Page 191 - Plant-Based Remediation Processes

10 Phytostabilization as Soil Remediation Strategy 183

• Mercury may be chemically or biologically transformed to methylmercury and
dimethylmercury, of which the former is bioaccumulative and the latter is
volatile and may be transported over long distances.
Mercury is not essential for plant or animal life. The main human exposure to Hg
is via inhalation of the vapor of elemental Hg and ingestion of methylmercury
compounds in food. This compound affects among other organs also the brain, and
it is documented that (as for lead) children in the embryonic stage receive mercury
via the placenta causing persistent effects on children’s mental development.
However, the Hg toxicity varies among the different types of Hg. Generally,
organic forms are much more toxic than the inorganic forms.

10.1.8 Arsenic

Arsenic is a silver-gray or white metallic solid element found in nature. Arsenic
combines with other elements to form organic and inorganic compounds, inorganic
arsenic compounds being more toxic than organic arsenic compounds. Soils and
waters containing high levels of arsenic compounds can easily contaminate plants,
animals, and human beings in contact with them, as they either produce toxic
effects or accumulate in plants and thereby enter animal and human food chains
(Nriagu 1994). Several thousand people consuming untreated groundwater might
have a considerable health risk and there would be a harmful influence on the
development of agriculture and cattle raising activities. The disease ascribed to
arsenic contamination was later called ‘chronic endemic regional hydroarsenism’
(HACRE, ‘hidroarsenicismo cro ´nico regional ende ´mico’, in Spanish).

10.2 Heavy Metals in Environment

Heavy metals that are introduced into soils accumulate mainly in the upper layers of
soils (Smith 1996). In general, this accumulation allows plants to uptake them, and
through a biomagnification process in the food chain, they can constitute a serious
health problem for animals and humans. Nevertheless, their mobility in soil could
be reduced due to high capacity of soil material to adsorb heavy metals. When
heavy metals are added to soils, some of them may chemically or physically interact
with the natural compounds of soils, being immobilized or forming compounds that
have low solubility. This degree of sorption is predominantly affected by environ-
mental factors, soil components, and properties as well as the amount of heavy
metals added (Jalali and Khanlari 2008). In fact, metal transport is dependent on the
physiochemical properties and the amount of the metals, but mostly on the physical
and chemical properties of the soil. As a result, the primary soil factors controlling
the potential bioavailability of metals are soil organic matter content, soil pH, the