The Adsorption Mechanism Of PCDD/Fs On Porous Carbon Materials

With the increasing proportion of municipal solid waste incineration in the total waste harmless-treatment in China, the emission problem of by-product dixoins is becoming more and more prominent. Activated carbon has still been the most widely used to abate dioxins emission in waste incineration plants, because of its relatively low capital cost and high removal efficiency. However, there are still some insufficiencies on the adsorption mechanism of dioxins on activated carbon, which result in blindness of selection and application of activated carbon for dioxin control. Thus, this paper focused on the topic of adsorption characteristics of dioxins on activated carbon and carbon nanotube. The research work and some main valuable conclusions in this paper are as followed.1) Adsorption of 17 toxic dioxin congeners on three kinds of commercial activated carbons was evaluated through bench-scale experiments. A dioxin generator was applied to generate dioxin containing gas stream with constant concentration. Physical properties of activated carbon, including its Brunauer-Emmet-Teller-surface, pore volume were considered. Among the three types of activated carbon tested, the coconut shell sample with wide pore size distribution reached the highest adsorption efficiency. Under the testing conditions used, the total toxic quivalent (TEQ) removal efficiency reached over 96.6%, much more than the 81.7% efficiency achieved by lignite activated carbon. The results showed that the removal efficiencies of dioxins correlate more strongly with pore volume (r2> 0.93) than with the surface area (r2< 0.48). In addition, adsorption efficiencies of 17 toxic congeners on lignite activated carbon were increasing with the number of substituted chlorines.2) On the basis of results obtained in the first research part, the modification of activated carbon was conducted using Fe(NO3)3 impregnation compled with CO2 activation under high temperatures. During the modification process, the effect of impregnation ratio, activation temperature, activation time was studied. Results showed that in the activation process, the loaded Fe-contained compounds functioned as catalyst for the carbon gasification (C-CO2 reaction), which maked the porosity of activated carbon larger. When the activated carbon with 1% impregnation ratio was catalytic activated under 900℃ for 2 hours, the ratio of mesopore volume in the modified activated carbon could reach as much as 97.5% of the total pore volume.3) Comparative studies of adsorption characteritics of 136 (toxic and non-toxic) tetra- to octa-chlorinated dioxin congeners on the original activated carbon, modified activated carbon and commercial multi-walled carbon nanotube (MWCNT) were conducted. Results showed that MWCNT with lower porosity reached the highest removal efficiency because of distinct adsorption mechnism to be compared with activated carbon. The adsorption efficiency of dioxins on MWCNT increased slowly from 82.4% to 93.5%, when the adsorption temperature was rising from 50℃ to 250℃. However, When the temperature continued rising to 300℃, the removal efficiency decreased to -62.3% due to the re-formation of dioxins. In addition, the removal efficiencies of dioxin homologue groups on the three carbon materials rised with the rising chlorine substitution number. The removal efficiencies vary approximately as a power function of vapor pressure (correlation coefficients R2=0.82,0.92 and 0.96, respectively).4) Commercial activated carbon samples were evaluated during tests to remove trace amounts of dioxins dissolved in n-hexane. The key variables selected were the concentration of dioxins in n-hexane and the dosage of activated carbon. Results showed the overall TEQ removal efficiency could be more than 99.5% when 1.0 g of activated carbon was used at 25℃ in the initial concentration range of 0.7 μg/L-3.6μg/L. The adsorption capacity linearly increased with the increased initial concentrations (R2>0.98). The linear correlation coefficient (R2> 0.99) showed that a linear relationship between the amount of adsorption equilibrium (Qe) and the solution equilibrium concentration (Ce), Henry linear adsorption isotherm was more suitable for the liquid adsoprtion of dioxins with trace concentration.