Other
Scientific paper
May 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49.1123k&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 49, Issue 5, pp.1123-1136
Other
2
Scientific paper
Two chemical processes can remove Mg 2+ from suspensions containing amorphous silica (am-SiO 2 ) at low temperatures: adsorption and precipitation of a Mg-hydroxysilicate resembling sepiolite. Mg 2+ removal from am-SiO 2 suspensions was investigated, and the relative role of the two removal processes evaluated, as a function of: pH, ionic strength, Mg 2+ concentration, and temperature. The extent of Mg 2+ adsorption onto am-SiO 2 decreases with increasing NaCl concentration due to displacement of Mg 2+ by Na + . At NaCl concentrations of 0.05 M and above, adsorption occurs only at pH values above 8.5, where rapid dissolution of am-SiO 2 gives rise to high concentrations of dissolved silica, resulting in supersaturation with respect to sepiolite. Removal of Mg 2+ , at concentrations of 40 to 650 M, from am-SiO 2 suspensions in 0.70 M NaCl at 25 °C occurs at pH 9.0 and above. Experiments show that under these conditions adsorption and Mg-hydroxysilicate precipitation remove Mg 2+ at similar rates. For 0.05 M Mg 2+ , at 0.70 M ionic strength and 25 °C, measurable Mg 2+ removal occurs down to ca . pH 7.5 but is primarily due to Mg-hydroxysilicate precipitation. For the same solution conditions at 5°C, Mg 2+ removal occurs above pH 8.0 and is primarily due to adsorption. Assuming that increasing pressure does not greatly enhance adsorption, Mg 2+ adsorption onto am-SiO 2 is an insignificant process in sea water. The surface charge of pristine am-SiO 2 in sea water is primarily controlled by interactions with Na + . The principal reaction between Mg 2+ and am-SiO 2 in marine sediments is sepiolite precipitation. The age distribution of sepiolite in siliceous pelagic sediments is influenced by temperatures of bottom waters and by geothermal gradients.
Kastner Miriam
Kent Douglas B.
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