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SPECHROM Report Summary

Project ID: 299302
Funded under: FP7-PEOPLE
Country: Greece

Final Report Summary - SPECHROM (Spectroscopic and computational investigation of chromium binding on pure minerals and Asopos aquifer soils)

Spectroscopic and computational investigation of chromium binding on pure minerals and Asopos aquifer soils
Maria Chrysochoou, Ph.D., Associate Professor, Department of Civil and Environmental Engineering, University of Connecticut. Email: Ph: +1 860 486 3594
Daniel Mamais, Ph.D. Associate Professor, School of Civil Engineering, National Technical University of Athens. Email: Ph: +30 210 772 2901
Project website:

Understanding the mobility of metal contaminants in the subsurface is essential to performing risk assessments for exposure of sensitive receptors, such as humans and ecosystems. The concentration of metals in groundwater is controlled both by the aqueous chemistry and by their interactions with the aquifer constituents; sorption and precipitation are both retardation mechanisms that can decrease metal concentrations to low enough levels that intervention is not required downstream from the pollution source. Thus, a quantitative understanding of these metal-soil interactions is crucial to the development of fate-and-transport models for risk assessment. Hexavalent chromium (Cr(VI)) is a human carcinogen and an important metal contaminant, since it is found both in the natural geologic background of many countries, and it has widespread industrial use. The overall goal of this project is to understand the behavior of Cr(VI) in Greek aquifers with elevated Cr(VI) concentrations due to the natural geologic background and in an area with significant industrial activity (Asopos river basin). SPECHROM is a sister project to the LIFE+ project “Chromium in Asopos River System: Remediation Technologies and Measures” (CHARM), which focuses on pollution control and remediation measures.
Accordingly, the SPECHROM project had two main research objectives:
1. Determine the surface complexation mechanisms of hexavalent chromium (Cr(VI)) to environmentally relevant minerals (iron and aluminium oxyhydroxides) using infrared and X-ray Absorption spectroscopy, coupled with sorption isotherms and computational chemistry calculations. This knowledge will be used to build sorption models that are sensitive to geochemical gradients and changing aquifer properties.
2. Determine the speciation of Cr in ultramafic soils, which have naturally high Cr concentrations. Several areas in Greece have such a geologic background, including Vergina, Thiva and Asopos. Of these, Vergina only has agricultural activity, while Thiva and Asopos are impacted by industrial activities. Understanding Cr mobility and oxidation mechanisms as a function of regional characteristics contributes towards risk assessment for groundwater pollution in any area with ultramafic background, in countries such as Italy, Spain, France, USA and Mexico, among others.
For the first objective, three minerals were targeted as the most important Cr(VI) sorbents in soils, specifically ferrihydrite (Fe2O3·0.5H2O), hematite (α-Fe2O3) and boehmite (γ-AlOOH). In order to determine the binding mechanisms on these minerals, Attenuated Total Reflectance – Fourier Transform Infrared (ATR-FTIR) spectroscopy and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies were employed, coupled with Density Functional Theory (DFT) calculations. The results provided new insights into Cr(VI) speciation on these surfaces, the most important of which are:
1. Cr(VI) forms both monodentate and bidentate complexes on ferrihydrite and hematite. Monodentate complexes forms first, at low surface coverage and high pH values (pH 6-8), while bidentate complexes prevail at high surface coverage and low pH values (pH <6). The inclusion of both species with separate thermodynamic constants in surface complexation modeling substantially improved fits to experimental sorption data on ferrihydrite.
2. Outer-sphere complexes with some inner-sphere (monodentate and bidentate) complexation were observed on boehmite. The inner sphere complexes were only present at high surface coverage and low pH. These data may used in similar fashion to ferrihydrite to build improved models for Cr(VI) sorption on boehmite, which are currently not available.
In addition, a variety of soil and rock samples obtained from several areas in Greece with geogenic Cr(VI) were analyzed via X-ray Absorption Near Edge Spectroscopy (XANES) to understand the speciation of both Cr and manganese (Mn). Mn is an important element because it facilitates the oxidation of non-toxic, immobile trivalent Cr (Cr(III)) to toxic and mobile Cr(VI). Thus, understanding the potential for Cr oxidation enables the prediction of potential Cr(VI) levels in groundwater. Samples were collected and analyzed from Asopos, Vergina, Thiva, Thermi and Avlona. All these areas have common geologic background, which includes serpentinite, an ultramafic rock type that contains high Cr levels and that is found in several populated areas around the world and especially Europe. Two proposals were prepared and submitted to acquire beamtime for XANES analyses on these soils and sediments, one for bulk XANES at the National Synchrotron Lightsource (NSLS) in Brookhaven, NY, USA and one for microXANES at the Advanced Light Source (ALS) in Berkeley, CA, USA.
The bulk XANES data from the five areas indicated that all Mn oxidation states (II, III and IV) were present in the aquifer soils, however Mn(III) and Mn(IV) were largely dominant over Mn(II). These oxidation states responsible for the oxidation of non-toxic Cr(III) to carcinogenic Cr(VI), which is mobile in groundwater. In Vergina, Mn(III) and Mn(IV) oxides were identified not only in shallow soils, but also in aquifer materials down to 57 m depth. Given that no industrial activity is present in the area, the observed Cr(VI) concentrations in groundwater that are up to 65 μg/L can only be attributed to the natural oxidation by the observed Mn oxides. Thus, the analyses showed that there is potential for continued production of Cr(VI) in the impacted areas, even in deep aquifers.
These results agreed with the microstructural analyses of the same samples, which investigated the spatial associations of Mn and Cr and their speciation on the grain scale. The results showed that the bulk of the Cr mass is present in larger chromite grains, while the remaining is finely dispersed within the soil matrix, most likely substituted in the primary mineral of serpentine soils, such as chrysotile. Several chromite grains with intimate spatial association with Mn oxides were identified. Previous literature indicates that Cr(III) leaches from chromite in the outer layer, leaving magnetite as rims, which was confirmed by the analyses. The physical association with Mn is conducive to quick sequestration of Cr(III) and subsequent oxidation to Cr(VI).
Overall, the synergy between the SPECHROM and the LIFE+ projects lead to an increased understanding of Cr(VI) production and binding mechanisms in serpentine soils, which are located in large populated areas in Europe and worldwide. The produced knowledge can aid in the development of guidelines and management strategies with regard to the presence of hexavalent chromium in aquifers located in serpentine geologic background.

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