Prospects of metal recovery from wastewater and brine

Abstract

Modern technology relies on an undisrupted supply of metals, yet many metals have limited geological deposits. Recovering metals from wastewater and brine could augment metal stocks, but there is little guidance on which metals to prioritize for recovery or on the techno-economic viability of extraction processes. Here we critically assess the potential for recovering metals from wastewater and brine. We first look at which metals are critical for recovery on the basis of their supply risks and the impacts of those supply restrictions. We then assess the feasibility of recovering these metals from various water sources by estimating the required operational costs to match market prices. Next we discuss the limitations of established separation technologies that may inhibit the practicality and scalability of metal recovery from water. We conclude by highlighting materials and processes that could serve as more sustainable alternatives to metal recovery with further research and development. Modern technology relies on the present and future availability of elements across the periodic table, especially metals and metalloids 1. The unique properties of each element impart distinct functionality to materials that allow them to perform effectively in a given technological application 2. For example, the path to global decarboniza-tion is contingent on sustainably sourced lithium, nickel and cobalt to manufacture lithium-ion batteries with high energy density, as well as platinum to produce polymer electrolyte fuel cells with high catalytic performance. The perpetual supply of these critical metals and metal-loids, however, is not guaranteed as supplies decrease and ore grades decline in the future 3,4. A circular resource economy with routine recycling could increase the long-term supply of metals and metalloids, which we collectively refer to as 'metals' for convenience 5. Some common metals are already recycled at rates near or exceeding 50% (for example, copper and lead), but are nevertheless projected to have supply deficits by 2050 (ref. 6). Specialty metals (such as rare-earth elements) largely cannot be recycled because they are used in small quantities in products with complex elemental compositions, such as computer chips, optoelectronic devices and high-strength magnets 2,7. Innovative recycling methods and improved reuse rates will be critical to meet rising demand for both common and specialty metals over the next few decades 6,7. Municipal and industrial wastewaters are increasingly regarded as potentially viable sources for recycling valuable elements 8. Apart from augmenting metal supply, the valorization of 'waste' metals from these waters would potentially offset wastewater treatment costs. Naturally occurring brines, such as seawater, salt lakes and geothermal aquifers are even larger repositories of valuable metals, with some material stocks surpassing their availabilities on land 9. A few metals are commercially extracted from these sources, including lithium from shallow brine beneath dry lakes and magnesium from seawater 10 , typically using one or more chemically intensive precipitation steps. The prospect of extracting other valuable commodities has led to extensive research to develop chemical-free highly selective methods to recover dilute target metals from complex water matrices 11,12. There is no consensus, however, about which metals and water sources should be the focus of recovery efforts, nor is there much guidance about the techno-economic viability of metal recovery from these water sources. Accordingly, a diverse collection of metals from various water sources has been studied-ranging from lithium and uranium in seawater 13,14 to mercury in groundwater 15 and selenium in industrial