Partial loading of performance reference compounds in passive samplers and its effect on the fractional approach to steady state

Abstract

Passive sampling for hydrophobic organics in soils and sediments is typically accomplished by measuring the equilibrium accumulation of the target compounds onto a sorbing polymer. Because the samplers do not often achieve equilibrium in the environment, the release of performance reference compounds (PRCs) pre-loaded on the sampler is used to estimate the extent of equilibrium uptake of a similar target compound. In this work, we evaluate the effect of non-equilibrium loading of PRCs on the estimated equilibration of target compounds. A numerical transport model considering both internal and external mass transfer resistances to the polymer during loading from a finite reservoir was constructed to predict PRC distribution polymer sheets. An analytical internal diffusion, external mass transfer limited model is then employed to simulate the release of the PRCs and predict both the apparent and actual approach to equilibration for the uptake of target compounds. Partial equilibration during loading of PRCs does not lead to inaccuracies in target compound equilibration except when internal resistances control both PRC release and target compound uptake, under which conditions the PRCs overestimate the extent of equilibration of target compounds. This is likely to occur during short PRC loading times (e.g., <1 month) and thick polymer sheets (e.g., >100 µm in thickness). If external mass transfer resistances control PRC release and target compound uptake, even partially loaded PRCs can accurately indicate the extent of equilibration of target compounds as long as the initial PRC loading is adequately characterized.