Aqua regia digestion cannot completely extract Hg from biochar: A synchrotron-based study. (October 2020)

• Record Type: Journal Article
• Authors: Liu, Wenfu; Feng, Yu; Zhong, Huan; Ptacek, Carol; Blowes, David; Liu, YingYing; Finfrock, Y. Zou; Liu, Peng; Wang, Sheng
• Is Part Of: Environmental pollution. Volume 265(2020)Part A
• Publication Date: 2020-10
• Subjects: Mercury -- Biochar -- Aqua regia -- Confocal micro-X-ray fluorescence imaging -- Extended X-ray absorption fine structure; Pollution -- Periodicals; Pollution -- Environmental aspects -- Periodicals; Environmental Pollution -- Periodicals; Pollution -- Périodiques; Pollution -- Aspect de l'environnement -- Périodiques; Pollution -- Effets physiologiques -- Périodiques; Pollution; Pollution -- Environmental aspects; Periodicals; Electronic journals; 363.73
• DOI: 10.1016/j.envpol.2020.115002
• Languages: English
• ISSNs: 0269-7491

Abstract

Abstract: Mercury (Hg) is commonly extracted from solid phase samples using aqua regia for total Hg (tHg) analysis. However, uncertainties exist regarding the complete extraction of Hg by aqua regia, especially from carbonaceous materials. To investigate whether aqua regia can completely extract Hg from biochars, batch-style experiments were carried out to evaluate extraction efficiency of aqua regia with respect to Hg-loaded biochar and to characterize the residual Hg speciation and spatial distribution. Different types of biochars (raw, FeCl3 -modified, and FeSO4 -modified, prepared at different temperatures) were reacted with Hg-spiked solution before the digestion experiments. Adsorption analyses indicate the biochars were successfully loaded with Hg and that the Hg content was higher in biochars pyrolyzed at higher temperature (900 versus 300 or 600 °C). The results of digestion experiments indicate Hg could not be completely extracted from the biochars tested, with a greater percentage of residual Hg in biochars pyrolyzed at 600 (60 ± 15%) and 900 (75 ± 22%) than 300 °C (7 ± 2%). Furthermore, the fraction of residual Hg in FeSO4 -modified biochars after aqua regia digestion was significantly lower than in FeCl3 -modified and unmodified biochars. Confocal micro-X-ray fluorescence imaging (CMXRFI) showed residual Hg in biochars is concentrated on surfaces prior to digestion, but more homogeneously distributed after digestion, which indicates Hg on biochar surface is more easily digested. Hg extended X-ray absorption fine structure (EXAFS) spectra modelling showed residual Hg in biochars mainly exists as Hg(II)–Cl. These results indicate extra caution should be paid for tHg determinations using aqua regia digestion method in soil (especially in forest), sediment, and peat samples containing black carbon, activated carbon, or biochar. Graphical abstract: Image 1 Highlights: Hg could not be completely extracted from the biochar by aqua regia. Hg on the surface of biochar particles is more easily extracted. The residual Hg was primarily present as Hg(II)–Cl for all of the biochars tested.