In situ measurement with diffusive gradients in thin films: effect of biofouling in freshwater

On-site evaluation of DGT passive sampling for quantitatively measuring per- and polyfluoroalkyl substances in a river-estuary-sea water system

The passive sampling technique of diffusive gradients in thin-films (DGT) is promising for monitoring emerging contaminants such as per- and polyfluoroalkyl substances (PFAS). It is urgent to evaluate the impacts of salinity and exposure time on DGT sampling before it can be set as a standard method. Herein, DGT sampler based on the binding gel of weak anion exchanger (WAX) resin was deployed in a representative water system of the Xiaoqing river-estuary-sea for representative sampling windows (<1 day to 28 days) with high pH (8.18 ± 0.04 to 8.51 ± 0.17) and wide ranges of salinity (0.95 ± 0.07‰ to 14.37 ± 3.92‰), total dissolved solids (1.20 ± 0.09 g/L to 15.29 ± 3.91 g/L) and dissolved organic matter (2.8-32 mg/L). The results showed that the WAX-DGT sampler exhibited good performance for most target PFAS except for short-chain perfluorocarboxylates (C ≤ 5) in 14 days. When the exposure time was over 14 days, biofouling of the sampler may deflect the mass accumulation of the PFAS in the sampler. Salinity played an important role in the mass binding of PFAS by DGT. The shorter the carbon chain of the compound, the greater the influence of the salinity. PFAS with carboxyl groups had greater affinities for the biofouled membrane filter than those with sulfonic groups. In the river-estuary-sea system, where PFAS concentrations changed dynamically, the temporal resolution of the monitoring strategy has been demonstrated to be more important than spatial resolution. DGT provided a better integral of PFAS exposure than grab sampling in the dynamic water system and offered equivalent sensitivity of grab sampling with exposure time <10 d and greater sensitivity with exposure time ≥10 d. Thus, DGT has the advantage of providing high temporal resolution monitoring. This study provided support for the standardization of the DGT technique.

Assessment of dissolved mercury by diffusive gradients in thin films devices in abandoned ponds impacted by small scale gold mining

In order to fulfil the Minamata Convention on Mercury, it is necessary to monitor the Hg contamination in freshwater ecosystems nearby artisanal and small scale gold mining (ASGM) areas. Since most of these ASGM communities are located in remote areas, a convenient method for sampling, preserving and transporting samples is needed. In this study we evaluated the feasibility of the diffusive gradient in thin-films (DGT) technique to detect and quantify the labile fraction of Hg and other metals (Pb, Cu, Zn, Cd, Ni, Mn and Cr) in a hard-to-reach gold mining district in the state of Chocó, Colombia. We deployed DGT at sampling sites along the Atrato river and abandoned mining ponds (AMPs) which were deserted in different periods since 1997 to 2019 (6-15 years). In average, the labile THg concentrations in AMPs (148.9 ± 43.2 ng L^-1) were a 50% higher than in the river water (99.9 ± 37.4 ng L^-1). In the ponds, no significant differences were found in labile Hg with respect abandonment period. Labile Ni (0.9-493.1), Mn (1.33-11.48), Cu (0.030-2.233), and Zn (0.67-10.29) (in μg L^-1) were found in higher amounts than for the rest of metals. Labile concentrations of metals are related with their downstream proximity to gold mining activities, being higher in devices deployed close to ASGM sites. Moreover, this study demonstrates the feasibility of the DGT technique to sample, transport, storage, and preserve labile Hg from hard-to-reach ASGM areas.

Effects of seasonal biofouling on diffusion coefficients through filter membranes with different hydrophilicities in natural waters

Biofouling is a major issue for the diffusive gradients in thin films (DGT) passive samplers during long-term deployment. Although biofilms have a negative effect on DGT samplers, the effects of seasonal biofilms on the flux of targets through different membranes are poorly understood. Herein, we evaluated the relationship between the biofilm growth and diffusion coefficients' decline rate through two membranes with different hydrophilicities during the four seasons in natural waters. Cu²⁺ and tetracycline were selected as the model metal and organic contaminant, respectively. Rapid biofilm growth on the membrane surface was observed, and the fouling growth rate increased in the order: winter < spring < autumn < summer. Biofouling had a negative effect on the diffusion coefficients of Cu²⁺ and tetracycline. Generally, the decreasing tendency of diffusion coefficients agreed with the increasing tendency of the fouling growth rate. Biofilms in a lag phase with little bacterial colonies had insignificant effect on the diffusion coefficients. After 30 days, the decline ratios of diffusion coefficients were in the range of 38.14%-53.05%, 69.63%-83.19%, 51.57-68.42%, and 19.43-35.84%, respectively, during the spring, summer, autumn and winter. The flux through the membrane with higher hydrophilicity was greater. Both the hydrophilicity of membrane and structure of target analytes had important effects on the diffusion coefficients through biofouled membranes. Owing to similar physical and chemical characteristics, there was insignificant difference in diffusion coefficients decline trend between the Yalu River water and Hunhe River water in the summer.

Concurrent sampling of transitional and coastal waters by Diffusive Gradient in Thin-films (DGT) and spot sampling for trace metals analysis

This protocol was developed based on the knowledge acquired in the framework of the Interreg MONITOOL project (EAPA_565/2016) where extensive sampling campaigns were performed in transitional and coastal waters covering eight European countries. It provides detailed procedures and guidelines for the sampling of these waterbodies by concurrent collection of discrete water samples and the deployment of Diffusive Gradient in Thin-films (DGT) passive samplers for the measurement of trace metal concentrations. In order to facilitate the application of this protocol by end-users, it presents steps to follow in the laboratory prior to sampling campaigns, explains the procedures for field campaigns (including in situ measurement of supporting parameters) and subsequent sample processing in the laboratory in preparation for trace metal analyze by inductively coupled plasma-mass spectrometry (ICP-MS) and voltammetry. The protocol provides a systematic, coherent field sampling and sample preparation strategy that was developed in order to ensure comparability and reproducibility of the data obtained from each project Partner in different regions. • Standardization of the concurrent sampling of transitional and coastal waters by DGT passive samplers and spot sampling. • Robust procedures and tips based on existing international standards and comprehensive practical experience. • Links to demonstration videos produced within the MONITOOL project.

Metal speciation of the Paraopeba river after the Brumadinho dam failure

On January 25, 2019, a tailings dam at the Córrego do Feijão iron ore mine (Brumadinho, Minas Gerais, southern Brazil) ruptured and released ~12 million m³ of mine tailings into the Paraopeba River, which is an important source of drinking water to a populous region. While water potability due to a strong increase in turbidity has been well documented, possible effects of metal contamination are yet to be addressed. We investigated the speciation of metals in the river water and desorption of metals from sediments as a means of supporting risk assessment, using the diffusive gradient in thin films (DGT) technique, desorption experiments and chemical speciation calculations. The results of the in-situ DGT monitoring revealed that the labile concentrations of metals were low in relation to the respective total and dissolved concentrations. Chemical speciation calculations showed that the heavy metals were not stable in the Paraopeba River. The desorption experiments suggested that sediments may release a limited amount of As and Cu, but large amounts of Mn into the river water. Higher concentrations of Fe and Mn indicated a possible association with the impact of mine tailings. In general, the total metal concentrations during the rainy season were higher than those during the dry season, whereas the reverse was generally the case for labile forms. This pattern reveals that metal speciation is intrinsically dependent on the seasonal variation of the hydrological conditions.

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.

Simultaneous determination of mercury, cadmium and lead in fish sauce using Diffusive Gradients in Thin-films technique

Fish sauce is a popular seasoning liquid originating from southeastern Asian cuisine, consisting of fermented fish, salt and additional ingredients. Fish can contain high amounts of metals, some of which are hazardous for human health. Therefore, authorities responsible for food safety and quality should monitor the levels of these contaminants in fish and fish deviated products. In this work, the passive sampling technique of Diffusive Gradients in Thin-films (DGT) containing Chelex-100 and Purolite S924 resin gels, is used for the determination of dissolved mercury (Hg), cadmium (Cd) and lead (Pb) in fish sauce. The DGT performance test showed linear accumulation of Hg, Cd and Pb on the binding gels versus deployment time. A wide range of pH and salt concentration did not affect the performance of the DGT. The effective diffusion coefficients of Hg, Cd and Pb in diffusive gels were determined by applying a series of deployments in fish sauce solution. Besides the direct sampling with the DGT technique, fish sauce samples were also digested using a microwave oven. Analyses of DGT and microwave oven digested samples were performed with Sector Field Inductively Coupled Plasma Mass Spectrometry (SF-ICP-MS). Both methods were then used for the analysis of fish sauces from local retail stores. Due to the preconcentration ability of DGT, lower detection limits of Hg, Cd and Pb could be achieved compared to the microwave digestion method. The DGT technique offers a more sensitive method for trace element analysis in complex food matrices.

Monitoring Organic Pollutants in Waters Using the Diffusive Gradients in the Thin Films Technique: Investigations on the Effects of Biofouling and Degradation

The diffusive gradients in thin films (DGT) passive sampling technique has been increasingly used to provide time-weighted average concentrations of the biorelevant fraction of organic contaminants in waters, with high spatial and temporal resolutions at low cost. This study was tested for the effects of biofouling and compound degradation/loss during sample handling/storage on the DGT measurement of a range of emerging organic pollutants. Biofouling was tested using biofilms collected from the influent and effluent of a typical urban wastewater treatment plant. Most (85%) target compounds showed no detectable effect on the DGT measurement when 8- and 15-day biofouled membrane filters were used. Four storage methods were designed to test for within-sampler degradation/loss for up to 2 months. Intact samplers can be simply stored in polyethylene bags at ambient temperature (18-26 °C) with most compounds stable (mass loss <20%) for at least 1 week. Keeping intact samplers at 4 °C or binding gels in solvent gave good recoveries, with most chemicals being stable for up to 2 months, although the best results were obtained when binding gels were kept in solvent at 4 °C. Recommendations are made for sample handling and storage of DGT samplers used for determination of trace organics in monitoring and surveillance campaigns.

POCIS Calibration for Organic Compound Sampling in Small Headwater Streams

Field-based atrazine sampling rates (R_s ) obtained by the polar organic chemical integrative sampler (POCIS) method were measured in 9 headwater streams over 3 yr covering 5 to 6 exposure periods of 2 to 3 wk/site/yr. Rates were best in line with the model R_s  = 148 mL/d, with a standard deviation of 0.17 log units (factor 1.5). The POCIS canisters reduced mass transfer coefficients of the water boundary layer by a factor of 2 as measured by alabaster dissolution rates. A mechanistic model that accounts for flow and temperature effects yielded a fair estimate of the effective exchange surface area (12.5 ± 0.8 cm² ). This model could only be tested for higher flow velocities because of uncertainties associated with the measurement of flow velocities <1 cm/s. Pictures of sorbent distributions in POCIS devices showed that the effective exchange surface area varied with time during the exposures. Error analysis indicated that sorbent distributions and chemical analysis were minor error sources. Our main conclusion is that an atrazine sampling rate of 148 mL/d yielded consistent results for all 3 yr across 9 headwater streams. Environ Toxicol Chem 2020;39:1334-1342. © 2020 SETAC.

Influence of some physicochemical parameters on the passive sampling of copper (II) from aqueous medium using a polymer inclusion membrane device

Recently polymer inclusion membranes (PIMs) have been proposed as materials for passive sampling, nonetheless a theoretical base to describe the mass transfer process through those materials, under such conditions of monitoring, has not been elucidated. Under the assumption that: (i) the transport of the metal ion occurs at steady state conditions, (ii) the concentration gradients are linear, and (iii) the kinetics of the chemical reactions in the extraction process on the membrane are elemental; an equation for the passive sampling of copper (II) using a PIM system containing Kelex-100 as carrier is derived. The prediction capacity of this sampler under different conditions of temperature, metal concentration, flow velocity, ionic strength and pH is analyzed as well. Among the dependencies of the PIM on the physicochemical conditions, effects of concentration, temperature and flow velocity tend to increment copper (II) flux across the membrane, being the parameter temperature the one with the most pronounced effect at T ≥ 30 °C. Ionic strength had no great effect on passive sampler response, however the sampler is dependent on the acidity of the medium. The comparable metal ion concentrations estimated from the PIM sampler to those obtained by direct measurements of the sampling medium suggest that PIMs can be robust materials when used as passive sampler devices.

Operational DGT threshold values for metals in seawater from protected coastal areas in Sardinia (Western Mediterranean)

Diffusive gradients in thin films (DGTs) were used for monitoring metal (Cd, Cu, Ni, and Pb) concentrations in protected and non-protected coastal areas in Sardinia (Western Mediterranean). The deployment of DGTs in relatively undisturbed areas enabled calculation of operational DGT threshold values, which can be used for assessments of the environmental quality of coastal areas. The DGT thresholds were defined as the median metal concentrations that were found in protected areas, which ensured consideration of the natural variability of the different study sites. The calculated DGT thresholds were 11.6 ng L^-1 for Pb, 5.1 ng L^-1 for Cd, 63 ng L^-1 for Cu and 152 ng L^-1 for Ni. A comparison of the calculated DGT thresholds with previous DGT studies in the area demonstrated their suitability for identifying sites of environmental concern in the Western Mediterranean.

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.