DGT-induced Fluxes In Sediments Research Articles

Coastal degradation regulates the availability and diffusion kinetics of phosphorus at the sediment-water interface: Mechanisms and environmental implications

Coastal wetlands have experienced considerable loss and degradation globally. However, how coastal degradation regulates sediment phosphorus (P) transformation and its underlying mechanisms remain largely unknown in subtropical coastal ecosystems. This study conducted seasonal field measurements using high-resolution diffusive gradient in thin films (DGT) and dialysis (Peeper) techniques, as well as a DGT-induced fluxes in sediments (DIFS) model, to evaluate the mobilization and diffusion of P along a degradation gradient ranging from pristine wetlands to moderately and severely degraded sites. We observed that sediment P is diminished by coastal degradation, and severely degraded sites exhibit a decline in the concentration of available P, despite the presence of distinct seasonal patterns. High-resolution data based on DGT/Peeper analysis revealed that labile P and soluble reactive P (SRP) concentrations varied from 0.0006 mg L−1 to 0.084 mg L−1 (mean 0.0147 mg L−1) and from 0.0128 mg L−1 to 0.1677 mg L−1 (mean 0.0536 mg L−1), respectively. Coastal degradation had a substantial impact on increasing SRP and labile P concentrations, particularly at severely degraded sites. Although severely degraded wetlands appeared to be P sinks (negative P flux at these sites), we did also observe positive diffusive flux in October, indicating that coastal degradation may accelerate the diffusion and remobilization of sediment P into overlying water. The simulations of the DIFS model provided compelling proof of the high resupply capacity of sediment P at severely degraded sites, as supported by the increased R and k-1 values but decreased Tc values. Taken together, these results suggest coastal degradation reduces the sediment P pool, primarily attributed to the strong remobilization of P from the sediment to porewater and overlying water by enhancing the resupply capability and diffusion kinetics. This acceleration induces nutrient loss which adversely impacts the water quality of the surrounding ecosystem. To reduce the adverse effects of coastal degradation, it is essential to adopt a combination of conservation, restoration, and management efforts designed to mitigate the risk of internal P loading and release, and ultimately maintain a regional nutrient balance.

Diffusion kinetic processes and release risks of trace metals in plateau lacustrine sediments

The ecological risk posed by trace metals in the plateau lacustrine sediments of China has attracted worldwide attentions. A better understanding of the kinetic diffusion processes and bioavailability of these metals in plateau lakes is needed. Using the diffusive gradient in thin films (DGT) and Rhizon, concentrations of Mn, Mo, Ni, Cr, and Co in the sediments, labile fractions, and interstitial water of Lake Fuxian were comprehensively analyzed. According to the DGT-induced fluxes in sediments (DIFS) model, fully sustained and unsustained resupplies are possible ways in which metals are released from solids to the solution. Moreover, the resupply characteristics of metals varied at different depths in the sediments and at different sites in the lake. Based on the DIFS model, the effective concentrations (CE) of the trace metals were calculated and all except Cr showed good linear relationships with the DGT-labile concentrations, indicating that the CE values were valuable for predicting metal bioavailability. According to the CE values, the metal contamination released from the sediments was relatively low based on the Monte Carlo simulation. This study provides a comprehensive solution for studying the environmental behavior and potential ecological risks of toxic metals in sedimentary environment.

The coupled effect of sediment resuspension and microbiota on phosphorus release and transformation in a simulated aquatic ecosystem

Plant microbial involvement in phosphorus (P) release and transformation during resuspension events in bottom aquatic systems remains poorly understood. Laboratory experiments were used in this study to explore the release and transformation of P from sediments and the microbiota involved. Dissolved and labile P were obtained by high-resolution dialysis and the diffusive gradients in thin films (DGT) technique, respectively. The bacterial community was investigated using a scanning electron microscope and 16S rRNA high throughput sequencing technique. Finally, the diffusion fluxes dynamics of phosphorus were calculated using the DGT-induced fluxes in sediments and soils (DIFS) model. These results showed that sustained disturbance can balance dissolved P concentrations in the resuspension layer (overlying water) and enhance the stability of P transformation and release processes. Notably, the bacterial community within the deeper sediment layer (Below 10 mm sediment depth) significantly influences P interception. Acidobacteria and Neobacteria were identified as the key bacterial genera responsible for phosphorus adsorption of plant roots at speeds of 150 rad/min (R150). In addition, as the sediment depths increased, a slower decrease in R curves fitted with the DIFS model, which indicated a continuous resupply process from solid phase to pore water, especially below R150. This study provides theoretical references for managing shallow lakes with frequent sediment resuspension.

Predicting the Kinetics of Resupply of Organic Pollutants from Sediments Using Diffusive Gradients in Thin Film Samplers and their Bioavailability to Aquatic Invertebrates.

The present study used diffusive gradients in thin film (DGT) samplers deployed in situ at a wastewater-impacted site (Clarkboro Ferry) for 20 days to develop a predictive model between time-weighted mean concentrations of seven selected antipsychotic compounds in water and those in resident benthic invertebrates, specifically crayfish (Faxonius virilis). The model was further combined with a model of desorption of antipsychotic compounds to predict kinetics at the sediment-water interface. Antipsychotic compounds were mostly detected in adult crayfish and internal concentrations were similar among targeted compounds, except for lesser concentrations of duloxetine. The model, based on the mass balance of organic chemicals, to predict uptake by organisms exhibited good agreement with measured values (R2 = 0.53-0.88), except for venlafaxine (R2 = 0.35). At the sediment-water interface, positive fluxes were observed for antipsychotic compounds and the results from DGT-induced fluxes in sediments (DIFS) coupled with equilibrium hydroxyl-β-cyclodextrin extraction further indicated partial resupply of antipsychotic compounds from sediments to the aqueous phase, despite the labile pool being relatively limited. The results of the present study affirm that DGT techniques can be used as a predictive tool for contamination in benthic invertebrates and can simulate the ability of contaminant resupply from sediments. Environ Toxicol Chem 2023;42:1696-1708. © 2023 SETAC.

Resupply, diffusion, and bioavailability of Hg in paddy soil-water environment with flood-drain-reflood and straw amendment

Mercury (Hg) poses a significant risk in paddy fields, particularly when it is converted to methylmercury (MeHg) and accumulates in rice. However, the bioavailability and resupply kinetics of Hg in the paddy soil-water environment are not well understood. In this study, the diffusive gradients in thin films (DGT) and the ‘DGT-induced fluxes in sediments’ model (DIFS) were first adopted to investigate the Hg resupply kinetics, diffusion fluxes and bioavailability in a paddy environment subjected to flood-drain-reflood treatment and straw amendment. Our results show that although the straw amendment limited the bioavailability of Hg (38.2%–47.9% lower than control) in porewater by decreasing its resupply capacity, especially with smaller straw particles, the net production of MeHg in paddy fields was significantly increased after straw amendment (73.5%–77.9% higher than control). The results of microbial sequencing indicate that enhanced methylators (e.g., family Geobacter) and non-Hg methylators (e.g., Methanosarcinaceae) played a crucial role in MeHg production following straw amendment. Moreover, Hg-containing paddy soils generally tend to release Hg into the overlying water, while drain-reflood treatment changes the direction of Hg diffusion fluxes in the paddy soil-water interface. The drainage-reflooded treatment decreases the Hg reactive and resupply capacity of the paddy soil, thereby hindering the release of Hg from soil into overlying water during the early stages of reflooding. Overall, this study provides novel insights into the behavior of Hg in paddy soil-water surface microlayers.

Diffusive gradients in thin films for transfer of phosphorus, nitrate and ammonium in lake sediments

There is a lack of the understanding of the transfer of phosphate (PO4-P), ammonium (NH4-N) and nitrate (NO3-N) at the millimeter scale in sediments related to internal loadings in the semi-arid and cold Lake Hulun in China. In this paper, a passive sampling technique-diffusion gradients in thin films (DGT) has been used for the biogeochemical behaviors and dynamic transfer of PO4-P, NH4-N and NO3-N in sediments in this lake. The two-sided DGT probe (AMP-TH&ZrO-Chelex) and ZrO DGT piston are designed for sediment/water interface. Based on change characters of DGT concentration profiles (nutrients, Fe and Mn) related to redox status and chemical properties, N and P mobilizations are derived. The reductive dissolution of Mn(IV) or Fe(III) (oxyhydr)oxides accounts for subsequent P release is corresponded with the oxic or anoxic status in sediment layers; Fe(III)-reduction is the main reason for P remobilization. The breakdown of algae biomass deposited on the uppermost sediment and phosphatase enzymes activity enhanced by NH4-N also lead to P remobilization. In sub-anoxic or anoxic layer, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) induce exchangeable ammonium (NH4ex) enrichment and NH4-N release, while, anammox depletes NH4-N mobility. Aerobic nitrification results in NO3-N remobilization. Organic matter favors NH4-N release. DGT induced fluxes in sediments (DIFS) model is used to derive the labile P pool (51.5–465.5 cm3 g−1), resupply constant (0.186–0.734), response time (54–11,070 s) and Dspt rate (32–382,517 nmol cm−3 d−1) for describing dynamic P transfers with two opposite types of “slow” and “fast” resupplies. The diffusive fluxes across sediment/water interface in ranges of 67.9–498.7 μg m−2 d−1 (PO4-P), −1416–106.8 (NH4-N) and –2147–301.1 μg m−2 d−1 (NO3-N) and DIFS parameters can be used for release risks in lake regions and the sediment remediation strategies are recommended.

Geochemistry and release risk for nutrients in lake sediments based on diffusive gradients in thin films.

A comprehensive understanding of the mobility of both nitrogen (N) and phosphorus (P) and the inter-relationships between P, N, and iron (Fe) in sediments is important for controlling the "internal loadings" of nutrients in lakes. In this research, diffusive gradients in thin film (DGT) assemblies with binding layers (ZrO-AT, chelex, and ZrO) were designed for PO4-P, Fe, ammonium (NH4-N), and nitrate (NO3-N) at sediment/water interface (SWI) in Western Lake Taihu (China). The biogeochemical processes of N and P related to the physicochemical properties, the dynamic P transfer, the distribution characteristics of P microniches, and the estimation of the release risks in sediments in Western Lake Taihu were simultaneously revealed by the passive sampling technique-DGT with the high spatial resolutions (millimeter and sub-millimeter). Based on DGT concentration (CDGT) related to physicochemical properties in sediments, (1) P biogeochemical reactions included P release from Fe-bound P during Fe reduction, algae biomass decomposition, and phosphatase enzyme activity increased by NH4-N; (2) denitrification and dissimilatory nitrate reduction to ammonium (DNRA) led to exchangeable ammonium (NH4ex) enrichment and NH4-N release; anammox depleted NH4-N transfer; organic matter (OM) mineralization favored NH4-N release; and (3) aerobic nitrification led to NO3-N remobilization; denitrification and DNRA reduced NO3-N release. Redox status, OM, Fe, aluminum, or calcium influenced mobilization of nutrients. The numerical model of DGT-induced fluxes in sediments was used for dynamic P transfers with resupply types ("slow" ~ "fast") controlled by labile P pool, resupply constant, response time, and Dspt rate. The formation of P microniches in two dimensions was revealed. Sediment P release risk index (0.49 ~ 36.85 [lg (nmol cm-3 d-1)]) with "light" ~ "high" risks and diffusive fluxes across SWI (µg m-2 d-1) of 15.0 ~ 639 (PO4-P), - 1403 ~ 5010 (NH4-N), and - 1395 ~ 149 (NO3-N) were derived and lake management strategies were provided. The DGT technique provides the characterization of the mobilization of nutrients and evidence for biogeochemical processes at the fine spatial scales for control of internal loadings in sediments.

Substantial increase in P release following conversion of coastal wetlands to aquaculture ponds from altered kinetic exchange and resupply capacity

The reclamation of wetlands and its subsequent conversion to aquaculture may alter regional nutrient (im)mobilization and cycling, although direct assessments of phosphorus (P) cycling and its budget balance following wetland conversion are currently scarce. Here, parallel field experiments were conducted to investigate and compare the availability and mobilization mechanisms of P from natural coastal wetlands and the adjacent converted aquaculture ponds based on high-resolution diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques and the DGT-induced fluxes in sediments (DIFS) model. The study found that the conversion of wetland to pond strongly reduced the sediment P pool by changing its forms and distribution. High-resolution data showed that concentrations of labile P and soluble reactive P across the sediment–water profiles were markedly enhanced by the converted aquaculture pond, although they exhibited large spatiotemporal heterogeneity. Moreover, the synchronous distribution of labile P, iron (Fe) and sulfur (S) across profiles in coastal wetlands indicated that the dissolution of Fe (III) oxyhydroxide–phosphate complexes coupled with sulfate reduction were the main mechanisms regulating sediment P mobilization in coastal areas. However, the converted aquaculture pond weakened or even reversed this dependence by decoupling the Fe-S-P reactions by changing the sediment structure and nutrient balance. Substantial increases in labile P, Fe and S fluxes in the pond suggested the conversion of wetland to aquaculture facilitated the internal release of P, Fe and S from sediment into water. The high resupply parameter (R) and desorption rate (k-1) combined with low response time (Tc) in the pond, as fitted by DIFS model, indicated the strong resupply capacity and fast kinetic exchange of sediment P across the sediment–water interface, which is consistent with the high P diffusion fluxes recorded in the pond. It was concluded that converted aquaculture ponds act as an important source of P release in coastal areas, potentially exacerbating water quality degradation and eutrophication. Specific initiatives and actions are therefore urgently needed to alleviate the internal P-loading during aquaculture.

Combining passive sampling with fraction transfer and toxicokinetic modeling to assess bioavailability of organic pollutants in a benthic invertebrate, Lumbriculus variegatus

This study investigated passive adsorption of nine antipsychotic drugs by the benthic oligochaete, Lumbriculus variegatus from spiked freshwater sediments. Based on a combination of diffusive gradients in thin-films (DGT) passive sampling and modelling, specifically the DGT-induced fluxes in sediments (DIFS) model and fractions-transfer models, an uptake and diffusion-induced model was established for oligochaetes. Lamotrigine and bupropion showed positive fluxes from sediment to overlying water. Oligochaetes accumulated both lamotrigine and carbamazepine. The simulation model for transfer showed that after 30 days antipsychotic compounds in sediment were present predominantly in stable or bound fractions, rather than labile fractions. Dissolved organic carbon and degradation were important drivers of dissipation rates of antipsychotic compounds in sediments and labile pool size. Uptake by oligochaetes and diffusion-induced models matched measured concentrations well ( R 2 = 0.83–0.95 and R 2 = 0.99–1.00, respectively). Processes of uptake by oligochaetes were comparable to DIFS results, where fast desorption to the labile pool in the short response time and adsorption processes, except carbamazepine and lamotrigine, in sediments dominated kinetics. It suggests a link between bioavailability of organic pollutants for sediment-dwelling organisms and desorption kinetics involved with fractions transfer within sediments, which has important implications for the environmental risk assessment of these chemicals. • Organic pollutants could be passively adsorbed by benthic worms. • Sediment porewater and DOC are the major sources for passive adsorption by benthic worms. • The contribution of worm passive uptake is < 1% from the dissipation in sediments.

The mechanistic investigation of geochemical fractionation, bioavailability and release kinetic of heavy metals in contaminated soil of a typical copper-smelter

Identifying the bioavailability and release-desorption mechanism of heavy metals (HMs) in soil is critical to understand the release risk of HMs. Simultaneously, the mechanistic investigation of affecting the bioavailability of HMs in soil is necessary, such as the grain-size distribution and soil mineralogy. Herein, the bioavailability of HMs (Cu, Cd, Ni, Pb, and Zn) in different area soils near a typical copper-smelter was evaluated by the sequential extraction technique (BCR), diffusive gradients in thin-films (DGT), and DGT-induced fluxes in sediments (DIFS) model. Results showed that the HMs proportion of the residual fraction in all soils was the highest. The average bioavailability concentration (CDGT) of Cu and Cd in industrial soil was the highest, with 45.12 μg· L−1 and 9.06 μg· L−1. The result of DIFS model revealed that the decreased order of the mean value of desorption rate constant (K-1) was Cd > Zn > Ni > Cu > Pb, 5.91 × 10−5, 4.96 × 10−5, 2.89 × 10−5, 9.64 × 10−6, and 8.69 × 10−6, respectively. According to the spatial distribution of release potential (R-value), the release potential of labile-Cu in agricultural soil was the highest, which was mainly attributed to fertilizer application in farmland. Simultaneously, the reduced hydroxyl was also related to the agricultural activities, resulting in the weakened adsorption capacity of HMs by soil. Redundancy analysis (RDA) results showed that the bioavailability of Cd, Ni, and Zn was mainly driven by soil pH, while the bioavailability of Cu and Pb was primarily driven by dissolved organic carbon (DOC). Meanwhile, carbonate minerals had a positive correlation with the bioavailability of Cd, Ni, and Zn, which could promote the release of HMs in mining soil as chemical weathering progresses. In conclusion, this study provides a structured method which can be used as a standard approach for similar scenarios to determine the geochemical fractionation, bioavailability, and release kinetics of heavy metals in soils.

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

Sequential extraction and in-situ diffusive gradients in thin films (DGT) techniques were used to determine phosphorus (P) fractions and high-resolution 2D fluxes of labile PDGT, Fe2+DGT, and S2−DGT in sediment systems. The diffusion fluxes were subsequently calculated for different scenarios. Dynamic diffusion parameters between solid sediment and solution were also fitted using the DIFS (DGT-induced fluxes in sediments) model. The results suggested that Fe-bound P (Fe-P) was the dominant pool which contributed to the resupply potential of P in the water–sediment continuum. Significant upward decreases of labile PDGT, Fe2+DGT, and S2−DGT fluxes were detected in pristine and incubated microcosms. This dominance indicated the more obvious immobilization of labile P via oxidation of both Fe2+ and S2− in oxidic conditions. Additionally, these labile analytes in the microcosms obviously decreased after a 30-day incubation period, indicating that water-level fluctuations can significantly regulate adsorption–desorption processes of the P bound to Fe-containing minerals within a short time. Higher concentrations of labile PDGT, Fe2+DGT, and S2−DGT were measured at the shallow lake region where more drastic water-level variation occurred. This demonstrates that frequent adsorption–desorption of phosphate from the sediment particles to the aqueous solution can result in looser binding on the solid sediment surface and easier desorption in aerobic conditions via the regulation of water levels. Higher R values fitted with DIFS model suggested that more significant desorption and replenishment effect of labile P to the aqueous solution would occur in lake regions with more dramatic water-level variations. Finally, a significant positive correlation between S2−DGT and Fe2+DGT in the sediment indicated that the S2− oxidization under the conditions of low water-level can trigger the reduction of Fe(III) and subsequent release of active P. In general, speaking, frequent water-level fluctuations in the lake over time facilitated the formation and retention of the Fe(II) phase in the sediment, and desorption of Fe coupled P into the aqueous solution when the water level was high.

Availability and diffusion kinetic process of phosphorus in the water–sediment interface assessed by the high-resolution DGT technique

Desorption of phosphorus (P) bound to iron-containing minerals (Fe-P) is a crucial component of the eutrophication process in lakes. However, the main process and regulation mechanism of iron (Fe) and sulfur (S) on P release is little known because of a lack of in situ high-resolution data. High-resolution measurement evidence is needed to assess the availability and diffusion kinetic process of P at the water–sediment interface. Soluble reactive phosphate (SRP), ferrous ion (Fe2+), and sulfide (S2−) fluxes through the water–sediment interface in a freshwater lake were detected using the novel double-sided diffusive gradients in thin films (DGT) technique. Different P forms in solid sediment were also measured using the sequential extraction procedure. The diffusion fluxes across the water–sediment interface and dynamic diffusion parameters between solid sediment and solution were calculated using the DGT-induced fluxes in sediments and soils (DIFS) model. There was a clear decrease of the SRPDGT, Fe2+DGT, and S2−DGT fluxes from ~5 cm sediment depth to the water–sediment interface. The significant positive correlation between SRPDGT and Fe2+DGT fluxes in the whole profile demonstrates that Fe-P was a vitally important source of labile P in the solution phase. The significant positive fluxes of SRPDGT and Fe2+DGT indicated upward diffusion from the sediment particle toward the overlying water. This process further indicated the desorption and resupply of SRP and Fe2+ from the solid sediment phase and the synergistic effect between these two parameters. In addition, a gentler decline of R curves fitted with the DIFS model was found as the sediment depths increased, which suggesting the continuous resupply process from solid phase to pore water, especially under anaerobic conditions. The novel DGT technique in combination with DIFS analysis confirmed the considerable remobilization and transport capacity of labile P fractions including loosely adsorbed MgCl2-P and reductive Fe-P. These pools can diffuse from sediment particles to the interstitial and the overlying water, and can be further assimilated by organisms in shallow lacustrine ecosystem.

Application of DGT/DIFS combined with BCR to assess the mobility and release risk of heavy metals in the sediments of Nansi Lake, China.

The heavy metal contamination of the aquatic ecosystem is still prevalent even after reduction of the external anthropogenic inputs of the metals. The release of labile heavy metals from the sediments into the water is a potential risk, responsible for the contamination of the aquatic system. Herein, samples of sedimentary column cores were collected in Nansi Lake, and the distribution profiles of the labile and soluble metals (Cd, Cu, Ni, Pb, and Zn) were obtained by the diffusive gradient in thin films (DGT) and the high-resolution dialysis (HR-peeper) technique. Furthermore, the mobility, bioavailability and release risk of the heavy metals were assessed using the results of geochemical sequential extraction, DGT as well as the DGT-induced fluxes in sediments (DIFS) model. The results showed that the profile characteristics of the DGT-labile and soluble heavy metals showed irregular distribution in the sediment cores and Cd, Pb, Zn had an obvious positive correlation with Fe/Mn (p < 0.05). Ni, Cu, and Zn existed primarily in the residual fraction (accounting for 58-76%), while Cd and Pb existed in the reducible fraction (accounting for 50-67%). The Cd and Ni (0.027-0.185) had higher mobility coefficients compared with Pb, Cu, and Zn (0-0.011), and positive diffusive fluxes also proved that Cd and Ni were easy to be released from the sediments. In addition, the R values of five metals (0.18-0.85) ranged between Rdiff to 0.95, indicating that all the metals had partially sustained case from the sediments solid phase. Based on the DIFS model, the five metals had weak mobility from the sediment to pore water, but the release risks in the Nansi Lake should also be of concern, especially for the highly mobile Cd and Ni in the Dushan Lake.

High resolution evidence of iron-phosphorus-sulfur mobility at hypoxic sediment water interface: An insight to phosphorus remobilization using DGT-induced fluxes in sediments model

The deterioration of reservoirs in southern China due to the kinetics of Iron (Fe), Phosphorus (P) and sulphide (S) at the sediment-water interface (SWI) is a major problem that needs urgent attention. Studies on the biogeochemistry of Fe, P, and S using high-resolution profile techniques in reservoirs in this region are limited. The diffusive gradient in thin films (DGT) technique, high-resolution dialysis, DGT-computer imaging densitometry (CID), DGT-induced fluxes in sediments (DIFS) and planar optode (PO) device were used to describe the dynamics Fe-P-S in SWI during hypoxia. The results showed the release of Fe-P-S in SWI was due to sulfate reduction and iron reduction influenced greatly by hypoxia. Positive apparent fluxes were recorded indicating that the sediments release Fe-P-S to the overlying water. High positive correlations (r2 > 0.7) for DGT-labile Fe and DGT-labile P in sediments revealed that iron-bound P controlled the release of P at SWI during reductive dissolution. The low correlation between DGT-labile Fe and DGT-labile S (r2 < 0.4) disclosed the combative nature between sulfate reduction and iron reduction process. The low correlation occurred because of the co-precipitation between Fe and S, forming black materials such as monosulfide (FeS) and pyrite (FeS2) in a hypoxic environment. The DIFS model showed the resupply ability (R-values) of P in sediments belonged to the partially sustained case with a steady state case of resupply at TB3 (Tc = 1088s, Kd = 1005.61 cm3/g R = 0.72, K−1 = 0.19 day−1) and TB4 (Tc = 712 s, Kd = 712.53 cm3/g, R = 0.78, K−1 = 0.46 day−1). The resupply rate belonged to the non-steady state case at TB1 (Tc = 10,990 s, Kd = 396.3 cm3/g, R = 0.35, K−1 = 0.07 day−1) and TB2 (Tc = 6097 s, Kd = 578.5 cm3/g, R = 0.45, K−1 = 0.10 day−1). The DGT-CID-PO-DIFS provided a deep insight on the mechanism of Fe-P-S and remobilization of P at SWI leading to Blackwater events and eutrophication.

Mechanism of phosphorus mobility in sediments with larval (Propsilocerus akamusi) bioturbation.

The effects of chironomid larval (Propsilocerus akamusi) bioturbation on sediment phosphorus (P) mobility were studied over the course of 34days using the indoor larval cultivation method on in situ sediment cores. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to record fine-scale changes of soluble and DGT-labile P and iron (Fe) concentrations in the sediment. The larval-driven irrigation of the overlying water into their burrows significantly increased the oxygen penetration depth (OPD) and redox state (Eh) in sediments. In addition, the soluble and DGT-labile P and Fe decreased with the increase of OPD and Eh in larval-bioturbated sediments. The greatest decrease in the mean concentration of SRP, soluble Fe, and DGT-labile P in the Propsilocerus group was observed on Day 15 of the experiment, with a decrease by over half of the mean concentration of the control group. Furthermore, two-dimensional measurements of DGT-labile P concentration showed notable reductions of DGT-labile P around larval burrows. The DGT-induced fluxes in sediments (DIFS) model also exhibited a much longer response time (420s) and a much higher rate of P adsorption (0.002s-1) in the bioturbation sediments than those in the control sediments (116s and 0.009s-1, respectively). A significant correlation was shown for DGT-labile P and DGT-labile Fe. We conclude that Fe(II) oxidation and its enhanced adsorption were the major mechanisms responsible for the decrease of soluble and DGT-labile P in sediments.

Using diffusive gradients in thin films (DGT) and DGT-induced fluxes in sediments model to assess the dynamic release of copper in sediment cores from the Three Gorges Reservoir, China

As one of the largest man-made reservoir, the Three Gorges Reservoir (TGR) brings great uncertainty and challenges regarding trace metal (e.g., Cu) remobilization in the sediment. Therefore, in this study, sediment cores were collected from the mainstream of the Yangtze River and its three tributaries in the TGR, with a focus on evaluating the Cu remobilization risk and release dynamics using conventional methods, diffusive gradients in thin films (DGT), and the DGT-induced fluxes in sediments (DIFS) model. The results showed that although total Cu concentrations were slightly higher than background values, Cu was mainly observed in the unreactive residual fraction. Additionally, assessment of sediment quality guidelines and the risk assessment code indicated low Cu contamination risk for all sampling sites. However, the results of DGT measurements at the sediment-water interface showed efflux of Cu from sediment to the overlying water at most sites, except for the upstream of the Meixi River and the mainstream of the Yangtze River. Interestingly, diffusion fluxes at the three tributaries displayed an increase trend from the upper to lower reaches. The DIFS model simulation further implied that the highest resupply capacity and desorption rate occurred in the Zhuyi River and the upstream of the Meixi River. In fact, the accumulation of Cu from the upper to the lower reaches of the Zhuyi River significantly elevated the Cu resupply capacity. Thus, more attention should be paid to Cu mobilization in the TGR, especially in the Zhuyi River and the upstream of the Meixi River.

A novel method for evaluating the potential release of trace metals associated with rainfall leaching/runoff from urban soils

The release of pollutants in soils owing to rainfall is a major challenge related to urbanization. Here, urban soils from different functional zones were collected to evaluate the release risk and estimate their annual release amounts of trace metals (Co and Ni) using multiple techniques, including diffusive gradients in thin-films (DGT), DGT-induced fluxes in sediments (DIFS) model, and Fourier-transform infrared (FTIR) spectroscopy. The results indicate that the average concentration of Co (6.55 mg kg−1) was slightly lower than that of the local soil background, whereas for Ni, the trend was reversed. Risk assessments based on total concentrations show that the soil samples were uncontaminated with Co while uncontaminated to moderately contaminated with Ni. However, the mobility coefficients indicate that Co posed low to medium risk, while Ni posed low risk. Hence, further investigation of DGT measurements and DIFS model show that the DGT-measured Co and Ni concentrations were lower than the corresponding concentrations in solution, leading to low R values and the partial resupply of Co and Ni from the solid phase. The FTIR spectra and elemental analysis suggest that because of the electrostatic attraction and complexation of the hydrosilicate minerals and organic matters, the metal resupply was restrained, resulting in them being only a partial resupply. Moreover, the mobility of Co was mainly controlled by the resupply ability, response time, and desorption rate; while for Ni, the particle concentration and porosity played important roles in determining mobility. In addition, the release amounts of the trace metals were quantified using the binary mixing equation. The estimated annual release amounts of Co and Ni in Pingshan District were 0.44–3.54 t and 1.93–16.47 t, respectively. This study provides an effective in-situ method for estimating the release amounts of trace metals in soils during rainfall combining DGT and DIFS model.

Effects Of Short-Term Aerobic Conditions On Phosphorus Mobility In Sediments

In this study, the effects of short-term aerobic conditions on phosphorus (P) mobility in sediment were investigated in the eutrophic lakes. To accomplish this, an indoor cultivation system was established and short-term cultivation (3, 8 and 15 days) was conducted under control and aerobic treatments. High-resolution diffusive gradients in thin films (DGT) and dialysis (HR-Peeper) techniques were used to determine the mobility patterns of P in sediments. The results showed that: (1) Aerobic treatment significantly increased oxygen concentrations (maximum 17.1%) in overlying water within 15 days, with no significant effects on sediment oxygen penetration depth or redox potential. Additionally, aerobic treatment inhibited soluble reactive phosphorus (SRP) and DGT-labile P release from sediment to overlying water. (2) Aerobic treatment decreased the pore water SRP and soluble Fe(II) on day 3 with no impact on sediment SRP/DGT-labile P and soluble/DGT-labile Fe on day 8 and 15. (3) Model simulation using DGT Induced Fluxes in Sediments (DIFS) showed that the pore water SRP replenishment ability of sediment particle P was lower in the aerobic group than the control group, and the sediment system equilibration time was elongated after pore water SRP decreased in the aerobic group on day 3. (4) On day 3, SRP and soluble Fe(II), DGT-labile P and DGT-labile Fe showed significant positive correlations (r > 0.888, p < 0.001), indicating that reduced pore water SRP concentration was related to decreased pore water soluble Fe(II).

Phosphorus availability changes in acidic soils amended with biochar, fly ash, and lime determined by diffusive gradients in thin films (DGT) technique.

The lime, biochar, and fly ash are widely used to ameliorate acidic soils. The purpose of our pot experiment was to understand whether these soil amendments affected the availability of phosphorus in soils using diffusive gradients in thin films (DGT) technique. Three soil amendments, biochar (B), fly ash (F), and lime (L), were applied alone or combined to Ultisol and Alfisol at the rate of 40, 40, and 6g/kg, respectively. The DGT and chemical extraction techniques were used to quantify the soil available (phosphorus) P dynamic in the rice growth period. The addition of amendments can effectively increase the phosphorus content extracted by 0.01M KCl solution. The biochar amendment showed the best effect on increasing the contents of DGT-P and Olsen-P in the soil. Combined with the DGT-induced fluxes in sediments (DIFS) model, it was found that soil amendments mainly increased the adsorption rate (k) and desorption rate (k-1), and reduced soil reflect time (Tc) of phosphorus released to the pore water. The P contents accumulated in rice grains correlated more closely with the effective P concentration measured by DGT technique, compared with those with soil total P, pore water P, and Olsen-P. These results suggested that the DGT technique had significant advantage over conventional chemical extraction techniques when assessing the bioavailability of phosphorus to rice growth. Application of soil amendments increased the contents of Al-P and Ca-P of both Ultisol and Alfisol, and decreased the content of Fe-P in soils. The addition of amendments could change the affinity of Fe and phosphorus decreasing the phosphorus fixation in soils. The increase of available phosphorus in soil is mainly due to the change of pH value and the direct diffusion of phosphorus in the amendments (biochar and fly ash). In conclusion, soil amendment can affect the availability and adsorption ability of phosphorus in soil, and biochar has the best effect on the availability of soil phosphorus.

Successful control of internal phosphorus loading after sediment dredging for 6 years: A field assessment using high-resolution sampling techniques

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6years, due to their geographical proximity (a line distance of approximately 9km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353mg/m2/d to 0.174, 1.58 and 0.048mg/m2/d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom.