Study On Adsorption Efficiency And Mechanism Of Phenolic Endocrine Disrupting Chemicals On Activated Carbons With Various Surface Modification

The problem of water environment of endocrine disrupting chemicals has attractted people's attention in recent years. Especially, some endocrine disrupting chemicals have been detected in drinking water. The adsorption of organic contaminants by activated carbons is the most effective and widely used methods to purify water. Therefore, it is imperative to establish the correlation between the surface physical and chemical properties of activated carbons and the adsorption capacity for the given contaminants, which may provide the scientific basis for the application of activated carbon.Two activated carbons, W20 (wood-based) and F20 (coal-based), were selected in this study. The activated carbons with different surface physical and chemical properties were obtained by modification. Bisphenol A was selected as the representative of endocrine disrupting chemicals in this study. A key objective was to study the relationship between the adsorption capacities of target pollutants and surface chemical characteristics of activated carbons. Therefore, the behaviors and mechanism of bisphenol A on various activated carbons were studied in detail and the factors affecting the adsorption of bisphenol A were simutaniously discussed. At last, two types of activated carbons with higher adsorption capacity were used to adsorbe phenolic endocrine disrupting chemicals spiked in Songhua River water and sand filtered water.The textural and chemical characteristics of the activated carbons were changed by modification. Nitric acid oxidation decreased the specific surface areas of the original activated carbons. But it increased the content of carboxyl and hydroxy functional groups. As a result, the zero point charges of adsorbents were decreased. In addition, the transformation of the structure of activated carbons, treated by thermic reduction under an atmosphere of H2 or N2, was influenced by the materials of carbon and the temperature of modification to a large extent. This process decreased the contents of surface acid oxygen-containing groups, which increased the zero point charge of adsorbents. However, the change of specific surface areas was small when activated carbons were modified by hydrochloric acid and sodium hydroxide. Where the former reduced the zero point charge of carbons and the latter was contrary to it. It is worth nothing that the changes of ash content were different for specific modification.The uptake of bisphenol A was mainly controlled by external mass transfer through the boundary film of liquid and intra-particular mass transfer. The behavior of adsorption was found to obey a pseudo-second order kinetic model. Further results indicated that the adsorption of bisphenol A on activated carbons was well described by the monolayer Langmiur model (except for carbon samples of nitric acid oxidation). However, the coverage of adsorbates on the surface of activated carbon was incomplete. In addition, the adsorption capacity of W20N, a thermal treatment sample under an atmosphere of N2, was the highest and its saturated adsorption capacity (qm) reached 526.32mg/g, while the nitric acid oxidation sample W20A represented the worst adsorption capacity and corresponding qm was only 175.44mg/g.The adsorption of bisphenol A on activated carbons was spontaneous and exothermic process which is due to physisorption reaction. The molecules of adsorbed bisphenol A were parallel to the surface of activated carbon. The adsorption capacities of bisphenol A on activated carbons were affected by the solution pH which determines the charge of both the carbon and the adsorbate. The initial sample W20 and its modified sample W20N indicated the worst capacities at pH =11, which attributed to the enhanced electrostatic repulsion. In addition, the ionic strength of solution can occupy the surface active sites, produce the screening effect and cause the salting-out effect, which influnced the adsorption of target pollutants on activated carbons.The analyzing results of mechanism indicated that the adsorption of bisphenol A on carbons mainly followedπ-πdispersion interaction. The water clusters, formed by the hydrogen bond between the carbon surface and water molecules, markedly restrained the adsorption capacity of activated carbons. It was favorable to improve the adsorption of bisphenol A by decreasing the content of surface acid oxygen-containing groups (especially for carboxylic groups). Moreover, the surface with zero charge density was the most favorable for the adsorption of bisphenol A in adsorptive system.At last, the adsorption capacities of selected phenolic endocrine disrupting chemicals on activated carbons increased with the increase logKow from the actual water (except for estrone). And the modified carbon represented a better efficiency for all tested water quality parameters. While the adsorption capacities of activated carbons were decreased by the natural organic matters and other organics. Therefore, the activated carbon with more mesopores could increase the purification efficiency. Especially, the activated carbon obtained by a certain modification, with the pore structure and surface chemistry favored the adsorption of organic compounds, were indispensable.