Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 73

The mechanisms responsible for the highly efficient arsenic translocation in P. vittata have
been gradually unraveled. Arsenite [As(III)] is consistently present as the major species in
P. vittata fronds with As(V) dominating the roots regardless arsenic species supplied (Kertulis
et al ., 2005; Mathews et al ., 2010; Singh and Ma, 2006; Wang et al ., 2002).This suggests
that efficient As(V) reduction and As(III) translocation as an important contributing factor
to arsenic hyperaccumulation in P. vittata .
Su et al. (2008) reported that 93-98% of arsenic in the xylem sap of P. vittata was As(III) with
either As(V) or As(III) being supplied, indicating a significantly higher mobility of As(III) than
As(V) in xylem transport. Mathews et al. (2010) investigated the location of As(V) reduction by
exposing P. vittata to 0.10 mM As(V) for 8 d. They found that As(III) concentrations increased
significantly from 7% in the roots up to 68% and 71% in the rhizome sap and rhizome tissue.
Along with upward translocation to the fronds, 86% As(III) in the frond sap and 90-100% As(III)
in the pinnae were recorded, indicating a remarkable reduction capacity of the fronds as well as the
rhizome. This is supported by Tu et al. (2004c) who has shown that upon exposing excised tissues
of P. vittata to 0.67 mM As(V) for 2 d, 86% and 24% As(III) was present in the excised pinnate and
roots. Taken together, it is conceivable that rhizomes and fronds are primarily responsible for As(V)
reduction in P. vittata with increasingly more reduction along upward translocation. Besides, little
arsenic efflux from the roots to external media and lack of a strong arsenic sequestration in roots
both facilitate the highly efficient arsenic translocation in P. vittata (Su et al ., 2008; Zhao et al .,
2009).
Arsenic phytoextraction with P. vittata has been demonstrated to be affected by plant age, with
young ferns exhibiting higher arsenic translocation than the older ones (Gonzaga et al ., 2007a;
Santos et al ., 2008; Tu et al ., 2004a). For example, 36% more arsenic was accumulated in 2-month
old P. vittata than 4-16 month old plants after 8-week growth in an arsenic-contaminated soil,
which is likely associated with the higher metabolic activity in younger plants (Gonzaga et al .,
2007a). In addition, arsenic TF was reduced from 3.2 to 1.6 from old to young ferns, indicating
decreased arsenic translocating ability of P. vittata with increasing plant age. Furthermore, frond
biomass after 8 weeks growth increased by 39, 6.9, 2.0 and 1.1 times for P. vittata of increasing
age of 2-, 4-, 10- and 16-month old, respectively. These results highlight the necessity to use young
ferns in phytoextraction and harvest fronds before they senesce to minimize the remediating time
and potential arsenic being leached from senesced fronds by rainwater in the field.
4.2.3 Potential improvement
4.2.3.1 Phosphorous amendment
To achieve more effective phytoextraction, it is essential to employ proper agronomic techniques
and plant management such as fertilizing and rhizosphere manipulation. One key strategy towards
efficient phytoextraction is to enhance arsenic availability in soils. P amendment can be used in
assisting arsenic uptake by P. vittata in the presence of toxic metals (e.g., Pb, Zn, and Cd) through
increasing plant biomass and soil arsenic bioavailability via competitive anion exchange together
with reduced metal toxicity by immobilization (Cao et al ., 2003; Fayiga and Ma, 2006; Tu and
Ma, 2003). A maximum of P/As ratio of 1.2 in soil solution or 1.0 in the fronds has been suggested
for an improved performance of P. vittata in arsenic phytoextraction by enhancing fern biomass
and arsenic uptake (Tu and Ma, 2003). For instance, with a range of P added to the tested soil con-
taining 400 mg As(V) kg − 1 , a maximum of 26% soil arsenic extraction by P. vittata was recorded
at water soluble P/As molar ratio of 2.3 after 20 weeks growth in a greenhouse experiment. Simi-
larly, for another arsenic hyperaccumulator Pityrogramma calomelanos discovered in Thailand, a
significant increment of frond arsenic content of 14 mg plant − 1 was reported after 8-week growth
in the field containing 136-269 mg As kg − 1 with the addition of 100 mg P kg − 1 soil (Jankong
et al ., 2007). In hydroponics, split P addition (134 + 66 µ M) during P. vittata acclimation and
after arsenic exposure (145 µ gL − 1 ) has been shown to induce 1.5-folder higher efficiency in
frond As accumulation in the younger ferns (45-d-old) compared to the older ones (90-d and
180-d-old) (Gonzaga et al ., 2008), suggesting the more efficient stimulation of P on As uptake in