Effect of ageing on mobility and sequestration of phenanthrene in an agricultural soil

We studied the effect of ageing on the mobility and sequestration of 14C-phenanthrene in an agricultural soil. We used autovial microcolumns incubated for a period ranging between 0 and 156 days. Our results showed that a large proportion of radioactivity remained extractable by methanol up to 64 days of incubation, amounting from 44.5 to 80.2% of initial radioactivity. Mineralisation of 14C-phenanthrene began tardily, after the decrease of mineralisation of the soil organic matter. Throughout incubation, mineralisation remained slow and weak, of co-metabolic type, and only 2% of the quantity applied was mineralised over a period of 156 days. The amount of radioactivity leached varied considerably during incubation. Under the effect of water percolation of 64 mm, the quantities exported in equivalent 14C-phenanthrene went from 0.90 to 0.48 ng/g of soil between 0 and 35 days, then increased up to 1.5 ng/g at 100 days. The radioactivity present in pore water displayed the same dynamic and the same variations. For pore water, this radioactivity represented, in equivalent phenanthrene, 0.09 ng/g of soil up to 46 days, then increased rapidly to reach 0.5 ng/g at 65 days. In both cases (leachates and pore water), enrichment in radioactivity due to the presence of a high proportion of polar compounds reached 89%. Lastly, the formation of non-extractable residues progressed very slowly up to 45 days (5.9 to 13% of the amount applied), after which there was a marked increase, i.e. to 20.7, 32.2 and 35.8% at 52, 64 and 156 days, respectively. These increases in radioactivity in different compartments of soil are correlated with mineralisation of phenanthrene due to preliminary production of degradation products. Thus, the capacity of a soil to degrade and mineralise phenanthrene plays a determining role in the distribution of its residues between the soil solution and the solid matrix. Overall, our results suggest that in agricultural soils contaminated by strongly adsorbed organic compounds, water pollution and sequestration are primarily controlled by degradation.