Page 192 - Plant-Based Remediation Processes

184 A. Branzini and M.S. Zubillaga

Fig. 10.2 Dynamics of heavy metals in soil. Adapted from Pierzynski et al. (2005)

accessibility and character of sorption sites on soil surfaces, the contents of Fe and
Al oxyhydroxides, clay fraction content, and the cation exchange capacity (miner-
alogical composition) (Alvarez et al. 2008). However, according to USEPA (1993),
the first two of these factors are the most important for controlling the release of
metals to pore water and their subsequent bioavailability. It should be noted that the
residence time of heavy metals in soil is directly related to their bioavailability.
Also, as we see in Fig. 10.2, pollution of the environment by heavy metals poses a
threat to surface water and groundwater, which are used as the main sources of
drinking water by many inhabitants in the world.
On the other hand, metals present in soil often come in mixtures of three to five,
depending on the source of contamination. That is why an increased concentration
and simultaneous application of metals into soil creates increased competition
between cations and metals for binding sites, thus controlling their solubility
(Ghosh and Singh 2005). Several works on the biological effects of heavy metals
have focused on the action of single contaminants against test organisms (Enserink
et al. 1991; Parrott and Sprague 1993). However, these works have disregarded the
fact that interactions can occur when two or more heavy metals are applied
simultaneously to the environment and that their combined effect may result in
an increase (synergism) or a decrease (antagonism) of the toxicity of the separate
heavy metals (Otitoloju 2003). Also, since the heterogeneity of soils makes it very
difficult to predict the potential mobility and distribution of single metals, experi-
mental data are essential (Cerqueira et al. 2011). Mixture toxicity experiments may
reflect the actual pollution of ecosystems in a more realistic way than experiments
in which toxicants are tested individually (Spurgeon et al. 1994).