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.

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.

Lead sorption by biochar produced from digestates: Consequences of chemical modification and washing.

The main objectives of this work are to investigate the consequences of different chemical treatments (i.e. potassium hydroxide (KOH) and hydrogen peroxide (H2O2)) and the effect of biochar washing on the Pb sorption capacity. Biochars derived from sewage sludge digestate and the organic fraction of municipal solid waste digestate were separately modified with 2 M KOH or 10% H2O2 followed by semi-continuous or continuous washing with ultrapure water using batch or a column reactor, respectively. The results showed that the Pb adsorption capacity could be enhanced by chemical treatment of sludge-based biochar. Indeed, for municipal solid waste biochar, the Pb maximum sorption capacity was improved from 73 mg g-1 for unmodified biochar to 90 mg g-1 and 106 mg g-1 after H2O2 and KOH treatment, respectively. In the case of sewage sludge biochar, it increased from 6.5 mg g-1 (unmodified biochar) to 25 mg g-1 for H2O2 treatment. The sorption capacity was not determined after KOH treatment, since the Langmuir model did not fit the experimental data. The study also highlights that insufficient washing after KOH treatment can strongly hinder Pb sorption due to the release of organic matter from the modified biochar. This organic matter may interact in solution with Pb, resulting in an inhibition of its sorption onto the biochar surface. Continuous column-washing of modified biochars was able to correct this issue, highlighting the importance of implementing a proper treated biochar washing procedure.

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.

Hydrophobic features of EPS extracted from anaerobic granular sludge: an investigation based on DAX-8 resin fractionation and size exclusion chromatography.

The hydrophobic fractionation of extracellular polymeric substances (EPS) extracted from anaerobic granular sludge was performed on the DAX-8 resin (two elution pH conditions, i.e., pH 2 and pH 5 were tested). The impact of seven different EPS extraction methods on EPS hydrophobicity features was assessed. The results showed that the extraction methods and bulk solution pH influenced dramatically the biochemical composition of the EPS, and in turn, the hydrophobicity determined. Besides, EPS extracting reagents i.e., formaldehyde, ethanol, sodium dodecyl sulfate (SDS), and Tween 20 not only introduced extra carbon content in the total organic carbon (TOC) measurement but also interacted with the DAX-8 resin. By comparing the apparent molecular weight (aMW) distribution of untreated and pH-adjusted EPS samples, more complete EPS aMW information was preserved at pH 5. Thus, elution at pH 5 was preferred in this study for the qualitative analysis of EPS hydrophobic features. The hydrophobic fraction of EPS retained by the resin at pH 5 was ascribed to a wide aMW range, ranging from >440 to 0.3 kDa. Within this range, EPS molecules ranging from 175 to 31 kDa were mostly retained by the DAX-8 resin, which indicates that these EPS molecules are highly hydrophobic.

Role of extracellular polymeric substances (EPS) production in bioaggregation: application to wastewater treatment.

This paper reviews the formation, structure, and stability of bioaggregates with an emphasis on the composition and distribution of extracellular polymeric substances (EPS) and their role in bioaggregation. Bioaggregation is ubiquitous in natural environment and is of great importance in biological wastewater treatment processes. It greatly influences the flocculability, settleability, and dewaterability for flocs and sludge retention and shear resistance for biofilms. The physico-chemical and microbial structures of bioaggregates are dependent on operational conditions as well as microbial diversity and spatial distribution. The formation of bioaggregates is mediated by the physico-chemical interactions as well as the microbial interactions such as EPS production and quorum sensing. EPS are composed of a mixture of macromolecules including proteins, polysaccharides, humic-like substances, and nucleic acids, which entrap the microbial cells in a three-dimensional matrix. The composition and physico-chemical characteristics of EPS have significant influence on the maintenance of the bioaggregate structure and the process performance of the wastewater treatment. However, the mechanisms of bioaggregation are still unclear and the conclusions on the role of EPS were mostly drawn from the established correlations and hypotheses. This paper expects to provide up-to-date knowledge on bioaggregation and insights for further studies and applications.

Proposal of a new empirical model with flow velocity to improve time-weighted average concentration estimates from the Polar Organic Chemical Integrative Samplers.

It is now widely recognized that the sampling rate of Polar Organic Chemical Integrative Samplers (POCIS) is significantly affected by flow velocity, which can cause a consequent bias when determining time-weighted average concentrations (TWAC). We already observed the desorption of deisopropylatrazine (DIA) over time when added to the receiving phase of a POCIS. This desorption rate was particularly influenced by flow velocity, in an agitated water environment in situ. In the method presented here, we calibrated 30 pesticides under controlled laboratory conditions, varying the flow velocity over four levels. We simultaneously studied the desorption rate of DIA-d5 (a deuterated form of DIA) over time. An empirical model based on a power law involving flow velocity was used to process the information from the accumulation kinetics of the compounds of interest and elimination of DIA-d5. This type of model makes it possible to consider the effect of this crucial factor on exchange kinetics, and then to obtain more accurate TWACs with reduced bias and more acceptable dispersion of results.

Impact of oxygen cut off and starvation conditions on biological activity and physico-chemical properties of activated sludge.

Physico-chemical and biological parameters were monitored both throughout different oxygen cut off and starvation (OCS) times (6 h-72 h) and after the restoration of normal operational conditions. Sludge apparent viscosity and soluble extracellular polymeric substances (EPS) characteristics were measured to determine the activated sludge (AS) properties. Oxygen transfer, biological activity with specific oxygen uptake rate (SOUR) measurements during endogenous/exogenous conditions (without any external substrate/with external substrate consumption) and chemical oxygen demand (COD) removal were measured to assess the AS performances. During the different stress times, AS deflocculated as a decrease of apparent viscosity was observed and microorganisms biodegraded the released EPS to survive. After aeration return, and under endogenous conditions, size exclusion chromatographic fingerprints of soluble EPS were modified and macromolecules probably of type humic-like substances appeared in significant quantities. These new macromolecules presumably acted as biosurfactants. Consequently, the liquid surface tension, as well as the oxygen transfer rate (OTR), decreased. Under exogenous conditions, high biological activity (SOUR = 11.8 +/- 2.1 mg(O2 x g(MLVSS)(-1) x h(-1)) compensated the decrease of oxygen transfer. Finally, AS biomass maintained a constant COD degradation rate (15.7 +/- 1.9 mg(O2) x g(MLVSS)(-1) x h(-1)) before and after the disturbances for all times tested. This work demonstrates that AS microorganisms can counteract concomitant oxygen and nutrients shortage when the duration of such a condition does not exceed 72 h. Dissociation of endogenous/exogenous conditions appears to offer an ideal laboratory model to study EPS and biomass activity effects on oxygen transfer.

Adaptation of the o-DGT for the sampling of 12 hormones: calibration, performance evaluation, and recommendation.

This work highlights the methodology for the development of diffusive gradients in thin films (o-DGT) through its adaptation for 12 natural and synthetic hormones belonging to three different families (estrogens, progestins, and androgens). A reliable strategy must be applied during o-DGT lab adaptation to avoid issues related to the analysis (i.e., presence of matrix effects in grab or passive samples) but also to the o-DGT configuration (i.e., undesirable sorption or desorption, lack of performance with insufficient elution or unreliable diffusion coefficient). To avoid analytical issues due to the presence of salts in grab samples, CaCl2 exposure solutions must be used on a lab-scale development to monitor the hormone concentration. The selected o-DGT was composed of an Oasis® HLB binding gel and a diffusive gel in agarose because they provided better performance than polyacrylamide gels (i.e., higher elution factors and more repeatable diffusion coefficients). The elution factors of the binding gel were then from 0.79 ± 0.13 to 1.04 ± 0.13 (RSD < 15%) and the diffusion coefficients at 25 °C were from 4.07 ± 0.24 to 5.49 ± 0.28 × 10-6 cm2 s-1 (RSD < 9%). A laboratory exposure to a synthetic solution was performed to check the consistency with the DGT quantification model validating the calibration parameters for all hormones (except 17α-ethinylestradiol with a bias of 40%). Therefore, the o-DGT configuration is suitable for sampling hormones in the natural environment with LOQDGT ranging from 0.3 to 6.6 ng L-1.

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.

Multivariate Tiered Approach To Highlight the Link between Large-Scale Integrated Pesticide Concentrations from Polar Organic Chemical Integrative Samplers and Watershed Land Uses.

This paper presents a multi-step methodology to identify relationships between integrative pesticide quantifications and land uses on a given watershed of the Adour-Garonne Basin (Southwestern France). In fact, a large amount of pesticide concentration data was collected from 51 sites located in the Adour-Garonne Basin for a 1 year monitoring period in 2016. The sampling devices used here were polar organic chemical integrative samplers (POCIS), which provided time-weighted average concentration estimates. For each study site, its associated watershed and land cover distribution were determined using Corine Land Cover 2012 (CLC 2012) and Geographic Information System (GIS). The large-scale data were analyzed using multivariate statistical analyses, such as hierarchical cluster analysis (HCA) and principal component analysis (PCA). HCA grouped the 51 sites into five clusters with similar primary land uses. Next, the integrated pesticide concentration and land use distribution data sets were analyzed in a PCA. The key variables responsible for discriminating the sample sites showed distribution patterns consistent with specific land uses. To confirm these observations, pesticide fingerprints from sites with contrasting land uses were compared using a waffle method. The overall multivariate approach allowed for the identification of contamination sources related to their likely initial use, at the watershed level, that could be useful for preventing or containing pesticide pollution beyond simply acting on areas at risk.

Development of a multi-hormone analysis method by LC-MS/MS for environmental water application using diffusive gradient in thin films.

An analysis method for four families of hormones (estrogens, progestins, androgens and prostaglandins), dedicated to an efficient water monitoring with passive sampling, was developed using a liquid chromatography tandem mass spectrometry with triple quadrupole coupling and universal electrospray ionisation. Thirteen natural and synthetic hormones in ultra-pure water could be analysed in a single run according to the French Standard NF T90-210: calibration range of 0.1 (except for 17ß-Estradiol, Estriol, Estrone and Diethylstilbestrol, from 0.5 µg/L; and Ethinylestradiol, from 1 µg/L) to 20 µg/L with linear regressions (R2 ≥ 0.96), maximum accuracy deviations of 30% at intermediate fidelity for three concentration references (1, 10 and 20 µg/L) and instrumental LOQs from 0.05 to 1 µg/L. The stability of 11 hormones (10 µg/L) was studied under several storage conditions and sample evaporation. All selected hormones were stable for 60 days at -18 °C, 7 days at 4 °C and 7 days at 20 °C but continued drying flow after evaporation should be avoided, especially for 17α-Estradiol, Estrone and Diethylstilbestrol. Observed matrix effects using o-DGT extracts (diffusive gradient in thin-film sampler for polar organics) containing an environmental matrix varied from 24 to 92% but all matrix effects were corrected with IS use. Therefore, the developed method, coupled with o-DGT, was tested with the o-DGT deployment in rivers. Using diffusion coefficients from the literature or lab determined, the concentrations in the rivers varied for Estrone from 1.8 ng/L to 2.5 ng/L, and for Androstenedione from 0.4 to 1.1 ng/L.

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.

Extraction of extracellular polymeric substances from dam lake fresh sediments derived from crystalline bedrock.

Extracellular polymeric substances (EPS) produced by microorganisms have a key role in the sedimentary compartment, e.g. promoting aggregation and biostabilisation of sediment particles and increasing chemical reactivity at the water/sediment interface. Therefore, proper extraction methods are needed to study this EPS matrix. In this work, nine extraction methods based on physical (centrifugation, sonication), chemical (sodium hydroxide, sodium pyrophosphate, sodium tetraborate), and both chemical and physical (cation exchange resins, i.e. CER) treatments and their combinations, as well as the solid:liquid ratio used for extraction, were compared based on the quantity and compositions of extracted EPS. The organic carbon extracted was quantified and the nature of biochemical macromolecules (proteins, polysaccharides, and humic-like compounds) was evaluated using colorimetric methods. The amount of ATP was used as an indicator of cell lysis and showed contamination with intracellular materials in EPS extracted with chemical methods. Moreover, chemical extraction presented a large quantity of impurities due to non-removal of reactant salts by ultracentrifugation. For the nine methods tested, humic-like substances represented the main fraction of the extracted EPS, but for chemical extraction, the presence of humic materials from the sediment organic fraction was due to non-specific extraction of the EPS fraction. Therefore, chemicals methods are not recommended to extract EPS from sediment. Despite their low extraction efficiency, physical methods and CER, i.e. 'soft' extraction methods, are preferred using a solid:liquid ratio 1:40.

Cadmium Selenide Formation Influences the Production and Characteristics of Extracellular Polymeric Substances of Anaerobic Granular Sludge.

Feeding cadmium (II) and selenium (IV) simultaneously to anaerobic granular sludge with the aim of synthesizing cadmium selenide (CdSe) nanoparticles induces compositional changes in the extracellular polymeric substances (EPS) matrix of this sludge. A methanogenic anaerobic granular sludge was repeatedly exposed to Cd(II) (10-50 mg L-1) and selenite (79 mg L-1) for 300 days at pH 7.3 and 30 °C in a fed-batch feeding regime for enrichment of Se-reducing bacteria and synthesis of CdSe nanoparticles. EPS fingerprints of the granular sludge, obtained by size exclusion chromatography coupled to a fluorescence detector, showed a significant increase in the intensity of protein-like substances with > 100 kDa apparent molecular weight (aMW) upon repeated exposure to Cd(II) and Se(VI). This was accompanied by a prominent decrease in protein-like substances of aMW < 10 kDa. The fingerprint of the humic-like substances showed emergence of a new peak with aMW of 13 to 300 kDa in the EPS extracted from the Cd/Se fed granular sludge. Experiments on metal(loid)-EPS interactions showed that the CdSe nanoparticles interact mainly with loosely bound EPS (LB-EPS). This study showed that the formation of Se(0) and CdSe nanoparticles occurs in the LB-EPS fraction of the granular sludge and repeated exposure to Cd and Se induces compositional changes in the EPS matrix.

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.

Extraction of extracellular polymeric substances (EPS) from anaerobic granular sludges: comparison of chemical and physical extraction protocols.

The characteristics of the extracellular polymeric substances (EPS) extracted with nine different extraction protocols from four different types of anaerobic granular sludge were studied. The efficiency of four physical (sonication, heating, cationic exchange resin (CER), and CER associated with sonication) and four chemical (ethylenediaminetetraacetic acid, ethanol, formaldehyde combined with heating, or NaOH) EPS extraction methods was compared to a control extraction protocols (i.e., centrifugation). The nucleic acid content and the protein/polysaccharide ratio of the EPS extracted show that the extraction does not induce abnormal cellular lysis. Chemical extraction protocols give the highest EPS extraction yields (calculated by the mass ratio between sludges and EPS dry weight (DW)). Infrared analyses as well as an extraction yield over 100% or organic carbon content over 1 g g(-1) of DW revealed, nevertheless, a carry-over of the chemical extractants into the EPS extracts. The EPS of the anaerobic granular sludges investigated are predominantly composed of humic-like substances, proteins, and polysaccharides. The EPS content in each biochemical compound varies depending on the sludge type and extraction technique used. Some extraction techniques lead to a slightly preferential extraction of some EPS compounds, e.g., CER gives a higher protein yield.

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.

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.

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.