Web Release Date: December 24,
Effects of Dissolved Carbonate on Arsenate Adsorption and Surface Speciation at the Hematite-Water Interface
and
U.S. Geological Survey, MS 465, 345 Middlefield Road, Menlo Park, California 94025, and Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717
Received for review July 21, 2003
Revised manuscript received November 4, 2003
Accepted November 17, 2003
Abstract:
Effects of dissolved carbonate on arsenate [As(V)]
reactivity and surface speciation at the hematite-water
interface were studied as a function of pH and two different
partial pressures of carbon dioxide gas [PCO2 = 10-3.5
atm and ~0; CO2-free argon (Ar)] using adsorption kinetics,
pseudo-equilibrium adsorption/titration experiments,
extended X-ray absorption fine structure spectroscopic
(EXAFS) analyses, and surface complexation modeling.
Different adsorbed carbonate concentrations, due to the
two different atmospheric systems, resulted in an enhanced
and/or suppressed extent of As(V) adsorption. As(V)
adsorption kinetics [4 g L-1, [As(V)]0 = 1.5 mM and I =
0.01 M NaCl] showed carbonate-enhanced As(V) uptake in
the air-equilibrated systems at pH 4 and 6 and at pH 8
after 3 h of reaction. Suppressed As(V) adsorption was
observed in the air-equilibrated system in the early stages
of the reaction at pH 8. In the pseudo-equilibrium adsorption
experiments [1 g L-1, [As(V)]0 = 0.5 mM and I = 0.01
M NaCl], in which each pH value was held constant by a pH-stat apparatus, effects of dissolved carbonate on As(V)
uptake were almost negligible at equilibrium, but titrant (0.1
M HCl) consumption was greater in the air-equilibrated
systems (PCO2 = 10-3.5 atm) than in the CO2-free argon system
at pH 4-7.75. The EXAFS analyses indicated that As(V)
tetrahedral molecules were coordinated on iron octahedral
via bidentate mononuclear (
2.8 Å) and bidentate
binuclear (3.3 Å) bonding at pH 4.5-8 and loading
levels of 0.46-3.10 
M m-2. Using the results of the pseudo-equilibrium adsorption data and the XAS analyses, the pH-dependent As(V) adsorption under the PCO2 = 10-3.5
atm and the CO2-free argon system was modeled using
surface complexation modeling, and the results are consistent
with the formation of nonprotonated bidentate surface
species at the hematite surfaces. The results also suggest
that the acid titrant consumption was strongly affected
by changes to electrical double-layer potentials caused by
the adsorption of carbonate in the air-equilibrated
system. Overall results suggest that the effects of dissolved
carbonate on As(V) adsorption were influenced by the
reaction conditions [e.g., available surface sites, initial
As(V) concentrations, and reaction times]. Quantifying the
effects of adsorbed carbonate may be important in
predicting As(V) transport processes in groundwater,
where iron oxide-coated aquifer materials are exposed to
seasonally fluctuating partial pressures of CO2(g).
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