Phytostabilization of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 55

However, it is difficult to determine with any certainty what factors will influence the arsenate-
arsenite speciation in soil. For example, in 40 soils treated similarly with regard to redox potential,
the amount of arsenite and the arsenite/arsenate ratio varied considerably between the soils, indi-
cating that a range of both abiotic and biotic processes influences the arsenate-arsenite speciation
(Ackermann et al ., 2010).
The pH may influence As availability. Generally, soil pH > 7 increases the availability of
anions such as arsenate and arsenite, while soil pH < 7 may favor the retention of anionic As due
to an abundance of positive charges that neutralize the negative exchange sites in soil (Moreno-
Jiménez et al ., 2012). However, basic pH may favor co-precipitation of As with, for example,
sulfate or calcium, leading to reduced availability, and in pH below 2.5, arsenate may become fully
protonated leading to increased availability (Moreno-Jiménez et al ., 2012). The availability also
differs between arsenate and arsenite. For example, adsorption of arsenate to ferrihydrite in the
pH range 4-9 decreased with increasing pH, while the opposite occurred for arsenite (Raven et al .,
1998). The stronger adsorption of arsenite with increasing pH was likely due to the formation of
ferric arsenite.
Small amounts of the organic As species, such as monomethylarsonic acid (MMA) and dimethy-
larsinic acid (DMA), are also commonly present in soil (Bowell, 1994) ( Fig. 3.3 ). The organic
As species result mainly from the biological activity of microorganisms, primarily bacteria but
also fungi (Wood, 1974). Under reduced conditions, bacteria can convert some of the organic As
species into the volatile and highly toxic As species dimethylarsine (McBride and Wolfe, 1971)
and trimethylarsine (Sadiq, 1997) ( Fig. 3.3 ). For example, up to 240 mg of volatile As, mainly in
the form of trimethylarsine, per hectare per year was released from a low-As-concentration paddy
soil (containing 11 mg As kg − 1 DW), leading to an estimated contribution from soils of 0.9-2.6%
of the total global atmospheric input of As (Mestrot et al ., 2011).

3.3 SOIL COMPOSITION AND ARSENIC AVAILABILITY

The total concentration of As in soils is not the only determinant of its toxicity to living organisms.
Instead, the availability and speciation of As are the most important factors determining its toxicity.
For example, a five-fold higher toxicity of inorganic As has been observed in sandy than clayey
soils (Sheppard, 1992). Cao et al . (2003) compared two soils containing similar amounts of total
As: one was collected from an old abandoned wood preservation site and the other was spiked with
As by the authors. The authors found that As was approximately five times more available in the
laboratory-spiked soil than in the soil from the field. In the spiked soil, the As was predominately
bound to labile aluminum oxides, whereas in the soil from the wood preservation site, As was
bound to stable calcium-As and iron-As fractions, leading to the difference in As availability (Cao
et al ., 2003). Ma et al . (2006) referred to the process of reduced availability of As in soil over
time, as in the laboratory-spiked versus field soil case, as aging.
The geochemical form of As in soil strongly influences the soil As. Arsenopyrite has a low
availability, whereas As trioxide has a high availability. Depending on the pentavalent As mineral
present in the soil, As availability might vary between 0 and 41% (Meunier et al ., 2011). Iron
strongly influences the availability of As through the formation of amorphous iron(III) arsenates
and by replacing the surface hydroxyl groups of iron oxides with As (Kumpiene et al ., 2008). The
availability of arsenate is usually low in soils due to the strong adsorption of arsenate to iron and
aluminum oxides (Zhao et al ., 2010). Arsenite is more weakly adsorbed to soil particles and is
therefore easily released into the soil solution or displays higher availability under low redox con-
ditions (p E < 6). Iron-As oxides are also dissolved in soils with low redox potential (p E < 6) with
a concomitant redox-induced transformation of As to arsenite, leading to an increased availability
of arsenite (Zhao et al ., 2010).
Due to its anionic nature, complexation between As and negatively charged organic soil colloids
is low, and the availability of As is therefore mainly dependent on adsorption to clay minerals,
calcium carbonates, or iron or aluminum oxides or hydroxides (Sadiq, 1997). Organic matter