Skip to main content
NCBI home page
Sage Choice logoLink to Sage Choice
. 2022 Nov 8;41(1):37–51. doi: 10.1177/0734242X221127167

Coconut shell and husk biochar: A review of production and activation technology, economic, financial aspect and application

Azrine Ajien 1,2, Juferi Idris 1,2,✉, Nurzawani Md Sofwan 3, Rafidah Husen 4, Hazman Seli 1,2
PMCID: PMC9925910  PMID: 36346183

Abstract

The coconut industry generates a relatively large amount of coconut shell and husk biomass, which can be utilized for industrial and environmental purposes. Immense potential for added value when coconut shell and husk biomass are turned into biochar and limited studies are available, making this review paper significant. This paper specifically presents the production and activation technology, economic and financial aspect and application of biochar from coconut shell and husk biomass. Pyrolysis, gasification and self-sustained carbonization are among the production technology discussed to convert this biomass into carbon-rich materials with distinctive characteristics. The surface characteristics of coconut-based biochar, that is, Brunauer-Emmett-Teller (BET) surface area (SBET), pore volume (Vp), pore diameter (dp) and surface functional group can be enhanced by physical and chemical activation and metal impregnation. Due to their favourable characteristics, coconut shell and husk-activated biochar exhibit their potential as valuable adsorption materials for industrial and environmental application including biodiesel production, capacitive deionization, soil amendment, water treatment and carbon sequestration. With the knowledge of the potential, the coconut industry can contribute to both the local and global biocircular economy by producing coconut shell and husk biochar for economic development and environmental remediation. The capital and operating cost for production and activation processes must be taken into account to ensure bioeconomy sustainability, hence coconut shell and husk biomass have a great potential for income generation.

Keywords: Coconut shell, coconut husk, biochar, production, activation, application

Introduction

Coconut shell and coconut husk biomass are generated by the coconut industry in some countries in Asia, Oceania, Caribbeans, Central and South America and West and East Africa (Perera, 2012). In 2019, Food and Agriculture Organization (FAO) reported that Indonesia (17.1 million tons), Philippines (14.8 million tons), India (14.7 million tons), Sri Lanka (2.5 million tons), Brazil (2.33 million tons), Vietnam (1.68 million tons), Mexico (1.29 million tons), Papua New Guinea (1.19 million tons), Thailand (0.80 million tons) and Malaysia (0.54 million tons) are the world’s top 10 coconut producers (FAO, 2019), which contributes to 659.12 million USD import value and 540.50 million USD export value, with 0.8902 USD/EUR exchange rate (as of 31 December 2019) (European Central Bank, 2019). Coconut waste biomass is commonly mismanaged by direct disposal and open burning, hence causing significant green energy waste (Goh, 2018), detrimental human health, environmental pollution, greenhouse effects, global warming and climate change issues (Ferronato and Torretta, 2019; Jain et al., 2014). Therefore, an alternative method of coconut waste management is highly desired.

A new practice of coconut waste biomass involved transforming coconut biomass into biochar, which is a black, carbon-rich and highly porous material with a high degree of aromatization and strong anti-decomposition properties (Lehmann and Joseph, 2009; Spokas, 2013). Pyrolysis, gasification and self-sustained carbonization are adopted to produce coconut shell and coconut husk biochar. Pyrolysis thermally degrades biomass at elevated temperature in an oxygen-free condition to produce biochar, bio-oil and synthetic gas (Lee et al., 2019; Narzari et al., 2015), whereas gasification directly converts biomass into synthetic gas and yields a small amount of biochar (Sikarwar and Zhao, 2017). Self-sustained carbonization involves the combustion of biomass while the carbonization temperature is sustained on its own during the biomass transformation into biochar (Samsudin et al., 2019). Among these thermochemical processes, pyrolysis is reportedly having a lower biochar production cost (272 USD ton−1) in comparison to gasification (380 USD ton−1) (Brown et al., 2011; Meyer et al., 2011). However, there is no record of biochar production cost for self-sustained carbonization. While mitigation of air pollutant emissions caused by biochar production technology is required to be carried out (Sekar et al., 2021), previous studies indicated that converting coconut biomass into biochar is an effective method for coconut biomass management. Coconut shell and coconut husk biochar have different proximate (i.e. moisture (MC), ash, volatile matter (VM), fixed carbon (FC)), ultimate (i.e. carbon (C), hydrogen (H), oxygen (O), nitrogen (N)) and surface characteristics (i.e. BET surface area (SBET), pore volume (Vp), pore diameter (dp), surface functional group) depending on the selection of biochar production technology. Physical activation, chemical activation and metal impregnation can enhance the surface characteristics of coconut shell biochar in terms of BET surface characteristics and surface functional groups. The characteristics of coconut shell biochar, coconut husk biochar and coconut-shell-activated biochar make these carbon-rich materials to be potentially useful for industrial and environmental purposes. The knowledge of the usefulness of biochar and activated biochar from coconut waste biomass from different production technology and activation process enables the global coconut industry to use coconut shell and coconut husk biomass as potential assets for their specific industrial development and environmental remediation. However, the cost of different biochar production technology and activation process must be considered by the coconut industry in managing coconut shell and coconut husk biomass.

Characteristics of coconut shell and coconut husk biomass

Coconut shell and coconut husk biomass are mainly composed of lignin, cellulose and hemicelluloses (Borel et al., 2021). Increasing the rate of cellulose breakdown improves the porous structure of biochar (Li et al., 2020), whereas increasing the rate of lignin breakdown contributes to the formation of biochar with a high specific area, high FC content and fine aromatic structure (Jiang et al., 2020). The composition of lignin, cellulose and hemicellulose in a coconut shell and coconut husk biomass influences the characteristics of coconut shell and coconut husk biochar.

Previous studies indicated that the proximate characteristics of coconut shell and coconut husk biomass are different (Table 1). Coconut shell biomass contains 6–10 wt% MC, 0–2 wt% ash, 72–77 wt% VM and 15–23 wt% FC, whereas coconut husk biomass contains 0–8 wt% MC, 1–5 wt% ash, 83–85 wt% VM and 15–16 wt% FC. As coconut-based biomass has high VM and low ash and FC, there are higher possibilities for coconut shell and coconut husk biomass to be transformed into biochar (Shukla et al., 2019). Furthermore, both coconut shell and husk biomass have different compositions of C, H, O and N (Table 1). Under the biomass conversion process, coconut-based biomass with high carbon content (45–51 wt%) will have a higher biochar yield. Also, coconut-based biomass with lower nitrogen content (1–2 wt%) can reduce the possibility of toxic greenhouse gas (e.g., nitrogen oxides (NO x )) emissions during the biomass carbonization process (Tripathi et al., 2016).

Table 1.

Characteristics of coconut shell and coconut husk biomass before biochar production.

Biomass feedstock Proximate analysis (wt%) Ultimate analysis (wt%) Reference
Moisture Ash Volatile matter Fixed carbon C H O N
Coconut shell 5.70 0.60 77.20 22.80 52.60 6.20 53.10 2.00 Windeatt et al. (2014)
10.1 3.20 75.50 11.10 64.23 6.89 27.73 0.77 Khuenkaeo and Tippayawong (2020)
– – – – 46.80 5.80 47.10 0.30 Millán et al. (2021)
7.85–7.93 1.34–1.53 71.50–75.93 14.61–19.31 45.20–50.50 – 46.61–52.71 1.04–1.14 Samsudin et al. (2019)
9.99 0.38 – – – – – – Nuryana et al. (2020)
Coconut husk – 0.92 82.94 16.14 47.00 6.07 46.60 0.21 Suman and Gautam (2017)
7.50 5.30 85.30 14.70 44.70 7.50 61.80 0.80 Windeatt et al. (2014)
Open in a new tab

Coconut-based biochar production technology

Pyrolysis is widely used to produce coconut shell biochar (Adorna et al., 2020; Baharum et al., 2020; Behera et al., 2020; Khuenkaeo and Tippayawong, 2020; Nuryana et al., 2020; Pituya et al., 2016; Sari et al., 2020; Windeatt et al., 2014), whereas gasification is an alternative technology used (Atienza et al., 2020; Millán et al., 2021). Self-sustained carbonization is the most current method to produce coconut shell biochar (Samsudin et al., 2019). As for coconut husk, by far, pyrolysis is the only technology selected for coconut husk biochar production (Isidoria et al., 2018; Suman and Gautam, 2017; Windeatt et al., 2014). While these production technologies can produce coconut shell and coconut husk biochar with relatively desirable characteristics for various biochar applications, pyrolysis, gasification and self-sustained carbonization have yet used heat waste to transform coconut shell and coconut husk biomass into coconut shell and coconut husk biochar (Marous, 2014).

Pyrolysis

Pyrolysis is a thermochemical process that thermally degrades biomass at elevated temperatures into biochar, bio-oil and syngas (Lee et al., 2019). This process can operate in various reactor types including a tube furnace (Adorna et al., 2020), muffle furnace (Baharum et al., 2020; Behera et al., 2020; Pituya et al., 2016; Suman and Gautam, 2017), fixed-bed reactor (Windeatt et al., 2014), microwave (Nuryana et al., 2020) or an ablative system (Khuenkaeo and Tippayawong, 2020) under different temperatures, heating rate, retention time and an inert gas flow rate. Pyrolysis is a two-stage process. In the first stage, the complex molecular bonds of lignin, cellulose and hemicellulose in the biomass undergo cleavage to form carboxyl, carbonyl and hydroxyl groups on the biochar surface (Patwardhan et al., 2011), which subsequently undergo decarboxylation, dehydration and dehydrogenation processes to form larger molecules or heavy compounds of biomass (Lee et al., 2019). In the second stage, by continuously applying thermal energy, the larger molecules or heavy compounds of biomass undergo several chemical reactions to produce biochar, bio-oil and syngas (Lee et al., 2019) such as methane (CH4), hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2) (Dhyani and Bhaskar, 2019; Lee et al., 2019; Narzari et al., 2015).

Slow pyrolysis

In slow pyrolysis, biomass is thermally degraded at 300–800°C with a 5–10°C minute−1 heating rate (Liu et al., 2015). This process takes hours or days to be thermally degraded to yield 30–40 wt% biochar, 25–30 wt% bio-oil and 25–35 wt% synthesis gas (Dhyani and Bhaskar, 2019). Slow pyrolysis is the most preferred method to produce biochar as this process exhibits the highest biochar yield in comparison to bio-oil and biogas yield (Daful and Chandraratne, 2020; Mohan et al., 2006; Tripathi et al., 2016), but this process is more costly when it requires a longer retention time and higher energy to have a higher biochar yield (Zaman et al., 2017).

Microwave-assisted pyrolysis

In a conventional heating process, heat transfer between the solid biomass feedstock and its surroundings occurs through conduction, convection and radiation, which limits the flexibility of the heating process to control the operating temperature, whereas microwave-assisted pyrolysis applies microwave radiation which involves selective and volumetric heating for the thermal degradation of biomass (Huang et al., 2016; Lo et al., 2017; Motasemi and Afzal, 2013; Shukla et al., 2019). In microwave-assisted pyrolysis, an electromagnetic field first enters to penetrate the biomass material. This causes the generation of thermal energy throughout the penetration depth by dielectric heating. The dielectric heating takes place due to the interaction with dipoles present in the biomass material, which results in volumetric heating from inside the biomass material. Microwave heating usually requires a material with high dielectric constant and microwave absorbers are applied along with the biomass to facilitate dielectric heating during the process. Microwave-assisted pyrolysis is usually conducted under an operating temperature of 400–800°C, and it is considered an innovative approach to the conventional pyrolysis process (Sahoo and Remya, 2020; Shukla et al., 2019; Vijayaraghavan, 2019).

Ablative pyrolysis

In ablative pyrolysis, the biomass is degraded by thermal energy as the biomass particles are in intimate contact with a hot solid or surface (Peacocke and Bridgwater, 1994). Ablative pyrolysis is firstly used to process wood waste by using an electrically heated wire, which forms a thin vaporizing liquid layer (Diebold, 1980). The application of ablative pyrolysis proceeds with studying the ablative heat transfer during wood pyrolysis (Lede et al., 1985; Lédé et al., 1987; Martin et al., 1986). Then, the ablative pyrolysis was applied by using multiple blades inside the reactor, which yields 67% of bio-oil (Peacocke and Bridgwater, 1994). Currently, ablative pyrolysis is used to determine biochar and bio-oil yield by using a rotating blade ablative reactor (Khuenkaeo and Tippayawong, 2020). In ablative pyrolysis, applying inert gas may not be necessary, but a costly reactor, moderate temperature and low reaction rate during the pyrolysis process are required, thus making ablative pyrolysis one of the rarest pyrolysis types applied for biochar production (Zaman et al., 2017).

Gasification

Gasification is a partial combustion process in which biomass is partially combusted at higher temperatures (600–1200°C) under 10–20 seconds retention time (Brewer et al., 2009; McKendry, 2002). As dried biomass is subjected to the gasification process by using air, CO2 or steam (H2O) as a gasifying agent, the biomass undergoes four stages of gasification: (1) drying, (2) devolatilization, (3) partial oxidation and (4) reduction (Gómez-Barea and Leckner, 2010; Richardson et al., 2015). In the first stage, the biomass is dried to reduce its MC content. In the second stage, the dried biomass undergoes devolatilization to produce tar, water, synthesis gas and biochar (Cha et al., 2016; Huang et al., 2021). At the next stage, synthesis gases, such as CO, CO2, H2 and CH4, as well as solid biochar undergo partial oxidization. Finally, biochar undergoes reduction or gasification that produces ash and more synthesis gases such as CO, CO2, CH4, N2, H2, H2O and various VM (C x H y O z ). As gasification primarily aims to change biomass into gaseous products (e.g. CO, CO2, CH4. H2 and H2S), the gasification process should not be used to produce biochar (Bridgwater, 1995; Kirubakaran et al., 2009; Puig-Arnavat et al., 2010; Sikarwar and Zhao, 2017). Despite its low biochar yields (<10%), gasification generates biochar with a highly desirable surface area and hence nominates itself as one of the biochar production technologies (Brewer et al., 2009; Millán et al., 2021).

Self-sustained carbonization

In self-sustained carbonization, the carbonization temperature is maintained by itself while the combusting biomass is changed into biochar inside the reactor (Idris et al., 2015a, 2015b; Samsudin et al., 2019). Dried biomass is fed into a self-sustained carbonization reactor, which is primarily made of high-temperature resistance bricks. Inside the reactor, the biomass undergoes the combustion process, and the brick reactor is closed to produce an oxygen-free environment in the reactor. The burning biomass used the generated heat energy to break down cellulose, hemicellulose and lignin in biomass to form a complex biochar network (Lee et al., 2019). Firstly, self-sustained carbonization was used for the production of palm kernel shell biochar (Idris et al., 2015a, 2015b). Self-sustained carbonization was then used to change coconut shell biomass into coconut shell biochar. The coconut shell biochar produced by one-step self-sustained, low-temperature carbonization technology using a pilot-scale brick reactor can produce biochar with a high BET surface area, which is suitable to produce an adsorption material (Samsudin et al., 2019).

The financial aspect of biochar production technology

The cost of pyrolysis, gasification and self-sustained carbonization is different depending on the chemical used, energy requirements and operating parameters involved in the biochar production. In pyrolysis, higher heat energy or longer retention time may be required to have a higher biochar yield with more enhanced surface characteristics, hence increasing the operational cost of the process (Dhyani and Bhaskar, 2019; Lee et al., 2017). Additionally, the involvement of nitrogen (N2) gas may add to the capital cost of a pyrolysis reactor (Narzari et al., 2015). Though an ablative reactor is costly, the operating cost of ablative pyrolysis is low due to its absence of inert gas, moderate temperature requirement and low retention time (Zaman et al., 2017). For the gasification process, steam is an economic gasifying agent for biochar production (Chan et al., 2019), but the involvement of steam and the generation of toxic gases, such as nitrogen oxides (NO x ) and sulphur oxides (SO x ), may increase the capital cost and operational cost of the gasification process (Elkhalifa et al., 2019). A novel self-sustained carbonization process involves natural gas for biomass combustion, which lowers the energy requirement for biochar production and subsequently its lower operating cost (Idris et al., 2015a, 2015b; Samsudin et al., 2019). In addition, the absence of N2 gas causes self-sustained carbonization to have low capital costs. Thus, self-sustained carbonization has lower capital and operating cost in comparison to the pyrolysis and gasification process.

Characteristics of coconut shell and coconut husk biochar

The properties of coconut shell biochar and coconut husk biochar produced by different biochar production technologies are summarized in Table 2. The highest yield can be observed when microwave-assisted pyrolysis was applied to produce coconut shell biochar (84–91 wt%) (Nuryana et al., 2020). In contrast, gasification which occurs in fluidized bed gasifiers has the lowest coconut shell biochar yield (13 wt%) as the main purpose of gasification is to change biomass into gaseous products (Millán et al., 2021). As for coconut husk biochar, pyrolysis which occurs in a top-lit updraft unit gives the highest biochar yield (45 wt%) (Isidoria et al., 2018), while the same process which occurs in a fixed bed reactor gives the lowest biochar yield (31 wt%) (Windeatt et al., 2014). Self-sustained carbonization also gives a relatively high coconut shell biochar yield (30–32 wt%) which exhibits its ability to produce a relatively large amount of coconut shell biochar.

Table 2.

Proximate, ultimate and BET surface characteristics of coconut shell and coconut husk biochar produced by different biochar production technology.

Biomass Technology (process variant) Reactor Operating conditions Yield (wt%) Proximate analysis (wt%) Ultimate analysis (wt%) BET surface characteristics Reference
Temp. (°C) Heating rate (°C minute−1) Retention
time (hour)		 Particle size (mm) Moisture Ash Volatile matter Fixed carbon C H O N SBET (m2 g−1) Vp (cm3 g−1) dp (nm)
Coconut shell Pyrolysis (slow pyrolysis) Tube furnace 550 5.0 2.0 0.074 – – – – – – – – – 97.40 0.115 – Adorna et al. (2020)
Muffle furnace 700 7.0 2.0 <1.00 – – – – – – – – – 434.83 0.174 2.23–2.44 mm Baharum et al. (2020)
300 10.0 1.0 <1.00 56.35 3.80 2.69 60.46 33.05 – – – – – – – Behera et al. (2020)
400 10.0 1.0 <1.00 44.87 4.40 4.30 52.45 38.83 – – – – – – –
600 10.0 30 minutes <1.00 25.80 2.60 5.63 35.76 56.01 – – – – – – –
Ablative system 500 7.0 minutes 8.00–10.00 – 6.86 1.66 27.19 69.47 77.19 3.40 18.06 0.10 55.69 0.25 9.13 Khuenkaeo and Tippayawong (2020)
Fixed bed reactor 600 5.0 1.0 3.35–10.0 28.20 7.10 4.10 8.10 91.90 93.90 3.00 2.60 0.40 222.50 0.150 – Windeatt et al. (2014)
Muffle furnace 300 10.0 1.0 – – – – – – 53.47 3.70 42.57 0.10 1.03 – 60.60 Pituya et al. (2016)
2.0 – – – – – – 48.32 3.93 47.56 0.01 1.03 – 95.26
3.0 – – – – – – 43.46 3.62 52.76 0.01 0.95 – 80.07
400 10.0 1.0 – – – – – – 31.84 3.08 64.87 0.02 4.01 – 48.21
2.0 – – – – – – 43.08 3.17 53.62 0.01 8.76 – 40.57
3.0 – – – – – – 54.28 3.16 42.54 0.01 11.32 – 40.30
500 10.0 1.0 – – – – – – 65.48 3.15 31.35 0.01 199.38 – 27.07
2.0 – – – – – – 64.77 2.93 32.29 0.01 347.96 – 22.06
3.0 – – – – – – 64.06 2.70 33.20 0.01 351.95 – 22.20
Pyrolysis (microwave-assisted) Microwave reactor 126–205.40 – 15 minutes 1.18–2.00 91.31 – – – – – – – – 0.199 0.0018 36.30 Nuryana et al. (2020)
20 minutes 1.18–2.00 83.88 – – – – – – – – 0.456 0.0014 12.39
Pyrolysis (unknown) Pyrolysis reactor with tar scrubber – – – – 28.00 0.26 1.36 – – – – – – – – – Sari et al. (2020)
Gasification Top lit updraft gasifier – – – – – 10.05 3.21 9.41 77.33 – – – – – – – Atienza et al. (2020)
Fluidized bed gasifier 850 20.0 1.0 1.00–3.00 13.00 – 8.20 16.20 – 79.20 1.30 11.30 0.10 1041.8 >0.34 – Millán et al. (2021)
Self-sustained carbonization Pilot-scale brick reactor 300–500 – – 130.0–150.0 30.00–32.00 – – – – – – – – 308.00 0.150 2.00 Samsudin et al. (2019)
Coconut husk Pyrolysis (slow pyrolysis) Top-lit updraft retort unit 500 – 1.0 – 45.00 16.50 10.10 15.00 75.00 79.80 2.21 7.42 0.42 77.20 – 3.91 Isidoria et al. (2018)
Muffle furnace 400 5.0–15.0 2.0 <212 μm 43.33 0.00 26.27 30.56 43.17 47.92 3.50 47.06 1.34 39.57 – – Suman and Gautam (2017)
600 36.00 0.00 21.99 20.82 57.19 58.43 3.01 37.38 1.02 120.73 – –
800 34.00 0.00 17.32 5.33 77.35 67.31 2.67 28.98 0.87 256.60 – –
1000 31.00 0.00 12.03 4.72 83.25 74.08 2.50 22.74 0.50 590.41 – –
Fixed bed reactor 600 5.0 1.0 3.35–10.0 30.80 10.40 13.50 25.10 74.90 82.60 2.70 12.80 2.40 23.20 0.04 – Windeatt et al. (2014)
Open in a new tab

Proximate and ultimate analysis shows that coconut shell and husk biochar are relatively dry, carbon-rich material with a considerable amount of hydrogen, oxygen, nitrogen and any inorganic material that exists on their biochar surface. The increment of FC and elemental carbon occurs when coconut shell biomass undergoes pyrolysis or gasification to be transformed into coconut shell biochar (Millán et al., 2021; Pituya et al., 2016). A similar trend can be seen for FC and elemental carbon when coconut husk biomass was thermally decomposed into coconut husk biochar (Suman and Gautam, 2018). Furthermore, carbon, hydrogen and oxygen explain the existence of different surface functional groups on the surface coconut shell biochar and coconut husk biochar produced by pyrolysis and self-sustained carbonization such as carboxyl acid (–COOH), esters (–C=OOR’), aromatic, ketone (C=OR’), iso-cyanide (–C–CH3–NC), hydroxyl (–OH), aryl ethers, quinone, organosilicon compounds (Si–O), alkyl (–CH3), amide (–C=O–N) and alkyl ether (–O–CH3) group (Nuryana et al., 2020; Samsudin et al., 2019; Sari et al., 2020; Suman and Gautam, 2017). Moreover, a considerable amount of ash found in both coconut shell and husk biochar produced by pyrolysis (Behera et al., 2020; Suman and Gautam, 2017) and gasification (Atienza et al., 2020; Millán et al., 2021) indicates the presence of minerals such as carbonates, oxides or phosphates of alkali or alkaline earth metals. As coconut biomass is changed into coconut biochar, alkali-earth elements, such as potassium (K) and sodium (Na), possibly nucleate, condense and coagulate together to form a certain amount of ash on the biochar surface (Jia and Lighty, 2012). Surface functional groups and minerals are important features of biochar that act as adsorption material (Gwenzi et al., 2021).

Another important feature of coconut shell and coconut husk biochar is the BET surface characteristics. BET surface area, pore volume and pore diameter of biochar are important to identify the adsorption capacity of biochar (Narzari et al., 2015). Commonly, the BET surface characteristics of coconut shell and husk biochar are greatly depending on the operating parameters of the pyrolysis process. As pyrolytic temperature increases, the BET surface area of coconut shell biochar and coconut husk biochar increases which in turn increases their porosity (Pituya et al., 2016; Suman and Gautam, 2017). Furthermore, at a certain temperature, the BET surface area of coconut shell biochar increases as the retention time for the pyrolysis process increases (Pituya et al., 2016). Moreover, the BET surface characteristics of biochar are also affected by selected biochar production technologies. A gasification system that uses steam as the gasifying agent causes coconut shell biochar to have the highest BET surface area and pore diameter (Millán et al., 2021). Coconut shell biochar which is produced by self-sustained carbonization also exhibits its potential as an adsorption material (Samsudin et al., 2019).

Activation of coconut-based biochar

Biochar activation involves the application of physical and chemical agents to improve the properties of characteristics of biochar in terms of BET surface characteristics and surface functional group (Sajjadi et al., 2018, 2019). The chemical reactions that occur during biochar activation also can be used for nutrient recovery (Marou and Gavurov, 2022; Stavkova and Maroušek, 2021). Coconut shell and coconut husk biochar activation are carried out to improve their porous structures by ash and uncarbonized biomass removal (Bushra and Remya, 2020). Therefore, the improvement of coconut-based biochar is carried out by employing several activation methods (Sajjadi et al., 2018, 2019) including physical activation (Guo et al., 2009; Koltowski et al., 2016), chemical activation (Adorna et al., 2020; Baharum et al., 2020; Prauchner and Rodríguez-Reinoso, 2012) and metal impregnation (Prauchner and Rodríguez-Reinoso, 2012).

Physical activation

Physical activation involves applying air, steam (H2O) and CO2 as activation agents (Guo et al., 2009; Koltowski et al., 2016; Sudaryanto et al., 2006). Physical activation is a favourable activation method as it is less costly and more environmentally friendly as compared to chemical activation (Bushra and Remya, 2020). Moreover, steam uses less energy than CO2 for biochar activation, hence making the gasification rate in steam activation higher than in CO2 activation (Fan et al., 2013; Hernández-Montoya et al., 2012; Nabais et al., 2008).

Steam activation

Steam activation applies to steam or water vapour as an activation agent (Koltowski et al., 2016; Sudaryanto et al., 2006). This process revolves around a series of partial gasification of carbon-rich material (Sajjadi et al., 2019). During partial gasification, some carbon atoms (C f ) on the biochar surface are exposed to steam (H2O) molecules, which eliminates the VM and forms surface oxides (C(O)) on the biochar surface. Meanwhile, some C f exposed to steam is changed into CO2 and H2 gas. From these gasification steps, some CO2 and H2 gas partially devolatilize other C f on the surface of biochar to produce CO and CH4.

In this series of steam activation reactions, the number of new pores and the size of existing pores increase due to the removal of uncarbonized biomass and trapped products which exist during biochar production (Dalai and Azargohar, 2007; Santos et al., 2015). Furthermore, the number of aromatic compounds will be greater and a few oxygenated functional groups are also developed on the biochar surface (Sizmur et al., 2017).

CO2 activation

Biochar is subjected to partial gasification by CO2 gas (Sudaryanto et al., 2006) which reacts with the biochar available amorphous carbon in a limited oxygen atmosphere to produce C(O) and CO gas (Bushra and Remya, 2020; Sajjadi et al., 2019). In turn, C(O) is desorbed from the biochar porous surface, which causes further development of the existing porous structure (Sajjadi et al., 2019; Sizmur et al., 2017). During further development, CO gas is absorbed by the carbon active site on the biochar surface, which retards the partial gasification by CO2 gas (Sajjadi et al., 2019).

Chemical activation

Chemical activation involves the application of acid or alkali reagents to improve the properties of biochar. Both acid and alkali activation can improve or develop the porosity, pore volume, surface area and FC by eliminating any impurities found on the biochar surface, such as mineral ash, metals, organic matter and volatile carbon (Bushra and Remya, 2020; Gao et al., 2020; Sajjadi et al., 2018; Wang and Wang, 2019). In addition, acid and alkali activation can improve biochar surface properties by the addition of surface functional groups. In acid activation, acid reagents introduce acidic surface functional groups, including the carboxyl group (–COOH), which improve the hydrophilicity of biochar (Liu et al., 2012; Yang et al., 2019; Zhao et al., 2017). Furthermore, acid activation removes oxygen, sulphur and hydrogen from biochar, thus increasing aromaticity and loss of aliphatic character which is promoted at lower activation temperatures (Wang and Wang, 2019). Alkali activation adds hydroxyl functional group (–OH) to the biochar surface (Sizmur et al., 2017; Vijayaraghavan, 2019), which improves biochar capacity as an adsorption material (Yang et al., 2019).

Acid activation

H3PO4 activation

There are six aspects of phosphorus acid (H3PO4) activation on biochar namely dehydration and elimination, swelling role, acceleration carbonization, framework role, oxidation and aromatic condensation (Danish and Ahmad, 2018; Gao et al., 2020; Olivares-Marín et al., 2006; Puziy et al., 2020; Shi et al., 2019; Z. Zhang et al., 2020). During the H3PO4 activation, the H3PO4 changes into the P2O5 compound. Subsequently, P2O5 reacts with carbon on the biochar surface to change the biochar pore structure and generate P4 and CO2 gas.

Alkali activation

KOH activation

In potassium hydroxide (KOH) activation, carbon (C f ) on the biochar surface reacts with KOH to produce metallic potassium (K), potassium carbonate (K2CO3) and H2 gas (Sajjadi et al., 2018). Meanwhile, potassium-containing constituents (K2O) react with the C f to produce K, K2CO3 and H2 (Gao et al., 2020). Then, the K2CO3 reacts with carbon on the biochar surface to produce another metallic K, CO and CO2, hence changing the BET surface area, pore volume and pore diameter of biochar (El-Hendawy, 2009; Foo and Hameed, 2012; Lillo-Rodenas et al., 2003; Lozano-Castelló et al., 2007; Otowa et al., 1997).

NaOH activation

In sodium hydroxide (NaOH) activation, C f reacts with NaOH to produce metallic sodium (Na), sodium carbonate (Na2CO3) and H2) gas (Sajjadi et al., 2018). The generated Na2CO3 reacts with C f to produce more metallic sodium (Na), sodium oxide (Na2O) and CO2. Meanwhile, Na reacts with CO2 to produce more Na2O and CO during the NaOH activation. These reactions create new pores, open previously inaccessible pores, widen the existing pores and merge existing pores due to pore wall breakage (Foo and Hameed, 2012; Yang et al., 2010).

Metal impregnation

In metal impregnation, metal ions from metal salts or metal oxides, such as magnesium (Mg), improve the biochar properties (Wang and Wang, 2019). Biochar activated by metal impregnation has nano-crystals of metal oxides and biochar matrix that enhance the adsorption capacity, magnetism and catalytic performance of biochar (Wang and Wang, 2019; Zhang et al., 2012).

MgCl2 activation

During magnesium chloride (MgCl2) activation, biomass is firstly loaded with MgCl2 solution, which then is carbonized to produce activated biochar (Liu et al., 2013). In activating biochar, MgCl2· 6H2O undergo dehydration and decomposition to form magnesium oxides (MgO) on the surface of biochar.

Effect of activation method on coconut-based biochar properties

Physical or chemical activation can influence the surface characteristics of coconut-based biochar (Table 3). During H3PO4 (acid) activation, the BET surface area (434.83–508.07 m2 g−1) and pore volume (0.174–0.203 cm3 g−1) in coconut shell biochar increases (Baharum et al., 2020). The effect of activation temperature on mesopore and micropore volume of coconut shell biochar during H3PO4 activation is also reported (Prauchner and Rodríguez-Reinoso, 2012). Under impregnation ratio of H3PO4: biochar of 0.30 and 2 hours activation time, the mesopore (0–0.37 cm3 g−1) and micropore (0–0.74 cm3 g−1 of coconut shell biochar change as activation temperature increases from approximately 150–600°C. As the impregnated ratio increases from 0 to 0.92, the maximum mesopore and micropore volume is 1.04 and 0.69 cm3 g−1, respectively (Prauchner and Rodríguez-Reinoso, 2012). In KOH (alkali) activation, the porous structure in coconut shell biochar improves as BET surface area increases from 97.40 to 486.00 m2 g−1 and pore volume increases from 0.115 to 0.236 cm3 g−1 (Adorna et al., 2020). Some studies, however, found that chemical activation does not necessarily enhance the porous structure. During NaOH activation, BET surface area and pore volume decrease from 434.83 to 405.98 m2 g−1 and 0.174 to 0.162 cm3 g−1, respectively (Baharum et al., 2020).

Table 3.

BET surface characteristics of coconut shell biochar before and after physical activation, chemical activation and metal impregnation.

Activation type Activation agent Technology Reactor Carbonization condition Activation Condition Yield (%) Before activation After activation Reference
Temp. (°C) Heating rate (°C minute−1) Retention time (h) Particle size (mm) Temp. (°C) Heating rate (°C minute−1) Activ. time (hours) Flow rate (mL minute−1) Impreg. Ratio (chem.: biochar) SBET (m² g−1) VP (cm³ g−1) dp (nm) SBET (m² g−1) VP (cm³ g−1) dp (nm)
Physical Carbon dioxide (CO2) Pyrolysis Tubular reactor 600 10.0 2.0 <2.00 750 – 4.0 600 – 72.00 186.00 0.268 – 613.00 0.437 – Guo et al. (2009)
800 64.67 860.00 0.590 –
850 50.00 1305.00 0.689 –
900 29.33 1391.00 0.838 –
950 11.33 1323.00 0.878 –
900 2.0 58.67 724.00 0.506 –
3.0 45.33 1075.00 0.744 –
5.0 24.00 1653.00 1.089 –
6.0 17.33 1331.00 0.819 –
4.0 200 38.67 1013.00 0.671 –
400 34.00 1192.00 0.768 –
800 23.33 1700.00 1.135 –
1000 10.67 1360.00 0.928 –
Steam (H2O) Fluidized bed reactor (activation reactor) 650 – – – 800 10.0 78 minutes 0.60 – – 3.10 0.0009 1.20 626.8 0.3361 1.70 Koltowski et al. (2016)
Chemical Phosphoric acid (H3PO4) Tubular furnace 850 2.0 2.0 2.00–2.83 150.65–573.86 1.0 2.0 – 0.30 – – – – – Vmeso., 0.00–0.37 – Prauchner and Rodríguez-Reinoso (2012)
150.65–575.49 1.0 2.0 – 0.30 – – – – – Vmic., 0.00–0.74 –
500 1.0 2.0 – 0.01–0.91 – – – – – Vmeso., 0.00–1.04 –
500 1.0 2.0 - 0 – 0.92 – – – – – Vmic., 0.00–0.69 –
Muffle furnace 700 7.0 2.0 <1.00 80 – 1.0 – – – 434.83 0.174 – 508.07 0.203 – Baharum et al. (2020)
Zinc chloride (ZnCl2) Tubular furnace 850 2.0 2.0 2.00–2.83 267.04–652.07 1.0 2.0 – 0.40 – – – – – Vmeso., 0.06–0.13 – Prauchner and Rodríguez-Reinoso (2012)
269.11–654.14 1.0 2.0 – 0.40 – – – – – Vmic., 0.31–0.66 –
500 1.0 2.0 – 0–1.20 – – – – – Vmeso., 0.00–0.79 –
500 1.0 2.0 – 0–1.20 – – – – – Vmic., 0.12–0.70) –
Potassium hydroxide (KOH) Tube furnace 550 5.0 2.0 0.15 675 5.0 4.0 – 2:1 – 97.40 0.115 – 486.00 0.236 – Adorna et al. (2020)
Sodium hydroxide (NaOH) Muffle furnace 700 7.0 2.0 <1.00 80 – 1.0 – – – 434.83 0.174 – 405.98 0.162 – Baharum et al. (2020)
Open in a new tab

The effect of metal impregnation for coconut shell biochar activation is also reported (Prauchner and Rodríguez-Reinoso, 2012). Under impregnation ratio, ZnCl2: biochar of 0.40 and 2 hours activation time, as the activation temperature increases from 267.04 to 652.07°C, the mesopore volume range of the coconut-shell-activated biochar is 0.06–0.13 cm3 g−1, whereas the micropore volume range of the coconut-shell-activated biochar is 0.31–0.66 cm3 g−1 when temperature increases from 269.11 to 654.14°C. Under activation temperature of 500°C and 2 hours of activation time, the range of mesopore volume and micropore volumes are 0–0.79 and 0.12–0.70 cm3 g−1, respectively, as the impregnation ratio increases from 0 to 1.20.

The effect of CO2 temperature, CO2 flow rate and activation time on the characteristics of coconut shell biochar produced by pyrolysis is also reported (Guo et al., 2009). Before activation, the coconut shell biochar has a 186 m2 g−1 BET surface area and 0.268 cm3 g−1 pore volume. After activation, the pore characteristics change significantly where the BET surface area and pore volume increase to 613.00 m2 g−1 and 0.437 cm3 g−1, respectively, after CO2 activation at the temperature of 750°C and 4 hours activation period. Furthermore, the BET surface area and pore volume increase as activating parameters increase. For example, the BET surface area increases from 1013 to 1360 m2 g−1 when the CO2 flow rate increases from 200 to 1000 cm3 minute−1. Applying CO2 for biochar activation is proven to be effective as CO2 activation can bring significant changes in coconut-based biochar pore characteristics. Steam activation can improve the pore characteristics of biochar from coconut biomass feedstock. A significant increase in BET surface area (3.10–626.80 m2 g−1), pore volume (0.0009–0.3351 cm3 g−1) and pore diameter (1.20–1.70 nm) is reported after employing steam activation on coconut shell biochar produced by pyrolysis process (Koltowski et al., 2016).

Cost estimation for physical and chemical activation process on coconut-based biochar

In estimating the cost of the physical and chemical activation process, capital cost, fixed capital investment, operating cost and net present value must be taken into consideration (Lai and Ngu, 2020). For physical activation, CO2 and steam activation have similar annual capital costs (1.63 USD million), fixed capital investment (2.12 USD million), operating cost (1.22 USD million) and net present value (1.54 and 1.53 USD million). As for chemical activation, the different chemical has different cost estimation. All chemical activation has an identical capital cost (4.88 USD million) and fixed capital investment (6.32 USD million). Furthermore, the operating cost of KOH (2.82 USD million) and NaOH (2.92 USD million) is lower than the operating cost of H3PO4 (3.09 USD million). However, the net present value of H3PO4 activation (2.93 USD million) is higher than the net present value of KOH (2.09 USD million) and NaOH (2.39 USD million). As for metal impregnation, the cost estimation for MgCl2 may be evaluated based on the cost estimation of ZnCl2 impregnation, in which the capital cost, fixed capital investment, operating cost and net present value are 4.88, 6.32, 3.11 and 2.97 USD million, respectively. By comparison, the capital cost (1.63 USD million) and fixed capital investment (2.12 USD million) of physical activation are lower than the capital cost (4.88 USD million) and fixed capital investment (6.32 USD million) of chemical activation. Furthermore, the net present value of chemical activation is higher (2.10–2.97 USD million) than the net present value of physical activation (1.53–1.54 USD million). The lower net present value, capital cost and fixed capital investment cause physical activation to be more desirable in activating coconut shell biochar for surface enhancement.

Application of coconut-based biochar

Biochar from various types of biomass feedstocks can be used for 55 useful applications in both the industrial and environmental sectors (Schmidt, 2012). In industrial applications, biochar can be used in the building industry as a low-cost cement substitute (Maroušek et al., 2020) and energy source (Mardoyan & Braun, 2016). In environmental applications, biochar can be used as a soil conditioner, fertilizer and water treatment material (Braghiroli et al., 2018). The usefulness of biochar for industrial and environmental applications is due to the biochar surface characteristics (Bushra and Remya, 2020). The surface characteristics of coconut shell and coconut husk biochar exhibit their potential as adsorption material for industrial application and environmental remediation. The industrial applications of coconut shell biochar include biodiesel production (Behera et al., 2020) and capacitive deionization (Adorna et al., 2020), whereas environmental application of coconut shell and coconut husk biochar involves soil amendment (Millán et al., 2021; Pituya et al., 2016; Suman and Gautam, 2017; Windeatt et al., 2014), water treatment (Nuryana et al., 2020) and carbon sequestration (Windeatt et al., 2014). However, any literature regarding wider applications of coconut-based biochar and activated biochar, such as gas emission treatment, heavy metal contaminants and building materials, is still scarce, and thus the potential of coconut shell biochar, coconut husk biochar and coconut-shell-activated biochar for the various applications is required to be widely explored.

Industrial application

Biodiesel production

Coconut shell biochar, activated by sulphuric acid (H2SO4) produced via pyrolysis, was subjected to sulfonation to evaluate its suitability as a catalyst for transesterification (Behera et al., 2020). The maximum sulfonic density achieved (0.35 mmol g−1) as coconut shell biochar was pyrolysed at 300°C, and the sulfonic density declined (0.12 mmol g−1) as the carbonization temperature rises to 600°C. As carbonization temperature increases, the number of acidic functional groups and the number of acidic sites decreases, which resulted in the reduction of sulfonic density (Cheng and Li, 2018; Konwar et al., 2019). Nevertheless, this data shows that coconut shell biochar exhibited its potential as a catalyst for biodiesel production.

Capacitive deionization

Similarly, coconut shell biochar, activated by KOH produced via pyrolysis, was subjected to an indirect co-precipitation method with magnesium dioxide (MnO2) to prepare coconut shell-derived activated biochar-MnO2 nanocomposite (Adorna et al., 2020). During the preparation, the BET surface area of coconut-shell-activated biochar decreases from 486 to 304 m2 g−1. The activated biochar-MnO2 nanocomposite derived from coconut shell, however, has a relatively high specific electrosorption capacity (33.90–68.40 mg g−1) at 1.2 V, which is caused by its high mesopore volume ratio, high capacitance retention, good hydrophilicity and suitable pore texture which shorten the diffusion distance of ions (Adorna et al., 2020). This finding indicates the coconut shell activated biochar is suitable for electrochemical applications material such as water purification, desalination, and energy storage.

Environmental application

Soil amendment

Soil amendment is a common environmental application of coconut shell and coconut husk biochar. This is due to their favourable characteristics such as cation exchange capacity (CEC), water-holding capacity (WHC), nutrients composition, mineral release rate and pH value. At certain pyrolytic parameters, coconut shell biochar has 61.23 cmol kg−1 maximum CEC and 75.30% maximum WHC (Pituya et al., 2016). Biochar with high CEC and WHC can maximize plant nutrient uptake, improve root development and enhance soil fertility which can maximize crop production (Hansen et al., 2016). In addition, coconut shell and coconut husk biochar contain a relatively high concentration of essential minerals including phosphorus (P), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), molybdenum (Mo), zinc (Zn), manganese (Mn), copper (Cu), nickel (Ni), iron (Fe) and silicon (Si) (Millán et al., 2021; Windeatt et al., 2014). Coconut shell biochar produced by the steam-gasification process contains 20393.80 mg kg−1 of K, 4188.70 mg kg−1 of Na, 387.70 mg kg−1 of Ca, 274.00 mg kg−1 of P, 954.60 mg kg−1 of Si and 561.20 mg kg−1 of Cu (Millán et al., 2021). Additionally, the amount of P, K, Ca and Mg is higher in coconut husk biochar in comparison to coconut shell biochar produced by pyrolysis (Windeatt et al., 2014). Meanwhile, the coconut husk biochar produced by pyrolysis at 8000°C contains C, Na, Al, Si, Cl and K (Suman and Gautam, 2017). Moreover, coconut shell biochar has relatively high mineral release under neutral condition, in which coconut shell biochar from the gasification process release approximately 99% of K, 80% of P, 20% of Ca, 10% of Mg, 75% of Na and 35% of Si to the soil under pH 7 (Millán et al., 2021). Higher mineral concentration or mineral release rate indicates higher CEC of coconut shell and coconut husk biochar, which promote higher nutrient uptakes for plant growth. Coconut-based biochar is alkaline. The pH value of coconut shell and coconut husk biochar produced by pyrolysis is 8.50 and 9.60, respectively (Windeatt et al., 2014), whereas the pH value of coconut shell biochar produced by gasification is 10.20 (Millán et al., 2021). The higher pH value of coconut-based biochar promotes higher microbial activity, higher organic matter mineralization, higher plant nutrient availability and higher acid neutralization (Millán et al., 2021; Windeatt et al., 2014). Steam activation can improve the performance of coconut shell biochar in soil remediation. A significant increase in pore characteristics of coconut shell biochar by steam activation promotes the significant increment of the removal efficiency of freely dissolved Cfree (10–84%) and bioaccessible fraction Cbioacc (50–99%) of polycyclic aromatic hydrocarbons (PAHs), hence effectively decrease the leachate toxicity in soil (Koltowski et al., 2016).

Water treatment

Water treatment is another environmental application for coconut-based biochar (Nuryana et al., 2020). Coconut shell biochar produced by microwave-assisted pyrolysis has different methylene blue removal efficiency and adsorption capacity at different pH, adsorbent dosage, retention time and chemical agent. Nuryana et al. (2020) evaluated the effect of retention time on the performance of the coconut shell biochar to remove methylene blue from water. At 550 W, the adsorption capacity of the coconut shell biochar decreases from 0.6875 (removal efficiency = 55.00%) to 0.5165 mg g−1 (removal efficiency = 41.32%) as the carbonization time increases from 15 to 20 minutes. Baharum et al. (2020) had evaluated the effectiveness of biochar and activated biochar from coconut shell in removing diazinon from water at different pH values. Under pH 3 and 2 hours contact time, 1.0 g L−1 of coconut shell biochar achieved the highest diazinon removal efficiency (92.16%) at pH 3, which was slightly higher than at pH 5. The diazinon removal efficiency increases at pH 7 but declines as the pH value of the treating water increases to 9. Meanwhile, coconut shell biochar activated by H3PO4 (84.55%) and NaOH (87.93%) achieved their maximum diazinon removal efficiency at pH 7. Baharum et al. (2020) also evaluated the effect of adsorbent dosage (1.0–10.0 g L−1) on the diazinon removal efficiency by coconut shell biochar and H3PO4- and NaOH-activated biochar from coconut shell. At 2.0 g L−1 of adsorbent dosage, coconut shell biochar, coconut shell biochar activated by H3PO4 and coconut shell biochar activated by NaOH exhibited more than 80–90% of diazinon removal efficiency. While increasing adsorbent dosage does not always result in higher removal percentage of diazinon compound, the removal efficiency of diazinon by coconut shell biochar (98.28%) and coconut shell biochar activated by NaOH (97.95%) became higher at 10.0 g L−1. Baharum et al. (2020) also evaluated the adsorption capacity of coconut shell biochar and the H3PO4- and NaOH−-activated biochar from coconut shell. Under pH 7, coconut shell biochar (5.85 mg g−1), H3PO4−-activated biochar from coconut shell (5.47 mg g−1) and NaOH--activated biochar from coconut shell (5.69 mg g−1) achieved their highest adsorption capacity in removing diazinon during the water treatment process.

Carbon sequestration

Coconut shell and coconut husk biochar also show their ability for carbon sequestration, which is defined as a process of capturing, and storing CO2, which causes global climate change and greenhouse effects, from the atmosphere (USGS, 2019). The theoretical potential of coconut shell and husk biochar for carbon sequestration was made to predict the amount of CO2 that can be stored by coconut shell and coconut husk biochar (Windeatt et al., 2014). According to the prediction, 4.90 million tons of coconut shell biochar and 2.50 million tons of coconut husk biochar can store up to 9.90 million tons and 2.90 million tons of atmospheric CO2, respectively.

Economic aspects of coconut-based biochar application

The information regarding the economic aspects and concerns of coconut-based biochar application is still limited. The biochar production mainly in rural areas can help develop the region while assisting small and medium-sized industries to produce sufficient energy, improve farmer income and give solutions for coconut waste management (Amalina et al., 2022). Besides, despite market price uncertainty, a review of biochar prices indicated that the biochar market is still beneficial for the economy (Campbell et al., 2018). In 2014, a survey conducted by International Biochar Initiative indicates that the cited price of wood biochar sold by U.S. biochar sellers and global biochar sellers are 2.47 and 2.06 USD kg−1 (Tomlinson and Jirka, 2014). These data prove that biochar production, activation and application also can be financially beneficial for the coconut industry, which in turn can contribute to the growth of the global economy.

Safety concerns and negative impacts of coconut-based biochar application

In industrial and environmental applications, several negative impacts of coconut-based biochar application, though found to be scarce, must be addressed. Coconut shell biochar, coconut husk biochar and coconut-shell-activated biochar contain small particles, which if inhaled, irritate the eyes, skin and respiratory system (Ravi et al., 2016). While coconut-shell-activated biochar has higher PAH removal efficiency (Koltowski et al., 2016), the contaminant from biochar may cause a detrimental effect on human health (Zhang et al., 2019). Therefore, biochar must be pretreated for PAH removal. Besides, applying coconut shell and coconut husk biochar for soil amendment (Millán et al., 2021; Suman and Gautam, 2017; Windeatt et al., 2014) may cause negative alteration to soil properties and soil biota (Zhang et al., 2019). In terms of soil properties, coconut shell and coconut husk biochar may not necessarily improve soil porosity due to the stimulation of microbial activity which causes pore blockages (Mukherjee et al., 2014). As for soil biota, applying coconut-based biochar to soil may change the native soil biota, which causes negative responses of the earthworm and hence retarding the plant growth (Lehmann et al., 2011; Li et al., 2011; Tammeorg et al., 2014). While coconut shell and coconut husk biochar were predicted to store a relatively large amount of CO2 gas (Windeatt et al., 2014), coconut-based biochar may also cause greater CO2 emissions, which is caused by the abiotic release of inorganic carbon, the decomposition of labile components of biochar and the decomposition of organic matters by biochar (Mukherjee and Lal, 2014; Spokas and Reicosky, 2009). As for water treatment, applying coconut shell biochar for water treatment to remove specific contaminants causes biochar to have contaminants with high concentrations (Hossain et al., 2011; Nuryana et al., 2020). With these possible negative impacts, an elevated level of coconut-based biochar amendment must be taken into consideration and require further assessment (Table 4).

Table 4.

Application of coconut shell biochar, coconut husk biochar and coconut-shell-activated biochar for industrial and environmental purposes.

Application Technology (process variance) Operating temperature (time) Activation temperature (time) Findings Reference
Biodiesel production (catalyst) Pyrolysis (slow pyrolysis) 300–600°C (30 minutes to 1–hour) – • Coconut shell biochar achieved its maximum sulfonic density (0.45 mmol g−1) at 300°C, and the sulfonic density decline (0.12 mmol g−1) at 600°C, may indicate its potential as a catalyst for transesterification to produce biodiesel. Behera et al. (2020)
Capacitive deionization Pyrolysis (slow pyrolysis) 550°C (2 hours) KOH activation
675°C
(4 hours)		 • As an electrode material, coconut shell-derived activated biochar-MnO2 nanocomposite prepared by indirect co-precipitation has a relatively high specific electrosorption capacity (33.90–68.40 mg g−1) at 1.2 V. Adorna et al. (2020).
Soil amendment Pyrolysis (slow pyrolysis) 300°C (3 hours) – • Coconut shell biochar has a 75.30% maximum water-holding capacity. Pituya et al. (2016)
500°C (3 hours) • Coconut shell biochar has 61.23 cmol kg−1 of the maximum cation exchange capacity.
600°C (1 hour) – • Coconut shell biochar contains approximately 2000 mg kg−1 of K, 1000 mg kg−1 of Ca and 100 mg kg−1 of Mg.
• Coconut husk biochar contains approximately 3000 mg kg−1 of P, 18,000 mg kg−1 of K, 8000 mg kg−1 of Ca and 4200 mg kg−1 of Mg.
• Coconut shell has high surface areas and porosities which indicate good water retention properties and potential habitat for microbial communities.
• Coconut-based biochar (i.e. shell and husk) is highly alkaline (pH = 8.5–9.6), which can increase microbial activity, organic matter mineralization, plant-nutrient bioavailability, acid neutralization in soil.		 Windeatt et al. (2014)
Gasification 850°C (1 hour) – • Coconut shell contains 20,393.80 mg kg−1 of K, 4188.70 mg kg−1 of Na, 387.70 mg kg−1 of Ca, 274.00 mg kg−1 of P, 954.60 mg kg−1 of Si and 561.20 mg kg−1 of Cu.
• Coconut shell biochar releases approximately 99% of K, 80% of P, 20% of Ca, 10% of Mg, 75% of Na and 35% of Si to the soil at pH = 7.		 Millán et al. (2021)
Pyrolysis (slow pyrolysis) 650°C (–) Steam activation 800°C (78 minutes) • Steam activation causes the coconut-shell-activated biochar to remove more PAH contaminants in soil, whereby the removal efficiency increases from 10 to 84% for immobilization of freely dissolved (Cfree) and from 50 to 99% for a bioaccessible fraction (Cbioacc) of polycyclic aromatic hydrocarbons in soil. Koltowski et al. (2016)
Water treatment Pyrolysis (microwave-assisted) 126–205.40°C (15–20 minutes) – • The methylene blue adsorption capacity of the coconut shell biochar decreases from 0.6875 (removal efficiency = 55.00%) to 0.5165 mg g−1 (removal efficiency = 41.32%) as the carbonization time increases from 15 to 20 minutes at 550 W. Nuryana et al. (2020)
Pyrolysis (slow pyrolysis) 700°C (2 hours) H3PO4, NaOH activation
80°C
(1 hour)		 • At pH 3, coconut shell biochar has 92.16% removal efficiency after treating 1000 mg L−1 of diazinon in an aqueous solution for 2 hours.
• At pH 7, the maximum diazinon removal efficiency for coconut shell biochar activated by H3PO4 and NaOH is 84.55% (adsorption capacity = 5.47 mg g−1) and 87.93% (adsorption capacity = 5.69 mg g−1) respectively.
• At 2000 mg L−1 of adsorbent dosage, biochar and H3PO4- and NaOH-activated biochar from coconut shell exhibited more than 80–90% of diazinon removal efficiency.
• At 10,000 mg L−1 of adsorbent dosage, coconut shell biochar removes 98.28% of diazinon from aqueous solution, whereas the diazinon removal efficiency for coconut-shell-activated by H3PO4 and NaOH are 97.95 and 98.2%, respectively.		 Baharum et al. (2020)
Carbon sequestration Pyrolysis (slow pyrolysis) 600°C
(1 hour)		 – • 4.90 million tons of coconut shell biochar can theoretically capture or store 9.900 million tons of atmospheric CO2.
• 5.00 million tons of coconut husk biochar, can theoretically capture or store 2.90 million tons of atmospheric CO2. 		Windeatt et al. (2014)
Open in a new tab

Conclusion

Coconut shell and husk biochar and coconut-shell-activated biochar were confirmed to have favourable characteristics, such as high alkalinity, porous structure, CEC, WHC and mineral composition, which causes the biochar and activated biochar to become potentially useful for various applications such as biodiesel production, capacitive deionization, soil conditioning, toxic remediation, water treatment and CO2 storage. Such potential enables the coconut industry to manage coconut waste biomass and mitigate environmental issues effectively while providing income for biochar producers and sellers. The production and activation technology, economic and financial aspect, as well as potential application of coconut shell and coconut husk biochar is significantly directed towards value-added biomass for income generation particularly in the coconut industry.

Footnotes

Declaration of conflicting interests: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding: The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors thank the Sarawak Research Development Council (SRDC), Ministry of Education, Innovation and Talent Development (Government of Sarawak, Malaysia) for funding the research study (grant no. RDCRG/CAT/2019/12; RMI File No: 100-TNCPI/PRI 16/6/2 (019/2020)), the School of Chemical Engineering, College of Engineering, Universiti Teknologi MARA (UiTM) Sarawak Branch, Samarahan Campus; School of Chemical Engineering, College of Engineering, Universiti Teknologi MARA (UiTM) Selangor Branch, Shah Alam; Faculty of Applied Sciences, Universiti Teknologi MARA (UiTM), Sarawak Branch, Samarahan 2 Campus; and Faculty of Health Sciences, Universiti Teknologi MARA (UiTM) Sarawak Branch, Samarahan Campus for the analytical laboratory facilities and excellent technical support towards this research.

References

  1. Adorna J, Borines M, Dang VD, et al. (2020) Coconut shell derived activated biochar – manganese dioxide nanocomposites for high performance capacitive deionization. Desalination 492: 114602. [Google Scholar]
  2. Amalina F, Razak ASA, Krishnan S, et al. (2022) A comprehensive assessment of the method for producing biochar, its characterization, stability, and potential applications in regenerative economic sustainability – a review. Cleaner Materials 3: 100045. [Google Scholar]
  3. Atienza AH, Orcullo J, Salamat C, et al. (2020) Coconut shell feedstock based top lit updraft gasifier for biochar and heat cogeneration. Journal of Physics: Conference Series 1519: 012014. [Google Scholar]
  4. Baharum NA, Nasir HM, Ishak MY, et al. (2020) Highly efficient removal of diazinon pesticide from aqueous solutions by using coconut shell-modified biochar. Arabian Journal of Chemistry 13: 6106–6121. [Google Scholar]
  5. Behera B, Selvam MS, Dey B, et al. (2020) Algal biodiesel production with engineered biochar as a heterogeneous solid acid catalyst. Bioresource Technology 310: 123392. [DOI] [PubMed] [Google Scholar]
  6. Borel LDMS, de Lira TS, Ataíde CH, et al. (2021) Thermochemical conversion of coconut waste: material characterization and identification of pyrolysis products. Journal of Thermal Analysis and Calorimetry 143: 637–646. [Google Scholar]
  7. Braghiroli FL, Bouafif H, Neculita CM, et al. (2018) Activated biochar as an effective sorbent for organic and inorganic contaminants in water. Water, Air, and Soil Pollution 229: 230. [Google Scholar]
  8. Brewer CE, Schmidt-Rohr K, Satrio JA, et al. (2009) Characterization of biochar from fast pyrolysis and gasification systems. Environmental Progress & Sustainable Energy 28: 386–396. [Google Scholar]
  9. Bridgwater AV. (1995) The technical and economic feasibility of biomass gasification for power generation. Fuel 74: 631–653. [Google Scholar]
  10. Brown TR, Wright MM, Brown RC. (2011) Estimating profitability of two biochar production scenarios: slow pyrolysis vs fast pyrolysis. Biofuels, Bioproducts and Biorefining 5: 54–68. [Google Scholar]
  11. Bushra B, Remya N. (2020) Biochar from pyrolysis of rice husk biomass—characteristics, modification and environmental application. Biomass Conversion and Biorefinery 199: 7–12. [Google Scholar]
  12. Campbell RM, Anderson NM, Daugaard DE, et al. (2018) Financial viability of biofuel and biochar production from forest biomass in the face of market price volatility and uncertainty. Applied Energy 230: 330–343. [Google Scholar]
  13. Cha JS, Park SH, Jung SC, et al. (2016) Production and utilization of biochar: a review. Journal of Industrial and Engineering Chemistry 40: 1–15. [Google Scholar]
  14. Chan YH, Cheah KM, How BS, et al. (2019) An overview of biomass thermochemical conversion technologies in Malaysia. Science of the Total Environment 680: 105–123. [DOI] [PubMed] [Google Scholar]
  15. Cheng F, Li (2018) Preparation and application of biochar-based catalysts for biofuel production. Catalysts 8: 1–35. [Google Scholar]
  16. Daful AG, Chandraratne MR. (2020) Biochar production from biomass waste-derived material. In: Hashmi S, Choudhury IA. (eds) Encyclopedia of Renewable and Sustainable Materials. Elsevier: Amsterdam, Netherlands, pp. 370–378. DOI: 10.1016/B978-0-12-803581-8.11249-4. [DOI] [Google Scholar]
  17. Dalai AK, Azargohar R. (2007) Production of Activated Carbon from Biochar Using Chemical and Physical Activation: Mechanism and Modeling. In: ACS Symposium Series. American Chemical Society: Washington D.C., United States of America, pp. 463–476. DOI: 10.1021/bk-2007-0954.ch029. [DOI] [Google Scholar]
  18. Danish M, Ahmad T. (2018) A review on utilization of wood biomass as a sustainable precursor for activated carbon production and application. Renewable and Sustainable Energy Reviews 87: 1–21. [Google Scholar]
  19. Dhyani V, Bhaskar T. (2019) Pyrolysis of Biomass. In: Pandey A, Larroche C, Gnansounou E, et al. (eds) Biofuels: Alternative Feedstocks and Conversion Processes for the Production of Liquid and Gaseous Biofuels, 2nd Edition. Elsevier: Amsterdam, Netherlands, pp. 217–244. DOI: 10.1016/B978-0-12-816856-1.00009-9. [DOI] [Google Scholar]
  20. Diebold J. (1980) Specialists’ workshop on fast pyrolysis of biomass. In: Steiberg M, Stevens D and Vorhees K (eds) Specialists’ Workshop on Fast Pyrolysis of Biomass. Solar Energy Research Institute: Colorado, United States of America. https://ntrl.ntis.gov/NTRL/dashboard/searchResults/titleDetail/SERICP6221096.xhtml (accessed 28 August 2021).
  21. European Central Bank (2019) Euro foreign exchange reference rates. European Central Bank. Available at: https://www.ecb.europa.eu/stats/exchange/eurofxref/shared/pdf/2019/12/20191231.pdf (accessed 5 April 2021). [Google Scholar]
  22. El-Hendawy A-NA. (2009) An insight into the KOH activation mechanism through the production of microporous activated carbon for the removal of Pb2+ cations. Applied Surface Science 255: 3723–3730. [Google Scholar]
  23. Elkhalifa S, Al-Ansari T, Mackey HR, et al. (2019) Food waste to biochars through pyrolysis: a review. Resources, Conservation and Recycling 144: 310–320. [Google Scholar]
  24. Fan D, Zhu Z, Na Y, et al. (2013) Thermogravimetric analysis of gasification reactivity of coal chars with steam and CO2 at moderate temperatures. Journal of Thermal Analysis and Calorimetry 113: 599–607. [Google Scholar]
  25. FAO (2019) Coconut. Food and Agricultural Organization of the United Nations (FAO). https://www.fao.org/faostat/en/#data/QCL (accessed 11 August 2021).
  26. Ferronato N, Torretta V. (2019) Waste mismanagement in developing countries: a review of global issues. International Journal of Environmental Research and Public Health 16: 1–28. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Foo KY, Hameed BH. (2012) Potential of jackfruit peel as precursor for activated carbon prepared by microwave induced NaOH activation. Bioresource Technology 112: 143–150. [DOI] [PubMed] [Google Scholar]
  28. Gao Y, Yue Q, Gao B, et al. (2020) Insight into activated carbon from different kinds of chemical activating agents: a review. Science of the Total Environment 746: 141094. [DOI] [PubMed] [Google Scholar]
  29. Goh CH. (2018, September 24) Green energy source going to waste. The Star, 1. https://www.thestar.com.my/business/smebiz/2018/09/24/green-energy-source-going-to-waste
  30. Gómez-Barea A, Leckner B. (2010) Modeling of biomass gasification in fluidized bed. Progress in Energy and Combustion Science 36: 444–509. [Google Scholar]
  31. Guo S, Peng J, Li W, et al. (2009) Effects of CO2 activation on porous structures of coconut shell-based activated carbons. Applied Surface Science 255: 8443–8449. [Google Scholar]
  32. Gwenzi W, Chaukura N, Wenga T, et al. (2021). Biochars as media for air pollution control systems: contaminant removal, applications and future research directions. Science of the Total Environment 753: 142249. [DOI] [PubMed] [Google Scholar]
  33. Hansen V, Hauggaard-Nielsen H, Petersen CT, et al. (2016) Effects of gasification biochar on plant-available water capacity and plant growth in two contrasting soil types. Soil and Tillage Research 161: 1–9. [Google Scholar]
  34. Hernández-Montoya V, García-Servin J, Bueno-López JI. (2012) Thermal treatments and activation procedures used in the preparation of activated carbons. In: Hernández-Montoya V, Bonilla-Petriciolet A. (eds) Lignocellulosic Precursors Used in the Synthesis of Activated Carbon - Characterization Techniques and Applications in the Wastewater Treatment. London: InTech, pp. 19–32. DOI: 10.5772/39365. [DOI] [Google Scholar]
  35. Hossain MK, Strezov V, Chan KY, et al. (2011) Influence of pyrolysis temperature on production and nutrient properties of wastewater sludge biochar. Journal of Environmental Management 92: 223–228. [DOI] [PubMed] [Google Scholar]
  36. Huang WH, Lee DJ, Huang C. (2021) Modification on biochars for applications: a research update. Bioresource Technology 319: 1–11. [DOI] [PubMed] [Google Scholar]
  37. Huang Y-F, Chiueh P-T, Lo S-L. (2016) A review on microwave pyrolysis of lignocellulosic biomass. Sustainable Environment Research 26: 103–109. [Google Scholar]
  38. Idris J, Shirai Y, Andou Y, et al. (2015. a) Self-sustained carbonization of oil palm biomass produced an acceptable heating value charcoal with low gaseous emission. Journal of Cleaner Production 89: 257–261. [Google Scholar]
  39. Idris J, Shirai Y, Anduo Y, et al. (2015. b) Improved yield and higher heating value of biochar from oil palm biomass at low retention time under self-sustained carbonization. Journal of Cleaner Production 104: 475–479. [Google Scholar]
  40. Isidoria M, Gonzaga S, Mackowiak C, et al. (2018) Positive and negative effects of biochar from coconut husks, orange bagasse and pine wood chips on maize (zea mays L.) growth and nutrition. Catena 162: 414–420. [Google Scholar]
  41. Jain N, Bhatia A, Pathak H. (2014) Emission of air pollutants from crop residue burning in India. Aerosol and Air Quality Research 14: 422–430. [Google Scholar]
  42. Jia Y, Lighty JS. (2012) Ash particulate formation from pulverized coal under oxy-fuel combustion conditions. Environmental Science & Technology 46: 5214–5221. [DOI] [PubMed] [Google Scholar]
  43. Jiang C, Bo J, Xiao X, et al. (2020) Converting waste lignin into nano-biochar as a renewable substitute of carbon black for reinforcing styrene-butadiene rubber. Waste Management 102: 732–742. [DOI] [PubMed] [Google Scholar]
  44. Khuenkaeo N, Tippayawong N. (2020) Production and characterization of bio-oil and biochar from ablative pyrolysis of lignocellulosic biomass residues. Chemical Engineering Communications 207: 153–160. [Google Scholar]
  45. Kirubakaran V, Sivaramakrishnan V, Nalini R, et al. (2009). A review on gasification of biomass. Renewable and Sustainable Energy Reviews 13: 179–186. [Google Scholar]
  46. Koltowski M, Hilber I, Bucheli TD, et al. (2016) Effect of steam activated biochar application to industrially contaminated soils on bioavailability of polycyclic aromatic hydrocarbons and ecotoxicity of soils. Science of the Total Environment 566–567: 1023–1031. [DOI] [PubMed] [Google Scholar]
  47. Konwar LJ, Mäki-Arvela P, Mikkola JP. (2019) SO3H-containing functional carbon materials: synthesis, structure, and acid catalysis. Chemical Reviews 119: 11576–11630. [DOI] [PubMed] [Google Scholar]
  48. Lai JY, Ngu LH. (2020). The production cost analysis of oil palm waste activated carbon: a pilot-scale evaluation. Greenhouse Gases: Science and Technology 10: 999–1026. [Google Scholar]
  49. Lédé J, Li HZ, Villermaux J, et al. (1987) Fusion-like behaviour of wood pyrolysis. Journal of Analytical and Applied Pyrolysis 10: 291–308. [Google Scholar]
  50. Lede J, Panagopoulos J, Li HZ, et al. (1985). Fast pyrolysis of wood: direct measurement and study of ablation rate. Fuel 64: 1514–1520. [Google Scholar]
  51. Lee J, Sarmah AK, Kwon EE. (2019) Production and formation of biochar. In: Yong SO, Bolan N, Tsang DCW, et al. (eds) Biochar from Biomass and Waste: Fundamentals and Applications (pp. 3–18). Amsterdam: Elsevier. DOI: 10.1016/B978-0-12-811729-3.00001-7. [DOI] [Google Scholar]
  52. Lee J, Yang X, Cho S-H, et al. (2017) Pyrolysis process of agricultural waste using CO2 for waste management, energy recovery, and biochar fabrication. Applied Energy 185: 214–222. [Google Scholar]
  53. Lehmann J, Joseph S. (eds) (2012) Biochar for Environmental Management. Oxfordshire: Routledge. DOI: 10.4324/9781849770552. [DOI] [Google Scholar]
  54. Lehmann J, Rillig MC, Thies J, et al. (2011) Biochar effects on soil biota – a review. Soil Biology and Biochemistry 43: 1812–1836. [Google Scholar]
  55. Li D, Hockaday WC, Masiello CA, et al. (2011) Earthworm avoidance of biochar can be mitigated by wetting. Soil Biology and Biochemistry 43: 1732–1737. [Google Scholar]
  56. Li Y, Xing B, Ding Y, et al. (2020) A critical review of the production and advanced utilization of biochar via selective pyrolysis of lignocellulosic biomass. Bioresource Technology 312: 123614. [DOI] [PubMed] [Google Scholar]
  57. Lillo-Rodenas M, Cazorla-Amoros D, Linares-Solano A. (2003) Understanding chemical reactions between carbons and NaOH and KOH: an insight into the chemical activation mechanism. Carbon 41: 267–275. [Google Scholar]
  58. Liu P, Liu W-J, Jiang H, et al. (2012) Modification of bio-char derived from fast pyrolysis of biomass and its application in removal of tetracycline from aqueous solution. Bioresource Technology 121: 235–240. [DOI] [PubMed] [Google Scholar]
  59. Liu WJ, Jiang H, Tian K, et al. (2013) Mesoporous carbon stabilized MgO nanoparticles synthesized by pyrolysis of MgCl2 preloaded waste biomass for highly efficient CO2 capture. Environmental Science and Technology 47: 9397–9403. [DOI] [PubMed] [Google Scholar]
  60. Liu WJ, Jiang H, Yu HQ. (2015) Development of biochar-based functional materials: toward a sustainable platform carbon material. Chemical Reviews 115: 12251–12285. [DOI] [PubMed] [Google Scholar]
  61. Lo S-L, Huang Y-F, Chiueh P-T, et al. (2017) Microwave pyrolysis of lignocellulosic biomass. Energy Procedia 105: 41–46. [Google Scholar]
  62. Lozano-Castelló D, Calo JM, Cazorla-Amorós D, et al. (2007) Carbon activation with KOH as explored by temperature programmed techniques, and the effects of hydrogen. Carbon 45: 2529–2536. [Google Scholar]
  63. Mardoyan A, Braun P. (2016) Analysis of Czech subsidies for solid biofuels. International Journal of Green Energy 12: 405–408. [Google Scholar]
  64. Maroušek J, Gavurov B. (2022) Recovering phosphorous from biogas fermentation residues indicates promising economic results. Chemosphere 291: 133008. [DOI] [PubMed] [Google Scholar]
  65. Maroušek J. (2014) Significant breakthrough in biochar cost reduction. Clean Technologies and Environmental Policy 16(8): 1821–1825. DOI: 10.1007/s10098-014-0730-y. [DOI] [Google Scholar]
  66. Maroušek J, Maroušková A, Kůs T. (2020) Shower cooler reduces pollutants release in production of competitive cement substitute at low cost. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 1–10. DOI: 10.1080/15567036.2020.1825560. [DOI] [Google Scholar]
  67. Martin H, Lede J, Li HZ, et al. (1986) Ablative melting of a solid cylinder perpendicularly pressed against a heated wall. International Journal of Heat and Mass Transfer 29: 1407–1415. [Google Scholar]
  68. McKendry P. (2002) Energy production from biomass (part 1): overview of biomass. Bioresource Technology 83: 37–46. [DOI] [PubMed] [Google Scholar]
  69. Meyer S, Glaser B, Quicker P. (2011) Technical, economical, and climate-related aspects of biochar production technologies: a literature review. Environmental Science and Technology 45: 9473–9483. [DOI] [PubMed] [Google Scholar]
  70. Millán LMR, Vargas FES, Nzihou A. (2021) Characterization of steam gasification biochars from lignocellulosic agrowaste towards soil applications. Waste and Biomass Valorization 12(7): 4141–4155. [Google Scholar]
  71. Mohan D, Pittman CU, Steele PH. (2006) Pyrolysis of wood/biomass for bio-oil: a critical review. Energy & Fuels 20: 848–889. [Google Scholar]
  72. Motasemi F, Afzal MT. (2013) A review on the microwave-assisted pyrolysis technique. Renewable and Sustainable Energy Reviews, 28, 317–330. [Google Scholar]
  73. Mukherjee A, Lal R. (2014) The biochar dilemma. Soil Research 52: 217. [Google Scholar]
  74. Mukherjee A, Lal R, Zimmerman AR. (2014) Effects of biochar and other amendments on the physical properties and greenhouse gas emissions of an artificially degraded soil. Science of the Total Environment 487: 26–36. [DOI] [PubMed] [Google Scholar]
  75. Nabais JMV, Nunes P, Carrott PJM, et al. (2008) Production of activated carbons from coffee endocarp by CO2 and steam activation. Fuel Processing Technology 89: 262–268. [Google Scholar]
  76. Narzari R, Bordoloi N, Chutia RS, et al. (2015) Biochar: An overview on its production, properties and potential benefits. In: Choudhury H. (ed) Biology, Biotechnology, and Sustainable Development (Issue September). Research India Publication: New Delhi, India, pp. 13–40. DOI: 10.13140/RG.2.1.3966.2560. [DOI] [Google Scholar]
  77. Nuryana D, Alima MFR, Yahayu M, et al. (2020) Methylene blue removal using coconut shell biochar synthesized through microwave-assisted pyrolysis. Jurnal Teknologi 82: 31–41. [Google Scholar]
  78. Olivares-Marín M, Fernández-González C, Macías-García A, et al. (2006) Thermal behaviour of lignocellulosic material in the presence of phosphoric acid. Influence of the acid content in the initial solution. Carbon 44: 2347–2350. [Google Scholar]
  79. Otowa T, Nojima Y, Miyazaki T. (1997) Development of KOH activated high surface area carbon and its application to drinking water purification. Carbon 35: 1315–1319. [Google Scholar]
  80. Patwardhan PT, Dalluge DL, Shanks BH, et al. (2011) Distinguishing primary and secondary reactions of cellulose pyrolysis. Bioresource Technology 102: 5265–5269. [DOI] [PubMed] [Google Scholar]
  81. Peacocke GVC, Bridgwater AV. (1994) Ablative plate pyrolysis of biomass for liquids. Biomass and Bioenergy 7: 147–154. [Google Scholar]
  82. Perera C. (2012) Coconut. In: Gupta SK. (ed) Technological Innovations in Major World Oil Crops, Volume 1: Breeding (Issue January 2012). Springer: New York, United States of America. DOI: 10.1007/978-1-4614-0356-2. [DOI] [Google Scholar]
  83. Pituya P, Sriburi T, Wijitkosum S. (2016) Properties of biochar prepared from acacia wood and coconut shell for soil amendment. Engineering Journal 21: 63–76. [Google Scholar]
  84. Prauchner MJ, Rodríguez-Reinoso F. (2012) Chemical versus physical activation of coconut shell: a comparative study. Microporous and Mesoporous Materials 152: 163–171. [Google Scholar]
  85. Puig-Arnavat M, Bruno JC, Coronas A. (2010) Review and analysis of biomass gasification models. Renewable and Sustainable Energy Reviews 14: 2841–2851. [Google Scholar]
  86. Puziy AM, Poddubnaya OI, Gawdzik B, et al. (2020) Phosphorus-containing carbons: preparation, properties and utilization. Carbon 157: 796–846. [Google Scholar]
  87. Ravi S, Sharratt BS, Li J, et al. (2016). Particulate matter emissions from biochar-amended soils as a potential tradeoff to the negative emission potential. Scientific Reports 6: 35984. [DOI] [PMC free article] [PubMed] [Google Scholar]
  88. Richardson Y, Drobek M, Julbe A, et al. (2015). Biomass gasification to produce syngas. In: Pandey A, Bhaskar T, Stöcker M, et al. (eds) Recent Advances in Thermo-Chemical Conversion of Biomass. Elsevier: Amsterdam, Netherlands, pp. 213–250. DOI: 10.1016/B978-0-444-63289-0.00008-9. [DOI] [Google Scholar]
  89. Sahoo D, Remya N. (2020) Influence of operating parameters on the microwave pyrolysis of rice husk: biochar yield, energy yield, and property of biochar. Biomass Conversion and Biorefinery 12(8): 3447–3456. DOI: 10.1007/s13399-020-00914-8. [DOI] [Google Scholar]
  90. Sajjadi B, Chen W, Egiebor NO. (2019) A comprehensive review on physical activation of biochar for energy and environmental applications. Reviews in Chemical Engineering 35: 735–776. [Google Scholar]
  91. Sajjadi B, Zubatiuk T, Leszczynska D, et al. (2018) Chemical activation of biochar for energy and environmental applications: a comprehensive review. Reviews in Chemical Engineering 35: 777–815. [Google Scholar]
  92. Samsudin MH, Hassan MA, Idris J, et al. (2019). A one-step self-sustained low temperature carbonization of coconut shell biomass produced a high specific surface area. Waste Management & Research 37: 551–555. [DOI] [PubMed] [Google Scholar]
  93. Santos RM, Santos AO, Sussuchi EM, et al. (2015) Pyrolysis of mangaba seed: Production and characterization of bio-oil. Bioresource Technology 196: 43–48. [DOI] [PubMed] [Google Scholar]
  94. Sari RM, Gea S, Wirjosentono B, et al. (2020). Improving quality and yield production of coconut shell charcoal through a modified pyrolysis reactor with tar scrubber to reduce smoke pollution. Journal of Environmental Study 29: 1815–1824. [Google Scholar]
  95. Schmidt H-P. (2012) 55 Uses of Biochar. Ithaka Journal 25(1): 13–25. [Google Scholar]
  96. Sekar M, Kumar PTR, Selva GKM, et al. (2021). Techno-economic review on short-term anthropogenic emissions of air pollutants and particulate matter. Fuel 305: 121544. [Google Scholar]
  97. Shi Y, Liu G, Wang L, et al. (2019). Heteroatom-doped porous carbons from sucrose and phytic acid for adsorptive desulfurization and sulfamethoxazole removal: a comparison between aqueous and non-aqueous adsorption. Journal of Colloid and Interface Science 557: 336–348. [DOI] [PubMed] [Google Scholar]
  98. Shukla N, Sahoo D, Remya N. (2019) Biochar from microwave pyrolysis of rice husk for tertiary wastewater treatment and soil nourishment. Journal of Cleaner Production 235: 1073–1079. [Google Scholar]
  99. Sikarwar VS, Zhao M. (2017) Biomass gasification. In: Abraham MA, Sikdar SK, Hong J. (eds) Encyclopedia of Sustainable Technologies. Amsterdam: Elsevier, pp. 205–216. [Google Scholar]
  100. Sizmur T, Fresno T, Akgül G, et al. (2017). Biochar modification to enhance sorption of inorganics from water. Bioresource Technology 246: 34–47. [DOI] [PubMed] [Google Scholar]
  101. Spokas K. (2013) Biochar: The science behind the hype. United States Minnesota: Department of Agriculture. http://hdl.handle.net/2142/106765 [Google Scholar]
  102. Spokas KA, Reicosky DC. (2009) Impacts of sixteen different biochars on soil greenhouse gas production. Annals of Environmental Science 3(612): 179–193. [Google Scholar]
  103. Stavkova J, Maroušek J. (2021) Novel sorbent shows promising financial results on P recovery from sludge water. Chemosphere 276: 130097. [DOI] [PubMed] [Google Scholar]
  104. Sudaryanto Y, Hartono SB, Irawaty W, et al. (2006) High surface area activated carbon prepared from cassava peel by chemical activation. Bioresource Technology 97: 734–739. [DOI] [PubMed] [Google Scholar]
  105. Suman S, Gautam S. (2017) Pyrolysis of coconut husk biomass: analysis of its biochar properties. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 39: 761–767. [Google Scholar]
  106. Suman S, Gautam S. (2018) Biochar Derived from agricultural waste biomass act as a clean and alternative energy source of fossil fuel Inputs. In: Tsvetkov PV. (ed) Energy Systems and Environment, 1st Edition. London: InTech, pp. 207–215. DOI: 10.5772/intechopen.73833. [DOI] [Google Scholar]
  107. Tammeorg P, Parviainen T, Nuutinen V, et al. (2014) Effects of biochar on earthworms in arable soil: avoidance test and field trial in boreal loamy sand. Agriculture, Ecosystems and Environment 191: 150–157. [Google Scholar]
  108. Tomlinson T, Jirka S. (2014) State of the Biochar Industry 2014: A Survey of Commercial Activity in the Biochar Sector (Issue May). >New York: International Biochar Initiative. https://biochar-international.org/wpcontent/uploads/2018/11/ibi_state_of_the_industry_2014_final.pdf [Google Scholar]
  109. Tripathi M, Sahu JN, Ganesan P. (2016) Effect of process parameters on production of biochar from biomass waste through pyrolysis: a review. Renewable and Sustainable Energy Reviews 55: 467–481. [Google Scholar]
  110. USGS (2019) What is carbon sequestration? United States Geological Survey. https://www.usgs.gov/faqs/what-carbon-sequestration?qt-news_science_products=0#qt-news_science_products [Google Scholar]
  111. Vijayaraghavan K. (2019) Recent advancements in biochar preparation, feedstocks, modification, characterization and future applications. Environmental Technology Reviews 8: 47–64. [Google Scholar]
  112. Wang J, Wang S. (2019) Preparation, modification and environmental application of biochar: a review. Journal of Cleaner Production 227: 1002–1022. [Google Scholar]
  113. Windeatt JH, Ross AB, Williams PT, et al. (2014) Characteristics of biochars from crop residues: Potential for carbon sequestration and soil amendment. Journal of Environmental Management 146: 189–197. [DOI] [PubMed] [Google Scholar]
  114. Yang K, Peng J, Srinivasakannan C, et al. (2010) Preparation of high surface area activated carbon from coconut shells using microwave heating. Bioresource Technology 101: 6163–6169. [DOI] [PubMed] [Google Scholar]
  115. Yang X, Zhang S, Ju M, et al. (2019) Preparation and modification of biochar materials and their application in soil remediation. Applied Sciences 9: 1365. [Google Scholar]
  116. Zaman CZ, Pal K, Yehye WA, et al. (2017) Pyrolysis: A sustainable way to generate energy from waste. In: Samer M. (ed) Pyrolysis, 1st Edition. London: InTech, pp. 3–36. DOI: 10.5772/intechopen.69036. [DOI] [Google Scholar]
  117. Zhang C, Zeng G, Huang D, et al. (2019) Biochar for environmental management: mitigating greenhouse gas emissions, contaminant treatment, and potential negative impacts. Chemical Engineering Journal 373: 902–922. [Google Scholar]
  118. Zhang M, Gao B, Yao Y, et al. (2012). Synthesis of porous MgO-biochar nanocomposites for removal of phosphate and nitrate from aqueous solutions. Chemical Engineering Journal 210: 26–32. [Google Scholar]
  119. Zhang Z, Lei Y, Li D, et al. (2020) Sudden heating of H3PO4-loaded coconut shell in CO2 flow to produce super activated carbon and its application for benzene adsorption. Renewable Energy 153: 1091–1099. [Google Scholar]
  120. Zhao L, Zheng W, Mašek O, et al. (2017). Roles of phosphoric acid in biochar formation: synchronously improving carbon retention and sorption capacity. Journal of Environmental Quality 46: 393–401. [DOI] [PubMed] [Google Scholar]

Articles from Waste Management & Research are provided here courtesy of SAGE Publications

RESOURCES