Diploma Thesis

Mobilisierung von Arsen von Eisenoxiden und Aquifermaterial durch natürliches organisches Material

Markus Bauer (01/2003)

Support: Christian Blodau

Arsenic is a threat for the water resources of the world. In some regions arsenic contamination in the groundwater can be ascribed to the input of organic matter into the aquifers. Microbial degradation of organic substances leads to reducing conditions in those aquifers, which result in the mobilisation of arsenic from the solid phase. In contrast to these indirect eects of organic matter the direct inuences are not properly understood. The aim of this thesis was therefore, to observe whether organic matter directly influences the mobility of arsenic. The laboratory studies were designed as Batch-experiments with dierent solid phases containing arsenic. Arsenite or arsenate was sorbed on synthetic iron oxides and the solid phase was then incubated with solutions containing natural organic matter (NOM) in concentrations up to 50 mg C/l. Additional experiments were conducted with natural samples from dierent arsenic contaminated sites (soil, aquifer, sediment). NOM from two different wetlands were used. The amount of arsenic release was measured by graphit furnace AAS. In order to determine the amount of redoxtransformations during the incubation process, measurements of arsenic species were conducted with a HPLC-ICP-MS. The experiments showed, that NOM-solutions is able to mobilize arsenic from nearly all the examined solid phases. At the maximum 20 % of the sorbed arsenate and 60 % the sorbed arsenite were released from the iron oxides. In the natural samples the NOM-mobilized fraction covered up to 5,8 % of the total arsenic content. The arsenic mobility in approaches with NOM-free solutions was lower. The release of arsenic varied for the two different NOM. During the incubation of iron oxides up to 25 % of the solute NOM was sorbed. In dierent experiments changes in the arsenic speciation were observed, which were caused by the NOM. The highest amount of redoxtransformation occured for the oxidation of arsenite and involved about 20 % of the total arsenic content. The results indicate, that under the given conditions arsenic release is not primarily induced by NOM through redox reaction. In fact competition between arsenic and humic anions about sorption sites seems to be the cause for high arsenic mobility. The arsenic released from iron oxides and natural samples derives mainly from the weakly sorbed fraction. Therefore high NOM-concentrations in contaminated sites probably do not automatically lead to a quick mobilisation of arsenic.