Published: October 25, 2011 r2011 American Chemical Society 10020 dx.doi.org/10.1021/es202487h | Environ. Sci. Technol. 2011, 45, 10020–10027 ARTICLE pubs.acs.org/est Speciation of the Ionizable Antibiotic Sulfamethazine on Black Carbon (Biochar) Marc Teixid o, † Joseph J. Pignatello, ‡,§, * Jos  e L. Beltr  an, † Merc  e Granados, † and Jordan Peccia § † Departament de Química Analítica, Universitat de Barcelona, Martí i Franqu es 1-11, 08028 Barcelona, Spain ‡ Department of Environmental Sciences, Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504, United States § Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States b S Supporting Information ’ INTRODUCTION Sulfonamide antibiotics are widely used in veterinary medicine to promote growth and control infectious diseases. Since they are poorly metabolized, a large fraction is deposited onto soils from grazing and manure application. 1,2 At field concentrations, anti- biotics impact soil microbial communities and functions, 3,4 and may contribute to the proliferation of antibiotic resistant bacter- ial pathogens. 1,2,5 Sorption is a critical process controlling the mobility and bioavailability of sulfonamide antibiotics in soil. The subject of our report is sulfamethazine (SMT; 4-amino-N-[4,6- dimethyl-2-pyrimidinyl]benzenesulfonamide) a heavily used swine antibiotic in the U.S. and European Union. 5 Sorption of sulfonamides by whole soils, soil minerals, and dissolved organic matter has been studied, but reports are absent on sorption of these compounds to black carbon, an important component of the soil carbon pool and a form of which (biochar) is proposed as a soil amendment. Sulfamethazine (Figure 1) is hydrophilic (octanolwater partition coefficient, log K ow = 0.27) and in water exists as the cation (SMT + ), anion (SMT  ), uncharged molecule (SMT 0 ), and zwitterion (SMT ( ) due to proton exchange at the aromatic amine and sulfonamide groups (pK a1 , 2.28; pK a2 , 7.42). 6 This complex speciation in water presages complex sorption behavior in soil. Several studies have probed chemical speciation of sulfonamide drugs adsorption on/in natural particles. 715 Sorp- tion increases with organic carbon (OC) content and soil surface area and decreases with increasing soil solution pH. 10 Organic carbon is the main, 11 but not the only, active sorbent phase in soil. 12 Reported log K oc at around pH 5, where SMT 0 predomi- nates, range from 1.91 to 3.02. 10,13,14 Sorption of SMT by reference clays (kaolinite and montmorillonite) decreases with increasing solution pH and increases with ionic strength and surface charge density, consistent with a cation exchange mechanism. 8 Lertpaitoonpan et al. 10 proposed that sorption of SMT 0 to dissolved humic acid occurs by “hydrophobic partitioning”. Richter et al., 11 on the other hand, proposed that the related compound, sulfathiazole, having similar acid- ity constants as SMT (pK a1 , 2.11; pK a2 , 7.21), sorbs to dissolved humic acid mainly by cation exchange from low pH to pH ∼ 7.7. This assertion was based primarily on the finding that sorption generally decreased with increasing [H + ] at constant Received: July 19, 2011 Accepted: October 7, 2011 Revised: October 4, 2011 ABSTRACT: 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 SMT 0 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 ππ electron donoracceptor interaction of the protonated aniline ring with the π-electron rich graphene surface, referred to as π + π 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 SMT 0 and a surface carboxylate or phenolate, classified as a negative charge-assisted H-bond, ()CAHB. At pH 5, SMT 0 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 π + π 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.