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MIR Report Summary

Project ID: 21120
Funded under: FP6-MOBILITY
Country: France

Final Activity Report Summary - MIR (Mineral-fluid interface reactivity)

The MIR early stage research training (EST) was aimed at improving our fundamental understanding of mineral-fluid reactions, including dissolution, adsorption, nucleation, precipitation and solid-solution formation. These processes are keys to solving pressing issues like the development of smart coatings on body implants or drug delivery systems, minimising risk in groundwater extraction, safer pesticide application, optimising carbon dioxide (CO2) sequestration, assuring drinking water quality, safe storage of radioactive waste products and minimising pollutant transport. The ability to accurately predict reactions in these systems is of utmost importance for municipalities and industry in Europe today, but it relies on a detailed description of mineral-fluid reactions.

Among the project keyworkds are:

1. carbonate mineral precipitation and growth
2. biomineralisation
3. green rust
4. trace element uptake by calcite
5. the dissolution mechanisms of mineral solid solutions
6. element partitioning in marine biogenic carbonates
7. the mechanism of 'ion-exchange' processes
8. phosphate minerals and their role in soil fertility.

Much of the research completed in the MIR network was aimed at a better understanding the reactivity of carbonate minerals. Much of this interest is because of their role in CO2 sequestration. The formation of stable carbonate minerals such as calcite and magnetite provide a safe, stable long-term form for sequestering CO2 in the subsurface during carbon storage efforts. To better assess the sequestration potential of magnesite for CO2 sequestration and improve the understanding of mineral nucleation and growth, we measured magnesite precipitation rates as a function of temperature, saturation state and aqueous solution composition. These rates were quantified through the use of transition state theory to obtain robust equations that could be used to predict magnesite growth during carbon sequestration efforts. Measured rates also suggested that the magnesite precipitated via a spiral growth mechanism. This mechanism was confirmed using atomic force microscopy (AFM).

The MIR early stage researcher (ESR) used barite (BaSO4) as a model compound to understand the dissolution and precipitation mechanism of minerals in general. The nucleation, growth and dissolution of barium sulfate (barite) in aqueous solutions of different background electrolytes were systematically measured using both flow reactors and AFM fluid flow cells. The dependence of rates and crystal morphology was found to depend strongly on the type of electrolyte present in solution. These observations suggested that the effect of electrolytes on water structure and solution dynamics were paramount in controlling rates, whereas the effects on rates of direct contact between dissolved aqueous species and mineral surfaces were negligible. The findings implied that modification of the morphological features, rates and mechanisms of reactions by additives present in solution could be often explained without assuming any direct interactions between dissolved species and the crystal surface. The theoretical principles developed for reaction of barite in solutions of simple inorganic electrolytes were extended to explain the influence of ionic salts on the mechanism and kinetics of dissolution of the mineral calcite and dissolution of barite in the presence of synthetic organic polyelectrolyte.

Further research efforts in MIR were aimed at the reactivity of phosphorous. Phosphorous is the element limiting vital productivity in most aquatic systems on the Earth's surface, but in case aqueous phosphorous concentrations are too high pollution results. Over the past several decades phosphorus concentrations were increased in natural waters due to increased domestic, industrial and agricultural inputs; global riverine phosphorus fluxes to the ocean increased by one to two orders of magnitude in the last two decades. To address this growing environmental challenge MIR studied phosphate mineral dissolution and growth rates and used these results to assess if these minerals could be used to control dissolved phosphorous concentration of natural waters. MIR fellows measured dissolution and precipitation rates of the major phosphate minerals, such as struvite (MgNH4PO4.6H20), fluorapatite Ca5(PO4)3F, variscite (AlPO4.2H2O), strengite (FePO4.2 H20) and rhabdophane (LaPO4.2 H20 and NdPO4.2 H20). Geochemical modelling calculations that were performed using these measured rates indicated that the dissolution and precipitation of these minerals was sufficiently fast to buffer the phosphorous concentrations of most natural waters. Further modelling results were ongoing by the time of the project completion to develop environmentally friendly methods to clean polluted water and develop more efficient fertilizers.

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