Photodegradation of hydroxychlorothalonil in aqueous solutions

Hydroxychlorothalonil is a known breakdown product of the fungicide chlorothalonil. It is frequently found in leachate from golf course greens at concentrations up to 2 ppm. Yet, it is rarely detected in surface waters like shallow ponds. Prior research has shown that hydroxychlorothalonil is relatively stable in soil but less so in water. This gap motivated further investigation into how sunlight affects its degradation. The absence of detectable levels in surface waters raised questions about its fate in aquatic environments. No prior work had resolved the mechanisms of its photodegradation. Existing models did not account for the full range of environmental variables affecting its breakdown. This uncertainty drove the need for a study combining laboratory experiments with environmental simulations.

The aim of the study was to investigate the photodegradation of hydroxychlorothalonil in various aqueous solutions under simulated sunlight. The specific problem addressed was the discrepancy between its presence in soil leachate and its absence in surface waters. The researchers sought to determine the half-life of the compound in different water types. They also aimed to identify the degradation products and mechanisms involved. The study focused on photonucleophilic substitution as a possible pathway. The goal was to model how these findings translate to real-world aquatic environments. Simulations were used to estimate degradation rates in pond and seawater settings. The results could help refine environmental risk assessments for this compound.

The researchers exposed hydroxychlorothalonil to simulated sunlight in distilled-deionized water, phosphate buffer, and pond water. They measured degradation rates by tracking concentration changes over time. The half-lives in these solutions were approximately 30 minutes. In NaCl and NaCN solutions, degradation was slower, indicating possible nucleophilic reactions. Hydroxylated products were detected in the first hour of irradiation. These products were further degraded to aliphatic acids like succinic, oxalic, and malonic acid. The study used the U.S. Environmental Protection Agency's pond model to simulate aquatic dissipation. The model predicted longer half-lives in deeper water and seawater compared to laboratory conditions.

Hydroxychlorothalonil degraded rapidly under simulated sunlight in distilled-deionized water, phosphate buffer, and pond water with half-lives of about 30 minutes. Slower degradation was observed in NaCl and NaCN solutions, suggesting photonucleophilic substitution. Hydroxylated products formed within the first hour of irradiation. These products were further broken down into small aliphatic acids within 96 hours. The most abundant acids detected were succinic, oxalic, and malonic acid. Simulations predicted a half-life of 2.6 days in a 2-meter-deep pond and up to 3.4 days in seawater. These findings suggest rapid photodegradation in surface waters. The results align with the observed absence of hydroxychlorothalonil in receiving waters.

The authors suggest that hydroxychlorothalonil is rapidly degraded by sunlight in surface waters. The half-lives measured in laboratory settings translate to longer durations in natural aquatic environments. The degradation products include hydroxylated intermediates and small aliphatic acids. These findings may explain the low detection rates of hydroxychlorothalonil in surface waters. The study supports the idea that photodegradation is a key process in its environmental fate. The results suggest that transport into aquatic systems leads to rapid breakdown. The simulations indicate that pond and seawater conditions slow degradation compared to laboratory settings. These data may help refine risk assessments for this compound in environmental contexts.