Modeling the transport of organic chemicals between polyethylene passive samplers and water in finite and infinite bath conditions

Unveiling the ecotoxicological impact of microplastics on organisms - the persistent organic pollutant (POP): A comprehensive review

Microplastics have been ubiquitous in our environment for decades, and numerous studies have revealed their extensive dispersion, reaching far beyond the surface of the land, soil, aquatic ecosystems. They have infiltrated the food-chain, the food web, even the air we breathe, as well as the water we drink. Microplastics have been detected in the food we consume, acting as vectors for hazardous chemicals that adhere to their hydrophobic surfaces. This can result in the transfer of these chemicals to the aquatic life, posing a threat to their well-being. The release of microplastics into different environmental settings can give rise to various eco-toxicological implications. The substantial body of literature has led scientists to the consensus that microplastic pollution is a global problem with the potential to impact virtually any type of ecosystem. This paper aims to discuss crucial information regarding the occurrence, accumulation, and ecological effects of microplastics on organisms. It also highlights the new and emerging disease named "Plasticosis" that is directly linked to microplastics and its toxicological effects like permanent scarring and long-term inflammation in the digestive system of the seabirds. By comprehending the behaviour of these microplastic pollutants in diverse habitats and evaluating their ecological consequences, it becomes possible to facilitate a better understanding of this toxicological issue.

Modelling passive sampling of hydrophilic compounds under time-variable aqueous concentrations

Passive sampling is a crucial method for evaluating concentrations of hydrophilic organic compounds in the aquatic environment, but it is insufficiently understood to what extent passive samplers capture the intermittent emissions that frequently occur for this group of compounds. In the present study, silicone sheets and styrene-divinyl benzene-reversed phase sulfonated extraction disks with and without a polyethersulfone membrane were exposed under semi-field conditions in a 31 m3 flume at three different flow velocities. Natural processes and spiking/dilution measures caused aqueous concentrations to vary strongly with time. The data were analyzed using two analytical models that account for these time-variable concentrations: a sampling rate model and a diffusion model. The diffusion model generally gave a better fit of the data than the sampling rate model, but the difference in residual errors was quite small (median errors of 19 vs. 25% for silicone and 22 vs. 25% for SDB-RPS samplers). The sampling rate model was therefore adequate enough to evaluate the time-integrative capabilities of the samplers. Sampler performance was best for SDB-RPS samplers with a polyethersulfone membrane, despite the occurrence of lag times for some compounds (0.1 to 0.4 days). Sampling rates for this design also spanned a narrower range (80 to 110 mL/day) than SDB-RPS samplers without a membrane (100 to 660 mL/day). The effect of biofouling was similar for all compounds and was consistent with a biofouling layer thickness of 150 µm.

Release of Additives from Agricultural Plastic Films in Water: Experiment and Modeling

Globally, more than 6 million metric tons of agricultural plastic films are used to increase crop yields and reduce the use of water and herbicides, resulting in the contamination of soil and water by plastic debris and additives. However, knowledge of the occurrence and release of additives from agricultural films is limited. In this study, suspect screening with high-resolution mass spectrometry, one-dimensional Fickian diffusion models, and linear free energy relationships (LFERs) were used to determine the occurrence and mass transfer of various additives from agricultural plastic films. A total of 89 additives were tentatively identified in 40 films, and 62 of them were further validated and quantified. The aqueous concentrations of 26 released additives reached mg L^-1 after a 28 day incubation at 25 °C. Diffusion models and LFERs demonstrated that the film-water partition coefficient and the diffusivity in the polymer, the two critical parameters controlling the mass transfer, could be predicted using Abraham descriptors. The findings of this study highlighted the need for future research on the environmental fate and risk assessment of previously neglected additives in agricultural plastic films and other similar products.

Passive Sampler Exchange Kinetics in Large and Small Water Volumes Under Mixed Rate Control by Sorbent and Water Boundary Layer

Exchange kinetics of organic compounds between passive samplers and water can be partly or completely controlled by transport in the sorbent. In such cases diffusion models are needed. A model is discussed that is based on a series of cosines (space) and exponentials (time). The model applies to mixed rate control by sorbent and water boundary layer under conditions of fixed aqueous concentrations (open systems, infinite water volumes, in situ sampling) and fixed amounts (closed systems, finite water volumes, ex situ sampling). Details on the implementation of the model in computational software and spreadsheet programs are discussed, including numerical accuracy. Key parameters are Biot number (ratio of internal/external transfer resistance) and sorbent/water phase ratio. Small Biot numbers are always indicative of rate control by the water boundary layer, but for large Biot numbers this may still be the case over short time scales. Application to environmental monitoring of nonpolar compounds showed that diffusion models are rarely needed for sampling with commonly used single-phase polymers. For determining sorption coefficients in batch incubations, the model demonstrated a profound effect of sorbent/water phase ratio on time to equilibrium. Application of the model to sampling of polar organic compounds by extraction disks with or without a membrane showed that moderate to major sorbent-controlled kinetics is likely to occur. This implies that the use of sampling rate models for such samplers needs to be reconsidered. Environ Toxicol Chem 2021;40:1241-1254. © 2021 SETAC.

Application of polyoxymethylene passive air sampler to monitor hydrophobic organics in air around a confined disposal facility

Volatile losses of hydrophobic organic contaminants from a confined disposal facility (CDF) containing dredged contaminated sediments is of substantial concern to surrounding communities. A partitioning passive sampling approach using polyoxymethylene (POM) was applied to measure long-term average (weeks to months) air concentrations resulting from evaporation at a CDF. Measurements at 10 locations surrounding the CDF using the POM air samplers indicated that the highest concentrations of ΣPCBs∼13 ng/m³ and ΣPAHs ∼65 ng/m³ were measured during an active dredge material placement period when the average temperature was 23 °C. The measurements were dominated by the more volatile, lower molecular weight compounds of each type. Partitioning to the POM during the post dredge material placement period with average temperature of 5 °C was corrected for temperature and the measured ∑PCBs and ∑PAHs were ∼3 ng/m³ and 45 ng/m³ respectively. The partitioning passive sampling measurements agreed well with the available weekly 24-h high-volume air samples (HVAS) averaged over the POM equilibration time for lower congener number PCBs (15, 18, 20/28 and 31) and naphthalene but were as much as 10 times lower than HVAS for high molecular weight PAHs. The difference was likely the result of the greater association of these PAHs with particulates and sources other than evaporation from the CDF. The POM air sampler achieved the goal of providing a long-term average air concentration without having to collect, analyze and average multiple HVAS samples although the technique is largely limited to the lower molecular weight PAHs and PCBs and different equilibration times for different compounds complicate its use and analysis.

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.

In Situ Investigation of Performance Reference Compound-Based Estimates of PCB Equilibrated Passive Sampler Concentrations and Cfree in the Marine Water Column

Low-density polyethylene sheets are used as passive samplers for aquatic environmental monitoring to measure the freely dissolved concentration (Cfree ) of hydrophobic organic contaminants (HOCs). Freely dissolved HOCs in water will partition into the polyethylene until a thermodynamic equilibrium is achieved; that is, the HOC's activity in the passive sampler is the same as its activity in the surrounding environment. One way to evaluate the equilibrium status or estimate the uptake kinetics is by using performance reference compounds (PRCs). A fractional equilibrium (feq ) can be determined for target HOCs, under the assumption that PRC desorption from the passive sampler occurs at the same rate as for the unlabeled target HOCs. However, few investigations have evaluated how effectively and accurately PRCs estimate target contaminant Cfree under in situ conditions. In the present study, polyethylene passive samplers were preloaded with 6 13 C-labeled polychlorinated biphenyls (PCBs) as PRCs; deployed in New Bedford Harbor, Massachusetts, USA; and collected after 30-, 56-, 99-, and 129-d deployments. Using this unique temporal sampling design, PRC results from each deployment were fit to a diffusion model to estimate the Cfree of 27 PCB congeners and compare the results between the different deployment times. Smaller PCBs had variable concentrations over the 4 deployments, whereas mid-molecular weight PCBs had consistent Cfree measurements for all deployments (relative standard deviation <20%). High-molecular weight PCBs had the largest Cfree estimates after 30 d; these estimates and their standard deviations decreased with longer deployment times. These findings suggest that when targeting PCBs with more than 6 chlorines or contaminants with a log octanol-water partition coefficient ≥6.5, a deployment time longer than 30 d may be prudent. Environ Toxicol Chem 2020;39:1165-1173. © 2020 SETAC.

Polyethersulfone as suitable passive sampler for waterborne hydrophobic organic compounds - Laboratory calibration and field test in the Sosiani river, Kenya

We report the application of polyethersulfone (PES) membrane as a cost-saving and less labour-intensive single-phase passive sampler for waterborne hydrophobic organic compounds (HOCs) like organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs). The uptake kinetics of 31 HOCs from water to porous polyethersulfone (PES) membranes and their partitioning behaviour were investigated in laboratory studies. Sampling rates (R_s) of HOCs with PES were determined in a range from 1.15 to 12.9 L/d. The uptake of test chemicals and the elimination of analogous (pre-loaded) performance reference compounds (PRCs) showed anisotropy, both under laboratory and field conditions, implying that PRCs are not suitable for determining in situ sampling rates with PES. The PES-water partition coefficients (K_pw) are, on average, ten times higher than the related K_ow. A Linear Solvation Energy Relationship for modelling the measured log K_pw with PES under inclusion of all available published data yields a poor fit in comparison to what is usually obtained with homogeneous polymers like polydimethylsiloxane or low-density polyethylene. At least a strong linear relationship was found between log R_s and log K_pw for the narrow log K_ow range of HOCs investigated in this work which can be used for interpolation to other HOCs in this range. The PES membranes were also tested in a field trial in a tropical river against the well-established silicone rubber (SR) sheets. With laboratory-based R_s for PES generated under field-relevant temperature and water flow velocity it was possible to obtain time-weighted average concentrations in the lower ng/L range which are comparable (within a factor of two) with those derived from accumulated amounts in SR sheets (using in situ sampling rates).

Sorption kinetics of parent and substituted PAHs for low-density polyethylene (LDPE): Determining their partition coefficients between LDPE and water (KLDPE) for passive sampling

Low-density polyethylene (LDPE) has been widely used as a sorbent for passive sampling of hydrophobic organic contaminants (HOCs) in aquatic environments. However, it has seen only limited application in passive sampling for measurement of freely dissolved concentrations of parent and substituted PAHs (SPAHs), which are known to be toxic, mutagenic and carcinogenic. Here, the 16 priority PAHs and some typical PAHs were selected as target compounds and were simultaneously determined by gas chromatography-mass spectrometer (GC-MS). Some batch experiments were conducted in the laboratory to explore the adsorption kinetics of the target compounds in LDPE membranes. The results showed that both PAHs and SPAHs could reach equilibrium status within 19-38 days in sorption kinetic experiments. The coefficients of partitioning between LDPE film (50 μm thickness) and water (K_LDPE) for the 16 priority PAHs were in good agreement with previously reported values, and the values of K_LDPE for the 9 SPAHs are reported in this study for the first time. Significant linear relationships were observed, i.e., log K_LDPE = 0.705 × log K_OW + 1.534 for PAHs (R² = 0.8361, p < 0.001) and log K_LDPE = 0.458 × log K_OW + 3.092 for SPAHs (R² = 0.5609, p = 0.0077). The selected LDPE film was also proven to meet the condition of "zero sink" for the selected target compounds. These results could provide basic support for the configuration and in situ application of passive samplers.

Microplastic-Contaminant Interactions: Influence of Nonlinearity and Coupled Mass Transfer

Microplastic particles are ubiquitously detected in the environment. Despite intensive public and scientific discussions, their potential to transport contaminants in rivers and oceans under environmental conditions is still under assessment. In the present study we measured sorption isotherms and kinetics in batch experiments using phenanthrene (as a typical hydrophobic wastewater contaminant) and microplastic particles of different sizes and materials. We observed a linear sorption isotherm for polyethylene, in contrast to nonlinear sorption of polyamide and polystyrene, which could be best described by the Freundlich and the Polanyi-Dubinin-Manes isotherms, respectively. We modeled sorption kinetics as a combination of external mass transfer governed by diffusion through an aqueous boundary layer and intraparticle diffusion within the plastic. Which of these processes controls the kinetics depends on the sorption strength, particle size, diffusion coefficients, and time. We used semi-analytical and numerical methods to simulate the coupled mass transfer for both linear and nonlinear sorption. We successfully applied the semi-analytical model to polyethylene and the numerical code to polyamide and polystyrene, reproducing the measured kinetics and obtaining reasonable values for mass transfer and intraparticle diffusion coefficients. Subsequently, we used these coefficients to estimate the transport potential and relevant time scales for microplastic-bound contaminants under environmental conditions. Environ Toxicol Chem 2019;38:1635-1644. © 2019 SETAC.

Characterization of sorption properties of high-density polyethylene using the poly-parameter linearfree-energy relationships

High-density polyethylene (HDPE) is a known sorbent for non-ionic organic compounds in technical applications. Nevertheless, there is little information available describing sorption to industrial HDPE for a broad range of compounds. With a better understanding of the sorption properties of synthetic polymers, environmental risk assessment would achieve a higher degree of accuracy, especially for microplastic interactions with organic substances. Therefore, a robust methodology for the determination of sorbent properties for non-ionic organic compounds by HDPE is relevant for the understanding of molecular interactions for both technical use and environmental risk assessment. In this work, sorption properties of HDPE material used for water pipes were characterized using a poly-parameter linear free-energy relationship (ppLFER) approach. Sorption batch experiments with selected probe sorbates were carried out in a three-phase system (air/HDPE/water) covering an aqueous concentration range of at least three orders of magnitude. Sorption in the concentration range below 10^-2 of the aqueous solubility was found to be non-linear and the Freundlich model was used to account for this non-linearity. Multiple regression analysis (MRA) using the determined distribution coefficients and literature-tabulated sorbate descriptors was performed to obtain the ppLFER phase descriptors for HDPE. Sorption properties of HDPE were then derived from the ppLFER model and statistical analysis of its robustness was conducted. The derived ppLFER model described sorption more accurately than commonly used single-parameter predictions, based i.e., on log K_o/w. The ppLFER predicted distribution data with an error 0.5 log units smaller than the spLFERs. The ppLFER was used for a priori prediction of sorption by the characterized sorbent material. The prediction was then compared to experimental data from literature and this work and demonstrated the strength of the ppLFER, based on the training set over several orders of magnitude.

An exponential model based new approach for correcting aqueous concentrations of hydrophobic organic chemicals measured by polyethylene passive samplers

Although low density polyethylene (PE) passive samplers show promise for the measurement of aqueous phase hydrophobic organic chemicals (HOCs), the lack of a practical and unsophisticated approach to account for non-equilibrium exposure conditions has impeded widespread acceptance and thus application in situ. The goal of this study was to develop a streamlined approach based on an exponential model and a convection mass transfer principle for correcting aqueous concentrations for HOCs deduced by PE samplers under non-equilibrium conditions. First, uptake rate constants (k₁), elimination rate constants (k₂), and seawater-PE equilibrium partition coefficients (K_PEWs) were determined in laboratory experiments for a diverse suite of HOCs with logK_ow range of 3.4-8.3. Linear relationships between log k₂ and logK_ow, and between log K_PEW and logK_ow were established. Second, PE samplers pre-loaded with ¹³C-labeled performance reference compounds (PRCs) were deployed in the ocean to determine their k₂in situ. By applying boundary layer and convection mass transfer theories, ratio (C) of k₂ values in field and laboratory exposures was estimated. This C value was demonstrated a constant that was only determined by water velocities and widths of PE strips. A generic equation with C and logK_ow as parameters was eventually established for extrapolation of non-equilibrium correction factors for the water boundary layer-controlled HOCs. Characterizing the hydrodynamic conditions indicated the sampler configuration and mooring mode should aim at sustaining laminar flow on the PE surface for optimal mass transfer. The PE estimates corrected using this novel approach possessed high accuracy and acceptable precision, and can be suited for a broad spectrum of HOCs. The presented method should facilitate routine utilization of the PE samplers.

Determination of PCB fluxes from Indiana Harbor and Ship Canal using dual-deployed air and water passive samplers

We have developed a method for measuring fluxes of PCBs from natural waters using air and water passive samplers deployed simultaneously in the Indiana Harbor and Ship Canal (IHSC). Net volatilization of ƩPCBs was determined for 2017, and ranged from 1.4 to 2.8 μg m^-2 d^-1, with a median of 2.0 μg m^-2 d^-1. We confirm earlier findings that the IHSC experiences constant release of gas-phase PCBs. Gas-phase and freely-dissolved water ƩPCB samples median were 4.0 ng m^-3 and 14 ng L^-1, both exhibiting increasing concentrations over the year of study, and with a strong positive correlation between them (R² = 0.93 for ƩPCBs). The relative concentrations of individual PCB congeners were very similar between air and water samples, and resemble Aroclor 1248, a mixture previously reported to contaminate the IHSC sediments. Monthly variability of the volatilization fluxes was primarily driven by the freely-dissolved water concentration changes (R² = 0.87). Although different sampling methods were performed to estimate air-water fluxes between the month of August of 2006 and 2017, ƩPCB net fluxes have decreased by more than 60%, suggesting that either dredging at IHSC from 2012 to 2017 or reduction of upstream sources have decreased the freely-dissolved water concentrations of PCBs, thus reducing the air-water net volatilization in IHSC. Finally, we have shown that this passive sampling approach represents a simple and cost-effective method to assess the air-water exchange of PCBs, increase analytical sensitivity, enable measurements over time, and reduce uncertainties related to unexpected episodic events.

Evaluation of passive sampling polymers and nonequilibrium adjustment methods in a multiyear surveillance of sediment porewater PCBs

Polymeric passive sampling devices are increasingly used to measure low-level, freely dissolved concentrations of hydrophobic organic contaminants in environmental waters. A range of polymers have been used for this purpose, and several different methods of accounting for nonequilibrium using performance reference compounds (PRCs) have been proposed. The present study explores the practical impacts of these decisions in an applied context using results from a multiyear passive sampling surveillance of polychlorinated biphenyl concentrations in sediment porewater at a contaminated marsh amended with activated carbon (AC) sorbent materials. In a series of 5 sampling events spanning almost 2 yr, we deployed polyoxymethylene and polyethylene samplers and calculated porewater concentrations with 5 different PRC adjustment methods. The results provide a basis for evaluating amendment performance by showing reductions of 34 to 97% in amended sediment porewater concentrations. They also provide a quantitative underpinning for discussions of the differences between sampling polymers, selection of PRCs, generation of high-resolution vertical profiles of porewater concentrations, and a comparison of PRC adjustment methods. For unamended sediment, older methods based on first-order kinetics agreed well with a recently developed method based on diffusion into and out of sediment beds. However, the sediment diffusion method did not work well for the sediments amended with AC. Environ Toxicol Chem 2018;37:2487-2495. © 2018 SETAC.

Performance of passive sampling with low-density polyethylene membranes for the estimation of freely dissolved DDx concentrations in lake environments

Laboratory and field studies were used to evaluate the performance of low-density polyethylene (PE) passive samplers for assessing the freely dissolved concentrations of DDT and its degradates (DDD and DDE, together referred to as DDx) in an Italian lake environment. We tested commercially available 25 μm thick PE sheets as well as specially synthesized, 10 μm thick PE films which equilibrated with their surroundings more quickly. We measured PE-water partitioning coefficients (K_pew) of the 10 μm thick PE films, finding good correspondence with previously reported values for thicker PE. Use of the 10 μm PE for ex situ sampling of a lake sediment containing DDx in laboratory tumbling experiments showed repeatability of ±15% (= standard deviation/mean). Next, we deployed replicate 10 μm and 25 μm PE samplers (N = 4 for 10 d and for 30 d) in the water and sediment of a lake located in northern Italy; the results showed dissolved DDx concentrations in the picogram/L range in porewater and the bottom water. Values deduced from 10 μm thick PE films compared well (95% of all comparison pairs matched within a factor of 5) with those obtained using PE films of 25 μm thickness when dissolved DDx concentrations were estimated using performance reference compound (PRC) corrections, whether left at the bed-water interface for 10 or 30 days. These results demonstrated the potential of this sampling method to provide estimation of the truly dissolved DDx concentrations, and thereby the mobile and bio-available fractions in both surface waters and sediment beds.

Investigating the Effect of Bioirrigation on In Situ Porewater Concentrations and Fluxes of Polychlorinated Biphenyls Using Passive Samplers

Polychlorinated biphenyl (PCB) fluxes from contaminated sediments can be caused by mechanisms including diffusion, bioirrigation, and resuspension, but it is often unclear which mechanisms are important. In the Lower Duwamish Waterway (Seattle, Washington), the presence of abundant benthic macrofauna suggests that porewater bioirrigation may be an important mechanism for PCB transport from the bed into the overlying water column. In this field study, the fluxes of PCBs due to bioirrigation were quantified by using (a) polyethylene (PE) samplers to quantify in situ and ex situ (i.e., equilibrium) PCB porewater concentration profiles and (b) measurements of the geochemical tracer ²²²Rn to quantify the rate of porewater exchange with overlying water. The results showed that bioirrigation caused sorptive disequilibrium with the surrounding sediment, which led to lower in situ porewater concentrations than expected from sediment concentrations. The combined fluxes of seven PCB congeners (Σ₇PCBs) were 1.6-26 ng/m²/day for the three field sites, similar in magnitude to the upper limit estimates of diffusive fluxes calculated assuming water-side boundary layer control (Σ₇PCBs = 1.3-47 ng/m²/day). Moreover, the depleted in situ porewater concentrations imply lower diffusive flux estimates than if the ex situ porewater concentrations had been used to estimate fluxes (Σ₇PCBs = 89-670 ng/m²/day). These results suggest that nondiffusive PCB fluxes from the sediment bed are occurring and that quantifying in situ porewater concentrations is crucial for accurately quantifying both diffusive and nondiffusive PCB fluxes.

Calibration of silicone rubber rods as passive samplers for pesticides at two different flow velocities: Modeling of sampling rates under water boundary layer and polymer control

There is a need to determine time-weighted average concentrations of polar contaminants such as pesticides by passive sampling in environmental waters. Calibration data for silicone rubber-based passive samplers are lacking for this class of compounds. The calibration data, sampling rate (R_s ), and partition coefficient between silicone rubber and water (K_sw ) were precisely determined for 23 pesticides and 13 candidate performance reference compounds (PRCs) in a laboratory calibration system over 14 d for 2 water flow velocities, 5 and 20 cm s^-1 . The results showed that an in situ exposure duration of 7 d left a silicone rubber rod passive sampler configuration in the linear or curvilinear uptake period for 19 of the pesticides studied. A change in the transport mechanism from polymer control to water boundary layer control was observed for pesticides with a log K_sw of approximately 3.3. The PRC candidates were not fully relevant to correct the impact of water flow velocity on R_s . We therefore propose an alternative method based on an overall resistance to mass transfer model to adjust R_s from laboratory experiments to in situ hydrodynamic conditions. We estimated diffusion coefficients (D_s ) and thickness of water boundary layer (δ_w ) as adjustable model parameters. Log D_s values ranged from -12.13 to -10.07 m²  s^-1 . The estimated δ_w value showed a power function correlation with water flow velocity. Environ Toxicol Chem 2018;37:1208-1218. © 2017 SETAC.

Passive sampling of DDT, DDE and DDD in sediments: accounting for degradation processes with reaction-diffusion modeling

Passive sampling is becoming a widely used tool for assessing freely dissolved concentrations of hydrophobic organic contaminants in environmental media. For certain media and target analytes, the time to reach equilibrium exceeds the deployment time, and in such cases, the loss of performance reference compounds (PRCs), loaded in the sampler before deployment, is one of the common ways used to assess the fractional equilibration of target analytes. The key assumption behind the use of PRCs is that their release is solely diffusion driven. But in this work, we show that PRC transformations in the sediment can have a measurable impact on the PRC releases and even allow estimation of that compound's transformation rate in the environment of interest. We found that in both field and lab incubations, the loss of the ¹³C 2,4'-DDT PRC from a polyethylene (PE) passive sampler deployed at the sediment-water interface was accelerated compared to the loss of other PRCs (¹³C-labeled PCBs, ¹³C-labeled DDE and DDD). The DDT PRC loss was also accompanied by accumulation in the PE of its degradation product, ¹³C 2,4'-DDD. Using a 1D reaction-diffusion model, we deduced the in situ degradation rates of DDT from the measured PRC loss. The in situ degradation rates increased with depth into the sediment bed (0.14 d^-1 at 0-10 cm and 1.4 d^-1 at 30-40 cm) and although they could not be independently validated, these rates compared favorably with literature values. This work shows that passive sampling users should be cautious when choosing PRCs, as degradation processes can affect some PRC's releases from the passive sampler. More importantly, this work opens up the opportunity for novel applications of passive samplers, particularly with regard to investigating in situ degradation rates, pathways, and products for both legacy and emerging contaminants. However, further work is needed to confirm that the rates deduced from model fitting of PRC loss are a true reflection of DDT transformation rates in sediments.

Shift in Mass Transfer of Wastewater Contaminants from Microplastics in the Presence of Dissolved Substances

In aqueous environments, hydrophobic organic contaminants are often associated with particles. Besides natural particles, microplastics have raised public concern. The release of pollutants from such particles depends on mass transfer, either in an aqueous boundary layer or by intraparticle diffusion. Which of these mechanisms controls the mass-transfer kinetics depends on partition coefficients, particle size, boundary conditions, and time. We have developed a semianalytical model accounting for both processes and performed batch experiments on the desorption kinetics of typical wastewater pollutants (phenanthrene, tonalide, and benzophenone) at different dissolved-organic-matter concentrations, which change the overall partitioning between microplastics and water. Initially, mass transfer is externally dominated, while finally, intraparticle diffusion controls release kinetics. Under boundary conditions typical for batch experiments (finite bath), desorption accelerates with increasing partition coefficients for intraparticle diffusion, while it becomes independent of partition coefficients if film diffusion prevails. On the contrary, under field conditions (infinite bath), the pollutant release controlled by intraparticle diffusion is not affected by partitioning of the compound while external mass transfer slows down with increasing sorption. Our results clearly demonstrate that sorption/desorption time scales observed in batch experiments may not be transferred to field conditions without an appropriate model accounting for both the mass-transfer mechanisms and the specific boundary conditions at hand.

The atmosphere as a source/sink of polychlorinated biphenyls to/from the Lower Duwamish Waterway Superfund site

Waterbodies polluted with polychlorinated biphenyls (PCBs) may cause the air in the surrounding area to become PCB-contaminated. Conversely, when a waterbody is located in or near an urban area, the deposition of atmospheric PCBs may act as a low-level, ongoing source of PCB contamination to that water. Distinguishing these situations is necessary to be protective of human populations and to guide efforts seeking to cleanup such aquatic ecosystems. To assess the situation at the Lower Duwamish Waterway (LDW) Superfund site, low-density polyethylene passive samplers were deployed in the summer of 2015 to quantify freely dissolved water and gaseous air concentrations of PCBs thereby enabling estimates of the direction and magnitude of air-water exchange of PCB congeners. For the sum of the 27 PCB congeners, average concentrations were 220 pg/m³ (95% C.I.: 80-610) in the air and 320 pg/L (95% C.I.: 110-960) in the water. The sum of air-water exchange fluxes of these PCB congeners was estimated to be 68 ng/m²/day (95% C.I.: 30-148) into the lower atmosphere, contrasting with the reported wet and dry depositional flux of only 5.5 ng/m²/day (95% C.I.: 1-38) from the air into the water. Therefore, the atmosphere was ultimately a sink of PCBs from the LDW Superfund site, at least under 2015 summertime conditions. However, we conclude that air-water exchange of PCBs is likely only a minor sink of PCBs from the LDW and only a minor source of contamination to the region's local atmosphere.

A method to account for the effect of hydrodynamics on polar organic compound uptake by passive samplers

Mass transfer coefficients of the water boundary layer (k_w ) were measured using alabaster dissolution kinetics in a diffusion cell that was operated at stirring rates between 90 min^-1 and 600 min^-1 , aiming to provide a more robust characterization of the effect of hydrodynamics on the uptake of polar compounds by passive samplers, as compared with characterizations in terms of stirring rates and water flow velocities. The measured k_w helped to quantitatively understand calcium sulfate transport through a poly(ethersulfone) membrane and 2 water boundary layers (at both sides of the membrane). Alabaster-based k_w value were used to understand atrazine transport in the diffusion cell, allowing the conclusion that atrazine transport in the membrane is via the pore space, rather than via the polymer matrix. The merits of measuring alabaster dissolution rates for passive sampler calibration and application in the field are discussed. The authors propose that passive sampler calibrations be carried out under controlled k_w conditions, rather than under controlled stirring rates or flow velocities. This would facilitate the interpretation of passive sampler calibration studies and the translation of laboratory-based water sampling rates to flow conditions that apply in the field. Environ Toxicol Chem 2017;36:1517-1524. © 2016 SETAC.

Evaluating the Relationship between Equilibrium Passive Sampler Uptake and Aquatic Organism Bioaccumulation

This Critcal Review evaluates passive sampler uptake of hydrophobic organic contaminants (HOCs) in water column and interstitial water exposures as a surrogate for organism bioaccumulation. Fifty-seven studies were found where both passive sampler uptake and organism bioaccumulation were measured and 19 of these investigations provided direct comparisons relating passive sampler uptake and organism bioaccumulation. Polymers compared included low-density polyethylene (LDPE), polyoxymethylene (POM), and polydimethylsiloxane (PDMS), and organisms ranged from polychaetes and oligochaetes to bivalves, aquatic insects, and gastropods. Regression equations correlating bioaccumulation (C_L) and passive sampler uptake (C_PS) were used to assess the strength of observed relationships. Passive sampling based concentrations resulted in log-log predictive relationships, most of which were within one to 2 orders of magnitude of measured bioaccumulation. Mean coefficients of determination (r²) for LDPE, PDMS, and POM were 0.68, 0.76, and 0.58, respectively. For the available raw, untransformed data, the mean ratio of C_L and C_PS was 10.8 ± 18.4 (n = 609). Using passive sampling as a surrogate for organism bioaccumulation is viable when biomonitoring organisms are not available. Passive sampling based estimates of bioaccumulation provide useful information for making informed decisions about the bioavailability of HOCs.

In situ passive sampling of sediments in the Lower Duwamish Waterway Superfund site: Replicability, comparison with ex situ measurements, and use of data

Superfund sites with sediments contaminated by hydrophobic organic compounds (HOCs) can be difficult to characterize because of the complex nature of sorption to sediments. Porewater concentrations, which are often used to model transport of HOCs from the sediment bed into overlying water, benthic organisms, and the larger food web, are traditionally estimated using sediment concentrations and sorption coefficients estimated using equilibrium partitioning (EqP) theory. However, researchers have begun using polymeric samplers to determine porewater concentrations since this method does not require knowledge of the sediment's sorption properties. In this work, polyethylene passive samplers were deployed into sediments in the field (in situ passive sampling) and mixed with sediments in the laboratory (ex situ active sampling) that were contaminated with polychlorinated biphenyls (PCBs). The results show that porewater concentrations based on in situ and ex situ sampling generally agreed within a factor of two, but in situ concentrations were consistently lower than ex situ porewater concentrations. Imprecision arising from in situ passive sampling procedures does not explain this bias suggesting that field processes like bioirrigation may cause the differences observed between in situ and ex situ polymeric samplers.

Determination of silicone rubber and low-density polyethylene diffusion and polymer/water partition coefficients for emerging contaminants

There is a growing interest in assessing the concentration and distribution of new nonregulated organic compounds (emerging contaminants) in the environment. The measurement of freely dissolved concentrations using conventional approaches is challenging because of the low concentrations that may be encountered and their temporally variable emissions. Absorption-based passive sampling enables the estimation of freely dissolved concentrations of hydrophobic contaminants of emerging concern in water. In the present study, calibration was undertaken for 2 polymers, low-density polyethylene (LDPE) and silicone rubber for 11 fragrances, 5 endocrine-disrupting compounds, 7 ultraviolet (UV) filters, and 8 organophosphate flame retardant compounds. Batch experiments were performed to estimate contaminant diffusion coefficients in the polymers (Dp ), which in general decreased with increasing molecular weight. The values for fragrances, endocrine-disrupting compounds, and UV filters were in ranges similar to those previously reported for polycyclic aromatic hydrocarbons, but were 1 order of magnitude lower for organophosphate flame retardant compounds. Silicone rubber had higher Dp values than LDPE and was therefore selected for further experiments to calculate polymer/water partition coefficients (KPW ). The authors observed a positive correlation between log KPW and log octanol/water partition coefficient values. Field testing of silicone rubber passive samplers was undertaken though exposure in the River Alna (Norway) for an exposure time of 21 d to estimate freely dissolved concentration. Some fragrances and UV filters were predominant over other emerging and regulated contaminants, at levels up to 1600 ng L(-1) for galaxolide and 448 ng L(-1) for octocrylene. Environ Toxicol Chem 2016;35:2162-2172. © 2016 SETAC.

Understanding the rates of nonpolar organic chemical accumulation into passive samplers deployed in the environment: Guidance for passive sampler deployments

Polymeric passive samplers have become a common method for estimating freely dissolved concentrations in environmental media. However, this approach has not yet been adopted by investigators conducting remedial investigations of contaminated environmental sites. Successful adoption of this sampling methodology relies on an understanding of how passive samplers accumulate chemical mass as well as developing guidance for the design and deployment of passive samplers. Herein, we outline the development of a simple mathematical relationship of the environmental, polymer, and chemical properties that control the uptake rate. This relationship, called a timescale, is then used to illustrate how each property controls the rate of equilibration in samplers deployed in the water or in the sediment. Guidance is also given on how to use the timescales to select an appropriate polymer, deployment time, and suite of performance reference compounds. Integr Environ Assess Manag 2016;12:486-492. © 2015 SETAC.