Copper mobilization and sequestration in mofettes – the role of redox conditions and soil

Copper had shown some quite interesting spatial distribution in study 1: Regarding total contents in SOM-rich topsoil, Cu was both enriched directly in the degassing center and in the non-CO2 -influenced areas, while soil contents were lowest in the transition zones with medium p(CO2). Pore water concentrations of Cu were highest in the upper 10 to 15 cm of these transition zones, i.e., also Cu mobility was highest there, especially pronounced in 2 to 4 m distance from the degassing center.

Interestingly, Cu mobility was lowest directly in the degassing center, with pore water concentrations being close to or even below the limit of detection. These observations indicate that two distinct Cu binding processes occurred inside and outside of the degassing center and that, in contrast to most other metal(loid)s considered in studies 1 and 2, CO2 degassing did not simply trigger Cu mobilization and leaching by soil acidification as might have been expected from studies on pH dependent Cu sorption in soil (Degryse et al., 2009).

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In order to clarify the distinct Cu binding mechanisms, sorption isotherms were determined in study 3 (Mehlhorn et al., 2018) for four SOM-rich topsoil samples of Site A with increasing soil p(CO2) (Reference A, Transition A2, Transition A1, and Mofette A) and two samples from Site B (Reference B and Mofette B).

Copper adsorption was lowest for Transition A1, followed by Transition A2, followed by the two References A and B, which showed almost similar adsorption isotherms. The isotherms of Mofette A and B, however, differed completely from the others: while references and transitions showed linear adsorption isotherms, the Cu adsorption to mofette soils could be fitted best using Freundlich isotherms. In addition, Cu adsorption was significantly higher for mofettes than for the other soils, with Mofette A showing the strongest Cu adsorption of all samples.

Since SOM is known to be the predominant binding partner for Cu in oxic soils (Karlsson et al., 2006, McBride et al., 1997, McLaren and Crawford, 1973, McLaren et al., 1983), both DOM and SOM were studied in more detail. The Cu sorption isotherms were replotted with respect to total DOC for Cu concentrations in liquid phase and with respect to total SOC for Cu adsorbed. Interestingly, the isotherms and linear adsorption coefficients determined for Transitions A1, A2, and Reference A were almost identical when referred to organic carbon. This indicates that SOM was the dominating binding partner for Cu in these soils. However, equal amounts of the spiked Cu complexed with DOM as with solid-phase SOM, resulting in different Cu mobilities in Transitions and References. As expected, SOC contents increased towards the mofette center with Reference A < Transition A2 < Transition A1 < Mofette A. However, also the amount of DOC, which was mobilized into the liquid phase during the course of the experiment, as well as the DOC:SOC ratio increased with increasing SOC. This indicates that the stability of SOM decreases with increasing p(CO2) as previously reported by Rennert and Pfanz (2015).

Differences in SOM quality were also detected in this study. Same as observed in other studies (e.g., Rennert et al., 2011, Ross et al., 2000) and in study 1 of this thesis, the C/N ratio increased towards the degassing center, indicating a lower decomposition state of mofettic SOM. While transitions and references were relatively similar in DOM composition determined by FTIR and in SOM composition determined by ¹³C NMR, both DOM and SOM of the mofette soils showed some distinct differences from the other soils. The relative fraction of aliphatic carbon in SOM was increased and bands indicative for aliphatic structures and polypeptides were more pronounced in DOM from mofettes.

Together with longer chain lengths of the aliphatic structures in DOM, as indicated by the ratio of bands indicative for asymmetric stretching of R−CH3 and R−CH2 (Ibarra et al., 1996), these observations confirmed the lower degradation state of DOM and SOM in mofettes.

A negative influence of geogenic CO2 on the interaction of SOM with soil minerals and thus on SOM stabilization due to accumulation of poorly degraded SOM has been first reported by Rennert and

Results and Discussion

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Pfanz (2015). Sorption isotherms determined in study 3 showed that the decreasing SOM stabilization with increasing p(CO2) significantly influenced Cu mobility, and regarding results of study 1, also the mobility of other metal(loid)s with a high affinity for complex formation with DOM. With increasing p(CO2), the DOC:SOC ratio increased and thus, also Cu mobility increased by formation of Cu-DOM complexes.

Determination of Cu solid-phase speciation by XAS confirmed the predominance of Cu binding to SOM for transition and reference soils. Using linear combination fitting, best fits could be achieved using reference compounds of Cu(II) adsorbed to organic substances. Binding of Cu(II) to SOM (or DOM) occurs via closely spaced hydroxylic (or amine) and carboxylic groups leading to the formation of very stable five- to six-membered ring chelates (Karlsson et al., 2006, Manceau and Matynia, 2010). Solid-phase speciation showed that also the spiked Cu predominantly adsorbed to organic matter and no significant differences in solid-phase speciation could be observed between transitions and references. This implies that structural changes in SOM composition, e.g., a lower abundance of carboxylic groups at elevated p(CO2) as recently described by Rennert (2018), did not influence Cu binding to a detectable extent. Only the DOC:SOC ratio determined Cu mobility in the sorption experiment of study 3.

In soils from the degassing center, another binding mechanism must have caused the strong Cu sequestration. This was indicated already by the differing shape of the sorption isotherms and further confirmed by the fact that Cu adsorption was strong despite high DOC concentrations and a low SOM degradation state in soil from Mofettes A and B. Copper solid-phase speciation determined by XAS further confirmed this hypothesis: according to linear combination fitting, 72% of the Cu in Mofette A was present in form of Cu(I) coordinated via sulfur. Even if no statistically significant distinction between Cu(I) bound to thiol groups of organic matter and Cu sulfide minerals was possible, it could be concluded from the strong Cu sequestration despite high DOC concentrations that Cu sulfide minerals were the dominating Cu compound in Mofette A. Furthermore, one fourth of the spiked Cu(II) must have been reduced and precipitated in form of Cu sulfide, besides adsorption of Cu(II) to SOM.

Copper(II) reduction and subsequent precipitation in form of sulfidic minerals is a process that is known from other anoxic soils, e.g., floodplains, wetlands, or paddy soils (Borch et al., 2009, Fulda et al., 2013a, 2013b, Weber et al., 2009a). Reduction can appear abiotically, e.g., by reaction with reduced humic substances (Maurer et al., 2013, Pham et al., 2012) or be triggered by microorganisms (Hofacker et al., 2015, Sugio et al., 1990). The formed Cu(I) is very reactive, thus, if it is not stabilized by complexation, e.g., with sulfide, it may disproportionate and form metallic Cu(0) and Cu(II) (Fenwick, 1926, Sharma and Millero, 1988). With SEM-EDS a small particle of metallic Cu was detected in a Cu-spiked sample from Mofette A, indicating that precipitation of Cu(0) following Cu(II)

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reduction and disproportionation might also occur in mofettes. However, Cu solid-phase speciation determined by XAS did not confirm the presence of Cu(0), thus, contributions must have been low.

The detected sulfide concentrations of 1.8 mg L^-1, which could be measured in pore water from the mofette center, imply that Cu(II) in mofettes is reduced either by microbes or abiotically, complexes with sulfide, and precipitates in form of sulfidic minerals. This can explain the strong Cu sequestration observed both in natural soils and in Cu spike experiments. In Mehlhorn et al. (2014), strong Cu accumulation was observed in a peat lens, which is located in approximately 2 m depth beneath mofette Site A. Solid-phase speciation of a sample from this peat lens revealed that the majority of the Cu is present as CuS, i.e., as covellite, reinforcing the hypothesis of Cu sequestration by sulfide mineral precipitation in mofettes.

Overall, this study helped to clarify the complex processes of Cu mobilization and sequestration processes along the redox and SOM gradient of a mofette. It could be shown that mofettes serve as sink for Cu and other chalcophilic elements by formation of or co-precipitation with sulfidic minerals.

The accumulation of Cu, Cd, Zn, and sulfur that was observed in the degassing center of Site A in study 1 was most probably also caused by metal sulfide precipitation. However, metal sequestration is only guaranteed under completely anoxic conditions, i.e., directly in the degassing center. In the surrounding transition area, with low amounts of oxygen available, increased mobility of Cu and other metal(loid)s due to complexation with DOM has to be expected.

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