Mercury is a highly toxic global pollutant, and its redox transformation is the key process controlling the global migration and bioaccumulation of mercury. Theoretically, mercury redox reactions mostly involve single-electron transfer, with monovalent mercury (Hg(I)) being an important intermediate. As a metastable form of mercury, Hg(I) is prone to transformation due to pre-treatment or analytical methods, making it difficult to detect directly. The understanding of its environmental presence and role in the mercury redox cycle remains unknown.

To understand and explore the environmental chemical behavior of Hg(I), a "new" ionic state of mercury, researchers have developed a method involving 2-mercaptoethanol complexation-mechanical oscillation extraction-liquid chromatography-inductively coupled plasma mass spectrometry (LC-ICP-MS), revealing the widespread presence of Hg(I) in environmental solid-phase media^[1]. Furthermore, Hg(I) has been confirmed as an important intermediate in the oxidation of zero-valent mercury (Hg(0)) in plant leaves^[2], and kinetic fitting results also indicate that the oxidation of Hg(0) to Hg(I) is the rate-limiting step in the dark oxidation of mercury droplets to divalent mercury (Hg(II))^[3]. Recent studies suggest that Hg(I) is not only an intermediate in mercury redox reactions but also plays a significant role in environmental mercury transformations, including comproportionation and disproportionation. For example, the team led by Baohua Gu from Oak Ridge National Laboratory in the United States posits that the comproportionation generation of Hg(I) may mediate the rapid isotopic exchange (˂1 h) between Hg(0) and Hg(II)^[4]; the team led by Feiyue Wang from the University of Manitoba in Canada points out that Hg(0) generated by the photolytic disproportionation of Hg(I) in soils from historical silver smelting mining areas is an important source of high-concentration gaseous Hg(0) in the region^[5]. The aforementioned studies preliminarily elucidate that Hg(I) may influence the migration and transformation of mercury through comproportionation-disproportionation reactions, but the stability of Hg(I) in the environment still needs clarification. Currently, traditional research still generally holds that low concentrations of Hg(I) in water can undergo rapid disproportionation and thus cannot stably exist^[6].

In 2024, the research team led by Yin Yongguang from the Research Center for Eco-Environmental Sciences of the Chinese Academy of Sciences was the first to confirm that Hg(I) at low concentrations (μg•L^-1) in natural water can stably exist, breaking the traditional academic understanding that low-concentration monovalent mercury cannot exist in environmental aqueous phases. On this basis, they revealed the generation and transformation of Hg(I) during the reduction of Hg(II) in natural water^[7].

The study first clarified through gravimetric methods that high concentrations of Hg(I) (mg•L^-1) can stably exist in the aqueous phase but found that Hg(I) undergoes a disproportionation reaction (2Hg(I)→ Hg(0)+Hg(II)) during LC-ICP-MS analysis. Traditional research suggests that when mercury concentration is low, Hg(I) is difficult to stabilize and usually undergoes disproportionation^[6]. The study used density functional theory calculations to discover that when OH^- and Cl^- are present, the Gibbs free energy for the formation of Hg(I) from Hg(0) and Hg(II) is -8 kcal•mol^-1 and -18.6 kcal•mol^-1, respectively, indicating that the comproportionation reaction of mercury can occur spontaneously. In summary, there exists a comproportionation-disproportionation equilibrium of Hg(I) in the aqueous phase, with comproportionation being dominant. However, chromatographic separation disrupts this balance; the chromatographic separation of Hg(I) disproportionation products Hg(0) and Hg(II) suppresses mercury re-comproportionation, ultimately leading to the complete disproportionation of Hg(I).

To verify this hypothesis, the study used enriched isotope tracing methods to confirm that the rapid isotope exchange between ¹⁹⁹Hg(0) and ²⁰²Hg(II) is related to the spontaneous comproportionation-chromatographic separation and disproportionation process of mercury (spontaneous comproportionation: ¹⁹⁹Hg(0) + ²⁰²Hg(II) → [¹⁹⁹Hg(I)−²⁰²Hg(I)], chromatographic separation and disproportionation: [¹⁹⁹Hg(I)−²⁰²Hg(I)] → ²⁰²Hg(0) + ¹⁹⁹Hg(II)). This result further confirms the spontaneous comproportionation reaction of mercury and indicates that low concentrations of Hg(I) can remain stable.

Notably, the study further detected the presence of Hg(I) (4.4~6.1 μg•L^-1) in streams near the mercury smelter, accounting for 54%~70% of the total dissolved mercury, indicating that Hg(I) is one of the predominant mercury species in this area. Additionally, the research found that natural water Hg(II) can be reduced to generate Hg(I), and environmental ligands (such as Cl^- and natural organic matter, etc.) may influence the reduction process of Hg(II) by controlling the stability of Hg(I).

The situation of mercury pollution in the post-Minamata era remains severe. Including Hg(I), a "new" form, into the framework of environmental mercury species is helpful for an in-depth understanding of the environmental occurrence, migration, and biological transformation of mercury, which can provide a scientific basis for predicting the environmental behavior of mercury and implementing mercury pollution prevention and control work. Therefore, the environmental chemical behavior of Hg(I) and its related environmental effects still need further consideration and exploration.