Speciation of the Ionizable Antibiotic Sulfamethazine on Black Carbon (Biochar)

Adsorption of ionizable compounds by black carbon is poorly characterized. Adsorption of the veterinary antibiotic sulfamethazine (SMT; a.k.a., sulfadimidine; pK(a1) = 2.28, pK(a2) = 7.42) on a charcoal was determined as a function of concentration, pH, inorganic ions, and organic ions and molecules. SMT displayed unconventional adsorption behavior. Despite its hydrophilic nature (log K-ow = 0.27), the distribution ratio K-d at pH 5, where SMT0 prevails, was as high as 10(6) L kg(-1), up to 10(4) times greater than literature reported K-oc. The K-d decreases at high and low pH but not commensurate with the decline in K-ow of the ionized forms. At pH 1, where SMT+ is predominant and the surface is positive, a major driving force is pi-pi electron donor acceptor interaction of the protonated aniline ring with the pi-electron rich graphene surface, referred to as pi(+)-pi EDA, rather than ordinary electrostatic cation exchange. In the alkaline region, where SMT- prevails and the surface is negative, adsorption is accompanied by near-stoichiometric proton exchange with water, leading to the release of OH- and formation of an exceptionally strong H-bond between SMT0 and a surface carboxylate or phenolate, classified as a negative charge-assisted H-bond, (-)CAHB. At pH 5, SMT0 adsorption is accompanied by partial proton release and is competitive with trimethylphenylammonium ion, signifying contributions from SMT+ and/or the zwitterion, SMT +/-, which take advantage of pi(+)-pi EDA interaction and Coulombic attraction to deprotonated surface groups. In essence, both pK(a1) and pK(a2) increase, and SMT +/- is stabilized, in the adsorbed relative to the dissolved state.