Variations of concentration and composition of polycyclic aromatic hydrocarbons in coals in response to dike intrusion in the Huainan coalfield in eastern China

Abstract

Although extensive studies have been conducted on coals influenced by igneous intrusion, the changes of concentration and composition of polycyclic aromatic hydrocarbons (PAHs) in coals responding to this kind of geological rapid heating are largely unknown. In the present study, thirteen coal samples, including unaltered coal, thermally altered coal and a dike-coal contact sample were collected along a transect approaching an igneous dike intrusion from the No. 3 coal seam of the Zhuji coal mine in Huainan coalfield, China. These samples represent a broad range of coal ranks ranging from high volatile A bituminous to anthracite. Vitrinite reflectance (%Ro) profiles indicate a progressive increase in %Ro value from ambient levels of 0.87–1.02% to 5.16% and include an alteration occurring out to ∼3.6m from the dike. Bulk chemical composition from proximate and ultimate analyses shows the typical trend with distance from the dike. The total amount of extractable PAHs (ΣPAH35), US-EPA priority pollutant PAHs (ΣPAH16) and carcinogenic PAHs (ΣPAH8) range from 3.4–81.1μg/g, 2.5–28.7μg/g and 0.7–16.9μg/g, respectively. PAH concentrations vary regularly with coal rank, reaching a maximum in low volatile bituminous coal (1.62%Ro), located 3.5m from the dike. This rank is higher than reported for coals not affected by contact metamorphism. Contact metamorphism has a dramatic effect on the coalification process and results in a different relationship between the PAH maximum concentration and coal rank. The igneous dike intrusion also has a significant effect on PAH distributions, where the proportions of 4–6 ring PAHs decrease toward the coal-dike contact and PAHs are dominated by those with 2 and 3 rings. The thermally altered coals show a decrease in alkylation compared with that of the unaltered coals, and the dealkylation process is apparently accompanied by the cross linking reaction of PAHs to form a more compact structure of organic macromolecule.