Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 113

likely at risk as well. As, a member of group V, occurs in nature in four states ( + 5, + 3, 0 and − 3),
but the pentavalent arsenate [As(V)] and trivalent arsenite [As(III)] are the most common forms.
Generally, iAs compounds are more toxic than organic As (org-As) compounds, and As(III) is
more broadly toxic than As(V) (NRC, 1999).
Arsenic contaminated groundwater is not just used for drinking but is also widely used for
irrigation of crops, and particularly for the staple food paddy/rice. The As content of lowland or
paddy/rice grain is generally much higher than that of upland cereal crops (Schoof et al ., 1999;
Williams et al ., 2007), because of the relatively high bioavailability of soil As under reduced
conditions. A global range of 0.08 to 0.2 mg As kg − 1 has been suggested for rice (Zavala et al .,
2008), but values as high as 1.8 mg As kg − 1 have been found in Bangladesh rice (Meharg and
Rahman, 2003). Compared to other countries, rice from Bangladesh and India had the highest
percentage of i-As 80%, compared to 42% in rice from the USA. This indicates that the percentage
of i-As in rice is not a constant factor geographically and probably depends on cultivar and growing
conditions (Williams et al ., 2005).
Several studies have estimated As in non-rice foods (Alam et al ., 2003; Das et al ., 2004;
Roy Chowdhury et al ., 2002; Williams et al ., 2006). They reported that As concentrations in
vegetables, fruits, spices and freshwater fish, from As contaminated areas of Bangladesh and
West Bengal, range from < 0.04 to 3.99 mg kg − 1 (dw). However, data from Taiwan showed that
fish contained high level of i-As (Huang et al ., 2003). It has become clear that dietary exposure
can contribute significantly to the total daily intake of i-As.
Several strategies exist to treat contaminated soils and water either through ex-situ or in-situ
technologies. The use of biological system has recently gained importance over conventional
methods. Bioremediation uses microorganisms to reduce, oxidize or eliminate contaminants. Its
biological processes rely on biochemical changes induced by microbes. The common processes
that are involved in bioremediation are chelation, compartmentalization, exclusion, sorption,
biomethylation, complexation, co-precipitation, transformation, uptake and immobilization of
different As species (Di et al ., 1999). In this chapter, the environmental and toxicological effects of
As are discussed and followed by an overview of bioremediation techniques that can be employed
for this element. Recent applications of As bioremediation are described in the chapter ( Fig. 6.1 ).
Figure 6.1. Diagrammatic representation of in-situ mitigation of arsenic pollutant by plants and microbes
including affecting factors and strategies for bioremediation.