Mechanistic investigation of mercury removal by unmodified and Fe-modified biochars based on synchrotron-based methods
Graphical abstract
Introduction
Natural and anthropogenic activities, such as geothermal activity, metallurgy, landfills, etc., can cause Hg release (Wang et al., 2012). Once transported into the biosphere, especially into a groundwater system, Hg is highly hazardous because of its toxicity and bioaccumulation in food chains (Driscoll et al., 2013; Wang et al., 2012). Many studies have evaluated the Hg removal efficiency and capacity of carbonaceous materials in consideration of their microporous structure, abundant surface area, and sufficient functional groups (Hadi et al., 2015; Wang et al., 2019a; Zabihi et al., 2010). The relevant removal mechanisms include complexation (Dong et al., 2013; Lv et al., 2012), reduction (Lloyd-Jones et al., 2004; Xu et al., 2016), ion exchange (Kılıç et al., 2008), and electrostatic interaction (Das et al., 2007).
As an environmental-friendly, low-cost, and efficient adsorbent, biochar has been extensively studied in recent years using different types of precursors and pyrolytic temperatures. Bagasse biochar removes Hg(II) by reduction and surface complexation, while Hg(II) is stabilized by hickory chip biochar through the formation of Hg-π binding (Xu et al., 2016). Dong et al. (2013) report Hg complexes with phenolic hydroxyl and carboxylic groups in biochar pyrolyzed at 300 and 450 °C but complexes with a graphite-like structure when pyrolyzed at 600 °C.
Subsequent to the exploration of new feedstocks and adjustments in pyrolytic temperature, efforts have been made to modify biochars to enhance their Hg-stabilizing ability (Tan et al., 2016; Tang et al., 2015). Modifying reagents, including sulfur-containing ligands (Kim et al., 2011; Park et al., 2019), Fe species (Gong et al., 2012; Wang et al., 2018a), and halides (Wang et al., 2018b), are extensively reported. As a soft Lewis acid, Hg(II) can complex with soft Lewis bases, such as reduced-S ligands (Bower et al., 2008) and halides. Sulfurization enhances Hg(II) removal by forming HgS (Liu et al., 2018), and the incorporation of halides promotes Hg(0) removal by combining chemisorption with physisorption (De et al., 2013). Oxygen-rich functional groups and Fe3O4 in Fe-modified biochars contribute to Hg stabilization (Yang et al., 2016). These modifications improve physico-chemical properties and introduce active sorbents as well as reactants.
Our previous study characterized biochars pyrolyzed from FeCl3- or FeSO4-soaked biomass (Feng et al., 2020), and the improvement in surface area, change in surface functional groups, and impregnation of various types of Fe, S, and Cl species makes Fe-modified biochars promising materials for Hg removal. Nano-scaled species, for example, Fe(0) and pyrrhotite (Fe1-xS), are formed inside the basic porous structure of Fe-modified biochars. Fe chips are efficient with respect to Hg remediation by adsorbing Hg to oxidized Fe (Bäckström et al., 2003). Combining functions of reduced Fe and S, binary compounds formed from Fe and S (e.g., mackinawite, Fe1-xS, and pyrite) are reported scavengers for aqueous Hg(II) (Bower et al., 2008; Jeong et al., 2007; Liu et al., 2008). However, the effectiveness of these Fe-modified biochars in Hg removal has not been investigated in detail.
Synchrotron-based methods, such as EXAFS and CMXRFI, are advanced tools for uncovering the removal mechanisms of heavy metals (Gibson et al., 2011; Liu et al., 2016, Liu et al., 2018). For example, as inferred from HgFe and HgAl based on EXAFS modeling, Hg(II) is bidentate and monodentate inner-sphere complexed to goethite, γ-alumina, and bayerite (Kim et al., 2004a, Kim et al., 2004b). Gibson et al. (2011) uncover mechanisms of Hg(II) treatment with the help of EXAFS modeling, indicating HgO and HgS bonds are formed in different materials. Liu et al. (2018) also model HgS bonds, with CMXRFI mapping indicating a surface-distributed tendency of Hg in sulfurized biochars. CMXRFI provides depth compositional information without destruction and EXAFS analysis provides local structural information (Liu et al., 2018), which are helpful for mechanistic research about Hg removal.
This study was designed to evaluate the potential of unmodified and Fe-modified biochars to immobilize aqueous Hg(II) in batch sorption experiments. Nine biochars produced from unmodified, FeCl3-modified, and FeSO4-modified biomass and pyrolyzed at 300, 600, or 900 °C were characterized with respect to Hg(II) removal ability. Mechanisms of interactions between Hg(II) and biochars were further investigated with synchrotron-based EXAFS and CMXRFI analyses.
Section snippets
Batch experiment
Unmodified and Fe-modified biochars produced from poplar (Populus adenopoda) wood were prepared following our previous work (Feng et al., 2020). In brief, ~3 × 3 × 3 mm3 biomass particles were soaked in FeCl3 or FeSO4 solutions (0.5 M) for 2 d, filtered out, and dried. Unmodified, FeCl3-modified, and FeSO4-modified biomass were pyrolyzed at 300, 600, or 900 °C under anaerobic conditions, resulting in unmodified, FeCl3-modified, and FeSO4-modified biochars. These biochars were anaerobically
Hg removal
The initial tHg concentration was 4700 μg L−1, and decreases in tHg concentration were observed for solutions with unmodified, FeCl3-modified, or FeSO4-modified biochars pyrolyzed at 300, 600, or 900 °C (Fig. 1). Hg removal efficiency increased with increasing pyrolytic temperature (P <0.05). The final tHg concentration of the solution treated with unmodified biochars was 140–660 μg L−1 for a removal efficiency of 86–97%. Compared to the unmodified biochars, removal efficiency of Fe-modified
Conclusions
Biochars pyrolyzed at higher temperatures have better Hg(II) removal ability than those pyrolyzed at lower temperatures. As such, this study verified that modifying biochars by pretreating biomass with Fe-reagents and pyrolyzing at 600 or 900 °C is a prospective strategy for Hg(II) remediation. Hg was bound to S and distributed corresponding to S in biochars with thiol or sulfide; otherwise, Hg was mainly removed by adsorption from solution with the distribution influenced by biochar pore
CRediT authorship contribution statement
Yu Feng: Conceptualization, Methodology, Data curation, Visualization, Software, Writing - original draft. Peng Liu: Conceptualization, Supervision, Funding acquisition, Resources, Project administration. Yanxin Wang: Supervision, Funding acquisition. Wenfu Liu: Data curation, Investigation, Formal analysis, Validation. Yingying Liu: Writing - review & editing. Y. Zou Finfrock: Methodology, Writing - review & editing, Resources.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Major Science and Technology Program for Water Pollution Control and Treatment [grant number 2018ZX07110], the National Natural Science Foundation of China [grant numbers 41521001, 41877478], the 111 Program [grant number B18049], the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) [grant number CUGGC06]. X-ray absorption spectroscopy analyses were performed at Beamline Sector 9 and 20 of the Advanced Photon Source,
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