EFFECTS OF IRON OXIDE MINERALOGY ON TRACE METAL AND NUTRIENT BEHAVIOR AND END USE OF MINE DRAINAGE PASSIVE TREATMENT SYSTEM RESIDUALS

Abstract

Management of produced mine drainage residuals (MDRs) formed in mine drainage (MD) passive treatment systems (PTS) can be costly for parties responsible for the long-term maintenance of these systems. Historically, MDRs were viewed as waste that needs to be disposed of, increasing the environmental footprint of PTS. MDRs are predominantly iron oxide minerals, making them favorable for use as phosphorus (P) sorbents to decrease anthropogenic P loadings into waterways that exacerbate eutrophication problems. Reusing MDRs as P sorbents can close the resource recovery loop to support a more circular economy. This research evaluated the efficacy of reusing MDRs from different MD chemistries as P sorbents. The physical and chemical properties of MDRs formed in different MD chemistries, how they change over time in situ, and how their different properties affect their P sorption capacity and metal desorption potential were investigated using laboratory batch sorption studies. Based on these results, a cost analysis was performed comparing the cost of reusing MDRs to disposing of MDRs. The first study of this research investigated the effect host MD chemistry has on the mineralogical and chemical properties of MDRs (Chapter 2). This study showed that MDRs formed in circum-neutral pH waters of PTS were less crystalline, giving them larger specific surface areas and higher points of zero net charge than MDRs formed in untreated net-acidic MD discharges. The pH and ionic strength of MD waters found in PTS make these MDRs more favorable for accumulating metals than MDRs formed in untreated discharges. The greater specific surface areas and points of zero net charge of PTS MDRs make them more favorable for reuse as P sorbents. However, those MDRs are also more likely to accumulate metals that may be released later, depending on how the metals sorbed and became incorporated into the crystal structure. The second study (Chapter 3) focused on the P sorption capacity of MDRs from three different types of MD chemistries: net-acidic coal MD, net-alkaline coal MD PTS, and net-alkaline hard-rock MD PTS. Benchtop sorption studies with lower concentrations of P found that the MDRs formed in net-acidic coal MD that had fewer ions sorbed onto their surface and sorbed more P than the MDRs formed in PTS. However, as the concentration of P increased, MDRs formed in PTS sorbed more P than the MDRs from untreated discharges due to their larger specific surface areas, higher points of zero net charge, and greater concentrations of Ca and Mg. All the MDRs evaluated released measurable concentrations of trace metals. However, the release of trace metals above the recommended guidelines occurred at unnatural concentrations of P (>500 mg L-1 P) and in acidic conditions (pH 3.5-6), which is less likely to happen in the field. Core samples of MDRs formed in PTS showed that the MDRs transform in situ over time from ferrihydrite and more amorphous goethite into more crystalline goethite, decreasing their SSA and P sorption capacity (Chapter 4). Despite the decrease in sorption capacity with age, all MDRs removed over 75% of the initial 50 mg L-1 P solution within 24 hours and did not release trace metal concentrations above acute and chronic criteria for freshwater systems. The cost-analysis study (Chapter 5) compared the costs associated with dewatering MDRs using Geotube® containers versus a drying basin and between landfilling MDRs or reusing the MDRs as a P sorbent. Disposing of MDR in a landfill was more expensive than leaving the MDR on-site for future reuse. However, transporting MDRs to a facility for additional drying and sieving was more costly than landfilling the MDR. Benchtop sorption studies showed that the P sorption capacities of MDR from two PTS were not statistically different from that of a commercially available P sorbent, Bayoxide® E33. Selling MDR at 5-10% of the market price of Bayoxide® E33 can offset the costs of recovering MDRs two times, 30 years of routine maintenance, and design and construction costs associated with the PTS. Reusing MDR as a P sorbent has the potential to offset PTS operation and maintenance costs, increasing the economic and environmental sustainability of PTS. Further research should be conducted analyzing the performance of these MDRs in a field setting with natural waters and engineering a design for applying these MDRs to make them more marketable.