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Summary Band 35

Hamer, Martin: Acid neutralizing capacity and mechanisms of proton buffering of defined minerals as well as selected rocks and soils from North Rhine-Westphalia. Bonner Bodenkundl. Abh. 35 (2000), 282 p.

 

Laboratory experiments were carried out to measure the processes of H+-buffering of nine defined minerals (carbonates, silicates, oxides), three selected rocks (loess, greywacke, clay shale), and 21 soils (selection criteria: acidification state, parent material, cultivation). The buffer capacity and buffer kinetics were measured by the proton consumption and by the cations released in equivalent amounts in batch- and pH-stat-experiments. The buffer mechanisms were characterized qualitatively using X-ray diffraction (XRD), light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
The extent of the acid neutralizing capacity (ANC) of the minerals determined by different methods depends mainly on the mineral structure, the chemical composition, and the specific surface of the minerals. The kinetics of the H+-buffering can be divided into three partial re-actions with different rates. The initial fast (0-24 h) and medium (24-400 h) buffer reactions can be attributed to desorption of cations adsorbed with various strengths, a protonation of active surface functional groups, a beginning dissolution of unstable parts of the surface with lattice defects and a dissolution of impurities containing CaCO3, Mn oxide and Fe oxide and of extremely fine mineral fragments on the mineral surfaces. The slow H+-buffering between 400 h and 1000 h is characterized by a constant reaction speed, which allowed the estimation of typical buffer rates for the different minerals. In this period of slow buffering a H+-consumption by mineral destruction occurs.
The buffer capacity of selected soil samples could be subdivided into a short-term, a medium-term and a long-term ANC. Depending on the pH-value, the buffer capacity of carbonates, exchangeable cations, pedogenic oxides and easily weatherable minerals of the soil samples were measured. The kinetics of the fast, medium and slow H+-buffer reactions of soils is mainly governed by the different reactivity of these buffer substances. The buffer kinetics of soil samples could also be divided into three reactions with different reaction kinetics.
The comparison of soil samples from sites under different use (arable land and forest) confirmed the influence of vegetation and cultivation on soil acidification. At forest sites, the total buffer capacity of soils with the same parent material is lower than under agricultural use (alluvial sand: 28 %, loess: 18 % and greywacke/clay shale: 5 %). Furthermore, the parent material and the depth of sampling affected the buffer capacities of individual soil horizons.
The long-term ANC of selected rocks increased in the following order: greywacke < loess < clay shale. Because of the high CaCO3 content of the loess the medium- and short-term  ANC increased in the order greywacke < clay shale < loess.

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