Sorption and desorption of phenanthrene and fluorene in mangrove forest soils of the Morrocoy National Park, Venezuelan Caribbean

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The research proved the capacity of mangrove soils (of Boca del Zorro Spout) to sorb and desorb two low molecular weight PAHs, fluorene and phenanthrene. In order to evaluate this capacity, we determined the total organic carbon (TOC wt%) and the maturity of soil organic matter (SOM) by calculating the maximum vitrinite reflectance (R_{o max}). Additionally, batch-reactor experiments, water leaching and ultrasonication were conducted to study the sorption and desorption process. The mangrove soil presents a TOC content of 25% and a humification range of SOM that has reached the range of peat. Sorption isotherms were nonlinear following the Freundlich model. The values of the isotherm nonlinearity index (N) and organic carbon normalized sorption coefficients (log K_oc) are similar for fluorene and phenanthrene; therefore, they follow the same sorption mechanism in the mangrove soil. The log K_oc values obtained suggest that the soil is composed of a mixture of both recent organic matter and organic matter with an advanced degree of maturity. For this reason, for both polycyclic aromatic hydrocarbons (PAHs), the process responded to a non-linear model with limited sorption sites with variable potential energy. Both fluorene and phenanthrene presented a desorption percentage between 6-7% in mangrove soil sample. This allows us to infer that the bonds presented in the sorption process are associated with the insertion of fluorene and phenanthrene into the pores of the humified mangrove soil material. The research into mangrove soil displayed significant heterogeneity in composition and strong sorption of PAHs, as a result, mangrove soil presents natural filters in this area of the Venezuelan Caribbean.