Polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) in density fractions of urban soils in Bangkok, Thailand

The distribution of polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) among differently reactive soil organic matter (SOM) pools may influence their mobility and bioavailability. Our objective was to examine the distribution of PAH and PCB among density fractions of urban soils of Bangkok. We determined the concentrations of 20 PAH and 7 PCB in bulk samples and in density fractions <1.6 g cm(-3), 1.6-2.0 g cm(-3), 2.0-2.2 g cm(-3), 2.2-2.4 g cm(-3), and >2.4 g cm(-3) of five urban surface soils (0-5 cm) from Bangkok. In the bulk soil, the sum of PAH concentrations ranged from 47 to 140 mug kg(-1) and that of the PCB concentrations ranged from 0.10 to 1.20 mug kg(-1). The two fractions with a density >2.2 g cm(-3) contained more than 90% of the total soil mass. The C and N concentrations increased with increasing density; the C/N ratio decreased, indicating increasing SOM alteration with increasing density. In the sum of the five density fractions we found, on average, 158% of the sum of PAH concentrations and 246% of the sum of PCB concentrations in the bulk soil. This indicated that density fractionation provided better access to sorption sites for the solvent. In general, the concentrations of PAH and PCB in the density fractions decreased with increasing density. The organic C concentration in the density fractions was significantly correlated with the sum of PAH concentrations (r = 0.84), with the sum of PCB concentrations (r = 0.64), and with the concentrations of each individual PAH (r >0.60) and PCB (r >0.54). There were no significant differences in the contributions of low-molecular weight PAH (< 203 g mol(-1)) and PCB (Nos. 28 and 52) to the sum of PAH and of PCB concentrations between the density fractions, indicating that the mixture of PAH and PCB was not altered in the same way as SOM. Our results suggest that PAH and PCB are partitioned to SOM in density fractions independent of SOM composition. We conclude that density fractions of soils do not represent specific PAH and PCB pools.