Pyrite Oxidation and Leaching in Excavated Lignite Soil

Publisher Website
Google Scholar
Abstract
The oxidation of soil pyrite and the leaching of the oxidation products was studied in lysimeter experiments. The study material was a nonoxidized lignite clay which contained 2.8% pyrite. The experiments showed that even in up to 1 m thick soil layers nearly all pyrite would oxidize in 1 yr. As a result the leachates collected became strongly acid (pH .apprx. 2) and the dissolved sulfate and Fe reached values of 0.75 and 0.39 M, respectively. As long as pyrite was present, most of the dissolved Fe consisted of Fe2+, but after that the oxidation conditions gradually changed and only Fe3+ was found. The leaching of the oxidized soil also led to lower ion concentrations and somewhat higher pH values of the leachates with the result that Fe3+ was precipitated in the soil and Fe no longer leached at the same rate as the sulfate. If the soil before oxidation was mixed with an excess of CaCO3, no leaching of Fe took place; at the slightly alkaline conditions oxidation of pyrite was impeded to such an extent that after 8 yr the soil still contained about 0.5% pyrite-S. In this soil, gypsum seemed to be formed together with amorphous iron oxides and the soil leachates were nearly saturated solutions of gypsum.