Comparative sorption of benzo[a]pyrene by soil and carbonaceous adsorbents

Benzo[a]pyrene (BaP) is one of the most dangerous polycyclic aromatic hydrocarbon, highly persistent and toxic and its remediation by the cost-effective adsorbents are of great importance. Although various technologies have been developed to remove BaP from the environment, its sorption through solid matrixes has received increasing attention due to cost-effectiveness. Studies regarding the absorption of PAHs by soil matrix have been focused mostly on non-carcinogenic compounds comprising two or three aromatic rings, such as naphthalene and phenanthrene. However, the BaP absorption by the soil matrix and different adsorbents is not yet well explored. The present research investigates the adsorption capacity of Haplic Chernozem, granular activated carbon and biochar in relation to BaP. The Haplic Chernozem properties has following properties : clay particles content was 53.1% for particles with diameter < 0.01 mm and 32.4% for particles < 0.001 mm; pHH2O - 7.3; Corg - 129 3.7%; CaСО3 - 0.1%; exchangeable cations Ca2+ - 31.0 and Mg2+ - 4.5 cmol(+) kg−1; cation exchange capacity (CEC) - 37.1 cmol(+) kg−1. Laboratory experiments with different initial BaP concentrations in the liquid phase, and different rations of both solid and liquid phases, show that Freundlich model describes well the adsorption isotherms of BaP by the soil and both adsorbents. Moreover, the BaP isotherm sorption by the Haplic Chernozem is better illustrated by the Freundlich model than the Langmuir equation. The results reveal that the sorption capacity of the carbonaceous adsorbents at a ratio 1:20 is orders of magnitude higher (13368 ng mL^-1 of activated carbon and 3578 ng mL^-1 of biochar) than that of the soil (57.8 ng mL^-1). The difference of the sorption capacity of the carbonaceous adsorbents and soil at a ratio 0.5:20 were 17-45 times. This is due to the higher pore volume and specific surface area of the carbonaceous adsorbents than soil particles, assessed through scanning electron microscopy. The results of sorption kinetics showed high sorption rates and achievement of sorption equilibrium after 1 h. Biochar adsorbed BaP more intensely than granular activated carbon. The sorption kinetic of  BaP by chernozem was compared with the adsorption kinetics by the carbonaceous adsorbents. Results indicate that the adsorption dynamic involves two steps. The first one is associated with a fast BaP adsorption on the large available surface and inside macro- and mesopores of the sorbent particles of the granular activated carbon and biochar. Then, the adsorption is followed by a slower process of BaP penetration into the microporous space, and/or redistribution into a hydrophobic fraction. Overall, the granular activated carbon and biochar are highly effective adsorbents for BaP, whereas the Haplic Chernozem has a rather limited capacity to remove BaP from contaminated solutions.  

The research was supported by RFBR, projects no. 19-29-05265 and 19-34-90185.