COBALT ADSORPTION ON BIRNESSITE AS A FUNCTION OF PH AND SULFATE CONCENTRATION UNDER COAL MINE DRAINAGE TREATMENT CONDITIONS

Demand for cobalt (Co) is increasing due its use in rechargeable batteries and super alloys vital for modern green energy technologies. Domestic Co sources are being explored as external Co supply chains present geopolitical risks and use extraction methods harmful to human and environmental health. Cobalt recovery from coal mine drainage (CMD) treatment solids, a waste stream consisting of iron, aluminum, and manganese (Mn) -rich oxy/hydroxides from passive remediation systems, is a potential low cost and environmentally sustainable source. In Appalachia, CMD solids in Mn-rich phases have been shown to contain up to 6000 mg/kg Co, comparable to low-grade Co ores (e.g. 2,000-10,000 mg/kg) [1]. During CMD treatment, the CMD solids accumulate as precipitates in limestone beds as the redox conditions and pH of CMD changes. The precipitation of CMD solids occurs over a wide range of pH values in sulfate-rich mine drainage. Cobalt sorption and precipitation mechanisms have not been studied under these conditions, thus making predictive modeling difficult. Exploring Co sorption mechanisms on different birnessite (d-MnO₂) structure types (hexagonal, H-birnessite and triclinic, Na-birnessite) that are common in CMD solids will offer insights into the Co retention mechanisms in CMD treatment systems. This study investigates Co adsorption on synthesized birnessites as a function of pH and sulfate concentrations to simulate CMD treatment conditions. Adsorption isotherms of Co at pH values of 3.5, 4.5, and 6.5 at 25°C onto Na-and H-birnessite will be presented, along with the mineralogy and morphology of post-extraction solids as characterized by X-ray diffraction and scanning electron microscopy. Our preliminary data showed that Co adsorption capacity on Na-birnessite peaked at pH 6.5. The results of these experiments can serve as calibration points for equilibrium and kinetic modeling approaches such as PHREEQ-N-AMDTreat+REYs [2] that are critical for developing effective and implementable remediation processes and to better inform future Co recovery efforts from CMD solids.

[1] Hedin et al., 2020; Int. J. Coal Geol., 231: 103610. [2] Cravotta III, C. A., 2021, Appl. Geochem. 126: 104845.