No abstract

The gradients of root exudates and of persistent organic pollutants, including polycyclic aromatic hydrocarbons (PAHs), in rhizosphere soil in proximity to the root surface are still not well elucidated. In this work, a greenhouse experiment was conducted to investigate the distribution gradients of root exudates and phenanthrene and pyrene, as selected PAHs, in rhizosphere soil close to (0-8 mm) the root surface of ryegrass [Ldium multiflorum Lam.) as the host plant. Rhizosphere soil from the root surface (0-8 mm) was divided into three layers: the rhizoplane and strongly and loosely adhering soil. Root exudates were characterized as soluble organic C, organic acids, and total soluble sugars. In PAH-spiked rhizosphere soils, the concentration of root exudates decreased with distance from the roots. In a sterilized treatment, the amounts of root exudates in the three rhizosphere layers were higher than when the soil was not sterilized, indicating that microbial consumption contributed significantly to the loss of root exudates in the rhizosphere. Tlie residual concentrations of phenanthrene and pyrene clearly increased in die order of rhizoplane to loosely adhering soil after 40 to 50 d, which was significantly and negatively correlated with the amount of root exudates in the rhizosphere. In total, 87 to 97% of phenanthrene and 69 to 79% of pyrene dissipated in all three layers of rhizosphere soil after 40 d, and >99% of phenanthrene and 93% of pyrene dissipated after 50 d. The degradation ratio of PAHs decreased in rhizosphere soils with distance from the roots.Abbreviations: OA, organic acid; PAH, polycyclic aromatic hydrocarbon: SOC, soluble organic carbon: TSS,) total soluble sugar.

No abstract

Soybean [Glycine max (L.) Merr.] stalk-based biochar was prepared using oxygen-limited pyrolysis. We evaluated phenanthrene (PHE) and Hg(II) sorption, from single and binary component solutions, onto prepared biochar. We found that the prepared biochar efficiently removed PHE and Hg(II) from aqueous solutions. The isotherms for PHE and Hg(II) sorption could be described using linear and Tóth models, respectively, both with high regression coefficients (R(2) > 0.995). When PHE and Hg(II) coexisted in an aqueous solution, we observed direct competitive sorption, each one suppressing another. Our results provide insight into the recycling of agricultural residues, and also a new application for removal of polycyclic aromatic hydrocarbons and heavy metals from contaminated water utilizing biochar from agricultural residue.

The impact of low‐molecular‐weight organic acids (LMWOAs) on the sorption–desorption of phenanthrene, used as a representative of polycyclic aromatic hydrocarbons, was investigated with a laboratory batch technique. Experiments were conducted with three types of soil samples containing various organic components: citric, oxalic, and malic acids. The sorption of phenanthrene by soils could be well described by a linear‐type model irrespective of the addition of LMWOAs. The simulated distribution constant (Kd) and C‐normalized distribution constant (Koc) for phenanthrene sorption decreased significantly with the addition of LMWOAs to the soils (P < 0.05). In contrast, the desorption amounts of phenanthrene were clearly enhanced by the addition of LMWOAs, and the impact of citric acid on the sorption–desorption of phenanthrene was generally more significant than those of oxalic or malic acids, probably due to their chemical structures and properties. Compared with the baseline soil samples, the desorption amounts of phenanthrene in the presence of LMWOAs were significantly decreased in soils after 60 d of cultivation (P < 0.05), whereby the level of the fractional soil organic C content (foc, %) increased proportionally with the inhibition strength of phenanthrene desorption in the presence of LMWOAs. Based on these observations, we suggest that the availability of polycyclic aromatic hydrocarbons can be enhanced with suitable types and concentrations of LMWOAs.