Phytostabilization of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 56

increases the mobility of As due to physical competition and electrostatic interactions with soil
particles, and through the formation of aqueous complexes containing As (Wang and Mulligan,
2009). Arsenic availability in soils with an organic matter content of more than 25% increased
with time due to an original high content of available As that became mobile during mineralization
(Meunier et al ., 2011). In soils with an organic matter content below 8%, aging did not influence
the availability of As due to the presence of arsenopyrite and pentavalent As forms, which were
not affected by organic matter (Meunier et al ., 2011). However, ternary complexes between ferric
complexes of humic substances and arsenate accounted for 25-70% of the total As in a laboratory
solution composed of humic acids, iron(III), and arsenate (Mikutta and Kretzschmar, 2011). The
complexation between humic substances, iron, and As with a concomitant reduced availability
of As, may lead to a revision of the general opinion of the relationship between organic matter
and As in the near future. In addition, phosphate increases As mobility. In the normal pH range
in soil (pH 4-8), arsenate exists mainly in its deprotonated form, as anions. In its anionic form,
arsenate acts as a physical analogue to phosphate, leading to competition for adsorption sites on
iron oxides/hydroxides in the soil (Zhao et al ., 2010). At an old chemical waste site, As mobility
was correlated with the content of both organic matter and phosphate (Hartley et al ., 2009).

3.4 PLANT TRAITS IN PHYTOSTABILIZATION

Plants reduce pollutant leaching and land erosion as well as providing additional economic values,
such as wood, bioenergy, dust control, and ecological services (Robinson et al ., 2009). On a mine
tailing site in northern Chile, approximately 70 colonizing plant species had potential economic
values for various uses (Orchard et al ., 2009). In addition, in moderately metal-contaminated
agricultural soils, profitable crops have been cultivated successfully for bioenergy production
(Fässler et al ., 2011; Greger and Landberg, 1999).
It is ideal to select and use native plant species for phytostabilization, rather than introduc-
ing potentially invasive plant species to remediation sites (Mench et al ., 2010). Plants used in
phytostabilization should have low accumulation of pollutants in shoots and high tolerance to
pollutants in the soil (Butcher, 2009; Cunningham, 1995). In hostile environments, such as mine
tailing deposits, plants may contribute organic material, promoting the growth of microorganisms,
which in turn will transform the tailings into a more soil-like structure (Mendez and Maier, 2008b).
Mycorrhizae may also help in maintaining viable plants by increasing the plants' phosphorous
uptake and reducing the As accumulation through the efflux of arsenite from the mycorrhizae
into the soil solution (Sharples et al ., 2000). Phytostabilization will be less successful if rainfall
exceeds evapotranspiration, a problem that can be addressed by recirculating the drainage water
(Robinson et al ., 2009). On the other hand, if rainfall is too scarce, for example, in semi-arid/arid
regions, supplementary irrigation may be needed at the onset of phytostabilization (Mendez and
Maier, 2008a).
Trees are often promoted in phytostabilization due to their high rates of evapotranspiration,
which reduce the flow of water through the soil, leading to a reduced leaching of pollutants
to the surrounding surface and groundwaters (Pulford and Watson, 2003). Due to the specific
characteristics of certain tree species, such as oak ( Quercus sp.), which increases soil acidification,
and poplar ( Populus sp.), which accumulates high levels of certain metals in its leaves, these
species should not be selected for phytostabilization in areas where such characteristics may
result in increased mobility of pollutants (Mertens et al ., 2007). If the contaminants are situated
below the root depth, plants will be unable to exert their phytostabilizing effect. To reach the
contaminants, plants, such as trees, able to send roots to greater depths are required. However,
deep rooting at desirable depths may not always develop, for example, if physical obstacles restrict
root growth or if surface soil water is abundant, making the plant reluctant to spend energy trying
to send its roots deeper (Negri et al ., 2003). In addition, if surface soil contains enough nutrients,
deep root growth does not occur (Stoltz and Greger, 2006).