Sorption-Desorption Characteristics Research Of Phenanthrene On Sedimets Of Lake

Polycyclic aromatic hydrocarbons (PAHs), one of persistent organic pollutants (pops), widely exists in the environment, with carcinogenic, teratogenic and mutagenic features, which can be enriched in organism easily and then be transported to the organisms at the top of the food chain. Therefore, it has been of great interest to researchers to study PAHs environmental behavior. As a complex aquatic ecosystem, lakes brings together various pollutants terrestrial ecosystem input. There is large amounts of organic matter and a variety of minerals in lake sediment, which has complex pore structure, and can adsorb hydrophobic organic pollutants easily, making it become the storeroom of polycyclic aromatic hydrocarbons in the lake. However, when the external environment changes, the accumulation of PAHs is released into water producing "secondary pollution". In recent years, domestic and foreign scholars have carried out the studies of PAHs contents and speciation in sediments, organisms and waters. But there are few studies of the adsorption characteristics and desorption process of sediment.In this study, we selected phenanthrene as representative of pollutants, taking sediments in Taihu Lake, Chaohu Lake, Xuanwu Lake as the research materials. Phenanthrene adsorption experiments on sediment were carried out in laboratory by simulation experiment. The characteristics of lake sediments sorption of phenanthrene at different concentrations (50,100,200 μg L-1) were studied. Freundlich and Henry isothermal sorption equilibrium models were applied to investigate the sorption dynamics process of different lake sediments. Furthermore, experiments of desorption of phenanthrene in sediments were carried out to study the effects of Calcium chloride, salicylic acid, surfactant on desorption of phenanthrene on sediments. X-ray diffraction (XRD) and scanning electron microscopy (SEM) technology were combined to analyze the mechanism of adsorption-desorption of phenanthrene on lake sediments. Conclusions were as follows:(1)14 kinds of PAHs measured by HPLC were identified as priority pollutants by the US EPA in No.1 Taihu sediment, Chr and BghiP were not detected in sediment; 15 kinds of PAHs measured by HPLC were identified as priority pollutants by the US EPA in No.2 Taihu sediment. The Chr was not detected in sediment; 14 kinds of PAHs measured by HPLC were identified as priority pollutants by the US EPA in No.3 Taihu sediment, Chr and Inp were not detected in sediment; 16 kinds of PAHs measured by HPLC were identified as priority pollutants by the US EPA in Lake Chaohu and Xuanwu sediments. The concentration of phenanthrene from high to low were as follows:the concentrations of phenanthrene in Taihu Lake is 520.60 ng g-1, significantly higher than ecological risk low value (ERL=240); the concentrations of phenanthrene in Chaohu Lake is 217.71 ng g-1, which closed to ecological risk minimum (ERL), 103.00 ng g-1 in No.1 Chaohu Lake,70.32 ng g-1 in No.3 Taihu Lake,59.93 ng g-1 in Xuanwu Lake,48.40 ng g-1 in No.2 Chaohu Lake, the lowest is 36.65 ng g-1 in No.1 Taihu Lake, except for No.2 Taihu Lake and No.2 Chaohu Lake, while low toxicological risk of phenanthrene was found in other sampling sites.(2) The process of adsorption of phenanthrene on sediments in Taihu, Chaohu, Xuanwu Lake can be divided into two stages:rapid and slow absorption processes, and it’s accorded with Freundlich isothermal sorption model, belonging to the exothermic reaction. The adsorption forces are mainly Van der Waals force, hydrogen bond force, dipole bond force, ionic bonding force and chemical adsorption force. The distribution coefficients(in logs)of adsorption phenanthrene on sediments range from 2.51 to 2.97. The values of n is not equal to 1, showing that intensity and capacity of phenanthrene adsorption on sediments is large. The adsorption quantity has a positive correlation with the organic carbon content.(3) The adsorption curves did not coincide with the desorption curves. The sorption and desorption of phenanthrene on sediments were hysteretic. The desorption rate was inversely proportional to the content of TOC. The higher organic carbon content could result in an irreversibly increasing impact on degree of desorption. The hysteresis index was positively correlated with the content of TOC (R2=0.9116). The capacities of three background solutions to the solubilization of phenanthrene in the sediments were as follows:Sodium dodecyl>sulfate acid>CaCl2. The desorption rate of phenanthrene was up to 74.65% in the sodium dodecyl solution,27.66% in the sulfate acid solution,19.36% in the CaCl2 solution.(4) The scanning electron microscope pictures of pore structure of lake sediment reflected the physical and chemical changes in the way of the combination between phenanthrene and sediment, which affects the process of adsorption and desorption of phenanthrene. The solvent impact on phenanthrene adsorpted on internal surface of micro-pores of lake sediment weaker than macro-pores ones, for that the phenanthrene molecular could not to be desorpted from the later ones. This is one of the main causes of desorption hysteresis. Absorption peak of Fourier infrared spectrum of sediment was enhanced significantly in 1400 cm-1-1600 cm-1 and 820 cm-1. After adsorption experiments, the content of aromatic carbon increased. The phenomenon elucidated that a reaction of chemical bonding occurred in the sediment during the process of phenanthrene adsorption. The discrepancy of adsorption performance mainly caused by the different content of organic carbon.