Silica pore water profile

Most commonly observed pore-water concentration profiles, (a) A nonreactive substance, such as chloride (b) a chemical, such as O2, which undergoes removal in the surface sediment as a result of aerobic respiration (c) a chemical that is consumed by a reaction that occurs in a subsurface layer, such as Fe2+(aq) precipitating with S2-(aq) to form FeS2(s) (d) a chemical released in surface sediments, such as silica via dissolution of siliceous hard parts (e) a chemical released into pore waters from a subsurface layer, such as Mn +(aq) by the reduction of Mn02(s) and (f) a chemical released at one depth (reactive layer 1), such as Fe2+(aq) by reduction of FeOOFI(s), and removal at another depth (reactive layer 2), such as Fe +(aq) precipitating as FeS2(s). Source From Schulz,... 

These diagenetic models have proven effective in understanding the effects of various sedimentary processes on silica dynamics in the seabed (Johnson, 1976 Ragueneau et al., 2000). For example, the effect of bioturbation by benthic fauna on pore-water silicate profiles has been examined (Schinck and Guinasso, 1978). Higher intensities of bioturbation lead to higher asymptotic silicate values at depth because the benthic fauna are mixing down particles from the surface... 

Silica concentrations in pore waters particularly and also in other subsurface and surface waters were determined in dozens of samples as part of this study, using atomic absorption spectrophotometry as well as inductive coupled plasma. This was essential for silica budget estimations. Present-day and paleothermal profiles were obtained from measured temperatures in boreholes, vitrinite reflectance (Rq) in interlayered-with-sandstones shales and from basins thermal modeling. 

To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po 0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). 

A. K Soper, Density profile of water confined in cylindrical pores in MCM-41 silica, J. Phys. Condens. Matter2i(6), 064107 (2012). 

The water density within the pore has been analysed by calculating density profiles perpendicular to the surface slab (Fig. 7). Close to the surface the density of water is low due to the fact that the alkyl chains, which connect the sulfonic-acid group to the silica surface, occupy the space and are, moreover, hydrophobic. A maximum in the water density is observed at a distance of 5-10 A from each wall around the hydrophilic sulfonic-acid groups. 

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