DISSOLUTION STOICHIOMETRY AND ADSORPTION OF ALKALI AND ALKALINE-EARTHELEMENTS TO THE ACID-REACTED WOLLASTONITE SURFACE AT 25-DEGREES-C

The Ca2+/H+ exchange reaction on the wollastonite surface was investig ated at 25-degrees-C with both short-term (<2.5 h) and long-term (>48 h) dissolution studies. In acidic solutions, the dissolution of wollas tonite is nonstoichiometric with a greater release of Ca than Si relat ive to the wollastonite stoichiometry. Both short-term and long-term C a2+/H+ exchange reaction stoichiometries are 0.5. Rapid desorption of Ca'' from the surface of untreated wollastonite caused a rise of the s uspension pH to about 10 in a couple of minutes. Therefore, potentiome tric titrations were performed with an acid-reacted wollastonite where most surface detachable Ca2+ had been removed. Addition of alkali and alkaline earth metal chloride solutions to the acid-reacted wollaston ite suspension results in a pH with K+ > Na+ > Ba2+ > Mg2+ > Ca2+ in e qual molal solutions. This suggests that the cations in these solution s are adsorbed to the wollastonite surface. Surface protonation proper ties of the acid-reacted wollastonite are found to be similar to those of microporous silica but with the point of zero salt effect (pzse) o f 4.5-5.5 rather than the 3.0 of microporous silica. The surface proto nation-deprotonation as a function of pH is modeled with a one-site do uble layer model which includes Na adsorption from the background elec trolyte to reasonable accuracy. The adsorption of CrO42-, MoO42-, Ca2, Mg2+, Ba2+, and Na+ from aqueous solutions to the acid-reacted wollo nite/water interface was determined as a function of the pH and ionic strength of the solution. CrO42- and MoO42- were not adsorbed to the w ollastonite surface at pH above 3. The extent of cation adsorption inc reases with increasing pH and decreases with increasing ionic strength . Ca2+ adsorption depends on both the surface area of wollastonite and total amount of Ca2+ in the suspension. For alkaline earth metals at the same concentration, the adsorption sequence is Ba2+ > Ca2+ > Mg2+. At pH 8.5, the maximum Ca2+ adsorption density on the acid-reacted wo llastonite is about 0.83 mumol m-2.