Effective Cleanup of Groundwater Contaminated with Radionuclides Using Permeable Reactive Barriers - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 164

FIGURE 9.8
Scanning electron microscopic image recorded in backscatter mode (heavy elements appear
bright) from HAP particles with uranium phosphate crystals (verified by the attached EDX
system). (From Simon, F.G., Biermann, V. and Peplinski, B. 2008. Applied Geochemistry 23(8),
2137-2145.)

1995). Surface site complexation can be described by different models with or
without electrostatic influence on charged surfaces (Allison et al. 1991). With
SOH as a notation for a surface site, the adsorption reaction of uranium onto
ferric oxyhydroxide as sorbent can be written as follows:

2 + +

SOHUO HO + +  SOHUOOH H − ( ) + (9.12)

2 2 2 3

Sorption, rather than precipitation, depends strongly on pH. If several con-
taminants should be removed from the groundwater, an optimum pH for
operation of the barrier is needed. This is difficult to achieve for uranium in
the presence of molybdenum because the latter is mobile at pH values above
8 while uranium exhibits low mobility (Morrison and Spangler 1993). The
distribution coefficient K d of uranium at different pH values in various soils
is displayed in Figure 9.9 (Office of Radiation and Indoor Air and Office of
Environmental Restoration 1999). K d is defined as the ratio of mass of adsor-
bate sorbed (mg/kg) to mass of adsorbate in solution (mg/L). High K d values
(l/kg) were derived from adsorption experiments with ferric oxyhydroxide
and kaolinite, low values from those with quartz which has low adsorptive
properties. The pH dependence arises from surface charge properties of the
soil and from the complex aqueous speciation of U(VI).