Article pubs.acs.org/IECR
Chelating Resin for Removal of Nickel Impurities from Gold Electroplating Solutions Siu-kwong Pang and Kam-chuen Yung* Department of Industrial and Systems Engineering, PCB Technology Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong ABSTRACT: Nickel impurities in pure gold deposits, because of the presence of nickel(II) ions in gold electroplating solutions, can cause adverse effects on the bonding of semiconductor chips. In order to remove the nickel impurities in gold electroplating solutions, the chelating resin, which is composed of styrene divinylbenzene copolymer as a backbone and also contains aminophosphonate ligands, has been studied for its nickel adsorption properties from gold electroplating solutions. Batch shaking adsorption experiments were performed, and the results showed that the resin could successfully purge gold electroplating solutions of nickel contamination, without a great influence on the concentration of gold in the form of Au(CN)2−. Approximately 98% of the nickel and a few percent of gold were adsorbed by the resin. The Ni adsorption percentage does not vary with the operational gold concentration. A small increment in the Ni adsorption percentage was observed when the pH of the gold electroplating solutions was increased from 6 to 8, since more active groups in the resin were deprotonated at higher pH. Because of the independence of Ni adsorption from operational gold concentrations and the independence of gold adsorption from pH, active sites for Ni adsorption differing from those for Au adsorption was proposed. The XPS data revealed that −PO32− in the aminophosphonate ligands in the resin participated into the chelation of Ni; however, −NH− in the aminophosphonate ligands in the resin could be protonated. The Ni adsorption on the resin in a gold electroplating solution obeys pseudo-secondorder kinetics rather than pseudo-first-order kinetics. Supporting by the kinetics results, the Sips (Langmuir−Freundlich) isotherm is suggested for a more adequate description of the Ni adsorption on the resin in a gold electroplating solution, although the experimental data could also satisfactorily fit to the Langmuir isotherm and the Freundlich isotherm.
1. INTRODUCTION Pure gold (99.9%) is used in selective areas on printed circuit boards (PCBs) for the direct attachment and bonding of semiconductor chips, and it also has better solderability.1,2 Gold electroplating is one of main techniques for depositing pure gold coatings on PCBs. The pure gold deposits are electroplated onto nickel undercoats, which have been deposited on copper substrates on PCBs by electroplating. Rinsing and a gold strike (a dilute version of a normal gold electroplating solution) are implemented before the deposition of a thick gold layer in a main gold plating tank in order to protect the main gold plating tank against contamination. Nickel contamination in the main gold plating tank still occurs when a gold strike and rinsing cannot completely prevent nickel(II) ions from being dragged into it (Figure 1). Consequently, the nickel(II) ions in gold electroplating solutions are electrochemically reduced to nickel, which is deposited with the gold on the nickel undercoats in the gold electroplating process. Nickel impurities in pure gold deposits can cause adverse effects on bonding.3 Nickel-contaminated gold electroplating solutions and failed products must be rejected, leading to an increase in the production cost. In addition, chemical steps need to be taken to treat the waste gold electroplating solutions, causing an increase in the risk of environment pollution and a lowering of safety in the workplace. Therefore, development of an effective way to purge gold electroplating solutions of nickel impurities is of a great interest to the electronic materials and electronics manufacturing industry. In © 2013 American Chemical Society
this study, chelating resin treatment is proposed to remove nickel(II) ions from gold electroplating solutions (Figure 1). Various types of ion-exchange resins are commercially available, and they are usually classified as cationic, anionic, and chelating resins. Chelating resins have been developed to selectively adsorb transition-metal ions from solutions, since the adsorption is achieved through the formation of transitionmetal complexes by covalent bonding between the ligands in the resin and the transition-metal ions in the solutions, instead of via simple electrostatic interaction, as in conventional cationic and anionic resins.4 The chelating resin bearing aminophosphonate ligands is known to effectively adsorb nickel,5−7 and it was employed in this study. The chemical structure of the resin bearing aminophosphonate ligands is displayed in Figure 2. Nickel is adsorbed by the resin through coordinating the aminophosphonate ligands to Ni2+. Previous studies have reported that this resin could effectively adsorbed nickel, whether the nickel was in the form of hydrated metal ions or metal-acid ligand complexes.6,7 In addition, Deepatana and Valix reported that such aminophosphonic acid-based chelating resin showed a higher Ni adsorption, in comparison to an iminodiacetic acidbased chelating resin in nickel citrate complex systems.5 According to their reported values of KR, αR, and β, the Ni Received: Revised: Accepted: Published: 2418
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Figure 1. Nickel contamination in gold electroplating solution and resin treatment.
phosphate was purchased from Acros. Phosphoric acid (85%) and potassium hydroxide for pH adjustment were provided by Riedel-de Haën and Sigma−Aldrich, respectively. The standard buffer solutions (pH 4.01, 7.01 and 10.01) for pH meter calibration were supplied by Hanna. The pure gold electroplating solution is usually a neutral solution type. The basic composition of the neutral gold electroplating solution without additives used in this work is listed in Table 1.2,3,8,9
Figure 2. Chemical structure of the resin bearing aminophosphonate ligands.
Table 1. Basic Composition of a Neutral Gold Electroplating Solution without Additives
adsorption capacity of 3.49 mg/g and 7.23 mg/g for the respective iminodiacetic acid and aminophosphonic acid-based chelating resins at 100 mg/L of the equilibrium Ni concentration and 0.01 M citric acid were calculated.5 Success in removal of Ni2+ in gold electroplating solutions by the chelating resin can increase the life of gold electroplating solutions, and may even eliminate the gold strike process, leading to a decrease in production cost and product rejection rate. In addition, it can reduce harm to the environment as the frequency of waste gold electroplating solution treatment will be decreased. The crucial point for success is whether the chelating resin can preferentially adsorb nickel rather than gold in gold electroplating solutions. The aim of this study is to explore the feasibility of using a chelating resin bearing aminophosphonate ligands to purge gold electroplating solutions of nickel impurities, without a great influence on the concentration of gold in the form of Au(CN)2−, which is usually used in gold electroplating solutions. Batch shaking adsorption experiments were performed to examine the effects of pH, contact time, and Au concentration on nickel removal by the resin. The Ni- and Auloaded resins were characterized by X-ray photoelectron spectroscopy (XPS). The nickel adsorption mechanism in the gold electroplating solutions was investigated by fitting the adsorption data to pseudo-first-order and pseudo-second-order reaction kinetics models, and adsorption isotherm models: Langmuir, Freundlich, and Sips (Langmuir−Freundlich).
component
value
Au (as KAu(CN)2) KH2PO4 pH
8−14 g/L 80 g/L 6.0−8.0
2.2. Instruments. The pH values were measured with a Checker pH meter (Hanna). An orbit shaker SK-330-Pro (DragonLab) was used in the batch shaking adsorption experiments. Inductively coupled plasma−atomic emission spectrometry (ICP-AES) (Optima 5300 DV; Perkin−Elmer) was used for the determination of the concentrations of Ni and Au. X-ray photoelectron spectrometry (XPS) (Axis Ultra DLD; Kratos) was employed for analyzing the resin before and after nickel and gold adsorption. 2.3. Batch Adsorption Experiments. Twenty milliliters (20 mL) of the neutral gold electroplating solutions (Table 1) containing Ni2+ ions were added into tubes, and 10 g/L of resin powder was dispersed into the solutions. The suspensions were shaken at 500 rpm at ambient temperature during batch tests. The suspensions were then filtered, and the nickel and gold concentrations of the filtrates were analyzed by ICP-AES. The adsorbed amount of nickel and gold in the resin (q, mg/ g) were obtained using the following equation: q=
2. EXPERIMENTAL SECTION 2.1. Materials. The chelating resin Amberlite IRC 747 UPS (Rohm and Haas, Philadelphia, PA), which contains aminophosphonate ligands in a poly(styrene-co-divinylbenzene) matrix, was obtained from Dow Chemicals. Potassium gold cyanide (KAu(CN)2) was supplied by Heraeus, Ltd. Nickel(II) sulfate was acquired from Fisher. Potassium dihydrogen
(C0 − Cf )V m
(1)
where C0 is the initial metal concentration (mg/L) in the solutions, Cf the final metal concentrations (mg/L) in the solutions, V the volume of the solutions (liters), and m the mass of the resins (grams). The percentage of nickel and gold adsorbed by the resin is equal to 2419
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C0 − Cf × 100 C0
Article
(2)
2.4. X-ray Photoelectron Spectroscopy (XPS) Analysis. After washing the nickel- and gold-loaded resin with absolute ethanol and then drying, the XPS spectra of the resin were recorded. The incident radiation was monochromatic Al Kα Xrays. Charge neutralization was used. The binding energy scale was calibrated using the C 1s peak at 285.0 eV.
3. RESULTS AND DISCUSSION 3.1. Effect of Gold Concentration. The effect of gold concentration on nickel and gold adsorption were studied. The gold concentrations examined could cover the usual operation range of gold concentration (8−14 g/L) in neutral gold electroplating solutions. The pH of the solutions was 7, which is the middle point of the operation range of pH of neutral gold electroplating solutions. The contact time between the resin and the solutions was 24 h. The initial nickel concentration in the solutions was 100 mg/L. Within the operation range of Au concentration (8−14 g/L), ∼5% of the gold was adsorbed by IRC 747 (Figure 3). One-way
Figure 4. Effect of gold concentration on the percentage of nickel adsorbed by IRC 747. The error bars represent the standard deviation of the nickel adsorption percentage.
the percentage of nickel adsorbed by IRC 747 is independent of the operational gold concentration. Although IRC 747 is saturated with gold at very high gold concentrations based on the above results, nearly all Ni2+ ions are still adsorbed by IRC 747 (∼98%), and the nickel adsorption percentage also shows independent of these very high gold concentrations. Thus, it is proposed that the active sites for nickel adsorption are different from those for gold adsorption; hence, nickel can be adsorbed by IRC 747 without any interference from Au(CN)2−. 3.2. Effect of pH. Since the operation pH range of the gold electroplating solutions is 6−8, the influence of the pH on nickel and gold adsorption was examined. The initial Au concentration of 11 g/L (the midpoint of the operation range) was chosen for study. The contact time between the resin and the solutions was 24 h. The initial nickel concentration in the solutions was 100 mg/L. From Figure 5, the nickel adsorption percentage increased slightly with pH, from ∼97% to ∼99%, and an enhancement of
Figure 3. Effect of gold concentration on the percentage of gold adsorbed by IRC 747. The error bars represent the standard deviation of the gold adsorption percentage.
ANOVA is a statistical method of comparing the means of two or more groups. There was no significant difference between the gold adsorption percentages at the operation range of gold concentration, since the p-value from one-way ANOVA analysis was greater than 0.05 (p-value = 0.837), and the p-values from the LSD post-hoc test for multiple comparisons ranged from 0.371 to 0.896 (>0.05). Thus, the percentage of gold adsorbed by IRC 747 does not vary with the operational gold concentration. It can be explained that the operational Au concentrations are sufficiently high to saturate the IRC 747 active sites for gold adsorption. Approximately 98% of nickel was adsorbed by IRC 747 when the operational gold concentration was varied from 8 g/L to 14 g/L (Figure 4). Through one-way ANOVA analysis, the p-value was greater than 0.05 (p-value = 0.615), and the p-values from the LSD post-hoc test for multiple comparisons were in the range of 0.207−1.000 (>0.05); therefore, there was no significant difference between the nickel adsorption percentages at the operation range of gold concentration. In other words,
Figure 5. Effect of pH (in the range of 6−8) on the percentage of nickel adsorbed by IRC 747. The error bars represent the standard deviation of the nickel adsorption percentage.
∼1% nickel adsorption was observed when the pH value was increased by 1. This small increment is statistically verified because the one-way ANOVA analysis (p-value = 0.000; 0.05). The results of the statistical analysis indicate that the percentage of gold adsorbed by IRC 747 is not dependent on the operation pH range. Active sites for gold adsorption appear to have no significant change in chemical nature when the pH varies from 6 to 8. It further supports the belief that the active sites for Au adsorption are different from those for nickel adsorption, which is pH-dependent. Since the polymer backbone of IRC 747 is styrene divinylbenzene copolymer, it is proposed that the formation of (arene)gold(I) complex, because of the reaction between the benzene rings on the resin backbone and Au(CN)2−, probably contributes to the chemistry of gold adsorption on IRC 747.10 3.3. XPS Analysis. In order to understand the molecular level information of nickel and gold adsorption on the resin bearing aminophosphonate ligands, XPS analysis was performed to characterize the surface of the resin, which had been subjected to nickel and gold adsorption. The nickel and gold concentrations in the gold electroplating solutions were 100 mg/L and 11 g/L, respectively. The pH of the solutions was 7, and the contact time was 24 h. A Ni 2p3/2 peak located at a binding energy of 855.8 eV was observed, as shown in Figure 7; therefore, the adsorbed Ni on the resin was identified. This signal can be attributed to the aminophosphonate ligands chelating with Ni2+ in the resin. Two signalsat binding energies of 85.7 and 89.4 eV, corresponding to Au 4f7/2 and Au 4f5/2, respectivelyare shown in Figure 8, and they confirm the adsorption of gold in the resin. Figure 9 shows the XPS O 1s spectra. Before Ni and Au adsorption, the peak located at a binding energy of 530.8 eV is a combination of the signals contributed by the oxygen in −P O, −P−OH, and −P−O− in the aminophosphonate ligands in
Figure 8. XPS Au 4f spectra of IRC 747 subjected and not subjected to nickel and gold adsorption.
Figure 9. XPS O 1s spectra of IRC 747 subjected and not subjected to nickel and gold adsorption.
the resin. After nickel and gold adsorption, the peak was shifted to 531.3 eV. The shift can be attributed to the change in the chemical environment of the oxygen in the aminophosphonate 2421
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ligands in the resin due to the formation of bonds between the oxygen and the nickel. The XPS N 1s spectra are displayed in Figure 10. The N 1s peak due to a combination of −NH and −NH2+ signals in the
Figure 11. Variation of the nickel adsorption percentage by IRC 747 with time in a gold electroplating solution.
12). The coefficient of determination (R2) can be calculated using the following equation:14 Figure 10. XPS N 1s spectra of IRC 747 subjected and not subjected to nickel and gold adsorption.
aminophosphonate ligands in the resin is located at a binding energy of 399.3 eV before the resin was subjected to nickel and gold adsorption. However, no shift of the N 1s peak could be observed after nickel and gold adsorption. It can be explained that the pKa value of −PO3H− in 2-aminoethanephosphonic acid is 6.21, whereas −NH3+ in 2-aminoethanephosphonic acid shows a pKa value of 10.92.11 These pKa values imply that the extent of deprotonation of −NH3+ is smaller than that of −PO3H−, while −NH3+ rather than −NH2 is a dominant species at pH 7. The infrared (IR) data also revealed that −NH2+− in the aminophosphonate ligands in the resin was the major form at pH 6, and −NH− dominated at pH 11.12 Therefore, it is suggested that most of the N atoms in the aminophosphonate ligands in the resin are protonated, resulting in a few N−Ni bonds in this condition. 3.4. Adsorption Kinetics. The nickel adsorption kinetics in a gold electroplating solution was examined. The adsorbed amounts of nickel were measured at different times. The initial nickel concentration for the kinetics study was 100 mg/L. The initial gold concentration was 11 g/L. The pH of the gold electroplating solutions was 7. The variation in nickel adsorption percentage with time is shown in Figure 11. After 15 h, a plateau level is approached, and ∼98% of the nickel is adsorbed after 24 h. The pseudo-first-order kinetic equation can be found in elsewhere13 and expressed as
dqt dt
= k1(qe − qt )
Figure 12. Plot of the adsorbed amount of nickel per unit mass of the resin at time t (qt) versus time (t) for the adsorption of nickel by IRC 747 in a gold electroplating solution. The solid line () is the experimental data; the dashed line (- - -) is the nonlinear regression result from the pseudo-first-order kinetics model.
R2 = 1 −
(5)
where SSE is the sum of the squares of the deviations of the experimental data from the regression curve and SStotal is the sum of the squares of the deviations of the experimental data from the mean of all the experimental data. Equation 6 represents the pseudo-second-order kinetic model:13−16 dqt dt
= k 2(qe − qt )2
(6)
where k2 is the rate constant for the pseudo-second-order adsorption. Integrating eq 6 for the boundary conditions of t = 0 to t = t and qt=0 = 0 to qt=t = qt gives
(3)
where qe and qt are the adsorbed amounts of nickel per unit mass of the resin at equilibrium and time t, respectively, and k1 is the rate constant for pseudo-first-order adsorption. After integration and applying the boundary conditions of t = 0 to t = t and qt=0 = 0 to qt=t = qt, eq 3 becomes ln(qe − qt ) = ln qe − k1t
SSE SStotal
qt =
qe2k 2t 1 + qek 2t
(7)
Equation 7 can be rearranged to obtain a linear form as
(4)
t 1 t = + qt qe k 2qe2
Nonlinear regression performed by Solver in the Excel program was applied for fitting the experimental data to eq 4 (see Figure 2422
(8)
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Therefore, linear regression performed by the Excel program can be used for fitting the experimental data to eq 8 (see Figure 13).
The Freundlich isotherm model can be applied to adsorption on heterogeneous surfaces and multilayer adsorption.17,18 The equation for the Freundlich isotherm is shown in eq 12.
qe = KFCf1/ n
(12)
where KF is a constant for the Freundlich isotherm related to adsorption capacity and n reflects the adsorption intensity. The linear form of eq 12 is log qe = log KF +
1 log Cf n
(13)
The Sips isotherm is used to describe an adsorbate occupying two sites, and its corresponding adsorption rate is proportional to (1 − θ)2, where θ is the fractional surface coverage.17 The Sips isotherm equation is as follows: Figure 13. t/qt against time t for the adsorption of nickel by IRC 747 in a gold electroplating solution. The data points represent the experimental data; the dashed line (- - -) represents the linear regression result from the pseudo-second-order kinetics model.
qe =
1 + (KSCf )1/ n
(14)
where KS is the Sips isotherm constant related to affinity, qmax the saturation capacity, and n the Sips exponent. Since both the Langmuir isotherm and the Freundlich isotherm have a linear form, linear regression carried out by the Excel program can be used for fitting the experimental data to eqs 11 and 13 (see Figures 14 and 15). For the Sips isotherm, nonlinear regression was carried out by Solver in the Excel program to fit the experimental data to eq 14 (see Figure 16), and R2 was determined by eq 5.
Obviously, the experimental data could fit the pseudosecond-order rate (eq 8) well, since the pseudo-first-order kinetics model showed a larger deviation from the experimental results (R2 = 0.903), relative to the pseudo-second-order kinetics model (R2 = 1.000). The values of qe and k2 in the pseudo-second-order kinetics model for nickel adsorption in a gold electroplating solution are 10.1 mg/g and 0.303 g mg−1 h−1, respectively. According to the pseudo-second-order kinetics model, the reaction between Ni2+ and the resin can be represented by the following equation:15 2A‐ + Ni 2 + → NiA 2
qmax (KSCf )1/ n
(9)
−
where A is the free active site on the resin. Based on the results of XPS, nitrogen in the aminophosphonate ligands in the resin cannot actively participate into the chelation of Ni2+, so the two active sites (A−) in eq 9 are dominantly contributed by the −PO32− moiety. 3.5. Adsorption Isotherms. Three adsorption isotherm models have been studied for describing the nickel adsorption characteristics of the resin bearing aminophosphonate ligands in a gold electroplating solution: the Langmuir isotherm, the Freundlich isotherm, and the Sips (or Langmuir−Freundlich) isotherm. The initial Ni concentrations ranged between 20 mg/ L and 100 mg/L. The initial Au concentration was 11 g/L. The pH of the solutions was 7. The adsorbed amounts of nickel were measured after 24 h of adsorption. The Langmuir isotherm model assumes that adsorption occurs at homogeneous sites, and one adsorbate occupies one site to form a monolayer on a surface of an adsorbent.14,17,18 The Langmuir isotherm equation can be expressed as qe =
Figure 14. Plot of Cf/qe against Cf for the Ni adsorption by IRC 747 in a gold electroplating solution. The points represent the experimental data; the dashed line (- - -) represents the linear regression result from the Langmuir isotherm.
qmax KLCf 1 + KLCf
(10)
where qe is the adsorbed amount of Ni per unit mass of the resin at equilibrium, qmax the monolayer saturation capacity, KL the Langumir constant, and Cf the final nickel concentration in a solution. Equation 10 can be rearranged to obtain a linear form: Cf C 1 = + f qe KLqmax qmax
Figure 15. Plot of log qe against log Cf for the nickel adsorption by IRC 747 in a gold electroplating solution. The data points are the experimental data. The dashed line (- - -) represents the linear regression result from the Freundlich isotherm.
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Notes
The authors declare no competing financial interest.
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Figure 16. Plot of qe against Cf for the adsorption of nickel by IRC 747 in a gold electroplating solution. The data points represent the experimental data; the dashed line (- - -) represents the nonlinear regression result from the Sips isotherm.
For the Langmuir isotherm, KL = 0.79 L/mg and qmax = 16.05 mg/g. For the Freundlich isotherm, KF = 6.70 mg1−(1/n) L1/n g−1, where n is 1.53. For the Sips isotherm, KS = 0.26 mg−1/n L1/n, qmax = 27.32 mg/g, and n = 1.25. The R2 values for the Langmuir isotherm, the Freundlich isotherm, and the Sips isotherm are 0.985, 0.997, and 1.000, respectively, and they all are very close to 1. Although the three isotherm models appear to adequately describe the Ni adsorption in a gold electroplating solution, it is suggested that the Sips isotherm is the most suitable model. Since the kinetics study revealed that the experimental data could well fit to the pseudo-second-order kinetics model, in which the nickel adsorption rate is dependent on the square of the number of the free active groups in the resin, better agreement is observed with the theory of the Sips isotherm model.
4. CONCLUSIONS Nickel adsorption by the resin bearing aminophosphonate ligands in gold electroplating solutions was studied. The results showed that the resin could successfully purge gold electroplating solutions of nickel contamination, without a great influence on the concentration of Au(CN)2−. The nickel adsorption approached a plateau level after 15 h, and ∼98% of the nickel was adsorbed after 24 h, while a small percentage of gold adsorption was achieved. The adsorbed amount of nickel did not vary with the operational gold concentration. A small increment in the amount of nickel adsorbed by the resin was observed when the pH of the gold electroplating solution was increased from 6 to 8. Because of the independence of nickel adsorption from operational gold concentrations and the independence of gold adsorption from pH, the active sites for nickel adsorption differing from those for gold adsorption was proposed. The XPS data revealed that −PO32− in the aminophosphonate ligands in the resin actively participated into the chelation of nickel; however, −NH− in the aminophosphonate ligands in the resin could be protonated. The Ni adsorption in a gold electroplating solution obeys pseudo-second-order kinetics and the Sips isotherm.
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REFERENCES
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