Article pubs.acs.org/est
Rapid and Efficient Removal of Microcystins by Ordered Mesoporous Silica Wei Teng,† Zhangxiong Wu,‡ Dan Feng,† Jianwei Fan,†,§ Jinxiu Wang,† Hao Wei,† Mingjuan Song,† and Dongyuan Zhao†,‡,* †
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, P. R. China ‡ Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia § College of Environmental Science and Engineering, State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, P. R. China S Supporting Information *
ABSTRACT: To alleviate the environmental and health threats from water resources polluted by large-sized microcystins (MCs), we demonstrate for the first time that ordered mesoporous silica materials with large pore sizes of 2−12 nm can be used as adsorbents for rapid and efficient removal of MCs. The obvious correlations between adsorption performance of MCs and physicochemical properties of adsorbents including pore mesostructure, texture and size, and surface chemistry have been well established. Accordingly, an excellent candidate, mesoporous silica SBA-15 templated from Pluronic P123 has been sorted out, exhibiting extremely rapid rate (one minute) as well as high capacities of 5.99 and 13 mg g−1 for removing high-concentration MC-LR and MC-RR, respectively, which are much higher than that of other silica-based adsorbents reported previously. The adsorption performance can be further improved from 50 to 95% at around pH 4 by grafting positively charged and/or hydrophobic groups onto pore surface of SBA-15. Furthermore, thermodynamic and kinetic evaluations provide additional valuable information for a better understanding of the adsorption process. Given the excellent adsorption performance, it is expected that mesoporous silica materials with unique characteristics are attractive for actual applications in removal of MCs from wastewater.
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carbons (ACs),13−15 clays,16 and peats.17 Unfortunately, few adsorbents have been developed for highly efficient removal of MCs, which is mostly due to the lack of a well understanding of the possible influences of the properties of adsorbents on adsorption and a better design for materials with suitable structure, texture, and surface chemistry for adsorption of MCs. MCs are monocyclic heptapeptides composed of seven amino acids including five invariant amino acids and two variable amino acids R1 and R2 (Supporting Information (SI) Figure S1).18 When these two variable positions are replaced with leucine and arginine, the molecule is named MicrosystinLR (MC-LR).19,20 It has a large molecular dimension with a volume of 2.63 nm3, an area of 1.8 nm2 and a molecular length up to ∼1.9 nm when being solvated in water.21 Microsystin-RR (MC-RR) with two arginine residues also has a similar molecular dimension. Therefore, a good adsorbent must have sufficiently large and accessible mesopores to accommodate
INTRODUCTION Microcystins (MCs) are a family of cyanobacterial toxins produced by Microcystis aeruginosa in many eutrophic waters, which are taking increasing adverse effects on both environment and human health worldwide.1−3 The primary threats of MCs originate from the high activity and acute lethal toxicity, leading to liver damage and tumor promotion.3,4 Therefore, it is of urgent need to develop a reliable method for rapidly and effectively removing them from water sources. This is of challenge because MCs are extremely stable due to their cyclic structures.5 Several strategies have been investigated for the removal of MCs. Traditional technologies including chemical coagulation, flocculation, and sand filtration can remove the particulate cyanobacterial cells but are ineffective against dissolved toxins.6−8 Chemical oxidation processes, such as chlorination and ozonation, are effective but they usually require high dosages and may result in formation of harmful byproducts such as tihalomethanes.8−10 Biological treatments generally exhibit a low efficiency because of a long operation time (from hours to days).11,12 By contrast, adsorption is regarded as a simple, effective and time-saving technology for the removal of MCs. Common adsorbents include activated © 2013 American Chemical Society
Received: Revised: Accepted: Published: 8633
February 13, 2013 June 23, 2013 June 23, 2013 June 24, 2013 dx.doi.org/10.1021/es400659b | Environ. Sci. Technol. 2013, 47, 8633−8641
Environmental Science & Technology
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MCs. Traditional adsorbents, such as ACs, mainly composed of micropores (6 nm) are convenient for entrance of MCs. It can be directly observed that the adsorption amount improves correspondingly with the increase of pore size from 6.8 to 10.2 nm on the pristine SBA-15. Although the total surface areas decline with enlarged mesopore size, the surface areas from the mesopores are not obvious changed, because the reduction in surface areas comes from micropores but not mesopores. The results demonstrate that at a certain range of pore diameter, the mesopore diameter or volume (increased with pore enlargement) plays a key role in improvement of the adsorption capacity, which is probably due to the easier mass transportation in larger spaces.49 That is, the adsorbent SBA15-10 with a large mesopore diameter or volume, straight pore channels, and high mesoporous surface area performs excellent adsorption ability toward MCs. In addition, there lots of small mesopores exist in the pore walls of SBA-15, which are interpenetrated with the mesopores. Although the utilization of the smaller mesopores is mostly impossible, the capillary force inside the micropores may facilitate the mass transfer of MCs.50 The capillary forces from both mesopores and micropores lead to a rapid adsorption rate. Influences of Surface Chemical Properties. The adsorption performances change a lot after functionalizing
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ASSOCIATED CONTENT
S Supporting Information *
Details in the syntheses and characteriztion of all ordered mesoporous silica adsorbents and MCs analysis. Adsorption isotherms and kinetic modelings. Nine figures, two schemes and four tables. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-21-51630307; e-mail:
[email protected]. Notes
The authors declare no competing financial interest. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program (2013CB934104 and 2012CB224805), the NSF of China (21210004), and Shanghai Leading Academic Discipline Project, Project Number: B108, the State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF12001) and the discovery project of Australian Research Council (DP120101194).
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