In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 3

Figure 1.3. Permeable reactive barrier. Funnel-and gate configuration (USEPA, 1998).

Both configurations have been used for extensions up to 300 m but, as they require excavation,
they are limited to depths up to 15-20 m.
The funnel-and-gate configuration uses classic impermeable barriers disposed as a funnel to
direct the plume to the gate constituted by the PRB. The pattern flow of the groundwater is
more altered with this system compared with that using the continuous configuration. In both
configurations, the permeability of the gate should be higher than that of the aquifer, to avoid
diversions of the groundwater around the reactive gate. PRBs are especially attractive for ground-
water remediation because they conserve the water energy, and are potentially more inexpensive
than the conventional pump-and-treat remediation due to lower operational and maintenance
costs. Another advantage is that the reactants are used in-situ , thus avoiding the need for large
installations and equipment on the surface.
When evaluating the suitability of a reactive medium it is necessary to account for its capac-
ity to transform the pollutant with an appropriate kinetics, keeping an adequate permeability
and reactivity during long times, and releasing only environmentally acceptable compounds as
by-products.
The main processes that control the immobilization and transformation of the pollutants in the
barrier include sorption on the reactive medium and precipitation, chemical reaction and biogenic
reactions (Diels et al ., 2003). The most usual mechanism for non-polar organic compounds is
sorption because of their hydrophobicity (Scherer et al ., 2000).
On the other hand, metals are usually adsorbed through electrostatic attraction or through a
superficial complexation reaction. The suitability of sorbent materials for PRBs depends mainly
on the strength of the sorbed complex and on the capacity of the material to sorb a specific
pollutant. These materials have also the advantage of not releasing adverse chemicals to the
groundwater, but their efficiency generally depends also on the groundwater geochemistry (e.g.,
pH and major anions and cations). Furthermore, metals can be immobilized increasing pH or
adding an excess of ions to form an insoluble mineral phase. Thus, the metal precipitation process
is a combination of a transformation process followed by an immobilization process (Chen et al .,
1997; Ma et al ., 1994).
As both sorption and precipitation are normally reversible processes, it may be necessary to
eliminate the reactive materials and the accumulated products, depending on the stability of the
immobilized components and the geochemistry of the groundwater.