Mercury adsorption kinetics on sulfurized biochar and solid-phase digestion using aqua regia: A synchrotron-based study

Abstract Various studies have focused on the performance and removal mechanism of aqueous Hg using biochars, but the specific Hg removal process has not been fully elucidated. Aqua regia digestion is considered a common Hg extraction method from solid samples, but uncertainties exist regarding Hg extraction from biochars. Aqueous Hg removal kinetic experiments followed by aqua regia digestion procedures and solid-phase analyses were conducted to investigate the Hg diffusion and transformation processes on oak biochar (BC) and evaluate the extraction efficiency of aqua regia as well as the stability of Hg adsorbed by biochars. Kinetic experiments indicated Hg adsorption capacity increased and adsorption rate decreased using oak biochar pretreated with calcium polysulfide (CP-BC) relative to unmodified BC. Confocal micro-X-ray fluorescence imaging illustrated Hg penetration into the interior of BC and accumulation on the CP-BC surface. Hg extended X-ray absorption fine structure spectra modeling indicated the Hg was first adsorbed as Hg(II)–O and transformed to Hg(II)–S forms on CP-BC, whereas only the Hg(II)–Cl forms of Hg were observed on BC. The results of aqua regia digestion suggest that Hg cannot be completely extracted from BC. These results provide information on the kinetic processes of Hg removal by biochars and incomplete extraction of Hg by aqua regia, which is critical for optimizing biochar modification methods and evaluating the long-term stability of adsorbed Hg in aqueous environments.