Recovery of Rare Earth Elements from Low-Grade Feedstock

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Recovery of Rare Earth Elements from Low-Grade Feedstock Leachates using Engineered Bacteria Dan Park, Aaron William Brewer, David W. Reed, Philip Hageman, Laura Nielsen Lammers, and Yongqin Jiao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02414 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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Recovery of Rare Earth Elements from Low-Grade Feedstock Leachates using Engineered Bacteria

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Dan M. Park1, Aaron Brewer1,2, David W. Reed3, Philip L. Hageman4, Laura N.

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Lammers5 and Yongqin Jiao1*

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Livermore, California 94550, United States; 2Univiersty of Washington, Earth and Space

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Sciences, Seattle, WA 98195, United States; 3Idaho National Laboratory, Idaho Falls,

Physical and Life Science Directorate, Lawrence Livermore National Laboratory,

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Idaho 83415, United States; 4United States Geological Survey, MS964D Denver Federal

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Center, Denver, CO 80225, United States; 5Department of Environmental Science,

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Policy, and Management, University of California, Berkeley, California 94720, United

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States.

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* Corresponding author: [email protected]

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Abstract

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many recent investigations. However, REE adsorption by bioengineered systems has

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been scarcely documented, and rarely tested with complex natural feedstocks. Herein,

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we engineered E. coli cells for enhanced cell surface-mediated extraction of REEs by

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functionalizing the OmpA protein with 16 copies of lanthanide binding tag (LBT).

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Through biosorption experiments conducted with leachates from metal-mine tailings and

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rare earth deposits, we show that functionalization of the cell surface with LBT yielded

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several notable advantages over the non-engineered control. First, the efficiency of REE

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adsorption from all leachates was enhanced as indicated by a 2 to 10-fold increase in

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distribution coefficients for individual REEs. Second, the relative affinity of the cell

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surface for REEs was increased over all non-REEs except Cu. Third, LBT-display

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systematically enhanced the affinity of the cell surface for REEs as a function of

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decreasing atomic radius, providing a means to separate high value heavy REEs from

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more common light REEs. Together, our results demonstrate that REE biosorption of

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high efficiency and selectivity from low-grade feedstocks can be achieved by engineering

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the native bacterial surface.

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TOC Art

The use of biomass for adsorption of rare earth elements (REEs) has been the subject of

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Keywords: mineral extraction, bioadsorption, biosorption, leaching, rare earth, rare earth elements, rare earth metals, REE, REEs

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Introduction

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Rare earth elements (REEs) are critical components of many clean energy

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technologies (e.g., wind turbines and hybrid car batteries) and consumer products (e.g.,

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mobile phones, laptops, appliances, and automotive sensors).1 As such, constraints or

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limitations in the supply of REEs could hinder the growth of technology industries that

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are critical for the transition to a low-carbon economy.2, 3 Greater than 90% of the global

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REE supply is obtained from China, leaving the global market vulnerable to supply

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restrictions as was observed recently when export quotas were tightened.4 To alleviate

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supply vulnerability and diversify the global REE supply chain, it is imperative to

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develop new extraction methodologies and explore alternative REE resources.

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Non-traditional REE resources, such as mine tailings, geothermal brines and coal

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byproducts are abundant and offer a potential means to diversify the REE supply chain.5-8

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However, given the low REE content and high concentrations of competing metals

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present in these feedstocks, conventional REE-extraction approaches are prohibitive at an

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industrial scale.9 These REE extraction approaches, particularly hydrometallurgy via

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solvent extraction, are also energy intensive and pose severe environmental burdens.10, 11

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Therefore, the development of alternative technologies that enable efficient recovery of

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REE from non-traditional feedstocks is highly desirable.

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Microbially mediated surface adsorption (biosorption) represents a potentially

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cost-effective and ecofriendly approach for metal recovery.12-15 Microorganisms exhibit

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high metal adsorption capacities because of their high surface area per unit weight and

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the abundance of cell surface functional groups (e.g., carboxylates and phosphates) with

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metal coordination functionality.15 Additionally, the reversibility and fast kinetics of

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adsorption enables an efficient metal extraction process,16-18 while the ability of cells to

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withstand multiple adsorption-desorption cycles avoids the need for frequent cell-

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regeneration,17, 19 decreasing operational costs. Biosorption is also expected to have a

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minimal environmental impact relative to traditional extraction techniques.12, 15

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Biosorption could be particularly well-suited for REE extraction given reports of

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selective adsorption of REEs over non-REEs.20-23 For example, Tm3+ was preferentially

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adsorbed over Fe2+ and Mn2+ by Bacillus subtilis,23 while Sm3+ was adsorbed over Cu2+,

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Mn2+, Co2+, Ni2+, Zn2+ and Cd2+ by Arthrobacter nicotianae.22 Additionally, the stability

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constant for the interaction of B. subtilis with Nd3+ was stronger than for all other tested

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metal cations, except the uranyl oxycation.21 Furthermore, despite the highly similar

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physicochemical properties of REEs, a general biosorption preference for heavy REEs

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over light REEs has been observed,16, 24, 25 highlighting the potential of biosorption for

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enrichment of individual REEs. Indeed, a recent study reported a pH-dependent REE

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desorption scheme that achieved separation factors for certain REE pairs that exceeded

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solvent extraction standards.26 Nevertheless, it remains an open question whether

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biosorption will be an effective means for selective extraction of REEs from low-grade

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feedstocks that contain high concentrations of competing metals.

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To further improve the adsorption capacity and selectivity of the cell surface for

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particular metals (e.g., Au and Pb), bioengineering approaches have been successfully

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used to display selective metal-binding peptides or proteins on the cell surface.12, 13, 17, 27-

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adsorbing strain with lanthanide binding tags (LBT)30 inserted at a permissive site of the

We recently reported the construction of a recombinant Caulobacter crescentus REE-

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S-layer protein, anchoring the LBT to the cell surface.17 This approach enhanced the

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ability of C. crescentus to extract REEs from a high-grade REE ore leachate.17 However,

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the concentrations of adsorbed non-REE metals were not quantified, precluding

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evaluation of the selectivity of REE adsorption. Furthermore, a maximum of eight copies

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of LBT per S-layer protein was achieved due to cellular toxicity, limiting the REE

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adsorption capacity of the Caulobacter system. In order to improve the REE adsorption

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capacity and adapt the LBT surface display to a broader range of microbial species, here

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we utilized the abundant surface protein OmpA31, 32 as an anchor for LBT display in E.

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coli. The resulting LBT-displayed strain was used directly as a whole cell adsorbent

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with feedstock leachates of complex matrix and low REE content. Our results

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demonstrate the feasibility of coupling bioengineering with biosorption for REE

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extraction from low-grade feedstocks.

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Methods

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copies of dLBT (double-LBT) were fused to the 3’ end of ompA (encoding the outer

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membrane protein A) and placed under the control of the arabinose-inducible promoter

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(PBAD) using the pBAD-ompA-pbrR vector28 as a template (see Supporting Information

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for details). The resulting lpp-ompA-LBT expression plasmids were transformed into the

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indicated E. coli strains (Table S1).

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Expression of lpp-ompA-dLBT constructs. E. coli strains harboring lpp-ompA-dLBT

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expression plasmids were grown in LB media supplemented with 50 µg/ml ampicillin.

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Expression of lpp-ompA-dLBT was induced at mid-exponential phase using 0.002%

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arabinose for 3 h at 37°C. The E. coli cells were harvested, washed once in 10 mM MES

Construction of plasmids for surface display of LBT in E. coli. Multiple, adjacent

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(2-(N-morpholino)ethanesulfonic acid) pH 6.0, normalized by OD600 and used in

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biosorption or luminescence experiments. Control cells contained the lpp-ompA-

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dLBTx8 expression plasmid but were not treated with arabinose. For C. crescentus,

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strains DMP146 (expressing 4 copies of LBT per RsaA protein) and JS4022 (Control)

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were grown overnight at 30°C in PYE supplemented with 2 mM CaCl2 as described

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previously17 and washed as described for E. coli.

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Tb adsorption capacity determination. Washed E. coli and C. crescentus cells were

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incubated at an OD600 of 0.25 in a solution containing 10 mM MES pH 6.0, 10 mM NaCl,

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and varying Tb3+ concentrations (0-400 µM) for 30 min at RT prior to centrifugation at

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20,000 x g for 8 min. The extracted supernatant was analyzed using inductively coupled

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plasma mass spectrometry (ICP-MS) to quantify Tb3+ concentrations. Total adsorbed

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Tb3+ was calculated by subtracting the Tb3+ concentration remaining in the supernatant

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from the concentration of Tb3+ in the control without bacterial cells. For dry cell weight

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determination, 6 ml of cells from reactions lacking Tb3+ were dried overnight at 65°C.

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Adsorption capacity was normalized to dry cell weight.

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Acid leaching of REE-containing source material. Bull Hill borehole samples (43.5 m

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below land surface) were obtained from Rare Element Resources, Sundance, Wyoming,

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USA). Samples from the Round Top Mountain mineral deposit (El Paso, TX)33 were

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crushed into fine particles using a mortar and pestle. Mine Tailings from Lower Radical

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and Togo Mines (near Montezuma, CO) were obtained as powder (