Activated bio-chars derived from rice husk via one- and two-step KOH-catalyzed pyrolysis for phenol adsorption
Graphical abstract
Introduction
Biomass can be converted to fuels and chemicals via thermochemical processes such as hydrothermal processes (e.g., hydrothermal carbonization/liquefaction), pyrolysis, and gasification (Wang et al., 2017). Among them, pyrolysis has been extensively applied as a promising platform to valorize various types of biomass (Wang et al., 2017; Shen et al., 2016). Generally, pyrolysis of biomass or wastes at relative higher temperatures produces biochar, bio-oil and syngas with higher heating values (Shen et al., 2016). In addition, the biochar can be fabricated into activated carbons that are used as adsorbents and catalyst supports (Shen, 2015).
As an abundant agricultural bio-waste, rice husk (RH) with low combustion value and potential damage to the environment, can bring the issue of its sustainable utilization to the research hotspot (Alvarez et al., 2014a; Satayeva et al., 2018). However, RH can be converted into biofuels (e.g., syngas) through the pyrolysis at high temperatures (Shen et al., 2014a, Shen et al., 2015b; Alvarez et al., 2014b; Zhao and Li, 2016). RH is composed mainly of lignin (20–30%), holo-cellulose (55–65%), SiO2 (15–20%) and extracts (2–5%), which can be regarded as a natural organic-inorganic composite (Shen, 2017a, Shen, 2017b). Therefore, carbonization or activation of the lignin-rich biomass (e.g., RH) in an inert atmosphere yields highly porous carbons with large surface areas (Shen et al., 2014b; Correa et al., 2017). The activation methods include physical activation (Alvarez et al., 2015, Alvarez et al., 2014b), chemical activation (Yahya et al., 2015), and integrated process (Prauchner et al., 2016). Additionally, the self-activation by the gases emitted from biomass pyrolysis not only saves the cost of activating agents, but decreases the environmental impact (Xia and Shi, 2016). In the physical activation, the biochar is activated at a very high temperature (>900 °C) in the presence of steam or CO2. The chemical activation usually involves the impregnation of activating agents, such as potassium hydroxide (KOH), potassium carbonate (K2CO3), zinc chloride (ZnCl2), and phosphoric acid (H3PO4) with biochar followed by activation in an inert atmosphere (Prauchner et al., 2016; Muniandy et al., 2014). Compared to the former, the latter is more efficient because of its lower consumption of energy and time. In particular, the chemical activation with the KOH has been extensively developed for the synthesis of activated bio-carbons (Singh et al., 2017; K. Yang et al., 2018; Laksaci et al., 2017).
Otowa et al. (1993) proposed the KOH activation mechanism as shown in (R1), (R2), (R3), (R4). As the 700 °C temperature condition is the point at which KOH activation occurs, the activated carbon was produced at 750 °C for RH char (Nam et al., 2018). At an initial activation stage, the K2O and K2CO3 below 700 °C, as indicated in Eqs. (R1), (R2), are formed over the combination of two reactions: water-gas reaction (C + H2O → CO + H2) and water-gas-shift (WGS) reaction (CO + H2O → CO2 + H2). After the complete consumption of KOH, the produced K2O and K2CO3 above 700 °C were substantially reduced by forming metallic K, K2O and CO as indicated in Eqs. (R3), (R4).
The KOH activation is a compelling method for creating highly microporous structure and functional groups on the surface of the carbons due to the intercalation of K between the lattices, joint oxidation of carbon and activation of carbon with in-situ formed CO2 during the high-temperature process (D. Zhang et al., 2016; C. Zhang et al., 2016; Li et al., 2015; Wang and Kaskel, 2012). The KOH activation process generally undergoes two steps, including activation of biomass with the KOH solution and subsequent carbonization via calcination at elevated temperatures (Li et al., 2015; Wang and Kaskel, 2012). Herein, the activation process can be simplified by replacing the liquid KOH with solid KOH. Table 1 presents the comparison of activated carbons derived from RH. From these works, some key points can be summarized: (1) Chemical activation under the inert environment (e.g., N2) is extensively used because of its efficient at relatively lower temperatures; (2) Chemical activation generally produces a relatively high SBET; (3) the KOH is widely used an activating agent due to its excellent catalytic effect on carbon gasification; and (4) Activation of RHC (i.e. two-step) is widely used, since the initial pyrolysis of biomass at a relatively lower temperature produces a high-yield of char with poor porosity, which is subsequent gasified to develop many new pores. However, the heavy tar can be produced by the low-temperature pyrolysis. Therefore, the one-step pyrolysis at relatively high temperatures may avoid this problem. Up to date, the comparative study has been rarely reported on the synthesis of activated bio-char via the one- or two-step pyrolysis process. The one-step pyrolysis refers to co-pyrolysis of biomass with the activating agents (i.e. in situ activation) (Yin et al., 2018), while the two-step pyrolysis refers to activation of bio-char with the activating agents (i.e. ex situ activation) (X. Yang et al., 2018).
Phenol is a common and highly toxic organic pollutant that widely presents in coking residues, papers, gases, oil-refining wastes, medical wastewater and other industrial wastewaters (Kim et al., 2016). Phenolic compounds are also the main components (e.g., tar) in the by-products of biomass pyrolysis or gasification (Shen et al., 2015a; Shen, 2017a, Shen, 2017b; Shen et al., 2017). As a volatile organic compound (VOC), it is difficult to degrade in nature. Noteworthy, human phenol poisoning is caused by contacting with respiratory tract and skin (Chen et al., 2016). The low concentration can induce protein denaturation, and the high concentration causes protein precipitation, thereby leading to a direct damage to cells and paralysis of central nervous system (Xiong et al., 2018). Effective removal of phenol from water has become a significant issue in energy application and environmental protection. This work comparatively studied the KOH-catalyzed activation of bio-chars from RH via one- and two-step pyrolysis. The physiochemical properties of activated bio-chars including porosity properties, specific surface areas and surface functional groups were characterized. Then, the activated bio-chars were evaluated for adsorption of phenol in the water.
Section snippets
Feedstocks and chemicals
The feedstock of RH was collected from a rice-milling factory at Jinhu, Jiangsu (China). The received RH was washed out by the distilled water to remove the impurities. Then, it was dried in an oven at 105 °C for 10 h. The clean RH was dry stored for the further use. The properties of RH mainly including proximate analysis and ultimate analysis were shown in Table 2. It was found that high-content of SiO2 in the ash. Besides, RH contained three main components of cellulose (23.8%), lignin (20.3%)
Yields of activated bio-chars
The yields of activated bio-chars were shown in Fig. 2. The yield of activated bio-char decreased with the increase of the pyrolysis temperature. The two-step pyrolysis can produce high yields of the activated bio-chars compared to the one-step pyrolysis. KOH in the RH had the catalytic effect on the pyrolysis process. And the pyrolytic gas could contribute to the interaction of char and gas (Blasi et al., 2017). At 750 °C, the yield of the AB2-1-750 (19.4%) was higher than that of AB1-1-750
Conclusions
The activated bio-carbons from RH were synthesized by one- and two-step pyrolysis. Two-step pyrolysis could produce high yields of the activated bio-carbons compared to one-step pyrolysis. At 750 °C, the yield of the AB2-1-750 (19.4%) was much higher than that of the AB1-1-750 (2.5%). The yield of activated bio-carbon decreased with the increase of the mass ratio of KOH, which had significant effects on developing the surface area and porosity of carbon at different activation temperatures.
Acknowledgements
The work is supported by the Startup Fund for Introducing Talent at NUIST (Grant 2243141501046). The financial supports by the National Science Foundation of China (Grant 21607079 and 91543115) are gratefully acknowledged. Besides, the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (Grant 16KJB610012) are appreciated.
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