Silicate minerals

About: Silicate minerals is a research topic. Over the lifetime, 1794 publications have been published within this topic receiving 67064 citations.

Use of silicate minerals for pH control during reductive dechlorination of chloroethenes in batch cultures of different microbial consortia

Abstract: In chloroethene-contaminated sites undergoing in situ bioremediation, groundwater acidification is a frequent problem in the source zone, and buffering strategies have to be implemented to maintain the pH in the neutral range. An alternative to conventional soluble buffers is silicate mineral particles as a long-term source of alkalinity. In previous studies, the buffering potentials of these minerals have been evaluated based on abiotic dissolution tests and geochemical modeling. In the present study, the buffering potentials of four silicate minerals (andradite, diopside, fayalite, and forsterite) were tested in batch cultures amended with tetrachloroethene (PCE) and inoculated with different organohalide-respiring consortia. Another objective of this study was to determine the influence of pH on the different steps of PCE dechlorination. The consortia showed significant differences in sensitivities toward acidic pH for the different dechlorination steps. Molecular analysis indicated that Dehalococcoides spp. that were present in all consortia were the most pH-sensitive organohalide-respiring guild members compared to Sulfurospirillum spp. and Dehalobacter spp. In batch cultures with silicate mineral particles as pH-buffering agents, all four minerals tested were able to maintain the pH in the appropriate range for reductive dechlorination of chloroethenes. However, complete dechlorination to ethene was observed only with forsterite, diopside, and fayalite. Dissolution of andradite increased the redox potential and did not allow dechlorination. With forsterite, diopside, and fayalite, dechlorination to ethene was observed but at much lower rates for the last two dechlorination steps than with the positive control. This indicated an inhibition effect of silicate minerals and/or their dissolution products on reductive dechlorination of cis-dichloroethene and vinyl chloride. Hence, despite the proven pH-buffering potential of silicate minerals, compatibility with the bacterial community involved in in situ bioremediation has to be carefully evaluated prior to their use for pH control at a specific site.

Global silicate weathering flux overestimated because of sediment-water cation exchange.

Abstract: Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes [Formula: see text] from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net [Formula: see text] consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth [Formula: see text] degassing fluxes need to be further investigated.

The evolution of atmospheric CO2 on Mars: The perspective from carbon isotope measurements

Abstract: In situ measurements of 12C/13C of atmospheric CO2 on Mars made by Viking are, within error, similar to values normally encountered on Earth. However, high precision isotopic measurements made on SNC meteorites show there to be a 40‰ fractionation between carbon associated with silicate minerals (δ13C ≈ −25‰, 12C/13C ≈ 91.3) and trapped CO2 gas or carbonate minerals (δ13C ≈ +15‰, 12C/13C ≈ 87.7). To a first approximation, it can be considered that the silicate-sited carbon in SNC meteorites carries the isotopic signature of the element present at depth in the Martian crust; herein it is considered that this magmatic carbon is representative of bulk Mars. On the other hand, carbon in the form of carbonate most probably results from secondary processes in operation at the surface of the planet. Trapped CO2 is thought to be a sample of Martian atmospheric gas. The significant difference in isotopic composition between the different components in SNC meteorites is difficult to interpret in terms of closed-system equilibrium or kinetic isotopic fractionation effects. Rather, it seems that the relative enrichment of 13C in surficial carbon is due to preferential removal of the lighter isotope during an atmospheric loss process. Between 30 and 70 mbar of CO2 may have been lost from the atmosphere to outer space.