Microbial Conversion of Wetland Plants to Organic Acids for Reductive Bioleaching of Metals (Manganese, Iron, and Nickel): Review

This review focuses on invasive aquatic and wetland plants, such as cattail (Typha latifolia) L., common reed (Phragmites australis (Cav.) Trin. ex Steud.), and water hyacinth (Eichhornia crassipes (Mart.) Solms), as feedstocks for microbial production of organic acids used in reductive bioleaching of manganese, iron, and nickel. These species are abundant, fast-growing, low in lignin relative to woody biomass, and often available at zero or negative acquisition cost as products of environmental mitigation, making them attractive alternatives to feedstocks that compete with food or established markets. As high-grade ore reserves decline, sustainable and low-cost alternatives for metal recovery have become vital, and organic acids such as citric, oxalic, and acetic acid derived from biomass fermentation offer a promising alternative to synthetic inorganic acids. This paper synthesizes current literature on the compositional characterization of these aquatic and wetland feedstocks, the low-input pretreatment strategies suited to their low-lignin structure (including brief physical pre-conditioning via microwave or ultrasonic exposure alongside biological pretreatment), and the microbial fermentation pathways that convert their carbohydrate content into organic acids for reductive bioleaching. Key research gaps specific to bioconversion of these underexplored feedstocks are highlighted to guide the development of an integrated, scalable bioprocess for sustainable metal recovery.