Zr-oxide DGT Research Articles

High-resolution imaging of labile P & S in coastal sediment: Insight into the kinetics of P mobilization associated with sulfate reduction

The internal P cycling in coastal sediments during early diagenesis is of great importance to aquatic ecosystems, but its relationship with sulfate reduction is still unclear and lacks visualized evidence. In this study, diffusive gradients in thin films (Zr-oxide DGT and AgI DGT) techniques at a two-dimensional (2D) submillimeter resolution were utilized to synchronously characterize the in situ labile P and dissolved sulfide in coastal sediments around Xiamen Bay, China. The distributions of dissolved sulfide formed from sulfate reduction showed large vertical and horizontal variations, which are mainly controlled by the reactive organic carbon (OC) and redox conditions at different sampling sites. Multiple correlation analysis revealed that significant and linear correlations exist between dissolved sulfide and DGT-labile P, demonstrating that P mobilization is predominantly regulated by sulfide-dependent reductive dissolution of Fe(III)-bound P and sulfate reduction-associated OC mineralization. Both labile P and dissolved sulfide showed lower values at intertidal sites, due to oxic/suboxic conditions favoring weak sulfate reduction and strong P sequestration. Labile P accumulation during flood tides and removal during ebb tides on tidal timescales indicate that the mobilization and detention of P in surface sediment are regulated by tidal cycles and consequential S and Fe redox cycling. The apparent diffusion flux of P implies that the internal P loading can act as a P source to aquatic ecosystems.

Submillimeter-scale heterogeneity of labile phosphorus in sediments characterized by diffusive gradients in thin films and spatial analysis

Sediments have a heterogeneous distribution of labile redox-sensitive elements due to a drastic downward transition from oxic to anoxic condition as a result of organic matter degradation. Characterization of the heterogeneous nature of sediments is vital for understanding of small-scale biogeochemical processes. However, there are limited reports on the related specialized methodology. In this study, the monthly distributions of labile phosphorus (P), a redox-sensitive limiting nutrient, were measured in the eutrophic Lake Taihu by Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) on a two-dimensional (2D) submillimeter level. Geographical information system (GIS) techniques were used to visualize the labile P distribution at such a micro-scale, showing that the DGT-labile P was low in winter and high in summer. Spatial analysis methods, including semivariogram and Moran’s I, were used to quantify the spatial variation of DGT-labile P. The distribution of DGT-labile P had clear submillimeter-scale spatial patterns with significant spatial autocorrelation during the whole year and displayed seasonal changes. High values of labile P with strong spatial variation were observed in summer, while low values of labile P with relatively uniform spatial patterns were detected in winter, demonstrating the strong influences of temperature on the mobility and spatial distribution of P in sediment profiles.

Release of reactive phosphorus from sediments in Dongting Lake linked with the Yangtze River

Environmental contextEutrophication caused by excessive inputs of phosphorus is a prevalent global environmental problem. Reactive phosphorus released from sediments was measured by two new in situ passive sampling techniques capable of high-resolution measurements of phosphorus concentration. The methods provide the scientific evidence for solving the problems associated with deteriorating surface water quality. AbstractInternal phosphorus (P) loading is regarded as a major eutrophication factor and may prevent improvements in lake water quality. Two new in situ passive sampling techniques, high-resolution pore-water equilibrators (HR-Peeper) and zirconium oxide-based diffusive gradients in thin films (Zr-oxide DGT), were combined to measure dissolved reactive phosphorus (DRP) (CPeeper) and labile phosphorus (CDGT) at five sites in South Dongting and West Dongting Lakes. The vertical distribution of CPeeper and CDGT displayed similarity, which demonstrated that the buffering capacity of the labile P in sediments was similar at different depths. The diffusion flux of P from the sediments at the sediment–water interface ranged from 1.9 to 88ng m–2 day–1, with an average value of 38ng m–2 day–1. The P flux at the entrances to the Yuan, Li and Zi Rivers was fairly large at all five sites. The sediments at the five sites released P into overlying water, indicating that the sediments are an important source of P for Dongting Lake.

High-resolution imaging of labile phosphorus and its relationship with iron redox state in lake sediments

A thorough understanding of the labile status and dynamics of phosphorus (P) and iron (Fe) across the sediment-water interface (SWI) is essential for managing internal P release in eutrophic lakes. Fe-coupled inactivation of P in sediments is an important factor which affects internal P release in freshwater lakes. In this study, two in-situ high-resolution diffusive gradients in thin films (DGT) techniques, Zr-Oxide DGT and ZrO-Chelex DGT, were used to investigate the release characteristics of P from sediments in a large freshwater lake (Dongting Lake, China; area of 2691 km2) experiencing a regional summer algal bloom. Two-dimensional distributions of labile P in sediments were imaged with the Zr-Oxide DGT without destruction of the original structure of the sediment layer at four sites of the lake. The concentration of DGT-labile P in the sediments, ranging from 0.007 to 0.206 mg L−1, was highly heterogeneous across the profiles. The values of apparent diffusion flux (Fd) and release flux (Fr) of P varied between −0.027–0.197 mg m−2 d−1 and 0.037–0.332 mg m−2 d−1, respectively. Labile P showed a high and positive correlation (p < 0.01) with labile Fe(II) in the profiles, providing high-resolution evidence for the key role of Fe-redox cycling in labile P variation in sediments.

Simultaneous Measurements of Eight Oxyanions Using High-Capacity Diffusive Gradients in Thin Films (Zr-Oxide DGT) with a High-Efficiency Elution Procedure.

A zirconium oxide binding gel-based diffusive gradients in thin films (Zr-oxide DGT) was developed for simultaneous measurements of P(V), As(V), Cr(VI), Mo(VI), Sb(V), Se(VI), V(V), and W(VI). All of the oxyanions were rapidly bound to Zr-oxide gel with differences in binding affinity. The eight bound oxyanions were successfully recovered by one-step elution using a mild reagent of 0.2 M NaOH-0.5 M H2O2 by overcoming the problems in analyses of the oxyanions. The optimized elution time was reduced to 3-5 h from 24-48 h required by other DGTs. DGT uptakes of all the oxyanions were independent of pH (4.42-8.45) and ionic strength (0.1-500 mM). The DGT capacities for six oxyanions detected in multioxyanion solution were only 0.19 to 0.35 times of those detected in single-oxyanion solution, reflecting a strong competition among the oxyanions during DGT uptake. Except for Se(VI) in seawater, Zr-oxide DGT accurately measured all of the oxyanions in synthetic freshwater and seawater, with the capacities ∼29 to >2397 times and ∼7.5 to 232 times those of two commonly used DGTs (Metsorb and precipitated ferrihydrite (PF) DGTs) in freshwater and seawater, respectively. Measurements by Zr-oxide DGT in contaminated sediments were in agreement with only two oxyanions with the two commonly used DGTs; the two DGTs accumulated less or no mass of other oxyanions. This study demonstrates significant advantage of Zr-oxide DGT over the other DGTs in simultaneous measurements of the eight oxyanions due to the former's high capacity and a wide tolerance to environmental interferences, together with a high efficiency in elution.

Fine-scale bioturbation effects of tubificid worm (Limnodrilus hoffmeisteri) on the lability of phosphorus in sediments

This study investigated the effects of tubificid worm bioturbation on the lability of phosphorus (P) in microcosm sediments. High-resolution dialysis (HR-Peeper) and two types of diffusive gradients in thin films (DGT) (Zr-oxide DGT and ZrO-Chelex DGT) were used to measure soluble P and Fe, and labile P and Fe at a millimeter spatial scale. The worm bioturbation promoted P release (up to 511% of the control) to the overlying water on the 6th day, but it was reduced compared to the control (up to 171% of the control) from the 22nd day to the 102nd day because of the adsorption by Fe(III) oxyhydroxides. The worm bioturbation reduced the pore water soluble P concentration up to 48% and the DGT-labile P concentration up to 29% of the control from a sediment depth of −10 mm to approximately −130 mm before the 22nd day of incubation due to worm ingestion of sediment particles. Two-dimensional measurements of DGT-labile P also showed a much lower concentration of labile P around the worm burrow. This effect disappeared on the 53rd and 102nd day. However, the soluble P and DGT-labile P decreased again up to 41% and 38%, compared to the control from the sediment depth of −20 mm and −10 mm to approximately −130 mm, respectively, on the 152nd day of incubation due to the adsorption by Fe(III) oxyhydroxides. Soluble Fe(II) and DGT-labile Fe did not show significant changes from the worm bioturbation on the 6th day, but decreased up to 31% and 47% of the control after the 6th day. The results that worm ingestion of sediment particles is a significant driver of soluble and labile P reduction in the sediments before the 22nd day. After that, soluble and labile P reduction was attributed to P adsorption by Fe(III) oxyhydroxides.

Zr oxide-based coloration technique for two-dimensional imaging of labile Cr(VI) using diffusive gradients in thin films

A novel Zr oxide diffusive gradients in thin films (DGT)-based measurement technique for high-resolution imaging of labile Cr (VI) is introduced in this study. The method is based on the diphenylcarbazide coloration technique for Cr (VI) combined with computer-imaging densitometry, which provides a relation between the accumulated mass of Cr (VI) and the grayscale intensity. The Zr oxide gels show good performance in reflecting the accurate measurement of Cr (VI), independent of the effects of pH, ionic strength, and PO43− concentration. Settling of the Zr-oxide gel was identified as a simple and effective method to suppress leaching of the claret-colored compound out of the gel. In addition, the optimal volume of added coloration reagent was 125 times that of the binding gel and 30min was selected as the optimal time for the chromogenic reaction. The relationship between the accumulated Cr (VI) and grayscale intensity was analyzed under the optimized conditions. The Zr oxide DGT technique could also obviously reflect the heterogeneity of sediment. Consequently, Zr oxide DGT-based coloration is certified to be a robust and suitable tool for providing effective and high-resolution information on bioavailable Cr (VI).

The diffusion fluxes and sediment activity of phosphorus in the sediment–water interface of Poyang Lake

ABSTRACTThe dissolved reactive phosphorous (DRP) in pore water releases into the water body through the sediment–water interface. The DRP concentrations in the water and sediment of Poyang Lake were determined by high-resolution pore diffusion balance (HR-Peeper) and Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) techniques. The range of the DRP concentrations in the pore water samples of different sampling sites was 30–360 ng/cm3. The diffusion fluxes of DRP in the sediment–water interface were calculated based on the DRP concentration gradients. The range of DRP flux in Poyang Lake was 4.78–40.5 ng m−2 d−1 with an average value of 14.7 ng m−2 d−1. The results showed that the P diffusion flux was higher in the middle of the Poyang Lake than both upstream and downstream of the lake.

In-situ characterization and assessment of arsenic mobility in lake sediments

In-situ characterization and assessment of arsenic (As) mobility in sediments was scarce. In this study, the distributions of labile As at a vertical resolution of 2 mm were obtained in the sediments of a large Lake Taihu through in-situ measurements using a Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique. The DGT-labile As, interpreted as DGT flux (FDGT), exhibited three different patterns in the lake, with all the patterns generally showing an increasing mobility followed by a decreasing mobility with sediment depth. The mobility of As could be characterized by the average FDGT (0.06–1.27 pg cm−2 s−1) in the top 10 mm surface sediments, the maximal FDGT (FDGT-M, 0.14–2.44 pg cm−2 s−1) in the end of the initial increasing phase of FDGT, and the diffusion length (ΔL, 28–66 mm) from the depth showing the FDGT-M to the sediment-water interface. The upward mobilization of labile As from the deep sediments to the surface sediments and overlying water became evident when FDGT-M > 1.7 pg cm−2 s−1 or ΔL < 41 mm. The results, for the first time, showed a prospect in in-situ risk assessment of the pollution of sediment As. It was suggested that the increasing mobility of As in the upper sediments was controlled by the reduction of As(V) and the reductive dissolution of Fe(III) (hydr)oxides, while the decreasing mobility in the deep sediments was attributed to immobilization of As(III) by secondary Fe(II)-bearing minerals.

In situ, high-resolution imaging of labile phosphorus in sediments of a large eutrophic lake

Understanding the labile status of phosphorus (P) in sediments is crucial for managing a eutrophic lake, but it is hindered by lacking in situ data particularly on a catchment scale. In this study, we for the first time characterized in situ labile P in sediments with the Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique at a two-dimensional (2D), submillimeter resolution in a large eutrophic lake (Lake Taihu, China, with an area of 2338 km2). The concentration of DGT-labile P in the sediment profiles showed strong variation mostly ranging from 0.01 to 0.35 mg L−1 with a considerable number of hotspots. The horizontal heterogeneity index of labile P varied from 0.04 to 4.5. High values appeared at the depths of 0–30 mm, likely reflecting an active layer of labile P under the sediment–water interface (SWI). Concentration gradients of labile P were observed from the high-resolution 1D DGT profiles in both the sediment and overlying water layers close to the SWI. The apparent diffusion flux of P across the SWI was calculated between −21 and 65 ng cm−2 d−1, which showed that the sediments tended to be a source and sink of overlying water P in the algal- and macrophyte-dominated regions, respectively. The DGT-labile P in the 0–30 mm active layer showed a better correlation with overlying water P than the labile P measured by ex situ chemical extraction methods. It implies that in situ, high-resolution profiling of labile P with DGT is a more reliable approach and will significantly extend our ability in in situ monitoring of the labile status of P in sediments in the field.

Improved Diffusive Gradients in Thin Films (DGT) Measurement of Total Dissolved Inorganic Arsenic in Waters and Soils Using a Hydrous Zirconium Oxide Binding Layer

A high-capacity diffusive gradients in thin films (DGT) technique has been developed for measurement of total dissolved inorganic arsenic (As) using a long shelf life binding gel layer containing hydrous zirconium oxide (Zr-oxide). Both As(III) and As(V) were rapidly accumulated in the Zr-oxide gel and could be quantitatively recovered by elution using 1.0 M NaOH for freshwater or a mixture of 1.0 M NaOH and 1.0 M H2O2 for seawater. DGT uptake of As(III) and As(V) increased linearly with deployment time and was independent of pH (2.0-9.1), ionic strength (0.01-750 mM), the coexistence of phosphate (0.25-10 mg P L(-1)), and the aging of the Zr-oxide gel up to 24 months after production. The capacities of the Zr-oxide DGT were 159 μg As(III) and 434 μg As(V) per device for freshwater and 94 μg As(III) and 152 μg As(V) per device for seawater. These values were 5-29 times and 3-19 times more than those reported for the commonly used ferrihydrite and Metsorb DGTs, respectively. Deployments of the Zr-oxide DGT in As-spiked synthetic seawater provided accurate measurements of total dissolved inorganic As over the 96 h deployment, whereas ferrihydrite and Metsorb DGTs only measured the concentrations accurately up to 24 and 48 h, respectively. Deployments in soils showed that the Zr-oxide DGT was a reliable and robust tool, even for soil samples heavily polluted with As. In contrast, As in these soils was underestimated by ferrihydrite and Metsorb DGTs due to insufficient effective capacities, which were likely suppressed by the competing effects of phosphate.

Investigation of potential interferences on the measurement of dissolved reactive phosphate using zirconium oxide-based DGT technique

A diffusive gradients in thin films (DGT) technique based on hydrous zirconium oxide (Zr-oxide) has been recently developed for the measurement of dissolved reactive phosphate (DRP). In this study, the detailed performance of the DGT technique is reported. Spiking experiments revealed that several orthophosphate monoester compounds contributed to the Zr-oxide DGT measurements of DRP. However, such a phenomenon is unlikely to occur during field conditions due to the low concentration of organic P in typical natural waters. The presence of Cl− (up to 106 g/L), SO2−4 (up to 16 g/L), HCO−3 (up to 817 g/L), and AsO−2 and AsO3−4 (both up to 1 mg As/L) in solutions had negligible effects on the measurement of DRP. The threshold concentrations of Cl−, SO2−4 and HCO−3 have been increased from previous reports for the measurements of DRP using other adsorbent-based DGT techniques. The capacity for DGT measurements of DRP decreased with increasing solution pH (4.2–9.2). The lowest capacity (95 μg P/cm2 at pH 9.2) was still greater than that of other DGT techniques that are usually used for the measurement of DRP (2–12 μg P/cm2). The Zr-oxide binding gel could be stored for up to 2 years without any aging effect. This period of validity was considerably longer than the ferrihydrite binding gel that is commonly used in present DGT devices (6 months). The field application revealed that the concentrations of DRP measured in three fresh water samples using the Zr-oxide DGT technique were in agreement with those of the traditional colorimetric method.

Evaluation of in situ capping with clean soils to control phosphate release from sediments

Evaluation of in situ capping with clean soils to control phosphate release from the sediments of a eutrophic bay in Lake Taihu was performed after 18months of capping. The concentrations of dissolved reactive phosphate (DRP) in pore waters and DRP resupply from native sediments and capped sediments were determined using high-resolution dialysis (HR-Peeper) and a Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique. The adsorption isotherm of these sediments was further investigated using a modified Langmuir model. The results showed low concentrations of DRP in pore waters with a low resupply from the sediments for sustaining pore water DRP concentration after capping. The calculated flux to the overlying water following the capping treatment was approximately half of that for the native sediments, implying that the capping reduced the release of phosphate from the sediments. The low resupply of the sediments after capping was further demonstrated by larger partitioning coefficient (Kp) values and greater adsorption capacity (Qmax) values, while zero equilibrium concentrations (EPC0s) were similar to those in native sediments. The larger Kp and Qmax were attributed to higher active Fe and Al introduced by the capping, indicating that the binding of phosphate onto the active Fe and Al played a critical role in reducing the internal loading of phosphorous.