Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 74

the young ferns. However, in addition to increasing As availability in soils, P also competes with
As for plant uptake so caution needs to be excised when adding P so plant arsenic uptake is not
adversely affected.

4.2.3.2 Mycorrhizal symbiosis
Regarding rhizosphere manipulation via mycorrhizal symbiosis, which has a well-documented
role in improving P uptake, it can increase arsenic accumulation and frond biomass in P. vit-
tata (Agely et al ., 2005; Fitz and Wenzel, 2002). Glomus microaggregatum, G. mosseae, G.
brohultii and G. geosporum represent the most common arbuscular mycorrhizal fungi (AMF)
in the rhizosphere of P. vittata (Wu et al ., 2009) At the exposure of 3.8-75 mg L − 1 As(V) in
hydroponics, inoculation of an uncontaminated isolate of G. mosseae almost doubled arsenic
influx into P. vittata roots relative to the control (25-105 vs . 14-55 mg As kg − 1 dw h − 1 ) during
20-min uptake experiment (Wu et al ., 2009). In a greenhouse experiment with total soil arsenic of
100 mg kg − 1 , 2-5 times more arsenic accumulation in the fronds was found in P. vittata colonized
by a community of AMF, with the enhancement being more evident with increasing soil P level
as compared to those without colonization (Agely et al ., 2005). Considering the fact that arsenic
acts as a chemical analog of P and the well documented role of AMF in enhancing P nutrition
for the host plants, it is not surprising to find enhanced arsenic accumulation by mycorrhizal
P. vittata with concurrent increase of plant biomass. However, the contribution of AMF to plant
growth and arsenic translocation in P. vittata has been shown to largely depend on the species of
AMF (Trotta et al ., 2006), implying arsenic phytoextraction with P. vittata could be optimized by
selected AMF symbiosis.

4.2.4 Potential environmental risks
4.2.4.1 Invasive risk
As a hardy and perennial fern species, P. vittata propagates quickly and is easy to maintain
in a humid tropic/subtropic climate, which facilitates arsenic phytoextraction using P. vittata .
However, from an ecological point of view, there is a concern regarding its invasive potential
considering it is classified as a type-II invasive species in Florida. It is important to employ young
ferns (e.g., 2 month-old), which exhibits higher capacity in arsenic accumulation than the old
ferns (e.g., 16 month-old) (Gonzaga et al ., 2007a). Such practice can reduce the potential of fern
invasion with little spores being produced during phytoextraction. In addition, to reduce its inva-
sion risk and overcome the geographical limitation of arsenic hyperaccumulators, exploration of
indigenous species with potential for arsenic phytoextraction from local sites rich in arsenic, e.g.,
mining areas, may provide alternative options, which are well-adapted to the local environment
(Antosiewicz et al ., 2008; Mahmud et al ., 2008; Visoottiviseth et al ., 2002).

4.2.4.2 Disposal of arsenic-rich biomass
Safe management and economical disposal of arsenic-loaded biomass remains unsolved. Improper
treatment of arsenic-rich plant materials may pose threats to ecosystem safety. For P. vittata ,
inorganic As(III) accounts for 94% of the total arsenic in the fronds after 18 weeks growth in

soil containing 50 mg As kg − 1 (Tu et al ., 2003). Furthermore, when the fronds were air-dried,
the amount of leached arsenic substantially increased, with arsenic concentration in the leachate
reaching 0.65 mg L − 1 after 5 d of drying. The same holds true for arsenic hyperaccumulating fern
P. calomelanos which grows naturally on arsenic-contaminated sites in southern Thailand, arsenic
in the fronds is primarily present as inorganic species with 86-93% being water-extractable with
the majority being As(III) (60-72%) (Francesconi et al ., 2002). Taken together, these results
suggest that arsenic concentrated in the hyperaccumulating ferns has high water solubility and
hence the arsenic-rich biomass should be properly managed and kept away from water supply to
minimize secondary contamination.
As for the possible disposable options, there is a paucity of data regarding the post-treatment
technology and associated cost-benefit analysis. To effectively reduce plant biomass, incinera-
tion has been considered and more than 90% of plant biomass can be reduced by this means

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