Partitioning of iron in organic and mineral phases: sequential extractions of lignite and bituminous coal

摘要

Pyrite (FeS_2), the most common sulfide mineral in Earth's surface environments, is a strong indicator of reducing conditions in aqueous environments (Descostes et al., 2004). The abundance of pyrite in nature and the important role of pyrite formation in geochemical cycles has spurred numerous experimental investigations addressing formation, mechanisms at low temperatures (< 100°C) and over a broad range of solution chemistries (Rickard and Luther, 2007 and references therein). The principal steps in sedimentary pyrite formation require the consumption of iron compounds to varying degrees through reaction with microbially-formed hydrogen sulfide (Luther 1991). The results presented here are part of an investigation of pyrite formation in organic-rich sediments, including coal. There are several working hypotheses for pyrite formation: a) iron is deposited diagenetically and pyrite formation is biologically driven; b) pyrite is being dissolved and residual iron is adsorbing to the surface; or c) pyrite is formed epigenetically, as iron attached to the coal surface reacts with sulfur compounds via percolating surface water or interactions with groundwater. The interaction of iron with both mineral and organic matter makes characterization of iron partitioning difficult, and thus it is poorly understood in modern and ancient organic-rich sediments. We have developed a sequential extraction allowing detailed information on the speciation of iron in coal. Five sediment iron fractions are characterized (1) surficially bonded Fe; (2) organically bound Fe (Fe_(org)); (3) carbonate-associated Fe, including siderite and ankerite; (4) reducible oxides, including ferrihydrite, lepidocrocite, goethite; (4) silicate Fe; and (5) pyrite Fe. Iron fractions were determined using a combination of pressurized fluid extraction, using EDTA and NMP, as well as leaching on a suite of carmel, lignite, and coal- samples collected from different coal regions within the United States. Preliminary data from a sample of bituminous coal collected from the Clarion Coal seam (Turkey City, PA) suggests that 90% of iron within the coal is bound to the coal surface. Additional samples are being processed and trends will be assessed between samples and coal quality groups. Ultimately, iron distribution within the coal seam has important implications for inferring formation and dissolution conditions. Understanding iron partitioning may assist in optimizing coal processing and combustion while minimizing environmental impacts.