Sulfide minerals buffering

The tailings comprise 5-10 wt. % pyrrhotite a highly reactive sulfide mineral that releases protons and Fe3+ into adjacent pore waters on oxidation. Further, the concentration of carbonate minerals in the tailings is low providing little buffering capacity above pH 5. Therefore, the tailings continued to acidify until they reach the pH of AI(OH)3 (pH 4-4.5) and Fe(OH)3 (pH 2.5-3.5) buffering. 

Respective weathering rates of sulfide minerals is pyrrhotite/sphalerite>pyrite. The buffering capacity of the tailings is low due to the lack of carbonates, allowing the rapid onset of low pH conditions. 

The reaction by-products of sulfide oxidation are distinct thus, evidence of sulfide oxidation can be found in the chemical signatures of ground water. In aquifers where sulfide oxidation occurs, ground water chemistry should show a positive correlation of arsenic with sulfate, iron, and trace metals contained in the sulfide minerals. Increases in total dissolved solids and specific conductance also result from sulfide oxidation, due to an increase of dissolved ions in the impacted waters. Ground water impacted by sulfide oxidation may reveal a negative correlation of arsenic with pH, provided that there is minimal buffering capacity provided by the host rocks. 

Grossman and Millet (1961) found that the free Fe-oxide concentration in noncalcareous soils was unchanged after contact with this buffer for nine weeks. Other researchers have shown that acetic acid at a concentration of 2.5% and pH 2.5 led to a partial attack of Fe and Mn oxides (Nissenbaum, 1972 Mclaren and Crawford, 1973 Tessier et al., 1979). Tessier et al. (1979) also indicated that this buffer solution at pH 5.0 was minimal in the attack of silicate minerals and sulfide. 

Virion templates of TMV were also used in combination with different synthetic routes for CdS, PbS, and Fe oxide nanoparticles. Nanoparticle-virion tubules were prepared by reacting a buffered solution of TMV in CdCl2 (pH 7) or TMV in Pb(N03)2 (pH 5) with H2S gas. The formation of metal sulfide nanoparticles occurred over 6 hours as observed by a uniform coating of CdS and PbS nanocrystals on the TMV surface from TEM analysis. Selected area electron diffraction of the mineralized products indicated a zinc blende crystal stracture for CdS particles and a rock salt structure for single domain PbS nanocrystals. The iron oxide nanoparticles were mineralized by the TMV templates by the oxidative hydrolysis of an Fe VFe acidic solution with NaOH. Consequently, a mineral coating of irregular ferrihydrite particles grew on the surface to a thickness of 2 nm. 

Figure 2 Partly schematic plot of the relationship between solubility of metal sulfides (curves with arrows showing down temperature deposition) and select initial metal concentrations (shown by horizontal hues at temperatures of mineral undersaturation). Drawn for 100 MPa pressure, and for fluids in equilibrium with the quartz-K-feldspar-muscovite buffer with a total chlorinity of 1 m (after Hemley and Hunt (1992)).

The acid generated through sulfide oxidation reacts with the nonsulfide gangue minerals within the mine wastes. The most significant pH-buffering reactions in mine settings are the dissolution of carbonate minerals, aluminum hydroxide and ferric oxyhydroxide minerals, and aluminosilicate minerals. 

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