Chromium(III)
Complexation to Natural Organic Matter:
Mechanisms and Modeling
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Abstract
Chromium is a common soil contaminant,
and it often exists as chromium(III).
However, limited information exists on the coordination chemistry
and stability of chromium(III) complexes with natural organic matter
(NOM). Here, the complexation of chromium(III) to mor layer material
and to Suwannee River Fulvic Acid (SRFA) was investigated using EXAFS
spectroscopy and batch experiments. The EXAFS results showed a predominance
of monomeric chromium(III)-NOM complexes at low pH (<5), in which
only Cr···C and Cr–O–C interactions were
observed in the second coordination shell. At pH > 5 there were
polynuclear
chromium(III)-NOM complexes with Cr···Cr interactions
at 2.98 Å and for SRFA also at 3.57 Å, indicating the presence
of dimers (soil) and tetramers (SRFA). The complexation of chromium(III)
to NOM was intermediate between that of iron(III) and aluminum(III).
Chromium(III) complexation was slow at pH < 4: three months or
longer were required to reach equilibrium. The results were used to
constrain chromium-NOM complexation in the Stockholm Humic Model (SHM):
a monomeric complex dominated at pH < 5, whereas a dimeric complex
dominated at higher pH. The optimized constant for the monomeric chromium(III)
complex was in between those of the iron(III) and aluminum(III) NOM
complexes. Our study suggests that chromium(III)-NOM complexes are
important for chromium speciation in many environments