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

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.

Remove mechanisms of sulfamethazine by goethite: the contributions of pH and ionic strength

“…This result indicates that H-bonding between the functional groups of SMT and goethite are significant [28]. Similar results have been found previously for SMT sorption on biochar, and we deem that the current results for SMT can be interpreted similarly [26].…”

“…When the pH decreased from 8.0 to 3.0, sorption decreased slightly and then sharply at pH below 3.0. We fitted the observed K d to a speciation model [25,26]:…”

“…As reflected in log K ow , SMT ? (-1.3) and SMT -(-2.09) are significantly less hydrophobic than SMT 0 (0.27) [26,27]. Normalizing K d 0 , K d ?…”

Removal of sulfamethazine antibiotics using cow manure-based carbon adsorbents

“…Thus, an enhancement of the p ? -p interactions between the polarized SMT molecules and electron-rich aromatic rings of the CM samples was expected at a higher carbonization temperature and under acidic conditions (Teixido et al 2011). Therefore, SMT adsorption was favorable on the hydrophobic surfaces of CM800 at pH 3.…”

Sorption of Atrazine in Tropical Soil by Biochar Prepared from Cassava Waste

“…The decrease in the O/C and (N+O)/C ratios indicated an increase in aromaticity and a reduction in polarity. Hydrophobic carbon can provide more sorption domains for HOCs, and aromaticity and pore-filling are positively correlated with aromatic carbon contents (Teixido et al 2011;Yang et al 2011).…”