**Characterization and Remediation of Soils and Sediments Polluted with Mercury: Occurrence, Transformations, Environmental Considerations and San Joaquin's Sierra Gorda Case**

I. Robles, J. Lakatos, P. Scharek, Z. Planck, G. Hernández, S. Solís and E. Bustos

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/57284

**1. Introduction**

Soil as important part of the ecosystems which must be protected in the environment context, and it is necessary be studied the possible overall impact of measures for protec‐ tion, with a very special attention from mining activities. The soil resource occupies a fundamental part of the ecosystems; when a soil is degraded, the others components of the ecosystems are degraded too.

The fate of the heavy metal in soils depends upon many soil processes that are governed by several soils properties of which soil pH and redox potential are known to be the most important parameters. Thus, the solubility of trace elements is often shown as a function of pH affected by amount and kind of organic matter. Trace elements are known to be accumu‐ lated in surface soils as a result of contamination from point sources as mining activities. An appreciable amount of the soils has been made unusable because of pollution. Highly conta‐ minated soils belong to a high healthy risk to human being and their environmentally harmful effects. That is why soil should be correctly understood and underestimated long range lethal effects that can have irreversible consequences. The improvement of soils damaged and contaminated by pollutants need of the particular soils, requires a full understanding of soil properties and of the deteriorating factors.

Mercury is one of the most toxic elements to human health and ecosystem; because of all mercury species are toxic. A wide variety of mercury species exist in the environment and its

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various chemical forms can differ in bioavailability, transport, persistence, and toxicity. Still, every mercury species is toxic with methyl mercury being the most toxic species. The World Health Organization (WHO) recommends a maximum methyl mercury intake of 1.6 µg Kg-1 per week, while the Environmental Protection Agency (EPA) lists a maximum recommended intake of 0.1 µg Kg-1 of body weight per day for adults. Due to high bioaccumulation, mercury is found on many levels of the food chain (Hinton and Veiga, 2001; Bengtsson, 2008). Any form of mercury in the environment may evolve into a more toxic species (methyl mercury) under biogeochemical transformation processes (Figure 1). Due to these processes and the high mobility of mercury species, a good understanding of how mercury species transform and accurate monitoring are essential for assessing the risk of mercury in the environment.

The impact of mercury depends strongly on its chemical species; understanding mercury transformations and the impact of its various chemical forms are vital to preventing harmful effects on humans and the environment. Nevertheless, the physicochemical characteristics of mercury are either useful or necessary for many industrial and agricultural applications, and mercury may be scattered over large area, depending on the source (Leopold et al, 2010).

**Figure 1.** Biogeochemical transformation processes of mercury (Leopold et al, 2010).

Mercury concentrations in ground water indicate that the highest concentration of mercury in groundwater comes from the soil and from aquifers. While simulating mercury predictions can often be difficult, mercury can be estimated in experiments conducted in batch mode or in columns. The percentage of Hg that can potentially leach from the soil was previously estimated in batch experiments. Distribution analyses of species in leachate confirmed the presence of inorganic species (Hg2+ and Hg0 ) ranging from 90 – 100 % (Bollen, 2008; Harvey, 2002).

The most commonly used techniques for the remediation of mercury contaminated soils have been classified as either excavation techniques or containment techniques, and are grouped as follows (Hinton and Veiga, 2001): (a) *ex situ* treatments: physical separation, thermal treat‐ ments, hydrometallurgical treatments; (b) *In situ* recuperation: vapor extraction coupled with evaporation (soil), permeable reactive barriers; (c) *In situ* leaching and extraction: electrokinetic separation, interceptor systems, phytoremediation, passive remediation; (d) *containment:* pump and treat impermeable barriers, sealed surfaces and drainage, stabilization and solidi‐ fication, sediment covering.
