Measuring Picogram per Liter Concentrations of Freely Dissolved Parent and Alkyl PAHs (PAH-34), Using Passive Sampling with Polyoxymethylene

Abstract

Passive sampling with nondepletive sorbents is receiving increasing interest because of its potential to measure freely dissolved concentrations of hydrophobic organic compounds (HOCs) at very low concentrations, as well as its potential for both laboratory use and field deployment. However, consistent approaches have yet to be developed for the majority of HOCs of environmental and regulatory interest. In the present study, a passive sampling method was developed which allows the freely dissolved concentrations of 18 parent and 16 groups of alkyl polycyclic aromatic hydrocarbons (PAHs) on the U.S. Environmental Protection Agency (USEPA)'s "PAH-34" target compound list to be measured. Commercially available 76-μm-thick polyoxymethylene (POM) was placed in sediment/water slurries and exposed for up to 126 days, with 28 days found to be sufficient to obtain equilibrium among the sediment, water, and POM phases for the target 2- to 6-ring PAHs. The POM/water partition coefficients (K(POM)) necessary to calculate freely dissolved concentrations for parent PAHs were determined in two separate laboratories (one using pure standards, and the other using coal tar/petroleum-contaminated sediments) and agreed very well. Since the so-called "16" alkyl PAHs on the PAH-34 list actually include several hundreds of isomers for which no standards exist, sediments impacted by coal tar, or spiked with a coal tar/petroleum nonaqueous phase liquid (NAPL) were also used to measure K(POM) values for each alkyl PAH cluster. The log K(POM) values ranged from ca. 3.0 to 6.2 for 2- to 6-ring parent PAHs, and correlated well with SPARC octanol/water coefficients (K(OW)) (correlation coefficient of r(2) = 0.986). However, log K(POM) values for alkyl PAHs deviated increasingly from SPARC log K(OW) values with increasing degree of alkylation. A simple empirical model that incorporates the number of carbon atoms in a PAH gave a better fit to the experimental log K(POM) values, and was used to estimate log K(POM) for alkyl PAHs that could not be directly measured. Detection limits (as freely dissolved concentrations) ranged from ca. 1 part per trillion (ng/L) for the 2-ring PAH naphthalene, down to <1 pg/L (part per quadrillion) for the 5- and 6-ring PAHs. Sorption isotherms were linear (r(2) > 0.99) over at least 4 orders of magnitude.

Extraction of sediment‐associated polycyclic aromatic hydrocarbons with granular activated carbon

“…Polyoxymethylene passive samplers were prepared by cutting the sheets into 300-mg strips and cold extracting them with hexane (30 min) and methanol (3 Â 30 min). Sorbent pretreatment was based on earlier reported procedures for POM strips [25,26]. After extraction, POM strips were air-dried and stored in a brown glass bottle until use.…”

“…The bottles differed in total suspension volume, but the solid-liquid ratio was identical (37 g/L) in the systems, except for the batch with the lowest mass of sediment, which had a solid-liquid ratio of 30 g/L. Polyoxymethylene samplers (0.07-6.7 g depending on the batch) were added, and the bottles were horizontally shaken at 160 strokes per min for 28 d. This period is sufficient to reach equilibrium in 76-mm POM strips [25]. Subsequently, the POM strips were removed, rinsed, and wiped with a tissue before extraction.…”

“…Subsequently, the POM strips were removed, rinsed, and wiped with a tissue before extraction. Equilibrium aqueous-phase PAH concentrations were calculated from the PAH concentrations in the POM samplers using previously published POM-water partitioning coefficients (K POM ) [25].…”

“…Polycyclic aromatic hydrocarbon partitioning to sediment was measured using the POM-SPE method [25,26] (Supplemental Data, Table S4). Replication between systems of equal volume was excellent with (n ¼ 3) standard deviation (SD) in log K SED of 0.01 to 0.1 for all PAHs.…”

“…Sorption of native PAHs to the best performing GAC was studied for mixtures at different GACsediment weight ratios and sediment only. For all these experiments, aqueous-phase PAH concentrations were accurately determined using 76-mm polyoxymethylene (POM) passive samplers [8,25,26]. The sorption to GAC in the sediment-GAC mixtures was assessed by subtracting the contribution of PAH sorption to sediment from total PAH sorption in the mixture.…”