Biochar has the ability to treat the polluted soil with organic and inorganic elements, decrease the soil nutrient leaching loss, and amend the physiochemical attributes of soil.

The soil improvement under biochar application is mostly reflected in the amendment of soil organic matter (SOM), the improvement of nutrient dynamics and utilization rate, and the amendment of acidic soil and soil erosion [14]. Moreover, biochar traits, such as higher surface area and porosity, enhance the water-holding capacity (WHC), the soil porosity and soil capacity, and the porous structure of biochar, which make it a better habitat for the soil microbial population (Figure 1). Biochar application can efficiently improve the structure of soil, decrease the moisture content loss because of structure runoff and filtration, and enhance soil-available water [17]. Montagnoli et al. [18] reported that the biochar’s higher porosity has strong water retention ability, and the slow discharge of water contents from biochars can significantly enhance the water conservancy properties of degraded soil. Biochar incorporation into the soil can enhance the pH levels, possibly due to the biochar’s higher base cation composition, such as Na⁺, K⁺, Mg²⁺, and Ca²⁺. Biochar ash content is comprised of carbonates and hydroxides, and these substances’ dissolution expedites the soil pH increase of enhancement [19]. The negatively charged surface functional groups of biochar can greatly adsorb cations and support an increase in the soil’s CEC [20]. Oni et al. reported that the composition of feedstocks determines the biochar CEC during pyrolysis mechanism [21]. Jain et al. [22] reported the immobilization of metal ions in the soil via substituting cations on the biochar, and these cations enter into the soil and enhance the pH. Therefore, the elevation of soil pH and CEC induced through biochar might be due to the decline of metal ions bio-availability. Therefore, biochar is frequently applied for soil reclamation polluted by cationic-trace components [22]. Biochar contains essential nutrients, such as Ca, Mg, K, P, and N. These elements are necessary for plant growth and development, and the release of these nutrients stimulates or accelerates the growth of plants [23]. Biochars produced from different feedstocks also varied in nutrient substances, for example, biochar derived from grass seed had a higher amount of P, whereas biochars derived from wood contained more Mg and Ca [24]. Moreover, the biochar’s strong WHC can decrease nutrient leaching, modify the nutrient dynamics, stimulate the root nodule, and accelerate the plant growth and immobilization of N [25]. Furthermore, the minimum dose of biochar needed to maintain plant growth also varies because of the diversity in the heavy metal and nutrient concentration of polluted soils [26]. The higher amount of organic and inorganic pollutants in soils causes a disturbance in soil enzyme activity or functionalities, and the microbial population may be seriously damaged [27]. Applying biochar can improve the habitats of the microbial community, by influencing the structure, diversity, microorganism’s activity, and nutrient availability [28]. Torabian et al. reported that compared to the biochars pyrolyzed at high temperatures, biochars derived at low temperatures were more contributive to soil microorganism’s growth because they comprised N and more DOC [29]. Gul et al. described that biochar can indirectly influence the P and N cycling reaction of microbes by altering the soil environment and structure of the microbial community, and can promote the plant rhizobial exchanges [30]. Therefore, adding biochar contributes to soil microbial activity, which may benefit plant growth and development [31]. In general, biochar improves the physicochemical traits of soil and is comprised of nutrients necessary for microbial and plant growth. Hence, biochar application is a potential material for the ecological reclamation of polluted soils with inorganic and organic elements.

Heavy metals persist for a long time and are not bio-degradable in polluted soils. The elimination of metals from contaminated soils is time-consuming and expensive. In situ metal stabilization through soil amendments, such as compost and lime, is usually employed to decrease the bio-availability of metals and decrease plant uptake [32]. Biochars can stabilize heavy metals, amend the quality properties of polluted soil, and significantly reduce the uptake of various metals in crops [33]. Thus, biochar application can be a potential solution for the reclamation of soils polluted with heavy metals. Metals stabilization in soils with biochar addition may involve different mechanisms, as explained in Figure 2. Taking lead ions (Pb²⁺) as an instance, many researchers proposed different mechanisms for the sorption of lead ions through biochar produced from sludge that may include: (i) the exchange of heavy metal with Mg²⁺, Ca²⁺, and other cations present in biochar, representing inner-sphere complexation and co-precipitation complexation with mineral oxide and complexed humic matter of biochar; (ii) surface complexation of heavy metals with various functional groups as well as inner-sphere complexation with free-hydroxyl of mineral oxides and other surface precipitation; and (iii) surface precipitation and van der Waals adsorption ensuring the Pb²⁺ stabilization [34]. In the case of acidic polluted soils, depending on the biochar type and presence of exchangeable cations, such as Ca²⁺, K⁺, Mg²⁺, and Na⁺ in biochar, these could govern the exchange of cations with heavy metals during the sorption process and may enrich the stabilization process [35]. Ennaji et al. [36] also illustrated that the exchange of heavy metal with K⁺, Na⁺, Mg²⁺, and Ca²⁺ from sludge-derived biochar was the main process responsible for this exchange in their work, but the contribution of monovalent cations (K⁺, Na⁺) was negligible. Thus, it could be stated that under actual field conditions, the biochar-derived sorption process in metal-polluted soils is mainly dependent on soil type and the cations present in both biochar and soils; consequently, metal remediation in polluted soils may differ. Mahmud et al. [37] demonstrated that the mineral constituents, e.g., phosphates and carbonates in the biochar, play a substantial role in stabilizing the metals in soil because these salts can precipitate with metals and lessen their bio-availability. Chen et al. suggested that the primary mechanism for dairy manure-based biochar to retain lead was the precipitation of insoluble lead phosphates [38]. Usually, during biochar preparation, water-soluble Mg, Ca, and P content increase when heated at 200 °C, but these reduced at high temperatures perhaps because of the higher crystallization of P-Mg-Ca. This was evident during the formation of whitlockite when the production temperature was elevated to 400 °C, thereby ensuring the smooth precipitation of lead. Biochar’s alkalinity can also stimulate metal precipitation in the soils [38]. In 2022, Palansooriya investigated the pH variation of the biochar and got a mean value of pH 8.0. With similar biomass materials, the pH value of biochar increases with the preparation temperature due to higher ash contents in the biochar [39]. Thus, many biochars are basic in nature, having a mulching effect that helps decrease the mobility of the heavy metals in polluted soils [40]. Conversely, the removal capacity of the same type of biochar differs with different kinds of heavy metals.

The application of biochar can decrease the mobility of various heavy metals in polluted soils (Table 1), which minimizes the risk of plant uptake. Various studies have presented that bamboo-derived biochar can remove chromium, nickel, mercury, cadmium, and copper from contaminated soil and water [41]. Biochar obtained from dairy residue prepared at a 300 °C pyrolysis temperature was more effective in sorbing lead than biochar prepared at 400 °C because biochar pyrolyzed at 300 °C had a greater concentration of soluble phosphate [42]. Since biochar properties depend on feedstock type and pyrolysis conditions, a single type of biochar cannot be universally used to reclaim polluted soils containing different heavy metals. Thus, when biochar is to be applied as an amendment for the reclamation of polluted soils, care must be taken about the type of heavy metals, biochar production temperature, residence time, moisture content, and the type of feedstock employed. The influence of biochar on metal bio-availability differs with biochar type and different kinds of heavy metals. Alipour et al. reported that when zinc and cadmium polluted soil was ameliorated by hardwood biochar, the concentration of zinc and cadmium in pore water decreased [8]. Concentrations of extractible zinc and arsenic in soil become higher with the biochar addition rate, whereas the concentration of extractible lead reduced, copper did not modify, and cadmium exhibited an inconsistent trend. They determined that the removal of metals on biochar with primary loadings up to 200 µmol at 7 pH took place in this order: lead > copper > cadmium > zinc > arsenic [43,44]. Singh et al. described that the biochar addition can decrease the discharge of heavy metals due to the redox reaction of heavy metals [44]. For instance, adding chicken manure-derived biochar in chromate-polluted soils increased the decline of mobile chromium hexavalent to less mobile chromium trivalent, thus reducing the leaching of chromium. The reduction in the leaching of chromium trivalent is accredited to adsorption as chromium hydroxide is produced from the release of hydroxide ions (OH⁻) during the chromium hexavalent reduction mechanism [44] (Figure 2).

The bio-availability of various metals indicates the toxicity in soils and the potential hazard of contaminating the human food-web. The bio-availability of contaminants regulates their degradation and eco-toxicology in polluted soils. Bio-availability is defined as a pollutant fraction representing the availability of a chemical agent to a living organism for eco-toxicology, assimilation, and degradation expression [33]. Many studies showed that applying biochar is more efficient in immobilizing heavy metals, thereby decreasing their phytotoxicity and bio-availability (Table 2). Liu et al. assessed the ability of biochar addition to amend the heavy metals toxicity in pit-tailings [45]. They used biochar prepared from orchard prune residues at 0%, 1%, 5%, and 10% rates. WHC, CEC, and pH level were increased with increasing biochar application rates, and the bio-availability of zinc, lead, and cadmium of mine-tailings was reduced, while cadmium showed the maximum reduction. According to Montagnoli et al. [18], applied biochar produced from cotton stalks improved the cadmium-polluted soil. The findings suggested that biochar obtained from cotton stalks can decrease the bio-availability of soil cadmium by co-precipitation or an adsorption mechanism. According to another study, the effects of sewage sludge-derived biochar on metals bio-availability and solubility in Mediterranean farming soil were compared with untreated sewage sludge (not charred). The biochar applications decreased the plant accessibility of lead, cadmium, zinc, and nickel when equated to sewage application [46]. Table 2 summarizes the outcome of various biochars on the uptake volume of pollutants and the bio-availability of contaminants. Biochar produced from green waste and chicken manure significantly reduced lead, copper, and cadmium uptake by Brassica juncea. It was also found that the decline in plant metal concentration was increased with increasing biochar rates, except for copper concentration. Biochar produced from rice proved more effective to immobilize lead and copper than cadmium [47]. Hence, when the sole objective of biochar addition is to immobilize various metals, special attention should be paid to selecting suitable feedstock and the production temperature of biochar. Gamboa et al. conducted a pot experiment and used activated biochar of wood in the soil spiked with metals to examine the biochar’s effect on the accessibility of zinc, lead, copper, and cadmium to corn [48]. Biochar addition reduced the concentration of copper, cadmium, and arsenic in corn shoots, but the effect of biochar addition was inconsistent on zinc and lead concentrations in corn shoots. Soil pH is closely associated with the bio-availability of metals in the soil. The addition of biochar can improve the CEC and pH of soil, and consequently increase the immobilization of various metals in the soil [49]. Siles et al. [50] conducted a study using biochar obtained from cow manure and mussel shell to decrease the lead toxicity in prominently lead-polluted soil in South Korea. Lead bio-availability in soil was reduced by 76% with biochar application. An increase in adsorption capacity and a rise in soil pH were considered the result of the reclamation effect of biochar. For instance, lead bio-availability in soil was reduced up to 93% with shell biochar, a mulching material. At present, many studies revealed that various kinds of biochars can decrease heavy metals’ bio-availability and their mobility. However, most of this research is carried out under controlled environments (under greenhouse and laboratory experiments). Therefore, to fully utilize the biochar potential as a reclamation agent, large-scale field studies should be conducted.