Effect of nonionic surfactants on the oxidation of carbaryl by anodic Fenton treatment

As a potentially promising technology, anodic Fenton treatment (AFT) has been shown to be very successful in pesticide removal. However, the influence of other constituents in the pesticide formulation, such as nonionic surfactants, has not been addressed. In this study, the effect of Triton X (TX) on the degradation kinetics and pathways of carbaryl undergoing AFT was investigated in an effort to facilitate its practical application. The presence of Triton X-100 was found to slow down the carbaryl degradation rate. This result can be attributed to the consumption of hydroxyl radicals (OH) by surfactants and the formation of a carbaryl⋯TX⋯Fe3+ complex, resulting in the unavailability of carbaryl to OH attack. The modified AFT kinetic model previously developed in this laboratory shows an excellent fit to the carbaryl degradation profile (R2>0.998), supporting the formation of a carbaryl⋯TX⋯Fe3+ complex. The carbaryl degradation rate decreased as Triton X-100 concentration increased from 20 to 1000 mg L−1. Both OH consumption by surfactants and complex formation are responsible for the degradation rate reduction below the critical micelle concentration (CMC), whereas the complex and micelle formation becomes a more dominant factor above the CMC. The effect of ethylene oxide (EO) numbers of a given nonionic surfactant mainly lies in the consumption of hydroxyl radicals, which increases with the length of the EO chain, but does not significantly affect the formation of the carbaryl⋯TX⋯Fe3+ complex. Based on the GC–MS and LC–ESI–MS results, no evidence was found that the carbaryl degradation pathway was affected. Carbaryl was typically oxidized to 1-naphthol and 1,4-naphthoquinone similar to what is observed in the absence of surfactants. Triton X-100 was degraded via the breakdown of EO chains and ω-oxidation of the terminal methyl group, which resulted in the production of a series of ethoxylate oligomers.