Arsenate–Ferrihydrite
Systems from Minutes
to Months: A Macroscopic and IR Spectroscopic Study of an Elusive
Equilibrium
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Abstract
Sorption by ferrihydrite is an important
control on As(V) concentrations
in many oxic aquatic systems. There are significant discrepancies
in reported sorption constants (log(<i>K</i><sub>As</sub>)), which presents a problem for quantifying and understanding this
important system. A review of reported ferrihydrite–As(V) sorption
studies indicated a positive correlation between reaction time used
in the experiments and the log(<i>K</i><sub>As</sub>) values
derived from the data. In this paper, we study the kinetics of As(V)
sorption over ≈3000 h in nine systems with varying pH and As(V)/Fe.
Ferrihydrite was stable in all systems containing As(V), and the [As(V)]
in solution decreased linearly as a function of log(<i>t</i>) (termed Elovich kinetics) over the full 3000 h in most systems.
A stable [As(V)] was only observed in systems with low As(V)/Fe and
low pH. Apparent As(V) sorption constants were derived from the data
at specific time intervals using the diffuse layer model and equations
describing log(<i>K</i><sub>As</sub>) values as a function
of time provide a way to describe this elusive equilibrium. IR spectra
support the hypothesis that slow interparticle diffusion is responsible
for the slow approach to equilibrium. This work resolves discrepancies
in previous studies of As(V)–ferrihydrite and provides equations
to allow for system appropriate log(<i>K</i><sub>As</sub>) values to be used
