In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 11

Table 1.3. Materials used in column laboratory experiments with polluted
waters from the alluvial Guadiamar aquifer (Seville, Spain).

Material Composition Size

Calcite CaCO 3 (s) 2 mm

Compost (i) Vegetable waste -

(ii) Urban solid waste and sewage sludge -

River sediment Sulfate reducing bacteria source

Fe aggregates Fe(s) (90%) 8-80 mesh

yielded 99.5% efficiency removal, attaining As levels below the limit of the WHO in drinking
water (10 µ gL − 1 ). For the reaction mixture in columns without Fe, there was an initial erratic trend
of variable As concentrations (up to 190 mg L − 1 ) during the first two months of the experiment.
Then, the concentrations were maintained between 10 and 20 µ gL − 1 , very close to the WHO
standard. The efficiency of both materials for removing reactive metals (Zn, Cu, Cd, Al and Fe)
(not shown in this text) was very high (greater than 99%). However, despite the high removal
of metal species and As, a net consumption of sulfate was not detected, suggesting that the
removal of As and metal ions is due to other processes not associated with precipitation as metal
sulfides.
These studies concluded that oxidation products of Fe(s) are the optimal phase for removing
As species. It can be concluded that organic matter used in vegetal municipal compost behaved
as a poor carbon source to support the processes of sulfate reduction. However, mixtures of
organic/calcite/Fe(s) were very efficient in removing metallic and As species, in this last case
with measured concentrations always below the value set by WHO of 10 gL − 1 . Both reaction

mixtures are being validated in a barrier of 120 m length, 1.4 m width and an average depth of
6 m in the aquifer of Guadiamar since 1999 (Bolzicco et al ., 2001).

µ

1.5 LIMITATIONS OF IRON REACTIVE BARRIERS:

USE OF REACTIVE ZONES

Permeable reactive barriers are tested presently to treat a growing number of contaminants in
groundwater, mainly organics. In fact, many places are being decontaminated with this technology.
However, despite the success of these barriers, there are still significant limitations that come
from physical and geochemical characteristics of the particular site (Wilkin et al ., 2003). Such
problems arise in the case of applications to inorganic contaminants [Cr(VI), As(V)], which means
that for these cases the iron barrier technology is not considered as proven for the USEPA.
The most important limitation is the lack of information on the long-term effectiveness of a
large-scale process. Even though some real scale studies appear promising and suggest means for
various durations in the range of decades, caution should be exercised when predicting clearance
rates scale studies from laboratory data of short duration. Removal rates in real scale applications
can be influenced by long processes such as aging of the reagent or decrease in permeability due
to precipitation, microbiological growth or accumulation of gas, which cannot be predicted in
short-term laboratory experiments (Lo et al ., 2007; USEPA, 1998).
In addition to the physical limitations imposed by the geology of the site and commercial
excavation techniques, there are many unresolved issues concerning the process of elimination.
Ideally, the contaminants are permanently immobilized or transformed into non-hazardous sub-
stances. For iron barriers, an unresolved issue is the role of ferrous iron precipitates and the
surface impurities in the reduction process. In theory, these compounds may serve not only as
reducers, but also as catalysts. Ferrous iron bound to the surface or complexed species can be