Characterization of narrow micropores in almond shell biochars by nitrogen, carbon dioxide, and hydrogen adsorption

Industrial Crops and Products 67 (2015) 33–40

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Characterization of narrow micropores in almond shell biochars by nitrogen, carbon dioxide, and hydrogen adsorption K. Thomas Klasson ∗ , Minori Uchimiya, Isabel M. Lima U.S Department of Agriculture, Agricultural Research Service, New Orleans, LA 70124, USA

a r t i c l e

i n f o

Article history: Received 22 September 2014 Received in revised form 7 January 2015 Accepted 8 January 2015 Keywords: Pore size distribution Nanopores Washing Normalized adsorption isotherm Characteristic curve

a b s t r a c t Characterization of biochars usually includes surface area and pore volume determinations by nitrogen adsorption. In this study, we confirmed that there is a substantial pore volume in biochars created via slow pyrolysis from low- and high-ash almond shells that cannot be characterized in this fashion due to the narrow pore sizes. Hydrogen and carbon dioxide were used, in addition to nitrogen, to characterize these narrow micropores. All isotherms, when appropriately normalized, fell on the same characteristic curve which aided in the characterization and understanding of the pore structure. Based on the characteristic adsorption isotherm, the average pore size diameter was estimated to be 0.4–0.8 nm. When washed, the biochars’ pore volume increased but the narrow micropore structure remained. While the biochars were alkaline, the pH of the wash solution had no or little impact on the adsorption of CO2 . Overall, the results suggest that N2 isotherms should be complemented with CO2 isotherms for proper characterization of biochars. Alignment of such normalized isotherms to characteristic curves can assist in generating a more complete understanding of the pore structure over the entire region of pore diameters. Published by Elsevier B.V.

1. Introduction Biochars from biomass have long been used by humans in agriculture (Lehmann et al., 2003). Charred biomass can help improve plant nutrient availability (Lehmann et al., 2003; Lehmann, 2007) and activated chars can provide remediation potential for soil and sediment (Chen et al., 2006; Zimmerman et al., 2008). Biochar soil application has also been discussed as a method of reducing the impact of global warming (Lehmann, 2007; Bracmort, 2009). In order to realize the benefits, Joseph et al. (2009) suggested that a classification system for biochar is needed and the International Biochar Initiative (IBI) recently published its standards for biochar classification (Anon, 2013). Both of the resources highlight the importance of biochar surface area and pore structure information. Joseph et al. (2009) stressed the importance of knowing the pore size distribution of the biochar as macro-pores (>50 nm diameter), meso-pores (>2 nm and <50 nm diameter), and micropores (<2 nm diameter). Biochars with large pore volumes in pores with diameters of >50 nm have a high water holding capacity, while adsorption of small diameter molecules (such as water contaminants) takes place in smaller diameter pores (Joseph et al., 2009).

∗ Corresponding author at: USDA-ARS, 1100 Robert E. Lee Blvd, New Orleans, LA 70124, USA. Tel.: +1 504 286 4511. E-mail address: [email protected] (K.T. Klasson). http://dx.doi.org/10.1016/j.indcrop.2015.01.010 0926-6690/Published by Elsevier B.V.

Biomass chars are microporous and the surface area and pore structure are impacted by raw material, pyrolysis temperature, pyrolysis time, and activation (Chen et al., 2008; Garrido et al., 1987; Jiang, 2004; Karaosmanoglu et al., 2000; Keiluweit et al., 2010; Klasson et al., 2014a,b; Lopez-Gonzalez et al., 1980; Lozano-Castello et al., 2004; McLaughlin and Shields, 2010; Ouensanga et al., 2003; Rodriguez-Reinoso et al., 1984, 1989; Silvestre-Albero et al., 2012). Biochars intended for soil applications will come in contact with rainwater that will alter their surface area properties as a result of ash removal (Klasson et al., 2014b), and it has been shown that ash removal by alkaline or acidic wash increases the specific surface area as measured by nitrogen adsorption (Klasson et al., 2014a,b; Lima et al., 2014; Zhang et al., 2013). Several gases have traditionally been used for surface area, pore volume, and pore structure characterizations (Brunauer, 1943; Brunauer et al., 1938), and nitrogen (at 77 K) became the gas adsorbate of choice (Sing et al., 1985) and the standard gas for surface area measurements and pore size distribution of some materials (Anon, 2010, 2012). However, it was found that the low diffusivity of N2 at 77 K and the presence of narrow micropores (or nanopores) in the range of approximately 0.4 nm required a different adsorbate, such as carbon dioxide (at 195 or 273 K), as N2 cannot penetrate these pores effectively or in a reasonable time period (Marsh and WynneJones, 1964). Some have argued that thermodynamic equilibrium of N2 in the narrow micropores may take weeks to achieve (Braida

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K.T. Klasson et al. / Industrial Crops and Products 67 (2015) 33–40

et al., 2003; Lenghaus et al., 2002). The technique of characterizing materials that has a wide range of pore sizes often requires the use of both N2 (at 77 K) and CO2 (at 273 or 298 K) for complete pore size characterization (Cazorla-Amoros et al., 1996; Garrido et al., 1987; Lozano-Castello et al., 2004; Rodriguez-Reinoso et al., 1989). The use of CO2 as adsorbate has not gone without challenges. While CO2 does not have a dipole moment, it does have greater electric quadrupole moment than N2 ; and CO2 has shown to interact with oxidized surface groups in molecular interaction simulations of adsorbates and model surfaces (Furmaniak et al., 2009; Tenney and Lastoskie, 2006). The enhanced interaction between CO2 and surface groups has experimentally been observed by several research groups (Meredith and Plank, 1967; Park and Kim, 1999). Thus, gases with lower (or no) quadrupole moment, such as argon, have been suggested and used (Furmaniak et al., 2009; Jagiello and Thommes, 2004; Silvestre-Albero et al., 2012) but it should be noted that CO2 can access smaller-diameter pores than both N2 and Ar (Braida et al., 2003) at adsorption equilibrium. Thommes et al. (2012) have provided a recent review of the benefits of each of the methods. The work presented herein characterizes biochars made from shells of two different almond varieties (with different ash content) and under two different pyrolysis conditions. The biochars were characterized by N2 , CO2 , and H2 adsorption to determine the extent of narrow micropores. Hydrogen was specifically included as it has a lower quadrupole moment than the other two gases (Orcutt, 1963). The materials were also subjected to water washes to determine if the narrow micropores were impacted by rainwater as well as washing at different pH (Klasson et al., 2014b).

2. Materials and methods Low-ash (1.0 wt%db ) containing almond shells were obtained from San Joaquin Valley Agricultural Sciences Center in Parlier, California, and high-ash (4.2%db ) almond shells were obtained from a sheller (Bakersfield, CA). The materials were ground with a cutting mill (SM 2000, Retsch Gmbh, Haan, Germany) and sieved to retain the 0.84–2 mm portion (No. 10–20 sieves). A commercial activated carbon, Filtrasorb 300, was obtained from Calgon Carbon (Pittsburgh, PA). Raw material (200 g) was placed in a ceramic crucible bowl and pyrolyzed at 800 ◦ C for 120 or 240 min in a box furnace (Lindberg, Type 51662-HR, Watertown, WI). The heating rate was not controlled but was consistently 8–10 ◦ C/min through the heat-up time. Nitrogen (1.6 L/min) was used as a sweep gas in the furnace to prevent oxygen from entering. After pyrolysis, the material was allowed to cool overnight in the furnace with nitrogen as sweep gas. Part of the material was washed with synthetic rainwater using a previous technique (Klasson et al., 2014b) to investigate the impact of rainwater exposure on the material. In addition, one of the biochars was subjected to washing in excess of 48 h with washing solutions adjusted to pH 2, 6, and 10 with HCl before briefly rinsed with deionized water and dried. Surface area and pore structure were investigated using a Quantachrome NOVA 2200e adsorption system (Quantachrome Instruments, Boynton Beach, FL). Samples were dried and outgassed at 473 K (200 ◦ C) for 3 h under vacuum followed by at least 12 h at room temperature under vacuum. Adsorption data was collected as 25 points on the nitrogen isotherm between 0.002 and 0.99 relative pressures at 77 K (liquid nitrogen), as 22 points on the carbon dioxide isotherm between 0.05 and 0.99 relative pressure at 273 K (ice water bath), and as 22 points on the hydrogen isotherm between 0.05 and 0.99 relative pressure at 77 K. All isotherms were collected in duplicates and the average values are presented in this manuscript.

Raw isotherms were normalized using the Polanyi theory (Manes, 1998; Polanyi, 1920) as modified by Dubinin (1960), which results in isotherms that correlate the adsorption space (volume, v) used by the adsorbate to the adsorption potential () and affinity coefficient (ˇ)

v = fn(/ˇ) = fn[(RT/ˇ)ln(pS /p)]

(1)

where T, p, and pS are the temperature, partial pressure, and saturation pressure of the adsorbate. When plotting v as a function of /ˇ for the gases at different temperatures and pressures on the same sorbent, the resulting curve passes through a common y-axis point, which corresponds to the total space available for adsorption, vtot . We used the relationship between v and , developed by Dubinin and co-workers (Dubinin, 1989; Dubinin and Astakhov, 1971; Stoeckli, 2001) in the form of

v = vtot exp{[B(RT/ˇ) ln(pS /p)]n }

(2)

also known as the Dubinin–Astakhov (DA) equation in which B and n are constants. By using multiple adsorbates, such as nitrogen and carbon dioxide, a characteristic curve made up by normalized isotherms over a greater range of conditions can be generated (Lozano-Castello et al., 2004). The general use of the relationships as described in Eqs. (1) and (2) to predict adsorptive properties of activated carbons has been summarized by Manes (1998). 3. Results and discussion The pore characterization method was initially tested with the reference activated carbon. The result of plotting the traditional and normalized isotherms (Eq. (1)) for N2 , CO2 , and H2 is shown in Fig. 1. As is noted, the total adsorbed N2 volume significantly exceeded both the adsorbed CO2 and H2 volumes under the experimental conditions at all values of p/pS (Fig. 1A). The total adsorbed H2 volume exceeded the adsorbed CO2 volume. Recalculated, using the Polanyi theory, the normalized adsorption data for the three adsorbates followed a general trend line (or characteristic curve) (Fig. 1B), once the affinity coefficients had been applied. The intercept of the curve with the y-axis, represents the total volume (vtot ) available for adsorption, which in the case of the reference carbon was approximately 0.4 mL/g (y-axis intercept in Fig. 1B). The behavior displayed in Fig. 1B has been shown by others for several adsorbates such as N2 , CO2 , benzene, isobutene, and isooctane (Garrido et al., 1987) and the co-incidence of the normalized isotherms to a characteristic curve has been noted in the absence of very narrow micropores (Lozano-Castello et al., 2004). Thus, we conclude that the activated carbon in Fig. 1 had an adsorption space accessible to N2 , CO2 , and H2 without diffusion limitation under the experimental conditions; i.e., very few narrow micropores were noted in the reference carbon. In Fig. 2, the raw isotherms for N2 , CO2 , and H2 are shown for the almond shell chars created under different pyrolysis conditions. As is noted in Fig. 2, the adsorption results using the experimental biochars were significantly different than with the activated carbon (Fig. 1A). The experimental biochars adsorbed more H2 than N2 for the same p/pS . Three of the experimental biochars (Fig. 2A, C and D) adsorbed more CO2 than N2 . In all cases, both activated carbon (Fig. 1A) and experimental biochars (Fig. 2) adsorbed more H2 than CO2 . The relatively higher quantities of adsorbed H2 and CO2 compared with N2 are indicative of diffusion limitation of nitrogen into pores that are very narrow (Braida et al., 2003; Lenghaus et al., 2002). The adsorbed N2 volume increased approximately five times, while the adsorbed CO2 and H2 volumes remained about the same, when the pyrolysis time was doubled in the case of low-ash almond shell biochars (Fig. 2A and B). This suggests the development of pores suitable for N2 adsorption with extended

K.T. Klasson et al. / Industrial Crops and Products 67 (2015) 33–40

pyrolysis time. Ouensanga et al. (2003) found that biochar from tropical almond shells adsorbed very little N2 when pyrolyzed for 2 h below 700 ◦ C but adsorption improved at higher temperatures. Other studies have shown insignificant N2 adsorption of almond shell biochar when pyrolyzed for 60 min at 350–800 ◦ C (Klasson

et al., 2014a). Thus, both pyrolysis temperature and time impact the nitrogen adsorption. In the case of high-ash almond shell biochars (Fig. 2C and 2D), the increased pyrolysis time improved the adsorption of all gases, suggesting a general expansion of adsorption space.

1.000

100 90

35

A

B

N2 CO2

F-300

H2

Adsobed Volume (mL/g)

Adsobed Volume (mLSTP/g)

80 70 60

N2 CO2

50

H2

40 30

0.100

0.010

20 10 F-300

0 0.0001

0.001

0.001

0.01

0.1

1

0

10

20

p/ps

30

40

50

(RT/β) ln(pS/p) (kJ/mol,K)

Fig.1. Traditional (A) and normalized (B) isotherms for Filtrasorb 300. The adsorbed volume (in graph B) was calculated from Rackett–Spencer–Danner molar volume correlation (Reid et al., 1987) for N2 and CO2 . As H2 experimentation was done above the critical temperature, the van der Waals co-volume was used as the molar “liquid” volume (Dubinin, 1960; Reid et al., 1987). The value for ˇ of 0.33 and 0.35 was used for N2 and CO2 (Cazorla-Amoros, 1996), and ˇ for H2 was set to 0.15 to fit the data. Saturation pressure (pS ) was estimated by the Wagner equation (Reid et al., 1987) for N2 and CO2 . For H2 (above its critical point), the value was obtained from Tr 2 Pc (Dubinin, 1960; Reid et al., 1987).

30 N2

Low-ash, 120 min

CO2

B Low-ash, 240 min

25

H2

Adsobed Volume (mLSTP/g)

Adsobed Volume (mLSTP/g)

25

30

A

20

15

10

5

20

15

10 N2 CO2

5

H2

0 0.0001

0.001

0.01

0.1

0 0.0001

1

0.001

p/ps

0.1

1

p/ps

30

30

C

N2

D

N2

High-ash, 120 min

CO2

High-ash, 240 min

CO2

25

25

H2

Adsobed Volume (mLSTP/g)

Adsobed Volume (mLSTP/g)

0.01

20

15

10

5

0 0.0001

0.001

0.01

p/ps

0.1

1

H2

20

15

10

5

0 0.0001

0.001

0.01

0.1

1

p/ps

Fig. 2. N2 , CO2 , and H2 isotherms for four experimental biochars made from low- and high-ash almond shells at 120 and 240 min pyrolysis times. The adsorbed volume at standard temperature and pressure (273 K and 101.3 kPa) is proportional to the number of moles adsorbed. The calculated N2 BET surface areas for these biochars have previously been reported as 44, 423, 81, and 201 m2 /g (Klasson et al., 2014b).

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K.T. Klasson et al. / Industrial Crops and Products 67 (2015) 33–40 1.000

1.000 N2

N2

B

CO2

CO2

H2

H2

Adsobed Volume (mL/g)

Adsobed Volume (mL/g)

A

0.100

0.010

0.100

0.010

Low-ash, 240 min

Low-ash, 120 min

0.001

0.001

0

10

20

30

40

50

0

10

(RT/β) ln(pS/p)

30

40

50

(RT/β) ln(pS/p) 1.000

1.000 N2

D

N2

CO2

CO2

H2

H2

Adsobed Volume (mL/g)

C

Adsobed Volume (mL/g)

20

0.100

0.010

0.100

0.010

High-ash, 240 min

High-ash, 120 min

0.001

0.001

0

10

20

30

40

50

0

10

20

30

40

50

(RT/β) ln(pS/p)

(RT/β) ln(pS/p)

Fig. 3. Normalized isotherms for N2 , CO2 , and H2 on four experimental biochars made from low- and high-ash almond shells at 120 min and 240 min pyrolysis times.

Transformation of the traditional isotherms (Fig. 2) to the normalized isotherms (Fig. 3) shows that the individual isotherms did not form a single characteristic curve for these biochars which was the case for the activated carbon (Fig. 1B). The lack of co-incidence of CO2 and H2 normalized isotherms and the fact that the CO2 isotherm is above the H2 isotherm suggests that there was a fraction of pores that were accessible to CO2 but not to H2 (and N2 ). Another explanation could be that CO2 , due to its higher quadrupole moment (Orcutt, 1963), interacted with surface groups, which may be incorrectly interpreted as narrow micropores (Samios et al., 2000). It should also be noted that increasing the pyrolysis time from 120 to 240 min probably increased the average pore diameter, as evidence of the merging of the individual isotherms toward a common characteristic curve. To quantify the pore fractions that were accessible to different adsorbents, we added a constant volume (representing pores not accessible to N2 ) to the measured adsorbed N2 volume in order to align the normalized N2 and CO2 isotherms. We did the same for the H2 adsorption data, aligning it with the normalized CO2 isotherm. Further information is provided in Appendix A. The results can be seen in Fig. 4. As is noted, the corrected normalized isotherms are fairly well aligned to characteristic curves. The correction needed can be viewed as adsorption volumes accessible to different types of adsorbates, which have been listed in Table 1. It is important to realize that the total accessible volume (vtot ) is only completely filled when the pressure of the adsorbate is equal to its saturation pressure, which may not occur in the actual experiments. As previously mentioned, the results in Table 1 show that increasing the pyrolysis time made the pores more accessible to N2 .

Table 1 Adsorption volumes in the experimental biochars, created from shells of two varieties of almonds (with low- and high-ash shells), accessible by different type of adsorbates. Accessible adsorption volume (mL/g) at p = pS Biochar type

N2

CO2

H2

Low ash 120 min Low ash 240 min High ash 120 min High ash 240 min

0.02 0.18 0.04 0.09

0.27 0.29 0.17 0.26

0.23 0.26 0.12 0.23

However, there was still a significant fraction of pores that were not accessible to N2 even after the longer pyrolysis time. The estimated volumes (vtot ) accessible to CO2 and H2 were strongly correlated (r2 = 0.99, Appendix A, Fig. A1). If the assumption is made that CO2 can access the narrow micropores that are not accessible by N2 (due to the lower diffusivity of N2 ), the narrow micropore volumes can be calculated from the difference between CO2 and N2 volumes in Table 1 as 0.25, 0.11, 0.13, and 0.17 mL/g or 93, 38, 76, and 65% of the total volume available for adsorption (by CO2 ). The strong correlation between estimated total accessible CO2 and H2 space suggests that surface chemistry does not appear to be a major contributor to CO2 adsorption when the adsorption space is completely filled. The small impact by surface chemistry on the total adsorption space for CO2 has been shown in modeling results as well (Furmaniak et al., 2010). The solid curves displayed in Fig. 4 correspond to the best fitted curve (by non-linear regression) based on the Dubinin–Astakhov relationship (Eq. (2)). The values of n (see Eq. (2)) were determined

K.T. Klasson et al. / Industrial Crops and Products 67 (2015) 33–40 1.000

37

1.000

A

B

N2

N2

CO2

H2

Adsorbed Volume (mL/g)

Adsorbed Volume (mL/g)

CO2

0.100

0.010

H2

0.100

0.010

Low-ash, 120 min

Low-ash, 240 min

0.001

0.001 10

0

20

30

40

50

10

0

(RT/β) ln(pS/p)

30

40

50

(RT/β) ln(pS/p)

1.000

1.000

C

D

N2

N2

CO2

CO2

H2

H2

Adsorbed Volume (mL/g)

Adsorbed Volume (mL/g)

20

0.100

0.010

0.100

0.010

High-ash, 120 min

High-ash, 240 min

0.001

0.001 0

10

20

30

40

50

(RT/β) ln(pS/p)

0

10

20

30

40

50

(RT/β) ln(pS/p)

Fig. 4. Corrected normalized isotherms for nitrogen, carbon dioxide, and hydrogen on four experimental biochars made from low- and high-ash almond shells at 120 and 240 min pyrolysis times.

to be equal to 2.6, 1.7, 2.5, and 2.3. Further characterization of the pore size distribution using Monte Carlo simulations kernels with the characteristic curves indicated that the average micropore size diameter of all the chars was in the range of 0.4–0.8 nm (see Appendix A, Fig. A2). This is equal to or less than the nitrogen bilayer thickness (0.7 nm) in adsorption theory (Thommes et al., 2012) and it is similar to what is considered the optimal pore size of 0.5–0.7 nm for H2 storage applications, where N2 and CO2 isotherms are often used to predict H2 storage capacity (Zhao et al., 2011). From the results presented in Figs. 2 and 3 and Table 1, it is noted that the biochar created from low-ash almond shells at a pyrolysis time of 120 min is unique having substantial adsorption space available for H2 adsorption (0.23 mL/g, Table 1), while very little adsorption space for N2 (0.02 mL/g). It is speculated that this material may be suitable for H2 separation from N2 . The H2 storage capacity for this material would be equal to 0.23 mL/g (at the saturation pressure of 7 MPa at 77 K) which corresponds to 0.017 g of H2 /g of biochar. This is approximately 30% of the capacity of some of the best H2 storage materials at the same temperature and pressure (Zhao et al., 2011). This same material may also be suited for separation of CO2 from N2 . When biochar is used as a soil amendment, it is likely to get saturated with water at some point and we have previously shown that exposure of biochar to aqueous solutions increases the surface area and pore volumes as measured by the N2 isotherm (Klasson et al., 2014a,b; Lima et al., 2014). Some impact of washing in the narrow micropore region was also noted in the biochars studied here (Fig. 5). Minor or no changes were noted in the normalized CO2 isotherm for the low ash almond shell biochar (Fig. 5A and B), while greater changes were noted for the

high ash almond shell biochar (Fig. 5C and D). In general, washing the materials improved the alignment of the normalized N2 and CO2 isotherms toward a common characteristic curve but significant differences were still noted between the isotherms indicating the continued presence of narrow micropores. Biochars made from almond shells are alkaline (Klasson et al., 2014b), causing an unbuffered washing solution (such as artificial rainwater) to also become alkaline. To investigate if the alkalinity had any impact on potential chemisorption of CO2 , we washed one of the biochars (high-ash, 120 min pyrolysis time) in solutions of constant pH (adjusted until constant pH with dilute HCl). The results showed that the normalized CO2 isotherms, regardless of pH, were identical (Appendix A, Fig. A3) indicating that neither the narrow-diameter micropores nor the surface chemistry changed in any meaningful way that would impact CO2 adsorption. Park and Kim (1999) applied a more severe surface treatment of activated carbon using 30% HCl and 30% NaOH and noted that CO2 adsorption increased in both cases (∼5–15%); N2 adsorption also increased but not to the same extent. The micropore (and narrow micropore/nanopore) characteristics of the biochars created here are not unique to the slow pyrolysis method or the starting raw materials. The presence of narrow micropores in biochars was indirectly shown by McLaughlin et al. (2012) who produced wood pellet biochar in a top lift updraft cookstove, which represents a different, partially oxic, pyrolysis method than the slow anoxic pyrolysis method used for our experiments. The amount of updraft affects the pyrolysis temperature and oxidation and, by varying the amount of draft in the stove, they created different chars with different adsorption capacities.

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K.T. Klasson et al. / Industrial Crops and Products 67 (2015) 33–40 1.000

1.000 Unwashed, N2

Unwashed, N2

B

Unwashed, CO2

Unwashed, CO2

Washed, N2

Washed, N2

Washed, CO2

Adsobed Volume (mL/g)

Adsobed Volume (mL/g)

A

0.100

0.010

Washed, CO2

0.100

0.010

Low-ash, 240 min

Low-ash, 120 min

0.001

0.001

0

10

20

30

40

50

0

10

(RT/β) ln(pS/p)

30

40

50

(RT/β) ln(pS/p)

1.000

1.000 Unwashed, N2

C

Unwashed, N2

D

Unwashed, CO2

Unwashed, CO2

Washed, N2

Washed, N2

Washed, CO2

Adsobed Volume (mL/g)

Adsobed Volume (mL/g)

20

0.100

0.010

Washed, CO2

0.100

0.010

High-ash, 120 min

High-ash, 240 min

0.001

0.001

0

10

20

30

40

50

0

10

(RT/β) ln(pS/p)

20

30

40

50

(RT/β) ln(pS/p)

Fig. 5. Normalized isotherms for N2 and CO2 on unwashed and rain-water-washed experimental biochars made from low- and high-ash almond shells at 120 and 240 min pyrolysis times.

1.000

1.000

A

B

N2

N2 CO2

Adsobed Volume (mL/g)

Adsobed Volume (mL/g)

CO2

0.100

0.010

0.100

0.010

Natural draft

Forced draft

0.001

0.001

0

10

20

30

40

50

(RT/β) ln(pS/p)

0

10

20

30

40

50

(RT/β) ln(pS/p)

Fig. 6. Characteristic isotherms for nitrogen and carbon dioxide on biochar produced with a top lift updraft stove in natural draft (A) and forced air draft (B) modes. Data were recalculated from McLaughlin et al. (2012).

While the chars were characterized by N2 and CO2 adsorption, no attempts were made to determine how well the data fit a characteristic curve. In Fig. 6, we show the isotherm data by McLaughlin et al. (2012) recalculated as normalized isotherms. The biochar produced with a natural draft (BET value of 216 m2 /g determined by McLaughlin et al., 2012) showed a lack of co-incidence of the normalized isotherms for N2 when compared to CO2 , while the forced air draft (BET value of 472 m2 /g) showed improved alignment to a characteristic curve, indicating more impact on N2 than

on CO2 adsorption. As previously stated, the lack of co-incidence between normalized N2 and CO2 isotherms is indicative of narrow micropores. Thus, the natural (low) draft conditions produced relative more nanopores that the updraft conditions. Garrido et al. (1987) also noted that increasing the burn-off resulted in increased adsorption space, with greater increases for N2 (vs. CO2 ). Olive stones have also been used for carbonization and there is evidence for narrow micropores in these materials as well. LopezGonzalez et al., 1980 noted that characterization with CO2 resulted

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in estimated micropore volumes an order of magnitude larger than those predicted by N2 isotherms. The researchers also noted that this discrepancy disappeared when the chars were activated with CO2 , suggesting enlargement of pores during this activation strategy. Salinas-Martinez de Lecea et al. (1988) used characteristic curves for N2 and CO2 in the form of the DA equation (with n = 2) to separate out micropores from what was termed wide micropores. Larger pores (e.g., wide micropores) cause the N2 adsorption isotherm to curve upward at higher p/pS values [=lower (RT/ˇ) ln(pS /p) values] as is noted in Fig. 1A but is absent in Fig. 2A–D. In their process of investigating the wide micropores they also suggested that the presence of narrow micropores in low burn-off activated carbons caused part of the normalized isotherm to deviate from a characteristic curve (Salinas-Martinez de Lecea et al., 1988). The deviation they presented suggests that some narrow micropores were present in their charred materials but probably not to the extent reported here. 4. Conclusions In conclusion, we have shown that biochar created from different types of almond shells (low- and high-ash) using different pyrolysis times contained significant pore volumes in the narrow micropore region that are not accessible for characterization using general N2 adsorption techniques. This is a characteristic often found in biochars. To characterize these materials properly, N2 isotherms should be complemented with CO2 and/or H2 isotherms as previously documented in literature. Alignment of such normalized isotherms to characteristic curves can assist in generating a more complete understanding of the pore structure over the entire region of pore diameters and allow estimation of pore volumes accessible to different gases. We also showed that manipulating the ash levels in the biochars by washing (intended to simulate natural exposure to water in soil biochar applications) can expose additional pores both in the micropore and narrow micropore regions. Washing had more impact on accessible pore volumes in the case of high-ash biochars. It is speculated that the presence of these narrow micropores suggests that these materials may be used to separate CO2 and H2 from N2 ; however, additional experimentation would be required for confirmation. Acknowledgements Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.indcrop. 2015.01.010. References Anon, 2010. Standard Test Method for Carbon Black – Total and External Surface Area by Nitrogen Adsorption, ASTM D6556-10. ASTM International, West Conshohocken, PA. Anon, 2012. Standard Practice for Calculation of Pore Size Distributions of Catalysts and Catalyst Carriers from Nitrogen Desorption Isotherms, ASTM D4641-12. ASTM International, West Conshohocken, PA. Anon, 2013. Standardized Product Definition and Product Testing Guidelines for Biochar That is Used in Soil. IBI-STD-01.1. International Biochar Initiative, Westerville, OH.

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