**2.1.1 Biochar and soil**

Biochar was produced from maize stover (stalks and other residues remaining after maize grain harvest) and switchgrass biomass collected from fields near Brookings, South Dakota, USA (44.31, -96.67). Briefly, the material was dried at room temperature and pulverized mechanically using a Thomas-Wiley laboratory mill (Model No. 3375-E15, Thomas Scientific, USA) to pass through a 4 mm screen. The ground materials were processed by microwave pyrolysis using the SDSU Ag and Biosystem Eng. Dept. microwave system (specific processing methods reported in Lei et al., 2009). Processing temperatures ranged from 5300 to 6700 C and microwave residence times ranged from 8 to 24 minutes with seven maize and nine switchgrass biochars produced (Table 1 and Figures 1 and 2). The energy output, product types, particle size distribution, and elemental analysis of the biochar recovered from maize stover using these processing conditions are reported in Lei et al. (2009).

For this study, the maize biochars were used alone or mixed with the A horizon soil of a Brandt silty clay loam (Fine-silty, mixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) soil at 1 or 10% (w/w) to examine their effect on solution pH, EC, and atrazine and 2,4-D Kds (sorption coefficients) for each biochar and biochar/soil combination. For switchgrass biochars, the 1 or 10% amendments to soil were used for pH and EC measurements, however, for herbicide sorption studies only biochar alone or soil mixed with 10% biochar were used, due to limited biochar supply. To maximize homogeneity, each soil/biochar combination was individually mixed by adding air-dry soil and biochar to each individual tube.

#### **2.1.2 Solution characteristics**

Biochars, soil, and soil with biochar amendments were analyzed for pH using a 0.01 *M*  CaCl2 slurry (1:1 w/v) and a standardized pH electrode. The solution pH was recorded after the reading had stabilized. Electrical conductivity (EC) was determined on a slurry that was mixed 1:1 (v/w) with 0.01 *M* CaCl2 and biochar, soil, or soil amended with biochar. The slurry was shaken for 0.5 hr and EC measured using a commercially available EC electrode.

### **2.1.3 Herbicide sorption**

62 Herbicides – Properties, Synthesis and Control of Weeds

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

This study examined atrazine and 2,4-D sorption to several biochars that were the result of microwave pyrolysis using varying temperatures and processing times of maize and switchgrass biomass. In addition, sorption characteristics of these two chemicals to soil

Biochar was produced from maize stover (stalks and other residues remaining after maize grain harvest) and switchgrass biomass collected from fields near Brookings, South Dakota, USA (44.31, -96.67). Briefly, the material was dried at room temperature and pulverized mechanically using a Thomas-Wiley laboratory mill (Model No. 3375-E15, Thomas Scientific, USA) to pass through a 4 mm screen. The ground materials were processed by microwave pyrolysis using the SDSU Ag and Biosystem Eng. Dept. microwave system (specific processing methods reported in Lei et al., 2009). Processing temperatures ranged from 5300 to 6700 C and microwave residence times ranged from 8 to 24 minutes with seven maize and nine switchgrass biochars produced (Table 1 and Figures 1 and 2). The energy output, product types, particle size distribution, and elemental analysis of the biochar recovered from maize stover using these processing conditions are reported in Lei et al.

For this study, the maize biochars were used alone or mixed with the A horizon soil of a Brandt silty clay loam (Fine-silty, mixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) soil at 1 or 10% (w/w) to examine their effect on solution pH, EC, and atrazine and 2,4-D Kds (sorption coefficients) for each biochar and biochar/soil combination. For switchgrass biochars, the 1 or 10% amendments to soil were used for pH and EC measurements, however, for herbicide sorption studies only biochar alone or soil mixed with 10% biochar were used, due to limited biochar supply. To maximize homogeneity, each soil/biochar combination was individually mixed by adding air-dry soil and biochar to each

Biochars, soil, and soil with biochar amendments were analyzed for pH using a 0.01 *M*  CaCl2 slurry (1:1 w/v) and a standardized pH electrode. The solution pH was recorded after the reading had stabilized. Electrical conductivity (EC) was determined on a slurry that was mixed 1:1 (v/w) with 0.01 *M* CaCl2 and biochar, soil, or soil amended with biochar. The slurry was shaken for 0.5 hr and EC measured using a commercially available EC electrode.

**2. Biochar influence on herbicide sorption to soil** 

amended with these biochars at two application rates were determined.

exploring for use.

**2.1 Materials and methods** 

**2.1.1 Biochar and soil** 

(2009).

individual tube.

**2.1.2 Solution characteristics** 

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

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 sorbent characteristics of the biochar.

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

#### **2.1.4 Statistical analysis**

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

#### **2.2 Results**

#### **2.2.1 Biochar pH and EC values**

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

The Influence of Biochar Production on Herbicide Sorption Characteristics 65

530ºC/16 min 550ºC/10 min 550ºC/22 min 600ºC/8 min

650ºC/10 min 650ºC/22 min 670ºC/16 min

530ºC/16 min 550ºC/10 min 550ºC/22 min 9600ºC/8 min 600ºC/16 min

 600ºC/24 min 650ºC/10 min 650ºC/22 min 670ºC/16 min Fig. 2. Examples of biochars formed after exposure of switchgrass (*Panicum virgatum)*

Electrical conductivity provides an indication of the amount of neutral soluble salts in the material or its salinity. High soil salinity often impedes the growth of most agricultural plants. Adding amendments that increase soil salinity, even though other beneficial

feedstocks to microwave pyrolosis at varying temperatures and times.

microwave pyrolosis at varying temperatures and times (see Lei et al., 2009).

Fig. 1. Examples of biochars formed after exposure of maize (*Zea mays*) stover feedstocks to


Table 1. The influence of seven maize stover and nine switchgrass biochars produced with microwave pyrolosis with different processing times and temperature conditions on 100% biochar and soils amended with 1% or 10% (w/w) biochar. The soil used for this study was the A horizon of a Brandt silty clay loam (Fine-silty, mixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) from Aurora, SD (44.31, -96.67) with an unamended pH in a 1:1 solution of 0.01 *M* CaCl2 of about 6.40 and an EC value of 1.63 mS cm-1. A '-' sign indicates significantly lower value and a '+' sign indicates significantly higher value compared with unamended soil.

~9.4 to 9.9. The pH of these biochars can be compared with other biochar data. A wood ash/biochar that was the by-product of a commercial ethanol plant (Chippewa Valley Ethanol Company, Benson, MN) was obtained and used for comparison purposes. The wood ash had a pH of over 11. In comparison, broiler litter biochar obtained from pyrolysis reactions at either 350 or 700ºC was found to have a fairly uniform acidic pH (5.5) (Uchimiya et al., 2010). These data indicate that the pH of different types of biochar are dependent on processing time, temperature, and initial feedstock material.

Fig. 1. Examples of biochars formed after exposure of maize (*Zea mays*) stover feedstocks to microwave pyrolosis at varying temperatures and times (see Lei et al., 2009).

530ºC/16 min 550ºC/10 min 550ºC/22 min 9600ºC/8 min 600ºC/16 min

64 Herbicides – Properties, Synthesis and Control of Weeds

parameters pH EC

°C min mS cm-1

soil + 1% biochar

Pyrolysis

Temp time Biochar

compared with unamended soil.

processing time, temperature, and initial feedstock material.

530 16 4.59 6.39 5.85-

550 10 4.77 6.38 6.04-

Maize (*Zea mays*)

Biochar soil +

0.3 2.4 1.4

2.3 1.8 2.2

1% biochar

soil + 10% biochar

soil + 10% biochar

22 7.60 6.47 6.61+ 1.9 1.8 1.8

22 9.43 6.43 6.76+ 2.0 1.8 1.9

22 4.06 6.49 5.71- 1.5 1.87 2.13

16 6.47 6.45 6.76+ 1.7 1.30 1.33 24 5.60 6.61 6.44 1.8 1.67 1.87

22 8.28 6.48 6.80+ 2.9 1.97 2.37+

Switchgrass (*Panicum virgatum*)

600 8 5.68 6.44 6.44 2.1 1.8 2.0 650 10 9.88 6.46 6.75+ 2.0 1.9 1.9

670 16 9.65 6.43 6.73+ 1.1 1.9 1.9

530 16 5.32 6.17 6.70 0.3 1.80 1.67 550 10 4.12 6.49 5.67- 2.1 2.13 1.97

600 8 4.15 6.60 5.90- 1.9 1.80 1.83

650 10 4.57 6.44 6.11- 2.0 2.07 2.20

670 16 9.10 6.48 6.85+ 2.5 1.67 1.90

Table 1. The influence of seven maize stover and nine switchgrass biochars produced with microwave pyrolosis with different processing times and temperature conditions on 100% biochar and soils amended with 1% or 10% (w/w) biochar. The soil used for this study was the A horizon of a Brandt silty clay loam (Fine-silty, mixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) from Aurora, SD (44.31, -96.67) with an unamended pH in a 1:1 solution of 0.01 *M* CaCl2 of about 6.40 and an EC value of 1.63 mS cm-1. A '-' sign indicates significantly lower value and a '+' sign indicates significantly higher value

~9.4 to 9.9. The pH of these biochars can be compared with other biochar data. A wood ash/biochar that was the by-product of a commercial ethanol plant (Chippewa Valley Ethanol Company, Benson, MN) was obtained and used for comparison purposes. The wood ash had a pH of over 11. In comparison, broiler litter biochar obtained from pyrolysis reactions at either 350 or 700ºC was found to have a fairly uniform acidic pH (5.5) (Uchimiya et al., 2010). These data indicate that the pH of different types of biochar are dependent on

Fig. 2. Examples of biochars formed after exposure of switchgrass (*Panicum virgatum)* feedstocks to microwave pyrolosis at varying temperatures and times.

Electrical conductivity provides an indication of the amount of neutral soluble salts in the material or its salinity. High soil salinity often impedes the growth of most agricultural plants. Adding amendments that increase soil salinity, even though other beneficial

The Influence of Biochar Production on Herbicide Sorption Characteristics 67

Atrazine sorption

**Kd (L kg-1)**

0

**Kd**(**L kg-1)**

soil.

0

10

20

Soil Kd +

30

40

50

60

70

10

Soil Kd

+


Biochar Soil + 10% biochar

+

+ + +

20

30

60

80

Biochar Soil + 1% biochar Soil + 10% biochar

+

+

+

+

**Pyrolysis parameters for producing corn biochar** 530C/16m 550C/10m 550C/22m 600C/8m 650C/10m 650C/22m 670C/16m

**Pyrolysis parameters for producing switchgrass biochar** 530C/16m 550C/10m 550C/22m 600C/8m 600C16m 600C/24m 650C/10m 650C/22m 670C/16m

Fig. 3 A and B. Atrazine sorption (Kd) values to biochar from maize (*Zea mays*) stover (A) and switchgrass (*Panicum virgatum*) (B) produced by microwave pyrolysis at various processing times and temperatures. Kd values of sorption for the A horizon of a Brandt silty clay loam (Fine-silty, fmixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) soil when amended with 1 or 10% maize biochar or 1% switchgrass biochar. Kd sorption value of atrazine to unamended soil averaged about 3.86 L kg-1. A "–" sign indicates lower sorption at P < 0.05 and a "+" sign indicates greater sorption at P< 0.05 than unamended

+

+ + + +

+

+

A

+

+

+ +

+

B


<sup>+</sup> <sup>+</sup>


properties such as water holding capacity would increase, would be counterproductive. Saline soils are recognized worldwide (Food and Agriculture Organization, FAO) as soils with an EC reading of >4 mS cm-1 (Richards, 1954; Abrol et al., 1988). In the U.S., the Soil Science Society of America (SSSA) uses a value of >2 mS cm-1 boundary for the saline classification. Woodchip biochar had an EC value of 3.6 mS cm-1. Biochar produced from maize stover had EC values ranging from 1.1 to 2.3 mS cm-1 with five out of the seven >1.9 mS cm-1. The switchgrass biochars had EC values ranging from 1.5 to 2.9 mS cm-1 with the highest EC when materials were processed at 650º C for 22 min.

#### **2.2.2 Influence on biochar amendment on soil pH and EC properties**

The Brandt soil chosen for this study was a silty clay loam with a pH of 6.4. Due to the inherent soil properties and buffering capacity of this soil, it was expected that even high applications of the most acidic or alkaline biochar would have minimal impact on soil pH. When 1% maize or switchgrass biochars were added to soil, pH changes were minimal (generally <3%) (Table 1). When soils were amended with 10% biochar, pH was influenced to a greater extent. The slurry pH decreased from 4 to 8% when low pH biochars were added and increased a maximum of 9% when high pH biochars were added.

Soil EC was 1.63 mS cm-1, well below the salinity values for saline soil. Adding either maize or switchgrass biochar to soil at 1% increased soil salinity, but with the exception of one switchgrass sample, did not increase the salinity to >2 mS cm-1. Amending soil with 10% with the maize biochar that had the greatest EC value (2.3 mS cm-1) was the only maize biochar that increased soil salinity above 2 mS cm-1. Adding switchgrass biochar at 10% had greater impact than maize stover biochar and increased EC values an average of 11% when compared with ECs of unamended soil. Three switchgrass biochars increased EC values from 23 to 36% (Table 1) with final soil slurry EC values above 2 mS cm-1, the SSSA value for saline soil classification. However, even with a 10% amendment, all final EC values were well below the FAO saline soil value of 4 mS cm-1. If significant amounts of these biochars were applied frequently to the same field, managers must be cognizant of the potential for changes to EC values. Saline soil remediation can be expensive and often requires long-term management interventions, rather than short-term programs.

#### **2.2.3 Atrazine sorption to biochar and soils amended with biochar**

Atrazine is a chemical in the triazine family and has a slightly positive charge in soil solutions (Laird and Koskinen, 2008). The positive charge on the molecule, when in solutions above its pKa, causes the molecule to be sorbed to materials that have a negative charge. Atrazine sorption to soil is considered moderate with Kd values ranging from 1 to 5 (Koskinen and Clay, 1997). The value is dependent on many soil properties including pH, organic matter, and clay content (Koskinen and Clay, 1997). In this study, atrazine sorption to biochar ranged from 7 to 92 L kg-1 (Figure 3). The sorption was dependent on feedstock type and processing method. These values ranged from 200 to 2300% greater than sorption to soil.

In general, the biochars from maize had much more variability in Kd values than switchgrass biochar (Figure 3). Three of the seven maize biochars had Kds less than 20 L kg-1 whereas the other four had values of 55 L kg-1 or greater. In general, the switchgrass biochars had lower

properties such as water holding capacity would increase, would be counterproductive. Saline soils are recognized worldwide (Food and Agriculture Organization, FAO) as soils with an EC reading of >4 mS cm-1 (Richards, 1954; Abrol et al., 1988). In the U.S., the Soil Science Society of America (SSSA) uses a value of >2 mS cm-1 boundary for the saline classification. Woodchip biochar had an EC value of 3.6 mS cm-1. Biochar produced from maize stover had EC values ranging from 1.1 to 2.3 mS cm-1 with five out of the seven >1.9 mS cm-1. The switchgrass biochars had EC values ranging from 1.5 to 2.9 mS cm-1 with the

The Brandt soil chosen for this study was a silty clay loam with a pH of 6.4. Due to the inherent soil properties and buffering capacity of this soil, it was expected that even high applications of the most acidic or alkaline biochar would have minimal impact on soil pH. When 1% maize or switchgrass biochars were added to soil, pH changes were minimal (generally <3%) (Table 1). When soils were amended with 10% biochar, pH was influenced to a greater extent. The slurry pH decreased from 4 to 8% when low pH biochars were

Soil EC was 1.63 mS cm-1, well below the salinity values for saline soil. Adding either maize or switchgrass biochar to soil at 1% increased soil salinity, but with the exception of one switchgrass sample, did not increase the salinity to >2 mS cm-1. Amending soil with 10% with the maize biochar that had the greatest EC value (2.3 mS cm-1) was the only maize biochar that increased soil salinity above 2 mS cm-1. Adding switchgrass biochar at 10% had greater impact than maize stover biochar and increased EC values an average of 11% when compared with ECs of unamended soil. Three switchgrass biochars increased EC values from 23 to 36% (Table 1) with final soil slurry EC values above 2 mS cm-1, the SSSA value for saline soil classification. However, even with a 10% amendment, all final EC values were well below the FAO saline soil value of 4 mS cm-1. If significant amounts of these biochars were applied frequently to the same field, managers must be cognizant of the potential for changes to EC values. Saline soil remediation can be expensive and often requires long-term

Atrazine is a chemical in the triazine family and has a slightly positive charge in soil solutions (Laird and Koskinen, 2008). The positive charge on the molecule, when in solutions above its pKa, causes the molecule to be sorbed to materials that have a negative charge. Atrazine sorption to soil is considered moderate with Kd values ranging from 1 to 5 (Koskinen and Clay, 1997). The value is dependent on many soil properties including pH, organic matter, and clay content (Koskinen and Clay, 1997). In this study, atrazine sorption to biochar ranged from 7 to 92 L kg-1 (Figure 3). The sorption was dependent on feedstock type and processing method. These values ranged from 200 to 2300% greater than sorption

In general, the biochars from maize had much more variability in Kd values than switchgrass biochar (Figure 3). Three of the seven maize biochars had Kds less than 20 L kg-1 whereas the other four had values of 55 L kg-1 or greater. In general, the switchgrass biochars had lower

highest EC when materials were processed at 650º C for 22 min.

management interventions, rather than short-term programs.

to soil.

**2.2.3 Atrazine sorption to biochar and soils amended with biochar** 

**2.2.2 Influence on biochar amendment on soil pH and EC properties** 

added and increased a maximum of 9% when high pH biochars were added.

Fig. 3 A and B. Atrazine sorption (Kd) values to biochar from maize (*Zea mays*) stover (A) and switchgrass (*Panicum virgatum*) (B) produced by microwave pyrolysis at various processing times and temperatures. Kd values of sorption for the A horizon of a Brandt silty clay loam (Fine-silty, fmixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) soil when amended with 1 or 10% maize biochar or 1% switchgrass biochar. Kd sorption value of atrazine to unamended soil averaged about 3.86 L kg-1. A "–" sign indicates lower sorption at P < 0.05 and a "+" sign indicates greater sorption at P< 0.05 than unamended soil.

The Influence of Biochar Production on Herbicide Sorption Characteristics 69

**Kd(L kg-1)**

0

**Kd(L kg-1)**

0

than unamended soil.

+

Soil Kd

20

40

60

80

5

10

15

20

Biochar Soil + 1% biochar Soil + 10% biochar

+

+

+

Biochar Soil + 10% biochar

+

<sup>+</sup> <sup>+</sup> <sup>+</sup>

**Pyrolysis parameters for producing corn biochar** 530C/16m 550C/10m 550C/22m 600C/8m 650C/10m 650C/22m 670C/16m

**Pyrolysis parameters for producing switchgrass biochar** 530C/16m 550C/10m 550C/22m 600C/8m 600C16m 600C/24m 650C/10m 650C/22m 670C/16m

Fig. 4 A and B. 2,4-D sorption (Kd) values to biochar from maize (*Zea mays)*stover and switchgrass (*Panicum virgatum*) produced by microwave pyrolysis at various processing times and temperatures; Kd values of sorption for the A horizon of a Brandt silty clay loam (Fine-silty, mixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) soil when amended with 1 or 10% maize biochar or 10% switchgrass biochar. Kd sorption value of unamended soil averaged about 1.0 L kg-1. A "+" sign indicates greater sorption at P< 0.05

<sup>+</sup> <sup>+</sup> <sup>+</sup>

+

+

2,4-D sorption

+

+

+

B

+

+

+

A

+

Soil Kd

Kd values for atrazine than maize, with only two of the nine samples having sorption values >18 L kg-1. Correlation analysis was conducted to examine pH of biochar vs Kd but these parameters were poorly to moderately correlated for maize (r = 0.4) and not correlated for switchgrass.

Amending soil with maize biochar at 1% increased the Kd with three biochars and decreased the Kd for one biochar. The maximum increase was 66% more sorbed than unamended soil. The 10% additions decreased the amount sorbed by soil in two samples by about 43%. This was surprising as one of the biochars alone had double the Kd of soil (Kd = 7 L kg-1) and a pH of 4.5 and the other had very high sorption (Kd = 82 L kg-1) value and pH of 7.6. It is unclear what properties of this biochar would result in lower atrazine sorption. The soil amended with three maize biochars used at 10% amendment had nearly 3 times as much atrazine sorbed (Kds ranging from 8.7 to 11.0 L kg-1) when compared with soil alone. Two switchgrass biochars with the highest atrazine sorption also increased atrazine sorption when added as a 10% soil amendment, and raised the Kds nearly 4-fold, with a Kd of about 15 L kg-1. Other switchgrass biochars had no or only a slight influence on atrazine sorption.

#### **2.2.4 2,4-D sorption to biochar and soils amended with biochar**

Unlike atrazine which has a positive charge in most soils, 2,4-D with a pKa of 2.8 is a weak acid in most soil solutions (Wauchope et al., 1992). This chemical was chosen as a model compound to explore the effect of biochar on these types of compounds. The negative charge on the 2,4-D, as well as other chemicals in this auxin-type chemistry, often results in low or no sorption to soil (Clay et al., 1988). If these types of chemicals have a long residence time in soil (e.g. picloram), there is a high potential for leaching, although, because 2,4-D often is reported to have a ½ life of 10 d or less, leaching of this chemical is not usually considered a problem.

The Kd sorption value of 2,4-D to unamended Brandt soil was about 1 L kg-1, a four-fold lower sorption than atrazine to this soil. All biochar samples had much greater sorption coefficients than soil alone (Figure 4), with switchgrass biochars generally sorbing more 2,4- D than maize biochars. The Kd values for all biochars, regardless of feedstock type ranged from about 3 to >80 L kg-1 and was much greater than soil. Kd values for soil amended with 1% maize biochars were similar to Kd of unamended soil (Figure 4). Amending soil with 10% biochar (either maize or switchgrass) resulted in a few treatment combinations that had increased sorption compared to soil. Maize biochar resulting from processing stover at 600ºC for 8 min increased 2,4-D sorption 3.3 times over unamended soils, whereas maize biochar formed from processing at 650ºC for 22 min increased 2,4-D sorption by 4.5 times. Switchgrass biochar added at 10% to soil had little impact on 2,4-D sorption with two exceptions. The first was the biochar formed when processed at 550ºC for 10 min where a 9.4- fold sorption increase was measured and the second when switchgrass was processed at 650ºC for 22 min where a 15-fold sorption increase was measured. These two switchgrass biochars also dramatically increased atrazine sorption. The char produced at the higher temperature did influence soil EC values at 10% addition (Table 1), however, it is not known what the exact properties of these biochars or their interactions with soil/solution resulted in these increased sorption amounts.

Kd values for atrazine than maize, with only two of the nine samples having sorption values >18 L kg-1. Correlation analysis was conducted to examine pH of biochar vs Kd but these parameters were poorly to moderately correlated for maize (r = 0.4) and not correlated for

Amending soil with maize biochar at 1% increased the Kd with three biochars and decreased the Kd for one biochar. The maximum increase was 66% more sorbed than unamended soil. The 10% additions decreased the amount sorbed by soil in two samples by about 43%. This was surprising as one of the biochars alone had double the Kd of soil (Kd = 7 L kg-1) and a pH of 4.5 and the other had very high sorption (Kd = 82 L kg-1) value and pH of 7.6. It is unclear what properties of this biochar would result in lower atrazine sorption. The soil amended with three maize biochars used at 10% amendment had nearly 3 times as much atrazine sorbed (Kds ranging from 8.7 to 11.0 L kg-1) when compared with soil alone. Two switchgrass biochars with the highest atrazine sorption also increased atrazine sorption when added as a 10% soil amendment, and raised the Kds nearly 4-fold, with a Kd of about 15 L kg-1. Other switchgrass biochars had no or only a slight influence on atrazine sorption.

Unlike atrazine which has a positive charge in most soils, 2,4-D with a pKa of 2.8 is a weak acid in most soil solutions (Wauchope et al., 1992). This chemical was chosen as a model compound to explore the effect of biochar on these types of compounds. The negative charge on the 2,4-D, as well as other chemicals in this auxin-type chemistry, often results in low or no sorption to soil (Clay et al., 1988). If these types of chemicals have a long residence time in soil (e.g. picloram), there is a high potential for leaching, although, because 2,4-D often is reported to have a ½ life of 10 d or less, leaching of this chemical is not usually

The Kd sorption value of 2,4-D to unamended Brandt soil was about 1 L kg-1, a four-fold lower sorption than atrazine to this soil. All biochar samples had much greater sorption coefficients than soil alone (Figure 4), with switchgrass biochars generally sorbing more 2,4- D than maize biochars. The Kd values for all biochars, regardless of feedstock type ranged from about 3 to >80 L kg-1 and was much greater than soil. Kd values for soil amended with 1% maize biochars were similar to Kd of unamended soil (Figure 4). Amending soil with 10% biochar (either maize or switchgrass) resulted in a few treatment combinations that had increased sorption compared to soil. Maize biochar resulting from processing stover at 600ºC for 8 min increased 2,4-D sorption 3.3 times over unamended soils, whereas maize biochar formed from processing at 650ºC for 22 min increased 2,4-D sorption by 4.5 times. Switchgrass biochar added at 10% to soil had little impact on 2,4-D sorption with two exceptions. The first was the biochar formed when processed at 550ºC for 10 min where a 9.4- fold sorption increase was measured and the second when switchgrass was processed at 650ºC for 22 min where a 15-fold sorption increase was measured. These two switchgrass biochars also dramatically increased atrazine sorption. The char produced at the higher temperature did influence soil EC values at 10% addition (Table 1), however, it is not known what the exact properties of these biochars or their interactions with soil/solution resulted

**2.2.4 2,4-D sorption to biochar and soils amended with biochar** 

switchgrass.

considered a problem.

in these increased sorption amounts.

Fig. 4 A and B. 2,4-D sorption (Kd) values to biochar from maize (*Zea mays)*stover and switchgrass (*Panicum virgatum*) produced by microwave pyrolysis at various processing times and temperatures; Kd values of sorption for the A horizon of a Brandt silty clay loam (Fine-silty, mixed, superactive, frigid Calcic Hapludoll, [Soil Survey Staff, 2011]) soil when amended with 1 or 10% maize biochar or 10% switchgrass biochar. Kd sorption value of unamended soil averaged about 1.0 L kg-1. A "+" sign indicates greater sorption at P< 0.05 than unamended soil.

The Influence of Biochar Production on Herbicide Sorption Characteristics 71

highly desirable if it can be used to increase water holding capacity or as a nutrient source. Biochars, if high in sorption capacity, may be applied strategically and could accomplish important roles in ecosystem health and environmental quality. Biochar, added in filter strips and waterways, eroded landscapes, or other areas where increased sorption is desired, may aid in cleaning water running off fields by sorbing undesirable contaminants. Increased sorption may also slow or stop herbicides from leaching, so highly sorbent biochar types may be desired over shallow aquifers or in areas low in native organic matter (Wang et al., 2010). Herbicide bioavailability in some cases may be reduced, protecting

Conversely, the effect of spreading biochars across entire fields may have negative results and be undesirable. One consequence may be that the materials increase soil EC values to saline levels. In addition, if the biochar reduces the efficacy of soil-applied herbicides or other pesticides this may have negative impacts. Reduced pesticide efficacy would require higher herbicide application rates to be as effective as lower rates. This would have monetary implications for growers and field managers by increasing management costs. Increased sorption, in some cases, also may increase the recalcitrance of pesticides leading to longer residence times in the environment. The occurrence of greater recalcitrance may be desirable if bioactivity was still acceptable and longer activity of the pesticide was desired to control the pest of interest. However, longer residence time may lead to other long-term environmental problems, such as greater leaching potential or carry-over problems into the

Prior to any regular field applications of any biochar, the biochar properties must be examined to determine the suitability of the material for the long-term management of a particular site. The reasons for the application should be defined clearly and the outcomes

Funding provided by South Dakota Maize Utilization Council, US USDA/Sun Grant Initiative, and South Dakota Agricultural Experiment Station. Undergraduate participation

Abrol, I.P., Yadav, J.S.P., & Massoud, F. (1988). Salt affected soils and their management,

Bridgewater, A.V. (2003). Renewable fuels and chemicals by thermal processing of biomass.

Bridgewater, A.V., Meier, D., & Radlein, D. (1999). An overview of fast pyrolysis of biomass.

Brown, C.D., Carter, A.D., & Hollis, J.M. (1995). Soils and pesticide mobility, In:

Cao, X., Ma, L., Gao, B., & Harris, W. (2009). Dairy-manure derived biochar effectively sorbs

*Chemical Engineering Journal,* 91, 2-3, (March 2003), pp. 87–102.

*Organic Geochemistry*, 30, 12, (December1999), pp. 1479–1493.

Food and Agricultural Organization of the United Nations (FAO), Soils Bulletin 39.

*Environmental Behaviour of Agrochemicals,* Roberts, T.R., & Kearney, P.C. (Eds.). pp.

lead and atrazine. *Environmental Science and Technology,* 43, 9, (May 2009), pp. 3285-

closely monitored to determine if expectations and results are synonymous.

included Mr. Mitch Olson, Mr. Dan Clay, and Ms. Kaitlynn Krack.

131-184, John Wiley & Sons, Chichester, England.

sensitive plants.

following season.

**4. Acknowledgments** 

**5. References** 

3291.
