Characterization of organic components in leachables from Bulgarian lignites by spectroscopy, chromatography and reductive pyrolysis

Abstract

Lignites from Thrace- and Sofia- coal basins, i.e. "Maritsa-East" and "Stanjanci" mines, were subjected to aqueous sequential extraction at 25 degrees C during 10 weeks. Each 7 days leachates were separated and pH and conductivity were measured. According to the extraction curves mu S vs. time two portions were prepared, enriched in salts, extracts from the first two weeks ("front" of the elution) and in organic matter, extracts from the rest of time ("tail" of the elution curve). Combined freeze-dried extracts were characterized by yields, technical and elemental analyses. They were studied by X-ray photoelectron spectroscopy (XPS) and Infrared spectroscopy (IR). According to XPS spectra the main species on the leachate surfaces were carbon atoms in aromatic and aliphatic structures, 48-53 at. % from the total C1s signal. The intensive N1s signal at 400 eV was assigned to pyrroles and amines (1-3 at. %). The most abundant sulphur form was inorganic sulphates while intensities for the other ones were < 1%. Respectively, the sulphatic sulphur content (13.6%) determined for Maritsa leachate was almost seven times higher than the same sulphur for Stanjanci leachate. FTIR spectra gave evidences for oxygen-containing functional groups, i.e. COOH groups in benzene carboxylic acids and their derivatives, in short-chain aliphatic fatty acids and for polyols. The appearance of mineral matter, mainly gypsum and kaolinite, made equivocal the IR spectra interpretation. Acetone soluble portions of "tail" leachates have represented relatively small parts of the organic matter (340 mg/kg Maritsa East and 80 mg/kg Stanjianci lignite) but have assigned reliable evidences for the polar constituents of leachates. Linear fatty acids, nC(12)-nC(32), nC(16)(max), n-alcohols, benzoic acids, i.e. hydroxy-, methoxy-, phthalic acids, were highly abundant. Sterols, stanols, ketosterols were present in both extracts. Phthalates were recognizable as well. AP-TPR-TD-GC/MS technique has proved the presence of phenols, PAHs and heteroatom containing components in leachates flue gases. From an environmental viewpoint it seems that the identified compounds do not represent an acute toxic risk. However, N-containing compounds could raise concerns and further attention is needed to be focused on them.