Influence of chemical and mineralogical soil properties on the adsorption of Sulfamethoxazole and Diclofenac in Mediterranean soils

Background: The irrigation with treated wastewaters can be a way for the introduction of organic contaminants in soils. However, their adsorption onto soils can allow a control of their bioavailability and leaching. The adsorption is influenced by properties of contaminants (water solubility, chemical structure) and soils (organic matter content, pH, mineralogy). This study aimed to investigate the effect of mineralogical composition, organic matter content and others parameters of soils on the adsorption of sulfamethoxazole (SMX) and diclofenac (DCF), two contaminants of emerging concerns (CECs), in real cases (Altamura, Sibari and Noci soils). Results: The isotherms data showed that the adsorption of the two CECs closely matched the Freundlich model, even if the DCF could also fit the linear one. The only exception was the adsorption of SMX on the soil of Sibari, for which Langmuir's model fitted better. In all cases, the Kd values were the highest for Altamura soil according mainly to its content of organic carbon. Positive correlations were found between Kd value of DCF and the soil organic carbon and Al oxyhydroxides content, suggesting their roles in its adsorption, while SMX showed only a slight positive correlation with the soil organic carbon content. Finally, between the two CECs studied, DCF was more adsorbed than SMX also because of the lower water solubility of the former. Conclusion: The good interaction between DCF and soil organic carbon suggests the organic amendment of soils before the application of treated watewaters. The low adsorption of SMX onto soils suggests greater leaching of this compound which is, therefore, potentially more dangerous than DCF. For this reason, the application of a filtration system with appropriate adsorbent materials before the application of wastewater to soils should be expected.