Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 67

CHAPTER 4

Recent advances in phytoremediation of arsenic-contaminated soils

Xin Wang & Lena Qiying Ma

4.1 INTRODUCTION

Arsenic contamination in soils occurs widely in a range of ecosystems resulting from geolog-
ical origins and anthropogenic activities. On average, arsenic concentration ranges from 5 to
10 mg kg − 1 in uncontaminated soils and above 10 mg kg − 1 in contaminated soils (Hossain, 2006).
Increased buildup of arsenic in irrigated soils has been widely recognized in South and South-east
Asia (Brammer and Ravenscroft, 2009), posing significant threats to agriculture sustainability.
In Bangladesh, long-term irrigation with arsenic-rich groundwater from shallow aquifers in dry
season adds > 1000 tons of arsenic to the agricultural soils (Ali et al ., 2003). In addition, arsenic
contamination in soils results from various anthropogenic activities, such as mining and smelting
(Williams et al ., 2009), and using arsenic-containing wood preservatives (Chirenje et al ., 2003),
pigment, pesticides, herbicide (Sarkar et al ., 2005) and feed additives (Arai et al ., 2003).
As a cost-effective and ecology-friendly technology, phytoremediation of arsenic-contaminated
soils has been widely studied. Among phytoremediation technologies, phytoextraction and phy-
tostabilization are two predominant approaches in remediation of soils contaminated with heavy
metals. Phytoextraction takes advantage of plants to remove contaminants from soils by con-
centrating the targeted contaminant to the harvestable tissues (Salt et al ., 1998). To achieve
effective arsenic removal from soils, the plant should be highly tolerant to arsenic and efficient in
accumulating arsenic into sufficient aboveground biomass. Therefore, phytoextraction efficiency
depends on both aboveground biomass yield and plant arsenic concentration. Bioconcentration
factor ( BF ), which is defined as the ratio of element concentration in plant shoots to that in soil,
has been used to measure a plant's efficiency in phytoextraction. Based on mass balance calcula-
tion, phytoextraction is feasible only by using plants with BF much greater than 1, regardless of
how large the harvestable biomass (McGrath and Zhao, 2003). Furthermore, to achieve efficient
removal of contaminant in a reasonable time frame with high plant survival and biomass yield, the
initial and target soil contaminant concentrations should be taken into account to predict the appli-
cability of phytoextraction, which is in most cases appropriate for soils with low contamination
(Zhao and McGrath, 2009).
For heavily contaminated sites (e.g., industrial and mining degraded sites), indigenous tolerant
species with extensive root system and low translocation factor ( TF , the ratio of contaminant con-
centration in shoots to that in roots) provide valuable plant resources to immobilize the pollutant in
the rhizosphere, and simultaneously stabilize the degraded sites by establishing vegetation cover.
Soil amendments, in some cases, are essential to assist the success of the survival of pioneering
species by mitigating contaminant toxicity and improving substrate conditions (Vangronsveld
et al ., 2009). In this way, ecological restoration of contaminated sites can be gradually achieved
through revegetation, which is termed as phytostabilization.
Beside these two major phytoremediation techniques, other methods include phytoexclusion
and rhizofiltration. To remediate large-scale agricultural soils contaminated by arsenic, phytoex-
clusion is more practical to reduce arsenic transfer from soil to crops. Based on the well-established
knowledge with regard to arsenic biogeochemistry and arsenic transport mechanisms in rice, a
range of strategies including water management, Si fertilization, and rhizosphere manipulation

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