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Environmental Processes
Copper mobilization and immobilization along an organic matter and redox gradient – insights from a mofette site Judith Mehlhorn, Johannes Besold, Juan S. Lezama-Pacheco, Jon Petter Gustafsson, Ruben Kretzschmar, and Britta Planer-Friedrich Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02668 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018
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Copper mobilization and immobilization along an
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organic matter and redox gradient – insights from a
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mofette site
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Judith Mehlhorn,† Johannes Besold,† Juan S. Lezama Pacheco, ‡ Jon Petter Gustafsson,§ Ruben
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Kretzschmar,∥ Britta Planer-Friedrich*,† †
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Environmental Geochemistry, Bayreuth Center for Ecology and Environmental Research
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(BayCEER), University of Bayreuth, D-95440 Bayreuth, Germany ‡
Department of Environmental Earth System Science, Stanford University, Stanford, California
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94305, United States §
Department of Soil and Environment, Swedish University of Agricultural Sciences, 75007
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Uppsala, Sweden ∥Soil
Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, CH-8092 Zurich, Switzerland
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ABSTRACT: Mofettes (natural geogenic CO2 exhalations) represent excellent sites to study the
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behavior of Cu in soils and the co-occurrence of different mobilization and immobilization
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processes since they exhibit both a gradient in redox conditions (oxic to permanently anoxic) and
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in soil organic matter (SOM; low to high contents). Soil and pore water samples from an 18 m-
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transect over a mofette showed a complex behavior of Cu, with highest mobility in the transition
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between oxic and anoxic conditions. Cu(II) sorption experiments on SOM-rich topsoil revealed
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that Cu mobility under oxic conditions was confined by adsorption to SOM while in the oxygen-
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free mofette center reduction and precipitation of sulfides was the dominating Cu-sequestering
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process. In transition areas with low amounts of oxygen (3 m at site A) caused SOM translocation from the mofette and
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accumulation. The previously described negative effect of high p(CO2) in mofettes on the
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formation of Fe (oxyhydr)oxides48,55-57 is reflected by strongly decreased Fe contents in samples
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from degassing centers and accumulation of Fe in transition zones (Table 1).
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Table 1. Overview of selected physical and chemical soil parameters of the untreated soil used in
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the experiments.a Samples were taken from approx. 5–15 cm depth. Contents of Cu, Fe, and S
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were determined in aqua regia digests. Mofette A distance [m]b
Transition A1 Transition A2 Reference A
Mofette B
Reference B
0
4
8
18
0
12
p(CO2) [-]c
0.972
0.556
0.072
0.016
n.d.
n.d.
p(O2) [-]c
0.000
0.089
0.198
0.205
n.d.
n.d.
4.0 ± 0.04
4.6 ± 0.01
pH [-]d C [g kg-1]
4.1 ± 0.04
3.7 ± 0.02
4.2 ± 0.03
202.0 ± 6.6
190.7 ± 2.5
148.3 ± 0.6
C/N ratio [-]
16.7 ± 0.1
15.7 ± 0.1
12.8 ± 0.1
Cu [mg kg-1]
23 ± 1
26 ± 1
36 ± 0
Fe [mg kg-1] 6,847 ± 164 75,697 ± 8,927 60,247 ± 889 S [mg kg-1]
4,086 ± 26
3,149 ± 141
2,981 ± 38
4.5 ± 0.02 112.3 ± 0.6
127.7 ± 4.5
161.7 ± 2.5
12.7 ± 0.03
16.0 ± 0.6
12.8 ± 0.1
33 ± 1
12 ± 0
21 ± 0
30,041 ± 722 6,679 ± 414 2,698 ± 41
2,651 ± 29
42,312 ± 1,794 3,052 ± 194
a
Mean ± standard deviation, n=3. bDistance from degassing center at Mofette A and B, respectively. cPartial pressure of CO2 and O2, respectively. dMeasured in 0.01 M CaCl2
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Copper Sorption Experiment. All experiments were conducted in triplicate inside an
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anaerobic chamber (Coy, 95% N2/5% H2) using nitrogen-purged solutions prepared with
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ultrapure deionized water (Millipore, 18.2 MΩ cm) and chemicals of analytical grade. Soil
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samples (approx. 300 g per sample) were thawed inside the anaerobic chamber overnight and
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roots and stones were removed by pressing the well-mixed soil through a nylon sieve (1 mm
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mesh size). The wet soil was weighted immediately into centrifuge vials (equivalent to approx.
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0.27 g dry weight) and 8 mL of 5 mM NaCl (Merck, p.a.) solution were added as background
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electrolyte leading to a soil-to-solution-ratio of approx. 1:30. All vials were wrapped in
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aluminum foil to exclude light influence. Suspensions were well-mixed, pH was adjusted to
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4.5±0.1 (approx. pore water pH of this mofette site52) using 1 M NaOH or 1 M HCl, and
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suspensions were pre-equilibrated for 24 h with occasional shaking (permanent shaking might
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promote colloid formation). Afterwards, 0–400 µL of 7.87 mM CuCl2 (Grüssing, p.a.) stock
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solution, pH 4.5, were spiked to the suspensions, yielding spike concentrations of 0, 0.23, 1.16,
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2.33, 5.83, and 11.65 mmol kg-1. The suspensions were well-mixed and pH was adjusted to
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4.5±0.1, then samples were left to equilibrate for 24 h with occasional shaking. The time
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required for achieving equilibrium conditions was determined in a kinetic study prior to this
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experiment (see section S.1, supporting information (SI)). After equilibration, pH was measured
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in soil suspensions, then vials were centrifuged (5 min at 4500 g, outside glovebox) and the
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supernatant was removed with needle and syringe, filtered (0.2 µm, cellulose-acetate), and
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stabilized for total Cu analysis in 0.45% H2O2 and 0.65% HNO3.
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Analyses of Collected Samples. The remaining soil from the sorption experiment and the
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additional 18 soil samples from Site A were freeze-dried, pestled, and digested in aqua regia
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(soil-to-solution-ratio 1:100, microwave assisted digestion, MARS Xpress, CEM). Total Cu
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concentrations in filtered and diluted liquid phase samples and aqua regia digests were measured
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with a quadrupole inductively coupled plasma mass spectrometer (ICP-MS, X-Series 2, Thermo
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Scientific) at m/z 65 to avoid interferences with ArNa+ at m/z=63. Aqua regia digests of the
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initial soil samples were additionally analyzed for total Fe and S concentrations by ICP-MS.
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Total C and N contents in the initial, freeze-dried and pestled soil samples were determined
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with a CHN elemental analyzer (Thermo Quest, Flash EA, 1112). The amounts of organic matter
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mobilized from solid into liquid phase were determined in triplicate for samples with highest
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Cu(II) spike. Due to limited sample amounts, the Cu sorption experiment was repeated for these
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samples, suspensions were centrifuged, and supernatants were filtered (0.45 µm, polyamide) and
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analyzed for total dissolved organic carbon (DOC) and nitrogen by thermo-catalytic oxidation
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with a TOC-VCPN Analyzer (Shimadzu).
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Dissolved CO2 concentrations in pore water samples collected from Site A were calculated
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from
head
space
concentrations
in
the
septum
bottles
applying
Henry's
law
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(kH,CO2=0.03344 mol L-1 atm-1 from Sander58, details in section S.3, SI). Gaseous head space
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concentrations were measured with a gas chromatograph (SRI Instruments 8610C, U.S.)
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equipped with a methanizer and a flame ionization detector.
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Sulfide concentrations in pore water samples were determined photometrically using the
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methylene blue method,59 measuring the absorbance at 650 nm with a multi-plate reader (Infinite
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200 PRO, Tecan). With the same method, we checked for sulfide formation in selected samples
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from Cu sorption experiment using filtered soil suspensions. In the Cu(II)-spiked samples we
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also checked whether Cu(I) formation occurred, using the bathocuproine method24,60 with
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absorbance measured at 492 nm.
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Characterization of SOM and Mineral Phases. Characterization of SOM was done for 13
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samples from Mofette A, Transition A1, and Reference A by solid-state
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resonance spectroscopy (NMR, details in section S.4, SI). Further, we characterized DOM by
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Fourier-transform infrared spectroscopy (FTIR). Extraction of DOM was done with ultrapure
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deionized water at a soil-to-solution-ratio of 1:5 for 24 h on a horizontal shaker. Absorption band
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assignments are based on Artz et al.61 and Rennert et al.48 Details on FTIR analysis can be found
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in section S.5, SI.
C nuclear magnetic
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Samples from Transition A1 and Reference A were analyzed by X-ray powder diffraction
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(XRD) to gain information on mineralogy using a Philips X’Pert Pro diffractometer operating in
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reflection mode with Fe filtered CoKα1 radiation, step size 0.03° 2θ, range 5–50°, and scan speed
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of 1.8°/min. Sample spectra can be found in section S.6, SI.
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One Cu-loaded sample of Mofette A was analyzed by scanning electron microscopy energy-
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dispersive X-ray spectrometry (SEM-EDS) in order to detect potential Cu precipitates (details in
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section S.7, SI).
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X-ray Absorption Spectroscopy. Copper K-edge X-ray absorption near edge structure
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(XANES) and extended X-ray absorption fine structure (EXAFS) spectra were collected at
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beamlines 4-1 and 4-3 at the Stanford Synchrotron Radiation Lightsource (SSRL, Stanford,
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USA). Spectra were collected for samples from Site A and for one sample from a peat lens at
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approx. 2 m depth beneath the degassing center of Site A (originating from the sampling in
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February, further information on physicochemical properties of this peat lens can be found in
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Mehlhorn et al.52). We collected XANES only for initial soil samples, XANES and EXAFS for
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samples with the highest Cu spike from sorption experiment and for the peat lens sample.
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Sample measurements were performed in fluorescence mode at 25 K using a 4-element solid-
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state Si drift detector (HTA Hitatchi) and a liquid He cryostat at beamline 4-3 (samples
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Transition A1, A2, Reference A, and peat lens) or at 77 K using a 32-element solid-state Ge
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detector (Canberra) and a liquid nitrogen cryostat (samples Mofette A). The following published
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spectra were used as reference compounds: Cu(II)-humic (Cu(II) adsorbed to a soil humic acid
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solution),30 Cu(II)-malate (Cu(C4H5O5)2(H2O)2),30 Cu(II)-carboxyl (Cu(II) adsorbed to carboxyl
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resin Bio-Rex® 70 (Bio-Rad)),22 Cu(II)-clay (Cu(II) adsorbed to montmorillonite),30 Cu(I)-thiol
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(Cu(I) adsorbed to thiol resin Ambersep GT74® (Supelco)),22 Cu(I)-GSH (Cu(I) adsorbed to L-
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Glutathione),30 Cu1.8S,20 CuS,30 and Cu(II)-ferrihydrite (30 µM Cu(II) adsorbed to 0.3 mM
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ferrihydrite,15 kindly provided by Charlotta Tiberg, Swedish Geotechnical Institute, Stockholm).
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Additionally, metallic Cu(0) (Cu foil), measured in transmission mode, was used as a reference
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compound. Further details on data collection and evaluation can be found in section S.8, SI.
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RESULTS AND DISCUSSION
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Natural Distribution of Solid and Liquid Phase Copper Along the Mofette-Reference Site
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Gradient. Compared to the non-CO2-influenced reference in 18 m distance, Cu soil contents in
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the degassing center (0 m distance) were similar or slightly increased while Cu pore water
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concentrations were decreased (Figure 1a-b). Similar trends were reported previously by
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Mehlhorn et al.52 who observed a relationship (r=0.59, P98.7% for XANES,
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Table S3, SI). The best fits with normalized sum-squared residuals (NSSR)
