Critical Metal Adsorption by Biotic and Abiotic Hydrous Manganese Oxides: Implications for Acid Mine Drainage Resource Recovery

Acid mine drainage (AMD) remediation facilities can produce
treatment
byproducts with near ore grade concentrations of rare-earth elements
(REEs), cobalt, and manganese. High concentrations of these critical
metals in treatment solids are often associated with hydrous manganese
oxides (HMOs) through adsorption and/or coprecipitation. Chemical
and microbial oxidation processes can influence HMO formation, mineralogy,
and sorption efficiency. Here, we investigate the adsorption of rare-earth
elements and yttrium (REY), cobalt, and nickel over 31 days by (1)
abiotic
HMO (δ-MnO2 and c-disordered H+ birnessite) produced
by chemical oxidation and (2) bitotic HMO produced by Mn-oxidizing
fungi, Paraphaeosphaeria sporulosa and Stagonospora sp. After 31 days, ∼70% of REY
was adsorbed by abiotic HMO, whereas >99% of REY was adsorbed by
biotic
HMO and/or fungal biomass within 7 days. Biotic HMO also adsorbed
∼30% Ni and ∼75% Co; however, Co and Ni adsorption by
abiotic HMO was negligible. Both biotic and abiotic HMOs were initially
poorly crystalline. However, over the course of the experiment, abiotic
HMO was transformed to more crystalline phases, resulting in a reduced
adsorption capacity and significant desorption of Co and Ni. In contrast,
the biotic HMO remained stable and resistant to structural changes
over time. This study demonstrates that biotic HMOs are highly efficient
at adsorbing Co and REY and that fungal biomass can also play a significant
role in this process, particularly for REY.