Distribution of dissolved iron in sediment pore waters at submillimetre resolution

Detecting antimony(III) on-site using novel gel-based techniques: Colorimetric diffusive equilibrium in thin films for two-dimensional imaging and surface-enhanced Raman scattering for sensitive quantification

Antimony (Sb) pollution has raised increasing public concerns and its rapid on-site screening is central for the risk assessment. Herein, we proposed two gel-based methods based on colorimetric diffusive equilibrium in thin films (DET) and surface-enhanced Raman scattering (SERS), for two-dimensional imaging and sensitive detection of Sb(III) by revisiting the phenylfluorone (PhF) complexation reaction. PhF was well dispersed in the polyvinyl alcohol (PVA) hydrogel and reacted with Sb(III) in the DET gel to form a strong PhF-Sb(III) complex. The distribution of Sb(III) was easily visualized at a submillimeter resolution using computer imaging densitometry, with a detection limit (LOD) of ∼100 nmol L-1. Field application in the Sb mine area reveals limited dissolved Sb(III) penetrating the redox barrier below the sediment-water interface by 20 mm in rivers and tailing pond sediments. To improve the detection sensitivity and apply the principle to trace Sb quantification, a SERS platform was established by anchoring PhF on the hydrogel-stabilized Ag nanoparticles via C-O-Ag bonding to specifically detect Raman-inactive Sb(III). Benefiting from the high SERS activity of PhF and enrichment ability of hydrogel, Sb(III) was quantified with a LOD of 1.2-10.7 nmol L-1 depending on the sample volume. The coexisting ions at a 100-fold higher concentration than Sb(III) resulted in only 3.3-10.4 % variation in SERS intensity, indicating a negligible interference on the SERS platform. The platform exhibited a RSD of 6.6-13.1 % and acceptable recoveries for various environmental matrices, highlighting its promise in on-site application.

Strong Substance Exchange at Paddy Soil-Water Interface Promotes Nonphotochemical Formation of Reactive Oxygen Species in Overlying Water

Photochemically generated reactive oxygen species (ROS) are widespread on the earth's surface under sunlight irradiation. However, the nonphotochemical ROS generation in surface water (e.g., paddy overlying water) has been largely neglected. This work elucidated the drivers of nonphotochemical ROS generation and its spatial distribution in undisturbed paddy overlying water, by combining ROS imaging technology with in situ ROS monitoring. It was found that H2O2 concentrations formed in three paddy overlying waters could reach 0.03-16.9 μM, and the ROS profiles exhibited spatial heterogeneity. The O2 planar-optode indicated that redox interfaces were not always generated at the soil-water interface but also possibly in the water layer, depending on the soil properties. The formed redox interface facilitated a rapid turnover of reducing and oxidizing substances, creating an ideal environment for the generation of ROS. Additionally, the electron-donating capacities of water at soil-water interfaces increased by 4.5-8.4 times compared to that of the top water layers. Importantly, field investigation results confirmed that sustainable •OH generation through nonphotochemical pathways constituted of a significant proportion of total daily production (>50%), suggesting a comparable or even greater role than photochemical ROS generation. In summary, the nonphotochemical ROS generation process reported in this study greatly enhances the understanding of natural ROS production processes in paddy soils.

Measurement of perfluoroalkyl substances in drinking water sources by DGT sampler with a novel fluorinated graphite binding gel

Extensive use of per- and polyfluoroalkyl substances (PFASs) has resulted in their widespread presence in natural waters. Concern for public health requires reliable measurement methods for determining their distribution and risks. Here, a sampling method based on diffusive gradients in thin films (DGT) was developed for measuring PFASs in drinking water sources. Fluorinated graphite (FG) particles were used to prepare the DGT binding gel for selective enrichment of trace PFASs in an aqueous environment. The FG-DGT method did not show sensitivity to relevant environmental parameters including pH (5.0-9.0), ionic strength (0.001-0.5 M), or DOM concentration (0-30 mg/L). The FG-DGT had enough capacity for deployment of up to four months. Six traditional and emerging PFASs including PFOS, PFOA, PFHpA, PFHxS, PFNA, and 6:2 FTSA at the ng/L level were detected in two major reservoirs serving as public drinking water sources by FG-DGT method coupled with liquid chromatography tandem mass spectrometry (LC-MS/MS). PFOA appeared at the highest observed concentrations in the drinking water sources. The research demonstrates that FG-DGT is an effective and efficient tool for monitoring PFASs in drinking water.

Evaluation of the distribution and mobility of labile phosphorus in sediment profiles of Lake Nansi, the largest eutrophic freshwater lake in northern China

Understanding various biogeochemical processes, especially in eutrophic sediments, necessitates fine-scale phosphorus (P) measurements in pore waters. To the best of our knowledge, the fine-scale distributions of P across the sediment profiles of Lake Nansi have rarely been investigated. Herein we evaluated the dynamic distributions of labile P and Fe across the sediment-water interface (SWI) of Lake Nansi at two-dimensional (2D) and sub-millimeter resolution, using well-established colorimetric diffusive gradients in thin films (DGT) methodology. The concentrations of labile P in all investigated sediment profiles exhibited strong spatial variations, ranging from 0 to 1.50 mg/L with a considerable number of hotspots. Lake Nanyang (0.55 ± 0.21 mg/L) had the highest mean concentration of labile P, followed by Lake Dushan (0.38 ± 0.19 mg/L), Lake Weishan (0.28 ± 0.21 mg/L), and Lake Zhaoyang (0.18 ± 0.09 mg/L). The highest concentrations of labile P were always detected in Lake Dushan, which had been subjected to excessive exogenous P pollution. The co-distributions of labile P and Fe in the majority of the sediment of Lake Nansi confirmed highly positive correlations (P < 0.01), suggesting that the mobility of labile P throughout the SWI was likely governed by iron redox processes. The apparent diffusion fluxes of P across the SWI ranged from -7.7 to 33.6 μg/m²·d, with a mean value of 5.26 ± 7.80 μg/m²·d. Positive apparent fluxes for labile P were recorded in most sediment cores, demonstrating the strong upward mobility of P from the sediment to the overlying water. Our results provided accurate and extensive information regarding the micro-distribution and dynamic exchange of labile P across the SWI. This allows for a better understanding of eutrophication processes and the implementation of P management strategies in Lake Nansi.

Mercury Accumulation in a Stream Ecosystem: Linking Labile Mercury in Sediment Porewaters to Bioaccumulative Mercury in Trophic Webs

Mercury (Hg) deposition and accumulation in the abiotic and biotic environments of a stream ecosystem were studied. This study aimed to link labile Hg in porewater to bioaccumulative Hg in biota. Sediment cores, porewaters, and biota were sampled from four sites along the Fourmile Branch (SC, USA) and measured for total Hg (THg) and methyl-Hg (MHg) concentrations. Water quality parameters were also measured at the sediment–water interface (SWI) to model the Hg speciation. In general, Hg concentrations in porewaters and bulk sediment were relatively high, and most of the sediment Hg was in the solid phase as non-labile species. Surface sediment presented higher Hg concentrations than the medium and bottom layers. Mercury methylation and MHg production in the sediment was primarily influenced by sulfate levels, since positive correlations were observed between sulfate and Hg in the porewaters. The majority of Hg species at the SWI were in non-labile form, and the dominant labile Hg species was complexed with dissolved organic carbon. MHg concentrations in the aquatic food web biomagnified with trophic levels (biofilm, invertebrates, and fish), increasing by 3.31 times per trophic level. Based on the derived data, a modified MHg magnification model was established to estimate the Hg bioaccumulation at any trophic level using Hg concentrations in the abiotic environment (i.e., porewater).

In situ high-resolution two-dimensional profiles of redox sensitive metal mobility in sediment-water interface and porewater from estuarine sediments

Metals in contaminated sediments may present high environmental risks and ecological threats to benthic organisms. Redox sensitive elements with different oxidation states show variations in solubility as a function of redox status of the sediment water environment. The novel high-resolution ZrO-Chelex-AgI diffusive gradients in thin film (HR-ZCA DGT) technique provided sensitive in situ mapping of metals in the estuarine sediments. The present study investigated the sub-millimeter two-dimensional distributions of DGT-labile S(-II), P(V), and six redox sensitive metals (Fe, Mn, V, Cu, Ni, and Zn) across sediment-water interface (SWI) severely influenced by anthropogenic activity. We for the first time used the V-turning value (the V/Fe ratios at ~0.03) to accurately identify the actual SWI. The diffusion boundary layer (DBL) thickness of Ni, Cu and Zn was consistent with those identified by the dissolved oxygen microelectrode method, and was 3-6 mm above the SWI. No significant release of dissolved Fe and P from sediments into the overlying water was found by diffusion process. The estimated fluxes (F_dif) of Ni, Cu, and Zn at DBL were 4.0-176, -1.1-235, and 5.0-108 μg m^-2 d^-1, respectively, and were significantly higher in sediments near the industrial effluent dumping sites than those in sediments impacted by domestic wastewater releases. Metal diffusion flux was mainly controlled by the particulate matter on the surface sediment and organic degradation. Traditional diffusion flux may have underestimated the flux of metals from the surface sediments. The discharge of hypoxic tributary was an important source of metal pollution in the contaminated estuarine sediments.

Toward In-Field Determination of Nitrate Concentrations Via Diffusive Gradients in Thin Films-Incorporation of Reductants and Color Reagents

Diffusive gradients in thin films (DGTs) have been established as useful tools for the determination of nitrate, phosphate, trace metals, and organic concentrations. General use of DGTs, however, is limited by the subsequent requirement for laboratory analysis. To increase the uptake of DGT as a tool for routine monitoring by nonspecialists, not researchers alone, methods for in-field analysis are required. Incorporation of color reagents into the binding layer, or as the binding layer, could enable the easy and accurate determination of analyte concentrations in-field. Here, we sought to develop a chitosan-stabilized silver nanoparticle (AuNP) suspension liquid-binding layer which developed color on exposure to nitrite, combined with an Fe(0)-impregnated poly-2-acrylamido-2-methyl-1-propanesulfonic acid/acrylamide copolymer hydrogel [Fe(0)-p(AMPS/AMA)] for the reduction of nitrate. The AuNP-chitosan suspension was housed in a 3D designed and printed DGT base, with a volume of 2 mL, for use with the standard DGT solution probe caps. A dialysis membrane with a molecular weight cutoff of <15 kDa was used, as part of the material diffusion layer, to ensure that the AuNP-chitosan did not diffuse through to the bulk solution. This synthesized AuNP-chitosan provided quantitative nitrite concentrations (0 to 1000 mg L-1) and masses (145 μg) in laboratory-based color development studies. An Fe(III)-impregnated poly-2-acrylamido-2-methyl-1-propanesulfonic acid/acrylamide copolymer hydrogel [Fe(III)-p(AMPS/AMA)] was developed (10% AMPS, and 90% AMA), which was treated with NaBH4 to form an Fe(0)-p(AMPS/AMA) hydrogel. The Fe(0)-p(AMPS/AMA) hydrogel quantitatively reduced nitrate to nitrite. The total nitrite mass produced was ∼110 μg, from nitrate. The diffusional characteristics of nitrite and nitrate through the Fe(III)-p(AMPS/AMA) and dialysis membrane were 1.40 × 10-5 and 1.40 × 10-5 and 5.05 × 10-6 and 5.15 × 10-6 cm2 s-1 at 25 °C respectively. The Fe(0)-hydrogel and AuNP-chitosan suspension operated successfully in laboratory tests individually; however, the combined AuNP-chitosan suspension and Fe(0)-hydrogel DGT did not provide quantitative nitrate concentrations. Further research is required to improve the reaction rate of the AuNP-chitosan nitrite-binding layer, to meet the requirement of rapid binding to operate as a DGT.

Development and evaluation of a new colorimetric DGT technique for the 2D visualisation of labile phosphate in soils

A new colorimetric technique for the measurement of labile phosphate in soils using the diffusive gradients in thin films (DGT) technique was developed in this study. This technique can determine the mass of phosphate accumulated on the precipitated Zr-oxide based binding gel by forming the blue colour following the standard molybdate-ascorbic acid method. The optimal reaction temperature and coloration time were 20 °C (room temperature) and 26 min. After determining a well-fitted calibration equation, the technique was able to measure phosphate concentration up to 2.5 mg/L for 24 h deployment with a detection limit of 10.1 μg/L. Two-dimensional quantitative visualisation of phosphate diffusion in three phosphorus (P) fertilised soils were obtained using the colorimetric technique. The results from the colorimetric DGT technique were compared to the elution DGT technique and Colwell P extraction. The DGT techniques (colorimetric and elution) and Colwell P measurements demonstrated similar patterns of phosphate diffusion in soil. Both DGT techniques showed similar phosphate concentration along the concentric rings around the fertiliser application. A new, convenient, and fast DGT colorimetric technique was developed, and successfully used to measure the distribution of potentially available phosphate in soils. The new technique is less laborious than current techniques as it does not require any pre-treatment of the binding gel layers or heating during scanning, thus providing faster results. Therefore, the technique may be more suitable for in-field applications and can be used to investigate the in situ diffusion of potentially available phosphate from fertilisers, and relate this to the plant uptake of P.

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.

Coprecipitation of Phosphate and Silicate Affects Environmental Iron (Oxyhydr)Oxide Transformations: A Gel-Based Diffusive Sampler Approach

Sorption of nutrients such as phosphate (P) and silicate (Si) by ferric iron (oxyhydr)oxides (FeOx) modulates nutrient mobility and alters the structure and reactivity of the FeOx. We investigated the impact of these interactions on FeOx transformations using a novel approach with samplers containing synthetic FeOx embedded in diffusive hydrogels. The FeOx were prepared by Fe(III) hydrolysis and Fe(II) oxidation, in the absence and presence of P or Si. Coprecipitation of P or Si during synthesis altered the structure of Fe precipitates and, in the case of Fe(II) oxidation, lepidocrocite was (partly) substituted by poorly ordered FeOx. The pure and P- or Si-bearing FeOx were deployed in (i) freshwater sediment rich in dissolved Fe(II) and P and (ii) marine sediment with sulfidic pore water. Iron(II)-catalyzed crystallization of poorly ordered FeOx was negligible, likely due to surface passivation by adsorption of dissolved P. Reaction with dissolved sulfide was modulated by diffusion limitations and therefore the extent of sulfidation was the lowest for poorly ordered FeOx with high reactivity toward sulfide that created temporary, local sulfide depletion (Fh < Lp). We show that coprecipitation-induced changes in the FeOx structure affect coupled iron-nutrient cycling in aquatic ecosystems. The gel-based method enriches our geochemical toolbox by enabling detailed characterization of target phases under natural conditions.

The method controls the story - Sampling method impacts on the detection of pore-water nitrogen concentrations in streambeds

Biogeochemical gradients in streambeds are steep and can vary over short distances often making adequate characterisation of sediment biogeochemical processes challenging. This paper provides an overview and comparison of streambed pore-water sampling methods, highlighting their capacity to address gaps in our understanding of streambed biogeochemical processes. This work reviews and critiques available pore-water sampling techniques to characterise streambed biogeochemical conditions, including their characteristic spatial and temporal resolutions, and associated advantages and limitations. A field study comparing three commonly-used pore-water sampling techniques (multilevel mini-piezometers, miniature drivepoint samplers and diffusive equilibrium in thin-film gels) was conducted to assess differences in observed nitrate and ammonium concentration profiles. Pore-water nitrate concentrations did not differ significantly between sampling methods (p-value = 0.54) with mean concentrations of 2.53, 4.08 and 4.02 mg l^-1 observed with the multilevel mini-piezometers, miniature drivepoint samplers and diffusive equilibrium in thin-film gel samplers, respectively. Pore-water ammonium concentrations, however, were significantly higher in pore-water extracted by multilevel mini-piezometers (3.83 mg l^-1) and significantly lower where sampled with miniature drivepoint samplers (1.05 mg l^-1, p-values <0.01). Differences in observed pore-water ammonium concentration profiles between active (suction: multilevel mini-piezometers) and passive (equilibrium; diffusive equilibrium in thin-film gels) samplers were further explored under laboratory conditions. Measured pore-water ammonium concentrations were significantly greater when sampled by diffusive equilibrium in thin-film gels than with multilevel mini-piezometers (all p-values ≤0.02). The findings of this study have critical implications for the interpretation of field-based research on hyporheic zone biogeochemical cycling and highlight the need for more systematic testing of sampling protocols. For the first time, the impact of different active and passive pore-water sampling methods is addressed systematically here, highlighting to what degree the choice of pore-water sampling methods affects research outcomes, with relevance for the interpretation of previously published work as well as future studies.

Influence of Physical Perturbation on Fe(II) Supply in Coastal Marine Sediments

Iron (Fe) biogeochemistry in marine sediments is driven by redox transformations creating Fe(II) and Fe(III) gradients. As sediments are physically mixed by wave action or bioturbation, Fe gradients re-establish regularly. In order to identify the response of dissolved Fe(II) (Fe²⁺) and Fe mineral phases toward mixing processes, we performed voltammetric microsensor measurements, sequential Fe extractions, and Mössbauer spectroscopy of 12 h light-dark cycle incubated marine coastal sediment. Fe²⁺ decreased during 7 days of undisturbed incubation from approximately 400 to 60 μM. In the first 2-4 days of incubation, Fe²⁺ accumulated up to 100 μM in the top 2 mm due to Fe(III) photoreduction. After physical perturbation at day 7, Fe²⁺ was re-mobilized reaching concentrations of 320 μM in 30 mm depth, which decreased to below detection limit within 2 days afterward. Mössbauer spectroscopy showed that the relative abundance of metastable iron-sulfur mineral phases (FeS_x) increased during initial incubation and decreased together with pyrite (FeS₂) after perturbation. We show that Fe²⁺ mobilization in marine sediments is stimulated by chemical changes caused by physical disturbances impacting the Fe redox distribution. Our study suggests that, in addition to microbial and abiotic Fe(III) reduction, including Fe(III) photoreduction, physical mixing processes induce chemical changes providing sediments and the inhabiting microbial community with Fe²⁺.

Comparison of DET, DGT and conventional porewater extractions for determining nutrient profiles and cycling in stream sediments

Determining inorganic nutrient profiles to support understanding of nitrogen transformations in stream sediments is challenging, due to nitrification and denitrification being confined to particular conditions in potentially heterogeneous sediment influenced by benthic microalgae, rooted aquatic plants and/or diel light cycles. The diffusive gradients in thin films (DGT) and diffusive equilibration in thin films (DET) techniques allow in situ determination of porewater concentration profiles, and distributions for some solutes. In this study, DGT, DET and conventional porewater extraction (sectioning and centrifugation) methods were compared for ammonium and nitrate in stream sediments under light and dark conditions. Two-dimensional distributions of Fe(ii) and PO4-P were also provided to indicate the degree of spatial and temporal heterogeneity in sediment porewater, which can explain the sources and sinks of ammonium at various depths in the sediments. Although the conventional porewater extraction method consistently measured higher NH4-N concentrations than the DGT and DET techniques, the study showed that the DET measurements were the most reliable indicator of porewater NH4-N concentrations, with the DGT data being usefully supplementary. However, a large proportion of the NO3-N concentrations measured by DGT and DET were close to or below the method detection limits. Therefore, further development of these techniques is required to reduce the blanks and detection limits to allow natural low sediment porewater NO3-N concentrations to be accurately monitored using DGT and DET. The study indicated that benthic microalgae had direct and indirect influences on porewater nutrient distributions over light-dark cycles. Overall, DGT and DET techniques can be useful for monitoring porewater nutrient concentrations and profiles and for determining how biological processes drive changes in sediment nutrient concentrations and distributions.

Mercury speciation, bioavailability, and biomagnification in contaminated streams on the Savannah River Site (SC, USA)

Water, sediment, and biota from two streams on the Savannah River Site were sampled to study mercury (Hg) biogeochemistry. Total and methyl- Hg (MHg) concentrations were measured for all samples, speciation models were used to explore Hg speciation in the water, and Diffusive gradients in thin films (DGT) were applied to indicate the vertical profiles of labile Hg (DGT-Hg). Trophic position (δ¹⁵N) was estimated for biota and used to establish MHg biomagnification model. The speciation model indicated Hg methylation in the water occurred on settling particles and the most bioavailable Hg species to bacteria were complexes of inorganic Hg and labile organic ligands. Correspondingly, dissolved organic carbon concentrations were positively related to MHg concentrations in the water. In the sediment, the sharp increase of DGT-Hg around the sediment water interface underscores the importance of this interface, which determines the differences in the accumulation and generation of labile Hg among different waterbodies. The positive correlation between sediment MHg and sulfate concentrations suggested possible methylation reaction by dissimilatory sulfate reducing bacteria in the sediment. The food web magnification factors of MHg were 9.6 (95% CI: 4.0-23.4) and 4.4 (95% CI: 2.5-7.7) for the two streams established with trophic data of biofilm, invertebrates, and fish. Meanwhile, DGT-Hg concentrations in the water were positively correlated to biofilm Hg concentrations, which can be combined with the MHg biomagnification model to generate a modified biomagnification model that estimate MHg bioaccumulation with only labile Hg concentrations in the water. With this approach, Hg accumulation in abiotic and biotic environmental compartments was connected and the different bioaccumulation patterns of Hg in different waterbodies were explained with both geochemical and biological factors.

Direct evidence for the enhanced acquisition of phosphorus in the rhizosphere of aquatic plants: A case study on Vallisneria natans

There are few studies about the processes and mechanisms for aquatic plants to take up phosphorus (P) in wetland soils and sediments. Direct observation of P mobilization in rhizosphere is lacking. In this study, high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to capture the small-scale changes of soluble reactive P (SRP) and soluble Fe, and labile P in the rhizosphere of Vallisneria natans (V. natans), respectively. The results showed 5.92- and 3.12-fold enrichments of P and Fe in the Fe plaques formed on the root surfaces, respectively, in comparison with the P and Fe concentrations in the non-rhizosphere sediments. Moreover, simultaneous releases of P and Fe appeared in rhizosphere and the SRP concentration showed up to 114-fold increases compared to the non-rhizosphere sediments. Five kinds of low-molecular weight organic acids (LMWOAs) were detected in the root exudates; oxalic acid accounted for 87.5% of the total. Extraction of Fe and P in the Fe plaques was greatly enhanced by root exudates compared to deionized water, and oxalic acid contributed to 67% and 75% of the total extracted Fe and P, respectively. The coupling processes of Fe plaque enrichment of P and oxalic acid complexation of Fe(III) led to significantly enhanced P acquisition in the rhizosphere of V. natans.

Carbon nanodot aqueous binding phase-based diffusive gradients in thin films device for measurement of dissolved copper and lead species in the aquatic environment

A reliable method for the determination of dissolved Cu²⁺and Pb²⁺species in waterviaa diffusive gradient in thin films (DGT) device using water-soluble carbon nanodots (CD) as the binding agent was developed (CD-DGT).

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.

Passive sampling methods for contaminated sediments: state of the science for metals

"Dissolved" concentrations of contaminants in sediment porewater (Cfree ) provide a more relevant exposure metric for risk assessment than do total concentrations. Passive sampling methods (PSMs) for estimating Cfree offer the potential for cost-efficient and accurate in situ characterization of Cfree for inorganic sediment contaminants. In contrast to the PSMs validated and applied for organic contaminants, the various passive sampling devices developed for metals, metalloids, and some nonmetals (collectively termed "metals") have been exploited to a limited extent, despite recognized advantages that include low detection limits, detection of time-averaged trends, high spatial resolution, information about dissolved metal speciation, and the ability to capture episodic events and cyclic changes that may be missed by occasional grab sampling. We summarize the PSM approaches for assessing metal toxicity to, and bioaccumulation by, sediment-dwelling biota, including the recognized advantages and limitations of each approach, the need for standardization, and further work needed to facilitate broader acceptance and application of PSM-derived information by decision makers.

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

This study presents a new approach combining diffusive equilibrium in thin-film (DET) and spectrophotometric methods to determine the spatial variability of dissolved iron and dissolved reactive phosphorus (DRP) with a single gel probe. Its originality is (1) to postpone up to three months the colorimetric reaction of DET by freezing and (2) to measure simultaneously dissolved iron and DRP by hyperspectral imaging at a submillimeter resolution. After a few minutes at room temperature, the thawed gel is sandwiched between two monospecific reagent DET gels, leading to magenta and blue coloration for iron and phosphate, respectively. Spatial distribution of the resulting colors is obtained using a hyperspectral camera. Reflectance spectra analysis enables deconvolution of specific colorations by the unmixing method applied to the logarithmic reflectance, leading to an accurate quantification of iron and DRP. This method was applied in the Arcachon lagoon (France) on muddy sediments colonized by eelgrass (Zostera noltei) meadows. The 2D gel probes highlighted microstructures in the spatial distribution of dissolved iron and phosphorus, which are most likely associated with the occurrence of benthic fauna burrows and seagrass roots.

Characterizing microbial communities and processes in a modern stromatolite (Shark Bay) using lipid biomarkers and two-dimensional distributions of porewater solutes

Modern microbial mats are highly complex and dynamic ecosystems. Diffusive equilibration in thin films (DET) and diffusive gradients in thin films (DGT) samplers were deployed in a modern smooth microbial mat from Shark Bay in order to observe, for the first time, two-dimensional distributions of porewater solutes during day and night time. Two-dimensional sulfide and alkalinity distributions revealed a strong spatial heterogeneity and a minor contribution of sulfide to alkalinity. Phosphate distributions were also very heterogeneous, while iron(II) distributions were quite similar during day and night with a few hotspots of mobilization. Lipid biomarkers from the three successive layers of the mat were also analysed in order to characterize the microbial communities regulating analyte distributions. The major hydrocarbon products detected in all layers included n-alkanes and isoprenoids, whilst other important biomarkers included hopanoids. Phospholipid fatty acid profiles revealed a decrease in cyanobacterial markers with depth, whereas sulfate-reducing bacteria markers increased in abundance in accordance with rising sulfide concentrations with depth. Despite the general depth trends in community structure and physiochemical conditions within the mat, two-dimensional solute distributions showed considerable small-scale lateral variability, indicating that the distributions and activities of the microbial communities regulating these solute distributions were equally heterogeneous and complex.

Transport zonation limits coupled nitrification-denitrification in permeable sediments

Measurement of biogeochemical processes in permeable sediments (including the hyporheic zone) is difficult because of complex multidimensional advective transport. This is especially the case for nitrogen cycling, which involves several coupled redox-sensitive reactions. To provide detailed insight into the coupling between ammonification, nitrification and denitrification in stationary sand ripples, we combined the diffusion equilibrium thin layer (DET) gel technique with a computational reactive transport biogeochemical model. The former approach provided high-resolution two-dimensional distributions of NO3(-) and (15)N-N2 gas. The measured two-dimensional profiles correlate with computational model simulations, showing a deep pool of N2 gas forming, and being advected to the surface below ripple peaks. Further isotope pairing calculations on these data indicate that coupled nitrification-denitrification is severely limited in permeable sediments because the flow and transport field limits interaction between oxic and anoxic pore water. The approach allowed for new detailed insight into subsurface denitrification zones in complex permeable sediments.

Millimeter-scale alkalinity measurement in marine sediment using DET probes and colorimetric determination

Constrained DET (Diffusive Equilibration in Thin films) probes equipped with 75 sampling layers of agarose gel (DGT Research(©)) were used to sample bottom and pore waters in marine sediment with a 2 mm vertical resolution. After retrieval, each piece of hydrogel, corresponding to 25 μL, was introduced into 1 mL of colorimetric reagent (CR) solution consisting of formic acid and bromophenol blue. After the elution/reaction time, absorbance of the latter mixture was read at 590 nm and compared to a calibration curve obtained with the same protocol applied to mini DET probes soaked in sodium hydrogen carbonate standard solutions. This method allows rapid alkalinity determinations for the small volumes of anoxic pore water entrapped into the gel. The method was assessed on organic-rich coastal marine sediments from Thau lagoon (France). Alkalinity values in the overlying waters were in agreement with data obtained by classical sampling techniques. Pore water data showed a progressive increase of alkalinity in the sediment from 2 to 10 mmol kg(-1), corresponding to anaerobic respiration in organic-rich sediments. Moreover, replicates of high-resolution DET profiles showed important lateral heterogeneity at a decimeter scale. This underlines the importance of high-resolution spatial methods for alkalinity profiling in coastal marine systems.

Application of DET (diffusive equilibrium in thin films) and DGT (diffusive gradients in thin films) techniques in the study of the mobility of sediment-bound metals in the outer section of Songkhla Lake, Southern Thailand

The techniques of diffusive equilibrium in thin films (DET) and diffusive gradients in thin films (DGT) were used in the outer section of Songkhla Lake, Thailand in order to obtain high-resolution profiles of total dissolved and labile trace metals in the sediment pore water and investigate benthic fluxes. Six DET probes and six DGT probes were deployed at the mouths of the Phawong, Samrong and U-Taphao canals. A close correspondence could be observed between the high-resolution profiles of Fe and As, revealing a close link between the reductive remobilization of Fe oxides and the reduction of As(V). Co and Ni DGT profiles showed a close correspondence with Mn, but a narrow mobilization zone. Reductive mobilization of Mn oxides and associated metals and sulfide precipitation control the behaviour of these metals. The DGT profiles of Cu, Zn, Cd and Pb show surface maximum, probably linked to organic matter degradation. Important benthic fluxes, especially for As, were found at the mouths of the U-Taphao and Phawong canals.

Natural organic matter quantification in the waters of a semiarid freshwater wetland (Tablas de Daimiel, Spain)

Dissolved organic matter (DOM) concentrations have been measured in the waters of a semiarid freshwater wetland, the Tablas de Daimiel, Spain, when the system-characterised by variable hydroperiodicity conditions, was completely flooded (February 2011). Fluxes of DOM from the wetland soils to the overlying waters were measured by using a passive diffusion sampler (peeper). Not only dissolved organic carbon (DOC) concentrations were measured but refractory organic matter (ROM, usually known as humic substances) was also quantified using a novel voltammetric method. Fluorescence spectra were recorded to help in selecting the appropriate standard for ROM quantification, test the homogeneity of DOM in the waters and get an indication of their source. The results obtained show a 7-fold increase in measured ROM concentrations from the Gigüela River to the outlet, which points to a net exportation of ROM from the wetland and to the existence of an internal source of ROM in the system, probably diffusion from the wetland soils. This hypothesis is confirmed by the flux of ROM from the soils to the water column measured with the peeper and by the common fluorescence characteristics of column and interstitial waters. The smaller increase in DOC concentrations along the wetland, in spite of the higher DOC fluxes from soils, suggests that there is significant turnover of organic carbon (OC) in the water column. The system acts as a major carbon sink but, when flooded, exports OC as DOM.

High-resolution simultaneous measurements of dissolved reactive phosphorus and dissolved sulfide: the first observation of their simultaneous release in sediments

The reassessments of environmental processes in sediments rely upon capturing the heterogeneous features of elements at a small scale and at the same location. In this study, a diffusive gradients in thin films (DGT) technique was developed for the high-resolution simultaneous measurements of dissolved reactive phosphorus (DRP) and dissolved sulfide. A new binding gel was used in this DGT technique, which was prepared by incorporating AgI particles into the zirconium oxide binding gel previously used in the DGT measurement of DRP. The concentrations of the DRP and sulfide loaded into the binding gel were determined by a routine procedure and a computer-imaging densitometry (CID) technique, respectively. The performance of this DGT technique was tested under laboratory conditions and applied to in situ measurements in sediments of a shallow lake. Simultaneous release of DRP and dissolved sulfide was observed in a sulfide microniche with a diameter of ∼3 mm and in locally aggregated zones with a length over 1 cm, which was attributed to the simultaneous reductions of Fe(III) and sulfate and the associated release of Fe-bound P in the zones of the reactive organic matter in sediments. The good performance of this technique implies that there is a great potential for the development of new DGT techniques capable of simultaneous measurements of more analytes.

Ion Beam Analysis: A Century of Exploiting the Electronic and Nuclear Structure of the Atom for Materials Characterisation

Analysis using MeV ion beams is a thin film characterisation technique invented some 50 years ago which has recently had the benefit of a number of important advances. This review will cover damage profiling in crystals including studies of defects in semiconductors, surface studies, and depth profiling with sputtering. But it will concentrate on thin film depth profiling using Rutherford backscattering, particle induced X-ray emission and related techniques in the deliberately synergistic way that has only recently become possible. In this review of these new developments, we will show how this integrated approach, which we might call "total IBA", has given the technique great analytical power.

A high-resolution dialysis technique for rapid determination of dissolved reactive phosphate and ferrous iron in pore water of sediments

Characterization of pore water chemistry necessitates measurements of the chemical components at a high spatial resolution due to the heterogeneous nature of sediments. In this study, a novel high-resolution dialysis technique was developed for rapid sampling of dissolved reactive phosphate (DRP) and ferrous iron (Fe) in pore water at a vertical resolution of 2mm. This technique employed a mini dialysis device (peeper) with a deployment time of 2 days in sediments for equilibration. Operational procedures following retrieval of the peeper include in situ freezing of the pore water samples through spraying commercial carbon dioxide snow, frozen storage of the pore water samples, and quick analysis of chemicals in pore water samples using 384-well microplate photometric methods. The sample consumption for each parameter can be minimized to 6μL, which allowed simultaneous determination of DRP and ferrous iron in a peeper device. Laboratory tests showed that the technique was robust enough for the in situ measurements. Field deployment was subsequently performed in three sites of a shallow, eutrophic bay in Lake Taihu of China. Simultaneous determination of DRP and ferrous Fe in all the sites demonstrated a similar behavior of both chemicals in the pore water profiles, verifying the control of P mobilization by Fe in sediments.

Links between bacterial communities in marine sediments and trace metal geochemistry as measured by in situ DET/DGT approaches

Our current view about the relationship between metals and bacteria in marine sediments might be biased because most studies only use ex situ approaches to quantify metals. The aim of the present research was to compare ex situ and in situ methods of metal measurement (DET and DGT--diffusive equilibration or diffusive gradients in thin-films) and relate the results with two commonly used microbiological variables (bacterial biomass and bacterial diversity as revealed by DGGE). No previous studies have used such in situ approaches in microbial ecology. For biomass and most of the investigated trace metals (Ag, Cd, Sn, Cr, Ni, Cu, Pb, and Al) no significant correlations were found. The exceptions were Fe, Mn, Co, and As which behave like micronutrients. For bacterial diversity, no relevant relationships were found. We conclude that in situ methods are more adapted tools for microbial ecologists but that ex situ approaches are still necessary.

Representative measurement of two-dimensional reactive phosphate distributions and co-distributed iron(II) and sulfide in seagrass sediment porewaters

The high degree of heterogeneity within sediments can make interpreting one-dimensional measurements difficult. The recent development and use of in situ techniques that measure two-dimensional distributions of porewater solutes have facilitated investigation of the role of spatial heterogeneity in sediment biogeochemistry. A colourimetric diffusive equilibration in thin films method has been developed that allows two-dimensional, high-resolution measurement of reactive phosphate in sediment porewaters. A method detection limit of 0.22 μM, an effective upper limit of ~1000 μM and relative standard deviations typically below 5% were achieved. This method was evaluated by deployment in seagrass (Zostera capricorni) colonised sediments, as part of combined probes with similar colourimetric methods for sulfide and iron(II). The two-dimensional, high resolution distributions obtained provide a highly representative measurement of the co-distributions of porewater solutes, allowing heterogeneous features and biogeochemical processes to be observed and interpreted. Microniches of high phosphate concentration >100 μM were observed throughout the distributions and were interpreted to be due to localised zones of rapid organic matter mineralisation, possibly using electron acceptors other than iron(III) oxyhydroxides (e.g. aerobic respiration) as often they did not correspond with microniches of higher Fe(II) concentration.

Interactions of trace metals with hydrogels and filter membranes used in DET and DGT techniques

Equilibrium partitioning of trace metals between bulk solution and hydrogels/filter was studied. Under some conditions, trace metal concentrations were higher in the hydrogels or filter membranes compared to bulk solution (enrichment). In synthetic soft water, enrichment of cationic trace metals in polyacrylamide hydrogels decreased with increasing trace metal concentration. Enrichment was little affected by Ca and Mg in the concentration range typically encountered in natural freshwaters, indicating high affinity but low capacity binding of trace metals to solid structure in polyacrylamide gels. The apparent binding strength decreased in the sequence: Cu > Pb > Ni approximately to Cd approximately to Co and a low concentration of cationic Cu eliminated enrichment of weakly binding trace metal cations. The polyacrylamide gels also had an affinity for fulvic acid and/or its trace metal complexes. Enrichment of cationic Cd in agarose gel and hydrophilic polyethersulfone filter was independent of concentration (10 nM to 5 microM) but decreased with increasing Ca/ Mg concentration and ionic strength, suggesting that it is mainly due to electrostatic interactions. However, Cu and Pb were enriched even after equilibration in seawater, indicating that these metals additionally bind to sites within the agarose gel and filter. Compared to the polyacrylamide gels, agarose gel had a lower affinity for metal-fulvic complexes. Potential biases in measurements made with the diffusive equilibration in thin-films (DET) technique, identified by this work, are discussed.

A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: I. Laboratory development

A gel probe equilibrium sampler has been developed to study arsenic (As) geochemistry and sorption behavior in sediment porewater. The gels consist of a hydrated polyacrylamide polymer, which has a 92% water content. Two types of gels were used in this study. Undoped (clear) gels were used to measure concentrations of As and other elements in sediment porewater. The polyacrylamide gel was also doped with hydrous ferric oxide (HFO), an amorphous iron (Fe) oxyhydroxide. When deployed in the field, HFO-doped gels introduce a fresh sorbent into the subsurface thus allowing assessment of in situ sorption. In this study, clear and HFO-doped gels were tested under laboratory conditions to constrain the gel behavior prior to field deployment. Both types of gels were allowed to equilibrate with solutions of varying composition and re-equilibrated in acid for analysis. Clear gels accurately measured solution concentrations (+/-1%), and As was completely recovered from HFO-doped gels (+/-4%). Arsenic speciation was determined in clear gels through chromatographic separation of the re-equilibrated solution. For comparison to speciation in solution, mixtures of As(III) and As(V) adsorbed on HFO embedded in gel were measured in situ using X-ray absorption spectroscopy (XAS). Sorption densities for As(III) and As(V) on HFO embedded in gel were obtained from sorption isotherms at pH 7.1. When As and phosphate were simultaneously equilibrated (in up to 50-fold excess of As) with HFO-doped gels, phosphate inhibited As sorption by up to 85% and had a stronger inhibitory effect on As(V) than As(III). Natural organic matter (>200 ppm) decreased As adsorption by up to 50%, and had similar effects on As(V) and As(III). The laboratory results provide a basis for interpreting results obtained by deploying the gel probe in the field and elucidating the mechanisms controlling As partitioning between solid and dissolved phases in the environment.

High resolution profiles of thallium, manganese and iron assessed by DET and DGT techniques in riverine sediment pore waters

High resolution profiles of Mn, Tl and Fe concentrations have been assessed in the pore waters of river Leie sediments at Warneton and Menen (at the border of Belgium and France) by DET (Diffusive Equilibrium in Thin Films) and DGT (Diffusive Gradients in Thin Films) techniques. The oxidized, solid Mn (IV), Tl (III) and Fe (III) compounds were reduced in the suboxic (+255 to -20 mV versus Standard Hydrogen Electrode (SHE)) riverine sediments and since these reduced species are much more soluble also they are released into the pore waters. The highest DET (total dissolved) concentrations of Fe (76 mg l(-1)), Mn (2 mg l(-1)) were observed at the station of Menen, while Tl maxima differed only slightly between the 3 surveys (21 to 27 microg l(-1)). The average ratios of Fe/Mn/Tl in the pore waters at the 3 sampling stations are fairly constant for both the DET and DGT samplings. However, the results indicate that compared to Fe and Tl a greater proportion of the Mn measured by DET is accumulated by DGT, reflecting the ready supply of Mn from solid phase to solution.

High-resolution profiles of trace metals in the pore waters of riverine sediment assessed by DET and DGT

The techniques of DET (diffusive equilibrium in thin films) and DGT (diffusive gradients in thin films) were applied to obtain high-resolution vertical profiles of trace metals in freshwater sediments. In the framework of the EU-Interreg project Stardust (http://www.vliz.be/projects/stardust/) between France and Belgium, in which the mobility of sediment bound metals is investigated, sediment samples were collected from the Upper Scheldt River (at Helkijn, Belgium) and the Leie River (at Warneton, located at the Belgian-French border). Intra- and inter-laboratory comparisons of the gel techniques were carried out between the two laboratories involved. In general, a good agreement was observed, taking sediment heterogeneity into account. At both stations, metal pore water profiles show more or less similar tendencies although the sediment at Warneton was more anoxic than at Helkijn. A strong correlation between Fe and Co was found at Helkijn as well as at Warneton. The metal gradients at the water/sediment interface were calculated from the high resolution profiles and the conventional, low resolution profiles. Significant differences were observed.

Trace Metal Measurements in Low Ionic Strength Synthetic Solutions by Diffusive Gradients in Thin Films

In view of conflicting reports regarding the performance of DGT in low ionic strength solutions (I < 1 mM), further investigations have been carried out. Minimal washing of the diffusive gel and deployment in 1.0 and 10 mM NaNO3 solutions containing Cu and Cd gave the theoretical response of 1 for [C](DGT)/[C](SOLN), where [C](DGT) is the concentration of metal measured by DGT and [C](SOLN) is the concentration of metal measured directly in the solution by an appropriate analytical method. Erroneously high values for [C](DGT)/[C](SOLN) were obtained when these same gels were deployed at I = 0.1 mM, presumably due to a net negative charge on the gel, attributable to the presence of initiation products of polymerization. However, washing the diffusive gels completely, where the storage solution pH equaled that of deionized water, gave values of approximately 0.5 for [C](DGT)/[C](SOLN) from deployments at I = 0.1 mM, consistent with the lower measured value of the diffusion coefficients at this ionic strength. These results can be explained by the presence of a net positive charge on the gel when it is exhaustively washed, which reduces the effective diffusion coefficient of metal ions by changing their concentration at the gel-solution interface (Donnan partitioning). Diffusive gel equilibration experiments showed the presence of low capacity sites capable of binding metals irrespective of ionic strength. This binding within the diffusive gel does not affect most DGT measurements, as short (4 h) deployments at concentrations of 10 ppb gave theoretical results. Incomplete washing of the resin-gel caused a 5-15% measurement error and a decrease in precision, even at ionic strengths of 10 mM. A high level of accuracy and precision (typically <5%) was maintained during all aspects of this work, even at ionic strengths of 0.1 mM, in contrast to previous results. This is attributable to three factors: (1) exhaustive washing and conditioning protocols, (2) improvements to the DGT sampling device, and (3) low and reproducible blanks due to ultraclean handling procedures. Provided effective diffusion coefficients measured at the same ionic strength are used, the established DGT theory is obeyed irrespective of ionic strength.

Distribution and behavior of trace metals in the sediment and porewater of a tropical coastal wetland

Vertical profiles (0-30 cm below surface) of four trace metals-Cadmium (Cd), Chromium (Cr), Lead (Pb) and Zinc (Zn)-in the sediment and sediment porewater of an ecologically important intertidal mudflat in the Mai Po and Inner Deep Bay Ramsar Site were thoroughly studied over a period of 10 months (from March 1999 to January 2000). Two surveys, one in summer and another in winter, involving a total of eight sampling stations were conducted to study the seasonal variation of the remobilization characteristics of these trace metals in the mudflat sediment. The range of depth averaged concentration of these trace metals in the mudflat sediment was: 0.3-0.8 microg/g (Cd); 9.8-91.0 microg/g (Cr); 7.3-69.1 microg/g (Pb); and 39.5-192.0 microg/g (Zn), while that in the sediment porewater was: 0.3-121.1 microg/l (Cd); 3.0-2704.1 microg/l (Cr); 2.6-105.6 microg/l (Pb); and 32.6-4238.3 microg/l (Zn). In general, levels of dissolved trace metals in the sediment porewater were much higher in the summer than in the winter while their concentrations in the sediment were more or less the same throughout the year. Enrichment of Cd, Pb and Zn in the sediment porewater of the upper oxic layer and that of Cr in the oxic-sub-oxic boundary was generally observed. Regions in the vicinity of the Mai Po mangroves and the river mouths of Shenzhen River and Shan Pui River were found to be hotspots of trace metal pollution. Benthic diffusive fluxes of trace metals from the mudflat sediment were also estimated. Of the four trace metals, cadmium showed the greatest tendency toward remobilization from the sediment phase to the more bio-available porewater phase.

Role of Leptothrix discophora in mediating metal uptake in the filter-feeding bivalve Mytilus trossulus (edulis)

The potential for filter-feeding bivalves to accumulate metals from a wide range of food sources is an important consideration when examining trophic transfer of metals up the food chain. The objective of this study was to determine the role of Leptothrix discophora in mediating metal uptake in the filter-feeding bivalve Mytilus trossulus. The bacterium L. discophora SP-6 was cultured in the absence or presence of Mn, allowing for a naturally formed Mn oxide sheath to develop. Secondary metals (Cd and Pb) were then added to the cultures, allowing for potential Cd and Pb adsorption to the Mn oxide sheath. Resulting bacterial aggregates of known diameter were then fed to the bivalve M. trossulus using a flow-through system. Initial concentrations of both Pb and Cd on the bacterium did not differ significantly in the presence or absence of the Mn oxide; conversely both Pb (F = 7.39, p < 0.0001) and Cd (F= 33.65, p < 0.0001) were found at lower concentrations in the mussel tissue when the Mn oxide was present. To determine whether these differences in metal uptake could be attributed to sorting by the mussel based on food quality, nutritional analysis was performed. Bacterial food matrixes containing Mn oxides were found to have significantly lower levels of carbon (F = 256, p < 0.0001). Particle clearance rates for the various food matrixes were positively correlated with organic content (R2 = 0.852, p > 0.008). The results of our study suggest that metal uptake in M. trossulus was significantly decreased for Cd with a similar trend for Pb when the SP-6 sheath contained Mn oxides. The mechanism mediating this differential uptake is best explained by food quality, in that a higher quality food source enhanced metal uptake due to an increased clearance rate of organic-rich particles by M. trossulus.

Corer-reactors for contaminant flux measurement in sediments

Design details and operating instructions are provided for a sediment corer that can be converted into a reactor for the measurement of the fluxes of contaminants from sediments to overlying waters. The corer-reactor permits measurements, under controlled laboratory conditions, on intact, largely undisturbed sediment cores, without significantly perturbing the physical and chemical conditions found in the field. The design can be constructed in-house for around US dollar 240 (A dollar 400) (excluding motor and corer lid), making it a relatively inexpensive system.

A new rechargeable dialysis pore water sampler for monitoring sub-aqueous in-situ sediment caps

A new rechargeable dialysis pore water sampler is proposed that is adapted to the requirements of monitoring in-situ sediment remediation techniques, in particular in-situ capping. Sampling and recharging of the sampler can be carried out from a boat or a pontoon by means of separate tubing and a peristaltic pump. The possibility of repeated sampling permits a temporal as well as a spatial resolution of pore water geochemistry. This aims to facilitate a monitoring of temporal variations in contaminant profiles within a cap matrix. To meet these particular requirements the basic peeper design is modified. In this manuscript constructive details and materials used are discussed as well as the feasibility and reliability of the sampling and recharging process. The peeper is designed for I m depth profile with spatial resolution of 1/5.5cm and the practical temporal resolution, that chiefly depends on sediment characteristics, is 1/3 weeks. Results from laboratory and field testing show that sample volumes of 20 cm3 can be obtained from a depth of 8 m without mixing of sample and recharge water. The field test results with an exposure time of 8 months indicate that no clogging of neither the membranes nor the sample tubing occurred. The temporal development of the concentration-vs. -depth profiles of sodium, iron, and nickle, chosen as examples from the investigated metals in the pore water, document the stability of the monitoring system. The results thus corroborate that this new type of sampler can be employed as a tool for monitoring contaminants at the sediment-to-water interface and particularly within an in-situ cap.

Metalloporphyrin molecular sieves based on Tin(IV)porphyrin phenolates

[reaction: see text] The crystal and molecular structures of two six-coordinate tin(IV)porphyrin phenolate complexes show that infinite cylindrical channels of uniform pore dimension are formed along the crystallographic c-direction. The underlying recognition event responsible for the porosity is an extremely tight intermeshing of the meso-tolyl units between layers which is a result of Sn-O...H interactions. Thermogravimetric analysis and differential scanning calorimetry have been used to characterize the sieve-like materials.

Pore water testing and analysis: the good, the bad, and the ugly

The increasingly common practice of collecting and assessing sediment pore water as a primary measure of sediment quality is reviewed. Good features of this practice include: pore water is a key exposure route for some organisms associated with sediments; pore water testing eliminates particle size effects; pore water analyses and tests can provide useful information regarding contamination and pollution. Bad features include: pore water is not the only exposure route; pore water tests lack chemical or biological realism: their "sensitivity" relative to other tests may be meaningless due to manipulation and laboratory artifacts; many sediment and surface dwelling organisms are not directly influenced by pore water. Bad features can become ugly if: other exposure pathways are not considered (for toxicity or bioaccumulation); manipulation techniques are not appropriate; pore water tests are inappropriately linked to population-level effects. Pore water testing and analyses can be effective tools provided their limitations are well understood by researchers and managers.

Two-dimensional small-scale variability of pore water phosphate in freshwater lakes: results from a novel dialysis sampler

Vertical concentration profiles of soluble reactive phosphorus (SRP) in the upper sedimentary zone of freshwater lakes are an important means for studying internal phosphorus (P) loading and to gain insight into early diagenetic processes. The interpretation of such pore water profiles generally neglects the occurrence of horizontal variability at a specific sampling site. To further examine this variability, we have designed a novel two-dimensional sampler (2D peeper) consisting of 2280 chambers at a spatial resolution of 9 mm providing a sampling area of 43 x 44 cm. This new device was deployed in three eutrophic lakes in north-eastern Germany. The resulting 2D images of the SRP concentrations, diffusive fluxes, and turnover rates revealed systematic vertical and horizontal structures with local niches of increased phosphorus release. Thus, the extrapolation of P flux calculations based on one-dimensional pore water profiles may lead to a considerable error. The observed small-scale horizontal heterogeneity, probably mainly caused by organisms, was larger in the biologically more active Lake Müllrose and Süsser See than in the deeper Arendsee where meio- and macrozoobenthos were missing. In all cases, the variability was highest at the sediment-water interface and diminished with sediment depth.