Speciation of arsenic in a Green River oil shale and in its retort oil and waters

Abstract

Oil shale, which geologically is a kerogen containing marlstone, contains arsenic in concentrations more than ten times greater than the earth's crustal concentration. Arsenic has been shown to rapidly and permanently deactivate commercial hydrotreating catalysts. Its removal is necessary to protect the catalysts during cracking and refining of shale oil. In addition, arsenic is a toxic element and presents a major environmental pollution problem when oil shale is retorted for the production of synthetic fuel. Hence, a detailed study of the distribution of this element in oil shale is necessary to help solve the problems it may cause. The distribution of arsenic among the various components of a Green River oil shale has been investigated using a variety of analytical techniques. The total arsenic in the oil shale was quantitated with a graphite-furnace atomic absorption spectrometer (GFAAS) and an inductively coupled argon plasma atomic emission spectrometry (ICPAES). The inorganic arsenic was fractionated using HCl, HF and HNO3. Approximately 90 percent of the total arsenic has been found to be present as inorganic arsenic. The soluble organic matter (bitumen) was separated by Soxhlet extraction using tetrahydrofuran as the solvent. This fraction was found to contain three percent of the total arsenic. In addition, the bitumen contains organoarsenic species such as CH3As- and (CH3)2As-. The kerogen (insoluble organic matter) contains five percent of the total arsenic. Approximately 50 percent of this arsenic is present as organoarsenic species such as CH3As- and (CH3)2As-. These arsenic species were separated and quantitated by the technique, hydride generator-DC helium plasma emission spectrometry (HG-DCHPES). Using scanning electron microscopy with energy-dispersive x-ray spectrometry (SEM-EDS) and x-ray diffraction procedures, two arsenic-bearing minerals, namely skutterudite (Co,Fe,Ni)As3 and safflorite (Co,Fe)As2 were identified. The presence of these two minerals in oil shale has not been reported in the literature. About 65 percent of the total arsenic in oil shale was found to be present in these minerals...