Metal Recovery using Tannin Resin: An
Alternative for the Treatment of Electronic Waste
Josiel Martins Costa*, Amanda Bensi, Gustavo Erdmann Barroso Romani, Hellynson Cássio Lana, Richard Silveira Pereira and Tania Regina Giraldi
Instituto de Ciência e Tecnologia, Universidade Federal de Alfenas (UNIFAL-MG), Brazil
Submission: February 14, 2019;Published: February 27, 2019
*Corresponding author: Josiel Martins Costa, Instituto de Ciência e Tecnologia, Universidade Federal de Alfenas (UNIFAL-MG), Rodovia José Aurélio Vilela, 11999, BR 267, Km 533, 37715-400, Poços de Caldas, MG, Brazil
How to cite this article: Josiel M C, Amanda B, Gustavo E B R, Hellynson C L, Richard S P, et al;. Metal Recovery using Tannin Resin: An Alternative for the Treatment of Electronic Waste. Int J Environ Sci Nat Res. 2019; 17(3): 555967. DOI:10.19080/IJESNR.2019.17.555967.
Electronic wastes are produced from all discarded electronic materials, such as computers, cell phones, printers and televisions. With the great generation of this solid residue, it became necessary to study an environmentally suitable technique, able to recycle the metals generated by it. In this study, black wattle tannin resin was used as a potential alternative for the treatment of this waste due to its metal adsorption capacity. Black wattle resin was synthesized by using different pH values (3, 4, 5 and 6) and particle sizes (80, 100, 140 and 200 mesh) from the standard solution of AgNO3 and solid silver dissolved into HNO3 solution. Absorption technique was used for the quantification of Ag+ from samples. It was observed that pH values closer to neutrality (5 - 6) favor a greater adsorption of Ag+ ion as well as smaller particle sizes (granulometry of 140 - 200 mesh). From the tests of atomic absorption, a high yield of the silver adsorption by the tannin was verified, becoming the resin a viable adsorbent, as much for its efficiency, as for being a vegetal compound which does not promote the degradation of natural resources.
Contemporary society is experiencing great and rapid technological growth. While advances in technology make it possible to improve the medical sciences, high-speed communication, and greater access to information, on the other hand, it generates an immense accumulation of electronic waste, i.e. tablets, smartphones, televisions, among other discarded electronics. The problem generated by the high consumption of those is electronic waste, which contains toxic and polluting substances. When electronic waste is not treated, or it is recycled irregularly, it causes serious damage to the environment and human health. However, there are also metals of economic interest in discarded printed circuit boards (PCB) . The recycling of electronic waste is of fundamental importance for the global ecosystem, due to the large volume generated, and for the economy, since metals of commercial value can be found in plates of cell phones and computers .
An alternative in the techniques of recycling metals is the use of tannin resin [3-6]. Tannins are polyphenols obtained from vegetable matrices, widely used in textile industries and found in many species plant, for example, black acacia (Acacia mearnsii De Wild) [7-10]. Since tannins contain an abundant amount of adjacent hydroxyl groups in their molecules, they form chelates with metals and can be classified as hydrolyzable and condensed [11,12]. These plant species are studied not only in the removal of metals from wastewater but also in the recovery of noble metals, such as gold, based on the complexing and reducing properties of
tannins . From the reaction of this substance with oxalic acid (H2C2O4), formaldehyde (H2CO) and water under heating and reflux, the resin is obtained [8,9]. By synthesizing the resin, it is possible to make metallic ions in solution to be adsorbed on the surface of these resins, thus extracting them from the solution .
The application of low-cost absorbent materials can be advantageous when compared to other traditional processes such as reverse osmosis, solvent extraction, chemical precipitation, filtration, electrode deposition and evaporation . In this context, it is necessary to investigate techniques for the metal recovery that are environmentally suitable and present satisfactory efficiency. For this purpose, the objective of this paper was to quantify the adsorption capacity of the tannin resin using a standard solution of silver ions (Part I) and subsequent use of silver ions solution from solid silver (Part II), in order to compare the results in different pH values and resin granulometry.
30g of tannin was weighed and solubilized in distilled water at 60°C. The mixture was transferred to a round bottom flask where it remained under heating and reflux. 0.5g of oxalic acid (H2C2O4) was added, remaining for 60min under heating and refluxing. It was added over 20mL of formaldehyde (H2CO) and 10mL of saturated
solution of oxalic acid remained for a further 30min under
reflux. After this time, 100mL of heated distilled water was
added to promote separation of the phases. This step remained
for an additional 60min under the above conditions. The product
was cooled, subjected to vacuum filtration, and then transferred
to a clock glass, for oven drying at 40°C for 24h [8,9,15]. Figure 1
shows the final product obtained.
Before the measurements of ions adsorption on the tannin
resin are carried out, silver standard solutions are prepared from
the aqueous silver nitrate solution concentration of 0.1g L-1 (analytical
reagent). In the analyzes performed, it was initially verified
the ability of the resin to absorb silver ions at different pH in order
to obtain the ideal pH for a better absorption. Thus, 4 samples
were used, varying the pH in the values of 3, 4, 5 and 6.
It was added 1g of tannin resin (granulometry obtained in the
100-mesh sieve) in 100mL of 0.1g L-1 AgNO3 solution. The pH was
then adjusted to 3 (adding 0.1M HNO3 solution) and the sample
was placed on the magnetic stirrer for 80min. 15mL of the sample
was removed and transferred to a vessel to be taken in the centrifuge
for 12min at 2500rpm. The supernatant was removed and
transferred to a labeled tube called Sample 1. The same procedure
was performed for Sample 2. For Sample 3 instead of adding 0.1
molar HNO3, 0.5 molar NaOH was added to regulate the pH, since
the pH of the resin with the silver nitrate solution was around 4.
From the analysis at different pH values, it was observed an
ideal pH for the adsorption of silver through the resin. Once the
optimum pH was defined, the other variable studied was the granulometry
of the resins. It used a same pH (the pH that promoted
better silver adsorption by the resin) for the 4 samples and only
changing the granulometry of the tannin resin.
In samples 5, 6 and 7 the same pH range (5.0) was used, since
the literature reports that this value is considered an ideal value
for adsorption of the silver ions in the tannin resin, since acidic
medium and were not beneficial [16,17]. The pH was controlled
by the addition of 0.5 molar NaOH solution. The other procedures
were also performed for samples 5, 6 and 7. The temperature of
the samples was 25.5°C during the experiment. The samples were
analyzed by the atomic absorption technique. The Table 1 containing
the variables analyzed in the experiment. For all samples,
the ratio was 1g of resin to 100mL of 5.88 10-4M AgNO3 solution.
The experiment consisted of using two 100mL beakers with
approximately 70mL of 0.1M AgNO3 solution in each. Several
copper wires were added in the beakers and the deposition of
a dark-looking solid on the wires was noted. This solid was silver
metal, since silver underwent oxidation by gaining electrons
from copper. In this way, copper oxidation occurred, presenting a
blue coloration in the characteristic solution when it is in ionized
solution. The wires remained for 10min in the solution. The wires
were removed and the solid deposited on the wire was scraped
using a spatula. The solid was washed through vacuum filtration
and dried in the oven at 60°C for one h. The reaction 1 occurred:
In order to obtain the ionic silver, HNO3 was used to open the
solid silver and to obtain 1L of 0.1g L-1 AgNO3 solution. Such concentration
is the same throughout the experiment, with the aim
of comparing results. The chemical reactions 2 and 3 describe the
obtaining of solid silver and the recycle of HNO3<./p>
The atomic absorption test was performed according to item
2.2. The 100-mesh sieve was used for the sieving of the resin of
tannin in 100 mL of 0.1 g L-1 AgNO3 solution. The pH of the samples
was adjusted by adding 0.1M HNO3 or 0.5M NaOH solution as
required. Samples were placed on the magnetic stirrer for 80min.
15mL was withdrawn from the sample and transferred to a vessel
to be taken in the centrifuge for 12min at 2500rpm. Subsequently,
the supernatant was removed and transferred to a labeled tube in
order to be observed by the atomic absorption technique. In samples
5, 6 and 7 the same pH range (5.0) was used and according
to item 2.2. The variations in grain size were the same, according
to Table 1. For all samples, the ratio was 1g of resin to 100mL of 5.88 x 10-4sup> molar AgNO3 solution. The samples were analyzed using
atomic absorption spectrometer with flame.
As previously mentioned, to verify the efficiency of the tannin
resin in capturing Ag+ ions of silver aqueous solutions, two variables
were used: the pH of the solution and the grain size of the
resin. Table 2 indicates the results obtained from the studied variables
from part I and II, being the measures of atomic absorption
of the solutions after remaining in contact with the tannin resin.
Since the result is the amount of mg L-1 Ag+ present in the supernatant
liquid, and it being known that the initial Ag+ ion concentration
was 0.1g L-1 i.e. 100mg L-1, one can obtain the amount that
was probably adsorbed by the resin, being subtracted from the
initial concentration by the result of the atomic absorption.
When analyzing the results of the samples with different pH,
it was possible to verify that the pH influences the adsorption of
the ions in the resin. It is observed that acid pH is not favorable for
the adsorption process, since the concentration of Ag+ at pH 3 is
34.8mg L-1. This indicates that in this condition, 65.2mg of Ag+ was
adsorbed by the resin, while at higher pH values, as in the case of
the pH 6 solution, the amount of Ag+ adsorbed by the resin was
79.9mg. At low pH levels, competition between the H+ ions and
the metal ions (Ag+) of the resin may occur, making it difficult to
adsorb . In the case of pH higher than 6, the Ag+ ions precipitate
and the phenolic groups of the tannin resin would be oxidized
[10,18], that is, basic pH is also not favorable for the adsorption
With these results, it was possible to obtain a graph (Figure 2)
that relates the pH of the solution to the amount of Ag+ adsorbed
by the resin. In this way, pH 5 was used as the reference for studies
of the influence of grain size, since the literature reports that it is
an ideal pH range . In this work, it was verified that solutions
with pH 6 presented better adsorption results of Ag+ ions. However,
with the experimental conditions, it is not possible to affirm
that this is a significant difference in relation to what the literature
reports, that is, better results in solutions with pH 5. As previously
mentioned, in addition to the assays at different pH, assays with
different particle sizes were carried out. The use of pH 5 solutions
for the four samples was standardized in these cases.
Comparing the adsorption result of Ag+ from part I to part II,
at different pH values, it was verified that there is a certain relation,
since the pH of greater adsorption in part I was also the one
with higher value in part II. However, the adsorption values of part
II are higher. This may have occurred because the Ag+ ions were
not adsorbed totally on the tannin resin, since the adsorption is an
estimate, and Ag+ ions can react with other compounds that were
present in the solution, as for example the resin itself, the acid and
the base that were used to regulate the pH of the solution. In this
second part, the sample that presented the highest adsorption by
analyzing the pH was sample 4, which has pH 6, with adsorption
in the resin of 87.6mgL-1 of Ag+, being this pH value with the highest
adsorption also in the result of part I. In this way it is not possible
to describe again a significant difference, since experimental
errors must be considered.
From the Table 2, a graph (Figure 3) with the behavior of the
Ag+ ion adsorption was obtained as a function of the granulometry
of the tannin resin. It can be seen in Figure 3 that the resin of 80
mesh granulometry provided a lower Ag+ adsorption. This fact can
be explained due to the contact surface of the particles. It is noteworthy
that in the 80 mesh sieve the particles were larger than in
the other sieves. The size of the grains of the phenolic resins is also
of great relevance, and according to the results obtained, it was noticed that smaller particles adsorb greater amounts of ions of
the metal. This is due to smaller particles have larger surface area,
and this favors the adsorption of a larger quantity of ions .
The amount of Ag+ adsorbed on the resins obtained with the other
sieves practically remained constant, with little oscillation of the
adsorbed amounts. The small decrease in the sieve from 140 to
200 mesh may have occurred due to experimental errors during
the execution of the experiment. Again, the adsorption values are
higher than in part I, even though the same concentration of Ag+
is in solution. This may have occurred according to the previously
described factors, noting that there was an attempt to perform
part II equal to part I, however experimental errors may have occurred
which affect the results. A literature divergence, which also
occurred in part I, in which a larger particle (80 mesh) has higher
adsorption (83.3mg L-1 Ag+) than a smaller particle (100 mesh)
has a larger contact surface (83.0mg L-1 of Ag+). This divergence
is not very significant and is worth the same justification of the
It was possible to synthesize the tannin resin from the powder.
Tests were performed with the resin at different pH and granulometry.
For pH values the adsorption efficiency was not efficient
as well as for larger particle size. Already for the pH value equal
to 5 - 6 and granulometry equal to 140 - 200 mesh as adsorption
efficiencies were high, due to the equilibrium between H+, OH- and
Ag+ ions and higher surface area. Thus, black wattle tannin resin
as adsorbent of metallic ions from printed circuit boards (PCB)
presents a viable option, since it is a vegetal compound and not
toxic to the environment.