Mechanistic Model Describing the Uptake of Chemicals by Aquatic Integrative Samplers: Comparison to Data and Implications for Improved Sampler Configurations

Abstract

Aquatic integrative passive samplers are used for determining aqueous concentrations of polar organic pollutants, yet their uptake mechanisms are poorly understood. We introduce a one-dimensional model to simulate uptake by a passive sampler, Chemcatcher. The model considers the uptake as molecular diffusion through a series of the aqueous boundary layer (ABL), the membrane filter (MF), and the sorbent disk with concurrent sorption by matrix of the MF and the disk. Uptake profiles of ca 20 polar chemicals measured over a week and a month were accurately modeled. Characteristic behavior such as lag-times, linear and curved uptake, and equilibrating behavior were well-explained by the model. As the model is mechanistically based, it was able to show the combined influences of the MF/water (KMF/w) and disk/water (Kdisk/w) partition coefficients, diffusion coefficients, and the ABL thickness on the sampling rates. On the basis of the model results, we offer three concrete recommendations to achieve linear uptake needed for measuring time-weighted average concentrations: (i) Use a MF that does not significantly sorb chemicals (e.g., log KMF/w < 3) to avoid lag times. (ii) Use a sorbent with strong sorption properties (e.g., log Kdisk/w > 6) for effective trapping of chemicals on the disk top layer. (iii) Make the ABL and/or the MF thicker so that the diffusion toward the disk slows down.