286
M. Pettine
15.4.1
Oxidation Processes
Because the redox potential of the Cr(VI)/Cr(III) couple is so high (Fig. 15.3), there
are few oxidants capable of oxidizing Cr(Ill) to Cr(VI). Dissolved oxygen and manganese oxides were first considered as the most important species responsible for the
oxidation of Cr(Ill) in aquatic environments (Eary and Rai 1987).
The kinetics of the oxidation of Cr{Ill) to Cr(VI) with O2 have been investigated in
laboratory and field studies, and the reaction was found to be slow with halflives ranging from 2 to 20 months (Schroeder and Lee 1975; Cranston and Murray 1978; Emerson
et al. 1979; Eary and Rai 1987).
The oxidation of Cr(Ill) by manganese oxides is reported to be more rapid than by
dissolved oxygen. These oxides are present in aquatic environments as grain coatings,
crack deposits or finely disseminated grains, related or not with bacterial activities
(Richard and Bourg 1991). The oxidation reaction occurs through three steps, including adsorption of Cr(Ill) onto Mn02 surface sites, oxidation of Cr(Ill) to Cr(VI) by
surface Mn(IV) and desorption of the reaction products, Cr(VI) and Mn(ll). The
Cr{Ill) oxidation rates were found to be strongly related to the amount and the surface area of Mn oxides, changing from initially rapid values to significantly lower rates
after 20-60 min (Schroeder and Lee 1975; Nakayama et al. 1981d; Eary and Rai 1987;
Fendorf and Zasoski 1992). Different theoretical stoichiometry and rates have been
suggested for different Mn oxides (Eary and Rai 1987; Richard and Bourg 1991)
Cr(OH)2+ + 3 j3-Mn02(S) + 3 H20~ HCr04" + 3 MnOOH(s) + 3 H+
(15.5)
Eary and Rai (1987) have reported that Cr(Ill) is readily oxidized to Cr(VI) by 13Mn02(S) over the pH range from 3.0 to 10.1 and calculated a half life of 95 days for the
oxidation of a 10- 5 M Cr{Ill) solution for 1 kg of a 20% porosity soil containing
0.05 % wt j3-Mn02(S) with a surface area of 5.0 m 2 g-l. Other forms of soil manganese
oxides such as birnessite (0-Mn02) and cryptomalene ( a -Mn02) that are likely to have
higher surface energies than j3-Mn02(S) may cause a more rapid oxidation of Cr(Ill)
than j3-Mn02(S)' Therefore, manganese oxides are likely to be the most important oxidants for Cr{Ill) in ground-water porous systems and at the sediment-water interface
of oxic aquatic systems. Under different field conditions where concentrations of Mn
particles are lower, and competition for surface sites and inorganic/organic coating
of reactive sites increase, the role of Mn oxide should be minor.
Recent findings (Cooper and Zika 1983; Zika et al.1985) on the photochemical production of H 2 0 2 in surface sea water have stimulated our interest in studying its role
in Cr speciation through the oxidation of Cr(Ill). The rates of oxidation of Cr{Ill) to
Cr(VI) with H 2 0 2 have been measured under pseudo first order conditions in borate
buffered NaCI0 4 and NaCI solutions (Pettine and Millero 1990; Pettine et al. 1991) as a
function of pH, temperature, ionic strength and H20 2 concentration. The oxidation
rates were found to be first order with respect to both 1/ H+ or OH- in the pH range
7-9 and H 2 0 2 in the range 112 to 2621 JlM. Fig. 15.4 shows the values of the logarithm
of the pseudo-first-order constant as a function of pH. Results showed a marked de-
M. Pettine
15.4.1
Oxidation Processes
Because the redox potential of the Cr(VI)/Cr(III) couple is so high (Fig. 15.3), there
are few oxidants capable of oxidizing Cr(Ill) to Cr(VI). Dissolved oxygen and manganese oxides were first considered as the most important species responsible for the
oxidation of Cr(Ill) in aquatic environments (Eary and Rai 1987).
The kinetics of the oxidation of Cr{Ill) to Cr(VI) with O2 have been investigated in
laboratory and field studies, and the reaction was found to be slow with halflives ranging from 2 to 20 months (Schroeder and Lee 1975; Cranston and Murray 1978; Emerson
et al. 1979; Eary and Rai 1987).
The oxidation of Cr(Ill) by manganese oxides is reported to be more rapid than by
dissolved oxygen. These oxides are present in aquatic environments as grain coatings,
crack deposits or finely disseminated grains, related or not with bacterial activities
(Richard and Bourg 1991). The oxidation reaction occurs through three steps, including adsorption of Cr(Ill) onto Mn02 surface sites, oxidation of Cr(Ill) to Cr(VI) by
surface Mn(IV) and desorption of the reaction products, Cr(VI) and Mn(ll). The
Cr{Ill) oxidation rates were found to be strongly related to the amount and the surface area of Mn oxides, changing from initially rapid values to significantly lower rates
after 20-60 min (Schroeder and Lee 1975; Nakayama et al. 1981d; Eary and Rai 1987;
Fendorf and Zasoski 1992). Different theoretical stoichiometry and rates have been
suggested for different Mn oxides (Eary and Rai 1987; Richard and Bourg 1991)
Cr(OH)2+ + 3 j3-Mn02(S) + 3 H20~ HCr04" + 3 MnOOH(s) + 3 H+
(15.5)
Eary and Rai (1987) have reported that Cr(Ill) is readily oxidized to Cr(VI) by 13Mn02(S) over the pH range from 3.0 to 10.1 and calculated a half life of 95 days for the
oxidation of a 10- 5 M Cr{Ill) solution for 1 kg of a 20% porosity soil containing
0.05 % wt j3-Mn02(S) with a surface area of 5.0 m 2 g-l. Other forms of soil manganese
oxides such as birnessite (0-Mn02) and cryptomalene ( a -Mn02) that are likely to have
higher surface energies than j3-Mn02(S) may cause a more rapid oxidation of Cr(Ill)
than j3-Mn02(S)' Therefore, manganese oxides are likely to be the most important oxidants for Cr{Ill) in ground-water porous systems and at the sediment-water interface
of oxic aquatic systems. Under different field conditions where concentrations of Mn
particles are lower, and competition for surface sites and inorganic/organic coating
of reactive sites increase, the role of Mn oxide should be minor.
Recent findings (Cooper and Zika 1983; Zika et al.1985) on the photochemical production of H 2 0 2 in surface sea water have stimulated our interest in studying its role
in Cr speciation through the oxidation of Cr(Ill). The rates of oxidation of Cr{Ill) to
Cr(VI) with H 2 0 2 have been measured under pseudo first order conditions in borate
buffered NaCI0 4 and NaCI solutions (Pettine and Millero 1990; Pettine et al. 1991) as a
function of pH, temperature, ionic strength and H20 2 concentration. The oxidation
rates were found to be first order with respect to both 1/ H+ or OH- in the pH range
7-9 and H 2 0 2 in the range 112 to 2621 JlM. Fig. 15.4 shows the values of the logarithm
of the pseudo-first-order constant as a function of pH. Results showed a marked de-