Abstract
Two Argonne premium coal samples, Illinois No. 6 (IL) and Wyodak−Anderson (WY), were extracted by 1-methynaphthalene for 1 h at 360 °C and under 1 MPa nitrogen (cold) protection. Elution of inorganic elements into coal extracts as well as their chemistry has been mainly investigated. An indirect method for metallic speciation was employed by initially washing coal with a variety of acids. Subsequently, the washed coals as well as the respective raw coal were extracted. For a given metal, elution of its ion-exchangeable fraction was defined as the difference between its amounts eluted into the extracts of raw coal and acetic acid-washed coal. Elution of submicrometer discrete particles was defined as the difference between the extracts of acetic acid-washed coal and nitric acid-washed coal. Elution of its fraction insoluble in nitric acid was assigned as organometals which are chemically associated with coal carbonaceous matrix and/or those incorporated into fine clay minerals. About 822 and 1110 ppm inorganic elements were eluted into the extracts of IL and WY coals, respectively. Fe was the most prevalent. The transition metals including Cr, Ni, Mn, Co, Cu, and Zn were also abundant. These metals were mostly nitric-acid insoluble. Electron spin resonance spectroscopy characterization suggested the high-spin Fe 3+ state for Fe that is virtually totally associated with coal functional groups. A portion of it may have a porphyrin-like structure that is soluble in sulfuric acid (7% in methanol, v/v). While the left Fe can be high-spin Fe 3+ stored in the organometallic complex having a distorted axial symmetry. The other transition metals mainly partitioned between organometallic complex and cations incorporated into fine clay minerals. Regarding the remaining metals in coal extracts, they are mainly submicrometer discrete particles in IL extract. Elution of the ion-exchangeable carboxylates was however prominent during WY coal extraction.
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