Immobilization and detoxification of chromium in the vadose zone is made possible by the existence of an effective reductant, SO2, that exists in a gaseous form at room temperature. Experimental studies were designed to characterize stoichiometry and
kinetics of chromium reduction both in aqueous solutions at pH values near neutrality and in soil.
First, batch experiments and elemental analyses were conducted to characterize the stoichiometry and kinetics of Cr(VI) reduction in water. The stoichiometric ratio of
S(IV) removed to Cr(VI) removed ranged between 1.6 and 1.8. The overall reaction is
believed to be the result of a linear combination of two reactions in which dithionate is
an intermediate and sulfate is the stable oxidized product. The reaction was also rapid,
with the half-time of about 45 minutes at pH 6 and about 16 hours at pH 7. A two-step
kinetic model was developed to describe changes in concentrations of Cr(VI), S(IV), and
S(V). Nonlinear regression was applied to obtain the kinetic parameters. The rate of
reaction was assumed to be second-order with respect to [Cr(VI)] and first-order with
respect to [S(IV)], and [S(V)]. The values for the rate coefficient for the first reaction
(k1) were found to be 4.5 (?10%), 0.25 (?9.4%) (mM-2h-1) at pH 6 and 7, respectively.
The values of the rate coefficient for the second reaction (k2) were 25 (?29%), 1.1 (?
30%) (mM-2h-1) at pH 6 and 7, respectively. The reaction rate decreased as pH
increased. Experiments showed that the rate at pH 7 was lower than that at pH 6 by one
order of magnitude.
Second, batch experiments and elemental analyses were conducted to characterize
the stoichiometry and kinetics of Cr(VI) reduction in soil. The stoichiometric ratio of
S(IV) removed to Cr(VI) removed was almost 2, which is slightly higher than that for
the reaction in water. This higher value may be due to S(IV) oxidation by soil-derived
Fe(III). The reaction was rapid, with the half-time less than 2 minutes, which is faster
than in water. The rate coefficients, k1 and k2, were 22 (?41%) and 13 (?77%) (M-2h-1), respectively.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/1183 |
| Date | 15 November 2004 |
| Creators | Ahn, Min |
| Contributors | Batchelor, Bill |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | 541385 bytes, 72618 bytes, electronic, application/pdf, text/plain, born digital |
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