An evaluation of ferrihydrite- and Metsorb™-DGT techniques for measuring oxyanion species (As, Se, V, P): Effective capacity, competition and diffusion coefficients

A new DGT technique based on nano-sized Mg2Al layered double hydroxides with DTPA for sampling of eight anionic and cationic metals

In this work, a new resin gel incorporated with layered double hydroxide nanoparticles modified with diethylenetriaminepentaacetic acid is developed for application in diffusive gradients in thin-film devices (abbreviated as LDHs DGT) to monitor eight anions and cations (such as Fe, Mn, Co, Ni, Cu, Cd, Pb, and As) in natural waters and soils. The accumulated anions and cations were quantitatively recovered by one-step elution using 0.5 mol·L^-1 HNO₃ with an optimized elution time of 30 min. The performance of the LDHs DGT was independent of solution pH (5-8) and ionic strengths (5-100 mmol·L^-1). The capacities of the LDHs DGT for Mn(II), Fe(II), Co(II), Ni(II), Cu(II), As(V), Cd(II), and Pb(II) individually are determined to be 202.9, 363.6, 246.9, 88.8, 99.5, 75.3, 159.8, and 671.7 μg·cm^-2. During the field deployments in a nature river, LDHs DGT measured concentrations of cations and anions were almost like those measured by the traditional sampling method (except Fe(II), Cd(II), and Co(II)). In addition, bioavailable Cd measured by LDHs DGT correlated well with Cd in rice grains (R² = 0.55), indicating that LDHs DGT is a reliable tool for assessing the risk of Cd.

Development of bromide-selective Diffusive Gradients in Thin-Films for the measurement of average flow rate of streams

Diffusive Gradients in Thin-Films (DGT) have traditionally been used to measure time-weighted average concentration in water. We tested whether Br^--DGT in combination with the trace-dilution flow rate method, could be used as a new approach for measuring water flow rate. A novel bromide selective DGT based on the Purolite Bromide Plus anion exchange resin (Br^--DGT) was developed, which provided environmental bromide concentrations comparable to grab samples. The Br^--DGT provided quantitative bromide concentrations at a range of pH, competing ion concentrations, and in synthetic natural solution. The uptake efficiency was 95.7 ± 3.4%, and the elution efficiency was 95.5 ± 4.7%. The absorption maximum/saturation point of each binding disk was 0.684 ± 0.001 mg. Bromide adsorption to the binding layer was linear to 44.1% of the total binding capacity, 0.302 mg. The determined diffusion coefficient through the agarose cross-linked polyacrylamide (APA) hydrogels was 1.05 × 10^-5 cm² s^-1 at 17.9 °C, temperature corrected to 25 °C was 1.29 × 10^-5 cm² s^-1. DGT flow rates were between -14.7 and 6.50% of the flow independently monitored flow rate (weir). In comparison, grab sample flow rates diverged by 5.52 to 58.9% from the weir flow rate.

Dissolved iodide in marine waters determined with Diffusive Gradients in Thin-films technique

For the first time, Diffusive Gradient in Thin-films (DGT) focuses on the inorganic iodine species iodate (IO₃^-) and iodide (I^-). A silver-doped Cl resin (AgdCl), which is known to selectively accumulate I^-, was used to make a binding gel. Laboratory investigations were designed to verify the suitability of the AgdCl-DGT method to measure the total I^- concentration in environmental waters. Total recovery of I^- was obtained using an elution solution containing 100 mmol L^-1 KCN. DGT validation experiments in 10 mmol L^-1 NaCl showed linear accumulation of I^- over time, contrary to IO₃^-, thus confirming the selectivity of AgdCl-binding gel. The AgdCl-DGT measurement of total I^- concentration was independent of pH (4.5-8.8) and was not impacted by the presence of bicarbonate (1-5 mmol L^-1). Finally, the performance of AgdCl-DGT samplers were tested in two continental waters and a synthetic seawater. The AgdCl-DGT samplers measured 27-33% of the total I^- concentration in the two continental waters up to 24 h of deployment time, whereas the AgdCl-DGT response retrieved the total I^- concentration in seawater up to 72 h (106 ± 7%). The difference in DGT response was attributed to the low ionic strength of the two continental waters, limiting the application of AgdCl-DGT method to media with higher ionic strength.

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 P_DGT, Fe²⁺_DGT, and S^2-_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 P_DGT, Fe²⁺_DGT, and S^2-_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 S^2- 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 P_DGT, Fe²⁺_DGT, and S^2-_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 S^2-_DGT and Fe²⁺_DGT in the sediment indicated that the S^2- 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.

Development and evaluation of a new colorimetric DGT technique for the 2D visualisation of labile phosphate in soils

A new colorimetric technique for the measurement of labile phosphate in soils using the diffusive gradients in thin films (DGT) technique was developed in this study. This technique can determine the mass of phosphate accumulated on the precipitated Zr-oxide based binding gel by forming the blue colour following the standard molybdate-ascorbic acid method. The optimal reaction temperature and coloration time were 20 °C (room temperature) and 26 min. After determining a well-fitted calibration equation, the technique was able to measure phosphate concentration up to 2.5 mg/L for 24 h deployment with a detection limit of 10.1 μg/L. Two-dimensional quantitative visualisation of phosphate diffusion in three phosphorus (P) fertilised soils were obtained using the colorimetric technique. The results from the colorimetric DGT technique were compared to the elution DGT technique and Colwell P extraction. The DGT techniques (colorimetric and elution) and Colwell P measurements demonstrated similar patterns of phosphate diffusion in soil. Both DGT techniques showed similar phosphate concentration along the concentric rings around the fertiliser application. A new, convenient, and fast DGT colorimetric technique was developed, and successfully used to measure the distribution of potentially available phosphate in soils. The new technique is less laborious than current techniques as it does not require any pre-treatment of the binding gel layers or heating during scanning, thus providing faster results. Therefore, the technique may be more suitable for in-field applications and can be used to investigate the in situ diffusion of potentially available phosphate from fertilisers, and relate this to the plant uptake of P.

Development and evaluation of a DGT sampler using functionalised cross-linked polyethyleimine for the monitoring of arsenic and selenium in mine impacted wetlands

Arsenic and selenium are both carcinogenic and their presence in fresh water has attracted the development of robust and accurate monitoring techniques. A new diffusive gradients in thin-films (DGT) sampler was developed and evaluated for the in situ measurements of arsenic and selenium. The binding layer was made from a mixture of sulphonated and phosphonated cross-linked polyethylenimine (SCPEI and PCPEI, respectively). The optimum ratio of a SCPEI and PCPEI resin mixture was determined. The DGT sampler was calibrated under laboratory conditions to determine the influence of sample turbulence, concentration and pH. The optimised DGT passive sampler was field deployed in a mine impacted dam for 12 days. Binding layer optimisation shows that the polymers had to be mixed in a specific ratio of 80% sulphonated and 20% phosphonated per 0.8 g of the resin mixture, in the loose polymer form. Embedding the resin mixture in agarose gel reduced the uptake of both arsenic and selenium dramatically. At sample pH 3.0 and 5.0, the DGT sampler did not show significant differences in uptake of the two elements during the 15 day deployment. The passive sampler had limited adsorption capacity and was found better suited for dilute solutions, with concentrations below 0.5 mg L^-1 of the target metals. This effect was more pronounced when exposed to dam water which had competing cations. Cations may have reduced the capacity by binding to the PEI backbone via the large number of amine groups. Nonetheless, these cations did not show linear uptake.

Elimination of interferences in the determination of platinum, palladium and rhodium by diffusive gradients in thin films (DGT) and inductively coupled plasma mass spectrometry (ICP MS) using selective elution

The analysis of platinum (Pt), palladium (Pd) and rhodium (Rh) in aquatic samples by the diffusive gradients in thin films (DGT) technique using chelating resins, specific designed for the accumulation of PGEs, namely Purolite S914, S920 and Italmatch Chemicals IONQUEST® MPX-317. may however, still be influenced by the accumulation of other elements such (Cu, Zn, Pb, etc.) which will be extracted simultaneously by the hot aqua regia extraction and interfere with the Inductively Coupled Plasma Mass Spectrometry (ICPMS) analysis of the Platinum Group Elements (PGEs). Selective extractions were investigated to release the interfering elements without loss of the Platinum Group Elements (PGEs) from the resin gels. . A rinse with deionized water removes over 95% of Sr and Rb and a second rinse with 0.05 mol L^-1 H₂SO₄ can be used to as a common eluent to remove an important fraction of the interfering elements from S920 and S914 without loss of PGEs but this results in loss of around 15% of the PGEs from MPX-317. It was shown that selective extractions can be used to remove specific interferences from each resin gel.

Impact of low ionic strength on DGT sampling with standard APA gels: Effect of pH and analyte

Only a limited and scattered knowledge is currently available on the conditions leading to the occurrence of sampling alteration at low ionic strength (<10^-3 mol L^-1) with DGT (diffusive gradients in thin films technique). In this study, the role of the pH and the charge of the analyte were comprehensively evaluated with DGT equipped with APA (polyacrylamide with agarose-derivative crosslinker) diffusive gels and ZrO or Chelex binding phases. The sampling of four cations (Cd^II, Cu^II, Ni^II and Pb^II) and two anions (As^V and Cr^VI) was compared for pH 4, 6 and 8 at common (10^-2 mol L^-1) and low (10^-4 mol L^-1) ionic strengths. Results showed that the sampling was modified at low ionic strength only in the most acidic condition (pH 4) for both anions and cations with an opposite incidence: cations' sampling was halved whereas anions' sampling was increased. Furthermore, cations sampling alteration was similarly reproduced using diffusion cell experiments, which requires only the APA gel, indicating that the binding layer does not participate in the low ionic strength effect. The intensity of DGT sampling modification was consistent with a prediction based on Donnan partitioning of analytes at gel/solution interface for several valences (from -I to + III). All these results strongly suggest that the APA diffusive gels carry positive charges that create a Donnan effect at low ionic strength. Since no ionic strength effect could be evidenced at pH 6 and 8, it can be reasonably assumed that this effect occurs only marginally for DGT deployments in most natural waters.

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 C_DGT 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 (K_d), response time (T_c), 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.

Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption

Development of novel adsorbents often neglects the competitive adsorption between co-occurring oxo-anions, overestimating realistic pollutant removal potentials, and overlooking the need to improve selectivity of materials. This critical review focuses on adsorptive competition between commonly co-occurring oxo-anions in water and mechanistic approaches for the design and development of selective adsorbents. Six "target" oxo-anion pollutants (arsenate, arsenite, selenate, selenite, chromate, and perchlorate) were selected for study. Five "competing" co-occurring oxo-anions (phosphate, sulfate, bicarbonate, silicate, and nitrate) were selected due to their potential to compete with target oxo-anions for sorption sites resulting in decreased removal of the target oxo-anions. First, a comprehensive review of competition between target and competitor oxo-anions to sorb on commonly used, nonselective, metal (hydr)oxide materials is presented, and the strength of competition between each target and competitive oxo-anion pair is classified. This is followed by a critical discussion of the different equations and models used to quantify selectivity. Next, four mechanisms that have been successfully utilized in the development of selective adsorbents are reviewed: variation in surface complexation, Lewis acid/base hardness, steric hindrance, and electrostatic interactions. For each mechanism, the oxo-anions, both target and competitors, are ranked in terms of adsorptive attraction and technologies that exploit this mechanism are reviewed. Third, given the significant effort to evaluate these systems empirically, the potential to use computational quantum techniques, such as density functional theory (DFT), for modeling and prediction is explored. Finally, areas within the field of selective adsorption requiring further research are detailed with guidance on priorities for screening and defining selective adsorbents.

Development and evaluation of the iron oxide-hydroxide based resin gel for the diffusive gradient in thin films technique

An ion-exchange resin Lewatit FO 36 was used for the preparation of a new resin gel for the diffusive gradient in thin films technique (DGT). The DGT method was optimized for the accumulation of four bioavailable arsenic species (As^III, As^V, monomethylarsonic acid, dimethylarsinic acid) in the aquatic environment. The total sorption capacity of Lewatit FO 36 resin gel was 535 μg As disc^-1. The microwave-assisted extraction in the presence of NaCl (10 g L^-1) and NaOH (10 g L^-1) was used for the isolation of arsenic species from the Lewatit FO 36 resin gel. The elution efficiency of arsenic was 98.4 ± 2.0%. Arsenic was determined by the optimized electrothermal atomic absorption spectrometry (ET-AAS) method using palladium modifier, pre-atomization cool-down step and tungsten carbides coating of graphite tube. The Lewatit FO 36 resin gel provides accurate results (c_DGT/c_SOL ratio 0.86-1.00) in the pH range 4-8. No significant influence of experimental conditions was observed in the presence of chlorides (0-0.5 mol L^-1) and humic acid (0-100 mg L^-1). Only a very high concentration of phosphates (10 mg L^-1) caused a slight decrease in the diffusion coefficients of MMA and As^V species (8.4% and 12.4%, respectively). The presence of iron (0-1 mg L^-1) caused a decrease in the diffusion coefficients, but with regard to the common concentrations of iron (less than 0.3 mg L^-1), the negative effect was considered not significant for As^III and DMA in natural water. The DGT-ET-AAS method was applied for the determination of bioavailable arsenic species in the spiked river water samples and also in-situ in the water reservoir. The new resin gel was characterized by a homogeneous gel structure with excellent reproducibility (< 5% variation of results between batches) and high sorption capacity which suggests its possible long-term application (up to 286 days in the environment with the arsenic concentration of 100 μg L^-1).

A novel Mg(OH)2 binding layer-based DGT technique for measuring phosphorus in water and sediment

Diffusive gradients in thin films (DGT) have gained wide attention for in situ measurement of reactive phosphorus species (PO4) in natural water, sediments and potentially soils. In this study, a novel Mg(OH)2 binding gel was formed using magnesium hydroxide obtained by in situ hydration of calcined magnesium oxide. Laboratory scale experiments showed that the novel Mg(OH)2 gel had a homogeneous dispersion of fine particles of Mg(OH)2 with a particle size of 2-5 μm. With 10 mL of 2.0 mol L-1 NaOH as the eluting agent, the optimal elution efficiency of PO4 on the Mg(OH)2 gel was 72 ± 5%. There were linear relationships between the accumulated PO4 mass and the applied PO4 concentration (0.1 to 20 mg P per L), time (0 to 24 h) and temperature (22 to 40 °C). The capacity of the Mg(OH)2 binding layer was determined to be 99.5 μg P per disc. Tests in synthetic seawater, Chaohu Lake and Yihai Pond confirmed that Mg(OH)2-DGT was able to accurately measure phosphorus up to 10 days. This was indicated by the good agreements between the concentrations measured by DGT (CDGT) technology and by an ex situ chemical method in solution (Csoln), with a CDGT/Csoln ratio between 0.91 and 1.09.

Evaluation of a mercapto-functionalized silica binding phase for the selective sampling of SeIV by Diffusive Gradients in Thin films

This study evaluates binding discs based on 3-mercaptopropyl-functionalized silica gel for the selective sampling of selenite (Se^IV) using Diffusive Gradients in Thin films sampler (DGT). Se^IV accumulation was quantitative and selective over Se^VI and followed the theoretical linear accumulation with the exposure time up to 0.7 µg. The sampling was not affected by ionic strength variations down to 10^-2 mol L^-1 (as NaNO₃) but Se^IV accumulation was found to decrease significantly for pH greater than 5 and was nearly zero at pH 9. Both the limited accumulation range and the pH dependence were unexpected because they have not been reported in the literature related to the Se^IV trapping by thiol-based solid phases. Our experiments showed that after Se^IV was bound to thiol functional groups, a further pH-dependent reaction occurred with free thiols, resulting in the reduction of Se^IV into elemental selenium (Se⁰) followed by its release and back-diffusion through the DGT sampler. Unfortunately, such a reversible accumulation is incompatible with the implementation of the mercapto-functionalized silica binding phase in DGT devices for Se^IV selective sampling.

Passive sampler measurements of inorganic arsenic species in environmental waters: A comparison between 3-mercapto-silica, ferrihydrite, Metsorb®, zinc ferrite, and zirconium dioxide binding gels

The performances of five Diffusive Gradients in Thin Films (DGT) binding gels, namely 3-mercapto-functionalized silica (3MP), ferrihydrite (Fh), Metsorb^®, zinc ferrite (ZnFe₂O₄)_, and Zirconium oxide (ZrO₂)_, were evaluated for in situ determination of As speciation in water and sediments. A combination of batch experiments at various pH (without addition of buffers) and in the presence of reduced species (Mn²⁺, Fe²⁺ and HS^-),time-series experiments in oxic waters, and in situ deployment in anoxic river sediments has permitted to evaluate the potential interferences among the binding gels. Firstly, the efficiency of each DGT binding gel dedicated to total As (i.e., Fh, Metsorb®, ZnFe₂O₄ and ZrO₂) or As(III) (i.e., 3MP) determination confirms that the determination of As species is possible in oxic freshwater and seawater over 96 h for a wide range of pH (5-9). Secondly, concerning the deployment in river sediment, high HCO₃^- concentrations have a little negative effect only on the DGT performances of the iron(III)-binding gels (i.e, Fh and ZnFe₂O₄). Thirdly, the presence of sulfides does not show any effect on the DGT uptake of As, but strongly affects the elution factor parameter. Discrepancies in elution between the different binding gels potentially result in precipitation of orpiment, especially in 1 mol L^-1 HNO₃. A correction of the classical elution factor derived from batch experiments was applied to provide more representative results. Finally, this study shows the difficulties to determine As speciation in anoxic sediments, and suggests that corrections of the elution factor may be required as a function of the species present in the deployment matrices.

Development of cerium oxide-based diffusive gradients in thin films technique for in-situ measurement of dissolved inorganic arsenic in waters

A diffusive gradients in thin films (DGT) method using a new high-capacity cerium oxide (CeO₂) binding gel, for the first time, was developed for measuring dissolved inorganic arsenic in freshwater and seawater. The capacities of the new CeO₂ binding gel were 682 μg and 375 μg for As^III and As^V, respectively. The masses of As^III and As^V accumulated by CeO₂-DGT device increased linearly with time and agreed well with the theoretical value calculated by DGT equation. The arsenic accumulation by CeO₂-DGT was independent of pH (4.05-9.04) and ionic strength (0.1-750 mM), and common anions including CO₃^2-, SO₄^2-, Cl^- and PO₄^3- had no obvious interference. CeO₂-DGT showed excellent long-term deployment performance in freshwater and synthetic seawater. Field trials with CeO₂-DGT achieved successfully the time-weighted-average concentrations of total inorganic arsenic in reservoir water (1.38 ± 0.09 μg/L) and coastal seawater (0.45 ± 0.06 μg/L). The results were comparable to those measured by grab sampling. The proposed method was reliable and robust for in-situ measurements of dissolved inorganic arsenic in environmental waters.

The impact of dam flushing event on dissolved trace elements concentrations: Coupling integrative passive sampling and discrete monitoring

Sediments accumulation in reservoirs induces water storage capacities reduction and flood risks increases rendering dam flushing or dredging events compulsory for security reasons. Short transient events like dam flushing monitoring is still a great challenge because the suspended sediments and contaminants concentrations increases could occur over only few hours/days and cover tens of kilometres. Since 1942, 21 dam flushing events have been performed on the Upper Rhône River (from Lake Geneva in Switzerland to Lyon in France) in order to evacuate accumulated sediments behind the Verbois dam (Switzerland). We designed an original sampling strategy to assess the 2016 dam flushing event consequences on the spatio-temporal dynamics of dissolved trace elements concentration and to reveal how passive sampling monitoring (Diffusive Gradient in Thin films, DGT) could improve this evaluation. Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, As and Hg dissolved concentrations were monitored by discrete and passive sampling at 3 stations over 160 km downstream the Verbois dam. Since dissolved Fe, Cu, Cd, Cr, Pb, Zn and Hg concentrations did not show great variations during the flushing event, the DGT efficiency was not fully demonstrated for these elements. In contrast, a sharp increase of Mn, Ni, Co and As dissolved concentrations (up to 22 times) was recorded, resulting mainly from a release from resuspended sediment. The dissolved As increase was mainly caused by reduced arsenic (As^III) increase, even monitored 160 km downstream the Verbois dam. The DGT measurements were highly representative of trace elements concentrations and As speciation dynamics in comparison with discrete sampling. Although relatively high dispersion was highlighted for some elements DGT measurements during the flushing event, we showed that DGTs are robust and powerful time-integrative tools to monitor many trace elements more efficiently than discrete sampling during a short transient event on a large spatial scale.

In Situ Selective Measurement of SeIV in Waters and Soils: Diffusive Gradients in Thin-Films with Bi-Functionalized Silica Nanoparticles

The speciation of selenium (Se) controls its fate and behavior, determining both its biological and environmental activities. However, in situ monitoring of Se^IV presents a significant challenge due to its sensitivity to redox change. A novel diffusive gradients in thin films (DGT) technique containing mercapto-, amino-bifunctionalized SBA15 mesoporous silica nanoparticles was developed and evaluated in a series of laboratory and field deployment tests. The SBA-DGT exhibited a linear accumulation of Se^IV ( r² > 0.997) over a 72 h deployment, with negligible accumulation of Se^VI(<5%). Consistent prediction of Se^IV occurred within ionic strength and pH ranges of 0.1-200 mmol L^-1 and 3.6-8, respectively. Limits of detection of the SBA-DGT were 0.03 μg Se^IV L^-1, which is suitable for natural waters. Moreover, the properties of the bifunctionalized SBA15 enable it to be fabricated within ultrathin (0.05 mm) gel layers for use in conjunction with O₂ planar optode imaging. This new sandwich sensor technology with SBA-DGT was validated by mapping the two-dimensional distribution of Se^IV and oxygen simultaneously in rice rhizospheres. This study shows that SBA-DGT provides a selective measurement of Se^IV in situ, demonstrating its potential for both environmental monitoring and as a research tool for improving our understanding of Se biogeochemical processes.

Passive sampling monitoring of PAHs and trace metals in seawater during the salvaging of the Costa Concordia wreck (Parbuckling Project)

Passive sampling techniques were used for monitoring trace metals and polycyclic aromatic hydrocarbons (PAHs) in the seawater surrounding the Costa Concordia shipwreck (Isola del Giglio, Italy). The monitoring lasted two and a half years (2012-2014) and considered all four phases of the "parbuckling project": stabilisation of the wreckage, installation of steel caissons on both sides of the wreck, parbuckling, and refloating. Dissolved trace metals (Cd, Cr, Cu, Hg, Ni, Pb, and V) were measured with diffusive gradients in thin films (DGT), while freely dissolved PAHs were measured with semi-permeable membrane devices (SPMDs). Passive sampling allowed to detect very low concentrations of contaminants, and indicated significant differences among the sampling stations and among the different steps for salvaging the wreck. The results suggested that the main source of contamination was the heavy working vessel traffic at the disaster site, rather than the release of contaminants from the wreck.

In situ sampling and speciation method for measuring dissolved phosphite at ultratrace concentrations in the natural environment

Phosphite (P^+III) is of emerging chemical interest due to its importance within the global phosphorus cycle. Yet, to date, precise/accurate measurements of P^+III are still lacking due to the inherent analytical challenges linked to its instability/ease of oxidation and ultra-trace concentration. Here, we present the first in-situ sampling and speciation analysis method, for dissolved P^+III, using the diffusive-gradients-in-thin-films (DGT) technique, combined with capillary-column-configured-dual-ion-chromatography (CC-DIC). Method optimization of the DGT elution regime, to simultaneously maximize desorption efficiency and CC-DIC sensitivity, along with the characterization of diffusion coefficients for P^+III, were undertaken before full method validation. Laboratory-performance testing confirmed DGT-P^+III acquisition to be independent of pH (3.0-10.0) and ionic strength (0-500 mM). The capacity for P^+III was 45.8 μg cm^-2, while neither P^+V (up to 10 mg L^-1) nor As^+V (up to 1 mg L^-1) impacted the DGT-P^+III measurement. This novel method's functionality stems from the herein confirmed speciation preservation and double pre-concentration of P^+III, resulting in quantification limits as low as 7.44 ng L^-1 for a 3-day deployment. Applications of this method in various terrestrial/aquatic environments were demonstrated and simultaneous profiles of P^+III and P^+V across a sediment-water interface were captured at mm resolution in two contrasting redox-mesocosm systems.

As and Sb are more labile and toxic to water spinach (Ipomoea aquatica) in recently contaminated soils than historically co-contaminated soils

Elevated concentrations of As and Sb impact environmental quality and human health. In this study total and bioavailable As and Sb were measured from recently and historically contaminated soils and the phytotoxicity of these soils was evaluated with Ipomoea aquatica (35-d exposure from germination) using biomass, length of plant tissues and photosynthetic efficiency. As and Sb were both present within the soil (co-contaminated). The bioavailable As and Sb in soils were determined by a Sequential Extraction Procedure (SEP) and compared to total soil concentrations and bioaccumulation in the edible parts of I. aquatica. For both As and Sb, bioavailable concentrations increased proportionally with the total soil concentrations and greater bioavailability in recently contaminated soil was observed. Tissue dry mass and length drastically reduced with increasing total and SEP-bioavailable As and Sb soil concentrations. The total soil concentration was a less sensitive measure of the phytotoxicity of As and Sb than the bioavailable fraction. Shoot length was inhibited by 50% (EC50) at bioavailable As concentrations of 80-96 mg kg-1 in both recently and historically contaminated soils; however, bioavailable Sb EC50 for shoot length was achieved at lower bioavailable concentrations, 96 (42-219) and 12 (7-19) mg kg-1 in recently contaminated soils and historically contaminated soils, respectively. Shoot biomass was inhibited by 50% (EC50) at bioavailable As concentrations of 11 (4-30) and 49 (37-65) mg kg-1 in recently and historically contaminated soils, respectively whereas this occurred at much lower bioavailable Sb concentrations, 2-5 mg kg-1 in both recently and historically contaminated soils. Aging is important in contaminated soils, it decreases the lability of As and Sb and hence their bioavailability to agricultural plants, thus posing a lower risk of exposure of these metalloids to humans through agricultural plants grown in contaminated soils.

Evaluating the diffusive gradients in thin films technique for the prediction of metal bioaccumulation in plants grown in river sediments

The diffusive gradients in thin films (DGT) technique is a useful tool for assessing metal bioavailability in sediments. However, the DGT technique has not been used to predict metal bioaccumulation in plants grown in sediments in river systems. In this study, the DGT technique was evaluated for predicting metal bioaccumulation in Phragmites australis growing in contaminated sediments. In sediments with high levels of contamination, release of DGT-labile Cr, Zn, Cu, and Cd occurred, which resulted in high bioaccumulation of these metals in P. australis. Bioaccumulation of Cr, Cu, Zn, and Cd was strongly correlated with the metal concentrations in the sediments measured by the DGT technique. By contrast, the correlation between sediment content and bioaccumulation for As was weak. There were significant negative correlations between the content of Ni in the plant tissues and the contents of the other metals. Overall, the DGT technique provided predictions of metal bioaccumulation similar to those obtained using total metal measurements in multiple polluted sediment samples. Therefore, DGT analysis could be used for assessing heavy metal bioavailability, and metal bioaccumulation in P. australis was not all significantly correlated with the bioavailability concentrations of metals in river sediments.

Evaluation of the DGT technique for selective measurement of aluminium and trace metal concentrations in an acid drainage-impacted coastal waterway

The performance of DGT-Chelex, DGT-Metsorb and DGT-MBL (Chelex-Metsorb mixed binding layer) with open and restricted diffusive layers for trace metal (Al, Cd, Co, Cu, Mn, Ni, Pb, Zn) and oxyanion (As, Mo, Sb, V) measurements, was evaluated in four natural waters with different pH (range 3.29-7.81). In moderately acidic (pH ≈ 5) and circumneutral (pH ≈ 6.3) waters, all three binding layers measured relatively similar concentrations of Al, while in more alkaline waters (pH ≈ 8) DGT-MBL measured higher concentrations than the other two binding layers. The measurements of DGT-Chelex and DGT-MBL for Co, Cu, Ni, Pb and Zn, and DGT-Metsorb and DGT-MBL for As, Sb and V were within 82-119% and not statistically different (p > 0.05) over the pH range 5-8. Mn measurements by DGT-Chelex and DGT-MBL were quite similar (95%) at pH 6.3, while DGT-MBL measured higher concentrations than DGT-Chelex at other pHs. The ratios of measured concentrations with different diffusive layers (C_restricted/C_open) were between 0.78 and 1.12 for all binding layers and no statistical differences (p > 0.05) were observed, except for Al at pH 7.81 and Cu at pH 6.28. DGT-MBL was comparable to DGT-Chelex for the measurement of most trace metals, and to DGT-Metsorb for the measurement of most oxyanions, over the pH range 5.05-7.81. Overall, DGT-MBL is superior to the other tested binding layers because it can simultaneously measure cations and anions, and accurately measure dissolved Al, across the greatest range of environmental conditions.

Determining time-weighted average concentrations of nitrate and ammonium in freshwaters using DGT with ion exchange membrane-based binding layers

Commercially-available AMI-7001 anion exchange and CMI-7000 cation exchange membranes were utilised as binding layers for DGT measurements of NO₃-N and NH₄-N in freshwaters. These ion exchange membranes are easier to prepare and handle than DGT binding layers consisting of hydrogels cast with ion exchange resins. The membranes showed good uptake and elution efficiencies for both NO₃-N and NH₄-N. The membrane-based DGTs are suitable for pH 3.5-8.5 and ionic strength ranges (0.0001-0.014 and 0.0003-0.012 mol L^-1 as NaCl for the AMI-7001 and CMI-7000 membrane, respectively) typical of most natural freshwaters. The binding membranes had high intrinsic binding capacities for NO₃-N and NH₄-N of 911 ± 88 μg and 3512 ± 51 μg, respectively. Interferences from the major competing ions for membrane-based DGTs are similar to DGTs employing resin-based binding layers but with slightly different selectivity. This different selectivity means that the two DGT types can be used in different types of freshwaters. The laboratory and field experiments demonstrated that AMI-DGT and CMI-DGT can be an alternative to A520E-DGT and PrCH-DGT for measuring NO₃-N and NH₄-N, respectively, as (i) membrane-based DGT have a consistent composition, (ii) avoid the use of toxic chemicals, (iii) provided highly representative results (C_DGT : C_SOLN between 0.81 and 1.3), and (iv) agreed with resin-based DGTs to within 85-120%.

Assessing cadmium and vanadium accumulation using diffusive gradient in thin-films (DGT) and phytoplankton in the Churchill River estuary, Manitoba

Diffusive gradient in thin films (DGT) and phytoplankton communities were evaluated for the measurement of Cd and V at environmentally relevant concentrations in laboratory settings and in the Churchill River estuary (Manitoba, Canada) during an annual spring melt. Despite rapid changes in hydrology and water quality, DGT samplers and intracellular Cd and V concentrations were positively correlated (0.79 < r(2) < 0.99), suggesting comparable accumulation trends between both DGT-labile and intracellular monitoring techniques. The largest accumulated concentrations of both Cd and V by DGT and phytoplankton accumulation methods were found later into the river discharge period. In controlled settings, accumulated Cd and V concentrations by the diatom Attheya septentrionalis displayed a strong correlation with metals accumulated by DGTs (r(2) > 0.99). Principal component analysis (PCA) reinforced similarities between both metal monitoring techniques and assessed how changing environmental variables during the river discharge period influenced each monitoring technique. Cd accumulation was influenced by DOC concentrations and protein-like DOM whereas ionic strength (i.e. conductivity) and humic-like DOM influenced V accumulation. The present findings suggest that (1) DGT is a versatile tool for monitoring bioaccumulation of Cd and V in highly dynamic environmental systems and (2) DOC concentration, DOM composition, conductivity, pH, and river discharge influence the bioavailability of Cd and V in estuarine and riverine waters.

High-resolution measurement and mapping of tungstate in waters, soils and sediments using the low-disturbance DGT sampling technique

Increasing tungsten (W) use for industrial and military applications has resulted in greater W discharge into natural waters, soils and sediments. Risk modeling of W transport and fate in the environment relies on measurement of the release/mobilization flux of W in the bulk media and the interfaces between matrix compartments. Diffusive gradients in thin-films (DGT) is a promising passive sampling technique to acquire such information. DGT devices equipped with the newly developed high-resolution binding gels (precipitated zirconia, PZ, or ferrihydrite, PF, gels) or classic/conventional ferrihydrite slurry gel were comprehensively assessed for measuring W in waters. (Ferrihydrite)DGT can measure W at various ionic strengths (0.001-0.5molL(-1) NaNO3) and pH (4-8), while (PZ)DGT can operate across slightly wider environmental conditions. The three DGT configurations gave comparable results for soil W measurement, showing that typically W resupply is relatively poorly sustained. 1D and 2D high-resolution W profiling across sediment-water and hotspot-bulk media interfaces from Lake Taihu were obtained using (PZ)DGT coupled with laser ablation ICP-MS measurement, and the apparent diffusion fluxes across the interfaces were calculated using a numerical model.

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

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

Novel speciation method based on Diffusive Gradients in Thin Films for in situ measurement of uranium in the vicinity of the former uranium mining sites

The Diffusive Gradients in Thin Films (DGT) technique using PIWBA resin (The Dow Chemical Company) was developed and validated for the measurement of uranium (U) concentration in natural and uranium mining influenced waters. The U uptake on the PIWBA resin gel was 97.3 ± 0.4% (batch method; Vsol = 5 mL; [U] = 20 μg L(-1); 0.01 M NaNO3; pH = 7.0 ± 0.2). The optimal eluent was found to be HNO3conc/70 °C with an elution efficiency of 88.9 ± 1.4%. The laboratory DGT investigation demonstrated that the PIWBA resin gel exhibits a very good performance across a wide range of pH (3-9) and ionic strength (0.001-0.7 M NaNO3) at different time intervals. Neither effect of PO4(3-) (up to 1.72 × 10(-4) M), nor of HCO3(-) (up to 8.20 × 10(-3) M) on the quantitative measurement of uranium by DGT-PIWBA method were observed. Only at very high Ca(2+) (2.66 × 10(-4) M), and SO4(2-) (5.55 × 10(-4) M) concentration, the U uptake on DGT-PIWBA was appreciably lessened. In-situ DGT field evaluation was carried out in the vicinity of three former uranium mining sites in France (Loire-Atlantique and Herault departments), which employ different water treatment technologies and have different natural geochemical characteristics. There was a similar or inferior U uptake on DGT-Chelex(®)-100 in comparison with the U accumulation on a DGT-PIWBA sampler. Most likely, the performance of Chelex(®)-100 was negatively affected by a highly complex matrix of mining waters. The high concentration and identity of co-accumulating analytes, typical for the mining environment, did not have a substantial impact on the quantitative uptake of labile U species on DGT- PIWBA. The use of the polyphenol impregnated anion exchange resin leads to a significant advancement in the application and development of the DGT technique for determination of U in the vicinity of the former uranium mining sites.

Influence of dissolved organic matter on dissolved vanadium speciation in the Churchill River estuary (Manitoba, Canada)

Diffusive gradients in thin films (DGT) devices were used to investigate the temporal and spatial changes in vanadium (V) speciation in the Churchill estuary system (Manitoba). Thirty-six DGT sets and 95 discrete water samples were collected at 8 river and 3 estuary sites during spring freshet and summer base flow. Dissolved V concentration in the Churchill River at summer base flow was approximately 5 times higher than those during the spring high flow (27.3 ± 18.9 nM vs 4.8 ± 3.5 nM). DGT-labile V showed an opposite trend with greater values found during the spring high flow (2.6 ± 1.8 nM vs 1.4 ± 0.3 nM). Parallel factor analysis (PARAFAC) conducted on 95 excitation-emission matrix spectra validated four humic-like (C1C4) and one protein-like (C5) fluorescent components. Significant positive relationship was found between protein-like DOM and DGT-labile V (r = 0.53, p < 0.05), indicating that protein-like DOM possibly affected the DGT-labile V concentration in Churchill River. Sediment leachates were enriched in DGT-labile V and protein-like DOM, which can be readily released when river sediment began to thaw during spring freshet.

In situ high-resolution evaluation of labile arsenic and mercury in sediment of a large shallow lake

The precise evaluation of arsenic (As) and mercury (Hg) bioavailability in sediment is crucial to controlling As and Hg contamination, but traditional ex situ measurements hamper comprehensive analysis of labile As and Hg in sediment. In this study, we characterized in situ labile As and Hg in sediment of Lake Hongze using the zirconium (Zr) oxide diffusive gradients in thin films (DGT) technique and 3-mercaptopropyl functionalized silica gel DGT, respectively. The concentrations of DGT-labile As and Hg in the sediment profiles were found to exhibit considerable variation, ranging from 0.15 to 4.15 μg L(-1) for As and from 0.04 to 1.35 μg L(-1) for Hg. As and Hg flux values, calculated based on the concentration gradients measured from the DGT profiles for both the overlying water and sediment close to the sediment-water interface, were used to determine the contamination status of As and Hg. Flux values of As and Hg were between -0.066 and 0.067 ng cm(-2)d(-1) and between -0.0187 and 0.0181 ng cm(-2)d(-1), respectively. The GNU's Not Unix R (GNU R) programming language was used to identify outliers of As and Hg at various depths at the sampling sites. The results indicate that the sites with the most outliers were all located in the regions that were seriously affected by contaminants from the Huai River. The DGT-labile As and Hg concentrations in the 0-30 mm layer were found to be significantly correlated with concentrations of labile As and Hg, total dissolved As and Hg, and total As and Hg in the overlying water, as indicated by ex situ measurements. Results show that DGT is a reliable and high-resolution technique that can be used for in situ monitoring of the labile fractions of As and Hg in sediment in fresh water bodies.

New holder configurations for use in the diffusive gradients in thin films (DGT) technique

This study reports two new holder configurations that can be used in the diffusive gradients in thin films (DGT) technique, including a dual-mode holder and a new flat-type holder.

Determination of total arsenic using a novel Zn-ferrite binding gel for DGT techniques: Application to the redox speciation of arsenic in river sediments

A new laboratory-made Zn-ferrite (ZnFe2O4) binding gel is fully tested using Diffusive Gradient in Thin films (DGT) probes to measure total As [including inorganic As(III) and As(V), as well as MonoMethyl Arsenic Acid (MMAA(V)) and DiMethyl Arsenic Acid (DMAA(V))] in river waters and sediment pore waters. The synthesis of the binding gel is easy, cheap and its insertion into the acrylamide gel is not problematic. An important series of triplicate tests have been carried out to validate the use of the Zn-ferrite binding gel in routine for several environmental matrixes studies, in order to test: (i) the effect of pH on the accumulation efficiency of inorganic As species; (ii) the reproducibility of the results; (iii) the accumulation efficiency of As species; (iv) the effects of the ionic strength and possible competitive anions; and (v) the uptake and the elution efficiency of As species after accumulation in the binding gel. All experimental conditions have been reproduced using two other existing binding gels for comparison: ferrihydrite and Metsorb® HMRP 50. We clearly demonstrate that the Zn-ferrite binding gel is at least as good as the two other binding gels, especially for pH values higher than 8. In addition, by taking into consideration the diffusion rates of As(III) and As(V) in the gel, combining the 3-mercaptopropyl [accumulating only As(III)] with the Zn-ferrite binding gels allows for performing speciation studies. An environmental study along the Marque River finally illustrates the ability of the new binding gel to be used for field studies.

Novel precipitated zirconia-based DGT technique for high-resolution imaging of oxyanions in waters and sediments

Water-sediment exchange is a fundamental component of oxyanion cycling in the environment. Yet, many of the (im)mobilization processes overlay complex spatial and temporal redox regimes that occur within millimeters of the interface. Only a few methods exist that can reliably capture these porewater fluxes, with the most popular being high-resolution diffusive gradients in thin films (HR-DGT). However, functionality of HR-DGT is restricted by the availability of suitable analyte binding agents within the sampler, which must be simple to cast and homogeneously distributed in the binding layer, exhibit adequate sorption capacities, be resistive to chemical change, and possess a very fine particle size (≤10 μm). A novel binding layer was synthesized to meet these requirements by in situ precipitation of zirconia into a precast hydrogel. The particle diameter≤0.2 μm of zirconia in this precipitated gel was uniform and at least 50-times smaller than the conventional molding approach. Further, this gel had superior binding and stability characteristics compared with the commonly used ferrihydrite HR-DGT technique and could be easily fabricated as an ultrathin gel (60 μm) for simultaneous oxygen imaging in conjunction with planar-optodes. Chemical imaging of anion and oxygen fluxes using the new sampler were evaluated on Lake Taihu sediments.

Evaluation of the diffusive gradients in thin films technique using a mixed binding gel for measuring iron, phosphorus and arsenic in the environment

Simultaneous measurements of dissolved iron (Fe), phosphorus (P), and arsenic (As) were made using the diffusive gradients in thin films technique equipped with a mixed binding gel impregnated with zirconium oxide and Chelex-100 (ZrO-Chelex DGT). The ZrO-Chelex binding gel exhibited rapid binding dynamics to Fe(II), P(PO4(3-)), and As(III)/As(V) in mixed solutions of the three elements. The bound Fe, P, and As could be quantitatively recovered using an established four-step elution procedure. Simultaneous measurements of the three elements with ZrO-Chelex DGT were validated through time-series experiments, and there was no influence of pH (4.1 to 7.1) and ionic strength (0.01 to 800 mM). The DGT capacities for As(III) and As(V) were 69.0 μg and 186 μg per device, respectively, which were much greater than those of the widely used ferrihydrite DGT. Applications in synthesized freshwaters and in sediments further confirmed its feasibility in simultaneous measurements in the environment. The present study offered that the ZrO-Chelex DGT will be a useful tool in in situ monitoring of Fe-P-As and assessment of their interactions in the biogeochemical processes.

Uncertainty evaluation of the diffusive gradients in thin films technique

Although the analytical performance of the diffusive gradients in thin films (DGT) technique is well investigated, there is no systematic analysis of the DGT measurement uncertainty and its sources. In this study we determine the uncertainties of bulk DGT measurements (not considering labile complexes) and of DGT-based chemical imaging using laser ablation - inductively coupled plasma mass spectrometry. We show that under well-controlled experimental conditions the relative combined uncertainties of bulk DGT measurements are ∼10% at a confidence interval of 95%. While several factors considerably contribute to the uncertainty of bulk DGT, the uncertainty of DGT LA-ICP-MS mainly depends on the signal variability of the ablation analysis. The combined uncertainties determined in this study support the use of DGT as a monitoring instrument. It is expected that the analytical requirements of legal frameworks, for example, the EU Drinking Water Directive, are met by DGT sampling.