Evaluation of mercury stabilization mechanisms by sulfurized biochars determined using X-ray absorption spectroscopy
Graphical abstract
Introduction
Mercury (Hg) is a global high-priority contaminant in water, sediments, and soils [1]. Extensive efforts have been devoted to removing Hg from water and to stabilize Hg in soils, sediments, and solid-wastes to decrease its impact on the environment [2,3]. Pyrolyzed carbonaceous materials, including activated carbon, charcoal, and biochar, have been used as reactive media in a variety of Hg treatment systems [[4], [5], [6], [7], [8], [9]]. For example, Hg removal from aqueous solution using biochar derived from malt spent rootlets [10] and activated carbons derived from sago waste [11] and coirpith [12] have been shown to be effective; however, aqueous Hg concentrations after treatment often remain in the μg L−1 to mg L−1 range, above those required for protection of the environment [13]. To overcome these limitations, functionalized resins have been used to achieve final aqueous concentrations of Hg in the ng L−1 range [[14], [15], [16], [17], [18]]. Synthetic chelating ligands have also been used to maximize Hg removal [19,20]. These specialized resins and ligands can be costly, potentially limiting their use for large-scale Hg treatment as would be required for remediation of watershed-scale contamination. There remains a need to evaluate the effectiveness of low-cost reactive materials that promote removal of Hg to ng L−1 levels.
The bond between Hg(II) and sulfide or thiol compounds is strong [[19], [20], [21], [22], [23], [24]], and has been extensively studied. Hg(II) is preferentially bonded to thiol functional groups compared to O/N ligands in organic matter [[25], [26], [27]]; Hg is likely complexed with two thiols in organic matter at a distance of 2.33 Å as indicated by Hg extended X-ray absorption fine structure (EXAFS) modeling. The binding with sulfide or thiol can result in a decrease in Hg bioavailability [28]. However, in some cases, the presence of polysulfide and low molecular weight-thiols in solution can lead to enhanced solubility of cinnabar and a potential increase in Hg bioavailability [29,30]. Impregnation of polysulfide into carbonaceous materials can potentially reduce this adverse effect.
Calcium polysulfide (CPS) has been shown to be an effective reagent for removing Cr(VI) from water in both laboratory and field scale applications [[31], [32], [33]], and dimercapto (DMC)-related compounds have been studied for Hg removal with efficient removal of Hg observed [21,34,35]. Elemental S and H2S have also been applied for activated carbon (AC) sulfurization [22,23], but the process requires high temperatures and the H2S gas is corrosive and toxic.
Characterization of the form and spatial distribution of Hg within biochar particles is important for understanding Hg removal mechanisms and estimating the stability of bonded Hg. Synchrotron-based techniques can be used to characterize Hg speciation and S species of the sulfurized sorbent, for example to observe Hg-Br, Hg-S, and Hg-C binding environments on brominated and sulfurized sorbents [22]. Feng et al. [23] report that elemental S, thiophene, and sulfate are likely responsible for Hg uptake in S-treated AC based on the results of S X-ray absorption near-edge structure (XANES) analyses.
Micro-X-ray fluorescence (μ-XRF) mapping has also been widely used to characterize the spatial distribution of Hg in various materials [[36], [37], [38], [39], [40]]. One drawback of μ-XRF mapping is that the fluorescence received by the detector is the sum of the signal along the incident beam path through the sample. Micro-XRF is often used to characterize elemental distributions in thin-sections, therefore measured distributions are representative of the sum of the elements over the thickness of a thin-section (usually ≥30 μm [8,38,40]). Confocal X-ray micro-fluorescence imaging (CXMFI), an emerging non-destructive technique, can overcome this drawback [41]. Depth compositional information can be obtained from precise locations of a particle using CXMFI [42]. The particle orientation can be adjusted using CXMFI, but for thin-sections, the slice of the particle is fixed.
In a previous study, Liu et al. [8] evaluated hardwood- (sp. Quercus) based biochar in its unmodified form and observed the relatively effective removal of Hg from the water. The current study is focused on improving Hg uptake by sulfurizing this biochar using CPS and DMC. A series of batch tests were conducted to evaluate the removal of dissolved Hg from aqueous solution using these sulfurized biochars. Solid-phase reaction products were examined using a range of synchrotron-based techniques, including S K-edge XANES, μ-XRF, CXMFI, and Hg EXAFS analyses, to evaluate the forms and distribution of Hg within the sulfurized biochar particles.
Section snippets
Biochar sulfurization
Biochar (CL2) was produced from oak wood at ∼700 °C (Cowboy Charcoal Co.). CPS (Green Earth Sure-Gro IP Inc.) and DMC (2,5-dimercapto-1,3,4-thiadiazole; 98%, Sigma-Aldrich) were used as biochar sulfurization reagents.
The biochar sulfurization was conducted under low-O2 conditions. CL2 was crushed and sieved to a size of 0.5–2 mm and rinsed six times with Ar-purged ultra-pure water to remove fine particulates. The targeted S contents for the CL2 were 0.5, 2, and 5%. To achieve this, CL2 (20 g)
Aqueous chemistry
Batch experiment pH values increased slightly after the addition of unmodified and sulfurized biochars. The initial pH values were 7.9, 7.6, and 7.3 in solutions with THg concentrations of 17,800 ng L−1, 245,000 ng L−1, and 4960 μg L−1, respectively. At the termination of the experiment, the pH values increased to ∼8.2 in all solutions mixed with washed and unmodified hardwood-based biochar (CL2), CPS-sulfurized CL2 (CL2-CPS), and DMC-sulfurized CL2 (CL2-DMC). The initial alkalinity was ∼95 mg L
Conclusions
Hg immobilization by reaction or complexation with S functional groups within sulfurized biochar is a potential strategy for remediating contaminated sites by limiting Hg transport and decreasing Hg bioavailability. Results of this study indicate that Hg removal is enhanced after sulfurization of an oak biochar compared with unmodified biochar. After treatment, the Hg is distributed mainly on the surface of sulfurized biochars and on the surface and within the unmodified biochar particles.
Acknowledgements
This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), E. I. du Pont de Nemours and Company, and the Canada Research Chair program. Synchrotron-based techniques were performed at GSECARS and PNC/XSD (CLS@APS) of APS, at SXRMB, CLS. This research was performed using optics provided by Cornell High Energy Synchrotron Source. The CLS is supported by the Canada Foundation for Innovation, NSERC, the University of Saskatchewan, and others. Peng Liu
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