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

Mercury extraction from solid materials is a common analytical task, but the reliability of aqua regia digestion remains uncertain in carbon-rich matrices. Prior research has shown aqua regia is widely used for total mercury analysis, but its effectiveness in carbonaceous materials is debated. No prior work had resolved whether aqua regia fully extracts mercury from biochars. This gap motivated a study to assess extraction efficiency and residual mercury speciation in biochars. The study aimed to clarify whether mercury remains after digestion and how its distribution and speciation change. The study also sought to compare mercury extraction from biochars modified with different iron salts. The research focused on mercury speciation and spatial distribution using synchrotron-based techniques. The findings could help improve mercury analysis in soils and sediments containing biochar.

The study aimed to determine whether aqua regia digestion fully extracts mercury from biochars and to characterize any residual mercury. The specific problem addressed was the uncertainty in mercury extraction completeness from carbon-rich materials. The motivation arose from the need for accurate total mercury measurements in environmental samples. The researchers tested biochars modified with iron salts and produced at different temperatures. They wanted to understand how pyrolysis temperature and modification affect mercury extraction efficiency. The study also aimed to identify the speciation of residual mercury after digestion. The researchers used synchrotron-based imaging and spectroscopy to analyze mercury distribution and chemical forms. The goal was to inform best practices for mercury analysis in biochar-containing environmental matrices.

The study used batch-style experiments to evaluate aqua regia digestion efficiency on mercury-loaded biochars. Biochars were prepared at 300, 600, and 900°C and modified with FeCl₃ or FeSO₄. Each biochar type was spiked with mercury before digestion. Confocal micro-X-ray fluorescence imaging (CMXRFI) was used to map mercury distribution before and after digestion. Extended X-ray absorption fine structure (EXAFS) spectroscopy was employed to determine mercury speciation in residual samples. Adsorption analyses confirmed successful mercury loading onto biochars. The extraction efficiency was quantified by comparing mercury concentrations before and after digestion. The study also compared mercury extraction from unmodified, FeCl₃-modified, and FeSO₄-modified biochars. The results were analyzed to determine how pyrolysis temperature and modification influenced mercury retention.

Aqua regia digestion did not fully extract mercury from biochars, with residual mercury remaining after digestion. Biochars pyrolyzed at 600°C retained 60 ± 15% of mercury, while those at 900°C retained 75 ± 22%. Biochars pyrolyzed at 300°C retained only 7 ± 2% of mercury. FeSO₄-modified biochars had significantly lower residual mercury than FeCl₃-modified and unmodified biochars. Confocal micro-X-ray fluorescence imaging showed mercury was concentrated on biochar surfaces before digestion but more evenly distributed afterward. This suggests surface-bound mercury is more easily extracted than internal mercury. EXAFS analysis revealed residual mercury primarily existed as Hg(II)-Cl complexes. The study found mercury extraction efficiency varied with pyrolysis temperature and biochar modification.

The study found aqua regia digestion does not fully extract mercury from biochars, especially those pyrolyzed at higher temperatures. Residual mercury remained after digestion, with higher retention in biochars produced at 600 and 900°C. The speciation of residual mercury was primarily Hg(II)-Cl, as shown by EXAFS analysis. The results suggest caution when using aqua regia digestion for total mercury analysis in biochar-containing samples. The study also showed FeSO₄-modified biochars retained less mercury than FeCl₃-modified ones after digestion. The spatial distribution of mercury changed from surface-concentrated to more homogeneous after digestion. These findings indicate the need for improved digestion methods for mercury analysis in carbon-rich materials. The authors propose further research to optimize mercury extraction techniques for environmental samples.