分级多孔 γ-Al2O3空心微球微波水热法制备及其对刚果红的快速吸附

聂龙辉 谭 侨 朱 玮 魏 琪 林志奎

(1湖北工业大学化学化工学院, 武汉 430068; 2湖北省催化材料协同创新中心, 武汉 430068)

分级多孔 γ-Al2O3空心微球微波水热法制备及其对刚果红的快速吸附

聂龙辉1,2,*谭 侨1朱 玮1魏 琪1林志奎1

(1湖北工业大学化学化工学院, 武汉 430068;2湖北省催化材料协同创新中心, 武汉 430068)

以KAl(SO4)2和尿素为前驱体, 通过微波水热法于180 °C反应20 min, 经600 °C 焙烧2 h制得分级多孔γ-Al2O3空心微球. 所制备的样品被用于吸附典型有机染料刚果红(CR)溶液. 结果表明, 制备的γ-Al2O3空心微球直径为0.8–1.0 μm, 厚度约为200 nm. 此γ-Al2O3空心微球具有高的比表面积(243 m2g–1)和分级大孔-中孔结构,此结构非常有利于液相过程中的质量传递. 微波水热法制备的γ-Al2O3空心微球比水热法制备的γ-Al2O3和商用的γ-Al2O3样品显示出更快和更强的吸附性能. 此样品的吸附数据很好地符合假二级速率方程和Langmuir吸附理论模型. 从Langmuir吸附理论模型计算得到微波水热法制备的γ-Al2O3空心微球的最大吸附量(qmax) 25 °C时高达 515.4 mgg–1. 由于具有分等级结构、高比表面积、大的孔容和吸附能力, 微波水热法制备的γ-Al2O3空心微球样品有望成为一种具有很好应用潜力的环境吸附剂.

分级多孔材料; γ-Al2O3; 刚果红; 吸附动力学

d oi: 10.3866/PKU.WHXB201507201

1 Introduction

Synthetic dyes are one of the most common pollutants in water from textile and printing industries. For the most of dyes, they easily give rise to the carcinogenicity, teratogenicity, and even mutagenicity due to the high toxicity, which will bring great threats to various livings and human health.1Even very low concentration of the dyes present in water can be highly toxic to aquatic systems.2So, the removal of dye pollutants from wastewater is urgent to satisfy environmental requirements. The efficient and fast removal of dyestuffs from water has attracted increasing attention due to the increasingly serious water-polluting problems. Adsorption is a widely used and investigated technology for the removal of toxic dyes from wastewater due to its simplicity, high-efficiency, and low cost compared to other treating methods, such as biological treatment, advanced oxidation process, and electrochemical technique. Various metal oxides were widely used as adsorbents for the dye removal, such as γ-Al2O3,3,4γ-Fe2O3and Fe3O4,5–7BiOI,8MgO,9CeO2,10NiO,11and MnO2,12among them, gamma alumina (γ-Al2O3) has been regarded as a promising inorganic adsorbent due to its excellent physical-chemical properties and low toxicity.3,13Recently, there have been a great number of interests in the development of inorganic oxide materials with hierarchically porous hollow microsphere structure as adsorbents, catalysts or catalyst supports due to their large specific surface area, low density, mciron size, and special porous structure system being benificial to transfer reactants to active sites.5,14,15For example, Xu et al.5prepared γ-Fe2O3and Fe3O4magnetic hierarchically nanostructured hollow microspheres by a solvothermal combined with precursor thermal conversion method. The prepared γ-Fe2O3hierarchically nanostructured hollow microspheres showed a better adsorption ability for salicylic acid (SA) than methylene blue (MB) and basic fuchsin (BF), and Fe3O4hierarchically nanostructured hollow microspheres had the best performance for adsorbing MB. Kong et al.16fabricated hierarchically porous TiO2/Pd composite hollow spheres by using solvothermal treatment. The as-prepared TiO2/Pd composite hollow spheres exhibited high electrocatalytic activity towards the reduction of H2O2. In our previous work,16hierarchically macro-mesoporous structured γ-Al2O3hollow spheres with open and accessible pores were firstly synthesized by hydrothermal method and then used as support to deposit Pt nanoparticles on their surface. The prepared Pt/γ-Al2O3composite hollow spheres showed higher catalytic activity in the oxidative decomposition of HCHO at room temperature than those prepared using the conventional nanoparticles as supports. Great efforts have been also made to prepare the hierarchically porous γ-Al2O3(or γ-AlOOH, the precursor of γ-Al2O3) hollow microsphere.3,16–20However, most of hierarchically porous γ-Al2O3(or γ-AlOOH) hollow microspheres were prepared by hydrothermal (HT) method.16,17,19–21One of the main drawbacks of this conventional method is the slow reaction rate and needs a long reaction time at a given temperature. Yet, microwave-assisted hydrothermal (MAH) can overcome the above disadvantage and is often used to fabricate various nano materials14,15,22because microwave-assisted heating is generally faster, eco-friendly, and very energy-saving.

Up to now, it is also a challenge to develop a facile, ecofriendly, and energy-saving method for preparing dye adsorbents with fast adsorption and large adsorption capacity. In our previous work,23the γ-Al2O3hollow microspheres with high surface and hierarchically meso-macroporous structure were suceessfully prepared for 40 min by the MAH method and following calcination using KAl(SO4)2and urea as raw materials. The obtained γ-Al2O3hollow microspheres showed the enhanced adsorption capacity for Cu2+in aqueous solution. In this study, the hierarchically nanostructured γ-Al2O3hollow microspheres was also prepared by the MAH method but in a shorter reactive time of 20 min from the viewpoint of energy saving. Here, the prepared γ-Al2O3hollow microspheres were used to remove the CR from water. The prepared γ-Al2O3hollow microspheres exibited a fast adsorption with a short absorption equilibrium time and enhanced adsorption performace for CR than that prepared by HT or the commercial γ-Al2O3product. The dye adsorption data were fitted to different kinetics and adsorption isotherm models.

2 Experimental

The preparation of the hierarchically porous γ-Al2O3by the MAH method (named MWAO) was similar to our previous report.23Only the difference is that the mixture solution of KAl(SO4)2and urea (AR, purchased from Sinopharm Chemical Reagent Co., Ltd.) was microwave-treated at 180 °C for 20 min. For comparison, the hierarchically porous γ-Al2O3by the HT method (named HTAO) was prepared by the same procedure with the literature23and the commercial γ-Al2O3(named CAO) was purchased from Sinopharm Chemical Reagent Co., Ltd.

2.1 Characterization

The as-synthesized samples were analyzed by X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2adsorptionadsorption isotherms. The types of instruments and experimental conditions are the same as those in the literature.23The zetapotential of the MWAO sample at different pH values was measured using a Zetaplus, Brookhaven Instruments Corporation, USA.

2.2 Adsorption experiments for Congo red

Adsorption measurements for Congo red (C32H22N6O6S2Na2) were performed by adding 80 mg of as-prepared samples into 100 mL of CR having a concentration of 45–360 mgL–1(under stirring conditions) at room temperature (25 °C) and pH 7.0.Analytical samples were taken from the suspension after various adsorption times (total adsorption time: 60 min) and separated by centrifugation. The CR concentration was analyzed using a UV-Vis spectrophotometer (UV-2550, Shimadzu, Japan). c/c0was evaluated and used to characterize the relative adsorption capacity (c0and c represent the initial concentration of CR and its concentration after treatment, respectively). The amount of CR adsorbed per unit mass of the adsorbent was also evaluated by using the mass balance equation as below:

where qt(mgg–1) is the amount adsorbed per gram of adsorbent at time t (min), c0is the initial concentration of CR in the solution (mgL–1), ctis the concentration of CR at time t of adsorption (mgL–1), m is the mass of the sample (g), and V is the volume of solution (L).

3 Results and discussion

3.1 Phase structures and morphology

The XRD patterns of the MWAO, HTAO, and CAO samples are shown in Fig.1a. It shows that all the diffraction peaks are indexed to γ-Al2O3(JCPDF card, No. 10-0425). No XRD peaks of other alumina phases can be observed, indicating pure γ-Al2O3phase is obtained by HT and MAH methods with following calciation at 600 °C for 2 h.

Figs.1b and 1c show the SEM images of the MWAO and HTAO samples, respectively. It can be seen from SEM images that the prepared MWAO and HTAO samples are hierarchically hollow microspheres. Also, it clearly shows that the preparation methods have an important effect on the microsphere size. The hollow microsphere diameters of γ-Al2O3are around 0.8–1.0 μm for the MWAO sample by MAH method (Fig.1b) and 4.0–6.0 μm for the HTAO sample by HT method (Fig.1c), respectively. The microsphere size of the former is much smaller than that of the latter, indicating the possible larger surface area for the former. Further observation indicates that the external surface of the MWAO hollow spheres consisted of randomly aggregated and interconnected nanoplatelets (inset of Fig.1b); consequently, the resulting microspheres are highly disordered porous superstructures with highly rough outer surface. TEM image (Fig.1d) of the MWAO sample further confirms that the MWAO sample is hollow sphere structure with a shell thickness of approximately 200 nm. The smaller products obtained in the MAH process is due to the more rapid volumetric heating and reaction rates for the MAH process than the HT method, which results in more fast and homogeneous deposition of the more and smaller aluminum oxyhydroxide (the precursor of γ-Al2O3) under the same reactant concentration.23

Fig.1 XRD patterns of the CAO, MWAO, and HTAO samples (a); SEM images of MWAO (b) and HTAO (c) samples; TEM image of the MWAO sample (d)

3.2 N2adsorption analysis

Fig.2 shows a contrast of nitrogen adsorption-desorption isotherms and pore-size distribution curves for the MWAO, HTAO, and CAO samples. The three nitrogen adsorptiondesorption isotherms for above three samples are of type IV according to the International Union of Pure and Applied Chemistry (IUPAC) classification,24indicating the existence of mesopores. Different from the CAO sample, the adsorption branches of isotherms for the MWAO and HTAO samples resemble type II, suggesting the presence of macropores. The shape of hysteresis loop for the CAO sample resembles type H4 at p/p0range of 0.5–1.0, indicating the presence of narrow slit-like pores.However, the shape of hysteresis loops for the MWAO and HTAO samples resembles type H3 at high p/p0range of 0.6–1.0, indicating the presence of slit-like pores. The isotherms of the MWAO and HTAO samples show high adsorption values at relative pressures approaching 1.0, which is for the typical materials with large mesopores and macropores.14At high p/p0of 0.9–1.0, the amount of adsorbed N2on MWAO is the biggest among the above three samples, implying the largest pore volume for the MWAO sample. The pore-size distribution curves calculated from the desorption branches of the nitrogen isotherms by the BJH method are shown in inset of Fig.2. The figure shows that the CAO sample has a narrow pore-size distribution with peak pore at ca 3.5 nm. However, the pore-size distribution curves of the MWAO and HTAO samples are quite broad with larger peak pores at ca 20 and 25 nm for MWAO and HTAO, respectively. N2adsorption-desorption isotherms do not provide information about macropores with sizes larger than 100 nm. Therefore, the macroporous structure of the MWAO sample was observed directly by SEM. The as-prepared MWAO sample exhibits a great amount of open slit-like pores with outer pore diameters of 50–150 nm in the shell wall of hollow spheres (inset of Fig.1b) and a cavity with diameter of 600 nm (Fig.1d). These open macroporous channels may serve as ideal liquid-transport routes for liquid molecules into the interior space of alumina. The BET surface area (SBET), pore volume (Vpore), and pore size (dpore) of the MWAO, HTAO, and CAO samples are listed in Table 1, further confirming the MWAO sample having the biggest specific surface area, pore volume, and pore size among the above three samples, which are very beneficial for adsorption of big molecule dye pollutants in aqueous solution.

Fig.2 Nitrogen adsorption-desorption isotherms and the corresponding pore-size distribution curves (inset) for the MWAO, HTAO, and CAO samples

Table1 Basic parameters for the as-synthesized and commercial samples

Fig.3 Adsorption rate (ct/c0, c0= 90 mgL–1) of CR on the MWAO, HTAO, and CAO samples (a), absorption spectra (b), and the corresponding picture (c) of a CR solution with time

3.3 Adsorption for Congo red

3.3.1 Comparison of adsorption capacity for Congo red

Hollow micro/nanostructured materials were usually used for removal of pollutants from water due to their desirable adsorption properties. A comparative study for CR adsorption was carried out over the hierarchical MWAO, HTAO, and commercial particle-like CAO samples (see Fig.3a). As shown in Fig. 3a, two stages of adsorption can be observed for the MWAO and HTAO samples. In the first stage, CR was immediately adsorbed within 1 min (c/c0= 0.04, 0.13 for the MWAO and HTAO samples, respectively), and in the second stage from 1 to 3 min, CR was slowly adsorbed. Then, after 3 min, CR concentration kept unchanged. It's worth noting that CR concentration became near to zero over the MWAO sample (c/c0≈ 0), indicating CR in solution was completely adsorbed on the surface of the MWAO sample, and the adsorption capacity of the MWAO sample for CR is about 112.5 mgg–1after 30 min; however, the HTAO sample was observed to reach adsorption eq uilibrium with the equilibrium adsorption capacity of 109.0 mgg–1. Comparatively, the commercial particle-like CAO sample showed lower adsorption capacity (94.7 mgg–1after 30 min, in this case, the equilibrium adsorption was not reached after 30 min. due to much smaller pore size). Obviously, the MWAO and HTAO samples with the hierarchical meso-macroporous structure showed a much faster adsorption equilibrium than thecommercial particle-like CAO sample, which is mainly due to the former's larger average pore size and hierarchical mesomacroporous structure facilitating transport of this relatively large organic molecule. Also, the MWAO sample shows the fastest and biggest adsorption for CR among the above three samples, which is due to its largest surface area, pore volume, and average pore size (see Table 1) as well as hierarchical porous structure. This observation is confirmed by the UV-Vis absorption curves and photo picture recorded at different times (see Figs.3b and 3c, respectively). Furthermore, the used MWAO sample containing CR could be regenerated by a simple thermal treatment in air at 350 °C for 2 h without diminishing its adsorption performance; its adsorption performance kept almost unchanged after the second regeneration (see Fig.4a). After adsorption test, the solid-liquid separation in suspension was rather easy by free sedimentation due to the submicron size of the hierarchical γ-Al2O3by MAH (see Fig.4b).

Fig.4 (a) Adsorption rate of CR on the MWAO sample for three recycled experiments (CR: 90 mgL–1, 25 °C) and (b) pictures of the CR solution with the MWAO sample just after the third-cycle adsorption for 30 min (1) and the above solution after free sedimentation for 120 min (2)

3.3.2 Adsorption kinetics

The adsorption kinetics describes the solute uptake rate, which, in turn governs the equilibrium time of adsorption process at the solid-solution interface and provides valuable information for understanding the mechanism of adsorption. The CR adsorption over MWAO was further studied on the variation of concentrations of CR (mgL–1) at pH = 7.0 with time ranging from 1 to 60 min (see Fig.5). It can be seen from Fig.5 that the adsorption occurs rapidly during the early stage of adsorption process, which may be due to the availability of abundant active sites on the adsorbents. Then, the uptake of CR becomes very slow due to a gradual decrease in the number of active sites in the bulk solution. The adsorption capacity increased along with time and reached a constant value after about 30 min, indicating adsorption equilibration for CR was achieved. Upon increasing the initial concentration of CR from 45 to 360 mgL–1, the adsorption capacity increased from 55.4 to 425.7 mgg–1.

Fig.5 Changes of adsorption capacity with adsorption time for Congo red on the MWAO sample at different initial CR concentrations

In general, the CR adsorption experimental data fit well with the pseudo-second order model.8,11,25The experimental data of CR adsorption over MWAO were also analyzed using pseudosecond order and intra-particle diffusion kinetic models. The uniformity between the experimental data and model-predicted values was expressed by correlation coefficients R2. The linearized form of pseudo-second order kinetic rate equation is:

where qeand qtare the adsorption capacity (mgg–1) at equilibriumand at time t (min), respectively, k2is the rate constant (gmg–1min–1). Fig.6a shows the fitting result using pseudosecond order model. The plots of t/qtvs t give straight lines under different initial concentrations. The calculated k2, qe, and R2are presented in Table 2. Table 2 shows that the obtained R2values are greater than 0.999 for the MWAO sample at different CR initial concentrations. It also shows a good agreement between the experimental and the calculated qevalues, indicating this model to describe well the adsorption process of CR onto the MWAO sample.

Intraparticle diffusion is a transport process which describes the movement of species from the bulk of the solution to the solid phase. The intraparticle diffusion model can be described as Eq.(3):

where kintrais the intra-particle diffusion rate constant (mgg–1min–1), which may be taken as a rate factor, i.e., percent CR adsorbed per unit time, ciis intra-particle diffusion rate constant related with the boundary layer thickness. The linear fits of the intraparticle diffusion model for the adsorption of CRwith different initial concentrations on the MWAO sample at 25 °C are shown in Fig.6(b). The plots show multi-linearity, implying that two or three different adsorption steps take place. The first sharper region is related to the instantaneous adsorption stage (kd1) that mainly occurred on the external surface of MWAO sample, which was completed within the first 1 min. The reason is that the initial CR concentration in the solutions is high and this large concentration gradient provides enough driving force for CR diffusing to the external surface of the MWAO sample from the solution. The second region (kd2) is the gradual or slow adsorption stage where intra-particle diffusion is the rate limiting step.2The third region (kd3) is the final equilibrium stage where intra-particle diffusion further slows down due to the extremely low concentrations of adsorbate left in the solutions. As described and discussed above, CR is slowly transported by intraparticle diffusion into the particles and finally reaches the wall surface of the nanopores. Table 3 presents a comparison of intraparticle diffusion rate parameters for the adsorption of CR with different initial concentrations on the MWAO sample at 25 °C. Further observation indicates kd1> kd2> kd3. It is easily understood because the concentration of CR left in the solutions gradually decreases with prolonging adsorption time.

Fig.6 Second-order kinetics (a) and intra-particle diffusion kinetics (b) of the MWAO sample for CR adsorption with different initial concentrations

Table2 Pseudo-second-order adsorption kinetic constants of the MWAO sample at different CR initial concentrations

3.3.3 Adsorption isotherms

To simulate the adsorption isotherm, two commonly used models, the Langmuir and Freundlich isotherms, were selected to explicate dye-MWAO interaction. According to the assumption of Langmuir model, all adsorption sites are identical and energetically equivalent, and the adsorption is localized in a monolayer, and once an adsorbate molecule occupies a site, no further adsorption can take place at that site. Then, the Langmuir isotherm can be expressed as:

where ce(mgL–1) and qe(mgg–1) are the equilibrium adsorbate concentrations in the aqueous and solid phases, qmax(mgg–1) is the maximum monolayer adsorption capacity, and kL(Lmg–1) is a constant related to the energy of adsorption. The values of qmaxand kLare calculated from the slope and intercept of the linear plot ce/qeversus ce(as shown in Fig.7(a)).

The essential characteristics of the Langmuir equation can be expressed in terms of a dimensionless separation factor, RL, defined as following:

where c0represents the highest initial solute concentration and kLis Langmuir's adsorption constant (Lmg–1).

Table3 Intra-particle diffusion model constants and correlation coefficients for the adsorption of CR with different initial concentrations on the MWAO sample

Fig.7 Langmuir isotherm of the MWAO sample for CR adsorption at 25 °C (a) and Freundlich isotherm of the MWAO sample for CR adsorption at 25 °C (b)

The Freundlich adsorption model assumes that adsorption takes place on heterogeneous surfaces. The Freundlich isotherm describes reversible adsorption and is not restricted to the formation of the monolayer. The Freundlich equation predicts that the CR concentration on the adsorbent will increase with increasing the CR concentration in the liquid. The Freundlich equation is an empirical equation and is written as follows:

where qerepresents the solid phase adsorbate concentration in equilibrium (mgg–1), cethe equilibrium liquid phase concentration (mgL–1), kFthe Freundlich constant (mgg–1)(Lmg–1)1/n, and 1/n is the heterogeneity factor. The values of kFand n are calculated from the intercept and slope of the linear plot lnqeversus lnce(as shown in Fig.7(b)).

Table4 Adsorption isotherm parameters of the MWAO sample

Fig.8 Zeta potential of the MWAO sample at different pH values

Table5 Comparison of CR adsorption capacities of different adsorbents

Table 4 shows the calculated values of Langmuir and Freundlich models' parameters. The RL(RL= 0.012, in the range of 0–1) and n (n > 1) values indicate that the CR adsorption on the MWAO sample is favourable for the above two adsorption isotherms under the experimental conditions.2From Fig.7, the CR adsorption over the MWAO sample exhibits a reasonable fit to the Freundlich model (R2= 0.9379). However, a better fit to Langmuir equation is confirmed by giving a greater R2value (R2= 0.9941). This implies that the Langmuir model may describe the adsorption isotherm for the MWAO sample better. Because the isoelectric point of the MWAO sample was measured to be about 8.9 (as shown in Fig.8), so, it is easy to understand that the Langmuir-type adsorption of CR on the MWAO sample is due to the strong electrostatic interaction between the positive charges on the surface of the MWAO sample and the negative charges of CR. Therefore, it is reasonable to infer that the adsorption is site-specific and thus a monolayer is formed. Additionally, from Table 4, the maximum adsorption capacity (qmax) calculated from Langmuir model for the MWAO sample is up to 515.4 mgg–1, which is bigger than most results reported in literature (see Table 5). The large CR adsorption capacity for the MWAO sample is mainly due to itslarge BET surface area, pore volume, and pore-size distribution. Also, a larger qmaxvalue (826.4 mgg–1) from Langmuir model than that (515.4 mgg–1) of the MWAO sample is reported for another hierarchically porous γ-Al2O3(for Al90-600),27which is mainly due to their acid adsorption condition (pH = 5.0, but in our experiment, pH = 7.0) and the larger surface area (320.6 m2g–1) for the Al90-600 sample. The lower pH value is benifical for CR adsorption because CR is an anionic dye.

4 Conclusions

Hierarchically porous γ-Al2O3hollow microspheres were synthesized by a MAH and followed by calcination method using KAl(SO4)2and urea as raw materials. The prepared γ-Al2O3hollow microspheres have the high surface area and hierarchically meso-macroporous structure. Therefore, this material also exhibits a faster adsoption and enhanced adsoption performance for Congo red in adsorption experiments than that prepared by HT and commercial γ-Al2O3, and it has a maximum Langmuir adsorption capacity of 515.4 mgg–1, which is much higher than many other oxide or mineral adsorbents. After adsorption test, the solid-liquid separation in suspension was rather easy by free sedimentation due to the sub-micron size of the hierarchical γ-Al2O3by MAH. A facile preparation condition of the hierarchical γ-Al2O3adsorbent by MAH together with its great CR adsorption performance and easy separation make this material be a promising candidate for wastewater treatment.

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Fast Adsorption Removal of Congo Red on Hierarchically Porous γ-Al2O3Hollow Microspheres Prepared by Microwave-Assisted Hydrothermal Method

NIE Long-Hui1,2,*TAN Qiao1ZHU Wei1WEI Qi1LIN Zhi-Kui1
(1School of Chemistry and Chemical Engineering, Hubei University of Technology, Wuhan 430068, P. R. China;2The Synergistic Innovation Center of Catalysis Materials of Hubei Province, Wuhan 430068, P. R. China)

Hierarchical nanostructured γ-Al2O3hollow microspheres were synthesized from KAl(SO4)2and urea precursors by the microwave-assisted hydrothermal (MAH) method at 180 °C for 20 min followed by calcination at 600 °C for 2 h. The as-prepared sample was used to remove the organic dye Congo red (CR) from aqueous solution. The results showed that the obtained γ-Al2O3hollow microspheres are about 0.8–1.0 μm in diameter with a shell thickness of approximately 200 nm. The γ-Al2O3hollow microspheres have a high surface area of 243 m2g–1and a hierarchical meso-macroporous structure, which is beneficial for mass transfer in liquid processes. Therefore, the prepared γ-Al2O3hollow microspheres exhibit faster adsorption and enhanced adsorption performance for CR than particles prepared by the hydrothermal method and commercial γ-Al2O3. The adsorption kinetic data follow the pseudo-second-order equation and the equilibrium data fit well to the Langmuir model. The maximum adsorption capacity (qmax) of the obtained γ-Al2O3hollow microspheres calculated by the Langmuir model is up to 515.4 mgg–1at 25 °C. The γ-Al2O3hollow microspheres prepared by the microwave-assisted hydrotherm method show promise as an adsorbent for environmental applications due to their hierarchical porous structure, high surface area, large pore volume, and adsorption capacity.

Hierarchically porous material; γ-Al2O3; Congo red; Adsorption kinetics

O647

Received: April 3, 2015; Revised: July 20, 2015; Published on Web: July 20, 2015.

*Corresponding author. Email: nielonghui@mail.hbut.edu.cn; Tel: +86-27-59750481.

The project was supported by the National Natural Science Foundation of China (51572074), Natural Science Foundation of Hubei Province, China (2011CDB079), Hubei College Student Innovation Training Project, China (201310500017), and Open Fund of Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, China (CHCL12003)

国家自然科学基金(51572074), 湖北省自然科学基金(2011CDB079), 湖北省大学生创新训练项目(201310500017)和中南民族大学催化与材料科学重点实验室开放基金(CHCL12003)资助

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