PAHs sorption to biochar colloids changes their mobility over time

Abstract

Biochar is an efficient sorbent for polycyclic aromatic hydrocarbons (PAHs). However, little is known about PAHs sorption kinetics and the effect on the mobility of biochar colloids in saturated porous media. In this study, naphthalene (NAP) and phenanthrene (PHE) were chosen as typical PAHs. In different stages, different sorption sites of PAHs on biochar played a distinct role in affecting the transport of biochar colloids in saturated porous media. The biochar colloids showed less negative surface charge as the contact time between biochar colloids and PAHs increased from 0.017 h to 96 h, which led to the mobility of biochar colloids decreasing over time. But after 168 h contact time, the surface charge of biochar colloids became more negative again, and the inhibition effect of PAHs on biochar colloid transport was weakened. This was related to the sorption kinetics of PAHs on biochar colloids: (1) PAHs sorption onto outer biochar surface; and (2) adsorbed PAHs diffusion into internal biochar pores. PAHs sorption onto the outer biochar surface shielded the negative surface charge and then decreased the mobility of biochar colloids, and then the adsorbed PAHs diffusion into internal biochar pores increased their mobility again. Our results suggested that porous colloids-facilitated PAHs transport in porous media might be highly related to interaction time between PAHs and porous colloids.