Physics
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.2587x&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 12, pp.2587-2598
Physics
1
Scientific paper
The Ca 2+ / H + exchange reaction on the wollastonite surface was investigated at 25°C with both short-term (<2.5 h) and long-term (>48 h) dissolution studies. In acidic solutions, the dissolution of wollastonite is nonstoichiometric with a greater release of Ca than Si relative to the wollastonite stoichiometry. Both short-term and long-term Ca 2+ / H + exchange reaction stoichiometries are 0.5. Rapid desorption of Ca 2+ from the surface of untreated wollastonite caused a rise of the suspension pH to about 10 in a couple of minutes. Therefore, potentiometric titrations were performed with an acidreacted wollastonite where most surface detachable Ca 2+ had been removed. Addition of alkali and alkaline earth metal chloride solutions to the acid-reacted wollastonite suspension results in a pH decrease with K + > Na + > Ba 2+ > Mg 2+ > Ca 2+ in equal molal solutions. This suggests that the cations in these solutions are adsorbed to the wollastonite surface. Surface protonation properties of the acid-reacted wollastonite are found to be similar to those of microporous silica but with the point of zero salt effect (pzse) of 4.5-5.5 rather than the 3.0 of microporous silica. The surface protonation-deprotonation as a function of pH is modeled with a one-site double layer model which includes Na adsorption from the background electrolyte to reasonable accuracy. The adsorption of CrO 4 2- , MoO 4 2- , Ca 2+ , Mg 2+ , Ba 2+ , and Na + from aqueous solutions to the acidreacted wollastonite/water interface was determined as a function of the pH and ionic strength of the solution. CrO 4 2- and MoO 4 2- were not adsorbed to the wollastonite surface at pH above 3. The extent of cation adsorption increases with increasing pH and decreases with increasing ionic strength. Ca 2+ adsorption depends on both the surface area of wollastonite and total amount of Ca 2+ in the suspension. For alkaline earth metals at the same concentration, the adsorption sequence is Ba 2+ > Ca 2+ > Mg 2+ . At pH 8.5, the maximum Ca 2+ adsorption density on the acid-reacted wollastonite is about 0.83 mol m 2- .
Walther John V.
Xie Zhixin
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