T113 Celebrating the Scientific Contributions of Kirk Nordstrom—Part 1: Acid to Neutral Mine Drainage, Geochemistry of Iron and Sulfur, Sulfate Minerals, Natural Background, and Geochemical Modeling

Monday, 28 October 2013
8:00 AM - 12:00 PM
Room 503

SCHWERTMANNITE SOLUBILITY

Paper number: T113-2
Program: Technical Sessions
Time: 8:10 AM – 8:25 AM

Authors

Abstract

Schwertmannite, Fe8O8(OH)8-2x(SO4)x, is a common mineral in acid sulfate environments. Reported equilibrium constants (Ks) for its solubility reaction,

Fe8O8(OH)8-2x(SO4)x + (24-2x)H+ = 8Fe3+ + xSO42- + (16-2x)H2O (where ~0.5 < x < ~2.0),

span a wide range of values and are often used without regard to schwertmannite stoichiometry. In order to determine how Ks varies with schwertmannite stoichiometry, we compiled 23 analyses (9 new and 14 from the literature) of schwertmennite and coexisting aqueous solution from natural settings and used the PHREEQC model with the MINTEQ database to calculate log Ks for each solution composition. These calculated log Ks values vary systematically with schwertmannite sulfate contents (x). Linear regression of log Ks versus x gives the relationship: log Ks = 40.40 – 15.10x (R2 = 0.68).

For these natural solutions, log (aFe(III)/aH+3) changes systematically with pH. Plotting the solution compositions on a log (aFe(III)/aH+3)–log (aH+2aSO4) diagram shows that the low pH the solutions are nearly in equilibrium with goethite and sulfate rich (x ≈ 2) schwertmannite and the high pH the solutions are nearly in equilibrium with ferrihydrite and sulfate poor (x ≈ 1) schwertmannite.