ReviewBiochar as potential sustainable precursors for activated carbon production: Multiple applications in environmental protection and energy storage
Graphical abstract
Introduction
Activated carbon is the carbonaceous material known as its large specific surface area, superior porosity, high physicochemical-stability, and excellent surface reactivity, which is widely employed as functional materials for various applications (Delgado et al., 2012, Sevilla and Mokaya, 2014, Shafeeyan et al., 2010). The commonly used feedstocks for traditional activated carbon production are wood, coal, petroleum residues, peat, lignite and polymers, which are very expensive and non-renewable (Chen et al., 2011). Therefore, many researchers have been focusing on preparing activated carbon using low-cost and sustainable alternative precursors, including agricultural residues (rice husk, corn straw, bagasse etc.) and solid wastes (sludge, food waste, garden waste etc.) (Chen et al., 2011, Yahya et al., 2015). Producing activated carbon from waste and by-products have gained attention since availability of low-cost precursors is necessary for the economic feasibility of large scale activated carbon production.
Recently, much attention has also been focused on the application of these biomass resources for biochar production via various thermochemical processes under oxygen-limited conditions and at relatively low temperatures (<700 °C), including pyrolysis, hydrothermal carbonization, flash carbonization, and gasification (Meyer et al., 2011). Considerable studies have highlighted the benefits of using biochar in terms of carbon sequestration, soil amendment, soil productivity improvement (Manyà, 2012, Sohi, 2012) and pollution control (Ahmad et al., 2014, Mohan et al., 2014, Tan et al., 2015). In addition, the thermochemical treatment of biomass has energy recovery potential, which can generate biofuels and syngas accompanied with biochar production (Manyà, 2012). The resultant biochar usually exhibit porous structure, maintained surface functional groups and mineral components due to the removal of the moisture and the volatile matter contents of the biomass by thermal treatment (Liu et al., 2015). These favorable properties lead to high reactivity of biochar, and hence, make it possible to be used as an alternative carbon material.
However, the applications of biochar in different fields are also restricted due to its limited functionalities, inherited from the feedstock after thermochemical treatment (Tan et al., 2016b). For instance, the un-activated biochar usually shows relatively lower pore properties (especially for micropore volume), which restricts its ability in CO2 capture and energy storage. In addition, the raw biochar has limited ability to adsorb various contaminants (Nair and Vinu, 2016, Yao et al., 2013), particularly for high concentrations of polluted water. Therefore, there is a growing interest of the scientific community on physical and chemical activation of biochar for expending its applications in various areas by improving its chemical/physical properties in the past few years (Ahmed et al., 2016b, Rajapaksha et al., 2016, Tan et al., 2016b). Biochar has been used as a renewable and low-cost precursor for activated carbon production. Globally, the mean price for biochar was $2.65 kg−1, which was highly variable depending on the origin of biochar production sites and ranged from as low as $0.09 kg−1 (Philippines) to $8.85 kg−1 (UK) (Ahmed et al., 2016a). Activated biochar appears to be a new potential cost-effective and environmentally-friendly carbon materials with great application prospect in many fields. Compared with traditional activated carbon, the main advantage of activated biochar is that the feedstocks of biochar production are abundant and low-cost, which mainly obtained from agricultural biomass and solid waste (Table S1) (Tan et al., 2015). The performances of activated biochar applied in various fields have also been reported to be equivalent to or even higher than that of commercial activated carbon and other much more expensive materials such as CNTs and graphene (Angın et al., 2013, Dehkhoda et al., 2014, Jung et al., 2015b, Nguyen and Lee, 2016).
According to the above-explained considerations, the production of biochar from low-cost and sustainable biomass appears to be a very attractive alternative precursor for activated carbon production, which integrates carbon sequestration and renewable energy generation into multiple applications including water pollution treatment, CO2 capture, and energy storage. The purpose of the current review is to review and summarize recent information concerning physical and chemical activation of biochar and their effects on the properties of resultant activated carbon. The influence of these activation methods on the water pollution treatment using activated biochar and the mechanisms of improved adsorption for various contaminants are discussed. In addition, the application of activated biochar for CO2 capture, and energy storage are also reviewed. Furthermore, knowledge gaps and future research needs that exist in the activation and application of activated carbon produced from biochar are highlighted.
Section snippets
Physical activation
Recently, many researches utilize physical methods for biochar activation, which could optimize surface structure of biochar. Significant physical changes in surface area, pore volume, and pore structures of biochar may be achieved by means of physical activations, which are the important parameters for biochar applications. In addition, physical activation may not only change the porosity of biochar but also affect its surface chemical properties (surface functional groups, hydrophobicity and
Application for water pollution treatment
As discussed above, physical and chemical activation could have significantly beneficial effects on biochar chemical/physical properties, including increasing biochar surface areas, improving pore structures, adding surface functional groups, and changing the hydrophobicity of biochar surface. These changes could result in the enhancement of adsorption ability of biochar for various contaminants (Table 1). As shown in the Tables 1 and S4, in most cases, physical and chemical activation usually
Application for CO2 capture
The reduction of anthropogenic CO2 release into the atmosphere has been recognized as the crucial matter due to its huge contribution to global climate change (Nasri et al., 2014, Toro-Molina et al., 2012). Adsorption is considered as a promising method for CO2 separation and the surface physical and chemical properties of adsorbent play a critical role during the adsorption process (Zhang et al., 2014a). Biochar produced from biomass waste followed by activation usually showed high surface
Application for energy storage
In addition to these applications mentioned above, biochar-based activated carbons have also been used in energy storage fields. For example, activated biochar have been employed as electrode materials for supercapacitors or as porous matrix to host active substances for cathodes (Table 2, Table 5).
Future perspectives
Thus it can be seen that, activated biochar appears to be a new potential cost-effective and environmentally-friendly carbon material with great application prospect in many fields. Despite recent researches on production and application of activated biochar in multiple areas are increasing, a number of research gaps still exist (Fig. 1). To close these knowledge gaps, the following recommendations are suggested:
- (i)
The feedstock with different compositions, production conditions and activation
Conclusions
This review presents a summary of biochar activation and the multiple applications of resultant activated carbon. The different functions of activation methods on the specific properties of biochar result in the different adsorption ability of activated biochar for various contaminants. The abundance, applicable physical/chemical properties, and ease of processability of activated biochar make it suitable to be employed as cost-effective and environmentally-friendly material for CO2 capture and
Acknowledgements
The authors would like to thank financial support from the National Natural Science Foundation of China (Grant Nos. 51609268, 41271332, 51521006, 41301339, and 51608208), and the Hunan Provincial Innovation Foundation for Postgraduate (Grant Nos. CX2015B090 and CX2015B092).
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