Characteristics of PAH tar oil contaminated soils-Black particles, resins and implications for treatment strategies

Tar oil contamination is a major environmental concern due to health impacts of polycyclic aromatic hydrocarbons (PAH) and the difficulty of reaching acceptable remediation end-points. Six tar oil-contaminated soils with different industrial histories were compared to investigate contamination characteristics by black particles. Here we provide a simple method tested on 6 soils to visualize and identify large amounts of black particles (BP) as either solid aggregates of resinified and weathered tar oil or various wood/coke/coal-like materials derived from the contamination history. These materials contain 2-10 times higher PAH concentrations than the average soil and were dominantly found in the sand fraction containing 42-86% of the total PAH. The PAH contamination in the different granulometric fractions was directly proportional to the respective total organic carbon content, since the PAH were associated to the carbonaceous particulate materials. Significantly lower (bio)availability of PAH associated to these carbonaceous phases is widely recognized, thus limiting the efficiency of remediation techniques. We provide a conceptual model of the limited mass transfer of PAH from resinated tar oil phases to the water phase and emphasize the options to physically separate BP based on their lower bulk density and slower settling velocity.