Application of a newly developed AC-DGT for predicting neonicotinoid insecticide (NNI) bioavailability in soils.

Phosphorus influence on arsenic bioavailability in soils and its toxicity to plants is widely recognized. This work compares competitive influence of P on As bioavailability in dry and flooded soils. Pot experiments were carried out in dry and flooded soils, respectively. Bioavailable As in soils was measured using diffusive gradients in thin films (DGT), soil solution concentration, and three single chemical extraction methods. P concentration at 50 mg/kg promoted wheat growth in dry soil. At concentrations above 50 mg/kg, P competition inhibited wheat growth and enhanced As toxicity. In flooded soil, the rice height and biomass decreased with the increase of P addition. P concentrations above 800 mg/kg were lethal to the rice. The content of As absorbed by wheat and rice roots as well as shoots increased with the increase of P concentration. The bioavailability of As in wheat- and rice-grown soils, determined by all methods, also increased with the increase of P concentration. The correlation analysis between the bioavailable As measured by the all three methods and the content of As in plants showed a significant positive correlation. The Pearson correlation coefficient for the DGT method was higher comparing to all other methods. DGT-induced fluxes in soils (DIFS) modeling further showed sharp decreases of Tc (the characteristic time to reach equilibrium between available solid As pool and soil solution As from DGT perturbation) and increases of desorption and adsorption rate constants (k1 and k−1) of As in P-amended soils, reflecting that the kinetic release of As from available solid As pools became much easy from P competition. P competition in both dry and flooded soils could significantly increase bioavailability of As and further increase its toxicity. Competition effect was more pronounced in flooded soil. DGT is a more accurate method for As bioavailability evaluation in both dry and flooded soils.

There has been increasing concern about the widespread occurrence and persistence of pesticides in the environment. Pesticides can transport among and between environmental compartments, causing pollution in water, soil and air, and posing potential risks to humans and the ecosystem. There is a need to study the fate and behaviour of pesticides in the environment. Over the last few decades passive sampling approaches have aroused attention in detecting pesticides, but they are still under development. In this thesis, the passive sampling technique of diffusive gradients in thin-films (DGT) was developed and validated for pesticides in water and soils for the first time. The DGT technique was developed for in situ measurement of 9 pesticides in water. The compounds were carefully selected to represent a wide range of properties and classes, so that the technique may have wider applicability in future. Two types of binding material (HLB and XAD 18) were used and compared. Laboratory testing was carried out with various controlled experiments. HLB showed higher binding capacity but with slower uptake than XAD 18. The principle of DGT was confirmed as the mass accumulated by DGT was inversely related to the thickness of diffusive layer and proportional to the deployment time. The performance of the DGT sampler was found to be independent of pH (4.7-8.2), dissolved organic matter concentration (<20 mg L-1) and ionic strength (0.01-0.25M). Several laboratory and field trials were conducted to confirm the usage of DGT for in situ measurement of pesticides in water and soils. DGT has great potential to be applied to trace organic contaminant studies in soils and sediments, but so far work research on this line has been very limited. DGT was therefore investigated for in situ measurement of atrazine (ATR) and its 5 metabolites in soils, and compared with other two approaches to predict bioavailability to maize and to assess the ATR degradation pathway. The results showed that DGT performed best in measuring the bioavailability of total ATR (ATR and its metabolites) to maize. Hydroxylation was the dominant degradation procedure during aging and maize growth in the test soils. This could be well characterized using DGT. DGT was also deployed in a group of aged soils with different pH, soil types and ATR contaminated levels, to explore the behaviour of atrazine in soils and its sorption/desorption. Soil properties had influence on the labile pool size (Kd) and re-supply capability of ATR (R), while aging affected the labile pool in some soils, but had only a slight influence on re-supply. The DIFS (DGT-induced fluxes in soil/ sediment) model was employed to further characterize the kinetics of desorption from the solid phase to the solution phase, this showed that desorption kinetics and the labile pool size commonly affected the re-supply. Owing to the frequently simultaneous occurrence of ATR and arsenic (As) in the environment, DGT equipped with precipitated ferrihydrite binding gel was deployed to investigate the effect of ATR on the availability of As in soils. The addition of ATR did not impact on the measurements of As availability in the test soils, in the concentration range (up to 50 mg kg-1 ) used. This research has demonstrated that DGT is an effective tool for measuring pesticides in soils and waters. It can be used for monitoring purposes, and in experiments designed to better understand pesticide fate, behaviour, availability and to help with assessment of their risk in the environment.

DGT and kinetic analyses differentiate Se and Cd bioavailability in naturally enriched paddy soils

Bioavailability and resupply dynamics of thallium in agricultural soils affected by lead-zinc mining: In-situ insights from DGT-DIFS modeling.

Prediction of heavy metal bioavailability in intact soil is important to manage soil pollution risks. We developed a regression model for representative Japanese soils to judge their potential vulnerability to cadmium (Cd) pollution. We added four rates of Cd to 17 sample soils to mimic artificial contamination. After aging the contaminated soils, we measured Cd’s bioavailability using the diffusive gradients in thin-films (DGT) technique. We then evaluated the relationships between bioavailability of Cd ([CdDGT]) and intact soil properties by statistical analyses. Cation exchange capacity (CEC) and pH emerged as significant factors to explain the cadmium bioavailability in Japanese soils. Specifically, lower CEC and lower pH were associated with higher [CdDGT], which poses a higher risk for soil ecosystems. The correlation between pH and [CdDGT] had a high dependence on [CdAdd], whereas that for CEC did not. Regression analysis also showed that the interaction between intact soil pH and spiked concentration ([CdAdd]) had a significant contribution to [CdDGT]. The regression model developed was rationally supported by a biotic ligand model. This simplified but realistic model would be useful in estimating the vulnerability of representative Japanese soils and determining the risk for Japanese soils in relation to Cd contamination.

Contamination of heavy metals (including the cadmium, Cd) in agricultural soils has become an increased issue, posing a threat to the crop safety and human health. In order to evaluate the contamination characteristics and bioavailability of Cd in the soil−crop systems from the East edge of the Dongting Lake, four kinds of agricultural products for typical crops (rice, peanut, sweet potato, and corn) and corresponding rhizosphere soils were collected and analyzed for the Cd concentrations. The technique of diffusive gradients in thin-films (DGT) was applied to evaluate the Cd bioavailability in the rhizosphere soils. Concentrations of Cd ranged from 0.04 to 2.95 mg/kg (average 0.24 mg/kg) with 73.9% sites above the background levels, especially for paddy soils. Cd concentrations in the agricultural products ranged from 0.01 to 2.19 mg/kg (average 0.18 mg/kg), with Cd enrichment observed in the peanut samples. No obvious correlations (R2 < 0.25) were observed between the Cd concentrations in the agricultural products and total Cd concentrations in the rhizosphere soils, this indicated that the total Cd concentrations in the soils cannot predict the concentrations in the agricultural products of crops. While the DGT measured Cd concentrations showed good correlations (R2 = 0.64−0.90) with the concentrations in the most agricultural products of crops, which may be used to evaluate the safety of the soil and further safety of the agricultural products of crops. Overall, DGT showed a good potential for prediction of heavy metal bioavailability in soil since the DGT technique can simulate the sustained supply of heavy metals from solid to liquid in the soils.

The theoretical basis for using measurements of metal uptake by the technique of diffusive gradients in thinfilms (DGT) to mimic processes in soils that affect uptake of metals by plants is examined. The uptake of metals by plants and DGT were compared conceptually and quantitatively by using the classic Barber model of plant uptake and the DIFS (DGT-induced fluxes in soils) model of uptake by DGT. For most metals and plants considered, uptake fluxes were similar to those induced by DGT using the most common gel layer thicknesses of 0.2 to 2 mm. Consequently DGT perturbs the chemical equilibrium of metals in the soil solution and between soil solution and solid phase, to a similar extent to plants, and therefore induces a similar balance in supply by diffusion and by release from the solid phase. DIFS was used to show that desorption kinetics, which are not considered by the plant uptake model, are likely important for uptake when the capacity of the soil solid phase is large. Model calculations showed that mass flow into a plant root would only contribute appreciably to the total flux of metal under circumstances when the solid phase reservoir of metal was very low. Generally, however, DGT is likely to emulate supply processes from the soil that govern uptake of metal by plants. Exceptions are likely to be found in poorly buffered soils (typically sandy and/or low pH), and at very high concentrations of metals in soil solution, such that the soil solution concentration at the plant root interface is higher than the Michaelis-Menten constant (Km).

Concentrations of Al, Fe, Mn, Ni, Cu, Cd, Pb, and Zn were measured using DGT (diffusive gradients in thin-films) devices deployed in situ in 34 headwater streams in Northern England. Mean values of filtered samples analyzed by ICP-MS (inductively coupled plasma mass spectrometry) were used, along with DOC (dissolved organic carbon), pH and major ions, to calculate the distribution of metal species using the speciation code WHAM. DGT-measured concentrations, [Me]DGT, of Zn and Cd were generally similar to concentrations in filtered samples, [Me]filt. For the other metals, [Me]DGT was similar to or lower than [Me]filt. Calculation of the maximum dynamic metal from the speciation predicted using WHAM showed that most of the lower values of [Cu]DGT could be attributed to the dominance of Cu-fulvic acid complexes, which diffuse more slowly than simple inorganic species. Similar calculations for Al, Pb, and Mn were consistent with appreciable proportions of these metals being present as colloids that are not simple complexes with humic substances. Differences between WHAM predictions and the measured [Ni]DGT indicated that WHAM used with the default binding parameters underestimates Ni binding to natural organic matter. Plots of [Me]DGT versus the ratio of bound metal to DOC provided slight evidence of heterogeneous binding of Pb and Cu, while results for Mn, Cd, and Zn were consistent with weak binding and complete lability.

The technique of diffusive gradients in thin films (DGT) has been suggested to sample an available fraction of metals in soil. The objectives of this study were to compare DGT measurements with commonly measured fractions of Zn in soil, viz, the soil solution concentration and the total Zn concentration. The DGT technique was used to measure fluxes and interfacial concentrations of Zn in three series of field-contaminated soils collected in transects toward galvanized electricity pylons and in 15 soils amended with ZnCl2 at six rates. The ratio of DGT-measured concentration to pore water concentration of Zn, R, varied between 0.02 and 1.52 (mean 0.29). This ratio decreased with decreasing distribution coefficient, Kd, of Zn in the soil, which is in agreement with the predictions of the DGT-induced fluxes in soils (DIFS) model. The R values predicted with the DIFS model were generally larger than the observed values in the ZnCl2-amended soils at the higher Zn rates. A modification of the DIFS model indicated that saturation of the resin gel was approached in these soils, despite the short deployment times used (2 h). The saturation of the resin with Zn did not occur in the control soils (no Zn salt added) or the field-contaminated soils. Pore water concentration of Zn in these soils was predicted from the DGT-measured concentration and the total Zn content. Predicted values and observations were generally in good agreement. The pore water concentration was more than 5 times underpredicted for the most acid soil (pH = 3) and for six other soils, for which the underprediction was attributed to the presence of colloidal Zn in the soil solution.

Extended application of diffusive gradients in thin films (DGT) in assessing arsenic bioavailability and human health risks in brownfield soils

BackgroundThe metals bioavailability in soils is commonly assessed by chemical extractions; however a generally accepted method is not yet established. In this study, the effectiveness of Diffusive Gradients in Thin-films (DGT) technique and single extractions in the assessment of metals bioaccumulation in vegetables, and the influence of soil parameters on phytoavailability were evaluated using multivariate statistics. Soil and plants grown in vegetable gardens from mining-affected rural areas, NW Romania, were collected and analysed.ResultsPseudo-total metal content of Cu, Zn and Cd in soil ranged between 17.3-146 mg kg-1, 141–833 mg kg-1 and 0.15-2.05 mg kg-1, respectively, showing enriched contents of these elements. High degrees of metals extractability in 1M HCl and even in 1M NH4Cl were observed. Despite the relatively high total metal concentrations in soil, those found in vegetables were comparable to values typically reported for agricultural crops, probably due to the low concentrations of metals in soil solution (Csoln) and low effective concentrations (CE), assessed by DGT technique. Among the analysed vegetables, the highest metal concentrations were found in carrots roots. By applying multivariate statistics, it was found that CE, Csoln and extraction in 1M NH4Cl, were better predictors for metals bioavailability than the acid extractions applied in this study. Copper transfer to vegetables was strongly influenced by soil organic carbon (OC) and cation exchange capacity (CEC), while pH had a higher influence on Cd transfer from soil to plants.ConclusionsThe results showed that DGT can be used for general evaluation of the risks associated to soil contamination with Cu, Zn and Cd in field conditions. Although quantitative information on metals transfer from soil to vegetables was not observed.

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

Chapter 16 Theory and applications of DGT measurements in soils and sediments

A diffusive gradients in thin-films technique for the assessment of bisphenols desorption from soils

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