Development of an optical fiber fluorescence setup for in-situ PAHs detection in porous media. Application to pyranine transport in sand columns

An optical method based on fluorescence spectroscopy was developed for in-situ non-destructive, real time, org. pollutant detection and quantification in soil. Optical fiber-based light-induced fluorescence probes allowing in-situ specific chem. detection were constructed. Pyranine was chosen as a model fluorescent polycyclic arom. hydrocarbon (PAH). The effect of sand particles on fluorescence measurements was established: the fluorescence intensity in water-satd. sand was 8 times lower than in aq. solns., due to light scattering by the sand particles. To adapt the method to dynamic pollutant concn. measurements in soil, 2 different designs of light diffusers were constructed and compared. A light distributor with a quartz window was chosen for its higher sensitivity and reproducibility. The probes were introduced into 2 different columns: short ones used to study the effect of the measurement location in the column and longer ones to study pyranine transport. It was shown that, in columns, the measurement location plays an important role; measurements near the walls, in particular, were different from those performed more towards the center of the column in a given section. As a consequence, one should avoid measurements near the circumference. Results were successfully compared to a chem. transport model and revealed that the methodol. is a powerful tool to measure in-situ concn. changes; on the other hand, fluorescent measurements can be used efficiently to det. transport parameters and give results comparable with those obtained with classical breakthrough curve fittings.