Environmental implications of dissolved black carbon released from biochar

Black carbon (BC) is a group of carbon materials that has drawn wide interest due to its critical role in many geological and environmental processes. Here, the chemical and structural properties of the bulk and dissolved fractions (< 0.45 μm in size) of BC derived from rice straw or bamboo were thoroughly and comprehensively characterized using elemental analysis combined with a variety of spectroscopic techniques. The dissolved BC was found to contain 30-40% more oxygen and more polar functional groups, but lower aromaticity and less condensed aromatic clusters than the bulk BC. Compared with some well-characterized humic substances, the dissolved BC contained less aliphatic carbon but higher aromatic and carboxylic carbon. The dissolved BC exhibited extraordinarily strong adsorption affinity for hydrophobic organic contaminants (HOCs). The sorption distribution coefficient (K_d) of phenanthrene to the dissolved BC derived from rice straw was nearly one order of magnitude higher than that to an extracted soil humic acid, despite the relative close organic carbon contents between them. The strong sorbing ability of the dissolved BC was attributable to its high aromatic carbon content and flexible structures to facilitate the formation of three-dimensional pseudomicelle structures for hydrophobic partitioning. Additionally, the dissolved BC derived from bamboo was found to generate reactive oxygen species (ROS) including singlet oxygen and superoxide during irradiation under simulated sunlight. The apparent quantum yield of singlet oxygen was 4.5 ± 0.2%, 2 to 3 fold higher than many well-studied humic substances. Accordingly, the dissolved BC could effectively mediate the phototransforamtion of 17β-estradiol under sunlight irradiation. The half-life of 17β-estradiol in solution was significantly reduced from 150 h to 4.8 h in the presence of 5 mg C/L dissolved BC. The triplet-excited states of dissolved BC were the predominant reactive intermediates involved in the photoreaction stemming from the unique properties of high content of oxygen-substituted aromatic structures and small molecular size. Results obtained from this work would shed new light on the environmental mobility, liability, and reactivity of BC, as well as transport of organic contaminants.