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Abstract Band 50

Markus Förster (2011): Sequestration of sulfadiazine in soil.
Bonner Bodenkundl. Abh. 50, 109 S., 11 Abb.,  13 Tab.



Sequestration is a key process influencing the environmental fate of xenobiotics in soil. In contrast to hydrophobic compounds, for which the extent of sequestration in soils is well studied, important information is missing for polar compounds like the sulfonamides. In order to increase the knowledge about the environmental fate of this class of substances, which is widely used in veterinary medicine, I studied the sequestration of sulfadiazine, partly in the presence of its main metabolites and manure, in soils and soil fractions.

At first, various tests with different solvent mixtures (two- to four-component mixtures with water, methanol, acetonitrile, acetone, and/or ethyl acetate), pH values (natural pH, pH 4 and 9), and extraction temperatures (up to 200°C) were conducted to extract aged sulfadiazine residues in soils. For this purpose the soils were blended with manure derived from 14C/12C-sulfadiazine-treated pigs and incubated at 10°C in the laboratory before the extraction tests. The soil extracts were measured by liquid scintillation counting and high-performance liquid chromatography coupled to tandem mass spectrometry. A cleanup via solid phase extraction was avoided to keep the method rapid and simple. With respect to sulfadiazine yields, stability of soil extracts, and the amount of coextracted matrix, a microwave extraction of soil (15 minutes, 150°C) using acetonitrile/water 1:4 (v/v) is the method of choice for the exhaustive extraction of aged sulfadiazine residues from soils.

To study the sequestration of sulfadiazine residues, contaminated manure (fresh and aged) of fattening pigs medicated with 14C/12C-sulfadiazine was applied to the soils. Afterwards the samples were incubated up to 218 d at 10°C in darkness during a laboratory experiment. The developed extraction method was then used as final step in a sequential extraction method that enabled me to approach sulfadiazine residues of different (bio)accessibility with 0.01 M CaCl2 (CaCl2 fraction), methanol (MeOH fraction), and finally acetonitrile/water (residual fraction, microwave extraction at 150°C). In each fraction, total radioactivity, sulfadiazine, and its major metabolites (4-hydroxysulfadiazine, N-acetylsulfadiazine) were quantified. The results showed that both sulfadiazine and, to a lesser extent, 4-hydroxysulfadiazine were rapidly (re)formed from N-acetylsulfadiazine during the first 2 to 4 weeks after application of fresh manure, i.e. the N-acetylated metabolite does not sequester in soil to a significant extent. Yet, the water- and methanol-extractable sulfadiazine and 4-hydroxysulfadiazine also dissipated rapidly (DT50=6.0-32 d) for the fresh manure treatment with similar rate constants for both soil types. In the residual fractions, however, the concentrations of both compounds increased with time indicating a strong sequestration of these compounds in soil in the presence of manure. Consequently, it can be concluded that the residual fraction comprises the sequestered pool of sulfadiazine and its hydroxylated metabolite.

As antibiotic-soil interactions are influenced by the physicochemical properties of the substance, the composition of the soil and, possibly, the presence of manure, a final experiment was conducted to determine the time-dependent extractability of sulfadiazine from soils and soil fractions. To this aim, sulfadiazine was applied to soils and soil fractions, each with or without manure-derived dissolved organic matter. The samples were incubated at 4°C in darkness for up to 72 d and afterwards sequentially extracted to quantify sulfadiazine fractions of different (bio)accessibility. Additionally, sorption kinetics and isotherms were determined for freshly-spiked sulfadiazine in soils and, partly, in soil fractions. The results show that sulfadiazine sorption to soil can be best described using the Freundlich equation. Yet, sorption exhibited substantial hysteresis, indicating an aging of sulfadiazine in soil within the observed time frame. In contrast, the affinity of sulfadiazine to mineral soil fractions was low and followed a linear sorption isotherm. In conclusion, organic carbon is the most important constituent influencing the sequestration of sulfadiazine in soil, while iron oxides participate in sorption reactions with sulfadiazine only to a minor degree. Additionally, the presence of manure-derived dissolved organic matter influences the sequestration of sulfadiazine in soils, especially in areas, where soil minerals are not covered with organic carbon. In summary, sulfadiazine and 4-hydroxysulfadiazine are quickly and to a large extend sequestered in soil, which makes their long-term fate in this environmental compartment a future research need.