Physically (CO₂) activated hydrochars from hickory and peanut hull: preparation, characterization, and sorption of methylene blue, lead, copper, and cadmium

Abstract

The effects of CO₂ activation temperature (600–900 °C) and time (1 and 2 h) on the physicochemical and sorptive characteristics of hickory and peanut hull hydrochars were investigated. The extent of burn-off increased with increasing activation times and temperatures, and ranged from 34–54% and 28–50% for activated hydrochars (AHCs), respectively. The surface area and pore volume of the AHCs also increased with activation time and temperature and were much higher than those of their corresponding non-activated parent hydrochars. In general, the physical activation improved the ability of all AHCs to sorb methylene blue, lead (Pb²⁺), copper (Cu²⁺), and cadmium (Cd²⁺) from aqueous solutions. AHCs created at 900 °C had the best sorption ability, and the highest sorption rate was usually observed in 900 °C 2 h AHCs. The sorption of methylene blue and the three heavy metals was strongly correlated with AHC surface area, suggesting that the adsorption occurred via site-specific interactions.

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1. Introduction

Hydrothermal carbonization (HTC) has attracted recent attention as a technique to convert biomass to useful carbonaceous products, due to several advantages it has over dry pyrolysis. Among these is its greater energy efficiency, because of higher yields of solids without need for energy-intensive drying compared to dry pyrolysis.¹ Hydrochar, the resulting solid carbonaceous product from HTC,¹ may be used for many of the same purposes as its dry pyrolysis counterpart, biochar, such as contaminant adsorption from soil and water, fuel, waste processing, and as a soil amendment for the purposes of carbon sequestration and fertility enhancement.^2–6 However, compared to biochar produced by dry pyrolysis, hydrochars generally have lower surface areas and pore volumes and higher volatile organic matter content, which are generally less desirable characteristics for most applications, particularly contaminant adsorption.^7,8

Activation, which commonly increases surface area and pore volume of pyrogenic biochar,⁹ might also improve the characteristics of hydrochar. Hydrochars often have high VOC contents that may not be removed through washing.^10,11 An additional benefit of activation may be the removal of volatile matter through evaporation during heating. In addition, the low ash content of hydrochar makes it a more attractive option for activation than char created through dry pyrolysis. This is because the presence of minerals may hinder pore and surface area development during the charring and activation processes.^12,13

Activated carbons can be produced either through physical or chemical activation. Physical activation is similar to gasification with an oxidizing agent, commonly CO₂ or steam, at relatively high temperatures, typically between 600 and 1200 °C.¹⁴ In contrast, chemical activation uses acids, bases, or salts to change the properties of a material's surface.^13,14 While coconut shells are often used in commercial production of activated carbon because of its low ash content, activated carbonaceous materials from a variety of alternative feedstocks, such as eucalyptus, coir pith, palm stones, and corn cobs, have been shown to be highly effective in the removal of contaminants such as dyes and heavy metals.^15–18

While several studies have examined the sorptive properties of hydrochar and activated hydrochars (AHCs) for various contaminants,^19,20 few have examined the effects of activation conditions on AHC physicochemical properties and its resulting sorption ability. Thus, the objectives of this study were to investigate the effects of activation time and temperature during physical activation using CO₂ on the physicochemical characteristics of hickory and peanut hull hydrochars. The AHCs were tested for their ability to sorb a variety of contaminants (methylene blue, lead, copper, and cadmium) from aqueous solutions in batch reactions. Finally, the relationships between the AHC physicochemical properties and their sorption characteristics were examined in order to identify predominant sorption mechanisms.

2. Materials and methods

2.1. Hydrochar production and activation

Hickory and peanut hull hydrochars were produced using the same procedure previously described.⁸ Briefly, feedstocks were milled to particle sizes of 0.5–1 mm and added to a pressurizable lidded stainless steel pot to a height of about one inch from the top (50 g of hickory and 55 g of peanut hull). Deionized (DI) water was added to the same level (290 and 313 mL of water, respectively). The pots were then sealed and heated on a hotplate to 200 °C for six hours (the HTC conditions to produce hydrochar with the highest yield and surface area⁸). The resulting hydrochars were rinsed for one hour by submersion in tap water and ten minutes in DI water to remove water soluble volatile matter, and oven dried for 24 hours at 70 °C. For activation, 5.0 g of the hydrochar were heated in a quartz tube furnace for either 1 or 2 hours, and at temperatures of 600, 700, 800 and 900 °C under CO₂ gas flowing at a rate of 150 mL min⁻¹. The AHCs were then allowed to cool to room temperature and were stored in an airtight container prior to characterization. The percentage of feedstock mass loss due to activation (burn-off degree) was calculated as:
where W₀ is the original mass of hydrochar and W is the mass yield of activated hydrochar.

2.2. Activated hydrochar characterization

A CHN Elemental Analyzer (Carlo-Erba NA-1500) was used to determine the total carbon, hydrogen, and nitrogen contents of the activated hydrochars. Samples were analyzed in duplicate and the averages are reported here. The surface areas (SA) of the samples were measured by N₂ sorptometry on a Quantachrome Autosorb I at 77 K and using the Brunauer–Emmett–Teller (BET) method in the 0.01 to 0.3 relative pressure range of the N₂ adsorption isotherm. Pore volumes (PV) were calculated from the desorption branch N₂ isotherms using Barrett–Joyner–Halenda (BJH) theory. Samples were de-gassed under vacuum at least 24 h at 180 °C prior to analysis.

2.3. Batch sorption experiments

Batch aqueous contaminant sorption experiments were carried out in triplicate at concentrations previously determined to be in the upper ends of their sorption ranges: methylene blue (700 ppm), lead (500 ppm), copper (20 ppm) and cadmium (20 ppm). All chemicals used were analytical grade and obtained from Fisher Scientific Co. and solutions were prepared using DI water (Nanopure water, Barnstead). Mixtures of 30 mL of each solution and 0.1 g of AHC in 68 mL digestion vessels were agitated on a mechanical shaker at 40 rpm for 24 hours. The solutions were then immediately filtered through 0.45 μm filter paper (Whatman). Aqueous lead, copper, and cadmium concentrations were analyzed using inductively coupled plasma spectroscopy (ICP-AES, Perkin Elmer Optima 2100 DV), and methylene blue was measured with a UV spectrometer (Thermo Scientific EVO 60) at a wavelength of 665 nm. The sorption rates of the AHCs were calculated as the difference between starting and final sorbate solution concentrations relative to the amount of sorbent (mmol kg⁻¹). One-way ANOVA tests were conducted using Minitab software to determine the statistical significance of the difference between each AHC's sorption ability and its nonactivated hydrochar counterpart.

3. Results and discussion

3.1. Physicochemical characteristics

Burn-off degree was generally greater for AHCs made from hickory versus peanut hull, varying from 50–72% and from 42–66%, respectively (Table 1). After activation, the hydrochars turned from dark brown to black (Fig. S1, ESI†), but no differences in color were observed between the different activation conditions. The differences in burn-off degree can partly be attributed to the structural composition of the two feedstocks. Peanut hull has a lignin content of 28%, compared to 18% for hickory.^21,22 Feedstocks with high lignin content yield more solids when carbonized, while the holocellulosic fraction is more volatile.¹⁴ Burn-off degree was generally similar for hydrochars activated at 600 and 700 °C for both activation times of 1 and 2 h, but increased when activated at 800 °C for 2 hours and at 900 °C for both 1 and 2 hours. These values are comparable to those found for other activated carbon materials. For example, activated carbons produced from oak, corn hulls, corn stover, eucalyptus, and wattle leaves had burn-off rates of 31.8–51.4% at 700–800 °C, 32.3–45.2% at 700–800 °C, 41.5–50.2% at 700–800 °C, 20.6–83% at 600–900 °C, and 21–67.1% at 600–900 °C, respectively.^15,16,23

Table 1 Bulk properties of activated hickory and peanut hull hydrochars^a

Feedstock	Activation temperature (°C)	Activation time (h)	Burn-off (%)	BET-SA (m^2 g^−1)	BJH-PV (mL g^−1)	C (wgt%)	H (wgt%)	N (wgt%)
a SA = surface area and PV = pore volume.
Hickory	Nonactivated	—	—	8	0.121	68.7	5.3	0.2
	600	1	50	445	0.038	85.1	2.8	0.2
	600	2	52	453	0.050	88.1	2.8	0.2
	700	1	54	441	0.044	90.1	1.6	0.2
	700	2	54	465	0.049	90.3	0.6	0.3
	800	1	56	474	0.040	91.0	0.4	0.3
	800	2	70	667	0.041	89.1	0.4	0.3
	900	1	66	703	0.053	90.6	0.7	0.4
	900	2	72	928	0.054	90.5	0.2	0.4
Peanut hull	Nonactivated	—	—	7	0.010	70.6	6.0	1.9
	600	1	46	310	0.079	70.6	6.0	1.9
	600	2	48	353	0.075	84.8	2.0	2.3
	700	1	48	349	0.067	86.5	2.2	2.3
	700	2	46	365	0.114	88.7	1.5	2.3
	800	1	48	345	0.056	87.7	1.4	2.2
	800	2	58	488	0.069	87.7	1.2	2.3
	900	1	66	988	0.121	86.6	2.0	2.6
	900	2	62	1308	0.114	86.5	2.9	2.2

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Hickory and peanut hull AHC surface areas varied from 441–928 m² g⁻¹ and from 309–1308 m² g⁻¹, respectively. Burn-off degree was strongly correlated with surface area (R² = 0.86 and 0.76 for hickory and peanut hull, respectively). The two parameters generally increased with an increased activation temperature or time (Fig. 1). As to the burn-off degree, SA increased most dramatically with an additional hour of treatment at 800 or 900 °C or with the temperature step increase from 800 to 900 °C. Pore volume, however, did not show a clear relationship with activation time or temperature, but was greater for all AHC made from peanut hull. Interestingly, activation dramatically increased the surface area of hickory hydrochar but decreased the pore volume almost three times, on average. The decrease in pore volume could be attributed to the fact that at higher activation temperatures and longer activation times, the walls between pores could be destroyed, causing adjacent pores to fuse and create ones with larger diameters.¹⁵ Sometimes, a sintering effect is created, where the pore walls collapse and decrease surface area due to the filling in of the pores.¹⁸ Other times, however, the destruction of walls increases the size of the pores and decreases microporosity, but total pore volume and surface area still increase. Thus, microporosity increases with increasing temperature and activation time, but after a certain point, pore diameters begins to increase and may or may not be accompanied by an increase in surface area. These patterns show that physical activation by CO₂ gasification has varying effects depending on biomass types. Apparently, removal of volatiles during activation increased the number of very small pores in the case of hickory hydrochar, while larger pores were opened during activation of peanut hull hydrochar. These volatiles of both hydrochars may be relatively H-rich and C-poor, as the C content increased and the H content decreased significantly following activation (Table 1).

Fig. 1 Relationship between hydrochar burn-off degree and surface area.

3.2. Hydrochar batch sorption

Sorption of methylene blue (MB), lead (Pb), copper (Cu), and cadmium (Cd) by the AHCs ranged from 109.5–597.8, 36–225.4, 17.4–72.3, and 1.1–17.9 mmol kg⁻¹, respectively (Fig. 2). While all the tested samples removed the contaminants from the solutions, the AHCs generally adsorbed more than the non-activated hydrochars. Previous studies have shown that the surfaces of the two non-activated hydrochars are negatively charges (with zeta potential values of −25.91 and −29.51 mV, respectively),⁸ enabling them to adsorb the cationic MB and heavy metals through electrostatic attractions. Further, the CO₂ activation process can introduce acidic functional groups to carbon surfaces,²⁴ which might make the surfaces of the AHCs more negatively charged to promote the electrostatic attractions.

Fig. 2 Removal of (a) methylene blue, (b) Pb, (c) Cu, and (d) Cd by activated hydrochars from aqueous solution. * indicates significantly different from the nonactivated.

ANOVA tests showed that all of the AHCs sorbed significantly more methylene blue and lead than their respective non-activated hydrochars (p < 0.05). For hickory AHCs activated for 2 h at 600 °C, 1 h at 700 °C, and 1 h at 800 °C and peanut hull AHCs activated at 600 °C, sorption of copper did not differ from their non-activated hydrochars. Hickory AHCs sorbed significantly more Cd only when activated for 2 h at 800 °C and for 1 or 2 h at 900 °C, and peanut hull AHCs only when activated for 2 h at 700 °C and for 1 or 2 h at 800 °C and 900 °C.

The methylene blue and lead sorption rates of all of the AHCs in this work were below those maximum capacities of the commercial activated carbons (which range from 656.6–931.7 and 231.7–395.8 mmol kg⁻¹ for methylene blue and lead, respectively;^25–28), however, the sorption rates of the AHCs made at 900° were only slightly lower. Because the sorption rates obtained in this work were obtained only at one initial sorbate concentration, they might be much lower than the maximum capacities obtained from the sorption isotherms. The sorption properties of the AHCs, particularly the ones activated at 900°, thus were comparable to that of the commercial activated carbons.

The sorption rates for all of the sorbates were positively correlated to AHC surface area (Fig. 3) and the relationship was strongest for methylene blue and lead. This result indicates that the mechanism of interaction was via site specific interactions, i.e. surface adsorption. The amount of methylene blue sorbed by the AHC increased most when the activating temperature was increased to 900 °C (Fig. 2a). And although hickory AHCs showed a higher adsorption rate than the corresponding peanut hull activated at lower temperatures peanut hull AHC sorbed more methylene blue when produced at 900 °C. This pattern corresponds to the pattern or surface area for these AHCs (Table 1). Other studies have found the surface area of activated carbons to be correlated with adsorption rate of methylene blue. For example, activated carbon produced from olive waste cakes and Posidonia oceanica (L.) leaves adsorbed more methylene blue when activation conditions were adjusted to increase surface area.^29,30 The lignin contents of peanut hull and hickory also likely play a role. Lignin is usually the component that contributes to microporosity, and with peanut hull having higher amounts of lignin, the number of micropores would also be lower.¹⁴ Thus, more methylene blue is adsorbed by hickory, which has less lignin and consequently has more pores with larger diameters than peanut hull. Methylene blue is a large molecule that would not be able to be fit into the smaller micropores on the surface of the AHCs.

Fig. 3 Relationship between surface area and amount of (a) methylene blue, Pb (b), Cu (c), and Cd (d) sorbed by activated hydrochars.

Hickory AHCs sorbed more lead than peanut hull AHCs for all activation conditions (Fig. 3b), and the increases in sorption rate at 900 °C were not as large as they were for methylene blue. For peanut hull, the sorption rates of copper increased gradually with activation conditions, despite the dramatic increase in surface area at 900 °C. In addition, peanut hull oftentimes sorbed more copper at activation temperatures less than 900 °C. Weaker trends in cadmium sorption patterns with surface area were also observed for AHC made from both feedstocks (Fig. 3d). These weaker correlations between AHC surface area and copper and cadmium sorption suggest that these metals did not bind at all the available AHC binding sites. The size of the hydrated ionic radii of the metals likely explains the differences in their ability to be sorbed by the AHCs, where sorption rate is inversely correlated with hydrated ionic radii. The hydrated ionic radii of the metals follow the order Cd²⁺ (4.26 Å) > Cu²⁺ (4.19 Å) > Pb²⁺ (4.01 Å). As the hydrated ionic radius increases, the ion becomes more distant from the surface it is to be adsorbed onto. Thus, the sorption ability is stronger for ions with a smaller hydrated ionic radii.³¹

AHC pore volume was not correlated with sorption rates for any of the contaminants, further suggesting that sorption occurred by a surface site-related, rather than pore-filling mechanism (Fig. 4). Mixed results in the relationship between pore volume and sorption rates have been observed; while methylene blue sorption increased with pore volume of activated carbons made from Posidonia oceanica (L.) leaves, there was no such relationship for activated carbons made from coal or coconut shells.^24,30

Fig. 4 Relationship between pore volume and amount of (a) methylene blue, Pb (b), Cu (c), and Cd (d) sorbed by activated hydrochars.

4. Conclusions

CO₂ activation of hydrochar resulted in an increase in surface area, which led to greater sorption of methylene blue and lead at all activation temperatures, but cadmium and copper sorption increased only at activation temperatures of 700–900 °C. In general, the higher the temperature and the longer the activation time, the greater the ACH ability to adsorb a range of contaminant from aqueous solution. Because hydrothermal carbonization is more energy efficient than dry pyrolysis, the use of ACHs may prove to be a more cost-effective and environmentally friendly alternative to traditional activated carbons for environmental applications.

Acknowledgements

This research was partially supported by the NSF through CBET-1054405.

References

1. J. A. Libra, K. S. Ro, C. Kammann, A. Funke, N. D. Berge, Y. Neubauer, M. M. Titirici, C. Fuhner, O. Bens, J. Kern and K. H. Emmerich, Biofuels, 2011, 2, 71–106.
2. X. Lu, B. Jordan and N. D. Berge, Waste Manage., 2012, 32, 1353–1365.
3. C. Kammann, S. Ratering, C. Eckhard and C. Mueller, J. Environ. Qual., 2012, 41, 1052–1066.
4. Y. Xue, B. Gao, Y. Yao, M. Inyang, M. Zhang, A. R. Zimmerman and K. S. Ro, Chem. Eng. J., 2012, 200, 673–680.
5. K. S. Ro, J. M. Novak, M. G. Johnson, A. A. Szogi, J. A. Libra, K. A. Spokas and S. Bae, Chemosphere, 2016, 142, 92–99.
6. K. Sun, K. S. Ro, M. Guo, J. M. Novak, H. Mashayekhi and B. Xing, Bioresour. Technol., 2011, 102, 5757–5763.
7. R. Becker, U. Dorgerloh, M. Helmis, J. Mumme, M. Diakite and I. Nehls, Bioresour. Technol., 2013, 130, 621–628.
8. J. Fang, B. Gao, J. Chen and A. R. Zimmerman, Chem. Eng. J., 2015, 267, 253–259.
9. D. Angin, E. Altintig and T. E. Kose, Bioresour. Technol., 2013, 148, 542–549.
10. W. Buss and O. Masek, J. Environ. Manage., 2014, 137, 111–119.
11. I. Bargmann, M. C. Rillig, W. Buss, A. Kruse and M. Kuecke, J. Agron. Crop Sci., 2013, 199, 360–373.
12. M. Ahmedna, W. E. Marshall and R. M. Rao, Bioresour. Technol., 2000, 71, 113–123.
13. C. F. Chang, C. Y. Chang and W. T. Tsai, J. Colloid Interface Sci., 2000, 232, 45–49.
14. Suhas, P. J. M. Carrott and M. M. L. R. Carrott, Bioresour. Technol., 2007, 98, 2301–2312.
15. T. Y. Zhang, W. P. Walawender, L. T. Fan, M. Fan, D. Daugaard and R. C. Brown, Chem. Eng. J., 2004, 105, 53–59.
16. Y. Ngernyen, C. Tangsathitkulchai and M. Tangsathitkulchai, Korean J. Chem. Eng., 2006, 23, 1046–1054.
17. C. Namasivayam and D. Kavitha, Dyes Pigm., 2002, 54, 47–58.
18. J. Guo and A. C. Lua, J. Anal. Appl. Pyrolysis, 1998, 46, 113–125.
19. A. S. Mestre, E. Tyszko, M. A. Andrade, M. Galhetas, C. Freire and A. P. Carvalho, RSC Adv., 2015, 5, 19696–19707.
20. W. M. Hao, E. Bjorkman, M. Lilliestrale and N. Hedin, Ind. Eng. Chem. Res., 2014, 53, 15389–15397.
21. C. Guler, Y. Copur and M. Tascioglu, Bioresour. Technol., 2008, 99, 2893–2897.
22. J. A. Maga, J. Agric. Food Chem., 1986, 34, 249–251.
23. A. C. Lua, T. Yang and J. Guo, J. Anal. Appl. Pyrolysis, 2004, 72, 279–287.
24. S. B. Wang, Z. H. Zhu, A. Coomes, F. Haghseresht and G. Q. Lu, J. Colloid Interface Sci., 2005, 284, 440–446.
25. N. Kannan and M. M. Sundaram, Dyes Pigm., 2001, 51, 25–40.
26. B. G. P. Kumar, K. Shivakamy, L. R. Miranda and M. Velan, J. Hazard. Mater., 2006, 136, 922–929.
27. M. K. Aroua, S. P. P. Leong, L. Y. Teo, C. Y. Yin and W. M. A. W. Daud, Bioresour. Technol., 2008, 99, 5786–5792.
28. M. Inyang, B. Gao, W. Ding, P. Pullammanappallil, A. R. Zimmerman and X. Cao, Sep. Sci. Technol., 2011, 46, 1950–1956.
29. A. Bacaoui, A. Yaacoubi, A. Dahbi, C. Bennouna, R. P. T. Luu, F. J. Maldonado-Hodar, J. Rivera-Utrilla and C. Moreno-Castilla, Carbon, 2001, 39, 425–432.
30. M. U. Dural, L. Cavas, S. K. Papageorgiou and F. K. Katsaros, Chem. Eng. J., 2011, 168, 77–85.
31. S. B. Chen, Y. B. Ma, L. Chen and K. Xian, Geochem. J., 2010, 44, 233–239.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01644h

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