2. Acid drainage is a persistent environmental problem in many mineralized areas, especially where mining has taken place. Not all drainage, however, is acidic or rich in metals. Primary controls on drainage pH and metal content seem to be Exposure of sulfide minerals to weathering Availability of atmospheric oxygen Ability of nonsulfide minerals to buffer acidity Acid drainage and buffering

3. Task 1 — Role of atmospheric oxygen Simulate pyrite dissolution into a hypothetical groundwater Consider systems isolated from and open to exchange with the atmosphere

4. 0.51 1 2 3 4 5 6 7 Pyrite reacted (cm 3 ) pH Open to atmospheric fO 2 Isolated Role of atmospheric oxygen

5. Water in the absence of O 2 supply becomes saturated with respect to a sulfide mineral after only a small amount of dissolution. As a result, pH and fluid composition change little. O 2 additionally promotes metabolism of bacteria that catalyze sulfide mineral dissolution and oxidation of dissolved iron. Role of atmospheric oxygen

6. 010203040 0 10 20 30 40 Pyrite reacted (mmoles) Reaction products (mmoles) FeSO 4 + H+H+ SO 4 2− HSO 4 − Fe(SO 4 ) 2 − Open to atmospheric O 2

7. FeS 2 + 15/4 O 2 (aq) + 1/2 H 2 O  FeSO 4 + + SO 4 2− + H + Pyrite Equilibrium with atmospheric oxygen FeS 2 + 15/4 O 2 (aq) + 1/2 H 2 O  FeSO 4 + + HSO 4 − Pyrite Initial reaction As pH decreases

8. Task 2 — Buffering by wall rocks Build upon the previous example React pyrite into a fluid in equilibrium with calcite

9. 0.51 1 2 3 4 5 6 7 Pyrite reacted (cm 3 ) pH No calcite Calcite Open to atmospheric O 2

10. 0.51 –4 –2 0 2 4 6 Pyrite reacted (cm 3 ) Volume Change (cm 3 ) Gypsum Fe(OH) 3 Calcite

11. No calciteCalcite pH 1.7 5.6* Fe (mg kg −1 )2300 0.03 SO 4 80001800 Ca 9.91100 HCO 3 746000 *CO 2 degassing, not accounted for here, would further increase pH Reaction of 1 cm 3 Pyrite in system open to atmospheric O 2 Resulting fluid chemistry

12. Effect of pH on ferric iron solubility Oxidation of ferrous iron Fe 3+ + 3 H 2 O  Fe(OH) 3 + H + Ferric hydroxide Fe 2+ +.25 O 2 (aq) + 2.5 H 2 O  Fe(OH) 3 + 2 H + Ferric hydroxide HFO precipitation

13. Initial stage of calcite dosing As pH increases CaCO 3 + 2 H +  Ca 2+ + CO 2 (aq) + H 2 O Calcite CaCO 3 + H +  Ca 2+ + HCO 3 − Calcite pH adjustment

14. Task 3 — Co-Precipitation React calcite into acidic, metal-rich fluid Assume iron is already oxidized HFO precipitates as pH increases What metals sorb to HFO?

15. 01234 0.2.4.6.8 1 Calcite reacted (mmoles) Jarosite-Na Minerals (mmoles) Calcite Fe(OH) 3 HFO precipitation

16. 34567 0.2.4.6.8 1 pH Fraction sorbed AsO 4 3− As(OH) 4 − Pb 2+ Cu 2+ Zn 2+ HFO complexation

17. ComponentMaximum sorbed (mg kg −1 ) In solution (mg kg −1 ) As(OH) 4 − 25.40.2 AsO 4 3− 24.71.0 Cu 2+ 11.60.5 Pb 2+ 37.80.2 Zn 2+ 11.918.0 Sorption capacity Capacity for each metal to sorb if it occupied every surface site. Account for amount of precipitate, number of sites, and site types accepting each metal.

18. Craig M. Bethke and Brian Farrell © Copyright 2016 Aqueous Solutions LLC. This document may be reproduced and modified freely to support any licensed use of The Geochemist’s Workbench® software, provided that any derived materials acknowledge original authorship.