Sorption Characteristic of Phenanthrene on Biochar-Amended Soils: Effect of feedstock, pyrolysis temperature, and aging duration

The high sorption capacity of biochar is widely known in environmental studies. Especially, biochar is effective for removal of hydrophobic organic compounds (HOCs) due to high surface area and porosity. In this study, the sorption characteristic of biochar-amended soil was evaluated by sorption kinetic experiment of phenanthrene (PHE). For PHE sorption test, the effect of biochar feedstock (sludge waste char (SWC), municipal waste char (MWC) and wood char (WC), Giant Miscanthus (GM)), pyrolysis temperature (400°C, 500°C and 700°C,), and duration of amending period (0, 3, 6, and 12 months) was assessed. Field Emission-Scanning Electron Microscopy (FE-SEM) and Fourier Transform-Infrared Spectroscopy (FT-IR) techniques were used to detect pore structure and the surface functional group of biochar amended soils. For all kinetic tests, apparent sorption equilibrium was attained in 24 hr. The result showed that sorption capacity of biochar amended soils was greatly influenced by biochar feedstock and pyrolysis temperature. For all samples, the sorption capacity of PHE by biochar amended soils decreased with aging period. This observation is due to the fact that the aromatic characters of biochar are different by feedstock and pyrolysis temperature and the amount of O-containing hydrophilic functional groups increased surfaces of biochar by natural oxidation (e.g. carboxyl groups) as confirmed by the result of FT-IR and FE-SEM. In addition, biochar pore blockage by inorganic minerals, which tended to increase with aging period, might attenuate the sorption capacity of samples. In conclusion, biochar derived from various feed stocks are all effective for PHE sorption. But the sorption capacity of biochar amended soils decreased with increasing aging duration most likely due to increasing hydrophilic functional groups of biochar surfaces and pore blockage by inorganic minerals in the weathering processes. Therefore, for the design of biochar amendment to attenuate environmental risk of hydrophobic chemicals (e.g., PHE) in soil system, the effect of weathering on its sorption capacity must be considered.