The literature search yielded 170 articles on Scopus, 268 on Web of Science, and 184 on Google Scholar. Out of them, 120 were removed before screening because of duplication, acceptability, and others, leaving 502 single citations. After title and abstract screening, 209 articles were rejected. By the use of inclusion and exclusion criteria, 42 articles were included in the qualitative synthesis (Figure 1).
Land uses
Climate change, socioeconomics (culture and population dynamics), and government policy are the key determinants of land use (Briassoulis et al. 2000). The use of the land would typically aim to promote social welfare, but its market value is far from being a reliable gauge of its social value, especially in developing nations where completely competitive land markets and rigorous legislative limitations of land use are uncommon. (Azqueta 1995). Land use has changed quite quickly in the second half of the 20th century as a result of the implementation of agricultural and economic policies (Tanrivermis 2003). Due to rural depopulation and the modernization of agriculture, significant changes occurred in human land use in the 20th century (second half). Plants, soil, nutrients, and water are all greatly affected by how the land is used and managed (Ghorbani et al. 2015).
The ramifications of changing agricultural land use described in the literature seem to vary by region (Tasser and Tappeiner, 2002), which is supported by the fact that studies concentrate on various geographic locations (Lasanta et al. 2001; Krohmer and Deil, 2003). Land-use change usually led to cultivating management change, which might deeply influence the soil quality (Fu et al. 2000; Guo et al. 2001; Hou et al. 2007). As a result of the direct effects that heavy metals in soils have on human health through the production of food, heavy metal content in soils is quickly becoming one of the key indicators of soil quality (Bai et al. 2010). Currently, researchers have focused on the consequences of land use patterns on soil heavy metal accumulation, with an emphasis on agricultural land uses which included farmland, uncovered vegetable land, orchards and forest land (Chen et al. 2005; Zheng et al. 2005; Shen et al. 2006). The growing of vegetables in the greenhouse has evolved extremely quickly in recent years and greatly aided in the supply of agricultural products, particularly vegetables, which is one of the significant land-use patterns in China. Some agricultural practices, like the excessive and incorrect application of chemical and organic fertilizers, have caused the soil quality in greenhouse vegetable fields to deteriorate and decline (Bai et al. 2010).
Potentially toxic metals or heavy metals
There have been numerous debates over the definition of the term "heavy metals" (Briffa et al. 2020). Heavy metals were proposed by some researchers based on their high atomic weight, while others proposed them based on density, chemical characteristics, toxicity, or density. However, the term "heavy metal" has recently been applied to metallic chemical substances and metalloids that impact the environment negatively and humans (Duffus 2002; Lenntech 2018; Tchounwou et al. 2012). Heavy metals, according to (Banfalvi 2011), are naturally occurring elements with a high atomic weight and a density five times greater than that of water. Examples of heavy metals that are seen in our everyday life are Titanium of atomic number 22, Vanadium (atomic number 23), Chromium (atomic number 24), Manganese (atomic number 25), Iron (atomic number 26), Cobalt (atomic number 27), Nickel (atomic number 28), Copper (atomic number 29) Zinc (atomic number 30), Arsenic, Molybdenum, Silver, Cadmium, Tin, Platinum, Gold, Mercury and Lead (Duffus 2002; Lenntech 2018; Tchounwou et al. 2012). Potentially toxic metals, such as aluminium, selenium, and arsenic, are not digested by the body but accumulate and cause damage to the soft tissues, and also become hazardous to the environment (Masindi and Muedi, 2018). Metalloids also called trace elements such as zinc and molybdenum, are not hazardous or harmful to the environment, they are referred to as micro elements because of their limited activities in the soil (Briffa et al. 2020).
Potentially toxic metal sources in soil
Sources of potentially toxic metals in the soil can be expressed as follows (Lombi and Gerzabek, 1998):
Where;
M is the potentially toxic metal
p is the parent material
a is the atmospheric deposition
f is the source of fertilizers
ag is the source of agrochemical
ow are the sources of organic waste
ip are other inorganic pollutants
cr is crop removal
l is the losses by leaching, volatilization, etc.
Naturally, potentially toxic metals occur in the soil ecosystem from the weathering of parent materials at stages that are regarded as trace (<1000 mg kg−1) and hardly toxic (Kabata-Pendias and Pendias, 2001). Cadmium has been reported to occur naturally in the soil at a concentration ranging between 0.1-1 mg kg-1 and is mostly found in sedimentary rocks (Traina 1999). Enhanced concentrations of arsenic are estimated from shales, clays and phosphates-bearing minerals whiles Cr is found in all rocks but high concentrations are reported in mafic and ultramafic rocks (Kotas and Stasicka, 2000).
Anthropogenic sources of heavy metals in the soil are more mobile and thus bioavailable than pedogenic heavy metals (Kaasalainen and Yli-Halla, 2003; Kuo et al. 1983). Potentially toxic metals linked to human activity leaded gasoline and lead-based paints, fertilizer application, animal manure application, biosolids (sewage sludge), compost, pesticides, coal combustion residues, petrochemicals, and atmospheric deposition are some of the sources of accumulation in the soil. Other sources include high metal waste disposal in unregulated landfills, leaded gasoline, lead-based paints, and biosolids. (Zhang et al. 2010; Khan et al. 2008; Basta et al. 2005). Heavy metals such as cadmium (Cd) chromium (Cr) mercury (Hg), arsenic (As), and lead (Pb) are associated with human activities.
Distribution of potentially toxic metals in soil
Li et al. (2015) studied the distribution and relationship with soil characteristics of the top 70 heavy metals under various land use and found that potentially toxic metals are distributed in soils depending on land use patterns, especially agriculture land use. Other related research has shown that potentially toxic metals in soil exhibit heterogeneous distribution in the vertical and horizontal directions in different ecosystems (Karbassi et al. 2018; Wang et al. 2017). Ye et al. (2013) argued, that the distribution of heavy metals in the soil near a river was high and decreased with increasing distance from the river. For example, Diane et al. (2010) compared the distribution of elemental Pb in the riparian and agricultural land use in southern Canada and found that the concentrations of Pb in the soil of the riparian area were almost 12 times that those in the agricultural area. Young et al. (2015) carried out descriptive statistics for the heavy metal distribution in the agricultural soils of Taiyuan and concluded that out of seven metals only Ni was normally distributed.
Potentially toxic metals are evenly distributed at the depth of 20 cm of the soil profile and most of them extended down between a soil depth of 20 to 40 cm, indicating highly varying characteristics nature of the soil profile (Zhou et al. 2008). Again, Atafar et al. (2010) stated that lead (Pb) distribution was in the range of 1.6– 6.05 mg kg-1 soil before fertilization and after harvesting reached the range of 2.75–12.85, an increase by 2 folds. The distribution of the Mn, Zn, Cu, Ni, Co, Cr, Pb and Cd in soil profiles and surface soils was investigated. It was shown that the soil-forming processes have brought about a separation of these elements between various soil components, which causes differences in the distribution patterns. He concluded that the ionic radius is of major significance for this distribution (Anderson 1977).
Agricultural land use and accumulation of potentially toxic metals
Agricultural soils have a major buildup of potentially toxic metals, which has contaminated the food chain and is a serious problem for human and livestock consumption despite the wide variances in the distribution of heavy metals in soils, according to numerous researchers (Atafar et al. 2010; Li et al. 2014; Xia et al. 2014). The accumulation of heavy metals mostly occurs in surface soils, and this may have deleterious effects on most plant species without any detectable effect on groundwater quality (Wu and Pan et al. 2013). Cadmium (Cd) and Lead (P) are the two types of heavy metals that accumulate more readily in surface soils but significant decrease in the lower horizons (Gimeno-García, et al. 1995). Recent studies in agricultural soils have indicated that potentially toxic metal accumulation has exceeded its thresholds (Midrar-ul-Haq et al. 2003; Tariq et al. 2006; Malik et al. 2010a; Mushtaq et al. 2010; Ali et al. 2014). Organic and inorganic fertilizer applications containing heavy metals were the main sources of potentially toxic metal accumulation in those soils. In addition, the application of pesticide-containing heavy metals, such as mancozeb, was implicated to contribute substantially to heavy metal accumulation in the soils (Bai et al. 2010). The variations in the concentration of the heavy metals could be attributed to the different rates of fertilizer application under different land-use patterns. Trace metals are essential nutrients but are required in small quantities. The deficiency of trace elements (Cu, Co, Fe, Mn, Mo, Ni and Zn) negatively affects the growth of plants (Lasat et al. 2000). The concentration of trace metals could be increased in deficient soils and improve crop yield using inorganic fertilizers (Lasat et al. 2000). Wuana and Okieimen (2011) posited that Cu deficient soils for cereal production are occasionally treated with Cu whilst Mn is supplied to root crops. Additionally, he and other researchers indicated that farmers apply a lot of fertilizers to the soil in intensive farming systems to provide adequate N, P, and K for crop growth (Wuana and Okieimen, 2011). Heavy metals like Cd and Pb are present in the compounds used to produce these elements at tiny levels. They may be much more abundant in the soil as a result of the continuous application of fertilizers (Jones and Jarvis, 1981). Wuana and Okieimen (2011) reported that most weedicides used in agriculture productivity and horticulture contain substantial concentrations of heavy metals. The chemicals used to formulate the pesticides contain Cu, Hg, Mn, Pb, or Zn. Therefore, continuous application of pesticides, weedicides, fungicides, insecticides, etc in various fields results in the accumulation of heavy metals. Copper accumulation in the soil is mainly attributed to agricultural activities such as the continuous application of copper-based fungicides and pesticides (Apori et al. 2018). Application of biosolids such as manures, compost, and municipal sewage sludge to the soil inadvertently leads to heavy metals accumulation (Wuana and Okieimen, 2011), though they serve as an organic amendment for soil fertility improvement.
Globally, twenty million hectares of cultivable land are thought to be irrigated with wastewater. According to studies, wastewater irrigation-based agriculture provides 50% of the urban areas' supply of vegetables in some Asian and African towns (Bjuhr 2007). In general, farmers are more concerned with increasing their yields and earnings than they are with the environmental advantages or risks. Despite the typically low levels of heavy metals in wastewater effluents, irrigation of the land for an extended period with them can eventually lead to heavy metal buildup in the soil (Wuana and Okieimen, 2011). Different land-use patterns (greenhouse field, vegetable field soils, forest field, and maize field soil) have a significant difference in the accumulation of potentially toxic metals of As, Cr, Ni, Cu, Cd, and Zn but did not show any effect on the accumulation of Pb (Bai et al. 2010). When heavy metal accumulation exceeds the standard, soil contamination occurs which negatively affects the sustainable development of the ecological environment and social economy (Zhao and Li. 2013; Wang et al. 2014). Huang et al. (2010) concluded in their work that the accumulation of heavy metals in soils is significantly affected by land use, especially in agricultural lands.
Barman et al. (2000) observed that the transfer of potentially toxic metals in the soil to part of a plant did not follow any trend and varied with the type of heavy metal, species and plant parts. Again, Barman et al. (2000) observed in their studies that out of 32 plant samples analyzed, the percentage of the sample showing metal accumulation ratio (soil to other parts of the plant) ≥1 is as follows in descending order; Fe (84.0%)>Cu (81.3%) >Ni (59.3%) > Cr (46.9%)>Zn (31.3%)>Pb (17.4%) >Cd (9.4%). The ratio >1 indicates a very high accumulation in the plant tissues than in soil.
Types of biochar
Agricultural and Forestry Waste biochar
Waste generated from agricultural activities and forestry can be used to produce biochar. This type of biochar has gained popularity in the area of pollution control due to its low cost and ease of acquisition (Zhang et al. 2013). Biochar made from agricultural wastes such as seed shells (Liu et al. 2018), corn cob (Eduah et al. 2019), corn straw (Guo et al. 2020), and potassium-iron rice straw (Guo et al. 2020) could effectively remove Cu2+ and Pb2+ ions from the soil, with removal capacities of 1.67, 2.08, and 0.41 mmol/kg, respectively.
Wood biochar
Studies have demonstrated that natural wood and waste wood are essential and common resources used to make biochar. Some trees can survive all year round, therefore can grow well even in hard situations, hence representing a plentiful source of the material. Recently, eucalyptus (Butphu et al. 2020) and mulberry (Lu et al. 2015) have been used to prepare biochar, and biochar made from these materials has been demonstrated to be effective adsorbents for removing harmful chemicals from contaminated soil (Ali et al. 2019). When utilizing biochar made from eucalyptus and pine, for example, rates of copper and lead removal from the soil were 93 % and 90 %, respectively (Debela et al. 2011; Lyu et al. 2020).
Industrial Waste biochar
Industrial organic waste such as sludge (Cea et al. 2012), and municipal solid trash make up the majority of industrial waste (Karimi et al. 2020). From 2010 to 2019, research into biochar production from industrial waste increased by 70 %. Biochar produced from industrial waste at a temperature of 750 oC was observed to be an efficient adsorbent for Cu2+ removal, with a maximum adsorption capacity of 18.5 mg g-1 (Wang et al. 2020).
Characteristics of biochar
Numerous studies have used the initial biomass feedstock and its biochar to illustrate the basic physicochemical properties of both raw and pyrolyzed content (Yaashikaa et al. 2020; Nartey and Zhao, 2014). Zhang et al. (2013) reported that chemical features have been shown to differ greatly, due to feedstock types and consequently biochar from the feedstock. The fundamental elements of biochar are carbon (C), hydrogen (H), oxygen (O), silicon (S), and nitrogen (N). It is also known that biochar contains fixed carbon. The fixed carbon is used to estimate how many carbonaceous compounds are present in the biochar solid. Van Krevelen's diagrams demonstrated the inconsistency of using the H/C and O/C ratios to calculate aromaticity and maturity degrees. (Nartey and Zhao, 2014). Basic oxygen-hydrogen, oxygen-carbon, and carbon-hydrogen rations have been established to determine the degree of pyrolysis and the amount of biochar oxidative modification in soil and solution systems (Lehmann et al. 2006; Yu et al. 2011). However, it has also been shown that biochar contains functional groups that are carboxylic, lactonic, and phenolic. Due to the presence of tube fractures that were initially created by plant cells, biochar produced at various pyrolytic temperatures has a characteristic shape resembling honeycombs. BET (Brunauer, Emmett, and Teller) is widely distributed in biochar as a result of these well-developed pores (Gao, Yue, and Gao, 2013; Cantrell et al. 2012). Applied biochar produced at a low pyrolysis temperature in combination with inorganic fertilizer is observed to be suitable because it regulates nutrient release (Gao, Yue, and Gao, 2013; Cantrell et al. 2012). Additionally, biochar made at low temperatures is much more stable than biochar made at high temperatures. Once integrated into the soil, however, the porous structure becomes unstable and the fine fractions are abraded (Nartey and Zhao, 2014).
Biochar for removing heavy metals accumulated in the soil
Heavy metals and their concentration can be removed from the soil following biochar application. Biochar is a stable material, more carbonaceous which is produced from pyrolysis and gasification of biomass (Paz-Ferreiro et al. 2014). Due to the polar functional groups, large surface area and transition metals of biochar, it can absorb heavy metals through a variety of adsorption processes (Paz-Ferreiro et al. 2014; Zhang et al. 2013). When it comes to heavy metal removal in soil, biochar is non-selective; hence it is good for heavy metals accumulated in soils due to land use patterns (Lyu et al. 2018). The biomass used to produce the biochar, pyrolysis temperature and the method of production has a major impact on the biochar properties (Paz-Ferreiro et al. 2014; Gholizadeh et al. 2019). Arsenic (As), Copper (Cu), Cadmium (Cd), Zinc (Zn), Chromium (Cr), Cobalt (Co), Nickel (Ni), Antimony (Sb), Mercury (Hg), Thorium (Th), Lead (Pb), Silicon (Si), and Selenium (Se) are the potentially toxic metals that can be found in the soil (Hayyat et al. 2016). Their accumulation in the soil can be toxic to human and plant life. To produce biochar that can be used to remove heavy metals from the soil, the producer or the researcher has to take into consideration the type of biomass and reactivity (Gholizadeh and Hu, 2021). Also, the physical properties of biochar can change if the temperature during the pyrolysis is increased (Paz-Ferreiro et al. 2014; Hu and Gholizadeh, 2019). When the pyrolysis temperature is increased between 400 oC to 900 oC, biochar surface area also increases from 0.1 to 3.2–100–500 m2/g (Zhao et al. 2017). The unique characteristic of biochar (ion exchange and sieve-like nature) is the ability to absorb and trap heavy metals accumulated in soils (Paz-Ferreiro et al. 2014).
The application of biochar for heavy metals removal can be done in three different ways including functional groups complexation, the release of cation and physical adsorption or surface precipitation (Gholizadeh and Hu, 2021). For functional group complexation, heavy metals react with the hydroxyl functionalities on the biochar surface (Gholizadah and Hu, 2021). A metal cation like Ca2+ or Mg2+, which may be present in the structure of the biochar, exchanges heavy metals like Pb2+ for the process of ion exchange (Gholizadeh and Hu, 2021). The amorphous nature of the biochar traps heavy metals during the physical adsorption or surface precipitation process (Gholizadeh and Hu, 2021). Biochar applied which is pyrolyzed at a temperature of 900 oC could decrease Cu, Zn, As, Pb, Cd, and Cr from 91.65 to 9.44 wt%, 98.82 to 63.34 wt%, 97.91 to 52.11 wt%, 55.91 to 4.87wt %, and 73.51 to 9.57 wt%, respectively (Gholizadeh and Hu, 2021, Xing et al. 2019). When the pyrolysis temperature of biochar increases, its adsorption capability gets better. This is because biochar pyrolyzed at higher temperatures has a higher concentration of functional groups that contain oxygen on their surface (Gholizadeh and Hu, 2021). Results from an experiment conducted by Liang et al. (2017) using rice husk biochar produced at a temperature of 500 oC to remove heavy metals accumulated in wetland surrounding soil showed that biochar can remove or reduce heavy metals. This is because the concentration of heavy metals such as Cd, Cu and Zn reduced from 5.59 to 4.73 mg kg-1, 53.9 to 51.57 mg kg-1, and 210.82 to 194.59 mg kg-1, respectively. Gholizadeh and Hu (2021) reported that as a result of the large number of polar functional groups in biochar, such as carboxyl, hydroxyl, and carboxyl groups, the heavy metal ions could be adsorbate by physical sorption and complexation mechanisms. They added that the immobilization of heavy metals in the soil by biochar was also improved by ion exchange, precipitation, and the trapping of the potentially toxic metals in the nanopores of the material.
Different materials used to produce biochar cannot have the same efficiency to remove heavy metals from the soil. This is because the biochar produced may have different structures and stability. This agrees with a study conducted by Wang et al. (2017) where biochar applied reduced heavy metals (Cd, Cr, Hg, and Pb) accumulated in the soil considerably. They used biochar produced from pig manure and corn straw and it was noticed that the concentration of Hg (0.79 mg/kg) was reduced by pig manure biochar to 0.34 mg/kg and corn straw biochar to 0.59 mg/kg. Biochar produced from pig manure was observed to remove more heavy metals from the soil systems due to its higher surface area (surface area for corn straw and pig manure biochar samples was 10.7 and 26.8 m2/g, respectively) (Gholizadeh and Hu, 2021).
Characteristics of compost
One of the most efficient ways to make good use of organic waste is by converting it into useful amendments such as compost for soil fertility improvement (Antil et al. 2014). Due to the growing demand for ecologically friendly methods of treating waste and organic agriculture products, interest in composting has recently surged (Antil et al. 2014). Composting is a more effective technique of waste disposal that enables the recycling of organic matter, not to mention it is an environmentally friendly way to dispose of waste (Ko et al. 2008). The utilization of compost prepared from manure has attracted attention recently. The management of the composting process depends on the succession of mesophilic and thermophilic microorganisms, both of which are active in the process (Ishii et al. 2000). The quality of composts made from various organic wastes varies, and it is dependent on the composition of raw material and the composting process utilized (Ranalli et al. 2001). The quality of compost is mostly dependent on its stability and maturity. Compost maturity and stability are related to phytotoxicity and the activities of microorganisms. Morel et al. (1985) reported that the population of microorganisms, monitoring biochemical properties of microbial activity and biodegradable ingredients analysis are some of the biological activities of compost which can be used to determine the compost’s maturity.
Compost application for heavy metal removal
The presence of humic substances, mineral ions, and microorganisms in compost has can decrease the risk of heavy metals' ecological and environmental effects and their immobilization in soils for agriculture productivity (de la Fuente et al. 2011; Udovic and McBride, 2012). Composting can generally help to lower the risk of agricultural failure, financial losses, and heavy metal exposure dangers to people. In the world, compost is thought to be a fantastic waste management choice. Following the composting process, organic wastes lose most pathogens and parasites, lose weight and have their phytotoxicity from heavy metals and organic contaminants discharged (Kapanen et al. 2013; Kulikowska and Gusiatin, 2015). In agriculture, compost has been used as an alternative to synthetic fertilizers. Evidence indicating the use of compost improved the soil's physical characteristics and fertility, enhanced microbial activity, improved crop biomass, and enhanced crop development (Calleja-Cervantes et al. 2015; Proietti et al. 2015; Tian et al. 2015). To reduce or get rid of heavy metals that have built up in the soil through agricultural land use, composting is a cheap, extremely useful, and environmentally beneficial method.
The majority of research found that adding compost to soils can reduce or immobilize heavy metals in agricultural soil by altering the physical and chemical properties of the soil and reacting with the heavy metals (Liu et al. 2009; Bolan et al. 2014). The majority of research found that adding compost to soils can immobilize or reduce heavy metals in agricultural soils by altering the physical and chemical properties of the soil and interacting with the heavy metals (González et al. 2012; Sharifi and Renella 2015). However, the risk associated with applying compost in agriculture cannot be overlooked. Muhammad et al. (2021) observed that the application of compost at the rate of 0.5, 1, 2, and 4 % to artificially contaminated soil reduced the presence of Pb, Cd, and Cr, respectively. The level of lead decreased from 18.26 to 7.13mg k/g when 4 % of compost was applied. Also, the concentration of cadmium in soil was decreased from 9.33 mg k/g to 5.36 mg k/g at 4% rate of compost application, and the chromium content was reduced at 4% compost application from 18.47 mg k/g to 7.34 mg k/g. The higher (amount) the compost application rate the more the remediate or reduction of heavy metals (Muhammad et al. 2021).
Mechanisms of biochar for heavy metal removal in soil
Immobilization of heavy metal activities in the soil is through exchange adsorption of biochar surfaces. The higher the cation exchange, the heavier metals are retained (Lehmann 2006; Reesa et al. 2014). Ion exchange occurs when positive modifications in the soil and negative charge groups on the biochar surface contact electrostatically (Wang et al. 2018). This type of reaction falls under nonspecific adsorption and is reversible due to its lower adsorption energy. Depending on how biochar is aromatized, the cationic function is determined (Wang et al. 2018). The ability to lose electrons from functional groups increases and the impact of adsorption becomes more substantial as their conjugate aromatic structure is present to a larger extent (Li et al. 2017). According to Wang et al. (2018), adsorption and dissolution-precipitation of mineral constituents in biochar can effectively lower heavy metal activity. Soil pH is can increase through biochar application and the reaction of heavy metal ions with -OH, PO43-, CO32- can form hydroxide, carbonate or phosphate precipitation, which effectively heavy metal concentrations (Reesa et al. 2014; Wang et al. 2018). Biochar complexation is important for the fixing of heavy metal ions with high affinity (Wang et al. 2018). Many researchers have discovered that the reactions of oxygenic functional groups like hydroxyl group (-OH), carboxyl group (-COOH) and amino group (-NH2) with heavy metals on the surface of biochar contribute significantly to the adsorption of heavy metal ions (Xu et al. 2012; Li et al. 2017). Biochar applied can absorb heavy metal contaminants more effectively and remove them from the soil because of their larger surface area and increased surface energy (Wang et al. 2018). Many factors influence the effect of adsorption of biochar on heavy metal ions, including biochar source materials, pyrolysis temperature, soil pH, physical and chemical properties of heavy metal ions, and the biochar application rate (Wang et al. 2018). All things being equal, despite having the biggest surface area, biochar produced from animal manure outperforms sludge biochar and plant biochar in terms of heavy metal ion adsorption. This is because P-rich biochar from animal faeces can precipitate or coprecipitate with particular heavy metal ions, making the biggest contributions to the healing process.
Table: 1: Removal of potentially toxic metals in soil with different biochar types and their mechanisms
References
|
Biochar type
|
Heavy metals
|
Matrix
|
Adsorption mechanism
|
Lu et al. (2012)
|
Sludge biochar
|
Pb2+
|
Soil
|
- Complex reaction with hydroxyl (-OH) and carboxyl (-COOH)
- Precipitation and complexation
|
Cao et al. (2010)
|
Dairy manure
|
Pb2+
|
Soil
|
Ion exchange, adsorption, and precipitation with PO43-, CO32-
|
Liang et al. (2014)
|
Dairy manure, rice straw
|
Pb2+
|
Soil
|
Electrostatic adsorption, ion exchange
|
Xie et al. (2016)
|
Walnut green husk
|
Pb2+
|
Soil
|
As a result of the aromatic structure, it reacts with the heavy metal and ion exchange with functional containing oxygen groups
|
Liu et al. (2016)
|
peanut shell, Chinese medicine residue
|
Pb2+
|
Soil
|
Complexation, ion exchange, electrostatic adsorption, and pH increase make the Pb2+ -carbonate bounded state to changed into Pb2+-insoluble phosphate and silicate state
|
Kong et al. (2011)
|
Beanpoles
|
Hg2+
|
Soil
|
Precipitation, forming Hg (OH)2, HgCl2
|
Dong et al. (2014)
|
Brazilian pepper
|
Hg2+
|
Soil
|
A composite reaction with hydroxyl (-OH) and carboxyl (-COOH) reacts with an aromatic structure to form Hg-π
|
Xu (2015)
|
Sugarcane, walnut wood chips
|
Hg2+
|
Soil
|
Composite reaction with hydroxyl (-OH) and carboxyl (-COOH), ion exchange, electrostatic adsorption
|
Zhao (2015)
|
Rice husk and rice straws
|
Hg2+
|
Soil
|
Composite reaction with hydroxyl (-OH) and carboxyl (-COOH), ion exchange
|
Ippolito et al. (2012)
|
Broiler litter
|
Cu2+
|
Soil
|
Complexation with functional groups to form Cu3(CO3)2 (OH)2, CuO
|
Mohan et al. (2007)
|
Oak biochar
|
Cr (VI)
|
Soil
|
Deoxidize Cr (VI) into Cr (VIII) complex reaction with hydroxyl (-OH) and carboxyl (-COOH)
|
Yang et al. (2015)
|
Sugarcane leaves, tapioca stem, rice straw, silkworm excrement
|
Cd2+
|
Soil
|
Electrostatic adsorption
precipitate with CO32-, OH-
|
Zhang (2012
|
Maize straw
|
Cd2+
|
Soil
|
Electrostatic adsorption, precipitation)
|
Guan et al. (2013)
|
Pine needle, maize straw, dairy manure
|
As5+
|
Soil
|
Electrostatic adsorption
|
Huang et al. (2014)
|
Maize straw
|
As3+
|
Soil
|
Non-electrostatic physical reversible adsorption and chemical irreversible adsorption with polar groups
|
Wang et al. (2015)
|
Hardwood
|
As3-
|
Soil
|
Increase the solubility of As3-
|
Wu et al. (2015)
|
Almond Putamina, reed straw
|
Ni2+
|
Soil
|
Composite reaction with hydroxyl (-OH) and carboxyl (-COOH)
|
Wang et al. (2017) Chen et al. (2011)
|
Water hyacinth Hardwood
|
Zn2+
|
Soil
|
Electrostatic adsorption and ion exchange
|
Xu (2015)
|
Dairy manure
|
Pb2+, Cu2+ Zn2+, Cd2+
|
Soil
|
Oxygenic functional groups and precipitate with PO4 3-, CO3 2-
|
Xu (2015)
|
Rice husk
|
Pb2+, Cu2+ Zn2+, Cd2+
|
Soil
|
Phenolic hydroxy group with surface complex with
|
Mechanism of compost for heavy metal removal in soil
The incorporation of compost into the soil can change the bioavailability, mobility, and harmful effects of potentially toxic metals on both plants and animals. Precipitation, adsorption, complexation and redox reactions are the mechanisms which are responsible for these effects. (Huang et al. 2016).
Adsorption and Complexation
Application of compost serves as a bio sorbent which can absorb potentially toxic metals (Soares et al. 2016; Anastopoulos and Kyzas, 2015), and the composting capacity of adsorption capacity can be assessed by using kinetics of adsorption and experiment involved with adsorption isotherms (Paradelo and Barral, 2012; Venegas et al. 2015; Zhang 2011). Simantiraki and Gidarakos (2015) reported that Compost has a better removal capacity than zeolite and is reported to reduce or remediate the availability of heavy metals in water through chemical adsorption by 85–89%. The availability of potentially toxic metals in soil can be remediated using compost by altering the physicochemical characteristics (such as pH, oxidation-reduction potential (Eh), and organic matter content of soils. This will also help soil particles to more effectively bind with potentially toxic metals (Kargar et al. 2015; Vaca-Paulin et al. 2006). Predominantly, the application of compost immobilizes potentially toxic metals through the inorganic component, substance of humus, and microorganisms (Caporale et al. 2013; Chang Chien et al. 2007; Tsang et al. 2014). They further explained that the high number of organic functional groups (carboxyl, carbonyl, and phenols) is due to the abundance of humus in the compost which binds metal ions through complexation. In addition to that, compost shows diverse affinities to different potentially toxic metals. According to Chien et al. (2006), the linkage of humic substances with potentially toxic metals followed an increasing order: Pb > Cu > Cd > Zn. Because of the composition of humic (hydrophilic and hydrophobic compounds) is it able to act as surfactants (Kulikowska et al. 2015).
Applied compost can immobilize potentially toxic metals through biosorption and biomineralization due to the availability of microorganisms in the compost (Lloyd 2002; Sudha Bai and Abraham, 2003). Some isolated microbes (Penicillium chrysogenum, Graphiumputredinis, Fusarium solani) that were present during the composting process was able to absorb more than 90 % of lead in the soil (del Carmen Vargas-Garcia et al. 2012). The presence of manganese, iron and aluminium in the compost can retain heavy metals regularly (Hettiarachchi et al. 2003; Tapia et al. 2010).
Precipitation
Precipitation is another method of immobilization of potentially toxic metals, it leads to low-soluble content in the soil. The abundant component of compost and important are the Humic substances (Huang et al. 2016). When compost is added to the soil, humic compounds react with heavy metals. It creates complexes which are humate or metal-humic very stable to hydrolyse and finally precipitate (Garcia-Mina 2006). When compost is incorporated into the soil, it can improve precipitation and complexation which can decrease acid-soluble and reducible Cu fractions (Lagomarsino et al. 2011). Another factor in the immobilization of heavy metals is the high phosphorus (P) concentration of some compost. Inorganic phosphorous compounds and soil-mobile heavy metals may react to produce insoluble precipitates (Katoh et al. 2014; Liu et al. 2009).
Redox Reaction
The redox reaction is an oxidation-reduction reaction between two species. Most times one species undergoes oxidation whiles the other species also undergo reduction. Potentially toxic metals can be transformed into a non-toxic state due to the reaction between organic matter and heavy metals in the compost (Huang et al. 2016). Due to the presence of Dissolved Organic Matter (DOM) in the compost, enhances the redox reaction of heavy metals (Banks et al. 2006; Chiu et al. 2009). There are functional groups especially quinone moieties which are redox-active contained in the DOM which accept electrons from electron donors and present electron transfer capacities, thereby improving the redox reaction of heavy metals (Huang et al. 2010; Yuan et al. 2011). Additionally, after adding compost, the sulfate reduction could be improved and some heavy metals could be converted into low accessible species of metal sulfide because the bacteria in the compost absorbed the dissolved oxygen, creating an anaerobic state (Hashimoto et al. 2011; Paul 2014).
Table 2: Removal of potentially toxic metals in soil with different types of compost and their mechanisms
Feedstock type
|
Heavy metals
|
Effect
|
Effective mechanisms
|
References
|
Municipal organic waste (MOW) and Domestic organic waste
|
Zn, Cu, Ni, Pb, Cd
|
Compost produced from DOM and MOW can absorb heavy metals
|
Process of adsorption
|
Venegas et al. (2015)
|
Municipal solid waste
|
Cu, Cd, Pb, Ni, Zn, Cr
|
No significant influence on mobility factor of metals
|
Both soluble and insoluble complexes were formed with organic compounds
|
Achiba et al. (2009)
|
Green waste
|
Cu, As Pb
|
Mobility and solubility of As and other metals were increased
|
Organic carbon and Fe in soil pore water were enhanced
|
Clemente et al. (2010)
|
Animal manure and Green waste
|
As, Cu
|
Was able to reduce the mobility and leachability of Cu but not As
|
Surface complexation
|
Tsang et al. (2013)
|
Market waste
|
As
|
As levels in soil pore water were improved
|
Availability of phosphorous improved and increase in water-soluble Fe and carbon in pore water
|
Hartley et al. (2009, 2010
|
Biochar and compost
The synergy between biochar and compost has the efficiency to remove heavy metals accumulated in soils (Tang et al. 2020). An experiment conducted by Tang et al. (2020) applied biochar (rice straw biochar at a pyrolysis temperature of 500 oC) and compost (compost produced from rice straw, vegetable leaves, etc) at the same rate and the result showed that integration of biochar and compost, sole application of compost, and sole application of biochar reduced Cd by 87.1 %, 69.6 % and 65.8 % respectively. Gholizadeh and Hu (2021) reported that through electrostatic interactions and chelation processes, the high surface area and high amount of polar functional groups in biochar could immobilize the heavy metals. Composting results in improved stable organometallic compounds with heavy metals due to the presence of humic substances.