Adsorption And Desorption Characteristics Of Endosulfan In Three Agricultural Soils In Southwest China

Endosulfan is a broad-spectrum and high-efficiency organochlorine pesticide, and has been widely used as a pesticide for cotton, wheat, tobacco, tea tree and apple. In 2011, endosulfan was listed in the persistent organic pollutants (POPs) owing to its strong toxicity and bioaccumulation, and long-distance transport. Our country began to produce and use endosulfan from 1994, the residue and accumulation of endosulfan could be found in the agricultural soils in many regions. Endosulfan can be degraded through biological and abiotic actions with the half life of 60～800 d in soils, and may have serious threat to the agroecological environment and human health. As a hydrophobic organic pollutant, endosulfan can be firmly adsorbed by soil colloids, which may decrease the microbial degradation efficiency. Therefore, to study the environmental behaviors of endosulfan in soil can provide scientific foundation for protection of soil environmental quality, safety assurance of agricultural products, and development of the restoration theory and experimental techniques for endosulfan contaminated soil.At present, some scholars studied the adsorption and desorption behaviors of endosulfan in soils, mainly involving in sandy loam, fluvisols, clay soil, red soil and compost soil, etc., and found that adsorption behaviors of endosulfan were directly related to soil type, organic matter content and soil pH, and also influenced by other factors, such as soil texture, soil moisture and the initial concentration of endosulfan. Purple soil, latosols and fluvisols are the three agricultural soils widely distributed in southwest China, and a large deal of endosulfan has been used to control pests, and is still used to tobacco cultivation in this region, which makes more endosulfan residue in the soils. However, the migration and transformation behaviors of endosulfan in purple soil, latosols and fluvisols are not clear, and no report on adsorption and desorption characteristics of endosulfan in these soils can be found. In this thesis, we studied the adsorption and desorption characteristics of endosulfan in purple soil, latosols and fluvisols by using static adsorption and desorption experiments, examined the influenc factors on the adsorption of endosulfan in the soils, and discussed the adsorption mechanism of endosulfan preliminarily. The main results are summarized as follows:1. The adsorption kinetic process of endosulfan in the three soils could be well described by second-order kinetic model, and the initial adsorption rate constants of a-endosulfan in purple soil, latosols and fluvisols were obtained as 0.157,0.104 and 0.125 mg/(g min), while those of β-endosulfan were 0.115,0.082 and 0.088 mg/(g min), respectively. Among them, the adsorption rate constants of α-,β-endosulfan in purple soil were the highest, while those in latosols were the lowest.2. The adsorption thermodynamic characteristics of endosulfan in the three soils could be well described by Langmuir isotherm equation, and the maximal adsorption capacities of a-endosulfan in purple soil, latosols and fluvisols were obtained as 0.088, 0.069 and 0.047 mg/g, while those of β-endosulfan were 0.024,0.017 and 0.020 mg/g, respectively. Among them, the maximal adsorption capacities of α-,β-endosulfan in purple soil were the highest.3. The adsorption processes of endosulfan in the three soils were the result of joint action of physical adsorption and chemical adsorption. The adsorption of endosulfan in soil colloids was mainly influenced by hydrophobic bonds and Vander Waals force, while hydrogen bond was relatively small, and dominated by physical adsorption. For the same soil, the adsorption capacity of a-endosulfan was higher than that of β-endosulfan, which is related to the molecular structure of endosulfan, the affinity of β-endosulfan to the soil was higher than that of a-endosulfan.4. The desorption experiments found that the desorption percents of a-endosulfan in purple soil, latosols and fluvisols were 12%-21%,14%-26% and 13%-24%, while those of β-endosulfan were 15%-22%,18%-27% and 16%-25%, respectively. The maximal desorption capacities of a-endosulfan in purple soil, latosols and fluvisols were 0.032,0.036 and 0.033 mg/g, while those of β-endosulfan were 0.011,0.012 and 0.011 mg/g, respectively. The desorption percents of endosulfan in the three soils were very low, indicating that endosulfan could be firmly adsorbed by soil colloids.5. Under the experimental conditions examined in this study, the initial pH of adsorption solution had a relative great influence on the adsorption capacity, whereas the temperature and adsorbent amount no significant influence. At room temperature and shaking 12 h, endosulfan could be effectively adsorbed in purple soil, latosols and fluvisols when the initial pH of the adsorption solutions were 8,7 and 7, respectively.