Dynamic passive sampling of hydrophobic organic compounds in surface seawater along the South Atlantic Ocean east-to-west transect and across the Black Sea
Graphical abstract
Introduction
Contamination of seawater by bio-accumulative hydrophobic organic compounds (HOCs) is a concern around the globe, posing a threat to organisms, the marine ecosystem and human health. HOCs, such as polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polybrominated diphenylethers (PBDEs), novel brominated flame retardants (NBFRs) or hexabromocyclododecanes (HBCDs), together with polycyclic aromatic hydrocarbons (PAHs), are toxic, and many of them undergo long-range transport around the globe (Stemmler and Lammel, 2009, Stemmler and Lammel, 2013; Xie et al., 2011; Möller et al., 2011; Lohmann et al., 2012b; Bigot et al., 2016; Luek et al., 2017; Carrizo et al., 2017). Due to HOCs lipophilic properties, persistence and bioaccumulation potential (Lavandier et al., 2013; Gu et al., 2017), the concentrations found in high trophic level animals or humans can reach toxic levels (AMAP, 2015). Chronic exposure to HOCs can lead to adverse health effects such as disruption of physiological systems, cancer or fetal development interference (El-Shahawi et al., 2010). Despite that part of HOCs, known as persistent organic pollutants (POPs), were banned or their global use was restricted (UNEP, 2019), they are still ubiquitous, and their concentrations only slowly decrease in the terrestrial and marine environment in response to abatement measures.
Seawater is one of the environmental compartments containing HOCs and representing their essential transport medium. The HOCs are mainly emitted to seawater from rivers, dominantly bound to suspended particles (Koelmans et al., 2010; Fernandez et al., 2014; Meng et al., 2017), and from the atmosphere via dry and wet deposition (Lammel et al., 2015), or released directly from ships (Endres et al., 2018). The distribution in seawater is governed by a complex system of ocean fronts and currents. Ocean fronts are narrow areas with enhanced horizontal gradients, which separate areas with possibly different vertical stratification of trace concentrations. Between these areas, along-frontal currents are then responsible for a convective mass transfer of the contaminants, either freely dissolved or bound to particles (Dachs et al., 2002; Belkin and Cornillon, 2007; Belkin et al., 2009; Lohmann and Belkin, 2014). Suspended particles can later-on settle to the seabed (Lohmann and Belkin, 2014). HOCs in seawater can also bioaccumulate in aquatic organisms and marine food webs (Arnot and Gobas, 2006). Semivolatile HOCs may be returned to the atmosphere in relaxation to chemical equilibrium (Jantunen and Bidleman, 1998; Stemmler and Lammel, 2009). Since the HOCs are distributed to seawater from other environmental compartments, undergo long-range marine transport, bioaccumulate and very little is known about their distribution in global waters, their monitoring is necessary to comprehend the underlying processes. Obtained data can be used for looking at large-scale spatial and temporal trends, but also to assess the distribution of HOCs between the atmosphere, seawater and other marine compartments (biota including plankton, sediment, sea-ice, human-made microplastics). Also, they can be used to study phenomena related to compound distribution across compartments (e.g. bioaccumulation), related contaminant mass fluxes, and for marine ecosystem risk assessment.
Sampling of HOCs in seawater is challenging since their concentrations are very low, typically in sub-ng-per-litre levels. The methods based on sampling and analysis of grab samples are generally not suitable due to insufficient sensitivity and a high potential for analyte losses or sample contamination during sampling, storage, transport, and processing. However, in situ large volume extraction methods (e.g. Infiltrex type solid-phase extraction; SPE) proved to be suitable for sampling HOCs in seawater. Several sampling cruises applied these methods for monitoring HOCs in sections of open oceans and marginal seas: Arctic Ocean (Möller et al., 2011; Carrizo et al., 2017), Atlantic Ocean (Nizzetto et al., 2008; Xie et al., 2011; Lohmann et al., 2012b; Luek et al., 2017), Indian Ocean (Huang et al., 2014), Pacific Ocean (Zhang and Lohmann, 2010; Cai et al., 2016), Southern Ocean (Xie et al., 2011; Bigot et al., 2016; Cai et al., 2016), Black Sea (Maldonado and Bayona, 2002). Most of these cruises did not address only measuring HOC concentrations in seawater but also their air-sea mass exchange fluxes. Nevertheless, all aforementioned sampling cruises used a combination of a glass fibre filter with polyurethane foam, or XAD-2, or PAD-2 resin for HOCs extraction from seawater. This approach is suitable for the measurement of total concentration in seawater samples. The total aqueous concentration is dependent on the abundance of suspended particulate matter and colloids present, and does not provide an unbiased measure of compounds' freely dissolved concentration. The freely dissolved concentration is an important parameter because it is proportional to chemical activity, needed in multi-phase distribution modelling, as well as for estimating organism exposure in marine ecosystem risk assessment (Reichenberg and Mayer, 2006; Gilbert et al., 2016). To our knowledge, only passive sampling techniques are suitable for measurements of freely dissolved concentrations of HOCs in water.
The passive sampling (PS) technique is based on diffusion-driven sorption of the freely dissolved compounds from the sampled medium to the receiving phase until the equilibrium is reached or the sampling is stopped (time integrative sampling) (Vrana et al., 2005). Reaching the equilibrium for HOCs with log KOW > 6 during a deployment is time-consuming (Vrana et al., 2019), and the PS of HOCs in water typically works in the time integrative sampling regime. The uptake of the quantifiable amount of compounds from seawater into the sampling material may take several weeks (Allan et al., 2009). Such long periods correspond to typically low effective sampling rates, in the order of litres of water extracted per day. The low sampling rate is limiting PS onboard moving research vessels since the time period available for sampling is limited. Higher sampling rates can be achieved by increasing samplers' surface area or water turbulence, as the water boundary layer (WBL) mostly limits HOCs uptake to polymer-based PS (Booij et al., 2007).
Different approaches using PS technique were tested to sample surface water in an open ocean: static deployment performed by Ma et al. (2018) with deployment over one year with sufficient uptake. However, existing infrastructure has to be available, and the sampling is done only at one spot; sampling from a research vessel by towing the sampler (Lohmann et al., 2012b), where a larger geographic area was captured. However, only low sampling rates were obtained. Sampling from moving vessels requires a PS with high sampling rates in order to sample sufficient amounts of compounds for later quantification within the short time period of several days available for sampling, while the ship covers a certain stretch along the ocean transect. Vrana et al. (2018) demonstrated the applicability of a novel “dynamic” passive sampling device (DPS) in a freshwater environment developed to maximise contaminants' sampling rates by forcing water at high flow rate along the passive sampler surface. For a sampler with a surface area of 400 cm2, they obtained an average efficient WBL-controlled sampling rate ~70 L d−1 for a model compound with a molar mass of 300 g mol-1. In this way, almost 500 L of water can be extracted within one week of integrative sampling. Even the achievable limit of quantification (LOQ) favours DPS use since units of fg L−1 up to units of pg L−1 can be obtained with LOQs comparable to or lower than those expected in seawater. Therefore, the DPS device has the potential to address the monitoring of HOCs' spatial distributions in surface seawater.
In this study, we used dynamic passive sampling from ships cruising along transects in two case studies: the South Atlantic Ocean, and the Black Sea to (i) demonstrate the applicability and performance of mobile DPS for efficient sampling of HOCs in surface seawater, (ii) indicate HOC levels and their spatial gradients along the sampled transects.
Section snippets
Sampler preparation
Passive samplers were prepared from AlteSil (Altec, UK) translucent silicone elastomer (silicone rubber, SR) sheets (7 × 28 cm or 5.5 × 9.0 cm; 0.5 mm sheet thickness). Before exposure, SR sheets were Soxhlet extracted with ethyl acetate for 100 h to remove impurities and silicone oligomers. Then, sheets were homogeneously dosed with 14 performance reference compounds (PRCs: D10-biphenyl and 13 PCB congeners absent in technical mixtures; detailed in Tables S4 and S5) according to the procedure
Passive sampler performance
In this study, passive sampling was used for in situ concentration of HOCs from seawater into SR polymer. A DPS device was applied onboard expedition ships in order to maximise the sampled volume of water within the short time available for sampling from a moving ship. The DPS was applied based on previous evidence that it can increase Rs by >5-fold in comparison with samplers deployed in cages (Vrana et al., 2018). The passive sampling performance, characterised for each sampled compound by Rs
Conclusions
In this study, we reported freely dissolved concentrations of HOCs in the South Atlantic Ocean (east-to-west transect) and across the Black Sea. The concentrations found in the SAO were at very low levels or <LOQ (hundredths to units of pg L−1). Specifically, PCBs and OCPs were found mostly <LOQ or in similar levels as reported earlier for the southern hemisphere. A spatial trend was found for endosulfans with an increasing concentration on the east-to-west transect. The opposite trend was
CRediT authorship contribution statement
Jaromír Sobotka: Investigation, Formal analysis, Data curation, Visualization, Writing – original draft. Gerhard Lammel: Conceptualization, Funding acquisition, Project administration, Resources, Investigation, Writing – review & editing. Jaroslav Slobodník: Conceptualization, Funding acquisition, Methodology, Visualization, Writing – review & editing. Anne Schink: Investigation. Roman Prokeš: Investigation, Methodology. Branislav Vrana: Conceptualization, Methodology, Writing – review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Research activities were carried out in the RECETOX Research Infrastructure which is supported by the Czech Ministry of Education, Youth and Sports (LM2018121) and the European Structural and Investment Funds, Operational Programme Research, Development, Education (CZ.02.1.01/0.0/0.0/16_013/0001761). The Black Sea investigations were supported by the EU/UNDP Project: Improving Environmental Monitoring in the Black Sea – Phase II (EMBLAS-II) – ENPI/2013/313 – 169. We thank the crews of RV
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