1 Introduction
1.2 Influence of carbon dioxide on metal(loid) mobility in soil
1.2.1 Influence of carbon dioxide induced soil acidification
The previously mentioned soil acidification, which is induced by CO2 dissolution in soil pore water, can influence metal(loid) binding mechanisms and mobility in several ways. Weakly bound metal cations can be replaced by protons, which are formed from carbonic acid dissociation, resulting in mobilization of these metal(loid)s. Additionally, intensified weathering of minerals, which is induced by soil acidification, can release adsorbed or incorporated metal(loid)s into the liquid phase. These processes have been studied in detail with regard to risk assessment at GCS sites and many researcher groups could prove CO2-induced mobilization of several metal(loid), e.g., Al, arsenic (As), cadmium (Cd), cobalt (Co), copper (Cu), chromium (Cr), Fe, Mn, nickel (Ni), lead (Pb), and zinc (Zn), by desorption and mineral dissolution processes (e.g., Jones et al., 2015, Kirsch et al., 2014, Lawter et al., 2016, Little and Jackson, 2010, Lu et al., 2010, Smyth et al., 2009, Terzi et al., 2014). However, also re-adsorption of certain metal(loid)s, e.g., shown for As, Cd, Cu, and Zn, or precipitation of new minerals can occur (e.g., Lawter et al., 2015, Lu et al., 2010, Mickler et al., 2013, Montes-Hernandez et al., 2013, Shao et al., 2015).
1.2.2 Influence of carbon dioxide induced anoxic conditions
Due to the permanently anoxic conditions, the formation of pedogenic (oxyhxdr)oxides is inhibited in mofette soils (compare section 1.1.4). This could not only increase the mobility of typical (oxyhydr)oxide forming metals like Fe, Al, and Mn, but also the mobility of metal(loid)s which are known to bind to or co-precipitate with these pedogenic minerals.
release into the atmosphere
adapted vegetation
soil surface
leaching of base cations inhibited formation of pedogenic oxides
poor crystallinity of pedogenic oxides
acidification accelerated silicate weathering
ascending geogenic CO2
shift of microbial communities to anaerobic and acidophilic ones inhibited decomposition of plant residues diminished stabilization of SOM by sorption and/or occlusion diminished bioturbation
accumulation of (particulate) soil organic matter (SOM)
Introduction
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A further consequence of the permanently anoxic conditions in mofettes is the previously mentioned shift in microbial communities. Microbial processes could on the one hand contribute to metal(loid) mobilization, e.g., by the alteration of existing equilibria or microbially induced dissolution of certain minerals, like the reductive dissolution of Fe (oxyhydr)oxides (Kirk et al., 2013). On the other hand, microorganisms can promote mineral precipitation by increasing alkalinity or by direct biological formation of carbonates (Harvey et al., 2016, Kirk et al., 2013, Lions et al., 2014 and references therein) and thus contribute to metal(loid) immobilization. From other anoxic soils like wetlands or floodplain soils, it is known that under sulfur-reducing conditions chalcophilic metal(loid)s, e.g., Cu, Zn, As, or Fe, can react with microbially produced sulfide and (co-)precipitate in form of sulfidic minerals (Fulda et al., 2013a, 2013b, Hofacker et al., 2013, Morse and Luther, 1999, Weber et al., 2009b). This process may also play a role in mofette soils as suggested by Blume and Felix-Henningsen (2009) and as indicated by the presence of pyrite in deeper sediments of the Hartoušov mofette (Bussert et al., 2017, Flechsig et al., 2008).
Furthermore, SOM, which accumulates in mofettes due to anoxic conditions, can influence the binding mechanisms and the mobility of metal(loid)s. Increased contents of solid-phase SOM could provide potential new sorption sites for metal(loid)s with a high affinity for organic matter and thus contribute to their immobilization. However, dissolved organic matter (DOM) concentrations might also be increased in the pore water of mofettes and DOM can compete with metal(loid)s for sorption sites or mobilize organic matter-affine metal(loid)s by complexation (Kirk, 2004).
1.2.3 Previous results on metal(loid) mobility at the mofette site studied in this thesis
In a pre-study for this thesis, soil contents and pore water concentrations of As, Cu, Fe, Mn, and Ni of the two mofette sites were compared with nearby, non-CO2-influenced soils (references) (Mehlhorn et al., 2014). It could be shown that the long-term CO2 ascent in mofette soils influenced metal(loid) contents considerably (Figure 4). The mofette soils had significantly lower contents of Fe and As compared to the references and the mobility, i.e., the distribution coefficient between soil solid phase and pore water, of these elements was increased in the mofettes. This was attributed to the significantly lower content of poorly crystalline (content reduced by 75%) and well-crystalline Fe (oxyhydr)oxides (content reduced by 91%).
Besides the CO2-induced increase in As mobility, also changes in As speciation were observed. In contrast to the references, up to 11% of methylated As and up to 9% of thiolated As could be detected in mofettes besides arsenite and arsenate. Thereby, the occurrence of methylated As species correlated with methane concentrations in the pore water. The formation of these As species was thus most probably related to microbial methane production by methanogenic archaea and to sulfide production by sulfur-reducing bacteria, which have been shown to predominate in these mofette soils (Beulig et al., 2015, 2016). Changes in speciation can also influence As mobility, since methylated As species
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are known to be more mobile than arsenate (Bowell, 1994) and thiolated As species are even more mobile than arsenite (Suess and Planer-Friedrich, 2012).
While the mobilizing effect of geogenic CO2 on Fe and As could clearly be shown in Mehlhorn et al.
(2014), the results for Mn, Ni, and Cu were less clear. Soil Mn contents were slightly decreased in mofettes compared to reference soils, but also Mn pore water concentrations remained low. The soil contents of Cu and Ni were relatively equal in mofettes and references (Site A) or showed lower contents in the upper 60 cm depth and higher contents in deeper soil (Site B) in mofettes compared to references. Pore water concentrations of Ni and Cu were lower in the mofettes than in the references.
The observed distribution patterns were attributed to a mixture of mobilization and re-adsorption processes under long-term CO2 influence, with SOM being the main candidate for re-adsorption due to significantly increased SOM contents in the mofettes (Figure 4). This hypothesis was reinforced by strong accumulation of Ni, Cu, and also As in a peat lens, which was detected in approximately 2 m depth at Site A (named ‘site 1’ in Mehlhorn et al. (2014)).
Figure 4. Conceptual model of metal(loid) binding processes and mobility at a mofette and a non-CO2 -influenced reference site as presented by Mehlhorn et al. (2014).
However, the exact processes that caused the distribution patterns observed for Cu, Mn, and Ni could not be explained entirely. Comparing samples from the degassing center of the mofettes with a p(CO2) of 1 with non-CO2-influenced reference soils has not been sufficient to completely understand the mobilization and immobilization processes in mofettes. Therefore, a more detailed investigation of the transition between permanently anoxic and oxic soil conditions is necessary.
Corg
Fe As
Mn
Ni Cu
Cu
Ni
partly CO2 oxic
O2
Reference
Cu Ni
Mn
As
Fe
As
Mn
Ni Cu
Cu Ni
Mofette
Ni Cu
Mn
As Fe
anoxic CO2
CO2
CO2 CO2
O2 CH4
Fe
Corg
Fe (hydr)oxides Organic matter
Solid phase/
in solution
As/As Net-mobilisation
Net-immobilisation pH
Fe
C
orgCH4
Introduction