Abstract
Given the increasing global demand for rare earth elements (REE),
prospects for REE recovery from both traditional and non-traditional
sources have been a focus of intense interest. Many have noted the need
for ecologically sustainable alternatives to conventional
pyrometallurgical and hydrometallurgical methods to recover REE. Among
the newer approaches that have garnered recent interest are those that
rely on microbiological processes or microbiologically produced reagents
to recover the rare earths. Biological approaches can often avoid many
of the environmental and or safety hazards associated with the corrosive
(e.g., strong acids) or toxic chemicals (e.g., organic solvents) often
used in hydrometallurgy as well as costs related to the high energy,
reagent and capital requirements and potential air emissions associated
with pyrometallurgy. Microbial processes are considered environmentally
friendly because they are “natural”, although opportunities also exist
to improve on native capabilities by the application of synthetic
biology. In this chapter we will focus on some important factors that
have not been as widely discussed but which should be considered in
planning actual deployment of biological approaches for recovery and
purification of rare earths, drawing on some of our own experience for
examples. In particular we will focus on geochemical and biogeochemical
constraints posed by the feedstocks from which REE may be extracted, for
both bioleaching and biosorption, and point out the importance of
aqueous equilibrium modeling as a tool for interpreting results and
supporting design of biological recovery methods. We will also discuss
some important cost factors for REE recovery that are specific to
biological processes.