| This paper describes the activation of pine and wheat straw biochar with KOH andmicrowave heating to obtain a product with high adsorption capacity for toluene and acetone.For this purpose, a microwave activation process for the preparation of activated biochar wasdeveloped and the effects of various KOH/biochar mass ratios on the structural properties andadsorption capacity of activated biochar were investigated. The preparation conditions, such asthe microwave power, the KOH/biochar ratio and the microwave heating time were investigated.The properties of the produced activated carbon were characterized using iodine number as wellas BET surface area and pore size distribution. Adsorption capacity for toluene/acetone onactivated biochar was obtained using breakthrough curves at low concentrations and a sorptionanalyzer at high concentration. The adsorption isotherms were fitted with theDubinin–Radushkevich (DR) model. In the regeneration, this paper presents a comparison ofmicrowave and conductive heating regeneration of toluene and acetone from activated biochar,in terms of regeneration ratio, desorption rate, heating time and energy consumption. Thefollowing conclusions may be drawn:(1) The optimized preparation conditions for activated biochar are that carbonization timeis2h and humid N2during the carbonization period; and microwave power, microwave heatingtime are600W and30min, respectively, as well humid N2during the microwave activationperiod. As for pine and wheat straw activated biochar, the BET surface area increased from1053m2/g and942m2/g to1250m2/g and2044m2/g with an increase in the KOH/biochar massratio from0.5to3.0, respectively; micropore surface area and micropore volume showedsimilar increases. Most of the pores developed in the activated biochar samples are within themicroporous range, indicating that the microporosity is independent of the KOH/biochar massratio. Microporosity was present due to the initial porosity of the precursor material (e.g. thetracheids) that aided KOH impregnation into the biochar and to chemical reactions betweenKOH and biochar during microwave heating. The FTIR spectra of biochar activated by microwave heating and KOH with ratio of3.0showed bands of–OH, C-H, C=O, and=CH2functional groups.(2) For toluene at200ppmv, the adsorption capacities of pine and wheat straw activatedbiochar increase from22.3%and17.1%to47.6%and32.7%as increasing of KOH/biocharratio from0.5-3.0. The same tendency of acetone at200ppmv, the adsorption capacities of pineand wheat straw activated biochar increase from15.4%and14.0%to21.7%and17.8%asincreasing of KOH/biochar ratio from0.5-3.0. Adsorption capacity was increased according tothe increment of inlet concentration from100to300ppmv. For toluene, adsorption capacity ofpine and wheat straw activated biochar at KOH/biochar ratio of3.0was increased from43.4%and20.2%to51.3%and24.4%, respectively with the increasing of inlet concentration from100to300ppmv. For acetone, adsorption capacity of pine and wheat straw activated biochar atKOH/biochar ratio of3.0was increased from17.8%and12.7%to24.4%and21.4%,respectively. Higher KOH/biochar mass ratios resulted in longer breakthrough times andproduced more micropores, leading to a higher adsorption capacity. Furthermore, the fasterbreakthrough was and the slope of breakthrough curve was gradually increased. Breakthroughcurves on pine activated biochar and wheat straw activated biochar corresponding toKOH/biochar mass ratios of0.5,1.5, and3.0, respectively. Pine activated biochar atKOH/biochar mass ratios of0.5,1.5, and3.0took8,12, and18hours, respectively, to reachbreakthrough during the toluene adsorption process. Wheat straw activated biochar atKOH/biochar mass ratios of0.5,1.5, and3.0took4.5,5.5and6hours, respectively. Theadsorption capacity and breakthrough time decreased as the adsorption temperature increasedfrom25to45℃. As for toluene, the adsorption capacity of pine and wheat straw activatedbiochar at KOH ratio of3.0decreased from47.6%and32.7%to25.0%and19.2%,respectively, when the temperature increased from25to45℃; for acetone, the adsorptioncapacity of pine and wheat straw activated biochar at KOH ratio of3.0decreased from21.7%and17.7%to15.3%and12.0%, respectively, when the temperature increased from25to45℃.(3) Adsorption capacity at high concentration was increased according to the increment ofKOH/biochar ratio from0.5to3.0. For toluene, adsorption capacity of pine and wheat strawactivated biochar were increased from32.2%and20.2%to71.9%and45.9%, respectively,when KOH/biochar ratio was increased from0.5to3.0. For acetone, adsorption capacity ofpine and wheat straw activated biochar were increased from24.7%and22.7%to57.8%and36.4%, respectively, when KOH/biochar ratio was increased from0.5to3.0. An increase oftemperature from25to45℃, the toluene/acetone adsorption capacity from the adsorptionisotherm of activated biochar at high concentration was decreased. DR Dubinin–Radushkevich(DR) model can be used to fit toluene/acetone adsorption isotherm data for pine and wheat straw activated biochars at various temperatures of25,35,45℃.(4) The adsorption capacity of activated biochar is not influenced by successivemicrowave swing regeneration. For toluene, it was found that the adsorption capacities of pineand wheat straw activated biochar are about445mg/g and231mg/g, respectively, and5cyclesover repetitive microwave heating did not reduce or increase adsorption capacities. For acetone,it showed the adsorption capacities of pine and wheat straw activated biochar are about330mg/g and185mg/g, respectively. A increasing of microwave output power, flow rate of purgegas, and humidity of purge gas, desorption rates using microwave heating was increased. Theoptimized parameters of microwave output power, flow rate of purge gas, and humidity ofpurge gas for microwave heating under this experiment condition are:600W,1L/min and humidN2. Comparison of pine and wheat straw activated biochar loaded with toluene/acetone onadsorption capacity, regeneration ratios, temperature rats, desorption rates, power and energyconsumption, microwave regeneration has been demonstrated on a lab-scale to be a rapider,more efficient and simpler regeneration technique than that of conductive regeneration method.The results showed that the regeneration ratio reached99.7%and99.8%under3min and1minof microwave heating with constant power of600W, when the pine and wheat straw activatedbiochar was loaded with toluene and acetone, respectively. For microwave heating withconstant temperature, it took30min and10min for pine activated biochar loaded with tolueneand acetone, respectively, while it took only10min and5min for activated activated biocharloaded with toluene and acetone, respectively. However, for conductive heating, it took120minand60min for both pine/wheat straw activated biochar loaded with toluene and acetone,respectively. To achieve99.6%desorption, microwave heating requires13.5KJ/g and9KJ/g fortoluene and acetone, while conductive heating requires40.5KJ/g and31.5KJ/g for toluene andacetone. Therefore, the energy consumption of conductive heating was3times higher than thatof microwave heating for the desorption of toluene and acetone. The BET surface area,micropore volume, micropore surface area and surface functional group of the microwave andconventional treated samples was found unchanged. Therefore, microwave heating withconstant power is recognized as the most efficient and feasible regeneration method foractivated biochar saturated with toluene or acetone. |
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