Biochar in Agriculture for Achieving Sustainable Development Goals

By Yong Sik Ok

Biochar in Agriculture for Achieving Sustainable Development Goals introduces the state-of-the-art of biochar for agricultural applications to actualize sustainable development goals and highlight current challenges and the way forward. The book focuses on scientific knowledge and biochar technologies for agricultural soil improvement and plant growth. Sections provide state-of-the-art knowledge on biochar production and characterization, focus on biochar for agricultural application and soil improvement, discuss the roles of biochar for environmental improvement in farmland to relieve water and waste management as well as climate change, highlight biochar used for boosting bioeconomy and clean energy, and discuss future prospects.

This book will be important to agricultural engineers and researchers as well as those seeking to improve overall soil and environmental conditions through the use of biochar.

• Focuses on biochar utilization in agricultural applications, targeting deeper elaboration of biochar as a cost-effective and renewable material in field-scale agriculture applications
• Highlights biochar’s role in boosting the bioeconomy which shows great potential for promoting a circular economy and maximizing environmental, social and economic benefits
• Connects biochar applications with sustainable development goals

• Language: English
• Publisher: Academic Press
• Release date: May 14, 2022
• ISBN: 9780323853446

Book preview

Preface

The widely recognized terminology of biochar was first introduced in 2006, emphasizing its carbon sequestration potential during soil amendment. The potential capacity of biochar to improve sustainable agriculture was quickly unveiled afterwards. Sixteen years on, numerous scientists have worked on biochar technology, and it is proven that biochar with a science-informed and fit-for-purpose design can address multiple challenges related to sustainable development goals. The booming biochar market in current years renders biochar a ready-to-implement technology for smart and sustainable agriculture.

In this book we integrate both fundamental knowledge and cutting-edge insights about the roles of biochar in agriculture for achieving sustainable development goals. Part I introduces agriculture waste–derived biochar and the relationships between biochar and sustainable development goals. Part II focuses on biochar production and its tunable properties, which are the key to customizing biochar performance for variable applications in agriculture. Selection of biomass, production conditions, and activation methods are elaborated on for the production of high-performance biochar for various applications. Part III emphasizes the roles of biochar in food production for sustainable agriculture, considering composting, fertilizer production, pesticides immobilization, mineral composition, and metals/metalloids (im)mobilization. Environmental improvement in farmland through biochar application is the main theme of Part IV, showing the potential advantage of biochar for fostering a circular economy and achieving greenhouse gas reduction, nutrient recovery, and soil health improvement. Part V aspires toward the circular bioeconomy and clean energy applications of biochar. New concepts are introduced about a sustainable pyrolysis approach, sustainable management of biochar, CO2 adsorption by biochar, and energy production accompanied with emerging biochar technology.

State-of-the-art knowledge of biochar technology is crucial to sustainable agriculture and the society. Given our global targets of carbon neutrality, sustainable blueprints, human well-being, and one health for the planet, we hope this book will inspire interdisciplinary stakeholders to join hands and transfer knowledge to new generations for the sake of our sustainable future.

Part I

Introduction

Outline

Chapter 1 Agricultural waste-derived biochar for environmental management

Chapter 2 Biochar and sustainable development goals

Chapter 1

Agricultural waste-derived biochar for environmental management

Babasaheb M. Matsagar and Kevin C.-W. Wu,    Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan

Abstract

The agricultural waste-derived biochar has myriad environmental applications that show up by lowering greenhouse gas emissions due to the carbon-negative biochar cycle. Besides, biochar production is an economically favorable and promising method for agriculture waste management. In this book chapter, biochar production methods, biochar properties, and the role of biochar production reaction conditions on the physicochemical properties of biochar are discussed. The unique properties of biochar, including functional groups, large surface area, high porosity, superior stability, availability of metal cation and minerals, high cation exchange capacity make it suitable for various environmental management applications. We describe the biochar utilization for soil amendment, carbon sequestration, air pollution control, agriculture waste management, water purification, and energy generation for environmental management to fledge out the opportunities for biochar applications and sustainable ecosystems. Biochar application for soil improvement has been beneficial for agro-ecosystems from the environmental and agronomical prospects. Converting agricultural waste into biochar is a sustainable strategy for agricultural waste management, besides the integrating of biochar for soil amendment promoting the proficiency of resource, and increasing sustainable agriculture and circular bio-economy.

Keywords

Agriculture waste; biomass; biochar; soil amendment; water purification; waste management; environmental management

1.1 Introduction

Since agricultural waste biomass/lignocellulosic biomass is a renewable energy source available abundantly, it is imperative to utilize lignocellulosic biomass for the synthesis of chemicals and carbon materials. The lignocellulosic biomass is mainly composed of cellulose, hemicellulose, and lignin (Matsagar and Dhepe, 2017). While realizing that biomass is an excellent carbon source easily converted into carbon materials such as biochar and value-added chemicals for environmental applications (Liao et al., 2018; Matsagar et al., 2017), nevertheless, the agricultural wastes are underused for bulk scale applications for biochar, liquid fuels, and value-added chemicals production due to slow decomposition rate, lower heating value, and lack of economically feasible processes, respectively. The utilization of lignocellulosic biomass in the synthesis of biomaterials, liquid fuels, and gases is a carbon-neutral process. Besides, the waste generated in the agro-processing industry and agriculture is a promising source for biochar production. The management of waste generated from the agricultural sector and related industries is challenging yet necessary to a beneficial environment. The conversion of agricultural waste into biochar can provide environmental and agronomic benefits due to various environmental applications of biochar (Cuong et al., 2021). Besides, biomass utilization for biochar synthesis showed economic benefits due to renewability, easy availability, and low cost of raw biomass. Therefore, recently, the industrial and the academic interest in biochar production and applications have been increasing steadily.

The agricultural wastes including bagasse, rice husk, wheat straw, corn stalk, and coconut coir are readily available sources for biochar production in large quantities and have been used in the synthesis of biochar for environmental applications (Ahmad et al., 2014; Li et al., 2019; Liu et al., 2015a). The conversion of biomass into value-added chemicals, fuels, and energy is a carbon-neutral process (Cao et al., 2019). However, agricultural waste conversion into biochar is a carbon-negative process, as presented in Scheme 1.1. This means carbon emission decreases in the atmosphere in the case of biomass-derived biochar processes at work. The unique properties of biochar, including functional groups, large specific surface area (SSA), high porosity, superior stability, high cation exchange capacity (CEC) make it suitable for various environmental applications. The utilization of biochar for soil amendment improves the soil quality, and sequestering the carbon in the form of biochar in soil reduces the CO2 concentration in the atmosphere (Paustian et al., 2019). The biochar cycle process can act as a carbon sink. The reported results suggested that biochar is effective in reducing 12% global greenhouse gas (GHG) emissions annually (Woolf et al., 2010). Consequently, future research has focused on optimizing biochar synthesis methods to tailor biochar physicochemical properties for environmental management applications.

Scheme 1.1 Illustration for the carbon and biochar cycle showing the carbon-negative process for biochar cycle. (A) Carbon cycle. (B) Biochar cycle.

1.2 Biochar production and properties

1.2.1 Production of biochar

General thermochemical methods employed for converting agricultural waste and other biomass into biochar are listed in Table 1.1 (Matsagar et al., 2021; Sangjan et al., 2020; Wang et al., 2018). The hydrothermal carbonization (HTC) of biomass carried out under low temperature (180°C–300°C) in water media makes it a cost-effective biochar production method. The product obtained in this process is called hydrochar. An increase in HTC temperature above 250°C–350°C increases bio-oil production, and the process is called hydrothermal liquefaction (Yang et al., 2020). Additionally, increasing temperature above 400°C produces syngas, and the process is called hydrothermal gasification (Yang et al., 2020).

Table 1.1

Pyrolysis is a highly used method and one of the promising ones for biochar production. In this process, the thermal decomposition of organic materials was performed under the 250°C–900°C temperature range in an oxygen-free environment (Qambrani et al., 2017). The pyrolysis converts agricultural waste biomass into solid, liquid, and gas products such as biochar, bio-oil, and syngas, respectively. The gaseous products are mainly composed of CO2, CO, CH4, H2, and syngas (Ahmad et al., 2014). Higher moisture content in biomass is not suited for efficient pyrolysis, while dry biomass was usually used for higher char yield. The lower pyrolysis temperature provides a higher biochar yield and decreases syngas production. Pyrolysis is classified as a slow or a fast pyrolysis process depending on the temperature, heating rate, and residence time employed (Table 1.1).

Torrefaction is a recent, still developing technique used for biochar synthesis. The classification of torrefaction methods and their details are provided in Table 1.1. Mainly, the torrefaction is classified as dry and wet torrefaction. The dry torrefaction is further divided into oxidative and nonoxidative torrefaction, whereas the wet torrefaction is divided into the steam explosion and dilute acid treatment torrefaction indicating, that for wet torrefaction, the biomass is treated in the steam or liquid phase. Dry torrefaction is also stated as mild pyrolysis due to it requires use of lower temperatures than conventional pyrolysis method. For dry torrefaction, biomass is treated in the gas phase and it is superior for commercialization. However, it requires high energy for reducing moisture content (<10 wt.%) during the biomass predrying step. The oxidative torrefaction provides biochar with higher oxygen/carbon (O/C) and hydrogen/carbon (H/C) ratios. With an increase in oxidative torrefaction severity, the biochar yield and high heat value (HHV) decreases. The nonoxidative torrefaction provides the biochar with high HHV and lowers O/C and H/C ratios. The physicochemical characteristics and elemental composition of biochar differ based on the biochar synthesis conditions, type of synthesis method, and types of biomass used.

1.2.2 Biochar engineering

The biochar synthesized from agricultural waste was modified by physical and chemical activation methods for tailoring physicochemical properties to specific environmental management applications. The biochar modification is performed during its production or even after biochar production. Recently, biochar modification was studied in depth to improve the biochar properties for expanding its applications. Biochar modification with physical activation methods such as steam/gas activation and ball milling activation is relatively low-cost and has lower environmental risks (Amusat et al., 2021). The ball milling increases the dispersibility of biochar in water, reduces the particle size of the biochar, and enhances the SSA (Lyu et al., 2018). For instance, the bamboo-derived biochar synthesized at 450°C with ball milling showed improved acidic functional groups favoring complexation and electrostatic interaction mechanisms with heavy metals (Lyu et al., 2018). The steam activation method enhances the SSA and increases surface reactivity showing higher sorption capacities than what the biochar synthesized by the pyrolysis method without activation shows. The physical activation using CO2 oxidant showed controllability for porous structure due to its enhanced stability under higher temperatures (Cuong et al., 2021). Biochar modified with physical activation increases adsorption for heavy metals, organic pollutants, and nutrients because of the increased SSA and porosity controllability. Furthermore, the physical activation method is safer, cleaner, and provides impurity-free biochar than what chemical activation does. Microwave and magnetic biochar modification methods were also reported for improving the physicochemical properties of biochar for specific applications (Kazemi Shariat Panahi et al., 2020).

Biochar engineered by chemical activation with appropriate oxidant increases heavy metals uptake and sorption capacity. For chemical activation, biochar can be treated with strong acids (H2SO4, HCl H3PO4), bases (NaOH and KOH), and oxidizing agents such as Fe (III) and KMnO4 (Cuong et al., 2021). Biochar treatment using strong acids increases the surface oxygen-containing functional groups, which increase the adsorption of Pb, Cu, Zn, Cd, and sulfamethazine (Kazemi Shariat Panahi et al., 2020). Biochar activation using alkali provides higher aromatic functional groups, π–π interaction, higher electrostatic attractions, surface precipitation, and/or surface complexation. Improved pore size distribution and higher SSA are realized with Fe (III) and KMnO4 oxidizing agents. Physicochemical methods have been used for biochar modification to generate a high ratio of microporosity (Cuong et al., 2021).

The biological method for biochar modification showed biofilm formation inside and outside in the porous structure. It is a promising method for removing pharmaceuticals from onsite sewage systems. Biochar with a microorganism colonized structure could help minimize the toxicity of dissolved elements in wastewater. The biochar with biofilm showed 75% Mn (II) removal capacity from the solution. More details on biochar modification/engineering using physicochemical methods were reported in previous articles (Cuong et al., 2021; Kazemi Shariat Panahi et al., 2020; Li et al., 2020). The biochar engineering methods showed great potential for improving the physicochemical properties of biochar for specific applications.

Recently biochar has been used as functionalized materials due to the accessibility of large surface functional groups, such as C=O, C–O, COOH, and –OH. Availability of these functional groups provides more scope for converting biochar into highly modifiable functionalized materials (Liu et al., 2015b). The functionalized biochar showed applications for energy storage, CO2 capture, air pollution control, and catalysis.

1.2.3 Biochar properties

Biochar is a porous carbon material containing inorganic components such as alkali and alkaline earth metals. It has higher recalcitrant properties, carbon content, and aromaticity than what hydrochar produced via HTC has. The pyrolysis (350°C–950°C) of agricultural waste biomass successfully produces biochar that usually contains 10%–45% oxygen (Tripathi et al., 2016). The cellulose, hemicellulose, and lignin components of biomass thermally break down into biochar, bio-oil, and noncondensable gases. Biochar can be microporous, mesoporous, and macroporous, depending on the use of synthesis conditions. The morphology and structural properties of the biochar are greatly changed with the variation of employed pyrolysis temperature. The higher pyrolysis temperature increases aromatic carbon, showed higher SSA, and generates microporosity on the biochar (Nanda et al., 2016; Zhao et al., 2018). Therefore, biochar prepared using a high temperature shows improved adsorptive capacity than what biochar using a low temperature shows (Srinivasan and Sarmah, 2015). Besides, it has a positive correlation with the biochar pH (Zhao et al., 2018). The lower pyrolysis temperature improves the biochar yield, electrical conductivity, and CEC. The synthesis parameters such as temperature, heating rate, residence time, pressure, rates of heat transfer, and interactions of vapor-solid, all, affect the properties of the desired biochar.

For understanding the physicochemical properties of biochar, various advanced characterization techniques were used, including thermogravimetric analysis, scanning electron microscope, N2-sorption, X-ray diffraction, Raman, Fourier-transform infrared, and nuclear magnetic resonance spectroscopy (Yaashikaa et al., 2020). Physicochemical characterization of biochar suggested that the biochar has superior fuel qualities compared to what agricultural waste biomass has because biochar has higher carbon content and lower volatile matter than biomass. The physicochemical properties of biochar mainly depend on the selection of biomass sources and the pyrolysis temperature employed in biochar synthesis. The properties of biochar produced from different raw biomass under identical pyrolysis conditions are not similar due to their structural and compositional differences. For example, biochar synthesized from rice straw and herb residue under slow pyrolysis at 600°C for 3 h showed a SSA of 156.2 m²/g (pH 9.7) and 51.3 m²/g (pH 10.1), respectively (Li et al., 2020; Lian et al., 2014; Sun et al., 2014). The ash content in biomass, particle size, compositions, and physical properties of biomass also influence the properties of final biochar. Therefore, biochar synthesis process parameters are optimized to achieve desired physicochemical properties and higher yield of biochar.

Various physicochemical properties of biochar are illustrated in Fig. 1.1. The biochar showed a range of physicochemical properties depending on the source of feedstock, pyrolysis temperature, and heating rate. The heating rate showed a slight effect on the biochar properties and yield of biochar. An increase in heating rate (5–15°C/min) exhibited a slightly lower biochar yield (Zhao et al., 2018). Mainly, the pyrolysis temperature changes the biochar properties significantly. Higher pyrolysis temperature decreases H and O contents and increases C content in the biomass (He et al., 2018; Tomczyk et al., 2020). This means lower O/C and H/C ratios with the increase in pyrolysis temperature are indicative of deoxygenation and dehydration reactions, respectively (Hassan et al., 2020). As with an increase in pyrolysis temperature, the H- and O- functional groups decreased, which lowers the polarity of the biochar. Besides, higher pyrolysis temperature increases the SSA of biochar. Nevertheless, the pyrolysis temperature above 700°C showed a drop in SSA. Generally, the biochar produced from agricultural waste biomass showed a higher surface area than what biochar produced from animal litter and solid waste showed (Tomczyk et al., 2020). The higher Brunauer–Emmett–Teller (BET) surface was observed mainly because of an increase in micropore volume, which results in a decrease in average pore diameter with the increase in pyrolysis temperature. Besides, an increase in pyrolysis temperature (400°C–800°C) increases the pH, aromaticity, and ash content of biochar.

Figure 1.1 Physicochemical properties of biochar.

The higher lignin content in lignocellulosic biomass showed a higher biochar yield (Tomczyk et al., 2020). Biochar contains bulk elements, mainly C, H, and O, along with heteroatoms including N, S, and P, and metal content (K, Na, Mg, Ca, Zn, and Cu). The effect of biomass feedstock affects the N content in the produced biochar (Ippolito et al., 2020). The ammonia (NH3) retention on the biochar can be increased by increasing the O and S containing functional groups. Besides, the NH3 retention increases the N content for the biochar, which influences CO2 adsorption uptake (Krounbi et al., 2020). Consequently, understanding the role of biochar functional groups is crucial for exploring biochar applications for environmental applications.

1.3 Biochar for environmental management

Recently significant research focuses on the utilization of agricultural waste-derived biochar for environmental management applications, including soil improvement, air pollution control, waste management, water purification management, mitigation of climate change, and energy production. Besides, biochar was applied to remove inorganic and organics contaminants from waste effluents from the industry and other sources. Biochar applications are linked to climate change and agro-ecosystems because biochar simultaneously helps to address the soil fertility issue and also decreases GHG emissions. Biochar has been extensively used for a variety of applications due to its versatile physicochemical properties. Systematic utilization of biochar can provide a promising solution for developing sustainable strategies for environmental management.

1.3.1 Soil management

Incessant development and growth in the agricultural sector are necessary where the majority population is dependent on agriculture. Although many advanced techniques are being implemented for efficient agriculture, sustainable agriculture is still a challenge. The biochar-treated soil improves the soil properties providing promising opportunities for Green Revolution to take effect as sustainable agricultural ecosystem practice. Besides, biochar utilization is economically favorable for the enrichment of soil to benefit the agricultural ecosystem. The biochar enhances the soil structure with stable organic carbon compounds. Besides, the improving of antibiotic fixation of soil reduces soil toxicity and increases accessibility to soil organisms thus strengthening microbial growth. Biochar-treated soil provides higher N, P, and K content that decreases the pH of acidic soil and increases soil CEC (Yaashikaa et al., 2020). Besides, biochar utilization technology aids GHG reduction in the atmosphere that will help to combat global warming to some extent. Biochar utilization shows several benefits in soil, energy, and climate change by improving the soil quality, energy coproduction, and by reducing CO2, respectively.

Excessive use of agrochemicals decreases soil productivity and creates an adverse effect on water resources. Conversely, biochar utilization improves soil fertility and lowers the adverse environmental impact on soil and water resources. Biochar can play a crucial role in expanding opportunities for sustainable soil management by altering the physical properties of soil that affect soil-to-water response and by showing a positive effect on plant growth. Moreover, biochar improves the soil moisture content, suggesting the reduction in adverse effects of drought on the crop. Besides, biochar increases the soil cation and anion exchange capacities and porosity, which increase O2 diffusion within the landfill resulting in microbial degradation (Lawrinenko and Laird, 2015). Functional groups with oxygen such as carbonyl, alcohol, and carboxylate contribute to CEC due to their tendency to carry a negative charge and act as Lewis bases for cation sorption (Lawrinenko and Laird, 2015). The improved nutrient exchange has been observed with increasing CEC and anion exchange capacity (AEC) for biochar treated soil, which has resulted in higher fertilizer utilization efficiency and cleaner run-off (Phares et al., 2020). The higher AEC and CEC allow cleaner water to drain from the soil facilitating the retention of dissolved nutrients enriching the deprived soil. Biochar surface contains various functional groups, including ketones, –COOH and –OH, which enhance the adsorption of toxic chemicals (Al, Mn, As Cd, Cu, Ni, and Pb) in contaminated soils (Tu et al., 2020). The acidity of the biochar is controlled using pyrolysis temperature, for example, biochar produced at a higher temperature (700°C) is alkaline. Conversely, the biochar produced at a lower temperature (300°C–400°C) is acidic (Qambrani et al., 2017). The alkaline soil fertility is improved by acidic biochar, while acidic soil fertility is increased by alkaline biochar.

The application of agricultural waste-derived biochar for soil improvement strategy also helps in mitigating climate change via the sequestering of carbon in the soil. In the carbon sequestration process, the carbon is captured and stored to inhibit its emission in the atmosphere. The use of biochar thus exhibits a positive effect on carbon sequestration in soil. However, the role of biochar in carbon sequestering mechanisms is still not clear. The higher stability of biochar in soil and its synthesis from biomass can help in decreasing the GHG emissions into the atmosphere. The biochar derived from agricultural waste, sewage sludge, and food waste show negative effects for the biochar cycles, suggesting that lower GHG is emitted than consumed (Alhashimi and Aktas, 2017). Recent research shows a reduction in nitrous oxide and methane emissions from the soil by biochar via biotic and abiotic mechanisms (Qambrani et al., 2017). Bioenergy produced during pyrolysis counterbalances energy consumption from fossil feedstocks. Additionally, during photosynthesis, the C is fixed in the biomass.

1.3.2 Air pollution control

The remediation of gas emission from various industrial processes using biochar is challenging due to a lack of knowledge of the effect of biochar properties on the remediation of different gases. For example, SO2 adsorption on biochar was investigated by Braghiroli et al. (2019) showed a nonlinear relationship of SO2 adsorption with the SSA, porosity (mesopore or micropore), and ultra-micropore volume of biochar. However, recent studies, that are helping to understand the effect of renewable biochar properties for these industrial applications of gas remediation, can create future opportunities in developing biochar technology for controlling air pollution. The gases released from various industrial processes increase air pollution and create health risks. The current methods (fabric filters and electrostatic precipitation) used to remove these gases are expensive and have shown lower efficiency for toxic metal vapors (Hg⁰). Therefore, of late, research has focused on developing low-cost biochar technologies for removing gases generated from industrial processes (Gwenzi et al., 2021). Biochar has been used efficiently for removing acidic gases (SO2, H2S, and CO2), NOx, ozone, Hg⁰, and volatile organic compounds (Braghiroli et al., 2019; Maurer et al., 2017). The mechanism for removing gases involves adsorption, hydrophobic interactions, electrostatic attraction, precipitation, complexation, and size exclusion, depending on the properties of biochar (Gwenzi et al., 2021).

Biochar modification process has been studied for its desired properties for improving biochar performance for remediation of gases. Various surface functional groups can provide suitable interactions and binding sites for the removal of flue gas. The oxygenated functional groups offer electrostatic interaction for inorganic pollutants. Besides, the polarity, aromaticity, CEC, ash, porosity, and mineral content of biochar can also affect the removal of gases. The biological and chemical stability of the biochar improved by its increasing aromaticity which means highly aromatic biochar is suitable for the removal of gases under high temperatures. Biochar with mineral constituents such as oxides, carbonates, and alkali and alkaline earth metals increases the sorption of acidic gases (Xu et al., 2017). The Hg⁰ is difficult to remove from air pollution due to its poor solubility in water and lower melting point. The animal and agricultural wastes-derived biochar were reported for the removal of Hg⁰ from the flue gas. Hg⁰ adsorption showed, mainly, the chemisorption mechanism. The biochar impregnated with ammonium bromide showed high Hg⁰ removal (80%) in the fixed-bed system. The halogen (I) impregnation of biochar increases chemisorption sites, which leads to an increase in Hg⁰ removal (32% for 0% KI and 89% for 3% KI). The C-I group that is formed on the biochar by I impregnation then lets the oxygen present in the reactor to generate chemisorbed site (oxygen bonded species). The adsorbed Hg on biochar that reacts with this chemisorbed site is oxidized to Hg-I (Liu et al., 2018).

The introduction of the heteroatom in biochar enhances the CO2 capture performance. Besides, Cha et al. (2016) reported on the higher CO2 sorption for biochar prepared under high temperature. Higher SSA and higher N-content are crucial for CO2 adsorption (Matsagar et al., 2021). The KOH engineered biochar with a SSA of 1400 m²/g has a high CO2 uptake than pristine biochar (Shahkarami et al., 2015). The modification of biochar with ammonium halide impregnation has exhibited high efficiency for Hg removal from flue gas (Zhu et al., 2016). The systematic biochar modification improves the biochar efficiency for the adsorption of various gases and, hence, creates future opportunity in developing technology for controlling air pollution.

1.3.3 Waste management

Agricultural and animal wastes are significant environmental burdens that may lead to ground and water pollution if not utilized properly. For example, the burning of agricultural wastes results in air pollution and emission of the GHG. Agricultural waste such as rice husk, wheat straw, soybean stover, bagasse, rice straw is produced every year in large quantities. The conversion of agricultural wastes into biochar is an efficient waste management strategy than are conventional waste management methods. Biomass waste utilization for the synthesis of biochar via pyrolysis is a promising environmental waste management approach, which reduces the environmental challenges caused by animal and agricultural waste disposal (Qambrani et al., 2017). A good example of waste management, with concurrent benefits of decrease in transportation cost and production volumes, was the pyrolysis of agricultural waste and sludge waste to biochar (Hossain et al., 2011; Lee et al., 2017). Converting underutilized agricultural wastes efficiently into biochar to improve soil quality and water purification applications can be the most suitable way for waste management and sustainable agriculture.

Using agricultural wastes as feedstocks for pyrolysis to produce biochar is an economically promising opportunity for environmental applications of agricultural waste and for waste management. Appropriate agricultural wastes and other biomass waste management help sustaining the environment in many ways, such as recovering energy from waste, decreasing energy required for waste transportation, waste reduction, improving carbon sequestration, and decrease in GHG emissions. Low-grade biomass, dry and wet biomass converted into biochar, and subsequently, the biochar cycle showed lower GHG emissions. The recycling of biochar to soil is confirming of a positive feedback loop to improve crop production.

1.3.4 Water purification

Wastewater purification is one of the emerging topics of biochar applications due to its desired physicochemical properties. Traditional methods such as membrane separation, ion exchange, and chemical precipitation for contaminants removal from wastewater are expensive and generate chemical waste. In contrast, the production of biochar from renewable agricultural waste and other biomass feedstocks for wastewater treatment is an economically viable method, which helps to manage agricultural and animal waste efficiently. Besides, biochar production is environmentally friendly, and, in many thermochemical processes of biomass, biochar is one of the by-products.

Recently, the removal of contaminants from wastewater and water, including of heavy metals, dyes, pesticides, antibiotics, inorganic ions, phenols, polycyclic aromatic hydrocarbons, and volatile organic compounds, was demonstrated using biochar (Godwin et al., 2019; Wang et al., 2019). The agricultural waste-derived biochar was employed for arsenic (As) removal from water by Mukherjee et al. (2021). Mukherjee et al. (2021) has demonstrated high As uptake capacity observed with 2 g/L biochar concentration, and, also, higher As removal efficiency observed at near-neutral pH. The biochar applications for the remediation of organic contaminants from water and soil also showed high performance. The higher SSA, higher microporosity, and hydrophobicity of biochar increase the sorption of organic contaminants of water. The biochar produced at higher temperatures is less polar and highly hydrophobic due to the loss of hydrophilic functional groups. The surface polarity, surface area, ionic strength and pH of the solution, and aromaticity are crucial characteristics of biochar as these directly affect the biochar adsorption capacity for aqueous organic contaminants (Ahmad et al., 2014). For example, the nonpolar compounds (trichloroethylene) were adsorbed on biochar via hydrophobic sites (Ahmad et al., 2012). In contrast, on the surface of biochar, the adsorption capacity of the polar 1-naphthol compound is higher compared to that of the nonpolar naphthalene compound (Chen and Chen, 2009).

The excess use of pesticides and herbicides in the agricultural sector leads to issues related to human health and ecological balance, and cause toxicity to some beneficial organisms (Dai et al., 2019). Biochar acts as a unique sorbent for pesticides through hydrophobic interaction, pore-filling, and π–π interactions (Jin et al., 2016). Biochar is also highly effective in the removal of antibiotics. Most of the research on antibiotic and other contaminants adsorption was reported in the water. Nevertheless, the development of remediation methods for contaminants with multiple pollutants compound in the soil is necessary. The pine sawdust-derived biochar produced using steam gasification at 700°C has shown high sorption capacity for sulfamethoxazole in pasture soil (Srinivasan and Sarmah, 2015). Besides, it offers the highest degree of aromatic condensation than green waste and corncob-derived biochar do. Liu et al. (2015a) reported the agricultural waste-derived biochar for atrazine removal from water. The results indicate that the soybean-derived biochar shows higher activity for the removal of atrazine than other agricultural waste-derived biochar. The adsorption capacity of the atrazine over soybean-derived biochar was mainly associated with the pH and pore volume of the biochar.

The sorption mechanism of heavy metals and organic contaminants vary on biochar depending on biochar properties, as presented in Fig. 1.2. Biochar-derived from biomass has various metal cations (Na, K, Mg, and Ca), which can be exchanged with heavy metals (Wang et al., 2020). Besides, minerals present in biochar act as sorption sites for heavy metals, improving the sorption degree of heavy metals. Four possible sorption mechanisms are proposed for inorganic contaminants on biochar: (1) ionic exchange, (2) ionic metal attraction, (3) precipitation, and (4) cationic-metal attraction, as presented in Fig. 1.2. For organic contaminants sorption, the sorption mechanisms are hydrogen bonds, hydrophobic effect, electrostatic interaction, and pore-filling mechanisms that play a crucial role (Ahmad et al., 2014; Yaashikaa et al., 2020). Moreover, the physicochemical properties of organic contaminants and biochar are the deciding factor for these mechanisms presented in Fig. 1.2.

Figure 1.2 Postulated sorption mechanisms (A) inorganic and (B) organic contaminants on biochar. Reproduced with permission from Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., et al., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19–33.

Efficient regeneration of biochar is crucial for its reusability to promote environmental management processes. Mainly two methods are employed for biochar regeneration (1) adsorbate decomposition and (2) adsorbate desorption (Dai et al., 2019). For industrial applications, the efficient adsorption and desorption cycles and reusability of biochar are crucial. Therefore, recent studies are focused on biochar regeneration using various methods, including thermal regeneration, solvent regeneration, use of inorganic acids or alkalis, microwave irradiation, and supercritical fluid regeneration (Sun et al., 2017). Thermal and solvent biochar regeneration methods are commonly used for biochar regeneration and show lower operating costs, while the supercritical fluid regeneration method is in its initial experimental stage.

1.3.5 Energy production

Unlike fossil fuels, lignocellulosic biomass converted into biochar can release energy nearly without mercury or sulfur. Consequently, biochar production from agricultural waste can be a promising method for energy generation (Kim et al., 2019; McHenry, 2009). During the pyrolytic biomass conversion, biochar produced is one of the valuable energy sources. Besides, volatile gases formed during pyrolysis are CO, CH4, H2, syngas, and other combustible gases. The GHG released and other pollutants typically linked with biomass burning are captured and condensed into solid carbon-rich biochar, liquid fuels such as bio-oil, and syngas by the pyrolytic method. All these products can be used on-site for energy production as a part of the process. Moreover, during the conversion of biomass into solid carbon, the energy lost in the conversion process can be used as a source of heat energy or captured as valuable gas products, indicating that pyrolysis is a highly efficient, cost-effective method for biochar applications and energy production. For example, syngas can be used as a fuel for electricity generation in gas turbines, heating, and diesel engines. Compared to dry pyrolysis, HTC of biomass at 180°C–300°C produces higher biochar and lower gas yield. Besides, biochar with a high N, P, and K value, and HHV were obtained via HTC. HTC of agricultural waste and other biomass in a controlled environment were converted into biochar as high carbon-containing fuels with HHVs (Qambrani et al., 2017). Thus, biochar production from agricultural wastes reduces disposal costs and provides an economically viable solution for energy for agriculture and other industries.

1.4 Summary

The main aim of this book chapter was to focus on the role of agricultural waste-derived biochar for environmental management. The physicochemical properties of biochar are discussed in detail considering biochar applications for surface sorption, soil improvement, water purification, and renewable energy production. Agricultural waste-derived biochar is a unique renewable and sustainable green source used in various environmental management applications. The production of biochar from agricultural waste is economically favorable. Besides, biomass has a higher sorption capacity for contaminants than activated carbon has. The conversion of agricultural waste into biochar will also help in waste management concurrently with value addition. The properties of biochar can be tailored for environmental applications, mainly, by the pyrolysis condition and type of feedstock, which help to design the desired biochar as a promising green sorbent for environmental applications.

Pyrolytic conversion of biomass is a superior agricultural waste management option producing biochar that can improve the soil properties. Biochar plays a crucial role in agricultural applications such as where it increases soil fertility, improves the physical properties of soil, and moisture content, which helps to increase the crop yield and enhance crop quality. Biochar application for soil improvement has been beneficial for agro-ecosystems from environmental and agronomical prospects. In composting, biochar improves the agronomic value of compost and decreases GHG emissions. Biochar cycle showed negative carbon emission when compared to carbon cycle/biorefinery processes. Agricultural wastes management via the biochar cycle is a sustainable strategy integrated with traditional fertilization practices promoting a circular economy in agriculture. Therefore, energy-efficient clean mass production of biochar from agricultural wastes and integration of biochar for environmental management are highly necessary for improving the circular economy.

Biochar is a promising material for removing gases to control air pollution due to its suitable physicochemical properties for gas adsorption. Biochar showed high efficiency for removing acidic gases such as CO2, H2S, and SO2. Biochar also exhibited high efficiency for the removal of Hg⁰. The relatively higher stability of biochar under high temperatures is suitable for gas removal under high temperatures. Consequently, biochar can create future opportunities for controlling air pollution technology. However, understanding the effect of biochar properties on the adsorption of various gases is necessary for improving the adsorption efficiency of gases on biochar. Besides, for controlling air pollution, developing selective biochar-based air filters is crucial.

Biochar is also highly effective for the sorption of organic and inorganic contaminants from polluted effluents. The use of biochar for water purification is superior than that of ion exchange, membrane separation, and chemical precipitation methods because of the higher cost of these methods and chemical waste generation. The sorption of organic contaminants increased by increasing microporosity, SSA, hydrophobicity, and aromaticity of the biochar. The development of an efficient method for the remediation of various types of contaminants from multiple industrial polluted effluents is necessary. Moreover, the regeneration of biochar is also crucial to further enhance the biochar reusability to promote environmental sorption processes.

Future research should focus on the long-term use of biochar for environmental management. For example, soil remediation on the ecosystem and understanding the mechanism of carbon sequestration can be cited as directions for future studies. Moreover, process integration of agricultural waste management by producing the biochar, and utilizing the biochar and the energy generated during pyrolysis on-site to make the whole process cost-effective are necessary.

Acknowledgments

This study was supported by Ministry of Science and Technology (MOST), Taiwan (108-2638-E-002-003-MY2).

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Chapter 2

Biochar and sustainable development goals

Xinni Xiong, Mingjing He, Shanta Dutta and Daniel C.W. Tsang,    Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, P.R. China

Abstract

According to the sustainable development goals set by the United Nations, great efforts should be made on improving soil health and developing sustainable green materials. As a renewable and economical carbonaceous material derived from biomass waste, biochar provides a nature-based solution in improving soil health and relieving energy crisis. Biochar has shown great environmental benefits by being extensively applied in soil amendment, soil remediation, and mitigation of greenhouse gas emissions. Novel and facile ways of various emerging modification strategies are being explored to synthesize cost-efficient biochar composites. In the future, interdisciplinary collaborations among multiple sides are suggested to boost the sustainable development in a green manner.

Keywords

Biochar; soil health; sustainable development goals; soil management

2.1 Introduction

Over the past years, the sustainable development goals (SDGs) become an attractive topic for sustainable development and international collaboration over the globe. The United Nations put forward 17 SDGs as the blueprint to build a greener and more sustainable future for the whole world. These SDGs aim to address various global challenges such as poverty, climate change, and environmental remediation (UN, 2015). In particular, soil health becomes a vital element among several SDGs, including SDG 1 (no poverty), SDG 2 (zero hunger), SDG 3 (good health and well-being), SDG 6 (clean water and sanitation), SDG 7 (affordable and clean energy), SDG 12 (responsible consumption and production), SDG 13 (climate action), and SDG 15 (life on land). The biochar soil application associated with SDGs can be illustrated in Fig. 2.1. Soil constitutes a significant section of the ecosystem and serves as a fundamental support for crop growth, thus acting as a determining factor for productivity and human well-being. Soil biodiversity closely affects crop diseases and assures the ordered working of ecosystem. The health of soil environment is not only the significant factor for eliminating poverty and hunger but also a critical condition to ensure water quality and greenhouse gas (GHG) emissions (Hou et al.,