Mechanochemical approach to remove phenanthrene from a contaminated soil

Soil samples collected from a cultivated soil of Southern Italy after artificial contamination with phenanthrene (PHE) were ground in a ball mill and compared with spiked (via acetone) sample as control. The mechanochemical treatment was also applied to a simple binary system birnessite (delta-MnO2)-PHE and to soil added with birnessite to evaluate the oxide role in removing the contaminant. Different extraction methods, such as Soxhlet, sonication, and desorption from resin beads were adopted to estimate the residual PHE analysed with HPLC. X-ray diffraction and TG-DTA analyses were performed to quantify mineralogical phases in soil and their possible modifications after grinding. The results showed that the grinding was more efficient in removal of PHE when the pollutant was in solid phase (similar to 50% of removal) than when it was spiked via acetone in the same soil (similar to 20% of removal). Addition of birnessite to soil did not change significantly the removal of PHE through time. Independently of the extraction methods used, the PHE recovered after the mechanochemical treatment in the presence of solid PHE was always lower, suggesting a higher efficiency of such a treatment in degrading PHE or forming bound residues in its original or transformed form. X-ray powder diffraction of milled solid PHE showed that order-disorder phase transition occurred in solid phenanthrene as consequence of the mechanochemical treatment. X-ray diffractometry and TG-DTA analyses were crucial in evidencing that interaction between PHE and birnessite occurred.