Combined Effects of Plant Cultivation and Sorbing Carbon Amendments on Freely Dissolved PAHs in Contaminated Soil

We report freely dissolved concentrations (C_free) of PAHs in soils amended with 2.5% biochar and activated carbon (AC) during a long-term (18-months) field experiment. The study evaluates also the impact of different plants (clover, grass, willow) on C_free PAHs. The cumulative effect of treatments on nitrogen and available forms of phosphorus, potassium, and magnesium is also assessed. The direct addition of biochar to soil did not cause any immediate reduction of the sum of 16 C_free PAHs, while AC resulted in a slight reduction of 5- and 6 ring compounds. The efficiency of binding of C_free PAHs by biochar and AC increased with time. For biochar, the maximum reduction of 4-6-ring PAHs (18-67%) was achieved within 6 months. For 2- and 3-ring PAHs, a gradual decrease of C_free was observed which reached 60-66% at 18 months. AC proved to be better in reducing C_free PAHs than biochar, though for 2- and 3-ring PAHs, the differences in AC and biochar performances were smaller than those for 4-6-ring PAHs. After 18 months, a significantly lower content of C_free PAHs was observed in the soil with plants compared to the unplanted soil. Except for potassium, AC or biochar did not negatively impact nutrient availability.