Low temperature–produced and VFA–coated biochar enhances phenanthrene adsorption and mitigates toxicity in marine sediments

Polycyclic aromatic hydrocarbons (PAHs) are widespread in marine environments, having negative implications on both ecological systems and human health. At the same time, research on highly–efficient and environmentally–friendly alternatives compared to conventional remediation techniques must be stepped up. In this work, for the first time, a novel modification procedure of willow–derived biochar based on volatile fatty acids (VFA) has been employed to enhance biochar structures aimed at avoiding phenanthrene (PHE) desorption. Batch adsorption–desorption experiments showed a PHE removal of 93% and a hysteresis index of 4.9 after 72 h with biochar pyrolyzed at 400 °C (BC400) and modified with VFAs (MBC400), respectively. A lower efficiency was obtained with higher temperature–produced biochars (i.e. 700 °C) due to lower surface area and functional group abundance compared to the biochars obtained at 400 °C. Physisorption and chemisorption mechanisms were mainly involved in a multilayer PHE adsorption onto the heterogeneous BC400 and MBC400 surfaces, as shown by pseudo–first– and –second–order kinetic (R2 = 0.988) and Freundlich isotherm (R2 = 0.951–0.995) models. A more complicated and advanced subsequent application of biochar in PHE–polluted marine sediments with a dosage of 0.1–5% (w/w) led to the reduction of pore water PHE up to 24% after 28 days. An ecotoxicological test showed a mitigated toxicity of the sediment remediated with BC400 and MBC400 towards Folsomia candida springtails due to a decrease in adult mortality and reproduction inhibition, which was directly correlated with the reduction of PHE bioavailability (rPHE–FC = 0.76).