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Recovery of Critical Metals from Leach Solution ofElectronic Waste using Magnetite ElectrospunCarbon Nano�bers CompositeAamir Iqbal 

University of PeshawarM. Rasul Jan 

University of PeshawarJasmin Shah  ( jasminshah2001@yahoo.com )

University of PeshawarBahroz Rashid 

University of Peshawar

Research Article

Keywords: Critical metals, Electronic waste, Carbon nano�bers, Solid phase extraction, Magnetic nano-hydrometallurgy

Posted Date: May 27th, 2022

DOI: https://doi.org/10.21203/rs.3.rs-1413993/v1

License: This work is licensed under a Creative Commons Attribution 4.0 International License.  Read Full License

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AbstractScarcity in mining and geo-political direction diverts attention toward critical metals recycling. Gallium(Ga), Indium (In) and Germanium (Ge) are among the critical metals consume approximately 80 percentof its world mining in innovative production of electrical and electronic equipment. The fast obsolescingrate generate large amount of electronic waste, which is now seen as secondary reservoir for criticalmetals. These metal resources need to be dealt with effective recycling capabilities. Based on solid phaseextraction, magnetic nano-hydrometallurgy is opening a new area of metallic contents recovery inconventional hydrometallurgy. In the present work, polyacrylonitrile (PAN) based electrospun nano�berswere synthesized and carbonized at 800°C in the inert environment. After surface oxidation, carbonnano�bers were decorated with magnetite particles through co-precipitation. The saturationmagnetization value (Ms = 23.6emu/g) con�rm high loading of magnetite particles. The selected criticalmetal ions are freely present in aqueous solution at pH 1 to 3, thus highest removal e�ciency wasobserved at pH 2. Pseudo second order kinetics con�rm the chemical/charges interaction betweensorbent and sorbate ions. Maximum sorption capacity calculated through Langmuir isotherm was 226,191, 171 mg/g for Ge(IV), Ga(III) and In(III) metal ions, respectively. The RL value (0 < RL>1) indicatefavourable sorption process. The sorbed target metal ions were collectively eluted using 1mol/Lhydrochloric acid. The preconcentration factor was calculated 1080 for Ge(IV) and In(III) while 1260 forGa(III). The method was validated with 5µg/mL spiked multi-element standards and applied to multipleacid leached electronic waste samples like PCBs, waste LCD panels and solar panels etc. Recoveries inthe range of 96.2% for Ga(III), 95.6% for In(III) and 97.4% for Ge(IV) in the presence of diverse ionsindicate the suitability of the proposed method for target metal ions even in complex matrix.

1. IntroductionTechnological innovation in electronic equipment relies upon the continuous mining of metals. More than60 elements at various composition are found in modern electronic devices (Hageluken 2006; Khaliq et al.2014). The European commission listed some metals critical, according to their risk in market supply,scarcity in mining & puri�cation and more importantly by their geo-political directions (Blengini et al.2017; Perez et al. 2019; Schluep et al. 2009). Among the 28% of non-ferrous metals, critical or scaresmetals makes smaller percent composition of the electronic equipment however share high in theireconomic cost. These metals are necessary for emerging green innovative production of electronicequipment to meet the market demands and often cannot be replaced. Gallium, germanium and indiumare among these metals mostly used in printed circuit boards (PCBs), integrated circuits, semiconductors,hard disk drives, �bre glass, light emitting diodes (LED), solar cells and liquid crystal displays (LCDs)(Bernhardt and Reilly 2019; Grandell et al. 2016; Hayes and McCullough 2018; Işıldar et al. 2019).Reported in 2015 that their availability in earth crust was estimated 20 ppm for gallium, 1 ppm forgermanium and 0.05 ppm for indium with annual global production of 400, 800 and 200,000 metrictonnes for gallium, indium and germanium, respectively. Only LCDs in the form of transparent electrodeaccount for more than 70% use of indium as indium tin-oxide (InSnO2) and found in concentration range

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of 100 to 400 ppm depending upon its origin. Similarly, semiconductor account for 80% consumption ofgallium as gallium arsenide (GaAs) and optical �bres 25% and infrared optics uses more than 30% ofannually produced germanium as germanium oxide (GeO2). 15% of the total annually producedgermanium is utilized in solar cells application and increasing drastically due to conversion on torenewable energy resources (Arvidsson and Sandén 2017; Nguyen and Lee 2020).The limited productionand high consumption of these metals put technological innovation and supply under risk. Keeping inview of these strategic resources and their high technological applications the recovery of selected criticalmetals from secondary sources is the need of the day.

The drastic innovation and fast development in electronic sector on the other hand also generate largeamount of electronic waste (e waste/WEEE). In 2006 United Nation reported 20–50 million tons of ewaste production per annum that is increased to 65.4 million tons in 2017 and it is predicted thatdeveloping countries will discard double to this e waste quantity to environment up to 2030 (Kandil 2013;Schwarzer et al. 2005). The market value of e waste estimated in 2009 was 11 billion USD with 8.8%annual growth. The booming market of importing second-hand electronic equipment in developingcountries became the source of higher e waste production compared to developed countries like Europe,USA, Korea and Japan etc. With no or limited regulation for waste management the developing countriesbecame the dumping sites for modern developed countries. The large production of waste electrical andelectronic equipment (WEEE) and their informal dealing not only detriment our environment but alsoeffect the economy (Sajid et al. 2019). Basal action network estimated 0.0013, 0.0016 and 0.0016% ofthe total weight of Personal Computer (PC) for gallium, germanium and indium, respectively with lessthan 1% end of life recycling rate of gallium and germanium and 60% recovery potential of indium. Aspent LED approximately contains 2.1 and 1.1 mg/Kg gallium and indium, respectively. Optical �bres andsolar panels waste are the important secondary source of germanium containing 4.1% germanium oftheir total mass having 50% germanium recycling e�ciency (Nguyen and Lee 2020; Ruiz et al. 2018).Applying circular economic 3Rconcept (Reduce, Reuse, and Recycle), e waste is now considered the mostsigni�cant secondary source for selected critical metals (Ueberschaar et al. 2017). These metal resourcesavailable from e waste cannot be overlooked and need to be dealt with effective recycling capabilities.

Magnetic nano-hydrometallurgy based on solid phase extraction (SPE) is the newly introduced extractiontechnique mostly applied for the metal ions extraction (Perez et al. 2019; Toma 2015). The techniqueutilizes super-paramagnetic nanoparticles or magnetic particle decorated sorbents for capturing,transporting and processing of metal ions present in liquid phase. Carbon based sorbents are the mostcommonly known sorbents magnetized so far (Aghaei et al. 2017; Iqbal et al. 2020). Carbon nano�bers(CNFs), after its synthesis from polyacrylonitrile (PAN) precursor by electrospinning process is broadlyapplied for metal ions extraction (Ahmad et al. 2020; Chen et al. 2007; Duan et al. 2012; Thamer et al.2019). The high surface area, porous texture and controlled chemical behaviour make CNFs surfacehighly attractive for attachment of nanomaterial and thus easily magnetized. The introduction ofmagnetic nanoparticles to carbon surface improves sorption capacity of the composite by introducingoxygen functional groups and providing charge exchange capacity thus combining sorption and redox

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reaction. The composite materials developed have high stability, large surface area, ease in regenerationand capability of automation (Huang et al. 2017; Mirzaei et al. 2015; Ren et al. 2012). The solid phaseafter mixing with sample liquid phase is then separated with the help of external magnet and washedwith selected eluent for extraction. The material is reused in the sustaining cyclic process of sorption anddesorption. Irrespective of pyrometallurgical high temperature treatment which give rise to environmentalpollution via emission of toxic gases or conventional hydrometallurgy utilizing high quantity of toxic and�ammable organic solvent and intensive chemical processing, the magnetic nano-hydrometallurgy ful�lthe important requisites of green chemistry (Toma 2015).

Limited or fewer recycling capabilities implemented up till now for Ga, In and Ge metals. This work aimsto demonstrate a suitable extraction methodology for recovery of critical metal ions Ga(III), Ge(IV) and In(III). The method uses magnetic nanoparticles decorated electrospun carbon nano�bers as pH selectivesorbent for extraction of critical metal ions even though present in complex metallic matrix. The materialhas the capabilities to be reused for multiple extraction cycles and retain both its physical and chemicalproperties. Furthermore, in contrast to conventional liquid extraction phenomenon, this article opens anew horizon for recycling of metallic waste based on magnetic nano-hydrometallurgy.

2. Materials And Methods

2.1. Reagents and instrumentsPure analytical grade chemicals were used throughout this research experiments. PAN powder (Mw ~150,000) was obtained from scienti�c polymer product, Inc. N,N-dimethylacetamide (38839) wassupplied by Fluka Chemika Switzerland. Nitric acid, hydrochloric acid and acetic acid were purchasedfrom Merck Darmstadt Germany. Similarly, Iron II and III salts of ammonium sulphate used as precursorfor magnetite nanoparticles synthesis were also purchased from MerckDarmstadt Germany. Ammoniasolution was obtained from BDH chemical England. 1000 µg/mL ICP standard solution of gallium,germanium and indium were supplied by VGH Labs INC., Manchester and were used throughout theexperiments. Deionized water (DI) with conductance less than 0.4µS/cm obtained from ALGA PURELABUltra. Controlled parameters electrospinning assembly were used at Material Research Laboratory,Department of Physics, University of Peshawar. Quarts tube furnace with continuous inert gas supply andtemperature range higher than 1000°C were used for carbonization of raw �bres.

The synthesized sorbent was characterized for surface morphology using �eld emission ScanningElectron Microscope (FESEM) Quanta FEG450. FTIR-6800 type A spectrometer with KBR disc was usedfor recording IR spectra between 4000-400cm− 1 for functional groups identi�cation. Thermaldecomposition behaviour was studied under oxygenated environment with pyres diamond series ofPerkin Elmer Thermograms USA. XRD pattern was obtained using X-ray Diffractometer (Jeol JDX 9C)with Cu radiation source at wavelength of 1.5478A°. Magnetic properties were investigated with 8600series VSM of Lake Shore cryotrons with magnetic �eld up to 3.2 T with fast �eld changes up to 10000Oe/s. Sonic Vibra VC750 ultrasonic bath and Gallen Kamp GB mechanical shaker were used for material

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dispersion. Jenway microprocessor pH meter was used for pH measurement. Metal ions analysis wascarried out using Liberty series AX sequential inductively coupled plasma optical emission spectrometer(ICP-OES) of Varian Australia with pre-set experimental parameters.

2.2. Sorbent Synthesis

2.2.1. Preparation and Carbonization of ESCNFsPAN based nano�bers were synthesized by an electrospinning technique. PAN powder was dissolved inN,N-dimethylacetamide using magnetic stirrer at 50°C till a clear viscous solution was obtained. Thesolution was loaded to 10 cc syringe with needle connected to positive terminal of high voltage powersupply ~ 20k V. The syringe was forced with controlled electric motor at optimized �ow rate of 1.5 mL/hwhile product was collected on aluminium foil placed 12 cm apart, connected to negative terminal ofpower supply. Electrospinning procedure was carried out under normal room temperature (25–30°C) andhumidity. The obtained nano�bers were stabilized in oven up to 150°C using temperature ramp of2°C/min for 2 hours in order to vaporize any remaining solvent and to achieve structurally stable �bers.Stabilized nano�bers were detached from aluminium sheets and a part of it was carbonized at 800°C ininert environment. Temperature was slowly increased at a rate of 5°C/min with continuous �ow ofnitrogen. The product was named as electrospun based carbon nano�bers (ESCNFs).

2.2.2. Oxidation of ESCNFsESCNFs were oxidized with 8% nitric acid solution for 30 min at 80°C for 4 hours in a closed reactionvessel. The product was then thoroughly washed with DI water. The process was repeated twice in orderto achieve maximum oxidation. Finally, the oxidized ESCNFs were �ltered, washed till neutral pH wasachieved and dried overnight.

2.2.3. Decoration of Oxidized ESCNFs with MagneticNanoparticlesMagnetic nanoparticles were synthesized by co-precipitation method. For 1 g of ESCNFs, 1.7 g of(NH4)2Fe(SO4)2.6H2O and 2.5 g of NH4Fe(SO4)2. 12 H2O were dissolved in 200 mL of DI water. Oxidizedcarbon-based material was dispersed in solution via ultrasonic treatment for 10 min. Iron oxide (Fe3O4)nanoparticles were precipitated on surface of carbon-based materials by drop wise addition of 8 mol/Lammonia solution with continuous stirring. The mixture was adjusted to pH 11 and allowed to react for 2hours. The whole experiment was carried out at 50°C in inert argon environment. The product wasseparated with external magnet, washed with DI water several times and dried in vacuum oven at 80°C.The resultant magnetic composite materials were named as M-ESCNFs.

2.3. Sample preparationE waste samples of various categories (Table 4) were collected from local electronic market, brought tothe laboratory, washed with ethanol followed by DI water and dried with hot air blow. As described in theliterature(Forsén and Aromaa, 2013; Schaeffer et al., 2018), the samples was manually dismantled and

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pre-processed for removal and segregation of specialized components. Each separated portion wasshredded to approximately uniform particle size. A pre-weighted portion of each sample was taken inTEFLON beakers. The optimized acid leaching condition (Cheng et al. 2019) was used in this study. 5mol/L HNO3 with 50 mL/g liquid to solid ratio was applied with continuous stirring on 30°C constanttemperature water bath for 1 hour. The process was repeated twice. Leachate was �ltered and heated tillvolume reduced to 10mL. The concentrated leached solution was diluted to known volume with DI water.

2.4. Sorption experimentA known volume of the sample solution was placed in a �ask having a pre weighted composite materialin it. The pH of the solution was adjusted to 2. The sample and composite was mixed mechanically usingwrist action shaker under sustainable conditions. The two phase mixture was brought into contact withexternal magnet and allowed to stand. The liquid phase was isolated and analysed for metallic content.In the next step, the metal loaded sorbent was eluted with selected eluent (10mL of 1 mol/L HCl) andagain isolated with external magnet. The eluent phase was carefully decanted and the concentration ofGa(III), In(III), and Ge(IV) ions were simultaneously analysed using ICP-OES. Sorption capacity (qe) andpercent removal e�ciency (%RE) ware calculated using the following equations 1 and 2(Iqbal et al.2020).

qe = (Ci − Ce)Vm

1

%RE =Ci − Ce

Ci× 100

2Where Ci is the initial target metal ion concentration in solution phase and Ce is the �nal concentrationafter sorption experiment.

3. Result And Discussions

3.1. Characterization of the composite material

3.1.1. SEM Morphology and structural analysisCarbon nano�bers were synthesized in slight variable diameter because of minor variations inelectrospinning parameters during synthesis. Figure 1 shows SEM images of PAN derived ESCNFs aftercarbonization and functionalization with magnetite nanoparticles. It is worth mentioning that the threador �brous structure is retained after carbonization at 800°C. The diameter of carbonized ESCNFs is much

( )

( )

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thinner than that of as synthesized PAN derived electrospun �bers. The diameter is ranging from 500 to850nm approx. Similarly, the apparent colour is changed from white to shiny black. In contrast tostatement reported in the literature(Chen et al. 2007), when the carbonized ESCNFs were oxidized withconcentrated nitric acid, for increase in surface oxygenated functional groups, the �brous morphology isdestroyed leaving carbonized material with no more �brous structure (Fig. 1d). Due to highly chargedsurface under electron beam, it was di�cult to focus at high resolution and target nano-structuredmagnetite particles, however agglomerated magnetite particles up to several micrometre is clearly seen atsome places embedded in ESCNFs (Fig. 1e and f).

3.1.2. FTIR AnalysisThe attachment of magnetite particles to ESCNFs is further con�rmed from FTIR analysis. A large valleyfor magnetic ESCNFs in the range of 3350 cm− 1to 3600 cm− 1 indicates the presence of –OH group dueto the presence of magnetite nanoparticles. The same smaller drift in ESCNFs is attributed to hydroxylgroup of carboxylic acid or aldehydes. The increase in absorption band observe in the range of 1580 cm− 

1 to 1780 cm− 1as compared to non-magnetized ESCNFs is attributed to the attachment of magnetiteparticles to C = O of carboxylic acid and ketonic groups. It further con�rms that the C = O group act asreducing agent and stabilizer for Fe3O4 nanoparticles. A well-de�ned strong absorption band observed

from 580 to 640 cm− 1 corresponds to Fe-O stretching band of magnetite particles.

3.1.3. Thermal AnalysisThermal behaviour of bare and magnetized ESCNFs was studied in oxygen atmosphere. It was observed(Fig. 3) that the thermal decomposition of magnetized ESCNFs takes place at relatively smallertemperature as compared to bare ESCNFs. Similarly, the decomposition take place in smooth way whileincreasing the temperature from 450 to 650°C for ESCNFs leaving less than 5% thermal residue (curve a).On the other hand, curve b, rough pattern of decomposition with approximately 63% thermal residue formagnetized ESCNFs indicate the presence of magnetite particles attached to the surface of ESCNFs.Curve c, indicate the decomposition pattern of magnetized ESCNFs after 6 consecutive adsorptiondesorption cycles. The curve shows that the thermal residue is now only ~ 38% and approximately 25% ofmagnetite particles are washed out while eluting analyte ions.

3.1.4. X Ray Diffraction (XRD) AnalysisCrystal structure of the attached magnetite particles were investigated using X-ray diffraction analysisutilizing Cu Kα radiation with wave length of 1.54A°. For ESCNFs curve (Fig. 4), a strong merged peak at2 =23.9 and 2 =25.4 corresponding to (120) and (002) crystal plane of carbon, respectively re�ectssuccessful carbonization of synthesized PAN based nano�bers. The result is consisted with JCPDSreference No. 500926 and 41-1487. Diffraction peak at 2 =42.9 is labelled to graphitic peak of carbon. Onthe other hand, for M-ESCNFs diffraction peaks at 2 =30.3, 35.6, 43.1, 54, and 57.4, respectivelycorresponds to (220), (311), (400), (422) and (511) crystal plane. These characteristic peaks con�rmpresence of magnetite nanoparticles in composite composition, as these diffraction peaks are indexed tocubic structure of magnetite (Fe3O4) particles (JCPDS 19–0629) (Soloveva et al. 2016).

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3.1.5. Magnetization studyThe attachment of magnetite particles to ESCNFs were further con�rmed from magnetic analysisutilizing vibrating sample magnetometery (VSM) analysis. Figure 5 indicate magnetization hysteresisloop for ESCNFs before and after magnetization. The saturation magnetization (Ms) value increase from0.02 emu/g with almost no hysteresis performed, to S shape hysteresis with high Ms value of 23.6 emu/gbetween applied �eld ranges from + 5500 to -5500 Oe when the material is magnetized. Similarly, thecoercivity (Hc) value is decreased from 105.7 to 40.3 Oe after magnetization. The result con�rms highloading of magnetite particles to ESCNFs surface.

3.2. Optimization study and Data Analysis

3.2.1. Effect of pHThe stable oxidation state of Ga and In is III, while for Ge is IV. That make their ionic radii comparablylarge to their respective group members present above them i.e., Al(III) and Si(IV). Thus higher charge andmoderate radii make them to fall in category of hard acid as described by Pearson HASAB concept (Woodand Samson 2006). Therefore, these metals prefer to form complexes with hard base like OH− and othergroups containing oxygen. Ga, In and Ge metal ions thus strongly attract to oxygen group of magnetiteparticles, carboxylic and ketonic groups of carbon nano�bers. These metals are only freely present intheir respective ionic form at lower pH of 1.5-3.However, above this pH they are present for example asGa(OH)2+, Ga(OH)2

+, Ga(OH)3 and at pH > 7 almost predominantly present as Ga(OH)4−(Wu et al., 2019;

Xiong et al., 2020; Zhang et al., 2011). On the other hand, when pH is too low (pH < 1.5) these metal ionsget complexed with chloride ions or any other anionic part of acid. Likewise, dissolution of magnetiteparticles at pH < 1 also destroy composite structure and cause decrease in sorption of target metal ions(Uheida et al. 2006). Figure 6 shows that the same sorption pattern is followed by Ga, In and Ge metalions when sorbed on magnetic ESCNFs. Sorption increases when pH is increased from 1 to 2 however,decrease suddenly with further increase in pH, because hydrolysis of cationic metal ions decreasesinteraction of sorbent and sorbate surface.

3.2.2. Effect of Sorbent DoseIt has been observed that increase in sorbent material increase the availability of active site thusaccommodate large amount of sorbate. For �x concentration of metal ions increase in sorbent increaseremoval e�ciency but once equilibrium established between the two interacting phases then no moreincrease occurs in the removal e�ciency. Figure 7 indicate that when sorbent dose increased from 20 to200 mg for 5 µg/mL solution of each targeted metals ion, sorption reached to equilibrium at dose of 150mg and no further increase in removal e�ciency was observed.

3.2.3. Effect of Initial Concentration and Isotherm ModelsSorption capacity is an important factor for quantitative extraction of sorbate ions. Figure 8 indicates thatsorption capacity increases for 150 mg of sorbent when sorbate concentration increased from 5 to 80

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µg/mL and remained constant with further increase of metal ions concentration. However, on the otherhand removal e�ciency decreases due to saturation.

Sorption isotherm is an invaluable source for describing retention or mobility of the liquid phase targetedsorbate on to sorbent surface at constant temperature and pH. The homogeneity of sorption process isdescribed by Langmuir sorption isotherm model while Freundlich isothermal model indicateheterogeneous sorption process and not restricted to monolayer sorption (Foo and Hameed 2010). Dataobtained for changing initial concentration is evaluated through the following linear Eq. 3and 5 forLangmuir and Freundlich sorption isotherms, respectively while RL value is calculated through Eq. 4.

Ceqe

=1

qmCe +

1KLqm

3

RL =1

1 + KLCo

4

logqe =1nlogCe + logKf

5Table 1 and Fig. 9a& 9b represents data derived parameters and data set for both isothermal models.Regression coe�cient (R2) indicate that sorption of selected critical metal ions (Ga(III), In(III) and Ge(IV))on M-ESCNFs is homogeneous monolayer sorption following Langmuir sorption isotherm. The RL value(0 < RL>1) indicate favourable sorption process. The slop (1/n) of the Freundlich isotherm was foundbetween 0–1, indicate heterogeneity of sorbent surface and implies chemisorption process. Maximumsorption capacity (Ga(III) = 190.8, In(III) = 171.2 and Ge(IV) = 226.8 mg/g) calculated through Langmuirisotherm was found close to the experimental values and suggests that the proposed sorbent is suitablefor recovery and extraction of target metal ions.

3.2.4. Effect of Contact Time and Sorption KineticsContact time effect on sorption process is evaluated by continuously mixing via mechanical shaking of150 mg of sorbent with 5µg/mL of each selected critical metal ion for a predetermined time period (20–200 min). Figure 10 represent effect of contact time which indicates that removal e�ciency increaseswith increase in contact time until the process attains equilibrium. Percent removal increase e�cientlywith increase in time up to 160 min until all available active sites became occupied and no remarkableincrease observed.

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Time dependent study gives us insight to the sorption mechanism. Kinetic investigation was carried outbased on pseudo 1st and pseudo 2nd order kinetic models using linear equations6 and 7 for both kineticmodels, respectively.

ln qe − qt = lnqe − K1t

6

tqt

=1

K2q2t

+t

qe

7Various parameters calculated are listed in Table 2. While linear �t of data is graphically shown inFig. 11a &11b. Based on regression coe�cient (R2) values, sorption of Ga(III), In(III) and Ge(IV) on M-ESCNFs is best described by pseudo 2nd order kinetic model which veri�es that the rate limiting step ischemisorption.

3.2.5. Effect of Sample VolumeConstant sorbent dose (150 mg) was applied to variable sample volume in the range of 20 to 240 mLhaving 5µg/mL concentration of each metal ion. Figure 12, shows the relation between applied samplevolume and obtained percent removal e�ciency. It indicates that e�ciency of sorbent decreases withincrease in sample volume. This is because the ions get hydrolysed and ionic interaction decreases withsorbent surface. For Ge(IV) and In(III) ions removal e�ciency decrease up to 90 percent when samplevolume reaches 120 mL while for Ga(III) ion the same value reaches at 140mL. Eluting sorbed metal ionswith 10 mL of eluent, preconcentration factor was calculated using the relationship (PF = Vi/Vf×%RE)(Jalilian et al., 2017), and was found 1080 for Ge(IV) and In(III) while 1260 for Ga(III).

3.2.6. Effect of Potentially Interfering IonspH plays an important role in selection of metal sorbent interactions. According to the literature, Cu(II)and Fe(II) are quantitatively extracted at pH = 6 and at strong acidic media (pH = 2) they remain in thesolution (Ahmad et al. 2020). Similarly, initial dedicated sorting for visually identi�ed components wherethe target metal ion is present and magnetic separation during sample preparation also decreasechances of interferences from these metals. Although, at very high concentration if found they stronglydecrease removal e�ciency of the target metal ions. Table 3 shows percent recovery of the target metalions (5 µg/mL each) in the presence of interfering ions. From results, it can be concluded that the percentrecovery is not heavily affected by the presence of interfering species. However, an important group ofelement that may strongly interfere the sorption of targeted metal ions are precious metal (PM) ions i.e.Ag(I), Au(III), Pd(II) and Pt(IV). In another parallel sorption study of PM ions using magnetic MWCNTscomposite (Iqbal et al. 2020) which indicated that these metal ions also sorbed at pH ~ 2. Anyhow, theirspeciation and complex ions formation mechanism make the targeted critical metal ions different from

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PM ions. Also reported in the literature (Haghighi et al. 2018) that elution of these metal ions withspeci�ed complexing reagent, one can achieve the goal of selective extraction.

3.2.7. Desorption and Regeneration of SorbentAt very low pH, the target metal ions interact with anionic component of the acid and thus remain in thesolution. Therefore, various acids like hydrochloric acid, nitric acid and acetic acid have been tested aseluent with variable concentration of 0.5 to 1.0 mol/L. From Fig. 13 it is clear that higher elution isachieved while increasing acid concentration. Similarly, bases like ammonia and sodium hydroxide alsoelute target metal ions with good percent recovery but it destroys composite structure by quickdissolution of magnetite particles. Therefore, 1.0 mol/L hydrochloric acid was selected as suitable eluentwith contact time of 5 min and 10 mL eluent volume.

The ideal sorbent has good regeneration and recycling ability. The synthesized sorbent was tested forsorption desorption cycle under optimized experimental condition for 5µg/mL of Ge(IV). It was found(Fig. 14) that the sorbent material retains its sorption ability up to 5 cycles of continuous sorptiondesorption process and decrease to 85% when subjected to 6th cycle. This most commonly occur due towashing out of magnetite particle and destruction of �brous texture while eluting target metal ion withacidic eluent. The phenomenon is supported by experimental evidence from SEM (Fig. 1d) and thermalanalysis (Fig. 3) where 38% loading of magnetite particles observed after six consecutive cycles ofsorption-desorption.

3.2.8. Analytical Performance of the Developed MSPEAnalytical performance of the developed method reported in statistical terms such as limit of detection(LOD) and relative standard deviation (%RSD) as measurement of precision and accuracy. Where LOD = 3Sb/m was calculated for 11 replicate measurements of the blank solution. Calculation of %RSD is basedupon 11 replicate measurements of 50 ng/mL of selected metal ions solution. For Ga(III) measured at417.206 nm, LOD was found 8.3 ng/mL with 3.7% RSD. Similarly, for Ge(IV) measured at 265.118 nm,LOD is 13.7 ng/mL with 2.6% RSD and In(III) measured at 303.963nm, LOD was found 16.7 ng/mL with3.2% RSD.

3.2.9. Method Validation and Performance for RealSamplesThe method validity and accuracy was tested with a multi-element ICP-OES reference standard (10µg/mL of Al(III), As(III), Ba(II), Cd(II), Cu(II), Mn(II), Pb(II), Se(IV), Zn(II) ions and 100 µg/mL of K(I) ion)spiked with 5 µg/mL of Ga(III), In(III) and Ge(IV) ions (Table 3). Maximum recovery of 96.2% for Ga(III),95.6% for In(III) and 97.4% for Ge(IV) ions indicate that the proposed method have good capacity toextract targeted critical metal ions even in complex matrix. The method was applied to numerous WEEEsamples physically identi�ed for selected critical metal ions traces and coded as S-1 to S-5 (Table 4).Su�cient amount of selected metal ions was extracted applying M-ESCNFs composite as sorbentutilizing optimized extraction parameters. Results summarized in Table 4 have a close agreement to other

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previous work reported in literature (Chen et al. 2018; Mortada et al. 2015; Virolainen 2013; Xiong et al.2020; Yang et al. 2013).

ConclusionThe magnetized sorbent composite was synthesised after PAN based electrospun nano�bers werecarbonized and decorated with magnetite particles through co-precipitation. The sorbent wassuccessfully applied for the recovery of critical metal ions Ga(III), In(III) and Ge(IV) from target WEEEsamples utilizing magnetic nano-hydrometallurgy. Magnetite composite of M-ESCNFs yielded long termstability, high reproducibility and ease of dispersion and recollection. The sorbed target metal ions werecollectively eluted using 1.0 mol/L hydrochloric acid. Maximum sorption capacity calculated throughLangmuir sorption isotherm was 226mg/g,191mg/g and 171 mg/g for Ge(IV), Ga(III) and In(III) metalions, respectively. In addition to physio-sorption on carbon surface pseudo second order kinetics alsocon�rm the chemical/charges interaction between sorbent and sorbate ions. Recoveries in the range of96.2% for Ga(III), 95.6% for In(III) and 97.4% for Ge(IV) in the presence of diverse ions indicate thesuitability of the proposed method for the selected critical metal ions even in complex matrix.

DeclarationsDeclarations of interest

The authors declare that they have no con�ict of interest.

Ethics approval and consent to participate

 Not applicable

Consent for Publication

Yes

Data availability

Not available

Funding

No funding available

Author contribution

Jasmin Shah contributed to the conception and design of the experiment. Aamir Iqbal conducted theexperiments and wrote the text of manuscript. Bahroz Rashid performed the FTIR, TGA, XRD and VSManalysis. M. Rasul Jan involved in the Writing, Review & Editing of the manuscript. All authors reviewedthe manuscript.

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TablesTable 1: Parameter of Langmuir and Freundlich sorption isotherm models for critical metalions (Ga(III), In(III) and Ge(IV) sorption on M-ESCNFs

Metal ions Langmuir model Freundlich Model  Qm(mg/g) KL (L/mg) RL R2 Kf n R2

Ga(III) 190.8 0.471 0.040 0.997 48.72 2.29 0.894In(III) 171.2 0.478 0.039 0.989 41.94 2.27 0.873Ge(IV) 226.8 0.413 0.046 0.990 64.01 2.54 0.969

Table 2:   Parameter of Pseudo first order kinetics and Pseudo second order kinetics modelfor sorption kinetics of critical metal ions (Ga(III), In(III) and Ge(IV)) sorption on M-ESCNFs

Metal ions Pseudo First Order Kinetics Pseudo Second Order Kinetics  qe(mg/g) K1 R2 qe(mg/g) K2 R2

Ga(III) 47.8 0.035 0.827 22.50 0.0006 0.989In(III) 41.7 0.032 0.897 23.91 0.0005 0.983Ge(IV) 32.0 0.032 0.892 19.74 0.0013 0.991

Table 3: Tolerance limit for possible interfering ions on sorption of  critical metal ions(Ga(III),     In(III) and Ge(IV)) sorption on M-ESCNFs

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Interferingspecies/ions

Max. Tolerance limit ([X]/[Ga, In andGe]) 

Removal efficiency (%±Sb)

    Ga In Ge

Cu(II) 110 93±1 94±1 96±2

Fe(II) 130 94±1 94±1 97±1

Ag(I) 30 93±1 96±1 95±2

Al(III) 80 92±1 93±1 93±2

Au(IV) 05 88±1 90±2 90±1

As(III) 70 94±1 92±1 95±2

Pb(II) 140 94±1 93±1 96±2

Pd(II) 10 92±1 92±1 95±2

Pt(IV) 15 91±2 94±1 93±2

Mn(II) 90 94±1 93±2 95±2

Ni(II) 100 89±1 91±1 93±2

Co(II) 55 91±1 91±2 93±1

Cr(III) 60 94±2 93±3 94±1

Cr(VI) 25 94±2 93±3 89±2

Si(IV) 60 92±1 93±1 96±2

Sn(IV) 100 96±1 89±2 90±1

Zn(II) 110 88±3 92±2 93±1

Ca(II) 1000 92±1 95±3 95±2

Mg(II) 1000 91±2 95±1 97±1

Na(I) 5000 89±3 92±2 96±1

K(I) 5000 92±1 93±1 93±2

[X], concentration of interfering ion Sb, Standard deviation

Table 4: Determination of target metal ions in multiple WEEE samples and reference                standard

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Samplereference

Nature of sample/standard Metal ion concentration found±Sb(µg/g)

    Ga(III) In(III) Ge(IV)S-1 Visually identified components from PCBs

(Integrating circuit, LEDs and transistors)17.6±2.6 4.85±1.7 2.94±2.4

S-2 Waste Solar panel 3.27±0.3 3.82±0.5 4.18±4.4S-3 Ge doped optical fi     ber ND ND 1858±12S-4 Waste LCD Panels of mobile smart

phones23.6±2.5 186±7.4 1.85±0.04

S-5 Waste LCD panels of laptop screen    41.4±4.1 202±4.8 3.42±0.8R-1 Multi-element reference standard

(spiked with 5µg/mL of Ga(III), In(III)and Ge(IV))

4.81±0.14 4.78±0.09 4.87±0.11

Figures

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Figure 1

PAN derived ESCNFs at different magni�cation (a,b and c), ESCNFs after oxidation with 30 mL ofHNO3(8%) (d) and magnetized ESCNFs (e and f)

Figure 2

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FTIR spectra of ESCNFs and magnetized ESCNFs

Figure 3

TGA thermograms of ESCNFs and magnetized ESCNFs

Figure 4

Diffraction pattern of ESCNFs and magnetized ESCNFs

Figure 5

VSM magnetization curve ESCNFs and M-ESCNFs

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Figure 6

Effect of pH on sorption of Ga(III), In(III) and Ge(IV) on M-ESCNFs Composite

Figure 7

Effect of Adsorbent Dose on Removal E�ciency of Ga(III), In(III) and Ge(IV)on M-ESCNFs Composite

Figure 8

Effect of initial concentration on sorption capacity and removal e�ciency of Ga(III), In(III) and Ge(IV) onM-ESCNFs composite

Figure 9

Linear form of Langmuir (a) and Freundlich (b) sorption isotherms for sorption of Ga(III), In(III) and Ge(IV)on M-ESCNFs composite

Figure 10

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Effect of contact time on removal e�ciency of Ga(III), In(III) and Ge(IV) on M-ESCNFs composite

Figure 11

Liner regression plot for Pseudo 1st and 2nd order kinetic models representing sorption of Ga(III), In(III)and Ge(IV) on M-ESCNFs

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Figure 12

Effect of sample volume on percent removal e�ciency of Ga(III), In(III) and Ge(IV) on M-ESCNFs

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Figure 13

Selection of eluent with optimised concentration for the elution of Ga(III), In(III) and Ge(IV) adsorbed onM-ESCNFs  

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Figure 14

Number of sorption-desorption cycles effecting quantitative recovery

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