Shiming Ding

Organic phosphorus species in surface sediments of a large, shallow, eutrophic lake, Lake Taihu, China

Organic phosphorus (P) fractions in surface sediments from a large shallow, eutrophic Lake Taihu, China, were extracted with 0.1 M NaOH after pre-treatment of the sediments with a solution composed of 0.1 M EDTA and 2% (w/v) Na(2)S(2)O(4). Composition of organic P in the extracts was then characterized by (31)P nuclear magnetic resonance spectroscopy ((31)P NMR). Several P species, including phosphonates, orthophosphate, orthophosphate monoesters, phospholipids, DNA, pyrophosphate and polyphosphate, were detected in the NaOH extracts. The proportion of extracted organic P to total P in sediments was negatively correlated with total P in the water column, as were the proportions for orthophosphate monoesters and DNA. This implies that the majority of organic P in surface sediments is likely stabilized in some way, and does not directly contribute to the internal loading of P from sediments.

Measurement of dissolved reactive phosphorus using the diffusive gradients in thin films technique with a high-capacity binding phase.

Measurement of dissolved reactive phosphorus (DRP) by the diffusive gradients in thin films (DGT) technique was investigated using a new binding phase. Half-dried amorphous zirconium oxide (with 50 ± 5% of water content) was mixed with acrylamide solution for the preparation of the new binding phase. The resulting binding gel had a high binding capacity (223 μg P cm(-2)) for phosphate. The solution of NaOH (1 M) was used for elution of phosphate from the gel, and an elution efficiency of 0.95 was obtained. A test of DGT uptake with this gel showed its dependence on temperature, and there was no influence of pH (3 to 10) and ionic strength (10 nM to 0.1 M). Its capacity for DGT response exceeded 100 μg P cm(-2), corresponding to a DRP concentration of more than 20 mg L(-1) for a 24 h deployment with a standard DGT device at 25 °C, which was at least 50 times of the Fe-oxide gel commonly used in the present DGT technique. Measurements with this high-capacity DGT technique in a laboratory microcosm of homogeneously mixed sediments gave smooth and reproducible mass-depth profiles. This technique was well demonstrated by in situ measurements in algal- and macrophyte-dominated regions of Lake Taihu. The DGT-measured concentrations of DRP were on average 20% and 40% of the DRP concentrations in pore waters, respectively, indicating a partial resupply of the sediments to the pore waters with DRP.

A review of fractionations of rare earth elements in plants

Abstract Studies were carried out on several aspects of rare earth elements (REEs), such as the theory and practice of their applications in agriculture, their geochemical behaviors in natural and agricultural ecosystems, the mechanisms for the increase of crop yield using REE fertilizer, and their toxicology. However, limited knowledge was available for the transfer processes and the features and mechanisms of distribution and fractionations of REEs inside plants. The characteristics of REE fractionations in plants can be used to “trace” the pathway of REE transportation from soils (solution) to plants. A better understanding of the mechanisms of REE fractionations was helpful to investigate the controlling factors, including both the internal and the external ones. The characteristics and mechanisms of REE fractionations in plants and their significance were reviewed. Furthermore, the prospect for these fields was discussed, in hope of providing a new way in studying the bioavailability of REEs and heavy metals.

Diffusive Gradients in Thin Films Technique Equipped with a Mixed Binding Gel for Simultaneous Measurements of Dissolved Reactive Phosphorus and Dissolved Iron

Developing a technique to track the release of phosphorus (P) and iron (Fe) simultaneously in sediments would be very useful in deepening our understanding of the internal loading process of P coupled with Fe cycling in aquatic systems. In this study, a new technique was established to measure simultaneously the dissolved reactive P (DRP) and dissolved Fe primarily released from sediment solids based on the diffusive gradients in thin films (DGT) theory. A mixed binding gel (ZrO-Chelex gel) used for assembling DGT was developed for simultaneous uptake of DRP and dissolved Fe(II) using amorphous zirconium hydroxide (Zr-oxide) and Chelex-100 resin as binding agents. Simultaneous measurements of DRP and dissolved Fe(II) with the ZrO-Chelex DGT were validated in solution and were independent of solution pH and ionic strength in normal environments. The capacities of the ZrO-Chelex DGT for measurements of DRP and dissolved Fe(II) were 90 μg P cm(-2) and 75 μg Fe cm(-2), with the latter being greater than that (45 μg Fe cm(-2)) observed with the Chelex-100 resin DGT commonly used in DGT measurements of metals. Microcosm experiments further confirmed the feasibility of the ZrO-Chelex DGT for simultaneous measurement of P and Fe in sediments, with a higher concentration of Fe being measured due to this methods higher capacity compared with the Chelex-100 resin DGT.

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.

Gel-Based Coloration Technique for the Submillimeter-Scale Imaging of Labile Phosphorus in Sediments and Soils with Diffusive Gradients in Thin Films

We report a highly promising technique for the high-resolution imaging of labile phosphorus (P) in sediments and soils in combination with the diffusive gradients in thin films (DGT). This technique was based on the surface coloration of the Zr-oxide binding gel using the conventional molybdenum blue method following the DGT uptake of P to this gel. The accumulated mass of the P in the gel was then measured according to the grayscale intensity on the gel surface using computer-imaging densitometry. A pretreatment of the gel in hot water (85 °C) for 5 d was required to immobilize the phosphate and the formed blue complex in the gel during the color development. The optimal time required for a complete color development was determined to be 45 min. The appropriate volume of the coloring reagent added was 200 times of that of the gel. A calibration equation was established under the optimized conditions, based on which a quantitative measurement of P was obtained when the concentration of P in solutions ranged from 0.04 mg L(-1) to 4.1 mg L(-1) for a 24 h deployment of typical DGT devices at 25 °C. The suitability of the coloration technique was well demonstrated by the observation of small, discrete spots with elevated P concentrations in a sediment profile.

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.

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 18 months 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 (K(p)) values and greater adsorption capacity (Q(max)) values, while zero equilibrium concentrations (EPC(0)s) were similar to those in native sediments. The larger K(p) and Q(max) 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.

Effects of hydrodynamics processes on phosphorus fluxes from sediment in large, shallow Taihu Lake

The turnover of phosphorus (P) in lake sediments, a major cause of eutrophication and subsequent deterioration of water quality, is in need of deep understanding. In this study, effects of resuspension on P release were studied in cylindrical microcosms with Y-shape apparatus. The results indicated that there was a positive correlation between flux of suspended substance across sediment-water interface (F(SS)) and the wind speed, and an increasing F(SS) during each wind process followed by a steady state. The maximal F(SS) under light, moderate, and strong wind conditions were 299.9 +/- 41.1, 573.4 +/- 61.7, and 2093.8 +/- 215.7 g/m2, respectively. However, flux of P across sediment-water interface (F(P)) did not follow a similar pattern as F(SS) responding to wind intensity, which increased and reached the maximum in initial 120 min for light wind, then decreased gradually, with maximal flux of 9.4 +/- 1.9 mg/m2. A rapid increase of F(P) at the first 30 min was observed under moderate wind, with maximal flux of 11.2 +/- 0.6 mg/m2. Surprisingly, strong wind caused less F(P) than under light and moderate wind conditions with maximal flux of 3.5 +/- 0.9 mg/m2. F(SS) in water column declined obviously during the sedimentation process after winds, but F(P) varied with wind regime. No obvious difference was detected on F(P) after 8 h sedimentation process, compared with the initial value, which means little redundant P left in the water column after winds.

Bioavailability assessment of phosphorus and metals in soils and sediments: a review of diffusive gradients in thin films (DGT)

This paper provides an overview of the principle and latest development of the diffusive gradients in thin films (DGT) technology and its applications in environmental studies with a focus on bioavailability assessment of phosphorus and metals in sediments and soils. Compared with conventional methods, DGT, as a passive sampling method, has significant advantages: in situ measurement, time averaged concentrations and high spatial resolution. The in situ measurement avoids artificial influences including contamination of samples and sample treatment which may change the forms of chemicals. The time averaged concentration reflects representative measurement over a period of time. The high-resolution information captures the biogeochemical heterogeneity of elements of interest distributed in microenvironments, such as in the rhizosphere and the vicinity of the sediment-water interface. Moreover, DGT is a dynamic technique which simultaneously considers the diffusion of solutes and their kinetic resupply from the solid phases. All the advantages of DGT significantly promote the collection of “true” information of the bioavailable or labile forms of chemicals in the environment. DGT provides potential for applications in agriculture, environmental monitoring and the mining industry. However, the applications are still at the early testing stage. Further studies are needed to properly interpret the DGT-measured results under complex environmental conditions, and standard procedures and guideline values based on DGT are required to pave the way for its routine applications in environmental monitoring.