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Here, we present a protocol to synthesize nitrogen/oxygen dual-doped mesoporous carbon from biomass by chemical activation in different pyrolysis modes followed by modification. We demonstrate that the microwave pyrolysis benefits the subsequent modification process to simultaneously introduce more nitrogen and oxygen functional groups on the carbon.
An environment-friendly technique for synthesizing biomass-based mesoporous activated carbon with high nitrogen-/oxygen-chelating adsorption for Cu(II) is proposed. Bagasse impregnated with phosphoric acid is utilized as the precursor. To pyrolyze the precursor, two separate heating modes are used: microwave pyrolysis and conventional electric-heating pyrolysis. The resulting bagasse-derived carbon samples are modified with nitrification and reduction modification. Nitrogen (N)/oxygen (O) functional groups are simultaneously introduced to the surface of activated carbon, enhancing its adsorption of Cu(II) by complexing and ion-exchange. Characterization and copper adsorption experiments are performed to investigate the physicochemical properties of four prepared carbon samples and determine which heating method favors the subsequent modification for doping of N/O functional groups. In this technique, based on analyzing data of nitrogen adsorption, Fourier transform infrared spectroscopy, and batch adsorption experiments, it is proven that microwave-pyrolyzed carbon has more defect sites and, therefore, time-saving effective microwave pyrolysis contributes more N/O species to the carbon, although it leads to a lower specific surface area. This technique offers a promising route to synthesis adsorbents with higher nitrogen and oxygen content and a higher adsorption capacity of heavy-metal ions in wastewater remediation applications.
Activated carbon has unique adsorption properties, such as a developed porous structure, a high specific surface area, and various surface functional groups; therefore, it is employed as an adsorbent in water treatment or purification1,2,3,4. Besides its physical advantages, activated carbon is cost-effective and harmless to the environment, and its raw material (e.g., biomass) is abundant and easily obtained5,6. The physicochemical properties of activated carbon depend on the precursors that are used in its preparation and on the experimental conditions of the activation process7.
Two methods are typically used to prepare activated carbon: a one-step and a two-step approach8. The term one-step approach refers to precursors being carbonized and activated simultaneously while the two-step approach refers to that sequentially. In view of energy conservation and environmental protection, the one-step approach is more preferred for its lower temperature and pressure demanding.
Besides, chemical and physical activation are utilized to improve the textural properties of activated carbon. Chemical activation possesses apparent advantages over physical activation because of its lower activation temperature, shorter activation time, higher carbon yield, and more developed and controllable pore structure in a certain degree9. It has been tested that chemical activation can be performed by impregnating biomass used as feedstock with H3PO4, ZnCl2, or other specific chemicals, followed by pyrolysis to increase the porosity of the activated carbon, because lignocellulosic components of biomass can be easily removed by a subsequent heating treatment, owing to the dehydrogenation capability of these chemicals10,11. Hence, chemical activation greatly enhances the formation of activated carbon's pores or improves the adsorptive performance to contaminants12. An acidic activator is preferred to H3PO4, due to its relatively lower energy demand, higher yield, and less impact on the environment13.
Microwave pyrolysis has the superiority in time savings, uniform interior heating, energy-efficiency, and selective heating, making it an alternative heating method to synthesis-activated carbon14,15. Compared with conventional electric heating, microwave pyrolysis can enhance thermo-chemical processes and promote certain chemical reactions16. Recently, extensive studies have focused on preparing activated carbon by chemical activation from biomass using one-step microwave pyrolysis9,17,18,19. So, it is considerably informative and environment-friendly to synthesis biomass-based activated carbon by microwave-assisted H3PO4 activation.
In addition, to improve the adsorption affinities of activated carbon toward specific heavy-metal ions, modification by heteroatom [N, O, sulfur (S), etc.] doping into carbon structures has been proposed, and this has proven to be a desirable method20,21,22,23,24,25,26. Defective sites in or at the edges of a graphite layer can be replaced by heteroatoms to generate functional groups27. Hence, nitrification and reduction modification are used to modify resultant carbon samples to dope N/O functional groups which play a crucial role in efficiently coordinating with heavy metal to form complexing and ion-exchange28.
Based on the findings above, we present a protocol to synthesize N/O dual-doped mesoporous carbon from biomass by chemical activation and two different pyrolysis methods followed up by modification. This protocol also determines which heating method favors the ensuing modification for doping of the N/O functional groups and, thus, enhancing the adsorption performance.
1. Preparation of Bagasse-based Activated Carbon
2. Modification of Electrical-furnace-pyrolyzed Bagasse-based Activated Carbon and Microwave-pyrolyzed Bagasse-based Activated Carbon
Note: The modification of the two samples was conducted according to the literature29.
3. Adsorbent Characterization
4. Cu(II)-adsorption Experiments
Nitrogen adsorption/desorption isotherms of four samples are presented in Figure 1. All adsorption isotherms show a rapid increase in low P/P0 region and these isotherms belong to type IV (IUPAC classification) demonstrating their pore structure that consists of micropores and dominant mesopores32.
The surface physical parameters for all samples obtained from the n...
In this protocol, one of the critical steps is the successful preparation of mesoporous carbon with better physicochemical properties by the one-step approach, where optimal experimental conditions need to be determined. So, in a previous study28, we have carried out orthogonal array microwave pyrolysis experiments, considering the effect of the impregnation ratio of bagasse and phosphoric acid, pyrolysis time, microwave oven power, and drying time. Besides, great care must be taken in tedious Cu(...
The authors have nothing to disclose.
The authors acknowledge the Fundamental Research Funds for the Central Universities of China (No.KYZ201562), China Postdoctoral Science Fund (No. 2014M560429) and the Key research and development plan of Jiangsu Province (No. BE2018708).
Name | Company | Catalog Number | Comments |
All chemicals and reagents (phosphoric acid, etc.) | Nanjing Chemical Reagent Co., Ltd | Analytical grade | |
Electric furnace | Luoyang Bolaimaite Experiment Electric Furnace Co., Ltd | ||
Microwave oven | Nanjing Yudian Automation Technology Co., Ltd | 2.45 GHz frequency | |
Surface-area and porosimetry analyzer | Beijing Gold APP Instrument Co., Ltd | Vc-Sorb 2800TP | |
Fourier transform infrared (FTIR) spectrometer | Nicolet | 6700 | |
Flame atomic absorption spectrophotometry | Beijing Purkinje General Instrument Corporation | A3 | |
Element Analyzer | Germany Heraeus Co. | CHN-O-RAPID |
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