Sorption of Phenols: Influence of Groundwater pH and of Soil Organic Carbon Content

Abstract

Problem statement: Physical and chemical properties of soil, groundwater and pollutants deeply influenced the type and the strength of sorption phenomena involving hydrophobic organic contaminants: Soil fraction of organic carbon (foc) and cation exchange capacity, groundwater pH and pollutants hydrophobicity were with undoubtedly the predominant issues. Approach: In this study an experimental study of the sorption of phenol and 2-nitrophenol on three soils was performed considering three initial pH values, with the aim to evaluate the soil/pollutant interactions as a function of the relative abundance of the neutral contaminants and phenolate species and of the solid phase properties. Results: The considered soil samples (a silty clay, a silt and a sand coming from northern Italy) underwent to a physical and chemical characterization: Particle-size, mineralogical and chemical analyses were performed. The experimental data coming from some batch tests, carried employing aqueous phases containing different concentrations of phenol and 2-nitrophenol at initial pH values equal to 4, 7 and 10, were fitted by means of several isotherm models. Three Dual Mode Models (DMM), employing different isotherm models to represent the sorption of the neutral and the anionic species of the pollutants, were evaluated by the researchers to simulate the sorption of phenol and 2-nitrophenol in the examined conditions. Conclusion: The soil/contaminant interaction mechanisms determining the isotherm shapes were discussed: foc appeared to be the main issue, although the nitro-group was able to promote sorption. In all considered soil samples and at all initial pH values, the highest foc (found in the silty soil) and the presence of the nitro-group determined the highest amounts of pollutant transferred on the solid phase. The two DMMs proposed by the researchers, implying nonlinearity, showed a higher reliability in simulating experimental data compared to a DMM based on linear sorption isotherms. Main sorption mechanisms were controlled by the partitioning of neutral pollutants in soil organic matter, but also the interactions between the pollutants, both in phenolate and in neutral forms and the soil mineral surface played a primary role.