Environmental relevance of adsorption of doxycycline, enrofloxacin, and sulfamethoxypyridazine before and after the removal of organic matter from soils.

In this work batch-type experiments were used to study the adsorption of the antibiotics doxycycline (DC), enrofloxacin (ENR), and sulfamethoxypyridazine (SMP) in cultivation soils, before and after the removal of soil organic matter. Organic matter removal by calcination resulted not only in C and N removal, but also in increased soil pH, exchangeable basic cations and surface area values. The results indicate a very different behavior depending on the type of antibiotic, showing the adsorption sequence DC > ENR > SMP. Specifically, DC adsorption was very high in untreated soil samples (with organic matter), and was still high (although decreased) after the removal of soil organic matter. Furthermore, the adsorption behavior of DC was clearly dependent on the pH of the medium. Regarding ENR, it also showed high adsorption, although to a lesser extent than DC. However, when soil organic matter was removed, ENR adsorption significantly decreased in all soil samples. As regards SMP, it was adsorbed to a much lesser extent, and the removal of soil organic matter caused an additional drastic decrease in adsorption, reaching negligible values in some samples. Desorption followed the reverse sequence of adsorption, specifically in the order DC < ENR < SMP. In the case of DC, desorption was negligible, both in samples with and without organic matter, while for ENR and SMP, desorption clearly increased for soil samples where organic matter was removed. These results may be of relevance as regards environmental quality and public health, especially to facilitate a correct use of soils and organic amendments in areas that receive the application of substances containing the investigated antibiotics.

Adsorption-desorption of doxycycline in agricultural soils: Batch and stirred-flow-chamber experiments.

With the aim of obtaining information about the environmental fate and dynamics of the antibiotic doxycycline (DC) when it reaches soil, adsorption and desorption processes were studied for this compound in 20 agricultural soils, by means of batch-type and stirred-flow-chamber experiments. The results indicate that the studied soils adsorbed high amounts of DC, with adsorption percentages >91% in all cases. In addition, adsorption results were satisfactorily modeled, with good fittings to the Langmuir and Freundlich equations, with the values for Langmuir's maximum adsorption capacity (qmax) varying between 14,692 and 26,141 µmol kg-1 (average 17,816), and between 1,906 and 13,120 Ln µmol1-n kg-1 (average 6,969) for the Freundlich affinity coefficient, which are very high. The soil variables most related to the adsorption of the antibiotic were linked to organic matter (specifically, soil organic carbon-SOC- and soil nitrogen -N- contents), and to the clay fraction, as well as to cation exchange capacity, being the soils with a greater content in these variables those that presented a greater adsorption. FTIR results shown that DC adsorption mechanisms were based on interactions such as hydrogen bonds and π-π interactions between the antibiotic and soils. Desorption was very low, reaching values between 1 and 2% in batch experiments, and between 5 and 15% in stirred flow chamber experiments, which indicates a strong hysteresis affecting adsorption and desorption processes. This fact can be considered positively, as these soils could retain DC very strongly, thus reducing risks to human and ecological health.

Adsorption/desorption of sulfamethoxypyridazine and enrofloxacin in agricultural soils.

Adsorption and desorption processes were studied for the antibiotics sulfamethoxypyridazine (SMP) and enrofloxacin (ENR) in 20 agricultural soils devoted to wheat-potato and vine cultivation. Batch experiments were used to conduct kinetic studies, as well as to evaluate adsorption and desorption for different concentrations of antibiotics. The results indicated that adsorption curves were linear for SMP, while presented a certain curvature in the case of ENR. The adsorption of both antibiotics was fitted to a linear equation and to the Freundlich model. In the case of the linear equation, the values of the distribution coefficient Kd were lower for SMP (0.9-26.0 L kg-1) than for ENR (121-2345 L kg-1). In the Freundlich model, the values of the adsorption constant KF ranged between 1.7 and 34.0 Ln µmol1-n kg-1 for SMP, and between 829 and 3019 Ln µmol1-n kg-1 for ENR. A multiple regression analysis showed that, in the case of SMP, 78% of the variance of the adsorption parameter Kd was explained by soil organic carbon (SOC) and exchangeable magnesium (Mge) contents, while for ENR no significant relation was found. In addition, for SMP, 66% of the variance of KF was explained by SOC, and for ENR 45% of the variability of KF was explained by nitrogen content. Desorption rates were higher for SMP (reaching up to 24%) than for ENR (which reached up to 7%). Desorption percentages showed a potential relation with the adsorption parameter Kd for both SMP and ENR. These results could be useful to elucidate the evolution and dynamics of these emerging pollutants in soils and other environmental compartments, which could be of aid in controlling public health risks associated to them.