M.Pettine
The Cr(VI)/Cr(I1I) ratios at seawater ionic strength become in this case 10 19 .4 and
101.1 with pe of 12.5 and 6.5, respectively. According to Reaction 15.3, Cr(VI) should be
about a factor of 10 higher than Cr(I1I) at a pe of 6.5. This Cr(VI)/Cr(I1I) ratio is about
an order of magnitude lower than that resulting from Relation 15.2 at the same pe. In
anoxic regions the ratio Cr(VI)/Cr(I1I) should be about 10- 20 or less, suggesting analytically insignificant concentrations of Cr(VI).
15.3
Environmental Concentrations
The chromium content of natural solids varies widely from tens of nanomoles to a
few micromoles per gram of solid (Richard and Bourg 1991); while the dissolved concentrations range from one to ten nanomoles per liter in unpolluted or slightly polluted aquatic systems. Typical concentrations recorded in oceanic, coastal and estuarine waters are given in Table 15.1. These available data indicate that Cr(VI)/Cr(III)
ratios range from <1 to >100 for oxic conditions (Chuecas and Riley 1966; Fukai 1967;
Fukai and Vas 1969; Kuwamoto and Murai 1970; Grimaud and Michard 1974; Emerson
et al. 1979; Cranston and Murray 1980; Nakayama et al. 1981b; Nakayama et al. 1981C;
Cranston 1983; Murray et al. 1983; Campbell and Yeats 1984; Ahern et al. 1985; Pettine
et a1.1992; Pettine et al.1997). Earlier results (Chuecas and Riley 1966; Fukai 1967; Fukai
and Vas 1969; Kuwamoto and Murai 1970; Grimaud and Michard 1974) giving values
<1 were probably affected by the alteration due to Cr(VI) reduction on storage at low
pH (Fukai and Vas 1969; Kuwamoto and Murai 1970; Grimaud and Michard 1974; Ahern
et a1.1985; Pettine et al.1988) or reflected variable extraction yields of organically bound
Cr(III) (Nakayama et al. 1981a; Jeandel and Minster 1984). Ratios of about 0.5 and 2
were found in Saanich Inlet below the 0iH2S interface (Emerson et al. 1979) and at
stations along a transect from Hawaii to the coast of Baja California at the top of oxygen minimum zone (Murray et al. 1983), respectively.
Thus, field distributions of Cr(I1I) and Cr(VI) appear in many cases to be far from
thermodynamic equilibria in both oxic and anoxic environments. However, the revision of the hydrolysis constants of Cr(III) (Rai et al. 1987) makes the extent of the discrepancy of the field results with thermodynamic expectations lower than was previously highlighted by Elderfield (1970) for oxic environments.
Many factors, including unreliable analytical data, existence of unknown redox
species, and kinetic control by purely chemical and biologically mediated reactions
may be invoked to explain the deviations of the field concentrations with respect to
thermodynamic predictions. This discrepancy has stimulated studies on the kinetics
and speciation of chromium aimed at elucidating processes responsible for Cr(III)Cr(VI) interconversions in environmental matrices. Data recently gathered on the kinetics of the main redox reactions affecting the cycle of chromium in natural waters
will be reviewed in the next section.
15.4
Kinetic Studies
Until recently, geochemical and environmental models have considered the fate of
metals in the environment as the result of equilibrium processes only, while it has now
The Cr(VI)/Cr(I1I) ratios at seawater ionic strength become in this case 10 19 .4 and
101.1 with pe of 12.5 and 6.5, respectively. According to Reaction 15.3, Cr(VI) should be
about a factor of 10 higher than Cr(I1I) at a pe of 6.5. This Cr(VI)/Cr(I1I) ratio is about
an order of magnitude lower than that resulting from Relation 15.2 at the same pe. In
anoxic regions the ratio Cr(VI)/Cr(I1I) should be about 10- 20 or less, suggesting analytically insignificant concentrations of Cr(VI).
15.3
Environmental Concentrations
The chromium content of natural solids varies widely from tens of nanomoles to a
few micromoles per gram of solid (Richard and Bourg 1991); while the dissolved concentrations range from one to ten nanomoles per liter in unpolluted or slightly polluted aquatic systems. Typical concentrations recorded in oceanic, coastal and estuarine waters are given in Table 15.1. These available data indicate that Cr(VI)/Cr(III)
ratios range from <1 to >100 for oxic conditions (Chuecas and Riley 1966; Fukai 1967;
Fukai and Vas 1969; Kuwamoto and Murai 1970; Grimaud and Michard 1974; Emerson
et al. 1979; Cranston and Murray 1980; Nakayama et al. 1981b; Nakayama et al. 1981C;
Cranston 1983; Murray et al. 1983; Campbell and Yeats 1984; Ahern et al. 1985; Pettine
et a1.1992; Pettine et al.1997). Earlier results (Chuecas and Riley 1966; Fukai 1967; Fukai
and Vas 1969; Kuwamoto and Murai 1970; Grimaud and Michard 1974) giving values
<1 were probably affected by the alteration due to Cr(VI) reduction on storage at low
pH (Fukai and Vas 1969; Kuwamoto and Murai 1970; Grimaud and Michard 1974; Ahern
et a1.1985; Pettine et al.1988) or reflected variable extraction yields of organically bound
Cr(III) (Nakayama et al. 1981a; Jeandel and Minster 1984). Ratios of about 0.5 and 2
were found in Saanich Inlet below the 0iH2S interface (Emerson et al. 1979) and at
stations along a transect from Hawaii to the coast of Baja California at the top of oxygen minimum zone (Murray et al. 1983), respectively.
Thus, field distributions of Cr(I1I) and Cr(VI) appear in many cases to be far from
thermodynamic equilibria in both oxic and anoxic environments. However, the revision of the hydrolysis constants of Cr(III) (Rai et al. 1987) makes the extent of the discrepancy of the field results with thermodynamic expectations lower than was previously highlighted by Elderfield (1970) for oxic environments.
Many factors, including unreliable analytical data, existence of unknown redox
species, and kinetic control by purely chemical and biologically mediated reactions
may be invoked to explain the deviations of the field concentrations with respect to
thermodynamic predictions. This discrepancy has stimulated studies on the kinetics
and speciation of chromium aimed at elucidating processes responsible for Cr(III)Cr(VI) interconversions in environmental matrices. Data recently gathered on the kinetics of the main redox reactions affecting the cycle of chromium in natural waters
will be reviewed in the next section.
15.4
Kinetic Studies
Until recently, geochemical and environmental models have considered the fate of
metals in the environment as the result of equilibrium processes only, while it has now