Investigating the Heavy Metal Adsorption of Mesoporous Silica Materials Prepared by Microwave Synthesis
© The Author(s). 2017
Received: 29 January 2017
Accepted: 11 April 2017
Published: 4 May 2017
Mesoporous silica materials (MSMs) of the MCM-41 type were rapidly synthesized by microwave heating using silica fume as silica source and evaluated as adsorbents for the removal of Cu2+, Pb2+, and Cd2+ from aqueous solutions. The effects of microwave heating times on the pore structure of the resulting MSMs were investigated as well as the effects of different acids which were employed to adjust the solution pH during the synthesis. The obtained MCM-41 samples were characterized by nitrogen adsorption–desorption analyses, X-ray powder diffraction, and transmission electron microscopy. The results indicated that microwave heating method can significantly reduce the synthesis time of MCM-41 to 40 min. The MCM-41 prepared using citric acid (c-MCM-41(40)) possessed more ordered hexagonal mesostructure, higher pore volume, and pore diameter. We also explored the ability of c-MCM-41(40) for removing heavy metal ions (Cu2+, Pb2+, and Cd2+) from aqueous solution and evaluated the influence of pH on its adsorption capacity. In addition, the adsorption isotherms were fitted by Langmuir and Freundlich models, and the adsorption kinetics were assessed using pseudo-first-order and pseudo-second-order models. The intraparticle diffusion model was studied to understand the adsorption process and mechanism. The results confirmed that the as-synthesized adsorbent could efficiently remove the heavy metal ions from aqueous solution at pH range of 5–7. The adsorption isotherms obeyed the Langmuir model, and the maximum adsorption capacities of the adsorbent for Cu2+, Pb2+, and Cd2+ were 36.3, 58.5, and 32.3 mg/g, respectively. The kinetic data were well fitted to the pseudo-second-order model, and the results of intraparticle diffusion model showed complex chemical reaction might be involved during adsorption process.
Heavy metal pollution has become a major environmental problem, threatening the environment and public health. Heavy metals can accumulate in the environment and cause heavy metal poisoning. They are not biodegradable and cannot be metabolized or decomposed. Moreover, they can easily enter the food chain and cause chronic toxic effects with gradual accumulation in living organisms. According to the World Health Organization drinking water guidelines, the acceptable concentration limits for Cu2+, Cd2+, and Pb2+ are 2, 0.003, and 0.01 mg/L, respectively . Several methods have been applied for the effective removal of heavy metal ions such as ion exchange , nano-filtration , solvent extraction , chemical precipitation , adsorption [6, 7], etc. Among the available methods, adsorption technology is the most promising and frequently used technique due to its simplicity, high efficiency, and low cost . Various kinds of adsorption materials have been used to remove heavy metal ions from aqueous solution, such as activated carbon  and clays . However, these materials have low adsorption efficiency for heavy metal ions because of their low pore volume and poor pore structure.
Mesoporous materials have been widely used for the adsorption of heavy metal ions [11–14] due to their exceptionally large specific surface area, regular pore structure, and tunable pore sizes. Since the discovery of M41S silica in 1992 , MCM-41 has become the most popular type of M41S silica materials and one of the most commonly used mesoporous adsorption materials. MCM-41 is characterized by highly uniform pore channels, large pore size, and high surface area . Synthesis of mesoporous MCM-41 materials attracts a lot of interest because of their potential applications in catalysis , ion exchange , biosensors , and drug delivery [20, 21]. Commercial reagents employed in the traditional preparation of MCM-41 are always expensive and toxic, such as tetraethylorthosilicate (TEOS) . Moreover, other silica precursors, such as agricultural waste, have also been used to produce mesoporous silica materials (MCM-41). For instance, using rice husk ash for the synthesis of SBA-15, MCM-41, and MCM-48 has been reported previously [23–25]. The use of mesoporous materials from industrial solid waste, such as coal combustion waste fly ash, as a silica source has also attracted attention due to economic advantages [26, 27]. Silica fume, a very fine amorphous silica powder, is obtained as a by-product during metal production processes. Silica fume mainly contains amorphous SiO2 particles (greater than 85 wt%), and it has been used as an inexpensive silica source in the synthesis of microporous and mesoporous materials . Thus, it further broadens the applications of silica fume which is generally used as an additive in cement and concrete , in bricks and ceramic tiles , as well as a filler in plastics and paints . The use of silica fume as silica source also allows the green synthesis of MCM-41, since it does not require the use of any harmful reagents.
The conventional hydrothermal synthesis process of MCM-41 requires a long crystallization time and a high crystallization temperature. In 1996, thermally stable molecular sieve MCM-41 with hexagonal channels was synthesized in a temperature-controlled microwave oven from aged precursor gels within about 1 h by Wu et al. . At present, microwave irradiation technique is widely applied to the synthesis of mesoporous molecular sieves [33–35]. Compared with the conventional hydrothermal synthesis method, microwave-assisted synthesis method employs microwave dielectric heating, uniform heating, or molecular selective heating. Thus, it offers the advantages of homogeneous heating and fast elevation of the temperature of synthesis system to crystallization temperature, resulting in more homogeneous nucleation, shorter crystallization times [36, 37], and products with uniform size [38, 39].
This work explores the preparation of mesoporous silica materials (of MCM-41 type) using a rapid microwave heating method and their application for Cu2+, Pb2+, and Cd2+ removal from aqueous solutions. The effects of different acids used for pH adjustment and different microwave heating times on the pore structure of MCM-41 were investigated. We also studied the influence of pH on adsorption capacity of the adsorbent. The metal adsorption isotherms were fitted using Langmuir and Freundlich adsorption isotherm models. In addition, kinetics and intraparticle diffusion model were also studied to understand the mechanism of the adsorption process.
Silica fume was used as the silica source and it was obtained from a local metallurgy-grade silicon factory. Its main chemical component (85 wt%) is SiO2. It was dried at 150 °C for 24 h and used after dissolution and purification; more details can be found in Additional file 1: Figure S1 and Table S1. Other reagents, such as sodium hydroxide (NaOH), cetyltrimethylammonium bromide (CTAB), hydrochloric acid (HCl), citric acid, oxalic acid, acetic acid, copper nitrate (Cu(NO3)2), lead nitrate (Pb(NO3)2), and cadmium nitrate (Cd(NO3)2) were purchased from Shanghai Chemical Reagent Company of China and used as received. All of the reagents were analytical grade.
Preparation of MCM-41
The sodium silicate solution was conveniently obtained using dissolution method. Then, the MCM-41 was synthesized by the following procedure: 5 g of CTAB was dissolved in 100 mL of deionized water, and the aqueous solution was continuously stirred for 30 min at room temperature. Then, 50 mL of pretreatment filtrate was slowly introduced into the above CTAB solution under stirring, and then one of several different acids (HCl, citric acid, oxalic acid, or acetic acid) was added dropwise to adjust the pH value of the solution to an optimum range of 10.87–10.88. Subsequently, the solution was stirred for 30 min–1 h and then subjected to microwave heating (SANYO EM-208MS1, China) for different times to allow the hydrothermal reaction to occur. After crystallization of the product, it was then cooled to room temperature, filtered, and washed with deionized water. The obtained solid product was placed in an oven and dried at 110 °C for 12 h. Finally, the synthesized material was calcined at 550 °C for 5 h to remove the template molecules. MCM-41 samples synthesized via this route were designated as x-MCM-41(y), where “x” represents the acid used to adjust reaction solution pH and “y” is the microwave heating time.
The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-3B diffractometer with Cu–Kα radiation with a voltage of 40 kV. Nitrogen (N2) adsorption–desorption isotherms were measured at 77.5 K by a NOVA 2200e surface area and pore size analyzer (Quantachrome Instruments). The specific surface area of the sample was calculated using BET (Brunauer–Emmett–Teller) method, and pore size distribution was determined using BJH (Berrett–Joyner–Halenda) model. The sample morphology was examined using a JEOL JEM-2100 transmission electron microscope (TEM).
Effect of Initial pH
The effect of pH on Cu2+, Pb2+, and Cd2+ adsorption was investigated over the pH range from 3.0 ± 0.1 to 7.0 ± 0.1. In a typical procedure, the adsorption experiments of Cu2+, Pb2+, and Cd2+ were carried out in a series of conical flasks containing 0.1 g of c-MCM-41(40) and 100 mL of Cu2+, Pb2+, and Cd2+ solutions of 40 mg/L. The pH was adjusted to the desired value by HCl (0.1 M) or NaOH (0.1 M), and the pH values were measured by a pH meter (OHAUS Starter 3C, China). Then, the conical flasks were continuously stirred at 25 °C for 24 h. After reaching adsorption equilibrium, the suspended adsorbent was easily collected from the aqueous solution by centrifugation at a speed of 5000 rpm for 5 min. Then, the supernatant was filtered with a syringe filter of 0.22 μm and analyzed via an AAS spectrophotometer after appropriate dilution.
Adsorption Isotherm Experiment
Kinetic Adsorption Experiment
Results and Discussion
Characterization of MCM-41
Textural properties of the samples prepared with microwave heating
S (m2 g−1)
V (cm3 g−1)
Effect of Solution pH
Comparison of Cu2+, Pb2+, and Cd2+ sorption capacities with other sorbents
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Water Research, 32(4) (1998) 1314–1322
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Activated poplar sawdust
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Proc. 1999 Conference on Hazardous Waste Research, St. Louis, 1999, pp. 121–130
The kinetics of Cu2+, Pb2+, and Cd2+ adsorption on c-MCM-41(40) are shown in Additional file 1: Figure S5. The adsorption of all heavy metals rapidly occurred within 50 min due to the large number of available sites at the initial stage. With the increase in adsorption time, there was a gradual decrease in the concentration of heavy metals in solution as well as the available active sites on adsorbent, due to the accumulation of metal ions on c-MCM-41(40), leading to the decrease in adsorption rate at the later stage.
Kinetic parameters and correlation coefficients of the two kinetic equations on c-MCM-41(40)
q e.exp (mg/g)
First-order kinetic model
Second-order kinetic model
a e,cal (mg/g)
q e,cal (mg/g)
Intraparticle Diffusion Model
In this study, an ordered mesoporous silica material MCM-41 has been conveniently synthesized using silica fume as the silica source under microwave heating. Microwave heating synthesis method significantly reduced the reaction time to 40 min. The sample c-MCM-41(40) prepared with citric acid under microwave heating for 40 min possesses the highest pore volume and pore diameter among all the samples, as well as well-defined crystallinity and regular hexagonal array of mesoporous channels. It also shows good performance for removing Cu2+, Pb2+, and Cd2+ in the pH region of 5.0–7.0. The adsorption data of Cu2+, Pb2+, and Cd2+ showed good fitting with the Langmuir isotherm, suggesting that the adsorption process is a homogeneous process. The maximum adsorption capacities of c-MCM-41(40) for Cu2+, Pb2+, and Cd2+ were 36.3, 58.5, and 32.3 mg/g, respectively. Kinetic data of c-MCM-41(40) were found to fit better with pseudo-second-order kinetic model than pseudo-first-order kinetic model, and the calculated equilibrium adsorption capacities (q e,cal) from the pseudo-second-order adsorption kinetics were very close to the experimental data (q e.exp). In addition, the results of intraparticle diffusion model indicate that the intraparticle diffusion is not the only rate-limiting step, and complex chemical reactions or redox reactions might also be involved.
Financial support from NSFC (21307046, U1137601, and U1402233), Natural Science Foundation of Yunnan Province (2015FB120), and personnel training funds of KMUST (KKZ3201422005) are gratefully acknowledged.
WJZ carried out the whole study, explained and discussed obtained results, and prepared the manuscript. JXW, DW, and XTL, realized the synthetic part. CYH coordinated the analytical part of the study. SFH carried out the physical–chemical measurements of the investigated materials. YML and WHM designed and coordinated the study. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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