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ROLE OF SURFACE CHEMISTRY AND MORPHOLOGY IN REACTIVE ADSORPTION OF H2S ON IRON (HYDR) OXIDES/GRAPHITE OXIDE COMPOSITES Javier A. Arcibar-Orozco, Rajiv Wallace, Joshua K. Mitchell, and Teresa J Bandosz Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504563z • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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ROLE OF SURFACE CHEMISTRY AND MORPHOLOGY IN REACTIVE ADSORPTION OF H2S ON IRON (HYDR) OXIDES/GRAPHITE OXIDE COMPOSITES
Javier A. Arcibar-Orozco, Rajiv Wallace, Joshua K. Mitchell, Teresa J. Bandosz* Department of Chemistry, The City College of New York, New York, NY 10031 USA *Whom correspondence should be addressed to. Tel.:(212)650-6017; Fax: (212)650-6107); Email:
[email protected] ABSTRACT Composites of magnetite and 2-line ferrihydrite with graphite oxide were synthesized and tested as hydrogen sulfide adsorbents. Exhausted and initial composites were characterized by adsorption of nitrogen, X-Ray diffraction, potentiometric titration, thermal analysis, and FTIR. The addition of GO increased the surface area of the composites due to a formation of new micropores. The extent of the increase depended on the nature of the iron (hydr)oxide and the content of GO. The addition of GO did not considerably change the crystal structure, but increased the amount of acidic functional groups. While for the magnetite composites an increase in the H2S adsorption capacity after GO addition was found, the opposite effect was recorded for the ferrihydrite composites. That increase in the adsorption capacity was linked to the affinity of the composites to adsorb water in mesopores of specific sizes in which the reaction with basic surface groups take place. Elemental sulfur, and ferric and ferrous sulfates were detected on the surface of the exhausted samples. A redox reactive adsorption mechanism is proposed as governing the retention of hydrogen sulfide on the surface of the composites. The incorporation of GO enhances the chemical retention of H2S due to the incorporation of OH reactive groups and an increase in surface heterogeneity.
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KEYWORDS Ferrihydrite, magnetite, graphite oxide, desulfurization, reactive adsorption, iron hydr(oxide)
Introduction Hydrogen sulfide (H2S) is a toxic gas with a foul odor that strongly affects human health,1 particularly the nervous system.2-3 The release of H2S to the atmosphere occurs naturally as a result of microbiological activity and geothermal processes. It can also be produced in industrial or municipal processes, such as a waste water treatment.4-5 An increase in industrial activities and more stringent environmental regulations has turned H2S removal into a critical issue in recent years.6 Therefore, there is an increasing demand for efficient H2S removal technologies, especially those which can be applied at ambient conditions.
Amongst the current removal technologies, the adsorption process stands as one of the most feasible due to is convenience of application, low energy demand, and the relatively low waste generation.7 H2S adsorbents that perform well at room temperature include pyrolyzed waste materials,8 activated carbons,9-13 zeolites,14 and metal oxides.15-17 The latter consist of a wide range of versatile adsorbents that can be employed for the reactive adsorption of H2S.7 Moreover, it has been reported that composites of metal oxides and carbonaceous phases such as graphite, graphene, and graphite oxide (GO) increase the ability of oxides to adsorb H2S.18-21 Such composites can be successfully synthetized by co-precipitation from a salt precursor in the presence of the dispersed graphite phase.22 Mabayoje and coworkers20 synthesized composites of cobalt (hydr)oxides with graphite oxide and/or graphene. The materials adsorbed a marked amount of H2S. The highest H2S adsorption capacity was found for the composite containing a small amount of graphite oxide (5%). The GO incorporation increased the performance of about 2 ACS Paragon Plus Environment
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four times. A different trend was observed for copper (hydr) oxychlorides, in which a five times increase in the adsorption capacity was recorded on the materials containing 10% graphene.18 Other examples of the positive effects of GO addition to metal (hydr)oxides were reported for zinc (hydr)oxide15 and zirconium hydroxide19 composites, where the GO addition increased the H2S uptake by 50 % and 20 %, respectively.
Iron oxides, hydroxides, or collectively named iron (hydr)oxides, either crystalline or amorphous, have been addressed in the literature as efficient adsorbents of H2S.7, 23-27 The adsorption capacity depends on the surface area, pore size, crystal structure, and coordination chemistry of the specific iron (hydr)oxide.28 Magnetite and ferrihydrite are two important iron (hydr)oxides. The former is an oxide that contains a mixture of FeIII and FeII that gives magnetic properties,29 and also can contribute to sulfur oxidation.30 The latter is hydrous ferric oxide of a microcrystalline structure1, 31
that can have high surface areas with reactive hydroxyl groups randomly dispersed.32 Despite
that, not many studies have described in detail the interactions of H2S with these two iron (hydr)oxides. To the best of our knowledge, no composites of these two crystal structures and GO have been synthesized to be used as desulfurization media. Since interactions of the graphite oxide with iron (hydr)oxide are expected,22, 33 the final properties of the composites will depend on the type of (hydr)oxide. This in turn might lead to the differences in the behavior of these composites as H2S adsorbents.
The objective of this paper is a synthesis of new composites consisting of iron (hydr)oxides and graphite oxide, and an evaluation of their performance as H2S desulfurization media. The effect of the GO addition was studied in two iron (hydr)oxides with different chemical/physical nature: highly amorphous 2-line ferrihydrite and crystalline magnetite. The materials were extensively characterized and their ability to remove H2S was linked to physicochemical features of their 3 ACS Paragon Plus Environment
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surfaces. Finally, based on the collected data, an adsorption mechanism for H2S on the composite materials was proposed.
Experimental Materials All chemicals used (FeCl3•6H2O, FeSO4•7H2O, NH4OH, Ethanol, NaOH, Graphite, and AgNO3) were reagent grade. All solutions were prepared with deionized water (
