Recovery and separation of precious metals using waste paper

Article abstract of DOI:10.1246/cl.2007.1254

An adsorption gel was prepared from waste paper by immobilizing iminodiacetic acid so as to investigate its adsorption behavior for various metal ions from chloride media. The gel was found to be selective for Au ^III and Pd^II over Pt

Full text of DOI:10.1246/cl.2007.1254

1254
Chemistry Letters Vol.36, No.10 (2007)
Recovery and Separation of Precious Metals Using Waste Paper
Chaitanya Raj Adhikari, Durga Parajuli, Hidetaka Kawakita, Rumi Chand, Katsutoshi Inoue,Ã and Keisuke Ohto
Department of Applied Chemistry, Saga University, 1-Honjo, Saga 840-8502
(Received July 4, 2007; CL-070712; E-mail: inoue@elechem.chem.saga-u.ac.jp)
and the recovery of precious metals from various secondary
resources, we have investigated the selective recovery of
precious metals using chemically modified waste paper.
The detailed process for the preparation of the gel has been
published elsewhere5 and can be summarized as follows: waste
newsprint paper was pretreated with a concentrated solution
(20%) of NaOH for a few hours followed by washing and drying.
It was further treated by chlorination with thionyl chloride in the
presence of pyridine. At the same time, iminodiacetic acid was
converted to diethyl iminodiacetate by treatment with ethanol
saturated with HCl gas. The reaction between the chlorinated
waste paper and diethyl iminodiacetate in acetonitrile in the
presence of K2CO3 and subsequent hydrolysis of the product
with 1 M (M = mol dmÀ3) NaOH in ethanol and 0.1 M HCl in
turn yielded the final product.
For the adsorption study, test solutions of various metal ions
were prepared by using analytical grade salts or acids of the
respective metals. To study the adsorption behavior of waste
paper gel batchwise, 15 mL of 0.2 mM of different metal chlo-
ride solutions at varying hydrochloric acid concentrations was
individually mixed together with 20 mg of the adsorption gel fol-
lowed by continuous shaking for 24 h at 30 ꢀC in a thermostatic
shaker. For the isotherm study, the concentration of the metal ion
was varied while keeping the gel weight and solution volume
constant. The concentrations of the metal ions before and after
adsorption were measured by using a Shimadzu model ICPS-
8100 ICP/AES spectrometer. The X-ray diffraction spectrum
was recorded using a Rigaku RINT-8829 X-ray diffractometer
while the SEM images were recorded using a JEOL model
JSM 5200 scanning microscope under an acceleration voltage
of 15 kV.
An adsorption gel was prepared from waste paper by immo-
bilizing iminodiacetic acid so as to investigate its adsorption be-
havior for various metal ions from chloride media. The gel was
found to be selective for AuIII and PdII over PtIV and other base
metal ions including CuII, FeIII, NiII and ZnII at varying concen-
trations of hydrochloric acid. Moreover, AuIII was reduced by
the gel giving rise to clearly recognizable beautiful elemental
gold particles which was confirmed by means of the XRD-spec-
trum and SEM-images of the adsorbent after adsorption. This
result has increased the prospect for mutual separation of AuIII
and PdII from coexisting PtIV and base metals.
The demand for precious metals like gold, palladium,
and platinum is ever increasing because of their newer and
broader applications in various sectors such as manufacturing
of electronic and electrical devices, automobile catalysts, and bi-
ochemical applications, i.e., pharmaceuticals, which have led to
the increase of the price of precious metals over the past years. In
addition, diminishing quality of the available mined ores and the
environmental degradation associated with mining have consid-
erably raised their production cost. Under these circumstances, a
world-wide interest in their recovery from secondary resources
has emerged.1 From the viewpoint of sustainable development,
this should be taken as a positive note as it will help to reduce
the alarming level of electrical and electronic wastes in the de-
veloped countries produced as a result of the cutthroat competi-
tion in the market of PCs, mobile telephones, and entertainment
electronics.
Various processes are in practice currently, and many alter-
natives have been reported in the literature for the effective and
efficient recovery of precious metals.2 Generally precious metals
contained in anode slimes generated during electrorefining of
nonferrous metals are totally dissolved in hydrochloric acid con-
taining chlorine gas to obtain a concentrated chloride solution,
from which each precious metal is separated and recovered by
means of solvent extraction and ion exchange.3 However, vari-
ous drawbacks such as slow kinetics, high cost, and the release
of environmentally unacceptable chemicals in waste streams
are associated with the solvent extraction technique. On the oth-
er hand, the application of commercially available ion-exchange
resins and chelating resins is limited owing to their nonselective
nature and low uptake capacity for precious metals.
The adsorption behavior of iminodiacetic acid type of
modified waste paper gel for a number of base and precious
metal ions in hydrochloric acid was studied batchwise. As shown
in Figure 1, the waste paper gel was found to be selective only
for AuIII and PdII, which suggests that the gel not only
100
AuIII
CuII
FeIII
80
60
NIII
In this context, we have investigated the utilization of waste
biomasses as sorption active materials for the recovery of valua-
ble metals from waste water. In our previous study, a recovery
process for gold from chloride media using an adsorption gel
generated from wood, lignophenol, as well as persimmon peel
gel has been investigated.4
It is evident that tons of waste paper is being generated
in various forms everyday. Unlike pure cellulose, amorphous
matrix of paper makes it easier for chemical modification. In this
paper, from the viewpoints of the effective use of waste paper
40
PdII
PtIV
ZnII
20
0
0
1
2
3
4
[HCl] / mol.dm-3
Figure 1. Adsorption of different metal ions on waste paper gel as a
function of hydrochloric acid concentration.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.10 (2007)
1255
Table 1. Maximum adsorption capacities of waste paper gel for AuIII and
4
PdII at different concentrations of hydrochloric acid
3.30
[HCl] (molÁmÀ3
AuIII (molÁkgÀ1
)
)
1
2
3
4
3
3.30
1.42
3.09
1.25
3.06
1.12
2.98
0.95
PdII (molÁkgÀ1
)
2
1.42
(a)
1
0
AuIII
PdII
Gold Aggregates
0
1
2
3
4
5
6
Ce / mmol.dm-3
Figure 2. The adsorption isotherms of AuIII and PdII ions on the waste
paper gel. [HCl] 1 M, weight of adsorbent 20 mg, volume of the solution
15 mL, shaking time 60 h, temperature 30 ꢀC.
discriminates against PtIV but is also inert toward base metals
such as FeIII, CuII, ZnII, and NiII which are major contaminants
of gold and palladium. Taking account of the fact that most of
these metal ions coexist in various industrial waste solutions
and leach liquors, the result is promising for the selective recov-
ery of AuIII and PdII from any base metal and most interestingly
even from PtIV.
(b)
Gold
Aggregates
Since the gel has exhibited strong adsorption only for AuIII
and PdII, the adsorption isotherm study was carried out for these
two metal ions. As is evident from Figure 2, the gel exhibits a
remarkably high adsorption capacity of 3.3 and 1.42 mol kgÀ1
dry weight of the gel for AuIII and PdII, respectively, in 1 M
hydrochloric acid medium. This difference in capacity for PdII
and AuIII is ascribed to the reduction of the AuIII ion to the
elemental form during adsorption as will be described in detail
subsequently.
Figure 3. (a) SEM image of the waste paper gel after the adsorption of
AuIII at 100Â magnification. Acceleration voltage: 15 kV. (b) Photograph
of gold aggregates floating on the surface of HCl solution after adsorption.
and denser with time and fell down to the bottom of the solution
in a few hours. These results prove that gold particles formed
by the reduction show a tendency to detach from the gel surface
and form clean aggregates. This kind of observation was also
found in our previous study.3
High selectivity of the waste paper gel for AuIII and PdII and
subsequent reduction of the AuIII ion to the metallic form leading
to its physical separation from the gel matrix has given a new
insight not only into the recovery and isolation of precious
metals but has also demonstrated potential application in the
mutual separation of precious metals.
In addition, considering the practical importance of the
selective adsorption of precious metals in stronger acidic medi-
um, it was preferred to observe the loading capacity of the waste
paper gel at higher concentrations of hydrochloric acid. Adsorp-
tion isotherm study for AuIII and PdII was carried out also at 2, 3,
and 4 M hydrochloric acid media, and the respective values are
given in Table 1. Regardless of the acid concentration, compara-
ble maximum loading capacities were observed for AuIII, and a
slightly decreasing order was found for PdII. This result can be
correlated well with that of Figure 1 where the adsorption
percentage of AuIII at higher acidic medium is only slightly low-
er than at 1 M and the values are in decreasing trend for PdII.
Nevertheless, the stable performance of the gel with comparably
high adsorption capacities at any concentrations of hydrochloric
acid provides strong advantage for the real life application.
In order to confirm the reduction of AuIII to Au0, the XRD
spectrum of the gel was taken after the adsorption of AuIII. Sharp
peaks at 2ꢀ values of 38.24, 44.46, 64.6, and 77.68 degrees were
observed which belong to the metallic gold. The observation
suggests the reduction of the AuIII ion to the elemental form
during adsorption. This fact was further reinforced from the
SEM image of the gel taken after the adsorption of AuIII as
shown in Figure 3a in which very fine aggregated particles of
gold (gray complexion) with various shapes are distinct. Also,
Figure 3b shows a photograph of the sample solution and the
gel in a glass vessel after shaking, from which the aggregated
gold particles floating on the surface of the HCl solution can
be seen. It was also observed that these aggregates became larger
This study was financially supported by the Industrial Tech-
nology Research Grant Program in 2006 (No. KS18000002)
from the New Energy and Industrial Technology Development
Organization (NEDO), Japan.
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Published on the web (Advance View) September 22, 2007; doi:10.1246/cl.2007.1254