Studies On The Adsorption Of VOCs By Activated Carbons And The Structure-function Relationship

Abstract:To improve the adsorption performance of activated carbons and make the adsorption more selective, it is necessary to comprehensively and systematically understand the mechanism of the physical differences between the activated carbons and adsorbates, and their interaction. What's more, it is of necessity to systematically and quantificationally investigate the inherent correlation between the adsorption performance of adsorbates by activated carbons and the micro structure-function relationship.This is a study from the perspective of interdiscipline, which is based on the adsorption phenomenon of the volatile organic compounds (VOCs) by activated carbons from the micro/macro point of view, and the effect of the micro structures and surface physicochemical properties of VOCs and activated carbons on the adsorption properties of VOCs from the perspective of interdiscipline. The experimental study of the adsorption of was conducted by the isothermal adsorption experimental apparatus. The complicated surface physicochemical properties of VOCs inside the activated carbons and the interaction mechanism between the adsorbents and adsorbates were studied deeply. The activated carbon was characterized by infrared spectroscopy, microscopic observation, specific surface and pore size analysis, elemental analysis and Boehm titration. Theoretical methods were employed to analyze the adsorption performance data, which were then combined with the structure characteristics parameters. The relationship between the adsorption of VOCs by activated carbons and their micro structure properties was examined by combining computer software, mathematical methods and experimental studies, which further renders an effective guidance on the preparation of novel activated carbons and the optimization of the corresponding purifying equipment.This dissertation achievement contains five sections:(1) Studies on the effect of VOCs physical properties on the adsorption performance of activated carbons. It was found that the saturated adsorption of VOCs is closely related to VOCs molecular dynamics diameter, molecular weight, density, boiling point, vapour pressure, polarity, with positive linear relationship between the first four and negative linear relationship between the last two.(2) Studies on the effect of carbon's pore structure on the adsorption of VOCs. The results shown that the more complex the internal structure of activated carbons is, the lower the mass transfer rate of adsorbates is. The good gradient distribution of pore structure benefits the diffusion of adsorbates inside the activated carbons. The adsorbates with smaller molecular diameters are much easier to diffuse in the adsorbents. The diffusion process inside the activated carbons can be well explained by the correlation between the pore volume and adsorption capacity in different pore diameter ranges.(3) Studies on the effect of the composite oxidation modifications of activated carbons on their physical properties and the adsorption of toluene. The results exhibit that the micropore volume of the hot-composite oxidation modified carbons increases when the carbons are pretreated by strong oxidants. Both the chemical oxidation modification and hot-composite oxidation modification generate the surface oxygenic functional groups. When the temperature of heat modification is low, the surface oxygenic functional groups are mainly from the chemical oxidation modification. When the temperature of heat modification is high, they are mainly from the oxidation reaction between oxygen and the surface of carbons. The modified carbons possess acidic groups, which are in favor of the adsorption of toluene. Suitable hot-composite oxidation modification can not only increase the surface acidic groups on the activated carbons but also enhance the micropore volume, further improving the adsorption of toluene.(4) The granular activated carbons were modified using oxalic acid, formic acid and sulfamic acid respectively. The effects of adsorption time, concentration and temperature on the dynamic adsorption of xylene were then investigated. Test results show that the activated carbons modified using organic acid have high capacity in the adsorption of xylene. The pore structures of the modified carbons were decreased to some extent, which was proved by FTIR to be caused by the production of hydroxyl, ketonic group, carboxyl and S=O groups. With the increase in xylene inlet concentration and the decrease in adsorption temperature, the adsorption rate of the modified carbons increase, the same as equilibrium adsorption capacity. Bangham's kinetic model well describes the adsorption process of the four modified carbons with correlation coefficients R2above0.997.(5) The optimization of adsorption parameters of VOCs and construction of micro/macro coupled structure-function relationship. Based on orthogonal analysis and multiple regression analysis, activated carbon modification and adsorption process were optimized by Design Expert7.0. The physical properties changes of activated carbon were studied under different modification conditions. The effects of physical properties of activated carbon on adsorption performance were discussed. By developing mathematical model, it has been found that the effects of modification factors, such as temperature, oxygen density and time, on the specific surface areas of activated carbons are not significant, while their effects on the micropore volume and surface oxygen content are pronounced. The activated carbons with large specific surface area, large micropore volume and high surface oxygen content can be obtained under the modification conditions of high oxygen density, high temperature and short time period, which will provide an important guidance on the preparation of activated carbons.The study results of this paper are benefit to develop the new activated carbon materials with high surface area, huge micropore volume and high surface functional groups.