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Agriculture, Dairy & Animal Science
Considerable interest is being shown in using biochar production from waste biomass with a variety of disciplines to address the most pressing environmental challenges. Biochar produced by the thermal decomposition of biomass under oxygen-limited conditions is gaining popularity as a low-cost amendment for agro-ecosystems. The efficiency of biochar formation is affected by temperature, heating rate, feedstock type, particle size and reactor conditions. Properties such as pH, surface area and ash content of produced biochar increases with increasing temperatures. Biochar produced at lower heating rates may have high porosity and be beneficial for morphological changes in the soil. Biochar can help to enhance soil health and fertility as well as improve agricultural yield. As a result, biochar can assist in increasing food security by promoting sustainable agricultural systems and preserving an eco-friendly environment. Biochar is also widely being used as a sorbent for organic and inorganic pollutants, owing to its large surface area, allowing it to be immobilized from soil with ease. The functional groups and charges present on the surface of biochar play an important role in pollutants removal.
- agro-ecosystem
- biochar
- food security
- pollutants
1. Biochar Production Process
The making of biochar is simple and suitable for many regions around the world; however, large-scale development requires optimization and economic evaluation. Furthermore, the process of producing biochar ranges from traditional kilns and earth mounds to engineered systems that depend on flat beds or fluidized reactors for pyrolysis, gasification or other methodologies [36].
1.1. Feedstocks
There are many potential feedstock materials used in the production of biochar, including wood and agricultural wastes, leaves, rice husks and straw, paper sludge, food waste, manure, bagasse, etc. [37,38,39,40,41] Feedstocks for biochar production are plentiful and inexpensive, primarily derived from agricultural biomass and solid waste [42,43]. Furthermore, agricultural waste feedstock materials (rice, wheat and maize straws), forest residues, etc. are used to make biochar. Biochar produced from invasive terrestrial plant species and different aquatic weeds can aid in invasion management while also protecting the environment [44]. Similarly, crop waste and wood are both energy-rich feedstocks that lend themselves well to pyrolysis [45,46]. Some wastes such as manure or sewage sludge may be appropriate for biochar production, if they can be produced at very high temperatures [47]. Various types of animal waste (chicken or poultry manure, sheep manure, duck manure) have proven to be valuable soil amendments after being pyrolyzed, because of their higher nutrients content [38]. In addition, the application of biochar improved waste management and provided valuable nutrients (such as N, P, K) to the soil as well as metal sorption on contaminated land [48]. Knowing how initial feedstock characteristics influence biochar properties is critical in terms of feedstock. Feedstocks have been found to have a significant impact in the production of biochars with distinctly varied chemical characteristics [49]. Wood-based biochars contain more C and less plant-available nutrients, whereas manure-based biochars exhibit the opposite trend, and grass-based biochars often fall midway between woody and manure biochars [50]. The total C content in biochars is frequently increased since most feedstocks include significant C concentrations; however, feedstock selection has a considerable impact on biochar C content. Wood-based biochars included more C than biochars manufactured from other feedstocks, owing to a lack of other elements (e.g., N, S, P, K, Ca, and P), resulting in a lesser C-dilution effect in wood-based biochars [50].
1.2. Methodology
In relation to temperature, residence time and heating rate, there are several methods for producing biochar [51,52].
These methods affect the chemical composition and physical state of the feedstocks used. Furthermore, during the valorization process, feedstocks (cellulose, hemicellulose, lignin, and pectin) are depolymerized and fragmented, from which small amounts of condensable bio-oil and non-condensable gases are produced [57,58].
1.2.1. Pyrolysis
Pyrolysis is a thermal decomposition of biomass material in the absence of oxygen at a specific temperature, pressure and resident time that is required for complete combustion [59,60]. However, the final products may be produced in significant quantities. Pyrolysis can be categorized into slow and fast pyrolysis based on temperature, pressure and residence time [61,62]. Fast pyrolysis is a direct thermochemical process that can emulsify feedstocks or biomass into biochar with liquid bio-oil, which has a greater energy potential [62,63]. Fast pyrolysis occurs under three conditions: first, when biomass is pyrolyzed at temperature >100 °C/min, the particles of the obtained biochar are distinct in size; second, particles and pyrolysis fumes are released in 0.5–2 s with less time at high temperatures; and third, temperatures in a moderate, fast pyrolysis treatment completed at 400–600 °C [64]. In view of this, fast pyrolysis advancement is obligatory to keep the fume residence time in the reaction chamber to a minimum in order to achieve high bio-oil quality [63]. Similarly, slow pyrolysis is carried out at higher temperatures (350–550 °C) in the absence of O2 [65]. As a result, the biomass containing cellulose, hemicellulose and lignin produces 30% more biochar than fast pyrolysis (12%) or gasification (10%) [66]. The mechanism is observed by significantly reducing the degree of polymerization, which consists of two reaction processes: (a) Slow pyrolysis, which includes cellulose decomposition at a higher residence time and temperature rate and (b) fast pyrolysis, which is done at a higher heating rate by rapid volatilization with formed levoglucosan [67]. Moreover, the hydroxymethyl furfural produced by the dehydration of levoglucosan can decompose to produce both liquid and gaseous products such as syngas and bio-oil [68]. Ref. [69] conducted an experiment to produce biochar from lignocellulosic feedstocks using a slow pyrolysis method and compared its sustainability impact to that of direct biomass combustion. Similarly, ref. [69] found that biochar obtained through pyrolysis produces higher quality bio-oil than alternative treatments. Therefore, the findings observed that the effect of pyrolysis is only dependent on the energy supplied during pre-treatment processes. Additionally, ref. [67] observed that the mechanism of hemicellulose decomposition is correlated with cellulose, because oligosaccharides are formed after hemicellulose depolymerization. This can happen when reaction occurs between intramolecular rearrangement, decarboxylation, depolymerization and aromatization. In accordance with this, the building blocks of lignin are linked with the β-O-4 bond that breaks in the lignin decomposition mechanism and produces free radicals. Such free radicals absorb protons from other species, resulting in the decomposition of organic compounds and move towards the other molecules, showing chain propagation in the produced biochar. The advantage of pyrolysis is that it is a zero-waste process and the disadvantage is that it is not being suitable for biomass with a high moisture content. Several more products can be produced through the pyrolysis method [70].
1.2.2. Hydrothermal Carbonization
Hydrothermal carbonization is one of the cost-effective methods to produce hydrochar. In this, hydrochar can be produced at low temperature (180–250 °C) [71,72]. In the hydrothermal process, the hydrochar is produced by dried feedstocks, which is different from pyrolysis [73]. The process involved in the hydrothermal method is as follows: (a) Feedstocks can be mixed with water and placed in a closed reactor to gradually provide temperature stability and (b) their various temperatures result in a variety of products such as biochar produced at a temperature below 250 °C, bio-oil in between 250–400 °C, which is known as hydrothermal liquefaction, and gaseous substances such as syngas (CO, CO2, H2 and CH4) at temperatures above 400 °C, which is known as hydrothermal gasification [74]. During the hydrothermal carbonization process, the intermediate products 5-hydroxymethylfurfural and its derivatives are formed as a result of the reactions involved such as fragmentation, dehydration and isomerization [75]. Additionally, the hydrochar is produced through the process of polymerization, condensation and intramolecular dehydration [76]. The feedstock with a high lignin content creates more complicated mechanisms due to its high molecular weight and complex nature. The decomposition of lignin commences in the reaction between dealkylation and hydrolysis, which produces phenolic product such as catechols, phenols, syringols, etc. [76,77] Therefore, the produced hydrochar is an intermediate of repolymerization and crosslinking [62,78]. However, the lignin content in biomass is not dissolved in the liquid phase and is converted into hydrochar. The advantages of hydrothermal carbonization include effective utilization of biomass as a clean and convenient solid fuels, and a disadvantage is that it is difficult to collect the products and higher requirements of equipment.
1.2.3. Gasification
Gasification is the breakdown of carbonaceous materials into gaseous products such as syngas, including CO, CO2, H2, CH4 and some hydrocarbons in the reactor that contain oxygen and steam at high temperatures [79]. Syngas production is solely reliant on the feedstock reaction stability at high temperatures [80]. However, as the temperature rises, CO and H2 levels increase while CO2 and CH4 levels decrease. The main product of the gasification process is syngas and char is the by-product with the lowest yield [81]. The processes involved in gasification are as follows: (a) Feedstock or biomass drying or complete moisture destruction, evaporation without energy recovery and the moisture content of biomass influences its drying process and (b) gasification involves a combustion or oxidation reaction with several feedstock materials that have reactive and combustible properties in a gasifier to produce CO, CO2 and water [82]. The advantage of gasification is that it has high efficiency energy recovery; the disadvantage lies with its complex technology and high investment and operating costs.
1.2.4. Torrefaction
Torrefaction is a new and advanced method for making biochar [83,84]. The biomass is gradually heated to a high temperature of 300 °C in an oxygen-deficient environment and it produces a solid, uniform biochar with less moisture and greater energy content than raw biomass [85]. During the torrefaction process, the moisture content and some volatile organic compounds are evaporated from the biomass in the reaction chamber [86]. There are three main techniques, which are as follows: (a) The biomass is dried in steam at temperatures no higher than 260 °C and a residence time of 10 min, which is referred to as steam torrefaction, (b) wet torrefaction, also known as hydrothermal carbonization, occurs when biomass is mixed with water at a temperature of 180–260 °C and a residence time of 40 min and (c) in oxidative torrefaction, in which biomass that can be treated with oxidizing agents such as gas is used in the combustion process to generate heat energy. Torrefaction is an incomplete pyrolysis process that requires a temperature of 200–300 °C, a residence time of more than 30 min, a heating rate of 50 °C/min and the absence of oxygen [87]. Several researchers explained that the torrefaction process is divided into four stages: heating, drying (including pre-drying and post-drying), torrefaction and cooling [53,88]. In heating, the biomass is heated until it is completely dry (like moisture and biomass evaporation) at a given temperature. In drying, the biomass is dried (complete drying of moisture and biomass evaporation) at a temperature of 100 °C and 200 °C, referred to as pre-drying and post-drying, respectively. As a result, by using the appropriate temperature, the volume of biomass is reduced. Torrefaction is the final process of producing biochar at a temperature of 200 °C and maintaining stability at that temperature throughout the manufacturing process. In cooling, the produced biochar is allowed to cool at room temperature. The advantages of torrefaction include the production of syngas and biofuels and biochar production, and the disadvantage is that it requires extensive gas cleaning.
1.2.5. Flash Carbonization
Flash carbonization is a process whereby biomass can burn at a high pressure (1 to 2 MPa) and temperature (300–600 °C) for a residence time of 30 min; as a result, about 40% solid carbon materials are released [24]. The methodology of flash carbonization seems to be very limited in the literature and is not widely used.
2. Application of Biochar in Agro-ecosystems
Declining soil fertility caused by infelicitous application of synthetic fertilizers is also a big worry for agricultural systems, especially in arid and semi-arid regions of the world [136]. Increasing the number of chemical fertilizers used during the green revolution initially increased agricultural yield, but it also caused a rapid deterioration in soil fertility and quality, which later disrupted the long-term viability of soil systems [12,35]. Thus, it is necessary to create new strategies that emphasize sustainability in terms of productivity, resource use, soil quality and accessibility for farmers [12]. Crop productivity could be increased by strengthening soil quality by increasing soil organic matter (SOM) through the application of biochar [137]. Earlier research supported the use of biochar to enhance soil quality and crop yield in various parts of the world [35,138]. The application of biochar has been viewed as a viable tool to address the complex issues of soil quality deterioration, waste management and boosting crop productivity [15,17,139]. In addition, researchers reported improvements in soil C sequestration and soil hydro-physical properties such as water holding capacity due to the modification of some of the soil physical properties such as porosity, texture, structure and aggregate stability under biochar-applied soils [17,137,138].
2.1. Effect of Biochar on Soil Nutrients Cycle
Biochar contributes to an improvement in the N cycle and/or additional N supply and increased soil N retention, and its use efficiency has been linked to increased plant productivity [140,141]. When biochar is added to soil, it not only improves soil fertility, but also provides micro and macro nutrients as required. According to a meta-analysis by [142], biochar may have a greater tendency to improve soil fertility through managing the N cycle than to supply nitrogen. The soil N cycle may be affected by biochar via a number of processes, including N adsorption or desorption by biochar, which can lower or raise the amount of inorganic nitrogen in the soil and alter the amount of soil mineralizable substrates, which in turn influences the microbial processes of N mineralization or immobilization (i.e., labile organic compounds) and shifts the equilibrium between the nitrification and denitrification processes by changing the characteristics of the soil (i.e., pH and aeration) [143]. However, the surface features of biochar (such as surface area, acidic functional groups, and CEC) as well as the species and quantity of NH4+ N and NO3- N in soils greatly influence the extent and dominating processes of N cycling as altered by biochar [140]. Ref. [144] documented that P-solubilizing bacteria (Pseudomonas and Bacillus) were shown to be more prevalent in biochar-amended soil, which indicated that the fixed P forms in soil minerals, SOM or biochars might be solubilized or changed into accessible P.
2.2. Effect of Biochar on Soil Biological Processes
Plant growth is directly impacted by changes in the soil biota [145]. For soil microorganisms, biochar’s porous structure makes a suitable shelter that guards against predation or desiccation [146]. According to several reports, biochar promotes the growth of mycorrhizal fungus. The aggregation and structural stability of soil are both influenced by mycorrhizal fungi, which also help plants absorb nutrients and water. However, biochar has the potential to attract bacteria, making them less susceptible to leaching. According to [147], adding biochar to paddy soil boosted bacterial abundance by 161%, with Gram-positive bacteria being more affected by biochar than Gram-negative bacteria. The application of biochar also alters the N2-fixing bacteria that produce ammonia (NH3) from atmospheric N2 [148]. The reported impact of biochar on biological nitrogen fixation has been attributed to a variety of different mechanisms [149]. It has been observed that raising biochar application rates increases biological nitrogen fixation [150,151].
2.3. Effect of Biochar on Crop Growth and Productivity
There has been a sharp rise in interest in biochar and its impact on crop productivity. Applications of biochar to soils have been found to increase plant growth and yield in several recently published sources [152,153,154]. Ref. [152] examined the association between biochar inputs and productivity (yield and above-ground biomass) for many crops and reported that biochar on average enhanced agricultural production by 10%. Further, authors reported that the liming impact (raised pH) and the better water holding capacity of the soil are the two key factors of biochar that account for increased productivity. Several studies have found that adding biochar to soil reduces bulk density while increasing WHC. The increase in WHC after biochar addition is ascribed to the biochar’s large surface area and porosity, which contribute to improved water usage efficiency and thus plant productivity [154]. The increase in WHC caused by biochar additions could be more noticeable in sandy soils, where the limited surface area of their particles and the presence of macro-pores limit their capacity to hold water. Ref. [154] proposed that biochar addition could improve the WHC of desert soils, resulting in greater plant growth. Ref. [153] articulated that the application of biochar increased the crop yields of soybean, maize, wheat and rice crops by 16%, 17%, 19% and 22%, respectively, over the control treatment without a biochar application. Similarly, ref. [155] stated that crop yield increased by 11% on average when the crop was cultivated with a biochar amendment. All of these studies are highly encouraging but there is a worry that applying fresh or pure biochar to soil, because of its high carbon content, may eventually cause the soil’s nitrogen to become immobilized, which would negatively affect plant growth and reduce crop output [156,157,158]. The crop response, however, varies from adverse to favorable depending on the properties of the biochar, how it is applied and the pre-existing soil conditions [13,15]. Furthermore, owing to the differences in soil buffering ability, different biochar application rates were recommended for different texture soils [159]. They found that a low application rate of Thai traditional kiln biochar derived from Eucalyptus camaldulensis (1%) was suitable for coarse-textured soil with low buffering capacity. However, a larger biochar rate (2%) was proposed for fine-textured soil, which had a higher buffering capacity than coarse-textured soil. These findings suggested that the function of biochar was strongly related to pyrolysis temperatures, soil and plant types and application rates.
2.4. Effect of Biochar on Plant Physiology
A number of physiological indicators responded to biochar treatments as a result of soil, biochar type and other factors. Biochar soil amendment, for example, reduced the leaf chlorophyll content in highland rice cultivated in nutrient-poor soils [160]. Ref. [161] conducted a pot study and found that the application of cotton-sticks-derived biochar increased photosynthetic rate, transpiration rate and sub-stomatal CO2 concentration, as well as the concentrations of chlorophylls, carotenoids, lycopene, anthocyanin, ascorbic acid and protein. Furthermore, ref. [162] reported that when soil treated with mixed-wood biochar at a level of 3 kg m−2 soil, the photosynthetic rate was enhanced three-fold, stomatal conductance was increased 1.7-fold and a 5% rise in chlorophyll fluorescence was observed. It was reported that the application of biochar in poor sandy soils improved plant growth by improving soil–plant water relations (enhanced relative water content and leaf osmotic potential) and photosynthesis (condensed stomatal resistance and stimulated photosynthesis rate by increasing the electron transport rate of photosystem II) under well-watered and drought conditions [163]. The increased water-holding capacity of biochar-amended soils can be used to predict the overall increase in plant accessible water [164]. Biochar amendment improved plant physiology in wheat and maize cultivated in sandy loam soil, whereas the addition of biochar had a significantly positive effect on photosynthetic rate, stomatal conductance and xylem K+ and Na+ in comparison to the control soil [165]. Furthermore, lettuce (Lactuca sativa L.) plants grown in biochar-treated soil had higher leaf water potential, absorption rates, transpiration rates and water usage efficiency [166]. According to [167], biochar improved soil adsorption of Na+ and raised plant xylem K+ content, enhancing potato tuber output. The study also observed that applying biochar had a favourable residual effect on lowering Na+ uptake in the next wheat crop [168]. As a result, biochar has the potential to ameliorate salinity-induced mineral absorption reductions and may be a novel approach for mitigating the impacts of salinization in arable and polluted soils [169].
3. Application of Biochar for Reclamation of Contaminated Agro-ecosystems
Biochar has a wide variety of environmental applications due to its abundance in feedstock, large surface area, microporosity and ion exchange capacity [177]. Biochar is gaining popularity as a soil amendment because it has the capacity to mitigate climate change by sequestering carbon from the atmosphere into the soil [178]. It also improves soil properties and fertility by increasing moisture, nutrient retention and microbial activity, resulting in increased crop productivity [179]. On the other hand, biochar has the ability to decontaminate polluted soils. These various potential benefits have been cost-effective and environmentally friendly for environmental restoration.
3.1. Reclamation of Inorganic and Organic Pollutants
In a recent study, the application of biochar has been suggested for the removal of metal contaminated soil and water [1]. Several studies have already been conducted or investigated into the quality of biochar and its ability to eliminate or reduce pollution loads from contaminated agricultural soil. According to a recent study, the mechanisms behind the removal of heavy metal with biochar amendments can be linked to electrostatic interactions and precipitation reactions. Because of the decreased zeta potential and increased CEC, there is greater negative charge on the soil surface when biochar is used [179]. In terms of precipitation, the significantly elevated soil pH resulting from biochar amendments could contribute to a decrease in heavy metal mobilisation. In different conditions, various oxidates, phosphates and carbonates would form. For example, a novel precipitate was observed on Pb-loaded biochar derived from sludge at an initial pH 5 and was used as a lead–phosphate silicate [180]. Ref. [181] conducted an experiment on the effects of biochar obtained from rice straw on the mobility and bioavailability of Cu(II), Pb(II) and Cd(II) in Ultisol soil [181]. By increasing the biochar amendment dose, acid extractable Cu(II) and Pb(II) fell by 19.7–100% and 18.8–77.0%, respectively. The reducible Pb(II) for treatments with 3% and 5% biochar was two and three times greater than that of samples without biochar when these heavy metals were supplied at 5 mmolkg−1. Another study [182] used microanalyses techniques to investigate the ability of biochar to immobilise and retain As, Cd and Zn from a multi-element contaminated sediment derived soil, finding that biochar reduced Cd and Zn concentrations by 300- and 45-fold, respectively. Heavy metals in the soil can be immobilised, allowing them to be held in the soil and released at a slower rate, resulting in less environmental damage.
The use of biochar to remove organic pollutants from soil is critical, notably for the removal of fungicides, herbicides and pesticides, as well as industrial chemicals such as volatile organic compounds, polycyclic aromatic hydrocarbons (PAHs) and other pollutants [183]. The interactions of these pollutants with various properties of biochar often direct the abstraction processes. Organic pollutants are removed primarily through chemisorption (electrophilic contact), physisorption (hydrophilic, electrostatic attraction/repulsion via-electron donor-acceptor, pore diffusion and H bonding), chemical transformation (through a reductive reaction or electrical conductivity) and through biodegradation (by diverse microorganisms located on the surface and in the micropores of biochar) [184]. Biochar interactions with organic pollutants are influenced by pH, pyrolysis temperature, feedstock type and pollutant ratios to biochar. Biochar is desirable for the removal of nonpolar organic contaminants due to its greater surface area and microporosity at higher pyrolysis temperatures [185]. As the pyrolysis temperature increases above 500 °C, aromaticity, low polarity and acidity of biochar increase, resulting in the loss of O– and H–containing functional groups. When O–bearing functional groups are reduced, hydrophobic interactions accelerate; however, biochar produced at temperatures below 500 °C may contain more O– and H– bearing functional groups, giving it a strong affinity for polar organic molecules [186].
3.2. Reclamation of Pesticides
The effect of biochar on heavy metal and organic pollutant remediation is proposed as a cost-effective and environmentally acceptable solution for managing polluted environments. Pesticides, on the other hand, are intentionally put in soil or other environmental compartments in agriculture to control pests and diseases. From the standpoint of humans and ecosystems, greater sorption and decreased dissipation of pesticides in the presence of biochar may reduce the risk of environmental pollution and human exposure. Furthermore, from an agricultural standpoint, decreased bioavailability and plant uptake may boost crop production and reduce chemical residues in crops. However, because a pesticide aimed at controlling specific pests or weeds must be accessible to be effective, lower pesticide efficacy due to biochar application is undesirable in agricultural soils [187]. Some studies have noticed that the efficiency of insecticides is compromised in the presence of biochar [188]. Ref. [189] found that the fumigant 1,3- dichloropropene had less efficacy in biochar-amended soil. The results showed that the dose of 1,3-dichloropropene that doubled in the soil amended with 1% biochar achieved full activity against nematode survival. Despite the lower efficacy, adequate nematode control was accomplished with 0.5% and 1% biochar at a 1,3- dichloropropene dose on the low end of the recommended rates range. However, if the biochar’s adsorption strength is too high, appropriate insect or weed control will be impossible to achieve. In this situation, biochar with a higher surface area would be regarded as undesirable, as biochar with a higher surface area has been found to have a higher sorption capacity. Ref. [188] discovered a strong influence of biochar application on the efficacy of artrazine in soil in the management of ryegrass weed and suggested that the dose in biochar supplemented soil (1% biochar by weight) may need to be increased by 3–4 times to attain the required weed control. They also mentioned that the impact of biochar on herbicide efficacy was determined by the chemistry of the herbicide molecule and its method of action. However, it was recently revealed that the sorption capacity of biochar decreases with age, which could be crucial for herbicide efficacy control in biochar-added soils [190]. It is critical to strike a balance between biochar’s potentially beneficial influence on pesticide clean-up and its detrimental impact on pesticide efficacy.
3.3. Reclamation of Other Pollutants
Biochar amendment can also immobilise other contaminants in the environment, such as radionuclides and nutritional elements [191]. Agricultural waste management has become one of the most pressing environmental challenges in recent years since significant amounts of organic waste are generated as a result of intensive agricultural activity. Agricultural wastes, such as crop straw and animal manure, are high in organic components and other elements that plants require, making them ideal for amending agricultural land with the goal of improving soil properties. These wastes can help recycle nutrients, increase soil organic matter levels and improve soil characteristics [192]. However, dumping agricultural waste into the soil without first treating it can cause a slew of issues. For example, manure application carries a high risk of runoff and leaching of manure-derived components such as N and P, which might endanger streams and lakes; uncooked sludge carries a risk of excessive heavy metal levels, which can harm the environment. Converting agricultural trash to biochar is a useful waste management approach, especially since agricultural wastes have little potential to slow down climate change. Composting is one of the most frequently accepted methods for recycling agricultural wastes, as it avoids some of the drawbacks associated with direct land application of raw wastes, such as phytotoxicity [193]. Biochar is used as a bulking agent that can aid this process as a structural and drying supplement as well as a source of carbon and energy for microbes [192].
This entry is adapted from the peer-reviewed paper 10.3390/agriculture13030512
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