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Understanding the long-term sequestration of veterinary antibiotics into soil fractions with different bioavailability is important in terms of assessing their eco-toxicological impact. We performed 60-d batch sorption experiments with radiolabeled sulfadiazine (SDZ) using samples from two agricultural soils. Sequential extraction with CaCl/MeOH (easily accessible fraction), microwave (residual fraction, RES), and combustion (nonextractable residues, NER) was used to quantify the sequestration dynamics of the C-derived SDZ-equivalent concentration. Multiple harsh extractions allowed us to mathematically extrapolate to the amount of SDZ equivalents that can be potentially extracted, resulting in halving the NER fraction after 60 d. A modified two-stage model with irreversible sorption combined with global parameter optimization was able to display the sequestration dynamics. We demonstrated this with sterilized samples in which no transformation of the parent compound was observed. This also showed that transformation was primarily biologically driven. These modeling results verified the procedure, which was then applied to nontreated samples from both soils to estimate effective parameter values for SDZ-derived equivalents. Observed initial sorption, to which up to 20% of the kinetic sorption sites attributed, was included in the model. Both the RES and NER fractions reached a sorption plateau, with NER occupying about 30% of the kinetic fraction (RES+NER) for all soils. The sorption and sequestration of SDZ were soil-specific and dominated by kinetics. Sequestration in the RES fraction was much slower (characteristic time: 60 d) than the redistribution in the NER fraction (characteristic time: <6 d). The work presented here contributes to the prediction of the dynamics of (bio-)availability., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

The environmental risks caused by the use of fluoroquinolone antibiotics in human therapeutics and animal husbandry are associated with their persistence and (bio)accessibility in soil. To assess these aspects, we administered difloxacin to pigs and applied the contaminated manure to soil. We then evaluated the dissipation and sequestration of difloxacin in soil in the absence and presence of plants within a laboratory trial, a mesocosm trial, and a field trial. A sequential extraction yielded antibiotic fractions of differing binding strength. We also assessed the antibiotic's effects on nitrogen turnover in soil (potential nitrification and denitrification). Difloxacin was hardly (bio)accessible and was very persistent under all conditions studied (dissipation half-life in bulk soil, >217 d), rapidly forming nonextractable residues. Although varying environmental conditions did not affect persistence, dissipation was accelerated in soil surrounding plant roots. Effects on nitrogen turnover were limited due to the compound's strong binding and small (bio)accessibility despite its persistence., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

Antibiotics reach soils via spreading of manure or sewage sludge. Knowledge on the transport behavior of antibiotics in soils is needed to assess their environmental fate. The effect of flow rate and applied mass, i.e., input concentration and pulse duration, on the transport of 14C-sulfadiazine (SDZ; 4-aminoN-pyrimidin-2-yl-benzenesulfonamide) was investigated with soil column experiments and numerical studies. Sulfadiazine was applied in pulses (6.8, 68 or 306 h) under steady-state (0.051 and 0.21 cm h(-1)) and intermittent flow conditions and at two input concentrations (0.57 and 5.7 mg L(-1)). Breakthrough curves (BTCs) of 14C were measured and for one experiment concentrations of SDZ, and its transformation products 4-(2-iminopyrimidin-1(2H)-yl)aniline (An-SDZ) and N(1)-2-(4-hydroxypyrimidinyl)benzenesulfanilamide (4-OH-SDZ) were determined. After finalizing the leaching experiments, 14C was quantified in different slices of the columns. A lower flow rate led to remarkably lower eluted masses compared with the higher flow rates. All BTCs could be described well using a three-site attachment-detachment model for which a common set of parameters was determined. However, the BTC obtained with the high input concentration was slightly better described with a two-site isotherm-based model. The prediction of the concentration profiles was good with both model concepts. The fitted sorption capacities decreased in the order SDZ > 4-OH-SDZ > An-SDZ. Overall, the experiments reveal the presence of similar mechanisms characterizing SDZ transport. The dependence of model performance on concentration implies that although the three-site attachment-detachment model is appropriate to predict the transport of SDZ in soil columns, not all relevant processes are adequately captured.

Antibiotics, such as sulfadiazine (SDZ), may enter arable soil by spreading of manure of medicated husbandry or directly by the excrement of grazing animals. Knowledge of the fate of antibiotics in soils is crucial for assessing the environmental risk of these compounds, including possible transport to ground water. Kinetic sorption of (14)C-labeled SDZ (4-amino-N-pyrimidin-2-yl-benzenesulfonamide) was investigated using the batch technique. The batch sorption-desorption experiments were conducted at various concentration levels (0.044-13 mg L(-1) initial solute concentration) and time scales (0.75-272 d). Sorption of (14)C-SDZ in the investigated silty loam was time dependent and strongly nonlinear in the solution phase concentration. The time to reach an apparent sorption equilibrium was about 20 d. However, desorption was very slow, and 41 d were insufficient to reach the desorption equilibrium. An inverse modeling technique was used to identify relevant sorption processes of (14)C-SDZ during the batch experiments. Among the investigated two- and three-domain sorption models, adsorption and desorption of (14)C-SDZ were best described with a new model defining two sorption domains and four parameters. Whereas sorption in the first sorption domain was nonlinear and instantaneous, solute uptake in the second sorption domain was rate limited following first-order kinetics. Desorption followed the same rate law until an equilibrium distribution was reached. After that, desorption was assumed to be impossible due to partly irreversible sorption. Although the proposed model needs further validation, it contributes to the discussion on complex sorption processes of organic chemicals in soils.