In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 4

In these terms, four different barrier types may be defined, taking into consideration the
involved physical, chemical and biological processes:

• Type 1 : Precipitation and acidity control: calcite and mixtures of calcite with siliceous gravel

or other similar materials with adequate porosity.

• Type 2 : Chemical reduction with acidity control and sulfide precipitation: calcite and zerovalent

iron.

• Type 3 : Biological reduction of sulfates with acidity control and precipitation of sulfides: a

source of organic material (activated sludge, compost, wood chips), a bacterial source (sludge
from anaerobic areas of local rivers and creeks) and an acidity neutralizer as for instance
limestone lime (Blowes et al ., 1996; Gubert et al ., 2004).

• Type 4 : Chemical and biological reduction of sulfate with acidity control and precipitation of

sulfides: the composition of this barrier is the same as the previous one, but elemental iron is
added to increase the capacity of sulfate reduction (Hammack et al ., 1994; Schneider et al .,
2001).

A correct design of a PRB implies the need to characterize the physical and chemical processes
that regulate the acidity of the waters, as well as the elimination of metallic and nonmetallic
species and the hydrodynamic features of these materials. For this, experiments in columns that
simulate the behavior of the PRB have to be made (Hammack et al ., 1994). The description of the
behavior of the barrier material at laboratory scale is of vital importance for a correct design and to
preview its alterations with time. This can be done by incorporation of the fundamental parameters
obtained experimentally using simulation models that incorporate the transport through a porous
media and the chemical reactions that occur between the media and the solutes in water (Benner
et al ., 1999; Bolzicco et al ., 2001).
The transformation of a pollutant in a less hazardous form through irreversible reactions does
not necessarily require the elimination of the reactive medium, unless the reactivity decreases
or becomes obstructed. An example of this type of transformation in a reactive barrier is an
irreversible redox reaction where the pollutant is reduced or oxidized; the media may pro-
vide directly electrons for the reaction or may indirectly stimulate microorganisms to mediate
the electron transfer either supplying an electron acceptor (e.g., oxygen) or an electron donor
(e.g., a carbon source). In order to be effective, the electron transfer between the medium and
the pollutant must be thermodynamically favorable and easy in kinetic terms (Morrison et al .,
2006).
Microorganisms frequently mediate redox reactions where the pollutants are either in a
reduced form (e.g., petroleum hydrocarbons) or in an oxidized form (e.g., chlorinated solvents or
nitrates), using the degradation of the pollutants as metabolic nest to obtain energy and material
for cellular synthesis (Alvarez, 2000).
Four types of barriers are commonly used: with a sorbent, with zerovalent iron (ZVI), iron
scraps and composite barriers with both organic material and elemental iron, as will be indicated
in Section 1.3.2.
The design of a reactive barrier takes into account several factors. These include the reaction
velocity for a specific pollutant concentration by unit of mass of reactive medium or by surface
area, and the geochemistry and hydrology of the system. These factors affect the residence time
of the contaminated water in the barrier necessary to reach the target pollutant concentrations.
The capacity to manipulate some of these factors in predominantly passive terms would keep
the cost-efficiency ratio, allowing a more flexible design and more confidence to achieve the
elimination of the pollutants (Burghardt et al ., 2007).
The main pollutant groups studied for elimination through PRBs are the halogenated aliphatic
organics (trichloroethylene, tetrachloroethylene and carbon tetrachloride), heavy metals and
metalloids (hexavalent chromium, lead, molybdenum, arsenic and cadmium) and radionuclides
(Blowes et al ., 2000; Chen et al ., 1997; Gotpagar et al ., 1997; Gu et al ., 1999).