Environ. Sci. Technol. 2008, 42, 5486–5491
Dimethylamine-Modified Waste Paper for the Recovery of Precious Metals CHAITANYA RAJ ADHIKARI, DURGA PARAJULI, HIDETAKA KAWAKITA, KATSUTOSHI INOUE,* KEISUKE OHTO, AND HIROYUKI HARADA Department of Applied Chemistry, Saga University, 1-Honjo, Saga 840-8502
Received January 16, 2008. Revised manuscript received April 16, 2008. Accepted May 8, 2008.
Waste newsprint paper was modified with dimethylamine (DMA) to obtain a tertiary amine type adsorption gel called DMApaper gel. This new derivative was investigated for adsorption, from hydrochloric acid medium, of gold, palladium, and platinum as well as some base metals. The gel exhibited selectivity only for precious metals with a remarkably high capacity for Au(III), i.e., 4.6 mol/kg dry gel and comparable capacities, i.e., 2.1 and 0.9 mol/kg for Pd(II) and Pt(IV), respectively. Also, Au(III) was reduced to the elemental form during adsorption. Furthermore, column adsorption and subsequent elution of the adsorbed metal ions by acidic thiourea revealed encouraging recoveries (approximately 90%), thus enhancing the scope of the gel for effective preconcentration, separation, and recovery of precious metals. The effectiveness of recovery of precious metals from real industrial liquor was also tested, and it showed highly encouraging results with respecttothestabilityofthegelintheharshmedium,andselectivity for the targeted metal ions in the presence of excess of other metal ions.
Introduction Because of our growing concern for environmental pollution, the impending metals crises, and the energy crisis, methods and processes for materials recycling and recovery have become very necessary for the creation of a sustainable world. Recycling and reuse of metals, in particular, is very important since it expands the supply of feedstock for advanced materials and prevents the practice of excessive mining. The market for electrical and electronic devices has experienced sharp growth recently due to the attractive minimodels and their user-friendly features that keep being updated day by day to keep pace with intense competition. Personal computers, mobile phones, and entertainment devices have brought unprecedented change to our lifestyle, and thus they have become an indispensable part of our daily life. However, these regularly updated amenities have come only at the cost of a growing amount of electrical and electronic waste (e-waste), creating negative impacts on the environment and bringing serious problems to the supply of raw materials, especially precious metals which are not only scarce but also very essential for these advanced devices. This new crisis needs to be addressed cautiously because it has both * Corresponding author tel.: +81-952-28-8671; fax: +81-952-288591; e-mail:
[email protected]. 5486
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environmental and economic significance (1–3), as in most cases the precious metal content of e-wastes is unexpectedly higher than that in the ore itself. The usual practice for the recovery of the metals from such wastes includes dismantling the devices and treating each part separately for metal recovery, after which parts such as printed circuit boards (PCBs) are leached with aqua regia or hydrochloric acid containing chlorine gas. The conventional methods for the recovery of precious metals from such leach liquors involve the repeated use of various low capacity and/or low selectivity synthetic reagents. During such operations, large amounts of waste effluents or secondary wastes are generated that require costly treatment. To improve this situation, some research work has been conducted by using adsorbents prepared from various biomass wastes, as they are cheap, environmentally benign, biodegradable, and, most importantly, exhibit special affinity or selectivity toward particular metal species because of the unique natural structure. Fortunately, several biomasses including trees, herbs, agricultural and marine byproducts, and also microorganisms have been found to be useful for the separation, preconcentration, and recovery of metals (4–7). However, in order to replace the conventional petrochemical-based adsorbents by the renewable biomass sorbents in real cases, selectivity, stability, reproducibility, and proper disposal schemes need to be verified. In the present study, the prospective application of chemically modified waste paper for the recovery of Au(III), Pd(II), and Pt(IV) has been tested under acidic conditions.
Experimental Section Materials. Waste newspaper was used as the feed material for the adsorption gel. Microcrystalline cellulose (Merck, Germany), thionyl chloride (Sigma Aldrich, St. Louis), and dimethylamine (50%) (Wako, Japan) were used without further purification. Analytical grade chloroauric acid and chloroplatinic acid (Wako, Japan) were used to prepare Au(III) and Pt(IV) test solutions, respectively. Real industrial leach liquor sample containing several metallic components including precious metals was kindly provided by Tanaka Kikinzoku Kogyo, K.K., Tokyo, Japan. All other chemicals used for the synthesis and for adsorption tests were of analytical grade and were used without further purification. Methodology. The amount of adsorption was calculated from the difference in metal concentration before and after adsorption and the weight of the adsorption gel. Metal ion concentrations were measured using a Shimadzu model ICPS-8100 ICP/AES spectrometer. The FTIR spectra of the adsorption gel were determined with a JASCO model FTIR410 using KBr disk base. A Rigaku RINT-8829 X-ray diffractometer was used to record X-ray diffraction spectra (XRD). Preparation of the Adsorption Gel. Waste newspaper was crushed and washed with soap and water and treated with 20% sodium hydroxide solution, washed again till neutral pH, and dried. As shown in Scheme 1, the pretreated waste paper was chlorinated by treating with thionyl chloride in pyridine medium under a N2-atmosphere. The product obtained was washed and dried. Finally, the chlorinated paper was treated with dimethylamine and formaldehyde. The product was washed with dilute hydrochloric acid, dilute sodium chloride solution, and finally with distilled water. It was dried and crushed to obtain 150 µm mesh sized black powder and named as DMA-paper gel. For comparison, commercially available microcrystalline cellulose was also modified with dimethylamine and named as DMA-cellulose. 10.1021/es800155x CCC: $40.75
2008 American Chemical Society
Published on Web 06/25/2008
SCHEME 1. Preparation of DMA-Paper Gel
Adsorption Tests. A preliminary kinetic study of Au(III), Pd(II), and Pt(IV) uptake on the DMA-paper gel was carried out to determine the optimum time needed to reach equilibrium. The adsorption behavior of the DMA-paper gel for Au (III), Pd(II), Pt(IV), Cu(II), Fe(III), Ni(II), and Zn(II) was individually examined at varying hydrochloric acid concentrations (0.1 to 4 M) (M ) mol/dm3). For each metal ion, the initial metal concentration was 0.2 mM. Test solutions (15 mL) were mixed together with 20 mg of adsorption gels in stoppered flasks and shaken for 24 h using a thermostatted shaker maintained at 30 °C to attain equilibrium. Similar batch adsorption tests were carried out for the adsorption of Au(III), Pd(II), and Pt(IV) on DMA-cellulose and unmodified waste paper. Adsorption isotherms of Au(III), Pd(II), and Pt(IV) on DMApaper gel were also obtained by using test solutions of varying concentrations of these metal ions, i.e., 0.5-10 mM in 1 M hydrochloric acid solution. Test solutions were (15 mL each) mixed together with 20 mg of adsorption gels and shaken for 60 h at 30 °C to attain equilibrium. Similarly, tests to check the effectiveness of DMA-paper gel for the recovery of precious metals from industrial liquors was tested in static mode. Column Experiment: Loading and Elution. Because of their coexistence in a number of practical applications, a flow experiment for the separation of low concentrations of Au(III), Pt(IV), and Pd(II) from a high concentration of Cu(II) was conducted. A column (height ) 24.5 cm, inner diameter ) 8 mm) packed with 0.1 g DMA-paper gel was prepared. Prior to passing the test solution, the column was conditioned with distilled water followed by 1 M hydrochloric acid solution for 24 h. Meanwhile, a test solution containing about 20 mg L-1 of each of Au(III), Pt(IV), and Pd(II) and 100 mg L-1 Cu(II) was prepared in 1 M hydrochloric acid. This solution was then pumped at a constant flow rate of 6 mL/h (mean residence time ) 4.52 min, superficial flow velocity ) 12 cm/h) through the column using an Iwaki model PST-100N peristaltic pump. The effluent solution was collected at 1 h time intervals using a Bio-Rad model 2110 fraction collector prior to measurement of the metal concentrations. To elute the adsorbed metal ions, the loaded column was first washed with water and then passed a mixture of 0.1 M thiourea and 1 M hydrochloric acid at a rate of 6 mL/h. The eluate solution was collected similarly at 30 min time intervals prior to determination of the metal concentration. Similarly, for adsorption-elution cycle tests, the column was loaded by passing the test solution containing about 20 mg L-1 each of Au(III), Pd(II), and Pt(IV) in 1 M hydrochloric acid for 24 h, and the adsorbed metals were eluted by the same method as described above. The adsorption/elution operation was repeated three times.
modification was calculated by dividing the observed % from the elemental analysis by the calculated % composition of nitrogen, and the density of the functional group was calculated from the observed nitrogen value. For DMA-paper gel, the observed value for nitrogen was 5.07%, and hence the extent of modification was calculated as 68.5% (3.6 mol/ kg functional group density), whereas, in the case of DMA cellulose, it was only 1.01% nitrogen or 13.7% (0.72 mol/kg functional group density). This difference in the extent of modification under identical experimental conditions is attributable to the difference in the structure of paper and commercial cellulose. Paper cellulose is highly amorphous and is easily chemically modified whereas commercial cellulose is crystalline in nature, and consequently reagents cannot easily penetrate into the inner parts of the matrix. Effect of Shaking Time. The adsorption kinetics of Au(III), Pt(IV), and Pd(II) on the DMA-paper gel was studied at 30 °C. Figure 1 shows the time variation of adsorption of these metals. As evident from the figure, in the case of Pd(II) and Pt(IV), equilibrium was reached within 5 h, whereas in the case of Au(III), after a distinct change within 5 h, only slightly increased adsorption took place after this time, indicating that the adsorption kinetics is fast for all three metal species but some other phenomenon might be occurring in the case of Au(III), resulting in a small but steady increase in adsorption. Effect of Hydrochloric Acid Concentration on the Adsorption Behavior of DMA-Paper Gel. The adsorption behavior of the DMA-paper gel for various metal ions is shown in Figure 2, from which it is clear that all three precious metal ions, i.e., Au(III), Pt(IV), and Pd(II), were quantitatively adsorbed over the whole region of hydrochloric acid concentration, whereas the gel showed no affinity for base metals such as Cu(II), Fe(III), Ni(II), and Zn(II). This result indicates that the precious metals can be selectively recovered from any other coexisting base metal ions in hydrochloric acid medium using the DMA-paper gel. According to our previous study, Diaion WA30, a commercially available resin having dimethylamine functional group, showed a sharp decrease in Pd(II) adsorption and a significant increase in Fe(III) adsorption with increasing concentration of hydrochloric acid. Similarly, considerable adsorption of Zn(II) has also
Results and Discussion Elemental Analysis. Cellulose units being the major component of paper, the % composition of individual elements in the functionalized paper and cellulose was calculated on the basis of the tentative structure as shown in Scheme 1. The tentative % composition of carbon, hydrogen, and nitrogen for DMA-paper gel and DMA-cellulose gel are calculated as 50.8, 7.9, and 7.4, respectively. The degree of
FIGURE 1. Kinetics of adsorption of metal ions on DMA-paper gel. Initial concentration of metal ions ) 0.5 mM, [HCl] ) 1 M, wt of gel ) 20 mg, temperature ) 30 °C. VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Adsorption behavior of DMA-paper gel for various metal ions as a function of hydrochloric acid concentration. Initial concentration of metal ions ) 0.2 mM, wt. of gel ) 20 mg, shaking time 24 h, temperature ) 30 °C.
FIGURE 3. The adsorption isotherms for Au(III), Pd(II), and Pt(IV) on DMA-paper gel. [HCl] ) 1 M, vol of solution ) 15 mL, shaking time ) 60 h, temperature ) 30 °C, wt of gel ) 20 mg. been observed at low acid concentration (8). The adsorption behavior of the DMA-cellulose was also studied under the same experimental conditions (Supporting Information Figure 1). As expected from its poor degree of modification, adsorption of Au(III), Pd(II), and Pt(IV) was quite insignificant. To understand this behavior, the empirical values of the crystallinity index (C.I.) for microcrystalline cellulose and waste paper were determined using Segal’s method and found to be 82.11 and 65.18, respectively (9). This indicates that the difference in crystallinity plays major role in the diffusion property of the materials leading to the remarkable difference in the extent of modification. The adsorption behavior of unmodified waste paper also was insignificant (not shown).
From this comparison, it can be concluded that the waste paper gel prepared by a simple modification process can be utilized as an advanced sorption active material for the recovery of precious metals from complex mixtures. Adsorption Isotherms for Precious Metal Ions. The adsorption isotherm study for Au(III), Pd(II), and Pt(IV) onto the DMA-paper gel was carried out taking various concentrations of individual metal solutions. As shown in Figure 3 adsorption increases with increasing metal concentration and tends to approach constant values corresponding to each metal species in the high concentration region, i.e., plateau region, suggesting a typical Langmuir type adsorption. From the constant values in the plateau region, maximum adsorption capacity was evaluated as 4.6 mol/kg of the dry gel for gold which means 1 kg of the DMA-paper gel is capable of recovering almost an equivalent amount of gold. Taking account of the selectivity over other metal ions and the simplicity of gel preparation, this result is outstanding. Similarly, the capacities for palladium and platinum were determined to be 2.1 and 0.9 mol/kg, respectively, which are also comparatively high compared with those for other adsorbents, as shown in Table 1 (11–19). Similar to our previous works on lignophenol gels, the very high capacity for gold is attributed to the reduction of Au(III) to the Au(0) during adsorption (8, 10). In order to confirm the reductive adsorption of gold, the XRD spectrum of the gel before and after adsorption of Au(III), Pd(II), and Pt(IV) was determined. Four peaks at 38°, 44.5°, 64°, and 77° assigned to elemental gold were distinctly observed whereas no such peaks corresponding to elemental palladium and platinum were noticed. From this difference, it is obvious that the adsorption of palladium and platinum is purely an ionic interaction, whereas for gold, the ionic interaction is followed by subsequent reduction to the elemental form. This phenomenon is considered to be responsible for the higher adsorption capacity for gold, even higher than the amount of available functional sites as evaluated from the elemental analysis. However, the XRD spectrum of Diaion WA30 taken after the adsorption of Au(III) did not exhibit any peaks for gold suggesting the absence of reduction of Au(III). Figure 3, Table 1 Mechanism of Adsorption. As all the adsorption tests were carried out in chloride media, Au(III), Pd(II), and Pt(IV) exist mostly as anionic chloro complexes such as AuCl4-, PdCl42-, and PtCl62-, respectively. Similarly, the tertiary amine group of the DMA-paper gel in hydrochloric acid medium is protonated as described by eq 1. Hence, the adsorption of metal chloro complexes takes place according to the anion exchange reactions as expressed by eq 2 for Au(III) and by eq 3 for Pd(II) and Pt(IV), where the coordination numbers of these metal ions (4 for Au(III) and Pd(II) and 6 for Pt(IV)) are satisfied by coexisting Cl- ions. Other metal ions such as
TABLE 1. Comparison of Loading Capacities of DMA-Paper Gel for Au(III), Pd(II), and Pt(IV) with Other Adsorbents loading capacity (mg/g) adsorbent lysine modified cross-linked chitosan glysine modified cross-linked chitosan resin 2-mercaptobenzothiazole-bonded silica gel poly(vinylbenzyl chloride-acrylonitryle-divinylbenzene) modified with tris(2-aminoethyl)amine Amberlite IRC 718 EN-lignin PA-lignin glutaraldehyde-cross-linked chitosan sulfur derivative of chitosan glutaraldehyde-cross-linked chitosan flakes DMA-paper gel
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Au(III)
Pd(II)
Pt(IV)
reference
70.3 170 4.5
109 120 18
129 122 6.5
11 12 13
190
280
245
14
136 607 384 565.5 624 906
58.5 22.7 40.4 180 352 224
66.3 105 43 346.3 176
15 8 16, 17 16, 18 19 this work
FIGURE 4. Breakthrough profiles for Cu(II), Au(III), Pd(II), and Pt(IV). Feed concentration: Cu(II) ) 100 ppm, Au(III), Pd(II), Pt(IV) ∼ 20 ppm, flow rate ) 6 mL/h (a). Elution profiles for loaded Pd(II) and Pt(IV) with 0.1 M thiourea in 1 M HCl. Flow rate 6 mL/h (b). Fe(III) and Zn(II) also give rise to anionic chloro complexes over a wide chloride concentration region, and it is wellknown that these are solvent extracted with high molecular weight amines and adsorbed on anion exchange resins. However, these are not adsorbed to any extent on the present gels, a result which requires clarification in future work. RN(CH3)2¯ + H+·Cl-¯ T RN(CH3)2H+·Cl-¯ +
-
-
RN(CH3)2H ·Cl ¯ + AuCl4 T RN(CH3)2H
+
(1)
·AuCl4-¯
+ Cl
-
(2) 2[RN(CH3)2H]+·Cl-¯ + MCln2- T [RN(CH3)2H]2+·MCln2-¯ + 2Cl- (3) where M ) Pt(IV), Pd(II). Column Experiment: Loading and Elution. Since the DMA-paper gel has shown high selectivity and capacity for gold, palladium, and platinum, the selectivity over other metal ions was tested by recovering these precious metals from a mixture containing excess of Cu(II), a representative base metal coexisting in practical waste solutions, by means of column operation. Figure 4a shows the breakthrough profile of the tested metal ions from the column packed with DMApaper gel. The breakthrough of Cu(II) took place immediately after the start of flow whereas it took about 250 bed volumes
for Pt(IV). Similarly, in the case of Au(III) and Pd(II), breakthrough occurred after more than 300 bed volumes. From the area of breakthrough curve, the maximum loading capacities of the DMA-paper gel for Au(III), Pt(IV), and Pd(II) were evaluated as 0.26, 0.31, and 0.60 mol/kg of the dry gel, respectively. These values are much lower than the values observed in the batchwise adsorption isotherm study, which is possibly attributable to the continuous flow of the solution, competitive adsorption, lower metal concentration compared to that in the isotherm test (increasing adsorption with increase in metal concentration), and slower reduction kinetics in the case of gold. Figure 4 Figure 4b shows the elution profiles of the loaded precious metal ions with acidic thiourea solution. It reveals that all the metals were eluted with high concentration factors of 35 for Pd (II), 28 for Pt (IV), and nearly 20 for Au (III). In the same way, two more consecutive adsorption-elution cycle tests were conducted. The results are shown in Table 2. In the first cycle the test solution was passed until complete saturation was reached (Figure 4) whereas in cycles 2 and 3 the test solution was passed only for 24 h. For this reason, a big difference in the amount loaded in cycle 1 and 2 is observed. It is clear from the table that, on average, 92.8% Au(III), 85.2% Pt(IV), and 88.5% Pd(II) were eluted and recovered. The recovery % of precious metals in this study appears quite high, and it is probably due to the optimal affinity of the waste paper gel for the metal ions. Almost complete recovery of the loaded metal ions including the reduced gold from the loaded gel verifies the easy regeneration of gel for repeated use. In addition, comparable loading and recovery was achieved in each cycle which means the DMA-paper gel can be effectively used for at least three consecutive cycles adding more economic and environmental benefit to the material. Table 2 The IR spectra of the DMA-paper gel was taken before and after Au(III) loading and elution. In Figure 5, the disappearance of a weak band at 1581 cm-1 (corresponding to R2NH) in the gel after adsorption and the appearance of new peaks in 2370-2380 cm-1 region (corresponding to R3NH+, though it may not be strong evidence) indicates the adsorption process. Some new peaks also appear after elution at 958 and 1112 cm-1 in the spectra. However, it is not easy to trace the source of electrons responsible for the reduction of gold to its elemental form by spectroscopic methods as described by Richardson et al. (20). Recovery of Precious Metals from Industrial Sample. Before carrying out the adsorption tests, the industrial metal liquor TK was qualitatively analyzed for its composition by using a Shimadzu Model ICPS-8100, ICP/AES spectrometer. Several metal ions and some other elements were found to be present at various proportions of concentration. Metal ions in the order of tentative concentration range in ppm are as follows. In thousands range: Cu, Zn, Fe, Ni, Cr, Al, and Cd; in hundreds range: Ir, Br, Ca, Mo, Au, Ti, Er, and Mn; in the tenths range: Sn, I, Pb, W, Ag, Pd, Pt, and so on. Similarly the concentrations (in ppm) of precious metals and some base metals were measured quantitatively and are found to be Cu(8400), Zn(2600), Fe(1900), Au(250), Pd(16), and Pt(11). It shows that sample TK contains several times excess of
TABLE 2. Performance of the DMA-Paper Gel in Consecutive Adsorption-Elution Cycles cycle 1 (mg)
Au(III) Pt(IV) Pd(II)
cycle 2 (mg)
cycle 3(mg)
adsorbed
eluted
% recovery
adsorbed
eluted
% recovery
adsorbed
eluted
% recovery
5.07 6.09 6.37
4.86 4.62 4.32
95.9 75.9 67.8
3.60 2.83 2.38
3.51 2.52 2.37
97.5 89.0 99.4
3.46 2.76 2.25
2.94 2.51 2.22
85.1 90.8 98.5
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FIGURE 5. IR spectra of the DMA-paper gel taken before adsorption, after Au(III) adsorption, and after elution with thiourea. test of individual model solution can be obtained also in the real solution. This result is strongly encouraging in terms of selectivity and consistent adsorption of metal ions even in the presence of nitrate of aqua regia, a common leaching agent in various metal refineries.
Acknowledgments
FIGURE 6. Solid-liquid ratio for the adsorption of precious metals and copper from sample TK on DMA-paper gel. copper, iron, and zinc than the targeted precious metals. As the aqueous chemistry of one metal ion may resemble with some other in the mixture, recovering any desired metal from such a complex mixture is a tough task. Also, any adsorption material or solvent extraction agent which is exclusively selective toward a single metal ion is rare. Taking this fact into consideration, any sorbent that will be useful to recover precious metals must be especially inert toward the other metal ions such as copper, iron, and zinc including other coexisting metal ions. The acid concentration of sample TK was found to be 2.5 M; hence, the adsorption test was carried out without dilution. Also, as the sample was generated after aqua regia treatment, it is expected to contain some nitrate ions. In order to know the affinity of DMA-paper gel for the precious metals in the TK mixture, solid-liquid ratio adsorption tests in static mode taking 10 mL of TK with varying mass of the gel was carried out. The observed result as summarized in Figure 6 shows almost complete recovery of precious metals at solid-liquid ratio as less as five. As explained earlier, the concentration of copper in sample TK is several folds higher than that of any given metals but its adsorption as observed in Figure 6 is almost negligible. Despite the concept that many metal ions including gold, palladium, platinum, and copper form anionic chloro complexes in acidic chloride medium, the selective adsorption of precious metals in comparatively low concentration over the dominating level of copper ion is very interesting. In addition, the adsorption of other metal ions such as iron or zinc, the concentration of which in TK are also several fold of that of the precious metals was also found to be negligible. Further, the selectivity pattern over other metal ions present in smaller concentration range was tentatively studied by performing the qualitative measurement of the samples after adsorption and no significant difference in concentration of any such metal ions was detected. All of these results confirm the fact that the selectivity pattern of DMA-paper gel observed in the batch 5490
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This study was financially supported by Industrial Technology Research Grant Program in 2006 (KS18000002) from New Energy and Industrial Technology Development Organization (NEDO) in Japan. The authors acknowledge Tanaka Kikinzoku Kogyo, K.K., Tokyo, Japan, for kindly providing the sample solution TK.
Supporting Information Available Details of the preparation of DMA-paper gel and the figure showing adsorption behavior of DMA-cellulose. This material is available free of charge via the Internet at http:// pubs.acs.org.
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(15) Park, C.; Chung, J. S.; Cha, K. W. Separation and preconcentration method for palladium, platinum and gold from some heavy metals using Amberlite IRC 718 chelating Resin. Bull. Korean Chem. Soc. 2000, 21, 121–124. (16) Arrascue, M. L.; Garcia, H. M.; Horna, O.; Guibal, E. Gold sorption on chitosan derivatives. Hydrometallurgy 2003, 71, 191–200. (17) Ruiz., M.; Sastre, A. M.; Guibal, E. Palladium sorption on glutaraldehydecrosslinked chitosan. React. Funct. Polym. 2000, 45, 155–173. (18) G., Guibal. E.; Von Offenberg Sweeney, N.; Vincent, T.; Tobin, J. M. Sulfur derivatives of chitosan for palladium sorption. React. Funct. Polym. 2002, 50, 149–163. (19) Guibal, E.; Larkin, A.; Vincent, T.; Tobin, J. M. Chitosan sorbents for platinum sorption from dilute solutions. Ind. Eng. Chem. Res. 1999, 38, 4011–4022. (20) Richardson, M. J.; Johnston, J. H.; Borrmann, T. Monomeric and polymeric amines as dual reductants/stabilisers for the synthesis of gold nanocrystals: a mechanistic study. Eur. J. Inorg. Chem. 2006, 13, 2618–2623.
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