**1.8 Biochar's potential role as a sorbent for organic pollutants like glyphosate**

Biochar can be defined as "the porous carbonaceous solid produced by the thermochemical conversion of organic materials in an oxygen-depleted atmosphere that has physicochemical properties suitable for safe and long-term storage of carbon in the environment" [34]. Biochar and activated charcoal are similar concerning production via pyrolysis, with medium to high surface areas [35]; however, biochar is not activated or treated like activated charcoal [35, 36]. Crop residues are pyrolyzed at high temperature (>500°C) in the absence of oxygen, followed by various activation processes to form activated charcoal [35]. In comparison to activated charcoal, biochar has a non-carbonized fraction that interacts with soil contaminants like glyphosate. Soil minerals can increase the surface area and pore size of biochar, which in turn increase the adsorption capacity of biochars for organic pollutants like glyphosate [37]. Biochar application can reduce the bioavailability and leachability of organic pollutants in soils through adsorption and other physicochemical reactions [38]. An increase in the surface area of biochars has been observed to increase the biochar's ability to adsorb organic contaminants [39, 40]. The addition of phosphorus fertilizer to biochar-amended soils can, however, remobilize glyphosate and damage non-target plants; therefore, improved understanding of this risk is important (**Figure 2**) [41].

The soil environment is a three-dimensional structure of water-filled pores, gas-filled pores, and soil particulates (organic matter, sand, silt, and clay) [42]. Biochar can be used as a sorbent for organic pollutants due to its highly aromatic

**109**

**Figure 2.**

*Glyphosate Resistance of* Chloris virgata *Weed in Australia and Glyphosate Mobility…*

nature, high surface area, micropore volume, and abundance of polar functional groups [43]. Factors affecting biochar's performance for adsorption include pyrolysis temperature and surface area. Pyrolysis temperature is one of the factors directly affecting biochar's performance. An increase in pyrolysis temperature of biochar generally increases the degree of carbonization and consequently surface area. Even with the increase in surface area of biochars, sorption sites can be blocked by organic matter, and this is the likely cause for the diminished capability of aged biochars to adsorb organic contaminants [44]. The behaviour of biochar changes with time after its application to soil, and this process is known as "aging". Aging can alter the behaviour of biochar. To increase the remediation efficiency of biochar concerning herbicides, more detailed research to explore the aspect of aging is warranted.

In a study related to herbicide terbuthylazine-biochar-soil interaction, there was higher adsorption of herbicide in soil with low organic matter than in soil with the high organic matter. The reason for this result was attributed to a high concentration of organic molecules competing with herbicide for sorption sites in the soil having a high amount of organic matter [40]. Availability of herbicides can be greatly reduced with the application of biochar [45]. Even a low application rate (0.1%) of biochar in

the soil can appreciably reduce the availability of herbicides like diuron [44].

specific surface area, high microporosity, and high aromatic carbon.

**1.10 The behaviour of glyphosate in a soil-biochar system**

In a comparative study [46], 42 times higher hexachlorobenzene sorption by biochar than that by control soil was observed, resulting in the reduction of volatilization and earthworm (*Eisenia foetida*) uptake of hexachlorobenzene from the soil. The extent of sorption of pesticides generally depends on the aromaticity of soil organic carbon. Properties that make biochars effective against herbicides are a high

Plant uptake of pesticides decreases markedly with increasing biochar content of the soil despite the greater persistence of the pesticide residues in biochar-amended soils [47]. In a similar study related to the effects of biochar, wood vinegar, and plants on glyphosate leaching and degradation, the addition of biochar to the soil

**1.9 Behaviour of herbicides in a soil-biochar system**

*DOI: http://dx.doi.org/10.5772/intechopen.90323*

*Phosphate and glyphosate adsorption by minerals [5].*

*Glyphosate Resistance of* Chloris virgata *Weed in Australia and Glyphosate Mobility… DOI: http://dx.doi.org/10.5772/intechopen.90323*

#### **Figure 2.**

*Sorption in 2020s*

and leaching of 14C-glyphosate in agricultural soils, non-extractable glyphosate residues become available eventually and take part in biodegradation and leaching. Empirical constants (KF) of Freundlich sorption isotherm were 16.6 for the clay loam, 33.6 for the silty clay loam, and 34.5 for the sandy clay loam indicating that it is the soil structure which dictates the glyphosate sorption behaviour [26]. Leaching of glyphosate was dependent on hydrodynamic and biodegradation properties of soils [26]. Application of char can be used as a strategy to increase the sorption of glyphosate [27]. Movement of pesticides and their bioavailability and biotransformation are controlled by adsorption/desorption mechanisms operating at the interface between organic and inorganic soil colloids. High-resolution magic angle spinning and nuclear magnetic resonance techniques can distinguish mobile and immobile phases of pesticides like glyphosate [28]. Another study on glyphosate transport parameters suggested that glyphosate sorption is a kinetic process that depends on

pore-water velocities and residence time of soil solution [29].

**1.7 Why is glyphosate application on field sites a concern?**

The International Agency for Research on Cancer (IARC) has reclassified that glyphosate is "probably carcinogenic to humans" [30]; however, the United States Environment Protection Agency (US EPA) concluded that there is no convincing evidence that "glyphosate induces mutations" [31]. The US EPA relied mostly on unpublished regulatory studies, 99% of which were negative, while IARC relied mostly on peer-reviewed studies, 70% of which were positive [31]. Glyphosatebased herbicides often contaminate drinking water sources, air, and precipitation in agricultural regions [30]. As the usage of glyphosate-based herbicides continues to increase, investment in epidemiological studies, biomonitoring, and toxicology studies based on the principles of endocrinology should be done [30]. Apart from cancer, glyphosate has been found to be a potential factor causing chronic kidney disease due to drinking water faced by Sri Lankan farmers [32]. The role of drinking water has also been reported in another study which caused ill health in Indian farmers [33].

**1.8 Biochar's potential role as a sorbent for organic pollutants like glyphosate**

Biochar can be defined as "the porous carbonaceous solid produced by the thermochemical conversion of organic materials in an oxygen-depleted atmosphere that has physicochemical properties suitable for safe and long-term storage of carbon in the environment" [34]. Biochar and activated charcoal are similar concerning production via pyrolysis, with medium to high surface areas [35]; however, biochar is not activated or treated like activated charcoal [35, 36]. Crop residues are pyrolyzed at high temperature (>500°C) in the absence of oxygen, followed by various activation processes to form activated charcoal [35]. In comparison to activated charcoal, biochar has a non-carbonized fraction that interacts with soil contaminants like glyphosate. Soil minerals can increase the surface area and pore size of biochar, which in turn increase the adsorption capacity of biochars for organic pollutants like glyphosate [37]. Biochar application can reduce the bioavailability and leachability of organic pollutants in soils through adsorption and other physicochemical reactions [38]. An increase in the surface area of biochars has been observed to increase the biochar's ability to adsorb organic contaminants [39, 40]. The addition of phosphorus fertilizer to biochar-amended soils can, however, remobilize glyphosate and damage non-target plants; therefore, improved understanding of this risk is important (**Figure 2**) [41]. The soil environment is a three-dimensional structure of water-filled pores, gas-filled pores, and soil particulates (organic matter, sand, silt, and clay) [42]. Biochar can be used as a sorbent for organic pollutants due to its highly aromatic

**108**

*Phosphate and glyphosate adsorption by minerals [5].*

nature, high surface area, micropore volume, and abundance of polar functional groups [43]. Factors affecting biochar's performance for adsorption include pyrolysis temperature and surface area. Pyrolysis temperature is one of the factors directly affecting biochar's performance. An increase in pyrolysis temperature of biochar generally increases the degree of carbonization and consequently surface area.

Even with the increase in surface area of biochars, sorption sites can be blocked by organic matter, and this is the likely cause for the diminished capability of aged biochars to adsorb organic contaminants [44]. The behaviour of biochar changes with time after its application to soil, and this process is known as "aging". Aging can alter the behaviour of biochar. To increase the remediation efficiency of biochar concerning herbicides, more detailed research to explore the aspect of aging is warranted.

#### **1.9 Behaviour of herbicides in a soil-biochar system**

In a study related to herbicide terbuthylazine-biochar-soil interaction, there was higher adsorption of herbicide in soil with low organic matter than in soil with the high organic matter. The reason for this result was attributed to a high concentration of organic molecules competing with herbicide for sorption sites in the soil having a high amount of organic matter [40]. Availability of herbicides can be greatly reduced with the application of biochar [45]. Even a low application rate (0.1%) of biochar in the soil can appreciably reduce the availability of herbicides like diuron [44].

In a comparative study [46], 42 times higher hexachlorobenzene sorption by biochar than that by control soil was observed, resulting in the reduction of volatilization and earthworm (*Eisenia foetida*) uptake of hexachlorobenzene from the soil. The extent of sorption of pesticides generally depends on the aromaticity of soil organic carbon. Properties that make biochars effective against herbicides are a high specific surface area, high microporosity, and high aromatic carbon.

#### **1.10 The behaviour of glyphosate in a soil-biochar system**

Plant uptake of pesticides decreases markedly with increasing biochar content of the soil despite the greater persistence of the pesticide residues in biochar-amended soils [47]. In a similar study related to the effects of biochar, wood vinegar, and plants on glyphosate leaching and degradation, the addition of biochar to the soil

decreased the leaching of glyphosate irrespective of plants. Hence, it was concluded that biochar can be used as an effective strategy to reduce the potential environmental risk to aquatic environments caused by glyphosate [27].

In a study related to the effects of wood-based biochar on the leaching of pesticides chlorpyrifos, diuron, and glyphosate, it was concluded that biochar can be used as an adsorptive layer directly on or close to the soil surface to prevent losses of pesticides [48]. In another study, biochar was found to limit glyphosate transport in soil systems; however, the addition of phosphatic fertilizer remobilized the glyphosate from biochar-amended soils. This phosphate-induced glyphosate desorption phenomenon is important to consider in soils having biochar amendment [41]. The type of biochar also plays an important role, as hardwood biochars were ineffective sorbents of glyphosate in high-phosphate soils [41]. Biochars produced at high temperature were effective sorbents of glyphosate [41]. Reduced glyphosate sorption on biochars was observed with the increase in pH from 6 to 9 [41, 49, 50].
