Redoxtrons – An experimental system to study redox processes within the capillary fringe

Spatiotemporal characterisation of the soil redox status within the capillary fringe (CF) is a challenging task. Air-filled porosities (ε), oxygen concentration (O2) and soil redox potential (EH) are interrelated soil variables within active biogeochemical domains such as the CF. We investigated the impact of water table (WT) rise and drainage in an undisturbed topsoil and subsoil sample taken from a Calcaric Gleysol for a period of 46 days. We merged 1D (EH and matric potential) and 2D (O2) systems to monitor at high spatiotemporal resolution redox dynamics within self-constructed redoxtron housings and complemented the data set by a 3D pore network characterization using X-ray microtomography (X-ray μCT). Depletion of O2 was faster in the organic matter- and clay-rich aggregated topsoil and the CF extended >10 cm above the artificial WT. The homogeneous and less-aggregated subsoil extended only 4 cm above the WT as indicated by ε–O2–EH data during saturation. After drainage, 2D O2 imaging revealed a fast aeration towards the lower depths of the topsoil, which agrees with the connected ε derived by X-ray μCT (εCT_conn) of 14.9% of the total porosity. However, small-scaled anoxic domains with O2 saturation <5% were apparent even after lowering the WT (down to 0.25 cm2 in size) for 23 days. These domains remained a nucleus for reducing soil conditions (EH < −100 mV), which made it challenging to characterise the soil redox status in the CF. In contrast, the subsoil aeration reached O2 saturation after 8 days for the complete soil volume. Values of εCT_conn around zero in the subsoil highlighted that soil aeration was independent of this parameter suggesting that other variables such as microbial activity must be considered when predicting the soil redox status from ε alone. The use of redoxtrons in combination with localised redox-measurements and image based pore space analysis resulted in a better 2D/3D characterisation of the pore system and related O2 transport properties. This allowed us to analyse the distribution and activity of microbiological niches highly associated with the spatiotemporal variable redox dynamics in soil environments

Soil aeration and redox potential as function of pore connectivity unravelled by X-ray microtomography imaging

Platinum (Pt)-tipped electrodes are frequently employed to measure the soil redox potential (EH). Thereby, the timely transition from reducing towards oxidising soil conditions is one of the most important biogeochemical changes that can occur in soil. This condition is mainly linked to the air-filled pore volume (ε) and pore geometries. However, even when the Pt electrodes are located in close vicinity to each other, EH readings behave non-uniformly, presumably due to the millimetre scaled heterogeneity of pore spaces controlling oxygen (O2) availability and transport. In this study, we examined the ε distribution and pore connectivity in the close vicinity of a Pt electrode during an artificial evaporation experiment using an undisturbed soil sample (Ah-horizon, Calcaric Gleysol). We combined physio-chemical methods with non-destructive X-ray computed microtomography (μCT) and 3D-image analysis. μCT scans were conducted at three-time points, that is, reducing conditions with EH &lt; −100 mV (CT-1), the transition from reducing towards oxidising conditions with an EH increase &gt; 5 mV h−1 (CT-2), and oxidising conditions with EH &gt; 300 mV (CT-3). We observed that the shift from reducing towards oxidising conditions took place at an air-filled porosity (εCT) of ~0.03 cm3 cm−3, which matches very with gravimetrically calculated data obtained by tensiometry of ε ~0.05 cm3 cm−3. Besides the relation of EH and ε, image analysis revealed that a connected εCT (εCT_conn) of ~0.02 cm3 cm−3 is needed to enable enhanced O2 diffusion from the soil surface towards the Pt surface and facilitate a straightforward EH response. We conclude that εCT_conn is a critical parameter to assess aeration processes in temporarily water-saturated soils to characterise a switch in redox conditions. Highlights: Usually, soil redox dynamics are related to the air-filled porosity (εCT) but here its connected portion (εCT_conn) was found more relevant. 3D X-ray computed microtomography imaging close to a redox electrode enabled us to understand the soil aeration process. Connected εCT (εCT_conn) of ~0.02 cm3 cm−3 facilitated oxidising soil conditions. εCT_conn is a critical parameter to assess the aeration process in temporarily water-saturated soils. © 2021 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science

Manganese and iron oxide-coated redox bars as a tool to in situ study the element sorption in wet soils

When studying redox conditions in soils with manganese (Mn) and iron (Fe) oxide-coated redox bars, we observed the formation of Fe oxides along the Mn oxide coating and assumed sorption of other elements from soil solution to oxide surface. The objective of this study was to investigate the formation of Fe oxides along Mn redox bars and to analyze element sorption from soil solution to either Mn or Fe oxide along redox bar coatings. We protruded Mn redox bars into solutions with defined Fe2+ concentrations and removed the bars at distinct time intervals. The Mn oxide coating and potential Fe oxides were extracted using dithionite-citrate-bicarbonate (DCB). To investigate in situ element sorption behavior, we used previously field-installed redox bars, protruding these Mn redox bars into acidified hydroxylamine hydrochloride (AAH) to selectively extract Mn oxide and afterwards into DCB to dissolve the remaining Fe oxide coating. This two-step extraction procedure enabled the differentiation of elements bonded to either Mn or Fe oxide. Additionally, we analyzed the redox bar coatings at a very small scale ( Pb > Zn, Fe oxide preferentially sorbed oxyanions with As > P > Mo > V, respectively. Field-Fe oxides precipitating along the Mn redox bars sorbed elevated levels of As and P compared with the action of synthesized lab-Fe oxides along Fe redox bars, a finding which we attribute to short-range-ordered Fe phases with elevated sorption capacity. Besides providing information regarding the monitoring of soil redox status, the developed sequential two-step extraction procedure enables the differentiation of the selective sorption of elements in the soil solution to the coating of Mn and Fe redox bars. The collection of Fe oxides formed naturally along the Mn redox bar coatings further enables the investigation of temporally and spatially diverse Fe oxide-forming processes

Mobility of Iron-Cyanide Complexes in a Humic Topsoil under Varying Redox Conditions

The potentially toxic Fe-CN complexes ferricyanide, [FeIII(CN)6]3−, and ferrocyanide, [FeII(CN)6]4−, undergo a variety of redox processes in soil, which affect their mobility. We carried out microcosm experiments with suspensions of a humic topsoil (pH 5.3; Corg 107 g kg-1) to which we added ferricyanide (20 mg l-1). We varied the redox potential (EH) from −280 to 580 mV by using O2, N2 and glucose. The decrease of EH led to decreasing concentrations of Fe-CN complexes and partial reductive dissolution of (hydrous) Fe and Mn oxides. The dynamics of aqueous Fe-CN concentrations was characterized by decreasing concentrations when the pH rose and the EH dropped. We attribute these dependencies to adsorption on organic surfaces, for which such a pH/EH behavior has been shown previously. Adsorption was reversible, because when the pH and EH changed into the opposite direction, desorption occurred. This study demonstrates the possible impact of soil organic matter on the fate of Fe-CN complexes in soil

Trace Element Solubility in a Multimetal-Contaminated Soil as Affected by Redox Conditions

Redox conditions play an outstanding role in controlling the behaviour of trace elements in soil environments. They are not only sensitive to water saturation but also to soil temperature because many redox reactions are mediated by microorganisms. In this study, we investigated the influence of oxidising (oxygen predominant), weakly reducing (Mn-III,Mn-IV reduction) and moderately reducing (Fe-III reduction) conditions at three temperature regimes (7, 15 and 25 degrees C) on the solubility of ten trace elements. Multimetal-contaminated topsoil (pH 5.8) from a floodplain in Germany was investigated with the following aqua regia-soluble concentrations: Zn 903, Cu 551, Cr 488, Pb 354, Ni 93.5, As 35.7, Co 22.4, Sb 20.5, Cd 8.3 and Mo 6.5 mg kg(-1). Soil suspensions were held at fixed redox potential in microcosm experiments, sampled at every third day and analysed for trace elements. Time to achieve weakly and particularly moderately reducing conditions was temperature dependent and increased in the order 25 degrees C<15 degrees C<7 degrees C. Under oxidising conditions, the solubility of the trace elements was low. Reductive dissolution of Mn oxides under weakly reducing conditions was accompanied by a release of Co and Mo. Reductive dissolution of Fe oxides (and of remaining Mn oxides) under moderately reducing conditions additionally led to a release of As, Cd, Cr, Ni and Pb, whereas Cu and Zn were hardly affected. Antimony revealed a different behaviour because, after a first increase, a continuous decrease in its concentration was observed soon after the onset of weakly reducing conditions. We conclude that soil temperature should be considered as a master variable, to distinguish between weakly and moderately reducing soil conditions, and that it is necessary to keep element-specific behaviour in mind when dealing with the effects of redox conditions in soils on trace element solubility

Manganese-Oxide-Coated Redox Bars as an Indicator of Reducing Conditions in Soils

Identification of reducing conditions in soils is of concern not only for pedogenesis but also for nutrient and pollutant dynamics. We manufactured manganese (Mn)-oxide-coated polyvinyl chloride bars and proved their suitability for the identification of reducing soil conditions. Birnessite was synthesized and coated onto white polyvinyl chloride bars. The dark brown coatings were homogenous and durable. As revealed by microcosm devices with adjusted redox potentials (E-H), under oxidizing conditions (E-H similar to 450 mV at pH 7) there was no Mn-oxide removal. Reductive dissolution of Mn-oxides, which is expressed by the removal of the coatings, started under weakly reducing conditions (E-H similar to 175 mV) and was more intensive under moderately reducing conditions (similar to 80 mV). According to thermodynamics, the removal of Mn-oxide coatings (225 mm(2) d(-1)) exceeded the removal of iron (Fe)-oxide coatings (118 mm(2) d(-1)) in soil column experiments. This was confirmed in a soil with a shallow and strongly fluctuating water table where both types of redox bars were inserted. Consequently, it was possible to identify reducing conditions in soils using Mn-oxide-coated bars. We recommend this methodology for short-term monitoring because tri- and tetravalent Mn is the preferred electron acceptor compared with trivalent Fe, and this additionally offers the possibility of distinguishing between weakly and moderately reducing conditions. If dissolved Fe2+ is abundant in soils, the possibility of nonenzymatic reduction of Mn has to be taken into account

Iron isotope composition of aqueous phases of a lowland environment

Environmental context Iron (Fe) isotope analysis is a powerful tool to understand the transport of Fe within and from soils to rivers. We determined Fe isotopes and Fe concentrations of soil solutions at different depths and found that the Fe isotope compositions are modified owing to adsorption onto Fe oxides, especially in the subsoil. Hence Fe-rich capillary rising groundwater or seeping Fe-rich surface water are depleted in Fe and potentially other metals in Fe oxide-rich soil horizons. Abstract The mobility of iron (Fe) in soils is strongly affected by redox conditions, which also affect Fe input into groundwater and rivers. Stable Fe isotope analyses allow further investigation of Fe translocation processes within, into and out of soils. Soil solutions taken from a Gleysol in a lowland area (NW Germany) at different depths revealed that Fe concentration and isotope ratios strongly varied with abundance of solid Fe oxides. Low Fe-56 values of -1.7 parts per thousand and minimum Fe concentrations of similar to 0.2mgL(-1) were recorded in soil solutions of Fe-rich horizons. Soil solutions of a Fe-poor horizon, however, yielded higher Fe-56 values (-0.39 parts per thousand) and Fe concentrations of up to 68mgL(-1). The water of an adjacent drainage ditch featured Fe-56 values of -1.1 parts per thousand, in strong contrast to +0.60 parts per thousand of short-range ordered Fe oxide deposits in the ditch bed. We attribute the coupled low Fe-56 values and Fe concentrations to combined adsorption and atom exchange between dissolved Fe and Fe oxides. Consequently Fe oxide-poor horizons had higher Fe-56 values and dissolved Fe concentrations. Outflow of Fe-rich groundwater and surface water during rainfall into rivers is responsible for high Fe-56 for Fe-oxide precipitates and low riverine Fe-56 values

Chromium Release from a COPR-Contaminated Soil at Varying Water Content and Redox Conditions

Many soils in the region of Kanpur, North India, are heavily affected by the leather industry and its upstream supplier sector, as indicated by elevated chromium (Cr) contents. Under reducing conditions-for instance, at water saturation after monsoon rain or flood irrigation-the dynamic and species distribution of Cr may be affected due to changes in redox potential (E-H). In this study, the influence of E-H on the speciation and release of Cr from a contaminated agricultural soil was investigated. A soil sample that was affected by hyperalkaline leachate from chromite ore processing residue, was taken and packed in soil columns, and subjected to a saturation-drainage-saturation cycle. After initial water saturation, the E-H dropped slowly to minimum values of around. 100 mV (calculated to pH 7), while E-H was controlled by CrO42-/Cr2O3(s), or CrO42-/(Fe,Cr)OOH redox couples. Soil drainage resulted in a quick return to oxidizing conditions; i.e., E-H >300 mV. The Cr species distribution and release showed a clear trend with E-H. At the beginning of the experiment, under oxidizing and weakly reducing conditions (E-H range from >100 to 300 mV), Cr(VI) was released in particular. However, under moderately reducing conditions (E-H range from 100 to -100 mV), Cr was gradually immobilized and irreversible sequestered via reductive precipitation. The results presented in this study provide an improved understanding of the mobility of Cr(VI) in contaminated soils at varying water contents, which is essential for the evaluation of environmental risks in this region