Solute displacement experiments with the antibiotic sulfadiazine (SDZ) and its main transformation products in pig manure were performed to investigate the influence of manure on SDZ transport. Either pig manure containing 14C-sulfadiazine (4-amino-N-2-pyrimidinyl-benzenesulfonamide), and its main transformation products 14C-4-OH-SDZ and 14C-N-Ac-SDZ, or a 14C-SDZ solution was incorporated in the first centimeter of undisturbed and repacked soil columns, which were then irrigated. Breakthrough curves (BTCs) of 14C, SDZ, 4-OH-SDZ, N-Ac-SDZ and 4-[2-iminopyrimidine-1(2H)-yl]-anilin were measured. The 14C distributions vs. depths were determined after the conclusion of the leaching experiments. An application of SDZ together with manure resulted in lower peak values of the 14C BTCs and a slightly lower amount of eluted mass. In the experiments with manure, the 14C concentrations in the uppermost layers of the soil columns were higher, probably due to the filtration of manure particles onto which SDZ or its transformation products were sorbed. The transformation products showed a relatively high leaching potential similar to SDZ. Cotransport with organic particles seemed to be of minor relevance for the eluted amounts of solutes. All BTCs were modeled using a numerical model that considered degradation chains from N-Ac-SDZ into SDZ and from SDZ into 4-OH-SDZ, as well as one reversible and one irreversible kinetic sorption site for each solute. The applied model fitted the BTCs of SDZ and its transformation products reasonably well. The fitting process revealed a high mobility of both SDZ and its transformation products. While N-Ac-SDZ degradation into SDZ was fast and no extended tailing of N-Ac-SDZ was observed, the transport behavior of 4-OH-SDZ was similar to that of SDZ.

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