We present a theoretical investigation on the properties of interstitial nickel and its related complexes in diamond. The atomic configurations, symmetries, spins, and energetics of interstitial nickel, nickel-boron and nickel-vacancy complexes were investigated by a total energy ab initio methodology. Those results are used to explain certain electrically active centers, commonly found in synthetic diamond.

O presente estudo apresenta as concentrações de Hg em cinco espécies abundantes de macrófitas aquáticas (Elodea densa, Sagittaria montevidensis, Salvinia auriculata, Pistia stratiotes e Eichhornia crassipes) coletadas em duas represas que recebem águas da transposição do rio Paraíba do Sul, SE do Brasil. A acumulação de Hg nessas espécies e seu papel no transporte de Hg ao longo do sistema fluvial devido ao manejo das macrófitas são discutidos. As concentrações de Hg foram maiores nas macrófitas flutuantes que nas enraizadas. Em geral, as raízes apresentaram maiores concentrações de Hg que as folhas para todas as espécies. As concentrações de Hg variaram de acordo com as espécies entre 46-246 ng.g-1 e 37-314 ng.g-1, respectivamente. Estas concentrações são maiores que aquelas relatadas para macrófitas coletadas em lagos não contaminados no Brasil e em outras regiões tropicais, e similares àquelas relatadas para áreas moderadamente contaminadas. As concentrações de Hg podem ser atribuídas ao transporte fluvial a partir da região industrializada do vale do rio Paraíba do Sul. Uma amostragem intensiva de Pistia stratiotes na represa do Vigário foi realizada para avaliar a capacidade de incorporação de Hg por esta macrófita. Os resultados mostraram uma correlação negativa, significativa, entre as concentrações de Hg e o tamanho dos indivíduos, demonstrando a importância de juvenis desta espécie na absorção de Hg. A retirada periódica de macrófitas da represa, seguida por sua disposição em áreas adjacentes, pode afetar a dinâmica do Hg. Os resultados mostram uma mobilização de 0,52 a 1,3 kg Hg por ano, uma fração significativa da carga total de Hg presente nas águas da represa. A disposição inadequada deste material poderá resultar em um aumento da mobilização de Hg na bacia.

This paper reports on a study to determine the Hg content in the five most abundant aquatic macrophyte species (Elodea densa, Sagittaria montevidensis, Salvinia auriculata, Pistia stratiotes and Eichhornia crassipes) in two artificial reservoirs flooded by water diverted from the Paraíba do Sul river, SE Brazil. The potential of these species for Hg accumulation and their role in Hg transport along the river system due to macrophyte management were evaluated. Mercury concentrations were higher in free-floating than in rooted species. Roots were also richer in Hg than were leaves. Dry weight Hg concentrations in leaves and roots from all species varied from 46-246 ng.g-1 to 37-314 ng.g-1, respectively. These values are higher than those reported for uncontaminated lakes in Brazil and in other tropical areas and similar to those reported for moderately contaminated sites. Mercury concentrations can be attributed to fluvial transport from the heavily industrialized Paraíba do Sul river basin. Intensive sampling of Pistia stratiotes from two sites in the Vigário reservoir was performed to evaluate the capacity of Hg incorporation in short periods of time. The results showed a significant negative correlation between Hg content and size class of individual plants, demonstrating the importance of juveniles, fast growing plants in absorbing Hg. The foremost impact related to Hg contents in the studied area concerns the periodic removal of macrophytes for reservoir management, followed by disposal in nearby areas. This results in the mobilization of 0.52 to 1.3 Kg of Hg per year, a significant fraction of the Hg burden present in reservoir waters. Disposal of such material may result in Hg leaching to river systems, affecting the Hg transfer throughout the basin.

We carried a theoretical investigation on the electronic and structural properties of titanium impurities in silicon, diamond, and silicon carbide. The calculations were performed using the spin-polarized full-potential linearized augmented plane wave method in the supercell approach. The atomic configurations and transition and formation energies of isolated Ti impurities were computed.

Isolated nickel impurities in diamond have been investigated using the spin-polarized full-potential linearized augmented plane wave total energy method. The electronic and atomic structures, symmetries, transition energies, and formation energies of substitutional and interstitial Ni impurities in diamond were computed. The results were discussed in the context of the electrically active centers in synthetic diamond.

We carried a theoretical investigation on the properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. Our calculations were performed using the ab initio spin-polarized full-potential linearized augmented plane wave method, and taking into account full atomic relaxation. The results on the structural, electronic, and optical properties were compared to available experimental data.