Leaching of PAHs from Coal Mining Heap Samples from the Saarland
P 1.4 in Contaminant leaching from mining and overburden dumps
After 250 years, coal mining stopped in the Saarland, Germany, in 2012, but a lot of mining heaps remained (ca. 80). The heaps, though unprofitable for extraction of coal, yet contain a significant amount of naturally occurring coal. The coals are known as a major source of polycyclic aromatic hydrocarbons (PAHs). PAHs are organic compounds, relatively non-volatile and of low water solubility. They are considered as pollutants with high persistence, toxic impact on living organisms and carcinogenic potential (CEPA, 1994). Pies et al. (2007) found PAH in the floodplain of the Saar River and downstream of the River Mosel and concluded that the PAHs found downstream resulted from former coal mining industry carried out in the near vicinity.
The aim of this study was to quantify the amount of PAHs which could be potentially and under almost real conditions leached from heap sediments. Sediment samples (top 10 cm) from the Saarland heaps of Duhamel, Göttelborn, Lydia, Reden and Viktoria, as well as 2 coal samples, were used for extraction and batch experiments. Leaching experiments with an accelerated solvent extraction unit (Dionex300) were executed, using acetone (potential leaching) and water at different temperatures (40°C and 80°C, "real" leaching). Additionally, batch experiments were conducted to investigate leaching under near-equilibrium conditions (10 days). The 16 EPA-PAHs and four additional PAHs (1-methylnaphthalene, 2-methylnaphthalene, benzo[e]pyrene and perylene) were analysed by gas chromatography with mass detection. Additionally total organic carbon (TOC) and physico-chemical parameters (pH and TDS) were analysed.
The heap samples contained a potential concentration in the range of 0,01 - 36 mg/kg. The highest value of 36 mg/kg was found in the heap Lydia (most abandoned PAH was naphthalene). In general, light PAHs (mass lower 202 AMU) were found in concentrations up to 40 times higher than heavy PAHs. Coal samples showed 4-times higher PAH concentrations (most abandoned light PAHs) than sediment samples. However, the water extractions showed only light PAHs. The batch experiments (3 samples per heap, 1 coal) showed only light PAHs in the water phase (concentrations from 0,1 – 0,5 µg/L), with 2-methylnaphthalene (0,5 µg/L) in the coal sample. The highest concentration of total PAHs of a heap was found at Lydia, ca. 6 times higher than the lowest concentration found in the heap Victoria.
Potential light PAH concentration in sediments (acetone extraction) were ca. 3 orders of magnitude higher than water extractions at 40°C and 80°C or in batch experiments. The extract at 80°C showed 20 times higher concentrations than at 40°C for the lighter PAHs. TOC content was found to be above 60% in coal samples (with 90% OC). Sediment samples showed TOC values in the range of 2% - 8%.
Light PAHs from heaps have been found to be mobile, but maybe immediately sorbed by natural TOC. However, dust emissions may pose a potential risk from heaps.
Pies C., Yang Y., Hofmann T. (2007): Distribution of Polycyclic Aromatic Hydrocarbons (PAHs) in Floodplain Soils of the Mosel and Saar River.- J Soils Sediments 7 (4) 216–222.
CEPA Canadian Environmental Protection Act, Government of Canada (1994): Priority Substances List Assessment Report, Polycyclic Aromatic Hydrocarbons.