Principles of passive sampling for ex situ measurement of hydrophobic organic compounds in sediment: Key considerations on dilution, depletion, and equilibrium

Passive sampling techniques have undoubtedly proven effective for determining freely dissolved concentrations (C_free) in sediment. However, the absence of a rationale-based, hands-on protocol for guiding practitioners in conducting ex situ exposure of passive samplers in sediment underscores the need for research and development in this area. The goal of this study was to address three critical issues for passive sampling in sediment ex situ: sediment dilution, depletion, and nonequilibrium. Polyethylene (PE) and polymethyl methacrylate (PMMA) film samplers were utilized in experiments involving hydrophobic organic compounds (HOCs, including PAHs, PCBs, PBDEs, and pesticides) spiked sediments. The results indicated that the influence of sediment dilution could be safely disregarded for HOCs with a larger K_d value (i.e., > 140) when moisture content of 80 % was selected for the exposure. Given some data deviations stemming from sample analysis, the depletion extent could significantly vary within a wide range (up to 36.4 %) rather than being fixed at a specific level (e.g., 5 %). Most HOCs reached equilibrium in a 10-d exposure for the PE sampler, and compounds with a log K_ow value <7.12 did not need correction for nonequilibrium. An equation for estimating the nonequilibrium correction factor and an approach for correcting nonequilibrium exposure were introduced. The passive sampling method with PE was applied to a set of field-contaminated sediments under the depletion extents ranged from 7.1 % to 77 %. Based on the relatively comprehensive understanding of the passive sampling in sediment ex situ, a practical and standardized protocol was provided for C_free measurement of HOCs.