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Cr(VI)/Cr(III) and As(V)/As(III) Ratio Assessments in Jordanian Spent Oil Shale Produced by Aerobic Combustion and Anaerobic Pyrolysis Tayel El-Hasan,† Wojciech Szczerba,‡ G€unter Buzanich,‡ Martin Radtke,‡ Heinrich Riesemeier,‡ and Michael Kersten§,* †
Geology Department, Faculty of Science, Taibah University, Al-Madinah Al-Munawwarah, KSA BAM Federal Institute for Materials Research and Testing, Department 1: Analytical Chemistry and Reference Materials, Berlin 12200, Germany § Geosciences Institute, Johannes Gutenberg-University, Mainz 55099, Germany ‡
ABSTRACT: With the increase in the awareness of the public in the environmental impact of oil shale utilization, it is of interest to reveal the mobility of potentially toxic trace elements in spent oil shale. Therefore, the Cr and As oxidation state in a representative Jordanian oil shale sample from the El-Lajjoun area were investigated upon different lab-scale furnace treatments. The anaerobic pyrolysis was performed in a retort flushed by nitrogen gas at temperatures in between 600 and 800 °C (pyrolytic oil shale, POS). The aerobic combustion was simply performed in porcelain cups heated in a muffle furnace for 4 h at temperatures in between 700 and 1000 °C (burned oil shale, BOS). The high loss-on-ignition in the BOS samples of up to 370 g kg 1 results from both calcium carbonate and organic carbon degradation. The LOI leads to enrichment in the Cr concentrations from 480 mg kg 1 in the original oil shale up to 675 mg kg 1 in the g850 °C BOS samples. Arsenic concentrations were not much elevated beyond that in the average shale standard (13 mg kg 1). Synchrotronbased X-ray absorption near-edge structure (XANES) analysis revealed that within the original oil shale the oxidation states of Cr and As were lower than after its aerobic combustion. Cr(VI) increased from 0% in the untreated or pyrolyzed oil shale up to 60% in the BOS ash combusted at 850 °C, while As(V) increased from 64% in the original oil shale up to 100% in the BOS ash at 700 °C. No Cr was released from original oil shale and POS products by the European compliance leaching test CEN/TC 292 EN 12457-1 (1:2 solid/water ratio, 24 h shaking), whereas leachates from BOS samples showed Cr release in the order of one mmol L 1. The leachable Cr content is dominated by chromate as revealed by catalytic adsorptive stripping voltammetry (CAdSV) which could cause harmful contamination of surface and groundwater in the semiarid environment of Jordan.
1. INTRODUCTION Oil shale is a fine-grained sedimentary rock containing relatively large amounts of kerogen from which it is possible to profitably extract oil and combustible gas suitable for energy production.1 Oil shale is therefore considered an important alternative prospective source of energy in the world and especially in Jordan.2 However, due to still relatively low cost of crude oil compared to the cost of mining and extraction of oil shale, only few oil shale deposits have been exploited in the world. Nonetheless, the utilization plans of the oil shale as a result of the steady increase in oil prices are eminent. By 2010, oil shale extraction was being undertaken in Estonia, Brazil, and China, while Australia, the U.S., Canada, and Jordan are planning to start or restore shale oil production. A recent report reveals that about 34 billion barrels of shale oil can be r 2011 American Chemical Society
produced from the proven reserves at the main five locations on north, central, and southern parts of Jordan (El-Lajjoun, Sultani, Ed-Darawish, Attarat, and Maghar), which put Jordan at rank 7 of the world oil shale containing countries.3 On the other hand, Jordan is considered one of the five most water scarce countries in the world. There is concern that a sizable oil shale industry could negatively impact Jordan’s limited water supply. Studies have proven the feasibility of using the oil shale as a power generating fuel for direct combustion in a circulating Received: March 1, 2011 Accepted: October 5, 2011 Revised: September 26, 2011 Published: October 05, 2011 9799
dx.doi.org/10.1021/es200695e | Environ. Sci. Technol. 2011, 45, 9799–9805
Environmental Science & Technology fluidized-bed (CFB) process at temperatures above 650 750 °C. However, a rough calculation suggests that a 300 MW thermal power plant would require 20 000 tons of oil shale, producing 11 000 tons of burned oil shale (BOS) ash annually,4 and even more if coupled with oil shale retorting facilities.5 There are plans to fill up the open pit mines with the ash and spent shale.2 However, there is concern that through their interaction with rain and groundwater, the deposits could form leachate that could resemble toxic mine drainage causing degradation of surface and groundwater quality. This is of particular importance for semiarid environments such as in the Jordanian El-Lajjoun area which is rich in oil shale resources but have also a medium to high susceptibility to groundwater pollution.4 It is therefore crucial to analyze these wastes for their content and speciation of leachable toxic elements in order to take precautionary measures. The high Ca/S ratio renders the ashes rich in free lime (portlandite) which results in highly alkaline pH values of the leachates (pH 10 13). Classical heavy metals like Ni, Cu, Zn, Cd, and Pb are not very mobile under such conditions due to strong surface adsorption.6 On the other hand, oxyanion forming redox-sensitive metals and metalloids like V, Mo, As, Se, and Cr may well be mobilized under such conditions.7 Previous studies of BOS ash leachates have revealed concentrations in the order of 20 μM for Cr and 4 μM for As,4,8 which are well above the respective WHO threshold of groundwater utilization for drinking water (1 μM for Cr(VI)). No speciation analysis have been performed of the dissolved Cr, but chromium identified in alkaline leachates is almost always hexavalent because equilibrium with insoluble Ca Cr(III) minerals causes Cr(OH)4‑ species to be present at vanishingly small concentrations levels. The main concern to be verified is, therefore, that the Cr could occur in the highly toxic hexavalent form. To assess potential environmental impact of the BOS ashes and in case to develop and improve on ash stabilization and toxic element immobilization methodology, both solute and solid Cr speciation analysis are clearly desirable. For solid speciation analysis of trace elements in environmental samples on a molecular level, synchrotron beamlines have the potential of deploying in parallel X-ray fluorescence, diffraction, and absorption spectroscopy.9 13 X-ray absorption near-edge spectroscopy (XANES) is extremely sensitive to the absorbing target atom oxidation state, site symmetry, and ligand covalence in solid samples. Much has been learned about chromium chemistry in coal combustion products utilizing such XANES measurements.11,12 XANES characterization of arsenic speciation in oil shale and its derivatives was first performed by Cramer et al.13 However, they have not studied BOS ash for which to our knowledge there is yet no report on such toxic trace element speciation measurements. This baseline study aims at characterizing the potentially harmful oxidation state of Cr and As in the original oil shale rock upon combustion or pyrolysis. These results are essential in order to evaluate the potential leachability of these trace elements from the spent oil shale tailings traced by classical leaching tests, and hence, the toxicity to the environment and potential remediation measures.
2. MATERIAL AND METHODS 2.1. Sampling and Sample Preparation. The El-Lajjoun area is located in western part of Central Jordan about 110 km south of the capital city of Amman, with an estimated area of 34 km2.4
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The oil shale deposit is relatively shallow and near to the surface which favors an open pit mining, with a proven reserve of 1.2 billion tons of oil shale having an oil content of 125 million tons.2 Bender14 first used the term “oil shale” when studying this deposit and described it as an euxinic facies containing thick lenses of black, strongly bituminous shale, and brown-black, bedded, bituminous limestone. The sample was taken from an outcropping lithological unit of the main oil shale stratum with a thickness reaching 86 m.15 It is composed of bituminous marl that has thin-bedded and laminated features with intercalations of bituminous limestone of gray to light gray color. The lower portions are phosphatic marl intercalated with slightly bituminous thin black chert layers. The oil shale sample from El-Lajjoun area (10 kg) was crushed, ground, and sieved
