124 JOUR.GEOL.SOC.INDIA, VOL.90, JULY 2017 ENVIRONMENTAL MINERALOGYAND GEOCHEMISTRY – THE CASE OF URANIUM CONTAMINATION RANEN SEN 1 and SHARADINDRA CHAKRABARTI 2* 1 Former Professor, Colorado School of Mines, USA. 2 Associate Professor and Head, Department of Geology, Sister Nibedita Government College, Hastings Houe, Kolkata, India. *E-mail: sharad_presi@rediffmail.com are prominent in clayey environment. Nano-pore surfaces can increase U (VI) sorption, more so in carbonate-bearing groundwater. In the absence of carbonates, the uranyl ion and its complexes sorb strongly onto surfaces of iron oxides and natural organic materials (NOM) as well as onto edge sites of clay minerals. In Xu and Jung’s (2012) opinion, the structural state of nanominerals is between ideal goethite and ferrihydrite, and uniform size of goethite indicates that they may be precursor to ferrihydrite. Nano-pores within nano-crystalline Fe oxides strongly influence the sorption of uranium and hence the extent of mobility and transport of U (VI) in sub-surface environments. Chemically speaking, the mobility of U depends on its speciation and redox state. It is present as mobile U (VI) in oxidizing conditions. Under reducing conditions, relatively insoluble and immobile U (IV) predominates typically as the mineral uraninite (UO 2 ) but non-uraninite phases may also be negotiated. Soluble U (VI) can be reduced by microbes ultimately to U (IV) and precipitated, thereby reducing the mobility of U (VI). Microbial cycling has significant impact on radionuclide behavior across a wide range of environment and are important in managing contaminated land sites and geological disposal scenarios, where biogeochemical processes are likely to occur and be considered in safety case development. U (V), on the other hand, is generally considered as transient though signs are emerging, which suggest that it might be stable for few weeks under certain conditions. NOM are known to be an important component of uranium fate and transport (Aiken et al., 2011). Uranium has high affinity for both dissolved and particle-associated NOM fractions and NOM is implicated in increased nano transport of uranium in natural waters. Complexation of U (VI) with dissolved NOM increases mobility in water, but when associated with solid phase, NOM can result in accumulation of uranium. In fact, hexavalent uranium may lose its mobility by adsorption. Common cations like Fe and Ca play significant role in uranium mobility through NOM-stabilized metal (Fe) nanoparticles. To fix the problem of actinide contamination of soils, sediments and water, microbial remediation may be utilized. Bacterial activity can fix these radio-nuclides into insoluble form that cannot be readily dispersed. Uraninite particles in sediments, formed by microbial reduction, are typically less than 2nm across and this small size has important implications for uraninite reactivity and fate. These fine particles can still be transported in aqueous environment and precipitation of uranium as insoluble uraninite cannot be presumed to immobilize it. Fe-oxidic mineral nano-particles/nano-minerals in high Fe(III) soils/sediments and water may have transport-facilitating and transport- impeding effects on uranium contaminants in a mining/milling area depending on the chemical ambience of the medium structure of nano- pores as well as the NOM characteristics. Most reactive surfaces in fact are nano-pore surfaces. The behavior of geological fluids and minerals in nano-pores (Sun et al., 2010) is significantly different from those in normal non-nanoporous environments. The effect of nano- The International Mineralogical Association has defined Environmental Mineralogy as ‘interdisciplinary field dealing with systems at/near the earth’s surface, where geo-sphere comes into contact with hydrosphere, atmosphere and biosphere’. This is the environment on which plants and animals (including humans) depend for survival but get disrupted by human activities due to exploitation and mobilization of earth’s resources. Uranium contamination in surface/subsurface soils/sediments and water is an environmental reality around the uranium mines. Understanding the fundamental processes that control U-mobility, reactivity and bioavailability help to identify impacts of land use (U- mining and milling), improve site management and remediation strategies for U-mining, milling and disposal. Basically uranium contamination is a multi-scale complex process, which controls the dynamics of components involved, components being a part of mineralogy.These are not considered to be part of classical mineralogy, but provide coherent view of the interplay amongst (U) pollutant release, sequestration, transport and functioning of geochemical and biogeochemical systems. Uranium contaminants, derived from mining and milling, and observed in surface/sub-surface soil/sediment/water, are present in forms of U (IV) and U (VI), depending on its redox state (Langmuir, 1997). Uranium in these forms is adsorbed onto Fe oxide nano- minerals/mineral nano-particles (like nano-ferrihydrite, nano-goethite and nano-hematite) and their nanopores greatly affect the sorption behavior of uranium. These iron oxide minerals, having nano- dimensions, are more essential sorbents for both U (VI) and U (IV). U (VI) is readily soluble in water, particularly in oxic environment, and highly mobile than U (IV). Moreover, sizable amount of U (VI) is irreversibly bound to nanopore surfaces of iron oxide minerals; the binding strength is found to be directly related to nano-pore size. The population of fine nano-particles is identified as mix of nano-porous goethite with ferrihydrites, associated with clay minerals (illite/ smectite) in the coating layers. These nano-particles with nano-pores are main carrier of U. The host mineral nano-particles are nano-porous in surface/sub-surface due to aggregation of nano-particles. Nano-pores are known (Wang et al., 2003) to provide the most reactive surface areas. However, the role of nano-pores in controlling the mobility of inorganic pollutants is not well understood. The mobility of U (VI) is governed by sorption-desorption processes (Wazne et al., 2003). Sorption-desorption are significantly controlled by nano-pores due to high surface area and nano-pore confinement effects (Fox et al., 2006; Hochella, 2008). Estimation of labile fraction of solid phase U (VI) in sorption-desorption and that of its non-labile fraction which is resistant to desorption, are very important (Wang et al., 2003; Xu and Jung, 2012) for nano-pore mobility and transport of U (VI). Sorption- desorption of U (VI) is time dependent and attributed to nano-pore/ micro-pore effect (Xu and Jung, 2012). Nano-porous structure of sub- surface materials significantly determines the mobility and transport of U (VI). Nano-pores are commonly associated with iron oxides in soil sediments. Effects of nano-pore surfaces on U (VI) sorption and reduction