1 research outputs found
Chromium(III) Complexation to Natural Organic Matter: Mechanisms and Modeling
Chromium is a common soil contaminant,
and it often exists as chromiumÂ(III).
However, limited information exists on the coordination chemistry
and stability of chromiumÂ(III) complexes with natural organic matter
(NOM). Here, the complexation of chromiumÂ(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 chromiumÂ(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
chromiumÂ(III)-NOM complexes with Cr···Cr interactions
at 2.98 Ã… and for SRFA also at 3.57 Ã…, indicating the presence
of dimers (soil) and tetramers (SRFA). The complexation of chromiumÂ(III)
to NOM was intermediate between that of ironÂ(III) and aluminumÂ(III).
ChromiumÂ(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 chromiumÂ(III)
complex was in between those of the ironÂ(III) and aluminumÂ(III) NOM
complexes. Our study suggests that chromiumÂ(III)-NOM complexes are
important for chromium speciation in many environments