A COMPARATIVE ASSESSMENT OF CRITICAL METAL REMOVAL BY BIOTIC AND ABIOTIC HYDROUS MANGANESE OXIDES

Microbial (biotic) oxidation of Mn can produce a wide variety of hydrous manganese oxides (HMOs) with high cation adsorption capacity. In passive acid mine drainage (AMD) treatment systems, biotic HMO may dominate over abiotic HMO, resulting in treatment precipitates with high concentrations of energy-critical metals such as rare earth elements (REE), Co, and Ni [1]. To understand the relative importance of abiotic and biotic processes in passive treatment systems, we conducted benchtop experiments to investigate the uptake of ten critical metals (Co, Ni, La, Ce, Nd, Pr, Gd, Dy, Yb, and Y) by (1) HMOs produced by two fungal species (Paraphaeosphaeria sporulosa and Stagonospora sp.) and (2) abiotic HMO, vernadite (δ-MnO₂), synthesized using a method described by Hinkle et al. [2]. Experiments involving the abiotic HMO are currently underway to assess critical metal uptake and to allow comparative analyses of biotic and abiotic uptake rates and efficiency.

Preliminary results of the biotic experiments show that in the presence of dissolved Mn, both fungi produced a highly disordered HMO similar to vernadite which becomes more ordered and crystalline with time. However, Stagonospora sp. was more efficient at removing dissolved Mn from solution compared to P. sporulosa. Critical metal uptake by biotic HMO (HMO + fungal biomass) and fungal biomass only was assessed via periodic sampling over 31 days to track changes in dissolved metal concentrations. Our results indicate that Co in solution decreased by 90% and 80% in the presence of HMO produced by Stagonospora sp. and P. sporulosa, respectively. In contrast, only 50-60% of Ni was removed after 31 days. The presence of fungal biomass only did not result in significant removal of Co and Ni. Near total removal (95-99%) of REE and Y was achieved within 18 days, both in the presence of biotic HMO and fungal biomass only. Normalized REE patterns show a preferential removal of light REE over 31 days with no apparent Ce anomaly.

[1] Hedin, BC et al., 2019, Int. J. Coal Geol. 208, 54-64. [2] Hinkle, MA et al., 2016, Geochim. Cosmochim. Acta 192, 220-234.