Weathering of Algae- and Humic-Type Organic Matter in the Spoil Banks of Brown-Coal Mine (Miocene, Sokolov Coal-Mining Area, Czech Republic)

Bohdan Kr˜íbek Czech Geological Survey, Klárov 3/131, 118 00 Prague 1, Czech Republic

Mojmír Strnad Czech Geological Survey, Klárov 3/131, 118 00 Prague 1, Czech Republic

Jirí C˜ejka J. Heyrovsky´ Institute of Physical Chem. AS CR, Dolejs˜kova 22,180 00 Praz˜e, Czech Republic

Zdene˜k Sobalík J. Heyrovsky´ Institute of Physical Chem. AS CR, Dolejs˜kova 22,180 00 Praz˜e, Czech Republic

Stanislav Hilgard Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2 Czech Republic

Václav Sedlác˜ek Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2 Czech Republic

Eva S˜ebestová Inst. of Rock Structure & Mechanics AS CR, V Holes˜ovic˜kách 41, 182 09 Prague 8, Czech Republic

The strippings of the mined coal seams of the Sokolov Basin consist of lacustrine Cypris formation (Eggenburg-Ottang), containing 0.5-7.0 % T.O.C. The rock sequence is formed in its stratigraphically lower part by illite-kaolinitic clays and in the upper part by illite-kaolinite-montmorillonitic clays with admixtures of analcine and siderite. The organic matter is mostly of the algae type (kerogene I-II), however, the humic-type organic matter (kerogene III) predominate in the vicinity of the coal seams. There are occasional beds up to several dm in thickness, consisting almost exclusively of alginite-type macerals with a very small mineral admixture.

The algae-type organic matter is characterised by high hydrogen contents (Rock-Eval hydrogen index (HI) = 700-1000); and C11-C31 n-alkanes greatly predominate in the GC spectra of their pyrolyzates. The infrared spectra are characterised by strong absorption bands in the 2923 cm-1 and 2859 cm-1 regions (attributed to aliphatic CH2 and CH3 groups) and by weak bands in the 1456 and 1384 cm-1 regions (sCH3 vibrations). The d12C values of the algae type organic matter vary from -28 to -26 ”.

The hydrogen contents in the humic-type organic matter are low (IH = 250-400); alkyl phenols, xylenols and alkyl naphthalenes predominate in the GC spectra of their pyrolyzates, with smaller amounts of n-C13 to n-C31 alkanes. The IR spectra are characterised by a predominant band at 1600-1640 cm-1 (C=O group and aromatic ring vibrations) and a broad band in the 1100-1250 cm-1 region, associated with various oxygen-bearing functions. The d12C values are lower that those for algae type organic matter (by 24-26 ”).

In the mining of brown coal, the roof sediments are placed on spoil banks, where they are gradually weathered. The weathering of both algae- and humic type organic matter was studied on spoil banks of various ages.

It was found that the weathering of algae-type organic matter leads to only small changes appearing as a decrease in IH to a value of 400-450, a slight increase in the intensity of the IR band in the 1710 cm-1 region (carbonyl and carboxyl functions) and a disappearance of n-C11 to n-C15 alkanes in the GC spectra of the pyrolysis products. The amount of extractable humic acids in weathered samples is very small and approximately the same as for the unweathered rock (less than 1% T.O.C.). These changes are interpreted as being a consequence of microbial activity involving preferential decomposition of low-molecular-weight alkanes. Microbial activity is also reflected in lower values of the carbon to nitrogen ratios in the rocks of spoil banks (C/N = 7-16), compared with the fresh, unaltered rocks (C/N = 19-30).

The weathering of coal-type organic matter is accompanied by a small decrease in the hydrogen index (IH = 200-250) and by a marked increase in the intensities of the IR bands in the 1710 cm-1 (carbonyl functions) and in 1100-1250 cm-1 regions (broad band associated with various oxygen-bearing functions). There is a substantial increase in the amount of extractable humic acids: from 10-40% T.O.C. in samples containing unweathered organic matter to 70% T.O.C in highly weathered samples. Considerable amounts of phenols, creosols and xylenols appear in the pyrolysis-GC spectra. The amount of aliphatic hydrocarbons decreases only slightly. These changes are interpreted as being the result of oxidation by atmospheric oxygen without substantial influence of bacterial activity. Low microbial activity is also indicated by the carbon to nitrogen ratios in samples containing highly weathered humic-type organic matter (C/N = 50-70), which are the same as samples that were not exposed to weathering (C/N = 55-70).

Independent of the type of organic matter, the weathering in waste heaps is much more intense than in natural weathering profiles.

The results of this work indicate that algal-type organic matter is relatively inert to weathering and has only small impact on the soil properties of the deposited substrate. In contrast, humic-type organic matter is weathered very rapidly with the formation of large amounts of humic acids, which considerably increase the sorption capacity of the material deposited on the spoil banks. The favourable effect of humic substances on the soil properties is, however, greatly limited by the production of antiseptic substances (phenols and creosols), which prevent the development of microbial activity.