Purpose The soil contamination by hydrophobic organic contaminants (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), poses great threats to human health and ecological security and attracts worldwide concerns. The total HOC concentrations overestimate its available fraction to the soil biota. Increased understanding of the availabilities of PAHs in soil environment will have considerable benefits for their risk assessment and be very instructive to food safety and remediation strategies in contaminated sites. However, the availability of PAHs in aging soils and particularly the correlations of the availabilities with their forms in soils have yet to be elucidated. In this work, the availabilities of PAHs in aging soils were evaluated using a sequential mild extraction technique. Materials and methods Four typical zonal soils in China previously free of PAHs were collected from A (0-20 cm) horizon, air-dried, and sieved. Soils were spiked with a solution of phenanthrene and pyrene as representative PAHs in acetone. After the acetone evaporated off, the treated soils were progressively diluted with unspiked soils and sieved again several times to homogenize the soil samples. The forms of PAHs in soils were experimented using microcosms that are similar to those reported in literature. Various treated soils were packed into amber glass microcosms (each with 25 g soil). Three replications were given for each treatment. NaN 3 solution (0.5%) was added to some microcosms in order to get the microbeinhibited treatments. The soil water contents were adjusted to be 20% of soil water-holding capacity. After incubation for 0, 2, 4, 8, 12, and 16 weeks in microcosms with a temperature of 25°C, the soils were sampled. PAHs were then extracted by a sequential mild extraction technique, and their forms and availabilities were determined. Results and discussion The available residual concentrations of phenanthrene and pyrene generally decreased with aging time, and the PAHs were more readily available at the start of the incubation, but their availabilities decreased rapidly with increasing the soil-PAH contact time. In addition, the degradation efficiency of the available PAHs in soils was generally higher for PAHs with low molecular weight. The available residues of PAHs in soils were fractionated into desorbing and non-desorbing fractions. The desorbing fractions were the largest portion of the available PAHs in soils. In addition, the desorbing fractions were the main portion to be readily biodegradable. The non-desorbing fractions of PAHs were less bioavailable and with less possibility to be biodegraded in soils. The dissipation of the desorbing PAH fractions accounted for the dominant contribution to the dissipation of the available fractions of tested PAHs. The formation of bound PAH residues was observed in soils. However, the concentrations of the bound residual PAHs were very low. Comparing with the microbial biodegradation, the transformation of PAHs from available fractions to bound residues was a negligible ...

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