Determining the Mass Transfer Coefficient of the Water Boundary Layer at the Surface of Aquatic Integrative Passive Samplers

Passive sampling devices (PSDs) offer key benefits for monitoring chemical water quality, but the uptake process of PSDs for hydrophilic compounds still needs to be better understood. Determining mass transfer coefficients of the water boundary layer (k(w)) during calibration experiments and in situ monitoring would contribute toward achieving this; it allows for combining calibration data obtained at different temperature and hydrodynamic conditions and facilitate the translation of laboratory-derived calibration data to field exposure. This study compared two k(w) measurement methods applied to extraction disk housings (Chemcatcher), namely, alabaster dissolution and dissipation of performance reference compounds (PRCs) from silicone. Alabaster- and PRC-based k(w) were measured at four flow velocities (5-40 cm s(-1)) and two temperatures (11 and 20 degrees C) in a channel system. Data were compared using a relationship based on Sherwood, Reynolds, and Schmidt numbers. Good agreement was observed between data obtained at both temperatures, and for the two methods. Data were well explained by a model for mass transfer to a flat plate under laminar flow. It was slightly adapted to provide a semi-empirical model accounting for the effects of housing design on hydrodynamics. The use of PRC-spiked silicone to obtain in situ integrative k(w) for Chemcatcher-type PSDs is also discussed

Automated summary

Passive sampling devices (PSDs) play a key role in monitoring chemical water quality, but further understanding is needed for the uptake process of hydrophilic compounds by PSDs. This study compared two methods of measuring water boundary layer mass transfer coefficients (k(w)), showing good agreement between data obtained at different conditions. The use of PRC-spiked silicone to obtain in situ integrative k(w) for Chemcatcher-type PSDs is also discussed in the study.