The impact of water-washing, biodegradation and self-heating processes on coal waste dumps in the Rybnik Industrial Region (Poland)

•Four sets of coal waste materials were analysed.•Rock Eval pyrolysis, GC/MS and petrography methods were used.•Different effects of secondary changes on coal waste materials were discussed.

Water-washing, biodegradation and self-heating impacts on coal waste features were investigated on four sample sets: fresh coal wastes (sampled less than one week after dumping), samples from gullies in coal-waste dumps, self-heated material and Bierawka river sediment mixed with coal/coal-waste particles. Rock Eval pyrolysis, petrography and gas chromatography–mass spectrometry (GC–MS) were applied to assess degree of organic-matter alteration. It was found that water-washing and self-heating did not influence the vitrinite reflectance. Rock-Eval pyrolysis results better reflected the secondary changes in organic matter, namely, S2-TOC and HI-Tmax allowed their extent to be defined. Changes were noted in other geochemical e.g., n-alkane parameters Σ2/Σ1 increased and n-C23/n-C31 decreased whereas, Pr/Ph, Pr/n-C17 and Ph/n-C18 ratios increased slightly compared to fresh coal waste. The Hunt diagram allows discrimination between water-washed- and biodegraded samples from unaltered, fresh coal waste. Similarly, changes in distributions of phenols, alkyl aromatic hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) reflect degree of water-washing. Self-heating strongly affects distributions of some biomarkers such as n-alkanes, Pr, and Ph, whereas pentacyclic triterpane distributions were generally well preserved. Phenols occur in greatest amount in self-heated wastes, probably released from heated vitrinite. Domination of 2–3 ring PAH rings indicated the bitumen precipitation zone and well-differentiated burnt-out wastes. Recent organic-matter input is seen in increased CPI (Carbon Preference Index) values.