Bioleaching of ion-adsorption rare earth ores by biogenic lixiviants derived from agriculture waste via a cell-free cascade enzymatic process

Biogenic organic acids are considered environmentally friendly alternatives to ammonium sulfate for leaching ion-adsorption rare earth ores. Fermentation is popular for their production, but undesired cell metabolites and residual medium components could dampen the leaching performance. High feedstock cost is another major barrier for their applications. To tackle these challenges, a cell-free cascade enzymatic process was developed to produce gluconate from low-cost rice straw. Gluconate production by fermentation and enzymatic catalysis was first compared, and it was found that the enzymatic process gave a 99% yield within 24 h, while it was only 60% for fermentation after 48 h. The gluconate from enzymatic process was superior to that from fermentation in terms of leaching efficiency (100% vs. 18.1%). The precipitation of rare earth elements (REEs) by residual phosphate was identified as the major contributor to the low leaching performance by gluconate produced via fermentation. The gluconate derived from rice straw via the cascade enzymatic process did not show a compromised leaching efficiency, reaching 100% as well. This study sheds lights on how enzymatic process could be adopted to circumvent interferences for REEs recovery by fermentative impurities, and opens a new avenue for potentially economical production of biogenic lixiviants from renewable and cheap feedstocks.