Effects of sediment manipulation on freely dissolved concentrations of hydrophobic organic chemicals

The freely dissolved concentration (C_free) of hydrophobic organic chemicals (HOCs) in pore water is an important parameter in the risk assessment and management of contaminated sediments and soils. It can be determined most conveniently through ex situ passive sampling, i.e., in the laboratory. For this purpose, samples are taken from the field and transported to the laboratory, where they are stored and possibly manipulated by sieving, freezing, drying and/or grinding. Although the objective of ex situ passive sampling often is to determine a C_free that reflects the metric under in situ conditions, hardly any information is available on possible effects of sample manipulation. Hence, the present study investigated the impact of freezing, freeze-drying, and grinding on C_free of HOCs in field sediments, as determined with solid phase microextraction (SPME). Freezing increased the C_free of 3- and 4-ring polycyclic aromatic hydrocarbons (PAHs) with up to a factor of 4, whereas for 5- and 6-ring PAHs hardly any effects were observed. Generally, additional freeze-drying did not further increase C_free, but subsequently grinding the samples (further) increased C_free of all PAHs with up to a factor of 4, leading to an overall maximum observed increase in C_free of a factor of 16. Probably, these effects are caused by a structural change in the sorption matrix, enhancing PAH availability. The results indicate that freezing, freeze-drying, and grinding prior to ex situ C_free determinations should be avoided, as these treatments may considerably increase the C_free of HOCs, leading to an overestimation of risks.