XPS study of pyrite-free coals subjected to different oxidizing agents

Surface composition of two of pyrite-free coals characterized by a high organic sulphur content subjected to oxidation by peroxyacetic acid, oxygen in 0.5 N Na2CO3 aqueous solution and air for 7 days at 125 °C were studied. Oxidation process followed the route: diffusion → chemisorption (formation of oxidized surface species) → further oxidation of surface species → gasification. The extent of each stage was dependent on the structure of initial coal. PAA acid was found to be the most efficient oxidizing and desulphurizing agent. The role of pyrite was discussed. The removal of pyrite influenced the results of the oxidation process following it. The oxidation ability of the used media formed a similar sequence (PAA > O2/Na2CO3 > air/125) for coal samples with or without FeS2, but the numerical values of several parameters (the surface content of elements C, O, S, N; the type and number of oxygen-or sulphur-containing surface species) differed in both cases. The presence of FeS2 increased oxidation rate but the overall effect depended on the starting amount of oxygen-containing surface species which in turn was influenced by the procedure of pyrite removal.