Abstract
To make use of rubidium salt resources in the Qinghai Salt Lake and to valorize Stellera chamaejasme L., mesoporous activated carbon was prepared by a chemical activation method using rubidium chloride as an activator. Using methylene blue and iodine as pollutant models, the adsorption capacity of mesoporous activated carbon was determined. The specific surface area, pore structure, and surface functional groups of mesoporous activated carbon were determined using a fully automated specific surface area analyzer, X-ray diffractometer (XRD), Raman spectrometer, scanning electron microscope (SEM), Fourier transform infrared spectrometer (FT- IR), and an X-ray electron spectrometer (XPS). The surface functional groups of mesoporous activated carbon were found to be mainly comprised of carbon and oxygen. The mesoporous activated carbon had a certain graphitized microcrystalline structure. The carbon yield of the mesoporous activated carbon was 13.55% ± 0.41%, the specific surface area was 877.02 m2/g, the adsorption amount of iodine, 891.35 mg/g, and the adsorption amount of methylene blue, 256.95 mg/g. The dominant pore size was 0.945 nm, the average pore size was 2.43 nm, and the pore volume was 0.26 cm3/g.
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Preparation of Stellera chamaejasme L. Mesoporous Activated Carbon by Rubidium Chloride Chemical Method
Shimin Dang, Jianxia Guo, Ze Liu, and Haichao Li *
To make use of rubidium salt resources in the Qinghai Salt Lake and to valorize Stellera chamaejasme L., mesoporous activated carbon was prepared by a chemical activation method using rubidium chloride as an activator. Using methylene blue and iodine as pollutant models, the adsorption capacity of mesoporous activated carbon was determined. The specific surface area, pore structure, and surface functional groups of mesoporous activated carbon were determined using a fully automated specific surface area analyzer, X-ray diffractometer (XRD), Raman spectrometer, scanning electron microscope (SEM), Fourier transform infrared spectrometer (FT- IR), and an X-ray electron spectrometer (XPS). The surface functional groups of mesoporous activated carbon were found to be mainly comprised of carbon and oxygen. The mesoporous activated carbon had a certain graphitized microcrystalline structure. The carbon yield of the mesoporous activated carbon was 13.55% ± 0.41%, the specific surface area was 877.02 m2/g, the adsorption amount of iodine, 891.35 mg/g, and the adsorption amount of methylene blue, 256.95 mg/g. The dominant pore size was 0.945 nm, the average pore size was 2.43 nm, and the pore volume was 0.26 cm3/g.
DOI: 10.15376/biores.19.4.8312-8323
Keywords: Stellera chamaejasme L.; Rubidium chloride; Mesoporous activated carbon
Contact information: Qinghai Nationalities University, Key Laboratory of Applied Physical Chemistry of Qinghai Province, Qinghai810007, China; *Corresponding author: lihaichao@vip.163.com
INTRODUCTION
A poisonous plant known as Stellera chamaejasme L. grows on the Tibetan Plateau in China (Li et al. 2009). Stellera chamaejasme, also known as broken intestines, steamed bread flowers, and mountain radish, has excellent resistance to drought and tenacious vitality in the Tibetan Plateau. It exhibits rapid growth as it continues to take over the local plant space, poisoning livestock and poultry. The spread of S. chamaejasme is a sign of local ecological degradation. It has been reported that S. chamaejasme is mainly used in the medical and paper industries (Li et al. 2009; Jo et al. 2020). To reduce the burden on the ecosystem and increase the utilization rate of S. chamaejasme, a targeted expansion of the fields of application of Stellera chamaejasme is needed.
Activated carbon is a general term for carbon materials with high specific surface area and strong adsorption capacity (Zhang et al. 2022). Activated carbon has a wide range of applications and is often used in medical, industrial production, food hygiene and other industries (Zhang et al. 2023). Chen et al. (2015) prepared activated carbon from S. chamaejasme by a chemical activation method. The quality of the S. chamaejasme is mainly concentrated in its root, which is structured by the epidermis, endodermis, and mesostyle. The root material mainly consists of dry matter, benzyl alcohol extract, cellulose, and other elements. From the carbon content of the elements contained in its roots, S. chamaejasme is a good raw material for the preparation of activated carbon (Zhao et al. 2023).
The dissolved content of Qinghai Salt Lake is a lake dominated by chloride salts, with a variety of salt minerals co-existing (Du 2014). The salt substances in Qinghai salt lake are mainly potassium salt and lithium salt. The proven lithium reserves in the salt lake reach 17.24 million tons (Chen et al. 2024). Qinghai Salt Lake contains rubidium and caesium resources that are scarce in the outside world. Using rubidium chloride as activator and lignin as raw material, Zhang (2024) successfully prepared a carbon adsorption material with a specific surface area of 433 m2/g under the activation temperature of 600 ℃. The study shows that rubidium chloride can be used as an activator.
In this study, the activated carbon of S. chamaejasme was prepared using salt lake rubidium chloride as an activator. This experiment provides fundamental research data for the protection of Qinghai’s environment, exploitation of salt lake resources, and subsequent research on the activation mechanism of chlor-alkali metals.
MATERIALS AND METHODS
Materials
Stellera chamaejasme (secondary treatment required) was picked from the Huangyuan Grassland in Qinghai. Rubidium chloride (AR, 99.9%), calcium hydroxide (AR, 99.9%), Iodine solution (1.2 g/L), and Methylene blue solution (400 mg/L) were purchased from Aladdin. The drugs were used directly without post-treatment, and the water used in the experiments was deionized water.
Preparation of Activated Carbons
The S. chamaejasme root was peeled. The middle layer of white bast was cleaned and added to the calcium hydroxide for cooking to extract crude fiber (cooking time 2 h, temperature 60 °C). The crude fiber was washed until the washing solution became neutral, and then the course fibers were repeatedly pounded into pulp. The pulp water was sieved and filtered, and the filtered product was placed in a cool place to dry.
According to the preset impregnation ratio (raw material: activator), the treated S. chamaejasme and rubidium chloride were weighed, with the mass of the pretreated S. chamaejasme always being 10 g. The next step was to add 30 mL of deionized water and mix the raw materials with the activator. The oven temperature was set to 70 °C and the mixture was dried for 40 minutes to remove excess moisture. The muffle furnace temperature was set to 700 °C, the heating rate to 10 °C/min, the holding time to 120 min, and calcination of the mixture was performed. This was followed by washing the calcined product with deionized water to neutrality, drying, and grinding to obtain activated carbon.
Characterization Methods
Carbon yield and ash content are important indicators used to judge the yield of activated carbon and raw material selection. The charcoal yield was calculated from the ratio of the mass of activated charcoal to the mass of the raw material. A blank control experiment was set up to calculate the ash content using the difference in mass before and after the raw material (Kim et al. 2023). The prefabricated waterproofing ratio was set to 1:3, 1:2, 1:1, 2:1, 3:1, 5:1, and 10:1. (Zhang et al. 2024; Chen et al. 2015).
The adsorption capacity of activated carbon for methylene blue was determined by the requirements of GB/T12496.10-2015. The specific operational steps were: mixing the sample with the configured methylene blue solution, determining the transmittance of light of the mixed solution with a spectrophotometer, and when the transmittance of light met the requirements, the number of milliliters of methylene blue solution titrated at this transmittance was the adsorption volume of activated carbon on methylene blue. The iodine value adsorption was determined with reference to GB/T12496.8-2015, and the determination process was the same as methylene blue (Ahmed et al. 2014; Ren et al. 2024).
The N2 adsorption-desorption experiments were carried out on the activated carbon using a TriStarI 3020 fully automated specific surface area and pore analyzer (Micromeritics, USA) at 77 k. The pore size and specific surface area size of the activated carbon were determined based on the N2 adsorption-desorption measurements (Doczekalska et al. 2022).
The activated carbon was gold-sprayed in a vacuum coater. The acquisition time was set at 100 s, the intensification voltage was 20 KV, the scanning spacing was 15 mm, and the surface of the activated carbon was photographed using a SU8010 field emission scanning electron microscope (Hitachi, Japan) (Columba et al. 2022).
The microcrystalline structure of the activated carbon was scanned using a D/MaX-2550PC X-ray diffractometer (RIGAKU, Japan) within a diffraction angle range of 2θ = 10 to 70° and a diffraction rate of 4°/min. The graphitized degree of activated carbon was determined using NEXUS-670 FTIR-Raman spectrometer (Nicolet, USA) at an excitation wavelength of 530 nm (Kunusa et al. 2021).
A Nicolet™ is50 Fourier transform infrared spectrometer (PerkinElmer, USA) was used to test the absorbance peaks of the functional groups on the surface of the activated carbon using the spectrometer by setting the scanning wavelength of the instrument to 400 to 4000 cm-1. The pressure of the analysis chamber was set at 3 × 10-10 mbar and the incident wavelength of X-rays was 500 μm, and the elements contained on the surface of the activated carbon were analyzed by using an ESCLAB Xi X-ray photoelectron spectrometer (Shenzhen Core Derun Electronic Technology Co Ltd.) (Wang et al. 2017).
All test methods were carried out three times for each condition.
RESULTS AND DISCUSSION
Characterization of Activated Carbon
High ash content affects the formation of pores on the surface of activated carbon (Kim et al. 2023). Ash content is one of the most important indicators of whether a substance can be used as a raw material for the preparation of activated carbon. The ash mass percentage of pretreated S. chamaejasme was 3.12% ± 0.15% as measured relative to a blank experiment. In terms of ash content, S. chamaejasme is similar to common raw materials for activated carbon processing, such as coconut shells and straw, and is a low ash content raw material (Buah et al. 2019). Figure 1 shows the graph of activated carbon yield versus impregnation ratio under the experimental conditions of activation temperature of 700 °C and activation time of 2 h. The activated charcoal yield was found to increase with the increase of the impregnation ratio; when the impregnation ratio is 1:1, the charcoal yield reaches a peak of 13.55% ± 0.41%.
Zhang et al. (2023) prepared activated carbon with a carbon yield of 7.42% using cellulose as the raw material and cesium chloride as the activator. Chen et al. (2015) used S. chamaejasme as the raw material and phosphoric acid as the activator to prepare activated carbon with a high carbon yield of 57.41%. By comparing the carbon yield, the carbon yield of the activated carbon produced in this study was classed as moderate.
Fig. 1. Activated carbon yield as a function of impregnation ratio (S. chamaejasme: Rubidium chloride)
Fig. 2. Adsorption of activated carbon dyes as a function of the impregnation ratio (S. chamaejasme: Rubidium chloride)
Methylene blue and iodine were taken as pollutant models, and the adsorption capacity of activated carbon was determined by observing the adsorption value of activated carbon to pollutants. Figure 2 shows the dye adsorption on activated carbon prepared with different impregnation ratios under the experimental conditions of activation temperature of 700 °C and activation time of 2 h.
As can be seen from Fig. 2, the non-activated S. chamaejasme did not have the ability to adsorb significant amounts of methylene blue and iodine. After activation, the adsorption capacity of S. chamaejasme toward iodine and methylene blue increased greatly, and the adsorption capacity of S. chamaejasme activated carbon toward dye increased with decreasing impregnation ratio. The adsorption value of activated charcoal for dyes reached a peak value when the impregnation ratio was 1:1. The adsorption capacity for iodine was 891 mg/g, and for methylene blue it was 257 mg/g.
Specific surface area and pore structure data of activated carbon are important indicators to judge the adsorption performance of activated carbon. The specific surface area and pore size of activated carbon can be determined by the adsorption-desorption test with nitrogen. Figure 3 shows the specific surface area of activated carbon versus the impregnation ratio, from which the specific surface area of activated carbon will increase with the impregnation ratio, first reaching a peak and then decreasing. The maximum value of the specific surface area of activated carbon was obtained at the impregnation ratio of 1:1, and the maximum specific surface area was 877 m2/g.
Fig. 3. Plot of activated carbon impregnation ratio versus specific surface area (S. chamaejasme: Rubidium chloride)
A comprehensive analysis of the carbon yield, dye adsorption capacity of activated carbon, and specific surface area showed that the best performance of the activated carbon was achieved under the experimental conditions of an activation temperature of 700 °C, activation time of 2 h, and impregnation ratio of 1:1. The activated carbon prepared under this experimental condition was named AC-R.
Figure 4 shows the nitrogen adsorption-desorption isotherm of AC-R. It can be seen that when P/P0 < 0.19, AC-R exhibited monolayer adsorption at this stage, and the adsorption of nitrogen by AC-R increased with the increase of P/P0. When P/P0 > 0.19, AC-R began to exhibit multilayer adsorption, and the rate of adsorption of nitrogen by AC-R began to slow down; when P/P0 = 0.43, the AC-R underwent nitrogen desorption; when 0.37 < P/P0 < 0.97, the adsorption-desorption isotherms of AC-R did not overlap, and according to the calculation, the average difference between the nitrogen adsorption value and the desorption value of AC-R in the interval was 23. This shows that the difference in the pore size of AC-R was large.
Fig. 4. TheAC-R nitrogen adsorption-desorption isotherm
Figure 5 shows the pore size distribution of AC-R. It can be seen that the pore size values of AC-R are distributed at 0.5 to 3.1 nm; the dominant pore size is 0.945 nm, and the average pore size is 2.43 nm. Thus, AC-R was classed as mesoporous activated carbon.
Fig. 5. The AC-R pore size distribution map
Table 1 shows the data comparison table between AC-R and related studies. The specific surface area of the activated carbon prepared by Zhang et al. (2024) using lignin as the raw material and rubidium chloride as the activator was 433 m2/g with an average pore size of 2.02 nm (Table 1). This can be seen from the data comparison in Table 1. The S. chamaejasme Linn activated carbon prepared in this study had high specific surface area and pore volume.
Table 1. Comparison Table of Data from AC-R and Related Studies
Figure 6 shows the scanning electron microscope (SEM) images of Stellera chamaejasme raw material (6a), S. chamaejasme charcoal (6b), and AC-R (6c-6d). As shown in Fig. 6, no obvious void structure was apparent on the surface of the S. chamaejasme raw material (6a); the surface of the inactivated S. chamaejasme charcoal was similar to that of the S. chamaejasme raw material (6b); and the activated S. chamaejasme Linn activated charcoal surface appeared to have an obvious void structure after the activation (6c, 6d). The activation mechanism of rubidium chloride has not yet been clarified, so it is not possible at this point to determine the cause of void formation on the AC-R surface. This study provides theoretical support for subsequent studies on the activation mechanism of rubidium chloride.
Fig. 6. Scanning electron microscope images of S. chamaejasme at 10 μm (a), SEM images of S. chamaejasme charcoal at 10 μm (b), SEM images of AC-R at 10 μm (c) and 1 μm (d)
Figure 7 shows the XRD analysis pattern of AC-R. As shown, there were obvious characteristic peaks in the interval of 2θ of 15 to 32.5°, which shows a certain crystal structure within the AC-R. There were irregular diffraction peaks at 2θ = 26.85° and 2θ = 29.4°. The miscellaneous peaks were fitted and compared with the standard PDF card, which shows that at 2θ = 29.4° is the characteristic peak eigen-peak of CaCO3, and 2θ = 26.85° is the characteristic peak of SiO2. The crucible powder remaining in the activated carbon led to the generation of the characteristic peak of SiO2; when extracting the crude fiber of S. chamaejasme, Ca(OH)2 remaining in the activated carbon reacted with CO2 and eventually formed CaCO3 leading to the generation of the characteristic peak.
Fig. 7. TheAC-R XRD diffraction pattern
Fig. 8. The AC-R Raman spectrum data map
The Raman spectra of AC-R are shown in Fig. 8, from which the D-peak peak value of AC-R was 1350 cm-1 and the G-peak peak value was 1590 cm-1. The D peak represents the irregular vibration peak of the carbon atom, which belongs to the sp3 vibration peak, and the G peak represents the regular vibration peak of the carbon atom, which belongs to the sp2 vibration peak. The degree of graphitization of activated carbon can usually be characterized by ID/IG, and when ID/IG ≥ 0.8, the activated carbon has a certain degree of graphitization. The larger the ID/IG, the higher the degree of graphitization of the activated carbon, and the ID/IG of AC-R = 0.85, which shows that AC-R has a certain degree of graphitization from the numerical analysis.
Figure 9 shows the surface functional group analysis of S. chamaejasme raw material, AC-R. Observation of the infrared curve of S. chamaejasme in Fig. 9a shows that S. chamaejasme exhibited obvious absorbance peaks of aromatic compounds and C-O groups in the infrared wavelength interval of 500 to 1250 cm-1 (Zhang et al. 2024); observation of the infrared curve of AC-R in Fig. 9a include an absorbance peak generated by vibration of C-O groups at 1220 cm-1 for AC-R and an absorbance peak generated by vibration of C = C groups at 1590 cm-1 for AC-R (Zhang et al. 2023). A comparison of the two FT-IR curves in Fig. 9a shows that S. chamaejasme lost many functional groups of alcohols, esters, and aromatic compounds upon activation (Chen et al. 2015).
Fig. 9. FT-IR diagram of activated carbon AC-R (a), AC-R XPS scanning overview (b), AC-R XPS C1s scanning general view (c), AC-R XPS O1s scanning general view (d)
Figure 9b shows the total XPS scanning spectra of AC-R, Fig. 9c shows the fitted XPS scanning spectra of AC-R for carbon, and Fig. 9d shows the fitted XPS scanning spectra of AC-R for oxygen. Carbon and oxygen are most prominent in the scanning peaks of AC-R XPS (9b). At the binding energies of: 284.15 eV, 285.46 eV, and 288.65 eV, the corresponding functional groups are C-C, C-O, and C=O, respectively (9c) (Burg et al. 2002). At the binding energies of 532.5 eV and 533 eV, the corresponding functional groups are C=O and C-O (9d) (Zhang et al. 2024).
CONCLUSIONS
- The ash content of S. chamaejasme was 3.12% ± 0.15%; thus, S. chamaejasme can be classified as a low ash material.
- S. chamaejasme was used as the raw material for the preparation of activated carbon, rubidium chloride was used as the chemical activator, and the S. chamaejasme mesoporous activated carbon was successfully prepared under the experimental conditions of an activation temperature of 700 ℃, activation time of 2 h, and impregnation ratio of 1:1.
- The carbon yield of S. chamaejasme mesoporous activated carbon reached 13.55% ± 0.41%, with a dominant pore size of 0.945 nm, an average pore size of 2.43 nm, a pore volume of 0.26 cm3/g, and a specific surface area of 877 m2/g, with adsorption of iodine reaching 891 mg/g, and of methylene blue reaching 257 mg/g.
- The S. chamaejasme mesoporous activated carbon exhibited a certain degree of graphitization and microcrystalline structure, and the surface functional groups were mainly carbon-containing and oxygen-containing functional groups.
ACKNOWLEDGMENTS
This research project was supported by the Qinghai Province Key R&D and Transformation Programme (2022-QY-210).
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Article submitted: July 17, 2024; Peer review completed: August 7, 2024; Revised version received: August 14, 2024; Accepted: September 1, 2024; Published: September 13, 2024.
DOI: 10.15376/biores.19.4.8312-8323