Bachelor Thesis
Mobilization of arsenic under abandoned waste disposal sites
Support: Britta Planer-Friedrich, Johannes Besold
Elevated concentrations of arsenic are often found in the groundwater downstream of abandoned waste disposal sites (e. g. former household waste or building rubble disposal sites), even though no arsenic-containing waste has been deposited knowingly. The arsenic concentrations in groundwater may even reach levels that require further investigation and possibly even further measures. The contamination is assumed to be caused by the mobilization of arsenic through organic-rich and often highly reducing leachate. Under reducing conditions, iron (oxyhydr)oxides dissolve and arsenic, which preferentially binds to them, is released. Such arsenic-containing iron (oxyhydr)oxides may be present both in the deposit material and in the geogenic subsoil. The high organic content in the leachate accelerates the microbially triggered reduction processes and can increase the mobility of arsenic by formation of soluble complexes. Furthermore, the arsenic itself is reduced from the surface-bound pentavalent species arsenate to the usually more mobile trivalent species arsenite. Methylated and thiolated arsenic species can also form. In soil protection and waste legislation, soil samples are evaluated using leachate from batch or column tests. However, these tests are carried out under completely oxic conditions. Due to the above-mentioned processes, it can be assumed that the potentially mobilizable arsenic and the resulting concentrations in the leachate are underestimated using these standard methods. For the bachelor thesis, anoxic incubation experiments will be carried out with various materials which comply with the limits for arsenic in the standard test procedures (test value: 10 µg/L for As) and which are therefore regarded as harmless according to soil protection standards. The liquid phase will be sampled and examined over time in order to obtain information on mobilizable arsenic and arsenic speciation. In parallel, redox conditions and dissolved iron concentrations are monitored in order to obtain information on reductive dissolution processes. The initial material and the solid material after the end of the experiment are also characterized in more detail (total arsenic and iron contents, oxalate and dithionite extraction as well as XRD to characterize the iron minerals). The aim is to clarify whether materials classified as harmless according to soil protection standards have the potential to release arsenic in relevant concentrations under the conditions found in abandoned waste disposal sites. The results of the project and the procedures developed will be very useful to clarify the origin of arsenic and to estimate the potentially mobilizable arsenic quantities in corresponding practical cases. Similar processes are sometimes also observed in backfills and waste disposal sites that are still in operation, so that the results may also be transferable to these areas. This bachelor thesis is a cooperation project with the Bavarian Environment Agency (Department 96: contaminated sites, harmful soil changes, certified experts).