The effects of pH regulation upon the release of sulfate from ferric precipitates formed in acid mine drainage

'Batch experiments' (25:1 v:w) were used to test the effects of pH upon the release of SO4-2 from ferric precipitates formed in acid mine drainage (AMD) in southeastern Kentucky. Analytical grade CaO ['quicklime'], Ca(OH)(2) [hydrated lime] and CaCO3 [referred to as 'limestone'] were used as alkalinity generating agents and were mixed with ferric precipitates composed of amorphous iron oxyhydroxides, jarosite and goethite. Regression analyses indicated that aqueous SO4-2 concentrations increased linearly with pH between values of 2.5 and 9.0. At pH = 7, 30-45% of the oxalate soluble SO4-2 was released from the ferric precipitates. The 3 types of alkaline neutralizing agents released similar, but not identical, quantities of SO4-2 at a given pH. The results of these experiments suggest that AMD remediation schemes should consider raising the pH of waste streams only to Values necessary to precipitate and/or sorb metals, in order to minimize SO4-2 desorption. Iron oxyhydroxides that precipitate within AMD often sorb high concentrations of SO4-2; a significant percentage pf which will be released in streams draining carbonate bedrock. (C) 1999 Elsevier Science Ltd. All rights reserved.