The MUlti-start optimization
algorithm for Surface complexation
Equilibrium (MUSE) algorithm has been developed to optimize the fitting
of thermodynamic constants for surface complexation modeling (SCM).
Although there is a plethora of software to perform data fitting and
determine intrinsic equilibrium constants, the algorithms used are
highly dependent on initial values and choice of parameters. This
limits their transferability to model other systems, for example,
reactive transport processes. With this in mind, a hybridized optimization
approach, based on a multistart algorithm combined with a local optimizer,
has been developed to allow the simultaneous optimization of SCM parameters
and to assess the sensitivity of these parameters to changes in the
model assumptions. In this study, the CD–MUSIC formalism with
a Basic Stern electrostatic model is utilized to model chromate adsorption
on ferrihydrite, although the MUSE algorithm can be applied to any
adsorption data set and be implemented in any model formulation. This
study offers two innovative components to the inverse SCM modeling
approach: (a) determination of the true global optimum by performing
multiple minimizations of the mean squared error between the simulated
and observed data using a large number of initial starting points
and (b) quantitative simulation of spectroscopic pH-dependent profiles
for two chromate surface complexes. We demonstrate that when MUSE
is implemented to determine chromate log Ks, their
dependence on other adjustable parameters such as specific surface
area (SSA) and capacitance is relatively small (i.e., less than one
unit difference for chromate log Ks on ferrihydrite)
and can be accounted by mathematical functions determined through
the MUSE algorithm. The robustness of the algorithm is demonstrated
in the absence of the spectroscopy data as well, with traditional
batch tests yielding similar thermodynamic constants as the spectroscopic
profiles.