Bachelor Thesis

Influence of CO2 on mobilizing Fe and As by abiotic and microbially triggered dissolution of ferrihydrite

Rouven Metz (03/2016-09/2016)

Support: Britta Planer-Friedrich, Judith Forberg

A possible risk of carbon storage sites are leakages through the cap rock. The release of CO2 into soil can influence the chemistry dramatically, e.g. lead to reductive dissolution of Fe (hydr)oxides and release of adsorbed toxic elements such as As. In this thesis, the CO2 influence on microbially triggered reductive dissolution of ferrihydrite and on adsorbed As was investigated. Ferrihydrite, loaded with Arsenite, was inoculated with the acidophilic, facultative anaerobic, Fe(III)-reducing bacterium Acidiphilium SJH (A.SJH) with different headspace gas phases. The gas phases represented oxic (air headspace), anoxic (100 % N2) and anoxic conditions at high CO2 concentrations (100 % CO2). Iron mobilization in abiotic treatments was negligible, and no difference between the gas phases was seen, even when pH was lowered to 3.0. In contrast, a significant Fe mobilization was observed in all treatments inoculated with A.SJH, independent of the headspace gas. Samples with CO2 in the headspace showed the highest Fe mobilization (up to 1.3 ± 0.5 % of Fe(tot) after 31 h). However, the difference was probably caused by a lower pH in CO2 treatments due to formation and dissociation of carbonic acid (pH 4.2 compared to a pH of 5.5 in N2 and air treatments). No clear difference in biotic Fe mobilization could be seen between the gas treatments when the pH was forced to the same low value (pH 3.0) indicating that the activity of acidophilic A.SJH and thus Fe mobilization were mainly influenced by pH. Accompanied with the ferrihydrite dissolution, adsorbed Arsenite was mobilized. However, this microbially triggered As mobilization was overlapped by fast abiotic desorption, that occurred within 1 h and that was highest at low pH. A CO2 leakage into soil will thus not lead to abiotic Fe (hydr)oxide dissolution reactions, but the induced anoxic conditions and soil acidification may increase the activity of Fe(III)-reducing microorganisms and thus cause Fe and As mobilization.