Phytostabilization of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 59

plaque formed in the oxidized rhizosphere of salt marsh plants (Doyle and Otte, 1997). In addition,
rice cultivars with high oxygen release ability from their roots accumulated less As in the straw
and grain than did cultivars with low oxygen release ability, probably due to the formation of iron
plaque with concomitant As adsorption to the plaque (Mei et al ., 2009).
Plants have active mechanisms for pumping organic acids into the rhizosphere to render phos-
phorus available. Due to chemical similarities to phosphate, arsenate may also become more
available due to these mechanisms, for example, plant strategies to attack iron oxides/hydroxides
(Moreno-Jiménez et al ., 2012). High levels of organic soil acids, such as phytic acid (380 µ gg − 1
root dry weight) and oxalic acid (50 µ gg − 1 root dry weight), released from plants increase the
mobility of As in soils contaminated with chromated copper arsenate and/or aluminum and iron
arsenates (Tu et al ., 2004). Due to its anionic nature, competition between As and organic
molecules containing phenol, -OH, and -COOH for sorption sites on soil particles leads to
increased As mobility (Grafe et al ., 2001). Hence, plants releasing high levels of organic acids in
root exudates are unsuitable for As phytostabilization.

3.5.2 Plant species suitable for arsenic phytostabilization
The general characteristics of plants suitable for As phytostabilization include a high As tolerance,
a low translocation factor (i.e., [As] shoot :[As] root ), and a self-sustaining existence. Several plant
species have been reported as interesting for As phytostabilization, and many of them were found
growing naturally in mine tailings. These include Eucalyptus , especially E . cladocalyx , where no
effect on As availability or soil pH was observed in tailings treated with this plant, topsoil, and
biosolid amendments compared with untreated tailings (King et al ., 2008). Three Viola species
growing at an abandoned mine site in Macedonia were found suitable for phytostabilization,
exhibiting a low As translocation factor ( < 0.2) (Stefanovic et al ., 2010). At an old gold mine
in New Zealand, several plant species, naturally occurring in the area, were found suitable for
phytostabilization purposes, including local shrubs but also grasses such as Agrostis capillaries
and Anthoxanthum odoratum and the reed Juncus articulatus (Craw et al ., 2007). Rhododen-
dron tomentosum and Veronica beccabunga occurring on mine tailings in Sweden had high root
accumulation ([As] root :[As] soil > 2.5) and low root-to-shoot translocation ( < 0.2) (Bergqvist and
Greger, 2012). In an abandoned tungsten mining area in Spain, Salix atrocinerea was found grow-
ing in the most polluted areas with low accumulation of As in the shoots ( < 7mgAskg − 1 ), while
Scirpus holoschoenus accumulated more than 3000 mg kg − 1 of As in the roots (Otones et al .,

2011).
Other examples come from plants growing in soil polluted from the accident at the Aznalcól-
lar pyrite mine in Southern Spain. Lupinus albus contained 3.75 mg As kg − 1 in the shoots and
40 mg As kg − 1 in the roots while reducing the soluble As fractions in the soil compared with soil
without Lupinus albus (Vázquez et al ., 2006). Reduced availability of As in the soil was also
demonstrated for Retama sphaerocarpa , which had a high survival rate and low shoot As
accumulation (1.5 mg As kg − 1 ) (Moreno-Jiménez et al ., 2011).
Whole plant families thought likely to be successful in As phytostabilization include Asteraceae
and Chenopodiaceae (Mendez and Maier, 2008b). Table 3.1 summarizes the plant species and
families suggested as suitable for As phytostabilization.

3.6 AMENDMENTS FOR ENHANCED ARSENIC STABILIZATION

3.6.1 Amendments for arsenic stabilization
The addition of amendment materials can help promote conditions for successful As phytostabi-
lization in soil. Iron-based amendments reduce As mobility (Kumpiene et al ., 2008). For example,
in brownfield soil, the availability of As was reduced by 98% using a water treatment residue
consisting mainly of ferrihydrite; furthermore, aging the residue for 103 days did not affect