POLYCYCLIC AROMATIC HYDROCARBONS (PAHS) AND POLYCHLORINATED BIPHENYLS (PCBS) IN PARTICLE-SIZE SEPARATES OF URBAN SOILS IN BANGKOK, THAILAND

Abstract

Bioavailability of PAHs and PCBs in soils depends on the strength of their sorption to soil organic matter (SOM), which exists in differently reactive pools. In order to assess the fate of PAHs and PCBs in tropical soils, we examined the distribution of 20 PAHs and seven PCBs among particle-size separates (coarse sand, fine sand, silt, clay, floatables) in 10 urban surface soils (0-5 cm) of Bangkok, Thailand. The quantitatively negligible floating fraction (<1.2 g kg⁻¹) has the significantly highest sum of PAH and PCB concentrations (on average: 2548 μg Σ PAHs kg⁻¹, 20375 ng Σ PCBs kg⁻¹). The average sum of PAH concentrations decreases in the order, silt (298 μg Σ PAHs kg⁻¹) ≥ clay (275) > fine sand (174) > coarse sand (71). The highest average PAH concentration in the silt fraction may be explained by an input of PAHs associated with silt-sized particles or by a particularly high affinity of PAHs to silt-sized SOM. The latter assumption is supported by the highest average organic C (C_org)-normalized PAH concentration in silt (12.3 ± 13.2 μg Σ20PAHs (g C_org)⁻¹) > fine sand (11.5 ± 9.6) = floatables (11.4 ± 5.8) > clay (5.9 ± 5.1) > coarse sand (5.7 ± 4.4). Average percentages of low molecular weight PAHs (<203 g mol⁻¹) decrease from coarse to fine fractions (coarse sand: 57% < fine sand: 48 < silt: 44 < clay: 38 < floatables: 12). This indicates a degradation or volatilization of the less persistent and more volatile PAHs during turnover of SOM, which shows an increasing state of alteration with decreasing particle size. The average sum of PCB concentrations in the particle-size separates decreases in the order, clay: 2349 ng Σ PCBs kg⁻¹ > silt: 1597 > fine sand: 803 > coarse sand: 261 and is more closely related to the C_org concentration than that of the PAHs. No indications of degradation of the low-chlorinated PCBs 28 and 52 with increasing state of SOM alteration were observed. The distribution of PAHs and PCBs among particle-size separates in the studied tropical soils is similar to that reported in the literature for temperate soils. Despite similar physicochemical properties, the markedly different distribution of PAHs and PCBs among the particle-size separates indicates different fates in soils.