Saturday, 15 July 2006

Alteration of secondary minerals along a time series in alkaline soils derived from carbonatic wastes of soda production.

Reinhold Jahn1, Gritta Grünewald1, Klaus Kaiser1, and Herbert Pöllmann2. (1) Institute of Soil Science and Plant Nutrition, Martin Luther University, Weidenplan 14, Halle(Saale), D-06108, Germany, (2) Institute of Geo Sciences, Von-Seckendorff-Platz 3, Halle (Saale), 06120, Germany

Industrial activities result in increasing amounts of technical substrates being deposited in landfills. These substrates are subject to weathering and soil forming processes, therefore offer excellent opportunities to study initial stages of soil formation. Here we studied the chemical and mineralogical transformations on waste deposits of soda industry. The four sites selected derived from lagooned CaO • H2O and CaCO3 and were under natural succession for 15, 19, 57, and 70 years. The soils, calcareous spolic Regosols, are weakly to strongly alkaline with pH values ranging from 8 to 11. After deposition, the substrate's initial pH of 12 drops rapidly in the topsoil due to the reaction of dissolved Ca either with CO2 from the atmosphere or evolved by microbial respiration and finally stabilizes at around 8.1. All soils show high electrical conductivity, up to 12.3 mS cm-1 at the youngest site. The electrical conductivity decreases with time due to leaching processes and the formation of less soluble secondary minerals. The binding of CO2 results in high amounts of carbonate, increasing with time. 70 years after deposition, calcite dominates the topsoil (0–30 cm depth), comprising about 80% of the soil material. The mineral composition was characterized by X-ray diffraction. Besides of calcite, we found the less common minerals ettringite, thaumasite, hydrocalumite and hydrotalcite at different quantities and different distributions. The formation and alteration of these minerals are basically influenced by changes in the soil pH. With progressing weathering neither thaumasite nor ettringite are stable due to the suitable soil reaction (~8.1). In contrast, hydrocalumite and hydrotalcite exist in all investigated soils. They are stable also under weakly alkaline conditions and thus may exist in all carbonatic soils. Results indicate a surprisingly rapid soil development driven by the highly dynamic formation and alteration of minerals in carbonatic substrates under alkaline conditions.

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