Phytostabilization of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 61

mobility could be the desorption of As from iron oxides due to competition from organic matter.
Similarly, amendments consisting of sewage sludge and fly ash from wood combustion increased
the mobility of As in drainage water from limed mine tailings (Stolz and Greger, 2002). In soil
from an old alkali works contaminated with As, Cu, and Pb, amendments consisting of green
waste compost led to an increase in As mobility with a concomitant increase in uptake of As
in lettuce and sunflower (Clemente et al ., 2010). In an agricultural soil, amendments of organic
matter did not reduce the mobility of As (De La Fuente et al ., 2010). Table 3.2 summarizes
the concentrations of As in leachates from soil before and after treatments with organic matter
amendments.
Treating As-polluted soil with phosphate and oxalic acid leads to a drastic increase in the
mobility of As. Phosphate and oxalic acid mobilize As by inhibiting the adsorption of arsenate
and arsenite and by dissolving As-associated aluminum and iron oxides/hydroxides in the soil,
respectively (Wovkulich et al ., 2010). In agricultural soil, adding an inorganic nutrient fertilizer
including nitrogen, phosphorous, and potassium (15:15:15 N:P:K) increased As mobility (De
La Fuente et al ., 2010). In soil contaminated with As and Pb, phosphate amendments, such as
calcium-magnesium-phosphate and rock phosphate, reduced the Pb mobility but increased the
As mobility, while ferrous sulfate amendments produced the opposite results (Cui et al ., 2010).
Phosphate is a widely accepted amendment for immobilizing Pb in soil and aqueous solutions
(Miretzky and Fernandez Cirelli, 2009). These results highlight the problems of treating sites with
multiple contaminants and the need to establish individual strategic management plans for each
individual site.

3.7 MANAGEMENT PLAN FOR ARSENIC PHYTOSTABILIZATION

How should phytostabilization in an As-polluted area be established to successfully encourage
both plant vegetation and As immobility? Soil quality and pH may have to be adjusted using
amendments to promote the hospitability and the desired physicochemical properties of the soil.
Plant species with desired abilities should be selected. If phytostabilization strategies are unable
to reduce the mobility of As and/or other contaminants, additional methods along with phytosta-
bilization may be employed to obtain effective stabilization of the contaminants. In Section 3.7,
we will discuss suggestions for a management plan for successful As phytostabilization.

3.7.1 Soil parameters that influence arsenic mobility
The first step when establishing As phytostabilization in an area is to characterize the soil to
determine the risks of As mobility and what actions to take. For example, adsorption of As
differs between soils, for example, As adsorption is higher on clayey than sandy soil (Kumpiene
et al ., 2008). Characteristics such as high soil organic matter ( > 25%) may lead to increased As
mobility, possibly due to the desorption of As by dissolved organic matter (Mench et al ., 2003).
Amendments could be added to address problems of high As mobility due to the physicochemical
properties of the soil (see Section 3.7.2 below).
To prevent increased As mobility due to anionic competition between hydroxyl ions and As,
and protonation of arsenate (pH < 2.5), the soil pH should be neither alkaline nor acidic (Moreno-
Jiménez et al ., 2012). A specific pH interval for optimal stabilization is impossible to determine
due to the specific characteristics of each soil; however, As sorption on solid phases such as
Fe oxides is promoted in the pH range around neutral (Renella et al ., 2008). The pH could be
adjusted by adding amendments such as carbonates or humic substances (Moreno-Jiménez et al .,
2012).
Redox potential should be high (p E > 10) to ensure that the dominant As species is the more
strongly adsorbed arsenate and not the more mobile arsenite (Zhao et al ., 2010). Redox poten-
tial could be increased by diverting and reducing the water flow and the groundwater level in
the area.