THE USE OF SANDSTONE TO AMELIORATE LIMESTONE ARMORING IN PASSIVE AMD REMEDIATION

Acid mine drainage (AMD) is the acidic, metal-laden solution produced by the oxidation of sulfide minerals exposed from mining activities. AMD is a significant environmental problem in most countries that have an extensive history of mining. Because of the large number of impacted sites there is considerable drive to develop low-cost remediation options. One of these options is the passive limestone trench. These trenches use the dissolution of limestone, which releases carbonate, as a sink for H⁺ to reduce the pH of AMD triggering precipitation of dissolved acid cations, further reducing the acidity of the AMD. While the removal of acid cations such as iron is necessary for AMD remediation, their precipitation can create a coating on limestone surfaces (known as armoring) that can inhibit further reactions, thereby limiting the effectiveness of the trench. The goal of this work was to investigate the possibility of using sandstone to ameliorate the armoring of limestone in a passive remediation scenario. We hypothesized that due to a lower point of zero charge (pzc), iron-precipitants would preferentially coat sandstone fragments leaving the limestone unarmored and resulting in quicker neutralization. Furthermore, we hypothesized that the reduction in the armoring of the limestone would be influenced by the mineralogy of the added sandstone.

The results of this investigation revealed that in the experimental set-up used, the addition of sandstone had only a minimal impact on the neutralization of AMD by limestone, regardless of the mineralogy of the sandstone. The armoring of the limestone by the precipitation of iron minerals concurrent with AMD neutralization was reduced by the addition of quartz-cemented sandstone, while the addition of iron-oxide cemented sandstone had no impact on limestone armoring. ICP-OES data indicated that dissolved iron decreased below detection limits in less than 10 hours in all experiments, with the addition of sandstone having no noticable impact. Comparison between batch experiments and sacrificial experiments indicated that the mixing regime greatly impacted the neutralization rates in the experiments. This in turn suggests that the results of these experiments may not accurately reflect the impact of adding sandstone to passive limestone trenches to remediate AMD.