QUANTIFYING TREMOLITE, SERPENTINE, AND TALC DISSOLUTION RATES: IMPLICATIONS FOR THE BIODURABILITY OF MG-SILICATE DUST

Dusts containing fibers of the Mg-silicate minerals serpentine (chrysotile) and tremolite can pose a threat to human health by leading to the development of asbestos-related disease. Previous studies on mineral fibers (both asbestos and synthetic vitreous fibers) have found a correlation between toxicity and biodurability (resistance to dissolution). These studies have, generally, focused on the narrow set of conditions found in the human body; however, a more comprehensive approach can provide additional information on dissolution mechanisms, as well as kinetic parameters. We have conducted a series of dissolution experiments over a relatively wide range of temperatures and solution compositions to quantify dissolution rates and identify dissolution mechanisms for these minerals.

Dissolution experiments have been conducted in mixed-flow reactors using both serpentine and tremolite as starting materials and at temperatures ranging from 25-150°C. The pH and the Mg, Si, and Ca concentration of the inlet solution was varied to measure the effect of solution chemistry on dissolution. The dissolution behavior of both of these minerals appears to be stoichiometric and is consistent with a dissolution mechanism that proceeds via a two-step process initiated by the substitution of Mg at the mineral surface with 2 H⁺ from solution, creating a partially detached silica tetrahedron. This tetrahedron can then be attacked and removed from the structure. Significant differences in dissolution behavior between the serpentine and tremolite were observed. The activation energy for serpentine dissolution is approximately 41kJ/mol, while the activation energy for tremolite is 80kJ/mol. In addition, the dissolution rate of serpentine appears to have a greater pH dependence.

Corresponding experiments were performed on 50nm to 100μm-sized talc particles (another hydrous Mg silicate mineral) to determine the affect of crystal size on dissolution rate. Results show that nano-talc dissolution rates are proportional to grain size at far-from equilibrium conditions for all sizes studied. This result suggests that mineral dust dissolution rates as a function of saturation state can be estimated using far from equilibrium dissolution rates and existing thermodynamic data.