In the last decades increased global environmental concerns to water and soils pollution. The main concerns are related to the contamination of the ecosystem, food security, and human health since many of the contaminants present in soil and water (residues of pesticides and antibiotics, genes of resistance to antibiotics, and heavy metals) are absorbed by plants and enter the food chain. Remediation of the contaminated water and soil to ensure sustainable water supply and food production is urgently needed. The use of biochar can have a positive effect on this remediation process. There are several studies that demonstrate the biochar’s ability to block/reduce the contaminating effect of pesticides, antibiotic residues, antibiotic resistance genes, and heavy metals. The objective of this chapter is to carry out a comprehensive review of the effect of using biochar on the availability/transmission of these contaminants to the soil and food supply chain.
- human health
- heavy metals
Currently, water and soil pollution is a global concern due to its negative effect on ecological safety and health risks [1, 2]. Soil contamination through inorganic and organic contaminants is a well-known problem [1, 3]. Contamination of agricultural soils is due to the long-term application of pesticides, fertilizers, plastic film, wastewater irrigation, sewage application, and other activities . Organic contaminants, such as organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), phthalate esters (PAEs), and polycyclic aromatic hydrocarbons (PAHs), are characterized by high toxicity, persistence, and bioaccumulation in the environment . Soil is a major reservoir for a variety of pollutants and is a secondary emission source of contaminants to groundwater and surface water .
In the last years has been increasing interest amongst the scientific community in the development of technologies to remediate contaminated sites . Soil remediation techniques are categorized as physical, chemical, or biological, based on the specific nature of the remediation mechanism employed . Physical and chemical methods are costly, inefficient, and result in further pollution, especially in the case of chemical methods . Adsorption is one of the most efficient biological methods for removing contaminants from water and wastewater . The mechanism and capacity of adsorption are influenced by many factors: physicochemical properties of the adsorbent; type and nature of the adsorbate; the related affinity of the adsorbate for the adsorbent; and process conditions.
Therefore, the use of biological materials derived from organic material is considered an eco-friendly and sustainable approach . The application of proper amendments such as biochar is a known efficacious and environmentally friendly method for reducing the availability and mobility of potentially toxic elements in contaminated soil via
For these organic wastes to be transformed into biochar, they are subjected to pyrolysis, gasification, or hydrothermal carbonization. These are the most common methods for biochar preparation [12, 13]. Biochar properties are highly dependent on the temperature (300–1000°C), time of pyrolysis, final acidity, and feedstock from which the biochar is made . According to the residues types, biochar can present diverse physiochemical properties biochar’s content of volatile matter, dissolved organic matter, ash, and carbon [1, 15]. Additionally, the pyrolysis condition may influence the physicochemical and chemical properties of biochar [16, 17]. However, there are characteristics that are always present, such as rich carbon content, high cation exchange capacity, large surface area, and stability structure [1, 13]. These characteristics, namely specific surface area, porosity, and cation exchange capacity, are responsible for the high adsorption capacity that gives it the ability to remove organic pollutants and heavy metals . Also, the solution pH has a great influence on the adsorption capacity of biochars . Moreover, biochar will improve the soil’s biological activities, nutrient retention, water-retention capacity, an increase of pH value, and amount of soil organic matter.
In last year’s, the utilization of biochar has been widely used in environmental applications such as soil remediation and water remediation. According to Krasucka et al.  the use of “green”, low cost, or sustainable biochar for contaminant sorption yields economic and environmental benefits, furthermore, agrees with global trends in generating a circular economy and sustainable development. Biochar has been a common material for environmental contamination due to the widespread availability of its raw materials, its simple preparation process, low cost, and strong adsorption performance .
The application of biochar can improve soils that pose abiotic stresses because of the presence of heavy metals, salt, or organic contaminants . Biochars produced from pyrolysis of biomass materials have received worldwide attention due to their broad usage in contaminant adsorption, soil remediation, and wastewater treatment . Biochar application can reduce the bioavailability and mobility of soil pollutants including pesticides, antibiotics, heavy metal, and antibiotic resistance genes in soil microorganisms .
2. Soil and water contaminants
Soil is one of the main resources that human beings rely on to survive, and also the material repository of biogeochemical cycles. The remediation of contaminated soils, in order to protect human health and achieve sustainable development, has become the goal of the scientific community. Remediating contaminated soils to protect human health and to achieve sustainable development has become a desirable goal .
The main soil contaminants that most concern society are the following: pesticides, antibiotic residues, antibiotic resistance genes (ARGs), and heavy metals.
Pesticide is any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest (insects, mice, and other animals, weeds, or microorganisms) . Pesticides are widely used to prevent or control the growth of crop pests in agricultural practices. Their leaching and accumulation in the soil and water is a threat to the environment and human health.
Pesticides are one of the persistent organic pollutants due to their recalcitrant, persistent, and bioaccumulative nature and due to their ability to be carried for long distances to affect remote uninhabited parts of the globe .
The strong adsorption potential of biochar has resulted in decreased availability of various agrochemicals (insecticides, fungicides, acaricides, and others) used for pest control in various crops . The effect of biochars on the fate of pesticide contaminants in soil depends on the biochars type and properties, which in turn is affected by the feedstock and pyrolysis conditions . Pesticide adsorption on biochars was affected by their aromaticity, polarity, pore-volume, pore diameter, pH, and surface acidity . More porous structures and higher surface area will result in higher sorption capacities . Studies carried out by Liu et al.  showed that an increase in pyrolysis temperature results in a larger pore volume and surface area. The pore volume of biochar increased from 0.056 to 0.099 cm3 g−1 with a temperature from 500 to 900°C, while surface area increased from 25.4 to 67.6 m2 g−1 . The composition of biochar also affects its properties. For example, the surface area of plant biochar (112–642 m2 g−1) such as oak wood, maize stover, and pine needle is generally much higher than that of pig manure (3.32–20.5 m2 g−1), and biosolid biochar (50.9–94.2 m2 g−1) such as poultry and turkey litter [25, 27, 28], similarly, pine needle biochar shows higher porosity (0.076–1.90 cm3 g−1) than biosolid biochar (0.053–0.068 cm3 g−1) derived at 500–700°C . In general, it can be stated that feedstock with high lignin content produces biochar with macroporous structures, while feedstock with high cellulose content mainly produces biochar with microporous structures .
The pyrolysis temperature of biochars produced from various biomasses (biosolids, agricultural residues, and animal manure) also interferes with its pH, generally, the increase in temperature results in a higher pH of the biochar, and higher pH of biochar can accelerate the hydrolysis of organophosphorus and carbamate pesticides in the soil through alkali catalysis mechanism .
Biochar increases microbial activity by providing available carbon and other nutrients which ultimately accelerate the biodegradation of pesticides in soil .
Several studies have been performed to investigate the effect of biochar addition for pesticide remediation. Biochar proved to be efficient in the sorption of pesticides such as atrazine, terbuthylazine, pyrimethanil, or bentazone [29, 30, 31, 32]. Using biochar irreversible adsorption of pesticides to soils has been as shown due to entrapment into biochar micropores, surface-specific adsorption, and partitioning into condensed structures . Adsorption of triazine herbicides by biochar was primarily affected by the mesoporosity and microporosity of biochars . Liu et al.  compared plant (soybean, corn stalk, and rice stalk) and manure (poultry, cattle, and pig) derived biochars for atrazine adsorption and concluded that, in general, plant-based biochars were more effective in removing atrazine and the removal efficiency increased with elevation in high temperatures treatments and biochar pH.
When studying the application of biochar for sulfamethazine absorption in soils treated with and without biochar derived from an invasive cucumber plant (
The use of biochar in water pesticides remediation also proved to be efficient. Mandal et al.  affirm that rice straw biochars are efficient to adsorb atrazine and imidacloprid from water and have great potential as the next-generation low-cost adsorbent to prevent contamination of groundwater and minimize the environmental impact caused by the pesticides.
However, biochar application studies for contaminated soil remediation have mainly been conducted in laboratories, greenhouses, or small plot experiments, so large-scale field experiments are needed before implementing remediation projects on an operational scale . Resuming, it can be concluded that pyrolysis temperature affects biochar properties and consequently pesticide sorption capacity. Biochar from high pyrolysis temperatures increases pesticide sorption in soils, reducing their soil mobility and bioavailability.
2.2 Antibiotic residues
The widely abuse of antibiotics has led to unpredictable residue releasing in the ecosystem and caused a series of environmental problems . The ecological toxicity of antibiotics is a subject of growing concern because residues of antibiotics in the environment may pose adverse effects on the ecosystem . Thus, antibiotics have a very low degradation rate in the natural environment and can persist for long periods . Residues of antibiotics are detected in wastewater, liquid manure, surface waters, groundwater, soil and plants, drinking water, and food . The application of animal manure in agricultural land is a major way through which antibiotics enter soils that are used for food production; subsequently, they can be transported to other environmental compartments and enter the food chain . The behavior of antibiotics and their transport in manure is related to their physicochemical properties as well as the abiotic properties of the environment.
The content of antibiotic residues in environmental samples (water, sediment, and soil) is seasonal, in the autumn the amounts of tested drugs were higher than in the summer because higher temperatures causing degradation of the drug and frequent rainfall increases the dilution . Irrespective of the different dissipation pathways for antibiotics in manure, adsorption is one of the key processes controlling the fate, mobility, and reactivity of antibiotics . Adsorption is predominantly, especially in the removal of antibiotics from the environment .
Really, the residues of the antibiotics cannot remove effectively in traditional biological wastewater treatment facilities . The methods for antibiotics removal from water include bio-electrochemical systems, heterogeneous photocatalytic method, advanced oxidation process, microbiological method, and adsorption method . Biochars are highly adsorptive with the potential for sequestration of environmental organic contaminants and proved suitable for mitigating effects of antibiotics residues in manure, which effects propagate when manure is added to the soil .
Biochar strongly sorbs several antibiotics, including oxytetracycline, ciprofloxacin, tetracycline hydrochloride, doxycycline hydrochloride, and fluoroquinolone  in an aqueous solution. The efficiency of removal of the three antibiotics increased with increasing biochar dosage up to 1.2 g L−1. In experimental studies developed by Peng et al. , on the adsorption of seven antibiotics in the environmental concentration of aqueous solutions by carbon-based materials have been observed and showed that the carbon-based materials have good adsorption properties for antibiotics in the actual concentration of environments, by which the highest removal efficiency of antibiotics can be up to 100%.
Biochar produced at high pyrolytic temperatures had greater adsorption capacity for antibiotic residues . The efficiency of removal of tetracycline hydrochloride, doxycycline hydrochloride, and ciprofloxacin was enhanced with increasing pyrolysis temperature and biochar dosage [7, 14]. In studies developed by Zeng et al. , the best adsorption was obtained with biomass pyrolyzed at 700°C. Regarding antibiotic adsorption, the characteristics of the biochar produced play an essential role, namely its sorption parameters, aromaticity, hydrophilicity, and group density surface oxygen functionals or ash content .
2.3 Antibiotic resistance genes
Antibiotic resistance genes (ARGs) are the means through which bacteria become super antibiotic-resistant bacteria . In last year’s dissemination and persistence of antibiotic resistance genes (ARGs), received increasing concerns like one of the biggest threats to global public health and food security [43, 44]. The spread of antibiotic-resistant pathogens is a growing problem in the world is considered a type of emerging contaminant.
According to World Health Organization  antibiotic resistance is one of the most critical human health challenges of the next century and heralded the need for “a global strategy to contain resistance”. In agreement with the last UN Global Environmental Outlook  ARGs as classified as the main pollutants for water sources, and call for the application of effective policies for its control in water bodies worldwide. Soil amendments with animal manure are potential sources of ARGs in soil, water, and plants . Animal manures, commonly applied to the soil, support the spread of ARGs from soil to humans via the food chains [44, 48]. Remediation of agricultural soils polluted with ARGs is important for protecting food safety and human health [20, 44].
Biochar amendments have proven able to decrease the relative abundance of multiple subtypes of antibiotic resistance genes (ARGs)  because could affect their dissemination and fate in the environment . Recently, a few studies also attempted to use biochar to alleviate ARG pollution in soil [49, 50]. Several studies demonstrated that biochar efficiently mitigated ARG pollution to some extent in soils  but not all the biochars consistently showed a positive effect.
In studies developed by Chen et al.  reported that biochar of rice straw application apparently reduced the abundance of 131 ARGs in non-planted soil, but little effect was found in a
Cui et al.  found that wheat straw biochar increased the abundance of tet and sul genes in a soil-plant system. These studies showed the uncertainties about the effectiveness of biochar in remediating soil ARG pollution. This uncertainty is related to the properties of the biochar, as the properties of biochar may affect the conjugative transfer of ARGs between bacteria namely feedstock and pyrolytic conditions . Also, the number of heavy metals and antibiotics present in the soil determines the evolution of ARGs in the soil [44, 52], being interesting the development of biochars with excellent adsorption capacities of heavy metals and antibiotics to prevent soil pollution by ARGs .
Relative to heavy metals and other organic pollutants the interaction between biochar and ARGs is much less explored . Therefore, further studies are needed to confirm the effect of different biochar on ARGs and even develop alternative strategies to improve the efficiency of biochar in the remediation of ARG soil pollution.
2.4 Heavy metals
Soil contamination with heavy metals has become a global environmental-health concern [1, 14, 54]. Their high level in the soil cause hazardous effects on soil quality, fertility, food safety, and human health . The entry of soil-borne heavy metals into the food chain depends on the amount and source of heavy metals input, the properties of the soil, the rate and magnitude of uptake by plants .
The main aim for researchers and environmentalists is to stop the entry of metals and metalloids into the food chain for better human health . Indiscriminate waste disposal practices have led to significant build-up in soils of a wide range of heavy metals . Sources of heavy metals in soil could be anthropogenic activities, such as mining and smelting, wastewater irrigation, exhaust emissions, and sludge applications [6, 55].
Heavy metals could impact soil fertility, microbial activities, biodiversity, crop yields and pose risks to human health due to dietary exposure . Biochar serves as a faster, more efficient, and environmentally friendly alternative to heavy metals in the agricultural soil .
The ability of biochar to remediate soil contaminated by heavy metals is linked to their facility to immobilize pollutants . Immobilization is the conversion of soluble and potentially soluble forms of heavy metals to geochemically stable solid phases. Adsorption, ion exchange, complexation, and precipitation are the major mechanisms reducing heavy metal bioavailability in soils [15, 56, 57]. Biochar could reduce the bioavailability of heavy metals in the soil through cation exchange, complexation, and other related effects . the addition of biochar to soil binds and/or precipitates heavy metals in soil and reduces their accumulation in plants .
The remediation effect depends on the characteristics of both biochar and soil and their interactions [6, 55]. Biochar applications could decrease the mobility/bioavailability of heavy metals in soils and their accumulation in plants .
The evolution of heavy metals in soil solutions altered by different biochars under vibrant redox conditions is a challenge and more investigation is necessary to comprehend the influence of the biochar on the dynamic forces of the heavy metals contaminated soils . There are several studies that demonstrate the effectiveness of biochar in the treatment of soils contaminated by heavy metals. Thus, studies developed by Zeng et al.  reported that maize straw biochar reduced the availability of Cd in soil by transforming this heavy metal into a state of lower availability. Using apricot shell and the apple tree derived biochar  found that the labile fractions of Cd in smelter-contaminated soil decreased. Also, in studies developed by  using biochar from sewage sludge, this one showed up effective in immobilizing non-essential heavy metals for plants. Li et al.  applied 3% of soybean-straw-derived biochar (hydrothermal carbon at 350°C) to the As and Cd co-contaminated farmland soil, which reduced the bioaccumulation of As in rice plants by 88% and the treatment effects on Cd were similar. A field experiment made by Zheng et al.  for Cd, showed that, when the application rates of soybean-straw-derived biochar and rice-straw-derived biochar were 20 ton ha−1, the content of Cd in rice roots, rice shoots, rice husks, and rice grains decreased by 25.0–44.1% and 19.9–44.2%, and 46.2–70.6% and 25.8–70.9%, respectively.
The ability of biochar to immobilize heavy metals is related to its ability to change soil pH, biochar typically contains alkaline components, which can considerably improve soil pH . For example, Van Poucke et al.  showed a significant negative correlation between the pH value increase and the soil exchangeable Cd content after biochar application. In studies developed by Fang et al.  was found that the addition of sludge-derived biochar to contaminated soil could increase the soil pH and reduce the effective concentration of Pb, Cd, Ni, and Cr in soil.
Studies developed by Li et al.  evidenced that the heavy metal concentrations in biochar were diluted by adding antibiotic mycelial residue, which led to lower toxic inputs to the environment, moreover, heavy metals were transformed to more stable fractions after co-pyrolysis. Liu et al.  confirm that the addition of dissolved biochar is an effective measure to control copper pollution in water, as the combination of humic acid-like components in dissolved biochar with Cu(ii) significantly reduces the concentration of Cu(ii) in water.
The hydrological conditions in the natural field can influence the effect of biochar on the stabilization of contaminated soil . Studies developed by dos Santos et al. , showed the biochar efficiency in removal methylene blue from water. About 83% of methylene blue removal was achieved within 30 minutes of equilibration time.
Braghiroli et al.  showed that biochar high aromaticity and porosity are essential for the sorption of organic contaminants, while the presence of oxygen-containing functional groups and optimum pH are crucial for the sorption of inorganic contaminants, especially metals. Thus, biochar can be proficiently adopted for metals adsorption from polluted water , showing to be a promising option for the treatment of contaminants in water, but further research is required to evaluate its performance with real effluents containing contaminants of emerging concern .
Biochar is widely used in wastewater treatment and soil remediation and shows great potential in ameliorating the toxicological effect of antibiotics, pesticides, antibiotic resistance genes, and heavy metals. Biochar efficacy for reducing the availability and mobility of potentially toxic elements in soil and water is dependent on the properties and chemical structure of the contaminants, as well as the adsorption process conditions; however, it is primarily dependent on biochar physicochemical properties. Biochar properties are predominantly determined by the feedstock type used and pyrolysis temperature. Hence, to obtain an efficient and selective biochar adsorbent, it is essential to select appropriate feedstocks and production conditions. However, research gaps still exist in the development of practical methods for preparing and applying different biochars that target specific heavy metals, and more research is needed to expand knowledge in this area.
This research was funded by National Funds by FCT—Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020 and UIDB/00681/2020, CERNAS-IPV.
Conflict of interest
The authors declare no conflict of interest.