Biochar Application in Soil to Immobilize Heavy Metals: Fundamentals and Case Studies

Biochar Application in Soil to Immobilize Heavy Metals: Fundamentals and Case Studies covers biochar’s application to soil heavy metal immobilization. The book covers biochar’s effect on soil micro-and macro-properties, assessment of heavy metal stability in biochar-treated soil, and long-term stability of heavy metals in biochar-treated soil. A notable feature of this book includes its extensive use of case studies. Chapters focus on small-scale field trials and medium to large-scale industrial applications of biochar to immobilize soil heavy metals. In addition, the flow of the whole book follows “mechanisms-to-applications-to-case studies, allowing readers to translate the fundamentals to practical applications.

This book provides soil and environmental scientists with the tools they need to build the links between micro-level surface chemistry and macro-level engineering performance.

• Covers the fundamentals and influence of pyrolysis temperature and feedstock on the fundamentals, which is very important for applications
• Includes sections that discuss the molecular interaction between biochar and heavy metals in soil
• Contains chapters with subsections that cover mechanisms, applications and case studies, thus allowing readers to quickly grasp content

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 28, 2023
• ISBN: 9780323854603

Book preview

Chapter 1

An overview of biochar application in soil to immobilize heavy metals

Zhengtao Shen,    School of Earth Sciences and Engineering, Nanjing University, Nanjing, P.R. China

Abstract

This chapter introduces the overview of soil heavy metal contamination and immobilization-based technologies. Biochar technology and its fitness for soil heavy metal immobilization are also introduced. This chapter provides the aims, objectives, and organization of the book.

Keywords

Biochar; heavy metals; immobilization; contaminated soil; soil remediation; overview; introduction

1.1 Soil heavy metal contamination and immobilization-based technologies

A large number of heavy metals are released to the environment annually due to human activities, such as mining, steel smelting, and fertilizing (Khan et al., 2021). The heavy metals can migrate and enter into soil through a range of pathways including atmospheric deposition, surface runoff, and groundwater flow. Soil itself has the capacity to retain a certain amount of heavy metals and avoids their exposure to the receptors (e.g., humans). However, when heavy metals continue to enter into soil from a source (e.g., mining), their concentrations will accumulate and exceed the sorption capacity of soil. The soil can no longer immobilize more heavy metals, and therefore the heavy metals migrated through a pathway (such as groundwater or surface waters) to a human environment or ecosystem. The receptors (typically humans) finally inhale or ingest the contaminants directly or indirectly and have health problems. This is the source–pathway–receptor model used to illustrate the soil heavy metal contamination process (Li et al., 2022). The heavy metal contaminated soil can cause risks to humans in a range of ways. The heavy metals can enter the water source of humans through underground water flow, and humans can ingest the contaminants from the water source directly (drinking) or indirectly (e.g., swimming and fishing). Moreover, the crops (e.g., wheat, rice, and corn) grown on the contaminated soil can uptake the heavy metals and accumulate them in their biomass to be consumed by humans, or soil particles attached to crops may be ingested. All of these can cause serious health problems to humans, such as cancer, skin irritations, reproductive damage, mental health problems, or death (Kan et al., 2021; Rajendran et al., 2022; Zerizghi et al., 2022).

The term heavy metal is rather an industrial than academic definition. Geochemists and a range of researchers prefer the term trace metal instead. When talking about heavy metals, they typically include lead (Pb), cadmium (Cd), nickel (Ni), zinc (Zn), copper (Cu), mercury (Hg), chromium (Cr), antimony (Sb), etc. Metalloids (e.g., arsenic (As)) are always studied together with heavy metals. When referring to heavy metals in this book, they may include metalloids although not academically correct.

Heavy contamination has been an increasing problem all overthe world. It has been reported that 3.5 million sites from industrial and mine land, landfills, energy production plants, and agricultural land are potentially contaminated in Europe (Petruzzelli, 2012). The United States Environmental Protection Agency estimated that there were more than 200,000 contaminated sites which will cost over US$187 billion to remediate in the United States. China is one of the most recent countries that conducted its national soil survey: according to the national soil survey report released by the Chinese government, 19.4% of agricultural land and 34.9% of former industrial land surveyed are contaminated, among which 82.8% has exceeding concentrations of heavy metals (Liu et al., 2022). According to United Nations Environment Programme, approximately 1/4 deaths in the world annually are caused by environmental problems directly or indirectly. Soil heavy metal contamination is one of the main environmental problems.

Immobilization-based technology is undoubtedly the number 1 remedy for heavy metal contaminated soil (Shen et al., 2019). Unlike organic contaminants, heavy metals cannot be destroyed. Although some technologies, such as soil washing and phytoextraction, try to remove heavy metals from the soil, their practicability remains questionable. Immobilization-based technology dominates the remedial market for heavy metals contaminated soil owes to its ability to achieve remediation objectives rapidly and at relatively low cost (Shen et al., 2019). It is also versatile, being applicable in situ or exsitu, and effective for a wide range of soil textures (Shen et al., 2018).

Immobilization-based technology originates from solidification/stabilization (S/S) technology. S/S aims to limit the release of harmful chemicals from hazardous wastes and was developed in the late 1950s for the management of sludge, and later adapted to soil remediation (Shen et al., 2019). It became the number 2 soil remedy in the US Superfund program, next only to physical separation, and subsequently gained popularity in Canada and the United Kingdom in the 1990s, and in France and the Netherlands in the 2000s (Shen et al., 2019). With the development of innovative remedial materials, such as commercial apatite, reactive magnesia and biochar, immobilization (stabilization)-based technology became more popular since approximately 2010 (Shen et al., 2019). In comparison to S/S, immobilization-based technology does not need cement, a highly energy-intensive product. The remedial materials (e.g., apatite, reactive magnesia, and biochar) are friendly to soil environment, sometimes even enhancing soil properties for plant growth. Typically, the dosage of remedial material is far less than S/S, saving a lot of materials and costs. Therefore immobilization-based technology is regarded as green and sustainable, and it is now the number 1 remedy in China (Shen et al., 2019). The key for immobilization-based technology is to develop cost-effective and high-performance remedial materials.

1.2 Biochar technology and its fitness for soil heavy metal immobilization

Biochar is the solid residue resulting from the pyrolysis of biomass. The term biochar was developed in 2006 with a purpose of explicitly referring to soil management and carbon storage using specifically designed charcoal (Lehmann et al., 2006). The use of charcoal by humans can date back to 500–2500 years ago (Woods and Denevan, 2009; Lehmann et al., 2004), while the use of designed charcoal (biochar) to mitigate climate change is recent (since 2006). The details of biochar technology and its link with soil management have been summarized in a range of books and reviews (e.g., the book Biochar for Environmental Management: Science And Technology, second edition edited by Joseph, S. and Lehmann, J.).

Biochar is one of the most promising materials for soil heavy metal immobilization. Because biochar is porous with relatively large surface area, aiding its physical adsorption of heavy metals; biochar is typically alkaline and can precipitate a range of heavy metals, including Cd, Pb, Cu, Ni, and Zn; biochar is typically negatively charged in soil with relatively large cation exchange capacity, aiding its electrostatic adsorption and cation exchange of a range of heavy metals including Cd, Pb, Cu, Ni, and Zn; the surface functional groups of biochar (e.g., carboxylic and phenolic groups) can complex with Cd, Pb, Cu, etc. In addition to immobilize heavy metals, the addition of biochar to soil inherently means the storage of carbon. Therefore the research applying biochar in soil to immobilize heavy metals exploded recently (Fig. 1.1).

Figure 1.1 The evolution of immobilization-based technology (Shen et al., 2019).

One major challenge for biochar to immobilize heavy metals in soil is its long-term effectiveness. From a geochemical view, sorption is not a strong binding compared with other mechanisms, such as mineralization. The adsorbed metals on biochar have the risks to be released to the environmental undervarious physical, chemical, and biological field aging conditions. We generally lack the knowledge of how the efficiency of biochar immobilization of heavy metals will evolve in terms of both fundamentals and case studies.

1.3 Aims, objectives, and organization of the book

The primary aim of this book is to provide the state-of-the-art progress of biochar remediation of heavy metal contaminated soil from the perspective of both fundamentals and case studies. The objectives of this book are to:

1. Provide the overview of biochar application in soil to immobilize heavy metals;

2. Reveal the mechanisms underlying biochar-induced heavy metal immobilization;

3. Summarize biochar’s effect on soil properties;

4. Introduce the assessment methods of heavy metal stability in biochar-treated soil;

5. Summarize the long-term stability of heavy metals in biochar-treated soil;

6. Introduce biochar-augmented binders for sustainable SS of wastes;

7. Introduce biochar field cases at contaminated sites; and

8. Provide the perspectives of biochar application in soil to immobilize heavy metals.

This book consists of eight chapters. The chapters are summarized as follows:

Chapter 1 An overview of biochar application in soil to immobilize heavy metals

This chapter introduces the overview of soil heavy metal contamination and immobilization-based technologies. Biochar technology and its fitness for soil heavy metal immobilization are also introduced. This chapter provides the aims, objectives, and organization of the book.

Chapter 2 Underlying mechanisms involved in biochar-induced metal stabilization

This chapter critically provides a comprehensive overview on biochar’s immobilization mechanisms, with special focus on the element-dependent interactions. The effects of biochar type on metal immobilization have also been assessed. Furthermore, it is suggested that adsorption mechanisms in the aqueous media may not be the same as soil immobilization mechanisms, as indirect interactions contribute to metal immobilization in soil. Finally, although biochar may fail in metal stabilization sometimes, it is proposed that this failure can be a blessing in disguise to some extent, since mobilized metals can be effectively remediated using plant-based remediation techniques, especially phytoextraction.

Chapter 3 Biochar’s effect on soil properties

This chapter summarizes the potential roles of biochar on a variety of soil properties. The effects of biochar on soil organic matters, soil biological carbon sequestration, soil organic matter mineralization, and soil methane emission are discussed. The effects of biochar on soil nutrients, microbiology, and chemistry are summarized. Among other properties, little attention has been paid to the potential influence of biochar on the engineering properties of soils. This chapter provides some insight into the effects of biochar on soil engineering properties from the perspective of soil structure and hydraulic characteristics.

Chapter 4 Assessment of heavy metal stability in biochar-treated soil

This chapter introduces conventional and advanced techniques to assess the stability of metals in biochar-treated soil, the advantages and limits of these techniques, and the prospects of new and novel techniques. This chapter covers the concept of metal and metalloid mobility in soils, discuss, how this property can be measured in chemical and physical ways and look at examples, where such techniques were applied for biochar-treated soils.

Chapter 5 Long-term stability of heavy metals in biochar-treated soil

This chapter discusses the influence from the environment and biochar property on the long-term stability of heavy metals in biochar-treated soil. The real performance of biochar in field and pot experiments to immobilize heavy metals is summarized and discussed. Aging methods to predict the longevity are introduced. Strategies to enhance the longevity of biochar in heavy metal immobilization are suggested.

Chapter 6 Biochar-augmented binders for sustainable stabilization/solidification of wastes

This chapter introduces the role of biochar with different properties in different cement-based S/S for contaminated soil, sediment and industrial waste. Contaminated wastes cause severe risks to human health and the environment. S/S is a widely accepted and cost-effective approach for the treatment of various wastes. The typically used cementitious binders in the S/S approach showed low compatibility with some specific pollutants. It is important to develop a sustainable remediation approach that can utilize low-carbon materials for S/S of contaminated wastes, such as biochar.

Chapter 7 Biochar field cases at contaminated sites

This chapter introduces three representative biochar field cases for heavy metal immobilization in three different regions. The first case is a 3-year field study on a sandy contaminated site that applied a British hardwood biochar and compost in the United Kingdom. The second case introduces a 4-year field study in China. Wheat straw biochar was produced and amended to Cd-contaminated agricultural land to reduce Cd accumulation in rice. The third case introduces a 1-year field study in Brazil. These three cases provide valuable insights into the performance of biochar in heavy metal immobilization at field conditions.

Chapter 8 Perspectives of biochar application in soil to immobilize heavy metals

This chapter summarizes the main findings of the book and provides the future directions from both fundamentals and case studies to facilitate the application of biochar in the remediation of heavy metal contaminated soils.

References

Kan et al., 2021 Kan XQ, Dong YQ, Feng L, Zhou M, Hou HB. Contamination and health risk assessment of heavy metals in China’s lead-zinc mine tailings: a metaanalysis. Chemosphere. 2021;267.

Khan et al., 2021 Khan S, Naushad M, Lima EC, Zhang SX, Shaheen SM, Rinklebe J. Global soil pollution by toxic elements: current status and future perspectives on the risk assessment and remediation strategies-a review. J Hazard Mater. 2021;417.

Lehmann, 2006 Lehmann J, Gaunt J, Rondon Mg. Bio-char sequestration terrestrial ecosystems–a review. Mitig Adapt Strateg Glob Change. 2006;11(2):403–427.

Lehmann et al., 2004 Lehmann, J., Kern, D.C., Glaser, B., Woods, W.I., 2004, Amazonian dark earths: origins properties managgement. 2003rd Edition. Springer. Petruzzelli, G., 2012. Soil contamination and remediation strategies. Current research and future challenge. In EGU General Assembly Conference Abstracts (p. 7963).

Liu et al., 2022 Liu F, Wu HY, Zhao YG, et al. Mapping high resolution national soil information grids of China. Sci Bull. 2022;67(3):328–340.

Li et al., 2022 Li K, Wang JY, Zhang YW. Heavy metal pollution risk of cultivated land from industrial production in China: spatial pattern and its enlightenment. Sci Total Environ. 2022;:828.

Petruzzelli, 2012 Petruzzelli G. Soil contamination and remediation strategies. Current Research Future Challenge 2012;:7963.

Rajendran et al., 2022 Rajendran S, Priya TAK, Khoo KS, et al. A critical review on various remediation approaches for heavy metal contaminants removal from contaminated soils. Chemosphere. 2022;287.

Reddy, 2013 Reddy KR. Electrokinetic remediation of soils at complex contaminated sites: Technology status, challenges, and opportunities Coupled Phenomena in Environmental Geotechnics–Manassero et al Eds. 2013. CRC Press, Taylor & Francis Group 2013;131–147.

Shen et al., 2019 Shen ZT, Jin F, O’Connor D, Hou DY. Solidification/Stabilization for soil remediation: an old technology with new vitality. Environ Sci Technol. 2019;53(20):11615–11617.

Shen et al., 2018 Shen ZT, Li Z, Alessi DS. Stabilization-based soil remediation should consider long-term challenges. Front Environ Sci Eng. 2018;12.

Woods and Denevan, 2009 Woods WI, Denevan WM. Amazonian Dark Earths: Wim Sombroek’s Vision Springer 2009;1–14.

Zerizghi et al., 2022 Zerizghi T, Guo QJ, Tian LY, Wei RF, Zhao CQ. An integrated approach to quantify ecological and human health risks of soil heavy metal contamination around coal mining area. Sci Total Environ. 2022;:814.

Chapter 2

Underlying mechanisms involved in biochar-induced metal stabilization

Liuwei Wang and Deyi Hou,    School of Environment, Tsinghua University, Beijing, P.R. China

Abstract

Biochar has been widely used for the purpose of heavy metal immobilization in soil. The high heterogeneity of biochar’s physicochemical properties as a result of variations in feedstock selection and pyrolysis parameters, the different chemical speciations, and geochemical fractions of metals, along with the distinct properties of different soil taxonomies, render dynamic interaction mechanisms between biochar and metals in soil. This chapter critically provides a comprehensive overview on biochar’s immobilization mechanisms, with special focus on the element-dependent interactions. The effects of biochar type on metal immobilization have also been assessed. Furthermore, it is suggested that adsorption mechanisms in the aqueous media may not be the same as soil immobilization mechanisms, as indirect interactions also contribute to metal immobilization in soil. Finally, although biochar may fail in metal stabilization sometimes, it is proposed that this failure can be a blessing in disguise to some extent, since mobilized metals can be effectively remediated using plant-based remediation techniques, especially phytoextraction.

Keywords

Immobilization; mechanism; surface complexation; pyrolysis; adsorption

2.1 Introduction

Biochar has long been acknowledged to be a soil amendment that can improve soil fertility (Hou 2021, 2020a; Mao et al., 2012). Due to the tunable surface functionality and the well-developed porous structure, this carbon-rich material has been widely used as a metal immobilization agent (Hou et al., 2020b; Rajapaksha et al., 2016; Wang et al., 2020c). The metal stabilization performance of biochar in soil is highly associated with the underlying immobilization mechanisms. Compared with aqueous adsorption mechanisms that have been extensively investigated, biochar-metal interactions in the soil media may be much more complicated. Direct interactions, such as physical adsorption, surface complexation, surface precipitation, ion exchange, and cation–π interactions are thought to be the dominant metal adsorption mechanisms in the aqueous solution (Li et al., 2017; Tan et al., 2015; Wang et al., 2021). However, for soil remediation, the indirect electrostatic interactions with heavy metals and the soil particles, as well as the ion exchange between metals and soil cation exchange sites may also contribute to metal immobilization in soil (He et al., 2019b; Jiang et al., 2012). It is also noteworthy that for different metals, application of biochar may result in either immobilization of mobilization. For instance, an increase in soil pH will promote the stabilization of cations, while enhancing the mobilization of oxyanions, such as As and Sb (Ahmad et al., 2017; Hua et al., 2019). Therefore, it is imperative to understand the element-specific biochar-metal binding mechanisms to effectively use biochar as an amendment in heavy metal-contaminated soils.

This chapter provides a mechanistic understanding of biochar interaction with soil heavy metals. Both direct and indirect immobilization mechanisms and element-specific interactions are explored. To guide the proper selection of biochar as an effective metal stabilization amendment, the effects of biomass feedstock and pyrolysis conditions on metal immobilization performances are assessed. In addition, the feasibility of extrapolating metal adsorption mechanisms from the aqueous media to soil is discussed. Finally, it is proposed that, although biochar is thought to immobilize metals, it may also mobilize soil metals, thus assisting the phytoremediation process.

2.2 Mechanistic understanding of biochar interaction with heavy metals in soils

Biochar interact with soil metals via various direct and indirect mechanisms. The immobilization mechanisms for different elements may also vary greatly. Therefore, this section first provides a comprehensive overview of general metal immobilization mechanisms, and then discusses the element specificity of these mechanisms in depth.

2.2.1 Immobilization mechanisms

2.2.1.1 Physical adsorption

Physical adsorption (physisorption) is a reversible, weak interaction between soil metals and the biochar (Bruch et al., 2007; Thommes and Cychosz 2014). Caused by intermolecular force that exists between the adsorbent and adsorbate, physical adsorption is a general phenomenon that occurs in any solid/fluid or solid/gas system (Ponec et al., 2018; Yang 2013). Compared with chemisorption that involves the formation of covalent or ionic bonds, this interaction does not change the chemical bonding structure. It may not be an effective immobilization mechanism, since physically sorbed metals can be easily desorbed and remobilized (Shaheen et al., 2019b; Zhang et al., 2013).

2.2.1.2 Electrostatic interactions

The electrostatic interactions between biochar and metals are determined by the electrical state of the biochar surface (He et al., 2019b; Tran et al., 2017). The point of zero charge (PZC) refers to the solution conditions under which the surface charge density is zero. When pH > pHPZC, metal cations will be electrostatically adsorbed. In comparison, when pH < pHPZC, oxyanions will be bound with electron-depleted sites on the biochar surface. Therefore, the electrostatic interaction is highly dependent on soil pH. If the acidity-alkalinity of the remediated soil changes, the immobilized metals will be released, posing threat to the environment (Houben et al., 2013).

As a special type of electrostatic attraction force, the cation-π interactions refer to the electrostatic interactions where the metal cations are chemisorbed by the aromatic rings of the biochar (Tran et al., 2017). A number of aromatic π donors and acceptors can dominate this process. For instance, the electron-rich π clouds of a single benzene molecule can electrostatically adsorb heavy metals (Fig. 2.1). For more condensed aromatic rings with a much larger aromatic lattice, a stronger electrostatic interaction will be achieved. It is noteworthy that the presence of oxygen-containing or nitrogen-containing functional groups on the edge of the aromatic rings will diminish this effect, since these groups will draw electrons away, thus decreasing the electron density of the aromatic ring (Fig. 2.1) (Keiluweit and Kleber 2009; Martinez and Iverson 2012; Tran et al., 2017).

Figure 2.1 Schemes for describing the electrostatic view of aromatic interactions. Cartoons are presented to describe qualitatively aromatic quadrupole moments in the more electron-rich aromatic rings, such as benzene and 1,5-dialkoxynaphthalene (DAN), as well as electron-deficient aromatics, such as 1,4,5,8-naphthalenetetracarboxylic diimide (NDI) that contain strongly electron withdrawing groups. Reproduced with permission from Martinez, C.R., Iverson, B.L., 2012. Rethinking the term pi-stacking. Chem. Sci. 3, 2191–2201. Copyright 2012 Royal Society of Chemistry.

2.2.1.3 Ion exchange

Generally, biochar has a high cation exchange capacity (CEC), being capable of releasing H+, K+, Ca²+, and Mg²+ to exchange with metal cations (Hailegnaw et al., 2019; Munera-Echeverri et al., 2018). The exchangeable cations of the biochar originate either from the oxygen-containing functional groups (e.g., carboxyl and phenolic) or the inherent alkaline minerals (e.g., K2CO3 and CaCO3) (Hou 2020; Xu et al., 2017). Apart from cation exchange, biochar can also adsorb anions through ion exchange. Potential anion exchange mechanisms include the pH-dependent nonspecific proton adsorption, pH-dependent adsorption onto pyridinium groups (N heterocycles), and the pH-independent adsorption onto oxonium groups (sp²-O heterocycles) (Fig. 2.2). It is noteworthy that the metals sorbed via ion exchange are readily bioavailable and labile (Alozie et al., 2018). They may desorb from the biochar when pH changes significantly. Therefore, this may not be the dominant mechanism leading to long-term effectiveness for the immobilization