Keywords

6.1 Introduction

Heavy metals contamination in soil and water has become an alarming issue in the recent times due to accelerated anthropogenic activities with increasing industrialization and decreasing land holdings leading to various health hazards as well as their adverse effect on the quality of soil and water (Singh et al. 2013; Bhatia et al. 2015; Matta et al. 2018; Gupta et al. 2019). Heavy metals are generally found in the soil environment in traces which are a result of weathering of rocks and minerals over the earth crust. These heavy metals become pollutant only when they cross a certain threshold limit due to anthropogenic activities which accelerate the release and addition of these metals into the environment, soils and water bodies. The sources of the heavy metals pollutions may be fertilizers, pesticides, bio solids, manures, metal mining, waste water, milling process, industrial wastes and airborne sources like dust or smoke from storage areas or waste dumps (Wuana and Okieimen 2011). The major heavy metal (loid)s pollutants commonly found in the contaminated environment are lead (Pb), chromium (Cr), arsenic (As), zinc (Zn), cadmium (Cd), copper (Cu), mercury (Hg), and nickel (Ni) (GWRTAC 1997). The common source of heavy metals includes cars, mining activities such as smelting, mine spoils and trailing, antiseptics, self-cleaning ovens, plastics, solar panels, mobile phones and particle accelerators (Koduru et al. 2017; Hubner et al. 2010). In addition to that the potential sources pyro-metallurgical industries, automobile exhausts, fossil fuel combustion are also the main sources of heavy metals (Lottermoser 2010; Prasad 2001), industries such as plastics, textiles, microelectronics, wood preservatives, refineries, agrochemicals (excessive use of fertilizers and pesticides) and waste disposal from sewage treatment plant, leachate from landfill and fly ash disposal some of the chief sources of the heavy metals (Singh and Kumar 2006; Kumar et al. 2016). Although, heavy metals content in domestic municipal sewage wastewater is mainly below threshold level (Sharma et al. 2014; Malla et al. 2015; Khan et al. 2019). The details about the sources of heavy metals and respective anthropogenic activities are given in Table 6.1.

Table 6.1 Anthropogenic sources of heavy metals
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Various research has been carried out all over the world towards remediation of the heavy metal contamination. This remediation may be by physical, chemical or biological means. Physical remediation is effective but cost-intensive and poses a risk of secondary diffusion while chemical methods cause secondary pollution to the environment. Biological methods, though safe to use but time consuming (Yang et al. 2019). Hence, in the last decade, the application of biochar has become the trend in addressing heavy metal contamination. Biochar based remediation are cost-effective as well as quick in action and raw material for biochar production (biomass) is easily available as biomass contributes 35% of the entire energy requirement of the world and it is the third-largest source of energy (Armynah et al. 2018). This chapter gives an insight into the action of biochar towards remediation of the heavy metal contamination.

6.2 Characteristics of Biochar

Biochar derived from two words, bio and char. Char means partially burnt to become blackened. So literally, biochar means charred biomass. It is carbonaceous or carbon-rich solid obtained by thermochemical conversion of biomass-derived from various sources like manure, leaves or wood at a temperature range of 300–1000 ° C (Xie et al. 2015) in a partial or anaerobic condition. The products of this conversion may be heat, power, fuel and/or chemicals. The thermochemical processes involved in the production of biochar are pyrolysis, gasification, hydrothermal carbonization and microwave carbonization. Biochar when produced from woody biomass, called as charcoal. Biochar is a recalcitrant and stabilized organic C compound. Biochar is produced from a varied range of sources or biomass. Many studies have been carried out towards the characterization of biochars derived from different sources. Biochar can be produced from forest species like acacia, gmelina, eucalyptus, pine (Suárez et al. 2017; Zhang et al. 2018) and animal manures like yak manure (Zhang et al. 2018) and chicken manure (Domingues et al. 2017) and from agricultural field crop by-products like coffee husk, sugarcane bagasse and rice husk (Armynah et al. 2018; Domingues et al. 2017). Biochar also has a range of application as wide as its sources. It is used in agriculture for fertility enhancement (Domingues et al. 2017) and amendment of soil acidity (Zhang et al. 2018), C sequestration (Domingues et al. 2017), wastewater treatment and environmental application for bioremediation. The application of biochar for any specific purpose depends upon its physicochemical properties. Physiochemical properties of biochar largely depend upon the type of feedstock from which it has been derived and the operation temperature conditions (Suárez et al. 2017; Yang et al. 2019; Armynah et al. 2018; Jindo et al. 2014; Zhang et al. 2018; Domingues et al. 2017). In most of the studies, the pyrolysis temperature has been maintained in the range of 300–800 ° C (Jindo et al. 2014; Zhang et al. 2018). Many studies have used pyrolysis technique for production of biochars (Yang et al. 2019; Jindo et al. 2014; Zhang et al. 2018; Domingues et al. 2017). Other than that, the process like flash carbonization and torrefaction have also been used (Yang et al. 2019).

6.2.1 Characterization of Biochar

For determination of morphological and physical properties like surface area and pore size distribution of biochar, Breuner Emmet and Teller (BET) physio-sorption method is mostly used (Suárez et al. 2017; Armynah et al. 2018; Jindo et al. 2014; Zhang et al. 2018; Nartey and Zhao 2014; Askeland et al. 2019). For surface characteristics and elemental analysis, Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) can be used (Jindo et al. 2014; Jechan et al. 2017; Bouraoui et al. 2015). Chemical bonding and structural properties of biochar can be determined by Fourier Transform Infrared (FTIR) Spectroscopy (Nartey and Zhao 2014; Askeland et al. 2019). Contaminant and heavy metals in biochars are determined by Atomic Absorption Spectrometer (AAS) and Inductively Coupled Plasma Mass Spectrometer (ICP MS) (Askeland et al. 2019). For analysis of structural elements C, H and O the CHN analyzer can be used. Biochars can also be characterized and differentiated based on elemental ratios like H/C, O/C or C/N.

The specific surface area of biochar (6.86 m2/g), micropore area (0.17 m2/g), total pore volume (22.29 mm3/g), and micropore volume (0.02 mm3/g). Ash content and pH were 42.25% and 10.09, respectively. Content of C element (48.45%), H (1.78%), N (1.47%), and S (0.78%) elements as shown in Table 6.2.

Table 6.2 Characterization of biochar
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6.2.2 Morphological Characteristics

The surface area and porosity of final biochar product depends on feedstock type and operation temperature. With increase in temperature, the surface area and total pore volume of the biochar increases and the particle size decreases for woody biochar (Suliman et al. 2017), biochar from agricultural products (Suárez et al. 2017) or from animal manures (Zhang et al. 2018). With increase in surface area the CEC of biochar also increases. Porosity and surface area of biochar material are important parameters for their use as remediation of contaminants.

6.2.3 Composition of Biochar

At low temperatures the biochar yield is higher and with increasing temperature the yield of biochar has been reported to decrease while ash and C contents have found to be increase (Jindo et al. 2014; Domingues et al. 2017). The increase in total C content of woody biochar at high temperatures may be contributed to higher degree of polymerization resulting to more condensed C structure (Lehman and Joseph 2009). Therefore, an increase in reaction temperature leads to an increase in carbon content, and decrease in the hydrogen and oxygen content (Jindo et al. 2014). In contrast, animal manure such as chicken manure (Domingues et al. 2017) and yak manure (Zhang et al. 2018) derived biochars show decrease in C content with increasing temperature which might be due to the high labile nature of organic compounds found in animal waste which are rapidly lost before biochar formation (Domingues et al. 2017). The different types of biochar on the basis of feedstock with removal of heavy metal from soil and water is given in Table 6.3. C and O are dominant elements present in biochar derived from any feedstock. Other than C and O the elemental composition varies according to feed stock and the percentage of elements also varies with operation temperatures. Apart from C and O biochar consists mostly of basic elements Ca, Mg, K (Armynah et al. 2018). For example, biochar derived from rice plant by products have Silicon as a dominant element (Armynah et al. 2018; Nwajiaku et al. 2018). With increase in pyrolysis temperature, the content of the basic elements Ca, Mg, K increases (Armynah et al. 2018; Zhang et al. 2018; Nwajiaku et al. 2018). Some studies also show that animal manure derived biochars contain traces of Fe, Cu, Mn and Zn (Zhang et al. 2018).

Table 6.3 The removal of heavy metals by biochar and their effects
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The amount of volatile compounds also gradually decreases with increasing pyrolysis temperature. The lignin containing biomass or woody biomass show a large change in volatile component with increasing temperature, compared to non woody species because of lignin decomposition at higher temperatures (Jindo et al. 2014).

6.2.4 Acidity/Alkalinity

The pH of biochar is an important property with regards to its use as soil amendment or as remediation of contaminated soils (Zhang et al. 2018). Because the changes in pH greatly influence some of the soil processes such as precipitation of minerals, nitrogen mineralization and ion exchange (Novak et al. 2016; Dai et al. 2018; Fidel et al. 2017). The resultant pH of biochar is dependent upon the temperature and type of feedstock. Generally, the pH value of biochar increase with increasing pyrolysis temperature (Jindo et al. 2014; Zhang et al. 2018; Suliman et al. 2017). Increase in pH at higher temperature may be due to increase in concentration of inorganic elements which are not pyrolysed and basic oxides formed at the surface under high temperature of pyrolysis (Zhang et al. 2018). The other reason may be the detachment of functional group during pyrolysis which is principally acidic in nature, such as hydroxyl, formyl or carboxyl groups (Weber and Quicker 2018; Tiwari et al. 2019a, b; Singh et al. 2019; Kour et al. 2019). Therefore removal of acidic functional group during pyrolysis shifted the pH to more basic condition. Hence, an increased pH-value is mainly contributed to increasing degree of carbonization (Quicker et al. 2016).

6.3 Mechanism of Biochar Interaction with Heavy Metals in Soil

The main reason behind application of biochar in soil is increasing the population of hydrocarbon-degrading microbes (Karppinen et al. 2017). Heavy metals cannot be degraded completely from the soil and water, but it can be transformed from more toxic to less toxic form. It can also be accumulated in plants and animals. Therefore, there are mainly four commonly used strategies for bioremediation of heavy metals.

  1. 1.

    Passive adsorption of ionic cations on the surface of biochar from the soil

  2. 2.

    Precipitation of anions like phosphate, carbonates, silicate, and chloride

  3. 3.

    Complexation with functional groups on the surface of biochar

  4. 4.

    Nutrients release such as N, P, K, Ca, Mg, S etc.

The processes passive adsorption, precipitation and complexation can reduce the bioavailability of heavy metals in the soil and further reduce the phytotoxicity to plants, on the other hand the nutrient release mechanism can increase the availability of nutrients to plants and microorganism in rhizosphere. The different mechanism of bioremediation and phytoremediation are given in Fig. 6.1. The mechanism of biochar application in phytoremediation is shown in Fig. 6.2. The mechanism of biochar application in phytoremediation is shown in Fig. 6.1. Phytoremediation includes the phytostabilization, phytoextraction, phytodegradation and phytovolatilization. Phytostabilization and phytoextraction are the commonly used technologies for the bioremediation of the contaminated agricultural soils. In phytoextraction, generally used the fast-growing hyper-accumulators or energy plants to uptake the heavy metals. The biochar amended phytoremediation from agricultural soils was performed using Brassica napus (Houben et al. 2013), Anthyllis vulneraria and Noccaea rotundifolium (Fellet et al. 2014), Brassica juncea (Rodrıguez-Vila et al. 2014), Spinacia oleracea and Brassica napus (Kelly et al. 2014), Amaranthus tricolor (Lu et al. 2015), Clitocybe maxima (Wu et al. 2016), Cassia alata (Huang et al. 2018), Solanum nigrum (Li et al. 2019) and Lycopersicon esculentam (Bandara et al. 2017). The impact of various dose of biochar and different plant species in bioremediation are mentioned in Table 6.4.

Fig. 6.1
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Mechanism of remediation of heavy metals absorbed from soil in plants

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Fig. 6.2
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Mechanism of combining effects of phytoremediation and biochar for bioremediation of heavy metal contaminated soil. Cationic metal ions expressed as M2+. (Source: Wenjie et al. 2018)

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Table 6.4 Effects of biochar application on bioremediation and phytoremediation of contaminated soils
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The alkaline nature and negatively charged surface of biochar help it to adsorb and retaining the heavy metals ions. Inspite of this, biochar also provide support and favourable micro environment for benificial microbes, nutrient supply to plant roots and contributes in the soil strcture development. Namgay et al. (2010) reported that application of biochar in contaminated soil reduces the availability of heavy metals which lead to reduction of absorption of heavy metal to plants. Apart from that biochar also increases the soil pH (Novak et al. 2009) hence might have improved sorption of these metals, consequently decreasing their bioavailability for plant uptake.

6.4 Interactions Between Biochar and Metals

These metals occur as their cations, anions or complexes. The form of occurrence depends upon the reaction (pH) (Park et al. 2019) and the aeration status (aerobic or anaerobic) of the environment. The mechanism of interaction of these metal (loid)s with biochar depend upon the form of their occurrence and the biochar characteristics (Ahmed et al. 2014) like surface area, porosity, content of exchangeable cations or anions and elemental composition of biochar which ultimately depends upon the feedstock type and operation temperature. Ion-exchange, electrostatic attraction and precipitation are predominant mechanisms for the remediation of inorganic contaminants by biochar (Ahmed et al. 2014). The increase in soil pH due to application of biochar also results in the immobilization of the metal cations (Egene et al. 2018).

Cd interacts by mechanisms of chemisorption, cation exchange, precipitation by carbonate (CO32−) and phosphate (PO43−) and complexation with surface functional groups like carboxyl (COO−) and hydroxyl (OH−) groups (Singh et al. 2017a, b, c, 2018; Tiwari et al. 2018; Park et al. 2019; Chen et al. 2019). In its compounds Cd occurs as divalent Cd (II). Adsorption of Cd2+is reported to be dominated by boundary layer diffusion and very small amount of intraparticle diffusion or adsorption due to diffusion by pores (Park et al. 2019). At the surface, the interaction is mainly by exchange with cations of Ca, Mg and K but intra particle interaction is due to precipitation by PO43−(Park et al. 2019; Chen et al. 2019). Pb2+ follows similar sorption mechanism as Cd2+and interacts with biochar by mechanisms of chemisorptions, cation exchange, precipitation by carbonate (CO32−), phosphate (PO43−) and silicate (SiO42−) (Wang et al. 2018; Chen et al. 2019). Pb is released as Pb2+, metal oxyanion complexes and its oxides and hydroxides in to the soil and water environment. But most stable forms are Pb2+ and lead hydroxy complexes (Wuana and Okieimen 2011). However, due to easy hydrolysis of Pb at low pH, biochar has been reported to have a higher affinity for Pb than Cd (Chen et al. 2019). In aqueous solution, the sorption of these divalent metal cations is pH dependant due to their strong tendency to be hydrated in aqueous solution (Dong et al. 2017). According to a study, adsorption of Cd increased with increasing pH up to 5 and maximum adsorption occurred at pH from 5–8 and the adsorption decreased gradually from pH 9 (Park et al. 2019). Similar behaviour has been reported for Pb adsorption with increasing pH. This behaviour is due to formation of hydroxide or carbonate complexes at higher pH (Park et al. 2019; Wang et al. 2018).

The dominant and toxic form of Cr found at contaminated sites is Cr (VI) and major species are chromate (CrO42−) and dichromate (Cr2O72−). At reduced condition it may occur as Cr (III) (Wuana and Okieimen 2011). The mechanism of Cr (VI) removal by biochar is governed by sorption due to electrostatic attraction of anionic Cr (VI) by protonated biochar surface, where it is reduced to Cr (III) by carboxylic and phenolic hydroxyl group. Other mechanism is sorption and reduction mediated by surface organic complexes (Cr (III) - DOM complex) of biochar, and direct aqueous reduction by dissolved organic matter (DOM), derived from biochar (Choudhary et al. 2017).

Arsenic is a metalloid contaminant. In aerobic environments the dominant form is As (V) and it occurs as arsenate ion. Cr and as occur as their anions (H3AsO4, H2AsO4−, HAsO42− and AsO43−) and hence, the positively charged biochar surfaces are important for their adsorption. Under reducing condition, dominant form is As (III). As may interact with biochar by anion exchange at surface with anions like PO42−, surface complexation with carboxylic and phenolic groups, adsorption on surface by metal bridging and DOM mediated reduction and physical adsorption (Vithanage et al. 2017).

6.5 Impacts of Biochar on Agriculture Productivity

The application of biochar has resulted in to improve in soil fertility, increase crop productivity, and water holding capacity of soil etc. as given below.

6.5.1 Soil Property and Crop Productivity

The excess use of fertilizer lead to leaching of excess nutrients, runoff losses of nutrients especially nitrogen to nearby water bodies, reduce soil fertility, acidification of soil, etc. On the other hand application of biochar in soil increases the organic carbon stock, helpful for maintaining the population of beneficial microorganism, improves physico-chemical properties, cation exchange capacity of soil, reduce the leaching of soluble macronutrients and therefore enhancing the soil fertility and crop yield. Biochar application also improves the water holding capacity of soil by improving soil quality in terms of physical chemical and biological properties of soil. An overview of combining biochar application with phytoremediation is given in Fig. 6.3.

Fig. 6.3
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A summary of the potential positive effects achieved by combining phytoremediation and biochar in heavy metal pollution remediation

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6.5.2 Carbon Sequestration

As biochar is rich source of organic carbon, so have a potential role in carbon sequestration in soil. Biochar degrade slowly in soil, hence play an important role in built up of organic carbon in soil. Carbon sequestration improves the organic carbon status of soil. As organic carbon is tightly bound to soil particle, results in to less emission of CO2 to the atmosphere from agriculture. Therefore biochar has an effective role in mitigating greenhouse gases emission and climate change.

6.5.3 Waste Management

Biochar is produced from waste material (municipal sewage waste, biomass, chicken waste etc.). Since the waste material is reuse for production of biochar, therefore the biochar production process is sustainable technique and economically viable.

6.6 Conclusions

Globally, heavy metal pollution is a severe problem. Several in-situ bioremediation methods are used for treating contaminated soil and water. Biochar application is one of the environment friendly and sustainable technology for treating heavy metal contaminated soil. Various types of biochar on the basis of feedstock used are hardwood, chicken manure, oak wood, rice straw, dairy manure etc. The heavy metals such as Cr, Zn, Pb, Cu, As are removed by biochar form contaminated soil and water through mechanism such as complexation, adsorption, precipitation and stabilization. In spite of decreasing bioavailability of heavy metal to soil, biochar application also improve soil physicochemical properties, organic carbon content, increase in pH and CEC of soil, maintaining the population of beneficial microorganism and waste management. Hence, it can be concluded that biochar application to contaminated soil and water plays a dual role of controlling soil and water pollution and improving the soil fertility.