199

The accuracy of the empirical equations (Eqs. 7.2 and 7.3) obtained from
the batch kinetic tests was evaluated using the column test data. Eq. 7.2 was
used to predict partition coefficients using data obtained from the column tests. A
comparison of the measured and estimated partition coefficients is shown in Fig.
7.16. Good agreement is shown when initial solution pHs are considered (Fig.
7.16a). However, there is little agreement between the estimated partition
coefficient and the partition coefficients from the column tests when solution pHs
obtained at breakthrough of zinc are used for the estimation.
Rate constants obtained from the column tests and the batch kinetic tests
are shown in Fig. 7.17. The rate constants obtained using both methods are
comparable, and appear to be modestly affected by solution pH.
7.4 PRB DESIGN
PRB thicknesses were computed using Eq. 3.21 to illustrate how PRBs
can be used for treating groundwater contaminated with zinc. The mechanical
dispersion coefficient was calculated by multiplying one-tenth of the barrier
thickness by the seepage velocity. The molecular diffusion coefficient for zinc
was calculated by multiplying the aqueous diffusion coefficient of zinc by the
tortuosity. The aqueous diffusion coefficient of zinc (7.37x10-6 cm2/s) was
obtained using an average aqueous diffusion coefficient (Li and Gregory 1974),
and a tortuosity of 0.4, which was obtained from typical values of tortuosity
ranging from 0.31 to 0.64 (Bear 1972). The molecular diffusion coefficient