**5. Mechanisms of PTMs adsorption into biochars**

The mechanisms proposed for PTMs immobilization are explored in [56, 57]. In summary, three mechanisms are mostly responsible for PTMs retention into BCs, among them: (1) PTMs exchange with calcium (Ca2+), magnesium (Mg2+), and other cations associated with BCs (**Figure 7**); (2) complexation of PTMs into different surface functional groups, as previously highlighted (see Section 4); and (3) the physical adsorption followed by surface precipitation contributing to PTMs immobilization.

In the first case, the PTMs/cations exchange is further attributed to the co-precipitation of PTMs and their innersphere complexation with complex humic matter and mineral oxides contained in the biochar [57]. First, the strengths of PTMs adsorption into BCs surface are low due to the presence of water molecules surrounding the ion (oscillation in the distance of the electrostatic retention). Latterly, water molecules are released and the affinity to complexation enhances depending on the composition and structure of the biochar reactive surface, thus a much stronger inner-sphere complex is formed disfavoring the PTMs release back to the soil solution. In the second case, the PTMs inner-sphere complexation is with the free hydroxyl of mineral oxides surface (OH) and other surface precipitation [57]. In the third case, the surface precipitation of PTMs occurring is designated as *amorphorse*, since biochars present an amorphous although highly reactive structure, similarly as the organic matter compounds [58].

### **Figure 7.**

*Mechanisms of cation exchange between positively charged ions contained in the reactive surface of biochars and potentially toxic heavy metals ('heavy metal') dissolved in soil solution. The graph was made by the authors.*

**187**

**Author details**

João Arthur Antonangelo\* and Hailin Zhang

Stillwater, OK, United States of America

provided the original work is properly cited.

Department of Plant and Soil Sciences (PSS), Oklahoma State University (OSU),

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

application rate and frequency to improve soil PTMs remediation efficiency.

\*Address all correspondence to: joao.antonangelo@okstate.edu

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

Most studies from the last decade have demonstrated the strong potential of BCs in reducing leachability and immobilizing PTMs pollutants in soils that were previously phytoavailable. By increasing soil pH, BCs act as liming materials and PTMs precipitate as insoluble PTMs hydroxides in a high pH environment. Additionally, the greater SOC attributed to the BCs addition to multi-metal contaminated soil contributes to the formation of PTMS-organic anion complexes that are precipitated out from the system, since the previous bioavailable PTMs contents were much higher than usual. The characteristics of BCs vary widely with different feedstocks (biomass materials) and pyrolysis conditions (low and high temperatures). Generally, there are three mechanisms related to the direct removal of PTMs (and other pollutants) towards BCs reactive surface, which are related to a strong sorption and weak desorption of cationic PTMs then indicating that BCs

The current works emphasizing the use of biochar for soil remediation purposes have mainly been conducted in laboratories and/or greenhouses on a small scale with controlled conditions. As pointed out by [57], large-scale field trials are essential before operational scale remediation projects are implemented. Since the BC properties are largely varying, and sometimes contrasting, it is important to design BC products for every specific remediation project. The BCs ability to sequestrate may lead to the accumulation of PTMs contaminants in the amended soils in the long-term, and yet the pollutant environmental fate over time is not well elucidated. It is well known that the capacity of BCs to adsorb and/or complex PTMs decreases with time as a consequence of the aging process and saturation. Therefore, it is strongly encouraged to research on the aging process of BCs activity in the future because such information would help in the decision-making of the BC

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

sequesters pollutants in itself.

**6. Conclusions and final considerations**

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