M. Pettine
A number of low molecular weight organic compounds including substituted carboxylic acids and phenols were also found to be able to reduce Cr(VI), and reactions were
found to be catalyzed by oxide surfaces (Deng and Stone 1996). Fe(II) and H2S appear
to be the most important candidates in the reduction of Cr(VI) in natural waters (Saleh
et al. 1989). Both these chemicals are widely used in treatment processes of industrial
wastes for Cr(VI) removal through its reduction to Cr(lII) and the subsequent precipitation of Cr(lII) ions (Eary and Rai 1988).
Reduced forms of iron and sulfur can be produced in the pore waters of sediments
and in anoxic basin waters due to chemical or biological processes as well as in hydrothermal waters due to geochemical processes. Fe(II) and sulfides may also be produced in surface sea waters by photo oxidation of organic matter (Collienne 1983; Miller
et al. 1995; Voelker et al' 1997) and hydrolysis of carbonyl sulfides. Recently, we have
studied the kinetics of the Cr(VI) reduction by H2S (Pettine et al. 1994). The overall
rate constant (k, M- I min-I) in
dCr(VI) / dt = -k[Cr(VI)][H 2 S1
was given by (SD = 0.07 in log k)
log k = 16.19 -1.06 pH - 2301 / T
and was shown to be independent of ionic strength. Based on these kinetic measurements, the half times of Cr(VI) in NaCI media range from 0.5 to 500 ays at 1 mM and
11lM, respectively (Pettine et al. 1994). The reduction of Cr(VI) with H2S was found to
be catalyzed by heavy metals (Pb 2 +, Cu 2 +, Cd2+, Ni2+), which caused large increases in
the reduction rates at micromolar concentrations (Pettine et al. 1998a). The effect was
attributed to the formation of MeCr04 complexes which react faster with sulfide than
free chromate and follow the sequence NiCr04> PbCr04 > CuCr04 > CdCr04' Fe(III)
was also found to exert a catalytic effect on the reduction of Cr(VI) with H2S (Pettine
et al. 1998a), but the effect was, in this case, attributed to the substitution of the slow
one step reaction of Cr(VI) with H 2 S with a fast two-step process consisting of
Fe(III)+H 2 S, followed by the reaction of Fe(II)+Cr(VI). The latter process involves
cyclic reduction and oxidation of Fe with electron transfer from H2S to Cr(VI).
The role of Fe(II) in the reduction of Cr(VI) in aqueous systems has been recently
investigated by various workers (Fendorf and Li 1996; Buerge and Hug 1997; Sedlak
and Chan 1997; Pettine et al. 1998b). Our recent paper (Pettine et al. 1998b) extended
the previous findings by describing the reaction rate dependence on chromate and
ferrous ion speciation, pH, temperature and ionic strength, and by allowing one to
predict chromium reactions under most naturally occurring conditions from fresh to
salt water environments.
The rate of Cr(VI) reduction is given by the general expression
-d[ Cr(VI) 1 / dt = k[ Cr(VI) 1 [Fe(II) 1
where k (M- I min-I) can be determined from the
log k = 6.74 - 1.01 pH - 188.5 / T
A number of low molecular weight organic compounds including substituted carboxylic acids and phenols were also found to be able to reduce Cr(VI), and reactions were
found to be catalyzed by oxide surfaces (Deng and Stone 1996). Fe(II) and H2S appear
to be the most important candidates in the reduction of Cr(VI) in natural waters (Saleh
et al. 1989). Both these chemicals are widely used in treatment processes of industrial
wastes for Cr(VI) removal through its reduction to Cr(lII) and the subsequent precipitation of Cr(lII) ions (Eary and Rai 1988).
Reduced forms of iron and sulfur can be produced in the pore waters of sediments
and in anoxic basin waters due to chemical or biological processes as well as in hydrothermal waters due to geochemical processes. Fe(II) and sulfides may also be produced in surface sea waters by photo oxidation of organic matter (Collienne 1983; Miller
et al. 1995; Voelker et al' 1997) and hydrolysis of carbonyl sulfides. Recently, we have
studied the kinetics of the Cr(VI) reduction by H2S (Pettine et al. 1994). The overall
rate constant (k, M- I min-I) in
dCr(VI) / dt = -k[Cr(VI)][H 2 S1
was given by (SD = 0.07 in log k)
log k = 16.19 -1.06 pH - 2301 / T
and was shown to be independent of ionic strength. Based on these kinetic measurements, the half times of Cr(VI) in NaCI media range from 0.5 to 500 ays at 1 mM and
11lM, respectively (Pettine et al. 1994). The reduction of Cr(VI) with H2S was found to
be catalyzed by heavy metals (Pb 2 +, Cu 2 +, Cd2+, Ni2+), which caused large increases in
the reduction rates at micromolar concentrations (Pettine et al. 1998a). The effect was
attributed to the formation of MeCr04 complexes which react faster with sulfide than
free chromate and follow the sequence NiCr04> PbCr04 > CuCr04 > CdCr04' Fe(III)
was also found to exert a catalytic effect on the reduction of Cr(VI) with H2S (Pettine
et al. 1998a), but the effect was, in this case, attributed to the substitution of the slow
one step reaction of Cr(VI) with H 2 S with a fast two-step process consisting of
Fe(III)+H 2 S, followed by the reaction of Fe(II)+Cr(VI). The latter process involves
cyclic reduction and oxidation of Fe with electron transfer from H2S to Cr(VI).
The role of Fe(II) in the reduction of Cr(VI) in aqueous systems has been recently
investigated by various workers (Fendorf and Li 1996; Buerge and Hug 1997; Sedlak
and Chan 1997; Pettine et al. 1998b). Our recent paper (Pettine et al. 1998b) extended
the previous findings by describing the reaction rate dependence on chromate and
ferrous ion speciation, pH, temperature and ionic strength, and by allowing one to
predict chromium reactions under most naturally occurring conditions from fresh to
salt water environments.
The rate of Cr(VI) reduction is given by the general expression
-d[ Cr(VI) 1 / dt = k[ Cr(VI) 1 [Fe(II) 1
where k (M- I min-I) can be determined from the
log k = 6.74 - 1.01 pH - 188.5 / T