Reactive adsorption of SO2 on activated carbons with deposited iron nanoparticles
Graphical abstract
Highlights
► Prehumidification of carbon surface increases two and half times the uptake of SO2. ► H2SO4 is the product of SO2 reactive adsorption on the unmodified carbon surface. ► Iron species react with H2SO4 forming sulfates. ► Small iron nanoparticles enhanced adsorption by providing well-dispersed reactive centers.
Introduction
Sulfur dioxide in air has been associated with several environmental problems such as acid rain [1], [2], destruction of foliage cover, abrasion of buildings and monuments [3]. Moreover, a broad range of health problems are also linked to sulfur dioxide emissions to the atmosphere [4]. Therefore, there is a great interest in either desulfurization of fossil fuels or removal of SO2 directly from emission sources. One of the most widespread technologies used for the removal of sulfur dioxide is adsorption. Several adsorbents have been investigated and modified for the effective removal of SO2 from a gas phase. They include alumina [5], zeolites [5], [6], metal surfaces [7], [8], [9] and carbonaceous materials [10], [11], [12], [13], [14], [15]. Activated carbon is often used as a SO2 removal medium. It has been demonstrated that SO2 can be oxidized to SO3 and then in a reaction with water, sulfuric acid is formed [10]. This leads to the acidification of the activated carbons surface. Therefore carbons of a basic surface and small pores [10], [16] were found as the most suitable for SO2 separation from air.
The reactions between SO2 and iron oxyhydroxides surfaces have been investigated by several research groups [8], [17], [18], [19], [20]. Baltrusaitis and coworkers [8] found that SO2 can be bound to the surface of hematite (α-Fe2O3) and goethite (α-FeOOH) forming adsorbed sulfite and sulfate. Furthermore, it has been reported that iron oxyhydroxides promote SO2 transformation by oxidation/photolysis [17]. Faust and coworkers [19] proposed a mechanism of surface complex formation by a ligand exchange with surface hydroxyl groups. Davini, in his studies of the adsorption capacity of materials consisting carbon and iron derivatives [21], found that the SO2 adsorption capacity increased from 35 mg/g to 79 mg/g with an increase in the iron content from 0 to 1.28%. This was attributed to the presence of basic surface sites on carbon and to iron that promotes SO2 transformation into more stable species. This latest finding highlights the benefits of carbon-iron adsorbents for SO2 adsorption. However, in the majority of the cases, when carbon materials containing iron have been studied as SO2 adsorbents, the particle size of iron oxyhydroxides was not discussed.
The size of metal particles can be considered as a determinant factor for the adsorption of various molecules on activated carbon. When very large metal particles are deposited on the surface, they can lead to the pore blocking and a low adsorption capacity. On the other hand, when nanoparticles are deposited on the surface, the pores can be still accessible and both physical adsorption and chemisorption can enhance a separation process. Yean and coworkers [22] reported that a decrease in the size of magnetite particles from 300 nm to 20 nm led to an increase of about 200% in the arsenite adsorption. Stara and Matolin [23] found that Pd particles of 27 and 2.5 nm supported on alumina affect differently the adsorption of CO. They reported that the activation energy for CO desorption is particle-size dependent and it decreases with a decrease in size. Zhang and co-workers [24] found a dramatic increase in the response and sensitivity of SnO2(CuO) sensors, when the particle size was in the range of nanometers. In our previous work [25] we demonstrated that it is possible to anchor iron nanoparticles of about 3–4 nm at the surface of carbon by using phosphate as a capping agent. The decrease in the particle size allowed increasing the uptake of arsenic from aqueous solution up to 40%, without an increase in the iron content of the materials. These iron-containing materials can be very efficient media for SO2 adsorption from ambient air since iron nanoparticles can exhibit special catalytic properties due to their quantum confinement [26].
Taking into account the above, the objective of this paper is to evaluate the effect of two different nanoparticle sizes of iron oxyhydroxides, anchored on activated carbons for the removal of SO2 from a gas phase. The introduction of very small nanoparticles is expected to enhance the SO2 adsorption capacity via providing highly dispersed active sites for the adsorption/reactive adsorption process. The interactions between the surface of the materials and SO2 are analyzed based on the extensive characterization of porosity and surface chemistry.
Section snippets
Materials
A commercial activated carbon used in this study was Filtrasorb 400 manufactured by Calgon (AC). For the modification the particle size of 250–500 μm was chosen and the initial carbon was washed with a 10% HCl solution and then rinsed with double deionized water until constant pH. Finally, the carbon was dried at 100 °C for 12 h to remove moisture.
The Filtrasorb carbon was treated to anchor two different particle sizes of iron oxyhydroxides on its surface. The following procedure was followed to
Results and discussion
Some details on the characterization of AC and AC-MP samples are presented in Ref. [31]. For the sake of comparison relevant results for the initial samples are included to the comparative graphs.
After modification, the iron content in AC-M increased to 1.39% and in AC-MP to 1.12%. As can be seen in the TEM images in Fig. 1, the iron nanoparticles in AC-M are in the range of 15–30 nm; however, when the particles are synthesized in the presence of a capping, there is a remarkable decrease in the
Conclusions
The results presented in this paper demonstrate that the anchorage of very small iron nanoparticles of about 3–4 nm on the surface of activated carbon increases the amount of SO2 adsorbed of about 25%. It happens owing to the high dispersion of the adsorption centers with the high affinity for SO2 bonding. On the surface of carbons sulfur dioxide is either weakly, as SO2 gas, or strongly, as sulfuric acid, deposited in the pore system. Water is needed to form the later. The sulfuric acid
Acknowledgments
This project was funded by CONACYT-Ciencia Basica (SEP-CB-2008-01-105920) and ARO grant W911NF-10-1-0039, NSF collaborative grant CBET 1133112. Javier Arcibar-Orozco appreciates the scholarship from CONACYT (232598). We thank to Dulce Partida and Guillermo Vidriales for their technical support in samples preparation, and to Dr. Miguel Avalos-Borja and Nicolas Cayetano for the microscopy studies. The authors are grateful to Dr. Jacek Jagiello for SAIEUS software.
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