Natural oxidation of a temperature series of biochars: opposite effect on the sorption of aromatic cationic herbicides.

The natural oxidation of biochar in the environment has been widely observed. However, its influence on the sorption of organic contaminants remains poorly understood. In the present study, a series of wood-based biochars prepared between 300 and 600°C (referred to as BC300-BC600) was abiotically incubated for one year to examine the aging effect of the temperature series of biochars on their sorption of aromatic cationic herbicides (ACHs, paraquat and diquat) as well as a nonpolar reference adsorbate (naphthalene). One year of oxidation showed no obvious effect on the surface area, but distinct increases in the O/C elemental ratio, density of the surface groups and cation exchange capacity (CEC) were observed. Therefore, these properties were significantly affected by the charring temperature. After incubation, high-temperature biochars (BC500 and BC600) displayed a 14.1-36.3% decrease in the sorption (qm) of ACHs. The alteration of their sorption tendency was similar to the reduced sorption of naphthalene on oxidized biochars, in which the increased surface groups lowered the surface area accessible to adsorbates because of blockage by adsorbed water molecule clusters. Conversely, a pronounced increase of ACHs sorption by 121.7-201.1% on the low-temperature biochar (BC300) was observed, presumably due to the increase of CEC values after oxidation. This result was further demonstrated by a significant linear relationship between the paraquat sorption (qm) and CEC values (R(2)=0.9895) of oxidized biochars. Interestingly, one year of oxidation simultaneously resulted in an enhanced sorption of paraquat and a reduced sorption of diquat on BC400, which indicated that the oxidation-induced sorption change of ACHs is a complex function of changes in the surface properties of the biochars as well as the molecular structure of the solute.