Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 77

3.9 to 6.5 after two weeks growth of white lupin in a pot experiment. Furthermore, soil soluble
concentrations of As and Cd were reduced by 56% and 86% after five-month growth in field
conditions with the roots being the major sink of As and Cd (Vazquez et al ., 2006).

4.3.3 Fe oxides and biochar
In some cases, amendments such as Fe oxides are essential to mitigate arsenic toxicity and hence
facilitate plant survival. For example, two highly-contaminated mine tailings in South Korea
contained arsenic levels up to 6670 and 56,600 mg kg − 1 , resulting in high arsenic toxicity to
plants. Reduction of ∼ 70-80% available arsenic was achieved by adding amorphous Fe with the
majority of arsenic bond to the stable Fe precipitates (Kim et al ., 2003). In a field trial involving
four arsenic-contaminated sites from UK containing 748 mg As kg − 1 , a mean reduction of 22-
32% arsenic uptake by tested vegetables was recorded upon the addition of 0.2-0.5% Fe as ferrous
sulfate in the top soils (Warren et al ., 2003), confirming the high capacity of iron in soil arsenic
immobilization. In a field trial of a grassland established on an abandoned chemical waste site,
soil amendment with Fe(III) plus lime was the most efficient treatment with the labile arsenic
being reduced by 92%. An 8% reduction in leached arsenic upon the application of lime was
observed, resulting from the binding of arsenic with Ca 2 + and resulting in reduced As mobility
(Hartley et al ., 2009a).
Biochar, as a promising soil amendment, has important environment implications for the bio-
chemical behaviors of metals in soils. Regardless the type of feedstock and pyrolysis conditions,
biochar with relatively high cation exchange capacity consistently shows adsorption capacity
towards metal cations but little binding ability for arsenic species at typical environmental pH
(Mohan et al ., 2007). It should be noted that arsenic liability in soils increased to varying extent
upon the application of biochar with higher arsenic concentrations in water-soluble and surface-
adsorbed pool, probably due to raised soil pH by biochar (Hartley et al ., 2009b; Namgay et al .,
2010). For example, in a pot experiment with maize, the extractable arsenic in soil increased from
5.16 mg kg − 1 in the control soil to 5.96 mg kg − 1 in the biochar treatment (15 g biochar kg − 1 soil)
(Namgay et al ., 2010). Similarly, in a 60-d field experiment, arsenic concentration in soil pore
water increased by 30 fold with biochar application (soil:biochar = 2:1, v/v). These results high-
light the potential risk of arsenic mobilization upon biochar application to arsenic-containing soils.
To effectively immobilize arsenic upon the application of biochar, iron oxides can be employed
considering their high affinity to arsenic (Hartley and Lepp, 2008). Therefore, the combination
of biochar and iron oxides can be a useful strategy to immobilize arsenic while improving soil
fertility.

4.3.4 Phosphate
As a chemical analog of As(V), P is an effective competitor of As(V) for binding sites in soils.
Due to the competitive anion exchange, increased bioavailability and plant uptake of arsenic has
been well-documented upon P application (Cao and Ma, 2004; Hartley et al ., 2009a). In a pot
experiment with soil arsenic at 0, 15, and 30 mg kg − 1 , elevated arsenic accumulation in both rice
grain and straw with lower grain yield was observed after P application of 50 mg kg − 1 P (Hossain
et al ., 2009). For example, arsenic concentrations in rice grain and straw increased from 0.64 and
5.77 mg kg − 1 to 0.71 and 6.21 mg kg − 1 upon P addition at 30 mg kg − 1 arsenic. As a result, the
grain yield (g pot − 1 ) was reduced by 33-66%. Furthermore, arsenic concentration associated with
Fe plaque was reduced by 20% on average, indicating higher arsenic solubility induced by P in rice
rhizosphere. This highlights the risk that P fertilization may induce arsenic mobilization in soils
and increase its uptake by food crops. Similar case has been reported during phytostabilization
of a gold mine tailings with elevated arsenic ( > 1000 mg kg − 1 ) in New Zealand, where P has
been employed to improve the hostile environment for better plant establishment (Mains et al .,
2006). As a result, the leached arsenic increased proportionally to the amount of P applied.
With P amendment at 3 g m − 2 , up to 0.5 and 0.9 mg L − 1 arsenic was leached from the bare and