Effects of soil pH, Total Organic Carbon and texture on fate of Polycyclic Aromatic Hydrocarbons (PAHs) in soils

Polycyclic aromatic hydrocarbons (PAHs) are a group of quotidian organic compounds having between two and six condensed aromatic rings braced in angular, cluster or linear forms possessing hydrogen and carbon atoms only. This work aims to evaluate the physicochemical characteristics of soil as determinants of PAHs distribution and fate. The study area is situated on the Benin River beneath the convergence of Jamieson River and River Ethiope. The weather and climatic conditions of the area are of the Niger Delta region. Sixteen priority PAHs were determined in ten sample stations. Upon extraction, purification and quantification of PAHs was done using GC-FID. According to the methods of Walkey-Blacks, Black and Bouyoucus, soil total organic carbon, pH and texture were analyzed respectively. Four replicate samples were chosen to create a composite and used as quality control. Kruskal-Wallis test with tied rank was used to determine a causative correlation between soil physicochemical parameters and ∑PAHs. Results showed that the study area is contaminated with high concentration of PAHs ranging from 60.76 – 271.11 239 µg.g-1 with higher percentages of HPAHs i.e. PAHs with LogKow > 4.0. It is observed that observed alkaline pH favour significant fungal, asperigellus and microbial degradation of PAHs, and hinder photo-catalytic degradation. However, acidification with elemental sulfur, ferrous sulfate and aluminum sulfate could enhance photo-catalytic degradation. Results of TOC and soil structure do not favour significant degradation of HPAHs. Kruskal-Wallis tied rank test revealed that there was a positive significant difference between pH, TOC, percentage clay, and silt fractions with ∑PAHs concentrations since calculated K is greater than critical K0.05, P<0.05 except percentage of sand fractions and ∑PAHs with P>0.05. A substantial percentage of these PAHs have no anthropic origin and are characterized as having high lipo-affinity, semi-volatility and resistance to biotic and/or abiotic transformations. Due to edaphic factors the observed HPAHs may partially degrade with the consequent formation of highly toxic by-products. Therefore, oral or dermal ingestion of soil in the study area may cause significant exposure of the human population to these PAHs. Risk assessment and probable toxicity effects on animal diversity should be carried out in soil samples of the study area.