Unveiling the Redox Behavior of Mercurous [Hg(I)] Species (Hg ₂ (OH) ₂ ) across an Environmental <i>E</i> _h Gradient Governed by 1,4-Benzoquinone and 1,4-Hydroquinone

Mercury (Hg) biogeochemical cycling involves redox transformations between Hg(0) and Hg(II), likely via a mercurous Hg(I) intermediate. Despite previously thought metastable, Hg(I) was found to exist stably in natural waters; yet, the redox behavior governing Hg(I) occurrence remains elusively studied. Here, we simulated an environmental <i>E</i>_h gradient (0.219-0.600 V, vs SHE) using noninterfering 1,4-benzoquinone and 1,4-hydroquinone as redox mediators, and demonstrate that Hg(I) redox transformations are strongly <i>E</i>_h-dependent. Hg(I) was stable at 0.302-0.339 V, contrasting significant oxidation (19-98%) at 0.369-0.600 V or reduction (81%) at 0.219-0.255 V. Speciation analysis and thermodynamic calculations confirm that varying proton level alters Hg(II) species abundance (HgHPO₄, HgPO₄^-, and Hg(OH)₂), driving this <i>E</i>_h-dependence. Specifically, HgHPO₄/HgPO₄^- exhibits lower reduction potentials (to Hg(I), as Hg₂(OH)₂) than Hg(OH)₂. Consequently, Hg(I) is subject to oxidation to HgHPO₄/HgPO₄^- at 0.369-0.440 V (pH 6.0), while Hg(OH)₂ preferentially undergoes stepwise reduction to Hg(I) and Hg(0) at 0.236-0.308 V (pH 7.5). Additionally, externally forced Hg(0) removal processes, e.g., purging used in conventional Hg(0) analysis, significantly affect Hg(I) transformations and enhance Hg reduction under reducing conditions, whereas natural degassing has negligible effects. This work provides valuable parameters for Hg redox thermodynamics and important implications for predicting Hg(I) occurrence in aquatic environments.