Abstract
Vietnam is known as an agricultural country with a variety of agricultural crops. In addition to agricultural by-products, bio-waste is the by-product from livestock waste, forestry, industry, and daily life. They affect the soil, water, and air environment by self-degradation processes in the environment. Therefore, researchers have come up with ideas for the usage of the by-products to decrease the amount of waste and minimize the environmental effects. In Vietnam, the by-products were used by researchers to produce bio-ethanol, fertilizers, composites, and activated carbon (AC). AC is one of the materials used to rapidly reduce the number of agricultural by-products by researchers. The synthesis process is optimized for the highest yield, while the physicochemical properties are also clearly analyzed through the X-ray diffraction, Fourier transform infrared, and Bacterial endotoxin testing assays. The average recorded specific surface area was about 300 m2 g−1. The functional groups and surface structure showed that the material has an amorphous structure with –OH, –CH, –CC, –C═C, –C═O groups. The AC from agricultural waste had been studied and applied to treat pollutants present in water.
1 Introduction
Vietnam is a country with a tropical climate in Southeast Asia. This country is known as an agricultural country with a variety of agricultural crops. According to the Food and Agriculture Organization (FAO), the output of agricultural products in Vietnam was about 47 million tons in 2020. Along with the economic development of the agricultural industry, the problem of agricultural waste after harvesting and preliminary processing was a matter of concern. Agricultural wastes had the ability to biodegrade in the natural environment, but it took a lot of time. While the agricultural output in Vietnam was growing and tended to increase rapidly in the coming years, it could be seen that the source of agricultural by-products in Vietnam also increased year-on-year. In addition to the agricultural by-products, bio-waste was the by-products from livestock waste, forestry, industry, and daily life [1,2], and the collection and treatment of biomass were applied only in a small amount.
The agricultural waste and bio-waste affect the soil, water, and air environment by self-degrading in the environment. The effects have been depicted in Figure 1. A large number of by-products would be decomposed in the environment after a period of time. This decomposition process produced a large amount of polluting gases such as methane, carbon oxide, carbon dioxide, nitrous oxide, and dinitrogen oxide [2,3]. They had a negative impact on the atmosphere. The soil environment was contaminated with a large amount of nutrients such as nitrogen, phosphorus, potassium, and other organic substances [2,4]. During the decomposition process, a large amount of leachate flows into rivers, streams, or groundwater, affecting the water environment [4,5]. Therefore, researchers had thought of using the by-products to decrease the waste amount to the environmental effect.
Impact of agricultural waste on the surrounding environment.
In Vietnam, by-products were applied by researchers to produce bio-ethanol, fertilizer, compost, and activated carbon (AC) [6,7,8]. AC was one of the materials used to rapidly reduce the number of agricultural by-products by researchers. Over the past decades, by-products such as rice husk ash, coir, corncob, and bamboo have been researched and used as AC [9,10,11,12,13]. But the applicability in practice was quite limited. There were also many by-products that were yet to be researched and developed. In this report, the research works in Vietnam are reviewed in general. Methods for synthesizing AC from agricultural by-products have been proposed. After the synthesis process, the physicochemical properties of the materials were also clearly synthesized through the X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Bacterial endotoxin testing (BET) analyses results. Besides, the applicability of the material was also considered and application orientation.
2 Potential of bio-waste in Vietnam
The output of agricultural products in Vietnam, according to the FAO, is shown in Figure 2a. The output of agricultural products to supply domestic and foreign markets was about 47 million tons in 2020. The main agricultural crops were rice, vegetables, fruits, or short-term crops (Figure 2b). After harvesting and transporting, a large amount of agricultural by-products were generated. This was about 10% of the total yield [1,2]. While the output of agricultural products was increasing in Vietnam, it can be seen that the source of agricultural by-products was also increasing year-on-year. They were affecting the soil, water, and air environment by self-degrading in the environment. To reduce the amount of waste affecting the environment, researchers have come up with ideas for the usage of the agricultural by-products. In the study by Thuong et al., they used pre-treated durian skins to adsorb methylene blue (MB) [14]. The functional groups identified through FTIR results include OH, CH, C═C, and CO. Besides, the adsorption capacity of the durian skin was 235.80 mg/g for MB, and was 527.64 mg/g for crystal violet (CV). The adsorption capacity of raw materials was also paid attention to by the researchers. This shows the great interest of researchers for by-products in Vietnam. In addition, Vietnamese authors had also used the orange peel, grapefruit peel, passion fruit peel, and rice husk to remove pollutants [14,15,16,17]. This shows that Vietnamese by-products were potential for interested researchers.
Production of Vietnamese agricultural products as per the FAO system (a) and percentage of main agricultural crops in Vietnam (b), data sources are reproduced from the FAO at https://www.fao.org under an open access Creative Common CC BY license.
3 Potential of AC from bio-waste in Vietnam
3.1 Physical activation methods
The physical activation method was one of the simplest AC synthesis methods used by the researchers. The conditions for synthesizing AC by the physical activation method are shown in Table 1, which shows that heat treatment process was the most used process by the researchers. There were also other methods such as hydrothermal, shaking, ultrasonic, and microwave methods. The synthesis temperature suitable for the synthesis process was proposed to be 500oC but studies were carried out from 300 to 900°C. Besides, use of nitrogen gas and the synthesis time of 120 min had also been proposed by the researchers. During the study, the inert gases used included nitrogen and oxygen. The time period tested by studies ranged from 30 to 360 min. Lots of by-products such as coconut, coffee, bamboo, and rice husk were used in the study [9,10,11].
Synthesis of AC by physical activation method
| AC material | Activator | Ref. |
|---|---|---|
| Coconut Husk | 900°C + N2 | [18] |
| Bituminous coal, bamboo, and coconut shell | 800°C + N2 + 240 min | [19] |
| Rice Husk | 133°C | [20] |
| Cashew nut shell | 850°C + N2 + 70 min | [21] |
| Cashew nut shell | 600°C + N2 + 30 min | [22] |
| Cashew nut shell | 850°C + N2 | [23] |
| Wattle bark | 500°C + O2 + 120 min | [24] |
| Mimosa plants, wattle bark, and coffee husks | 500°C + O2 + 120 min | [25] |
| Spent coffee | 500°C + 120 min | [26] |
| Eichhornia crassipes and Phragmites australis | 500°C + 120 min | [27] |
| Rice husk | 500°C + O2 + 180 min | [28] |
| Mimosa pigra | 500°C + 120 min | [29] |
| Coffee husk derived biochar | 450°C | [30] |
| Biochar from urban | 500°C + O2 + 180 min | [31] |
| Biochar production from cattle waste | 600°C | [32] |
| Sugarcane bagasse (SB) and Cassava root husks (CRHs) | 400°C + O2 + 120 min | [33] |
| CRH-derived biochar | 400°C + O2 + 120 min | [34] |
| Rice straw | 300°C + 360 min | [35] |
| Coffee husk | 350°C + 60 min | [11] |
| Bamboo | 600°C + N2 + 120 min | [10] |
| Magnetic rice straw biochar | 500°C + O2 + 120 min | [9] |
| ACHC-KOH 1:1 and ACHC-KOH 1 M | KOH + 130°C + 120 min | [36] |
| Rice straw | 600°C + N2 | [17] |
3.2 Chemical activation methods
Chemical activation was considered a method that was not of interest for most researchers. In 2021, coconut coir was synthesized with the activator NaOH to treat MB and methyl orange dyes [37], in which, NaOH was used to activate at a concentration of 25%. It can be seen that the researchers had selected the method of material synthesis. The selection helped to bring the best quality material.
3.3 Physicochemical activation methods
In recent years, physicochemical methods interested researchers and were used to synthesize AC from agricultural by-products. Common activators such as ZnCl2, KOH, NaOH, and H3PO4 were used [38,39,40,41], among which, the activators ZnCl2, KOH, and NaOH were widely used. Synthesis temperatures had also been reported to range from 400 to 800°C. The best synthesis temperature was 500oC and was used in most of the research studies. It can be seen that AC was easily synthesized by different methods, in which the activation process was used heat as the main parameter. The applicability of the material would be revealed after the assessment of the material’s physicochemical properties (Table 2).
Synthesis of AC by physicochemical activation method
| AC material | Activator | Ref. |
|---|---|---|
| SB | ZnCl2 + 500°C | [12] |
| Banana peel | KOH + 500°C + N2 | [41] |
| SB | ZnCl2 + 500°C | [42] |
| SB | ZnCl2 + N2 + 400°C | [43] |
| Corncob wastes | H3PO4 + 400°C + O2 | [13] |
| Durian shell | KOH + 600°C + N2 | [44] |
| Rice husk | K2CO3 + 600°C + N2 | [45] |
| Pomelo peels | KOH + 800°C + N2 | [8] |
| Coffee husk | NaOH + 500°C + N2 | [38] |
| Spent coffee | NaOH + 500°C + N2 | [46] |
| Spent coffee grounds | KOH + 500°C + N2 | [47] |
| Iron–manganese rice husk silica (FMRS) | NaOH + 650°C | [39] |
| Corn stalk | H3PO4 + 600°C | [40] |
| Modified biochar | AlCl3 + 500°C | [48] |
| Citrus maxima peel | KOH + 700°C + CO2 | [49] |
4 Physicochemical properties of AC from bio-waste in Vietnam
The physicochemical properties were noted, such as specific surface area, functional groups, and crystal structure. They were analyzed by methods such as BET, FTIR, and XRD. The analysis results gave a comprehensive view of AC from agricultural by-products. For AC activation with KOH, the surface area was quite low about 40 m2 g−1. The surface area was improved for AC activation with NaOH. The activator that gave the largest specific surface area was K2CO3 (1,583 m2 g−1) [45]. When activated by heat, the surface area of the material was quite stable. With an activation temperature of 500°C, the recorded surface area was about 300 m2 g−1 for various materials [24,25,38]. Besides, the material’s crystal was also recorded in 2 (theta) = 20–30°, 45°. This has shown stability in the network structure. The recorded active functional groups of the materials include OH, CO, C═C, C═O, CC, and CH. Representative functional groups for AC were well-proven by previous studies (Table 3).
Physical properties of AC from bio-waste in Vietnam
| AC source | Activator | Surface area (m2 g−1) | Functional groups | Angle of crystalline | Ref. |
|---|---|---|---|---|---|
| Banana peel | KOH | 63.5 | OH, C═O, NO, CH, CO, C═C, and C≡C | 20° and 30° | [41] |
| Corncob wastes | H3PO4 | 1,097 | OH, CH, C═C, C═O, and CO | — | [13] |
| Coconut husk | 900°C + N2 | 890 | — | 22o and 44o | [18] |
| Sugarcane bagasse | ZnCl2 | 1,500 | OH, ON, CO, and CC | 32.2o, 34.8o, and 36.1o | [12] |
| Rice husk | K2CO3 | 1583.6 | — | 22.5o and 43o | [45] |
| ZnCl2-AC | ZnCl2 | 1495.57 | OH, CO, C═O, CC, CN, ON, and CH | 20–30o | [43] |
| Wattle bark | 500°C + O2 | 393.15 | OH, CH, C≡C, C═C, C═O, and C≡N | — | [24] |
| CFBC | 500°C + N2 | 338.4 | C═O, OH, and CH | — | [38] |
| NaOH-CFBC | NaOH + 500°C + N2 | 987.9 | |||
| Mimosa plants | 500°C + O2 | 285.53 | C≡C, C═C, C═O, and C≡N | — | [25] |
| Coffee husks | 2.62 | ||||
| Wattle bark | 393.15 | ||||
| NaOH-SCG | NaOH | 116.591 | OH, CH, CC, C═C, and C═O | — | [46] |
| Magnetite-Eichhornia crassipes biochar (MECB) | 500°C | — | OH, CH, CC, C═C, C═O, CO, and FeO | 31.62o, 35.78o, 43.84o, 59.67o, 62.53o, and 74.58o | [27] |
| Magnetite-Phragmites australis biochar (MPAB) | |||||
| 2 M Al-modified biochar | AlCl3 + 500°C | 255.85 | OH, CH, C═O, and Al═O | — | [48] |
| Rice husk biochar | 500°C + O2 + 180 min | 42.22 | OH, CH, C═O, C═C, and CO | — | [28] |
| Coconut coir | NaOH | 364.22 | OH, CO, and C═O | 22.5° and 43° | [37] |
| Rice husk | 329.71 | ||||
| Silver-biochar material | KOH + 700°C + CO2 | 79.2 | OH, C═C, C═O, CH, and CO | 38.2°, 43.6°, 64.4°, and 77.2° | [49] |
| FMRS | NaOH + 650°C + 300 min | 366.1 | OH, SiO, FeO, and MnO, | 35.4°, 45.5°, and 56.5° | [39] |
| SB | 400°C + O2 + 120 min | 4.0703 | OH, CO, C═C, C═O, and CH | — | [33] |
| CRHs | 2.3831 | OH, C═C, CO, and CH |
(—) unrecognized.
5 Application potential of AC
5.1 Application in dye treatment
In recent years, AC had been applied in color treatment quite a lot. The most treated pigments are cation dyes. Langmuir and pseudo-second-order (PSO) were recorded as suitable kinetic and isothermal models. Adsorption capacities were recorded from 70 to 90 mg g−1. It can be seen that the AC’s adsorption capacity was quite good. AC was activated by KOH for a much superior adsorption capacity of 316 mg g−1 [36]. The influencing factors were also evaluated by the researchers including temperature, time, concentration, pH, and content. The pH factor affects most of the adsorption processes. The alkaline pH medium was suitable for the cation dyes adsorption process [27,36], while the acidic pH medium was suitable for anion dyes [10,33]. The temperature factor was compatible at room temperature [27]. Pigment concentration and time were recorded as 50 mg L−1 and 60 min, respectively. Besides, the combination with magnetic nanoparticles caused the adsorption process to be significantly reduced. The researchers were also interested in the desorption and reuse after the adsorption process. In a study by Huu Tap Van and colleagues, the AC materials from SB and CRHs were combined with ZnO to treat RR24 pigment. The materials had shown the potential to be reused five times [33]. It can be seen that AC has relatively high reusability, which showed the potential of using AC in organic pigment treatment (Table 4).
Optimum conditions for AC from bio-waste in dye treatment
| Adsorbent | Pollutants | Adsorption conditions | Adsorption capacity (mg g−1) | Adsorption model | Ref. |
|---|---|---|---|---|---|
| ACHC-KOH 1:1 | MB | Temperature = 30°C, pH = 7, 960 min, dosage = 1 g L−1, and C o = 50 mg L−1 | 316.46 | Langmuir and PSO | [36] |
| ACHC-KOH 1 M | MB | Temperature = 30°C, pH = 7, 960 min, dosage = 1 g L−1, and C o = 50 mg L−1 | 357.38 | Langmuir and PSO | [36] |
| MECB | MB | Dosage = 10 g L−1, temperature = 25°C, and C o = 30 mg L−1 for MB and 20 mg L−1 for RB 19 | 88.89 | Langmuir and PSO | [27] |
| RB 19 | 44.78 | Freundlich and PFO | |||
| MPAB | MB | 72.56 | Langmuir and PSO | ||
| RB 19 | 39.36 | Freundlich and PSO | |||
| Silver-biochar material (Ag-CMPB) | MB | C o = 50 mg L−1 and time = 120 min | 95.5 | — | [49] |
| MO | 51.5 | ||||
| RB | 69.3 | ||||
| SB | RR24 | C o = 250 mg L−1, dosage = 1 g L−1, pH = 3, and time = 60 min | 75.13 | Langmuir | [33] |
| CRHs | 71.13 | ||||
| Ultrasound-assisted biochar of water bamboo | RB5 | C o = 60 mg L−1, dosage = 10 g L−1, pH = 2, and time = 2,880 min | 3.486 | Langmuir and PSO | [10] |
5.2 Application in heavy metal treatment
In addition to the application in the removal of organic colorants, researchers have also noticed the application of AC in heavy metals. The contaminated water with heavy metals was commonly known for the leaking wastewater process from metallurgical factories or existed in groundwater for a long time. Heavy metal ions such as iron, copper, zinc, arsenic, mercury, lead, etc., were commonly encountered. Currently, the studies on the application of AC application in heavy metal treatment are still quite limited. This is due to the lack of equipment during the assessment process making the assessment process incomplete. Therefore, the researchers studying the effect and treatment capacity of AC on heavy metals are still limited. Studies on heavy metal treatment by AC are shown in Table 5. AC was synthesized from the banana peel, SB, durian peel, and rice husk ash. They were used to treat copper, nickel, lead, and arsenic ions. The adsorption capacity was recorded in the range of 20–30 mg g−1 at fairly high concentrations (>60 mg L−1). The neutral pH value was evaluated for the pH factor of the solution. Besides, the RSM model was used to optimize the adsorption conditions.
Optimum conditions for AC from bio-waste in heavy metal treatment
| Adsorbent | Pollutants | Adsorption conditions | Adsorption capacity (mg g−1) | Adsorption model | Ref. |
|---|---|---|---|---|---|
| Banana peel | Cu2+ | C o = 85 mg L−1, dosage = 2.4 g L−1, and pH = 6.5 | 14.3 | RSM and Langmuir | [41] |
| Ni2+ | C o = 90.3 mg L−1, dosage = 1.8 g L−1, and pH = 6.4 | 27.4 | |||
| Pb2+ | C o = 74.4 mg L−1, dosage = 0.9 g L−1, and pH = 6.1 | 34.5 | |||
| SB | Cu2+ | C o = 75.0 mg L−1, dosage = 5.1 g L−1, and pH = 6.0 | 13.24 | RSM | [42] |
| Ni2+ | C o = 22.5 mg L−1, dosage = 5.0 g L−1, and pH = 6.8 | 2.99 | |||
| Pb2+ | C o = 65.7 mg L−1, dosage = 3.4 g L−1, and pH = 6.5 | 19.30 | |||
| Durian shell | Cu2+ | C o = 61.6 mg L−1, dosage = 5.0 g L−1, and pH = 5.2 | 76.92 | RSM | [44] |
| FMRS | Arsenic | C o = 2 mg L−1, dosage = 0.5 g L−1, pH = 7, and time = 1,440 min | 20.3 | Freundlich | [39] |
5.3 Application in antibiotic treatment
In addition to the handling of pigments and heavy metals, antibiotics are also of great concern. In Vietnam, antibiotic contamination in water is caused by wastewater treatment from hospitals. The most commonly treated antibiotics were norfloxacin and tetracycline. Antibiotic adsorption capacity was about 60–100 mg g−1 for AC from coffee grounds [26,46]. The used optimal models were RSM and Langmuir. The suitable pH medium was neutral. It can be seen that the research on antibiotics with AC was quite limited. This was a money for researchers to develop more about the applicability of AC in Vietnam.
5.4 Application in wastewater treatment
In the study by Bui Quoc Lap, Corn stalk was used to treat livestock wastewater. The values of BOD, COD, pH, and total nitrogen content were studied by the author. Livestock wastewater was collected from an experimental pig farm of Nong Lam University. At the optimum conditions of contact time of 3.0 h, biochar mass of 4.0 g L−1, and pH of 9, the COD and BOD removal reached 40%. The adsorption process took place in column adsorption medium [35]. Besides, corncob and coffee grounds are also used to adsorb ammonium in water. The adsorption capacity of corncobs was recorded at 8.69 mg g−1 with a suitable Thomas model for the adsorption process. Optimum savings were observed at flow = 2 mL min−1, C o = 40 mg L−1, and H = 15.8 cm [13]. The adsorption capacity of Coffee grounds was recorded at 51.52 mg g−1 with Langmuir and PSO models describing the adsorption process. Optimal conditions were also recorded at C o = 60 mg L−1, dosage = 2 g L−1, pH = 7, and time = 30 min [47]. It can be seen that the actual application of AC to adsorb wastewater was in need of further research. This was a potential for domestic researchers to develop more applied research.
6 Conclusion
The AC has been extensively studied in recent years. AC was synthesized by using many methods, and their physicochemical properties were also analyzed. The average specific surface area was about 300 m2 g−1. The functional groups and surface structure showed that AC has an amorphous structure with –OH, –CH, –CC, –C═C, and –C═O groups. XRD results also showed the typical structures of activated carbon (2 (theta) = 20–30°C and 45°C). AC from agricultural waste has been studied and applied to treat pollutants present in water. The research focused mainly on treating pigments and heavy metals using AC, while limitedly focused in antibiotic treatment. Treatment of organic pigments showed that AC treated positive pigments better than negative ones with the compliance of Langmuir and PSO models. For heavy metal treatment, AC is not processed with an adsorption capacity of about 20 mg g−1. The processing has also been optimized using the RSM model. It can be seen that antibiotics are one of the potential research directions that need to be developed. In addition, the optimization of adsorption conditions is also a research direction that needs attention before practical application. The practical application of the as-synthesized AC in wastewater treatment has also shown significant potential.
Acknowledgments
The authors would like to thank Nguyen Tat Thanh University for supporting them to carry out this review.
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Funding information: This study was funded by Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam.
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Author contributions: N.P.T. and T.K.N.T. – writing – original draft; L.V.T. and T.C.Q.N. – data curation and investigation; N.B.H. – writing – review and editing.
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Conflict of interest: All authors have read and agreed to the published version of the manuscript.
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Ethical approval: The conducted research is not related to either human or animal use.
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Data availability statement: Not applicable.
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