Mechanistic Characterization of Adsorption and Slow Desorption of Phenanthrene Aged in Soils

Abstract

Long-term adsorption of phenanthrene to soils was characterized in a silt-loam (LHS), a sandy soil (SBS), and a podzolized soil (CNS) by use of the Polanyi−Manes model, a Langmuir-type model, and a black carbon−water distribution coefficient (K BC) at a relative aqueous concentration (C e/S w) of 0.002−0.32. Aqueous desorption kinetic tests and temperature-programmed desorption (TPD) were also used to evaluate phenanthrene diffusivities and desorption activation energies. Adsorption contribution in soils was 48−70% after 30 days and 64−95% after 270 days. Significant increases in adsorption capacity with aging suggest that accessibility of phenanthrene to fractions of SBS soil matrix was controlled by sorptive diffusion at narrow meso- and micropore constrictions. Similar trends were not significant for LHS silt-loam or CNS podzol. Analysis of TPD profiles reveal desorption activation energies of 35−53 kJ/mol and diffusivities of 1.6 × 10-7-9.7 × 10-8 cm2/s. TPD tests also indicate that the fraction of phenanthrene mass not diffusing from soils was located within micropores and narrow width mesopores with a corresponding volume of 1.83 × 10-5-6.37 × 10-5 cm3/g. These values were consistent with the modeled adsorption contributions, thus demonstrating the need for such complimentary analytical approach in the risk assessment of organic contaminants.

Relationships between aging of PAHs and soil properties

“…On the contrary, the fraction of PAHs adsorbed by soft OC can be relatively easy to be released by the polar extractant and thus no significant relationship was observed between the extractable PAHs and the soft OC contents. However, it is surprising that no relationship between the extractable PAHs and recalcitrant BC content was discernable although a variety of investigations have indicated that BC have a strong affinity for PAHs (Abu and Smith, 2006;Agarwal and Bucheli, 2011). This phenomenon could be ascribed to the fact that BC content accounted for only about 10% of the TOC content, much lower compared with the data reported in the previous studies (Xiao et al, 2004).…”

“…S2 in SI). Furthermore, the comparable pore diameter (about 4 times larger than the PAH molecular size) was believed to be sensitive to HOC sequestration and diffusion in geomaterials (Farrell et al, 1999;Abu and Smith, 2006). In addition, a large portion of the cumulative pore volumes (about 35e65%) in the soils came from the pores with diameters smaller than 6 nm as shown in Fig.…”

“…Correlation analysis revealed that k rap was negatively dependent on TOC to a certain extent while no significance level was obtained. The pore distribution also exerted an important effect on the diffusion of HOCs (Pignatello and Xing, 1996;Farrell et al, 1999;Abu and Smith, 2006). Particularly, it was proposed that the pore fraction with diameters below 6 nm (hereafter specifically called PF 6 nm for short) predominately controlled the sorption of HOCs (Farrell and Reinhard, 1994;Pignatello and Xing, 1996).…”

“…Some investigations demonstrated that dissolved organic carbon (DOC) can exert a positive effect on the extractability of polycyclic aromatic hydrocarbons (PAHs) in soils (Tao et al, 2006;Luo et al, 2008). On the other hand, Abu and Smith (2006) indicated that the nonlabile fraction of phenanthrene in soils was retained within narrow width mesopores and micropores. In addition, clay can also exert an appreciable effect on PAH sequestration in soils (Luo et al, 2008).…”

Soil mobility of surface applied polyaromatic hydrocarbons in response to simulated rainfall

“…The differences in the soil behaviours are greatest for the lower molecular weight PAHs with pyrene demonstrating the greatest discrimination in terms of its behaviour in soils A and B despite the similarities in the log K oc values for fluoranthene (4.48) and pyrene (4.23). In addition to sorption onto outer humic surfaces, it has been postulated that PAH diffusion is also controlled by the narrow meso-and micro-pores of geomaterials where these are of a comparable molecular size (Abu and Smith, 2006;Nam and Alexander, 1998). It would appear that this factor becomes important in the case of the heavier PAHs (benzo(e)pyrene and benzo(ghi)perylene) enabling them to penetrate further into soil B due to its higher coarse sand fraction (Table 1) and resulting in similar removal depths for both soils despite the higher organic content of soil B.…”