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Hexavalent Chromium Generation within Naturally Structured Soils and Sediments Debra M. Hausladen, and Scott Fendorf Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04039 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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Environmental Science & Technology

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EST Article

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Hexavalent Chromium Generation within Naturally Structured Soils and Sediments

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Debra M. Hausladen and Scott Fendorf*

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Earth System Science Dept.

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Stanford University, Stanford, CA 94305. USA

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*corresponding author. Email: [email protected]; Phone: (650) 723-5238; Fax: (650) 725-2199

ACS Paragon Plus Environment


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ABSTRACT

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Chromium(VI) produced from the oxidation of indigenous Cr(III) minerals is increasingly being

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recognized as a threat to groundwater quality. A critical determinant of Cr(VI) generation within

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soils and sediments is the necessary interaction of two low solubility phases - Cr(III) silicates or

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(hydr)oxides and Mn(III/IV) oxides - that lead to its production. Here we investigate the

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potential for Cr(III) oxidation by Mn oxides within fixed solid matrices common to soils and

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sediment. Artificial aggregates were constructed from Cr(OH)3- and Cr0.25Fe0.75(OH)3-coated

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quartz grains and mixed either with synthetic birnessite or inoculated with the Mn(II)-oxidizing

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bacteria Leptothrix cholodnii. In aggregates simulating low organic carbon environments, we

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observe Cr(VI) concentrations within advecting solutes at levels more than twenty-times the

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California drinking water standard. Chromium(VI) production is highly dependent on Cr-mineral

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solubility; increasing Fe-substitution (x=0 to x=0.75) decreases the solubility of the solid and

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concomitantly decreases total Cr(VI) generation by 37%. In environments with high organic

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carbon, reducing conditions within aggregate cores (microbially) generate sufficient Fe(II) to

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suppress Cr(VI) efflux. Our results illustrate Cr(VI) generation from reaction with Mn oxides

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within structured media simulating soils and sediments and provide insight into how fluctuating

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hydrologic and redox conditions impact coupled processes controlling Cr and Mn cycling.

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INTRODUCTION Anthropogenic chromium is a well-known pollutant that often results from industrial

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processes including leather tanning, metal plating, stainless steel production, and chrome

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pigment manufacturing.1,2 Geogenic Cr(III) is widespread3–9, however, and may represent an

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important source of Cr(VI) if an oxidation pathway exists capable of producing appreciable


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Cr(VI) despite the low solubility of Cr(III)-bearing minerals10. Inhalation, ingestion, and dermal

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exposure to Cr(VI) can result in severe adverse health effects to humans, inclusive of respiratory

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and non-respiratory cancers.11 As Cr(VI) structurally parallels phosphate and sulfate, chromate

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anions are actively transported to cells throughout the body and organs.12 In contrast to Cr(VI),

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Cr(III) is an essential nutrient for humans that is thought to help with glucose transport into

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cells.13 Further differentiating Cr(III) and Cr(VI) are the solubilities of their mineral phases and

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propensity for transport within surface and subsurface environments. Chromium(III) forms low

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solubility hydroxide precipitates10 and strong mineral complexes14. The more toxic Cr(VI)

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species resides as the chromate (HCrO4-) anion that binds less extensively to soil and sediment

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minerals, and thus has both greater dissolved concentrations and propensity for transport within

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water systems.15–17

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Owing to the potential for oxidation, naturally occurring Cr(III) residing within geologic

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strata poses a widespread threat to water quality and human health. Chromium is the tenth most

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abundant element in Earth’s mantle14, and Cr-bearing minerals cover ca. 1% of Earth’s land

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surface, principally found in serpentenized and ultramafic rocks that are concentrated around

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convergent plate boundaries3,18. Weathering of the primary minerals within soil and sediments

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commonly results in Cr(III) hydroxide precipitates, often coprecipitated with Fe(III)- and Al(III)-

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hydroxides, and may coincide with Fe and Mn oxides.19,20 While concentrations in unaltered

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bedrock may reach 2 g/kg, Cr(III) can become further enriched during weathering; Berger and

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Frei reported Cr enrichment within lateritic soil profiles in Madagascar with concentrations

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reaching 60 g/kg20, and Oze et al. found up to 10 g/Kg soil in a Californian serpentine soils21.

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Natural occurrence of hexavalent chromium has been reported in groundwater in pristine

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aquifers far from anthropogenic sources.5,7,22–25 High Cr(VI) levels are often present in aquifers



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surrounded by ophiolites and other ultramaphic rocks and have been reported along the western

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coast of North America4,5,7,8,24–29, southern Africa30, South America31, and Europe 6,9,32–34.

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Aqueous Cr(VI) has been speculatively linked to geogenic Cr(III) being oxidized by Mn

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oxides.35–37 Field-scale studies have showed Mn concentrations to be a good predictor of an

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aquifer’s capacity to form and solubilize Cr(VI)29,35, while microscale XRF and XANES

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spectroscopic approaches have revealed close spatial associations between Mn oxides and

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hexavalent chromium36. In most natural systems, Mn oxides are the only known compound

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capable of oxidizing Cr(III) to Cr(VI) at pH < 9.38,35,39–41 Contrasting Cr(III) oxidation, reduction of Cr(VI) in soils and sediments is common under

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oxygen-limiting (anaerobic) conditions. Iron(II), which is nearly ubiquitous in anaerobic soils

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and sediments, is a facile reductant of Cr(VI) that results in Cr(III)-Fe(III) hydroxides of limited

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solubility.42–44 The solubility of these phases decreases with increasing Fe substitution.10,45 In

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addition to Fe(II), sulfides and organic matter are also potential reductants of Cr(VI).45–47 Finally,

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a range of microorganisms (i.e., Pseudomonas, Desulfovibrio, Shewanella, Bacillus species) are

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capable of enzymatic Cr(VI) reduction under both aerobic and anaerobic conditions (ref 48, and

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references therein).

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Biogeochemical Constraints Imposed by Soil/Sediment Structure Having physical structure that results in a range of pore sizes, soils and sediments are

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often viewed as dual-pore domains, with inter-aggregate channels governed by advective flow

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and intra-aggregate flow dominated by diffusion.49,50 Diffusion and advection control

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biogeochemical networks and influence the spatial distribution and extent of redox processes.51–

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Due to high oxygen demand, microbial respiration limits O2 in all but the exterior few


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millimeters of the diffusive, intra-aggregate zones55,49,53, leading to anaerobic conditions even

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within seemingly well-oxygenated environments. The redox zones resulting from soil

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architecture control the cycling and fluxes of Mn, Fe, and Cr throughout soil and sediments.

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Under aerobic conditions, Fe and Mn precipitate as minerals of limited solubility. As redox

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conditions shift to anaerobic conditions, often due to increased inputs of organic matter,

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reductive dissolution mobilizes Fe(III) and Mn(III/IV) phases. Thus, within anaerobic aggregate

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interiors, conditions conducive to Cr(VI) reduction may prevail, including production of Fe(II)

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via dissimilatory iron reduction; outward diffusion of Fe(II) therefore has the potential to reduce

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Cr(VI).

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In order for Cr(III) to be oxidized by Mn oxides, one of the two phases must dissolve and

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migrate to the other solid.18 Cr(III) oxidation rates are proportional to the dissolved concentration

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of Cr(III) predicted from estimated mineral solubility.18 Therefore, within the structured media of

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soils and sediments, diffusion rates of Cr(III) phases dictate that the two solids must therefore be

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in close proximity (except under acidic conditions). The Cr-oxidizing capacity of Mn oxides is

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substantiated for well-mixed systems where transport limitations are minimized or

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eliminated.18,56–58 However, there are no studies on Cr(III) oxidation within structured,

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physically-rigid conditions found in soils and sediments, despite evidence that Cr(VI) genesis

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likely occurs within such environments.8,24 Understanding Cr cycling within these structured

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systems is vital for predicting the potential oxidation (or reduction) of chromium. The variation

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in geochemical conditions across the aggregated structure of soils and sediments leads to

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conditions potentially conducive to Cr(III) oxidation in the exterior region along flow paths

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while Cr(VI) reduction may proceed distal from oxygen supply, provided sufficient organic

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carbon is present.



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Here, we seek to understand the propensity for Cr(VI) formation by reaction of Cr(III)

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minerals with Mn oxides in fixed structured media characteristic of soils and sediments. Further,

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we investigate the propensity of anaerobic microsites to limit Cr(VI) concentration by promoting

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reduction.

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MATERIALS AND METHODS To assess physical constraints of Cr(VI) genesis by Mn oxides, synthetic aggregates of

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Cr(OH)3- and Cr0.25Fe0.75(OH)3-coated quartz grains were constructed and placed in flow-

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through reactors with synthetic groundwater medium. To mirror diffusion constraints within

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natural systems, we use synthetically constructed architecture to investigate whether low

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solubility Cr(III) and Mn(III/IV) minerals can interact within a fixed matrix to release Cr(VI) to

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advecting porewater at concentrations consistent with those observed in groundwater of Cr-

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bearing sediments. Previous studies illustrate that mass transfer within these synthetic aggregates

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is diffusion controlled, and that flow within the reaction cell does not result in advection within

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the aggregates.51–54 Variation in solubility between Cr(OH)3 and Cr0.25Fe0.75(OH)3 allowed us to

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compare the extent of mineral dissolution on Cr(III) oxidation within fixed media characteristic

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of soils and sediments. Most Mn oxides in the environment are layer-type MnO2 (e.g., birnessite)

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formed by biologically catalyzed reactions with oxygen.59–61 In this study, we investigate the

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oxidizing capacity of both synthetic birnessite and biogenic Mn oxides generated in situ by

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Leptothrix cholodnii, a well studied beta-proteobacteria capable of enzymatic oxidation of

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Mn(II) and Fe(II).62–64 Leptothrix sp. are most commonly found at aerobic/anaerobic interfaces

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where Fe and Mn are cycled between soluble and insoluble forms.65


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Mineral Synthesis. Chromium hydroxide was synthesized by titrating 20 mM CrCl3 to

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pH 6 with 0.1M NaOH and maintaining at this pH for 24 h. Cr0.25Fe0.75(OH)3 was synthesized by

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titrating stoichiometric concentrations of FeCl3 and CrCl3 solutions with 0.1M NaOH to pH 7

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and maintaining the pH value for 3 d, similar to the procedure described in Hansel et al.66.

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During titration, the pH was kept below 7.5. Suspensions were stored at 4° C and not allowed to

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age for more than 48 h. Cr(OH)3 and Cr0.25Fe0.75(OH)3 gels were centrifuged, rinsed three times

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with doubly deionized (DDI) water, and mixed with ground Iota quartz sand (Unimin

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Corporation, Spruce Pine, NC) (Cr(OH)3: 0.22mmol Cr g-1 sand; Cr0.25Fe0.75(OH)3: 0.05mmol Cr

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and 0.16mmol Fe g-1 sand; 106-125 μm quartz grain size). Sand grains were coated in 100 g

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batches and stirred over a 3-d period until completely dry before being rinsed repeatedly with

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DDI water until rinse water was free of particulates; the sand was then left to dry for an

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additional 3-d. Birnessite was synthesized by reducing KMnO4 based on McKenzie67, as outlined

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by Ying et al.5454 Minerals were confirmed with X-ray diffraction analysis on a rotating sample

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using a Rigaku Miniflex 600 diffractometer with Cu-Kα radiation fitted with a 1D silicon strip

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detector, and Cr(III):Fe(III) ratios were confirmed with X-ray fluorescence spectrometry

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(Spectro Xepos HE XRF Spectrometer) (Supporting Information, Figure S3).

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Microbial Inoculum. To investigate the effect of different functional microbial

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communities on Cr(VI) generation, two functionally diverse organisms were introduced to

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Cr0.25Fe0.75(OH)3-aggregates: Leptothrix cholodnii, an obligate aerobic heterotrophic bacterium

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capable of Mn(II) oxidation and known for its ability to precipitate Mn oxides68, and Shewanella

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sp. strain ANA-3, a facultative anaerobe that couples lactate oxidation with the reduction of a

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wide-variety of terminal electron acceptors (TEAs), including Fe(III) and Mn(III/IV) oxides69.

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Shewanella sp. ANA-3 was grown aerobically in autoclaved tryptic soy broth (30 g L-1 DDI



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water) at 25°C until late log phase from frozen seed culture (stored in 20% glycerol at -80°C).

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Leptothrix cholodnii was grown aerobically in liquid mineral salts-vitamin-pyruvate (MSVP)

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medium at 25°C containing the following ingredients (g/L): (NH4)2SO4 0.24; MgSO4*7H2O

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0.06; CaCl2*2H2O 0.06; KH2PO4 0.02; Na2HPO4 0.03; HEPES 2.383; FeSO4 0.002; C3H3NaO3

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1; and 1mL filter-sterilized Wolfe’s vitamin solution.70 All cells were harvested, washed by

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centrifuging liquid cultures (5000 x g; 15 min; 25°C), and re-suspended in 50 mL of sterile

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30mM HEPES- and 10mM sodium bicarbonate-buffered basal salts medium (BSM (g/L): KCl

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0.2; MgCl 0.05; NaCl 0.46; CaCl2*2H2O 0.06; KH2PO4 0.007) at pH 7.1 three times.

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Aggregate Synthesis and Biological Treatments. Three different biotic aggregate

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treatments were investigated: (1) Cr0.25Fe0.75(OH)3-coated sand inoculated with ~8x108 cells L.

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cholodnii g-1 sand, (2) Cr0.25Fe0.75(OH)3-coated sand mixed with birnessite (1:2.5 Mn:Cr molar

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ratio) and inoculated with ~8x108 cells S. sp. ANA3 g-1 sand, and finally (3) Cr0.25Fe0.75(OH)3-

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coated sand inoculated with ~8x108 cells L. cholodnii and ~8x108 cells S. sp. ANA3 g-1 sand.

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Abiotic aggregates were composed of either Cr(OH)3- or Cr0.25Fe0.75(OH)3-coated sand mixed

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with birnessite (1:10 Mn:Cr and 1:2.5 Mn:Cr molar ratio, respectively). In order to promote

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particle aggregation, all aggregates were made with 1% agarose (0.1g UltraPure agarose

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dissolved in 10 mL DDI water) and mixed thoroughly so all mineral phases and/or bacterial cells

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were homogenously distributed.52–54 The agarose sand mixture was then poured into sterile

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molds and formed into 30 x 15mm (height x diameter) cylinders.

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Flow-through reactor setup. Aggregates were placed in flow-through reactors with a

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volume of 77 mL (38x51mm; height x diameter) with 0.2 µm filters at inflow (bottom) and

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outflow (top) (Supporting Information, Figure S1). Styrene-butadiene rubber (ø = 17mm) capped

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circular planes of aggregates (3M™ Scotch-Weld™ Instant Adhesive CA5) in order to preclude


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vertical flow through aggregate cylinders. Sterile 5mm glass beads (80 (±2) g) were added to

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stabilize aggregate position. Two abiotic and three biotic treatments were investigated

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(Supporting Information, Table S1). Synthetic groundwater medium was pumped into reactor

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cells and advected around aggregates before being collected and analyzed. For all reactors, a

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synthetic groundwater media was used as the advecting solution consisting of: (in mg/L)

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CaCl2•2H2O 60; MgCl2•7H2O 50; KCl 200; NaCl 460; KH2PO4 7; NH4Cl 0.95. After

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autoclaving the groundwater media, the following filter sterilized solutions were added for a final

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concentration of: 30 mM HEPES, 10 mM NaHCO3, 0.23 mM MnCl2, and pH adjusted to 7.6

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with 6 M NaOH. A dual-buffered system was necessary to stabilize pH in the presence of

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microbial activity and was also used in abiotic systems for consistency. For the influent solution

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concentrations, rhodochrosite is oversaturated (SI = 2.1). However, to ensure precipitation of

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Mn(II) solids did not change Mn concentrations within influent solution prior to entering the

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reactor, Mn concentrations in the influent media were analyzed over the course of the

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experiment; concentrations were constant at 0.21 (±0.007) and 0.23 (±0.007) mM for the biotic

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and abiotic systems, respectively. For biotic reactors, 1 mL Wolfe’s trace mineral and vitamin

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solutions were added along with dissolved organic carbon as electron donor. Pyruvate and lactate

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were added for a final concentration of 3 mM each to represent dissolved organic carbon (DOC)

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levels of carbon-rich environments. Based on previous studies we expect little impact on Mn

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oxide dissolution at these DOC concentrations.71 The surrounding solute of all reactors was

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continuously sparged with filtered air. The solute flow eluted at a rate of 0.8 mL h-1 for biotic

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reactors and 0.6 mL h-1 for abiotic reactors. After 22 days, an acidified groundwater medium

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with sodium acetate buffer (30 mM, pH 5) consisting of: (in mg/L) CaCl2•2H2O 60; NaCl 460;



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KCl 200; NaCl 460; KH2PO4 7; NH4Cl 0.95, was pulsed through abiotic aggregates for 4 days

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before resuming initial synthetic groundwater composition (pH 8).

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Aqueous and solid phase analysis. Total Mn, Fe, and Cr concentrations were measured

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from filtered effluent using inductively coupled plasma mass spectrometry (Thermo Scientific

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XSERIES 2 ICP-MS). Quality control standards were analyzed every 15 samples to ensure a

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≤5% deviation from the standard curve was maintained. Unacidified filtrate was measured

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immediately for aqueous Cr(VI) concentrations using the diphenyl carbazide (DPC)

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spectrophotometric method.35 Aggregates for solid phase analysis were removed from reactors

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after aqueous Cr(VI) production had subsided (15 and 11 days for abiotic and biotic treatments,

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respectively). Additionally, separate abiotic aggregates (Cr(OH)3 (n=3); Cr0.25Fe0.75(OH)3 (n=2))

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were run for 50 days before being harvested and analyzed. Before removal from the flow cell,

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submerged aggregates were rotated 90° (after removing glass beads) and dissolved oxygen (DO)

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concentrations were measured in the middle of the aggregate (from exterior to interior) using a

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microsensor with a tip diameter of 10 μm (OX-10, Unisense). The tip, mounted on a motor-

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driven micromanipulator stage (MMS, Unisense) positioned via a motor controller (MC-232,

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Unisense), and connected to a picoammeter (Microsensor Multimeter, Unisense), was slowly

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lowered from aggregate exterior to interior. Linear calibrations were performed before each

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measurement in 0.1M sodium ascorbate in 0.1M NaOH (0% O2 saturation) and air bubbled water

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(100% O2 saturation). Each cylindrical aggregate was sectioned into three concentric semi-

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circles labeled as ‘E’ for exterior (0-2 mm), ‘M’ for midsection (2-4.5 mm), and ‘I’ for interior

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(4.5-7.5 mm) for three 4 mm slices representative of the bottom (2-6 mm), middle (18-22 mm),

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and top (24-29 mm) of the aggregate; a similar approach was used in previous studies to examine


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spatial variation in dissimilatory Fe(III) reduction52, As(V) reduction72, and Mn(IV)-Fe(III)

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reduction54.

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X-ray absorption spectroscopic (XAS) analysis was performed on dry sediments to

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determine Cr, Mn, and Fe speciation and quantify Fe phases. Acid digestion with 6 M HCl was

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used to quantify solid phase Cr, Mn, and Fe concentrations. All solids from Shewanella-

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inoculated aggregates were processed anoxically within a glove-bag atmosphere of

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95%N2:5%H2.

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Micro-X-ray fluorescence (μ-XRF) analysis of radial slices of each aggregate was carried out

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on beamline 2-3 and 10-2 at the Stanford Synchrotron Radiation Lightsource (SSRL) to map the

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spatial distribution of Cr, Mn, and Fe from the interior to exterior of the aggregate. Manually

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sliced 1 mm samples placed between Kapton tape were measured on BL10-2. Samples mapped

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on BL2-3 were dried, embedded in EPOTEK301-2FL epoxy, and then sectioned to 30 µm

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thickness and mounted on a quartz slide. The beam was calibrated by setting the position of the

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pre-edge absorption peak of Na2CrO4 at 5993eV. Cr maps were taken at three energies (5993,

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6003, 6010 eV) at 2-4 μm steps for high-resolution mapping. A Fe-beta window was used to

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subtract interfering intensities between Cr, Fe, and Mn from spectra collected with a vortex

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detector.

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For bulk XAS measurements, samples were dried, mixed with BN and pressed into

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pellets. Spectra were then collected on beamlines 11-2 and 4-1 at SSRL. A double-crystal, Si

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(220) LN-cooled monochromator was used for energy selection. Cr K-edge XANES spectra

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were collected with energy steps of 0.3 eV from 5969 to 6019 eV. Coarser steps were taken

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outside of this region for normalization purposes. Solid-phase iron was investigated using the

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extended portion (EXAFS) of the Fe K-edge spectrum; scans were obtained from 6882 to 7922



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eV, which is equivalent to a maximum k=15.2 Å-1. The XANES portion of the Fe spectra was

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taken with a resolution of 0.3 eV, with the monochromator detuned 20% to minimize higher-

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order harmonics, and spectra collected with 30-element Ge detector. Spectra were background

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subtracted, normalized, converted to k-space (Å-1) and k3 weighted. The x(k)k3 spectrum was

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Fourier-transformed over 0 to 10 Å-1. Then peaks were individually isolated and

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backtransformed. Qualitative analysis was performed by comparing unknowns to reference

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compounds.

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RESULTS Aqueous Cr(VI) effluent concentrations Synthetic soil aggregates (abiotic or bacterially inoculated) were placed within reactors

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and flow was initiated. Within reactors having abiotic aggregates composed of Cr(OH)3, or

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Cr0.25Fe0.75(OH)3, and birnessite, Cr(VI) was detected in advecting solutes within 5 h (Figure 1).

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Concentrations increased for the first 44 h of reaction, reaching a maximum concentration of ca.

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4 μM Cr(VI); concentrations then decreased steadily. Despite differences in expected solubility,

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abiotic aggregates having birnessite and Cr(OH)3 or Cr0.25Fe0.75(OH)3 generate similar amounts

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of Cr(VI) within the first 2 d of reaction; thereafter, the concentrations of Cr(VI) produced via

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oxidation of Cr(III) diverge, with the greater solubility of Cr(OH)3 leading to higher Cr(VI)

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concentrations (Figure 1). The cumulative mass of Cr(VI) eluted from the Cr0.25Fe0.75(OH)3

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reactor was 0.45 (± 0.05) μmole while the cumulative mass for the Cr(OH)3-containing reactor

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was almost double, 0.72 (± 0.02) μmole, prior to an acid injection (Figure 1).

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After an initial peak in Cr(VI) concentration at ca. 2 to 4 d, the Cr(VI) production

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steadily decreased. To test whether the decline in Cr(VI) production resulted from a passivating


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surface layer on the Mn oxides, potentially MnCO3, an acidified synthetic groundwater (pH 5)

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pulse was introduced after 23 days (Figure 1, grey bar). During the period of acidified influent,

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effluent Cr(VI) concentrations continue to decrease as the pH drops. Once effluent pH values

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decrease to that of influent pH (pH = 5), effluent Mn(II) concentrations spike, resulting in a

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concentration of 0.48 (± 0.14) mM for Cr(OH)3 and 0.37 (± 0.19) mM for Cr0.25Fe0.75(OH)3

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aggregates (Figure 1a). A secondary pulse of Cr(VI) then occurs subsequent to the Mn(II) pulse

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as the reactor pH returns to the initial pH (pH = 8).

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Within soils and subsurface sediments, Mn oxides are likely formed through biologically

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mediated processes. Using the Mn oxidizing bacterium L. cholodnii (hereafter referred to

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generally as Leptothrix), we tested for the production of Cr(VI) within the Cr(OH)3- and

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Cr0.25Fe0.75(OH)3-aggregate reactors having influent Mn(II) concentrations of 0.2 mM. Cr(VI)

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production within the Leptothrix-inoculated Cr0.25Fe0.75(OH)3 –aggregate quickly (41.4 h)

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reaches a peak concentration of 1.1 μM (Figure 2). Despite releasing only 11% of the cumulative

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Cr(VI) mass eluted from the abiotic birnessite-aggregate, peak Cr(VI) concentrations from

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microbial-inoculated aggregates reach over 26% those of the abiotic treatment (Figures 1 and 2).

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Owing to soil/sediment architecture, redox heterogeneity commonly prevails, leading to

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aggregates having anaerobic interiors and aerobic exteriors. To test for the influence of anaerobic

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zones proximal to aerobic regions on the production and efflux of Cr(VI), we utilized aggregates

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having either synthetic birnessite with the metal reducing bacterium Shewanella sp. ANA3

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(hereafter referred to more generally as Shewanella) or co-cultures of Leptothrix and Shewanella.

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The presence of metal reducing bacteria suppresses Cr(VI) elution within both abiogenic and

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biogenic Mn-oxides. When Shewanella is added to synthetic birnessite, only 18.5 nmoles of

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cumulative Cr(VI) is released (only 4.7% of Cr(VI) release from synthetic birnessite without iron



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reduction) (Figure 2b). In Leptothrix and Shewanella co-inoculated Cr0.25Fe0.75(OH)3-aggregates,

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Cr(VI) concentrations remain below the California drinking water standard of 10 µg/L

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throughout the experiment and cumulative release of Cr(VI) is just over half of the

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birnessite/Shewanella aggregate at 11.3 nmoles (Figure 2b).

292 293 294

Oxygen Profiles Localized geochemical gradients created by diffusion-limited transport within soil

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aggregates restrict oxygen supply and can induce anaerobic conditions. Within oxygenated

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regions proximal to advective flow paths, obligate aerobic communities of Mn-oxidizing bacteria

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and fungi are able to colonize. Radial heterogeneity in Mn oxides and secondary Fe-minerals

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correlate with this geometric redox zonation resulting from oxygen gradients and associated

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metabolic activity (Figure 3; dissolved oxygen profiles are given in Supporting Information,

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Figure S4). In abiotic aggregates, with no microbial oxygen consumption, oxygen levels remain

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near saturation throughout the profile (Figure 3a; Figure S4). Oxygen penetration into aggregates

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decreases when Shewanella (a facultative anaerobe capable of Fe(III) and Mn(III/IV) reduction)

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is present (Figure 3c,d). The extent of oxygenation is shallowest when only Shewanella is

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present, suggesting that in the absence of competing obligate aerobes, Shewanella may consume

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oxygen more efficiently thereby limiting an aggregate’s oxic zone. In these aggregates, dissolved

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oxygen (DO) levels drop below 200 μM just 1.5 mm below the exterior aggregate surface

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(Figure 3d) and drop below detection limit (0.3 μM) 2.3 mm from the surface. With the addition

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of Leptothrix (aerobic, Mn-oxidizing bacteria) and Shewanella, DO levels greater than 200 μM

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are maintained only in the outer 3.9 mm of the aggregate (Figure 3c) and DO levels drop to

Hexavalent Chromium Generation within Naturally ... - ACS Publications

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