**1. Introduction**

Biochar is a carbon-rich by-product produced from the thermochemical conversion of biomass feedstock under partial or total absence of oxygen [1, 2]. Feedstocks used in biochar (BC) production are mostly wood, municipal and agriculture wastes [3–6]. Amending soil with biochar has received increased attention as a method for carbon sequestration in soils, thereby reducing carbon dioxide (CO2) emissions [7–9] and improving soil quality due to the vital role of carbon (C) in soil physical, chemical and biological processes [10].

Amending soil with biochar has been practiced for a long time. The high fertility of anthropogenic dark earth soils known as 'Terra Preta de Indio' in the Amazon basin has been related to the high content of charred materials [11–13]. Historically, the source of char in these soils has been considered as a disposal of charcoal from domestic fires and the practice of slash and char agriculture by Pre-Columbian Amazonian Indians [11, 14]. Hence, these soils have remained fertile and rich in biochar derived C stock for hundreds to thousands of years after they were abandoned.

In addition to the role of biochar in increasing the C sequestration and influencing the reduction of CO2 emissions, biochar has been shown to enhance soil quality and to stabilize PTMs [15]. Biochar has a potential benefit for improving soil fertility [16, 17], improving soil properties such as pH [11–13, 18], cation exchange capacity (CEC) and water holding capacity [19], enhancing plant growth [20], and reducing nutrient leaching losses [21]. The significant amount of calcium (and magnesium) carbonate (Ca/MgCO3) in BCs enables them to function as lime materials providing Ca and Mg to plants and neutralizing acidity when applied to acid soils.

The role of biochar in improving soil pH, organic carbon (OC), and CEC was also highlighted by [16]. Moreover, biochar can immobilize PTMs (immobilization is the reduction of the potential migration of PTMs to plants, or reduction of phytoavailability) such as cadmium (Cd), lead (Pb), and zinc (Zn) and thereby to reduce the phytoavailability of PTMs (concentration of PTMs in plant parts, or contents of PTMs in soils available to plants) to plants in contaminated soils, notably because it raises the soil pH [18, 22] and increases CEC and OC [23]. Many studies also found biochar application promotes the ability to remove organic contaminants [24, 25]. Because of its porous structure and diverse functional groups [26], biochar has been widely used in the field of agriculture and environmental protection [27] due to its ability to improve soil health and crop yields, and sequestering carbon, immobilizing PTMs and adsorbing organic pollutants such as polycyclic aromatic hydrocarbons (PAHs).

For these reasons, studies on biochar land-application have exponentially increased in the last 20 years (**Figure 1**). During the same period (1999–2018), the word 'potentially toxic metal' or 'heavy metal' places itself in the **top 5** within the 25 keywords used in biochar researches, numbering **308** publications [6]. Therefore, this chapter is to provide a summary of the most recent studies on biochar use to improve soil quality and to immobilize the phytoavailability of PTMs to plants of agricultural importance. The main goal is to improve our

### **Figure 1.**

*Number of publications (NP) of biochar studies since 1999 (adapted from [6]). RMSE: Root mean square error value of the exponential model adopted.*

**179**

**Table 1.**

*The Use of Biochar as a Soil Amendment to Reduce Potentially Toxic Metals (PTMs)…*

understanding of biochar production and application as a soil remediation

**2. Biochar increases soil pH and soil organic carbon content and affects** 

The effects of pH rising in soils are significantly influenced by biochar addition more than by other practices such as liming [28]. **Table 1** shows some of the main characteristics of BCs (pH included) as affected by feedstock sources and pyrolysis temperature. Biochar is superior to lime to remediate PTMs-polluted areas, mainly because acidic conditions can lead to the leaching of metals and threatening of groundwater [28]. Biochars can supply OC and raise soil pH, but lime only increases soil pH. Hence, poultry litter-derived biochar (PLB) proves itself as very effective in immobilizing Cd, even under strong acidic conditions, thus preventing Cd leaching

> **TC (wt%)**

Poultry litter 450 — 38 — 2 9.9 [16]

4-Wood pellets — 28.2 — 0.05 7.2

2-Poultry litter 350 2.7 38.4 — 4.1 7.4

2-Eucalyptus 600 — 81 — 1.1 10.4

*Summary of some biochar properties as affected by feedstock sources and pyrolysis temperature.*

*TC: total carbon. TOC: total organic carbon. TN: total nitrogen. "—": not given.*

1-Swicthgrass 350 1.4 42.6 — 0.9 5.2 [23]

1-Pig manure 350 — 31.6 — 3.8 8.3 [25]

1-Poultry litter 400 — 16.8 — 1.4 10 [28]

700 1.7 31.4 — 0.7 10

700 3.9 27.8 — 1.6 10

700 — 25.2 — 2.1 9.5

350 — 66.7 — 7.6 6.2 700 — 74.2 — 5 6.4

550 — 33 — 0.85 13

**TOC (wt%)**

330 55 82 80 — 9.7 [13]

600 — 53.5 — 0.31 10 [18]

500 — 23.7 — 0.8 9.4 [19]

— 25 — 0.85 9.3

— 28.4 — 0.14 7.6

**TN (wt%)** **pH Reference**

technique and to serve as the basis for future research work.

*DOI: http://dx.doi.org/10.5772/intechopen.92611*

**Pyrolysis temperature (°C)**

**Water (wt%)**

**PTMs phytoavailability**

**Biochar feedstock**

Wood of 'Quaresmeira' (*Tibouchina arborea*)

Miscanthus (Miscanthus × giganteus) straw

1-Switchgrass straw

2-Anaerobically digested fiber

3-Softwood bark

2-Deashed pig manure-biochar *The Use of Biochar as a Soil Amendment to Reduce Potentially Toxic Metals (PTMs)… DOI: http://dx.doi.org/10.5772/intechopen.92611*

understanding of biochar production and application as a soil remediation technique and to serve as the basis for future research work.
