**The Influence of Biochar Production on Herbicide Sorption Characteristics**

S.A. Clay and D.D. Malo *South Dakota State University, Plant Science Dept. Brookings, South Dakota USA*

#### **1. Introduction**

58 Herbicides – Properties, Synthesis and Control of Weeds

Yang, C., Li, Y., Zhang, K., Wang, X., Ma, C., Tang, H., Xu, P. (2010). Atrazine degradation

Ying, G.G., Kookana, R. S., Mallavarpu, M. (2005). Release behavior of triazine residues in stabilised contaminated soils. *Environmental Pollution*, Vol.134, 71-77.

enriched medium. *Biodegradation*, Vol.21, 97-105.

by a simple consortium of *Klebsiella* sp. A1 and *Comamonas* sp. A2 in nitrogen

Biochar is the by-product of a thermal process conducted under low oxygen or oxygen-free conditions (pyrolysis) to convert vegetative biomass to biofuel (Jha et al., 2010). There are a wide variety of end-products that can be manufactured depending on processing parameters and initial feedstocks (Bridgewater, 2003). The pyrolytic process parameters such as temperature, heating rate, and pressure can change the recovery amounts of each end-product, energy values of the bio-oils, and the physico-chemical properties of biochar (Yaman, 2004).

Biochars are recalcitrant forms of carbon and, depending on properties, can remain in the soil for greater than 1000 years (Skjemstad et al., 2002). The long-term persistence of this carbon form is due to slow microbial degradation and chemical oxidation rates (Sanchez et al., 2009). In addition, biochar interacts with soil materials such as ions, organic matter, and clays that generally increase the persistence of biochar within the soil. However, biochars, unlike commercial fertilizers, are not precisely defined materials and vary widely in properties depending on organic material source and manufacturing process (Karaosmanoglu et al., 2000; McHenry, 2009; Sohi et al., 2010). Increasing pyrolytic temperature decreases biochar recovery but increases C concentration of the char compared with char recovered at lower temperatures (Daud et al., 2001; Katyal et al., 2003). For example, as temperature increased from 3000 to 8000 C, biochar C content increased from 56 to 93% whereas biochar yield decreased from 67 to 26% (Okimori et al., 2003). Other pyrolytic parameters, such as sweep gas flow, can influence biochar particle size with higher flows reducing the particle size but increasing heating values (Katyal et al., 2003; Demirbas, 2004). Biochar also can be influenced by reactor design and other reaction parameters including heating rate, residence time, pressure, and catalyst used. Feedstock type, quality, and initial physical characteristics of the material (e.g. particle size, shape, and structure) can impact the bio-oil yield and properties, as well as the type and amounts of biochar formed (Bridgewater et al., 1999).

Landspreading biochar for a soil amendment is suggested to improve crop production efficiency because regardless of the initial manufacturing process, biochars have a high charge density and surface area. The use of biochar as a soil amendment is not a new concept. Dark earths (terra preta) discovered in the Amazon Basin were found to have

The Influence of Biochar Production on Herbicide Sorption Characteristics 61

biochar applied, the changes in soil properties associated with the application (e.g. soil pH, EC) as well as the physio-chemical properties of the char itself, may impact the use, rates, efficacious properties, and fates of agrichemicals used in agronomic management. The environmental fate (e.g. leachability, rate of decomposition, etc.) and efficacy of soil applied pesticides are influenced strongly by their reaction and retention with soil particles and organic matter (Brown et al., 1995). Agrichemical molecules can be removed from soil solution through attraction or attachment to the surfaces of organic materials and soil particles (adsorption) or movement into the matrix (like water into a sponge) (absorption). Often, experiments cannot distinguish between these processes so that the general term

Sorption is controlled by properties of the chemical of interest including the water solubility, pH, dissociation constant (pKa), octanol/water partition coefficient, and other factors (Weber, 1995) and can be used to help describe the fate of an herbicide in the environment (Wauchope et al., 2002). The sorption of the chemical also is affected by soil properties including water, organic matter, clay, sand, and oxide contents, and soil pH (Koskinen and Clay, 1997; Laird and Koskinen, 2008). Soils high in sand generally sorb much less chemical than loamy or clay type soils. Agricultural practices that involve modifying soil organic matter content often increase chemical retention. Indeed, studies have shown that adding biochar to soil can result in greater sorption of pesticides (Cao et al., 2009; Spokas et al., 2009; Yu et al., 2009). The distribution of chemical between a solution and solid phase gives an indication of the amount of chemical available in solution and is defined using a sorption

mass of herbicide sorbed per g of solid K

Large Kd values (typically over 100) indicate that a high amount of the chemical originally in solution is sorbed to the solid interface, with low amounts of chemical remaining in solution. Sorption of a chemical from the liquid phase of soil may result in the chemical being: 1) less available to plants, so there may be less uptake; 2) less available to soil organisms, thereby increasing the chemical's residence time and slowing degradation; and 3) less available to leach with water percolating through the soil, which could result in

The biochar source-processing combination provides a rich diversity of biochars to evaluate for soil amendment use (Lehmann et al., 2009). The potential of a specific biochar for a specific use will depend on the physical and chemical properties of the biochar, as well as soil characteristics. The challenge of amending soil with biochar is to identify the benefits that biochar can provide (e.g. fertility, increased water holding capacity) (Lehmann, 2007) and balance these against any negative effects that the char may have. Site-specific application recommendations of specific biochars require an examination of the products of different production and processing scenarios. Much of the biochar research has been based on slow pyrolysis with a goal to optimize biochar properties for a specific goal such as improved soil fertility, greenhouse gas mitigation, or heating value. Little work has been done with biochar produced from fast pyrolysis processes and even less with biochar

amount of chemical remaining in solution at equilibrium (1)

sorption is used.

coefficient (Kd) where:

d

improved groundwater quality.

produced from microwave pyrolysis reactors.

received deliberate land applications of charred materials and residues of biomass burning by Amer-indian populations before European arrival (Erickson, 2003; Sombroek et al. 2003). Pyrogenic C in terra preta is very resistant to microbial decay over centuries due to its complex aromatic structure and acts as a significant C sink (Glaser et al., 2001).

The benefits of biochar application have been hypothesized to include: increasing plant available soil water; building soil organic matter; enhancing nutrient cycling; lowering soil bulk density; acting as a liming agent if high in pH; and reducing transfer of pesticides and nutrients to surface and ground water (Laird, 2008) thereby improving water quality. The application of biochar to soil has been reported to have a positive impact on physical properties such as soil water retention and aggregation (Piccolo et al., 1996) and may decrease erosion potential. Glaser et al. (2002) observed an increase in field water holding capacity by 18% in charcoal enriched Anthrosol due to an increase in surface area. Biochar application has been shown to improve other soil physical, chemical, and biological properties (Glaser et al., 2002; Lehmann and Rondon, 2006) leading to positive impacts on plant growth and development. For example, Chidumayo (1994) observed enhanced seed germination (30%), shoot height (24%), and biomass production (13%) of seven indigenous woody crops with the application of charcoal compared with the crops on undisturbed Zambian Alfisols and Ultisols. Kishimoto and Sugiura (1985) also found increases in height (26 to 35%) and biomass (2.3 X greater) production of sugi trees (*Cryptomeria japonica L.*). Similar enhancement was observed in yields of annual crops such as maize (*Zea mays L.*) on Nigerian Alfisols and Inceptisols with the application of charcoal (Mbagwu and Piccolo, 1997) due to an increase of soil pH that resulted in greater micro-nutrient availability and decreased deficiencies. However, biochars also have been shown to have an extreme affinity for essential plant nutrients (Sanchez et al., 2009) that can provide a slow release mechanism.

Some biochars that have high pH (e.g. >9.5) can provide liming capacity and increase the soil pH (Sanchez et al., 1983; Mbagwu and Piccolo, 1997). For example, application of coal ash at the rate of 110 Mg ha-1 increased the pH of an eroded Palouse soil from 6.0 to 6.8 (Cox et al., 2001). Exchangeable bases also were observed to increase in sandy and loamy soils with the additions of hardwood and conifer charcoals (Tryon, 1948). Application of charcoal to highly weathered soils with low-ion retention capacities increased the cation exchange capacity (CEC) by 50% compared to unamended soil (Mbagwu and Piccolo, 1997). Oguntunde et al. (2004) reported a significant increase in soil pH, base saturation, electrical conductivity (EC), exchangeable Ca, Mg, K, Na, and available P in charcoal kiln sites and reported an increase in grain and biomass yield of maize of 91% and 44% respectively, with a coal char application. Leaching of NH4+ from an unfertilized Ferralsol was reduced with the application of charcoal due to its high C content, although the retention properties of chars may differ for other ionic species (e.g. K, Ca, Mg) if the char already contains high concentrations of the ion of interest (Lehmann et al., 2002). Because of biochar's diverse properties and potential for high reactivity in soils, a 'one-recommendation-fits-all situations' mentality for the use as of biochar as a soil amendment needs to be avoided. To date, the greatest positive impacts of biochar have been primarily observed on degraded soils and those with low fertility whereas applications on highly productive soils have been reported to have low or minimal impacts (Woolf et al., 2010).

Agrichemicals such as pesticides, growth regulating chemicals, and nutrients are applied to crops to control pests and increase yield potential. Depending on the type and amount of

received deliberate land applications of charred materials and residues of biomass burning by Amer-indian populations before European arrival (Erickson, 2003; Sombroek et al. 2003). Pyrogenic C in terra preta is very resistant to microbial decay over centuries due to its

The benefits of biochar application have been hypothesized to include: increasing plant available soil water; building soil organic matter; enhancing nutrient cycling; lowering soil bulk density; acting as a liming agent if high in pH; and reducing transfer of pesticides and nutrients to surface and ground water (Laird, 2008) thereby improving water quality. The application of biochar to soil has been reported to have a positive impact on physical properties such as soil water retention and aggregation (Piccolo et al., 1996) and may decrease erosion potential. Glaser et al. (2002) observed an increase in field water holding capacity by 18% in charcoal enriched Anthrosol due to an increase in surface area. Biochar application has been shown to improve other soil physical, chemical, and biological properties (Glaser et al., 2002; Lehmann and Rondon, 2006) leading to positive impacts on plant growth and development. For example, Chidumayo (1994) observed enhanced seed germination (30%), shoot height (24%), and biomass production (13%) of seven indigenous woody crops with the application of charcoal compared with the crops on undisturbed Zambian Alfisols and Ultisols. Kishimoto and Sugiura (1985) also found increases in height (26 to 35%) and biomass (2.3 X greater) production of sugi trees (*Cryptomeria japonica L.*). Similar enhancement was observed in yields of annual crops such as maize (*Zea mays L.*) on Nigerian Alfisols and Inceptisols with the application of charcoal (Mbagwu and Piccolo, 1997) due to an increase of soil pH that resulted in greater micro-nutrient availability and decreased deficiencies. However, biochars also have been shown to have an extreme affinity for essential plant nutrients (Sanchez et al., 2009) that can provide a slow release

Some biochars that have high pH (e.g. >9.5) can provide liming capacity and increase the soil pH (Sanchez et al., 1983; Mbagwu and Piccolo, 1997). For example, application of coal ash at the rate of 110 Mg ha-1 increased the pH of an eroded Palouse soil from 6.0 to 6.8 (Cox et al., 2001). Exchangeable bases also were observed to increase in sandy and loamy soils with the additions of hardwood and conifer charcoals (Tryon, 1948). Application of charcoal to highly weathered soils with low-ion retention capacities increased the cation exchange capacity (CEC) by 50% compared to unamended soil (Mbagwu and Piccolo, 1997). Oguntunde et al. (2004) reported a significant increase in soil pH, base saturation, electrical conductivity (EC), exchangeable Ca, Mg, K, Na, and available P in charcoal kiln sites and reported an increase in grain and biomass yield of maize of 91% and 44% respectively, with a coal char application. Leaching of NH4+ from an unfertilized Ferralsol was reduced with the application of charcoal due to its high C content, although the retention properties of chars may differ for other ionic species (e.g. K, Ca, Mg) if the char already contains high concentrations of the ion of interest (Lehmann et al., 2002). Because of biochar's diverse properties and potential for high reactivity in soils, a 'one-recommendation-fits-all situations' mentality for the use as of biochar as a soil amendment needs to be avoided. To date, the greatest positive impacts of biochar have been primarily observed on degraded soils and those with low fertility whereas applications on highly productive soils have been

Agrichemicals such as pesticides, growth regulating chemicals, and nutrients are applied to crops to control pests and increase yield potential. Depending on the type and amount of

reported to have low or minimal impacts (Woolf et al., 2010).

complex aromatic structure and acts as a significant C sink (Glaser et al., 2001).

mechanism.

biochar applied, the changes in soil properties associated with the application (e.g. soil pH, EC) as well as the physio-chemical properties of the char itself, may impact the use, rates, efficacious properties, and fates of agrichemicals used in agronomic management. The environmental fate (e.g. leachability, rate of decomposition, etc.) and efficacy of soil applied pesticides are influenced strongly by their reaction and retention with soil particles and organic matter (Brown et al., 1995). Agrichemical molecules can be removed from soil solution through attraction or attachment to the surfaces of organic materials and soil particles (adsorption) or movement into the matrix (like water into a sponge) (absorption). Often, experiments cannot distinguish between these processes so that the general term sorption is used.

Sorption is controlled by properties of the chemical of interest including the water solubility, pH, dissociation constant (pKa), octanol/water partition coefficient, and other factors (Weber, 1995) and can be used to help describe the fate of an herbicide in the environment (Wauchope et al., 2002). The sorption of the chemical also is affected by soil properties including water, organic matter, clay, sand, and oxide contents, and soil pH (Koskinen and Clay, 1997; Laird and Koskinen, 2008). Soils high in sand generally sorb much less chemical than loamy or clay type soils. Agricultural practices that involve modifying soil organic matter content often increase chemical retention. Indeed, studies have shown that adding biochar to soil can result in greater sorption of pesticides (Cao et al., 2009; Spokas et al., 2009; Yu et al., 2009). The distribution of chemical between a solution and solid phase gives an indication of the amount of chemical available in solution and is defined using a sorption coefficient (Kd) where:

$$\mathbf{K\_d} = \frac{\text{mass of herbicideorbed per g of solid}}{\text{amount of chemical remaining in solution at equilibrium}} \tag{1}$$

Large Kd values (typically over 100) indicate that a high amount of the chemical originally in solution is sorbed to the solid interface, with low amounts of chemical remaining in solution. Sorption of a chemical from the liquid phase of soil may result in the chemical being: 1) less available to plants, so there may be less uptake; 2) less available to soil organisms, thereby increasing the chemical's residence time and slowing degradation; and 3) less available to leach with water percolating through the soil, which could result in improved groundwater quality.

The biochar source-processing combination provides a rich diversity of biochars to evaluate for soil amendment use (Lehmann et al., 2009). The potential of a specific biochar for a specific use will depend on the physical and chemical properties of the biochar, as well as soil characteristics. The challenge of amending soil with biochar is to identify the benefits that biochar can provide (e.g. fertility, increased water holding capacity) (Lehmann, 2007) and balance these against any negative effects that the char may have. Site-specific application recommendations of specific biochars require an examination of the products of different production and processing scenarios. Much of the biochar research has been based on slow pyrolysis with a goal to optimize biochar properties for a specific goal such as improved soil fertility, greenhouse gas mitigation, or heating value. Little work has been done with biochar produced from fast pyrolysis processes and even less with biochar produced from microwave pyrolysis reactors.

The Influence of Biochar Production on Herbicide Sorption Characteristics 63

Atrazine solution was diluted to a final concentration of 13 µM in 0.01 CaCl2 using technical grade atrazine. This solution was spiked with about 0.4 kBq of uniformly-ring-labeled [14C] atrazine (specific activity of 1000 MBq mmol-1 with > 99% purity; Sigma Chemical Co., St. Louis, MO). The 2,4-D solution was made in a similar manner, with technical grade 2,4-D added to 0.01 M CaCl2 to have a final concentration of 13 µM. This solution was spiked with uniformly-ring-labeled [14C]-2,4-D (specific activity of 1000 MBq mmol-1 with > 99% purity;

A 4-mL aliquot of herbicide solution was added to 2 g soil or soil amended with 1 or 10% biochar (final slurry solution 2:1 v/w) in glass centrifuge tubes sealed with a Teflon-lined cap. A 5-mL aliquot of herbicide solution was added to 0.5 g biochar when biochar was used as the sorbent, with the final solution/biochar ratio of was 10:1 v/w, due to the highly

After solution addition, the mixtures were shaken or vortexed to form a slurry. Tubes containing the slurries were shaken for 24 hr, centrifuged, and a 250-µL aliquot of supernatant removed. The amount of 14C remaining in the supernatant solution was determined by liquid scintillation (Packard Model 1600TR) counting after the addition of scintillation cocktail. The amount of radioactivity sorbed was determined by comparing the counts in the supernatant samples with counts recorded from the original soil-free blank solution samples. The sorption coefficients (Kd) of the samples were then calculated as

Experimental treatments were run in triplicate and studies were repeated in time. Results were combined for the studies due to similarity of means and homogeneity of variance between studies. Means presented were averaged over all treatment replicates and

The biochars produced in this study ranged in pH from acidic (4.06) to alkaline (9.88), and were dependent on feedstock, pyrolysis temperatures, and processing times (Table 1). Differences were observed among maize and switchgrass feedstocks. For maize stover, three of the microwave pyrolysis reactions at high temperatures (>650ºC), regardless of processing time, resulted in biochars that were very alkaline (pH>9). Two processes at lower temperatures (530ºC and a processing time of 16 min or 550ºC with a processing time of 10 min) resulted in biochars with pH <5. The 22 min processing time at 550ºC resulted in a biochar with a more neutral (7.6) pH. For switchgrass, four processes resulted in biochars that were acidic (pH < 4.6) and the biochars were more acidic than biochars from maize at the same time and temperature. The acidic biochars were formed from processes that had low temperatures (<600ºC) or shorter times at 600ºC (8 min), or 10 min at 6500C. The most alkaline switchgrass biochar was the result of processing at 670ºC for 16 min. This biochar had a pH of ~9.1, which was lower than the alkaline maize biochars that ranged in pH from

L kg-1, correcting for the differences in volume added g-1 of material.

statistically separated by least significant difference calculation at P< 0.05.

**2.1.3 Herbicide sorption**

Sigma Chemical Co., St. Louis, MO).

sorbent characteristics of the biochar.

**2.1.4 Statistical analysis**

**2.2.1 Biochar pH and EC values**

**2.2 Results**

Feedstock is a key factor governing the status of physio-chemical properties of biochar. All types of materials including, but not limited to, palm shells, rapeseed (*Brassica rapa*) stems, sunflower (*Helianthus annuus*), and wood have been used or are being proposed as potential feedstock sources for use in the biofuel industry. In the Midwestern U.S., maize stover and switchgrass (*Panicum virgatum*) biomass are feedstocks that bioenergy companies are exploring for use.