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Selective recovery of tungsten from printed circuit board recycling unit wastewater by using emulsion liquid membrane process
Journal of Water Process Engineering 8 (2015) 75–81
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Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Selective recovery of tungsten from printed circuit board recycling unit wastewater by using emulsion liquid membrane process Avinash B. Lende, Prashant S. Kulkarni ∗ Energy and Environment Laboratory, Department of Applied Chemistry, Defence Institute of Advanced Technology (DU), Ministry of Defence, Pune 411025, India
a r t i c l e
i n f o
Article history: Received 2 May 2015 Received in revised form 11 September 2015 Accepted 11 September 2015 Keywords: Emulsion liquid membrane Aliquat 336 PCB recycling unit wastewater Recovery of W(VI)
a b s t r a c t Aqueous waste containing low concentrations of tungsten are generated during printed circuit board (PCB) or e-waste recycling process. Study has been initiated to develop a suitable emulsion liquid membrane (ELM) technique for the separation and recovery of tungsten [W(VI)] from such waste using Aliquat 336 in hexane as a carrier and sodium hydroxide as a stripping agent. The waste, having a composition of nearly 600 ppm W(VI), 150 ppm Pb(II) and below 5 ppm concentrations of Cu(II), Mn(II), Zn(II), Co(II), Ca(II), Mg(II), Na(I) and K(I) at pH 5, was used as the feed phase. Various factors that affect the emulsion stability, as well as percentage extraction of W(VI), have been optimized to obtain maximum enrichment of W(VI). An attempt was made to recover W(VI) without making any substantial changes to the waste composition. Under optimized conditions, the extraction percentage of W(VI) was found to be 80% with 4 times enrichment in stripping phase. The separation factor for W(VI) vs. other co-ions was found to be very high thereby indicating selective recovery of W(VI) from the wastewater by using ELM process. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten [W(VI)] is one of the valuable elements with versatile applications in various fields. Many industries such as an electrical, television-tube, bulb-light, explosives manufacturing, steel and iron are employing W(VI) in their production [1]. Notably, pure W(VI) is required in lighting filaments, electronic tools, satellites, creep resistant steels and alloys. One of its most important use is, in the form of W(VI) mono-carbide (WC) which has a hardness close to diamond [2]. W(VI) may be less toxic to the human body depending on the route of exposure and is not considered to be an important human health hazard up to a certain limit by WHO and USEPA. Selective separation of W(VI) from secondary sources is required in terms of their economic values and limited resources.
Abbreviations: ELM, emulsion liquid membrane; PCB, printed circuit board; DPP, direct plating process; WC, tungsten mono-carbide; WHO, World Health Organization; USEPA, United States Environmental Protection Agency; PPM, parts-per millions; SPAN 80, surfactant, sorbitan monooleate; TOC, total organic compound; dia, diameter; PAR, 4-(2-pyridylazo) resorcinol; rpm, rotation per minutes; w/o/w, water–oil–water; R3CH3 N+ Cl− , aliquat 336; mPa S, millipascal-second; min, minutes; M, molar. ∗ Corresponding author. Fax: +91 20 2691533. E-mail addresses: ps [email protected], [email protected] (P.S. Kulkarni). http://dx.doi.org/10.1016/j.jwpe.2015.09.003 2214-7144/© 2015 Elsevier Ltd. All rights reserved.
Numerous methods are reported for the recovery of W(VI) from the wastewater. For instance, solvent extraction [3–5], ultrafiltration [6], precipitation [7–9], thin layer chromatography [10], ion-exchange separation [11] and adsorption [12,13]. Membrane based separation processes are gaining importance due to their simplicity and easy scalability. The emulsion liquid membrane (ELM) process is an extensively used method for the separation of metal ions [14], hydrocarbons [15], amino acids [16] and biological compounds [17]. ELM is one of the finest methods for the simultaneous extraction and enrichment of the solute from feed phase within a single step. The basic properties of ELM operations such as low-solvent inventory and high surface area make them ideal for the separation of solutes from dilute aqueous streams [18]. An ELM is the resultant emulsion membrane phase consisting of a homogeneous mixture of surfactant, extractant, organic diluent and inside to which contain an aqueous strip phase. The solute species from the external feed phase gets dissolve in the organic phase and diffuses into the internal strip phase. The efficiency and selective transport across the membrane phase may be remarkably enhanced by the presence of extractant concentration in the organic phase. Finally, the enriched solute can be recovered by breaking of the emulsion [19]. Therefore, the overall performance of the ELM is dependent on several parameters such as surfactant and extractant concentration, treat ratio, contact time, feed phase pH and internal phase concentration.
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Fig. 1. A schematic of emulsion liquid membrane (ELM) process.
In the present study, the feed phase was taken as wastewater from the printed circuit board (PCB) or e-waste recycling unit [20]. The effluent was characterized by using ion chromatography instrument. It was found contain various metal ions such as Cu(II), Mn(II), Zn(II), Co(II), Ca(II), Na(II), K(I), Mg(II), W(VI) and Pb(II). Amongst them the concentration of W(VI) and Pb(II) was significant viz. 600 and 150 ppm, respectively, whereas the concentration of other co-metals in the effluent stream was below 5 ppm. An ELM process was designed by taking suitable and appropriate amount of extractant, surfactant and stripping agent. Several important parameters of ELM were investigated, and the kinetics was measured. Efforts were made in achieving selective recovery of W(VI) from the wastewater without making any substantial changes to its composition.
2.2. Development of ELM During the development of ELM, the emulsion was prepared by adding an appropriate quantity of extractant (0.02–0.15 M, Aliquat 336) and surfactant (3%, Span 80) with an organic diluent (Hexane). The mixture along with the aqueous stripping agent (0.1–1 M, NaOH) was stirred at 6000 rpm for 15 min by using high-speed homogenizer [21]. A fixed ratio of 1:1 was maintained between the aqueous and the membrane phase. The prepared emulsion was of water-in-oil (w/o) type having milky white uniform mixture. The internal particle size of the water-in-oil emulsion was found to be within the range of 4–8 m. The emulsion swelling was measured by comparing the internal strip phase volume before and after treatment with the feed phase. Swelling (%) =
Vi,t − Vi,0 × 100 Vi,0
(1)
2. Experimental 2.1. Materials and methods Surfactant, sorbitan monooleate (SPAN 80), was a gift sample from Mohini Organics (Pvt.) Ltd., Mumbai, India. The extractant, Aliquat 336, was purchased from Sigma–Aldrich, Steinheim, Germany. The effluent, wastewater containing W(VI) was obtained from PCB recycling unit industries. The pH adjustment of wastewater solution was carried out using 1 M of H2 SO4 and NaOH. All the chemicals used were of AR grade and used without any further purification. The ELM was developed by a Wise Tis (HG-15D) homogenizer (2000–27.000 rpm), and the speed was adjusted with a digital stroboscope (Lutron DT-2249). The internal particle size of the emulsion was measured by a particle size analyzer (Nanophox, 219F-2009, Germany). The recovery of metal runs was performed using a Heidolph RZR 2012 overhead stirrer (200–2000 rpm). The recovered concentration of W(VI) and other co-ions in the sample were analyzed by Ion Chromatography (Metrohm, 883 Basic IC Plus, Switzerland). The viscosity of the emulsion and pH of the feed phase samples were measured by Ostwald viscometer and pH meter (Hanna Instrument, HI 2211 pH/ORP Meter), respectively.
where Vi,0 and Vi,t are the initial and at time ‘t’ internal phase volumes. The most stable emulsion was obtained when there was neither swelling nor breakage during the extraction run. A schematic of the ELM process constituting emulsion preparation, permeation of W(VI) and breaking of the emulsion are depicted in Fig. 1. 2.3. Recovery of W(VI) Most of the metal ions exist in the form of hydrated ions in an aqueous stream, before; it can be extracted into the non-polar organic phase. In the extraction process, the water molecule must be replaced by ionic charge generated by the extractant. The developed emulsion was treated with PCB recycling unit wastewater (feed phase) by taking various treat ratios. The extraction run (mixture of emulsion and feed phase) was performed at 300 rpm using an overhead stirrer. Samples at different time intervals were taken out for the analysis. Later, the emulsion phase was separated from the raffinate phase. Finally, the emulsion was broken by heating in a closed vessel at 80 ◦ C for the analysis and recovery of W(VI). The recovery of tungsten by using Aliquat 336 (R3 CH3 N+ Cl− ) in ELM
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Table 1 Detection of solutes by using ion chromatograph. Analytes
Ions
Column
Eluent mixture
Detector
Cations
Ca(II), Na(II), K(I), Mg(II), Pb(II)
Metrosep C4, Metrohm
Conductivity detector
Anions
W(VI)
Metrosep A supp5, Metrohm
Transition metal ions
Cu(II), Mn(II), Zn(II), Co(II)
Metrosep C4, Metrohm
2.75 mmol/L HNO3 and 0.05 mmol/L pyridine-2,6-dicarboxylic acid 15 mmol/L Na2 CO3 , 5 mmol/L NaHCO3 and 100 mmol/L H2 SO4 1.75 mmol/L Oxalic acid + PAR
Conductivity detector
UV detector
process can be represented by Eqs. (1) and (2). −2 + − + M2+ WO−2 4(aq) + 2(R3 CH3 N Cl ) ↔ (R3 CH3 N )2 WO4
+ MCl2(aq) (Extraction)
(2)
(R3 CH3 N+ )2 WO4 −2 + 2Na+ ↔ Na2 WO4(aq) (aq) + (R3 CH3 N+ )2 (Stripping)
(3)
where M stands for co-metal ion from PCB recycling unit wastewater. Here, the extractant reacts as an anion exchanger, forming an ion-pair with the metal oxyanions from the aqueous solution. In the present case, as only W(VI) forms oxyanions (WO4 2− ) in water, it get exchanged very fast with the carrier. The initial stability of the emulsion was calculated by monitoring the phase separation with time. In the extraction process, the emulsion phase stability played a significant role for the efficient transport of solute from the feed phase. In this work, extraction, enrichment, swelling and breakage were calculated by using reported method [22]. 3. Results and discussion The primary focus of this investigation was selective separation and enrichment of W(VI) from the wastewater. Consequently, the study was undertaken for evaluating the critical parameters of ELM process such as the effect of extractant concentration, feed phase pH, treat ratio, contact time and internal phase concentration. Other parameters such as surfactant concentration = 3% (v/v), emulsification speed = 6000 rpm, emulsification t = 15 min and agitation speed during extraction = 300 rpm were kept constant [21,22].
Fig. 2. Effect of contact time on extraction of W(VI) from the wastewater.
were then injected into the ion chromatograph after being filtered again by using 0.22 m filter paper (dia. 13 mm). The sample A, B, and C were analyzed by following the cation, anion and transition metal analysis methods. The details of the method used and the results are reported in Table 1. The study showed that the PCB recycling unit wastewater contains nearly ten types of metal ions including W(VI). The dissolved solutes and their concentrations (in ppm) are Cu(II) = 5, Mn(II) = 1, Zn(II) = 1, Co(II) = 1, Ca(II) = 5, Na(II) = 5, K(I) = 0.5, Mg(II) = 4, W(VI) = 600 and Pb(II) = 150.
3.2. Contact time 3.1. Wastewater characterization The recycling of PCB waste from obsolete electronic devices is a relatively new activity and, therefore, offers numerous opportunities to recover expensive metals by means of the selective separation process. The PCB used in electronic devices are composed of different materials such as organics, polymers, ceramics and metals. The e-waste can be treated by thermal or nonthermal methods. The nonthermal method uses leaching as one of the main stages. Initially, the e-wastes are collected and grinded into a fine powder. The metallic and non-metallic parts may be separated by using a magnetic separator. Thereafter, most of the metals are leached out by adding HNO3 , HCl, HF, or aqua regia. Later, caustic can be added to the leached liquor for the precipitation of metals. The wastewater samples were collected from the PCB recycling unit and stored in polypropylene bottles having capacity of 500 mL. Total organic content (TOC) of the collected samples was found to be 4.73 mg/L. Next, they were filtered using a 0.22 m filter paper (dia. 47 mm). The filtrates were kept at 10 ◦ C in polypropylene tubes (50 mL) naming sample A, B and C. The pH of the entire filtrate was found to be around 5. To analyze the precise concentrations of the available cations and anions in wastewater; the samples were neither acidified nor diluted. The samples
Establishing a proper contact or residence time between the feed and emulsion phase minimizes the swelling and breakage of emulsion. Hence, understanding the effect of contact time helps in achieving optimum recovery of the solute from feed phase. ELM permeation studies of W(VI) were planned by varying the contact time and keeping other parameters constant. The effect of contact time on the extraction of W(VI) is shown in Fig. 2. Initially, the contact time was varied from 2 to 8 min. Samples of about 2–3 mL were withdrawn at different time intervals from the reaction mixture, for the analysis of raffinate phase. It is observed that as the contact time increased, the extraction efficiency of metal also gets increased [23]. Within 2 min, an extraction percentage of greater than 50% was achieved. However, all the co-metal ions (except Pb) got 100% extracted because of their minimum concentration (≤5 ppm) in wastewater. Further, the extraction efficiency of W(VI) slowly rose, and a maximum of 80% extraction was reached within 6 min. Additional increase in the contact time (beyond 6 min), rose the swelling of the organic phase from 12 to 35%. Due to which leakage of the solute from the organic phase was observed. The leakage decreased the extraction percentage of W(VI) to nearly 65% at 8 min. Therefore, a contact time of 6 min was kept in the future experiments.
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Fig. 4. Effect of feed phase pH on extraction % of W(VI) from the wastewater.
Fig. 3. Effect of extractant concentration on the extraction % of W(VI) from the wastewater.
3.3. Extractant concentration A selective extraction of the desired solute from wastewater is entirely dependent on the selection of membrane phase components and their concentration. Tertiary amines are frequently used for the solvent extraction process [24,25]. It has been reported that the extractants like Aliquat 336 and Alamine 336 were active reagents for the removal of heavy metals [26]. For instance, Salazar et al. reported the extraction procedure for the recovery of Cr(VI) from industrial effluents using Aliquat 336 [27]. Alamine 336 contains a basic nitrogen atom, it typically can react with a variety of inorganic and organic acids to form amine salts, which are capable of undergoing ion exchange reactions with a host of other anions. Herein, Aliquat 336 was selected as a mobile carrier in solvent hexane, and its concentration was varied from 0.02 to 0.15 M. The effect of carrier concentration on the extraction of W(VI) is depicted in Fig. 3. The extraction efficiency increased with increase in the concentration of Aliquat 336 from 0.02 to 0.06 M and nearly, remained same up to 0.1 M. In the ELM process, a higher extractant concentration at the interface between the membrane and feed phase promotes the transport of metals. However, it is observed that when the carrier concentration exceeds beyond 0.1 M, it results in the decrease in extraction percentage of W(VI). Higher content of an extractant in the membrane phase directly affects the viscosity of the emulsion, which leads to larger globules in the membrane phase [28]. The addition of more than 0.1 M of the extractant increases the viscosity of the emulsion from 215 to 388 mPa s; the outcome was mass transfer limitation. The high viscosity of membrane phase affects the extraction of solute, and a less stable membrane has low mass transfer efficiency. It is, therefore, a challenge to recover maximum solute in minimum extractant concentration. In the
present work, it was observed that approximately 80% extraction of W(VI) was achieved by using 0.06–0.1 M of extractant concentration whereas co-metals were completely extracted. However, it is important to note that the selected carrier is non-selective for the Pb(II) which remained behind in the raffinate. Therefore, the most optimized value of extractant concentration was found to be 0.06 M. 3.4. Feed phase pH Considering the presence of several co-ions in the wastewater the feed phase pH could be an essential criterion for investigation. The original pH value of the wastewater was 5 and hence, it was varied from 3 to 9. The pH was adjusted with either 1 M H2 SO4 or NaOH. Fig. 4 exhibits the effect of feed phase pH on the percentage of extraction of W(VI). It was noticed that as the pH increases towards the acidic region, the solute containing metal ions undergo protonation. Also, it was observed that, at highly acidic pH the properties of surfactant as a stabilizer were reduced, as it has a strong tendency to destabilize the emulsion [29]. On the contrary, the rise in pH towards the alkaline region, the hydroxide formation of extractant takes place. As the pH was increased above 5, the extraction efficiency was also found to be reduced because of the swelling of the emulsion [30]. These things have been found to hamper the separation of W(VI) at lower and higher pH of the feed phase. Therefore, the percentage of extraction was decreased from 80 to 68% at pH 3 and from 80 to 40% at pH 9. It was noticed that the percentage of swelling increased from 8 to 45% at pH 9. The resulted swelling makes the way for leaching of the solute. Therefore, pH 5 was found to be optimum for the recovery of W(VI) from wastewater. No significant effect was observed in the extraction of co-metal ions.
Table 2 Effect of treat ratio on extraction of solute from the wastewater. Metal ions
Cu
Treat ratio
Feed phase concentration (ppm) 5 1 1 5 1 1 5 1 1 5 1 1 Raffinate phase concentration (ppm) 0 0 0 0 0 0 0 0 0 0 0 0 Strip phase concentration (ppm) 0 0 0 0 0 0 0 0 0 0 0 0
1:1 1:5 1:10 1:15 1:1 1:5 1:10 1:15 1:1 1:5 1:10 1:15
Mn
Zn
Co
Ca
Na
K
Mg
W
Pb
1 1 1 1
5 5 5 5
15 15 15 15
0.5 0.5 0.5 0.5
4 4 4 4
600 600 600 600
150 150 150 150
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
468 216 120 270
150 150 150 150
0 0 0 0
0 0 0 0
1 1 2 1
0 0 0 0
0 0 0 0
245 1200 2400 600
0 0 0 0
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5
Enrichment factor
4 3 2 W
1
Pb
0
0.1
0.3
0.5
Internal phase concentraon (M)
1
Fig. 5. Effect of internal phase concentration on enrichment factor of W(VI) from the wastewater.
Fig. 6. First order plot for the recovery of W(VI) using optimised parameters of ELM.
3.5. Treat ratio
found to be decreased. At higher concentrations, internal phase concentration (NaOH) reacts with surfactant, Span 80 which is an ester (sorbitan monooleate). Finally, it results in the reduction of effective number of surfactant molecules and, therefore, membrane becomes unstable [34]. Moreover, the pH difference between internal phase and feed phase induces a significant amount of osmotic pressure difference [35]. The osmotic pressure difference is mostly responsible for the swelling of the emulsion which could also lead to the breakage of emulsion [36]. It can be concluded that 0.5 M of NaOH concentration was found to be optimum for the maximum recovery of W(VI). It is important to note that the selection of stripping agent was crucial, as it allowed stripping of only W(VI) and no traces of other co-ions, as depicted in Fig. 5.
A treat ratio is the ratio between emulsion and solute containing wastewater (feed phase). It can play an important role for the final recovery and enrichment of the solute from feed phase. From the economic and environment points of view, the use of minimum quantity of the emulsion phase for the feed phase treatment is preferable [28]. The treat ratio was varied from 1:1 to 1:15 by keeping the volume of emulsion constant and changing the amount of feed phase. The effect of treat ratio on percentage of extraction is shown in Table 2. The increase in treat ratio from 1:1 to 1:10, the extraction efficiency of W(VI) was found to be risen from 22 to 80% and the enrichment from 0.5 to 4. It is because, at very low treat ratio (1:1), less dispersion of emulsion in the feed phase occurs. Subsequently, it leads to reduction in the areas of contact between the two phases and resulted in the lower recovery of W(VI) [30]. Further increase in treat ratio to 1:15, the extraction of solute was found to decrease from 80 to 55%. It could be due to the high amount of the aqueous phase which accelerates the swelling and causes destabilization of the emulsion [31,32]. In the present study, the treat ratio of 1:10 was found to be better for the extraction and enrichment of W(VI) from the feed phase.
3.7. Extraction kinetics of W(VI) by ELM process The extraction kinetics of W(VI) in the ELM system can be defined by the following first order rate equation [37]: ln
Ct = −Kobs t C0
(4)
where (C0 ) and (Ct ) are the initial and time (t) concentration of W(VI) in the feed phase. Fig. 6 shows the first order plots for the optimised parameters of ELM (0.06 M of extractant, 0.5 M of stripping agent and pH 5 of the feed phase) which affect the recovery of W(VI). The slope obtained from the plot is negative and therefore, it can be concluded that the rate of extraction follows first order kinetics. It confirms that the kinetics of extraction is very quick (contact time of emulsion to the aqueous phase), and the removal rate of W(VI) reaches up to 80%.
3.6. Internal phase concentration The effect of internal phase concentration in the emulsion is an important parameter in deciding the transport efficiency of solutes. The increase in internal phase concentration up to a certain limit will increase the emulsion stability [33]. A variation of internal phase concentration was studied to understand the changes in the recovery of W(VI). The NaOH concentration was varied from 0.1 to 1 M by keeping other variables constant. It was observed that with an increase in the internal phase concentration from 0.1 to 0.5 M, the enrichment of W(VI) rose from 0.5 to 4 (Fig. 5). It is due to the rapid complexation of W(VI) with OH− ions inside the interface. However, further rise in the internal phase concentration beyond 0.5 M, the extraction percentage and enrichment of W(VI) were
3.8. Application of the optimized parameters for selective transport of W(VI) from the PCB recycling unit wastewater The results obtained from the above studies have provided valuable information about the right experimental conditions for
Table 3 Separation factor for W(VI) over other co-ions existing in the PCB recycling unit wastewater. No.
Metal co-ions
Feed phase concentration (ppm)
Stripping phase concentration (ppm)
Separation factor [w.r.t W(VI)] (ˇa/b )
1 2 3 4 5 6 7 8 9
Cu(II) Ca(II) Mn(II) Zn(II) Co(II) Na(II) K(I) Mg(II) Pb(II)
5 5 1 1 1 15 0.5 4 150
0 0 0 0 0 0 0 0 0
20 20 4 4 4 30 2 16 600
80
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the selective recovery of W(VI) from the wastewater. The study mainly focuses on achieving a high percentage of extraction and enrichment of W(VI) from the lean aqueous solutions. Various parameters optimised through which maximum recovery of W(VI) was achieved are: Aliquat 336 concentration = 0.06 M, feed phase pH 5, treat ratio = 1:10, contact t = 6 min and internal phase NaOH concentration = 0.5 M. A typical experiment, consisting of all the above parameters was carefully planned. The wastewater generated from the PCB recycling unit, can be treated with the ELM process, as shown in the flow-sheet (Fig. 2S). Throughout the experiment, the concentration of metal ions was monitored by using an ion chromatograph. Fig. 3S shows the actual chromatograms of all the ions (cation, anion and transition metals) studied by using separate columns and methodologies, as given in Table 1. Initially, the feed phase contained metal ions having concentrations viz. Cu(5 ppm), Mn(1 ppm), Zn(1 ppm), Co(1 ppm), Ca(5 ppm), Mg(4 ppm), Na(15 ppm), Mg(4 ppm), K(0.5 ppm), Pb(150 ppm) and W(600 ppm). After the treatment with the ELM process except Pb(II) (as appeared in raffinate phase), all the metal ions along with W(VI) get transported into the membrane phase. After that, a selective stripping of W(VI) occurs due to the alkaline stripping agent. The stripping concentration of W(VI) was obtained as 2400 ppm within 6 min, in one step. Following steps determine the transport of W(VI) ions inside the liquid membrane: diffusion of W(VI) ions to the external interface of the emulsion globule, interfacial reaction between the W(VI) ions and Aliquat 336 at the external interface, diffusion of the W(VI)-extractant complex into the emulsion globule, and stripping reaction of the complex with the alkaline solutions at the interface of the internal water droplets of the emulsion globule. The selectivity of W(VI) over the other metals was measured at this parametric composition. The selective separation and stripping of W(VI) over other ions was calculated using following transport model (separation factor) Eq. (5):
ˇa = b
Ca Cb
Ca Cb
strip (5) feed, 0
where subscripts (a) and (b) refer to the two metal species. (Ca ) and (Cb ) are the overall mass transfer concentrations. The separation factor (ˇa/b ) is defined as a ratio of the total mass transfer coefficients of the metal species [38]. The separation factor of W(VI) with respect to other co-ions present in the wastewater are shown in Table 3. It clearly shows that very high separation factors of W(VI) over the co-ions can be achieved. Particularly, with Pb(II) a separation factor of 600 is obtained with present ELM composition. 4. Conclusion The reported studies mainly focus on achieving high extraction percentage, enrichment and selective recovery of W(VI) from the PCB recycling unit wastewater. Various parameters that are optimised through which maximum recovery of W(VI) was achieved are: extraction concentration = 0.06 M, feed phase pH 5, treat ratio = 1:10, contact t = 6 min and internal phase concentration = 0.5 M. The selection of extractant played an important role by not allowing the transport of Pb(II) ions inside the membrane. On the contrary, the selection of stripping agent allowed only transport of W(VI) ions. The ELM process for recovery of W(VI) has numerous advantages over the conventional extraction method such as gaining high recovery by eliminating emulsion formation during extraction and stripping stages, and less solvent inventory. The selected components of ELM has achieved 80% of extraction and 4 times of enrichment of W(VI) from the wastewater. Along with
the recovery of W(VI), this process was found to be useful for the simultaneous purification of Pb(II) by leaving it in the feed phase. Supporting information Supporting information for this article containing calibration curves for all the metal ions along with the correlation coefficients (R2 ), detection limits (LODs), quantification limits (LOQs), precision (RSD%) and the concentrations of the separated ions analyzed in the wastewater using ion chromatograph is available free of charge via the Internet. Acknowledgements This research has been funded by DRDO (ERIP/ER/1003883/M/01/908/2012/D, R&D/1416, dated 28/3/2012) New Delhi, India for which the authors are grateful. Avinash B. Lende is thankful for the fellowship provided by DRDO. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jwpe.2015.09. 003. References [1] International Tungsten Industries Association, Newsletter. http://www.itia. org.uk/news/newsletter.dec02.pdf (2002). [2] E.M. Dubensky, E.E. Timm, High hardness, wear resistant materials, US Patent. 07 (1990) 403–411. [3] A.A. Palant, V.A. Petrova, N.A. Iatsenko, Solvent extraction of tungsten by diisododecylamine, J. Met. 3 (1998) 23–26. [4] W. Guan, G. Zhang, C. Gao, Solvent extraction separation of molybdenum and tungsten from ammonium solution by H2 O2 complexation, Hydrometallurgy 127 (2012) 84–90. [5] P. Ning, H. Cao, Y. Zhang, Selective extraction and deep removal of tungsten from sodium molybdate solution by primary amine N1923, Sep. Purif. Technol. 70 (2009) 27–33. [6] Z. Jianxian, S. Xiahui, Z. Lifeng, H. Qincheng, L. Shu, Recovery of tungsten(VI) from aqueous solution by complexation ultrafiltration process with the help of polyquaternium, Sep. Sci. Eng. 20 (2012) 831–836. [7] C. Cao, Z. Zhao, X. Chen, Selective precipitation of tungstate from molybdate-containing solution using divalent ions, Hydrometallurgy 110 (2011) 115–119. [8] Z. Zhao, C. Cao, X. Chan, G. Huo, Separation of macro amounts of tungsten and molybdenum by selective precipitation, Hydrometallurgy 108 (2011) 229–232. [9] Z.Z. Wei, C.C. Fang, C.X. Yu, Separation of macro amount of tungsten and molybdenum by precipitation with ferrous salt, Trans. Nonferrous Met. Soc. 21 (2011) 2758–2763. [10] R. Kuroda, K. Kawabuchi, T. Ito, Separation of rhenium, tungsten and molybdenum by thin-layer chromatography, Talanta 15 (1968) 1486–1488. [11] V. Inglezakis, H. Grigoropoulau, Modelling of ion exchange of Pb(II) in fixed beds of clinoptilotite, Micropor. Mesopor. Mater. 61 (2003) 273–282. [12] N. Chiron, R. Guilet, E. Deydier, Adsorption of Cu(II) and Pb(II) on to a grained silica: isotherm and kinetic models, Water Res. 37 (2003) 3079–3086. [13] R.R. Srivastava, N.K. Mittal, B. Padh, B.R. Reddy, Removal of tungsten and other impurities from spent HDS catalyst leach liquor by an adsorption route, Hydrometallurgy 127 (2012) 77–83. [14] P.S. Kulkarni, V.V. Mahajani, Application of liquid emulsion membrane (LEM) process for enrichment of molybdenum from aqueous solutions, J. Membr. Sci. 201 (2002) 123–135. [15] C.M. Das, G. Rungta, S.D. Arya, S. De, Removal of dyes and their mixtures from aqueous solution using liquid emulsion membrane, J. Hazard. Mater. 159 (2008) 365–371. [16] M. Matsumoto, T. Ohtake, M. Hirata, T. Hano, Extraction rates of amino acids by an emulsion liquid membrane with tri-n-octylmethylammonium chloride, J. Chem. Technol. Biot. 73 (1998) 237–242. [17] T. Kaghazchi, A. Kargari, R. Yegani, A. Zare, Emulsion liquid membrane pertraction of l-lysine from dilute aqueous solutions by D2EHPA mobile carrier, Desalination 190 (2006) 161–171. [18] N.N. Li, Separating hydrocarbons with liquid membranes, US Patent 3. 410 (1968) 794–798. [19] P.S. Kulkarni, S. Mukhopadhyay, S.K. Ghosh, Liquid membrane process for the selective recovery of uranium from industrial leach solutions, Ind. Eng. Chem. Res. 48 (2009) 3118–3125.
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Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Selective recovery of tungsten from printed circuit board recycling unit wastewater by using emulsion liquid membrane process Avinash B. Lende, Prashant S. Kulkarni ∗ Energy and Environment Laboratory, Department of Applied Chemistry, Defence Institute of Advanced Technology (DU), Ministry of Defence, Pune 411025, India
a r t i c l e
i n f o
Article history: Received 2 May 2015 Received in revised form 11 September 2015 Accepted 11 September 2015 Keywords: Emulsion liquid membrane Aliquat 336 PCB recycling unit wastewater Recovery of W(VI)
a b s t r a c t Aqueous waste containing low concentrations of tungsten are generated during printed circuit board (PCB) or e-waste recycling process. Study has been initiated to develop a suitable emulsion liquid membrane (ELM) technique for the separation and recovery of tungsten [W(VI)] from such waste using Aliquat 336 in hexane as a carrier and sodium hydroxide as a stripping agent. The waste, having a composition of nearly 600 ppm W(VI), 150 ppm Pb(II) and below 5 ppm concentrations of Cu(II), Mn(II), Zn(II), Co(II), Ca(II), Mg(II), Na(I) and K(I) at pH 5, was used as the feed phase. Various factors that affect the emulsion stability, as well as percentage extraction of W(VI), have been optimized to obtain maximum enrichment of W(VI). An attempt was made to recover W(VI) without making any substantial changes to the waste composition. Under optimized conditions, the extraction percentage of W(VI) was found to be 80% with 4 times enrichment in stripping phase. The separation factor for W(VI) vs. other co-ions was found to be very high thereby indicating selective recovery of W(VI) from the wastewater by using ELM process. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten [W(VI)] is one of the valuable elements with versatile applications in various fields. Many industries such as an electrical, television-tube, bulb-light, explosives manufacturing, steel and iron are employing W(VI) in their production [1]. Notably, pure W(VI) is required in lighting filaments, electronic tools, satellites, creep resistant steels and alloys. One of its most important use is, in the form of W(VI) mono-carbide (WC) which has a hardness close to diamond [2]. W(VI) may be less toxic to the human body depending on the route of exposure and is not considered to be an important human health hazard up to a certain limit by WHO and USEPA. Selective separation of W(VI) from secondary sources is required in terms of their economic values and limited resources.
Abbreviations: ELM, emulsion liquid membrane; PCB, printed circuit board; DPP, direct plating process; WC, tungsten mono-carbide; WHO, World Health Organization; USEPA, United States Environmental Protection Agency; PPM, parts-per millions; SPAN 80, surfactant, sorbitan monooleate; TOC, total organic compound; dia, diameter; PAR, 4-(2-pyridylazo) resorcinol; rpm, rotation per minutes; w/o/w, water–oil–water; R3CH3 N+ Cl− , aliquat 336; mPa S, millipascal-second; min, minutes; M, molar. ∗ Corresponding author. Fax: +91 20 2691533. E-mail addresses: ps [email protected], [email protected] (P.S. Kulkarni). http://dx.doi.org/10.1016/j.jwpe.2015.09.003 2214-7144/© 2015 Elsevier Ltd. All rights reserved.
Numerous methods are reported for the recovery of W(VI) from the wastewater. For instance, solvent extraction [3–5], ultrafiltration [6], precipitation [7–9], thin layer chromatography [10], ion-exchange separation [11] and adsorption [12,13]. Membrane based separation processes are gaining importance due to their simplicity and easy scalability. The emulsion liquid membrane (ELM) process is an extensively used method for the separation of metal ions [14], hydrocarbons [15], amino acids [16] and biological compounds [17]. ELM is one of the finest methods for the simultaneous extraction and enrichment of the solute from feed phase within a single step. The basic properties of ELM operations such as low-solvent inventory and high surface area make them ideal for the separation of solutes from dilute aqueous streams [18]. An ELM is the resultant emulsion membrane phase consisting of a homogeneous mixture of surfactant, extractant, organic diluent and inside to which contain an aqueous strip phase. The solute species from the external feed phase gets dissolve in the organic phase and diffuses into the internal strip phase. The efficiency and selective transport across the membrane phase may be remarkably enhanced by the presence of extractant concentration in the organic phase. Finally, the enriched solute can be recovered by breaking of the emulsion [19]. Therefore, the overall performance of the ELM is dependent on several parameters such as surfactant and extractant concentration, treat ratio, contact time, feed phase pH and internal phase concentration.
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Fig. 1. A schematic of emulsion liquid membrane (ELM) process.
In the present study, the feed phase was taken as wastewater from the printed circuit board (PCB) or e-waste recycling unit [20]. The effluent was characterized by using ion chromatography instrument. It was found contain various metal ions such as Cu(II), Mn(II), Zn(II), Co(II), Ca(II), Na(II), K(I), Mg(II), W(VI) and Pb(II). Amongst them the concentration of W(VI) and Pb(II) was significant viz. 600 and 150 ppm, respectively, whereas the concentration of other co-metals in the effluent stream was below 5 ppm. An ELM process was designed by taking suitable and appropriate amount of extractant, surfactant and stripping agent. Several important parameters of ELM were investigated, and the kinetics was measured. Efforts were made in achieving selective recovery of W(VI) from the wastewater without making any substantial changes to its composition.
2.2. Development of ELM During the development of ELM, the emulsion was prepared by adding an appropriate quantity of extractant (0.02–0.15 M, Aliquat 336) and surfactant (3%, Span 80) with an organic diluent (Hexane). The mixture along with the aqueous stripping agent (0.1–1 M, NaOH) was stirred at 6000 rpm for 15 min by using high-speed homogenizer [21]. A fixed ratio of 1:1 was maintained between the aqueous and the membrane phase. The prepared emulsion was of water-in-oil (w/o) type having milky white uniform mixture. The internal particle size of the water-in-oil emulsion was found to be within the range of 4–8 m. The emulsion swelling was measured by comparing the internal strip phase volume before and after treatment with the feed phase. Swelling (%) =
Vi,t − Vi,0 × 100 Vi,0
(1)
2. Experimental 2.1. Materials and methods Surfactant, sorbitan monooleate (SPAN 80), was a gift sample from Mohini Organics (Pvt.) Ltd., Mumbai, India. The extractant, Aliquat 336, was purchased from Sigma–Aldrich, Steinheim, Germany. The effluent, wastewater containing W(VI) was obtained from PCB recycling unit industries. The pH adjustment of wastewater solution was carried out using 1 M of H2 SO4 and NaOH. All the chemicals used were of AR grade and used without any further purification. The ELM was developed by a Wise Tis (HG-15D) homogenizer (2000–27.000 rpm), and the speed was adjusted with a digital stroboscope (Lutron DT-2249). The internal particle size of the emulsion was measured by a particle size analyzer (Nanophox, 219F-2009, Germany). The recovery of metal runs was performed using a Heidolph RZR 2012 overhead stirrer (200–2000 rpm). The recovered concentration of W(VI) and other co-ions in the sample were analyzed by Ion Chromatography (Metrohm, 883 Basic IC Plus, Switzerland). The viscosity of the emulsion and pH of the feed phase samples were measured by Ostwald viscometer and pH meter (Hanna Instrument, HI 2211 pH/ORP Meter), respectively.
where Vi,0 and Vi,t are the initial and at time ‘t’ internal phase volumes. The most stable emulsion was obtained when there was neither swelling nor breakage during the extraction run. A schematic of the ELM process constituting emulsion preparation, permeation of W(VI) and breaking of the emulsion are depicted in Fig. 1. 2.3. Recovery of W(VI) Most of the metal ions exist in the form of hydrated ions in an aqueous stream, before; it can be extracted into the non-polar organic phase. In the extraction process, the water molecule must be replaced by ionic charge generated by the extractant. The developed emulsion was treated with PCB recycling unit wastewater (feed phase) by taking various treat ratios. The extraction run (mixture of emulsion and feed phase) was performed at 300 rpm using an overhead stirrer. Samples at different time intervals were taken out for the analysis. Later, the emulsion phase was separated from the raffinate phase. Finally, the emulsion was broken by heating in a closed vessel at 80 ◦ C for the analysis and recovery of W(VI). The recovery of tungsten by using Aliquat 336 (R3 CH3 N+ Cl− ) in ELM
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Table 1 Detection of solutes by using ion chromatograph. Analytes
Ions
Column
Eluent mixture
Detector
Cations
Ca(II), Na(II), K(I), Mg(II), Pb(II)
Metrosep C4, Metrohm
Conductivity detector
Anions
W(VI)
Metrosep A supp5, Metrohm
Transition metal ions
Cu(II), Mn(II), Zn(II), Co(II)
Metrosep C4, Metrohm
2.75 mmol/L HNO3 and 0.05 mmol/L pyridine-2,6-dicarboxylic acid 15 mmol/L Na2 CO3 , 5 mmol/L NaHCO3 and 100 mmol/L H2 SO4 1.75 mmol/L Oxalic acid + PAR
Conductivity detector
UV detector
process can be represented by Eqs. (1) and (2). −2 + − + M2+ WO−2 4(aq) + 2(R3 CH3 N Cl ) ↔ (R3 CH3 N )2 WO4
+ MCl2(aq) (Extraction)
(2)
(R3 CH3 N+ )2 WO4 −2 + 2Na+ ↔ Na2 WO4(aq) (aq) + (R3 CH3 N+ )2 (Stripping)
(3)
where M stands for co-metal ion from PCB recycling unit wastewater. Here, the extractant reacts as an anion exchanger, forming an ion-pair with the metal oxyanions from the aqueous solution. In the present case, as only W(VI) forms oxyanions (WO4 2− ) in water, it get exchanged very fast with the carrier. The initial stability of the emulsion was calculated by monitoring the phase separation with time. In the extraction process, the emulsion phase stability played a significant role for the efficient transport of solute from the feed phase. In this work, extraction, enrichment, swelling and breakage were calculated by using reported method [22]. 3. Results and discussion The primary focus of this investigation was selective separation and enrichment of W(VI) from the wastewater. Consequently, the study was undertaken for evaluating the critical parameters of ELM process such as the effect of extractant concentration, feed phase pH, treat ratio, contact time and internal phase concentration. Other parameters such as surfactant concentration = 3% (v/v), emulsification speed = 6000 rpm, emulsification t = 15 min and agitation speed during extraction = 300 rpm were kept constant [21,22].
Fig. 2. Effect of contact time on extraction of W(VI) from the wastewater.
were then injected into the ion chromatograph after being filtered again by using 0.22 m filter paper (dia. 13 mm). The sample A, B, and C were analyzed by following the cation, anion and transition metal analysis methods. The details of the method used and the results are reported in Table 1. The study showed that the PCB recycling unit wastewater contains nearly ten types of metal ions including W(VI). The dissolved solutes and their concentrations (in ppm) are Cu(II) = 5, Mn(II) = 1, Zn(II) = 1, Co(II) = 1, Ca(II) = 5, Na(II) = 5, K(I) = 0.5, Mg(II) = 4, W(VI) = 600 and Pb(II) = 150.
3.2. Contact time 3.1. Wastewater characterization The recycling of PCB waste from obsolete electronic devices is a relatively new activity and, therefore, offers numerous opportunities to recover expensive metals by means of the selective separation process. The PCB used in electronic devices are composed of different materials such as organics, polymers, ceramics and metals. The e-waste can be treated by thermal or nonthermal methods. The nonthermal method uses leaching as one of the main stages. Initially, the e-wastes are collected and grinded into a fine powder. The metallic and non-metallic parts may be separated by using a magnetic separator. Thereafter, most of the metals are leached out by adding HNO3 , HCl, HF, or aqua regia. Later, caustic can be added to the leached liquor for the precipitation of metals. The wastewater samples were collected from the PCB recycling unit and stored in polypropylene bottles having capacity of 500 mL. Total organic content (TOC) of the collected samples was found to be 4.73 mg/L. Next, they were filtered using a 0.22 m filter paper (dia. 47 mm). The filtrates were kept at 10 ◦ C in polypropylene tubes (50 mL) naming sample A, B and C. The pH of the entire filtrate was found to be around 5. To analyze the precise concentrations of the available cations and anions in wastewater; the samples were neither acidified nor diluted. The samples
Establishing a proper contact or residence time between the feed and emulsion phase minimizes the swelling and breakage of emulsion. Hence, understanding the effect of contact time helps in achieving optimum recovery of the solute from feed phase. ELM permeation studies of W(VI) were planned by varying the contact time and keeping other parameters constant. The effect of contact time on the extraction of W(VI) is shown in Fig. 2. Initially, the contact time was varied from 2 to 8 min. Samples of about 2–3 mL were withdrawn at different time intervals from the reaction mixture, for the analysis of raffinate phase. It is observed that as the contact time increased, the extraction efficiency of metal also gets increased [23]. Within 2 min, an extraction percentage of greater than 50% was achieved. However, all the co-metal ions (except Pb) got 100% extracted because of their minimum concentration (≤5 ppm) in wastewater. Further, the extraction efficiency of W(VI) slowly rose, and a maximum of 80% extraction was reached within 6 min. Additional increase in the contact time (beyond 6 min), rose the swelling of the organic phase from 12 to 35%. Due to which leakage of the solute from the organic phase was observed. The leakage decreased the extraction percentage of W(VI) to nearly 65% at 8 min. Therefore, a contact time of 6 min was kept in the future experiments.
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Fig. 4. Effect of feed phase pH on extraction % of W(VI) from the wastewater.
Fig. 3. Effect of extractant concentration on the extraction % of W(VI) from the wastewater.
3.3. Extractant concentration A selective extraction of the desired solute from wastewater is entirely dependent on the selection of membrane phase components and their concentration. Tertiary amines are frequently used for the solvent extraction process [24,25]. It has been reported that the extractants like Aliquat 336 and Alamine 336 were active reagents for the removal of heavy metals [26]. For instance, Salazar et al. reported the extraction procedure for the recovery of Cr(VI) from industrial effluents using Aliquat 336 [27]. Alamine 336 contains a basic nitrogen atom, it typically can react with a variety of inorganic and organic acids to form amine salts, which are capable of undergoing ion exchange reactions with a host of other anions. Herein, Aliquat 336 was selected as a mobile carrier in solvent hexane, and its concentration was varied from 0.02 to 0.15 M. The effect of carrier concentration on the extraction of W(VI) is depicted in Fig. 3. The extraction efficiency increased with increase in the concentration of Aliquat 336 from 0.02 to 0.06 M and nearly, remained same up to 0.1 M. In the ELM process, a higher extractant concentration at the interface between the membrane and feed phase promotes the transport of metals. However, it is observed that when the carrier concentration exceeds beyond 0.1 M, it results in the decrease in extraction percentage of W(VI). Higher content of an extractant in the membrane phase directly affects the viscosity of the emulsion, which leads to larger globules in the membrane phase [28]. The addition of more than 0.1 M of the extractant increases the viscosity of the emulsion from 215 to 388 mPa s; the outcome was mass transfer limitation. The high viscosity of membrane phase affects the extraction of solute, and a less stable membrane has low mass transfer efficiency. It is, therefore, a challenge to recover maximum solute in minimum extractant concentration. In the
present work, it was observed that approximately 80% extraction of W(VI) was achieved by using 0.06–0.1 M of extractant concentration whereas co-metals were completely extracted. However, it is important to note that the selected carrier is non-selective for the Pb(II) which remained behind in the raffinate. Therefore, the most optimized value of extractant concentration was found to be 0.06 M. 3.4. Feed phase pH Considering the presence of several co-ions in the wastewater the feed phase pH could be an essential criterion for investigation. The original pH value of the wastewater was 5 and hence, it was varied from 3 to 9. The pH was adjusted with either 1 M H2 SO4 or NaOH. Fig. 4 exhibits the effect of feed phase pH on the percentage of extraction of W(VI). It was noticed that as the pH increases towards the acidic region, the solute containing metal ions undergo protonation. Also, it was observed that, at highly acidic pH the properties of surfactant as a stabilizer were reduced, as it has a strong tendency to destabilize the emulsion [29]. On the contrary, the rise in pH towards the alkaline region, the hydroxide formation of extractant takes place. As the pH was increased above 5, the extraction efficiency was also found to be reduced because of the swelling of the emulsion [30]. These things have been found to hamper the separation of W(VI) at lower and higher pH of the feed phase. Therefore, the percentage of extraction was decreased from 80 to 68% at pH 3 and from 80 to 40% at pH 9. It was noticed that the percentage of swelling increased from 8 to 45% at pH 9. The resulted swelling makes the way for leaching of the solute. Therefore, pH 5 was found to be optimum for the recovery of W(VI) from wastewater. No significant effect was observed in the extraction of co-metal ions.
Table 2 Effect of treat ratio on extraction of solute from the wastewater. Metal ions
Cu
Treat ratio
Feed phase concentration (ppm) 5 1 1 5 1 1 5 1 1 5 1 1 Raffinate phase concentration (ppm) 0 0 0 0 0 0 0 0 0 0 0 0 Strip phase concentration (ppm) 0 0 0 0 0 0 0 0 0 0 0 0
1:1 1:5 1:10 1:15 1:1 1:5 1:10 1:15 1:1 1:5 1:10 1:15
Mn
Zn
Co
Ca
Na
K
Mg
W
Pb
1 1 1 1
5 5 5 5
15 15 15 15
0.5 0.5 0.5 0.5
4 4 4 4
600 600 600 600
150 150 150 150
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
468 216 120 270
150 150 150 150
0 0 0 0
0 0 0 0
1 1 2 1
0 0 0 0
0 0 0 0
245 1200 2400 600
0 0 0 0
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5
Enrichment factor
4 3 2 W
1
Pb
0
0.1
0.3
0.5
Internal phase concentraon (M)
1
Fig. 5. Effect of internal phase concentration on enrichment factor of W(VI) from the wastewater.
Fig. 6. First order plot for the recovery of W(VI) using optimised parameters of ELM.
3.5. Treat ratio
found to be decreased. At higher concentrations, internal phase concentration (NaOH) reacts with surfactant, Span 80 which is an ester (sorbitan monooleate). Finally, it results in the reduction of effective number of surfactant molecules and, therefore, membrane becomes unstable [34]. Moreover, the pH difference between internal phase and feed phase induces a significant amount of osmotic pressure difference [35]. The osmotic pressure difference is mostly responsible for the swelling of the emulsion which could also lead to the breakage of emulsion [36]. It can be concluded that 0.5 M of NaOH concentration was found to be optimum for the maximum recovery of W(VI). It is important to note that the selection of stripping agent was crucial, as it allowed stripping of only W(VI) and no traces of other co-ions, as depicted in Fig. 5.
A treat ratio is the ratio between emulsion and solute containing wastewater (feed phase). It can play an important role for the final recovery and enrichment of the solute from feed phase. From the economic and environment points of view, the use of minimum quantity of the emulsion phase for the feed phase treatment is preferable [28]. The treat ratio was varied from 1:1 to 1:15 by keeping the volume of emulsion constant and changing the amount of feed phase. The effect of treat ratio on percentage of extraction is shown in Table 2. The increase in treat ratio from 1:1 to 1:10, the extraction efficiency of W(VI) was found to be risen from 22 to 80% and the enrichment from 0.5 to 4. It is because, at very low treat ratio (1:1), less dispersion of emulsion in the feed phase occurs. Subsequently, it leads to reduction in the areas of contact between the two phases and resulted in the lower recovery of W(VI) [30]. Further increase in treat ratio to 1:15, the extraction of solute was found to decrease from 80 to 55%. It could be due to the high amount of the aqueous phase which accelerates the swelling and causes destabilization of the emulsion [31,32]. In the present study, the treat ratio of 1:10 was found to be better for the extraction and enrichment of W(VI) from the feed phase.
3.7. Extraction kinetics of W(VI) by ELM process The extraction kinetics of W(VI) in the ELM system can be defined by the following first order rate equation [37]: ln
Ct = −Kobs t C0
(4)
where (C0 ) and (Ct ) are the initial and time (t) concentration of W(VI) in the feed phase. Fig. 6 shows the first order plots for the optimised parameters of ELM (0.06 M of extractant, 0.5 M of stripping agent and pH 5 of the feed phase) which affect the recovery of W(VI). The slope obtained from the plot is negative and therefore, it can be concluded that the rate of extraction follows first order kinetics. It confirms that the kinetics of extraction is very quick (contact time of emulsion to the aqueous phase), and the removal rate of W(VI) reaches up to 80%.
3.6. Internal phase concentration The effect of internal phase concentration in the emulsion is an important parameter in deciding the transport efficiency of solutes. The increase in internal phase concentration up to a certain limit will increase the emulsion stability [33]. A variation of internal phase concentration was studied to understand the changes in the recovery of W(VI). The NaOH concentration was varied from 0.1 to 1 M by keeping other variables constant. It was observed that with an increase in the internal phase concentration from 0.1 to 0.5 M, the enrichment of W(VI) rose from 0.5 to 4 (Fig. 5). It is due to the rapid complexation of W(VI) with OH− ions inside the interface. However, further rise in the internal phase concentration beyond 0.5 M, the extraction percentage and enrichment of W(VI) were
3.8. Application of the optimized parameters for selective transport of W(VI) from the PCB recycling unit wastewater The results obtained from the above studies have provided valuable information about the right experimental conditions for
Table 3 Separation factor for W(VI) over other co-ions existing in the PCB recycling unit wastewater. No.
Metal co-ions
Feed phase concentration (ppm)
Stripping phase concentration (ppm)
Separation factor [w.r.t W(VI)] (ˇa/b )
1 2 3 4 5 6 7 8 9
Cu(II) Ca(II) Mn(II) Zn(II) Co(II) Na(II) K(I) Mg(II) Pb(II)
5 5 1 1 1 15 0.5 4 150
0 0 0 0 0 0 0 0 0
20 20 4 4 4 30 2 16 600
80
A.B. Lende, P.S. Kulkarni / Journal of Water Process Engineering 8 (2015) 75–81
the selective recovery of W(VI) from the wastewater. The study mainly focuses on achieving a high percentage of extraction and enrichment of W(VI) from the lean aqueous solutions. Various parameters optimised through which maximum recovery of W(VI) was achieved are: Aliquat 336 concentration = 0.06 M, feed phase pH 5, treat ratio = 1:10, contact t = 6 min and internal phase NaOH concentration = 0.5 M. A typical experiment, consisting of all the above parameters was carefully planned. The wastewater generated from the PCB recycling unit, can be treated with the ELM process, as shown in the flow-sheet (Fig. 2S). Throughout the experiment, the concentration of metal ions was monitored by using an ion chromatograph. Fig. 3S shows the actual chromatograms of all the ions (cation, anion and transition metals) studied by using separate columns and methodologies, as given in Table 1. Initially, the feed phase contained metal ions having concentrations viz. Cu(5 ppm), Mn(1 ppm), Zn(1 ppm), Co(1 ppm), Ca(5 ppm), Mg(4 ppm), Na(15 ppm), Mg(4 ppm), K(0.5 ppm), Pb(150 ppm) and W(600 ppm). After the treatment with the ELM process except Pb(II) (as appeared in raffinate phase), all the metal ions along with W(VI) get transported into the membrane phase. After that, a selective stripping of W(VI) occurs due to the alkaline stripping agent. The stripping concentration of W(VI) was obtained as 2400 ppm within 6 min, in one step. Following steps determine the transport of W(VI) ions inside the liquid membrane: diffusion of W(VI) ions to the external interface of the emulsion globule, interfacial reaction between the W(VI) ions and Aliquat 336 at the external interface, diffusion of the W(VI)-extractant complex into the emulsion globule, and stripping reaction of the complex with the alkaline solutions at the interface of the internal water droplets of the emulsion globule. The selectivity of W(VI) over the other metals was measured at this parametric composition. The selective separation and stripping of W(VI) over other ions was calculated using following transport model (separation factor) Eq. (5):
ˇa = b
Ca Cb
Ca Cb
strip (5) feed, 0
where subscripts (a) and (b) refer to the two metal species. (Ca ) and (Cb ) are the overall mass transfer concentrations. The separation factor (ˇa/b ) is defined as a ratio of the total mass transfer coefficients of the metal species [38]. The separation factor of W(VI) with respect to other co-ions present in the wastewater are shown in Table 3. It clearly shows that very high separation factors of W(VI) over the co-ions can be achieved. Particularly, with Pb(II) a separation factor of 600 is obtained with present ELM composition. 4. Conclusion The reported studies mainly focus on achieving high extraction percentage, enrichment and selective recovery of W(VI) from the PCB recycling unit wastewater. Various parameters that are optimised through which maximum recovery of W(VI) was achieved are: extraction concentration = 0.06 M, feed phase pH 5, treat ratio = 1:10, contact t = 6 min and internal phase concentration = 0.5 M. The selection of extractant played an important role by not allowing the transport of Pb(II) ions inside the membrane. On the contrary, the selection of stripping agent allowed only transport of W(VI) ions. The ELM process for recovery of W(VI) has numerous advantages over the conventional extraction method such as gaining high recovery by eliminating emulsion formation during extraction and stripping stages, and less solvent inventory. The selected components of ELM has achieved 80% of extraction and 4 times of enrichment of W(VI) from the wastewater. Along with
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