title: Cation binding of antimicrobial sulfathiazole to leonardite humic acid.
abstract: Sorption of sulfathiazole (STA) and three structural analogs to Leonardite humic acid (LHA) was investigated in single- and binary-solute systems to elucidate the sorption mechanism of sulfonamides to soil organic matter (SOM). Cation binding of STA+ to anionic sites A- in LHA governed sorption up to circumneutral pH, based on the following findings: (i) From pH 7.7 to 3.3, the increase in extent and nonlinearity (i.e., concentration dependence) of STA sorption paralleled the increase in STA+. (ii) From pH 3.3 to 1.7, sorption decreased and nonlinearity increased, consistent with strong competition of STA+ and H+ for A-. (iii) Replacement of the protonable aniline group in STA by an apolar methylbenzene group resulted in much weaker, linear, and pH-independent sorption. (iv) Only analogs with aniline moieties displaced STA from LHA in binary-solute systems. Displacement occurred up to pH 5.4, at which <1% of STA in solution was cationic. (v) STA sorption was well-described (R2 = 0.98) by the NICA-Donnan cation-binding model, yielding high median affinities for STA+ to carboxylic and phenolic A- (log K(STA+,1) = 3.25 +/- 0.08 log (L mol(-1)) and log K(STA+,2) = 8.76 +/- 0.11 log (L mol(-1)), respectively). High affinity cation binding explains sorption of polar sulfonamides in agricultural soils and the strong dependence of sorption on SOM content and pH.
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Duebendorf Switzerland, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland.
authors.forename: Merle K
authors.initials: MK
authors.lastname: Richter
authors.forename: Michael
authors.initials: M
authors.lastname: Sander
authors.forename: Martin
authors.initials: M
authors.lastname: Krauss
authors.forename: Iso
authors.initials: I
authors.lastname: Christl
authors.forename: Manuel G
authors.initials: MG
authors.lastname: Dahinden
authors.forename: Manuel K
authors.initials: MK
authors.lastname: Schneider
authors.forename: René P
authors.initials: RP
authors.lastname: Schwarzenbach
mesh_terms: D000327:Adsorption; D000890:Anti-Infective Agents / Q000032:analysis / Q000737:chemistry*; D002412:Cations; D006812:Humic Substances / Q000032:analysis*; D008956:Models, Chemical; D015394:Molecular Structure; D012989:Soil Pollutants / Q000032:analysis / Q000737:chemistry*; D000077589:Sulfathiazole; D013432:Sulfathiazoles / Q000032:analysis / Q000737:chemistry*
chemical_list: D000890:Anti-Infective Agents; D002412:Cations; D006812:Humic Substances; D012989:Soil Pollutants; D013432:Sulfathiazoles; D000077589:Sulfathiazole