Assessing the Effective Reactive Surface Area in Heterogeneous Media Through the Use of Conservative and Reactive Tracer
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211 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
UO2t2+ on Oyster, VA Sands
Oyster, VA Sands 4
160
.
140 120
Cd
A
""_•100 2
AA A
E
n
0
i60 -
A
A
140
0
20
Ak
A
rhPL 201
00"0 0 40
60
80
100
11
00 0
Total Oxides (pMollg)
2 3 2 Surface Area (M Ig)
4
Figure 1. The relationship between reactive surface area and abundance of oxide coatings and surface area and the mass-based Kd,m for the adsorption of UO 22+ at pH 5 for sands collected from Oyster, Virginia [4]. Smith and Schafer [5] have previously proposed from theoretical considerations that surface area should be an anisotropic property in a heterogeneous porous media. However, routine static surface area characterization methods such as BET will not capture the anisotropy of the media. To measure effective reactive surface area for intact media, an approach that uses conservative and adsorbing tracers in solution to determine effective reactive surface area in advecting systems is proposed. The separate use of advecting systems and adsorption from solutions are established surface area characterization techniques with well developed theoretical basis [6]. The unique aspect of the multiple-tracer approach described here is to apply these techniques simultaneously (adsorption from an advecting solution) to intact heterogeneous porous media in both column studies and at the field scale. The in situ determination of the effective reactive surface area will allow more robust assessment and prediction of the transport of radionuclides, metals, and contaminants in the subsurface. APPROACH The use of tracers to estimate hydrological and reactive parameters of columns and aquifers is well established. Typically, one or more tracers with differing characteristics are introduced into the system being investigated and the breakthrough of the tracers is monitored. The multiple-tracer approach can be used to estimate the in situ effective reactive surface area (i.e., the surface area that a packet of advecting water interacts with) for intact cores and aquifers as described below. In the case of pure advective transport of a tracer (i.e., ignoring the effects of dispersion), the velocity of a tracer exhibiting linear reversible adsorption is given by Vr =
Vw Rf,
R:,r =c Vr
T
D / Tr
Tc
(1)
where Rfr is the retardation factor for the reactive tracer. Vr, Vw, and Vc are the velocities of the reactive tracer, water, and a conservative tracer introduced simultaneously with the reactive tracer (i.e., Vw = VA), respectively. Tr and Tc are the times required for the reactive and 212
conservative tracer, respectively, to travel distance D. Smith and Schafer [5] have previously shown that Rf, is related to the surface area along the flow path by 1-0
Rf,rl+KdL
+ Kd,AAS
(2)
where p is the grain density of the medium, 0 is the porosity, and Kd.m and Kd,.A are mass-based and surface area-based distribution coefficients, respectively. Equations (1) and (2) allow in situ or effective reactive surface area to be estimated from m
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