| Reduction of CO2, SO2 and NOxemissions is global issues.Biochar receives wide attention as alternative adsorbents for CO2, SO2 and NOx. benefited from its advantages including excellent renewability, abundant porosity and low costs. Nevertheless, the adsorption ability of raw biochar is poor compared to commercial adsorbents, ascribed to its poorer physiochemical properties. Modification of biochar to improve its pore structures and enrich its basic nitrogen containing functional groups is greatly propitious to selective adsorption of CO2, SO2 and NOx. In this work, three kinds of biomass (rice husk, soybean straw, camphor wood) were adopted as precursors for preparation of nitrogen-enriched biochar. Different modification atmospheres were compared and the influence of pre-deashing of biomass was investigated. The physiochemical properties of the prepared biochar were analyzed, as well as their relationships between CO2/SO2/NOx adsorption characteristics. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) and generalized 2D perturbation correlation method are combined to reveal mechanisms of CO2/SO2/NOx adsorption-desorption on the nitrogen enriched biochar. The main conclusions are as follows.(1) The physiochemical properties of biochar derived from three kinds of biomass demonstrates great differences because of the different in components and structures of the precursors. Ash in precursors, especially Si, Ca, Mg minerals are disadvantageous for the modification of biochar. Because the formation of stable Si, Ca, Mg oxides during modification process not only obviously impedes the development of pore structure, but also reduces the active sites for the introduction of nitrogen functional groups by consuming oxygen-containing groups on biochar surface through formation of Si, Ca, Mg oxides. Biochar derived from rice husk demonstrates poorest modification performance among the three ascribed to its highest ash contents. Pre-deashing of rice husk with HF and HC1 can successfully remove Si, Ca, Mg minerals and promote the development of pore structure and introduction of nitrogen functional groups, which improve the adsorption characteristics of biochar.(2) CO2-NH3 modification is successful in improve the physiochemical properties of soybean biochar. The result shows that CO2-NH3 modification of commendably combines the advantages of both CO2 activation and high temperature NH3 treatment. The presence of CO2 is not only conducive to the micropore structure improvement, but also can introduce some heteroatom groups (such as C-O and C=O), which can provide more active sites for the introduction of nitrogen functional groups. In the temperature range of 500 ℃ to 900 ℃, the micropore surface area, micropore volume, the nitrogen content and N/C ratio of soybean biochar all first increase and then decrease with the increase of modification temperature, and reach the maximums at 800 ℃ (533.83 m2/g,0.214 cm3/g, 6.61 wt.% nd 0.103, respectively). With the increase of modification temperature, the adsorbing capacity of CO2/SO2/NO also showed a similar change of physical and chemical characteristics, which is a first increases and then decreases trend, and reach the maximums at 800 ℃ (when the adsorption temperature is 30 ℃, the maximums of CO2/SO2/NO are 88.9 mg/g,201.9 mg/g and 43.7 mg/g, respectively; at 120 ℃, the maximums of CO2/SO2/NO are 49.9 mg/g,145.7 mg/g and 6.2 mg/g, respectively).(3) The linear fitting was used to analyze relationships between the CO2/SO2/NO adsorption capacities and the micropore volume/nitrogen content. The results show at the lower adsorption temperature, the adsorption of CO2/SO2/NO is mainly the micropore filling, and the physical adsorption is the dominant driving force for the adsorption. At the higher adsorption temperature, the adsorption of CO2/SO2/NO is mainly the chemical adsorption by the nitrogen functional groups, and the surface chemistry characteristics might be the main factor to control the adsorption. Generalized two-dimensional correlationinfrared spectroscopy analysis reveals that the hydroxyl, primary amide, amines, azo compound N=N, secondary amide groups and aliphatic C-N/C-O are all effective active sites of nitrogen-enriched biochar to adsorb CO2. When the nitrogen-enriched biochar was used to adsorb SO2, it can be found that there are not only the hydrogen bond interaction, but also the the covalent bond interaction of the sulfamide, thioketone, isosulfocyanide, thiocarbonyl, sulfoxides S=O and thioethers C-S. In addition, during the process of NO adsorption, the hydrogen bond and covalent bond interaction both exist, such as amides, aromatic amines, aliphatic amines, nitro and C≡N groups.(4) During the pair-wise adsorption of CO2 SO2 and NO. it can be found that at the lower adsorption temperature, three of them all scrabble for the micropore structure of the nitrogen-enriched biochar, and beween SO2 and NO, they also show a competition for the hydrogen bond interaction (such as polymer, pi bonds and polyhydric alcohols). At the higher adsorption temperature, CO2 and SO2 rival with each other in the nitrogen functional groups, CO2 and NO also show a competition for the nitrogen functional groups, but SO2 and NO promote each other. In addtion, during the co-adsorption of CO2, SO2 and NO, there are still the competitive relationship among three of them. However, comprehensively compared with other adsorbent in the co-adsorption of CO2, SO2 and NO, it can be found that there is still a great application prospect for the nitrogen-enriched biochar. |
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