Simultaneous 2D imaging of dissolved iron and reactive phosphorus in sediment porewaters by thin-film and hyperspectral methods

In-situ measurement of dissolved sulfide in surface sediment porewater using diffusive gradients in thin films (DGT) coupled with digital imaging

Dissolved sulfide in sediment porewater significantly influences aquatic ecosystems. Conventionally, sulfide determination in sediment porewater relies on ex-situ analytical methods, susceptible to measurement errors due to sulfide oxidation and volatilization during sample analysis. In this study, we introduced an innovative in-situ method for assessing dissolved sulfide in surface sediment porewater, leveraging the integration of diffusive gradients in thin films (DGT) with digital imaging. The DGT device effectively concentrates sulfide in sediment porewater, inducing observable color changes in the binding gel. Recordings of these changes, captured by imaging equipment, facilitated the establishment of calibration curves correlating grayscale value alterations in the binding gel to sulfide concentrations. Under optimal conditions, the developed method demonstrated a linear detection range of 3.0-200 μmol L-1 at 20 °C, particularly when the exposure time exceeded 180 min. The developed method is insensitive to salinity and suitable for measuring sulfide concentrations in various natural water environments. Compared to traditional ex-situ methods, our approach circumvents challenges linked to intricate pre-treatment, prolonged analysis duration, and significant systemic errors. This proposed method presents a real-time solution for sulfide concentration assessment in surface sediment porewater, empowering researchers with an efficient means to monitor and study dynamic sulfide levels.

Influence of different cyanobacterial treatment methods on phosphorus cycle in shallow lake microcosms

Cyanobacterial bloom is a pressing issue affecting water supply security and ecosystem health. Phosphorus (P) released from cyanobacterial bloom during recession is one of the most important components involved in the lake P cycle. However, little is known about the consequences and mechanisms of the P cycle in overlying water and sediment due to the anthropogenic treatments of cyanobacterial blooms. In this study, treatment methods using hydrogen peroxide (H2O2), polyaluminum chloride (PAC), and the feces of silver carp were investigated for their influence on the P cycle using microcosm experiments. Results showed that H2O2 treatment significantly increased the internal cycle of sediment-related P, while PAC treatment showed minor effects. H2O2 and PAC treatment suppressed the release of P from sediment before day 10 but promoted the release of P on day 20, while silver carp treatment suppressed the release of P during the whole experiment. The reductive dissolution of iron oxide-hydroxide was the major factor affects the desorption of P. Path analyses further suggested that overlying water properties such as dissolved oxygen (DO) and oxidation-reduction potential (ORP) play critical roles in the treatment-induced sediment P release. Our results quantify the endogenous P diffusion fluxes across the sediment-water interface attributed to cyanobacterial treatments and provide useful guidance for the selection of controlling methods, with silver carp being the most recommended of the three methods studied.

Iron bound phosphorus predominates the contribution of phosphorus to lake system from terrigenous source: The evidence from the small watershed scale

The reduction of exogenous emissions of phosphorus (P) is a crucial measure for resolving eutrophication in lakes. However, the input of terrigenous materials still potentially contributes to an increase of P load in lake systems. In this study, we examined the phosphate oxygen isotope (δ18OP) of various P fractions in soils and sediments in a small lake watershed, namely, Shijiuhu watershed. The high-resolution in-situ diffusive gradients in thin films (DGT) technology was also used to survey the dynamic processes of P diffusion from sediment particles to the water. The results demonstrated that lighter δ18OP values (16.2-19.5‰) for individual P fractions in lake sediments were detected compared to other land-use patterns, indicating the cumulative biological P recycling on anaerobic condition. Fe bound P (Fe-P) overall had heavier δ18OP values (17.3-24.8‰) than some of Ca bound P (Ca-P) and equilibrium values, suggesting that Fe-P conserved the parental isotope signatures from terrigenous source and could act as the ideal tracer for the lake sediments. The mixing effect of terrigenous detrital input and biological mineralization made the source identification uncertain by using Ca-P, which had a wider range of δ18OP values (13.0-26.6‰). Additionally, significantly positive correlation (r = 0.551-0.913, p<0.05) between soluble reactive P (SRP) and Fe2+ in interstitial water obtained using DGT measurement revealed the conspicuous release and desorption of solid Fe-P toward the water. High diffusion fluxes from the sediments toward the overlying water further demonstrated that the desorption of Fe-P in the soil-originated sediments toward the solution conspicuously facilitated the accumulation of SRP in lake water. The first-time application of δ18OP isotope combined with in-situ DGT techniques certified that it's feasible for the contribution confirmation from terrigenous to lacustrine environments, and presented the direct evidence for management strategy making about P control and eutrophication restoration at the catchment scale of lakes.

Two-Dimensional Mapping of Arsenic Concentration and Speciation with Diffusive Equilibrium in Thin-Film Gels

We present a new approach combining diffusive equilibrium in thin-film gels and spectrophotometric methods to determine the spatial distribution of arsenite, arsenate, and phosphate at submillimeter resolution. The method relies on the simultaneous deployment of three gel probes. Each retrieved gel is exposed to malachite green reagent gels differing in acidity and oxidant addition, leading to green coloration dependent on analyte speciation and concentration. Hyperspectral images of the gels enable mapping the three analytes in the 2.5-20 μM range. This method was applied in a contaminated brook in the Harz mountains, Germany, together with established mapping of dissolved iron. The use of two-dimensional (2D) gel probes was compared to traditional porewater extraction. The gels revealed banded porewater patterns on a mm-scale, which were undetectable using traditional methods. Small-scale correlation analyses of arsenic and iron microstructures in the gels suggested active iron-driven local redox cycling of arsenic. Overall, the results indicate continuous net release of arsenic from contaminant particles and deepen our understanding of arsenate transformation under anaerobic conditions. This study is the first fine-scale 2D characterization of arsenic speciation in porewater and represents a crucial step toward understanding the transfer and redox cycling of arsenic in contaminated sediment/soil ecosystems.

In Situ Simultaneous Analysis of Nitrogen and Phosphorus Migration in Urban Black Odorous Runoff

The extremely serious urban runoff eutrophication and black odorous phenomenon pose a significant threat to the lake aquatic ecosystem, resulting in a significantly increased frequency, magnitude, and duration of algal blooms in lakes. However, few investigations focus on small tributaries of the lakes, despite the ubiquity and potential local importance of these runoffs. Thus, the labile sediments NH₄⁺-N, NO₃^--N, PO₄^3-, Fe²⁺, and S^2- in black odorous runoff at Wuxi were overall analyzed at high resolution using diffusive gradients in thin films (DGT). The variations in labile N, P, Fe, and S distribution profiles at different sampling sites indicated high heterogeneity in sediments. The concentrations of labile P, Fe, and S showed synchronous variation from the sediment-water interface (SWI) up to -20 mm along sediment profiles. Moreover, there existed a significant positive correlation among labile P, Fe, and S concentrations (p < 0.05), which might represent typical odor compounds' FeS and H₂S synchronous release process in urban runoff. Furthermore, the apparent diffusion fluxes of labile P, Fe, and S across the SWI were all released upward, while fluxes of NH₄⁺-N and NO₃^--N release downward, indicating the sediments act as source and sink of P and N, respectively. Sediments' potential for endogenous P and N fractions release results in the black-odorous water, and sediment finally abouchement the Taihu, which intensifies further lake eutrophication phenomenon.

Dissimilatory nitrate reduction to ammonium (DNRA) potentially facilitates the accumulation of phosphorus in lake water from sediment

Nitrogen (N) and phosphorus (P) are crucial nutrients for eutrophication in the lacustrine ecosystem and attract the attention worldwide. However, the interaction between them need further clarification. This study aimed to assess the influence of dissimilatory nitrate reduction to ammonia (DNRA) on the cycle of P in lacustrine sediment. Different fractions of N and P in the pore water were measured using high-resolution in-situ measurement techniques, HR-Peeper and DGT, coupling with sequential extraction for solid sediment from a shallow freshwater lake. The results showed that elevated nitrate (NO₃^-) reduction via DNRA rather than denitrification was verified at deeper sediment layer, suggesting the generation of inorganic ammonia (NH₄⁺) as electron donor under anaerobic episodes. High abundance of DNRA bacteria (nrfA gene) obtained using high-throughput sequencing analysis were detected at upper layer and responsible for the accumulation of NH₄⁺ in the sediment coupling with chemolithoautotrophic metabolism. Additionally, significant desorption of ionic ferrous iron (Fe²⁺) and dissolved reactive phosphate (DRP) from solid phase and the enrichment in the solution was simultaneously detected. Higher concentration of solid Fe bound P (Fe-P) at deeper layer indicated the potential re-oxidation of Fe²⁺ as electron donor during DNRA process and sorption of DRP toward the Fe-containing minerals. However, obvious evidence of desorption proved by DGT indicated that higher NH₄⁺ concentrations favored the reduction of Fe(III) oxy(hydr)oxides and the desorption of DRP into the pore water and diffusion toward the overlying water. Finally, noteworthy S^2- release from solid sediment was speculated to stimulate the DNRA and facilitated the accumulation of NH₄⁺ in the solution, which further induced the enrichment of DRP in water from the solid phase. Overall, DNRA potentially facilitates the accumulation of P in lake water, and the synchronous control of N and P is important for the eutrophication management and restoration of lake eutrophication.

Linking pollution to biodiversity and ecosystem multifunctionality across benthic-pelagic habitats of a large eutrophic lake: A whole-ecosystem perspective

Biodiversity loss is often an important driver of the deterioration of ecosystem functioning in freshwater ecosystems. However, it is far from clear how multiple ecosystem functions (i.e., ecosystem multifunctionality, EMF) relate to biodiversity across the benthic-pelagic habitats of entire ecosystems or how environmental stress such as eutrophication and heavy metals enrichment might regulate the biodiversity-EMF relationships. Here, we explored the biodiversity and EMF across benthic-pelagic habitats of the large eutrophic Lake Taihu in China, and further examined abiotic factors underlying the spatial variations in EMF and its relationships with biodiversity. In our results, EMF consistently showed positive relationships to the biodiversity of multiple taxonomic groups, such as benthic bacteria, bacterioplankton and phytoplankton. Both sediment heavy metals and total phosphorus significantly explained the spatial variations in the EMF, whereas the former were more important than the latter. Further, sediment heavy metals mediated EMF through the diversity of benthic bacteria and bacterioplankton, while nutrients such as phosphorus in both the sediments and overlaying water altered EMF via phytoplankton diversity. This indicates the importance of pollution in regulating the relationships between biodiversity and EMF in freshwater environments. Our findings provide evidence that freshwater biodiversity loss among phytoplankton and bacteria will likely weaken ecosystem functioning. Our results further suggest that abiotic factors such as heavy metals, beyond nutrient enrichment, may provide relatively earlier signals of impaired ecosystem functioning during eutrophication process.

Water-level fluctuations regulate the availability and diffusion kinetics process of phosphorus at lake water-sediment interface

Sequential extraction and in-situ diffusive gradients in thin films (DGT) techniques were used to determine phosphorus (P) fractions and high-resolution 2D fluxes of labile P_DGT, Fe²⁺_DGT, and S^2-_DGT in sediment systems. The diffusion fluxes were subsequently calculated for different scenarios. Dynamic diffusion parameters between solid sediment and solution were also fitted using the DIFS (DGT-induced fluxes in sediments) model. The results suggested that Fe-bound P (Fe-P) was the dominant pool which contributed to the resupply potential of P in the water-sediment continuum. Significant upward decreases of labile P_DGT, Fe²⁺_DGT, and S^2-_DGT fluxes were detected in pristine and incubated microcosms. This dominance indicated the more obvious immobilization of labile P via oxidation of both Fe²⁺ and S^2- in oxidic conditions. Additionally, these labile analytes in the microcosms obviously decreased after a 30-day incubation period, indicating that water-level fluctuations can significantly regulate adsorption-desorption processes of the P bound to Fe-containing minerals within a short time. Higher concentrations of labile P_DGT, Fe²⁺_DGT, and S^2-_DGT were measured at the shallow lake region where more drastic water-level variation occurred. This demonstrates that frequent adsorption-desorption of phosphate from the sediment particles to the aqueous solution can result in looser binding on the solid sediment surface and easier desorption in aerobic conditions via the regulation of water levels. Higher R values fitted with DIFS model suggested that more significant desorption and replenishment effect of labile P to the aqueous solution would occur in lake regions with more dramatic water-level variations. Finally, a significant positive correlation between S^2-_DGT and Fe²⁺_DGT in the sediment indicated that the S^2- oxidization under the conditions of low water-level can trigger the reduction of Fe(III) and subsequent release of active P. In general, speaking, frequent water-level fluctuations in the lake over time facilitated the formation and retention of the Fe(II) phase in the sediment, and desorption of Fe coupled P into the aqueous solution when the water level was high.

Coupling between sediment biogeochemistry and phytoplankton development in a temperate freshwater marsh (Charente-Maritime, France): Evidence of temporal pattern

In freshwater systems, sediment can be an important source for the internal loading of PO₄. The limiting character of this element in such system leads to consider this phenomenon in terms of eutrophication risks and water quality stakes. A four-months follow-up (January, March, April and May 2019) was carried out in a strong phosphate (PO₄) limited secondary channel from an artificial irrigation system of Charente Maritime (France) to link the mobilization of remineralization products in the upper 6 cm layer of sediment (conventional core slicing/centrifugation and DET probes) and the phytoplankton biomass dynamics in the water column. Results showed congruent patterns between the temporal succession of the organic matter mineralization processes in the sediment and the primary biomass dynamics in the water column. In January and March (considered in winter), PO₄ proved to be retained by adsorption onto iron oxides in anoxic sediment since pore water nitrate inhibited for about a month the respiration of metal oxides in the first cm of sediment, thus limiting PO₄ availability and the phytoplankton growth. In April and May (early spring), after exhaustion of pore water nitrate, the dissolutive reduction of iron oxides released PO₄ into pore water generated a significant diffusive outgoing flux from the sediment to the water column with a maximum in April (-1.10E-04±2.81E-05 nmol cm^-2 s^-1). This release coincided with the nanophytoplankton bloom (5.50 µg Chla L^-1) and a potential increase of PO₄ concentration in the water column. This work provides some insight on the importance of benthic-pelagic coupling in anthropogenic systems. This conceptual model has to be deployed on other sites of interest where internal loading of P takes precedence over external inputs and nitrate mitigation drives its benthic recycling and ultimately its bioavailability. This is to be essential to characterize the aquatic environment quality in order to limit eutrophication risks.

In situ, high-resolution measurement of labile phosphate in sediment porewater using the DET technique coupled with optimized imaging densitometry

Obtaining two-dimensional distributions of reactive phosphorus in sediment porewater is very important for understanding fine-scale phosphorus mobilization and sequestration processes in sediments. In this study, the diffusive equilibrium in thin films (DET) measurement based on computer imaging densitometry (CID) was studied in detail with optimal conditions described. This study focuses on evaluating the two-dimensional colorimetric DET method coupled with CID (DET-CID method) for porewater labile phosphate measurements. The result shows that the red channel filter is the optimum channel for sensitivity to process the image. Additionally, staining time and temperature have great influence on the method, and 20 min staining time and ≥25 °C staining temperature were recommended. The minimum detection limit of labile phosphate of this method was 0.300 mg P/L, and the maximum detection limit could reach 50.00 mg P/L. The DET-CID technique can be used to measure labile phosphate in a wide range of acidic and alkaline water bodies (pH = 2-10 and water hardness from 0 to 2000 mg/L as CaCO₃). The linear regression analysis shows that this technique presents very similar results compared with other two existing methods (R² = 0.999). Our results would give insights into the precisely measurements of labile phosphate in field applications.

A new DGT technique comprised in a hybrid sensor for the simultaneous measurement of ammonium, nitrate, phosphorus and dissolved oxygen

A new diffusive gradients in thin films technique (ZrO-AT DGT) with zirconium oxide, A-62 MP and T-42H resins containing in a single binding gel was developed for simultaneous measurement of nitrate (NO₃-N), ammonium (NH₄-N) and phosphate (PO₄-P). The DGT uptake was found to be independent of pH variation from 3.2-8.7. Ionic strengths below 5, 10 and 750 mmol·L^-1 NaCl did not affect DGT uptake of NH₄-N, NO₃-N and PO₄-P, respectively. This new DGT was deployed in natural freshwater environments, with in situ measurements of the three nutrients found to be accurate. It ensured that rinsing the exposed surface of the DGT device at 3-day intervals can prevent biofouling. Additionally, a hybrid sensor comprising the novel DGT binding layer overlying an O₂ planar optrode was tested in sediments to evaluate the dynamics of O₂ and the three nutrients. Results showed that PO₄-P and NO₃-N fluxes decreased while fluxes of NH₄-N increased under aerobic conditions. Nearly simultaneous variation in O₂ and NO₃-N was observed at the sediment-water interface (SWI) and transformation of NO₃-N and PO₄-P was found to be sensitively influenced by O₂ dynamics.

A study of synchronous measurement of liable phosphorous and iron based on ZrO-Chelex (DGT) in the sediment of the Chaiwopu Lake, Xinjiang, Northwest China

The water-sediment interface of lakes is an important and unique area of the water environment; the geochemical behavior of nutrients in this area has a significant impact on the quality of the water environment and ecosystems, especially in shallow lakes. However, most studies do not provide direct in situ evidence for this in shallow lakes in arid regions; in order to explore the coupling relationship between phosphorus (P) and iron (Fe) in a sediment profile, we conducted a high-resolution analysis of liable Fe and P in sediments taken from the Chaiwopu Lake using ZrO-Chelex thin film diffusion gradient technology (ZrO-Chelex DGT). The results show that (1) the vertical spatial distribution trend of the liable P and Fe in the sediments from each sampling site is essentially similar. The contents of the liable P and Fe ranged from 0.004-0.125 mg/L and 0.050-0.190 mg/L, respectively, and the synchronous distribution of the micro-interface concentration reflects the coupling relationship between them. (2) The correlation analysis of the liable P and Fe concentrations showed that there were significant linear correlations between them (P < 0.05, bilateral). (3) The diffusion fluxes of P and Fe were - 51.76~65.12 μg (m² d)^-1 and - 451.27~457.06 μg (m² d)^-1, respectively, and were shown to be negative at the sediment-water interface for most of the samples, which showed that P and Fe were released from the overlying water into the sediments. (4) This research showed that the diffusive fluxes at the different sites are quite different, which indicates that the phosphorus and iron pollution in the sediments of the Chaiwopu Lake is affected by exogenous inputs. There was no significant correlation between P release flux and pH, ORP, conductivity (EC), the TDS of the overlying water, or the pH, salinity (Ca²⁺, Mg²⁺), and nutrient (organic matter) content of the sediment. The release flux of Fe is affected by the pH of the sediment. The results of this study provide references for the research of elements in the water-sediment interface of shallow lakes in arid regions, as well as other areas.

A multi-parameter in-situ water quality analyzer based on a portable document scanner and 3D printed self-sampling cells

This research introduced a new low-cost and multi-parameter analyzer for in-situ measurements of typical nutrients in water bodies. The analyzer consisted of color detection and chromogenic reaction modules. The self-sampling action of the 3D printed sampling/reaction cells was achieved with the cooperative application of rubber bands and dissolvable thread. The target analytes in the collected water sample reacted with the chromogenic reagents that were diffused from the pre-placed glass wool in the cell, producing color compounds. A portable document scanner was employed as a multi-parameter in-situ detector to record the image of the colored solutions in all five cells simultaneously. Based on the image, the corrected grayscale values were derived for target analyte quantitation. The relationships between grayscale values and concentrations of target analytes were established, and the temperature effects were studied. In addition, the practicability of the analyzer was demonstrated by in-situ experiments carried out in four different sites, including a creek, a river dock, a reservoir and a secondary settling tank in a wastewater treatment facility. The results indicated that the analyzer could be used for in-situ measuring of nutrients at μmol/L levels in the water. The nutrient concentrations obtained with the analyzer were comparable with those obtained with the standard methods. The presented analyzer provided new complementary ideas and methods for in-situ rapid measurement of nutrients and other target analytes in various water systems.

Sulfur, iron, and phosphorus geochemistry in an intertidal mudflat impacted by shellfish aquaculture

Dissolved sulfide, iron (Fe), and phosphorus (P) in a mudflat (Jiaozhou Bay, China) impacted by shellfish aquaculture were measured in situ by the diffusive gradients in thin films (DGT) technique. A combination of porewater and solid-phase chemistry was used to characterize the interplays of Fe and S, and their control on P mobilization. Below the subsurface layer, two times higher fluxes (F_DGT) of dissolved Fe²⁺ from porewater to the DGT device than those of dissolved sulfide indicate that dissimilatory iron reduction (DIR) dominates over sulfate reduction (SR). Spatial coupling of dissolved Fe²⁺ and P points to P release driven mainly by reductive dissolution of Fe. Much higher F_DGT values of dissolved Fe²⁺ relative to dissolved P imply that oxidative regeneration of Fe oxides at the sediment-water interfaces (SWIs) of the transitional mudflat serves as an effective "iron curtain" of upward diffusing P. In the mudflat sediments of DIR prevalence, the accumulation of total reduced inorganic sulfur (TRIS) is dampened, which can largely ascribed to enhanced oxidative loss of sulfide and/or limited availability of degradable organic carbon in the dynamic regimes. Low dissolved sulfide concentrations in the sediments leave the majority of reactive Fe unsulfidized and thus abundantly available to buffer newly produced sulfide.

Two-dimensional ammonium distribution in sediment pore waters using a new colorimetric diffusive equilibration in thin-film technique

This study presents a new gel based technique to describe the pore water ammonium distribution through the sediment-water interface in two dimensions at a millimeter scale. The technique is an adaptation of the classical colorimetric method based on the Berthelot's reaction. After the thin film of the gel probe was equilibrated by diffusion either in standard solutions or in pore waters, a colorimetric reagent gel was set on the gel probe, allowing development of the characteristic green color. A flatbed scanner and subsequent densitometry image analysis allowed to determine the concentration distribution of ammonium. The gel probe was tested in the laboratory for two media, deionized water and seawater, within the range 0-3000 μM in NH₄ ⁺. Detection limit is about 20 μM and accuracy about ±25 μM. The field validation was realized in a tidal mudflat of the French Atlantic coast by comparison with conventional pore water extraction and colorimetric analysis. The large range of concentrations and its applicability in continental and marine sediments suggest a wide range of applications of the technique for a reasonable cost.

A novel hybrid sensor for combined imaging of dissolved oxygen and labile phosphorus flux in sediment and water

A novel sensor assembled by a hybrid film was developed for 2D combined measurements of DO dynamic and labile P flux in sediment and water at sub-millimeter resolution based on PO and DGT techniques. The hybrid film is comprised of a transparent polyester membrane supporting two ultrathin sensing layers, i.e., a P binding layer (PBL) overlying a DO sensing layer (DSL). A robust, straightforward measuring strategy based on the referenced RGB and coloration-computer imaging densitometry (CID) methods was developed. Sensing properties for DO show a considerable homogeneity (RSD < 5%) and rapid response (<24 s) in fluorescent response. Calibration experiments reveal the sensitivity values for the DSL without/with PBL are 2.12/1.95, with an acceptable bias of less than 8%. The optimized PBL possesses a uniform distribution of zirconium-oxide microparticles at a relatively high DGT capacity (10.8 μg P cm^-2), in which the distribution of adsorbed-P can be imaged by the coloration-CID method. The performance of the sensor is compared to two conventional PO and DGT sensors. The hybrid sensor was successfully deployed in three types of benthic micro-interface and showed significant small-scale heterogeneity, providing new opportunities for advancing investigations into relevant biogeochemical processes.

Simultaneous Nitrite/Nitrate Imagery at Millimeter Scale through the Water-Sediment Interface

The present study describes new procedures to obtain at millimeter resolution the spatial distribution of nitrite and nitrate in porewaters, combining diffusive equilibrium in thin films (DET), colorimetry and hyperspectral imagery. Nitrite distribution can be easily achieved by adapting the well-known colorimetric method from Griess (1879) and using a common flatbed scanner with a limit of detection about 1.7 μmol L(-1). Nitrate distribution can be obtained after reduction into nitrite by a vanadium chloride reagent. However, the concentration of vanadium chloride used in this protocol brings coloration with a wide spectral signature that creates interference only deconvolvable by imaging treatment from an entire visible spectrum for each pixel (spectral analysis). This can be achieved by hyperspectral imaging. The protocol retained in the present study allows obtaining a nitrite/nitrate image with micromolar limit of detection. The methods were applied in sediments from the Loire Estuary after different treatments and allowed to precisely describe two-dimensional millimeter features. The present technique adds to the combination of gel-colorimetry and hyperspectral imagery a very promising new application of wide interest for environmental issues in the context of early diagenesis and benthic fluxes.

Phosphorus losses from agricultural land to natural waters are reduced by immobilization in iron-rich sediments of drainage ditches

Redox reactions involving iron (Fe) strongly affect the mobility of phosphorus (P) and its migration from agricultural land to freshwater. We studied the transfer of P from groundwater to open drainage ditches in an area where, due to Fe(II) rich groundwater, the sediments of these ditches contain accumulated Fe oxyhydroxides. The average P concentrations in the groundwater feeding two out of three studied drainage ditches exceeded environmental limits for freshwaters by factors 11 and 16, but after passing through the Fe-rich sediments, the P concentrations in the ditch water were below these limits. In order to identify the processes which govern Fe and P mobility in these systems, we used diffusive equilibration in thin films (DET) to measure the vertical concentration profiles of P and Fe in the sediment pore water and in the ditchwater. The Fe concentrations in the sediment pore water ranged between 10 and 200 mg L(-1) and exceeded those in the inflowing groundwater by approximately one order of magnitude, due to reductive dissolution of Fe oxyhydroxides in the sediment. The dissolved P concentrations only marginally increased between groundwater and sediment pore water. In the poorly mixed ditchwater, the dissolved Fe concentrations decreased towards the water surface due to oxidative precipitation of fresh Fe oxyhydroxides, and the P concentrations decreased more sharply than those of Fe. These observations support the view that the dynamics of Fe and P are governed by reduction reactions in the sediment and by oxidation reactions in the ditchwater. In the sediment, reductive dissolution of P-containing Fe oxyhydroxides causes more efficient solubilization of Fe than of P, likely because P is buffered by adsorption on residual Fe oxyhydroxides. Conversely, in the ditchwater, oxidative precipitation causes more efficient immobilization of P than of Fe, due to ferric phosphate formation. The combination of these processes yields a natural and highly efficient sink for P. It is concluded that, in Fe-rich systems, the fate of P at the sediment-water interface is determined by reduction and oxidation of Fe.

Two decades of chemical imaging of solutes in sediments and soils--a review

The increasing appreciation of the small-scale (sub-mm) heterogeneity of biogeochemical processes in sediments, wetlands and soils has led to the development of several methods for high-resolution two-dimensional imaging of solute distribution in porewaters. Over the past decades, localised sampling of solutes (diffusive equilibration in thin films, diffusive gradients in thin films) followed by planar luminescent sensors (planar optodes) have been used as analytical tools for studies on solute distribution and dynamics. These approaches have provided new conceptual and quantitative understanding of biogeochemical processes regulating the distribution of key elements and solutes including O2, CO2, pH, redox conditions as well as nutrient and contaminant ion species in structurally complex soils and sediments. Recently these methods have been applied in parallel or integrated as so-called sandwich sensors for multianalyte measurements. Here we review the capabilities and limitations of the chemical imaging methods that are currently at hand, using a number of case studies, and provide an outlook on potential future developments for two-dimensional solute imaging in soils and sediments.