Environ. Sci. Technol. 2005, 39, 3630-3638
Triplet-Sensitized Photodegradation of Sulfa Drugs Containing Six-Membered Heterocyclic Groups: Identification of an SO2 Extrusion Photoproduct ANNE L. BOREEN,† WILLIAM A. ARNOLD,‡ AND K R I S T O P H E R M C N E I L L * ,† Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, and Department of Civil Engineering, University of Minnesota, 500 Pillsbury Street SE, Minneapolis, Minnesota 55455
The aquatic photochemical behavior of a class of sulfa drugs containing six-membered heterocyclic substituents (sulfamethazine, sulfamerazine, sulfadiazine, sulfachloropyridazine, and sulfadimethoxine) was investigated. Photodegradation of the sulfa drugs in a natural water sample was significantly enhanced relative to the degradation in deionized water, with the exception of sulfadimethoxine. This indicated an indirect photochemical process that was identified through the use of quenchers to be attributable to interaction with triplet excited-state dissolved organic matter (3DOM). The direct photolysis rate constant and quantum yield for both the neutral and anionic species of each sulfa drug were calculated using matrix deconvolution methods. The quantum yield values range from 0.01 × 10-3 for the neutral form of sulfadimethoxine to 5 × 10-3 for the anionic form of sulfamethazine and are significantly lower than those observed in a previous study for sulfa drugs containing five-membered heterocyclic substituents, although the rate constants are of similar magnitude. The primary product formed in both direct and indirect photodegradation for all five compounds was identified as a sulfur dioxide extrusion product. The predicted environmental half-lives solely attributable to direct photolysis range from 8.6 h in midsummer at 30° latitude in pH 7 surface water for sulfachloropyridazine to 420 h in midwinter at 45° in pH 7 surface water for sulfadimethoxine. These half-lives, except for sulfadimethoxine, will be decreased by interaction with 3DOM.
Introduction Pharmaceuticals and personal care products (PPCPs) in the aquatic environment have been the subject of an increasing number of studies, including numerous accounts of their detection in various waterways (1-12). Although several classes of compounds have been detected, including nonsteroidal antiinflammatory drugs (NSAIDS), fragrances, dermatologicals, and antihistamic compounds, of particular * Corresponding author phone: (612) 626-0781; fax: (612) 6267541; e-mail:
[email protected]. † Department of Chemistry. ‡ Department of Civil Engineering. 3630
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 10, 2005
concern is the detection of antibiotics in the aquatic environment due to the possibility of increased bacterial resistance (13-17) or other adverse effects these biologically active compounds may exert on the aquatic environment. Sulfa drugs are one class of antibiotics that are frequently detected in surface waters at concentrations ranging from 0 to 1.9 µg/L (8). Higher concentrations have been detected in wastewater effluent and groundwater downstream from landfills (6, 9, 18). The general structure of a sulfa drug is shown in Table 1. The various analogues differ in the N-bound substituent of the sulfonamide linkage. The environmental fate of these compounds is currently unclear, although one likely degradation pathway that may play a role in controlling the fate of PPCPs is photodegradation (19-36). Two classes of photochemical mechanisms, both direct and indirect photodegradation, may operate in natural waters. Indirect photolysis may include reaction with transient excited species such as singlet oxygen (1O2), hydroxyl radical (•OH), triplet excited-state dissolved organic matter (3DOM), or other radical species (37-40). Previous work from our laboratory on the photolysis of sulfa drugs focused on compounds containing five-membered heterocyclic substituents, which were found to photodegrade solely by direct photolysis (35). We report here the photochemical behavior of the sulfa drugs containing six-membered heterocyclic substituents, including the identification of an indirect photochemical process occurring in natural water. Direct photolysis and •OH reaction rate constants are reported along with calculated surface water environmental half-lives. The primary photoproduct arising from photolysis of each sulfa drug is identified, and the product of sulfamethazine is quantified. The mechanism of photodegradation observed for the sulfa drugs in this study is markedly different than previously observed for the sulfa drugs containing five-membered heterocyclic substituents (35).
Experimental Section Chemicals. Sulfachloropyridazine, sulfadiazine (99.0%), and sulfamerazine (99.0%) were purchased from Sigma. Poly(ethylene glycol) (average Mn ) 200), perinaphthenone (97%), 4-amino-2,6-dimethoxypyrimidine (97%), 4-methylpyrimidine, deuterium oxide (D2O), furfuryl alcohol (FFA), sodium molybdate dihydrate (MoO42-, 99.99%), and pyridine were obtained from Aldrich. FFA was distilled prior to use. Hydrogen peroxide (30%) and sodium bicarbonate were purchased from Mallinckrodt. Ferrous sulfate (FeSO4·7H2O) and sodium carbonate were obtained from Fisher Scientific. Acetophenone, p-nitroanisole (PNA), sulfanilic acid (99%), sulfanilamide (98%), sulfamethazine (99%), 2-aminopyrimidine (99%), 3-methoxyacetophenone (3-MAP), sodium azide (99%), and isoprene were purchased from Acros Organics. Isoprene was purified by vacuum distillation to remove radical inhibitors prior to use. 3-Amino-6-chloropyridazine (98%), 2-amino-4-methylpyrimidine (98%), 4,6dimethylpyrimidine, and 2-amino-4,6-dimethylpyrimidine (98%) were obtained from Lancaster Synthesis. Pyrimidine free base was obtained from TCI America. Sulfadimethoxine sodium salt and 2,4-dimethoxypyrimidine were purchased from ICN Biomedicals. N-(4,6-Dimethylpyrimidin-2-yl)benzene-1,4-diamine was supplied by Peakdale Molecular and 1,4-diazabicyclo(2.2.2)octane (DABCO) was purchased from Matheson, Coleman, and Bell. Absolute ethanol (200 proof) was purchased from Pharmco, and methanol-d4 was obtained from Cambridge Isotope Laboratories. Solvents were of HPLC grade. 10.1021/es048331p CCC: $30.25
2005 American Chemical Society Published on Web 04/14/2005
TABLE 1. General Structure of a Sulfa Drug, Structure of Five Sulfa Drugs Containing Six-Membered Heterocyclic R Substituents, and Measured pKa Values
a pK values were calculated using a spectrophotometric titration as a described previously (35). Errors represent the 95% confidence levels and both the pKa values and the associated errors were obtained from fits of the data using Scientist for Windows (v. 2.01).
pKa Measurements. pKa values were determined through a spectrophotometric titration as previously described (35), with all UV-vis absorbance spectra measured on a Jasco V-530 spectrophotometer. Natural Water Photolysis Experiments. Sulfa drug solutions (10 µM) in deionized water (DI H2O) were photolyzed alongside solutions of the sulfa drugs (10 µM) in a natural water sample (LJW) (from Lake Josephine, an urban lake in St. Paul, MN, with a surface area of 441 000 square meters (41); 0.2 µm filtered, DOC ) 5.9 mg/L, pH ) 8.0; see Supporting Information for water characterization details) on a turntable apparatus under four Pyrex-filtered 175 W medium-pressure Hg-vapor lamps in uncapped quartz test tubes (o.d. ) 1.3 cm, i.d. ) 1.1 cm, V ) 10 mL). Additional photolyses of both the natural water and the DI H2O samples were conducted under similar conditions with the addition of a triplet quencher (isoprene, 0.1% v/v) or the elimination of oxygen (
