- Email: [email protected]
The potential applications of using compost chars for removing the hydrophobic herbicide atrazine from solution
Available online at www.sciencedirect.com
Bioresource Technology 99 (2008) 5673–5678
The potential applications of using compost chars for removing the hydrophobic herbicide atrazine from solution Lo Tsui a,*, William R. Roy b a
Departmental of Safety, Health and Environmental Engineering, Mingchi University of Technology, 84 Gungjuan Road, Taishan, Taipei 24301, Taiwan b Illinois State Geological Survey, 615 East Peabody Drive, Champaign, IL 61820, USA Received 23 August 2007; received in revised form 12 October 2007; accepted 16 October 2007 Available online 20 February 2008
Abstract One commercial compost sample was pyrolyzed to produce chars as a sorbent for removing the herbicide atrazine from solution. The sorption behavior of compost-based char was compared with that of an activated carbon derived from corn stillage. When compost was pyrolyzed, the char yield was greater than 45% when heated under air, and 52% when heated under N2. In contrast, when the corn stillage was pyrolyzed under N2, the yield was only 22%. The N2-BET surface area of corn stillage activated carbon was 439 m2/g, which was much greater than the maximum compost char surface area of 72 m2/g. However, the sorption affinity of the compost char for dissolved atrazine was comparable to that of the corn stillage activated carbon. This similarity could have resulted from the initial organic waste being subjected to a relatively long period of thermal processes during composting, and thus, the compost was more thermally stable when compared with the raw materials. In addition, microorganisms transformed the organic wastes into amorphous humic substances, and thus, it was likely that the microporisity was enhanced. Although this micropore structure could not be detected by the N2-BET method, it was apparent in the atrazine sorption experiment. Overall, the experimental results suggested that the compost sample in current study was a relatively stable material thermally for producing char, and that it has the potential as a feed stock for making highquality activated carbon. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Char; Compost; Surface area; Yield
1. Introduction Atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)1,3,5-triazine) is one of the most commonly occurring herbicides in agricultural tile drainage systems. Substantial research has focused on the environmental fate of this chemical (Kolpin et al., 1996; Kladviko et al., 1999). Our previous study has shown that atrazine can be removed from solution by using a compost bioreactor mainly through adsorption, but that the compost itself may also contribute undesirable qualities, such as color to the receiving water (Tsui and Roy, 2007). It has been suggested that *
Corresponding author. Tel.: +886 2 29089899x4652; fax: +886 2 29041914. E-mail address: [email protected] (L. Tsui). 0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.10.026
any inexpensive material with relatively large carbon content could be potential material to produce activated carbon (Bansal et al., 1988), and activated carbon is one of the most effective adsorbents for removing hydrophobic pollutants. Because compost made from agricultural wastes will likely have a significant carbon content, there is a potential to produce activated carbon from compost. Making activated carbon from compost would not only increase its surface area, it might also reduce the potentially adverse effects of using untreated compost as an adsorbent. The activated carbon can be manufactured through two activation methods: physical activation and chemical activation (Carrott and Carrott, 2007). In the physical activation method, the activation is carried out in a two-step process: carbonization and activation. Carbonization (pyrolysis) is sometimes called charring, which consists of
5674
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
heating the feed material under a controlled temperature. During carbonization, the carbon content of the char increases with temperature accompanied by a simultaneous decrease in oxygen content. The char then undergo the activation process at elevated temperature to enhance the internal pore structures. In the chemical activation method, the precursor is usually impregnated with a strong dehydrating agent, and then heated under a nitrogen-flow to simultaneously form and activate the carbon. However, several studies have suggested that carbonization may be a necessary step in order to produce high surface area activated carbon (Diao et al., 1999, 2002; Zou and Han, 2001), because it creates new cross-links within the precursor and hence enhances the porosity development. Because both physical and chemical activation could involve a two-step process, it would be reasonable to examine the quality of first-stage char first. Hence, the primary goal of this study was to examine the efficacy of creating chars from commercial compost. The compost samples were pyrolyzed under air or nitrogen conditions, and the pyrolyzed chars were then examined for their ability to sorb atrazine from solution. For comparison, one activated carbon produced from a corn-to-ethanol byproduct (corn stillage) was also included in this study to compare its sorption affinity for atrazine. 2. Methods 2.1. Materials Unlabeled atrazine (purity of 99%) was obtained from ChemService (West Chester, PA). Uniformed ring-labeled 14 C-atrazine (specific activity 15.1 mCi/mmol) (purity of 99%) was purchased from Sigma Chemical Co. (St. Louis, MO). The atrazine stock solution was prepared by dissolving 14C-labeled and nonlabeled atrazine in methanol, and then deionized water was added to prepare different concentrations of atrazine. The largest atrazine concentration was 1.7 mg/L, which was about 100 times greater than the typical atrazine concentration found in tile drainage water (Kolpin et al., 1996). The commercial compost sample was collected from one compost facility, and the initial feedstock material was a mixture of yard waste, horse bedding and food waste. Before conducting this study, the compost sample had been stored at room temperature for 3 years. The measured carbon, nitrogen, and ash content of the compost were about 51.5%, 3.5%, and 7%, respectively (Tsui et al., 2003). The activated carbon produced from corn stillage was kindly provided by the Illinois State Geological Survey (Desai, 1999). The corn stillage was first pyrolyzed in flowing N2, followed by physical activation in steam at 800 °C. 2.2. Procedure The bulk sample was screened through a 4-mm sieve before the pyrolysis process. A mass of 50 g of the compost
sample was heated in a 5 cm diameter horizontal quartz tube by a Lindberg type 59344 furnace. The samples were heated in flowing air (1 L/min) at temperatures ranging from 120 °C to 420 °C, and in flowing ultra-high purity N2 from 220 °C to 820 °C for 1 h. After heating, the samples were allowed to cool to room temperature under continuous flowing air or N2 conditions, and the remaining sample mass was recorded. The yield of compost char was defined as the ratio of the weight of the pyrolyzed char to that of the original compost, with both weights on a dry basis. The N2-BET surface areas of the prepared samples were measured in static volumetric apparatus (Monosorb, Quantachrome Corporation). Before N2 adsorption, each sample was outgassed (30% N2 with He balance) for 45 min at 120 °C. Nitrogen adsorption measurement was measured at the temperature of liquid N2 77 K, and the surface area was calculated from the linearlized BET equation (Greg and Sing, 1982). The adsorption study for atrazine was carried out using the batch methods developed by Roy et al. (1992). A mass of 0.5 g of unheated compost or pyrolyzed sample was added to 25 mL solution containing a known amount of 14 C-labeled atrazine in a polyethylene centrifuge tube. The centrifuge tubes were mixed in a rotating tumbler at 23 ± 2 °C until the system reached equilibrium, after which they were centrifuged at 12,000g for 20 min. The 14Clabeled atrazine concentration was determined by using a Packard 2000CA TriCarb liquid scintillation counter, and the amount of adsorbed atrazine was calculated as the difference between the initial atrazine concentration in the original solution and the equilibrated solution. Sorption isotherms were used to describe the sorption behavior. The activated carbon sample produced from corn stillage as described earlier was used for comparisons of sorption performance for atrazine with pyrolyzed compost samples.
3. Results and discussion 3.1. Compost char properties When the compost sample was pyrolyzed under air, the N2-BET surface areas of the resulting chars increased with increasing temperature up to 22.19 m2/g at 370 °C, but decreased to 2.31 m2/g when heated at 420 °C (Table 1), probably because the organic structure of the compost structure when heated at that temperature. The N2-BET surface area of compost chars continued to increase with an increase in temperature up to 720 °C when compost was pyrolyzed under N2. When comparing the compost chars heated under air and N2 at the same temperature, the compost chars produced under air had a greater surface area than the corresponding chars under N2 at temperature less than 420 °C. The results were similar to other studies showing that the chars pyrolyzed with the presence of oxygen exhibited a greater burn-off rate and larger surface area
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678 Table 1 Yield and surface area of pyrolyzed compost chars and corn carbons Sample type Compost Untreated Pyrolysis, air
Pyrolysis, N2
Corn stillage Pyrolysis, N2 Activation, steam
Temperature (°C)
Yield (%)
25 120 220 320 370 420 220 320 420 520 620 720 820
92 80 60 54 46 88 72 66 64 60 56 52
900 800
21.8 15.8
N2-BET surface area (m2/g) 2.93 3.13 10.21 17.72 22.19 2.31 7.62 10.38 16.27 22.39 51.21 72.39 71.63 35 439
when compared with chars heated in pure N2 (Teng et al., 1997; Sharma et al., 2004). The surface areas of the pyrolysis chars and activated carbon produced from corn stillage are compared in Table 1. The results showed that the pyrolysis of compost at 620 °C under N2 yielded a larger N2-BET surface area when compared with corn stillage chars pyrolyzed at 900 °C. During composting, microorganisms converted organic waste into humic substances, which were amorphous, partly aromatic, polyelectrolyte materials (Stevenson, 1994). Therefore, compost would likely have greater micropore structure when compared with the initial organic wastes. Although it appeared that the surface areas of all the compost chars were less when compared with corn stillage activated carbon (Table 1), several studies suggested that traditional BET-N2 measurement might underestimate the surface area of compost because of the inability of N2 to diffuse into organic matter at the temperature of N2 adsorption (De Jonge and Mittelmeijer-Hazeleger, 1996; Kaiser and Guggenberger, 2003; Makris et al., 2006). Therefore, the compost char might have much greater surface area than the reported measurements shown in Table 1. The char yield was calculated using the weight of resultant compost char divided by the weight of initial dried compost. The yield decreased with increasing temperature when the compost samples were pyrolyzed under both air and N2 conditions (Table 1). When the compost was heated under air at the maximum temperature (420 °C), there was still 46% of the initial mass remaining in the residue, suggesting a substantial mineral content in the compost. When the compost was heated under N2 at 820 °C, the char yield was 52%, which was at least a twice greater yield when compared with the corn stillage char yield of 21.8%. The higher yield of compost char could have been the result of the greater mineral content of the compost when com-
5675
pared with the corn stillage. The higher yield of compost char could have also resulted from the carbon in the compost sample being more thermally stable when compared with that in the corn stillage sample. The compost carbon had greater thermal stability than the initial organic wastes was also suggested by other studies using thermal analysis methods (Dell’Abate et al., 1998; Pietro and Paola, 2004), showing that the organic composition of some easily biodegradable fraction would be converted into aromatic structures during composting.
3.2. The atrazine adsorption by compost chars The atrazine could have been converted into other metabolites in solution, but those portions were relatively small during the initial stage of incubation (Tsui and Roy, 2007). Therefore, the dissipation of atrazine from solution during a 1-day incubation experiment was attributed mainly to adsorption. When compost samples were pyrolyzed under air, the results showed that the unheated compost (25 °C) had greater affinity for atrazine when compared with compost char heated to 120 °C (Fig. 1), although unheated compost sample had a smaller surface area (Table 1). In general, surface area measured by gas adsorption is based on the assumption that organic matter in compost has a fixed-pore volume. However, the unheated compost organic matter in aqueous solution should be visualized as a three-dimensional flexible polymer matrix (Xing and Pignatello, 1997; Pignatello, 1998), so the surface area of unheated organic matter could not be well defined. Therefore, the sorption capacity of unheated compost could not be evaluated simply through its measured surface area. When the compost samples were heated between 220 °C and 370 °C under air, the compost chars sorbed more atrazine when compared with the unheated compost sample (Fig. 1). The greater sorption affinity for atrazine by the
Fig. 1. The sorption isotherms (at 25 °C) for compost chars pyrolyzed under air at temperature ranging from 25° to 370 °C.
5676
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
3.3. The comparison of compost char and corn stillage activated carbon
Fig. 2. The sorption isotherms (at 25 °C) for compost chars pyrolyzed under nitrogen at temperature ranging from 25° to 720 °C.
compost chars could have resulted from the greater surface area; the N2-BET surface area of unheated compost was 2.93 m2/g, while the maximum surface area of pyrolyzed char in air was 22.19 m2/g (Table 1). The higher sorption affinity could have also resulted from the pyrolyzed chars being more hydrophobic because the oxygen content in compost would generally decrease with increased pyrolyzed temperature (Liou, 2004). The compost char heated at 420 °C under air had the least sorption affinity for atrazine among all the samples, corresponding to its smallest surface area (Table 1). When the compost samples were heated under N2, the sorption affinity for atrazine increased with increasing pyrolysis temperature up to 720 °C, and then the affinity decreased afterwards (Fig. 2).
The amount of atrazine sorbed from a 1.7 mg/L solution by the compost, compost chars, and corn stillage activated carbon are compared in Table 2 and Fig. 3. Although compost chars yielded a greater sorption affinity for atrazine when compared with the unheated compost sample, the sorption capacities on per gram of raw material (precursor) base were not significantly larger than the unheated sample (Fig. 3). For the compost chars pyrolyzed under air, all of them sorbed less atrazine per gram of raw material base when compared with the unheated sample, because greater mass was lost during the pyrolysis process. Except for the compost chars heated at 420 °C, the chars pyrolyzed under air had greater sorption capacities for atrazine when compared with the corresponding chars pyrolyzed under nitrogen, but the calculated capacities for atrazine were less on per gram of raw material base (Table 2). The above observations showed that the unheated compost had a greater sorption affinity for atrazine based on per gram of raw material weight. Therefore, if there is sufficient volume available to accommodate filter materials in the biofilter to remove atrazine, it would be more economical to use compost but not compost chars to remove atrazine from solution. The corn stillage activated carbon had N2-BET surface area of 439 m2/g, which was much larger than the compost chars with the maximum surface area of 72 m2/g (Table 1). However, the compost chars that pyrolyzed at 620 °C under N2 had a similar sorption affinity for atrazine on per-gram of heat-treated sample base (Table 2). As mentioned above, it has been reported that N2-BET measurement may not be the correct method to measure the
Table 2 The sorption capacity for atrazine per gram of compost sample (based on initial atrazine concentration of 1.71 mg/L) Temperature (°C)
Adsorption capacity per gram of heat-treated compost (mg/kg)
Adsorption capacitya per gram of raw compost (mg/kg)
Pyrolysis, N2
25 120 220 320 370 420 220 320 420 520 620 720 820
48.79 44.23 55.08 70.35 75.31 36.42 51.68 61.86 70.65 77.23 83.32 84.59 83.64
48.79 40.69 44.16 42.21 40.66 16.75 45.48 44.54 46.63 49.43 49.99 47.37 43.49
Activated carbonb Physical activation
800
83.06
18.09
Sample type Compost Untreated Pyrolysis, air
a
The adsorption per gram of raw compost was calculated by using the sorption capacity per gram of pyrolyzed sample multiple the percentage yield calculated in Table 2. b The corn stillage activated carbon was obtained from Desai (1999).
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
5677
4. Conclusions This study demonstrated that greater char yields were obtained when compost is used as a precursor when compared with corn stillage, suggesting that compost might have been more thermally stable than was the agricultural waste. Although the compost chars had less measured surface area than the corn stillage activated carbon, the compost chars exhibited a comparable sorption affinity for the environmental pollutant atrazine, possibly because of the micropore structure of compost. These results suggested that there are some potential advantages in producing compost chars as an inexpensive alternative to activated carbon for removing other similar environmental pollutants, but more work needs to be done to examine this hypothesis. Acknowledgements We are grateful to Dr. Anthony A. Lizzio formerly with the ISGS for providing corn stillage activated carbon and allowing us to access to his laboratory. We also thank Ms. Sheila Desai for performing the surface area analysis. References
Fig. 3. The comparison sorption capacity for atrazine of compost chars and unheated compost sample: (A) compost chars were pyrolyzed under air, (B) compost char were pyrolyzed under nitrogen.
surface of organic substances. Therefore, the actual surface area of compost chars may have been much larger than the values reported in this study, and thus might help to explain their comparable sorption capacities for atrazine when compared with corn stillage activated carbon. It should be noted that, in order to mimic the atrazine concentration in tile drainage water, the largest atrazine concentration used was 1.71 mg/L in this study. Therefore, it was not possible to compare the extent of atrazine sorption by the compost chars with the corn stillage activated carbon. Nevertheless, the experimental results demonstrated that the compost chars had a comparable sorption behavior when compared with activated carbon within the atrazine concentration range used in this study. Because it was much easier to produce char than activated carbon, it might be economically feasible to use compost chars as an alternative for activated carbon to remove environmental pollutants from solution.
Bansal, R.C., Donnet, J., Stoeckli, F., 1988. Activated Carbon. Marcel Dekker Inc., New York. Carrott, S.P.J.M., Carrott, M.M.L.R., 2007. Ligin-from natural adsorbent to activated carbon: a review. Bioresour. Technol. 98, 2301–2312. De Jonge, H., Mittelmeijer-Hazeleger, M.C., 1996. Adsorption of CO2 and N2 on soil organic matter: nature of porosity, surface area, and diffusion mechanisms. Environ. Sci. Technol. 30, 4008–4013. Dell’Abate, M.T., Canali, S., Trinchera, A., Benedetti, A., Sequi, P., 1998. Thermal analysis in the evaluation of compost stability: a comparison with humification parameters. Nutr. Cycling Agroecosys. 51, 217–224. Desai, S. 1999. Activated carbon from corn-to-ethanol by-product for air pollution control applications. Master Thesis. University of Illinois at Urbana-Champaign. Diao, Y., Walawender, L.T., Fan, L.T., 1999. Production of activated carbon from wheat using phosphoric acid activation. Adv. Environ. Res. 3, 333–342. Diao, Y., Walawender, W.P., Fan, L.T., 2002. Activated carbons prepared from phosphoric acid activation of grain sorghum. Bioresour. Technol. 81, 45–52. Greg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area and Porosity, second ed. Academic Press, London. Kaiser, R., Guggenberger, G., 2003. Mineral surfaces and soil organic matter. Eur. J. Soil Sci. 54, 219–236. Kladviko, E.J., Grochulska, J., Turco, R.F., Van Scoyoc, G.E., Eigel, J.D., 1999. Pesticide and nitrate transport into tile drains in different spacing. J. Environ. Qual. 28, 997–1004. Kolpin, D.W., Thurma, E.M., Goolsby, D.A., 1996. Occurrence of selected pesticides and their metabolites in near-surface aquifers of the Midwestern United States. Environ. Sci. Technol. 30, 335–340. Liou, T.-H., 2004. Evolution of chemistry and morphology during the carbonization and combustion of rice husk. Carbon 42, 785–794. Makris, K.C., Sarkar, D., Datta, R., Ravikovitch, P.I., Neimark, A.V., 2006. Using nitrogen and carbon dioxide molecules to probe arsenic (V) bioaccessibility in soils. Environ. Sci. Technol. 40, 7732–7738. Pietro, M., Paola, C., 2004. Thermal analysis for the evaluation of the organic matter evolution during municipal solid waste aerobic composting process. Thermochim. Acta 413, 209–214.
5678
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
Pignatello, J.J., 1998. Soil organic matter as a nanoporous sorbent of organic pollutants. Adv. Colloid Interf. Sci. 76/77, 445–467. Roy, W.R., Krapac, I.G., Chou, S.F.J., Griffin, R.A. 1992. Batch-Type Procedures for Estimating Soil Adsorption of Chemicals. US Environmental Protection Agency, Technical Resource Document, US EPA/530/-SW-87-006-F. Sharma, R.K., Wooten, J.B., Baliga, V.L., Lin, X., Chan, W.G., Hajaligol, M.R., 2004. Characterization of chars from pyrolysis of lignin. Fuel 83, 1469–1482. Stevenson, F.J., 1994. Humus Chemistry: Genesis, Composition, Reactions, second ed. John Wiley & Sons Inc., New York.
Teng, H., Ho, J.-A., Hsu, Y.-F., 1997. Preparation of activated carbons from bituminous coals with CO2 activation-influence of coal oxidation. Carbon 35, 275–283. Tsui, L., Roy, W.R., 2007. Effects of compost age and composition on atrazine removal from solution. J. Hazard. Mater. B139, 79–85. Tsui, L., Roy, W.R., Cole, M.A., 2003. Removal of dissolved textile dyes from wastewater by a compost sorbent. Color Tech. 1, 14–18. Xing, B., Pignatello, J.J., 1997. Dual-model sorption of low-polarity compounds in glassy poly(vinyl chloride) and soil organic matter. Environ. Sci. Technol. 31, 792–799. Zou, Y., Han, B.-X., 2001. Preparation of activated carbon from Chinese coal and hydrolysis lignin. Adsorp. Sci. Technol. 19, 59–72.
Bioresource Technology 99 (2008) 5673–5678
The potential applications of using compost chars for removing the hydrophobic herbicide atrazine from solution Lo Tsui a,*, William R. Roy b a
Departmental of Safety, Health and Environmental Engineering, Mingchi University of Technology, 84 Gungjuan Road, Taishan, Taipei 24301, Taiwan b Illinois State Geological Survey, 615 East Peabody Drive, Champaign, IL 61820, USA Received 23 August 2007; received in revised form 12 October 2007; accepted 16 October 2007 Available online 20 February 2008
Abstract One commercial compost sample was pyrolyzed to produce chars as a sorbent for removing the herbicide atrazine from solution. The sorption behavior of compost-based char was compared with that of an activated carbon derived from corn stillage. When compost was pyrolyzed, the char yield was greater than 45% when heated under air, and 52% when heated under N2. In contrast, when the corn stillage was pyrolyzed under N2, the yield was only 22%. The N2-BET surface area of corn stillage activated carbon was 439 m2/g, which was much greater than the maximum compost char surface area of 72 m2/g. However, the sorption affinity of the compost char for dissolved atrazine was comparable to that of the corn stillage activated carbon. This similarity could have resulted from the initial organic waste being subjected to a relatively long period of thermal processes during composting, and thus, the compost was more thermally stable when compared with the raw materials. In addition, microorganisms transformed the organic wastes into amorphous humic substances, and thus, it was likely that the microporisity was enhanced. Although this micropore structure could not be detected by the N2-BET method, it was apparent in the atrazine sorption experiment. Overall, the experimental results suggested that the compost sample in current study was a relatively stable material thermally for producing char, and that it has the potential as a feed stock for making highquality activated carbon. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Char; Compost; Surface area; Yield
1. Introduction Atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)1,3,5-triazine) is one of the most commonly occurring herbicides in agricultural tile drainage systems. Substantial research has focused on the environmental fate of this chemical (Kolpin et al., 1996; Kladviko et al., 1999). Our previous study has shown that atrazine can be removed from solution by using a compost bioreactor mainly through adsorption, but that the compost itself may also contribute undesirable qualities, such as color to the receiving water (Tsui and Roy, 2007). It has been suggested that *
Corresponding author. Tel.: +886 2 29089899x4652; fax: +886 2 29041914. E-mail address: [email protected] (L. Tsui). 0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.10.026
any inexpensive material with relatively large carbon content could be potential material to produce activated carbon (Bansal et al., 1988), and activated carbon is one of the most effective adsorbents for removing hydrophobic pollutants. Because compost made from agricultural wastes will likely have a significant carbon content, there is a potential to produce activated carbon from compost. Making activated carbon from compost would not only increase its surface area, it might also reduce the potentially adverse effects of using untreated compost as an adsorbent. The activated carbon can be manufactured through two activation methods: physical activation and chemical activation (Carrott and Carrott, 2007). In the physical activation method, the activation is carried out in a two-step process: carbonization and activation. Carbonization (pyrolysis) is sometimes called charring, which consists of
5674
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
heating the feed material under a controlled temperature. During carbonization, the carbon content of the char increases with temperature accompanied by a simultaneous decrease in oxygen content. The char then undergo the activation process at elevated temperature to enhance the internal pore structures. In the chemical activation method, the precursor is usually impregnated with a strong dehydrating agent, and then heated under a nitrogen-flow to simultaneously form and activate the carbon. However, several studies have suggested that carbonization may be a necessary step in order to produce high surface area activated carbon (Diao et al., 1999, 2002; Zou and Han, 2001), because it creates new cross-links within the precursor and hence enhances the porosity development. Because both physical and chemical activation could involve a two-step process, it would be reasonable to examine the quality of first-stage char first. Hence, the primary goal of this study was to examine the efficacy of creating chars from commercial compost. The compost samples were pyrolyzed under air or nitrogen conditions, and the pyrolyzed chars were then examined for their ability to sorb atrazine from solution. For comparison, one activated carbon produced from a corn-to-ethanol byproduct (corn stillage) was also included in this study to compare its sorption affinity for atrazine. 2. Methods 2.1. Materials Unlabeled atrazine (purity of 99%) was obtained from ChemService (West Chester, PA). Uniformed ring-labeled 14 C-atrazine (specific activity 15.1 mCi/mmol) (purity of 99%) was purchased from Sigma Chemical Co. (St. Louis, MO). The atrazine stock solution was prepared by dissolving 14C-labeled and nonlabeled atrazine in methanol, and then deionized water was added to prepare different concentrations of atrazine. The largest atrazine concentration was 1.7 mg/L, which was about 100 times greater than the typical atrazine concentration found in tile drainage water (Kolpin et al., 1996). The commercial compost sample was collected from one compost facility, and the initial feedstock material was a mixture of yard waste, horse bedding and food waste. Before conducting this study, the compost sample had been stored at room temperature for 3 years. The measured carbon, nitrogen, and ash content of the compost were about 51.5%, 3.5%, and 7%, respectively (Tsui et al., 2003). The activated carbon produced from corn stillage was kindly provided by the Illinois State Geological Survey (Desai, 1999). The corn stillage was first pyrolyzed in flowing N2, followed by physical activation in steam at 800 °C. 2.2. Procedure The bulk sample was screened through a 4-mm sieve before the pyrolysis process. A mass of 50 g of the compost
sample was heated in a 5 cm diameter horizontal quartz tube by a Lindberg type 59344 furnace. The samples were heated in flowing air (1 L/min) at temperatures ranging from 120 °C to 420 °C, and in flowing ultra-high purity N2 from 220 °C to 820 °C for 1 h. After heating, the samples were allowed to cool to room temperature under continuous flowing air or N2 conditions, and the remaining sample mass was recorded. The yield of compost char was defined as the ratio of the weight of the pyrolyzed char to that of the original compost, with both weights on a dry basis. The N2-BET surface areas of the prepared samples were measured in static volumetric apparatus (Monosorb, Quantachrome Corporation). Before N2 adsorption, each sample was outgassed (30% N2 with He balance) for 45 min at 120 °C. Nitrogen adsorption measurement was measured at the temperature of liquid N2 77 K, and the surface area was calculated from the linearlized BET equation (Greg and Sing, 1982). The adsorption study for atrazine was carried out using the batch methods developed by Roy et al. (1992). A mass of 0.5 g of unheated compost or pyrolyzed sample was added to 25 mL solution containing a known amount of 14 C-labeled atrazine in a polyethylene centrifuge tube. The centrifuge tubes were mixed in a rotating tumbler at 23 ± 2 °C until the system reached equilibrium, after which they were centrifuged at 12,000g for 20 min. The 14Clabeled atrazine concentration was determined by using a Packard 2000CA TriCarb liquid scintillation counter, and the amount of adsorbed atrazine was calculated as the difference between the initial atrazine concentration in the original solution and the equilibrated solution. Sorption isotherms were used to describe the sorption behavior. The activated carbon sample produced from corn stillage as described earlier was used for comparisons of sorption performance for atrazine with pyrolyzed compost samples.
3. Results and discussion 3.1. Compost char properties When the compost sample was pyrolyzed under air, the N2-BET surface areas of the resulting chars increased with increasing temperature up to 22.19 m2/g at 370 °C, but decreased to 2.31 m2/g when heated at 420 °C (Table 1), probably because the organic structure of the compost structure when heated at that temperature. The N2-BET surface area of compost chars continued to increase with an increase in temperature up to 720 °C when compost was pyrolyzed under N2. When comparing the compost chars heated under air and N2 at the same temperature, the compost chars produced under air had a greater surface area than the corresponding chars under N2 at temperature less than 420 °C. The results were similar to other studies showing that the chars pyrolyzed with the presence of oxygen exhibited a greater burn-off rate and larger surface area
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678 Table 1 Yield and surface area of pyrolyzed compost chars and corn carbons Sample type Compost Untreated Pyrolysis, air
Pyrolysis, N2
Corn stillage Pyrolysis, N2 Activation, steam
Temperature (°C)
Yield (%)
25 120 220 320 370 420 220 320 420 520 620 720 820
92 80 60 54 46 88 72 66 64 60 56 52
900 800
21.8 15.8
N2-BET surface area (m2/g) 2.93 3.13 10.21 17.72 22.19 2.31 7.62 10.38 16.27 22.39 51.21 72.39 71.63 35 439
when compared with chars heated in pure N2 (Teng et al., 1997; Sharma et al., 2004). The surface areas of the pyrolysis chars and activated carbon produced from corn stillage are compared in Table 1. The results showed that the pyrolysis of compost at 620 °C under N2 yielded a larger N2-BET surface area when compared with corn stillage chars pyrolyzed at 900 °C. During composting, microorganisms converted organic waste into humic substances, which were amorphous, partly aromatic, polyelectrolyte materials (Stevenson, 1994). Therefore, compost would likely have greater micropore structure when compared with the initial organic wastes. Although it appeared that the surface areas of all the compost chars were less when compared with corn stillage activated carbon (Table 1), several studies suggested that traditional BET-N2 measurement might underestimate the surface area of compost because of the inability of N2 to diffuse into organic matter at the temperature of N2 adsorption (De Jonge and Mittelmeijer-Hazeleger, 1996; Kaiser and Guggenberger, 2003; Makris et al., 2006). Therefore, the compost char might have much greater surface area than the reported measurements shown in Table 1. The char yield was calculated using the weight of resultant compost char divided by the weight of initial dried compost. The yield decreased with increasing temperature when the compost samples were pyrolyzed under both air and N2 conditions (Table 1). When the compost was heated under air at the maximum temperature (420 °C), there was still 46% of the initial mass remaining in the residue, suggesting a substantial mineral content in the compost. When the compost was heated under N2 at 820 °C, the char yield was 52%, which was at least a twice greater yield when compared with the corn stillage char yield of 21.8%. The higher yield of compost char could have been the result of the greater mineral content of the compost when com-
5675
pared with the corn stillage. The higher yield of compost char could have also resulted from the carbon in the compost sample being more thermally stable when compared with that in the corn stillage sample. The compost carbon had greater thermal stability than the initial organic wastes was also suggested by other studies using thermal analysis methods (Dell’Abate et al., 1998; Pietro and Paola, 2004), showing that the organic composition of some easily biodegradable fraction would be converted into aromatic structures during composting.
3.2. The atrazine adsorption by compost chars The atrazine could have been converted into other metabolites in solution, but those portions were relatively small during the initial stage of incubation (Tsui and Roy, 2007). Therefore, the dissipation of atrazine from solution during a 1-day incubation experiment was attributed mainly to adsorption. When compost samples were pyrolyzed under air, the results showed that the unheated compost (25 °C) had greater affinity for atrazine when compared with compost char heated to 120 °C (Fig. 1), although unheated compost sample had a smaller surface area (Table 1). In general, surface area measured by gas adsorption is based on the assumption that organic matter in compost has a fixed-pore volume. However, the unheated compost organic matter in aqueous solution should be visualized as a three-dimensional flexible polymer matrix (Xing and Pignatello, 1997; Pignatello, 1998), so the surface area of unheated organic matter could not be well defined. Therefore, the sorption capacity of unheated compost could not be evaluated simply through its measured surface area. When the compost samples were heated between 220 °C and 370 °C under air, the compost chars sorbed more atrazine when compared with the unheated compost sample (Fig. 1). The greater sorption affinity for atrazine by the
Fig. 1. The sorption isotherms (at 25 °C) for compost chars pyrolyzed under air at temperature ranging from 25° to 370 °C.
5676
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
3.3. The comparison of compost char and corn stillage activated carbon
Fig. 2. The sorption isotherms (at 25 °C) for compost chars pyrolyzed under nitrogen at temperature ranging from 25° to 720 °C.
compost chars could have resulted from the greater surface area; the N2-BET surface area of unheated compost was 2.93 m2/g, while the maximum surface area of pyrolyzed char in air was 22.19 m2/g (Table 1). The higher sorption affinity could have also resulted from the pyrolyzed chars being more hydrophobic because the oxygen content in compost would generally decrease with increased pyrolyzed temperature (Liou, 2004). The compost char heated at 420 °C under air had the least sorption affinity for atrazine among all the samples, corresponding to its smallest surface area (Table 1). When the compost samples were heated under N2, the sorption affinity for atrazine increased with increasing pyrolysis temperature up to 720 °C, and then the affinity decreased afterwards (Fig. 2).
The amount of atrazine sorbed from a 1.7 mg/L solution by the compost, compost chars, and corn stillage activated carbon are compared in Table 2 and Fig. 3. Although compost chars yielded a greater sorption affinity for atrazine when compared with the unheated compost sample, the sorption capacities on per gram of raw material (precursor) base were not significantly larger than the unheated sample (Fig. 3). For the compost chars pyrolyzed under air, all of them sorbed less atrazine per gram of raw material base when compared with the unheated sample, because greater mass was lost during the pyrolysis process. Except for the compost chars heated at 420 °C, the chars pyrolyzed under air had greater sorption capacities for atrazine when compared with the corresponding chars pyrolyzed under nitrogen, but the calculated capacities for atrazine were less on per gram of raw material base (Table 2). The above observations showed that the unheated compost had a greater sorption affinity for atrazine based on per gram of raw material weight. Therefore, if there is sufficient volume available to accommodate filter materials in the biofilter to remove atrazine, it would be more economical to use compost but not compost chars to remove atrazine from solution. The corn stillage activated carbon had N2-BET surface area of 439 m2/g, which was much larger than the compost chars with the maximum surface area of 72 m2/g (Table 1). However, the compost chars that pyrolyzed at 620 °C under N2 had a similar sorption affinity for atrazine on per-gram of heat-treated sample base (Table 2). As mentioned above, it has been reported that N2-BET measurement may not be the correct method to measure the
Table 2 The sorption capacity for atrazine per gram of compost sample (based on initial atrazine concentration of 1.71 mg/L) Temperature (°C)
Adsorption capacity per gram of heat-treated compost (mg/kg)
Adsorption capacitya per gram of raw compost (mg/kg)
Pyrolysis, N2
25 120 220 320 370 420 220 320 420 520 620 720 820
48.79 44.23 55.08 70.35 75.31 36.42 51.68 61.86 70.65 77.23 83.32 84.59 83.64
48.79 40.69 44.16 42.21 40.66 16.75 45.48 44.54 46.63 49.43 49.99 47.37 43.49
Activated carbonb Physical activation
800
83.06
18.09
Sample type Compost Untreated Pyrolysis, air
a
The adsorption per gram of raw compost was calculated by using the sorption capacity per gram of pyrolyzed sample multiple the percentage yield calculated in Table 2. b The corn stillage activated carbon was obtained from Desai (1999).
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
5677
4. Conclusions This study demonstrated that greater char yields were obtained when compost is used as a precursor when compared with corn stillage, suggesting that compost might have been more thermally stable than was the agricultural waste. Although the compost chars had less measured surface area than the corn stillage activated carbon, the compost chars exhibited a comparable sorption affinity for the environmental pollutant atrazine, possibly because of the micropore structure of compost. These results suggested that there are some potential advantages in producing compost chars as an inexpensive alternative to activated carbon for removing other similar environmental pollutants, but more work needs to be done to examine this hypothesis. Acknowledgements We are grateful to Dr. Anthony A. Lizzio formerly with the ISGS for providing corn stillage activated carbon and allowing us to access to his laboratory. We also thank Ms. Sheila Desai for performing the surface area analysis. References
Fig. 3. The comparison sorption capacity for atrazine of compost chars and unheated compost sample: (A) compost chars were pyrolyzed under air, (B) compost char were pyrolyzed under nitrogen.
surface of organic substances. Therefore, the actual surface area of compost chars may have been much larger than the values reported in this study, and thus might help to explain their comparable sorption capacities for atrazine when compared with corn stillage activated carbon. It should be noted that, in order to mimic the atrazine concentration in tile drainage water, the largest atrazine concentration used was 1.71 mg/L in this study. Therefore, it was not possible to compare the extent of atrazine sorption by the compost chars with the corn stillage activated carbon. Nevertheless, the experimental results demonstrated that the compost chars had a comparable sorption behavior when compared with activated carbon within the atrazine concentration range used in this study. Because it was much easier to produce char than activated carbon, it might be economically feasible to use compost chars as an alternative for activated carbon to remove environmental pollutants from solution.
Bansal, R.C., Donnet, J., Stoeckli, F., 1988. Activated Carbon. Marcel Dekker Inc., New York. Carrott, S.P.J.M., Carrott, M.M.L.R., 2007. Ligin-from natural adsorbent to activated carbon: a review. Bioresour. Technol. 98, 2301–2312. De Jonge, H., Mittelmeijer-Hazeleger, M.C., 1996. Adsorption of CO2 and N2 on soil organic matter: nature of porosity, surface area, and diffusion mechanisms. Environ. Sci. Technol. 30, 4008–4013. Dell’Abate, M.T., Canali, S., Trinchera, A., Benedetti, A., Sequi, P., 1998. Thermal analysis in the evaluation of compost stability: a comparison with humification parameters. Nutr. Cycling Agroecosys. 51, 217–224. Desai, S. 1999. Activated carbon from corn-to-ethanol by-product for air pollution control applications. Master Thesis. University of Illinois at Urbana-Champaign. Diao, Y., Walawender, L.T., Fan, L.T., 1999. Production of activated carbon from wheat using phosphoric acid activation. Adv. Environ. Res. 3, 333–342. Diao, Y., Walawender, W.P., Fan, L.T., 2002. Activated carbons prepared from phosphoric acid activation of grain sorghum. Bioresour. Technol. 81, 45–52. Greg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area and Porosity, second ed. Academic Press, London. Kaiser, R., Guggenberger, G., 2003. Mineral surfaces and soil organic matter. Eur. J. Soil Sci. 54, 219–236. Kladviko, E.J., Grochulska, J., Turco, R.F., Van Scoyoc, G.E., Eigel, J.D., 1999. Pesticide and nitrate transport into tile drains in different spacing. J. Environ. Qual. 28, 997–1004. Kolpin, D.W., Thurma, E.M., Goolsby, D.A., 1996. Occurrence of selected pesticides and their metabolites in near-surface aquifers of the Midwestern United States. Environ. Sci. Technol. 30, 335–340. Liou, T.-H., 2004. Evolution of chemistry and morphology during the carbonization and combustion of rice husk. Carbon 42, 785–794. Makris, K.C., Sarkar, D., Datta, R., Ravikovitch, P.I., Neimark, A.V., 2006. Using nitrogen and carbon dioxide molecules to probe arsenic (V) bioaccessibility in soils. Environ. Sci. Technol. 40, 7732–7738. Pietro, M., Paola, C., 2004. Thermal analysis for the evaluation of the organic matter evolution during municipal solid waste aerobic composting process. Thermochim. Acta 413, 209–214.
5678
L. Tsui, W.R. Roy / Bioresource Technology 99 (2008) 5673–5678
Pignatello, J.J., 1998. Soil organic matter as a nanoporous sorbent of organic pollutants. Adv. Colloid Interf. Sci. 76/77, 445–467. Roy, W.R., Krapac, I.G., Chou, S.F.J., Griffin, R.A. 1992. Batch-Type Procedures for Estimating Soil Adsorption of Chemicals. US Environmental Protection Agency, Technical Resource Document, US EPA/530/-SW-87-006-F. Sharma, R.K., Wooten, J.B., Baliga, V.L., Lin, X., Chan, W.G., Hajaligol, M.R., 2004. Characterization of chars from pyrolysis of lignin. Fuel 83, 1469–1482. Stevenson, F.J., 1994. Humus Chemistry: Genesis, Composition, Reactions, second ed. John Wiley & Sons Inc., New York.
Teng, H., Ho, J.-A., Hsu, Y.-F., 1997. Preparation of activated carbons from bituminous coals with CO2 activation-influence of coal oxidation. Carbon 35, 275–283. Tsui, L., Roy, W.R., 2007. Effects of compost age and composition on atrazine removal from solution. J. Hazard. Mater. B139, 79–85. Tsui, L., Roy, W.R., Cole, M.A., 2003. Removal of dissolved textile dyes from wastewater by a compost sorbent. Color Tech. 1, 14–18. Xing, B., Pignatello, J.J., 1997. Dual-model sorption of low-polarity compounds in glassy poly(vinyl chloride) and soil organic matter. Environ. Sci. Technol. 31, 792–799. Zou, Y., Han, B.-X., 2001. Preparation of activated carbon from Chinese coal and hydrolysis lignin. Adsorp. Sci. Technol. 19, 59–72.