Numerous ionizable organic micropollutants contain positively charged moieties at pH values typical of environmental systems. Describing organic cation and zwitterion interaction with dissolved natural organic matter requires explicit consideration of the pH-dependent speciation of both sorbate and sorbent. We studied the pH-, ionic strength-, and concentration-dependent binding of relatively large, organic cations and zwitterions (viz., the antibiotics clarithromycin and tetracycline) to dissolved humic acid in the absence and presence of Ca 2+ and evaluated the ability of the NICA-Donnan model to describe the data. Clarithromycin interaction with dissolved humic acid was well described by the model including the competitive effect of Ca 2+ on clarithromycin binding over a wide range of solution conditions by considering only the binding of the cationic species to low proton-affinity sites in humic acid. Tetracycline possesses multiple ionizable moieties and forms complexes with Ca 2+ . An excellent fit to experimental data was achieved by considering tetracycline cation interaction with both low and high protonaffinity sites of humic acid and zwitterion interaction with high proton-affinity sites. In contrast to clarithromycin, tetracycline binding to humic acid increased in the presence of Ca 2+ , especially under alkaline conditions. Model calculations indicate that this increase is due to electrostatic interaction of positively charged tetracycline-Ca complexes with humic acid rather than due to the formation of ternary complexes, except at very low TC concentrations.

A variety of peptidic and proteinaceous contaminants (e.g., proteins, toxins, pathogens) present in the environment may pose risk to human health and wildlife. Peroxymonosulfate is a strong oxidant (E H 0 = 1.82 V for HSO5 –, the predominant species at environmental pH values) that may hold promise for the deactivation of proteinaceous contaminants. Relatively little quantitative information exists on the rates of peroxymonosulfate reactions with free amino acids. Here, we studied the oxidation of 19 of the 20 standard proteinogenic amino acids (all except cysteine) by peroxymonosulfate without explicit activation. Reaction half-lives at pH 7 ranged from milliseconds to hours. Amino acids possessing sulfur-containing, heteroaromatic, or substituted aromatic side chains were the most susceptible to oxidation by peroxymonosulfate, with rates of transformation decreasing in the order methionine > tryptophan > tyrosine > histidine. The rate of tryptophan oxidation did not decrease in the presence of an aquatic natural organic matter. Singlet oxygen resulting from peroxymonosulfate self-decomposition, while detected by electron paramagnetic resonance spectroscopy, was unlikely to be the principal reactive species. Our results demonstrate that peroxymonosulfate is capable of oxidizing 19 amino acids without explicit activation and that solvent-exposed methionine and tryptophan residues are likely initial targets of oxidation in peptides and proteins.

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