Phosphoric acid pretreatment enhances the specific surface areas of biochars by generation of micropores☆
Graphical abstract
Introduction
Biochars are carbon-rich materials prepared by pyrolysis under limited oxygen condition, and have attracted increased attention for potential application in carbon sequestration and soil quality improvement (Wu et al., 2013; Xiao et al., 2014). Although biochars are proposed as pollution remediation materials to adsorb various contaminants (Chu et al., 2017; Uchimiya et al., 2010), they generally have low surface area in the range of pyrolytic temperatures below 500 °C (Lian and Xing, 2017), which limited their sorption of contaminants as an adsorbent (Gomez-Eyles et al., 2013). In addition to surface area, other factors, such as pore structure, contribute to the sorptive properties (Li et al., 2017b). It is therefore useful to explore effective techniques to further optimize biochar properties to facilitate their applications.
Various chemical treatment techniques, such as H2SO4, KOH and ZnCl2 methods, have been proposed and tested to modify biochars with improved sorption capacity (Aghababaei et al., 2017; Lau et al., 2017; Xia et al., 2016). Compared to these methods, phosphoric acid (H3PO4) treatment exhibits several advantages: relatively low pyrolysis temperature, low corrosivity to the equipment, minor pollution, and low costs (Girgis and El-Hendawy, 2002; Li et al., 2010). Previous studies have investigated the impact of H3PO4 treatment on biochar characteristics, mostly on the application as adsorbent for contaminants. For instance, Fernandez et al. (2015) characterized activated hydrochars (char substrates prepared by hydrothermal carbonization) from orange peels after H3PO4 modification at 600 °C. They observed better developed porosity and investigated sorption of emerging organic contaminants on hydrochars after H3PO4 modification. Wu et al. (2017) studied the sorption of Cr(VI) on modified biochar from pomelo peel, and also pointed out that modified biochar possessed the developed porous structure after H3PO4 treatment at 450 °C. Taha et al. (2014) studied the removal of a mixture of 15 different pesticides from water using biochars treated with H3PO4 at 80 °C. The surface functional groups and aromatization of treated biochars was increased and pesticides could be easily removed from contaminated water using biochars treated by H3PO4.
These papers, however, did neither discuss possible modifying mechanisms of H3PO4 treatment, nor the effect of precursor structural characteristics on the producted biochars. A lot of papers reported that pore structure of H3PO4-treated biochars was dominantly mesoporous (Guo and Rockstraw, 2006; Jagtoyen and Derbyshire, 1998). But, Zhao et al. (2017) recently reported that micropores were largely generated on modified biochars through the interaction between H3PO4 and carbon structure. It was generally agreed that the type of pore structure depended on the nature of feedstock and pyrolysis temperature (Lian and Xing, 2017). The observation was the generation of pore structure, the decrease of pore size and the increase of porosity as a result of the escape volatile matter during carbonization (Tan et al., 2015). These pore structure in biochars plays an important role in sorption process (Chu et al., 2017). However, the discussion of which kind of pores (macro-, meso-, or micro) are central in the general sorption properties of biochars is under debate. For instance, Phuong et al. (2015) reported that the macropores (≥50 nm) apparently have a significant effect on the water sorption capacity of the biochar. Contrarily, Li et al. (2017b) attributed the sorption capacity of biochars to mesopores (2–50 nm), rather than micropores. A third group of researchers claim that micropores (<2 nm) are central in the sorption process of inorganic and organic pollutants as reported by Pituello et al. (2015) and Liu et al. (2012).
A great deal of researches has demonstrated that sorption is an effective treatment for organic contamination in soils (Ghaffar et al., 2015; Jin et al., 2016). Carbamazepine (CBZ), an antiepileptic drug, can be frequently detected at notable concentrations in environment (Wang et al., 2013). Bisphenol A (BPA) is widely applied for plastic production and is well known for its endocrine disrupting potential (Pan et al., 2008). Both chemicals have been paid wide attention due to their health risk to humans (Gomez et al., 2007; Wanda et al., 2017). They can be discharged into the agricultural soils in different ways, including fertilization and irrigation (Knight et al., 2017; Novo et al., 2018). Therefore, CBZ and BPA were selected as representatives of organic compounds. Both chemicals were selected as the adsorbates also owing to their comparable hydrophobicity and molecular masses, but different molecular structures and functional groups (Table S1).
The main objective of this study was to illustrate the development of porous structure of the biochars as affected by H3PO4 treatment. Specifically, different precursors of different structural characteristics will be modified and thus the mechanisms of H3PO4 treatment will be discussed, with a major focus on the formation of porous structure of treated biochars. We hypothesize that particularly micropores are beneficial for the sorption of CBZ and BPA.
Section snippets
Materials
Pine sawdust (PS), obtained from the local wood processing mill (Kunming, Yunnan, China), were used to produce biochars. The collected PS were air-dried for 3 days, oven-dried overnight at 60 °C, and then ground to pass through a 60 mesh sieve. The representative compositions of biomass, cellulose (CL) and lignin (LG), were purchased from Sigma-Aldrich Chemical Co. for biochar preparation. CBZ and BPA were purchased from Beijing Chemicals Reagent Company (Analytical grade reagents). Selected
Pore structure development
With increases in the pyrolysis temperature, the SSA increased for all three types of charred samples, BCPS, BCPS-H, and PS-H (Fig. 1a). Clearly, without H3PO4 treatment, the SSA of BCPS was much lower than that of the other two types of H3PO4-treated biochars at the same temperature. For instance, BCPS3 has a SSA of 9.15 m2/g, while the SSA of BCPS3-H3 and PS-H3 increased to 795 m2/g and 1148 m2/g, respectively. The elevated SSA of modified biochars from H3PO4 treatment can be associated with
Conclusions
The phosphoric acid treatment was used to prepare modified biochars from pine sawdust. The two types of modified biochars had much larger specific surface area than that of the pristine biochars, which was attributed to the micropore development. Micropores were generated extensively on modified biochars through hydrogen protons catalysis effect. The specific surface area as well as the micropores of modified biochars from pine sawdust may greatly be attributed to the crosslink of cellulose
Acknowledgements
This research was supported by National Natural Scientific Foundation of China (41663013, U1602231 and 41703121), and Yunnan applied basic research project (2016FA040).
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This paper has been recommended for acceptance by Joerg Rinklebe.