PhD Thesis

Characterization of binding mechanisms and mobility of metals and metalloids under the influence of increased carbon dioxide in mofette soils

Judith Forberg (03/2014-11/2018)

Support: Britta Planer-Friedrich

Mofettes are natural exhalation sites of geogenic carbon dioxide (CO2), which mainly occur in seismically active regions. Soil conditions at such sites are strongly influenced by extreme CO2 partial pressure. The soil is anoxic up to the surface and CO2 dissolution in pore water causes soil acidification. Formation of pedogenic iron (oxyhydr)oxides is inhibited and decomposition of organic material is decelerated in such soils leading to accumulation of poorly degraded soil organic matter (SOM). These changes in soil conditions can affect metal(loid) binding mechanisms and thus the mobility of metal(loid)s in soil. Carbon dioxide triggered mobilization may become dangerous if large amounts of potentially toxic metal(loid)s are mobilized into aquifers, while immobilization can constitute a risk for plants and soil organisms by limitation of essential trace elements.

The aim of the present thesis was to investigate CO2-induced changes in binding processes and the mobility of several (trace) metal(loid)s. Besides aluminum (Al), cadmium (Cd), cobalt (Co), chromium (Cr), nickel (Ni), and zinc (Zn), special attention was payed to the elements iron (Fe), arsenic (As), manganese (Mn), and copper (Cu). The studied mofette site is located in the Cheb Basin in northwestern Czech Republic.

In a first study, the spatial distribution of the mentioned metal(loid)s around a main degassing feature of the mofette site was investigated both for soil and pore water. Sequential extraction of selected soil samples was used to assign distinct groups of hypothetical metal(loid) binding. The spatial distribution and mobility of Fe and As were mainly determined by the presence of Fe (oxyhydr)oxides, which could form as soon as traces of oxygen were available. Aluminum and Cr were predominantly incorporated into aluminosilicates and showed almost no mobilization. Depletion around the main degassing center could be explained by decreased mineral contents due to SOM accumulation. The metals Co, Mn, and Ni were depleted within the whole CO2-influenced area due to release from increased weathering of silicates and leaching, following long-term CO2-induced soil acidification. The only elements that showed enrichment directly in the degassing center were Cd, Cu, and Zn. Effective sequestration of these chalcophilic metals was attributed to the formation and (co-)precipitation of sulfidic minerals. The highest metal(loid) pore water concentrations correlated with dissolved organic carbon concentrations and were observed close to the degassing center for Al, As, Cr, Cu, Fe, and Zn. Due to anoxic conditions, poorly degraded, easily mobilized SOM accumulates in mofettes. Complexation with dissolved organic matter (DOM) prevents re-adsorption and leads to increased mobility of these metal(loid)s.

In a second study, short-term mobilization processes of Fe, As, Mn, and Cu following CO2 intrusion into a hitherto non-CO2-influenced soil were studied in laboratory batch experiments, using non-CO2- influenced soil from close vicinity of the mofette. Fast, abiotic mobilization of metal cations (shownfor Mn) occurred due to surface protonation, however, overall mobilization remained low. After depletion of other electron acceptors, microbially triggered reductive dissolution of Fe (oxyhydr)oxides began, mobilizing large amounts of Fe and incorporated metal(loid)s like As.

In a last study, Cu mobility was investigated both by studying a transect over the mofette site and by conducting a Cu-spike experiment with natural, SOM-rich topsoil from this transect. Sorption isotherms for Cu were determined and Cu solid-phase speciation was investigated using X-ray absorption spectroscopy. Copper mobility was highest in soils from the transition between oxic (reference) and anoxic (mofette) conditions, while strong Cu sequestration and high adsorption coefficients were determined for soil from the degassing center. Solid-phase speciation revealed that Cu reduction and precipitation of Cu sulfides was the main sequestration process in the permanently anoxic degassing center. In transition and reference soils, Cu binding to SOM was the dominating process. The lower degradation state of SOM in the mofettes has a negative effect on SOM stability, which could be seen by an increasing dissolved-to-solid-phase ratio of organic carbon with increasing CO2. Thus, also Cu mobility increased with increasing CO2 influence since Cu both complexed with DOM and solid-phase SOM. Mobility was highest in some meters distance from the degassing center, where micro-oxic conditions prevented Cu sulfide precipitation.

Overall, both metal(loid) mobilization and immobilization were found to occur in CO2-influenced soils, necessitating risk assessment with regard to potential ground water contamination or trace element limitations at such sites. Desorption and mineral dissolution are the main mobilization processes while sulfide mineral precipitation of chalcophilic metals is the main immobilizing process in mofette soils. Carbon dioxide influence on these processes is primarily indirect, via soil acidification and creation of anoxic conditions. Fast metal(loid) mobilization via desorption and reductive dissolution of Fe (oxyhydr)oxides are dominating processes on a short-term scale. Ongoing CO2 exhalation will induce further metal(loid) release via silicate weathering and subsequent leaching. The highest mobilization risk on a long-term scale arises from metal(loid) complexation with DOM, which can exhibit strongly elevated concentrations close to the degassing center.