Determination of the distribution coefficient (log Kd) of oxytetracycline, tylosin A, olaquindox and metronidazole in manure

Abstract

Olaquindox (log K ow=−2.3) and metronidazole (log K ow=−0.1) both have low tendencies to sorp to particles in manure. This corresponds with the negative log K ow values of these antibiotics. Tylosin (log K ow=1.63) and oxytetracycline (log K ow=−1.12) sorp relatively strongly to the manure particles and have log K d values between 1.5 and 2.0. The tendency to bind to manure was ranked after increasing binding as follows: metronidazole<olaquindox≪tylosin A and oxytetracycline. This order of ranking is consistent with results of sorption in soil. Our experiments illustrate that for some antibacterial agents estimation of the partitioning coefficients, K d, cannot be made from K ow and f oc alone. Sorption of oxytetracycline to manure is much higher than expected from the negative log K ow value of the compound. It is believed that sorption of oxytetracycline to manure is influenced by ionic binding to divalent metal ions as such Mg 2+ and Ca 2+ as well as other charged compounds in the matrix. Binding of oxytetracycline to soil is stronger than the binding to manure. This is most likely due to the strong mineral related metal complexes formed between soil, metal ion and oxytetracycline. These complexes are not known to exist in manure. The relatively strong sorption of tylosin A to manure corresponds with data found for soil sorption of tylosin. Tylosin has a log K ow value of 2.5, thus it is not surprising that this drug binds strongly to manure.