Achievements and challenges with equilibrium and kinetic passive sampling of hydrophobic and hydrophilic organic compounds in surface waters.

Passive sampling in surface waters is an important method in the monitoring and risk assessment of hydrophobic and hydrophilic organic chemicals. Sampler designs can be optimized for fast equilibrium attainment (equilibrium sampling) or improved time-integrative capability (kinetic sampling). We argue that both equilibrium and kinetic sampling can be applied when aqueous concentrations do not vary with time, whereas kinetic sampling also yields useful results for time-variable concentrations that are often observed with hydrophilic compounds in surface waters. We show that these methods have similar accuracy in principle, but that their dominant error sources are different: sampler-water sorption coefficients for equilibrium sampling and sampling rates for kinetic sampling. In contrast to passive sampling of hydrophobic compounds, passive sampling of hydrophilic compounds is not as well-established, but major progress has been made over the last decade in the modeling of transport through the water boundary layer, membrane, and sorbent, while less progress has been made for transport in the biofouling layer. We recommend a more extensive use of diffusion cells as a research tool to gain a better understanding of transport through the respective subphases, leading to a greater maturity of passive water sampling of hydrophilic compounds.