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Investigation on the efficiency and mechanism of Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles
Accepted Manuscript Title: Investigation on the efficiency and mechanism of Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles Author: Chunmei Xiong Wei Wang Fatang Tan Fan Luo Jianguo Chen Xueliao Qiao PII: DOI: Reference:
S0304-3894(15)00627-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.08.008 HAZMAT 17017
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
1-5-2015 1-8-2015 4-8-2015
Please cite this article as: Chunmei Xiong, Wei Wang, Fatang Tan, Fan Luo, Jianguo Chen, Xueliao Qiao, Investigation on the efficiency and mechanism of Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation on the efficiency and mechanism of Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles
Chunmei Xiong, Wei Wang*, Fatang Tan, Fan Luo, Jianguo Chen, Xueliao Qiao
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, 430074, Hubei, P.R. China * Corresponding authors. Wei Wang, Tel/fax: +86 27 87541540, E-mail: [email protected]
Highlights
MgO nanoparticles exhibited high removal abilities towards Cd(II) and Pb(II). The mechanism for Cd(II) and Pb(II) removal was precipitation and adsorption. The adsorbed heavy metals on MgO were difficult to be desorbed by water washing.
Abstract
In this study, the removal of Cd(II) and Pb(II) from aqueous solutions using MgO nanoparticles prepared by a simple sol-gel method was investigated. The efficiency of Cd(II) and Pb(II) removal was examined through batch adsorption experiments. For the
single adsorption of Cd(II) and Pb(II), The adsorption kinetics and isotherm data obeyed well Pseudo-second-order and Langmuir models, indicating the monolayer chemisorption of heavy metal ions. The maximum adsorption capacities calculated by Langmuir equation were 2294 mg/g for Cd(II) and 2614 mg/g for Pb(II), respectively. The adsorption process was controlled simultaneously by external mass transfer and intraparticle diffusion. In the binary system, a competitive adsorption was observed, showing preference of adsorption followed Pb(II) > Cd(II). Significantly, the elution experiments confirmed that neither Cd(II) nor Pb(II) could be greatly desorbed after water washing even for five times. XRD and XPS measurements revealed the mechanism of Cd(II) and Pb(II) removal by MgO nanoparticles was mainly involved in precipitation and adsorption on the surface of MgO, resulting from the interaction between active sites of MgO and heavy metal ions. Easy preparation, remarkable removal efficiency and firmly adsorptive ability make the MgO nanoparticles to be an efficient material in the treatment of heavy metal-contaminated water.
Keywords: MgO nanoparticle; cadmium ion; lead ion; adsorption; precipitation 1.
Introduction Nowadays, heavy metal ions, such as Pb(II), Cd(II), and Hg(II), especially in water,
have posed a serious threat to public health and ecological system due to their high toxicity, non-biodegradation and long-term damage by accumulation [1]. Therefore, the efficient removal of heavy metal ions from aqueous systems is of great significance in protecting the earth’s ecosystems. Up to now, various methods such as chemical
precipitation, adsorption, evaporation, ion exchange, membrane filtration, electrodialysis and reverse osmosis, have been developed to treat water containing heavy metals [2-5]. Among them, adsorption has been universally accepted as one of the most commonly practiced techniques for the removal of heavy metal ions because of its simple and stable handling process, high efficient wastewater treatment, absence of secondary pollution and low operating cost [6]. A number of materials including activated carbon [7], bentonite [8], magnetite [9], chitosan [10], etc., have been reported to be capable of adsorbing heavy metal ions from aqueous solutions. However, these adsorbents usually suffer from low adsorption capacities or removal efficiencies of heavy metal ions. Therefore, a lot of efforts are still continued to explore highly efficient adsorbents for the removal of heavy metal ions. Recently, some metal oxides or metal hydroxides as adsorbents have attracted much attention due to their low cost, environmentally benign nature and excellent adsorption properties for environmental applications [11-13]. Magnesium oxide (MgO), a typical metal oxide adsorbent, is a promising candidate for the treatment of water pollutants [14, 15]. Particularly, nanosized MgO has exhibited higher efficiency and faster adsorption rate in wastewater treatment, owing to its higher surface areas and much more surface active sites than bulk materials. Feng et al. [16] reported mesoporous MgO nanosheets had the maximum adsorption capacity of 1684.25 mg/g for Ni(II) removal from aqueous solutions. Tian et al. [17] found porous hierarchical MgO exhibited superb adsorption performance towards Congo red dye, with the maximum adsorption capacity of 2409 mg/g. In short, micro/nanosized MgO can efficiently remove heavy metal ions [18-20], fluoride [21],
toxic dyes [22, 23], etc. The pioneering work inspires us to further explore the adsorption capacities of MgO nanoparticles for heavy metal ions in solutions. On the other hand, the mechanism of heavy metal ions removal using MgO as an adsorbent is still controversial. Campbell and Starr [24] proved that metal (Cu, Ag and Pb) could combine with the oxygen or magnesium of MgO in the form of covalent bond. Zhu and Li [14] found that the dominant mechanism of Cd(II) removal was precipitation of Cd(OH)2 on the surface of MgO. Cao et al. [25] proposed a removal mechanism involving solid−liquid interfacial cation exchange between MgO and Pb(II) or Cd(II) to form PbO or CdO. Mahdavi et al. [26] proposed MgO nanoparticles had a much greater metal adsorption capacity than TiO2 and Al2O3 nanoparticles due to the occurrence of adsorption and precipitation of heavy metal ions with MgO. Thus, an in-depth investigation into the mechanism of heavy metal ions removal using MgO nanoparticles is needed urgently. In this work, the removal capacity and mechanism of Cd(II) and Pb(II) from aqueous solutions using MgO nanoparticles prepared by a simple sol-gel method were systematically investigated. The influences of contact time, initial metal ion concentration and solution pH on removal of Cd(II) and Pb(II) from single and binary metal solutions were examined in details. Besides, the adsorbed products were further analyzed by XRD, XPS and SEM equipped with an EDX analyzer to study the mechanism of heavy metal removal and the adsorption action between heavy metal ions and MgO nanoparticles.
2.
Materials and methods
2.1. Materials
Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) was obtained from Shanghai Experimental Reagent Co., Ltd. Citric acid (C6H8O7) was purchased from Sinopharm Chemical Reagent Co., Ltd. Cadmium chloride(CdCl2·2.5H2O) and lead nitrate (Pb(NO3)2) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All the chemical reagents were of analytical grade and used without further purification in this study. Doubly distilled (DI) water was used throughout the experiments.
2.2. Preparation of MgO nanoparticles The preparation procedure for MgO nanoparticles was similar to our previous work [27]. Typically, 0.02 mol of Mg(NO3)2·6H2O was dissolved in 20 mL of DI water. Subsequently, an aqueous solution containing equimolar amount of C6H8O7 was added into the above solution under stirring. Then the mixture was placed in a water bath at 80 °C with continuous stirring until it completely turned into a gel. Finally, the wet gel was dried at 150 °C to obtain a white fluffy precursor and followed by calcination at 600 °C for 1 h to yield MgO nanoparticles.
2.3. Characterization of MgO nanoparticles Crystal structure of samples was characterized by XRD (Philips/X’ Pert PRO). Morphology of samples was observed using transmission electron microscope (TEM, FEI Tecnai G2 20) and scanning electron microscope (SEM, FEI/Nova NanoSEM 450) equipped with an energy-dispersive X-ray (EDX) analyzer (Oxford/X-Max 50). Molecular structure was analyzed by Fourier tranaform infrared (FTIR) spectrometer (Bruker
VERTEX70). Brunauer-Emmett-Teller (BET) surface area measurements were carried out on a Micromeritics ASAP 2020M surface area analyzer. X-ray photoelectron spectroscopy (XPS) analysis was acquired on a Kratos/Axis Ultra DLD-600W spectrometer.
2.4. Batch adsorption experiments Stock solutions with different concentration of Cd(II) and Pb(II) were prepared using Pb(NO3)2 and CdCl2·2.5H2O as the sources of heavy metal ions, respectively. The pH of the stock solutions was adjusted using NaOH (0.1 M) and HNO3 (0.1 M). Adsorption kinetics experiments were conducted in a series of 250 mL beakers containing 0.01 g (100 mg/L) of MgO nanoparticles and 100 mL (100 mg/L) metal ion solutions with continuous stirring at pH 5 and 25 °C. After different time intervals, aliquots of the samples were collected and filtered through a 0.25 µm membrane. A PDV6000 plus (Cogent Environmental Ltd.) analyzer was used to measure the concentrations of metal ions in the remaining solutions. Adsorption isotherms experiments were conducted following similar procedure except using metal ion solutions with different concentrations. In binary metal system, the solutions containing the same initial concentrations of Cd(II) and Pb(II) were used to investigate the competitive adsorption of Cd(II) and Pb(II) on MgO nanoparticles. The amounts of Cd(II) and Pb(II) adsorbed on MgO nanoparticles, qt (mg/g) are calculated by using Equation (1): (1) where C0 (mg/L) is the initial metal ion concentration and C (mg/L) is residual metal ion
concentration in solution at time t, V (L) and W (g) are the volume of the solution and the weight of MgO nanoparticles, respectively. In the equation (1), Ce and qe are the equilibrium concentration and the adsorption amount at equilibrium time, respectively. Effects of initial pH values (pH= 2.0, 3.0, 4.0 and 5.0) were also studied in the single and binary systems in the similar procedure except using 250 mg/L of metal ion solutions. All the above experiments were performed in duplicate.
2.5. Elution experiments of the adsorption products Firstly, 0.03 g (1000 mg/L) of MgO nanoparticles was preloaded by 30 mL (1000 mg/L) of metal ion solutions according to the above adsorption procedure. Then, the resulting powders were dispersed in 30 mL of DI water under stirring for 4h at 25 °C. Finally, the mixture was centrifuged and the concentrations of heavy metal ions in the supernatant were measured to calculate the eluted amounts of Cd(II) and Pb(II) from the products after adsorption. 3.
Results and discussion
3.1. Structures and morphologies of MgO nanoparticles The XRD pattern and SEM image of the as-prepared MgO nanoparticles were shown in Fig. 1(a). Five diffraction peaks at 2θ= 36.8°, 42.8°, 62.1°, 74.5° and 78.4° could be indexed to the (111), (200), (220), (311), and (222) planes of MgO with a periclase structure (JCPDS 87-0653). No impurity peaks were observed, indicating that the products were pure MgO nanoparticles. Moreover, the average grain size of MgO was calculated to be 20.9 nm by using the Scherrer equation (Table 1S), according to the full width at
half-maximum (FWHM) of all the peaks. From the SEM image and TEM image (Fig. 1S(a)), the prepared MgO nanoparticles were found to exhibit plate-like nanoparticles with relatively uniform size distribution. The average particle size of MgO nanoparticles was measured to be 20.23 nm by using Nano Measurer 1.2 software, which corresponded to the result of XRD analysis. FTIR spectra of the as-prepared MgO nanoparticles was shown in Fig. 1S(b). A absorption band at 3438 cm-1 was assigned to the adsorbed water during sample preparation. Bands in the range of 1458-1640 cm-1 were attributed to the -OH stretching mode of water molecule [28]. Two characteristic bonds at 415 and 695 cm-1 occurred which confirmed the presence of Mg-O groups. The N2 adsorption-desorption isotherms and pore size distribution of the as-prepared MgO nanoparticles were shown in Fig. 1(b). The results revealed that the MgO nanoparticles possessed high BET surface area of 101.95 m2/g and an average pore diameter of 4.9 nm. The single point surface area at P/P0 = 0.299 was 112.19 m2/g and the pore volume at P/P0 =0.995 was 0.12 cm3/g. 3.2. Effect of contact time The effect of contact time on the single and binary adsorption of Cd(II) and Pb(II) on MgO nanoparticles was examined. As shown in Fig. 2(a) and (b), initially the adsorption amount qt increased quickly, then reached equilibria in about 30 min and 15 min for the single adsorption of Cd(II) and Pb(II), respectively. Clearly, the time to reach equilibrium adsorption is much shorter than those of zeolite [29, 30], activated carbon [31], and coal fly ash [32]. Such fast adsorption rate could be attributed to the abundance of active sites on the surface of MgO nanoparticles. Also, the binary adsorption of Cd(II) and Pb(II)
exhibited a similar trend (Fig. 2(c)). Hence, a contact time of 60 min was sufficient to reach equilibrium for MgO nanoparticles in the adsorption of heavy metal ions, which was selected in the further experiments. In addition, the change of solution pH was also measured during the adsorption process and shown in Fig. 2. The solution pH values gradually increased with time and reached equilibria in about 180 min, which was assigned to the hydration of MgO nanoparticles in aqueous solutions. In order to understand the characteristics of the adsorption process, three kinetic models detailed in the supplementary data, including pseudo-first-order [33], pseudo-second-order [34], and Weber-Morris kinetic models [35] were applied to fit the experimental data obtained from the single adsorption system (Fig. 2(d) and Fig. 2S), and the kinetic parameters were listed in Table S2. In the both case of Cd(II) and Pb(II), the linear regression correlations (R2) from pseudo-second-order model were higher than those from pseudo-first-order model. In addition, the calculated qe values (qe,cal) from the pseudo-second-order model were much closer to the experimental ones (qe,exp). Therefore, the pseudo-second-order model was more appropriate for describing the single adsorption behavior of Cd(II) and Pb(II), indicating that the adsorption behavior of heavy metal ions on MgO nanoparticels mainly involved the chemical adsorption [34]. The analysis of Weber-Morris model (Fig. 2S(b)), was applied to discuss the actual rate-controlling step in the single adsorption of Cd(II) and Pb(II), and three linear portions was observed. The first line portion represented external mass transfer. The second linear portion, intra-particle diffusion, showed sorption. And the third linear portion showed adsorption/desorption equilibrium. The plots did not through the origin, suggesting that
intra-particle diffusion was not the only rate-controlling step, and the external mass transfer also contributed significantly in the rate-controlling step due to the large intercepts of the second linear portion of the plots. So the adsorption process was collectively controlled by external mass transfer and intra-particle diffusion.
3.3. Effect of initial metal ion concentrations To explore the adsorption capacities of MgO nanoparticles towards Cd(II) and Pb(II), the adsorption of metal ions with different initial concentrations was investigated. Fig. 3(a) showed that single adsorption isotherms with the initial concentrations ranging from 50 to 400 mg/L. It was observed that the adsorption amount of MgO nanoparticles towards Cd(II) and Pb(II) increased gradually at low concentrations (0-250 mg/L), then reached a plateau and stayed steady despite a continuing increase in the initial concentrations of metal ions. The adsorption data of MgO nanoparticles towards single Cd(II) and Pb(II) were analyzed using three different isothermal adsorption models, namely Langmuir model [36], Freundlich model [37] and Dubinbin-Radushkevich model [38], respectively (Fig. 3(b) and Fig. 3S), and the calculated isothermal parameters were presented in Table 3S. Compared with the Freundlich and Dubinbin-Radushkevich models, the Langmuir model fitted better with the experimental data due to the higher correlation coefficients (R2>0.999). In particular, the maximum adsorption capacities (qmL) calculated by Langmuir model were 2294 and 2614 mg/g for Cd(II) and Pb(II), respectively, which are higher than the reported results of micro/nano structured MgO [14,18], activated carbon
[31] and coal fly ash [32]. As documented, the Langmuir model assumes monolayer coverage of the adsorbent surface, on which the binding sites have the same affinity for the adsorption [36]. Therefore, the formed monolayer was possibly due to the chemical interaction between MgO nanoparticles and heavy metal ions. In order to further verify chemisorption was dominant in the adsorption process of Cd(II) and Pb(II), the Dubinbin-Radushkevich model was applied to distinguish between physisorption and chemisorption. The mean free energies (E) of adsorption for Cd(II) and Pb(II) were 392.9 and 292.5 kJ/mol, respectively, suggesting that chemisorption was dominating in the single adsorption process of Cd(II) and Pb(II) (Table 3S). The competitive adsorption isotherms of Cd(II) and Pb(II) were also shown in Fig. 4(a). As expected, MgO nanoparticles simultaneously adsorbed Cd(II) and Pb(II) ions at low concentrations (0-100 mg/L), whereas a clear affinity order of Pb(II) > Cd(II) was observed at high concentrations. The adsorption capacity of MgO nanoparticles for Pb(II) increased rapidly and attained saturation at the initial concentration of 300 mg/L, while for Cd(II) it decreased gradually after a period of elevation. This could be attributed to the fact that the active adsorption sites of MgO particles were occupied mainly by Pb(II) rather than Cd(II) at high initial concentration [39, 40].For the binary metal system, the three models also were used to analyze the adsorption data of MgO nanoparticles towards Cd(II) and Pb(II), respectively (Fig. 4(b) and Fig. 4S), and the model parameters were summarized in Table 4S. Obviously, the experimental data for Pb(II) adsorption fitted well with Langmuir model (R2 ≈ 1), while the data for Cd(II) adsorption did not fit any of the three models. That is to say, the three models could not be used to describe the Cd(II)
adsorption in binary metal system. Moreover, the E for Pb(II) adsorption calculated by Dubinbin-Radushkevich model was 656.6 kJ/mol, implying the involvement of a chemisorption process. Combined with the results of single metal system, the monolayer chemisorption represented well the adsorption process of Pb(II) on MgO nanoparticles whether in the single or binary metal systems, while it wasn’t suitable for the adsorption of Cd(II) in the binary metal system. Additionally, in binary metal system the adsorption capacities of MgO nanoparticles towards Cd(II) and Pb(II) followed the order of Pb(II) > Cd(II), which is in agreement with the results reported in literatures [39, 41].
3.4. Effect of initial solution pH In practice, wastewater may be acidic or alkaline, so the solution pH is one of the most important factors in the adsorption process. According to the solubility product of Cd(OH)2 and Pb(OH)2, when the concentrations of Cd(II) and Pb(II) in solutions are less than 400 mg/L, they will not generate insoluble species at pH<8 and pH<6, respectively. Hence, the pH values in the range of 2-5 were chosen in the following experiments. Fig. 5(a) and (b) showed the effect of initial solution pH on the single adsorption of Cd(II) and Pb(II), respectively. Apparently, the pH value had a considerable influence on the adsorption capacity of MgO nanoparticles. It was found that the higher adsorption capacity for heavy metal ions was achieved at higher pH values. Note that the adsorption capacity for Cd(II) and Pb(II) increased rapidly in the pH range of 2-3. This result was attributed to that at very low initial pH (pH=2) the high concentration of H+ in solution could react with MgO nanoparticles, which would consume most of the adsorbents and
cause a decrease of the adsorption capacity. In addition, at low pH value Cd(II) and Pb(II) had low affinity to Mg(OH)2 generated from hydration of MgO nanoparticls, resulting in a decrease in the adsorption capacities for Cd(II) and Pb(II) [42, 43]. The effect of initial solution pH on the competitive adsorption of Cd(II) and Pb(II) on MgO nanoparticles was shown in Fig. 5(c). The adsorption capacities for Cd(II) and Pb(II) were both very low at the pH value of 2. As the initial solution pH increased, the adsorption capacity for Pb(II) increased sharply, and achieved almost complete adsorption when the initial solution pH value was more than 3, while only a small increase was observed for Cd(II) adsorption. The higher adsorption capacity for Pb(II) than that for Cd(II) could be attributed to their different affinities to MgO nanoparticles.
3.5. Mechanisms of Cd(II) and Pb(II) removal To study the removal mechanism of MgO nanoparticles towards Pb(II) and Cd(II), the changes of MgO nanoparticles after adsorption (Table 1) were also analyzed by SEM-EDX, XRD and XPS. First, elemental compositions of the adsorbed products were analyzed by EDX spectra, and the results were shown in Fig. 6. From the EDX spectra, the signals from C, O, Mg, Pt, Pb and Cd elements were detected. Here the C element could be from the adsorption of CO2 or organics, while the Pt element resulted from the sprayed platinum for SEM observation. Hence, it can confirm that the adsorbed products are mainly composed of O, Mg, Pb and Cd elements. Notably, the signal from Cd atom in binary metal system was very weak, which indicated the adsorbed products (Sample 3) contained a small amount of Cd. Furthermore, the weight ratios of heavy metal ions to
MgO were calculated according to the molar ratios of Cd/Mg and Pb/Mg from quantitative analysis of EDX spectra. Interestingly, the calculated adsorption capacities (heavy metal ions/MgO) were 2286.2 mg/g for single Cd(II) (Sample 1), 2547.4 mg/g for single Pb(II) (Sample 2), and 18.2 mg/g for Cd(II) and 2626.4 mg/g for Pb(II) in the binary metal system (Sample 3), respectively. These values were nearly consistent with the results of the above adsorption experiments, which indicated MgO nanoparticles could almost completely removed Cd(II) and Pb(II) in single system, and Pb(II) in the binary system from aqueous solutions. The SEM image and EDX mapping of MgO nanoparticles after adsorbing Cd(II) and Pb(II) (Sample 3) were shown in Fig. 7. The obtained product after adsorption displayed plate-like morphology with an average diameter of 1.5 µm (Figure 7a). Clearly, the particle size of the obtained product after adsorbing heavy metals was larger than that of the sample without heavy metal ions in solution (Sample 4, Fig. 5S(a)), which can be attributed to the adsorption of heavy metals ions besides the hydration of MgO. Moreover, EDX mapping showed that Cd and Pb elements were uniformly dispersed in the adsorbed product, implying MgO nanoparticles had a large number of active sites to be combined with heavy metals. Note that the density of Cd was far lower than that of Pb, indicating a small amount of Cd contained in the adsorbed product and adsorption preference of MgO nanoparticles towards Pb(II), which were consistent with the above analysis.
The adsorbed products (Sample 1 and Sample 2) for single Cd(II) and Pb(II) removal were further analyzed by XRD, shown in Fig. 8. All the diffraction peaks of the adsorbed
product for Cd(II) could be assigned to Cd(OH)2 (JCPDS 84-1767), CdO (JCPDS 05-0640), Mg(OH)2 (JCPDS 84-2163) and MgO (JCPDS 87-0653), while the diffraction peaks for Pb(II) could be indexed to Pb3(CO3)2(OH)2 (JCPDS 01-0687), PbO (JCPDS 72-0094), Mg(OH)2 (JCPDS 84-2163) and MgO (JCPDS 87-0653), which were quite different from the results reported in the literature [25]. As we all know, MgO nanoparticles are easily hydrated by water to generate Mg(OH)2 [19], which is further proved by the experimental result (Sample 4, Fig. 5S(b)). Hence, the Mg(OH)2 phase in the adsorbed products resulted from the hydration of MgO nanoparticles in aqueous solution. The generated Mg(OH)2 would partially dissociate to produce OH- ions, which were near the surface of MgO nanoparticles. As a result, heavy metal ions in aqueous solutions could combine with the produced OH- to form insoluble hydroxides (Cd(OH)2, Pb(OH)2), which nucleated and grew on the surfaces of Mg(OH)2/MgO. Because the resulting Pb(OH)2 was unstable, it could react with dissolved CO2 to form Pb3(CO3)2(OH)2 [18]. As for the presence of CdO and PbO phases, it could be attributed to the adsorption of Cd and Pb ions on the surface defect sites of MgO nanoparticles. It is well known that MgO nanoparticles have plenty of vacancy defects on the surface of MgO, especially magnesium vacancies, which can combine heavy metal ions (Cd2+ and Pb2+) to form metal oxides (CdO and PbO). Significantly, when Mg(OH)2 was used instead of MgO to remove Pb(II), the obtained product was composed of only Pb3(CO3)2(OH)2 and Mg(OH)2, and no PbO phase was found (Fig. 6S). Consequently, the mechanism of Cd(II) and Pb(II) removal from aqueous solutions by MgO nanoparticles was proposed to be cooperative effects of the precipitation and adsorption of heavy metal ions on the surface
of MgO. To further confirm the composition of the absorbed products, the MgO nanoparticles after absorbing Cd(II) and Pb(II) were also analyzed by XPS technique. Pronounced signals from C, O, Mg, Pb and Cd could be observed from the survey spectra (Fig. 7S), further validating the existence of these elements in the adsorbed products, which was in good accordance with the results of EDX analysis. Moreover, the relatively high intensities of Cd and Pb suggested a lot of heavy metal ions on the surface of MgO/Mg(OH)2. The high resolution scan of Cd 3d and Pb 4f regions were shown in Fig. 9(a) and (b), respectively. Fig. 9(a) displayed two peaks centered at 405.57 and 412.31 eV corresponding to Cd 3d5/2 and Cd 3d3/2, respectively. The two peaks were asymmetric, and each could be deconvoluted into two subpeaks: 405.43 and 406.21 eV for Cd 3d5/2, 412.10 and 412.94 eV for Cd 3d3/2. These deconvoluted subpeaks with a peak separation of ~6.7 eV between Cd 3d5/2 and 3d3/2 states could be assigned to Cd(OH)2 and CdO, respectively [44], which were in agreement with the above XRD analysis (Fig. 8(a)). The high-resolution XPS spectrum of Pb 4f also exhibited two distinguishable peaks located at 138.33 and 143.22 eV, originating from Pb 4f7/2 and Pb 4f5/2. Similarly, the peaks of Pb 4f7/2 and Pb 4f5/2 could be fitted individually into two subpeaks: 138.26 and 138.90 eV for Pb 4f7/2, 143.10 and 143.79 eV for Pb 4f5/2. The fitted subpeaks with a peak separation of ~4.8 eV between Pb 4f7/2 and Pb 4f5/2 could be attributed to Pb3(OH)2(CO3)2 and PbO, respectively [45, 46], which was consistent with the above XRD analysis (Fig. 8(b)). Additionally, the Mg 1s spectra of the absorbed products with (Sample 2) and without (Sample 4) heavy metal ions in solutions were examined, and shown in Fig. 8S.
Comparatively, the binding energy of Mg1s was observed to shift to high energy side after adsorption. It could be due to the formation of Mg–O–Pb bond in the adsorbed product, resulting in the decrease of electron cloud density of Mg–O bond. This result also indicated indirectly some heavy metal ions could be adsorbed on the surface of MgO/Mg(OH)2 in the form of oxides.
3.6. Cd(II) and Pb(II) elution experiments To further explore the adsorption action between heavy metal ions and MgO nanoparticles, elution experiments were performed by washing with water. The tested data showed that neither Cd(II) nor Pb(II) could be washed largely out from the adsorbed products by water washing, and the maximum elution rates of Cd(II) and Pb(II) were only about 0.07% and 0.05%, respectively (Table 2), indicating that both Cd(II) and Pb(II) could be adsorbed tightly on the surface of the adsorbent and suggesting a strong interaction between heavy metal ions and MgO nanoparticles. In other words, MgO nanoparticles have a fixing action on heavy metal ions to keep heavy metal ions from moving with water, which exhibited promising potentials for the practical application in wastewater treatment.
4. Conclusion In summary, this study investigated the Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles prepared by a simple sol-gel method. Batch adsorption experiments were carried out at different contact time, initial concentrations of heavy
metal ions and pH values, respectively. It was found that the adsorption rates were very fast and adsorption equilibriums were obtained in about 30 min and 15 min for the adsorption of Cd(II) and Pb(II), respectively. The best models to describe the kinetics and isotherms of single adsorption were the pseudo-second-order kinetic and Langmuir models, respectively, indicating the monolayer chemisorption of Cd(II) and Pb(II) on MgO nanoparticles. The maximum adsorption capacities calculated by applying the Langmuir equation were 2294 mg/g for Cd(II) and 2614 mg/g for Pb(II), respectively. The competitive system showed that the affinity order of these two metal ions was Pb(II) > Cd(II). Moreover, the solution pH was found to be a key factor in the adsorption of heavy metal ions on MgO, because the solution pH will influence precipitation of heavy metal ions and dissolution of the adsorbent in solutions. The analysis of the adsorbed products indicated the main mechanism for Cd(II) and Pb(II) removal was the precipitation and adsorption of heavy metal ions on the surface of MgO. Significantly, the elution experiments confirmed that neither Cd(II) nor Pb(II) could be greatly desorbed largely after water washing even for five times. Considering the advantages of low cost, mild condition, high adsorption capacity as well as eco-friendly to environment, MgO nanoparticles have very promising applications for removal of heavy metals from aquatic ecosystems.
Acknowledgements The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 50902054). The authors also acknowledge the experimental
help from Huazhong University of Science & Technology Analytical and Testing Center.
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277-283.
Figure Captions
Fig. 1. XRD pattern, SEM image (a), and N2 adsorption-desorption isotherms (b) of MgO nanoparticles. Fig. 2. Effect of contact time on single Cd(II) (a), Pb(II) (b), and competitive (c) adsorption on MgO nanoparticles; (d) Pseudo-second-order model for single adsorption of Cd(II) and Pb(II) on MgO nanoparticles. Fig. 3. Adsorption isotherms (a) and Langmuir isotherm plots (b) for single adsorption of Cd(II) and Pb(II) on MgO nanoparticles. Fig. 4. Adsorption isotherms (a) and Langmuir isotherm plots (b) for competitive adsorption of Cd(II) and Pb(II) on MgO nanoparticles. Fig. 5. Effect of initial solution pH on single Cd(II) (a), Pb(II) (b), and competitive (c) adsorption on MgO nanoparticles. Fig. 6. Quantitative analysis and EDX spectra of MgO nanoparticles after adsorbing heavy metal ions. Fig. 7. SEM image (a) and SEM-EDX elemental mapping (b-e) of MgO nanoparticles after adsorbing Cd(II) and Pb(II). Fig. 8. XRD patterns of MgO nanoparticles after absorbing Cd(II) (a) and Pb(II) (b). Fig. 9. High-resolution XPS spectra of Cd 3d (a) and Pb 4f (b).
Fig1(a) .
Fig1(b) .
Fig2(a) .
Fig2(b) .
Fig2(c) .
Fig2(d) .
Fig3(a) .
Fig3(b) .
Fig4(a) .
Fig4(b) .
Fig5(a) .
Fig5(b) .
Fig5(c) .
Fig6 .
Fig7(a) .
Fig7(b) .
Fig7(c) .
Fig7(d) .
Fig7(e) .
Fig7(f) .
Fig8(a) .
Fig8(b) .
Fig9(a) .
Fig9(b) .
Table 1 Experimental parameters for the mechanism investigation of metal ion removal using MgO nanoparticles. Sample
MgO dose
Cd(II) concentration
Pb(II) concentration
(mg/L)
(mg/L)
(mg/L)
1
100
250
0
2
100
0
250
3
100
250
250
4
100
0
0
Table 2 Elution rates of the adsorbed Cd(II) and Pb(II) on MgO nanoparticles. Metal
Adsorption
Elution rate of heavy metal ions (%)
capacity(mg/g)
1
2
3
4
5
Cd
999.9
0.060
0.065
0.068
0.072
0.066
Pb
995.2
0.053
0.048
0.052
0.050
0.042
S0304-3894(15)00627-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.08.008 HAZMAT 17017
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Journal of Hazardous Materials
Received date: Revised date: Accepted date:
1-5-2015 1-8-2015 4-8-2015
Please cite this article as: Chunmei Xiong, Wei Wang, Fatang Tan, Fan Luo, Jianguo Chen, Xueliao Qiao, Investigation on the efficiency and mechanism of Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation on the efficiency and mechanism of Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles
Chunmei Xiong, Wei Wang*, Fatang Tan, Fan Luo, Jianguo Chen, Xueliao Qiao
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, 430074, Hubei, P.R. China * Corresponding authors. Wei Wang, Tel/fax: +86 27 87541540, E-mail: [email protected]
Highlights
MgO nanoparticles exhibited high removal abilities towards Cd(II) and Pb(II). The mechanism for Cd(II) and Pb(II) removal was precipitation and adsorption. The adsorbed heavy metals on MgO were difficult to be desorbed by water washing.
Abstract
In this study, the removal of Cd(II) and Pb(II) from aqueous solutions using MgO nanoparticles prepared by a simple sol-gel method was investigated. The efficiency of Cd(II) and Pb(II) removal was examined through batch adsorption experiments. For the
single adsorption of Cd(II) and Pb(II), The adsorption kinetics and isotherm data obeyed well Pseudo-second-order and Langmuir models, indicating the monolayer chemisorption of heavy metal ions. The maximum adsorption capacities calculated by Langmuir equation were 2294 mg/g for Cd(II) and 2614 mg/g for Pb(II), respectively. The adsorption process was controlled simultaneously by external mass transfer and intraparticle diffusion. In the binary system, a competitive adsorption was observed, showing preference of adsorption followed Pb(II) > Cd(II). Significantly, the elution experiments confirmed that neither Cd(II) nor Pb(II) could be greatly desorbed after water washing even for five times. XRD and XPS measurements revealed the mechanism of Cd(II) and Pb(II) removal by MgO nanoparticles was mainly involved in precipitation and adsorption on the surface of MgO, resulting from the interaction between active sites of MgO and heavy metal ions. Easy preparation, remarkable removal efficiency and firmly adsorptive ability make the MgO nanoparticles to be an efficient material in the treatment of heavy metal-contaminated water.
Keywords: MgO nanoparticle; cadmium ion; lead ion; adsorption; precipitation 1.
Introduction Nowadays, heavy metal ions, such as Pb(II), Cd(II), and Hg(II), especially in water,
have posed a serious threat to public health and ecological system due to their high toxicity, non-biodegradation and long-term damage by accumulation [1]. Therefore, the efficient removal of heavy metal ions from aqueous systems is of great significance in protecting the earth’s ecosystems. Up to now, various methods such as chemical
precipitation, adsorption, evaporation, ion exchange, membrane filtration, electrodialysis and reverse osmosis, have been developed to treat water containing heavy metals [2-5]. Among them, adsorption has been universally accepted as one of the most commonly practiced techniques for the removal of heavy metal ions because of its simple and stable handling process, high efficient wastewater treatment, absence of secondary pollution and low operating cost [6]. A number of materials including activated carbon [7], bentonite [8], magnetite [9], chitosan [10], etc., have been reported to be capable of adsorbing heavy metal ions from aqueous solutions. However, these adsorbents usually suffer from low adsorption capacities or removal efficiencies of heavy metal ions. Therefore, a lot of efforts are still continued to explore highly efficient adsorbents for the removal of heavy metal ions. Recently, some metal oxides or metal hydroxides as adsorbents have attracted much attention due to their low cost, environmentally benign nature and excellent adsorption properties for environmental applications [11-13]. Magnesium oxide (MgO), a typical metal oxide adsorbent, is a promising candidate for the treatment of water pollutants [14, 15]. Particularly, nanosized MgO has exhibited higher efficiency and faster adsorption rate in wastewater treatment, owing to its higher surface areas and much more surface active sites than bulk materials. Feng et al. [16] reported mesoporous MgO nanosheets had the maximum adsorption capacity of 1684.25 mg/g for Ni(II) removal from aqueous solutions. Tian et al. [17] found porous hierarchical MgO exhibited superb adsorption performance towards Congo red dye, with the maximum adsorption capacity of 2409 mg/g. In short, micro/nanosized MgO can efficiently remove heavy metal ions [18-20], fluoride [21],
toxic dyes [22, 23], etc. The pioneering work inspires us to further explore the adsorption capacities of MgO nanoparticles for heavy metal ions in solutions. On the other hand, the mechanism of heavy metal ions removal using MgO as an adsorbent is still controversial. Campbell and Starr [24] proved that metal (Cu, Ag and Pb) could combine with the oxygen or magnesium of MgO in the form of covalent bond. Zhu and Li [14] found that the dominant mechanism of Cd(II) removal was precipitation of Cd(OH)2 on the surface of MgO. Cao et al. [25] proposed a removal mechanism involving solid−liquid interfacial cation exchange between MgO and Pb(II) or Cd(II) to form PbO or CdO. Mahdavi et al. [26] proposed MgO nanoparticles had a much greater metal adsorption capacity than TiO2 and Al2O3 nanoparticles due to the occurrence of adsorption and precipitation of heavy metal ions with MgO. Thus, an in-depth investigation into the mechanism of heavy metal ions removal using MgO nanoparticles is needed urgently. In this work, the removal capacity and mechanism of Cd(II) and Pb(II) from aqueous solutions using MgO nanoparticles prepared by a simple sol-gel method were systematically investigated. The influences of contact time, initial metal ion concentration and solution pH on removal of Cd(II) and Pb(II) from single and binary metal solutions were examined in details. Besides, the adsorbed products were further analyzed by XRD, XPS and SEM equipped with an EDX analyzer to study the mechanism of heavy metal removal and the adsorption action between heavy metal ions and MgO nanoparticles.
2.
Materials and methods
2.1. Materials
Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) was obtained from Shanghai Experimental Reagent Co., Ltd. Citric acid (C6H8O7) was purchased from Sinopharm Chemical Reagent Co., Ltd. Cadmium chloride(CdCl2·2.5H2O) and lead nitrate (Pb(NO3)2) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All the chemical reagents were of analytical grade and used without further purification in this study. Doubly distilled (DI) water was used throughout the experiments.
2.2. Preparation of MgO nanoparticles The preparation procedure for MgO nanoparticles was similar to our previous work [27]. Typically, 0.02 mol of Mg(NO3)2·6H2O was dissolved in 20 mL of DI water. Subsequently, an aqueous solution containing equimolar amount of C6H8O7 was added into the above solution under stirring. Then the mixture was placed in a water bath at 80 °C with continuous stirring until it completely turned into a gel. Finally, the wet gel was dried at 150 °C to obtain a white fluffy precursor and followed by calcination at 600 °C for 1 h to yield MgO nanoparticles.
2.3. Characterization of MgO nanoparticles Crystal structure of samples was characterized by XRD (Philips/X’ Pert PRO). Morphology of samples was observed using transmission electron microscope (TEM, FEI Tecnai G2 20) and scanning electron microscope (SEM, FEI/Nova NanoSEM 450) equipped with an energy-dispersive X-ray (EDX) analyzer (Oxford/X-Max 50). Molecular structure was analyzed by Fourier tranaform infrared (FTIR) spectrometer (Bruker
VERTEX70). Brunauer-Emmett-Teller (BET) surface area measurements were carried out on a Micromeritics ASAP 2020M surface area analyzer. X-ray photoelectron spectroscopy (XPS) analysis was acquired on a Kratos/Axis Ultra DLD-600W spectrometer.
2.4. Batch adsorption experiments Stock solutions with different concentration of Cd(II) and Pb(II) were prepared using Pb(NO3)2 and CdCl2·2.5H2O as the sources of heavy metal ions, respectively. The pH of the stock solutions was adjusted using NaOH (0.1 M) and HNO3 (0.1 M). Adsorption kinetics experiments were conducted in a series of 250 mL beakers containing 0.01 g (100 mg/L) of MgO nanoparticles and 100 mL (100 mg/L) metal ion solutions with continuous stirring at pH 5 and 25 °C. After different time intervals, aliquots of the samples were collected and filtered through a 0.25 µm membrane. A PDV6000 plus (Cogent Environmental Ltd.) analyzer was used to measure the concentrations of metal ions in the remaining solutions. Adsorption isotherms experiments were conducted following similar procedure except using metal ion solutions with different concentrations. In binary metal system, the solutions containing the same initial concentrations of Cd(II) and Pb(II) were used to investigate the competitive adsorption of Cd(II) and Pb(II) on MgO nanoparticles. The amounts of Cd(II) and Pb(II) adsorbed on MgO nanoparticles, qt (mg/g) are calculated by using Equation (1): (1) where C0 (mg/L) is the initial metal ion concentration and C (mg/L) is residual metal ion
concentration in solution at time t, V (L) and W (g) are the volume of the solution and the weight of MgO nanoparticles, respectively. In the equation (1), Ce and qe are the equilibrium concentration and the adsorption amount at equilibrium time, respectively. Effects of initial pH values (pH= 2.0, 3.0, 4.0 and 5.0) were also studied in the single and binary systems in the similar procedure except using 250 mg/L of metal ion solutions. All the above experiments were performed in duplicate.
2.5. Elution experiments of the adsorption products Firstly, 0.03 g (1000 mg/L) of MgO nanoparticles was preloaded by 30 mL (1000 mg/L) of metal ion solutions according to the above adsorption procedure. Then, the resulting powders were dispersed in 30 mL of DI water under stirring for 4h at 25 °C. Finally, the mixture was centrifuged and the concentrations of heavy metal ions in the supernatant were measured to calculate the eluted amounts of Cd(II) and Pb(II) from the products after adsorption. 3.
Results and discussion
3.1. Structures and morphologies of MgO nanoparticles The XRD pattern and SEM image of the as-prepared MgO nanoparticles were shown in Fig. 1(a). Five diffraction peaks at 2θ= 36.8°, 42.8°, 62.1°, 74.5° and 78.4° could be indexed to the (111), (200), (220), (311), and (222) planes of MgO with a periclase structure (JCPDS 87-0653). No impurity peaks were observed, indicating that the products were pure MgO nanoparticles. Moreover, the average grain size of MgO was calculated to be 20.9 nm by using the Scherrer equation (Table 1S), according to the full width at
half-maximum (FWHM) of all the peaks. From the SEM image and TEM image (Fig. 1S(a)), the prepared MgO nanoparticles were found to exhibit plate-like nanoparticles with relatively uniform size distribution. The average particle size of MgO nanoparticles was measured to be 20.23 nm by using Nano Measurer 1.2 software, which corresponded to the result of XRD analysis. FTIR spectra of the as-prepared MgO nanoparticles was shown in Fig. 1S(b). A absorption band at 3438 cm-1 was assigned to the adsorbed water during sample preparation. Bands in the range of 1458-1640 cm-1 were attributed to the -OH stretching mode of water molecule [28]. Two characteristic bonds at 415 and 695 cm-1 occurred which confirmed the presence of Mg-O groups. The N2 adsorption-desorption isotherms and pore size distribution of the as-prepared MgO nanoparticles were shown in Fig. 1(b). The results revealed that the MgO nanoparticles possessed high BET surface area of 101.95 m2/g and an average pore diameter of 4.9 nm. The single point surface area at P/P0 = 0.299 was 112.19 m2/g and the pore volume at P/P0 =0.995 was 0.12 cm3/g. 3.2. Effect of contact time The effect of contact time on the single and binary adsorption of Cd(II) and Pb(II) on MgO nanoparticles was examined. As shown in Fig. 2(a) and (b), initially the adsorption amount qt increased quickly, then reached equilibria in about 30 min and 15 min for the single adsorption of Cd(II) and Pb(II), respectively. Clearly, the time to reach equilibrium adsorption is much shorter than those of zeolite [29, 30], activated carbon [31], and coal fly ash [32]. Such fast adsorption rate could be attributed to the abundance of active sites on the surface of MgO nanoparticles. Also, the binary adsorption of Cd(II) and Pb(II)
exhibited a similar trend (Fig. 2(c)). Hence, a contact time of 60 min was sufficient to reach equilibrium for MgO nanoparticles in the adsorption of heavy metal ions, which was selected in the further experiments. In addition, the change of solution pH was also measured during the adsorption process and shown in Fig. 2. The solution pH values gradually increased with time and reached equilibria in about 180 min, which was assigned to the hydration of MgO nanoparticles in aqueous solutions. In order to understand the characteristics of the adsorption process, three kinetic models detailed in the supplementary data, including pseudo-first-order [33], pseudo-second-order [34], and Weber-Morris kinetic models [35] were applied to fit the experimental data obtained from the single adsorption system (Fig. 2(d) and Fig. 2S), and the kinetic parameters were listed in Table S2. In the both case of Cd(II) and Pb(II), the linear regression correlations (R2) from pseudo-second-order model were higher than those from pseudo-first-order model. In addition, the calculated qe values (qe,cal) from the pseudo-second-order model were much closer to the experimental ones (qe,exp). Therefore, the pseudo-second-order model was more appropriate for describing the single adsorption behavior of Cd(II) and Pb(II), indicating that the adsorption behavior of heavy metal ions on MgO nanoparticels mainly involved the chemical adsorption [34]. The analysis of Weber-Morris model (Fig. 2S(b)), was applied to discuss the actual rate-controlling step in the single adsorption of Cd(II) and Pb(II), and three linear portions was observed. The first line portion represented external mass transfer. The second linear portion, intra-particle diffusion, showed sorption. And the third linear portion showed adsorption/desorption equilibrium. The plots did not through the origin, suggesting that
intra-particle diffusion was not the only rate-controlling step, and the external mass transfer also contributed significantly in the rate-controlling step due to the large intercepts of the second linear portion of the plots. So the adsorption process was collectively controlled by external mass transfer and intra-particle diffusion.
3.3. Effect of initial metal ion concentrations To explore the adsorption capacities of MgO nanoparticles towards Cd(II) and Pb(II), the adsorption of metal ions with different initial concentrations was investigated. Fig. 3(a) showed that single adsorption isotherms with the initial concentrations ranging from 50 to 400 mg/L. It was observed that the adsorption amount of MgO nanoparticles towards Cd(II) and Pb(II) increased gradually at low concentrations (0-250 mg/L), then reached a plateau and stayed steady despite a continuing increase in the initial concentrations of metal ions. The adsorption data of MgO nanoparticles towards single Cd(II) and Pb(II) were analyzed using three different isothermal adsorption models, namely Langmuir model [36], Freundlich model [37] and Dubinbin-Radushkevich model [38], respectively (Fig. 3(b) and Fig. 3S), and the calculated isothermal parameters were presented in Table 3S. Compared with the Freundlich and Dubinbin-Radushkevich models, the Langmuir model fitted better with the experimental data due to the higher correlation coefficients (R2>0.999). In particular, the maximum adsorption capacities (qmL) calculated by Langmuir model were 2294 and 2614 mg/g for Cd(II) and Pb(II), respectively, which are higher than the reported results of micro/nano structured MgO [14,18], activated carbon
[31] and coal fly ash [32]. As documented, the Langmuir model assumes monolayer coverage of the adsorbent surface, on which the binding sites have the same affinity for the adsorption [36]. Therefore, the formed monolayer was possibly due to the chemical interaction between MgO nanoparticles and heavy metal ions. In order to further verify chemisorption was dominant in the adsorption process of Cd(II) and Pb(II), the Dubinbin-Radushkevich model was applied to distinguish between physisorption and chemisorption. The mean free energies (E) of adsorption for Cd(II) and Pb(II) were 392.9 and 292.5 kJ/mol, respectively, suggesting that chemisorption was dominating in the single adsorption process of Cd(II) and Pb(II) (Table 3S). The competitive adsorption isotherms of Cd(II) and Pb(II) were also shown in Fig. 4(a). As expected, MgO nanoparticles simultaneously adsorbed Cd(II) and Pb(II) ions at low concentrations (0-100 mg/L), whereas a clear affinity order of Pb(II) > Cd(II) was observed at high concentrations. The adsorption capacity of MgO nanoparticles for Pb(II) increased rapidly and attained saturation at the initial concentration of 300 mg/L, while for Cd(II) it decreased gradually after a period of elevation. This could be attributed to the fact that the active adsorption sites of MgO particles were occupied mainly by Pb(II) rather than Cd(II) at high initial concentration [39, 40].For the binary metal system, the three models also were used to analyze the adsorption data of MgO nanoparticles towards Cd(II) and Pb(II), respectively (Fig. 4(b) and Fig. 4S), and the model parameters were summarized in Table 4S. Obviously, the experimental data for Pb(II) adsorption fitted well with Langmuir model (R2 ≈ 1), while the data for Cd(II) adsorption did not fit any of the three models. That is to say, the three models could not be used to describe the Cd(II)
adsorption in binary metal system. Moreover, the E for Pb(II) adsorption calculated by Dubinbin-Radushkevich model was 656.6 kJ/mol, implying the involvement of a chemisorption process. Combined with the results of single metal system, the monolayer chemisorption represented well the adsorption process of Pb(II) on MgO nanoparticles whether in the single or binary metal systems, while it wasn’t suitable for the adsorption of Cd(II) in the binary metal system. Additionally, in binary metal system the adsorption capacities of MgO nanoparticles towards Cd(II) and Pb(II) followed the order of Pb(II) > Cd(II), which is in agreement with the results reported in literatures [39, 41].
3.4. Effect of initial solution pH In practice, wastewater may be acidic or alkaline, so the solution pH is one of the most important factors in the adsorption process. According to the solubility product of Cd(OH)2 and Pb(OH)2, when the concentrations of Cd(II) and Pb(II) in solutions are less than 400 mg/L, they will not generate insoluble species at pH<8 and pH<6, respectively. Hence, the pH values in the range of 2-5 were chosen in the following experiments. Fig. 5(a) and (b) showed the effect of initial solution pH on the single adsorption of Cd(II) and Pb(II), respectively. Apparently, the pH value had a considerable influence on the adsorption capacity of MgO nanoparticles. It was found that the higher adsorption capacity for heavy metal ions was achieved at higher pH values. Note that the adsorption capacity for Cd(II) and Pb(II) increased rapidly in the pH range of 2-3. This result was attributed to that at very low initial pH (pH=2) the high concentration of H+ in solution could react with MgO nanoparticles, which would consume most of the adsorbents and
cause a decrease of the adsorption capacity. In addition, at low pH value Cd(II) and Pb(II) had low affinity to Mg(OH)2 generated from hydration of MgO nanoparticls, resulting in a decrease in the adsorption capacities for Cd(II) and Pb(II) [42, 43]. The effect of initial solution pH on the competitive adsorption of Cd(II) and Pb(II) on MgO nanoparticles was shown in Fig. 5(c). The adsorption capacities for Cd(II) and Pb(II) were both very low at the pH value of 2. As the initial solution pH increased, the adsorption capacity for Pb(II) increased sharply, and achieved almost complete adsorption when the initial solution pH value was more than 3, while only a small increase was observed for Cd(II) adsorption. The higher adsorption capacity for Pb(II) than that for Cd(II) could be attributed to their different affinities to MgO nanoparticles.
3.5. Mechanisms of Cd(II) and Pb(II) removal To study the removal mechanism of MgO nanoparticles towards Pb(II) and Cd(II), the changes of MgO nanoparticles after adsorption (Table 1) were also analyzed by SEM-EDX, XRD and XPS. First, elemental compositions of the adsorbed products were analyzed by EDX spectra, and the results were shown in Fig. 6. From the EDX spectra, the signals from C, O, Mg, Pt, Pb and Cd elements were detected. Here the C element could be from the adsorption of CO2 or organics, while the Pt element resulted from the sprayed platinum for SEM observation. Hence, it can confirm that the adsorbed products are mainly composed of O, Mg, Pb and Cd elements. Notably, the signal from Cd atom in binary metal system was very weak, which indicated the adsorbed products (Sample 3) contained a small amount of Cd. Furthermore, the weight ratios of heavy metal ions to
MgO were calculated according to the molar ratios of Cd/Mg and Pb/Mg from quantitative analysis of EDX spectra. Interestingly, the calculated adsorption capacities (heavy metal ions/MgO) were 2286.2 mg/g for single Cd(II) (Sample 1), 2547.4 mg/g for single Pb(II) (Sample 2), and 18.2 mg/g for Cd(II) and 2626.4 mg/g for Pb(II) in the binary metal system (Sample 3), respectively. These values were nearly consistent with the results of the above adsorption experiments, which indicated MgO nanoparticles could almost completely removed Cd(II) and Pb(II) in single system, and Pb(II) in the binary system from aqueous solutions. The SEM image and EDX mapping of MgO nanoparticles after adsorbing Cd(II) and Pb(II) (Sample 3) were shown in Fig. 7. The obtained product after adsorption displayed plate-like morphology with an average diameter of 1.5 µm (Figure 7a). Clearly, the particle size of the obtained product after adsorbing heavy metals was larger than that of the sample without heavy metal ions in solution (Sample 4, Fig. 5S(a)), which can be attributed to the adsorption of heavy metals ions besides the hydration of MgO. Moreover, EDX mapping showed that Cd and Pb elements were uniformly dispersed in the adsorbed product, implying MgO nanoparticles had a large number of active sites to be combined with heavy metals. Note that the density of Cd was far lower than that of Pb, indicating a small amount of Cd contained in the adsorbed product and adsorption preference of MgO nanoparticles towards Pb(II), which were consistent with the above analysis.
The adsorbed products (Sample 1 and Sample 2) for single Cd(II) and Pb(II) removal were further analyzed by XRD, shown in Fig. 8. All the diffraction peaks of the adsorbed
product for Cd(II) could be assigned to Cd(OH)2 (JCPDS 84-1767), CdO (JCPDS 05-0640), Mg(OH)2 (JCPDS 84-2163) and MgO (JCPDS 87-0653), while the diffraction peaks for Pb(II) could be indexed to Pb3(CO3)2(OH)2 (JCPDS 01-0687), PbO (JCPDS 72-0094), Mg(OH)2 (JCPDS 84-2163) and MgO (JCPDS 87-0653), which were quite different from the results reported in the literature [25]. As we all know, MgO nanoparticles are easily hydrated by water to generate Mg(OH)2 [19], which is further proved by the experimental result (Sample 4, Fig. 5S(b)). Hence, the Mg(OH)2 phase in the adsorbed products resulted from the hydration of MgO nanoparticles in aqueous solution. The generated Mg(OH)2 would partially dissociate to produce OH- ions, which were near the surface of MgO nanoparticles. As a result, heavy metal ions in aqueous solutions could combine with the produced OH- to form insoluble hydroxides (Cd(OH)2, Pb(OH)2), which nucleated and grew on the surfaces of Mg(OH)2/MgO. Because the resulting Pb(OH)2 was unstable, it could react with dissolved CO2 to form Pb3(CO3)2(OH)2 [18]. As for the presence of CdO and PbO phases, it could be attributed to the adsorption of Cd and Pb ions on the surface defect sites of MgO nanoparticles. It is well known that MgO nanoparticles have plenty of vacancy defects on the surface of MgO, especially magnesium vacancies, which can combine heavy metal ions (Cd2+ and Pb2+) to form metal oxides (CdO and PbO). Significantly, when Mg(OH)2 was used instead of MgO to remove Pb(II), the obtained product was composed of only Pb3(CO3)2(OH)2 and Mg(OH)2, and no PbO phase was found (Fig. 6S). Consequently, the mechanism of Cd(II) and Pb(II) removal from aqueous solutions by MgO nanoparticles was proposed to be cooperative effects of the precipitation and adsorption of heavy metal ions on the surface
of MgO. To further confirm the composition of the absorbed products, the MgO nanoparticles after absorbing Cd(II) and Pb(II) were also analyzed by XPS technique. Pronounced signals from C, O, Mg, Pb and Cd could be observed from the survey spectra (Fig. 7S), further validating the existence of these elements in the adsorbed products, which was in good accordance with the results of EDX analysis. Moreover, the relatively high intensities of Cd and Pb suggested a lot of heavy metal ions on the surface of MgO/Mg(OH)2. The high resolution scan of Cd 3d and Pb 4f regions were shown in Fig. 9(a) and (b), respectively. Fig. 9(a) displayed two peaks centered at 405.57 and 412.31 eV corresponding to Cd 3d5/2 and Cd 3d3/2, respectively. The two peaks were asymmetric, and each could be deconvoluted into two subpeaks: 405.43 and 406.21 eV for Cd 3d5/2, 412.10 and 412.94 eV for Cd 3d3/2. These deconvoluted subpeaks with a peak separation of ~6.7 eV between Cd 3d5/2 and 3d3/2 states could be assigned to Cd(OH)2 and CdO, respectively [44], which were in agreement with the above XRD analysis (Fig. 8(a)). The high-resolution XPS spectrum of Pb 4f also exhibited two distinguishable peaks located at 138.33 and 143.22 eV, originating from Pb 4f7/2 and Pb 4f5/2. Similarly, the peaks of Pb 4f7/2 and Pb 4f5/2 could be fitted individually into two subpeaks: 138.26 and 138.90 eV for Pb 4f7/2, 143.10 and 143.79 eV for Pb 4f5/2. The fitted subpeaks with a peak separation of ~4.8 eV between Pb 4f7/2 and Pb 4f5/2 could be attributed to Pb3(OH)2(CO3)2 and PbO, respectively [45, 46], which was consistent with the above XRD analysis (Fig. 8(b)). Additionally, the Mg 1s spectra of the absorbed products with (Sample 2) and without (Sample 4) heavy metal ions in solutions were examined, and shown in Fig. 8S.
Comparatively, the binding energy of Mg1s was observed to shift to high energy side after adsorption. It could be due to the formation of Mg–O–Pb bond in the adsorbed product, resulting in the decrease of electron cloud density of Mg–O bond. This result also indicated indirectly some heavy metal ions could be adsorbed on the surface of MgO/Mg(OH)2 in the form of oxides.
3.6. Cd(II) and Pb(II) elution experiments To further explore the adsorption action between heavy metal ions and MgO nanoparticles, elution experiments were performed by washing with water. The tested data showed that neither Cd(II) nor Pb(II) could be washed largely out from the adsorbed products by water washing, and the maximum elution rates of Cd(II) and Pb(II) were only about 0.07% and 0.05%, respectively (Table 2), indicating that both Cd(II) and Pb(II) could be adsorbed tightly on the surface of the adsorbent and suggesting a strong interaction between heavy metal ions and MgO nanoparticles. In other words, MgO nanoparticles have a fixing action on heavy metal ions to keep heavy metal ions from moving with water, which exhibited promising potentials for the practical application in wastewater treatment.
4. Conclusion In summary, this study investigated the Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles prepared by a simple sol-gel method. Batch adsorption experiments were carried out at different contact time, initial concentrations of heavy
metal ions and pH values, respectively. It was found that the adsorption rates were very fast and adsorption equilibriums were obtained in about 30 min and 15 min for the adsorption of Cd(II) and Pb(II), respectively. The best models to describe the kinetics and isotherms of single adsorption were the pseudo-second-order kinetic and Langmuir models, respectively, indicating the monolayer chemisorption of Cd(II) and Pb(II) on MgO nanoparticles. The maximum adsorption capacities calculated by applying the Langmuir equation were 2294 mg/g for Cd(II) and 2614 mg/g for Pb(II), respectively. The competitive system showed that the affinity order of these two metal ions was Pb(II) > Cd(II). Moreover, the solution pH was found to be a key factor in the adsorption of heavy metal ions on MgO, because the solution pH will influence precipitation of heavy metal ions and dissolution of the adsorbent in solutions. The analysis of the adsorbed products indicated the main mechanism for Cd(II) and Pb(II) removal was the precipitation and adsorption of heavy metal ions on the surface of MgO. Significantly, the elution experiments confirmed that neither Cd(II) nor Pb(II) could be greatly desorbed largely after water washing even for five times. Considering the advantages of low cost, mild condition, high adsorption capacity as well as eco-friendly to environment, MgO nanoparticles have very promising applications for removal of heavy metals from aquatic ecosystems.
Acknowledgements The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 50902054). The authors also acknowledge the experimental
help from Huazhong University of Science & Technology Analytical and Testing Center.
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Figure Captions
Fig. 1. XRD pattern, SEM image (a), and N2 adsorption-desorption isotherms (b) of MgO nanoparticles. Fig. 2. Effect of contact time on single Cd(II) (a), Pb(II) (b), and competitive (c) adsorption on MgO nanoparticles; (d) Pseudo-second-order model for single adsorption of Cd(II) and Pb(II) on MgO nanoparticles. Fig. 3. Adsorption isotherms (a) and Langmuir isotherm plots (b) for single adsorption of Cd(II) and Pb(II) on MgO nanoparticles. Fig. 4. Adsorption isotherms (a) and Langmuir isotherm plots (b) for competitive adsorption of Cd(II) and Pb(II) on MgO nanoparticles. Fig. 5. Effect of initial solution pH on single Cd(II) (a), Pb(II) (b), and competitive (c) adsorption on MgO nanoparticles. Fig. 6. Quantitative analysis and EDX spectra of MgO nanoparticles after adsorbing heavy metal ions. Fig. 7. SEM image (a) and SEM-EDX elemental mapping (b-e) of MgO nanoparticles after adsorbing Cd(II) and Pb(II). Fig. 8. XRD patterns of MgO nanoparticles after absorbing Cd(II) (a) and Pb(II) (b). Fig. 9. High-resolution XPS spectra of Cd 3d (a) and Pb 4f (b).
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Fig5(b) .
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Table 1 Experimental parameters for the mechanism investigation of metal ion removal using MgO nanoparticles. Sample
MgO dose
Cd(II) concentration
Pb(II) concentration
(mg/L)
(mg/L)
(mg/L)
1
100
250
0
2
100
0
250
3
100
250
250
4
100
0
0
Table 2 Elution rates of the adsorbed Cd(II) and Pb(II) on MgO nanoparticles. Metal
Adsorption
Elution rate of heavy metal ions (%)
capacity(mg/g)
1
2
3
4
5
Cd
999.9
0.060
0.065
0.068
0.072
0.066
Pb
995.2
0.053
0.048
0.052
0.050
0.042