Comprehensive evaluation of the distribution, transport and ecological risk of heavy metals in intra-urban river sediments using high-resolution techniques.

Determining the distribution trends, transport mechanisms, and ecological risks of heavy metals (HMs) in urban river sediments is essential for the government to conduct appropriate remediation work. In this study, we collected sediment cores from the Yayao Waterway in Foshan City, China. The vertical distribution profiles of dissolved and labile Fe, Mn, Cd, Zn, Cu, Cr, Ni, Pb, As, and Co in the sediments were obtained using the thin-film diffusive gradient (DGT) and high-resolution peeper (HR-Peeper) techniques. In addition, the transport rates, contamination levels, and ecological concerns of the HMs were evaluated using the European Community Bureau of Reference (BCR) sequential extraction technique, the DGT-induced sediment fluxes (DIFS) model, and multiple contamination evaluation metrics. The results showed that most of the DGT-labile HMs were associated with Fe/Mn (hydrogen) oxides, and in particular, Zn, Ni, and Cr showed a significant negative correlation with Fe/Mn (p < 0.001). Additionally, Cd had the highest bioavailability (89.17%), and its net diffusive flux at the sediment-water interface (SWI) was positive, which indicated a high release risk from the sediment. However, the R-value of Cd based on the DGT-induced sediment fluxes (DIFS) operation was extremely low, suggesting that although Cd had the biggest supply pool of releases, its release rate was slow. The majority of sampling sites had significantly higher total HM contents in the surface sediments than the background values. The HM contamination in the sediments originated from human activities, primarily from industrial enterprises and with a large contribution from both agricultural and domestic sources. The most polluted HM with the highest ecological danger was Cd, followed by Cu, Zn, Ni, and As when the results of the four pollution evaluation indicators were combined. Consequently, the risk of contamination by HMs in inner-city river sediments should receive more attention.

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.

Principles, applications, and limitations of diffusive gradients in thin films induced fluxed in soils and sediments.

The diffusive gradients in thin films (DGT) technique serves as a passive sampling method, inducing analyte transport and concentration. Its application is widespread in assessing labile components of metals, organic matter, and nutrients across various environmental media such as water, sediments, and saturated soils. The DGT devices effectively reduce the porewater concentration through irreversible binding of solutes, consequently promoting the release of labile species from the soil/sediment solid phase. However, the precise quantification of simultaneous adsorption and desorption of labile species using DGT devices alone remains a challenge. To address this challenge, the DGT-Induced Fluxes in Soils and Sediments (DIFS) model was developed. This model simulates analyte kinetics in solid phases, solutions, and binding resins by incorporating factors such as soil properties, resupply parameters, and kinetic principles. While the DIFS model has been iteratively improved to increase its accuracy in portraying kinetic behavior in soil/sediment, researchers' incomplete comprehension of it still results in unrealistic fitting outcomes and an oversight of the profound implications posed by kinetic parameters during implementation. This review provides a comprehensive overview of the optimization and utilization of DIFS models, encompassing fundamental concepts behind DGT devices and DIFS models, the kinetic interpretation of DIFS parameters, and instances where the model has been applied to study soils and sediments. It also highlights preexisting limitations of the DIFS model and offers suggestions for more precise modeling in real-world environments.

Mercury distribution, mobilization and bioavailability in polluted sediments of Scheldt Estuary and Belgian Coastal Zone.

In this study, the vertical distribution of mercury (Hg) in estuarine and marine sediment porewaters and solid phases was assessed by conventional and passive sampling techniques in the historically polluted Scheldt Estuary and Belgian Coastal Zone (BCZ). The Diffusive Gradients in Thin-films (DGT) measured labile Hg concentrations (HgLA) were mostly lower than the porewater Hg concentrations (HgPW), and they also presented different vertical distribution patterns. Still high Hg concentrations in the sediment solid phases, comparable to the historical ones, were observed. Even though pH, redox potential and dissolved sulfide concentration could influence the Hg biogeochemical behaviour, organic matter (OM) played a key role in governing Hg mobilization from sediment solid phase to porewater and in its partitioning between porewater and solid phase over depth. In the marine sediments, where OM had a marine signature, higher labile Hg concentrations in the porewater and faster resupply from the solid phase were observed. The DGT technique showed significant potential not only for the measurement of bioavailable Hg fractions in porewater, but also for the assessment of kinetic parameters governing the release of labile Hg species from the solid phase with the assistance of the DGT Induced Fluxes in Sediments (DIFS) model.

Insight into the availability and desorption kinetics of Se and Cd in naturally-rich soils using diffusive gradients in thin-films technique.

Cadmium (Cd) contamination of selenium (Se)-rich soils may jeopardize the nutritional benefits of Se-biofortified crops. This study employed diffusive gradients in thin-films (DGT) technique and DIFS (DGT-induced fluxes in soils) model to understand the interdependency and driving factors of Se and Cd distribution and desorption kinetics across 50 soils from south China with naturally elevated levels. DGT-labile Se was the highest (up to 2.66 µg L-1) in non-carbonate/shale-derived soils, while Cd was maximal (5.53 µg L-1) in carbonate-based soils, reflecting soil background concentrations and soil characteristics. Over one-third of the soils showed labile Se:Cd molar ratio below 0.7, suggesting Cd phytotoxicity risks. The DIFS-derived response times (Tc) and desorption rate constants (k-1) suggested that Se was resupplied to the soil solution faster than Cd in soils with higher pH and SOM level, but Se resupply was still restricted due to the rapid depletion of its labile pool. As the first study of Se and Cd release kinetics in soils, our results reveal dependence on soil parent materials, with low labile Se:Cd soils presenting greater Cd hazards. By elucidating Se and Cd lability and interactions in soils, our findings help to inform management strategies to balance reduced Cd risk with adequate Se availability.

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.

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.

Predictive and estimation model of Cd, Ni, and Zn bioaccumulations in maize based on diffusive gradients in thin films.

Consumption of maize contaminated with heavy metals such as cadmium, nickel, and zinc threaten human health. For situ measuring the bioavailability of heavy metals, the diffusive gradients in thin films (DGT) is superior to other traditional methods. It is also important to find a method for predicting heavy metal enrichment in maize based on the DGT method. In this study, field surveys were conducted in the main maize producing areas of Tianjin, China. Heavy metal concentrations in maize grains were predicted by coupling DGT with traditional extraction methods. The results show that coupling DGT with soil solution can significantly improve prediction accuracy (Cd-R2 = 0.908, Ni-R2 = 0.903, and Zn-R2 = 0.904). This indicated that DGT and soil solution were feasible predictors of heavy metal concentration in maize. The DGT induced fluxes in soil/sediment (DIFS) model was used to simulate the uptake process of heavy metals by DGT, and better reveal the desorption processes of heavy metals in soils. DIFS-based desorption processes were employed to characterize the resupply ability of heavy metals in soils. The coupling of DGT and DIFS parameters provided the best prediction accuracy in this study (Cd-R2 = 0.920, Ni-R2 = 0.928, and Zn-R2 = 0.908). Predictions are slightly weaker for Zn than for Cd and Ni (Cd-P < 0.01, Ni-P < 0.01, and Zn-P < 0.05). The reason is that the average resupply type of Cd and Ni in soil is partially sustained while Zn is resupplied via diffusion only. The desorption rate k-1 can excellently improve the prediction accuracy of DGT, which avoids the disadvantage that soil solution does not consider desorption. The coupling of DGT and DIFS parameters provides an accurate and reliable method for predicting heavy metal enrichment in maize.

New insight for the diffusion-resupply kinetics of Cr(VI) in contaminated soil using DGT/DIFS.

Chromium (Cr) is a widespread pollutant with high toxicity and mobility. However, the diffusion-resupply kinetics of Cr(VI) between the solid phase and solution in the soils remain unclear. Here, we quantified the contributions of the soil solution and solid phase to the diffusion-resupply process of Cr(VI) in the contaminated soils using the diffusive gradients in thin-films (DGT) and DGT-induced fluxes in soils model. Based on the solution extraction result, Cr(VI) was the main available Cr species in the contaminated soils. Comparing the two diffusion-resupply stages of the kinetic process, the potential hazards due to the resupply from the solid phase can reach 10.71-50.66 %, although the soil solution accounted for the largest proportion of the effective concentration of Cr(VI) (49.34-89.29 %), which was ignored in the traditional equilibrium method. The kinetic parameters can be used to interpret the dynamic process. The resupply ability of the solid phase was closely related to the response time (Tc). The longer Tc was consistent with the low desorption constant, indicating a kinetic limitation. The magnitude of the resupply from the solid phase was related to labile pool size of Cr(VI) and soil organic carbon content. This study established a new quantification method for assessing diffusion-resupply kinetics of Cr(VI) in the soil， indicating the underestimation of Cr(VI) risk based on the use of traditional equilibrium methods. Our data provided a scientific basis for ecological risk assessment, pollution prevention, surface- and groundwater control, and environmental governance in areas with Cr contaminated soil.

Biogeochemical behavior of P in the soil and porewater of a low-salinity estuarine wetland: Availability, diffusion kinetics, and mobilization mechanism.

Estuarine wetlands, which typically store large amounts of phosphorus (P), are experiencing increased salinity as well as changed environmental factors caused by rising sea levels. In this study, the seasonal dynamics of P speciation, availability, and biogeochemical couplings with iron (Fe)-sulfur (S) in soil and porewater were measured in a low-salinity estuarine wetland using in situ high-resolution diffusive gradients in thin films (DGT) and dialysis (HR-Peeper) techniques. The diffusion kinetics and resupply capacity of P from the soil phase to solution were simulated using a DGT-induced fluxes in soils (DIFS) model. The transition from freshwater to brackish wetlands reduced soil P pools and shifted to more recalcitrant speciation. The concentration of DGT-labile P across the soil-water profiles ranged from 0.002 to 0.039 (mean: 0.015) mg L-1, which increased with increasing salinity in both the field and mesocosm experiments. The distributions of labile and soluble P showed high heterogeneity across the profiles, and there were some sharp peak values below the soil-water interface (SWI), which significantly increased the concentration and lability of P. The strong coupling between labile P and Fe (S) provided direct evidence for the coexistence of iron reduction (IR) and sulfate reduction (SR) in the estuary, while IR might predominate in P mobilization in the brackish environment because of higher labile Fe concentrations and stronger Fe-P couplings. The diffusion fluxes of P were positive at both sites, demonstrating that the kinetics of P were from the soils to the overlying water. Higher R and k-1 values fitted in the DIFS model implied that a stronger resupply capacity and desorption rate and thus faster remobilization kinetics of P occurred with increasing salinity. Our findings indicated that increased salinity (even at low levels) can alter the desorption rate and resupply capacity of soil P in estuarine wetlands and accelerate P remobilization and release by regulating the IR and SR processes, thereby leading to the deterioration of water quality.

Desorption kinetics of antipsychotic drugs from sandy sediments by diffusive gradients in thin-films technique.

Dynamic processes of organic contaminants in sediments can have important toxicological implications in aquatic systems. The current study used diffusive gradients in thin-films (DGT) devices in sandy sediments spiked with nine antipsychotics and in field sandy sediments. Samplers were deployed for 1 to 30 days to determine the flux of these compounds to DGT devices and the exchange rates between the porewater and sediment solid phase. The results showed a continuous removal of antipsychotics to a binding gel and induced a mobile flux from the DGT device to the adjacent sediment solution. A dynamic model, DGT-induced fluxes in soils and sediments, was used to derive rate constants of resupply of antipsychotics from solid phase to aqueous phase (response time, Tc) and distribution coefficients for labile antipsychotics. The largest labile pool was found for lamotrigine and carbamazepine in spiked sediments. Carbamazepine, clozapine, citalopram, and lamotrigine were resupplied rapidly by sediments with Tc (25-30 min). Tc values of bupropion and amitriptyline were the longest (≈5 h), which exhibited slow desorption rates in sediments. In field sediments, high resupply was found for carbamazepine and lamotrigine, which did not show higher labile pool. The Tc values were obviously higher in the filed sediments (52-171 h). Although the adsorption process is dominant for most studied antipsychotics in both spiked sediments and field sediments, the kinetic resupply of antipsychotic compounds may not be accurately estimated by laboratory-controlled incubation experiments. More studies are needed to explore the mechanisms of desorption kinetics by using in situ DGT technique in the field.

A novel passive sampling and sequential extraction approach to investigate desorption kinetics of emerging organic contaminants at the sediment-water interface.

Forms of organic contaminants is an important driver of bioavailable fraction and desorption kinetics of pollutants binding to sediments. To determine fluxes and resupply of nine environmentally-relevant antipsychotic drugs, which are emerging pollutants that can have adverse effects on aquatic organisms, interface passive samplers of diffusive gradients in thin films (DGT) were deployed for 21 days, in situ at the sediment-water interface in submerged sandy riverbank sediments. At each deployment time, samples of sediment were collected and subjected to consecutive extraction of pore water, as well as rapidly-desorbing (labile), stable-desorbing, and bound residue fractions. Concentrations of antipsychotic drugs decreased with sediment depth with the greatest concentrations observed in the top 2 cm. Positive fluxes of antipsychotic drugs were observed from sediment to surface water. The dynamic fraction transfer model indicated that the labile fraction can be resupplied with a lag time (> 21 d). When results were further interpreted using the DGT-induced fluxes in soils and sediments (DIFS) model, partial resupply of antipsychotic drugs from sediment particles to porewater was demonstrated. Desorption occurred within the entirety of the observed 15 cm depth of sediment. Fastest rates of resupply were found for carbamazepine and lamotrigine. Size of the labile pool estimated by the DIFS model did not fully explain the observed resupply, while a first-order three-compartment kinetic model for the fast-desorbing fraction can be used to supplement DIFS predictions with estimations of labile pool size.

Application of DGT/DIFS to assess bioavailable Cd to maize and its release in agricultural soils.

Tianjin, as an important maize production region in China, has a long history of sewage irrigation resulting in the soil cadmium (Cd) contamination. In this study, single extractions of CaCl2 and HNO3, BCR sequential extraction and the diffusive gradients in thin films technique (DGT) were used to measure the bioavailable Cd content in soils. The Cd content in soil samples all exceeded the background values, with 14.3% and 33.3% of sites in the Baodi District (BDD) and Jinghai District (JHD) exceeding the risk control values, respectively. The average content of Cd in maize samples is lower than the pollution control values, which may be related to the higher pH (8.53) and organic matter (OM) content (15.01 g kg-1) in soils. Bioavailable Cd measured by DGT correlated well with Cd in maize grains (R2 =0.92). The DGT and DIFS model predicted the metals release from the agricultural lands, the total concentration of Cd in soil was relatively low, but the labile Cd in the soils has adequate metal release capability. This study shows that DGT is efficient in predicting Cd accumulation in grains from contaminated soils.

Assessment of human health risk due to lead in urban park soils using in vitro methods.

Beijing parks always have a large flow of local residents and tourists, and the soil Pb could threaten human health by incidental ingestion. Soil samples from eleven parks in Beijing were collected to assess the human health risk associated with Pb. Lead bioaccessibility in these parks ranged from 3.2 ± 0.4% to 12.1 ± 0.5% in the physiologically based extraction test (PBET) gastric phase and increased when approaching the city center. The chemical forms and soil properties (Fe, organic matter, and grain size) were important factors affecting the soil Pb bioaccessibility. The geo-accumulation index of Beihai Park (BH, near the city center) reached 1.3 ± 0.1 indicating moderate contamination. Lead health risk to children in BH should be of concern though its hazard quotient was below one. Results obtained from the Diffusive Gradients in Thin-films (DGT)-induced fluxes in the soils (DIFS) model showed that Pb-release in some parks farther from the city center was a "partially sustained case" (Rdiff < R < 0.95) indicating that soil particles could partially replenish effective Pb to the soil solution. A relatively higher desorption rate constant (k1) and shorter characteristic response time (Tc) were also found in these parks, indicating non-negligible release risk. Soil Pb based on the PBET method and DIFS model could provide a reliable reference to park managers for the health risk management of Pb pollution.

Diffusion kinetic process of heavy metals in lacustrine sediment assessed under different redox conditions by DGT and DIFS model.

Different fractions and variations of Mn, Co, Ni, Cu, Cd, Pb, Zn, and Fe in sediment via oxic and anaerobic treatments were investigated using BCR sequential extraction methods, DGT technique, and DIFS model. The results indicated that reducible fraction was the considerable pool apart from residual fraction, suggesting the high desorption potential of heavy metals. The high-resolution DGT measurement indicated that CDGT significantly rose after anaerobic condition and characterized by the relative high R value. Significantly increasing positive fluxes varying from 0.64 to 339.4 µg cm-2 s-1 except Ni suggested that apparent diffusion upward occurred over time from the sediment to the overlying water on anaerobic episode. High proportion of reducible Fe fraction and concurrent reduction of Fe(III) to Fe(II) during anaerobic condition were responsible for the increase of labile metals. The diffusion kinetic parameters including the equilibrium distribution coefficient (Kd), response time (Tc), and rate constant (k1 and k-1) were obtained using DIFS model. These parameters confirmed the partially sustained resupply capacity of heavy metals from solid sediment particle to pore water because of the considerable reducible fractions. Additionally, planar optode (PO) imaging approach demonstrated that low pH accompanied with decreasing dissolved oxygen (DO) concentration on anaerobic condition enhanced the release of labile metal fraction. Generally, anoxia facilitated the reduction of reducible fraction of heavy metals and further strengthened the desorption, resupply and diffusion in the aquatic ecosystems.

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.

Kinetic process of Cr(III) in contaminated soils characterized by diffusive gradients in thin films technique.

Trivalent chromium has historically been considered as an environmentally benign micronutrient due to its low mobility; however, its kinetic process in soil remains poorly understood. Here, the labile fraction and kinetics of Cr(III) in contaminated soils were explored using diffusive gradients in thin films (DGT) and the DGT-induced fluxes model. In contrast to the low mobility of Cr(III) in soils reported by the classic equilibrium partitioning method, we observed steady resupply from the solid phase through a dynamic process, wherein Cr(III) in the soils were maintained by an intermediate resupply rate with the R values at their maximum (Rmax). The resupply of Cr(III) was influenced by the kinetic parameters and soil properties: (i) the resupply ability (R-Rdiff) was influenced by pH and response time (Tc); (ii) Rmax, was controlled by pH, Tc, and the desorption rate (k-1); (iii) k-1 was influenced by pH and soil texture. This study presents the new information regarding the kinetics of Cr(III) in soils and demonstrates that Cr(III) is steadily resupplied by soil, which is not captured by equilibrium-based methods, furthering our insight of the geochemical behavior of Cr(III). This information was essential for understanding the toxicity of Cr and improving remediation.

Release risk assessment of trace metals in urban soils using in-situ DGT and DIFS model.

Urbanization and urban construction lead to entensive environmental deterioration. Trace metals in urban soils pose a threat to urban water bodies and local populations. However, the release ability of labile metals and their release risk in urban soils remains unclear. Here, soils were collected from different functional zones in the Pingshan District (PSD) of Shenzhen. Based on results of soil properties, total contents of trace metals, geochemical index (Igeo), and risk assessment code (RAC), diffusive gradients in thin films (DGT) and DGT-induced fluxes in soil (DIFS) model were further used to assess the release risk of trace metals in urban soils. The results showed that the average total concentrations of trace metals (As, Cr, Cu, Pb, and V) were higher than the local soil background values, implying that trace metals accumulated in urban soils. However, the distributions of labile metals determined by DGT were not similar to those of total metal concentrations. Except for As, urban soils from PSD sites exhibited "uncontaminated to moderately contaminated" levels based on the average values of Igeo. Moreover, the pollution and migration of Cu in urban soils are problematic as evidenced by the Igeo and RAC assessments. Release ability of Cu was assessed using parameters of DIFS model (i.e., bioavailability concentrations (CE), resupply ability (R), response time (Tc), desorption rate (k-1), and sorption rate (k1)). Residential areas showed high CE values for Cu, while the resupply ability was low. Furthermore, considering the influences of R, Tc, k-1, and k1, membership function value was used to re-calculate the order of CE in urban soils. The final results suggested that the agricultural zone exhibited the highest release risk among soils from various functional zones. Therefore, DGT and DIFS model should be effective tools to assess the release risk of trace metals in urban soils.

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.

Phosphorus availability and dynamics in soil affected by long-term ruzigrass cover crop.

The use of grasses as cover crops in the off-season of cash crops under no-till has been largely adopted. However, soil phosphorus (P) uptake was previously shown to be reduced when ruzigrass is introduced in the rotation, affecting the viability and sustainability of this cropping system. The objective of this study was to assess the effect of ruzigrass on soil P availability and desorption kinetics under different P fertilizer application rates. A long-term field experiment where soybean (Glycine max) has been grown in rotation with ruzigrass (Urochloa ruziziensis) or fallow for 10 years, with the application of 0, 13, and 26 kg ha-1 of P, was evaluated for two consecutive years. Soil P desorption kinetics was assessed using diffusive equilibrium (DET) and gradient in thin films (DGT) techniques, as well as the DGT-induced fluxes in soils model (DIFS). Microbial biomass P (MBP) was assessed to verify if soil solution P (PDET) was reduced due to immobilization by microorganisms. Ruzigrass reduced MBP and PDET especially when P fertilizer was applied. The concentration of labile P (PDGT) was also lower after ruzigrass than in fallow. The soil ability to resupply P to soil solution was lower after ruzigrass regardless of P rates due to a slower desorption in response to the perturbation imposed by DGT. Growing ruzigrass as cover crop in the soybean off-season decreases soil P availability regardless of P fertilizer application rates by fundamentally reducing P mobility and P resupply from soil solid phase into soil solution.