Environ. Sci. Technol. 2021, 55, 9601−9608
Jack Garnett, Crispin Halsall,* Max Thomas, Odile Crabeck, James France, Hanna Joerss, Ralf Ebinghaus, Jan Kaiser, Amber Leeson, and Peter M. Wynn
ABSTRACT: Poly- and perfluoroalkyl substances (PFAS) are contaminants of emerging Arctic concern and are present in the marine environments of the polar regions. Their input to and fate within the marine cryosphere are poorly understood. We conducted a series of laboratory experiments to investigate the uptake, distribution, and release of 10 PFAS of varying carbon chain length (C4−C12) in young sea ice grown from artificial seawater (NaClsolution). We show that PFAS are incorporated into bulk sea ice during ice formation and regression analyses for individual PFAS concentrations in bulk sea ice were linearly related to salinity (r 2 = 0.30 to 0.88, n = 18, p < 0.05). This shows that their distribution is strongly governed by the presence and dynamics of brine (high salinity water) within the sea ice. Furthermore, long-chain PFAS (C8− C12), were enriched in bulk ice up to 3-fold more than short-chain PFAS (C4− C7) and NaCl. This suggests that chemical partitioning of PFAS between the different phases of sea ice also plays a role in their uptake during its formation. During sea ice melt, initial meltwater fractions were highly saline and predominantly contained short-chain PFAS, whereas the later, fresher meltwater fractions predominantly contained long-chain PFAS. Our results demonstrate that in highly saline parts of sea ice (near the upper and lower interfaces and in brine channels) significant chemical enrichment (ε) of PFAS can occur with concentrations in brine channels greatly exceeding those in seawater from which it forms (e.g., for PFOA, εbrine = 10 ± 4). This observation has implications for biological exposure to PFAS present in brine channels, a common feature of first-year sea ice which is the dominant ice type in a warming Arctic. KEYWORDS: PFAS, sea ice, chemical enrichment, brine, biological exposure, Arctic
1. INTRODUCTION Poly- and perfluoroalkyl substances (PFAS) are present in the Polar regions due to their long-range environmental transport (via atmosphere and ocean) and are considered “contaminants of emerging Arctic concern” (CEACs)1−6 The chemical structure of many PFAS consist of a hydrophilic moiety (e.g., COO−, SO3 −) along with a hydrophobic perfluorocarbon chain backbone of varying length which complicates the understanding of their environmental behavior and fate. Perfluoroalkyl acids (PFAA) are one major subgroup of PFAS that have received considerable regulatory attention, with “long-chain” perfluoroalkyl carboxylic acids (≥C8, PFCA) and perfluoroalkyl sulfonic acids (≥C7, PFSA) shown to bioaccumulate more than their “short-chain” analogues and hence pose a greater risk to higher trophic level organisms and polar marine ecosystems.7,8 PFAS have been observed in the sea-ice snowpack and in sea ice in the Arctic2 indicating deposition from the atmosphere with accumulation in the snowpack, as well as possible entrainment into sea ice from seawater during sea-ice growth in winter. Several studies have also observed some organochlorine persistent organic pollutants (POPs) in young or firstyear sea ice.9−12 Pucko et al, (2010a) measured the levels of ́ αand γ-HCH in first year sea ice in the Canadian Arctic and demonstrated significantly higher concentrations of these chemicals in the sea ice brine compared to under-ice seawater. Furthermore, they determined that their distribution and concentration in sea ice appears to be a function of circulating brine (a concentrated salt solution present within young sea ice).
Recently, experimental studies in artificial sea ice also showed elevated levels of organic pollutants in brine and demonstrated their distribution in newly formed bulk sea ice was primarily due to the movement of brine.13 Furthermore, αHCH was released at a faster rate from melting sea ice compared to less soluble chemicals (i.e., BDE-47, BDE-99), suggesting that partitioning of chemicals between internal solid ice surfaces and liquid brine was also an important process. Due to their known surface acting properties, we anticipate PFAS to display partitioning within sea ice which could result in their enrichment, presenting a motivation to undertake similar experiments to investigate their behavior during sea ice formation and subsequent melt. During sea-ice growth, brine convection causes most salts and other solutes to be rejected into the underlying seawater (e.g., Notz & Worster, 2008; Thomas et al., 2020). This process, referred to as gravity drainage, is the dominant process causing desalination during sea ice formation.14 The relationship between salinity and other solutes present in sea ice is indicative of the way in which chemicals are entrained and rejected from sea ice. Salinity-normalized concentrations have been used to study the behavior of nutrients,15 metals16 and dissolved organic matter (DOM)17 during sea ice formation and melt processes.
Understanding the behavior of PFAS in growing and melting sea ice will allow better predictions of contaminant fate during winter (freeze) and spring (thaw) periods in polar marine environments, and hence the timing and extent of PFAS exposure to ice−associated biota. Undertaking process-based studies to resolve contaminant fate in natural sea ice is challenging. Therefore, we used an artificial sea-ice chamber to conduct controlled experiments to quantify chemical transfer between seawater and sea ice. We investigate the behavior of several PFAS in sea ice during ice formation (freeze) and melt (thaw), testing the hypothesis that the uptake and distribution of PFAS (like like chloro- and bromo-POPs; see Garnett et al., 2019) in sea ice are controlled largely by the movement of brine.
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