DTDGA-Impregnated XAD-16 Beads for Separation of Gold from

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DTDGA Impregnated XAD-16 Beads for Separation of Gold from Electronic Waste Solutions Anant Babasaheb Kanagare, Krishan Kant Singh, Manmohan Kumar, manoj yadav, Ritesh Ruhela, Ajoy K. Singh, Asheesh Kumar, and Vaishali Sanjay Shinde Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03350 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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Industrial & Engineering Chemistry Research

DTDGA Impregnated XAD-16 Beads for Separation of Gold from Electronic Waste a,c

Anant B. Kanagare, aK.K. Singh, a*M. Kumar, bM. Yadav, b*R. Ruhela, bA.K. Singh, cA.

Kumar, dV. S. Shinde. a

Radiation and Photochemistry Division, bMaterials Processing Division, cChemistry Division,

Bhabha Atomic Research Centre, Trombay, Mumbai-400 08, India d

Savitribai Phule Pune University, Pune-411007, India

*Author to whom any correspondence should be addressed Dr. Manmohan Kumar E-mail: [email protected] Dr. Ritesh Ruhela E-mail: [email protected] Phone: (+) 91-22-25593994, (+) 91-22-25592605, Fax: (+) 91-22-25505151

Abstract DTDGA extractant-impregnated XAD-16 polymeric beads (DTGA–XAD16) were synthesized and evaluated for separation of gold from electronic waste solutions. The batch sorption studies were carried out, to understand the effect of various physical parameters onto the recovery of gold from aqueous medium. These synthesized beads were characterized by various techniques viz. FT-IR, optical microscopy, SEM and TGA analysis to gain insight into the composition and morphology of the beads. The kinetics measurement showed that about 180 min of equilibration time was enough to remove saturation amount of gold from the solution. Further, various kinetic modelling analysis of the extraction results was carried out using pseudo-first-order, pseudo-

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second-order and intra-particle diffusion equations, and the corresponding rate constants were determined. The maximum experimental sorption capacity of the beads is found to be ~35mg g-1. The equilibrium sorption data were fitted into different isotherm models and is found to be represented well by the Langmuir sorption isotherm equation. Stripping of the sorbed metal from the beads can be easily achieved by using 0.01 M thiourea in 0.1 M HCl. Reusability of the beads was also established by multiple sorption - desorption experiments. The synthesized beads have shown highly selective extraction of gold from simulated electronic waste. Keywords: Gold, XAD-16, Dithiodiglycolamide, DTDGA, Sorption.

1.

Introduction Gold is one of the noble metals, which is well known even from ancient times for its different important applications such as coinage, ornaments, gilding, metallurgy, textile industry, and in various biomedical fields, like dental gold etc. [1]. It also has several, not so ancient, industrial applications like electroplating for house hold and decorative purposes, wear resistant contacts and printed circuit boards in electronics, radiation resistant coatings, communication systems, satellite, etc. Suitable methods for recycling or refining of such precious metals are essential. Loss of gold in the effluents generated by industries needs attention, because, although the effluents contain gold in low concentrations, the net value of gold lost is significant, if large volumes of effluents generated are taken into consideration. Although gold poisoning is not very common, the toxicity of gold to humans has been reported [2]. The declining resources of gold, increasing demand and high cost, are some of the factors

responsible for development of new sorbents for its recovery from waste materials and industrial


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effluents containing even low concentrations of gold. It may be mentioned that the gold plating or jewellery manufacturing industries, in developing countries, operate on a small scale and therefore, the amount of effluent generated is not very high. Under these circumstances, the methods commonly used in industrialized countries for treating large volumes of effluents may become economically unattractive [3]. Clearly, there is a need for the development of suitable technologies of gold recovery from waste solutions, which may be selective, efficient and economical even at a smaller scale. The recovery of gold from waste solution is usually carried out by conventional methods such as precipitation [4], solvent extraction with carbitol [5, 8], adsorption [9] and ion exchange [10]. Many solid sorbents have been proposed for gold recovery, including activated carbon, persimmon tannin gel, neem leaf broth, tannin, fungal biomass, ion-exchange resins, etc [11]. Several authors have reported adsorption of gold using ion exchange resins such as Lewatit MP64 [12] and chitosan resin [13]. The main advantage of adsorption using solid sorbents over other techniques is that it can be employed even for dilute aqueous waste solutions [14]. In addition, high selectivity, less sludge volume produced, ease of regeneration, ability to meet strict discharge specifications, etc., are some of the other benefits. Several studies have been carried out on the adsorption of gold using Amberlite XAD 2000 [15], XAD-4, XAD-7, and XAD-8 [16, 17] based resins. The novel idea of impregnating selective liquid ligand in a suitable porous polymeric support provides environment friendly means of separating metals [18]. Recently, we have reported the synthesis of a similar novel composite sorbent resin, for recovery of palladium from acidic medium [19]. We hypothesised that a soft donor ligand like N,N,N’,N’-tetra-(2ethylhexyl)-dithiodiglycolamide (DTDGA) can also be used effectively for uptake of gold (soft metal) from acidic medium.



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In the present study DTDGA extractant-impregnated XAD-16 polymeric beads (DTGA– XAD16) were synthesized by wet chemical method and used for the extraction of gold from aqueous waste. Further characterization of the sorbent beads and effect of various physical parameters on the sorption of gold were also studied. 2. Experimental 2.1. Materials Amberlite XAD-16 resin beads were procured from local market. DTDGA extractant was synthesized by the condensation reaction of potassium salt of ethane-1, 2-dithiol with N,N-bis-(2ethylhexyl)-2-chloroacetamide according to the procedure described earlier [19]. The purity of the product was ~ 99 % and the yield of the reaction was ~ 95%. Auric acid (HAuCl4xH2O), obtained from Spectrochem, was used as received. All the other chemicals used were of GR grade, procured from the local market. The aqueous solutions were prepared, using water obtained from Millipore-Q water purification system, with conductivity of 0.6 µS cm-1 or lower. A standard solution of gold [1000 ppm (w/v)] was prepared as a stock solution, and diluted to the required concentrations, for the different extraction experiments. A simulated electronic waste (SEW) solution containing different metal ions, as given in Table 1, was also used for the extraction study. Table: 1 Composition of simulated electronic waste (SEW) Acidity: 3.0 mol L-1 HCl. Constituent Element Cr

Concentration (mg L-1) 142.5

Constituent Element Se

Concentration (mg L-1) 60.0

Ni Ba Sn Fe Au

126.5 153.5 347.5 735.0 175.0

Zn Pb Cu As Y

3.0 32.2 127.5 185 1.8


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2.2. Synthesis of Dithiodiglycolamide impregnated XAD-16 beads The wet impregnation method was used for the preparation of DTDGA-impregnated XAD-16 (DTDGA-XAD 16) beads. The Amberlite XAD-16 beads were washed successively with dilute HCl and Na2CO3 solutions and then with deionised water, and the washed beads were vacuum dried until a constant weight is attained. Now these dry beads were equilibrated with 0.1 M DTDGA solution in methanol for 8 hours, at 70oC and 300 rpm agitation rate for complete impregnation of the extractant. Then, these beads were successively washed with water and acetone and vacuum dried. The impregnated extractant is present both on the surface and the internal pores of the base polymer. The synthesized extractant impregnated polymeric beads (EIPBS) were used for the study of gold sorption from the waste. The structure of the DTDGA extractant is shown in Fig.1. It is expected to form 1:1 complex with Au (III) coordinating via S and O atoms [19].

Fig.1. N,N,N’,N’ -tetra-(2-ethylhexyl)-dithiodiglycolamide (DTDGA)

2.3 Batch method for the sorption-desorption study The effect of various physical parameters onto the sorption of gold by the EIPBs was investigated by the batch sorption method. A definite amount of the synthesized resin beads was equilibrated with the aqueous solution of known gold concentration, for a given time duration



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using a rotospin rotor at 25 rpm. The distribution coefficient (D, L g-1) and equilibrium sorption capacity (qe, mg g-1) were calculated using the following formulae: =

.

.

. 10

(1)

. 10

(2)

Where Co and Ce are respectively the initial and the equilibrium (after 180 min of equilibration) metal ion concentrations (mg L-1), is the total volume of the solution (L); and m is the mass of the sorbent beads (g). The metal sorbed onto the synthesized beads was desorbed by using 0.01 M thiourea in 0.1 M HCl as stripping agent. Now these beads were washed with water and used again for the sorption-desorption experiments.

2.4. Instrumentation Quantitative determination of gold in the aqueous solutions was carried out using Atomic Emission Spectrometry technique (ICP-AES). An Ultima-2 sequential scan instrument with axial ICP (Horiba Jobin Yvon, France), operated at 40.68 MHz was used. The error on the measured concentrations was within ± 5.0 %. The FT-IR spectra of the synthesized bead samples were recorded using diamond ATR holder, employing IR Affinity-1 FT-IR spectrophotometer, in the range 600-4000 cm−1. Elemental analysis of treated and untreated beads was carried out on a Flash elemental analyzer of Thermo make. The thermogravimetric analysis (TGA) was carried out using STARe System METLER TOLEDO instrument. A few mg of the sample was taken in an alumina sample holder, and thermograms were recorded at the heating rate of 15 °C min–1, from 30 to 900 °C, under dynamic


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condition, and in N2 atmosphere (50 ml min–1). The composition, loading of the extractant onto the beads and thermal stability of the beads were determined from the obtained thermo gram. Morphology and topography of the beads was found out by simple microscopy, using QX5 DIGITAL BLUE computer microscope and field emission scanning electron microscopy (FESEM) using NOVA NANO SEM 600 (FEI), operated at 15 kV. The compositional analysis of the synthesized beads was done by Energy Dispersive X-ray (EDX) using oxford instrumentation UK (model No. INCAE350). Also, the EDXRF spectra of the synthesized beads were measured in air atmosphere using a Jordon Valley, Israel, Ex-3600 TEC spectrometer. The specimens were excited by Rh anode X-ray tube and Rh filter was used to reduce the background. Tube voltage and currents used were 40 kV and 500 µA, respectively. The diameter of the X-ray beam falling on the specimens was approximately 7 mm. The X-rays were detected using a Si-PIN diode detector having a resolution of 140 eV (FWHM) at 5.9 keV (Mn Kα). The measurement time was 1000 s live.

3. Results and discussion 3.1 Characterization of the synthesized beads The beads of diameter about 1mm were synthesized during the wet impregnation method. The optical image analyser and scanning electron microscopy were used to examine the surface morphology of the beads. The optical image of synthesized DTDGA impregnated XAD-16 beads (Fig.2A) confirms diameter of the beads as ~ 1mm, whereas the FE-SEM image (Fig.2B) shows roughness of the bead surface which is due to the porous nature of the base beads and is responsible for efficient metal ion sorption.



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A

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B

Fig.2. (A) The optical photograph of surface at 10X magnification, (B) FE-SEM image of outer surface of the synthesized beads. The results of the elemental analysis of the DTDGA-XAD16 resin beads showed the presence of C (81.86%), H (8.48 %), N (1.12%) and S (1.40 %). These data confirm the impregnation of DTDGA extractant onto the XAD-16 resin. The presence of gold in the beads after sorption was confirmed from the EDX pattern as shown in Fig.3A and 3B. The compositional studies of the beads have also been done at different locations, to confirm the homogeneity of the samples. The EDX mapping of the un-doped beads shows the presence of carbon, oxygen and very small amount of sulphur also. While the EDX mapping of the doped beads shows the presence of gold (Au) along with carbon and oxygen. The elemental composition at different location of the bead shows similar atomic wt% of the detected elements, indicating almost uniform loading of Au metal on the beads. The presence of a major carbon peak is as expected because of the organic nature of the used blank beads. The presence of gold in the beads after sorption was also confirmed from the EDXRF spectra as shown in Fig.3C and 3D. The EDXRF spectra were processed using the IAEA QXAS software after format conversion. The relative sensitivities of the elements to obtain the concentrations were determined using multi element solutions having elemental concentrations of 1.56 and 6.25 µg/mL. These spectra clearly indicate the sorption of gold by the polymeric beads.


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A

B

2500 Au Lα

C

D

2000

Au Lβ

1500

Fe Kα

Ar Kα

500

Ca Kα

1000

S Kα

Fe Kα

Ca Kα

100

Zn Kα

S Kα Ar Kα

Intensity (counts)

1000

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 0

0

10

20

30

0

Energy (keV)

10

20

Energy (KeV)

Fig.3. EDX mapping of the (A) Blank & (B) Au-Loaded beads and the EDXRF spectra of the (C) Blank & (D) Au-Loaded beads. The FTIR spectra of the pure DTDGA extractant and the synthesized DTDGA-XAD16 beads are shown in Fig.4A and Fig.4B respectively. The hydroxyl group stretching was observed at around at 3375 cm–1, which was fairly broad due to the presence of hydrogen bonding in the beads. The absorption peak at around 1483 cm−1 was observed due to symmetrical bending movement within the aromatic rings of C–H in the plane. Fig.4B shows the presence of DTDGA, as evident from the bands at 1631 cm–1 (C=O stretch), 2957 cm–1 (C–H stretching of methyl). The IR spectrum of the synthesized DTDGA XAD 16 beads shows all the characteristic peaks of pure DTDGA extractant, shown in Fig.4A. This further confirms the impregnation of the DTDGA extractant into the Amberlite XAD-16 beads.



100

80

DTDGA liq. 60

40 4000

100

A

1483

3500

3000

2500

2000

1500 -1

Wave number (cm )

B % transmittance

% transmittace

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1000

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80

3375 2957 1631

DTDGA XAD-16 beads 60

4000

3500

3000

2500

2000

1500

1000

500

-1

Wave number (cm )

Fig.4.FTIR spectra of the (A) DTDGA pure liquid and (B) the synthesized EIPBs.

The TGA of pure DTDGA liquid and the DTDGA-XAD 16 resin beads are shown in Fig.5A and Fig.5B respectively. The thermogram of the DTDGA pure extractant shows main weight loss in the temperature range of 200 to 328oC as shown in Fig.5A. The thermo gram of the synthesized DTDGA-XAD 16 resin beads shows the initial weight loss ~20% of the starting beads weight, in the same temperature range, as shown in Fig.5B. Comparison of both the thermograms indicates that the observed initial weight loss is due to the decomposition of DTDGA extractant. Further weight loss of 45%, observed around 350 to 460oC is because of the decomposition of the Amberlite XAD base polymer. These results suggest that the synthesized beads contain around 20% of DTDGA and remaining (80%) base polymer (Amberlite XAD-16). These results also validate the good thermal stability, upto about 200oC, of the synthesized beads.


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100

100

A DTDGA pure liquid % Weight loss

60

40

20

B DTDGA-XAD 16 beads

80

80

% wieght loss

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0 100

0 100

200

300

400

500

600

700

800

200

900

Temp (o C )

300

400

500

600

700

800

900

o

Temp. ( C)

Fig.5.TGA curves of (A) pure DTDGA liquid, and (B) the synthesized EIPBs.

3.2 Study of sorption of gold on the synthesized beads The synthesized beads were tested for the sorption of Au from aqueous medium by batch sorption method. The effects of different experimental parameters / conditions on the sorption of the metal have been investigated. 3.2.1 Effect of contact time The effect of contact time on the sorption of gold onto the DTDGA-XAD 16 beads was investigated, to understand the mechanism of sorption kinetics. A fixed quantity (10 mg) of resin and 2 mL of 100 ppm acidic gold solution were equilibrated in different closed tubes for different time intervals at room temperature, using rotospin rotor at 30 rpm. The sorption capacity is plotted against time as shown in Fig.6. With increasing the contact time from 5 to 180 minutes, the sorption capacity of gold, from the waste solution, was found to increase from, 2 to11mg g-1. As the saturation value was achieved in about 180 minutes, this contact time was



used in further experiments to study the effect of different experimental condition on the equilibrium sorption of gold. 12

Effect of Contact time 10

8 -1

qe(mg g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6

4

2 0

20

40

60

80

100

120

140

160

180

Time (min)

Fig.6. The effect of equilibration time on the sorption of gold ions by the DTDGA-XAD 16 beads.

3.2.2 Sorption kinetics The kinetics of sorption expresses the rate of gold uptake on the synthesized beads, and also the equilibrium time. The data for gold sorption onto the DTDGA-XAD 16 beads at various contact times were analyzed into well known kinetic models, namely, pseudo-first and second order models. The pseudo-first order rate expression of Lagergren [20] is generally described by the following equation:

= −

(3)

Where qt (mg g-1) is the amount of metal ion sorbed at time t (min), qe (mg g-1) is the sorption capacity at equilibrium, and KF (min−1) is the rate constant for pseudo-first-order model.


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Integrating and applying the limiting conditions that qt=0 at initial time and it is qe at equilibration or infinite time, the equation becomes − = −

". #

(4)

The plot for fitting of the first order rate is presented in Fig.7A. The rate constant KF was obtained from the slope of the linear plot of log (qe – qt) against t and was determined to be 0.0304 min-1. The value of the correlation coefficient (R2) for linear fit was 0.9676. The pseudo- second order reaction is greatly influenced by the amount of metal on the sorbent’s surface and the amount of metal sorbed at equilibrium [21]. The pseudo-second-order rate reaction can be described by the following equation:

= $ − "

(5)

Where KS is the rate constant of pseudo-second-order model (in g mg-1 min-1), by definite integration of Eq. (5) with boundary conditions, the following equation is obtained:

=

%

&

'

%

+ ) * +

(6)

The value of Ks =2.91 (g mg-1 min-1) and qe (12.46 mg g-1) were determined respectively from the intercept and the slope of the linear plot of (t/qt) against t, as shown in Fig.7B. The correlation coefficient (R2) value for the fit was found to be 0.9889. The correlation coefficient (R2) for the second order rate equation was greater than that of the first order. Thus, the second order rate expression fits the data more satisfactorily than the first order rate expression. Hence we conclude that the rate of reaction depends upon the concentration of both, the sorbent as well as the sorbate.



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Various kinetic parameters calculated from pseudo first order and pseudo second order kinetic models are summarized in Table 2. The experimental qe value matches quite well with the value obtained from both the models, but it is more close to that from the pseudo first order model. Table 2: Pseudo-first- order and pseudo-second-order constants and R2 values for the sorption of gold onto the DTDGA-XAD beads (2 ml of 100 ppm Au solution equilibrated with 10 mg beads) Gold

Pseudo first- order

Pseudo second- order

qe experimental

conc. 100 mg L-1

KF

R12

min-1 0.0304

0.9676

qe

Ks

mg g-1

g mg-1min-1

11.34

2.91

R22

0.9889

qe

qe

mg g-1

mg g-1

12.46

11.5

The rate limiting step plays a very important role in the sorption mechanism. To understand the mechanism, usually the data is analyzed by intra-particle diffusion model. For a solid-liquid sorption process, the solute transfer usually occurs by external mass transfer (boundary layer diffusion) or intra-particle diffusion (pore diffusion), or by both [22]. According to Weber and Morris, the intra-particle diffusion model can be expressed by the linear equation.

= ,- +%/" + /

(7)

Where kid is the intra-particle diffusion rate constant and the value I give the idea about the thickness of boundary layer. The plot of + versus +1/2 as shown in Fig.7C exhibits multi-linearity, indicating that, in addition to the intra-particle diffusion other mechanisms are also involved in the sorption process. The first linear segment, though does not pass through the origin, has very


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low thickness of boundary layer (I value of only 172). Thus, it can be assigned to intraparticle diffusion (kid=1054.38). The second linear segment may be attributed to intra-particle diffusion, but the fact that the line does not pass through the origin indicates that the adsorption mechanism is complex, with the contribution of both surface adsorption and intra-particle diffusion (kid=463.497 and I=3379.769) and the third linear section represents the final equilibrium stage with kid=123.87 and I=9067.897. In order to further confirm whether the sorption mechanism is through intra-particle diffusion or film diffusion, the kinetic information can be examined, employing the kinetic expression set by Boyd et al. [23, 24] 1

0 = 1 − 2' exp −6

(8)

Where F is the fraction of solute sorbed on different time, and Bt is a mathematical function of F.

0 =

(9)

Where, qe and qt correspond to the amount sorbed (mg g-1) at infinite time (in the present study 120 min.), and at any given time t respectively. Solutions to the equation (8), depending on the value of F, are as follows. 6 = 28 −

2'

− 28 1 −

2 %9 "

6 = −0.4977 − ln 1 − 0

(10) (11)

The value of Bt can be calculated for each value of F, using equation (10) for F values up to 0.85 and, equation (11) for higher F values.



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The linearity of Bt vs. t plot gives valuable information to differentiate between the intraparticle diffusion and the film diffusion mechanism of the sorption process. A straight line passing through the origin will indicate that the sorption process is regulated only by intraparticle-diffusion mechanisms; or else, it is regulated by film diffusion. The Boyd’s plot of Bt against time (t), shown in Fig.7D, has very poor correlation coefficient value for a linear fit. It may be divided into two sections, the first one (upto 90 min) passing through origin, indicating that the sorption is controlled by the intraparticle diffusion mechanism. The second section, above 90 min, may correspond to the equilibrium stage. Though, it indicates that the intraparticle diffusion is not be the only rate controlling step in the removal of the sorbate, due to the poor R2 value, it is difficult to predict the mechanism conclusively based on the Boyd’s plot. Thus, these kinetic results suggest that the sorption of gold ions onto the DTDGA-XAD 16 beads can be satisfactorily explained by the pseudo second-order and the intra-particle diffusion models as evident from the higher ?2 values for these kinetic models. The pseudo second-order rate equation suggests the presence of numerous active sites on sorbent surface. The multi-linear graphs obtained for the intra-particle diffusion model confirm that initially there will be an instantaneous sorption of gold ions onto the surface of the sorbent system through surface diffusion. With increase in surface coverage there is a gradual sorption through controlled intra-particle or pore diffusion, where gold ions move into the interior of the beads and at a later stage the intra-particle diffusion starts to slow down.


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0.018 4.0

Pseudo first order

0.016

3.6

0.014

3.4

0.012

3.2

0.010

t/qt

log(qe-qt)

3.8

3.0

Pseudo Second Order

0.008

2.8 0.006 2.6 0.004

2.4

A

2.2

0.002

2.0

B

0.000 0

20

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20

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Time (min)

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100 120 140 160 180 200

Time (min)

12

Intraparticle Diffusion Model

4.0

Boyds Plot

3.5

10

3.0 8

2.5 2.0

Bt

-1

qt (mg g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

1.5 1.0

4

0.5 2

C 2

4

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8 1/2

Time

D

0.0 10

12

14

0

20

40

(min)

60

80

100

120

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Time (min)

Fig.7. (A) Pseudo first-order, (B) Pseudo second-order, (C) Intra-particle diffusion and (D) Boyd’s plots for the Au ion sorption onto the DTDGA-XAD 16beads, at a fixed initial concentration of gold ions.

3.2.3 Effect of amount of beads The effect of sorbent amount on gold sorption from aqueous solution was investigated at a constant contact time of 180 minutes. A 2 mL gold solution of 100 ppm concentration was equilibrated with different amount of beads ranging from 0.01 to 0.05 g. The results presented in the Fig.8 indicate that the % gold sorption increases with increase in sorbent dose, at a fixed initial metal concentration. In this study, an increase of beads dosage from 0.01 to 0.03 g resulted in the sorption of gold increasing from 77% to 93%, while further increase of the beads dose



from 0.04 to 0.05 g gave only a smaller increase in gold sorption probably due to very small amount of gold left in the aqueous phase. 100

Effect of varying beads conc.

90

% Sorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

80

70

60

50 0.01

0.02

0.03

0.04

0.05

0.06

Amount of beads (in g)

Fig.8. The effect of varying beads on the sorption of gold by the beads The decrease in sorption density can be attributed to the fact that some of the sorption sites remain unsaturated during the sorption process as the number of available sorption sites increase with an increase in the sorbent amount, thereby resulting in an increase in removal efficiency.

3.2.4 Effect of HCl concentration Extraction of gold by the DTDGA-XAD 16 beads has been carried out from solutions with different hydrochloric acid concentrations, to analyze the effect of acidity on the sorption. The %E values for gold, from the different hydrochloric acid solutions, are shown in Fig.9. The extraction of gold, with good efficiency (60% or more), could easily be accomplished by maintaining HCl concentration between 1.0 and 3.0 M. The higher %E values of gold were obtained at higher concentration of hydrochloric acid. The increasing Cl– ion concentration in solution could be the driving force for the complexation of the metal ions with the sorbent


Page 19 of 32

molecule, thereby increasing the extraction of gold onto the polymeric beads. The observed % E values vary from 60 to 85% at the studied acid strength of 1.0–3.0 M HCl indicating that these beads can be used with minimum adjustment of feed HCl concentration in the usual acidic waste. 90

E ffect o f H C l co n c. 85 80

% Extraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


75 70 65 60 1.0

1.5

2.0

2.5

3.0

3.5

H C l co n c (in M o lar)

Fig.9. The effect of HCl concentration on the sorption of gold

3.2.5 Effect of temperature The temperature at which a sorption process is carried out will influence both the sorption rate and the extent of sorption. Fig.10 shows the influence of the temperature on the sorption of gold onto the synthesized DTDGA-XAD sorbent beads. The data of Fig.10 reveals a general trend of increase in the % sorption of gold ions with temperature; this indicates endothermic nature of the sorption reaction. This increase may have been due to the enhanced rate of intraparticle diffusion of sorbate and changes in the size of the pores of the beads with temperature.



100

Effect of Temperature 80

% Sorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

60

40

20

30

40

50

60

o

Temperature ( C)

Fig.10. The effect of temperature on the sorption of gold by the beads

3.2.6. Effect of concentration of gold Fig. 11 shows the effects of initial concentration of Au onto the sorption capacity of the DTDGA- XAD 16 polymeric beads at pH=5. These clearly indicate that, with an increase in the initial gold concentration, the amount of sorbed gold ions increases significantly. At lower initial metal ions concentration, the sorption increases almost linearly, suggesting that the sorption sites on the DTDGA-XAD 16 beads are sufficient, and in this case, the amount of gold ions sorbed is dependent on the number of the metal ions transported from the bulk solution to the surfaces of the beads. At higher concentrations, however, the sorption no longer increases proportionally with the initial metal ion concentration, indicating that the number of sorption sites on the surfaces of the DTDGA-XAD 16 actually limits the amount of sorbed gold ions. The maximum experimental sorption capacity is found to be ~35 mg g-1.


Page 21 of 32

35

Equilibrium Sorption Capacity

30 25 -1

qe(mg g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15 10 5 0 -5 0

100

200

300

400

500

Co (ppm)

Fig.11. The effect of initial gold concentration on the sorption capacity of beads

3.2.7. Sorption isotherm models The sorption isotherm expresses the amount of solute sorbed per unit weight of sorbent as a function of equilibrium concentration in bulk solution at a constant temperature. The different sorption isotherm plots are presented in Fig.12.

3.2.7.1. Langmuir sorption isotherm model Langmuir model is the simplest theoretical model for monolayer sorption onto a surface with finite number of identical sites. The widely used Langmuir isotherm, in the form of equation (12), has been successfully applied in many real sorption processes [25] EF

q A = q BCD )%HEFG * G

(12)

Here, b (L mg-1) is the Langmuir equilibrium constant, which is related to the affinity of the binding sites, and qmax (mg g-1) is the maximum sorption capacity (theoretical monolayer



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

saturation capacity). The main characteristics of the Langmuir equation, the constants b and qmax, can be determined from the linear form of the Langmuir equation, as follows; IG JG %

= J =

%

KLM E

%

NOP

+ J

+ Q

IG KLM

%

NOP

(13)

(14)

A linear plot of (1/qe) versus (1/Ce) as shown in Fig.12A indicates that the sorption behaviour follows the Langmuir sorption isotherm. The values of qmax and b were found to be 70.34 mg g-1 and 6.2 L mg-1, from the intercept and the slope, respectively. The correlation coefficient (R2) was found to be 0.998, indicating that the Langmuir sorption model can be applied in this system. The theoretical value of qmax is much higher, almost double, than the observed experimental value of 35 mg g-1. Probably, incomplete utilization of all the sorption sites of the beads, can be one of the reasons.

3.2.7.2. Freundlich sorption isotherm model Freundlich isotherm is applicable to non-ideal sorption on heterogeneous surface as well as multilayer sorption. The equation for the model is given as [26]

= R %⁄S

(15)

where KF is relative indicator of sorption capacity, while the dimensionless, 1/n suggests the favourability and capacity of the sorbent/sorbate system. According to the theory, n > 1 represents favourable adsorption conditions. Eq. (15) is linearized into logarithmic form for data fitting and parameter evaluation, as follows:


Page 23 of 32

%

= + S R

(16)

Freundlich isotherm plot is shown in Fig.12B. The plot is linear, with the correlation coefficient (R2) of 0.984, indicating that the Freundlich model can also explain the sorption mechanism. The values of the Freundlich constants, KF and n, are given in Table 3. The obtained n value of 1.28 indicates favorable sorption, generally larger is the value of n (1 to 10) stronger is the interaction between sorbent and sorbate.

0.0004

Langmuir sorption isotherm

4.6

Freundlich sorption isotherm

4.4 0.0003

log qe

1/qe

4.2

0.0002

4.0 3.8 3.6

0.0001

3.4

A 0.0000 0.00

0.04

0.08

0.12

0.16

B

3.2 0.6

0.8

1.0

1.2

1/Ce

35

1.4

1.6

1.8

2.0

2.2

2.4

log Ce

Temkin Sorption Isotherm

10.5

D-R sorption Isotherm

10.0

30

9.5

25

-1

qe(mg g )

9.0

20

ln qe

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

8.5 8.0

10

7.5

5

7.0

C

0 0.6

0.8

1.0

1.2

1.4

(D)

6.5

1.6

1.8

2.0

2.2

2.4

-0.02 0.00

0.02

log(Ce)

0.04

0.06

0.08

0.10

0.12

0.14

0.16

2 Ε KJ/Mol

Fig.12. (A) Langmuir isotherm, (B) Freundlich isotherm, (C) Temkin isotherm and (D) DubininRadushkevich (D-R) Isotherm plots for sorption of gold on to the DTDGA-XAD beads

3.2.7.3. The Temkin isotherm



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

The Temkin isotherm [27] has been used in many sorption processes. A linear form of the Temkin isotherm can be expressed as:

= ) 2.303

VW Q

* X + ) 2.303

VW Q

* R

(17)

It can be simplified to equation (18), by substituting B for 2.303RT/b.

= 6 X + 6 R

(18)

The sorption data can be analysed according to Eq. 18. Therefore a plot of qe versus ln Ce enables one to determine the constants A and B respectively from the intercept and the slope. The values of the Temkin constants A and B as well as the correlation coefficient are listed in Table 3. It is clear from the plot with R2 values of 0.978 that the Temkin isotherm cannot be used to describe the sorption isotherm suitably.

3.2.7.4. Dubinin-Radushkevich (D-R) Isotherm. Dubinin-Radushkevich (D-R) isotherm assumes that the porosity of the sorbent has an effect on the sorption process. Dubinin suggested the isotherm to estimate the mean free energy of sorption which is given by the equation [28].

= YZ × exp −\ ] "

(19)

The linearized form of above equation can be given as ^ = ^ YZ − \ _ "

(20)

Where (mol2 kJ−2) is D-R model constant related to the mean adsorption energy and € is Polanyi potential given by


Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


%

_ = ?` ln 1 +

(21)

where ? is the gas constant (kJ mol−1K−1) and ` is the absolute temperature (K). The plot for DR model yields low correlation coefficient value of 0.853 (Fig.12D), confirming nonapplicability of this model. According to the above results, it is predicted that the initial monolayer sorption occurs (as confirmed from Langmuir model) leading to further multilayered interactions between the gold ions and the beads, as explained by the Freundlich isotherm. Table 3: Different Isotherm constants and corresponding R2 values for the adsorption of gold onto DTDGA-XAD beads Metal

Langmuir parameter

Freundlich parameter

Temkin parameter

D-R parameter

ion

Au

b

qmax

L mg-1

mg g-1

0.628

70.34

R2

KF

n

R2

B

mg g-1 0.998

704.0

A

R2

KD

R2

L mg-1 1.28

0.984

22.37

0.1406

0.978

-18.86 0.853

3.2.8. Thermodynamic studies The thermodynamic parameter, Gibb’s free energy change (∆ao) is essential to know the spontaneity of the sorption process. The Gibb’s free energy for the sorption (∆ao) was calculated from the equation ∆a # = −?` ^ c

(22)

By using the equilibrium constant (d) value obtained from Langmuir isotherm for the sorption experiment carried out at 298 K, where ? is the universal gas constant (8.314 J mol−1K−1) [29],



the change in free energy (∆ao) value obtained for uptake of gold ions is −15.16 kJ mol−1. This negative value reveals the feasibility of sorption and confirms the spontaneity of the sorption process.

3.3 Reusability of Beads

The reusability of a sorbent is very important for its effective utilization for recovery of a desired solute. Stripping studies with 0.01 M thiourea in 0.1 M HCl has shown that sorbed gold can be back extracted almost quantitatively in 2-3 contacts. The reusability of the DTDGA-XAD 16 beads was therefore tested by first sorbing Au from 100 ppm stock solution, followed by back extraction using 0.01 M thiourea in solution. Again the same beads were washed and put in the fresh solution of Au, followed by stripping with 0.01 M thiourea in 0.1 M HCl. This process was repeated upto four successive cycles.

100 R eusability study 80

% Sorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

60

40

20

0 1

2

3

4

no. of Cycles

Fig.13. Reusability study of the synthesized EIPBs for sorption of gold from acidic wastes

The stability and reusability of the beads was evaluated by calculating % sorption by the same set of beads after every cycle. The result of the reusability study is shown in Fig.13. It was


Page 27 of 32

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observed that even after four consecutive cycles of stripping and loading, there was only a small decrease in the sorption efficiency of the beads. The observed small decrease in the sorption efficiency is probably due to some incomplete stripping of the sorbed solute. No significant deformity in size or shape of the beads was seen after the studied 4 cycles. These results confirm multicycle reusability of the synthesized beads

3.4 Sorption studies with SEW solution In order to evaluate the efficacy of DTDGA-XAD 16 beads to selectively separate gold form electronic waste, these were contacted batch wise with simulated electronic waste (SEW) solution. Table 4 shows the distribution of SEW metal ions, it is evident that except gold any other metal is hardly sorbed on to DTDGA-XAD 16 beads. This shows the excellent selectivity of the beads for gold over other metal ions in SEW. Table 4: Distribution ratio of various metal ions present in SEW, Sorbent: SEW feed solution. Constituent Element Au Ni Cu Sn Fe Cr Se Zn Pb Ba As Y

Distribution Ratio (DM) L g-1 908

DTDGA-Impregnated XAD-16 Beads for Separation of Gold from

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