Bioleaching assisted foam fractionation for recovery of gold from the printed circuit boards of discarded cellphone

Waste Management 101 (2020) 200–209

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Bioleaching assisted foam fractionation for recovery of gold from the printed circuit boards of discarded cellphone Gang Zhou a, Huixin Zhang a, Wei Yang a, Zhaoliang Wu a, Wei Liu a,⇑, Chunyan Yang b,* a Tianjin Key Laboratory of Chemical Process Safety, School of Chemical Engineering and Technology, Hebei University of Technology, No.8 Guangrong Road, Dingzi Gu, Hongqiao District, Tianjin 300130, China b National-Local Joint Engineering Laboratory for Energy Conservation in Chemical Process Integration and Resources Utilization, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China

a r t i c l e

i n f o

Article history: Received 3 July 2019 Revised 1 October 2019 Accepted 7 October 2019

Keywords: Printed circuit boards Gold Bioleaching Foam fractionation Foam drainage

a b s t r a c t Present work was focused on recovering gold (Au) from the printed circuit boards (PCBs) of discarded cellphone by bioleaching assisted continuous foam fractionation. First, the cyanide-producing strains of Pseudomonas putida and Bacillus megaterium were co-cultured in order to supply a high cyanide concentration in the nutrient solution for mobilizing Au from waste PCBs (WPCBs). Bioleaching conditions were optimized by using response surface methodology. Under the suitable bioleaching conditions of pH of 10.0, pulp density of 5 g/L and leaching time of 34 h, the Au mobilization percentage was 83.59%. The leaching liquor with an Au concentration of 1.34 mg/L could be used as the feeding solution of continuous foam fractionation after removing solid particles and cell biomass. In order to strengthen foam drainage, a novel internal component of foam fractionation column was developed. Under the suitable operation conditions of CTAB concentration of 0.2 g/L, volumetric air flow rate of 100 mL/min and feed flow rate of 10 mL/min, the enrichment ratio and recovery percentage of Au were 43.62 and 87.46%, respectively. This study is expected to provide an effective strategy to recover Au from WPCBs, and to supplement the depleting natural resources. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, electronic industry has become the world’s largest and fastest growing manufacturing industry. Electronic products are used extensively in almost every aspect of human life, from portable applications (e.g. toys, mobile phones and iPads) to large-scale applications (e.g. televisions, computers and vehicles) (Singh et al., 2016). With the rapid advent of changing technology, the early obsolescence of the gadgets and devices leads to the generation of electronic waste (e-waste), which is recognized as a fastgrowing waste stream in the world (Lee et al., 2018). Every year, about 2.5 million tons of e-waste (both self-generated and imported from developed countries) appear in Chinese mainland, accounting for 15% of that generated world-wide (Ruan et al., 2014). Chinese government has introduced a set of e-waste management regulations, in response to its speedy e-waste generation and to regulations and actions on electronic equipment in other countries (e.g. Japan, USA and Canada) which have substantial ⇑ Corresponding authors. E-mail addresses: [email protected] (W. Liu), [email protected] (C. Yang). https://doi.org/10.1016/j.wasman.2019.10.016 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.

impacts on the exports of Chinese electronic products (Chi et al., 2011a). E-waste contains various substances, i.e. heavy metals, (copper, chromium and lead, etc.), brominated flame retardants (BFRs), polybrominated diphenyl ethers (PBDEs) and chlorofluoro-carbons (CFCs), and it has posed a serious threat to the environment and public health when disposed improperly (Ilankoon et al., 2018). E-waste has also been considered as a ‘‘secondary ore” in ‘‘urban mining” due to the presence of precious metals with a high quality (Ongondo et al., 2015). It has been reported that the ore deposit of silver (Ag) was below 10 g/ton compared to the e-waste deposits of Ag at 1000 g/ton (Cayumil et al., 2016). Currently, there has been increasing interest in ewaste recycling, especially with the aim of recovering precious metals (Akcil et al., 2015). Printed circuit boards (PCBs) are the basal components in electronic instruments with a high content of 10 wt%, while they have been widely regarded as the most recalcitrant e-waste due to their heterogeneity and complexity (Xiu et al., 2019). Many studies on the value distribution for different waste PCBs (WPCBs) samples show that the primary economic driver for recycling of WPCBs is the precious metal, especially gold (Au) (Cui and Zhang, 2008). Au is a transition metal and it has been widely used in the manufacture of PCBs serving as the contact

G. Zhou et al. / Waste Management 101 (2020) 200–209

material due to the high chemical stability, low resistivity and good corrosion resistance (Alzate et al., 2017). Previous studies have reported that the content range of Au was 0.010–0.035 g/kg in WPCBs and its grade was more dozen times than that of the natural mineral ore (Vats and Singh, 2015). Regrettably, most Au in WPCBs was destroyed due to mismanagement of the unorganized sector, such as landfill disposal and incineration (Choubey et al., 2015). Therefore, the recovery of Au from WPCBs is necessary in order to relieve environmental pressure of e-waste and supplement the depleting natural resources while contributing to the transition towards a circular economy. Many processing strategies, such as physical techniques followed by chemical processes like pyrometallurgy, hydrometallurgy and electrometallurgy, have been used to recover precious metals from WPCBs (Wang et al., 2017a). The traditional pyrometallurgical procedure was highly dependent on investment and it easily leaded to atmospheric pollution due to emission of toxic dioxins, furan gases and carcinogenic compounds. Metal recovery through hydrometallurgical techniques (leaching, solvent extraction, ion exchange, etc.) was gaining importance because it was cost-effective and affable to the environment in comparison to other available recovery processes (Agarwal et al., 2010). However, the hydrometallurgical treatment also generated a huge amount of acidic effluents which caused harmful diseases as well as ecological imbalance if discharged into the environment. Moreover, the large electric energy consumption and highly standardized devices of electrometallurgy resulted in a high operation cost. Thus, developing new technology for recovering precious metals is demanded urgently for improving added value of WPCBs recovery. The most commonly used reagents in Au extraction from ore through hydrometallurgical treatment were cyanide, thiocyanate and thiourea (Joda and Rashchi, 2012). However, the slow dissolving-out rate of Au and lethal toxicity of cyanide made cyanidation leaching be no longer recommended. The autooxidation of thiocyanate and thiourea in acidic medium would result in the dramatic increase in the operation cost (Liu et al., 2019). Bioleaching is a promising technology for dissolving metals and semi-metals from mineral ore and concentrate through microorganisms mediation, and it has attracted the increasing attention of researchers and environmentalists owing to the advantages of compact route, energy conservation and environmental friendliness (Funari et al., 2017). Chi et al. (2011b) had examined the leaching of Au from WPCBs using a cyanide-producing strain of Chromobacterium violaceum under the conditions of pH of 11.0, pulp density of 15 g/L and supplementing oxygen with 0.004% (v/ v) H2O2. Isßıldar et al. (2016) had investigated the Au bioleaching from electronic scrap by Pseudomonas putida using glycine as the substrate. 44% of Au was mobilized in the alkaline conditions at a pH range of 7.3–8.6 and temperature of 25 °C in two days. However, the Au mobilization efficiency by biogenic cyanide was much lower than that by chemical cyanide. The reported techniques for recovering metal ions from an aqueous solution mainly contain chemical precipitation, membrane electrolysis and foam fractionation. Among these techniques, foam fractionation is the more desirable one owing to its advantages of simple operation, easy scale up and high enrichment efficiency. Foam fractionation is an adsorptive bubble separation method and it has been used for removing trace surfactants from the aqueous solution based on their favorable thermodynamics on bubble surface (Shi and Wu, 2016). Currently, this technique has also been used to recover some materials without surface activity by using a surfactant as the collector (Lu et al., 2015). During foam fractionation, the surfactant dosage is very large for achieving the high collecting capacity of metal ions and generating a stable foam phase. Regrettably, the high liquid holdup of the foam results in a low enrichment efficiency of the desired metal

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ion due to the sufficient adsorbed surfactant mass flux. Foam drainage is the reflux of the entrained liquid through the interstitial spaces between the adjacent bubbles by gravity and plateau border suction effects, and it plays a critical role in determining the performance of foam fractionation, especially the enrichment ratio (Li et al., 2016). Foam drainage can be strengthened by elevating temperature and modifying the foam fractionation column. Although elevating temperature is conducive to decreasing the kinetic viscosity of a solution, thermal energy consumption leads to high operation cost (Wang et al., 2017b). Compared with the low space utilization of the columns with anomalous external channels (e.g. sphere and ellipsoid), the placement of internal components in the column has been considered as the more economical method to strengthen foam drainage (Linke et al., 2005). Dickinson et al. (2010) had constructed a single-stage column with parallel inclined internal channels and they found that a lower liquid holdup of the foam was obtained at the inserted plates with an inclined angle of 20° from the vertical. Li et al. (2015) had developed a novel column with wire gauze structured packing to accelerate bubble coalescence through supplying a large gas liquid contact area and creating anfractuous channels. In this work, the recovery of Au from WPCBs of discarded cellphone by bioleaching assisted continuous foam fractionation was studied. Two cyanide-producing strains of Pseudomonas putida (P. putida) and Bacillus megaterium (B. megaterium) were used for cooperative leaching of Au from WPCBs. Actually, the coexistence of mixed bacteria could occur in nature. First, cyanide production and growth of P. putida and B. megaterium in mixed culture were contrasted with those in pure cultures. Subsequently, the operation conditions of Au bioleaching from the WPCBs with or without oxidative leaching pretreatment (OLP) were optimized by using response surface methodology (RSM) with three independent factors of pH, pulp density and leaching time. Foam fractionation was performed in a continuous mode. Cetyltrimethyl ammonium bromide (CTAB) was used as the collector. In order to strengthen foam drainage, a novel foam fractionation column was developed by constructing internal components of vertical sieve tray internal peaked caps (VSTC). Finally, the drainage performance of VSTC was evaluated and the suitable operation conditions of foam fractionation for separating Au from the leaching liquor were determined, respectively.

2. Materials and methods 2.1. Materials and reagents WPCBs of discarded cellphone were obtained from a collection center of e-waste in Tianjin, China. No physical or mechanical separation processes were used before transporting them to the laboratory. WPCBs were shredded by using stainless steel blades after removing manually the main electronic components, such as connectors, capacitors and integrated chips. Then, the pieces were crushed and fined by using a high-speed universal grinder (800Y, Yongkang Boou hardware products Co. Ltd., Zhejiang, China). The fine powder with a size below 40-mesh was collected and dried in an oven (DHG-9078A, Wuxi Marit Electronic Technology Co. Ltd., Jiangsu, China) at 90 °C for 2 h before bioleaching. CTAB, hydrochloric acid (HCl), silver nitrate (AgNO3), sodium hydroxide (NaOH), sulfuric acid (H2SO4), hydrogen peroxide (H2O2, 30 wt%) and sodium chloride (NaCl) were purchased from Fengchuan Chemical Reagent Factory, Tianjin, China. Peptone, meat extract, yeast extract and glycine were supplied by Lianxing Biotechnology Co. Ltd., Tianjin, China. Ultrahigh-purity water was prepared in our laboratory by using a Millipore Milli-Q system from Barnstead International, Dubuque, IA, USA.

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2.2. Microorganisms and culture conditions P. putida (CGMCC 1.1820) was purchased from General Microbiological Culture Collection Center, Beijing, China. B. megaterium (ACCC10011) was obtained from the Agricultural Culture Collection of China, Beijing, China. Above two strains were cultured in the nutrient broth medium, containing peptone of 2.0 g/L, meat extract of 1.0 g/L, yeast extract of 2.0 g/L, NaCl of 5.0 g/L and glycine of 5.0 g/L. In order to obtain the best inoculation time, experiments were performed in 250 mL Erlenmeyer flasks at 25 °C and 170 rpm in a shaker incubator (HZQ-QA, Suzhou Well Experimental Supplies Co. Ltd., Jiangsu, China). The bacterial growth was persuaded in 50 mL solution with 5% (v/v) inoculum for 40 h. All the experiments were duplicated. 2.3. Determination of Au concentration and purity The concentration and purity of Au were determined by using an atomic absorption spectrophotometer (wfx-120, Beijing Rayleigh Analytical Instrument Corp., China) at 242.8 nm and inductively coupled plasma-optical emission spectrometry (ICP-OES, PerkinElmer, Optima 5300 V). Samples were filtered and centrifuged at 8000 rpm for 10 min to remove solid particles and cell biomass. The supernate was passed through a glass fiber filter (0.45 lm) to ensure particle-free suspension. The linear fitting equation is A = 9.6171CAu + 0.11722, where CAu (mg/L) is Au concentration. The linear correlation coefficient was 0.9992. 2.4. Determination of cyanide concentration The cyanide concentration was determined by potentiometric method with ion selective electrode (ELIT 8291, Nico 2000, UK) and titrated against standard AgNO3 solution as described by a previous work (Zlosnik et al., 2012). The ion selective electrode was connected to a pH meter (PHSJ-3F, INESA Instrument, Shanghai, China) set to read on mV. In order to minimize the contamination of the electrode, the samples were treated by centrifugation (CTK132C, Xiangyi Centrifuge, Hunan, China) at 8000 rpm for 10 min and then passed through a glass fiber filter (0.45 lm). The standard curve for calculating cyanide concentration is A = 0.0468Ccyanide
0.0496, where Ccyanide (mg/L) is cyanide concentration. The linear correlation coefficient was 0.9997. 2.5. Oxidative leaching pretreatment WPCBs were pretreated to remove Cu by oxidative leaching. Leaching tests were performed in 250 mL Erlenmeyer flasks which were placed in an electro-thermostatic water bath (SHA-BA, Changzhou Zhongbei Instrument Co. Ltd., Jiangsu, China) with a mechanical stirring rate of 200 rpm at 25 °C for 3 h. 10 g WPCBs powder were immersed in 100 mL of 2 M H2SO4 solution as leaching agent and 20 mL of H2O2 as oxygen source. The suspension was filtered and the resulting cake was washed three times by ultrahigh-purity water. The powder was collected to use for leaching Au after drying overnight and sterilization. 2.6. Bioleaching experiments RSM was used for modeling and analyzing the bioleaching process of Au from WPCBs. A five-level, three-variable central composite design (CCD) with six replicated at the central point was employed in this regard (as seen in Table 1). The response variables were Au mobilization percentages (MP, %) from WPCBs with or without OLP. Active growing cultures (2.1 ± 0.5  109 CFU/mL) of P. putida and B. megaterium were inoculated with 1% (v/v) in growth medium as described in section 2.2. Then, bioleaching

Table 1 Experimental design of five-level and three-variable CCD. Test set

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

X1, pH

8.5 10 8.5 8.5 7 10 7 8.5 10 7 8.5 8.5 10 11.02 8.5 7 8.5 8.5 5.98 8.5

X2, Pulp density, g/L

11.5 5 11.5 22.43 18 18 5 11.5 5 18 11.5 11.5 18 11.5 11.5 5 11.5 0.57 11.5 11.5

X3, Leaching time, h

32.5 50 32.5 32.5 15 15 50 3.07 15 50 32.5 61.93 50 32.5 32.5 15 32.5 32.5 32.5 32.5

Au mobilization percentage, MP (%) WPCBs

WPCBs with OLP

33.69 68.13 33.97 0.14 0.35 0.17 2.50 8.70 73.12 0.17 33.42 29.89 0.17 82.93 33.97 1.25 33.42 38.38 0.27 34.62

37.77 78.75 36.96 0.14 0.52 0.17 1.88 9.51 75.00 0.17 36.68 39.67 0.17 86.74 39.40 1.25 36.14 32.89 0.27 39.57

experiments were carried out by adding the sterilized powder of WPCBs into cultures of cyanide-producing bacteria at the point of maximal cyanide generation. Leaching temperature and agitation rate were 25 °C and 150 rpm, respectively. Au mobilization percentage can be calculated by Eq. (1).

MPð%Þ ¼

CL  V L  100% C Au  M

ð1Þ

where CAu is the Au content in WPCBs powder, mg/kg; CL is the Au concentration in the leaching liquor, mg/L; M is the weight of WPCBs powder, kg; VL is the volume of the leaching liquor, L. 2.7. Foam fractionation equipment and procedure The schematic diagram of continuous foam fractionation is illustrated in Fig. 1. The foam fractionation column was constructed by a transparent polycarbonate tube with a height of 1000 mm and an inner diameter of 44 mm. A sintered glass filter with an average pore size of 0.18 mm was installed at the bottom of the column serving as the gas distributor. An inlet port was arranged at 420 mm from the bottom of the column. The feeding solution was injected into the column from the inlet port by using a peristaltic pump (YW03, Changzhou Yuanwang Fluid Technology Co. Ltd., China). Simultaneously, the bulk solution was discharged from the outlet port near the bottom of the column. In each experiment, the initial loading liquid volume was 400 mL. The gas was pumped into the column by using a gas compressor (ACO-318, Guangdong Hailea Group Co. Ltd., China). A rotameter (LZB-3WB, 30–300 mL/min, Wuhuan Instrument Factory, Tianjin, China) was used to control volumetric air flow rate. The foam was collected in a receiver at the top of the column. In the current work, a novel internal component of VSTC had been developed and it was made up of four parts, involving plate, vertical sieve tray, horizontal cap and inclined cap. The plate was a transparent plexiglass annulus with 2 mm in thickness, 44 mm in external diameter and 14 mm in inner diameter. The external diameter and height of vertical sieve tray were 14 mm and 50 mm, respectively. The VSTC had some rows of aligned pores with a diameter of 3.4 mm. Each row had four pores and the horizontal distance between the adjacent two pores was 10.1 mm.

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Fig. 1. Schematic diagram of foam fractionation.

Moreover, the vertical distance between the adjacent two pores in two rows was 10.0 mm. The horizontal cap was a transparent plexiglass plectane with 0.5 mm in thickness and 30 mm in diameter. The inclined cap was a taper ring with 12 mm in edge distance. The included angle between the horizontal cap and inclined cap was 120°. VSTC internals were installed at the given installation sites of 500 mm (first stage), 600 mm (second stage), 700 mm (third stage), 800 mm (fourth stage) and 900 mm (fifth stage) from the bottom of the column. The flow direction of the horizontally flowing foam in VSTC would be changed due to the induction of the inclined cap. The large flow resistance of the rising foam at the gap between the wall and the inclined cap led to the enhance of the pressure around the external space between vertical sieve tray and caps, thereby generating the circumfluence of rising foam (as shown in Fig. 1). 2.8. Determination of mass flux Mass flux (u, g/min) is an important parameter for determining the adsorption property of the bubbles during foam fractionation. By assuming that the concentration of the desired material in the interstitial liquid inside the foam was equal to that in the residual solution, u can be calculated by Eq. (2).

u ¼ V f L Cf
V f LCr

ð2Þ

where VfL is the volumetric flux of the foamate, L/min; Cf and Cr are the concentrations of desired material in the foamate and the residual solution (g/L), respectively. 2.9. Determination of liquid holdup and drainage efficiency of each VSTC Liquid holdup of the foam out of the column (eout) can be calculated by Eq. (3).

eout ¼

V out V out þ V G

ð3Þ

where Vout is the volume of the foamate collected at the container in 5 min, L; VG is the volume of air in 5 min, L.

The drainage efficiency of each VSTC (g) can be calculated by Eq. (4).

gð%Þ ¼

eoutN
eoutðN
1Þ  100% eoutðN
1Þ

ð4Þ

where N is the number of VSTC internals in the column and it is larger than or equal to 1; eoutN and eout(N
1) are the values of liquid holdup of the foam out of the column with N and N
1 of VSTC internals, respectively. 2.10. Determination of bubble size and expansion rate of the bubbles The bubble size could be measured by photographing the foam with a digital camera (Nikon CooLPIX P6000) when the system reached the steady state. The bubble size can be obtained by measuring the Sauter mean bubble diameter (D32, mm) by the software of Reconverted and Scion Image using Eq. (5) (Yang et al., 2011).

Pn 3 Di D32 ¼ Pi¼1 n 2 i¼1 Di

ð5Þ

where n is the number of the bubbles and Di is the diameter of the ith bubble. In each photograph, at least 200 bubbles were measured for giving a Sauter mean bubble diameter. The expansion rate (x, %) of the bubbles can be calculated by Eq. (6).

xð%Þ ¼

4 3 4 3

pðD322 N Þ

3

pðD32ðN
1Þ Þ 2

 100% ¼ 3

D332N D332ðN
1Þ

 100%

ð6Þ

where D32N and D32(N
1) are the bubble sizes (mm) at the given photo locations of the Nth and (N
1)th stages of VSTC, respectively; N is larger than or equal to 1. 2.11. Evaluation of foam fractionation performance The performance of foam fractionation can be evaluated by the enrichment ratio (E) and the recovery percentage (R, %), and they are expressed as Eq. (7) and Eq. (8), respectively.

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E¼

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Cf C0

Cf V f Rð%Þ ¼  100% C 0 ðV 0 þ V 1 Þ

ð7Þ

ð8Þ

where Cf and C0 are the Au ion concentrations (g/L) in the foamate and the feeding solution, respectively; Vf , V0 and V1 are the foamate volume (L), the initial loading volume of the feeding solution (L), and the volume of the feeding solution fed into the column from the inlet port (L), respectively. 2.12. Statistical analysis All experiments were conducted under the statistical framework of duplicate experiments with appropriate control. Experimental data were interpreted statistically with analysis of variance. The statistical significance was evaluated at a probability level of P < 0.05. 3. Results and discussion

Table 2 The content of metals in WPCBs powder. Metals

Contents (mg/g)

Cu Zn Fe Pb Al Ni Ti Cr Au Mn K Cr Ag Sr Zr As Co V Ce Mo Cd Th Sc

392 ± 2.1 203.4 ± 1.6 19.8 ± 0.6 15.4 ± 0.5 14.7 ± 0.4 6.3 ± 0.4 0.706 ± 0.009 0.473 ± 0.0023 0.326 ± 0.0015 0.2402 ± 0.0015 0.1932 ± 0.0013 0.1743 ± 0.0012 0.1651 ± 0.0012 0.1577 ± 0.0010 0.0354 ± 0.0005 0.095 ± 0.0014 0.0261 ± 0.0008 0.0252 ± 0.0008 0.0167 ± 0.0005 0.0167 ± 0.0005 0.0128 ± 0.0003 0.0115 ± 0.0003 0.0051 ± 0.0001

3.1. Total metal characterization in WPCBs The metal content of WPCBs was determined by using atomic absorption spectrometer (wfx-120, Beijing Rayleigh Analytical Instrument Corp., China) and inductively coupled plasma-optical emission spectrometry (ICP-OES, PerkinElmer, Optima 5300V). Table 2 shows metals’ content in WPCBs powder. The major metal was found to be Cu (392 ± 2.1 mg/g) with a considerable amount of Zn (203.4 ± 1.6 mg/g), Fe (19.8 ± 0.6 mg/g) and Al (14.7 ± 0.4 mg/g). The content of Au was 0.326 ± 0.0015 mg/g, which was much higher than that in the PCBs of laptops (0.238 ± 0.0011 mg/g) or computer parts (0.031 ± 0.0005 mg/g) (Joda and Rashchi, 2012). This might be associated to the design and compact size of cellphone. Above results are consistent with the work of Arshadi and Mousavi (2015). 3.2. Cyanide production and growth of P. putida and B. megaterium The cyanide production and growth of two strains were investigated. The results have been placed in Fig. 2. It could be seen that the cyanide concentrations in nutrient solutions of the single strain and mixed strains first increased and then decreased with the increase of incubation time. Biogenic cyanide was produced as a secondary metabolite during the oxidative decarboxylation of glycine and the reaction processes were as follows.

ð9Þ It had reported the timeframe of the cyanide production that the maximal cyanide production yield could be reached at the late logarithmic and early stationary phases, and then it dropped at the late stationary and decay phases (Isßıldar et al., 2016). Obviously, a long stationary phase was conducive to obtaining a high cyanide concentration in the nutrient solution. It was found that approximately 20 h after inoculation, the strain of P. putida almost reached the end of the logarithmic phase. The maximum cyanide concentration was 13.14 ± 0.66 mg/L. For B. megaterium, the stationary phase was reached at 8 h after inoculation and it would be continued for 5 h. Subsequently, the cyanide concentration in the nutrient solution decreased to 5.82 ± 0.23 mg/L at 32 h. In the cocultivation of P. putida and B. megaterium, the stationary phase was continued for 20 h and the maximum cyanide concentration

was 13.92 ± 0.79 mg/L. At the end of stationary and decay phases, the decrease of cyanide concentration was caused by the conversion of cyanide into b-cyanophenylalanine or ammonia (Kebeish et al., 2017). Decomposition of cyanide by bacteria might have possibilities for overcoming the toxicity issues. The specific growth rates of P. putida, B. megaterium and their mixture were 0.069 s
1, 0.087 s
1 and 0.066 s
1, respectively. These results were consistent with the reports of Shin et al. (2013) and Chi et al. (2011a). Thus, it is feasible to mobilize effectively of Au from WPCBs in a short leaching period through the co-cultivation of cyanide-producing strains. 3.3. Mobilization of Au from WPCBs In bioleaching process, cyanides ions (CN–) were used to dissolve Au in an electrochemical process according to the following reaction.

4Au þ 8CN
þ O2 þ 2H2 O2 ! 4AuðCNÞ
2 þ 4OH


ð10Þ

It could be found in Fig. 3 that the Au mobilization percentage in the leaching liquors of WPCBs with and without OLP first increased and then decreased. The reason for this phenomenon was that Au ions could be adsorbed on the surface of strain cells (Natarajan and Ting, 2014). It had been reported that other heavy metal ions, such as Cu2+ and Ni2+, had an adverse effect on Au mobilization (Zhu et al., 2019). However, in the current work, the Au mobilization percentage in the leaching liquor of WPCBs without oxidative leaching pretreatment (83.35%) was lower than that in the leaching liquor of WPCBs with oxidative leaching pretreatment (86.17%). In WPCBs, there were mainly four layers of materials from the outer side to inside, including Au-Ni alloy, Ni, Cu and plastic (Ha et al., 2014). It was clear that the Au mobilization in initial stages would not be affected by other metals in samples due to their inadequate contact with the leaching liquor. Thus, in order to simplify operation, WPCBs could be used to leach Au without oxidative leaching pretreatment. In Fig. 3, the mobilization efficiency of Au increased significantly with the increase of pH. The pKa of HCN was 9.4 (Akcil et al., 2015). When pH was higher than 9.4, the dominant form of cyanide in the solution was CN–. At the lower pH values, the dominant form of cyanide was gaseous HCN.

G. Zhou et al. / Waste Management 101 (2020) 200–209

205

Fig. 3. Response surface graphs for the effects of pH, pulp density, leaching time on Au mobilization percentage from WSPC (A) and WSPC with OLP (B).

Fig. 2. Cyanide production and growth of P. putida (a), B. megaterium (b) and their mixture (c).

Then, large amounts of gas were released out from the solution before reaction (Luque-Almagro et al., 2016). The Au mobilization percentage was about 85% at pulp density of 5 g/L, while no Au was extracted as pulp density increased to 20 g/L. Natarajan and Ting (2015) had reported that pulp density had a linear negative effect on the metals recovery from PCB. The high environmental toxicity would destroy the growth of strain cells as pulp density increased. A higher cyanide concentration was required for attacking the massive gold by increasing pulp density for achieving the same percentage of Au mobilization. However, in fact, the cyanide-producing capacity of bacterium was limited. Thus, strategy for the Au mobilization using biologically produced cyanide should consider the balance between the chemical stability of the complexing/lixiviating agent and bacterial physiological requirements.

Table 1 shows the experimental conditions and Au mobilization percentages according to the factorial design. Multiple regression analysis was performed and the coefficients of the model were evaluated for significance. The regression model was significant at the considered confidence level because the p value was less than 0.0001 (as shown in Table 1S). At the predicted conditions of pH of 10.0, pulp density of 5 g/L and leaching time of 34 h, a mean Au mobilization percentage of 83.59% was obtained. Then, the leaching liquor with an Au concentration of 1.34 mg/L could be used as the feeding solution of foam fractionation after removing solid particles and cell biomass. 3.4. Evaluation of drainage efficiency of VSTC 3.4.1. Effect of VSTC on liquid holdup of the foam In order to investigate the effect of VSTC on liquid holdup of the foam, experiments were performed at the conditions of CTAB concentration of 0.2 g/L and feed flow rate of 10 mL/min. Volumetric air flow rate ranged from 50 mL/min to 150 mL/min, and the number of VSTC ranged from 1 to 5. The control column was the foam fractionation column without VSTC (N = 0). The results are presented in Fig. 4.

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As shown in Fig. 4(A), liquid holdup of the foam decreased with increasing the number of VSTC. This result indicated that foam drainage was strengthened significantly by the column with internal component of VSTC. At the volumetric air flow rate of 50 mL/ min, only three values of liquid holdup of the foam were measured. Then, it was difficult to collect continuously the foam from the top of the column when the number of VSTC was equal to or higher than 4. Moreover, the irregular decreasing trend in liquid holdup of the foam suggested that there were differences in the drainage efficiency of each VSTC in the column at different volumetric air flow rates. In foam fractionation, a certain volume of bulk solution would be entrained into the foam phase due to the polarity ‘‘head” of surfactant molecules around the bubble surface. At a given volumetric air flow rate, the maximum liquid holdup of the foam nearby the gas-liquid level between the bulk solution and the foam phase resulted in the largest drainage velocity (Ghosh et al., 2019). Thus, the drainage efficiency of the first stage VSTC was lower than those of other stages at any volumetric air flow rates (as seen in Fig. 4 (B)). As the foam rose in the column, liquid holdup decreased gradually until it reached equilibrium. Obviously, a low volumetric air flow rate contributed to reaching quickly equilibrium liquid holdup of the foam owing to the weak drainage resistance and slow bubble generating velocity. Then, the neutralization between gravity and drainage resistance at the equilibrium liquid holdup of the foam made the further stripping of interstitial liquid inside the foam even more difficult. Thus, the drainage efficiency of VSTC was low. Based on Laplace law, foam drainage would induce the occurrence of bubble marginal regeneration, disproportionation (the gas diffusion from smaller to larger bubbles) and bubble coalescence

(the rupture of the foam bubble) after reaching equilibrium liquid holdup of the foam (Perez et al., 2010). Thus, the drainage efficiency of the third stage was very high at the volumetric air flow rate of 50 mL/min. Furthermore, unstable foam resulted in a low recovery percentage of the desired material due to the insufficient interfacial adsorption. It could be found in Fig. 4(B) that the maximum drainage efficiency of VSTC and equilibrium liquid holdup of the foam at the volumetric air flow rate of 100 mL/min would be reached at the fourth and fifth stages, respectively. 3.4.2. Effect of VSTC on expansion rate of the bubbles The experiments for investigating the effect of VSTC on expansion rate of bubbles were performed under the conditions of CTAB concentration of 0.2 g/L, feed flow rate of 10 mL/min and VSTC number of 5. Volumetric air flow rate ranged from 50 mL/min to 150 mL/min. The results are presented in Fig. 5. As shown in Fig. 5, at 50 mL/min volumetric air flow rate, the expansion rate of bubbles in the experimental column was higher than that in the control column at the same height. The photographs of bubbles at the heights of 650 mm and 850 mm had been presented. The morphologic changes of bubbles from regular sphere to irregular polyhedron suggested that liquid holdup of the foam at the height of 850 mm was lower than equilibrium value. The large marginal pressure made air diffuse easily from smaller to larger polyhedron bubbles, thereby reducing foam stability (Tummino et al., 2018). Due to the low relative radius of curvature and high rigidity of gas-liquid interface, the large polyhedron bubbles would be broken up by the compress deformation as they flowed through the pores on the VSTC. At the volumetric air flow rate of 50 mL/min, foam was difficult to be continuously collected at the top of the column, which was installed with five VSTC components. With the increase of volumetric air flow rate, the short residence time of the foam in the column resulted in a low expansion rate of bubbles. The expansion rates of bubbles in both experimental column and control column at the height of 55 mm were zero at the volumetric air flow rates of 100 mL/min and 150 mL/min (Fig. 5). This result indicated that the first stage VSTC did not induce the deformation of bubbles. However, at other stages, the expansion rate of bubbles in the experimental column was larger than that in the control column. Thus, foam drainage could be strengthened effectively by installing VSTC components in the column within a specific range of volumetric air flow rate. 3.5. Foam fractionation for recovering Au from the leaching liquor 3.5.1. CTAB concentration In order to perform continuous foam fractionation, the total volume of feeding liquid was 2 L, which contained the initial loading

Fig. 4. Effects of VSTC on liquid holdup of the foam (A) and the drainage efficiency of each VSTC (B).

Fig. 5. Effect of VSTC on expansion rate of the bubbles.

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liquid volume of 400 mL. As shown in Fig. 6(A), with increasing CTAB concentration from 0.1 g/L to 0.5 g/L, the enrichment ratio of Au ion decreased from 51.33 to 1.21, while its recovery percentage increased from 65.14% to 95.22%. Meanwhile, the mass flux of Au ion in the foam increased from 0.0218  10
3 g/min to 0.1269  10
3 g/min. CTAB played a role of the collector for achieving the attachment of AuðCNÞ
2 on the gas-liquid interface based on electrostatic interaction. A high CTAB concentration was conducive to obtaining a large mass flux of Au ion in the foam. However, the high surface excess of CTAB molecules would increase the thickness of the liquid film around the bubbles, resulting in a high liquid holdup of the foam. It had been reported that the critical micelle concentration (CMC) of CTAB was 0.3 g/L at 25 °C (Liu et al., 2013). When the CTAB concentration was higher than its CMC, the self-aggregation of CTAB molecules would weaken interfacial adsorption. Considering both enrichment ratio and recovery percentage of Au ion, 0.2 g/L (typical dosage of CTAB was 0.4 g) was chosen as the suitable CTAB concentration for the following experiments.

3.5.2. Volumetric air flow rate As shown in Fig. 6(B), with increasing volumetric air flow rate from 50 mL/min to 250 mL/min, the enrichment ratio of Au ion decreased from 63.09 to 1.26, while its recovery percentage increased from 51.52% to 95.31%. Meanwhile, the Au ion concentration in the residual solution first decreased from 0.66 mg/L to 0.12 mg/L, and then increased to 0.31 mg/L. When volumetric air flow rate was low, the slow generating velocity of the bubbles in the bulk solution created a small available surface area for adsorbing the complex of CTAB and AuðCNÞ
2 . Then, the insufficient interfacial adsorption resulted in a high Au ion concentration in the residual solution and a low liquid holdup of the foam. With increasing volumetric air flow rate, the short residence time of the foam in the column would reduce the drainage efficiency of VSTC. A large volume of interstitial liquid would be entrained into the foam receiver, resulting in a low Au ion concentration in the foamate. The concentrating characteristics of foam fractionation were difficult to be exhibited when volumetric air flow rate was higher than 200 mL/min. Thus, 100 mL/min was chosen as the suitable volumetric air flow rate for the following experiments.

3.5.3. Feed flow rate As shown in Fig. 6(C), with increasing feed flow rate from 5 mL/ min to 15 mL/min, the enrichment ratio of Au ion decreased from 57.94 to 22.81, while its recovery percentage first increased from 61.08% to 87.46%, and then decreased to 65.32%. Meanwhile, the Au ion concentration in the residue solution first decreased from 0.53 mg/L to 0.18 mg/L, and then increased to 0.48 mg/L. At a given volumetric air flow rate, the available adsorption surface area was constant (Khalesi et al., 2013). With increasing feed flow rate, the short residence time of the feeding solution in the liquid phase led to decrease the adsorbed mass of the complex of CTAB andAuðCNÞ
2 . In addition, the mass rate of CTAB at a high feed flow rate was higher than that at a low feed flow rate. It had been reported that a high mass rate of CTAB was conducive to improving the surface excess of CTAB, leading to a high liquid holdup of the foam (Wouters et al., 2017). Thus, the recovery percentage of Au ion increased with the increase of feed flow rate. When feed flow rate was higher than 10 mL/min, the decrease in the recovery percentage of Au ion might be due to the fact that massive CTAB and AuðCNÞ
2 had not been transported into the foam phase. Based on above results, 10 mL/min was chosen as the suitable feed flow rate for recovering Au ion from the leaching liquor by foam fractionation.

Fig. 6. Effects of operation parameters (CTAB concentration (A), volumetric air flow rate (B) and feed flow rate (C)) of foam fractionation on the recovery efficiency of Au ion.

After foam fractionation, CTAB would be removed from the foamate by ion exchange. Then, zinc rod was added into the eluent for precipitating Au. The precipitate was collected and incinerated at 400 °C. The sinter was the recovered Au and its purity was 93.65%. 4. Conclusion The objective of this work was to recover Au from WPCBs of discarded cellphone. The innovative points of this work were listed as follows: (1) an effective strategy of bioleaching assisted continuous foam fractionation was proposed; (2) two cyanide-producing microorganisms were co-cultured; (3) in order to strengthen foam drainage, a novel internal component of VSTC was developed; (4) foam fractionation was performed to concentrate Au from the leaching liquor by using CTAB as the collector. Experimental results indicated that the co-cultivation of two cyanide-producing strains

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could maintain a long stationary phase to shorten leaching period through supplying a high cyanide concentration in the nutrient solution. Under the suitable bioleaching operation conditions of pH of 10.0, pulp density of 5 g/L and leaching time of 34 h, the Au mobilization percentage was 83.59%. The leaching liquor with an Au concentration of 1.34 mg/L could be used as the feeding solution of continuous foam fractionation after removing solid particles and cell biomass. A novel internal component of VSTC was developed to strengthen foam drainage. Under the suitable operation conditions of CTAB concentration of 0.2 g/L, volumetric air flow rate of 100 mL/min and feed flow rate of 10 mL/min, the enrichment ratio and recovery percentage of Au ion could reach as high as 43.62 and 87.46%, respectively. Finally, the total recovery percentage of Au from WPCBs of discarded cellphone was 73.11%. These results are expected to provide a cost-effective strategy for recovering precious metals, and to shed new light on the e-waste recycling system design. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21908041), Foundation of Hebei Educational Committee, China (No. QN2018079) and Key Basic Research Program of Hebei, China (No. 16964002D). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.10.016. References Agarwal, S., Ferreira, A.E., Santos, S.M., Reis, M.T.A., Ismael, M.R.C., Correia, M.J.N., Carvalho, J.M., 2010. Separation and recovery of copper from zinc leach liquor by solvent extraction using Acorga M5640. Int. J. Miner. Process. 97, 85–91. https://doi.org/10.1016/j.minpro.2010.08.009. Akcil, A., Erust, C., Gahan, C.S., Ozgun, M., Sahin, M., Tuncuk, A., 2015. Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants–a review. Waste Manage. 45, 258–271. https://doi.org/10.1016/j. wasman.2015.01.017. Alzate, A., López, E., Serna, C., Gonzalez, O., 2017. Gold recovery from printed circuit boards by selective breaking of internal metallic bonds using activated persulfate solutions. J. Clean. Prod. 166, 1102–1112. https://doi.org/10.1016/j. jclepro.2017.08.124. Arshadi, M., Mousavi, S.M., 2015. Enhancement of simultaneous gold and copper extraction from computer printed circuit boards using Bacillus megaterium. Bioresource Technol. 175, 315–324. https://doi.org/10.1016/j. biortech.2014.10.083. Cayumil, R., Khanna, R., Rajarao, R., Mukherjee, P.S., Sahajwalla, V., 2016. Concentration of precious metals during their recovery from electronic waste. Waste Manage. 57, 121–130. https://doi.org/10.1016/j.wasman.2015.12.004. Chi, X., Streicher-Porte, M., Wang, M.Y., Reuter, M.A., 2011a. Informal electronic waste recycling: a sector review with special focus on China. Waste Manage. 31, 731–742. https://doi.org/10.1016/j.wasman.2010.11.006. Chi, T.D., Lee, J.C., Pandey, B.D., Yoo, K., Jeong, J., 2011b. Bioleaching of gold and copper from waste mobile phone PCBs by using a cyanogenic bacterium. Miner. Eng. 24, 1219–1222. https://doi.org/10.1016/j.mineng.2011.05.009. Choubey, P.K., Panda, R., Jha, M.K., Lee, J.C., Pathak, D.D., 2015. Recovery of copper and recycling of acid from the leach liquor of discarded Printed Circuit Boards (PCBs). Sep. Purif. Technol. 156, 269–275. https://doi.org/10.1016/j. seppur.2015.10.012. Cui, J., Zhang, L., 2008. Metallurgical recovery of metals from electronic waste: a review. J. Hazard. Mater. 158, 228–256. https://doi.org/10.1016/j. jhazmat.2008.02.001. Dickinson, J.E., Laskovski, D., Stevenson, P., Galvin, K.P., 2010. Enhanced foam drainage using parallel inclined channels in a single-stage foam fractionation column. Chem. Eng. Sci. 65, 2481–2490. https://doi.org/10.1016/j. ces.2009.12.027.

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