Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 75

(Sas-Nowosielska et al ., 2004). As a volatile metalloid, arsenic tends to be easily released by
vaporization during thermal processing. Recent study has revealed that 24% of the accumulated
arsenic in P. vittata could be emitted during incineration at 800 ◦ C with a 94% loss of biomass (Yan
et al ., 2008). The residual ash provides an enriched and recoverable ore with arsenic concentration
being increased by 11-fold. While the development of a highly efficient capture system targeting
arsenic-containing flue gas seems essential for further application of this technology.
In addition, water extraction of the harvested biomass may be feasible considering its high water
solubility of arsenic. The resulting arsenic-rich extract could be recycled by an industry. Further-
more, decreased arsenic level in plant biomass could make the residue non-hazardous waste, with
lower fee in landfill disposal. However, there is still large uncertainty for this option regarding
arsenic extraction efficiency, residual arsenic concentration, and economic effectiveness. Alterna-
tively, high-arsenic biomass could be disposed in marine system (Francesconi et al ., 2002), which
has a high detoxification capacity by transforming inorganic arsenic into essentially non-toxic
organic forms by natural biochemical processes. While preliminary test and thorough assessment
with regard to the ecological effect of this scenario is needed to minimize potential environmental
risks. In addition, recent study has shown polymeric encapsulation technique could efficiently
stabilize arsenic-bearing solid residual (e.g., iron-based sorbent) producing more than an order
of magnitude lower concentration of leached arsenic than the conventional cement encapsulation
(Shaw et al ., 2008). However, the applicability of polymeric matrices to encapsulate arsenic-rich
biomass needs both standard and landfill simulation leaching tests. In brief, there is a long way to
go to establish a technically feasible, economically acceptable and environmentally safe disposal
method.

4.3 PHYTOSTABILIZATION

4.3.1 Indigenous tolerant species with low TF
In sites contaminated with high-level of arsenic and other toxic metals (e.g., Cu, Zn, and Cd),
phytostabilization is advantageous by immobilizing arsenic and reducing its exposure risks
to ecological receptors ( Table 4.3 ) . High adaptability of spontaneous species with multiple
metal/metalloid tolerance has been widely reported to serve as potential candidates for phy-
tostabilization of the contaminated sites (Antosiewicz et al ., 2008; Vamerali et al ., 2009; Whiting
et al ., 2004). For instance, in a recent phytoremediation trial on a contaminated site with 0.15 m
layer of gravelly soil over a 0.7 m layer of cinders containing As, Co, Cu, Pb and Zn, 100%
survival of indigenous Populus and Salix were obtained after two-year of experiments with soil
amelioration (e.g., mixing with imported soils, tillage, and fertilization) in spite of 16-92% lower
tissue biomass than the control (Vamerali et al ., 2009). The fact that trace metals were prefer-
entially accumulated in the woody roots (84-89%) with marginal shoot translocation suggests
the potential utilization of native Populus and Salix in phytostabilization of arsenic-contaminated
sites.

4.3.2 Substrate improvement by legumes
To overcome the general poor nutritional status in degraded sites with arsenic contamination,
legume plants with good N fixation capacity and strong root system serve as promising pioneering
colonizer species for substrate improvement and revegetation. Furthermore, most species of
legume generally exhibited low capability for shoot translocation of metals and hence pose low
risks of exposure to other organisms in the food chain. For example, by growing white lupin in
soils contaminated by the spill of acid pyrite sludge with co-occurrence of As and Cd, symbiosis
with rhizobia were successfully established by lupin plants with root nodule formation, although
the efficiency of N-fixation were reduced by 30-40% due to metal toxicity and inhospitable
substrate. Simultaneously, soil acidity was mitigated with a marked increase of soil pH from