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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 KSTA+,1 = 3.25 ± 0.08 log (L mol-1) and log KSTA+,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. [ABSTRACT FROM AUTHOR]