Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES
Article
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 32
Environmental Science & Technology
1 2 3
Recovery of Rare Earth Elements from Low-Grade Feedstock Leachates using Engineered Bacteria
4
Dan M. Park1, Aaron Brewer1,2, David W. Reed3, Philip L. Hageman4, Laura N.
5
Lammers5 and Yongqin Jiao1*
6 7
1
8
Livermore, California 94550, United States; 2Univiersty of Washington, Earth and Space
9
Sciences, Seattle, WA 98195, United States; 3Idaho National Laboratory, Idaho Falls,
Physical and Life Science Directorate, Lawrence Livermore National Laboratory,
10
Idaho 83415, United States; 4United States Geological Survey, MS964D Denver Federal
11
Center, Denver, CO 80225, United States; 5Department of Environmental Science,
12
Policy, and Management, University of California, Berkeley, California 94720, United
13
States.
14 15
* Corresponding author:
[email protected] 16 17 18 19 20 21
1
ACS Paragon Plus Environment
Environmental Science & Technology
22 23 24
Abstract
25
many recent investigations. However, REE adsorption by bioengineered systems has
26
been scarcely documented, and rarely tested with complex natural feedstocks. Herein,
27
we engineered E. coli cells for enhanced cell surface-mediated extraction of REEs by
28
functionalizing the OmpA protein with 16 copies of lanthanide binding tag (LBT).
29
Through biosorption experiments conducted with leachates from metal-mine tailings and
30
rare earth deposits, we show that functionalization of the cell surface with LBT yielded
31
several notable advantages over the non-engineered control. First, the efficiency of REE
32
adsorption from all leachates was enhanced as indicated by a 2 to 10-fold increase in
33
distribution coefficients for individual REEs. Second, the relative affinity of the cell
34
surface for REEs was increased over all non-REEs except Cu. Third, LBT-display
35
systematically enhanced the affinity of the cell surface for REEs as a function of
36
decreasing atomic radius, providing a means to separate high value heavy REEs from
37
more common light REEs. Together, our results demonstrate that REE biosorption of
38
high efficiency and selectivity from low-grade feedstocks can be achieved by engineering
39
the native bacterial surface.
40
TOC Art
The use of biomass for adsorption of rare earth elements (REEs) has been the subject of
41
2
ACS Paragon Plus Environment
Page 2 of 32
Page 3 of 32
Environmental Science & Technology
42 43
Keywords: mineral extraction, bioadsorption, biosorption, leaching, rare earth, rare earth elements, rare earth metals, REE, REEs
44
Introduction
45
Rare earth elements (REEs) are critical components of many clean energy
46
technologies (e.g., wind turbines and hybrid car batteries) and consumer products (e.g.,
47
mobile phones, laptops, appliances, and automotive sensors).1 As such, constraints or
48
limitations in the supply of REEs could hinder the growth of technology industries that
49
are critical for the transition to a low-carbon economy.2, 3 Greater than 90% of the global
50
REE supply is obtained from China, leaving the global market vulnerable to supply
51
restrictions as was observed recently when export quotas were tightened.4 To alleviate
52
supply vulnerability and diversify the global REE supply chain, it is imperative to
53
develop new extraction methodologies and explore alternative REE resources.
54
Non-traditional REE resources, such as mine tailings, geothermal brines and coal
55
byproducts are abundant and offer a potential means to diversify the REE supply chain.5-8
56
However, given the low REE content and high concentrations of competing metals
57
present in these feedstocks, conventional REE-extraction approaches are prohibitive at an
58
industrial scale.9 These REE extraction approaches, particularly hydrometallurgy via
59
solvent extraction, are also energy intensive and pose severe environmental burdens.10, 11
60
Therefore, the development of alternative technologies that enable efficient recovery of
61
REE from non-traditional feedstocks is highly desirable.
62
Microbially mediated surface adsorption (biosorption) represents a potentially
63
cost-effective and ecofriendly approach for metal recovery.12-15 Microorganisms exhibit
64
high metal adsorption capacities because of their high surface area per unit weight and
65
the abundance of cell surface functional groups (e.g., carboxylates and phosphates) with
3
ACS Paragon Plus Environment
Environmental Science & Technology
66
metal coordination functionality.15 Additionally, the reversibility and fast kinetics of
67
adsorption enables an efficient metal extraction process,16-18 while the ability of cells to
68
withstand multiple adsorption-desorption cycles avoids the need for frequent cell-
69
regeneration,17, 19 decreasing operational costs. Biosorption is also expected to have a
70
minimal environmental impact relative to traditional extraction techniques.12, 15
71
Biosorption could be particularly well-suited for REE extraction given reports of
72
selective adsorption of REEs over non-REEs.20-23 For example, Tm3+ was preferentially
73
adsorbed over Fe2+ and Mn2+ by Bacillus subtilis,23 while Sm3+ was adsorbed over Cu2+,
74
Mn2+, Co2+, Ni2+, Zn2+ and Cd2+ by Arthrobacter nicotianae.22 Additionally, the stability
75
constant for the interaction of B. subtilis with Nd3+ was stronger than for all other tested
76
metal cations, except the uranyl oxycation.21 Furthermore, despite the highly similar
77
physicochemical properties of REEs, a general biosorption preference for heavy REEs
78
over light REEs has been observed,16, 24, 25 highlighting the potential of biosorption for
79
enrichment of individual REEs. Indeed, a recent study reported a pH-dependent REE
80
desorption scheme that achieved separation factors for certain REE pairs that exceeded
81
solvent extraction standards.26 Nevertheless, it remains an open question whether
82
biosorption will be an effective means for selective extraction of REEs from low-grade
83
feedstocks that contain high concentrations of competing metals.
84
To further improve the adsorption capacity and selectivity of the cell surface for
85
particular metals (e.g., Au and Pb), bioengineering approaches have been successfully
86
used to display selective metal-binding peptides or proteins on the cell surface.12, 13, 17, 27-
87
29
88
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-
4
ACS Paragon Plus Environment
Page 4 of 32
Page 5 of 32
Environmental Science & Technology
89
S-layer protein, anchoring the LBT to the cell surface.17 This approach enhanced the
90
ability of C. crescentus to extract REEs from a high-grade REE ore leachate.17 However,
91
the concentrations of adsorbed non-REE metals were not quantified, precluding
92
evaluation of the selectivity of REE adsorption. Furthermore, a maximum of eight copies
93
of LBT per S-layer protein was achieved due to cellular toxicity, limiting the REE
94
adsorption capacity of the Caulobacter system. In order to improve the REE adsorption
95
capacity and adapt the LBT surface display to a broader range of microbial species, here
96
we utilized the abundant surface protein OmpA31, 32 as an anchor for LBT display in E.
97
coli. The resulting LBT-displayed strain was used directly as a whole cell adsorbent
98
with feedstock leachates of complex matrix and low REE content. Our results
99
demonstrate the feasibility of coupling bioengineering with biosorption for REE
100
extraction from low-grade feedstocks.
101 102 103 104
Methods
105
copies of dLBT (double-LBT) were fused to the 3’ end of ompA (encoding the outer
106
membrane protein A) and placed under the control of the arabinose-inducible promoter
107
(PBAD) using the pBAD-ompA-pbrR vector28 as a template (see Supporting Information
108
for details). The resulting lpp-ompA-LBT expression plasmids were transformed into the
109
indicated E. coli strains (Table S1).
110
Expression of lpp-ompA-dLBT constructs. E. coli strains harboring lpp-ompA-dLBT
111
expression plasmids were grown in LB media supplemented with 50 µg/ml ampicillin.
112
Expression of lpp-ompA-dLBT was induced at mid-exponential phase using 0.002%
113
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
5
ACS Paragon Plus Environment
Environmental Science & Technology
114
(2-(N-morpholino)ethanesulfonic acid) pH 6.0, normalized by OD600 and used in
115
biosorption or luminescence experiments. Control cells contained the lpp-ompA-
116
dLBTx8 expression plasmid but were not treated with arabinose. For C. crescentus,
117
strains DMP146 (expressing 4 copies of LBT per RsaA protein) and JS4022 (Control)
118
were grown overnight at 30°C in PYE supplemented with 2 mM CaCl2 as described
119
previously17 and washed as described for E. coli.
120
Tb adsorption capacity determination. Washed E. coli and C. crescentus cells were
121
incubated at an OD600 of 0.25 in a solution containing 10 mM MES pH 6.0, 10 mM NaCl,
122
and varying Tb3+ concentrations (0-400 µM) for 30 min at RT prior to centrifugation at
123
20,000 x g for 8 min. The extracted supernatant was analyzed using inductively coupled
124
plasma mass spectrometry (ICP-MS) to quantify Tb3+ concentrations. Total adsorbed
125
Tb3+ was calculated by subtracting the Tb3+ concentration remaining in the supernatant
126
from the concentration of Tb3+ in the control without bacterial cells. For dry cell weight
127
determination, 6 ml of cells from reactions lacking Tb3+ were dried overnight at 65°C.
128
Adsorption capacity was normalized to dry cell weight.
129
Acid leaching of REE-containing source material. Bull Hill borehole samples (43.5 m
130
below land surface) were obtained from Rare Element Resources, Sundance, Wyoming,
131
USA). Samples from the Round Top Mountain mineral deposit (El Paso, TX)33 were
132
crushed into fine particles using a mortar and pestle. Mine Tailings from Lower Radical
133
and Togo Mines (near Montezuma, CO) were obtained as powder (
