Photocatalytic degradation of endosulfan in contaminated soil with the elution of surfactants Bailian Xiong, Anhong Zhou, Guocan Zheng, Jinzhong Zhang & Weihong Xu
Journal of Soils and Sediments ISSN 1439-0108 Volume 15 Number 9 J Soils Sediments (2015) 15:1909-1918 DOI 10.1007/s11368-015-1139-x
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Author's personal copy J Soils Sediments (2015) 15:1909–1918 DOI 10.1007/s11368-015-1139-x
SOILS, SEC 2 • GLOBAL CHANGE, ENVIRON RISK ASSESS, SUSTAINABLE LAND USE • RESEARCH ARTICLE
Photocatalytic degradation of endosulfan in contaminated soil with the elution of surfactants Bailian Xiong 1,2 & Anhong Zhou 3 & Guocan Zheng 4 & Jinzhong Zhang 1,5 & Weihong Xu 1
Received: 21 January 2015 / Accepted: 13 April 2015 / Published online: 2 May 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Purpose 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. Materials and methods The endosulfan-contaminated soil, photocatalyst (nitrogen-doped anatase TiO2), and three eluents composed of Tween 80, SDS, and Na2SiO3 were pre-
Responsible editor: Huijun Zhao Electronic supplementary material The online version of this article (doi:10.1007/s11368-015-1139-x) contains supplementary material, which is available to authorized users. * Jinzhong Zhang
[email protected] 1
College of Resources and Environment, Southwest University, Key Laboratory of Eco-environments in Three Gorges Reservoir Region, Ministry of Education, Chongqing 400715, China
2
Department of Resources and Environment, Zunyi Normal College, Zunyi, Guizhou 563002, People’s Republic of China
3
Department of Biological Engineering, Utah State University, Logan, UT 84322-4105, USA
4
Chongqing Entry-Exit Inspection and Quarantine Bureau, Chongqing 400020, People’s Republic of China
5
Chongqing Key Laboratory of Agricultural Resources and Environment, Chongqing 400716, People’s Republic of China
pared, respectively. The degradation efficiencies of endosulfan in soil were examined by the photocatalyst under visiblelight 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. Results and discussion 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 firstorder kinetic model. Conclusions 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.
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Keywords Endosulfan . Nitrogen-doped TiO2 . Photocatalytic degradation . Soil . Surfactant
1 Introduction As an organochlorine pesticide (OCP), endosulfan is widely used to control many insects and mites for field crops such as paddy, cotton, sorghum, rape, beans, vegetable, and fruit around the world (Kathpal et al. 1997; Weber et al. 2010). Commercial endosulfan is a mixture of two isomers named as α- and β-endosulfan with the ratio of 7:3. Endosulfan is a highly toxic and bioaccumulative chemical to most living organisms, and was demonstrated to transport from its original source to a long distance in the environment, affecting remote human and wildlife populations (Preud’homme et al. 2015), and thus it was listed in persistent organic pollutants (POPs) by Stockholm Convention in 2011. It was reported that the cumulative global use of endosulfan was estimated to be 338 kilotons (Li and Macdonald 2005), and soil is the major sink of endosulfan. Simonich and Hites (1995) investigated the global distribution of 22 potentially harmful organochlorine compounds and found that endosulfan content in soil was the highest in the southeastern coast in China. Jia et al. (2010) reported that the geometric mean concentration of endosulfan was 1.60×10−3 mg/kg dry weight (dw) in Chinese rural soil with the highest total concentration (the sum of α-, β-endosulfan and endosulfan sulfate) around 0.19 mg/kg dw, and endosulfan content in soil of a discarded plant was even more than 100 mg/kg (Xiong et al. 2013). Endosulfan in soil can be degraded through biotic and abiotic processes (Kafilzadeh et al. 2015; Shah et al. 2015a) and persists for a relatively long period with half-lives of 60–800 days (Rao and Murty 1980). Endosulfan sulfate has been identified as the dominant metabolite in soil (Jia et al. 2010), which is more toxic than endosulfan (Shanahan 2003). Therefore, the residue of endosulfan and endosulfan sulfate in soil poses a serious threat to agroecological environment and human health; it is very urgent to develop remediation technology for endosulfan-contaminated soil. At present, the commonly used method to remedy endosulfan-contaminated soil is biodegradation. However, many works are still limited in the laboratory owing to its low degradation efficiency with high toxic metabolite (Shivaramaiah and Kennedy 2006; Goswami and Singh 2009; Kamei et al. 2011). Photochemical transformation, including direct and indirect photodegradation, is an effective abiotic transformation process of pesticide in surface soil. However, the electronic absorption spectra of most pesticides show little overlap with the spectrum of terrestrial sunlight, only a few pesticides are affected by direct photodegradation (Burrows et al. 2002). Endosulfan is fairly resistant to direct photodegradation, which may not be a main transformation
J Soils Sediments (2015) 15:1909–1918
process on soil surface (Tolan and Ensari 2006). Peñuela and Barceló (1998) found that endosulfan showed high stability in liquid phase when exposed to sunlight and xenon arc lamp, whereas the photocatalytic degradation in the presence of FeCl3/H2O or TiO2/H2O2 was very rapid with half-lives varied from 59 to 98 min. Shah et al. (2015a) also indicated that endosulfan and its by-products could be quickly degraded under the photocatalysis of iron-mediated oxidative systems in aqueous solution. These studies provide a new idea to remedy endosulfan-contaminated soil by photocatalytic degradation. Among many photocatalysts, anatase titanium dioxide (TiO 2 ) is commonly used in pollution control field. However, the use of TiO2 is limited by its wide band gap (3.2 eV) (Konstantyinou and Albanis 2004); the photocatalytic activity need be excited by ultraviolet (UV) light. Compared with visible light (45 %), UV light accounts for only a small fraction (8 %) in sunlight. Kuo et al. (2011) and Zeng et al. (2014) reported that the spectral responses of nitrogen-doped TiO2 could be extended to visible-light region to yield high reactivity, and thus this material displayed high photocatalytic activity under visible-light irradiation (Jagadale et al. 2008). Owing to the strong shielding action of soil to solar radiation, Hebert and Miller (1990) found that direct and indirect photodegradation were restricted to the depths of 0.2–0.4 and 2 mm on topsoil, respectively. Moreover, endosulfan is hydrophobic and can be readily adsorbed on soil (Kumar and Philip 2006), which makes photocatalyst difficult to contact endosulfan; thus, the photocatalytic degradation may be hindered. For these reasons, it is infeasible to degrade endosulfan in soil by adding photocatalyst directly. Occulti et al. (2008) found that the soil polluted by alkylphenols and polychlorinated biphenyls (PCBs) could be effectively remedied by two-phase process consisting of surfactant elution followed with photocatalytic degradation. Huang and Hong (2000) also obtained satisfactory results using photodegradation technique to remedy PCBs-contaminated soil with the elution of fluorinated surfactant. Our previous work indicated that endosulfan in an aged contaminated soil could be effectively eluted by three mixed surfactant eluents (The compositions were given in Materials and methods) (Xiong et al. 2013). In this work, the objectives were to use two strategies to degrade endosulfan in soil under visible-light radiation, i.e., surfactant elution followed with photocatalytic degradation, and simultaneous elution and photocatalytic degradation, and to obtain the photodegradation efficiencies and kinetic characteristics of endosulfan in soil.
2 Materials and methods 2.1 Reagents and materials The standard samples of α-endosulfan (98.0 %) and β-endosulfan (98.7 %) were purchased from Sigma-Aldrich, and
Author's personal copy J Soils Sediments (2015) 15:1909–1918
endosulfan sulfate (98.5 %) and endosulfan ether (99.6 %) from Dr. Ehrenstorfer Co. (Augsburg, Germany). The technical grade endosulfan (96.0 %) with the mass ratio of α- and βendosulfan 62.66:37.34 was purchased from Spring Co. Ltd., Henan, China. Toluene, ethyl acetate, and n-hexane (HPLC grade) were purchased from Thermo Fisher Corp. Polyoxyethylenesorbitan monooleate (Tween 80; molecular weight, 1,309; CMC, 15.7 mg/l), sodium dodecyl sulfate (SDS; molecular weight, 288; CMC, 2,100.0 mg/l), and the other chemicals (analytical reagent) were purchased from Chengdu Kelong Chemical Co. Ltd., China. Florisil solidphase extraction (SPE) column (500 mg/6 ml, Welchrom, USA) was activated with n-hexane prior to use. Pure TiO 2 and nitrogen-doped anatase TiO 2 were synthesized by sol–gel method based on the work of Khalid et al. (2012). For nitrogen-doped TiO2, tetrabutyl titanate, and urea were used as the sources of Ti and N, respectively, and the molar ratio of Ti to N was 1:4 in the synthetic reaction. The morphology of the two particles was observed using scanning electron microscopy (SEM) (Hitachi S-300 N, Japan) and X-ray diffractometer (Pgeneral XD-2/3, Beijing, China), and the granularity was measured by laser granularity analyzer (Brookhaven BI-200SM, USA). Chemical compositions and valence band spectra of the photocatalysts were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA) with monochromatic Al Kα X-ray source, and the UV–vis diffuse reflectance spectra (DRS) were measured by UV–vis spectrometer (Hitachi U-4100, Japan) equipped with an integrating sphere. 2.2 Endosulfan-contaminated soil The soil sample (neutral purple soil) was collected from the topsoil (0–20 cm depth) in an experimental field of Southwest University, Chongqing, China, air-dried, ground, and sieved through a mesh with 0.25-mm pore size. The basic physicochemical properties of the soil were as follows: pH, 6.23 (soil/water, 1:2); organic matter, 23.51 g/kg; cation exchange capacity (CEC), 28.02 cmol/kg; the contents of coarse silt (0.05–0.02 mm), fine silt (0.02–0.005 mm), and clay (
