The rate and pathway of ferrihydrite (Fh) transformation at oxic conditions to more stable products is controlled largely by temperature, pH, and the presence of other ions in the system such as nitrate (NO 3 − ), sulfate (SO 4 2−), and arsenate (AsO 4 3−). Although the mechanism of Fh transformation and oxyanion complexation have been separately studied, the effect of surface complex type and strength on the rate and pathway remains only partly understood. We have developed a kinetic model that describes the effects of surface complex type and strength on Fh transformation to goethite (Gt) and hematite (Hm). Two sets of oxyanion-adsorbed Fh samples were prepared, nonbuffered and buffered, aged at 70 ± 1.5 °C, and then characterized using synchrotron X-ray scattering methods and wet chemical analysis. Kinetic modeling showed a significant decrease in the rate of Fh transformation for oxyanion surface complexes dominated by strong inner-sphere (SO 4 2− and AsO 4 3− ) versus weak outer-sphere (NO 3 − ) bonding and the control. The results also showed that the Fh transformation pathway is influenced by the type of surface complex such that with increasing strength of bonding, a smaller fraction of Gt forms compared with Hm. These findings are important for understanding and predicting the role of Fh in controlling the transport and fate of metal and metalloid oxyanions in natural and applied systems.

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Weathering of ultramafic rocks has been linked to the occurrence of elevated concentrations of hexavalent chromium (Cr(VI)) in soils, sediments, and groundwater. Ultramafic rocks and the derived serpentine soils and sediments are encountered in populated areas around the world and present high Cr concentrations, with an average of 2200 and 2650 mg/kg for rocks and soils, respectively. Groundwater concentrations between 0.2 and 180 μg/L have been reported for Cr(VI) in ultramafic areas, exceeding occasionally the most prevalent drinking water limit of 50 μg/L Cr tot , the 5 μg/L Cr(VI) limit established in Italy, and the 10 μg/L Cr(VI) limit proposed in California. Cr release in groundwater occurs through the dissolution of trivalent chromium (Cr(III)) from its mineral hosts, followed by sorption of Cr(III) onto high-valence Mn oxides and oxidation to Cr(VI), which desorbs and is mobile at alkaline pH. Recent findings indicate that hydrogen peroxide and birnessite produced on the surface of Cr(OH) 3 by heterogeneous oxidation are two additional potential mechanisms. Thus, groundwater c o n c e n t r a t i o n s a r e c o n t r o l l e d b y a v a r i e t y o f geoenvironmental factors, including climate, soil mineralogy, pH, organic matter, and others. To provide a basis for the evaluation of Cr mobility in ultramafic environments, this paper presents an overview of the mineralogy and geochemistry of Cr-rich rocks, sediments, and soils, along with the weathering and geochemical processes that control the fate and transport in the subsurface.

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.

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