Chemical and Biochemical Characteristics of Acid Sedimentary Soft Rock in the Process of Weathering

Recently, the acid sedimentary soft rock that caused a collapse of the cutting ground has appeared in the surface with the road construction and the estate development. The acid soil at construction, however, must be effectively utilized as the land reclamation. The purposes of this study are to verify the importance of the biochemical oxidation of pyrite in the weathering process and to investigate the optimum pH condition for faster oxidation than in nature. First, pH, sulfate content and pyrite remaining in the acid soil that collected in the field were measured. When the ratio of pyrite remaining in the soil was less than 70 percents, pH was also lowered with the decrease in the pyrite remaining. Then, samples that added the commercial pyrite or sulfur powder were sterilized by the autoclave in order to clarify the biochemical characteristics of the acid sedimentary soft rock in the weathering. The result showed that sulfate ion was detected only in non-sterilized samples at pH below 3.5. On the other hand, sulfur powder was hardly converted at pH above 2.5. Therefore, it was confirmed that ferrous iron oxidation, given as Fe2++1/4O2+H+→Fe3++1/2H2O was the most important process in the biochemical oxidation of pyrite and its optimum pH was 2.5-3.5.