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.

The addition of biochar, sodium silicate, or earthworm is a feasible practice to repair soils disturbed by mining activities, and the reclamation is largely based on the alteration of the diversity and structure of soil bacteria. The objectives of this study were to assess the relative importance of these supplements on soil bacterial community diversity and structure in reclaimed mine areas. A field experiment with soybean was carried out in mining areas to assess the efficiency of nitrogen, phosphorus and potassium (NPK) fertilizers plus those supplements on soil bacterial community structure and diversity by the 16S rRNA sequencing method. Soil chemical properties were analyzed to their effects on the bacterial community structure. The results showed that the application of nitrogen, phosphorus and potassium (NPK) fertilizers significantly increased bacterial diversity, and a further increase was observed in NPK plus biochar, sodium silicate or earthworm addition. Furthermore, a higher number of genera were found in the NPK plus biochar and NPK plus earthworm treatments than that in the control, NPK and NPK plus sodium silicate treatments. The bacterial community was significantly associated with nutrients, such as carbon (C) and nitrogen (N). Moreover, soil organic carbon (SOC) and pH were the most dominant factors in shaping the soil bacterial community structure and diversity. Our data indicate that the addition of earthworms to soil rather than biochar and sodium silicate was the best strategy to mitigate the detrimental effects of mining activities on soil bacterial diversity.

The individual and combined effects of biochar (B) and inorganic fertiliser (F) have all been widely proofed to improve soil fertility and enhance crop growth and yield under irrigation (I) and rain fed conditions. However, the strength of their individual and combined effects on crop productivity has been scarcely reported. In addition, few studies have assessed their individual and co-application effects on economic returns. Therefore, a 2-year field experiment which consisted of factorial combination of irrigation (I) [100% full irrigation (FI), 80% FI and 60% FI], biochar (0 and 20 t/ha) and fertiliser (0 and 300 kg/ha) was conducted. According to the results, irrigation was the dominant factor that influences maize grain yield, followed by inorganic fertiliser and biochar, and they were all significant in their main effects. The strength of interaction effects among, I, F and B on maize grain yield follow the sequence F ×  I >  B ×  F >  B ×  I. The economic analysis showed that the ternary combination of B, F and I was more economical than the binary combination of B plus I, and F plus I (in that order), when compared with the standalone application of I at maximum production in the field experiment. In addition, combined applications of biochar and fertiliser improved soil nutrients, nutrient uptake in all irrigation treatments, compared to the standalone applications of biochar or fertiliser. Further research is, therefore, recommended for long-term evaluation of the economic viability of integrating biochar with fertiliser under irrigation.

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.

One-pot chemical activation and pyrolysis process was developed for biochar production from red macroalgae residue of Gelidium sesquipedale. The macroalgae residue was activated by various catalysts (KOH, NaOH, H₃PO₄, and CH₄ON₂) with the two concentrations (2.5 wt% and 5 wt%) using a pulverization system followed by slow pyrolysis at 500 °C. The activated biochars showed a porous morphology with an increase of water holding capacity compared to the unactivated one. The properties of activated biochar observed by further characterization (i.e., FTIR, SEM, TGA) revealed their feasibility to be used as an adsorbent. The results of adsorption experiment confirmed that adsorption was dependent not only on the surface area but also on the surface charge, and functional groups. The sorption performance of activated biochars (AcBC), in terms of the adsorption of methylene blue, was comparable to commercial activated charcoal (Norit^®). NaOH (2.5 wt%)-activated biochar had the removal efficiency of 87% versus 97% for commercial activated charcoal.