Phytostabilization of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 52

CHAPTER 3

Phytostabilization of arsenic

Claes Bergqvist & Maria Greger

3.1 INTRODUCTION

Pollutants from natural and anthropogenic sources can contaminate areas, having severe effects
on natural ecosystems and human land use. Sites contaminated due to military or industrial
activities, including wood impregnation, oil refining, gas or coal processing, and ammunition
and pesticide production, are often contaminated with arsenic (As) and in need of restoration. In
certain sedimentary rocks, such as shales, with a high organic content, As levels can be elevated.
Certain areas that contain high levels of toxic pollutants including As are so extensive that other
forms of remediation than stabilization are unrealistic. At such sites, natural vegetative growth
is often scarce due to high pollutant and/or low nutrient levels. Unvegetated areas may cause
problems for both human and animal populations, for example, due to inhalation of contaminated
airborne particles or through food chains (Mench et al ., 2000).
Phytotechnologies may offer a cost-effective alternative to established remediation techniques
for the restoration of contaminated areas (Mench et al ., 2010). Phytostabilization can be defined
as a method to immobilize pollutants using plants and plant-associated microbes (Cunningham
et al ., 1995; Ward and Singh, 2004). The ultimate goal of phytostabilization is to create a self-
sustaining, vegetative cap for the long-term control of polluting agents, reducing the availability
of pollutants by preventing dispersal by wind and water. Phytostabilization has advantages over
conventional mechanical remediation technologies, including being economically feasible and
less destructive. The primary focus of phytostabilization is to prevent the pollutants from translo-
cating to aboveground plant tissues and to sequester the pollutants in the rhizosphere through
precipitation, i.e., accumulated by the plant roots and immobilized onto soil particles (Mendez
and Maier, 2008a) ( Fig. 3.1 ).
Successful phytostabilization of heavy metals such as Cd, Pb, Zn, and B has been reported in
diverse materials, such as mine tailings and wood wastes (Brown et al ., 2005; Robinson et al .,
2007; Stoltz and Greger, 2002). In this chapter, we review current knowledge in the area of As
phytostabilization.

3.2 ARSENIC

Arsenic is present as a ubiquitous trace element. Several forms or species of As exist in soil
and water ( Fig. 3.2 ). Arsenic is carcinogenic and its toxicity is dependent on speciation. The
inorganic species of As are regarded as more harmful than the organic species to a wide range of
living organisms, with arsenite being more toxic than arsenate (Meharg and Hartley-Whitaker,
2002). The toxicity of organic As compounds such as arsenobetaine, found, for example, in fish,
is generally considered low (Kaize et al ., 1985). However, trivalent methylated intermediates,
such as methylarsonous acid and dimethylarsinous acid produced in the metabolic detoxification
pathways of As in the human liver, are more toxic than inorganic As species (Dopp et al ., 2010).
The major route for As intake by humans is through drinking water. Based on the risk of developing
cancer due to lifetime As exposure, most countries have set the limit for As in drinking water

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