Biochar nanoparticles-mediated transport of organic contaminants in porous media: dependency on contaminant properties and effects of biochar aging

Abstract Land application of biochar has been recommended as an effective soil amendment measure. Nonetheless, the applied biochar can accumulate co-existing contaminants. Meanwhile, nanoparticles formed due to biochar disintegration may facilitate contaminant transport in vadose zone and groundwater, posing a potential risk to the subsurface environment. Here, we show that the presence of pinewood- and rice straw-derived biochar nanoparticles (BCNPs) at parts per million level (~ 20 mg/L) can result in significant mobilization of hydrophobic, nonpolar contaminants (naphthalene and pyrene) and positively charged polar contaminants (trimethoprim and ciprofloxacin) in saturated sandy soil, but slightly inhibits the transport of negatively charged or neutral hydrophilic compounds (sulfamethoxazole and bisphenol A). With supplemental adsorption and desorption experiments we show that the ability of BCNPs in mediating contaminant transport (either enhancing or inhibiting) relies primarily on the extent of irreversible binding of a contaminant to the BCNPs. Sulfide reduction and leaching of organic carbon, two relatively mild (in terms of modification of physicochemical properties, e.g., surface O/C ratio) but widely occurring aging processes, facilitate co-transport of pyrene and bisphenol A with the BCNPs. However, this is mainly the result of increased mobility of the BCNPs (i.e., the carrier), rather than enhanced interactions between the BCNPs and the contaminants being carried. The findings underline the significant effects of BCNPs on the fate and transport of environmental contaminants, and further highlight the important role of aging in affecting environmental behaviors and effects of biochar materials. Graphical Abstract