Polycyclic Aromatic Hydrocarbons (PAHs) are one of the well-known organic pollutants and should be monitored continuously. PAHs have high organic carbon-water partition coefficients (logKoc), and thus exhibit a relatively high concentration in organisms and sediments due to bioaccumulation and adsorption to particles. Conventional method of measuring PAHs in sediment is Soxhlet extration, which extracts all types of PAHs and presents the total concentration (Ctotal). However, some PAHs in the sediment are strongly adsorbed to the sediment particles and so cannot be used by organisms. So, it may be overstimated in sediment risk assessment including an actual biological effects or bioaccumulation using Ctotal. Therefore, passive sampling methods (PSMs) have been developed to measure only bioavailable PAHs. In the case of in-situ PSMs, it takes a long time to reach the equilibrium between matrix and passive sampler (PS), and there are disadvantages such as biofouling or vanishment of the passive sampler (PS). In this study, we tried to optimize the ex-situ method to shorten the equilibrium time by exposing the passive sampler to sediments under various conditions in the laboratory.
When the stirring speed was set to 130 rpm in the first optimization experiment for determining the optimization conditions, the equilibrium time of 13 PAHs excluding the higher molecular PAHs (7<logKow) was 30 days. In order to shorten the equilibrium time more, the stirring speed was increased to 180 rpm in the second optimization experiment and then the equilibrium time of 13 PAHs excluding the higher molecular compound (7<logKow) was 10 days. Therefore, the optimized conditions of stirring speed (180 rpm) and exposure time (10 days) were applied in the contaminated sediments collected from Gampo, Jangseungpo, Guryongpo, Chuksan, Sacheon, Jinhae, Haengam, Gwangyang. The freely dissolved concentration (Cfree) predicted from PSMs ranged from 12 ng/L to 28 ng/L. On the other hand, the highest Ctotal obtained by Soxhlet extraction was at 1601 ng/g dw, and the lowest at 14 ng/g dw.
From the application of optimized ex-situ method in the contaminated sediments, the ratio of the freely dissolved concentration (Cfree) to the dissolved concentration (Cdissolved) in the sediment pore water was about 1~9%. It was confirmed that even if the total concentration (Ctotal) in the sediment is very high, the freely dissolved concentration (Cfree) bioavailable for the actual organism may be very low. Accordingly, for sediment risk assessment, it is considered that the freely dissolved concentration (Cfree) from passive sampling would be much more useful rather than the total concentration (Ctotal) from the conventional method.