Contact Behavior between Cells and Particles in Bioleaching of

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Contact Behavior between Cells and Particles in the Bioleaching of Precious Metals from Waste Printed Circuit Boards Zhihui Yuan, Zhe Huang, Jujun Ruan, Yaying Li, Jian Hu, and Rong-Liang Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01742 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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ACS Sustainable Chemistry & Engineering

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Contact Behavior between Cells and Particles in the Bioleaching of

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Precious Metals from Waste Printed Circuit Boards

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Zhihui Yuan1, Zhe Huang1, Jujun Ruan1∗, Yaying Li1, Jian Hu2,

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Rongliang Qiu1*

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1. Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation

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Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, 135

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Xingang Xi Road, Guangzhou, 510275, People’s Republic of China

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2. School of Environmental Science and Engineering, Yangzhou University

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Corresponding author: Jujun, Ruan Tel: +86 20 84113620; Fax: +86 20 84113620; E-mail: [email protected]; Rongliang, Qiu, [email protected]

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Bioleaching of precious metals from waste printed circuit boards (WPCBs) such

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as using cyanogenic cultures for extraction of gold and silver has gained considerable

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attention. It has the advantages of low cost and environment friendliness. However,

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low efficiency hindered its industrialization. In this study, a novel strategy has been

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proposed to improve bioleaching efficiency. We applied optical microscopy to

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investigate the surface interactions between bacteria and silver particles in two-step

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bioleaching. Results showed that bacterial adsorption and extracellular polymeric

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substances (EPS) binding were the primary negative behavior that hindered the

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leaching reaction. Functional group analysis by Fourier-transform infrared

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spectroscopy (FTIR) indicated that carboxyl, hydroxyl, and amine groups were the

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main chemical structures responsible for the negative influences. Bacteria–metal

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interactions were reduced in the presence of polyvinylpyrrolidone (PVP) and the

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silver recovery increased 1.8 times. Imaging analysis showed bacteria and EPS were

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dispersed from silver surface in the case of PVP addition. Zeta potential analysis

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indicated that PVP exhibited high affinity binding to silver particles and suppressed

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the attachment of the microbial materials onto silver surface. Our results demonstrated

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the utility of PVP addition for the bioleaching of precious metals.

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Key

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polyvinylpyrrolidone

32

word:

E-waste,

precious

metal,

bioleaching,

2


diffusion

control,

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INTRODUCTION

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E-wastes recycling is a worthwhile project not only from the perspective of

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waste disposal but also from the context of resource recovery.1–3 Among recycled

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materials, valuable metals in waste printed circuit boards (WPCBs) account for more

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than 95% of the recovery value of e-wastes. Furthermore, precious metals contribute

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to 80% of the monetary value of the recovered metals, and Au has the highest priority

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to be recovered.4 Therefore, precious metal recovery has gained increased attention.

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High concentrations of precious metals are found in electronic scrap samples.5

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Physical methods, such as mechanical disruption, magnetic separation, and

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electrostatic separation, are the preferred pre-treatment methods for WPCBs.6,7 These

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methods have properties of low cost and low pollutant emission and can efficiently

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obtain homogeneous materials. However, such methods present limitations in terms of

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purification of various valuable metals from metal composites. Precious metals

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exhibit high melting point and strong stability. Pyrometallurgical process requires

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high energy consumption and creates pollution to the environment, and traditional

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hydrometallurgical method produces large quantities of secondary pollutants.8

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Therefore, alternative environment-friendly technologies must be developed for

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reclamation of precious metals from e-wastes.

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In precious metal bioleaching, hydrocyanic acid produced by some bacteria

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serves as biogenic lixiviant. Precious metal dissolution can be achieved during cell

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cultivation. Initially, metal composites obtained after physical separation were directly 3



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subjected to bioleaching. This method exhibited low efficiency due to interference

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from exogenous metal. Pretreatment of metal components through biological

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pre-oxidation to remove base metals in the leached concentrates leads to high precious

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metal recovery.9,10 In addition, the researchers explored two-step leaching pattern and

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spent medium leaching.11–13 In two step bioleaching, solid metal samples were added

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when the lixiviant (cyanide) concentration reached maximum levels. In spent medium

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bioleaching, cells were separated from the culture after it reached maximum cyanide

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concentration, and only cell-free metabolites were used for bioleaching.13 However,

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spent medium leaching is not easy to achieve for industrial upscaling by means of

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centrifugation, filtration, and natural sedimentation. Bioleaching can also be enhanced

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through other measures, included adding hydrogen peroxide, co-cultivation of

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cyanogenic bacteria,14 changing leaching conditions,13 and using alkali tolerant

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bacteria or metabolically engineered bacteria.9,15 Cyanide concentration produced by

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bacteria is too low, though Au recovery has been nearly doubled using metabolically

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engineered strain.15 Thus, increasing the availability of lixiviant (biogenic cyanide and

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oxygen) will make important significance. However, investigations on mutual effect

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between bacterial and metal particles are rarely conducted. Thus, potential

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improvement strategies for high leaching efficiency are often overlooked. In our

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previous study,3 the medium components used in cyanide production were optimized,

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and a novel model was developed to predict cyanide production. However,

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bioleaching kinetics are often determined by diffusion process.17 In this paper, 4


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two-step bioleaching was conducted and contact behavior between bacteria and metal

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particles was studied. Effect of PVP on enhancing metal leaching was reported and

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the potential mechanisms were discussed.

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METHODS AND MATERIALS

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Materials

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In the present work, pure silver particles instead of crushed WPCBs were used to

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simulate the bioleaching process. Ag powder (200 < mesh < 300, purity 99.99%) was

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obtained from Changsha Tian Jiu Metal Material Co., Ltd. (Changsha, China). PVP

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(average molecular weight 24,000 Da) was purchased from Aladdin Chemistry Co.,

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Ltd. (Shanghai, China). Tryptone and yeast extract were obtained from Oxoid Ltd.

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(Hampshire, UK). Other reagents used were all of GR grade. Distilled water was used

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throughout the experiment.

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Microorganism and culture conditions

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Pseudomonas fluorescens P13,18 was used for cyanide production and silver

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bioleaching. Cells were mixed in potassium phosphate buffer containing 50% glycerol

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and stored at –20 °C. It was revived in 250 mL shaken flasks containing 60 mL of

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Luria–Bertani (LB) medium (10 g/L tryptone, 5 g/L yeast extract, and 10 g/L NaCl).

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Log phase bacteria (optical cell density at 600 nm wavelength ~0.6) was sampled and

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inoculated in 250 mL flasks each containing 60 mL of the cyanogenic medium and 5



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with 5% (v/v) inoculum. The cyanogenic medium contains 6 g/L tryptone and 5 g/L

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yeast extract.3 All the flasks used in the cell preparation and cyanide production were

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shaken at 150 rpm and 25 °C. These conditions could produce the maximum

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concentration of cyanide.

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Leaching experiments

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Two-step bioleaching was conducted. Bioleaching experiments of silver particles

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was performed in flasks. 100 mg of silver particles were fed into 250 mL flasks. Then

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the flasks were autoclaved at 121 °C for 15 min. Based on our previous study (ref. 3),

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cyanide amount in the cultures reached the maximum (approximately 7 mg/L) after

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15–18 hours of incubation (during early stationary phase). Thus, 50 mL of the cultures

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cultivated for 15 hours were transferred into the Ag containing flasks. To investigate

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the effect of PVP on silver leaching, PVP was immediately added into the flasks to a

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final concentration of 240 mg/L. The flasks without PVP addition acted as control

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group. The leaching condition was 25 °C and rotation speed of 200 rpm. After 96

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hours, the biologically leached solutions were sampled and centrifuged at 10,000 rpm

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10 min. The supernatant was passed through a membrane filter with pore size 0.45 µm.

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The filtrate was digested in triplicates using nitric acid and perchloric acid. After acid

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digestion, the metal composition was analyzed for Ag+ using Inductively Coupled

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Plasma Optical Emission Spectrometer (ICP-OES, Perkin Elmer Optima 5300 DV).

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Free cyanide and oxygen analysis

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Free cyanide produced in the cultures was tested using 4-pyridinecarboxylic acid 6


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color comparison method (Pack Test, WAK-CN; Kyoritsu Chemical Check Lab. Corp.)

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For details of cyanide measurement, please refer to ref. 3. Free oxygen of the leaching

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cultures in the silver containing flasks was measured by oxygen meter (YSI 550A).

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FTIR measurements

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Infrared spectra of the cyanogenic bacteria were recorded on an FTIR instrument

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(Nicolet, iS10) to identify the functional groups on the bacteria. Cultures at 3, 12, 48

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hours of bioleaching were extracted and thoroughly dried. Sample was prepared using

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the method described in ref. 19.

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Contact angle measurements

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The hydrophobicity of bacteria was determined by water contact angel

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measurement on Dataphysics OCA40 Micro. Bacterial layers were obtained by

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filtering the bacterial suspension on a 0.45 µm microporous membrane. Contact angle

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was measured after air drying the bacterial sediments for about 10 min to remove

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excess water. Three measurements were conducted, and the average value was

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obtained.

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Zeta potential measurements

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The surface property of bacteria treated with PVP was investigated using zeta

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potential measurements (Brookhaven Instruments Corporation, New York, NY, USA).

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Bacteria at the end of exponential phase (15 hours of incubation) was harvested,

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centrifuged at 5000 rpm for 5 min, and washed three times with saline. Then the

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sedimented bacteria were resuspended in various concentrations of PVP solutions (30, 7



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60, 120, 240, 480 and 960 mg/L). The final optical cell densities of the suspension at

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400 nm wavelength were approximately equal to 0.78, an appropriate bacterial

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concentration in favor of zeta potential measurement. The pH of the suspension was

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adjusted by 0.01 M NaOH or 0.01 M HNO3 solution to pH 7.9 (pH of bacterial

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culture after 15 hours of incubation increased to 7.9). In addition, to investigate the

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zeta potential change affected by silver addition, 100 mg of silver particles were

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dispersed in 50 mL of the prepared suspension. All the samples were measured five

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times and the average value was obtained.

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Microscopic analysis

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Optical microscopy imaging of silver particles was conducted using a

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fluorescence microscope (Leica AF6000). Sampling was conducted at 3 min, 1 h, 2 h,

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3 h, and 4 h after bioleaching started. Samples were extracted by the micropipet

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technique and dropped on cleaned microscope slide. Coverslip was pressed on the

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surface enough to disperse particles. Photomicrographs were taken immediately. The

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transmitted light intensity was adjusted correspondingly to obtain optimum imaging

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results.

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SEM measurements

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The collected bacterial cells (end log phase) were fixed with 2% glutaraldehyde at

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room temperature. After eliminating the remaining glutaraldehyde, the dehydration

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process was conducted with 20, 50, 80 and 100% of alcohol. The fixed cells was then

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observed by scanning electron microscopy (SEM) (Quanta 400F). Silver particles 8


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were also determined by SEM.

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To investigate the morphological properties of bacterial cells attached on silver

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surface, 200 μL of leaching solutions (1-hour leaching) containing silver particles

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were collected from the bottom of flasks. Then silver particles together with bacterial

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solution were dropped on a tin foil and naturally air-dried. Images were obtained

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using SEM.

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Calculation of wet weight of bacteria

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Force analysis of bacteria absorbed on silver surface was performed assuming

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that bacteria does not move anymore when it starts to dehydrate. Thus bacteria density

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was approximately equal to culture density. Wet weight of bacteria was estimated by

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multiplying bacteria size by culture density. Bacteria size was estimated from

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measurements of length and width at high magnification (20,000 fold enlargement,

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Figure 1A). Culture density was obtained by determining the quality of a certain

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volume of bacterial culture.

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RESULTS AND DISCUSSION

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A typical example of P. fluorescens in log phase was shown in Figure 1A. P.

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fluorescens is rod shaped and has size of 0.5 µm (width) × 0.8–1.5 µm (length).

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According to ref. 20, the size of P. fluorescens during the exponential phase was

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0.7–0.8 µm × 2.3–2.8 µm, which decreased with age. In addition, growth of P.

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fluorsescens generally experienced a lag phase (approximately 0-2 hours of 9



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incubation), a log phase (2-15 h), and a stationary phase (15-24 h).3 Figure 1B showed

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the image of silver particles (round shaped, ~50 µm in diameter) used in this

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investigation. For the density of silver is significantly greater than the density of

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bacterial culture, silver particles would gravitate toward the bottom of the flasks. At

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high shaking speed (150–200 rpm), silver particles tended to cluster at the center of

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the flask’s bottom (Figure 2A). At lower agitation speed, silver particles would spread

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out from the center and form a circle (Figure 2B). This behavior was obviously not

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conducive to rapid leaching reaction. Thus, specially designed reactor such as an

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end-to-end tubular allowing full contact of solid solution interface was recommended

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in further investigation of bioleaching. Most importantly, in the current work, we

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presented an alternative method for improving availability of cyanide lixiviant for the

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settling silver particles, namely addition of PVP, as will be shown in the next

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paragraphs.

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Optical microscopy images of silver particles sounded by bacteria

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Further observations on aggregate behavior of silver particles were performed

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using optical microscopy. Figure 3 showed the microscopic view of the distributed

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silver particles immersed in the bacterial bath. In its entirely, silver particles exhibited

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high propensity to agglomerate due to the high interfacial energy. Figures 3A and 3B

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showed there was little cellular material around the silver particles compared with

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Figures 3D, and 3E, which were imaged after 1 and 2 h of bioleaching, respectively.

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The dispersed silver particles in Figure 3B were likely caused by covering the 10


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coverslip. A magnified view of Figure 3B, namely (Figure 3C), showed silver

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particles were continuously surround by bacteria and it implied there existed specific

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adsorption of bacteria on silver particles. The brown and opaque bacterial fragments

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in Figures 3D and 3E were thought to be aggregated form of biofilm. Biofilm is

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commonly composed of structured micro-colonies and exopolysaccharide products. In

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a biofilm, microbes are usually irreversibly bind to the biofilm surface and

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encapsulated into a exopolysaccharide matrix.21 Closer observation of the stuck

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particles (Figure 3F) showed the bacterial agglomerates consisted of loosely attached

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EPS and bacteria. Acceleration of bacterial decline reinforced the biofilm formation

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and exhibited tight adhesion to silver particles (Figure 3G). Figure 3H showed the

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inner area of Figure 3G, double layer of silver particles formed as compared with that

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showed in Figure 3A. In Figure 3H, the average spacing between particles in the

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upper layer was smaller than that observed in Figure 3I. Increased EPS formation was

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the primary cause of particle agglomeration. The production of EPS might be a

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natural physiological response to the deterioration of the growth environment.

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However, it caused undesirable effects on bioleaching.

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SEM micrograph of bacteria attachment and force analysis

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The definition, production, and function of EPS from microbes were extensively

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studied. Here, only morphologies of EPS and bacteria adsorbed on the particles were

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examined using SEM (Figure 4).Few materials existed on the top of silver particles

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(Figures 4A and 4B). Emergence of surface roughening from the middle to the top 11



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half part of the particles might be attributed to the accretion of the downward

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translocation of the attached bacterial material. Figures 4C and 4D showed that the

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cells were monolayerly adsorbed on the upper hemispherical surface of silver particle.

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The static friction that associated with bacteria attachment played a dominant role and

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was analyzed in Figure 5. The calculated wet weight of a single bacterium ranged

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from 0.159 × 10−15 to 0.299 × 10−15 kg, approximately 1.6 × 10−6 to 2.9 × 10−6 nN (g

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= 9.8 N/kg). Magnitude of static friction force F1 = G = mg × cosθ. m refers to the wet

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weight of bacteria. The static friction and gravity of the same bacteria were within the

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same order of magnitude (0 < cosθ < 1). And it (about 10−6 nN) was far less than that

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could be measured by atomic force microscopy (AFM). The forces measured by AFM

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generally changed within ± 10 nN(+ represents the repulsion force, – represents the

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attraction force) during the bacterial approach and retraction from the mineral (metal)

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surface.22–24 Though the force interacted between bacteria and silver surface was very

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little, it significantly influenced the adsorption, and reliable method could be found to

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decrease the interaction. The approximate analysis might provide some useful

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information in understanding the bacteria-material interaction.

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Contact angle of bacteria

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The aforementioned results demonstrated that the attachment greatly increased

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due to the formation of biofilm during bioleaching. The contact angle of bacteria

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before and after bioleaching decreased from 13.5 ± 2.2 to 10.7 ± 1.6 °.It was

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demonstrated that bacterial hydrophilicity was slightly enhanced. Thus the force 12


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driving bacteria to silver surface might increase. In addition, silver dissolution might

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give suitable sites for bacteria adhesion. These bio-chemical mechanisms would

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enhance the surface passivity of silver during bioleaching.

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Surface characteristic of P. fluorescens during bioleaching

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The functional groups in P. fluorescens during bioleaching were investigated using

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FTIR analysis to identify the substances that associated with adsorption (Figure 6).

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All the spectrums showed similar absorption feature, which indicates the similar EPS

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composition of the bacteria. The adsorption peak ranged from 3300 to 3500 cm−1

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generally contained the characteristic peaks of the amine groups (−NH, from proteins)

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and hydroxyl groups (−OH, from saccharides).19 But in this study it suffered a

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significantly intensity loss revealing PVP had strongly interacted with bacterial

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surface. The adsorption peaks at 2925 and 2960 cm−1, at 1456 cm−1 were assigned as

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the −CH and –CH2 groups, respectively, and both are common components in the

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phospholipid, peptidoglycan, and teichoic acid of bacteria.25 The peaks at 1654 cm−1

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for the 3 h bacterial leaching, 1650 cm−1 for 12 h, 1650 cm−1 for 48 h were

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characteristic of the amide groups (−CON−). The peaks around 1455 cm−1 were

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regarded as amine groups (−NH−, −CN−, and peptidoglycan).19 The peaks around

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1400 and 1312 cm−1 were assigned to the carboxylic acid groups (−COOH). In

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addition, the unchanged peaks at 1238 and 1083 cm−1 represented the asymmetric and

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symmetric phosphate (−PO2). These identified polar functional groups were all

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available for biosorption,19 although it was possible that surface adsorption was not 13



264

the main mechanism. In general, bacterial adsorption and EPS binding to solid

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substrates were an important process in bioleaching, no matter this behavior was

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positive or negative. However, in the bioleaching of precious metals, inward cyanide

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and oxygen diffusion and outward metal-cyanide complexation might be impeded due

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to the biological components strongly attached to the metal surfaces. Thus, precious

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metal bioleaching efficiency decreased.

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Effect of PVP as an additive on silver leaching

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Many efforts have been devoted to increase recovery efficiency of precious metal

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regardless of the extent of diffusion control on bioleaching. In this work, the addition

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of PVP was shown to be effective in enhancing the recovery of silver (Figure 7). It

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almost increased silver recovery by 1.8 times. Further experiments showed that PVP

275

had no effect on cyanide production in two step bioleaching process (data not

276

shown).Meanwhile PVP was unable to dissolve silver. According to equation 1, silver

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could be dissolved by cyanide with the assistance of oxygen. Appropriate ratio of

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cyanide/oxygen would benefit reaction. At the beginning of bioleaching, cyanide

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concentration was approximately 7 mg/L, and the free oxygen concentration in the

280

leaching solutions ranged from 6.1 to 6.7 mg/L. Oxygen concentration was not a

281

limiting factor for leaching. In addition, PVP addition might cause damage to

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bacterial cells, but the impact would be negligible because cell density and cyanide

283

concentration had reached maximum in two-step bioleaching. Thus, alleviation of

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diffusion control was the most likely to occur. 14


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4Ag + 8CN − + O 2 + 2H 2 O → 4Ag(CN) 2− + 4OH −

286

Possible mechanisms in enhancing bioleaching by PVP

(1)

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Potential beneficial changes occurred in the bacteria-silver surface could explain

288

the enhanced bioleaching performance upon the addition of PVP. (1) Binding

289

characteristic. The components of EPS, such as proteins, glycoproteins, lipoproteins,

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and polysaccharides, contain both hydrophobic and hydrophilic sites that enable the

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adsorption of cells onto the particles to form a biofilm. PVP is capable of suppressing

292

nonspecific binding, and can also disperse biological units, such as mycelia.27 Zeta

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potential of the suspensions containing bacteria and PVP, with and without silver were

294

shown in Figure 8. PVP is positively charged in aqueous solutions and tend to interact

295

with the negatively charged bacteria surface.28 Thus a large increase in zeta potential

296

was observed when high concentrations of PVP was added. When silver particles

297

were added, the zeta potential of bacterial suspensions containing 120, 240, and 480

298

mg/L of PVP all decreased. This effect might be attributed to PVP’s affinity for silver

299

particles, which liberated many negative sites on the bacterial surface and lead to a

300

slight zeta potential reduction.

301

PVP has a strong anion binding property and the binding constants were 0.5, 3.3,

302

and 5.3 for Cl−, I−, and SCN−, respectively.29 Hence, PVP was expected to suppress

303

the negative adsorption of bacteria and EPS and liberate active surface areas on silver.

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As shown in Figure 9, the biofilms aggregated and did not mix with silver particles.

305

PVP might have acted as a bridge and enhanced the binding interaction to promote 15



306

Ag−CN− contact. (2) Dispersion ability. PVP is a nonionic dispersant that commonly

307

serves as capping agent in nanomaterial manufacturing.30,31 In the bioleaching process,

308

PVP might reduce the contact of silver particles to each other by specific silver

309

binding. Silver particles were mainly located at the center at the bottom of the flask

310

(Figure 2A) with a few dispersed particles spinning around the center. Particle

311

dispersion would enhance the conversion rate of the reactive components. (3)

312

Increased viscosity of the solution. At room temperature, PVP has a major impact on

313

the viscosity of aqueous solutions.32,33 In this study, 240 mg/L of PVP was added. This

314

concentration was effective in increasing the culture viscosity. Fluid shear stress is

315

proportional to fluid viscosity when it reached a steady state.34 A simplified formula

316

suitable for describing the shear force in shaken flasks is shown as γ = τt/µL, where γ

317

is the average shear rate, τt is the average shear stress, and µL is the fluid viscosity.

318

Thus, shear stress was speculated to increase when PVP was added. Meanwhile it

319

would offer a strong abrasion at the solid/liquid interface, resulting in the attenuation

320

of diffusion control. Recovering precious metals from WPCBs with bioleaching

321

technology occurred in the presence of oxygen (O2) and cyanide (CN−). The primary

322

site of metal particle dissolution was at the solid-liquid interface, and the interface

323

was immediately formed upon immersing the metal particle in the culture. Adsorption

324

of cell and EPS on the particle’s surface prevents direct collision and contact of active

325

components, which results in undesirable effects for the metal leaching. Though PVP

326

is demonstrated effective in increasing metal recovery, the solid-liquid interfaces 16


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deserves more attention.

328 329 330

CONCLUSIONS

331

In this study, we performed two-step bioleaching process for silver recovery. Contact

332

behavior between cells and silver particles were observed. After bioleaching, water

333

contact angle of the hydrophilic bacteria showed a minor decrease. This variation in

334

wettability indicated an enhanced bacterial affinity for polar materials. The

335

characterization analyses of functional groups on bacterial surface showed the

336

presence of carboxyl, hydroxyl, and amine groups, which were responsible for the

337

bacterial adsorption. Further observation found that bacteria formed aggregates on the

338

surface of silver. Biofilm formation caused negative effect on silver leaching reaction.

339

Subsequently, it was found that leaching of silver could be enhanced by the addition

340

of PVP. Considering that the positively charged PVP (in aqueous media) interacted

341

strongly with silver particles and bacteria, the overall mechanism can be summarized

342

as: (1) PVP promoted CN- migrate to silver. (2) PVP suppressed the bacterial

343

attachment. (3) PVP improved the interface exchange.

344 345

ACKNOWLEDGEMENTS

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This work was supported by the 111 Project (B18060), the National Natural Science

347

Foundation of China (Grant No. 51308488, 51741409), the Science and Technology 17



of

Guangdong

Province

(Grant

No.

2015B020237005

Page 18 of 29

348

Programs

and

349

2016A020221014), the Pearl River Star of Science and Technology (Grant No.

350

201710010032) and the Fundamental Research Funds for the Central Universities

351

(Grant No. 17lgzd22).

352 353

AUTHOR CONTRIBUTIONS

354

Z.Y. and J.R. designed the experiments; Z.Y. performed the experiments; Z.Y. and J.R.

355

wrote the manuscript; H.Z., Y.L. and R.Q. revised the manuscript.

356 357

COMPETING FINANCIAL INTERESTS

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The authors declare that they have no conflict of interest.

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REFERENCES

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FIGURES

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Figure 1. SEM images of P. fluorescens (A) and silver particles (B)

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Figure 2. Distribution of silver particles in the flasks. To clearly observe the particle movement,

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the bacteria were removed through centrifugation. The arrow points to the clarified solution

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Figure 3. Micrograph of bacterial and silver particle mixtures sampled at different time points

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Figure 4. SEM micrograph of naturally dried silver particles

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468 469

Figure 5. Simulation of static friction force of adsorbed bacteria

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Figure 6. FTIR spectra of P. fluorescens

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Figure 7. Bioleaching of silver with and without the addition of PVP

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Figure 8. Zeta potential of the bacterial solutions affected by the addition of PVP and silver

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particles

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Figure 9. Effects of PVP on silver particle distribution

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TOC graphic

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Recovery precious metals from WPCBs is a meaningful work in the area of

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sustainable development of the world.

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Contact Behavior between Cells and Particles in Bioleaching of

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