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Technical note

Magnetic nanohydrometallurgy: A promising nanotechnological approach for metal

production and recovery using functionalized superparamagnetic nanoparticles

Ulisses Condomitti, Andre Zuin, Alceu T. Silveira, Koiti Araki, Henrique E. Toma

Instituto de Quimica, Universidade de Sao Paulo, So Paulo, SP, Brazil

a b s t r a c ta r t i c l e i n f o

Article history:

Received 8 May 2012

Received in revised form 1 June 2012

Accepted 7 June 2012

Available online 16 June 2012

Keywords:

Magnetic nanohydrometallurgy

Superparamagnetic nanoparticles

Copper electrodeposition

We report on a new, promising nanotechnological approach for hydrometallurgy based on recyclable, chem-

ically functionalized superparamagnetic nanoparticles. In this process, the metal ions (e.g. Cu 2+) are cap-

tured by the nanoparticles and conned at the electrode surface by means of an external magnet. Due to

the pre-concentration effect the electrodeposition process is greatly improved, yielding the pure metal in a

much shorter time in comparison with the conventional electrodeposition process. After the electrolysis,

the magnetic nanoparticles are ready to return to the process. The proposed strategy can advantageously

be incorporated in hydrometallurgy, reducing the number of steps associated with complexation, organic sol-

vent extraction, metal release and diffusional electroprocessing, leading to a more sustainable technology.

2012 Elsevier B.V. All rights reserved.

1. Introduction

In the past two decades the global production of strategic metals

such as copper has trended away from pyrometallurgical processes,

in part because of the rich ores depletion and global warming con-cerns. New initiatives have been directed by legislation towards the

supposedly greener hydrometallurgical technologies. In particular,

heap leaching of oxide copper ores and cathode copper recovery by

solvent extraction (SX) and electrowinning (EW) is well established

as a primary low-cost hydrometallurgical copper recovery method.

Presently, more than 20%of total world production of copper is achieved

through the solvent extraction route (Rotuska and Chmielewski, 2008).

The process involves the lixiviation of the metal ions from the ores

after their treatment with acids or microorganisms (Petersen, 2010),

followed by their complexation with a selective organic reagent, and

by a sequence of extraction steps using organic solvents (Gouvea

and Morais, 2010; Habashi, 2009; Scott et al., 1997; Veit et al., 2006 ).

The extracted metal complexes are decomposed with acids and trans-

ferred into the aqueous phase. After adjusting the conditions, themetal ions are deposited electrochemically. The overall process in-

volves a large number of steps. Nowadays, in addition to the metallif-

erous mineral industry, SX-EW hydrometallurgy has also been used in

the recovery of copper from electric/electronic wastes, including the

printed circuits boards (Dudek and Fedkiw, 1999; Dutra et al., 2008;

Fornari and Abbruzzese, 1999; Lemos et al., 2006; Panda and Das,

2001). Considering the increasing world demand of copper, any im-

provement in the production process becomes highly relevant for

the economy and sustainability.

It has already been mentioned that hydrometallurgy has typically

an interdisciplinary nature (Han, 2003). Along this line, in this work,

we are introducing functionalized superparamagnetic nanoparticles in

the hydrometallurgy process, not only for capturing and transporting

metal ions, but also to promote their connement at the electrode sur-face, and to perform their direct electrochemical deposition, in a single

step. The economy of process, chemicals, energy and solvents are im-

portant green features of this process, here denoted as magnetic

nanohydrometallurgy.

We are employing nanoparticles composed by magnetite. They are

exceptional magnetic carriers exhibiting a very large magnetization re-

sponse as a consequence of the predominance of single magnetic do-

mains, in addition to a very large collective area to interact with the

chemical species in solution. Many interesting bioelectroanalytical and

catalytical applications have already been developed using magnetically

modied enzymes and substrates (Castilho et al., 2011; Hirsch et al.,

2000; Jeong et al., 2007; Katz et al., 2005; Latham and Williams, 2008;

Laurent et al., 2008; Liebana et al., 2009; Netto et al., 2011). By using an

external commercial miniature magnet (e.g. Nd2Fe14B, 1 cm, 11 kOe,from MagTek), the superparamagnetic nanoparticles can be readily con-

centrated at the electrode surface. Normally, by using directly the mag-

netic nanoparticles in electrochemistry, the observed result would be

the blockage of the electrode, since the electrochemical response

would be precluded by the local lm formed by the nanoparticles. How-

ever, we have shown that when the superparamagnetic nanoparticles

incorporate electroactive species, the lms become rather conducting,

exhibiting, in contrast, an enhanced electrochemical response due to

the pre-concentration effect (Condomitti et al., 2011a,b,c). This is one

of the most important aspects associated with the present work.

As a proof of concept, we employed the nanomagnetic hydromet-

allurgy process for the capture and electrodeposition of copper from

Hydrometallurgy 125126 (2012) 148151

Corresponding author. Tel.: +55 11 3091 3887.

E-mail address:henetoma@iq.usp.br(H.E. Toma).

0304-386X/$ see front matter 2012 Elsevier B.V. All rights reserved.

doi:10.1016/j.hydromet.2012.06.005

Contents lists available at SciVerse ScienceDirect

Hydrometallurgy

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h y d r o m e t

http://dx.doi.org/10.1016/j.hydromet.2012.06.005http://dx.doi.org/10.1016/j.hydromet.2012.06.005http://dx.doi.org/10.1016/j.hydromet.2012.06.005http://dx.doi.org/10.1016/j.hydromet.2012.06.005http://dx.doi.org/10.1016/j.hydromet.2012.06.005mailto:henetoma@iq.usp.brhttp://dx.doi.org/10.1016/j.hydromet.2012.06.005http://www.sciencedirect.com/science/journal/0304386Xhttp://www.sciencedirect.com/science/journal/0304386Xhttp://dx.doi.org/10.1016/j.hydromet.2012.06.005mailto:henetoma@iq.usp.brhttp://dx.doi.org/10.1016/j.hydromet.2012.06.005

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aqueous solution. The quantitative recovery of the metal element

was conrmed by parallel energy dispersive X-ray uorescence mea-

surements (EDXRF) from the copper solutions and from the super-

paramagnetic nanoparticles collected at the electrode surface.

2. Methodology

The superparamagnetic nanoparticles of magnetite were obtained

by the aqueous co-precipitation method, and coated with SiO2 (Kimet al., 2003; Sen and Bruce, 2005; Yamaura et al., 2004 ). After this, the

nanoparticles were functionalized with ethylenediaminepropylsilane

or EAPS (PIERCE). Typically, 500 mg sample of silica coated magnetic

nanoparticles were dispersed in 50 mL of tetrahydrofuran and after

5 min of sonication, 500 L of EAPS and 1 mL of water were added.

The system was stirred for 1 day and the material was magnetically

separated and dried under vacuum. The solvent was recycled. This

chemical coating was specially employed because of the well known

facility of ethylenediamine to bind copper(II) ions, forming stable

chelate complexes, as shown in Fig. 1. Such functionalized nanoparticles

were here denoted MagNP/EAPS.

Their superparamagnetic behaviour was conrmed by magnetiza-

tion experiments and relatedeld cooling (FC) and zero eld cooling

(ZFC) measurements using a Cryogenic Sx600 superconducting quan-tum interference device (SQUID) based magnetometer. Typically, no

hysteresis was observed above 115 K, corresponding to the blocking

temperature. At room temperature, a saturation magnetization of

49 emu g1 was determined from the extrapolation to very high

magneticelds. The starting superparamagnetic nanoparticles were

also monitored by dynamic light scattering and SEM, exhibiting an

average size distribution of 20 nm. The modied nanoparticles were

stored dry, and the solutions were always freshly prepared by

redispersing the solids in water containing the analytes, using an

ultrasonic bath for 30 min. In the dry form, the functionalized

nanoparticles are mainly in the form of aggregates. After mild sonica-

tion, stable colloidal solutions of nanoparticle clusters of about

80100 nm can be obtained. Such colloidal solutions are suited for

chemical purposes, since they respond more rapidly to the appliedmagneticelds, facilitating the transport and deposition of the nano-

particle clusters.

For academic purposes, the electrochemical experiments were

carried out using an AUTOLAB PGSTAT30 potentiostat/galvanostat,

with a rectangular electrochemical cell (Fig. 2) containing, in addition to

a platinum wire counter electrode, a Luggin capillary with the Ag/AgCl

(1 mol dm3 KCl) reference electrode and a 3 mm diameter gold

disc working electrode. This electrode was placed in contact with the

cell window, using a convenient experimental setup for the application

of an external magnetic eld as shown in Fig. 2. Then, a precise amount

(e.g. 5 mg) of nanoparticles was suspended into 20 mL of 0.1 mol dm3

KNO3solution containing 2.0 103 mol dm3 of Cu2+ ions for 5 min

and transferred to the electrochemical cell. By applying an externalmagnetic eld at the back side of the gold working electrode, the

superparamagnetic nanoparticles were attracted, forming a visible

coating on the electrodesurface. Thisprocess was essentially quantita-

tive, since after the transfer there was no evidence of nanoparticles in

solution.

Energy dispersive X-ray uorescence measurements, EDXRF, were

carried out using an EDX720 instrument from Shimadzu, equipped

with a X-ray tube with Rh target and a Si(Li) detector, working at

515 kV.

3. Results and discussion

The superparamagnetic nanoparticles containing the electroactive

species forma redox conductinglayer, allowing efcient electron transferto the electrode, as shown by the square wave voltammograms in Fig. 2

The conditions employed were frequency=20 Hz, amplitude=50 mV,

andEstep=5 mV. There is a sharp enhancement of the signals at 0.3 V

due to the high local concentration of the Cu2+ ions. The intensities

followed a linear with respect to the scan rates. The observed

voltammograms exhibited a prole coherent with a diffusion con-

trolled process for a nanoparticle lm thicker than the diffusion length

of the electroactive species. A similar behaviour has also beenobserved

for the modied electrodes coated with a layer of magnetic, or den-

drimeric nanoparticles encompassing a large number of ferroceny

groups, at the external surface (Qiu et al., 2009; Wang et al., 2009)

The deposition of copper at the electrode was conrmed by magneti-

cally removing the particles from the electrode, and scanning the

potential in the anodic direction, as in a typical stripping analysis

Fig. 2. Square wave voltammograms (150 mV s1) of MagNP/EAPS (a) magnetically

conned at the gold disc electrode after their interaction with Cu2+ ions (2.0

103 mol dm3) and (b) the corresponding reverse stripping analysis, conrming

the electrodeposition of the metal.

Fig. 1.Representation of MagNP/EAPS interacting with copper(II) ions.

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(Fig. 2B). The observed peak at 0.5 V reects the presence of copper

metal at the electrode surface.

For comparison purposes, the superparamagnetic nanoparticles

containing copper(II) ions were removed from the solution, and the

dry powder analysed using EDXRF, as shown in Fig. 3. Afterwards

they were transferred to the electrochemical cell containing the

supporting electrolyte solution, and the electrolysis was performed

under magnetically conned conditions. After the electrolysis, the

EDXRF analysis of the gold electrode exhibited the characteristic signalsof the copper metal deposited on the surface (Fig. 3), while the initial

copper content from the nanoparticles practically disappeared, cor-

roborating its complete transference from the magnetic nanoparticles.

In addition, the agreement between the copper content estimated

from the electroanalytical data and EDXRF was also better than 95%.

3.1. Simulating the hydrometallurgy process in the laboratory

In a typical laboratory experiment we employeda more concentrated

copper solution, as illustrated in Fig. 4, starting from 200 mL of

2.0102 mol dm3 Cu2+ and treating with 700 mg of MagNP/EAPS,

under stirring, at pH 7. After 10 min the nanoparticles were conned at

the electrode surface with a magnet and the amount of copper in solu-

tion wasprobed by EDXRF.The amount of MagNP/EAPS wasjust enough

to form a compact magnetic coating on the copper electrode (12 cm 2).

Theinitial amounts of copperin solution were in large excess in relation

to the magnetic nanoparticles. In this way the MagNP/EAPS became

rapidly saturated with copper(II) ions (encompassing about 8% from

the global mass), and were magnetically conned at the electrode in

order to perform the metal electrodeposition. The electrodes were con-

nectedto a sourcepower, and1.2 V potentialand 0.5 A current were ap-

plied forabout 3 min (Fig. 4a). According to the voltammograms shown

inFig. 2, the electrochemical process involved is rather fast and all the

conned metal ions are deposited in a single step, as in a reverse

stripping analysis. After this step, the nanoparticles were released

again from the electrodes, and allowed to interact with the remaining

copper(II) ions in solution. The process is also rather fast and the con-

nement canbe repeated several times.The process was carefully mon-

itored after each step by EDXRF (Fig. 4c). We have shown that it isessentially quantitative, and can proceed, if necessary, up to the com-

plete depletion of copper ions from the solution, as shown in Fig. 4b.

Comparatively, under identical conditions, electrodeposition proceeds

very slowly even under stirring, because of the large volume involved.

On the other hand, in typical copper hydrometallurgy plants the elec-

trodeposition times are measured in days, using extremely concentrat-

ed solutions of copper(II) ions and very large electrodes. Therefore, the

great advantages of our process are associated with the fact that the

nanoparticles are very effective carriers and good electrochemical

mediators. In addition, they can be magnetically manipulated and

recycled, being effective even at highly diluted solutions, where con-

ventional hydrometallurgy is no longer feasible.

These experiments were here reported as a proof of concept. Our

current research is focusing on the process optimization, pursuing

Fig. 4.Starting Cu2+ solution(2.0102 mol dm3) with the MagNP/EAPSmagnetically

conned at the copper electrode at the bottom (a), and after 7 successive capture/

electrolytical cycles (b), as monitored by EDXRF, (c) showing the gradual decay (ag)

and the depletion (h) of the metal ions from the solution.

Fig. 3.EDXRF analysis of copper(II) containing MagNP/EAPS, after removing from the

electrolyte solutions (top) and of the gold electrode surface (bottom) before (a) and

after performing the magnetic conned electrolysis (b).

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new functional groups for many different metals, and a better cell

design.

4. Conclusions

Functionalized superparamagnetic nanoparticles can be employed

for the capture, transport and electrodeposition of copper from aqueous

solution, leading to a strong enhancement of the electrochemical sig-

nals due the magnetic con

nement at the electrode surface. Based ontheir versatile performance, a promising alternative approach for the

conventional hydrometallurgy process can be devised, allowing metal

production under competitive and sustainable conditions provided by

the possibility of recovering and recycling the superparamagnetic

nanoparticles. Thanks to the pre-concentration effect, another impor-

tant application will be the commercial exploitation of diluted solu-

tions, and the selective metal removal for environmental remediation

purposes.

Acknowledgements

The nancial support from FAPESP, CNPq and PETROBRAS is grate-

fully acknowledged.

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