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 SO−24 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 SO−24 concentrations increased linearly with pH between values of 2.5 and 9.0. At pH=7, 30–45% of the oxalate soluble SO−24 was released from the ferric precipitates. The 3 types of alkaline neutralizing agents released similar, but not identical, quantities of SO−24 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 SO−24 desorption. Iron oxyhydroxides that precipitate within AMD often sorb high concentrations of SO−24; a significant percentage of which will be released in streams draining carbonate bedrock.

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