Effects of increasing acidity on metal(loid) bioprecipitation in groundwater: column studies.
Large-scale column experiments were carried out over a period of 545 days to assess the effect of increasing acidity on bacterial denitrification, sulfate reduction, and metal(loid) bioprecipitation in groundwater affected by acid mine drainage. At a groundwater pH of 5.5, denitrification and Cu2+ removal, probably via malachite (Cu2(OH)2CO3) precipitation, were observed in the ethanol-amended column. Sulfate reduction, sulfide production, and Zn2+ removal were also observed, with Zn2+ removal observed in the zone of sulfate reduction, indicating likely precipitation as sphalerite (ZnS). Se6+ removal was also observed in the sulfate reducing zone, probably as direct bioreduction to elemental selenium via ethanol/acetate oxidation or sulfide oxidation precipitating elemental sulfur. A step decrease in groundwater pH from 5.5 to 4.25 resulted in increased denitrification and sulfate reduction half-lives, migration of both these redox zones along the ethanol-amended column, and the formation of an elevated Cu2+ plume. Additionally, an elevated Zn2+ plume formed in the previous sulfate reducing zone of the ethanol-amended column, suggesting dissolution of precipitated sphalerite as a result of the reduction in groundwater pH. As Cu2+ passed through the zone of sphalerite dissolution, SEM imaging and EDS detection suggested that Cu2+ removal had occurred via chalcocite (Cu2S) or covellite (CuS) precipitation.
Davis
Alexander C
AC
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
Patterson
Bradley M
BM
Grassi
Michelle E
ME
Robertson
Blair S
BS
Prommer
Henning
H
McKinley
Allan J
AJ
D000143:Acids / Q000737:chemistry*; D011232:Chemical Precipitation; D006863:Hydrogen-Ion Concentration; D008670:Metals / Q000737:chemistry*; D010084:Oxidation-Reduction; D013431:Sulfates / Q000737:chemistry; D014874:Water Pollutants, Chemical
D000143:Acids; D008670:Metals; D013431:Sulfates; D014874:Water Pollutants, Chemical