Physics
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.3583l&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 17, pp.3583-3593
Physics
Scientific paper
By monitoring the change of the solution pH value, the dissolution kinetics of gel-derived Zn 2 SiO 4 powders (amorphous, and -phase nanometers in size) at various solution pH ranges and temperatures were studied. At 30°C, all the polymorphs showed a near-linear pH dependence on logarithmic dissolution rates in the range of pH = 1-4 and the slopes (i.e., - n value in the formula of rate = ka H + n ) of logarithmic dissolution rate-pH plots are -0.53, -0.63, and -0.51 for , and amorphous phases, respectively. The similarity of the pH dependencies for the polymorphs is due to the same leading cation (i.e., Zn 2+ as active site) for dissolution and similar structural linkage and hence, a similar protonation-detachment process of dissolution. The apparent activation energies for dissolution of all the powders at pH = 2 and 30-60°C indicated a diffusion-controlled process. Smaller particles of -Zn 2 SiO 4 caused a higher acidic dissolution rate, a lower activation energy, but nearly the same pH dependence of dissolution. At 30°C and a given pH, a critical concentration of Fe 2+ in solution is required to significantly suppress the acidic dissolution of -Zn 2 SiO 4 powders. This has been attributed to the adsorption equilibrium of Fe 2+ , which depends on pH and surface structure.
Lin Chung-Cherng
Shen Pouyan
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