Effects On Characteristics Of Modified ACFs Adsorption For Toluene Gas

The volatile organic compounds(VOCs)mainly involved hydrocarbon,as well as oxygen-containing hydrocarbon,nitrogen-containing hydrocarbon and sulphur-containing hydrocarbon,etc.They jeopardized human health and environmental quality seriously,so the token of character and reaearch of control of VOCs was always one of the pop problem in the area of environmental science and engineering.The methods of VOCs pollution control could be divided into two aspects:one kind was the destructive method including oxidation method and biology method;the other was the recycle method including absorption method,adsorption method,condense method and membrane separation method.Thereinto the adsorption method embodied many advantages such as the simple technology,no secondary pollution and easy regeneration,so it was used widely in VOCs control.Among the adsorption methods,sorbent selection is especially critical.In comparison to conventional activated carbons(AC),the activated carbon fibers(ACFs)were a new kind of multifunctional adsorption materials;these materials possessed higher surface areas,narrow and uniform pore size distribution,small and uniform fiber diameter, graphitic and high strength and elasticity.So as a new kind of adsorption materials,ACFs were gradually used to VOCs pollution control.But non polar and non hydrophilic properties on the original ACFs surface did not favor the adsorption of polar molecule of VOCs(such as Toluene).Research showed that the adsorption property of ACFs to target adsorbate could be improved by modification of their porous structure and surface chemistry.In this degree paper,the ACFs were modified by H₂O₂ and on this base,the changes and effects of porous structure and surface chemistry of the modified and original ACFs samples on the adsorption of Toluene gas were investigated.(1)According to Boehm's titration method,the quantities of three kinds of oxygen-con -taining functional groups including carboxyl groups,lactone groups,hydroxyl constants groups of phenolic character of ACFs modified under different impregnated concentration, pH and time of H₂O₂ were characterized.The results showed in comparison to the original ACFs,the total quantity of oxygen-co -ntaining functional groups on the surface of the modified samples was increased by 1.93mm/g at most;the impregnated concentration and pH affected the three kinds of functional groups obviously,however,the effect of the impregnated time was not notable; the three kinds of functional groups presented different variational trends.(2)According to ropy method,the ratio of quantity loss of ACFs modified under different impregnated concentration,pH and time of H₂O₂ were characterized.The data showed that the impregnated concentration and time affected the ratio of quantity loss of the modified ACFs slightly.Generally speaking,the ratio of quantity loss of all the samples modified under different concentration was about 10.35%;toward different impregnated time,the ratio of quantity loss of ACFs equaled to 8.86%;the impregnated pH affected the ratio of quantity loss of the modified samples obviously and maximal ratio of quantity loss was about 8.77%when the pH was 7.(3)When the relative pressure and concentration of Toluene respectively were 0.0934 mg/L and 11.361mg/L,the breakthrough curves of Toluene adsorption on ACFs modified under three ingredients were examined at 25℃.The results indicated that the equilibrium amounts adsorbed of Toluene of all the modified samples were higher than the original sample and the most equilibrium amount adsorbed could be increased by 42.86%;meanwhile,the concentration was the most critical factor to equilibrium amount adsorbed of Toluene within the three ingredients;with the increase of the impregnated concentration,the equilibrium amount adsorbed of Toluene of all the modified samples gradually decreased;when pH≤7,the relationship between equilibrium amount adsorbed of Toluene of all the modified samples and pH was negative, however,when pH below 7,the relationship between equilibrium amount adsorbed of Toluene of all the modified samples and pH was positive;the effect of the impregnated time on the equilibrium amount adsorbed of Toluene of the modified samples was not obvious.(4)During the three groups of single ingredient experiment,the three samples respectively modified with 1.47mol/L,4.41 mol/L and 8.82 mol/L of H₂O₂ in connection with the original sample were tested under nitrogen adsorption at low temperature experiment,Temperature-Programmed Desorption experiment and Toluene adsorption isotherm curves for ACFs at 25℃.The results indicated compared with the original sample, the porous structure of all the three modified samples were improved;the differences between the three modified samples were not evident but the trends of CO-evolving groups and CO₂-evolving groups were different.According to donor-acceptor electron mechanism, the amount of CO-evolving groups was reduced and favored the Toluene adsorption, however,because ofÏ€-Ï€dispersion force,the amount of CO₂-evolving groups was increased and was detrimental for it;in general,the total number of oxygen surface groups(i.e.,both CO and CO₂-type groups)was high,but the Toluene adsorption capacity was low.(5)Orthogonal experimenatal results indicated that the optimum modified conditions were concertration-1.47mo/L,pH-6,time-2h according to the standard of Toluene equilibrium adsorbed amount;while according to the standard of Toluene adsorbed velocity, the ptimum modified conditions were concertration-2.94 mo/L,pH-6,time-2h.(6)The experimental results revealed that activated carbon fibers was a potential adsorbent for capturing VOCs,as the breakthrough times were observed to considerably decrease with increase in the concentration levels.The simulation results from the mathematical model developed to predict the breakthrough profiles were found to be in good agreement with the data.The model assumes a finite kinetic rate for adsorption and desorption and incorporates two resistances to mass transfer,between the gas and the adsorbents surface,and within the pores of the adsorbents bed temperature of 50℃was found to be favorable in terms of longer breakthrough and adsorption times and were 7.5min and 80min respectively.From the breakthrough curves obtained over the temperature range of 35～50℃,the exothermic heats of adsorption were determined to be 4.2218kJ/mol.