Mercury is one of the most hazardous contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Depending on the prevailing environmental conditions, inorganic mercury species may be converted to many times more toxic methylated forms such as methylmercury, a potent neurotoxin that is readily accumulated by aquatic biota. Despite a considerable amount of literature on the subject, the behavior of mercury and many of the transformation and distribution mechanisms operating in the natural aquatic environment are still poorly understood. This review examines the current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms.

Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation

Interactions between mercury and dissolved organic matter––a review

In contrast to recent rapid growth in the field data on mercury flux over soils, our knowledge and understanding of the emission processes and controlling mechanisms still remains limited. Many processes could control emission of mercury from soils. The emission processes may involve both Hg(0) and Hg(II) pools, both homogeneous and heterogeneous distributions and reactions, and both dark and light processes. The physical and chemical controlling processes mainly involve (1) distribution of different mercury species among different soil phases, largely associated with adsorption and desorption of Hg(0) and Hg(II) species, and (2) mercury redox reactions in soils, some of which could be induced by sunlight, especially in the UV. The distribution (sorption versus desorption) of Hg(0) in soils could be the rate-limiting process in the case where Hg(0) is the dominant mercury pool, and the redox reactions could be rate-limiting in the case where Hg(II) is the major pool. Various hypothetical scenarios of Hg(0) and Hg(II) transformation in soils are discussed here. However, the specific processes that actually occur in soils under natural conditions are far from clear and need to be identified and quantified in future research.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JD900194

Processes influencing the emission of mercury from soils: A conceptual model

The bioavailability of humic substance-bound mercury (HS-Hg) in relation to soil properties was investigated using lettuce (Lactuca sativa var. Angustana irish). Results showed fulvic acid-bound Hg (FA-Hg) was an important source of Hg for lettuce. The availability of added Hg to the lettuce depends on the supply power of added Hg and the absorption capacity of the lettuce. Acidic soil, light soil texture, and the amount of humic substances and available nutrients enhance Hg availability. This suggests that humic substance-bound Hg poses greater risk to human health through food chain in sandy, acidic, and fertile soils.

Bioavailability of Humic Substance-Bound Mercury to Lettuce and its Relationship with Soil Properties

Mercury is one of the priority pollutants of our time, but also a unique element that can form organic and inorganic compounds of various degrees of toxicity. The toxicity of mercury is influenced by the form of metal finding, environmental factors, ways of metal and its compounds entering the organisms of living beings, and concentrations. In this paper, we consider the factors that determine the migration and transformation of mercury in complex hydrochemical areas of rivers-estuaries. The research is based on the results of observations carried out since 2004 at the mouths of the Northern Dvina, Pechora and Kyanda Rivers. The hydrological and hydrochemical conditions of waters and bottom sediments have been analyzed. Various forms of the occurrence and migration of mercury in the waters and bottom sediments of the objects under consideration have been studied. Conclusions about the priority factors that determine the behavior of mercury for each water body and, in general, for the estuarine areas of this natural zone have been drawn.

Factors Determining Migration and Transformation of Mercury in the Mouths of Northern Rivers

The data about the role of natural organic substances in dissolution, transport and concentrating of mercury in environment are generalized. The interaction of mercury (II) with fulvic acids (FA) and humic acids (HA) have been studied. It has been shown that in the reaction of mercury ions with FA stable soluble high-molecular mercury (II) fulvate complexes are formed. These complexes are predominating form of mercury in surface fresh waters. The interaction of mercury (II) with FA leads to the abrupt increase of mercury mobility in waters and sails. On the contrary HA behave as complexing sorbents promoting mercury concentrating in soils and bottom sediments. Another direction of mercury (II) interaction with humus acids is the formation of organomercury compounds. Principal possibility of abiological methylation of mercury by means of interaction of mercury (II) with FA has been proved. The output of methylmercury increases with concentration of FA and pH.

D. N. Chkhetia

Papers

The role of organic matter in mercury cycle