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.

Urban stormwater transports hydrophilic trace organic contaminants that need study to ensure safe stormwater discharge or stormwater harvesting for water supply.

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Commercial hardwood biochars ranging in N2 specific surface area of 0.1-427 m(2) · g(-1) were added to an agricultural soil at 0, 1, or 2% levels to determine whether they would predictably reduce the pore water concentration of sulfamethazine (SMT). The soil and biochar-soil mixtures were preweathered under mild (2 d, 20 °C) or more severe (28 d, 40 °C) conditions before spiking. The carbon-normalized biochar-water distribution coefficient (KBC) of the biochars varied by a factor of up to 10(4), depending on biochar properties and SMT concentration. Except for the fast-pyrolysis biochar, KBC greatly exceeded the soil organic carbon-water distribution coefficient KOC. Sorption in the mixtures increased as expected with biochar and dose. However, sorption was dramatically overpredicted (by up to 10(2.5)) by the sum of sorption to the individual components, indicating a strong weathering effect even under the mild conditions. The soil-subtracted weathered biochar-water isotherms were more linear, and the KBC values approached or lay within the range of KOC values reported for SMT in 19 soils. Biochars both in intimate contact with soil and placed in a membrane bag suspended in the solution showed reduced N2-B.E.T. surface area after weathering, implicating fouling of the biochar surface by humic substances transferred through water. The results indicate that only highly surfaceous, carbonaceous biochars would be useful for stabilizing soil contaminated with compounds such as SMT. They also suggest that weathering may attenuate the contribution of native (environmental) black carbon to sorption of such compounds in soils and sediments.