Activation Of Polycyclic Aromatic Hydrocarbons In Soils By Root Exudate

Organic pollutants entering into soils cause serious problems to ecological risk and to human health. How to remediate the contaminated soils is of worldwide concerns. Polycylic aromatic hydrocarbons (PAHs) is readily fixed in soils, so understanding the activation of PAHs by the root exudate is paramount of determining the remediation strategies for soil contamination.In this work, the activation, sorption and desorption of phenanthrene and pyrene as representative PAHs in three typical zonal soils were studied. Main novel findings are shown as follows:(1) The effect of root exudates (ARE) on the availability of PAHs was investigated. A butanol-extraction technique was used to evaluate the availability of phenanthrene and pyrene in three typical zonal soils (yellow-brown soil, brown-red soil and red soil) in China. Soils spiked with phenanthrene and pyrene were aged for0-60days, and the extractable phenanthrene and pyrene were detected. The extractable phenanthrene and pyrene increased more than11.2%and7.3%with the addition of ARE, respectively. The fraction of butanol-extractable phenanthrene and pyrene decreased52.5%and58.9%after aging. Due to the higher molecular weight and larger numbers of benzene rings, pyrene was more recalcitrant to be degradated. The extractable phenanthrene increased by337.4%in yellow-brown soil with the ARE concentration at300mmol·kg-1, while the value for pyrene was only108.1%with the same concentration of ARE. Microbes also had strong influence on the availability of PAHs.(2) The effect of root exudates on the PAHs sorption was also investigated in soils using a batch approach. The sorption of phenanthrene could be well described by a liner isotherm (r>0.96) irrespective of the presence of ARE. The addition of ARE inhibited the sorption of phenanthrene, resulting in the decreased Kd and Koc values. The Kd and Koc values for phenanthrene sorption in yellow-brown soil decreased from508.4L·kg-1and35602L·kg-1to295.9L·kg-1and20721L·kg-1, respectively. A lower soil foc value corresponds to a greater reduction of phenanthrene sorption with the addition of ARE. Furthermore, the addition of ARE could result in the decrement of soil organic matters (<2.98%) and the increment of DOM concentration in solution, which may be the main reason that the ARE inhibited the phenanthrene sorption in soils.(3) The effect of low-molecular-weight organic acids (LMWOAs)-one of the most important active component of ARE, on the sorption-desorption of PAHs was also investigated in tested soils. The data showed that the addition of LMWOAs inhibited the phenanthrene sorption, representing the decreasing Kd and Koc values. The K&and Koc values for phenanthrene sorption in tested soils decreased from483.1～855.8L·kg-1and59930～97893L·kg-1to125.9～163.0L-kg-1and11415～25512L～kg-1with the addition of1000mmol～L-1citric acid, respectively. The lower soil foc value corresponds to the greater reduction of phenanthrene sorption with the addition of LMWOAs. The desorption amounts of phenanthrene and pyrene increased from5.44,5.94, and7.22mg·kg-1to10.88,12.76and16.02mg·kg-1with LMWOAs concentration increased from0to1000mmol·L-1. Otherwise, the PAH desorption amounts decreased after aging. Compared with ARE, LMWOAs had stronger effects on phenanthrene and pyrene desorption. The DOM concentrations also increased with the increment of LMWOAs concentration. The higher DOM concentration in solution leads to a stronger promotion of PAH desorption in soils.