Green conversion of crop residues into porous carbons and their application to efficiently remove polycyclic aromatic hydrocarbons from water: Sorption kinetics, isotherms and mechanism
Graphical abstract
Introduction
With the continuous discharge of countless household and industry wastes into rivers, lakes and oceans, the earth’s water resources have been significantly polluted, substantially threatening the health of humans and aquatic biota (Schwarzenbach et al., 2006). Hence, developing efficient water treatment methods to remove contaminants has been deemed both significant and urgent, especially for highly toxic and persistent organic micropollutants (OMPs) (Alsbaiee et al., 2016). Polycyclic aromatic hydrocarbons (PAHs), a typical class of OMPs originating from the incomplete combustion of organic matter, e.g., volcanic eruption, forest fires and iron and steel smelting, are genotoxic and carcinogenic to humans and animals and easily transferred to unpolluted environments, change microbial activity and diversity, accelerate the propagation of antibiotic resistance genes, and are deemed priority pollutants by the United States Environmental Protection Agency (Montuori et al., 2016). Unfortunately, PAHs are already distributed in water resources globally, particularly the highly volatile and water-soluble light PAHs, such as naphthalene (NAP), acenaphthene (ACE), and phenanthrene (PHE) (Meng et al., 2019, Wang et al., 2017). Moreover, because of human activities, the pollution level of light PAHs is dramatically increasing (Meng et al., 2019). Hence, efforts should be made to prevent environmental pollution and avoid potential health risks by removing NAP, ACE and PHE from water environment.
In recent years, numerous approaches, including chemical and biological oxidation, sorption, and filtration, have been used to remove OMPs from water (Ahmed et al., 2017, Hamann et al., 2016, Lam et al., 2017). Among them, the sorption method is a well-developed technology with facile, economic, rapid and efficiency advantages (Xiao et al., 2017). The environment-friendly nature of this process, which does not produce harmful substances, is another advantage (Zhang et al., 2017). Furthermore, given their stable physicochemical properties, low cost and easy recyclability, many carbon-based sorbents derived from crop residues have recently been synthesized using pyrolysis techniques (Lam et al., 2018a, Lam et al., 2016, Lam et al., 2018b, Liu et al., 2018, Pandiarajan et al., 2018). However, undeveloped porous structures with limited surface areas, pore size distributions and pore volumes have produced low sorption capacities (e.g., below 100 mg g−1 for PHE) (Chen et al., 2008). Hence, many studies have explored the activation of crop residues to enhance their pore structure to achieve a high sorption capacity (Liew et al., 2018a). However, the routine activation agents, e.g., NaOH, KOH, and FeCl3, are strongly corrosive, which causes economic and environmental challenges during industrial applications (Deng et al., 2015, Sevilla et al., 2017). For this reason, environmentally friendly alternatives to corrosive agents for the development of pore structure must be explored and further applied to the treatment of OMPs-polluted water.
An emerging environmentally friendly activator, potassium bicarbonate (KHCO3), was recently proposed for the green synthesis of porous carbons (PCs) for enhanced supercapacitors; however, limited attention has been paid to its environmental application (Deng et al., 2015, Gong et al., 2017). Moreover, another method, pyrolysis hydrochar, has also been demonstrated to improve pore structure and enhance the removal efficiency of pollutants (Liu et al., 2018). However, little is known about whether PCs synthesized by a combined method, i.e., KHCO3 activation of hydrochar, are potential efficient adsorbents to remove OMPs from water. In addition, it has been reported that the physical and chemical properties of PCs and their removal capacity for pollutants are influenced by the type of crop residue used as precursor (Yahya et al., 2015). Rape straw has recently become a major concern because of the high demand for renewable plant oil. Approximately 1.96 × 107 tons of rape straw is generated in China annually (Xu et al., 2017). Corn cob, another typical agricultural waste from crop production, also urgently needs to be removed due to the mass production of corn, exceeding 1.03 billion metric tons annually worldwide (Sewsynker-Sukai et al., 2018). Nevertheless, there are few studies on the efficient recycling of rape straw and corn cob, especially in the synthesis of PCs and further applications in water treatment.
According to the above findings, the aim of this study was to propose an innovative and green route incorporating hydrothermal treatment and KHCO3 activation to convert crop residues, rape straw and corn cob, into PCs. This was followed by characterizing the structure and surface properties of the PCs and investigating the sorption kinetics and isotherms of PAHs onto PCs to understand the removal process, capacity and mechanism. There have been studies reported on the synthesis of PCs for the sorption of OMPs; however, most of them used environmentally unfriendly activation agents (Jung et al., 2017, Pandiarajan et al., 2018, Xiao et al., 2017). In addition, studies have been conducted on the pyrolysis of rape straw and corn cob but without high temperature activation to develop pore structure (Feng et al., 2012, Velmurugan et al., 2016). To the best of our knowledge, there are no studies reported in the literature on the green synthesis of PCs from rape straw and corn cob, and the application of these PCs to remove PAHs. This work provides a new sustainable method for converting crop residues into valuable PCs for the removal of OMPs from water.
Section snippets
Materials and reagents
Rape straw and corn cob were obtained from local farmland in Jiangsu, China. KHCO3 (≥99%) and HCl (guaranteed reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). Acetonitrile (chromatographic grade) was obtained from Accustandard Inc. (New Haven, CT, USA). NAP (≥99%), ACE (≥99%), and PHE (≥99%) were obtained from Dr. Ehrenstorfer (Augsburg, Germany) (E-supplement file). Ultra-pure water (R ≥ 18 MΩ) was prepared by a Millipore water purification system (MilliQ
Characterization of the PCs
From the SEM images, many macropores (2–5 μm) were found on RC but not on CC (E-supplement file). The micro-/mesopore texture of the PCs is evident from the TEM images (white points) (E-supplement file). Moreover, the graphene stripes in the TEM images indicate the presence of graphene structures in both RC and CC. The N2 adsorption/desorption isotherms of RC and CC are shown in Fig. 1a. The Brunauer-Emmett-Teller (BET) specific surface areas were calculated as 1281 m2 g−1 for RC and 1069 m2 g−1
Conclusions
PCs converted from crop residues by hydro-carbonization and activation with the non-corrosive agent KHCO3 showed extremely high sorption capacity for PAHs due to their large specific surface area, wide pore size distribution, high hydrophobicity and graphene structure. The sorption of PAHs onto PCs was dominated by a physical sorption process. Pore filling, hydrophobic effects and π-π stacking interactions on the heterogeneous surface were the main sorption mechanisms. This study provides an
Acknowledgements
This study was financially supported by National Key Research and Development Program of China (2017YFD0800704, 2018YFC1801005), the National Natural Science Foundation of China (41671236, 41877032), and Key Program of Frontier Sciences, Chinese Academy of Sciences (QYZDJ-SSW-DQC035).
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