Field Evaluation of Bauxite Residue Neutralization by Carbon Dioxide, Vegetation, and Organic Amendments

The objective of this study was to investigate the mechanisms and extent of bauxite residue (red mud) neutralization at the residue surface in field impoundments as a result of long-term reaction with atmospheric CO2 and addition of amendments to promote vegetation. The results showed that carbonation from atmospheric carbon dioxide reduces the pH of red mud from 12.5 to about pH 9.3 at the surface of storage cells, with the depth of neutralization dependent on the age of the stored residue (up to 1.2 m in 35 years). The presence of vegetation further lowered residue pH to about pH 8.5, with the depth of neutralization dependent on depth of root penetration. Sewage and yard-waste amendments accelerated neutralization and the establishment of vegetation and further lowered the residue pH to about pH 6.7, likely due to organic acid leaching. For vegetated areas, the root density (g roots/g soil) was proportional to the extent of neutralization of residue, with root density higher in near-surface residue than in deeper residue.