Characterization and pH neutralization products of efflorescent salts from mine tailings of (semi-)arid zones

Acidic water-soluble efflorescent salts in mining residues commonly form from sulfide oxidation and are particularly prevalent in semi-arid to arid environments. These salts are easily carried by winds and may contaminate the environment and affect human health, especially because they contain potentially toxic metals and have high solubilities. In this work, efflorescent salts from four mining sites in Mexico were characterized by X-ray diffraction and chemical analysis. They produced acidic pH values upon dissolution, and were found to be composed mostly of soluble sulfates of Ca(II), Mg(II), Zn(II) and/or Cu(II), and to a lesser extent of Fe(II/III) and/or Al, of variable stoichiometries, crystal water contents and crystallinity. Their physical and chemical characteristics varied highly depending on the underlying geochemical matrix and on the composition of the original mining residues, but gypsum was present in all samples. Two selected samples were dissolved and the solution neutralized, causing substantial precipitation of solids. The precipitates, at pH 7 were found to contain the Cu and/or Zn -rich hydroxysulfates: lahnsteinite [Zn-4(OH)(6)SO4 center dot 3H(2)O], or schulenbergite [Cu3Zn4(OH)(10)(SO4)(2)]. Solubility product constants were determined on these, and on other solids produced in the absence of certain major components [such as sulfate or Zn(II)], by a combination of chemical measurements and geochemical code calculations to speciate some of the components found. These results should enhance thermodynamic properties of databases and help to predict what phases form if pH neutralization occurs or is used for remediation purposes.