CHAPTER 15 . Redox Processes of Chromium in Sea Water
293
Cr(III)-Cr(VI) interconversions in the photic layer of aquatic systems appear to be
strongly controlled by the concentrations of photochemically produced Fe(1I) and H20 2
(Fig. 15.8). By assuming pH = 8.2, I = 0.74, Fe(II) = 1 nM and H20 2 = 100 nM, which are
conditions typical of surface sea water (Zika et al. 1985; Voelker et al. 1997), the kinetic
balance between the reduction of Cr(VI) by Fe(II) and the oxidation of Cr(I1I) with
H 2 0 2 is reached when Cr(I1I) has approximately the same concentration as Cr(VI).
Complexation of Cr(lII) by organic ligands and sorption to surfaces also occur in the
water column. Cr(I1I) scavenged from the water column may be reoxidized by Mn
oxides at the sea/sediment interface of oxic systems producing Cr(VI) species which
diffuse into the bottom waters.
Precipitation and sorption processes of Cr(III) explain the relatively short residence
time of chromium (_10 4 years) in oceans compared to sulfur (-10 7 years) and molybdenum (_10 6 years). These three elements behave similarly at their highest oxidation
state: they form tetracoordinate oxyanions of identical charge and nearly identical size
in oxygenated surface waters, are actively acquired by marine plankton by using the
same anion channel and neither strongly sorb onto inorganic surfaces nor give rise to
strong bioaccumulation processes (Kieber and Helz 1992). However, the kinetic control of the Cr(VI)/Cr(III) ratios in seawater, which provides Cr(IlI) concentrations
higher than those expected on a thermodynamic basis, favours the removal of chromium from the water column through sorption and precipitation processes and lowers its residence time.
Further data on organic Cr species are needed for understanding their role in the
cycle of chromium in natural waters.
Acknowledgements
The author wishes to thank Prof. EJ. Millero of the University of Miami for the revision of the English text. I wish also express my gratitude to Frank Millero for his continuous scientific stimulation during a twenty-year friendship.
References
Ahern F, Eckert JR, Payne NC (1985) Speciation of chromium in seawater. Anal Chim Acta 175:
147-151
Baes CF, Mesmer RE (1976) The hydrolysis of cations. Wiley, London
Buerge IJ, Hug SJ (1997) Kinetics and pH dependence of chromium{VI) reduction by iron{II). Environ
Sci Technol 31:1426-1432
Campbell JA, Yeats P (1984) Dissolved chromium in the St. Lawrence Estuary. Estuar Coastal Shelf Sci
19:513-522
Chuecas L, Riley JP (1966) The spectrophotometric determination of chromium in seawater. Anal Chim
Acta 35:240
Collienne RH (1983) Photoreduction of iron in epilimnion lakes. Limnol Oceanogr 28:83-100
Cooper WJ, Zika RG (1983) Photochemical formation of hydrogen peroxide in surface and ground
waters exposed to sunlight. Science 20:711-712
Cranston RE (1983) Chromium in Cascadian basin, northeast Pacific Ocean. Mar Chern 13=109-125
Cranston RE, Murray JW (1978) The determination of chromium species in natural waters. Anal Chim
Acta 99:275-282
Cranston RE,Murray JW (1980) Chromium species in the Columbia River and Estuary. Limnol Oceanol
25{ 6):1104-1112
Deng B, Stone AT (1996) Surface catalyzed chromium{VI) reduction: Reactivity comparisons of different organic reductants and different oxide surfaces. Environ Sci Technol 30:2484-2494
293
Cr(III)-Cr(VI) interconversions in the photic layer of aquatic systems appear to be
strongly controlled by the concentrations of photochemically produced Fe(1I) and H20 2
(Fig. 15.8). By assuming pH = 8.2, I = 0.74, Fe(II) = 1 nM and H20 2 = 100 nM, which are
conditions typical of surface sea water (Zika et al. 1985; Voelker et al. 1997), the kinetic
balance between the reduction of Cr(VI) by Fe(II) and the oxidation of Cr(I1I) with
H 2 0 2 is reached when Cr(I1I) has approximately the same concentration as Cr(VI).
Complexation of Cr(lII) by organic ligands and sorption to surfaces also occur in the
water column. Cr(I1I) scavenged from the water column may be reoxidized by Mn
oxides at the sea/sediment interface of oxic systems producing Cr(VI) species which
diffuse into the bottom waters.
Precipitation and sorption processes of Cr(III) explain the relatively short residence
time of chromium (_10 4 years) in oceans compared to sulfur (-10 7 years) and molybdenum (_10 6 years). These three elements behave similarly at their highest oxidation
state: they form tetracoordinate oxyanions of identical charge and nearly identical size
in oxygenated surface waters, are actively acquired by marine plankton by using the
same anion channel and neither strongly sorb onto inorganic surfaces nor give rise to
strong bioaccumulation processes (Kieber and Helz 1992). However, the kinetic control of the Cr(VI)/Cr(III) ratios in seawater, which provides Cr(IlI) concentrations
higher than those expected on a thermodynamic basis, favours the removal of chromium from the water column through sorption and precipitation processes and lowers its residence time.
Further data on organic Cr species are needed for understanding their role in the
cycle of chromium in natural waters.
Acknowledgements
The author wishes to thank Prof. EJ. Millero of the University of Miami for the revision of the English text. I wish also express my gratitude to Frank Millero for his continuous scientific stimulation during a twenty-year friendship.
References
Ahern F, Eckert JR, Payne NC (1985) Speciation of chromium in seawater. Anal Chim Acta 175:
147-151
Baes CF, Mesmer RE (1976) The hydrolysis of cations. Wiley, London
Buerge IJ, Hug SJ (1997) Kinetics and pH dependence of chromium{VI) reduction by iron{II). Environ
Sci Technol 31:1426-1432
Campbell JA, Yeats P (1984) Dissolved chromium in the St. Lawrence Estuary. Estuar Coastal Shelf Sci
19:513-522
Chuecas L, Riley JP (1966) The spectrophotometric determination of chromium in seawater. Anal Chim
Acta 35:240
Collienne RH (1983) Photoreduction of iron in epilimnion lakes. Limnol Oceanogr 28:83-100
Cooper WJ, Zika RG (1983) Photochemical formation of hydrogen peroxide in surface and ground
waters exposed to sunlight. Science 20:711-712
Cranston RE (1983) Chromium in Cascadian basin, northeast Pacific Ocean. Mar Chern 13=109-125
Cranston RE, Murray JW (1978) The determination of chromium species in natural waters. Anal Chim
Acta 99:275-282
Cranston RE,Murray JW (1980) Chromium species in the Columbia River and Estuary. Limnol Oceanol
25{ 6):1104-1112
Deng B, Stone AT (1996) Surface catalyzed chromium{VI) reduction: Reactivity comparisons of different organic reductants and different oxide surfaces. Environ Sci Technol 30:2484-2494