Article pubs.acs.org/est
Chromium Reaction Mechanisms for Speciation using Synchrotron in-Situ High-Temperature X‑ray Diffraction Fiona Low,† Justin Kimpton,‡ Siobhan A. Wilson,§ and Lian Zhang*,† †
Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia § School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia ‡
S Supporting Information *
ABSTRACT: We use in situ high-temperature X-ray diffraction (HT-XRD), ex-situ XRD and synchrotron X-ray absorption near edge structure spectroscopy (XANES) to derive fundamental insights into mechanisms of chromium oxidation during combustion of solid fuels. To mimic the real combustion environment, mixtures of pure eskolaite (Cr3+2O3), lime (CaO) and/or kaolinite [Al2Si2O5(OH)4] have been annealed at 600−1200 °C in air versus 1% O2 diluted by N2. Our results confirm for the first time that (1) the optimum temperature for Cr6+ formation is 800 °C for the coexistence of lime and eskolaite; (2) upon addition of kaolinite into oxide mixture, the temperature required to produce chromatite shifts to 1000 °C with a remarkable reduction in the fraction of Cr6+. Beyond 1000 °C, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr6+ in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000 °C has a glassy disposition that prevents its water-based leaching; and (4) Increasing temperature facilitates the migration of eskolaite particles into bulk lime and enhances the extent to which Cr3+ is oxidized, thereby completing the oxidation of Cr3+ to Cr6+ within 10 min.
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INTRODUCTION Cr has been designated as an element of major environmental concern by several regulating bodies.1 For the most part, Cr is found in either of two forms, benign Cr3+ and toxic and carcinogenic Cr6+. The latter form is highly mobile in soil and groundwater whereas Cr3+ is scarcely soluble.2 The rest of the Cr oxidation states are typically observed as unstable intermediates.3 There is a pressing need to clarify Cr speciation mechanisms owing to the adverse environmental and health impacts of Cr6+ release. This is particularly pertinent for oxidation of Cr3+-bearing compounds such as chromite (Fe2+Cr3+2O4) during the combustion of solid fuels (e.g., coal, biomass and industrial waste). Atmospheric emission from fossil fuel alone amounts to 3.4−22 Gg Cr/yr, with even more substantial mobilization from fuel to bottom and fly ash, ranging from 160−470 Gg Cr/yr.4 Compared to other heavy metals and metalloids, such Hg and As, Cr has a low volatility and thus, during combustion, it mostly remains in the solid ash with little transfer to the fugitive flue gas.5 In addition to Cr quantification, speciation of its oxidation states in solid ashes has been touched upon by previous studies (refs 6−14 summarized in Table 1) for the combustion of a variety of fuels in different types of combustors. As can be seen, the fraction of Cr6+ varies noticeably with the type of fuel, coal rank and type of combustor; however, there is a lack of a clear trend revealing the mechanisms that underpin the oxidation of Cr3+ to Cr6+. Of © XXXX American Chemical Society
the many variables that could affect this reaction in a combustion environment, it is unknown which are the crucial factors that should be controlled to minimize the formation of toxic Cr6+. It has been demonstrated using tests with pure compounds, including K2O, CaO, Na2SO4, Fe2O3, and MgO, that the extent of Cr3+ oxidation is strongly related to the reduction potential of the metal oxide.15 Thus, the oxides of K, Ca, and Na in ash promote this reaction, whereas the oxides of Fe and Mg do not produce any effect due to their low reduction potential for the activation of bulk oxygen into free radicals. Of the metal oxides commonly found in ash, the reaction between CaO and Cr3+ has received significant attention.5 This focus is appropriate because CaO is commonly present in ash irrespective of coal rank and solid fuel type. CaO is also a common sorbent that is mixed with coal and injected into the furnace for sulfur capture. The global reaction between CaO and Cr3+-bearing phases, generalized here as Cr2O3, is proposed as in the following eq 1.16,17 Cr2O3 + 2CaO + 1.5O2 → 2CaCrO4
(1)
Received: March 27, 2015 Revised: May 29, 2015 Accepted: June 9, 2015
A
DOI: 10.1021/acs.est.5b01557 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
1.1
5.6 4.5−29.3
4.9−51.7
10.1
brown coal
brown coal + silica-based additive subbituminous coal
bituminous coal
bituminous coal
199
international standard samples: WP976, EPA quality control BCR143, sewage sludge amended soil BCR144, sewage sludge BCR145, sewage sludge municipal solid waste
166 740 46.8
12
