Water Holding Capacity and Absorption Properties of Wood Chars

Article pubs.acs.org/EF

Water Holding Capacity and Absorption Properties of Wood Chars Jun Zhang and Changfu You* Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: The application of biomass char as a kind of soil amendment has an important role in soil water holding capacity (WHC), which has a close relationship with its own surface area, total pore volume, and porosity structure. In this research, the WHC and absorption properties of the chars were investigated. Two kinds of wood (poplar and pine) were pyrolyzed at both 450 and 550 °C to produce the chars for the experiments. The Boehm titration was used to measure the concentration of the functional groups. The surface area was determined through the Brunauer−Emmett−Teller (BET) method, while the morphological characteristic of the chars was studied by scanning electron microscopy (SEM). Furthermore, the total pore volume, average pore diameter, and porosity structure of the chars were measured by a mercury porosimeter. On the basis of the pore size distribution of the chars, the definition of the Soil Science Society of America was used as the classification standard. The results showed that there was a significant positive correlation between the WHC of the chars and the total pore volume. However, there was no obvious relationship between the surface area and the WHC of the wood chars. The water absorption rate (WAR) of chars was affected by both the total pore volume and the average pore diameter. The classification of the pore size was needed to further explain the differences of the WAR of the chars. The large pores can not only hold the water in it but also act as the passages to the small pores. The relatively small pore volume of mesopores seriously affected the WAR of the chars.

1. INTRODUCTION

Biomass char can also mitigate the effects of global warming by increasing and sequestering carbon in soils.3,10,14,15 However, research on the water holding capacity (WHC) and water absorption properties of biomass char remains quite limited. In previous studies, the contents of wood char have been found in aquifer materials.16 When biomass char is added to soil, the WHC of the soil can be significantly increased.17,18 There are two ways in which the soil water can be affected. The direct influence is that the biomass char as a kind of porous media can hold the water in its pores and then increase the water content of the soil. The indirect influence is that the biomass char added to soil will bind to other soil constituents and then improve the structure of the soil, which in return increase the soil WHC. In this research, only the direct influence will be investigated. There are some major factors that may influence the WHC and water absorption rate (WAR) of biomass char: surface functional groups, total pore volume, porosity structure, and specific surface area.19−21 Surface functional groups, especially oxygen-containing groups, have an important relationship to water adsorption capacity; this is because the mechanism of adsorption is believed to be hydrogen bonds.4 The hydrogen bonds can form between the oxygen-containing functionalities on the surface of the char (primary adsorption centers) and between the adsorbed molecules themselves (secondary adsorption centers). A large specific surface area can provide more adsorption centers that will enhance adsorption ability.19 The porosity structure is also an important factor. As previous studies have illustrated, most of the adsorption takes place in the micropores, while the

Since the beginning of the 21st century, energy issues and environmental problems have become increasingly serious. The use of bioenergy is thought to be an effective way to solve the energy crisis and other related environmental problems, and many countries around the world are now giving greater attention to the development of bioenergy. In consideration of the world’s increasing population and limited food production capabilities, bioenergy should not use food crops or farmland as sources of raw materials. In China, large areas of land are not fit for food cultivation because of the aridity and low fertility. Because some energy plants need less water and fertilizers, there is an opportunity for the large-scale cultivation of bioenergy plants. However, there is crucial need to minimize the soil constraints (desertification, salinization, and water deficit) before the growing of the energy plants in these regions could be scaled up. Therefore, improvements of soil water retention properties are critical to the successful development of biomass energy resources in these large areas of barren land, especially in arid and semi-arid regions. Biomass char is the solid product of biomass pyrolysis. Some researchers also use the terms biochar or charcoal to describe it.1 It usually has a large specific surface area, high porosity, and a high level of resistance to be mineralized to CO2.2,3 With these unique properties, biomass char can be used in many different ways. Many studies have examined its use as an alternative adsorption material to activated carbons4−6 for separating gas or adsorbing metal ions from wastewater.7−9 Other studies investigated its use as a soil amendment for improving soil quality by increasing the cation exchange capacity (CEC), which will retain the cations in an available form for the plant, because of the large surface area, negative surface charge, and charge density of the biomass char.2,10−13 © 2013 American Chemical Society

Received: January 14, 2013 Revised: April 24, 2013 Published: April 24, 2013 2643

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Table 1. Proximate and Ultimate Analysis Results proximate analysis (%, air-dry basis)

ultimate analysis (%, air-dry basis)

sample

moisture

ash

volatile matter

fixed carbon

C

H

O

N

S

poplar pine Y450 S450 Y550 S550

9.4 8.7 5.1 4.0 4.1 2.7

1.2 8.7 3.8 0.8 3.6 1.1

75.9 68.1 23.3 25.3 14.9 16.5

13.5 14.4 67.8 69.9 77.4 79.7

45.42 43.83 78.05 78.82 83.69 85.97

5.69 5.69 3.44 3.66 2.74 2.94

38.05 32.44 9.42 12.54 5.7 7.11

0.19 0.17 0.18 0.20 0.10 0.22

0.05 0.70 0.01 0.05 0.03 0.03

Figure 1. SEM micrograph of the chars: (A) Y450, (B) S450, (C) Y550, and (D) S550. groups of chars. The Brunauer−Emmett−Teller (BET) surface area of the chars was measured by the NOVA 4000 (Quantachrome, Boynton Beach, FL) surface area analyzer using nitrogen adsorption at 77 K, and the pore size distribution was analyzed using the AutoPore IV 9500 (Micromeritics, Norcross, GA) mercury porosimeter. Each measurement of the chars was performed only once, considering the accuracy of the equipment. 2.3. WHC and WAR. The dried chars were weighed (M1) before the experiment and then immersed in water for 48 h. After that, the chars were taken out of the container and the wet weight of the chars (M2) was determined by the vacuum method.25 The WHC of the chars (n = 3) was calculated using the following formula: WHC = (M2 − M1)/M1. For the measurement of the WAR (n = 2), the container with the accuracy of 0.2 mL was used. Before the experiment, the apparent densities of the chars (ρc) were measured by the mercury porosimeter. The mass of the chars (Mc) was weighed before the experiment, and the apparent volume of the chars (Vc) could be calculated by the formula: Vc = Mc/ρc. When the experiment started, the chars were mixed with a certain volume of water (VH2O) in the container and started timing. The volume of the mixture (Vmix) was written down at different time intervals. The specific water content in the chars could be calculated by the formula: vab = (Vc + VH2O − Vmix)/Mc. The power fitting with the

meso- and macropores serve as the passageways of the adsorbate to the micropores.5,22,23 The main purpose of this research is to analyze the effects of the characteristics of different kinds of biomass char on WHC and WAR. It would play an important role in guiding the use of biomass char as a kind of soil amendment in increasing the WHC of the soil.

2. EXPERIMENTAL SECTION 2.1. Char Preparation. Pine (Pinus sylvestris var. mongolica Litv.) and poplar (Populus davidiana) were used as the raw materials for the pyrolysis, because both materials are very common and easy to obtain in China. The proximate and ultimate analyses of the wood are shown in Table 1. Pyrolysis was carried out in an experimental electric furnace. Before the experiments, the wood used for the pyrolysis was cut into small cubes, which were about 1 cm3. The samples were then pyrolyzed in an experimental electric furnace at 450 or 550 °C for 40 min. The carrier gas was nitrogen in the pyrolysis runs. After the pyrolysis, the chars were stored in sealed bags to prevent them from absorbing any water vapor. In this research, the poplar chars that were produced at 450 and 550 °C were labeled as Y450 and Y550. The pine chars were labeled as S450 and S550, respectively. 2.2. Characteristics of the Analysis of Char. The Boehm titration24 was used to determine the acidic oxygen surface functional 2644

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Figure 2. Pore size distribution for (A) Y450, (B) S450, (C) Y550, (D) S550, (E) poplar, and (F) pine. form of y = axb was used to fit the experimental data of the specific water content. 2.3. Statistical Analysis. SPSS statistics (SPSS for Windows, SPSS, Inc., Chicago, IL; release 18.0.0) was used for analyzing whether there were correlations between the WHC of the chars and the total pore volume, surface functional groups, or surface area. The Pearson correlation analysis was performed to analyze the pore size distributions of the chars produced from the same raw material. One-way analysis of variation (ANOVA) was used for analyzing the significance of the differences between the means of the WHC. The correlation coefficient (R2) (Origin, OriginLab Corporation, Northampton, MA; release 8.5.0) has been reported (Figure 3) to characterize the precision of optimal data fitting analysis for WAR.

mesopores (30−75 μm), micropores (5−30 μm), ultramicropores (0.1−5 μm), and cryptopores (50 nm), mesopores (2−50 nm), and micropores (75 μm), 2645

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Table 2. Apparent Density, BET Surface Area, Total Pore Volume, Average Pore Diameter, and WHC of the Chars sample

apparent densitya (g/cm3)

BET surface areaa (m2/g)

total pore volumea (mL/g)

average pore diametera (nm)

Y450 S450 Y550 S550

0.20 0.24 0.18 0.21

4.17 1.33 210.57 220.43

4.20 2.85 4.78 3.22

1303.7 497.3 1231.1 1363.7

WHCb (mL of water/g of sample) 3.44 1.19 3.98 1.37

(±0.36) c (±0.10) (±0.35) c (±0.09)

a

n = 1 for the apparent density, BET surface area, total pore volume, and average pore diameter. bData are obtained from triplicate experiments and presented as the mean ± standard deviation (SD). The means of the WHC followed by the same lowercase letter c do not differ significantly by least significant difference (LSD) (0.05).

of Figure 2). On the basis of the analysis above, the definition of the IUPAC was not fit for this research because almost all of the pores would be classified as macorpores, which could not allow for a comparison of pore volumes between different pore size ranges. Thus, the definition of the Soil Science Society of America was used in this research. 3.2. WHC and WAR Analyses. The WHC of the chars that were produced from the poplar was ca. 300% higher than that of the chars produced from the pine (Table 2). As the pyrolysis temperature increased, the WHC of the char also increased. Both the WHC and the total pore volume of the chars decreased in the order Y550 > Y450 > S550 > S450. The Pearson correlation coefficient between the WHC and the total pore volume was 0.986 (p < 0.01), which means that the WHC had a significant positive correlation with the total pore volume. Conversely, there was no obvious relationship between the surface functional groups and the WHC of the chars (r = −0.659). The concentration of functional groups decreased as the pyrolysis temperature changed from 450 to 550 °C (Table 3). However, the trend of the concentration of functional

However, the pore size and total pore volume of the chars used in this experiment were larger than those of activated carbon.31 The water held in the pores by capillary force was greater than that absorbed to the surface of the chars. The WAR experiment lasted 9 h, and the results were presented in Figure 3. The experimental data fitted quite well with power fitting. Moreover, this equation has the same form as an empirical model used for soil infiltration,32 because both wood char and soil are porous media with similar pore size distributions. Thus, the absorption of water could be described for both media using the same equation. However, the exponent in the soil infiltration equation was 0.5, but the exponent for the char in the current experiment was less than 0.5. One possible reason was that the water absorption of the char occurred in all directions because the chars were fully immerged in water, while for the soil, the infiltration in the model was just in one direction. The WAR of the chars decreased in the order Y550 > Y450 > S550 > S450. The trend of the WAR was the same as the WHC results, indicating that the total pore volume was also important in WAR. In addition to the total pore volume, the pore structure was another factor that had an important influence on WAR. The WAR of the char with large pores exceeds that of char with small pores with the same total pore volume. Larger pores not only hold water but act as a passage to small pores. Thus, an average pore diameter may be used to describe the difference in the pore structure of the chars. However, an average pore diameter on its own cannot be used to accurately estimate WAR. The average pore diameter of the chars was in the order S550 > Y450 > Y550 > S450. This result was not consistent with the order of the WAR of the chars. Analysis of the pore size distribution more accurately reflects the WAR of the chars. Pore volumes of Y550 char exceeded other chars for all pore size ranges (Table 4). Furthermore, the volumes of all pore sizes in the Y550 char were large enough that there would be no restriction for the water to flow into relatively small pores. On the contrary, this restriction can be clearly seen from S450 and S550. The very small mesopore volume not only limited the WAR at the beginning of the penetration but also reduced the passages to small pores.

Table 3. Concentration of Surface Functional Groups sample Y450 S450 Y550 S550

carboxylic (mmol/g)a b

ND 0.07 ND 0.03

lactonic (mmol/g)a

phenolic (mmol/g)a

0.08 0.07 ND 0.01

0.15 0.23 ND 0.10

a n = 1 for the concentration of surface functional groups. bND = not detected.

groups was very different from that of the WHC of the chars. The pine char had a relatively higher concentration of the functional groups, but the WHC of the pine chars was much smaller than that of poplar chars. Although the hydrogen bond between the oxygen-containing functional groups and water molecules is very important in activated carbon adsorption, it had less effect in the biomass chars, which had a much larger pore size. The difference of the BET surface area between Y550 and S550 was very small (210.57 and 220.43 m2/g), but Y550 had a much higher WHC value. The difference between the WHC of Y450 and Y550 was quite small in comparison to the difference between their BET surface areas (4.17 and 210.57 m2/g, respectively). This was similar to S450 and S450 (1.33 and 220.43 m2/g, respectively). The Pearson correlation coefficient between the WHC and the surface area was 0.128 (p = 0.436). These results differed from those of studies in which activated carbon was used to adsorb gas or water vapor.29 The adsorption process of activated carbon mainly occurs on the surface of the activated carbon. Thus, the larger surface area and high concentration of the surface functional groups can increase the adsorption by offering more adsorption centers.30

4. CONCLUSION In this research, the WHC and WAR of four kinds of wood chars were measured and analyzed on the basis of the properties of chars, such as surface functional groups, surface area, total pore volume, average pore diameter, and pore size distribution. The total pore volume played a more important role than the surface area or surface functional groups in WHC for the chars with a relatively large pore size. The WHC of chars was consistent with the trend of the total pore volume. The WAR of chars was affected by both the total pore volume and the average pore diameter. Larger pores not only hold 2646

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Figure 3. WAR of chars (A) Y450, (B) S450, (C) Y550, and (D) S550.

Table 4. Volume of Different Pore Sizes

a

sample

macroporesa (mL/g)

mesoporesa (mL/g)

microporesa (mL/g)

ultramicroporesa (mL/g)

cryptoporesa (mL/g)

Y450 S450 Y550 S550

0.15 0.09 0.23 0.13

0.92 0.05 1.00 0.07

0.93 0.69 0.92 0.85

2.20 1.99 2.61 2.15

0.01 0.03 0.03 0.02

n = 1 for the volume of different pore sizes. (4) Pastor-Villegas, J.; Meneses Rodríguez, J. M.; Pastor-Valle, J. F.; Rouquerol, J.; Denoyel, R.; García García, M. Adsorption−desorption of water vapour on chars prepared from commercial wood charcoals, in relation to their chemical composition, surface chemistry and pore structure. J. Anal. Appl. Pyrolysis 2010, 88 (2), 124−133. (5) Pastor-Villegas, J.; Pastor-Valle, J. F.; Rodríguez, J. M. M.; García, M. G. Study of commercial wood charcoals for the preparation of carbon adsorbents. J. Anal. Appl. Pyrolysis 2006, 76 (1−2), 103−108. (6) Azargohar, R.; Dalai, A. K. Production of activated carbon from Luscar char: Experimental and modeling studies. Microporous Mesoporous Mater. 2005, 85 (3), 219−225. (7) Choy, K. K. H.; McKay, G. Sorption of metal ions from aqueous solution using bone char. Environ. Int. 2005, 31 (6), 845−854. (8) Mohan, D.; Rajput, S.; Singh, V. K.; Steele, P. H.; Pittman, C. U., Jr. Modeling and evaluation of chromium remediation from water using low cost bio-char, a green adsorbent. J. Hazard. Mater. 2011, 188 (1−3), 319−333. (9) Lin, Y.-R.; Teng, H. Mesoporous carbons from waste tire char and their application in wastewater discoloration. Microporous Mesoporous Mater. 2002, 54 (1−2), 167−174. (10) Glaser, B.; Lehmann, J.; Zech, W. Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoalA review. Biol. Fertil. Soils 2002, 35 (4), 219−230. (11) Lehmann, J.; da Silva, J. P., Jr.; Steiner, C.; Nehls, T.; Zech, W.; Glaser, B. Nutrient availability and leaching in an archaeological Anthrosol and a Ferralsol of the Central Amazon basin: Fertilizer, manure and charcoal amendments. Plant Soil 2003, 249, 343−357.

water but act as a passage to smaller pores. Thus, the pore size distribution analysis was needed to further explain the differences of the WAR of the chars. The application of biomass char as a soil amendment has many potential advantages, including the increase of soil WHC in arid and semi-arid regions of China. On the basis of the analysis in this research, biomass char with large total pore volumes and average pore diameters would be best for use in these regions.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-62785669. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Lehmann, J.; Joseph, S. Biochar for Environmental Management: Science and Technology; Earthscan: London, U.K., 2009; pp 2−3. (2) Lehmann, J. Bio-energy in the black. Front. Ecol. Environ. 2007, 5 (7), 381−387. (3) Roberts, K. G.; Gloy, B. A.; Joseph, S.; Scott, N. R.; Lehmann, J. Life cycle assessment of biochar systems: Estimating the energetic, economic, and climate change potential. Environ. Sci. Technol. 2009, 44 (2), 827−833. 2647

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(12) Gaunt, J. L.; Lehmann, J. Energy balance and emissions associated with biochar sequestration and pyrolysis bioenergy production. Environ. Sci. Technol. 2008, 42 (11), 4152−4158. (13) Liang, B.; Lehmann, J.; Solomon, D.; Kinyangi, J.; Grossman, J.; O’Neill, B.; Skjemstad, J. O.; Thies, J.; Luizão, F. J.; Petersen, J.; Neves, E. G. Black carbon increases cation exchange capacity in soils. Soil Sci. Soc. Am. J. 2006, 70, 1719−1730. (14) Lehmann, J. A handful of carbon. Nature 2007, 447 (7141), 143−144. (15) McHenry, M. P. Agricultural bio-char production, renewable energy generation and farm carbon sequestration in Western Australia: Certainty, uncertainty and risk. Agric., Ecosyst. Environ. 2009, 129 (1− 3), 1−7. (16) James, G.; Sabatini, D. A.; Chiou, C. T.; Rutherford, D.; Scott, A. C.; Karapanagioti, H. K. Evaluating phenanthrene sorption on various wood chars. Water Res. 2005, 39 (4), 549−558. (17) Dugan, E.; Verhoef, A.; Robinson, S.; Sohi, S. Bio-char from sawdust, maize stover and charcoal: Impact on water holding capacities (WHC) of three soils from Ghana. Proceedings of the 19th World Congress of Soil Science, Soil Solutions for a Changing World; Brisbane, Queensland, Australia, Aug 1−6, 2010. (18) Karhu, K.; Mattila, T.; Bergström, I.; Regina, K. Biochar addition to agricultural soil increased CH4 uptake and water holding capacity Results from a short-term pilot field study. Agric., Ecosyst. Environ. 2011, 140 (1−2), 309−313. (19) Pulido-Novicio, L.; Hata, T.; Kurimoto, Y.; Doi, S.; Ishihara, S.; Imamura, Y. Adsorption capacities and related characteristics of wood charcoals carbonized using a one-step or two-step process. J.. Wood Sci. 2001, 47 (1), 48−57. (20) Brown, R. A.; Kercher, A. K.; Nguyen, T. H.; Nagle, D. C.; Ball, W. P. Production and characterization of synthetic wood chars for use as surrogates for natural sorbents. Org. Geochem. 2006, 37 (3), 321− 333. (21) Ismadji, S.; Sudaryanto, Y.; Hartono, S. B.; Setiawan, L. E. K.; Ayucitra, A. Activated carbon from char obtained from vacuum pyrolysis of teak sawdust: Pore structure development and characterization. Bioresour. Technol. 2005, 96 (12), 1364−1369. (22) Asai, H.; Samson, B. K.; Stephan, H. M.; Songyikhangsuthor, K.; Homma, K.; Kiyono, Y.; Inoue, Y.; Shiraiwa, T.; Horie, T. Biochar amendment techniques for upland rice production in northern Laos: 1. Soil physical properties, leaf SPAD and grain yield. Field Crops Res. 2009, 111 (1−2), 81−84. (23) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by active carbons. Adsorption by Powders and Porous Solids; Academic Press: London, U.K., 1999; Chapter 9, pp 237−285. (24) Boehm, H. P. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 1994, 32 (5), 759−769. (25) Schrecker, S. T.; Gostomski, P. A. Determining the water holding capacity of microbial cellulose. Biotechnol. Lett. 2005, 27 (19), 1435−1438. (26) Soil Science Glossary Terms Committee. Glossary of Soil Science Terms 2008; Soil Science Society of America: Madison, WI, 2008. (27) International Union of Pure and Applied Chemistry (IUPAC). Compendium of Chemical Terminology Gold Book; IUPAC: Research Triangle Park, NC, 2012. (28) Wildman, J.; Derbyshire, F. Origins and functions of macroporosity in activated carbons from coal and wood precursors. Fuel 1991, 70 (5), 655−661. (29) Rodríguez-Reinoso, F.; Nakagawa, Y.; Silvestre-Albero, J.; Juárez-Galán, J. M.; Molina-Sabio, M. Correlation of methane uptake with microporosity and surface area of chemically activated carbons. Microporous Mesoporous Mater. 2008, 115 (3), 603−608. (30) Freeman, J. J.; Tomlinson, J. B.; Sing, K. S. W.; Theocharis, C. R. Adsorption of nitrogen and water vapour by activated Nomex chars. Carbon 1995, 33 (6), 795−799. (31) Lu, Q.; Sorial, G. A. Adsorption of phenolics on activated carbonImpact of pore size and molecular oxygen. Chemosphere 2004, 55 (5), 671−679.

(32) Philip, J. R. The theory of infiltration: 1. The infiltration equation and its solution. Soil Sci. 1957, 83 (5), 345−358.

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