stressed heterotrophic microbial community (Mendez et al., 2007). Further pollution of soil and tailings can be curbed by deploying pollution by employing environmentally sound technologies as alternatives (Beesley et al., 2011). For instance use of compost as soil amendment has been embraced by rural farmers due to its cost effectiveness (Umeobika and Onmonya, 2020). Many disadvantages of widespread use of chemical fertilizers include increase in soil acidity, mineral imbalance and soil degradation (Ayoola and Makinde, 2008). Also, in Europe Petruzzelli (2012) has reported that soil contamination has been known as a vital point for action within the European Economic Community, because it affects a large expanse of the available land. The economy in China has experienced progressive growth within the past few years; this has led to increased environmental problems (Xi et al., 2011). Standard strategies are enforced to reduce major metals from contaminated water. However, most of the main strategies are ineffective and undesirable due to high costs, high sludge production, and incomplete removal (Lara et al., 2016). Several studies aiming at reducing operating costs and increasing efficiency in water treatment have been conducted. Some of the biomass already worked on include: shells of aquatic animals and egg, fruit peels, vegetable oil and its residues, nuts, zeolites and husks of some roots and tuber crops cocoa and corn cobs. The efficiency of the process is hinged on the nature of the biomass used (Tejada-Tovar et al., 2016). Biochar is produced by thermochemical breakdown of biomass under restricted environments (Cha et al., 2016; Gondim et al., 2018). Temperature, type of biomass and atmosphere (often slowly oxidizing) are the most critical variables considered in this process (Kim et al., 2012). Water correction Contamination of aquatic systems may be a serious environmental issue and so the event of associate economical and appropriate technology to get rid of significant metals from binary compound solutions is important. Many strategies are often employed to remove significant metals from contaminated water. They include chemical precipitation, action, adsorption, membrane filtration; reverse diffusion, solvent extraction, and chemistry treatment with several of those strategies suffering from high capital and operational prices (Khatri et al., 2017).

Soil correction was done by removal of significant metal by screening followed by Soil washing from Contaminated Soil. In this technique the contaminated soil is removed from contaminated sites (ex-situ) and washed, the limitation of this method; the operation cannot be performed for a really massive volume of soil. Benefits of excavation involve the entire removal of the contaminants and also the comparatively fast cleanup of a contaminated site. Disadvantages embrace the actual fact that the contaminants square measure merely affected to a special place, wherever they need to be monitored; the danger of spreading contaminated soil and dirt particles throughout removal and transport of contaminated soil; and also the comparatively high value. Excavation is often the foremost high-ticket choice once massive amounts of soil should be removed or disposal as risky or toxic industrial waste is needed (Khatri et al., 2017). Stabilizing metals within the soil, significant metals are often left on the site and treated during a means that reduces or eliminates their ability to adversely have an effect on human health and also the environment. This method is usually referred to as stabilization. Eliminating the bioavailability of significant metals on the site has several benefits over excavation. A technique of stabilising significant metals consists of adding chemicals to the soil that cause the chelation of minerals that contain the significant metals during a form that's not simply absorbed by plants, animals, or people. This technique is termed in-place fixation or stabilization. This method doesn't disrupt the setting or generate dangerous wastes. Instead, the significant metal combines with the additional chemical to make a less harmful compound. The significant metal remains within the soil, however in a form that is abundant and less harmful, the disadvantages is that it permits incomplete neutralization of metals (Khatri et al., 2017). Uses of plants growing plants will facilitate contain or scale back significant metal pollution often referred to as phytoremediation. It has the advantage of comparatively low value and wide public acceptance. It is often but 1 / 4 of the price of excavation or in-place fixation. Phytoremediation has the disadvantage of taking longer to accomplish than different treatments. Plants are often utilized in other ways. Generally, a contaminated site is just revegetated during a method referred to as phytostabilization (Liu et al., 2017).

Heavy metals stay for years and not easily biodegradable in soils that are polluted. For heavy metals to be removed from polluted soils it is expensive and takes a lot of time (Cui and Zhang, 2004). Heavy metals are stabilized in situ by amending the soil with organic additives; this is mostly done to decrease the mineralization of metals and reduce absorption by plants (Komárek et al., 2013). Heavy metals can be stabilized in polluted soils by biochar. Biochar can lead to the improvement of polluted soil (Ippolito et al., 2012) and can greatly reduce the adsorption of heavy metals by crops. Thus, biochar has the potential of providing a novel remedy for soils polluted with heavy metals. There could be possible a large number of mechanisms involved in stabilizing heavy metals in soils using biochar. Using Pb2+ for instance, the research suggested different mechanisms for Pb2+ sorption with sludge-derived biochar as follows: 

Cheap source and waste management

Biochars are normally produced from inexpensive and abundantly existing waste biomaterials. Precisely, biochar feedstocks are mainly manufactured from the biomasses and solid wastes of agricultural works. Agricultural remains are seen in large amounts and are often difficult to dispose. For example, producing biochar from invasive plant can solve disposal problems and waste management. Also, aquatic algae are normally many and can block waterways; thus, other uses like the synthesis of biochar can benefit local people.

Nutrient

Pyrolysis of feedstock, leads to concentration of elements such as P, K, Ca, and Mg in biochar. Soluble organic substances are also formed during the pyrolysis process. Currently, using chemical fertilizers to improve soil is costly and out of reach for small farmers. Some disadvantages of the widespread use of chemical fertilizers are the soil acidity increase, mineral imbalance, and soil degradation (Ayoola and Makinde, 2008; Onmonya and Umeobika, 2020). Research has also shown that soils and residues can be characterized by total deficiency or low levels of organic matter and macronutrients and usually have an acidic pH, although some soil residues can be alkaline (Krzaklewski and Pietrzykowski, 2002; Gbadebo and Ekwue, 2014). In addition, tailings should also be without a normal soil structure and support a highly stressed heterotrophic microbial community (Mendez et al., 2007; Southam and Beveridge, 1992). The addition of biochar can provide plants and microrganisms with bioavailable nutrients. However, the nature of the starting materials and the conditions of pyrolysis determines the quality of biochar produced. The levels of C and N differed significantly in biochar when made from chicken droppings, pine and groundnut shells at 400 against 500°C. In addition, pyrolysis at 500°C yielded higher level of P, K, Ca, and Mg in biochar than at 400°C. This was due to the higher pyrolysis temperature, which decreased the CEC but enhanced mineralization of the feedstocks. In this context the goal is to maintain high quality of biomass, as well as the biochar given the right conditions for the process. In general, the nutrient content of plant derived vegetable biochar is comparatively lower than that of manure (Woolf et al., 2010). In line with this, Onmonya and Umeobika (2020) recommended that researchers do more research on processed cow dung to improve soil quality.

Soil stability

Erosion effects in Nigeria is huge, especially gully erosion in the south and southeast of the country. The high water flow with the undulating topography creates a high water flow. The lack of drainage channels to control fluid flow and soil structure has contributed to the severe effects of erosion in this area (Hillili et al., 2011). Moreso, wind erosion predominates in some parts of the northern states. In 1995 it was estimated that over 700 million kg of metals were dumped in land mine debris each year (Warhurst, 2000). The global impact of such tailings dumps is enormous, since unused tailings piles typically lie fallow for several decades and exposed tailings piles can spread over several tens of hectares due to aeolian dispersion and water erosion (González and González-Chavez, 2006). This has the potential to contaminate nearby communities and ecologically sensitive areas (Gbadebo and Ekwue, 2014). It is therefore imperative to research for alternatives such as biochar for promotion of stable soils (Trazzi et al., 2018). Biochar acts as an isolated particle, distinct from other stable organic matter that trapped in aggregates, soil pores or adsorbed on mineral surfaces. Biochar makes it easier for carbon to be sequestrated in the soil, because it is highly stable in its organic form. Sun et al. (2018) reported a half-live of 102 – 107 years for carbon in biochar and stated that biochar mineralizes very slowly. Woolf et al. (2010) also reported that fine particles of biochar have been in soils in climates with low heat levels, like Amazon.

Effect of biochar on heavy metal mobility

Biochar can reduce the movement of heavy metals in polluted soils, resulting in a low risk uptake by plants. Research has shown that bamboo-derived biochar can absorb Cu, Hg, Ni and Cr from soil and water and Cd in contaminated soils. Cao et al. (2009) reported that biochar derived from dairy manure at 200 °C pyrolyzes Pb more effectively than biochar produced at 350°C due to the higher concentration of soluble phosphate in the biochar at 200°C. A remediation process can utilize different biochar and mechanism for multi-element polluted soils.Therefore, when using biochar to improve soils polluted with heavy metals, the types of heavy metals in the polluted soil and the temperature used in the production of biochar must be considered because their properties depend on the pyrolysis conditions such as the water content of the feedstock, highest treatment temperature, type of starting material used and residence time. The influence of biochar on the bioavailability of metals varies depending on the type of biochar products and heavy metals. The ratio of Cd and Zn in pore water of contaminated soil was reduced when biochar from hardwood was applied (Beesley et al., 2010). Similarly, addition of biochar to contaminated soil reduced the concentrations of Cd and Zn in the pore water by 300 and 45-fold in a column leaching experiment (Beesley et al., 2011). Namgay et al. (2010) showed that the ratios of As and Zn that can be extracted in soil became high with the application of biochar; extractable Pb reduced; Cu was unchanged and Cd was not constant. They also described that there was sorption of trace elements on biochar with initial loadings up to 200 mol at pH 7 in the order: Pb > Cu > Cd > Zn > As. Biochar can decrease the leakage of metals via its effect of redox reactions of metals (Choppala et al., 2012). The significant reduction in the leaching of Cr(III) is due to the uptake of Cr(III) onto cation exchange sites and precipitation as Cr(OH)3 resulting from the discharge of OH ions during the process of Cr(VI) reduction (Bolan et al., 2013) as illustrated in Figure 1.

Biochar production and identification of the main biomasses used in this process

The two main techniques for converting biomass are: the use of enzymes and microorganisms (biochemical conversion) which is less expensive and environmentally friendly, although has a lower yield (Tripathi et al., 2016); and break down of biomass using heat (thermochemical conversion) (Kubilay et al., 2014). The thermochemical process includes conventional carbonization or pyrolysis, hydrothermal carbonization, incineration and gasification (Kubilay et al., 2014). The processes in this type of biochar production are mainly determined by the pyrolysis temperature, the residence time of the material in the reactor and the heating rate (Trazzi et al., 2018). Using biomass in combination with thermochemical synthesis has advantages in obtaining new carbonaceous materials with different applications, low cost, high availability in nature and fast regeneration (Santos, 2016). For (Novotny et al., 2015) various organic materials are suitable as raw materials in thermal processing, from agricultural and wood biomass to all available agricultural and industrial waste (husks, straw, seeds, bagasse, nut shells, wood chips, etc.) and even municipal waste (Table 2). The biochar produced in the pyrolysis process is high in energy comparable to the coal used in industry, owing to its microporous structure and high carbon content. In agriculture, it is used to improve soil quality and increase carbon storage. It slows down nutrient degradation and consequently improves soil quality. In the adsorption industry it is used to remove heavy metals such as Cr, Cd, Ni, Hg, Pb and organic compounds (Tripathi et al., 2016). Accordingly, low-cost alternative sources for the production of biochar are being researched into. Sofar agricultural residues such as rice husk (Doria et al., 2016), orange peel (Tejada-Tovar et al., 2015), corn cobs (Lopes et al., 2013), sugar cane bagasse (Ferreira et al., 2015) (Table 3) orange peel (Tejada-Tovar et al., 2015) (Table) 4 and the coir-chitosan composite (Costa et al., 2017) have shown great potentials for adsorption of pollutants.

Heavy metals are generally found in small amounts in agricultural soils. However, due to their cumulative behavior and toxicity, they not only have a potentially harmful effect on crops but also on human health (Ekwue et al., 2012; Das et al., 1997). Heavy metal contamination of soil, water and crops and its health impact on local residents is an ongoing social problem, and several studies have identified health risks for local residents living near abandoned mines (Chung et al., 2005). Man-made pollutants can threaten human health and harm the natural ecosystem and environment (Hilli et al., 2021). The bioavailability of heavy metals determines toxicity in soil and potential risk upon entry into the environment. Fellet et al. (2011) reported that increased application of biochar resulted in increased pH, cation exchange capacity, and water-holding capacity and decreased bioavailability of some metals in mine tailings. In a study Zhou et al. (2008), used biochar derived from cotton stalks to improve Cd-contaminated soil, the biochar decreased the bioavailability of Cd in the soil. Mendez et al. (2012) also reported that biochar treatments reduced the plant availability of Ni, Zn, Cd and Pb compared to sewage sludge treatments in a Mediterranean agricultural soil. A reduced Cd, Cu and Pb uptake by Indian mustard was reported by Park et al. (2011) when they applied chicken manure and green waste derived biochar. The study also recorded reduced metal concentrations in plant except for Cu, with increased biochar application. Furthermore, biochar is highly effective in adsorption of many natural and anthropogenic sourced organic compounds (Sarmah et al., 2010). Owing to its highly aromatic nature, large surface area, micropore volume and abundant polar functional groups, biochar is effective in absorbing a wide range of organic chemicals including pesticides, PAHs and new emerging contaminants such as steroid hormones (Kookana et al., 2011). The level of aromaticity, type of biochar and organic carbon play a major role for effective removal of contaminants (irrespective of the other properties of the biochar) (Sarmah et al., 2010). 

Bioremediation supplemented with biochar is one of the most important remediation technologies for the remediation of soil and water bodies contaminated with heavy metals. Biochar-enhanced remediation has great potential for immobilizing cationic heavy metals in mining tailings, tailings piles and water bodies, especially those with high acidity. Biochar can reduce the bioavailability and leachability of cationic heavy metals in soil and water, improve soil fertility and greening, and create a suitable environment for soil microbial diversity. To reduce the bioavailability of the organic pollutants and the risk of the pollutants entering the human food chain or leaching into groundwater, biochar could be of immense benefit. However, the long-term environmental fate of the deposited pollutants is still unknown and further research is warranted to fill this gap, especially under realistic field conditions through biochar-mediated remediation trials. Furthermore, it is important to select appropriate biochar to develop an effective strategy to immobilize anionic metals in situ. Future research should focus on: biochar stability and its impact on the fate and transport of metals in mining tailings and large-scale soils and waters; and understand the mechanisms of biochar-assisted bioremediation.

The authors have not declared any conflict of interest.