Photodegradation of sulfasalazine and its human metabolites in water by UV and UV/peroxydisulfate processes.

The widespread occurrence of pharmaceuticals and their metabolites in natural waters has raised great concerns about their potential risks on human health and ecological systems. This study systematically investigates the degradation of sulfasalazine (SSZ) and its two human metabolites, sulfapyridine (SPD) and 5-aminosalicylic acid (5-ASA), by UV and UV/peroxydisulfate (UV/PDS) processes. Experimental results show that SPD and 5-ASA were readily degraded upon UV 254 nm direct photolysis, with quantum yields measured to be (8.6 ±â€¯0.8) × 10-3 and (2.4 ±â€¯0.1) × 10-2 mol Einstein-1, respectively. Although SSZ was resistant to direct UV photolysis, it could be effectively removed by both UV/H2O2 and UV/PDS processes, with fluence-based pseudo-first-order rate constants determined to be 0.0030 and 0.0038 cm2 mJ-1, respectively. Second-order rate constant between SO4•- and SSZ was measured as (1.33 ±â€¯0.01) × 109 M-1s-1 by competition kinetic method. A kinetic model was established for predicting the degradation rate of SSZ in the UV/PDS process. Increasing the dosage of PDS significantly enhanced the degradation of SSZ in the UV/PDS process, which can be well predicted by the developed kinetic model. Natural water constituents, such as natural organic matter (NOM) and bicarbonate (HCO3-), influenced the degradation of SSZ differently. The azo functional group of SSZ molecule was predicted as the reactive site susceptible to electrophilic attack by SO4•- by frontier electron densities (FEDs) calculations. Four intermediate products arising from azo bond cleavage and SO2 extrusion were identified by solid phase extraction-liquid chromatography-triple quadrupole mass spectrometry (SPE-LC-MS/MS). Based on the products identified, detailed transformation pathways for SSZ degradation in the UV/PDS system were proposed. Results reveal that UV/PDS could be an efficient approach for remediation of water contaminated by SSZ and its metabolites.

Photophysical and photochemical insights into the photodegradation of sulfapyridine in water: A joint experimental and theoretical study.

For organic pollutants, photodegradation, as a major abiotic elimination process and of great importance to the environmental fate and risk, involves rather complicated physical and chemical processes of excited molecules. Herein, we systematically studied the photophysical and photochemical processes of a widely used antibiotic, namely sulfapyridine. By means of density functional theory (DFT) computations, we examined the rate constants and the competition of both photophysical and photochemical processes, elucidated the photochemical reaction mechanism, calculated reaction quantum yield (Φ) based on both photophysical and photochemical processes, and subsequently estimated the photodegradation rate constant. We further conducted photolysis experiments to measure the photodegradation rate constant of sulfapyridine. Our computations showed that sulfapyridine at the lowest excited singlet state (S1) mainly undergoes internal conversion to its ground state, and is difficult to transfer to the lowest excited triplet states (T1) via intersystem crossing (ISC) and emit fluorescence. In T1 state, compared with phosphorescence emission and ISC, chemical reaction is much easier to initiate. Encouragingly, the theoretically predicted photodegradation rate constant is close to the experimentally observed value, indicating that quantum chemistry computation is powerful enough to study photodegradation involving ultra-fast photophysical and photochemical processes.

Insights into photolytic mechanism of sulfapyridine induced by triplet-excited dissolved organic matter.

The ubiquity of sulfonamide antibiotics (SAs) in natural waters urges insights into their fate for ecological risk assessment in the aqueous euphotic zone. In this study, we investigated the effect of dissolved organic matter (DOM) on the photolysis of SAs with sulfapyridine as a reprentative. Results show that excited triplet state DOM ((3)DOM(∗)) is largely responsible for the photodegradation of sulfapyridine. The reaction of (3)DOM(∗) with a substructure model compound of SAs confirmed that sulfapyridine has one reaction site (aniline-N). Density functional theory (DFT) calculation was performed, which indicates that the anionic sulfapyridine has higher (3)DOM(∗) reactivity than that of the neutral form, which was also confirmed by steady state photolytic experiments. In the reaction, electrons of the aniline-N transfer to the carbonyl oxygen atom of (3)DOM(∗) moiety, followed by proton transfer, and leading to the formation of sulfapyridine radicals. The photolytic mechansim of sulfapyridine initiated by (3)DOM(∗) is helpful in understanding the photochemical fate and assessing the ecological risks of SAs in the aquatic environment.

Advanced oxidation of the antibiotic sulfapyridine by UV/H2O2: Characterization of its transformation products and ecotoxicological implications.

The aim of the present work is to investigate, under lab-scale conditions, the removal and transformation of the antibiotic sulfapyridine (SPY) upon advanced oxidation with UV/H2O2. High resolution mass spectrometry (HRMS) analyses by means of an ultra-high pressure liquid chromatography (UHPLC)-linear ion trap high resolution Orbitrap instrument (LTQ-Orbitrap-MS) were carried out in order to elucidate the different transformation products (TPs) generated. The abatement (>99%) of the antibiotic was only achieved after 180 min, highlighting its resilience to elimination and its potential persistence in the environment A total of 10 TPs for SPY were detected and their molecular structures elucidated by means of MS(2) and MS(3) scans. Finally, the combined ecotoxicity at different treatment times was evaluated by means of bioluminescence inhibition assays with the marine bacteria Vibrio fischeri.

Photodegradation of sulfapyridine under simulated sunlight irradiation: kinetics, mechanism and toxicity evolvement.

In this study, the photoinduced degradation of sulfapyridine (SPY) was investigated under simulated light irradiation (λ>200nm). The effect of pH and main water constituents including nitrate ion, bicarbonate, dissolved organic matter (DOM) and iron(III) on the photodegradation was explored. SPY was effectively removed in aqueous solution at pH 8 under UV-vis irradiation, with removal efficiency of 100% within 120min. DOM and iron(III) had retarding influence on the SPY removal, whereas nitrate ion and bicarbonate did not show any obvious effect. Under UV-vis irradiation, the formation of singlet oxygen ((1)O2) accelerated the SPY photodegradation and the contribution of indirect photolysis due to reaction with (1)O2 was up to 42%. The transformation products of SPY were identified by HPLC-MS and the possible photoreaction pathways were proposed. It showed that photoinduced hydrolysis, photo-oxidation via (1)O2 and desulfonation were the main degradation ways for SPY decomposition. Toxicity assays by Vibrio fischeri proved that the transformation products were more toxic than the parent compound.