In recent years, due to the broad application of biochars, the preparation, environmental behavior and aging processes of biochars have attracted wide attention globally, especially the modification of biochars. However, most of the studies only consider the improvement of biochar properties right after the modification, but neglect a complete evaluation of the long-term stability and eco-toxicity of these newly developed materials after entering the environment. With the development and utilization of biochars, engineered biochars (EngBCs) will soon enter the market, but its environmental risk still remains unclear. The literature does not provide adequate information on how aging of EngBCs will affect their properties, and indirectly impact the properties of soils (cycle of elements and organic matter). Therefore, this review paper summarizes the aging process and environmental risk of biochars, aiming at better understanding the interactions between EngBCs and soil components or pollutants. More importantly, this review is to point out the contradictory speculations of environmental behavior of EngBCs studied at the present stage. Due to the modification, the EngBCs stability may be significantly reduced. However, the formation of functional group on EngBCs will enhance their interaction with soil minerals to form biochars–mineral complex, and thus EngBCs could be protected. The impacts of EngBCs after entering the environment are also ambiguous. Therefore, understanding EngBCs environmental behavior is critical, which is helpful to reduce the potential risk and to produce EngBCs following the rule of sustainable development and safety to the environment.
| • | Sand size can affect microbial retention in biochar-amended sand |
| • | Effect of sand size on bacterial retention varied depending on bacterial strain |
| • | Minimal retention of microspheres within biochar-amended sand columns indicates minimal physical straining |
In the Amazon region, several residues that have been misused can serve as feedstocks for biochar production with the aim of recovering soils contaminated by heavy metals. However, these biochars need to be firstly tested for their adsorption capacity as well as their physicochemical attributes prior to field application. Therefore, this study aimed to characterize and evaluate the adsorption capacities of Cd2+ and Cu2+ of biochars produced from acai (BA), Brazil nut (BN), and palm kernel cake (BK) residues. Biochars were produced by slow pyrolysis at four different temperatures (400, 500, 600, and 700 °C). The physicochemical properties of the biochars, such as cation exchange capacity, ash, recalcitrance index, and aromaticity were enhanced with increased pyrolysis temperature. The adsorption capacities of Cd2+ and Cu2+ showed high correlations with the physicochemical properties of biochar, indicating the importance of these characteristics in the adsorption process. Furthermore, the adsorption of Cd2+ and Cu2+ also increased with the increase in the pyrolysis temperature. In a competitive system, Cd2+ exhibited higher adsorption capacity than Cu2+ for all biochars. In general, BN showed the highest adsorption capacity, followed by BK and BA. Biochars produced from the Amazonian residues have the potential to improve soil quality when used as amendments in the recovery of soils contaminated with Cd and Cu, representing an environmentally sound technology for the reuse of these residues.