Photocatalytic degradation of endosulfan in contaminated soil with the elution of surfactants

Abstract

Microbial degradation has been commonly used to remedy endosulfan-contaminated soil. However, this technique is limited by low degradation efficiency with high toxic metabolite. This work investigated the photodegradation efficiencies of endosulfan in soil with the elution of surfactants and revealed the kinetic characteristics of photocatalytic degradation with two strategies, i.e., surfactant elution followed with photocatalytic degradation, and simultaneous elution and photocatalytic degradation. The endosulfan-contaminated soil, photocatalyst (nitrogen-doped anatase TiO2), and three eluents composed of Tween 80, SDS, and Na2SiO3 were prepared, respectively. The degradation efficiencies of endosulfan in soil were examined by the photocatalyst under visible-light irradiation with two strategies, i.e., surfactant elution followed with photocatalytic degradation, and simultaneous elution and photocatalytic degradation. The kinetic characteristics of photocatalytic degradation of endosulfan in soil were studied at certain time intervals. The XRD, X-ray photoelectron spectroscopy, and UV–vis diffuse reflection spectra revealed that nitrogen was doped into TiO2 lattice, and the optical absorption of nitrogen-doped TiO2 shifted toward visible-light region. The residual ratios of α-, β-endosulfan in soil eluates obviously decreased with the irradiation time increasing and could not be detected at 240 min; this process could be well described by first-order kinetic model, and the half-lives of α-, β-endosulfan were 43.0–71.7 and 51.1–85.3 min, respectively; the concentrations of endosulfan sulfate showed a trend of increase, decrease, and till complete degradation. For simultaneous elution and photocatalytic degradation, the residual ratios of α-, β-endosulfan in soil were 0.21–0.25, 0.41–0.43 at 120 h with eluent 1 to eluent 3 treatments, respectively; the degradation process of endosulfan in soil-aqueous systems could be well described by four-parameter biphasic first-order kinetic model. Endosulfan in the soil eluates could be completely degraded at 240 min under visible-light irradiation and could be described by pseudo first-order kinetic equation. The degradation process of endosulfan in soil-aqueous systems obeyed four-parameter biphasic first-order kinetic model. As an intermediate of photocatalytic degradation, endosulfan sulfate could be rapidly degraded. It was concluded that surfactant elution followed with photocatalytic degradation was more effective than simultaneous elution and photocatalytic degradation.