Adsorption Behavior Of VOCs Onto Activated Carbons Modified With Organic Acids

Abstract:To study the adsorption behavior of Volatile Organic Compounds onto activated carbons (ACs) modified with organic acids, based on the pore structure, surface morphology and surface functional groups, specific surface area and pore size analyzer, scanning electronic microscope, Fourier transformed infrared spectroscopy were used to measure the physicochemical properties of ACs respectively. Using modified ACs as adsorbents and four kinds of VOCs:methanol, acetone, toluene, isopropanol were as adsorbates, Fixed-bed adsorption experiments were conducted under different temperature and inlet concentration. Combined with the physical properties of VOCs and ACs after modification, effects of physical properties of ACs and VOCs on adsorption properties were discussed. The adsorption thermodynamics parameters were calculated, and the AC regeneration experiments were conducted, the adsorption properties of ACs after regeneration were also tested. The primary conclusions are listed as follows:(1) After organic acids modification:The pore structure parameters ACs are reduced in different degrees, the surface of AC has etched traces, the surface morphology is rough and uneven, new more oxygen-containing functional groups are formed on the AC surface.(2) The analysis of adsorption properties of VOCs onto ACs shows: The adsorption capacities of AC-FA, AC-OA, AC-SA, AC-TA for the four kinds of VOCs are larger than AC-0, both the Langmuir model and Freundlich model can well describe the adsorption of the four kinds of VOCs on ACs. The order of adsorption energy of ACs is:toluene> acetone>isopropanol>methanol. With an increase of the inlet concentration of organic gas, the adsorption rate and equilibrium adsorption capacity of ACs increase, but the equilibrium adsorption time shorten. The fitting curves of Bangham kinetics model can better coincide with the experimental data, and the model predicted values agree well with the experimental values, the imitative effect of Elovich kinetics model is the worst. (3) The effects of physical properties of ACs and VOCs on adsorption properties show:The adsorption capacities of ACs increase with the BET specific surface area, micropore specific surface area, total pore volume and micropore volume increasing. The adsorption capacities of ACs present positive correlation with the molecular weight, molecular diameter, boiling point and the liquid density of the adsorbates, and present negative correlation with polarity index and dipole moment.(4) The adsorption thermodynamics and regeneration research of the adsorption of ACs for VOCs show:The changes of enthalpy AH and change of Gibbs free energy AG are negative, this suggests the adsorption process is exothermic and spontaneous. The changes of entropy AS of AC-0, AC-OA and AC-SA are negative, this suggests that the adsorption process is a entropy decreasing process, the freedom degree of the system is reduced. While the AS of AC-FA is positive, this illustrates the adsorption is a entropy increasing process, the freedom degree of the system increases. The average desorption rates of organic acids modified ACs are all smaller than AC-0, the greater the equilibrium adsorption capacity of ACs, the greater their desorption rates. The adsorption capacities of ACs after regeneration reduced in different degrees, but the decreasing amplitudes are smaller, the biggest decreasing amplitude is less than5.9%. For the methanol adsorption activated carbon in this study, heating regeneration is a basic feasible method.