption, the final solid was mixture of HAP and hydroxypyromorphite (Pb5(PO4)3OH). The dominated mechanism in
The authors wish to thank the Jiangsu Technological Support Plan of China for financial support (No. BE2011834).
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