The Study Of Adsorption Behaviour And Vertical Distribution Of Nonylphenol On Soils In Sewage Irrigation Area

Nonylphenol, an endocrine disrupter contaminants with high lipotropy, toxicity and persistent characteristics, is defined as persistent organic pollutants. It is widely used in the industrial production process as main raw material. However, such pollutants were universal detected in various matrix, especially in the urban sewage treatment plant effluent and sludge, and the concentrations were higher. Sewage irrigation is an effective way to alleviate the intension of agriculture irrigation in the regional with shortage water resources. But the irreasonable irrigation could lead to a series of ecological problems, such as heavy mental and organic pollutants in the soil and groundwater. The farmland is the essential places for the sewage flow process, which plays an important role in the transport and fate of pollutants. This course is influenced by various factors in the soil, including physicochemical properties and microbial activity of soil. In order to evaluate the migration mechanism of pollutants in the environmental, the study of adsorption distribution behavior is of great significance. Therefore, this study discusses the adsorption behavior of nonylphenol in the irrigation soils, and investigates the vertical distribution characteristics of nonylphenol in the profile soils and drill sediments. Meanwhile, to certain extent, the results provide definite guiding significance to reasonable irrigation farmland using urban sewage.To research the adsorption behavior and vertical distribution characteristic of nonylphenol, sewage irrigation soils as study object were selected in Xiaodian sewage irrigation area, Taiyuan City. The adsorption activity on the different solvents were completed by the batch laboratory experiments, meanwhile study the influence factors on the sorption, such as pH, ionic strength, black carbon, mineral and temperature. The concentration of NP in water solution was detected using liquid-liquid extraction followed high resolution liquid chromatography (LLE-HPLC). Six profile soil and three drill sediment samples were collected from three different irrigation styles in the study area. The method of ultrasonic extraction followed gas chromatography with mass spectrum was utilized to analysis the contaminant compounds. The vertical distribution characteristics of nonylphenol were analyzed in samples. Through study and analysis, a series of significant conclusions were drawn accordingly.(1) The adsorption kinetics of nonylphenol on soils are seperated two steps:quick adsorption and slow adsorption. The desorption activity in the sorption process is apt to observe in high initially concentration of nonylphenol. The adsorption kinetics are well fitted with linear pseudo-second-order model. The adsorption kinetics is further fitted using Weber-Morris model, and the sorption processes are separated by three successive adsorption steps. The intraparticle diffusion is not the rate-controlling step for adsorption. The rate of initial adsorption on humus acid is much higher than on soil and pure mineral (SiO2).(2) The freundlich model is fitted the experiment data on humus acid, however, the linear model are employed to fit the result on soil and silica. The order of adsorption capacity for nonylphenol in different sorbents is as follows:humus acid> soil> mineral. The adsorption capacity of nonylphenol on groundwater is higher than on sewage irrigation, and the soil organic matter (SOM) dominated over other influent factors. The distribution coefficient of normalized organic carbon for wastewater irrigation soil and groundwater irrigation soil were3.1×104L·kg-1and3.2×104L·kg-1, respectively, which limit the migration ability of nonylphenol in irrigation soils. Considering small value for both desorption amount and hysteresis coefficient, the desorption behavior doesn’t easily happen on soils. Compared with groundwater irrigation soil, the nonylphenol molecules were much easier to desorption by eluviations process on wastewater irrigation soils that could migrated to deeper soils.(3) The value of pH and ion strength on soils can influence the nonylphenol adsorption behavior. Compared with ion strength, the change of pH value brought greater influence on adsorption behavior. The adsorption mechanism is more complicated in the factor of ion strength. It is possibly ascribed to the salting-out effect and the charge transfer mechanism. The adsorption isotherm on black carbon and mineral in the soil treated by combustion were employed by linear model..Both the values of distribution coefficient are declined to some degree. Mineral composition constituent of the soil had some influence on adsorption behavior for nonylphenol. The negative values of the Gibbs free energy indicated the adsorption reaction occurs spontaneously. With the increase in temperature, the adsorption capacity decrease. The heat of sorption reaction was the result of synergistic effect with varieties forces such as hydrophobic bond, hydrogen bonding, Van der Waals force, dipolar bond and so on.(4) The compound of nonylphenol was detected in both profile soils and drill sediments. The concentration of nonylphenol in surface samples was79.6～1017.2ng·g-1(wastewater irrigation area);60.4ng·g-1(transition area);37.2ng·g-1(groundwater irrigation area). In generally, the order of mean concentration of nonylphenol in the irrigation surface soils: wastewater irrigation area>transition area>groundwater irrigation area. The concentration of nonylphenol in wastewater irrigation, transition, and groundwater irrigation profile soil samples were in the range of31.2～1017.2ng·g-1,8.4～3384.8ng·g-1and28.4～209.6ng·g-1, respectively. With the increase of depth in the profile soils, the contents of nonylphenol were decreased. The trend of migration to deeper soil layer for nonylphenol was observed in the partial sewage and groundwater irrigation profile soils.(5) The advection percolation of sewage in the irrigation channels had a clearly impact on the vertical distribution of nonylphenol. The vertical distribution of nonylphenol in drill sediments was influenced by sewage irrigation, and the change style was more frequent with the increasing of depth. The influence of pollutant vertical distribution was obvious in the swamp area due to the lithology characters of the drill sediments. The correlations between the concentration of nonylphenol and total organic carbon were positively significant in profile soils, and a negative correlation between the concentration of nonylphenol and pH, and not significantly correlated between the concentration of nonylphenol and moisture content. But there were not notably correlated in drill sediments between the concentration of nonylphenol and physicochemical properties included pH and total organic carbon. The distribution of nonylphenol in deeper soils was more complicated in our study area.