RESUMO
The presence of antibiotics in soils is increasing drastically in last decades due to the intensive farming industry and excessive human consumption. Clay minerals are one of the soil components with great adsorption capacity for organic pollutants. The study of interactions between antibiotics and mineral surfaces will give us scientific knowledge of these pollutants through soils. In this work, we study the adsorption of the antibiotic ciprofloxacin in the clay mineral fraction of soils from the Argentinian zone of Santa Rosa (Corrientes), in a collaborative research of experiments and atomistic modelling calculations of the intercalation of ciprofloxacin in the interlayer space of montmorillonite. Adsorption and desorption isotherms were performed and compared with different isotherm models. Additionally, enthalpy, entropy, and free energy were determined from equilibrium constants at a function of temperature. All these experiments and calculations lead to the conclusions that two adsorption types of ciprofloxacin are found on clay minerals: one weakly sorbed that is released during the desorption experiments, and other one strongly joined that remains in the soil.
RESUMO
There is a growing concern about glyphosate's behavior in the environment. Herbicide behavior in soils greatly depends on adsorption-desorption phenomena, which have shown great variability in soils, although the reversibility of this process has been less examined. The adsorption-desorption behavior of glyphosate was measured on representative soil profiles of the northeast region of Argentinean Pampas, a semi-arid crop cultivating region. Two soil profiles samples (P1 and P2, both Entic Haplustoll) were collected and segmented into depth increments of 0-10, 10-20, 20-40, 40-60, 60-80, and 80-100 cm. Batch adsorption/48 h-desorption isotherms were performed in a controlled setup. Soil samples had a high sand content (77-92%), and a low content of clay (<3%), but markedly differing in the available P content, especially in the upper layers of soil profiles (0-40 cm, P1 range 133-170 ppm; P2 range 7-43 ppm). Adsorption-desorption isotherms showed a similar range of variation, between 150 and 1400 L kg -1for KFads and 450-1400 L kg -1for KFdes, without appreciable evidence of hysteresis (0.95 ± 0.05). Sorption capacity parameters showed a distinct behavior with depth, P1 exhibiting a U-shape with minimum values at intermediate depths (20-60 cm), while P2 decreases sharply between 0 and 40 cm. General linear models considering the specific surface area (SSA) of each sample and the spatial correlation structure for soil profiles showed a main positive association of KFads and KFdes with the soil organic matter, together with a positive association with iron content (KFads), and a negative association of KFdes with available P content. These results indicate high adsorption extents and sorption reversibility of glyphosate to sandy loam soils of the region, which implies the potential for the herbicide to be available for leaching or degradation under a scenario of intensive use.