Arsenic Contamination After Wood Impregnation: Speciation, Sorption and Leaching - Highway and Urban Environment - page 266

ionized form H 2 AsO 3 − in basic environments at pH > 9.2. Arsenic is phytotoxic and
hazardous to living organisms, the most toxic and mobile forms being inorganic
As(III) species [7] . Inorganic As is transformed into organic As by the process
of methylation, by which methylarsenic acid (MMA) and dimethylarsenic acid
(DMA) are formed. The organic forms are less toxic and more stable compared to
the inorganic forms and can bioaccumulate [8] . The organic forms are present at
low concentrations in soil, water and sediment.
Arsenic binds strongly on soil colloids and is highly dependent on the content of
clay, hydr/oxides surfaces of Al, Fe and Mn, calcium carbonates and/or organic
matter [9] . It changes with pH, and the various forms have a different adsorption
capacity; only smaller amount of As binds to soil particles at high pH since it forms
anions in solution. It is therefore important to determine the form of As in order to
understand leaching processes. In general, Fe hydr/oxides have the highest degree
of involvement in the adsorption of As in both acidic and alkaline soils, but Al, Mn
and organic matter may be important in the former. Arsenite binds more weakly and
is less dependent on pH than As(V) [6] . The adsorption of As(V) on iron hydr/
oxides is higher at low pH while that of As(III) is higher at higher pH. The formation
of ferric hydr/oxides contributes to a natural reduction of the dispersal of As(V) in
the environment [10] . In Sweden, Fe oxides are common in soil while pure Al oxides
are less frequent [11] . In reducing conditions, iron hydr/oxides are dissolved, making
the adsorbed As dissolve and become more mobile [6, 12] . Ferric hydr/oxides are
metastable under most groundwater conditions and tend to dissolve in a discordant
way, forming ferrous iron and releasing As into the solution [13] . In addition, the
As form changes in reducing conditions; from As(V) to As(III) [6] . Furthermore, it
is probable that salinity also affects the solubility of As, since both dissolved and
particulate Fe concentrations decrease with increasing salinity [14] .
The general leachability (K d value) of As is estimated to 30 L/kg [15] . The K d
value is the ratio between the concentration of the substance in the solid and the
dissolved phase:

solid phase concentration mg kg DS ( / )

( /)

(1)

K =

d

dissolved phase concentration mg l

However, the concentration of As in soil affects the K d value and more As is dissolved
at high concentrations. A comparison of batch and column tests showed that higher
concentrations yield up to three times higher K d values [16] . For CCA-polluted
soil, the K d obtained in experimental conditions was lower compared to the
calculated K d .
The objectives of this study were to (i) determine concentrations of the different
As species in arsenic contaminated soil and groundwater, (ii) determine the correla-
tion of As concentration to the chemical soil characteristics, and (iii) study the
leachability of As under different conditions and compare the laboratory results
with As concentrations found in groundwater at the site. The results of the study
will be used as a basis for decisions about further investigations and remediation
actions at the site.