Trace elements fractionation in anaerobic digestates: Recommendation for using diffusive gradients in thin films passive samplers.

This study proposes an evaluation of the Diffusive Gradients in Thin films (DGT) technique to assess the labile fraction of trace metals and metalloids in anaerobic digestates. Experiments were performed in presence and absence of air to determine whether maintaining anaerobic conditions is mandatory during DGT deployments. A theoretically expected linear accumulation profile was observed for Fe, Mn, Ni, Mo, and As(III) in a manure-derived digestate and for Mn in distillery waste-derived digestate, whereas Al and Cu were detected without a consistent trend over time. The DGT technique can thus be used to evaluate the labile fraction of some trace elements in these digestates. The labile fraction of some elements was shown to evolve over 72 h when deployments were performed in the presence of air. We thus strongly recommend to systematically perform time-series deployments to identify and consider only the elements with a linear accumulation trend and to maintain anaerobic conditions.

Interest of a new large diffusive gradients in thin films (L-DGT) for organic compounds monitoring: On-field comparison with conventional passive samplers.

In this work, the performances of a Large Diffusive Gradients in Thin films (L-DGT, i.e., a DGT based on a Chemcatcher® holder with a 5-fold larger sampling area) were compared on-field with the conventional DGT and the Polar Organic Chemical Integrative Sampler (POCIS) for the monitoring of a wide range of organic contaminants (i.e., 65 pesticides and metabolites, 53 pharmaceuticals and 12 hormones). These three passive samplers were simultaneously deployed in four rivers during 14 days. Their performances were then evaluated according to their detection and quantification capacities and their physical robustness. The results obtained confirm the advantages of the L-DGT over the conventional DGT regarding its sensitivity but also its robustness during field deployment. The POCIS provides the higher sensitivity, allowing the detection of more organic compounds compared to the DGT and, to a lesser extent, the L-DGT. However, both L-DGT and DGT reduces the uncertainty on the determination of the time-weighted average concentrations (CW), mainly due to the narrow range of variation of their calibration parameters. Indeed, for a given compound, CW can vary up to only a 3-fold factor with DGT and L-DGT compared to a 2 to 10-fold factor (up to 50) with POCIS. Thus, the L-DGT appears to be more suitable than DGT in low-contaminated contexts, which require higher sensitivity, or than POCIS when a CW determination is needed. For a qualitative evaluation however, the POCIS remains the most suitable passive sampler.

Diffusive gradients in thin films (DGT): A suitable tool for metals/metalloids monitoring in continental waterbodies at the large network scale.

The contribution of Diffusive Gradients in Thin films (DGT) passive sampling to continental water quality monitoring was assessed in a real measurement network (6 sampling campaigns, 17 stations). Ten metals/metalloids (Al, Zn, Ni, Cd, Cu, Pb, Cr, As, Se and Sb) were studied using the control laboratory's working conditions with grab and DGT passive sampling. The DGT field deployments were robust, with a 3% sampler loss rate and a <65% average relative deviation between duplicates. Compared to grab sampling, DGT showed a similar quantification frequency for half of the targeted elements but showed a higher frequency for the other half (e.g., Cd quantification at 20% with grab sampling vs. 97% with DGT). Similar concentration trends were established using DGT and grab sampling at most sites throughout the year. Notably, for some elements, trends were only provided by DGT sampling. A study of several DGT blanks showed that the device contamination was occasional and originated primarily from cross-contamination during the disassembly step. Considering this contamination, the operational sensitivity by DGT was at least between 1 and 5 times greater in comparison to that by grab sampling. Estimations of the economic cost revealed that measurement networks cost 2 to 3 times more when monitored by DGT compared to standard grab monitoring. However, the information obtained based on each type of sampling method is different. Grab sampling is easy to implement and can highlight high contamination peaks. The DGT concentrations are averaged over time and are relevant to chronic exposure evaluations. Considering the good performance of the DGT sampling highlighted in this study and its complementarity with grab sampling in terms of water quality assessments, a combination of these two types of sampling, which can be affordable, should improve the water quality evaluation within monitoring networks.

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 (CdII, CuII, NiII and PbII) and two anions (AsV and CrVI) 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.

Adaptation of diffusive gradients in thin films technique to sample organic pollutants in the environment: An overview of o-DGT passive samplers.

The adaptation of the diffusive gradients in thin films technique (DGT) to sample organic pollutants in the environment, called o-DGT has been performed since 2011 for various types of organic compounds (e.g. pesticides, pharmaceuticals, hormones, endocrine disrupting chemicals, household and personal care products). To sample these different compounds, configuration of the samplers (mainly receiving phase and diffusive gel) has to be adapted. Up-to-date, sampling of 142 organic compounds by this passive sampler have been tested. This review provides the state-of-art of o-DGT passive sampler development, describing theory and modelling, calibration, configuration of the devices, and field applications. The most used configurations were agarose-XAD-18 and agarose-HLB configuration. o-DGT can be used to sample soils and most of natural waters (range of pH 4-9 and ionic strength 0.001-0.1 M). This review discusses current limitation of o-DGT in light of the feedback of DGT use to sample inorganic contaminants. It mainly concern the low sampling rates currently obtained by o-DGT compared to other passive samplers. This weakness could be compensated in the future with new sampler's design allowing an increase in exposure area.

A simultaneous assessment of organic matter and trace elements bio-accessibility in substrate and digestate from an anaerobic digestion plant.

This study evaluates a simultaneous assessment of organic matter (OM) and trace elements (TE) bio-accessibility in substrate and digestate from a full-scale anaerobic digester by a sequential OM extraction method. Simultaneous release of TE was determined along with the extraction of different OM fractions and the effects of extracting reagents on characteristics of OM were evaluated by nuclear magnetic resonance (NMR) spectroscopy. The reagents used for sequential extraction of OM were not enough selective. However, proteins were particularly removed by 0.1 M NaOH, while 72% H2SO4 mainly extracted hemicellulose and cellulose. The OM fractionation allowed for simultaneous extraction of >60% of total As, Cd, Co, Fe, Mn, Ni and Zn, while the extraction was limited for Al, Cr, Cu, Mo, and Pb. In substrate, >50% of total As, Co, Mn and Ni and <40% of total Fe, Zn and Mo were identified in bio-accessible fractions. In digestate, all elements demonstrated poor bio-accessibility except for As.

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 (SeIV) using Diffusive Gradients in Thin films sampler (DGT). SeIV accumulation was quantitative and selective over SeVI 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 SeIV 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 SeIV trapping by thiol-based solid phases. Our experiments showed that after SeIV was bound to thiol functional groups, a further pH-dependent reaction occurred with free thiols, resulting in the reduction of SeIV 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 SeIV selective sampling.

Distribution trend of trace elements in digestate exposed to air: Laboratory-scale investigations using DGT-based fractionation.

The use of digestate as amendment for agricultural soils has already been proposed as an alternative to mineral fertilizers or undigested organic matter. However, little information is available concerning the effect of digestate atmospheric exposure on trace elements speciation and, consequently, on their mobility and bio-accessibility when digestate is stored in open tanks or handled before land spreading. In this study, we investigated at laboratory-scale the effect of digestate aeration on the distribution of Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se and W using the diffusive gradients in thin films technique (DGT)-based fractionation. For this purpose, experiments were performed to assess the variation in distribution between the labile, soluble and particulate fractions over time in digested sewage sludge during passive and forced aeration. Results showed that aeration promoted a dissolution of Al, As, Co, Cr, Cu, Fe, Mn, Mo and Pb, suggesting a possible increase in their mobility that may likely occur during storage in open tanks or handling before land spreading. Labile elements' fraction increased only during forced aeration (except for Fe and Mn), suggesting that their short-term bio-accessibility can increase only after significant aeration as the one assumed to occur when land spreading takes place.

Limitation of flow effect on passive sampling accuracy using POCIS with the PRC approach or o-DGT: A pilot-scale evaluation for pharmaceutical compounds.

Flow velocity is known to alter passive sampling accuracy. We investigated the POCIS (Polar Organic Chemical Integrative Sampler) with PRC (Performance Reference Compounds) approach and Diffusive Gradients in Thin Films samplers (o-DGT) to limit the effect of flow on the quantification accuracy of ten model pharmaceuticals compounds (0.16 ≤ log KOW ≤ 4.51). POCIS and o-DGT samplers were exposed for seven days in controlled pilot-scale (hundreds of liters) experiments under quiescent or flowing (2 < V < 18 cm s-1) conditions. Under flowing conditions, both POCIS-PRC and o-DGT efficiently limited the flow effect and led, in most cases, to biases within analytical uncertainty (20%). Under quiescent conditions, o-DGT performed accurately (bias < 30% for most compounds) whereas the PRC approach was unsuitable to improve upon the accuracy of POCIS (PRC was unable to desorb). Therefore, both approaches are helpful in limiting the effects of flow on accuracy, but only o-DGT is efficient in quiescent conditions. However, o-DGT currently suffers from poorer sensitivity compared to POCIS, but the future development of o-DGT devices with wider windows could overcome this limitation.

Assessment of the DGT technique in digestate to fraction twelve trace elements.

This study proposes an evaluation of the diffusive gradients in thin films technique (DGT) for studying trace elements in digested sewage sludge samples. Twelve elements were monitored by Chelex (Al, Cd, Co, Cr (III), Cu, Fe, Mn, Ni, Pb) and zirconia-DGT (As, Mo, Se) samplers exposed from 4 h to 9 days. Twenty-four hours' deployment time was suitable for most of the studied elements. However, short deployment led to insufficient element accumulation or non-establishment of steady state while long deployment (from 18 to 144 h depending on the element) led to saturation of the binding gels and/or competing effects with other major elements. In addition, this study showed that the matrix of the digested sewage sludge lowers the accumulation of some trace elements in the DGT samplers, leading to labile concentrations underestimation of roughly 10-30% (depending on the element). Moreover, compared to the conventional total dissolved elements measurement, DGT technique allowed to quantify 7 out of 12 labile elements whereas only 3 out of 12 dissolved elements were quantified. These results highlight the potential of DGT technique to assess labile trace elements in digestate samples, provided a careful adaptation of the deployment time as well as an evaluation of the matrix effect is performed.

Improving elution strategies for Chelex®-DGT passive samplers.

Elution of Chelex® binding layers, commonly used for the diffusive gradients in thin films technique (DGT), is recognized as the most important contributor to the uncertainty of DGT measurements. Limiting uncertainty requires the use of optimized procedures and suitable elution recoveries (f e ). This work therefore investigated elution robustness to propose improved strategies. A wide range of conditions were investigated for the main elution parameters (Chelex® particle size, elution time, Chelex® loading, and eluent concentration and volume) on Al(III), Cd(II), Co(II), Cr(III), Cu(II), Ni(II), Pb(II), and Zn(II). Results showed that the choice of elution conditions should be a compromise driven by study constrains in terms of accuracy, repeatability, sensitivity, and targeted elements. Using experimentally determined recoveries should improve accuracy by approximately 5 to 10% compared to the use of recoveries from the literature. Fast elution of 1 h can be achieved without significant loss of recovery and repeatability except for Cr(III) (8 h minimum). Elution recovery depended on Chelex® loading for Zn and Cr and introducing recoveries adapted to the loading could improve accuracy up to, respectively, 11 and 27%. When standard recoveries are used, a 0.85 f e value would be more appropriate than the common value of 0.8 to minimize inaccuracy (except for Cr). Some flexibility can be applied to elution conditions without a significant change in recovery for most elements: HNO3 concentration of 1-15 M, volume of 1-2 mL, duration of 8-48 h. Cr(III) was unique in its sensitivity to elution condition variations; thus, choice is more restricted for this element. Graphical abstract Decisional tree for choosing elution procedure and recoveries for Chelex®-DGT.

Key role of the sorption process in alteration of metal and metalloid quantification by fouling development on DGT passive samplers.

The DGT technique (diffusive gradients in thin films) is widely used for passive sampling of labile trace metals and metalloids in natural waters. Although development of fouling on the protective membranes is frequently observed, its effect on DGT sampling has been barely investigated. This study evaluates the influence of fouling on sampling of trace cationic metals Cd(II), Cu(II), Ni(II) and Pb(II) and oxyanions As(V), Cr(VI), Sb(V) and Se(VI). Fouling was developed in situ on polycarbonate membranes in four diverse natural freshwater environments and sampling alteration was assessed in controlled laboratory experiments. Accumulation of oxyanions and Ni was unaltered in the presence of fouling whereas significant alteration occurred in sampling of Cd, Cu and Pb (at pH ∼5.4). Characterization of the fouled membranes highlighted the intervention of sorption phenomenon as sampling alteration was systematically observed alongside element sorption onto fouled membrane. A preliminary flowchart for identifying potentially biased quantifications linked to fouling development during in situ DGT deployment in natural waters is proposed.

Passive sampling of anionic pesticides using the Diffusive Gradients in Thin films technique (DGT).

DGT passive samplers using Oasis® HLB or Oasis® MAX sorbent were developed for anionic pesticides sampling. They were tested using four model compounds (i.e. bentazon, chlorsulfuron, ioxynil and mecoprop). Polyacrylamide diffusive gel was found to be more suitable than agarose gel for most anionic pesticides sampling. An elution procedure was optimized and diffusion coefficients were determined for quantitative use of the samplers. Depending on the DGT configuration used (HLB or MAX), accuracies better than 30% were demonstrated in laboratory for pH from 3 to 8 and ionic strengths from 10-2 to 1 M. Combined with the effective binding capacities of samplers (≥9 µg for each pesticide) and limits of quantification of the method (≤13 ng.L-1 using Q-TOF detector) monitoring of numerous aquatic systems can be expected. Except for ioxynil, accurate quantifications were demonstrated in laboratory using a spiked natural water for HLB-DGT whereas MAX-DGT did not give satisfactory results. A further in situ validation was performed in two rivers and showed identical detection frequency between HLB-DGT and POCIS of anionic pesticides (bentazon and mesotrione) whereas calculated concentrations, although within the same order of magnitude, could differ (<70%). HLB-DGT could therefore constitute an interesting alternative to other passive samplers for the monitoring of several anionic pesticides in aquatic systems but more work is required for quantification of molecules from hydroxybenzonitrile chemical group (ioxynil).

Simultaneous measurement of Cr(III) and Cr(VI) in freshwaters with a single Diffusive Gradients in Thin Films device.

Few attempts have been made to sample labile chromium with the DGT passive sampler (Diffusive Gradients in Thin Films) and, currently, no single device allows the simultaneous determination of both Cr(III) and Cr(VI). In this work, a procedure based on only one device combined with innovative selective elution is evaluated to assess chromium speciation. A zirconium binding gel is used to accumulate both Cr(III) and Cr(VI). Cr(VI) is quantitatively and selectively eluted by NaOH, allowing the subsequent determination of Cr(III). Accurate quantification of both species is demonstrated in synthetic solutions for pH ranging from 4 to 6 and ionic strength ranging from 10(-3) to 5×10(-2)M. Cr(VI) quantification is altered only for [SO4(2-)]≥5×10(-3)M. The method allows successful quantification of labile Cr(III) and Cr(VI) in spiked natural water. The limit of quantification of the procedure is suitable for trace level monitoring (0.03µgL(-1) for Cr(VI) and 1µgL(-1) for Cr(III), for a one-week deployment at 20°C) and the effective capacity of the sampler (∼25µg for each Cr oxidation state) should allow long term deployments. These results highlight the potential of this new procedure for a simple and effective chromium speciation analysis in natural waters.

Overview of the Chemcatcher® for the passive sampling of various pollutants in aquatic environments Part B: Field handling and environmental applications for the monitoring of pollutants and their biological effects.

The Chemcatcher(®) has been used for a wide range of environmental applications in various media (river water, seawater, sewage water, and treated wastewater). The aim of part B of this review is to compile and discuss the results obtained during these applications, from a screening or quantitative monitoring of water contamination, to a comparison with biomonitoring and bioassays. Special attention will also be paid to, firstly, the influence of environmental factors on analyte uptake and, secondly, the use of Performance and Reference Compounds for the in situ correction of sampling rates.

Overview of the Chemcatcher® for the passive sampling of various pollutants in aquatic environments Part A: Principles, calibration, preparation and analysis of the sampler.

The passive sampler Chemcatcher(®), which was developed in 2000, can be adapted for various types of water contaminants (e.g., trace metals, polycyclic aromatic hydrocarbons, pesticides and pharmaceutical residues) depending on the materials chosen for the receiving phase and the membrane. The Chemcatcher(®) has been used in numerous research articles in both laboratory experiments and field exposures, and here we review the state-of-the-art in applying this passive sampler. Part A of this review covers (1) the theory upon which the sampler is based (i.e., brief theory, calculation of water concentration, Performance and Reference Compounds), (2) the preparation of the device (i.e., sampler design, choice of the membrane and disk, mounting of the tool), and (3) calibration procedures (i.e., design of the calibration tank, tested parameters, sampling rates).

Estimates of pesticide concentrations and fluxes in two rivers of an extensive French multi-agricultural watershed: application of the passive sampling strategy.

In this study, the passive sampling strategy was evaluated for its ability to improve water quality monitoring in terms of concentrations and frequencies of quantification of pesticides, with a focus on flux calculation. Polar Organic Chemical Integrative Samplers (POCIS) were successively exposed and renewed at three sampling sites of an extensive French multi-agricultural watershed from January to September 2012. Grab water samples were recovered every 14 days during the same period and an automated sampler collected composite water samples from April to July 2012. Thirty-nine compounds (pesticides and metabolites) were analysed. DEA, diuron and atrazine (banned in France for many years) likely arrived via groundwater whereas dimethanamid, imidacloprid and acetochlor (all still in use) were probably transported via leaching. The comparison of the three sampling strategies showed that the POCIS offers lower detection limits, resulting in the quantification of trace levels of compounds (acetochlor, diuron and desethylatrazine (DEA)) that could not be measured in grab and composite water samples. As a consequence, the frequencies of occurrence were dramatically enhanced with the POCIS compared to spot sample data. Moreover, the integration of flood events led to a better temporal representation of the fluxes when calculated with the POCIS compared to the bimonthly grab sampling strategy. We conclude that the POCIS could be an advantageous alternative to spot sampling, offering better performance in terms of quantification limits and more representative data.

Can POCIS be used in Water Framework Directive (2000/60/EC) monitoring networks? A study focusing on pesticides in a French agricultural watershed.

In this study, the main current limitations in the application of the Polar Organic Chemical Integrative Sampler (POCIS) in regulatory monitoring programmes were evaluated. POCIS were exposed from March to December by successive periods of 14 days in the River Trec (Lot et Garonne, France) and analysed for 34 pesticides. The study of the uncertainty related to the POCIS data was performed and we concluded that it might be up to 138%, which is higher than European Union requirements but this issue was adequately counterbalanced by the gain of temporal representativeness. Comparison with data from the official monitoring programme from the French Water Agency showed that the POCIS is already suitable for both operational and investigative monitoring. The sampled fraction issue, and then compliance with Environmental Quality Standards, was also addressed. It was confirmed that POCIS samples only the dissolved fraction of dimethenamid and showed that for compounds like atrazine, desethylatrazine and metolachlor, the POCIS concentration is equivalent to the whole water concentration. For dimethenamid, which exhibited a tendency to adsorb on suspended matter, a method was suggested to assess the raw water concentration from the POCIS measure. Finally, an innovative procedure for using passive sampler data for compliance checks in the framework of surveillance monitoring is proposed.

DGT-labile As, Cd, Cu and Ni monitoring in freshwater: toward a framework for interpretation of in situ deployment.

The use of the Diffusive Gradient in Thin Film sampler (DGT) as a monitoring tool for regulatory programs is currently evaluated. In this context, the impact of commonly followed procedures on the accuracy of DGT-labile As, Cd, Cu, and Ni quantification was studied. Initial sampler contamination yields to define quantification limits instead of using blank subtraction, thus avoiding artifact concentrations. Not considering the alteration of element diffusion by the filter membrane leads to significant underestimation. However, diffusion coefficients determined on a non-fouled membrane were found to be suitable for the studied site, making it possible to use data from the literature. When diffusive boundary layer formation is neglected, no loss of accuracy is recorded provided the layer is thinner than 0.5 mm. Finally, exploration of potential biases allowed initiating a framework that might help limit inaccuracies in DGT-labile concentration estimation and interpretation, especially in a low contamination context.

Dissolved and bioavailable contaminants in the Seine river basin.

Diffusive Gradient in Thin Films (DGT) and Semi-Permeable Membrane Devices (SPMDs) were deployed in the Seine river basin in order to assess labile metals and truly dissolved Polycyclic Aromatic Hydrocarbons. We show that the tools are reliable in aquatic environments to assess the speciation of dissolved contaminants and hence provide a good insight into the potential bioavailability of contaminants. The deployment of the DGT and SPMDs in contrasting environments in the Seine river basin allowed distinction to be made of availability of contaminants between headwater streams and much more impacted river reaches and an assessment of bioavailability. At the stations under urban influence, the impact of dissolved organic matter on both copper and PAHs bioavailability is less pronounced than at upstream stations, where humic substances dominate.