characterization and surface analysis of commercially ... · 1 characterization and surface...

1

Characterization and Surface Analysis of Commercially Available Biochars for Geoenvironmental Applications

Erin N. Yargicoglu1, S.M. ASCE and Krishna R. Reddy2, P.E., D.GE, F.ASCE

1Graduate Research Assistant, University of Illinois at Chicago, Department of Civil and Materials Engineering, 842 West Taylor Street, Chicago, Illinois 60607, USA; [email protected] 2Professor, University of Illinois at Chicago, Department of Civil and Materials Engineering, 842 West Taylor Street, Chicago, Illinois 60607, USA; [email protected] ABSTRACT: Biochar is a solid byproduct generated during anaerobic biomass pyrolysis or gasification that has recently gained interest for both agricultural and environmental uses owing to its unique physical and chemical properties. These properties include high surface area and porosity, and ability to adsorb a variety of compounds, including nutrients, organic contaminants, and some gases. Physical and chemical properties of biochar are dictated by the feedstock and production conditions (i.e. pyrolysis temperature and conversion technology), which vary widely across commercial biochar producers. In this study, several commercially available biochars are characterized for key physical and chemical properties relevant to the common uses of biochars in environmental applications, specifically as an amendment to soil landfill covers for enhanced methane oxidation. SEM images of each biochar type were analyzed using Pore Crack Analysis Software (PCAS)© in order to quantify differences in surface porosity and structure. Trends in these properties are related to feedstock and production method to aid in the proper selection of biochar for geoenvironmental applications. A high variability in chemical composition, surface properties, and pore structure among commercially available biochars was observed, highlighting the importance of pre-screening biochars prior to use in applications such as a landfill cover or soil amendment. INTRODUCTION Biochar – the solid byproduct of biomass pyrolysis or gasification in an anoxic or micro-oxic environment – has recently been promoted as a soil amendment for improved soil fertility (Lehmann et al., 2011), carbon sequestration (Laird, 2008), and for methane mitigation in landfill covers (Yaghoubi, 2011; Reddy et al., 2014). This material has several unique properties that may make it amenable for a number of environmental applications, including its high internal porosity, high water-holding capacity, and nutrient retention and sorption properties (Glaser et al., 2002; Brown et al., 2006). Ongoing research in our laboratory has shown that biochar amended to landfill covers can promote the physical adsorption and microbial oxidation of CH4

IFCEE2015, San Antonio, TX, March 17-21, 2015

2

into CO2, a potent greenhouse gas with a global warming potential 25 times that of CO2 (Yaghoubi 2011; Reddy et al., 2014). In addition to improving landfill gas retention within the surface cover, the high internal porosity of biochars enhances methane oxidation by providing additional habitable sites for indigenous methane-oxidizing bacteria. Thus, biochar-amended soils may be useful for methane mitigation at landfills that cannot achieve complete methane removal via landfill gas extraction and collection systems. Increased interest in using sustainable materials for environmental applications has prompted research on the influence of feedstock and production conditions on biochar properties. Biochar can be considered a sustainable material for carbon sequestration in that it is a waste product from biomass pyrolysis that allocates labile organic carbon into a more stable carbon pool (biochar), preventing its release into the atmosphere as CO2. Desired qualities of biochar will vary depending on the application, such that a biochar optimized for use an agricultural amendment may not have the same properties as a char produced for use as a filter media for waste treatment. Several researchers have characterized the physicochemical properties of laboratory-produced biochars with respect to source materials and pyrolysis technology employed (Brewer et al., 2009; Lee et al., 2010; Koide et al., 2011; Kloss et al., 2012). Prior studies have found that the physical properties and chemical composition of the source material, as well as the production conditions and post-production treatments applied, govern important functional properties of the resultant biochar, such as sorption characteristics, surface area, porosity and structural properties, surface charge and alkalinity, and organic carbon content (Brewer et al., 2009; Spokas et al., 2011; Uchimiya et al., 2011; Kloss et al., 2012). However, these studies have primarily studied only biochars produced under controlled laboratory conditions, with less emphasis on commercially-available biochars that may actually be used in practice. In this study, the surface properties of six biochars are characterized relative to granular activated carbon (GAC) to provide further insight on the effects of production and post-production processes on relevant physicochemical properties of commercial, wood-derived biochars.. METHODS Biochar Selection and Production Processes Six different wood-derived biochars and granular activated carbon (GAC) were obtained from commercial vendors and selected for detailed characterization tests in this study. Table 1 summarizes the feedstock sources, production processes and conditions, and type of post-treatment applied (if any) for each of the studied biochars. The residence time in the pyrolysis or gasification chamber refers to the total length of heat treatment as reported by the vendor. All characterization tests were performed using each biochar “as is” from the vendor unless otherwise stated; detailed methodologies on biochar characterization tests are given in Yargicoglu and Reddy (2014) and Yargicoglu et al. (2014).


3

Physical-Chemical Characterization All selected biochars and granular activated carbon (GAC) were subjected to a battery of physical and chemical analyses following standard methods as listed in Table 2. Additional details on analytical methods for which no standard method was referenced (i.e. measurement of zeta potential, CHN content, SEM imaging and image analysis) are provided in Yargicoglu et al. (2014). In addition to parameters listed in Table 2 determined via standard methods, SEM images were taken and utilized for SEM image analysis as described in the following section. TABLE No. 1: Production conditions and source materials of studied biochars. Granular activated carbon (GAC; not shown) obtained from Fisher Scientific was also characterized. NR: not reported. Sample ID

Feedstock Treatment Process

Treatment Temp.

Residence Time

Post-Treatment

BS Pine wood Slow pyrolysis

350 – 600°C

6 hrs Sieved (< 3 mm)

CK 90% pine; 10% fir wood

Fast pyrolysis > 500°C < 1 hr Activated with O2

AW Aged oak & hickory wood

Pyrolysis – Missouri type concrete kiln

~500°C NR

Mixed with inocula & sieved (1/4”)

CE-WP1

Pinewood pellets

Gasification ~520°C

NR Fine ash retained

CE-WP2 NR Fine ash sieved

CE-AWP NR Stored for 2 years

Surface Area Analysis, SEM Imaging and Image Analysis Surface areas were determined via N2 adsorption on a BET Surface Area Analyzer (Micromeritics ASAP 2020). BET and Langmuir adsorption isotherms were generated to determine the single-point surface area. SEM images were taken by the Electron Microscopy Service at the University of Illinois at Chicago. Dry, biochar samples were first coated with 3 to 6 nm of Pt/Pd coating using a Cressington HR208 sputter coater in order to minimize sample charging. Images were captured using a Hitachi S-3000N Variable Pressure Scanning Electron Microscope operated in high vacuum mode with 2 to 10 kV accelerating voltage (voltage applied varied based on extent of sample charging) using a secondary electron detector. Images were taken at several magnifications ranging from x50 to x4000.


4

The micro-porosities of biochars and GAC were quantified using Pores (Particles) and Cracks Analysis System (PCAS) image processing software, developed and validated by Liu et al. (2011). PCAS first converts imported SEM images into black and white binary images with voids distinguished from solid surfaces based on the grey-level threshold values (T) entered. The average T values used for this study ranged from 107 – 138 for the biochars and GAC. Error analysis was conducted for individual biochars by varying the T values from T-4 to T+4 at two-step intervals. The minimum pore area (S0) was set to a default value of 50 pixels and the division radius (r) was set to 2.1 pixels. The segmentation process for the SEM images was repeated for each threshold value prior to the auto analysis. The statistical parameters corresponding to a pre-set probability range number (n=7) was extracted from the software for the pore area range analyzed. The average porosity value is then displayed from the resulting tabular output along with other micropore characteristics corresponding to pore geometry. TABLE No. 2: Parameters characterized for GAC and each biochar type and standard methods used. Parameters Characterization methods Physical Properties

Moisture Content ASTM D 1762-84 (wood charcoal) Specific Gravity ASTM D 854 Dry density Harvard miniature setup Bulk (moist) density ASTM D 7263-09 Particle Size Distribution ASTM D 422-63 Specific Gravity ASTM D 854

Chemical Properties

CHN Elemental analysis Perkin-Elmer Elemental Analyzer pH/ORP/EC ASTM D4972 – 01 Organic Matter content ASTM D2974 Ash, volatile matter, and fixed C ASTM D1762-84 Zeta-Potential Zeta-Meter 3.0+ System PAH content EPA Method SW8270C Trace Metal content EPA Method SW6020

RESULTS & DISCUSSION Physical Properties of Biochars A summary of physical and chemical properties characterized is given in Table 3. Visual observation in addition to particle size analyses indicated clear differences in size distributions and average particle size of biochars, with some biochars in a loose, granular form (AW, BS, CK) or formed into pellets (CE-WP1, CE-WP2, CE-AWP). As observed in previous studies, the fast pyrolysis char had a finer texture and smaller effective particle size (D50 of 0.22 mm) due to more rapid combustion in a fast pyrolysis chamber, and also exhibited a higher degree of carbon enrichment than the slow pyrolysis or gasification chars (indicated by its relatively low H:C molar ratio of


5

0.18). All average dry densities of these biochars were less than 1 g/cm3, likely due to the presence of large internal void spaces, which can be observed in SEM images (Fig. 1). These void spaces are largest in the CK biochar, with some greater than 25 microns in diameter. The distinct physical properties of the fast pyrolysis biochar (CK) were associated with certain chemical properties of the biochar, such as pH and metal content, as described in the following section. TABLE No. 3: Physical and chemical properties of studied biochars.

Property BS CK AW CE-

WP1 CE-WP2

CE-AWP

GAC

Dry density (g/cm3) 0.73 0.54 0.48 0.56 0.52 0.53 0.67 Specific gravity 1.36 1.51 1.19 0.77 0.59 0.91 1.65 Surface Area (m2/g) 40.6 155.1 5.4 0.4 0.1 - 611.9 Avg. Porosity (%) 46.3 54.7 29.4 44.4 41.4 39.9 59.4 D10 (mm) 0.09 0.08 0.33 0.24 1.29 2.68 3.3 D50 (mm) 0.71 0.22 0.89 1.13 3.15 5.75 2.97 Organic Matter Content (%)

33.9 32.3 74.5 96.0 97.5 87.8 91.4

Volatile Matter Content (%)

28.0 28.1 74.1 61.8 62.7 55.4 64.2

Ash Content (%) 65.7 61.6 25.4 4.6 1.5 4.3 2.9 Fixed C Content (%) 4.6 3.7 ND 33.2 35.0 40.3 17.9 Elemental Analysis C (%) H (%) N (%)

53.2 1.6 0.4

23.5 0.4 0.01

51.9 2.2 0.4

70.7 3.8 0.3

74.0 3.8 0.3

78.1 1.8 0.4

76.5 0.8 0.2

Molar Ratios H:C 0.35 0.18 0.51 0.63 0.61 0.27 0.12 C:N 144 5514 152 291 294 262 427 pH 8.5 9.0 7.9 6.2 6.8 7.6 8.9 ORP 74.2 -116.1 -63.5 35.1 2.3 -48.7 -120.8 EC 0.04 0.007 0.14 4.15 1.1 0.54 0.01 Zeta Potential (mV) -23.7 -15.8 -15.4 -25.6 -24.4 -18.6 -31.0 Chemical Properties of Biochars Overall, considerable variability in chemical properties were observed among pyrolysis-derived biochars; moreover, biochars produced via gasification (CE chars) tended to have distinct properties from chars produced via pyrolysis. Biochars with higher C:N and lower H:C ratios likely underwent greater thermal alteration due to the greater loss of H and N relative to C. The pH of biochars with a greater extent of carbonization (as inferred from H:C molar ratios) were slightly alkaline, with the highest pH observed in the biochar with the greatest degree of carbonization (CK),


6

consistent with prior reports documenting a liming effect as biomass is pyrolyzed (Lehmann et al. 2011). By contrasts, the CE biochars had pH values closer to neutral (ranging from pH 6.24 to 6.78) coupled with higher H:C ratios and fixed C contents relative to CK biochar. Of the biochars included in this study, both the lowest H:C ratio and highest C:N ratio was observed in the CK biochar (0.18 and ~5514, respectively). As with other properties, these data suggest that a greater extent of carbon enrichment occurred during fast pyrolysis of this biochar relative to other tested biochars. Other properties in the CK biochar, such as elevated pH (pH~9) and a relatively high surface area (~155 m2/g), were also apparently related to this increased degree of carbonization, consistent with observations in prior biochar characterization studies (e.g. Bruun et al. 2011; Brewer et al., 2011). It was generally observed that biochars from the same vendor cluster together, likely because the extent of biomass pyrolysis is controlling the development of alkaline pH due to the formation of insoluble salts (i.e. alkali metals), which are generally more abundant in hardwood ash as compared to lignin-poor feedstock, such as corn stover or switchgrass (Brewer et al., 2009). The highest pH values for commercial biochars (CK and BS) are also associated with the highest elemental fractions of metals, such as K and P (i.e. CK and BS biochars; Yargicoglu et al., 2014), reinforcing the hypothesis that biochar pH and metal salt content are directly related and resultant from the degree of biomass carbonization. Because pH is an important parameter for microbial systems, these differences in pH may significantly impact the microbial response to biochar amendments in soil systems. Surface Properties Physical and chemical surface properties of biochars are affected by the extent of thermal alteration, often times resulting in an enrichment in various ionic species, such as alkali metals, on the char surface as organic carbon and volatile matter are removed during pyrolysis (Keiluweit et al., 2010; Brewer et al., 2011). Typically, higher production temperatures lead to high biochar surface areas (SA), with significant increases in SA beginning at temperatures from 460 to 525°C (Kloss et al. 2012). Brown et al. (2006) reported maximum specific surface areas of ~ 400 m2/g for biochars produced at 750°C, regardless of the heating rate (ramp rate) applied. Production temperatures of biochars in this study ranged from 350-600°C for the slow pyrolysis biochar, and were at 500°C or above for the other biochars (gasification, fast and traditional kiln pyrolysis). Surface areas for the biochars were low relative to GAC, which had a surface area of 611.87 m2 g-1 (Table 3). Single point surface areas for biochars ranged from 0.0953 (CE-WP2 biochar) to 155.1 m2 g-1 (CK biochar; Table 3). The relatively low surface area values reported for the CE biochars are thought to be underestimates of the actual surface area due to difficulty in obtaining accurate measurements for the CE biochars. The likely explanation is the use of binding agent to form the pellets, which can prevent uniform N2 adsorption to the surface during analysis. SEM images were taken at several magnifications ranging from x50 to x4000; Figure 1 shows representative images of each of the biochars at x250 magnification; Figure 2 shows an example of the segmented SEM images used in the PCAS analyses


7

results (CE-WP2 biochar). With the exception of CK biochar, the biochars appear to retain many of the structural components of the wood fibers, indicating materials had not experienced complete thermal degradation. The SEM images for all samples captured at a magnification of x2000 were used for PCAS analysis; average porosity estimates obtained from these analyses for all biochars and GAC are shown in Table 3. The results of error analysis using PCAS (Table 4) indicated that the error values associated with estimates of average porosity fell within the acceptable range of ±5% (Liu et al. 2011). At the default gray level threshold used, the average porosities of biochars and GAC ranged from 30% (AW) to ~ 60% (GAC); CE biochars produced via gasification had porosities in the range of 36 to 44%. According to these analyses, GAC had higher porosity than all biochar samples (~60%) and CK biochar had the highest porosity (~55%) among tested biochars. These results are consistent with earlier studies reporting increases in porosity and surface area of biochars with increasing treatment temperatures and surface activation (Brown et al., 2006). Furthermore, it is thought that the microporous surfaces of these biochars are highly preferable for gas adsorption, and could act to improve gas retention within a landfill cover or biofilter system. It is important to note however, that image-based analyses are limited by image quality, software capabilities and user expertise. As such, experimentally-validated approaches (e.g. mercury intrusion porosimetry) should be used whenever possible in conjunction with PCAS analysis.

TABLE No. 4: Average porosity values obtained from PCAS analysis of SEM images set at the default gray level threshold (T) and at threshold level 2 and 4 points above and below the default gray-level threshold used. Biochar Gray Level Threshold

T-4 T-2 T T+2 T+4 BS 43.56% 48.23% 46.25% 56.59% 59.48% CK 47.99% 51.17% 54.68% 58.37% 64.01% AW 26.03% 28.26% 29.42% 30.68% 33.14% CE-WP1 38.76% 41.61% 44.37% 47.02% 49.55% CE-WP2 34.42% 36.63% 41.43% 43.88% 48.46% CE-AWP 36.00% 38.62% 39.85% 42.58% 43.92% GAC 54.37% 59.33% 59.35% 62.48% 40.23% All biochars had negative zeta potential, indicating a negative surface charge for all tested samples, similar to previously reported negative surface charges measured in biochar (e.g. Liang et al. 2006; Mukherjee et al., 2011). This negative charge is the primary mechanism by which cationic nutrients are adsorbed and retained by the biochar, a process thought to improve soil fertility in biochar-amended soils (Glaser et al. 2001; Lehman et al. 2007). Thus it would appear these biochars have favorable surface characteristics for application as an agricultural amendment, or for sorption of cationic nutrients and metals. Additionally, biochars with a higher portion of fine particles (AW, CK) had a higher (i.e. less negative) zeta potential than the pelleted CE biochars. Differences can also arise due to differences in the amount of sorbed


catineg

FIGA:

FIG CO OwaslowandTheproproof bal.,

ions (e.g. K+

gative) zeta p

G. 1: SEM iBS; B: CK;

G. 2: SEM i

ONCLUSIO

Overall, a higste wood-de

wer H:C ratid concentratiese findings

oduction parocessing treabiochar (Bre2011). The

+, Ca2+), whipotentials ob

images of bi; C: AW; D

image segme

NS

gh variabilityerived biochios generallyions of toxicare consisterameters (e

atments and ewer et al., 2desired qua

ich is a plaubserved in th

iochars teste: CE-WP1;

entation for

y in chemicahars includedy had higherc constituentent with priore.g. heat trstorage) on

2009; Keiluwlities will de

usible explanhose higher c

ed in this stE: CE-WP

r CE-WP2 u

al and physid in this stur surface ars (metals anr studies thareatment tem

the surfaceweit et al., 20epend on the

nation for thecation contai

tudy at x250P2; F: CE-A

using PCAS

cal propertieudy. Amonreas, surfaced PAHs) rel

at have undermperature,

e chemistry a010; Lee et e end use of

e relatively hining biocha

0 magnificatAWP.

S.

es was obserng these, bioe porosities, lative to the rscored the iresidence tand physicaal., 2010; Mthe biochar

8

higher (less ars.

tion.

rved among ochars with pH values, other chars. influence of time, post-

al properties Mukherjee et

(i.e. energy

f


9

use, agricultural amendment, or carbon sequestration), and thus should be considered in light of the available feedstocks and biochar production technologies employed. For carbon sequestration purposes, including as a landfill cover amendment for enhanced microbial CH4 oxidation, it appears that woody feedstocks are more favorable given their higher fixed carbon content and thus greater stability in soil and biocover systems. However, even among wood-derived biochars, significant variability in fixed carbon content was observed. Whatever the intended application, pre-screening of biochars for key functional properties is highly recommended given the variability observed in commercially available, wood-derived biochar properties. ACKNOWLEDGMENTS The authors of this study would like to gratefully acknowledge the National Science Foundation (Grant CMMI #1200799) for providing financial support for this work. REFERENCES Brewer, C. E., Schmidt-Rohr, K., Satrio, J. A., and Brown, R. C. (2009).

"Characterization of biochar from fast pyrolysis and gasification systems." Environmental Progress & Sustainable Energy, 28(3), 386-396.

Brewer, C. E., Unger, R., Schmidt-rohr, K., and Brown, R. C. (2011). "Criteria to Select Biochars for Field Studies based on Biochar Chemical Properties." Bioenergy Research, 4(4), 312-323.

Brown, R. A., Kercher, A. K., Nguyen, T. H., Nagle, D. C., and Ball, W. P. (2006). "Production and characterization of synthetic wood chars for use as surrogates for natural sorbents." Org Geochem, 37(3), 321-333.

Bruun, E. W., Hauggaard-Nielsen, H., Ibrahim, N., Egsgaard, H., Ambus, P., Jensen, P. A., and Dam-Johansen, K. (2011). "Influence of fast pyrolysis temperature on biochar labile fraction and short-term carbon loss in a loamy soil." Biomass and Bioenergy, 35(3), 1182-1189.

Glaser, B., Lehmann, J., and Zech, W. (2002). "Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal – a review." Biol Fert Soils, 35(4), 219-230.

Keiluweit, M., Nico, P. S., Johnson, M. G., and Kleber, M. (2010). "Dynamic molecular structure of plant biomass-derived black carbon (biochar)." Environ Sci Technol, 44(4), 1247-1253.

Kloss, S., Zehetner, F., Dellantonio, A., Hamid, R., Ottner, F., Liedtke, V., Schwanninger, M., Gerzabek, M. H., and Soja, G. (2012). "Characterization of slow pyrolysis biochars: Effects of feedstocks and pyrolysis temperature on biochar properties." J. Environ. Qual., 41(4), 990-1000.

Koide, R. T., Petprakob, K., and Peoples, M. (2011). "Quantitative analysis of biochar in field soil." Soil Biology and Biochemistry, 43(7), 1563-1568.

Laird, D. A. (2008). "The charcoal vision: a win–win–win scenario for simultaneously producing bioenergy, permanently sequestering carbon, while improving soil and water quality." Agron J, 100(1), 178-181.


10

Lee, J. W., Kidder, M., Evans, B. R., Paik, S., Buchanan Iii, A. C., Garten, C. T., and Brown, R. C. (2010). "Characterization of biochars produced from cornstovers for soil amendment." Environ Sci Technol, 44(20), 7970-7974.

Lehmann, J., Rillig, M. C., Thies, J., Masiello, C. A., Hockaday, W. C., and Crowley, D. (2011). "Biochar effects on soil biota – A review." Soil Biology and Biochemistry, 43(9), 1812-1836.

Liang, B., Lehmann, J., Solomon, D., Kinyangi, J., Grossman, J., O'Neill, B., Skjemstad, J. O., Thies, J., Luizao, F. J., Petersen, J., and Neves, E. G. (2006). "Black Carbon increases cation exchange capacity in soils." Soil Sci Soc Am J, 70(5), 1719-1730.

Liu, C., Shi, B., Zhou, J., and Tang, C. (2011). "Quantification and characterization of microporosity by image processing, geometric measurement and statistical methods: Application on SEM images of clay materials." Applied Clay Science, 54(1), 97-106.

Mukherjee, A., Zimmerman, A. R., and Harris, W. (2011). "Surface chemistry variations among a series of laboratory-produced biochars." Geoderma, 163(3–4), 247-255.

Reddy, K.R., Yargicoglu, E.N., Yue, D., and Yaghoubi, P. (2014). “Enhanced microbial methane oxidation in landfill cover soil amended with biochar.” J. of Geotechnical and Geoenvironmental Engineering, ASCE, 140(9), 04014047.

Spokas, K. A., Novak, J. M., Stewart, C. E., Cantrell, K. B., Uchimiya, M., DuSaire, M. G., and Ro, K. S. (2011). "Qualitative analysis of volatile organic compounds on biochar." Chemosphere, 85(5), 869-882.

Uchimiya, M., Wartelle, L. H., Klasson, K. T., Fortier, C. A., and Lima, I. M. (2011). "Influence of pyrolysis temperature on biochar property and function as a heavy metal sorbent in soil." J Agr Food Chem, 59(6), 2501-2510.

Yaghoubi, P. (2011). "Development of biochar-amended landfill cover for landfill gas mitigation." Ph.D. Thesis, University of Illinois at Chicago, Illinois.

Yargicoglu, E. N. and Reddy, K. R. (2014). “Evaluation of PAH and metal contents of different biochars for use in climate change mitigation systems.” Proc. of International Conference on Sustainable Infrastructure (ICSI), ASCE, Reston, VA.

Yargicoglu, E. N., Sadasivam, B. Y., Reddy, K. R., and Spokas, K. (2014). “Physical and chemical characterization of waste wood derived biochars.” Waste Management, In Print.


characterization and surface analysis of commercially ... · 1 characterization and surface...

Documents