Effect Of Low-Molecular-Weight Organic Acids On The Adsorption Of Phenanthrene In Purple Soil

With the rapid development of industry, agriculture and transportation industry, a large amount of polycyclic aromatic hydrocarbons(PAHs) continuously enter in the soil environment, which has aggravated the pollution of PAHs in soil. In recent years, 16 priority monitoring PAHs can be detected in soils of China, especially in northeast, southeast coastal and sichuan basin region, and the mainly detected PAHs are phenanthrene and naphthalene. The average content of PAHs in the surface soil can reach 3654.97 g/kg, and the cumulative extent is still increasing. The migration and transformation processes of PAHs in soil mainly include adsorption-desorption, volatile diffusion, photodegradation and biological degradation. Among these processes, the adsorption-desorption behavior of PAHs can directly influence on their species, bioavailability and ecological toxicity in soil. Therefore, it is of great importance to study the sorption-desorption behavior of PAHs in soil for understanding the environmental chemical behavior of PAHs, and protecting soil ecological environment and human health.At present, some researches focus on the adsorption behavior of phenanthrene, pyrene and naphthalene in yellow brown soil, black soil and red soil. It is thought that the adsorption behavior of PAHs is directly related to soil type, organic matter content and dissolved organic matter content. Purple soil is a typical agricultural soil distributed in Southwest China, and 16 priority monitoring PAHs can be detected with different levels. However, few report on the adsorption of PAHs in purple soil can be found while the adsorption of PAHs in is rarely reported. Low molecular weight organic acids(LMWOAs) is a kind of active component in soil root exudates, and can affect the migration and transformation of organic pollutants in soil. As for the effects of LMWOAs on the adsorption of PAHs in soil, some studies focused on citric acid, acetic acid, oxalic acid and malic acid. The difference of chemical properties of LMWOAs may bring about different effects on the adsorption of PAHs in purple soil. In this thesis, citric acid, malic acid and oxalic acid and their mixed acids were selected to investigate the effects of LMWOAs on the adsorption behavior of phenanthrene in purple soil, and the influence mechanism were preliminarily discussed. The main research results are obtained as follows.1. The adsorption of phenanthrene in purple soil reached equilibrium in 24 h, this process included rapid adsorption process and slow adsorption process, and can be well described by second-order kinetic equation. The equilibrium adsorption capacity was obtained as 96.15 mg/kg, and the adsorption rate constant as 1.0816 mg/(kg?h). This process was an exothermic physical adsorption process, and was linear adsorption dominated by distribution role.2. LMWOAs with low concentration(< 10 mmol/L) can promote the adsorption of phenanthrene in purple soil, wheareas LMWOAs with high concentration(> 10 mmol/L) can inhibit the adsorption, and the inhibition ability is related to the type and concentration of LMWOAs. The inhibition ability of citric acid was the strongest among the three LMWOAs, while that of the mixed acid A2(citric acid : malic acid : oxalic acid = 1:1:3) was the strongest among the three mixed acids, which is dependent on the molecular structure and acidity of the three LMWOAs. Moreover, the inhibition ability of the mixed acids was stronger than that of single acid, which was related to the ratio of the three LMWOAs.3. The content of dissolved organic matter(DOM) released from purple soil was related to the added LMWOAs concentration. When the initial concentrations of LMWOAs were in thr range of 2~20 mmol/L, the adsorption capacity of phenanthrene showed a trend of first increase and then decrease with the increase of LMWOAs concentration, and LMWOAs with high concentration(> 10 mmol/L) can promote the dissolution of soil organic matter. The adsorption capacity was negatively correlated to the content of DOM.4. The carboxyl and hydroxyl in LMWOAs can interact with soil clay minerals, and form LMWOAs-metal cation-inorganic complex, and promote the release of organic matter bound to purple soil. On the other hand, the acidity of LMWOAs can influence on the adsorption ability of purple soil to DOM, and thus influence on the distribution equilibrium of DOM between solid phase and liquid phase in soil. This is the main reason of the effect of LMWOAs on phenanthrene adsorption in purple soil.