Chemical-Free Recovery of Elemental Selenium from Selenate

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Chemical-free recovery of elemental selenium from selenatecontaminated water by a system combining a biological reactor, a bacterium-nanoparticle separator, and a tangential flow filter Zhiming Zhang, Comfort Adedeji, Gang Chen, and Youneng Tang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Environmental Science & Technology

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Chemical-free recovery of elemental selenium from selenate-contaminated water by a

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system combining a biological reactor, a bacterium-nanoparticle separator, and a

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tangential flow filter

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Zhiming Zhang a, Comfort Adedeji a, Gang Chen a, Youneng Tang a,*

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a

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Florida State University, 2525 Pottsdamer Street, Tallahassee, Florida 32310, United States

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*Phone: +1(850)410-6119; e-mail: [email protected]

Department of Civil and Environmental Engineering, FAMU-FSU College of Engineering,

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1 ACS Paragon Plus Environment


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ABSTRACT

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Biological selenate (SeO42-) reduction to elemental selenium nanoparticles (SeNPs) has been

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intensively studied, but little practiced owing to additional cost associated with separation of

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SeNPs from water. Recovery of the SeNPs as a valuable resource has been researched to make

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the approach more competitive. Separation of the intracellular SeNPs from the biomass usually

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requires the addition of chemicals. In this research, a novel approach that combined a biological

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reactor, a bacterium-SeNP separator, and a tangential flow ultrafiltration module (TFU) was

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investigated to biologically reduce selenate and separate the SeNPs, biomass, and water from

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each other. This approach efficiently removed and recovered selenium while eliminating the use

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of chemicals for separation. The three units in the approach worked in synergism to achieve the

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separation and recovery. The TFU module retained the biomass in the system, which increased

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the biomass retention time and allowed for more biomass decay through which intracellular

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SeNPs could be released and recovered.

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aggregates due to their different interactions with a tilted polyethylene sheet in the bacterium-

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SeNP separator. SeNP aggregates stayed on the polyethylene sheet while bacterial aggregates

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settled down to the bottom of the separator.

SeNP aggregates were separated from bacterial

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Table of Contents

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INTRODUCTION

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Selenium (Se), a naturally occurring element, is commonly found in agricultural drainage and

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wastewater from industries such as oil refining, mining and power generation.1 It exists in four

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oxidation states: selenate (SeO42-, +Ⅵ), selenite (SeO32-, +Ⅵ), elemental selenium (Se0, 0), and

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selenide (Se2-, -Ⅵ).2 As the most oxidized form and the most common form in contaminated

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water, SeO42- is highly water soluble and toxic to organisms at low concentrations.2-4 The typical

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total Se concentration in agricultural drainage is about 84 - 4,200 µg/L, and the value is larger in

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some industrial wastewater (up to 74 mg/L).5-7 The maximum contaminant level set for total Se

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by the U.S. Environmental Protection Agency for drinking water is 50 µg /L.8

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Existing selenate removal processes fall into two categories: 1) physicochemical processes that

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directly separate selenate from the water and 2) combined processes that use biological processes

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to reduce selenate to elemental selenium and physicochemical processes to separate elemental

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selenium from water. The physicochemical processes in category 1) such as reverse osmosis, ion

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exchange, zero-valent iron treatment, and ferrihydrite co-precipitation produce large volumes of

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residue such as brine and sludge that may cause secondary contamination and require expensive

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further treatment or disposal.9, 10 The combined processes in category 2) rely on the microbial

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reduction of selenate to elemental selenium nanoparticles (SeNPs),11 and the following

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separation of selenium particles and biomass from the water using physicochemical processes

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such as filtration, centrifugation or coagulation–flocculation.12 The combined processes produce

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less residue because the separation of the solid SeNPs and biomass from water is easier than the

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separation of the dissolved selenate.13 However, the application of the combined processes is

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very limited due to the higher capital cost associated with the combined processes.


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To offset the capital cost of the combined processes, researchers in recent years have been

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studying strategies to recover and reuse SeNPs produced from the combined process. Elemental

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selenium is a valuable resource, which is extensively used in industries such as the

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manufacturing of glass, electronics and alloys.14 However, its separation from the biomass is

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very challenging since the intracellular SeNPs are more common than the extracellular SeNPs.15

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The current selenium recovery methods fall into two categories. The first category breaks

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microbial cells to release SeNPs using autoclave or liquid nitrogen and then separates and

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purifies SeNPs using chemicals (e.g., n-octyl alcohol, chloroform, and ethyl alcohol).16, 17 To

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avoid breaking cells, the other category of methods uses microbes to reduce SeNPs to volatile

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methylated selenides and then uses chemicals (e.g., nitric acid) to collect the selenides, which are

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toxic.18 The significant consumption of chemicals in both categories makes them economically

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prohibitive and may cause secondary contamination. There is a need for novel SeNPs separation

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and recovery methods that rely on less or no chemical use.

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This work meets this need by testing a novel approach that does not use any chemicals during the

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SeNP separation process. The approach biologically reduces selenate to SeNPs, and separates

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the treated water, SeNPs, and biomass from each other in a system that involves three treatment

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units in synergism: 1) a fixed-bed biofilm reactor, 2) a novel bacterium-nanoparticle separator

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containing a tilted polyethylene sheet, and 3) a tangential flow ultrafiltration module (TFU). The

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fixed-bed biofilm reactor is a commonly used biological reactor for converting selenate to

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SeNPs.11 TFU was used for separating nanoparticles such as nickel nanoparticles and silver

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nanoparticles from water, but has not been used for separating selenium nanoparticles.19, 20 We



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hypothesize that it can also separate the SeNPs (typically 50 - 500 nm) and the bacteria

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(typically > 500 nm ) from water (Hypothesis 1).21, 22 The TFU consists of hollow fibers that

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allow the water to pass but retain and concentrate biomass and SeNPs. We hypothesize that the

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concentrated biomass increases the biological selenate removal rate which thereby allows a very

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high selenate loading rate (Hypothesis 2). The longer biomass retention is also beneficial in that

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it promotes biomass decay, which not only produces less biomass residue, but also releases the

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intracellular SeNPs for recovery and eliminates the need of chemical addition for breaking the

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cells as in the conventional Se recovery methods. To separate the SeNPs from the biomass in the

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system, we include into the system a bacterium-SeNP separator that has a tilted polyethylene

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sheet. We hypothesize that SeNPs and bacteria can be separated since they interact with the

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tilted polyethylene differently (Hypothesis 3). Bacteria and SeNPs can be potentially separated

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since SeNPs have much larger surface free energy (200 - 300 mJ·m-2) than bacteria (typically
0 means that the aggregate rolls off the polyethylene sheet. T versus R is plotted in Figure 8.

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A SeNP aggregate rolls off when its diameter is larger than 7.2 µm, and a bacterial aggregate

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rolls off when its diameter is larger than 24 µm. This explains the observation of bacterium-

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SeNP separation in Figure 3: The selenium aggregates had diameters less than 5.0 µm and they

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stayed on the polyethylene sheet (Figures 3c and 3d); on the other hand, most bacterial

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aggregates had diameters of more than 50 µm and they were only found at the bottom of the

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bacterium-SeNP separator (Figures 3c and 3e).

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The diameter larger than which the aggregate rolls off (e.g., 7.2 µm for the SeNP aggregate and

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24 µm for the bacterial aggregate according to Figure 8) is of particularly interest in practice and

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thereby is defined as the critical diameter. The critical diameters of the SeNP aggregates and the

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bacterial aggregates directly affect the bacterium-SeNP separation and can be controlled by

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varying the tilted angle (θ) of the polyethylene sheet. Figure S4 shows the critical diameters of

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the bacterial and SeNP aggregates versus θ. A θ smaller than 5° is not good for bacterium-SeNP

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separation since bacterial aggregates (d > 50 µm) also stays on the polyethylene sheet as well as

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all SeNP aggregates. A θ larger than 30° θ is good for separation since all SeNP aggregates (< 5

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µm) stays on the polyethylene sheet while most bacterial aggregates roll off as they have

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diameters larger than 50 µm. Further increase of θ has no significant impact on the separation of

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bacteria and SeNPs. 16 ACS Paragon Plus Environment

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Practical implications

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During the three-month test, clogging did not occur. Considering that scaled-up systems are

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usually operated in a much longer term, periodic backwash of the biological reactor should be

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included in the design to remove extra biomass. The bacterium-SeNP separator and the TFU

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should be disconnected from the biological reactor during backwash to eliminate the impact of

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backwash on the separator and TFU.

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In the test, the impact of dissolved oxygen on biological selenate removal was minimized since

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the system influent was purged with nitrogen gas and the system was operated in an anaerobic

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box. In real world applications, dissolved oxygen may exist in the influent and/or intrude into

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the system. One way to deal with a small concentration of dissolved oxygen is adding electron

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donor to the influent according to the stoichiometric requirement for simultaneous removal of

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dissolved oxygen and selenate.51 When the dissolved oxygen in the influent is much higher than

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the selenate concentration, a two-stage system may be used so that the first stage removes

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dissolved oxygen and the second stage removes selenate.52

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The elemental selenium transformed from SeNPs to nanorods after 58 days in the test,

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suggesting that the selenium recovery frequency affects the crystal structure of the recovered

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selenium. More research on the crystal structure is needed to connect the recovery frequency to

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the use of the recovered selenium. While the selenide production was negligible in the test (See

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Figure 2), it may be produced when some microbial species such as Bacillus selenitireducens are

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present.53 Selenide precipitates with some metals such as zinc to form metal sulfides, which may



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also be recovered. Our work focused on the recovery of selenium, but the same system may also

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be applied to other metals and metalloids such as palladium.54

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Major findings

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A combination of a biological reactor, a bacterium-SeNP separator, and a tangential flow

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ultrafiltration module was used for the first time for selenate removal from contaminated water

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and elemental selenium recovery. The SeO42- was reduced to SeNPs at a surface loading rate of

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280 mg Se·m-2·day-1, which was comparable to the highest loading rate reported in the literature

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for biofilm reactors. The tangential flow ultrafiltration module kept almost all of the SeNPs in

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the system, which were separated from the biomass in the bacterium-SeNP separator. The

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separation was well explained by torque analysis of the SeNP aggregates and bacterial

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aggregates on the tilted polyethylene screen in the separator. The torque analysis also predicts

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that the polyethylene sheet is suitable for bacterium-SeNP separation when the sheet is placed at

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a tilted angle of larger than 30°. A small portion of SeNPs on the polyethylene screen were

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transformed to Se nanorods, a more stable form.

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ACKNOWLEDGEMENTS

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This work was supported by a planning grant at Florida State University. The authors greatly

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thank Dr. Eric Lochner for providing technical support in the SEM imaging.

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Supporting Information Available

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This information is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. A schematic diagram of the selenium-recovery system

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80 [SeO42-]in [SeO42-]eff [SeO32-]eff

Se species (mg·L-1)

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[Se2-]eff [Se]solid in effluent [Se]solid in system

40

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0 0 508 509

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40 Day

Figure 2. Se species in the system

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a) Retentate collected after passing the system effluent through a 15-nm poresize membrane

b) Retentate collected after passing deionized water through a 15-nm poresize membrane (control of Figure 3a)

c) Polyethylene sheet (low magnification)

d) Polyethylene sheet (high magnification)

e) Precipitate at the bacterium-SeNP separator bottom (low magnification)

f) Precipitate at the bacterium-SeNP separator bottom (high magnification)

Figure 3. SEM images of solid samples from the bacteria-SeNPs separator and the system effluent at scenario 1. Note: EDX spectra of the particles are in Figure S1. 29 ACS Paragon Plus Environment


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a) Biofilm on the plastic media in the biological reactor (low magnification)

b) Biofilm on the plastic media in the biological reactor (high magnification)

Figure 4. Biofilm-coated plastic media in the biological reactor in scenario 1. Note: EDX spectra of the SeNPs are in Figure S1c.

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Day 1

Day 18

Day 20

Day 30

Day 45

Figure 5. Comparison of the biological reactor and the bacterium-SeNP separator in scenario 1

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a) Polyethylene sheet (low magnification)

b) Polyethylene sheet (high magnification)

Figure 6. Solids on polyethylene sheet in scenario 2

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Figure 7. A schematic diagram of forces applied to the bacterial aggregates (1) and SeNP

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aggregates (2) on the polyethylene sheet

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1e-17 Bacterial aggregates SeNP aggregates

T value

0

-1e-17

-2e-17

-3e-17 0 530 531

5

10

15

20

25

Diameter of bacterial or SeNP aggregates (µm)

Figure 8. The effects of the bacterial or SeNP aggregate diameter on the torque balance

532


Chemical-Free Recovery of Elemental Selenium from Selenate

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