Chemical modifications of activated carbons prepared from different Ganoderma residues, their adsorption, and catalytic application

ABSTRACT

This study aimed to prepare Ganoderma-based activated carbons for chemical adsorption and catalyzing N-(phosphonomethyl)iminodiacetic acid to glyphosate. The activated carbons were prepared by applying different activation conditions. These carbon samples were applied to catalyze N-(phosphonomethyl)iminodiacetic acid to glyphosate. The results showed that the black Ganoderma char activated at 800 °C for 90 min using 40% phosphoric acid had the highest methylene blue adsorption capacity and DPPH scavenging activities, but the methylene blue adsorption capacity of the black Ganoderma sample activated at 800 °C for 90 min using 40% acid-base solution was lower than the red AC sample. These carbon samples had high thermal stability. Chemical modifications of these chars with the carbon modifiers at an optimized activation condition increased the Langmuir-specific surface areas of these carbon samples up to 2055.09 m²/g. The carbon sample of black Ganoderma activated using the 40% acid-base solution at 800 °C for 90 min and coupled with the addition of hydrogen peroxide during ultrasonication and microwave-assisted reactions had the highest glyphosate catalyzing rate. The glyphosate yield obtained from the catalyzing at atmospheric pressure was 58.78 ± 0.28%. These Ganoderma-based activated carbons can replace other carbon materials for catalysis applications and as adsorbents in food and pharmaceutical industries.

Keywords:
Activated charcoal; antioxidant capacity; herbicide; decolorization; reishi mushroom

1. INTRODUCTION

Ganoderma is a medicinal and edible fungus. Ganoderma is the genus of this fungus, which is in the Ganodermataceae family. It is widely distributed in tropical and subtropical regions. This medicinal fungus usually grows on rotten wood under shady and humid conditions. The fruiting body of Ganoderma contains polysaccharides, protein, fat, polyphenols, saponin, and other phytochemicals. Its polysaccharides consist of glucans and water-soluble heteropolysaccharides [¹]. Among the Ganoderma phytochemicals, Ganoderma triterpenoids have a liver-protective effect [²]. It is a traditional medicine used widely for the prevention of cancers. Ganoderma polysaccharides and saponin have been studied and determined for their physicochemical and antioxidant properties [³]. The Ganoderma residues produced from the extraction of these bioactive substances are regarded as waste. The yields of Chinese medicine residues are closely related to the booming development of Chinese medicine and health industries, and it is estimated that the annual discharge of the residues exceeds a million tons. The management of these solid wastes includes waste burning, environmental pollution, and health risks.

Activated carbons (ACs) have been prepared from several sources, including biomass, bamboo fiber, and fruit peel. Various treatment methods and conditions are used for modifying and activating carbon samples [⁴]. ACs were developed from passion fruit peels using chemical activations [⁵]. The impregnation agents are acids and bases. The development of ACs from different parts of Ganoderma has been reported in the literature. Several activators applying a range of activation parameters were used to activate chars prepared from the Ganoderma lucidum bran residue [⁶]. These activators were potassium hydroxide (KOH), potassium permanganate, zinc chloride, and nitric acid. The study found that the bran residue char with an impregnation ratio of 1:6 and activated using KOH at 700 °C for 5 h had the highest Brunauer-Emmett-Teller (BET)-specific surface area (S_BET of 3147 m²/g) and selective Cu²⁺ detection range between 10 and 50 mM. The Chinese medicine residue containing G. lucidum as its main ingredient was used for preparing ACs and electrode material [⁷]. The AC sample prepared using the following activation condition (an impregnation ratio of 1:1, 1.5 h at 750 °C) had a high BET-specific surface area (1263.6 m²/g) and a capacitance of 252 F/g. The AC prepared from the cotton stalks-biomass using potassium hydroxide with an impregnation ratio of 1:1 at 800 °C for 30 min had the highest BET-specific surface area of 950 m²/g and methylene blue adsorption of 222 mg/g [⁸]. Previous studies also reported that the AC samples prepared from crops and agricultural biomasses and activated using the optimized activation condition with phosphoric acid activation had BET-specific surface areas of up to 1022 m²/g and a range of chemical adsorption rates [⁹, ¹⁰]. Therefore, there is a need to develop AC samples with higher specific surface areas.

ACs are promising adsorbents for removing unwanted chemical in wastewater [¹¹]. The adsorption of substrates by AC involves several known process pathways. They are electrostatic, hydrophobic interactions, hydrogen, and π-π bondings [¹²]. The occurrence of physical adsorption takes precedence over chemical adsorption because the activation energy required by physical adsorption (5–40 kJ/mol) is lower than the energy needed for chemical adsorption (40–800 kJ/mol) [¹³]. Thus, increasing the reaction temperature improves the adsorption rate. AC has good physical and chemical properties. The physical properties are high specific surface areas, high-temperature resistance, and acid and alkali resistance; their chemical properties involve interactions between several functional groups. These carbon properties are a good prerequisite for using AC as a strong adsorbent. ACs have been widely used for adsorbing harmful components in liquids and gases and as catalysts or catalyst carriers in chemical reactions [¹⁴]. The application of AC as a catalyst in the pyrolysis process to produce several compounds has been reported in the literature [¹⁵]. ACs have also been used in catalyzing glyphosate (GP) synthesis.

The literature shows that the AC with a specific surface area of 700-900 m²/g has been used to catalyze GP synthesis from N-(phosphonomethyl)iminodiacetic acid (PMIDA) in the reactor with reaction pressures between 0.35 and 0.45 MPa [¹⁶]. The AC sample had methylene blue, iodine, and phenol adsorption values greater than 158 mg/g, 749 mg/g, and 179.5 mg/g, respectively, and the GP conversion yield was as high as 98.2%. Due to the carbon surface containing transitional metals, these metals are potent catalysts. The transitional metals undergo autoxidation, rapidly oxidize, and then convert water molecules in the environment [¹⁷]. These metals, such as Fe and Pt, have more than two oxidation states. The surface of AC also contains these transitional metals. Moreover, the nitrogen functional groups from the pyridinic, pyrrolic, and pyridonic structures on the AC surface can adsorb transitional metal ions [¹⁸]. Therefore, they can freely initiate the oxidation of PMIDA to form GP.

GP is one of the most widely used herbicides in the world. It has the characteristics of high efficiency, low toxicity, low residue, and easy decomposition by microorganisms without damaging the soil environment. The literature reported that carbon samples catalyzed the oxidation of bisphosphine to synthesize GP with high selectivity [¹⁹]. It is of great significance to study the catalysis effect of AC in synthesizing GP. The catalyzing reaction is enhanced due to the multiporous structure of AC. The ACs prepared from different plant sources vary in their catalyzing activities, especially the synthesis of GP from PMIDA. Due to ACs having a high specific surface area and a wide range of chemical elements on the carbon surface, they are potent catalysts for synthesizing GP. There are limited studies on using AC as a catalyst for synthesizing GP, especially catalyzing GP synthesis at atmospheric pressure (~0.1 MPa) and enhanced catalytic activity by applying ultrasonication and microwave-assisted extraction. Also, no study has reported the elemental composition on the AC surface, especially the transitional metals. Preparing ACs from Ganoderma residues with high specific surface areas was new in carbon research. Chemical modification and activation of carbon samples have been reported to increase their specific surface areas and introduce new functional groups to the carbon surface. Chemically modifying the carbon surface with potassium hydroxide could also add potassium to its carbon surface. Therefore, this study aimed to develop new carbon-based catalysts for chemical adsorption and synthesizing GP using two different Ganoderma residues.

2. MATERIALS AND METHODS

2.1. Materials

Ganoderma residues collected were red G. lucidum and black G. atrum. The red and black Ganoderma residues were applied as the precursor materials for producing carbon samples. These Ganoderma residues were collected as by-products of polysaccharide extraction. The 200-mesh industrial-based AC was purchased from a local carbon material supplier. Anhydrous ethanol, 1-diphenyl-2-trinitrohydrazine (DPPH), Tris-HCl, Trolox (6-hydroxychromane), 85% phosphoric acid (H₃PO₄) solution, KOH, potassium bromide (KBr), methylene blue, N-(phosphonomethyl)iminodiacetic acid (PMIDA), 99.5% glyphosate (GP) of analytical standard, potassium bromide, sulfuric acid, nitric acid, sodium nitrite were purchased from Rhawn Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. AC sample preparation

The red and black Ganoderma residues were washed using distilled water and absolute ethanol at 60 °C. They were then oven-dried at 65 °C for 24 h. The dried Ganoderma residues were placed in crucibles and carbonized in a Muffle furnace at 500 °C for 2 h. The carbonized Ganoderma samples were pulverized and passed through a 50-mesh sieve. At a room temperature of 25 °C, the char powder was initially impregnated with different concentrations of H₃PO₄ solution (0–60%) or KOH solution (10–50%) for 12 h at a 1:5 impregnation ratio (w/v). The char powder was also impregnated in a mixture of H₃PO₄ and KOH, where the impregnation solution contained the respective percentages of acid and base. The chars impregnated in distilled water were used as control samples. The impregnated Ganoderma carbon powders were placed in the ceramic crucibles and activated in the Muffle furnace at different temperatures.

The set temperatures were 400 °C, 450 °C, 500 °C, 600 °C, 700 °C, and 800 °C. The activation times were 30, 60, and 90 min. After the high and extra high-temperature activations, the carbon samples were washed 4-5 times with distilled water, oven-dried, and pulverized using a 200-mesh sieve. The influence of the carbon modifiers on AC was also studied by impregnating the red and black Ganoderma chars with 40% H₃PO₄ and 40% H₃PO₄/KOH. In brief, 150 Ganoderma AC samples were initially prepared for screening purposes, where these AC samples were subjected to only methylene blue adsorption to evaluate their adsorption ability. Eight Ganoderma and passion fruit peel-based AC samples were prepared, along with the industrial AC, which was used to determine their catalytic ability and other analyses. At this point, both Ganoderma residues were prepared by activating with distilled water (control), 40% H₃PO₄ solution (40% AM), and a mixture of acid-base (40% H₃PO₄ and 40% KOH) solution (40% ABM) at 800 °C for 90 min. The control group was labeled as 800 °C-H₂O.

2.3. Methylene blue adsorption

Methylene blue adsorption assay was performed for the red and black Ganoderma carbon samples according to the method described in the literature [²⁰] with minor improvements. The methylene blue reagent of 100 mg/L was prepared for the adsorption assay. The adsorption assay was performed by adding 100 mg carbon powder to 50 mL of methylene blue solution. A 10-mg industrial AC powder was used as a comparative AC sample. After reacting at room temperature for 30 min, the dye solution was filtered using a 0.45 μm syringe filter. The absorbance was measured at 664 nm.

2.4. BET analysis

The specific surface areas of the red and black Ganoderma carbon samples were analyzed by ASAP-2460 Automatic Specific Surface and Porosity Analyzer (Micromeritics, Norcross. GA, USA) [²¹]. Carbon samples of 100 ± 30 mg were weighed and placed in the sample tube. The carbon samples were degassed by heating at (300 °C) and vacuumed for 8 h to remove the adsorbed impurity gas from the surface. The nitrogen adsorption capacity and isotherm were determined in an inert nitrogen atmosphere of −196 °C. The adsorption isotherm was used to calculate the specific surface areas, mean pore sizes, and pore size distributions.

2.5. FTIR analysis

Functional groups of the Ganoderma carbon samples were determined using a Nicolet iS10 Fourier Transform infrared (FTIR) spectrometer (Thermo Scientific, Waltham, USA). The adsorption spectra of the carbon samples were obtained from the FTIR analysis [²²]. The spectrum scanning range was from 400 cm⁻¹ to 4000 cm⁻¹. The carbon samples (1 ± 0.2 mg) and KBr powder (50 ± 2 mg) were mixed and pulverized into fine powder under yellow light irradiation. The carbon samples were then compressed into thin slices by applying 5–6 tons of pressure using a conventional tablet compression machine. The carbon-free KBr was used for background correction.

2.6. DPPH radical scavenging activity

DPPH radical scavenging assay was performed according to the method described in the literature [²³] with slight modification. The scavenging activity of hydroxyl groups on the carbon structures of the Ganoderma-based carbon samples and industrial AC was determined. A 2-mg carbon sample was mixed with 1 mL of DPPH working solution (1.0 mM in anhydrous ethanol) and 0.8 mL Tris-HCl buffer (0.1 M, pH 7.4). It was left to stand at room temperature for 30 min. The upper layer of the sample solution was pipetted, and the absorbance was measured at 517 nm. The DPPH radical scavenging activity of the carbon samples was expressed as Trolox equivalent antioxidant capacity (mM).

2.7. SEM and SEM-EDX analysis

Scanning electron microscopic (SEM) analysis of the Ganoderma carbon samples was performed using a SU5000 field emission scanning electron microscope (Hitachi, Tokyo, Japan) [²⁴]. Before the SEM analysis, the carbon sample was uniformly spread on the surface of the conductive adhesive as a thin layer, and the layer was coated with platinum for conductive treatment. The carbon sample was subjected to amplification of 1 K, 2 K, and 5 K; it was observed under the 5 KV acceleration voltage. The SEM-EDX analysis was performed using the microscope under the 15 KV acceleration voltage with 1000 times amplification. The 19 metals and non-metal elements were selected for the SEM-EDX analysis, and their percentages were determined.

2.8. X-ray diffraction analysis

X-ray diffraction (XRD) patterns of the Ganoderma carbon structures were evaluated using the methods described in the literature [⁵]. The carbon samples were determined for their crystallinity using XRD. The XRD was operated at 40 kV and 40 mA using CuKα (λ = 1.54056Å) as the radiation source. The carbon sample was pulverized into a 300-mesh fine powder before being subjected to XRD analysis. The X’Pert³ Powder X-ray diffractometer (Malvern Panalytical, Almelo, The Netherlands) was used to analyze the degree of carbon graphitization. The diffraction angle (2θ) used ranged from 5º to 90º.

2.9. Thermogravimetric and differential thermal gravimetric analysis

An STD Q600 thermogravimeter (TA Instruments, Co., Ltd., Newcastle, DE, USA) was used to determine the pyrolysis characteristics and thermal stability in the Ganoderma carbon samples [²⁵]. A 10-mg carbon sample in an alumina crucible was weighed and heated from 30 °C to 800 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. The nitrogen flow rate was 100 mL/min.

2.10. Glyphosate synthesis by carbon-based catalysts

GP was synthesized using the Ganoderma carbon samples as catalysts. The GP synthesis involved the conversion of PMIDA to GP using carbon-based catalysts [¹⁹]. In brief, 5.0 g PMIDA and 0.5 g carbon samples were weighed and added with 20 mL distilled water in a mini reactor and reacted under atmospheric pressure. A 50-mL centrifuge tube was used as the mimicking mini reactor. The tube was then placed in a water bath with its temperature set at 90 °C; the reaction times were 30, 60, and 120 min. At the end of the reaction, the sample solution was filtered using a 0.45 μm syringe filter. The filtrate was freeze-dried, and the lyophilized powder was collected as GP crystal.

The GP synthesis using the selected AC samples as catalysts was further determined by comparing the catalytic reactions and subjected to two different reaction modes. In brief, 5.0 g PMIDA and 0.5 g carbon samples were mixed and added with 20 mL distilled water, with or without 3 mL of 30% hydrogen peroxide (H₂O₂), and then reacted under atmospheric pressure and at room temperature (25 °C). The mixture was ultrasonicated for 15 min using a KQ-400KDE ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd, Kunshan, China) at a maximum power of 400 W. It was then heated in a conventional 0.9 Cu Ft air fry microwave oven (Guangdong Galanz Group Co., Ltd., Foshan, China) at 900 W for 100 sec. It was finally filtered and freeze-dried using a Martin Christ Alpha 1–2 LD plus freeze dryer (Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany). H₂O₂ was initially added to the reaction chamber before the reaction. The reaction was also performed using coupled ultrasonic and microwave-assisted reactions after adding H₂O₂ and AC samples. Their conversion rate was determined and expressed as the GP yields.

2.11. Determination of glyphosate yield

Quantitation of GP was performed using the nitrosylation reaction method by referring to a previously described method [²⁶]. This method has been validated for its repeatability and reproducibility. It formed a GP nitroso derivative when reacted with sodium nitrite in an acidic medium. This compound has a maximum absorption peak of 243 nm. The nitrosylation of GP was performed by mixing 1 mL of 1 mg/mL of sample solution and 100 μL of 50% sulfuric acid solution in a test tube. After adding 50 μL of 250 g/L KBr solution and 100 μL of 6.9 g/L sodium nitrite solution, the sample mixture was homogenized using a benchtop vortex mixer for 10 sec. The test tube was then placed in a water bath set at 25 °C and reacted for 30 min. After the reaction, the liquid was diluted using distilled water before the absorbance was measured at 243 nm. GP standard, at concentrations ranging from 0 to 1.0 mg/mL, was used to plot a standard calibration curve for calculating the yield of GP synthesis. The linearity of the calibration curve was high, with an R² value of 0.99.

2.12. Statistical analysis

All data were expressed as mean ± standard error (SE) of three replicates, except for SEM, BET, XRD, FTIR, and thermogravimetric analyses. These results were statistically analyzed using SPSS version 26.0 (SPSS Inc., Chicago, IL, USA). Analysis of variance coupled with the Tukey range test was used to compare the mean differences between different groups, and p < 0.05 was considered statistically significant.

3. RESULTS AND DISCUSSION

3.1. Development of ACs based on adsorption performance

The adsorption capacity of the red and black Ganoderma carbon samples was initially determined using the methylene blue removal assay. The carbon samples were prepared based on different chemical modifiers (H₃PO₄ and KOH), H₃PO₄ concentrations (0–60%), and temperatures (400–800 °C). As shown in Figure 1, the ACs prepared from these Ganoderma residues had moderate to high methylene blue adsorption capacities. When these Ganoderma chars were impregnated with different H₃PO₄ percentages and activated at 400 °C, 450 °C, and 500 °C for 30 min, their methylene blue adsorption capacities were not markedly different, except for the black AC activated at 500 °C (Figure 1A). The activation of these chars could be incomplete at lower activation temperatures (400–500 °C).

Figure 1
Methylene blue adsorption capacity of AC samples prepared from the red and black Ganoderma residues after activating at different activation times. (A) 30 min, (B) 60 min, (C) 90 min, (D) 90 min, and (E) 90 min activation times. Note: The AC samples presented in (A)–(D) were activated using H₃PO₄, whereas the AC samples shown in (E) were first activated using 40% H₃PO₄ and then with different KOH percentages. R: red Ganoderma carbon sample; B: black Ganoderma carbon sample; AC: activated carbon.

The methylene blue adsorption capacity of the red and black Ganoderma chars activated with H₃PO₄ at 500 °C for 60 min was concentration-dependent but not for the carbon samples prepared using the other activation conditions (Figure 1B). The results also showed that higher H₃PO₄ concentrations and activation temperatures improved the methylene blue adsorption capacity of the Ganoderma ACs. The possible explanation is that the AC surface was fully corroded due to the high H₃PO₄ concentration. The higher activation temperatures also enhanced the acid reaction. The adsorption capacity of the industrial AC was 254.19 ± 11.65 mg/g. It was comparable to the Ganoderma ACs.

An increase in the pore sizes of these ACs could be due to the longer activation times, thus enhancing their methylene blue adsorption capacity. The methylene blue adsorption capacity of the AC samples activated for 90 min was notably increased, especially for the 500 °C-activated black Ganoderma AC (Figure 1C). When the activation temperatures increased to 800 °C, about ten times higher methylene blue adsorption capacities were observed for the H₃PO₄-impregnated Ganoderma ACs (Figure 1D). The result showed that the 40% and 45% H₃PO₄-modified black Ganoderma ACs activated at 800 °C for 90 min had equally high methylene blue adsorption capacity. Moreover, most of the AC samples subjected to 40% H₃PO₄ and 40% KOH modifications (40% ABM) had methylene blue adsorption capacities increased to higher than 350 mg/g (Figure 1E). The impregnation of the 40% H₃PO₄-modified ACs with 40% KOH had a notable increase in their methylene blue adsorption capacities, especially those ACs that activated at temperatures higher than 600 °C.

Methylene blue is a cationic dye widely existing in the wastewater of the textile industry. Methylene blue adsorption is one of the indicators for evaluating AC performance [²⁷]. The high methylene blue adsorption capacity of the Ganoderma AC denoted that the AC sample had a higher ability to adsorb industrial chemicals compared with the other ACs. The adsorption capacity is attributed to its larger pore sizes and specific surface areas. Due to methylene blue being the most commonly used dye in the fabric industry [²⁸], the ACs prepared Ganoderma residues are efficient and environmentally friendly dye-removing agents. The development of these new types of AC can also be used as catalysts.

3.2. Specific surface areas and pore sizes

Chemical modifications and extremely high-temperature activation of Ganoderma-based carbon samples effectively increased their specific surface areas. Different types of specific surface areas determined in this study were BET-specific surface area, Langmuir-specific surface area, and t-plot micropore area (Table 1). Ganoderma-based char samples had specific surface areas of less than 500 m²/g. Activation of these chars at a high temperature of 800 °C had their BET-specific surface areas increased to two times higher than the chars. The results showed that chemical modification of these carbon samples effectively increased their Langmuir-specific surface areas to over 2000 m²/g. The Langmuir-specific surface area of the industrial AC was 1510.12 m²/g. It is considered a high-performance and increased specific surface area of the ACs.

Table 1
Specific surface areas, micropore areas, and pore sizes of the carbon samples prepared from red and black Ganoderma residues.

RED GANODERMA RESIDUE
		R	800 °C -H2O	800 °C -40% AM	800 °C -40% ABM	INDUSTRIAL
BET-specific surface area (m2/g)		375.44	775.46	899.20	886.17	1,143.65
Langmuir-specific surface area (m2/g)		448.95	1,055.22	1,292.84	1,300.29	1,510.12
t-Plot micropore area (m2/g)		359.77	625.91	701.61	482.75	753.88
Cumulative pore area (m2/g)	<2 nm	51.10 (50.41%)	204.12 (21.53%)	276.36 (26.17%)	383.05 (23.03%)	750.45 (18.74%)
	2–50 nm	49.75 (49.08%)	742.68 (78.34%)	777.90 (73.68%)	1278.34 (76.86%)	3247.64 (81.11%)
	>50 nm	0.52 (0.51%)	1.26 (0.13%)	1.58 (0.15%)	1278.34 (0.11%)	5.93 (0.15%)
Adsorption average pore diameter (nm)		1.73861	2.11860	2.11860	2.40179	3.46751
BLACK GANODERMA RESIDUE
		B	800 °C -H2O	800 °C -40% AM	800 °C -40% ABM
BET-specific surface area (m2/g)		311.46	768.36	1,087.63	1,441.99
Langmuir-specific surface area (m2/g)		357.16	951.84	1,559.78	2,055.09
t-Plot micropore area (m2/g)		299.44	634.27	888.19	1,002.43
Cumulative pore area (m2/g)	<2 nm	20.10 (52.67%)	220.78 (36.20%)	291.27 (26.90%)	471.12 (24.90%)
	2-50 nm	17.65 (46.25%)	388.41 (63.69%)	790.55 (73.01%)	1419.70 (75.03%)
	>50 nm	0.41 (1.07%)	0.63 (0.10%)	0.99 (0.09%)	1.35 (0.07%)
Adsorption average pore diameter (nm)		1.67258	1.91232	1.91232	2.17879

• R: red Ganoderma char; B: black Ganoderma char; AM: acid modification (40% H₃PO₄); ABM: acid-base modification (40% H₃PO₄ + 40% KOH).

Using different carbon modifiers remarkably altered the specific surface areas of the ACs. Among the two carbon modifiers used, the ACs prepared from black Ganoderma residue had higher specific surface areas than the red Ganoderma residue. Chemical modification of black Ganoderma-based char using KOH had a better effect than that of the H₃PO₄ modification, where using KOH to modify black Ganoderma-based char had a notably higher specific surface area than the other AC samples. The BET-specific surface areas of these carbon samples were lower than the industrial AC (1143.65 m²/g), except for the black Ganoderma AC sample activated at 800 °C with 40% KOH. This carbon sample had a Langmuir-specific surface area of 2055.09 m²/g, which was higher than the value determined for the industrial AC.

The results indicated that activation temperature is one of the factors for altering the carbon-surface area. A high-temperature treatment to a carbon sample oxidized and cracked the carbon fiber and increased its specific surface area. When H₃PO₄ was used as a carbon modifier, the weak acid corroded the carbon surface and enlarged its micropores. The chemical modification of the carbon sample using weak acid alone could not further increase its specific surface area. A combination of H₃PO₄ and KOH (40% ABM) further increased the specific surface area of the carbon sample after the extremely high-temperature activation. Applying combined carbon modifiers for activating the black Ganoderma carbon sample further increased its Langmuir-specific surface area to about 500 m²/g higher than the single modifier. However, it did not affect the red Ganoderma carbon sample. It could be because the red Ganoderma char has high thermal stability. The carbon fiber of red Ganoderma residue could withstand acid corrosion better than the black Ganoderma sample. Moreover, the specific surface areas of the ACs were pore-size dependent.

The extra high-temperature (800 °C) activation remarkably increased the percentages of mesopores (2-50 nm) of the carbon samples; chemical modifications with H₃PO₄ and KOH further increased the mesopore percentage of the black Ganoderma AC (Table 1). Its mesopore percentage was lower than the industrial AC. On the contrary, the mesopore percentage of red Ganoderma AC activated at 800 °C (78.34%) without chemical modification was far higher than the black Ganoderma AC (63.69%). Chemical modification of the red Ganoderma char caused a reduction in its mesopore percentage. These Ganoderma ACs had a higher proportion of microporous structure (<2 nm). The literature showed that chemical modification of mushroom lignin using KOH was performed, where KOH reduced the energy required for decomposing the residual lignin [²⁹]. The suggested reaction pathways for using H₃PO₄ and KOH as carbon modifiers during the chemical modifications of carbon samples are depicted in Figure S1.

The difference between Langmuir and BET-specific surface areas is due to the difference in the adsorption capacity of the carbon layer calculated using the two different models. According to Langmuir’s theory, the adsorption of adsorbent molecules on the adsorbent surface is limited to a single layer [²⁸], so the adsorption amount of a single layer is the saturated adsorption amount. The BET theory explains that the adsorption of an adsorbent on the adsorbent surface is not limited to a single layer but can be multilayer adsorptions. When the adsorbent reaches the saturated adsorption capacity, the saturated adsorption capacity is greater than the monolayer adsorption capacity. The literature reported that the BET theory was unsuitable for determining the microporous surface area of a solid material [³⁰]. Although BET theory is related to the assumption that gas adsorption is multilayer, the overlapping between single and multilayers could have impaired adsorption coverage.

In this study, the Langmuir-specific surface areas of the AC samples determined were higher than the BET-specific surface areas (Table 1). Langmuir’s equation could overestimate the nitrogen adsorption on a single-layer AC surface with higher interfacial tension than those with lower surface tension [³¹]. The microporous carbon structure with higher percentages of meso and micropores has a higher surface tension than the macroporous structure, and the prepared ACs had lower percentages of macropores, especially the AC sample activated using the optimized activation conditions. Therefore, the overestimation of the Langmuir-specific surface areas of these ACs could be attributed to the meso and microporous AC structures with a higher interfacial tension. A previous report also revealed that the membrane permeability was pore size-dependent [³²]. The adsorption capacity of an adsorbate like AC samples with different pore sizes is closely related to the particle sizes of the adsorbents. Adsorbents with a high proportion of micropores have a low ability to adsorb the adsorbates of larger particle sizes [³³]. Due to the industrial AC having a higher proportion of mesoporous structure, it is an ideal adsorbate for adsorbing many types of contaminants with sizes ranging between 2 nm and 50 nm. The Ganoderma chars subjected to an extra high-temperature treatment with chemical modifications had increased mesopore proportions and thus reduced the percentages of micropores.

After chemical impregnation and extremely high-temperature activation, the AC samples prepared from red and black Ganoderma residues had physicochemical properties comparable to the industrial AC. These AC samples can adsorb chemical substances, including contaminants in water and air and cholesterol in liquid foods. The AC prepared from fruit peel had been determined for its in vitro cholesterol adsorption effect. The cholesterol adsorption capacity of the fruit peel-based AC was comparable to the industrial AC [⁵]. If the Ganoderma ACs are used as catalysts or catalyst carriers, their larger specific surface areas can provide more loading sites for chemical reactions.

3.3. FTIR data

The chemical adsorption capacity of the Ganoderma carbon samples was dependent on the surface functional groups of the carbon sample besides its specific surface area [³⁴]. The FTIR spectra of the Ganoderma chars and AC samples treated with different carbon modifiers are shown in Figure 2. The spectral peaks of these samples were mainly distributed between 3700–2700 cm^-1, 1700–1250 cm^-1, and 1250–750 cm^-1. The result showed that the spectral peaks around 3500 cm^-1 in both Ganoderma carbon samples were the OH stretching in the carbon structures. This result is supported by a previous study that it was due to the OH stretching in the lignin, cellulose, and hemicellulose structures [³⁵]. The vibration peak at 2950 cm^–1 was the stretching vibration of multiple =CH bondings, the vibration peak at 2830 cm^–1 was the stretching vibration of −OH of the carboxyl group, and the vibration peak at 2720 cm^–1 was the stretching vibration of multiple −CH bondings. The vibration peak near 1600 cm^-1 was related to the stretching vibration of C=C bondings of the carbon structures. The C=O stretching characteristic of the carboxyl group was found at the vibration peak of about 1700 cm^-1, but the IR peak was weak.

Figure 2
Fourier transform infrared (FTIR) spectra of (A) red and (B) black Ganoderma-based carbon samples. Industrial AC was used for comparison. R: red Ganoderma char; B: black Ganoderma char; AM: acid modification (40% H₃PO₄); ABM: acid-base modification (40% H₃PO₄ + 40% KOH); AC: activated carbon.

The sharp peak of the carbon samples at 1360 cm^–1 was due to the −CH₃ stretching vibrations. The small IR peak at 1400 cm^–1 could be due to the weak stretching vibrations of −CH₂. The acid-base-treated carbon sample (40% ABM) had weaker stretching vibrations of −CH₂ than the other carbon samples. The small peak at 1400 cm^-1 could be due to the weak stretching vibrations of −CH₂. The vibration peaks close to 1150 cm^–1 were either the stretching vibrations of the C−C or C−O bonds. The vibration peaks between 1000 cm^-1 and 800 cm^–1 were associated with =CH bondings, and these characteristic peaks show a higher peak intensity for the chemically modified carbon samples. These FTIR data confirmed that the carbon structure was of polysaccharide origin. The vibration peak at 770 cm^–1 could be due to the stretching vibration of C−Cl, whereas the vibration peaks close to 550 cm^–1 could be the tensile changes of the CBr or CCl bonding. As shown in Figure 2, the functional groups of the industrial AC are similar to the Ganoderma carbon samples. However, the industrial AC had a stronger vibration of OH stretching.

These oxygen-containing functional groups give the carbon surface a more negative charge density [³⁶]. Adsorption of chemicals by AC occurs through forming a donor-acceptor complex between adsorbent molecules and carbonyl groups of AC. These oxygen-containing groups act as electron donors, whereas the aromatic rings of these chemicals are the electron acceptors. After the structural modification with 40% H₃PO₄, the C=O tensile vibration of the lactone group in the carbon structure enhanced. After the KOH treatment, the C=O tensile vibrations weakened. The FTIR spectra of the red Ganoderma char showed that it contained a higher proportion of hydroxyl group than the black Ganoderma char. A high-temperature activation of the black Ganoderma char destroyed the hydroxyl groups in its carbon structure but not the red Ganoderma char. Chemical modifications of the Ganoderma chars effectively altered their functional groups. Also, AC should have oxygen-containing functional groups such as carboxyl, phenolic hydroxyl, and lactone groups.

An AC with a larger specific surface area is more likely to increase the number of available functional groups on its carbon surface. Due to the existence of oxygen-containing functional groups, AC has polar sites and non-polar sites. Since most dye molecules have polar functional groups, they can preferentially be adsorbed on the polar region of AC rather than the base sites. Therefore, identifying AC surface chemicals promoted the specific application of AC. The information on the chemical properties of carbon surfaces can be obtained using Fourier transform infrared spectroscopy.

3.4. Hydroxyl group confirmation by DPPH radical scavenging assay

DPPH radical scavenging assay has been used to determine the reducing ability of an organic substance via an electron-transfer reaction between its hydroxyl group and DPPH· [⁵]. The DPPH radical scavenging effect of the Ganoderma AC samples could be related to the functional groups on the carbon surface, such as the hydroxyl or carboxylic group. As shown in Figure 3_, the red Ganoderma carbon samples possessed DPPH radical scavenging activities, where the scavenging activities were expressed as Trolox equivalents (TE) values. The chemically modified black Ganoderma AC samples had TE values similar to the industrial AC, but not for the red Ganoderma AC samples. This result showed that the black Ganoderma ACs contained certain functional groups, where these functional groups had strong reducing capacity. The functional group with reducing ability is the hydroxyl group.

Figure 3
Trolox equivalent antioxidant capacity (mM) of the Ganoderma-based carbon samples assessed by DPPH radical scavenging assay after activation at 800 °C for 90 min. R: red Ganoderma carbon samples; B: black Ganoderma carbon samples; AM: acid modification (40% H₃PO₄); ABM: acid-base modification (40% H₃PO₄ + 40% KOH).

Compared to the FTIR data, the black Ganoderma AC samples had a higher proportion of hydroxyl group than the red Ganoderma AC samples, especially the OH from the carboxyl group. The TE values of the Ganoderma AC samples chemically modified by H₃PO₄ and KOH were significantly greater than those of non-modified AC samples (p < 0.05). Therefore, chemical modifications of these Ganoderma chars effectively altered its functional groups. Moreover, the chemically modified AC samples had a higher proportion of metal elements, especially magnesium, aluminum, and calcium. These metals are potent reducing agents [³⁷].

3.5. Microstructures and elemental composition

Microstructure images of the Ganoderma ACs treated with different carbon modifiers are shown in Figure 4 and Figure 5. The fibrous structure of the Ganoderma chars was a cross-branch network with different thicknesses (Figure 4A and Figure 5A). The carbon dendritic was smooth and flat, without notable pores. After carbonizing at 500 °C, the Ganoderma chars remained relatively intact, with little or no fiber fragmentation. After being treated with 40% H₃PO₄ and activated at 400 °C (Figure 4B and Figure 5B), the carbon surface of the Ganoderma fiber had partially broken. An increase in the activation temperature to 800 °C caused the fracture in the carbon fiber of the Ganoderma char (Figure 4C and Figure 5C). The carbon samples treated with 40% H₃PO₄ and activated at 800 °C corroded the carbon fiber further (Figure 4D and Figure 5D). The 800 °C and 40% ABM-activated Ganoderma chars had notable mesopores on their carbon surface. The carbon fiber was also severely fractured or torn (Figure 4E and Figure 5E).

Figure 4
SEM images of red Ganoderma-based carbon samples activated at different activation temperatures for 90 min with two carbon modifiers. (A) Red Ganoderma char, (B) red Ganoderma AC activated at 400 °C with 40% H₃PO₄ (40% AM), (C) red Ganoderma AC activated at 800 °C with distilled water as carbon modifier, (D) red Ganoderma AC activated at 800 °C with 40% H₃PO₄ (40% AM), and (E) red Ganoderma AC activated at 800 °C with a mixture of 40% H₃PO₄ and 40% KOH (40% ABM). The SEM images of these samples were taken at 2,000, 5,000, and 10,000 magnifications. AC: activated carbon.

Figure 5
SEM images of black Ganoderma-based carbon samples activated at different activation temperatures for 90 min with two carbon modifiers and industrial AC. (A) Black Ganoderma char, (B) black Ganoderma AC activated at 400 °C with 40% H₃PO₄ (40% AM), (C) black Ganoderma AC activated at 800 °C with distilled water as carbon modifier, (D) black Ganoderma AC activated at 800 °C with 40% H₃PO₄ (40% AM), (E) black Ganoderma AC activated at 800 °C with a mixture of 40% H₃PO₄ and 40% KOH (40% ABM), and (F) industrial AC sample. The SEM images of these samples were taken at 2,000, 5,000, and 10,000 magnifications. AC: activated carbon.

The carbon pore sizes of the Ganoderma chars might be less than 2 nm, but the pore sizes of the Ganoderma ACs were from 5 to 20 nm. The chemical activation remarkably increased the pore sizes. Due to the low SEM magnifications, the carbon mesopores were hardly seen in the SEM images. The structure of the carbon fiber of the chemically modified Ganoderma chars was severely ruptured compared to the non-chemically modified char. As shown in Figure 5F, the particles of the industrial AC are flaky and porous. Therefore, the particle structures of these Ganoderma carbon samples were markedly different from the industrial AC.

Elemental analysis of the Ganoderma carbon samples was used to determine the metal and non-metal compositions of these carbon samples (Table 2). Among the non-metal elements, carbon is the dominant element in the carbon sample, followed by oxygen. The red Ganoderma char had lower carbon content than the black Ganoderma char, but not for the oxygen content. The carbon content of the industrial AC was similar to the red Ganoderma char. On the contrary, the black Ganoderma char had the lowest oxygen content. The oxygen percentage in the carbon sample increased after the extremely high-temperature activation. The extra high temperature (800 °C) treatment caused oxygen depletion in black Ganoderma char compared with the high-temperature (400 °C) treatment. The chemical modifications did not markedly alter the carbon and oxygen content of the carbon samples after the high-temperature activation, except for the black Ganoderma AC without chemical modifications. It could be because carbon and oxygen of the carboxylic and lactone groups of the Ganoderma ACs do not decompose at 400 °C [³⁸]. After the 800 °C activation, the chemical modification reduced its carbon content.

Table 2
Elemental compositions of carbon samples prepared from red and black Ganoderma residues.

ATOMIC NUMBER	ELEMENT (%)	INDUSTRIAL	R	R-400 °C -40% AM	R-800 °C -H2O	R-800 °C -40% AM	R-800 °C -40% ABM
6	C	87.49 ± 0.54	87.22 ± 0.38	81.27 ± 0.69	81.41 ± 0.23	80.01 ± 0.92	81.47 ± 2.96
8	O	9.98 ± 0.28	11.49 ± 0.19	14.50 ± 0.69	15.68 ± 0.05	14.91 ± 0.75	15.22 ± 2.26
12	Mg	0.02 ± 0.01	0.10 ± 0.02	0.03 ± 0.02	0.05 ± 0.00	0.20 ± 0.02	0.52 ± 0.13
13	Al	0.18 ± 0.06	0.01 ± 0.01	0.02 ± 0.02	0.24 ± 0.02	0.29 ± 0.02	0.54 ± 0.21
15	P	1.97 ± 0.30	0.36 ± 0.03	3.93 ± 0.51	1.86 ± 0.13	3.54 ± 0.12	0.35 ± 0.04
16	S	0.10 ± 0.02	0.03 ± 0.01	0.05 ± 0.01	0.06 ± 0.01	0.04 ± 0.01	0.02 ± 0.01
17	Cl	0.02 ± 0.02	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.00	0.01 ± 0.01	0.01 ± 0.01
19	K	0.09 ± 0.01	0.46 ± 0.20	0.02 ± 0.01	0.04 ± 0.01	0.17 ± 0.02	0.65 ± 0.19
20	Ca	0.05 ± 0.04	0.24 ± 0.04	0.13 ± 0.02	0.45 ± 0.05	0.59 ± 0.05	0.92 ± 0.25
24	Cr	0.01 ± 0.01	–	–	0.01 ± 0.00	–	–
26	Fe	0.03 ± 0.01	0.02 ± 0.02	0.02 ± 0.01	0.16 ± 0.01	0.16 ± 0.01	0.12 ± 0.05
28	Ni	0.02 ± 0.02	0.01 ± 0.01	0.01 ± 0.01	–	0.01 ± 0.01	0.00 ± 0.01
29	Cu	0.02 ± 0.01	-	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.01
30	Zn	0.02 ± 0.01	0.02 ± 0.01	–	–	0.02 ± 0.01	0.01 ± 0.01
32	Ge	–	0.01 ± 0.01	–	–	0.01 ± 0.01	0.02 ± 0.01
33	As	–	0.02 ± 0.01	–	0.01 ± 0.00	0.04 ± 0.01	0.10 ± 0.02
34	Se	–	–	0.00 ± 0.01	–	0.01 ± 0.01	0.01 ± 0.01
48	Cd	0.00 ± 0.01	0.01 ± 0.01	0.00 ± 0.01	–	–	0.01 ± 0.01
82	Pb	0.01 ± 0.01	0.00 ± 0.01	0.00 ± 0.01	–	0.01 ± 0.01	0.02 ± 0.01
			B	B-400 °C -40% AM	B-800 °C -H 2 O	B-800 °C -40% AM	B-800 °C -40% ABM
6	C		91.89 ± 0.13	79.24 ± 0.10	85.32 ± 0.09	83.27 ± 0.62	84.56 ± 1.01
8	O		6.45 ± 0.20	17.25 ± 0.33	12.27 ± 0.14	12.71 ± 0.68	12.73 ± 0.64
12	Mg		0.14 ± 0.02	0.00 ± 0.01	0.05 ± 0.01	0.11 ± 0.01	0.23 ± 0.02
13	Al		0.07 ± 0.04	0.03 ± 0.01	0.16 ± 0.012	0.23 ± 0.04	0.34 ± 0.06
15	P		0.50 ± 0.05	3.28 ± 0.21	1.53 ± 0.058	2.88 ± 0.17	0.37 ± 0.05
16	S		0.08 ± 0.01	0.04 ± 0.01	0.07 ± 0.01	0.07 ± 0.02	0.04 ± 0.01
17	Cl		0.01 ± 0.00	0.01 ± 0.01	0.01 ± 0.01	0.02 ± 0.01	0.02 ± 0.01
19	K		0.24 ± 0.02	0.01 ± 0.01	0.02 ± 0.01	0.05 ± 0.01	0.82 ± 0.15
20	Ca		0.49 ± 0.08	0.09 ± 0.01	0.44 ± 0.04	0.50 ± 0.04	0.70 ± 0.12
24	Cr		0.00 ± 0.01	0.01 ± 0.01	–	0.01 ± 0.01	–
26	Fe		0.03 ± 0.01	0.01 ± 0.01	0.06 ± 0.01	0.11 ± 0.01	0.07 ± 0.03
28	Ni		0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.01	0.00 ± 0.01	0.01 ± 0.01
29	Cu		0.01 ± 0.01	0.01 ± 0.01	0.03 ± 0.01	0.00 ± 0.01	0.01 ± 0.01
30	Zn		0.02 ± 0.01	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.01
32	Ge		0.00 ± 0.01	–	–	–	0.02 ± 0.01
33	As		0.03 ± 0.01	0.00 ± 0.01	0.01 ± 0.01	0.01 ± 0.01	0.06 ± 0.01
34	Se		0.00 ± 0.01	–	–	0.01 ± 0.01	0.01 ± 0.01
48	Cd		0.00 ± 0.01	–	–	–	0.00 ± 0.01
82	Pb		0.01 ± 0.01	–	–	0.00 ± 0.01	0.00 ± 0.01

• R: red Ganoderma char; B: black Ganoderma char; AM: acid modification (40% H₃PO₄); ABM: acid-base modification (40% H₃PO₄ + 40% KOH).

Hydrogen, nitrogen, phosphorus, and sulfur were non-metal elements, but we did not determine hydrogen and nitrogen in the carbon samples. The reason is that glucan-based chars do not contain nitrogen unless the chars contain protein as the main component. Potassium and calcium are the alkaline earth metals in the Ganoderma carbon samples. These elements accounted for a relatively large proportion of the Ganoderma chars in addition to phosphorus, where the percentages were as high as 0.5%. Phosphorus content in the Ganoderma carbon samples was significantly increased after the extremely high-temperature activation (p < 0.05). The chemical modification of these carbon samples using 40% H₃PO₄ remarkably increased their phosphorus content. The high and extra high-temperature activations of these Ganoderma chars caused a significant decomposition of potassium, except for the chemically modified ACs using 40% KOH. The extremely high-temperature treatment and chemical modifications of the Ganoderma chars further increased their calcium content, except the Ganoderma ACs activated at 400 °C after being treated with 40% H₃PO₄. Some of the other metal elements were not notably affected by these treatments.

Among the other metal elements determined (Table 2), magnesium and aluminum were the essential metals detected in the Ganoderma carbon samples. These two metals remarkably increased in the red and black Ganoderma ACs after being activated at 800 °C and chemically modified with 40% H₃PO₄ and 40% KOH. Chromium, copper, and selenium were not detected only in the red Ganoderma char. The extra high-temperature activation of the Ganoderma of AC samples at 800 °C caused an increase in the iron and arsenic content of the carbon samples. The high-temperature (400 °C) activation of red Ganoderma char caused a loss in zinc, germanium, nickel, selenium, cadmium, and lead. Similarly, germanium, nickel, selenium, cadmium, and lead were not detected in the extra high-temperature (800 °C) activated black Ganoderma char. Only chromium was not detected in the Ganoderma ACs activated at 800 °C after chemically modified using 40% H₃PO₄ and 40% KOH (40% ABM).

The increased phosphorus content of the AC samples was probably due to the formation of phosphate-containing functional groups such as −PH₂ and −PO₃H₂. The high potassium content could also be due to the reaction of potassium hydroxide with the hydroxyl group from alcohol, phenol, carboxylate, or sulfonate. The elemental composition of the Ganoderma ACs was similar to the industrial AC; however, their percentages of metals like magnesium, aluminum, iron, and arsenic are higher than the industrial AC. The carbon content of the Ganoderma ACs was also lower than that of the industrial AC sample. A carbon activating temperature of 800 °C was the extra-high temperature because a previous study reported that alkali metal vaporized at a temperature higher than 800 °C [³⁹]. This result may contribute to the development of new carbon-based catalysts. Metal elements should be the most prominent catalyst particles. The catalytic effect of an AC is also attributed to its metal-containing active sites [⁴⁰].

3.6. XRD patterns

X-ray diffraction (XRD) patterns of the Ganoderma carbon samples are depicted in Figure 6. The two broad diffraction peaks centered at 2θ of 22° and 44° corresponded to the crystalline graphite. The diffraction patterns of the Ganoderma carbon samples were similar to the industrial AC. It indicates that the graphitization process is successful; a high graphite phase appeared in the carbon sample [⁴¹]. The appearance of the diffraction peak indicated a predominantly amorphous carbon structure for the Ganoderma carbon samples. Also, reducing the carbon crystallinity of lignocellulosic material is beneficial for pore development [⁴²]. Chemical modification has somehow altered the forms of these amorphous carbon structures. A mixture of 40% H₃PO₄ and 40% KOH (40% ABM) used to modify the ACs somehow reduced their graphite phase. The reduced carbon crystallinity was observed for the chemically modified black Ganoderma AC compared to the red Ganoderma AC sample.

Figure 6
X-ray diffraction patterns of the Ganoderma-based carbon samples and industrial AC. (A) X-ray diffraction patterns of carbon samples prepared from red Ganoderma residue, (B) X-ray diffraction patterns of carbon samples prepared from black Ganoderma residue. R: red Ganoderma char; B: black Ganoderma char; AM: acid modification (40% H₃PO₄); ABM: acid-base modification (40% H₃PO₄ + 40% KOH); AC: activated carbon.

In addition to the amorphous phase of crystalline graphite, a few sharp diffraction peaks at 10-70° are shown in Figure 6. These diffraction peaks indicated that the Ganoderma AC samples contained other crystalline compounds besides graphite crystal. The red Ganoderma AC activated at 800 °C without chemical modification had diffraction peaks at (100), (101), (102), (110), (112), and (200) planes of 21°, 27°, 37°, 39.5°, 42.5°, and 50°, respectively, indicating the hexagonal phase of the graphite crystalline [⁴³]. The diffraction peaks of the H₃PO₄-modified red Ganoderma AC (40% AM) activated at 800 °C at (111), (200), (210), (211), and (410) planes corresponded to the Bragg diffraction [⁴⁴]. Therefore, the carbon crystalline of this H₃PO₄-modified AC was in monoclinic form.

3.7. Pyrolysis characteristics and thermal stability

The thermogravimetric (TG) analysis was investigated to evaluate the thermal stability of the Ganoderma carbon samples. As shown in Figure 7, heterogeneous patterns are observed for the TGA curves of different Ganoderma carbon samples. DTA curves recorded the relationship between the mass change rate of the carbon samples and temperature. The DTA curves showed that the mass decomposition patterns of Ganoderma carbon samples varied between these samples. The increase in temperature during the drying phase dehydrated the carbon samples.

Figure 7
Thermogravimetric (TG) and differential thermogravimetric (DTG) analysis of the Ganoderma-based carbon samples and industrial AC. (A) TGA of the red Ganoderma carbon samples and industrial AC, (B) TGA of the black Ganoderma carbon samples, (C) DTG of the red Ganoderma carbon samples and industrial AC, and (D) DTG of the black Ganoderma carbon samples. R: red Ganoderma char; B: black Ganoderma char; AM: acid modification (40% H₃PO₄); ABM: acid-base modification (40% H₃PO₄ + 40% KOH); AC: activated carbon.

The carbon samples prepared from the red Ganoderma residue had a maximum mass loss rate between 60 °C and 75 °C; the carbon samples prepared from the black Ganoderma residue had a maximum mass loss rate between 55 °C and 80 °C. The maximum mass loss rate of the industrial AC reached 69 °C. The initial mass reduction occurred mainly between 30 °C and 100 °C. At a heating rate of 10 °C/min, the moisture and volatile substances adsorbed on the carbon surface evaporated. It was the reason for the first heat loss. The mass loss rates of the red Ganoderma char and AC samples activated at 800°C, at 800°C with 40% AM, and at 800°C with 40% ABM in the first stage were 11.06%, 23.98%, 31.95%, and 17.11%, respectively. The mass loss rate of the black Ganoderma char, AC samples activated at 800°C, activated at 800°C with 40% AM, and activated at 800°C with 40% ABM had a mass loss rate in the first stage of 8.84%, 21.56%, 25.01%, and 15.62%, respectively. Also, the mass loss rate of the industrial AC was 8.29%.

The second stage of mass loss occurred between 100 °C and 700 °C. The different activation conditions of the Ganoderma ACs led to higher mass loss rates. At this stage, the red and black Ganoderma carbon samples had maximum mass loss rates at temperatures lower than 590 °C (Figure 7), whereas the industrial AC had a maximum mass loss rate at 600 °C. The maximum mass loss rates of the char samples were determined at temperatures ranging between 395 °C and 467 °C. The red Ganoderma AC showed a maximum mass loss at 580 °C, whereas the black Ganoderma AC had its mass loss at 565 °C.

The red and black Ganoderma chars activated at 800 °C with 40% AM showed a maximum mass loss rate at about 590 °C. The thermal stability of the AC sample after being chemically modified with 40% ABM was higher than that of the non-chemically modified ACs. The thermal decomposition of organic components occurs during the increase in heating temperature, and the branch and side chains of oxygen-containing functional groups break at this stage. The Ganoderma chars with oxygen-containing functional groups, especially hydroxyl, carbonyl, and carboxylic groups, fully decomposed in the second stage but not C-O-C [⁴⁵]. The high temperature causes an increased level of free radicals, eventually forming stable small molecules such as CO, CO₂, and H₂O. The loss of these compounds was the main reason for mass loss in the second stage. In addition to the decomposition of organic components, these carbon modifiers might undergo chemical reactions via the suggested pathways during the thermal decomposition (Figure S2).

The mass loss rates of the red Ganoderma char and AC samples activated at 800°C, at 800°C with 40% AM, and at 800°C with 40% ABM in the second stage were 84.36%, 54.45%, 54.43%, and 54.25%, respectively. Also, the mass loss rates of these black Ganoderma carbon samples were 72.73%, 72.46%, 67.68%, and 76.99%, respectively. Moreover, the mass loss rate of the industrial AC was 74.61%.

At the third heating stage, the maximum mass loss occurred at 700-900 °C. All organic components in the carbon samples decomposed at this stage, and the mass gradually became stable. The mass loss rates of the red Ganoderma char, red Ganoderma AC sample activated at 800 °C, red Ganoderma AC sample activated at 800 °C with 40% AM, and red Ganoderma AC sample activated at 800 °C with 40% ABM in this stage were 0.15%, 2.24%, 3.26%, and 0.11%, respectively, whereas the mass loss rates of the black Ganoderma carbon samples were 0.07%, 2.80%, 4.12%, and 0.13%, respectively. At this stage, the mass loss rate of industrial AC was 0.42%.

The literature shows that the thermogravimetric method is a simple and accurate method to study the thermal stability of AC. The experimental results showed that the thermal stability of the red Ganoderma char improved after the carbon activation with and without chemical modifications. On the contrary, the thermal stability of the black Ganoderma AC samples was decreased compared with the unmodified black AC. The result was supported by a previous study that these chemical-treated AC samples had lower thermal stability than untreated AC samples [⁴⁶]. In this study, the black Ganoderma char had a higher carbon content than the red Ganoderma char. The black char also contained a lower oxygen percentage than the red char. Since the red Ganoderma char contained a higher oxygen percentage than the black Ganoderma char, it decomposed and reduced to less than 5% of its original mass at the end of the third stage. The chemical activation improved the thermal stability of red Ganoderma char, especially the char modified using 40% ABM. The acid treatment rapidly broke the carbon chains and caused a higher carbon decomposition rate at the first and third stages of mass loss but not during the second stage. The rapid carbon decomposition in the black Ganoderma AC developed using 40% AM in the first and third stages was similar to the red Ganoderma AC. The red and black AC prepared using 40% ABM had improved thermal stability, especially the red Ganoderma AC developed using 40% ABM. Adding KOH to the AC during the chemical modification enhanced the carbon structure with potassium ions. Therefore, the 40% ABM remarkably increased the thermal stability of the AC samples. The data generated from this study can provide a basis for the industrial production of Ganoderma ACs to be used as catalysts.

3.8. Catalytic performance of Ganoderma ACs

The Ganoderma carbon samples were used as catalysts for converting PMIDA to GP. As depicted in Figure 8, using Ganoderma and other ACs as catalysts significantly increased the GP yield (p < 0.05). The catalytic effect was notably higher than the control group, which is the blank group. After adding industrial AC into the self-modified mini reactor, the catalytic effect increased with the extension of reaction time. The AC prepared from passion fruit peel (PFAC) had lesser catalytic effects than the industrial AC (Figure 8A). This result could be due to PFAC having a smaller specific surface area than the industrial AC [⁵]. The catalytic effect of the industrial AC was time-dependent, but not for the fruit-based and Ganoderma carbon samples (Figure 8B and Figure 8C).

Figure 8
GP yields of the catalytic reaction using the red and black Ganoderma ACs activated using different activation conditions and carbon modifiers. (A) Comparison blank groups, (B) red Ganoderma carbon samples, (C) black Ganoderma carbon samples, (D) addition of H₂O₂ and a combination of ultrasonic (UL) and microwave (M/W)-assisted methods. Note: The blank group is without adding an AC sample during the catalytic reaction. R: red Ganoderma char; B: black Ganoderma char; PFAC: passion fruit peel-based AC; AM: acid modification (40% H₃PO₄); ABM: acid-base modification (40% H₃PO₄ + 40% KOH); GP: glyphosate; AC: activated carbon; H₂O₂: hydrogen peroxide.

Among the Ganoderma carbon samples, the GP yields obtained after adding the Ganoderma ACs chemically modified with 40% ABM and reacted for 30 min were significantly increased (p < 0.05). The GP yields of the Ganoderma chars were the markedly lowest. The possible reason for the high GP yields is that the AC sample has a higher proportion of catalytic metals. These exposed metals could facilitate the reaction. The increase in the specific surface areas of the Ganoderma ACs could also be linked to a higher proportion of the metals adsorbed on their carbon surfaces. These metals, like magnesium and aluminum, are reducing agents, which could catalyze the oxidation reaction for converting PMIDA to GP.

Due to the catalytic reaction determined at atmospheric pressure, the GP yields obtained were low. Therefore, a new method was developed to improve the GP yield. Adding H₂O₂ to the reaction chamber was aimed to increase the oxygen level in the mini reactor to support the catalytic reaction. The results showed that adding H₂O₂ solely or using only the combined ultrasonic and microwave-assisted methods did not further increase the GP yield (Figure 8D). The reaction subjected to the ultrasonic-microwave-assistant and the addition of H₂O₂ remarkably enhanced the catalyzing process using the Ganoderma ACs, and the GP yield increased up to ten times higher than the initial method. The GP yield obtained from the application of PFAC was similar to the control without using AC as a catalyst, where their GP yields were lower than 10%. The literature showed that the GP synthesis catalyzed using different carbon samples yielded specific activities ranging from 0.1 to 4.0 mmol/h/g [⁴⁷]. In this study, the calculated specific activities of the Ganoderma ACs were 2.4-7.7 mmol/h/g. The innovative catalytic method that used ultrasonic-microwave-assisted reaction with adding H₂O₂ should have far higher specific activity (>10 mmol/h/g) than the non-assisted catalytic reactions.

The modified AC can act as a catalyst or play a catalytic role in converting PMIDA to GP [⁴⁷]. The synthesis of GP was influenced by using different AC samples, reaction times, and catalytic modes. H₂O₂ is an oxidizing agent for the catalyzing process, whereas metal elements determined on the AC surface are the reducing agent. The synthesis of GP from PMIDA can be easily achieved using AC as a catalyst. The oxidation process can occur under atmospheric pressure. The reaction temperature is another catalyzing factor because the GP synthesis at a higher reacting temperature gives a lower GP yield. The reason is that prolonged oxidation of PMIDA will further oxidize the GP to aminomethylphosphonic acid (AMPA) [¹⁹]. AMPA is the oxidation product or metabolite of GP breakdown. Therefore, a short reaction of about 30 min effectively converted most of the PMIDA to GP.

The conversion of PMIDA to GP is essentially an oxidation reaction. Adding an oxidizing agent or changing the reaction condition will affect the yield of the target product. Applying microwave irradiation in chemical reactions has become popular [⁴⁸]. Added only H₂O₂ into the reaction chamber and reacting at 90 °C did not increase the GP yield because the excessive O₂ coupled with the high temperature rapidly oxidized GP further into AMPA, thus reducing GP yield. A room temperature of 25 °C and a high ultrasound intensity increased the ultrasonic vibrations and enhanced the catalyzing process [⁴⁹]. At high temperatures, the microwave also rapidly altered the orientation of the polar PMIDA molecules for rapid conversion to GP. The airtight mini-reaction chamber inside the microwave oven had an increased estimated internal pressure between 10 and 100 kPa. The literature showed that catalyzing the conversion of PMIDA to GP reached a maximum conversion rate of 95% at a pressure of 0.3 MPa and 3-h reaction time with continuous O₂ flow [⁵⁰]. The GP conversion rate of this simple and innovative method was increased to about 60%. A small proportion of PMIDA could have also directly oxidized to AMPA, thus reducing the GP yield.

4. CONCLUSIONS

The red and black Ganoderma ACs developed from their residues after the chemical modification with the acid-base solution (a mixture of 40% H₃PO₄ and 40% KOH), also known as 40% ABM, had a maximum increase in the specific surface areas, especially their Langmuir-specific surface area, with a maximum value of 2055.09 m²/g. The maximum value of their methylene blue adsorption capacity was 379.78±2.47 mg/g. These AC samples had a high proportion of mesopores, where their physicochemical characteristics were comparable to the industrial AC. The possible functional groups detected for the carbon structure of these AC samples were carboxyl, hydroxyl, and lactone groups. The chemically modified Ganoderma ACs had increased thermal stability with a higher proportion of metal elements like magnesium, aluminum, potassium, and calcium on their carbon surface. These crystalline graphite structures were amorphous.

They were the potential catalysts for catalyzing the conversion of PMIDA to GP under atmospheric pressure because of their better physicochemical characteristics. Applying an ultrasonic-microwave-assisted reaction in synthesizing GP using the chemically modified black Ganoderma AC activated at 800 °C for 90 min significantly improved the GP conversion to about 60%. The methylene blue adsorption capacity and GP catalyzing ability of these AC samples were attributed to their specific surface areas, functional groups of the carbon structure, and metal ions on the carbon surface. The AC had carboxyl, hydroxyl, or lactone groups containing oxygen. Developing these economically and highly efficient carbon-based adsorbents for removing the dye in wastewater and catalyzing the conversion of PMIDA to GP warrants new alternatives for industrial usage.

SUPPLEMENTARY MATERIAL

The following online material is available for this article:

Figure S1 - Suggested reaction pathways for using H₃PO₄ and KOH as carbon modifiers during the chemical modifications of carbon samples. HT: high temperature; C: carbonization.

Figure S2 - Suggested reaction pathways for possible reactions of the carbon modifiers during the thermal decomposition.

5. ACKNOWLEDGMENTS

This project was funded by the Science and Technology Department of Guangxi Zhuang Autonomous Region (Guike AC22080005) and the Research Start-up Fund from Guilin University of Technology (RD2000002363). The Special Funds for Guiding Local Scientific and Technological Development by the Central Government (Guike ZY22096025) is also acknowledged.

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Publication Dates

• Publication in this collection
  12 Feb 2024
• Date of issue
  2024

History

• Received
  18 Oct 2023
• Accepted
  18 Dec 2023