Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 69

by P. vittata with frond arsenic up to 7,230 mg kg − 1 and BF of 74 (Tu et al ., 2002). In a separate
greenhouse study investigating the effect of repeated harvests on arsenic removal by P. vittata , ferns
were grown in six arsenic-contaminated soils with total arsenic ranging from 22.7 to 640 mg kg − 1
(Gonzaga et al ., 2008). With the application of extended time-release base fertilizer (N:P:K ratios
of 18:6:12) at a rate of 2 g kg − 1 soil, the ferns were well-established after 4 months of growth
and produced a good frond biomass ranging from 24.8 to 33.5 g plant − 1 with frond arsenic
being 66 to 6,151 mg kg − 1 and BF being 4.7 to 48 from the first harvest (October, 2003, 4
months after transplant). During the harvest, all aboveground biomass was removed, making it
difficult for the plants to re-grow under a cooler climate in the second growing period. As a
result, 34-75% lower frond arsenic concentrations and 40-84% less frond biomass were obtained
upon the second harvest (April, 2004). The results indicate that, though P. vittata is effective in
arsenic extraction, proper growth timing and harvest method (leaving some fiddleheads for faster
regrowth) is important to achieve optimum phytoextraction.
This conclusion is supported by a two-year field study of Kertulis-Tartar et al . (2006) who
found that it is necessary to leave the fiddleheads (young fronds) as well as few live fronds
at harvest to facilitate the regeneration and survival of ferns in winter season. Furthermore,
compared to senesced fronds, live fronds accumulated 25-49% higher arsenic concentrations
in field conditions, suggesting the necessity of frond harvest before they senesce to maximize
arsenic extraction. In the field trial in North Central Florida, surface soil arsenic was reduced
from 190 to 140 mg kg − 1 by P. vittata after 2 years with planting density of 0.09 m 2 per fern
(Kertulis-Tartar et al ., 2006). Based on the projected removal capacity, 7-8 years is needed to
remove arsenic in top 15 cm soil below the cleanup level for residential site (2.1 mg kg − 1 )or
commercial site (12 mg kg − 1 ) established by Florida Department of Environmental Protection.
If reasonable remediation time can be achieved using P. vittata , in some cases, phytoextraction
could be competitive in comparison with other conventional technologies for soil remediation
( Table 4.2 ).
The applicability of phytoextraction using P. vittata has been further tested in 21 contaminated-
soils from England with different soil types, arsenic contamination sources and concentrations as
well as the coexistence of Cu, Cd, Zn and Pb (Shelmerdine et al ., 2009). After three sequential
growth and harvest of P. vittata over 9 months, 0.1-13% of total soil arsenic was removed from
the soils where arsenic removal efficiency varied by up to 130-fold. Higher arsenic depletion
was found under conditions with relatively low available P and low contamination of Pb, Cd,
Cu and Zn. For instance, with comparable total arsenic being in soil-1 (367 mg kg − 1 ) and soil-4
(330 mg kg − 1 ), 1.7% of soil arsenic was removed after three harvests of fern fronds from soil-1
compared to only 0.26% from soil-4, which contained 3-24 times higher concentrations of Pb, Cd,
Cu and Zn than soil-1. This confirmed that P. vittata performed better in soils with only arsenic
contamination while exhibited apparent phytotoxicity and low arsenic uptake when grown in
multiple metal/metalloid co-contaminated soils (Caille et al ., 2004).
To predict the performance and success of arsenic phytoextraction with P. vittata , a com-
bined solubility-uptake model was established by Shelmerdine et al. (2009). According to the
model, arsenic phytoextraction by P. vittata is only suitable for marginally contaminated sites
with relatively high soil pH ( > 6.0). For example, for a heavily-contaminated soil (pH 5.6, total
As concentration 1250 mg kg − 1 , and labile As fraction of 3%), assuming an annual harvestable
yield of 3 t ha − 1 , no significant decrease in total soil arsenic content over 30 years is expected.
Therefore, arsenic phytoextraction using P. vittata is applicable in lightly contaminated soils
and can be used as a soil-polishing tool in combination with other conventional remediating
strategies.
4.2.2 Arsenic hyperaccumulation mechanisms
Based on a number of studies, the unique mechanisms of As hyperaccumulation have been
gradually unraveled, which appears to involve efficient As mobilization in the rhizosphere, rapid
root uptake and enhanced frond translocation of As by P. vittata ( Fig. 4.1 ).