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Gold sorption on weak base anion exchangers with aminoguanidyl groups
European Polymer Journal 38 (2002) 2239–2246 www.elsevier.com/locate/europolj
Gold sorption on weak base anion exchangers with aminoguanidyl groups Dorota Jermakowicz-Bartkowiak, Bo_zena N. Kolarz
*
Institute of Organic and Polymer Technology, Wrocław University of Technology, Wyspianskiego 27, 50-370 Wroclaw, Poland Received 25 October 2001; received in revised form 1 April 2002; accepted 17 April 2002
Abstract Two resins with different matrices, both bearing aminoguanidyl ligands, were obtained and used for gold sorption from hydrochloric acidic and from alkali solutions. Resin 1 was a aminoguanidine derivative of poly(acrylonitril-covinyl acetat-co-divinyl benzene) terpolymer (AN/VA/DVB), (67:27:10 wt.%), and Resin 2 the same derivative of gel expanded poly(vinylbenzyl chloride-co-divinylbenzene) copolymer (VBC/DVB). The resins sorption capacity and sorption isotherms of both resins were determined for AuCl 4 and Au(CN)2 anions. Resin 2 revealed the best sorption of gold from acidic and alkali solutions: 68 and 23 mg/g, respectively. The effect of the HCl concentration on AuCl 4 removal from solution was investigated. Gold was recovered quantitatively from both resins using a 5% thiourea solution in 0.1 HCl. Both resins remained ability of multiple gold sorption and desorption under acidic conditions. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aminoguanine resin; Gold sorption; Dicyanoaurate sorption; Tetrachloroaurate sorption; Gold desorption
1. Introduction Polymeric ion exchangers for gold recovery are subject of research although so far active carbons have been used for the extractions of gold on an industrial scale from the gold-bearing pulps for more than 30 years. There exist three processes of gold recovery in which Au is present either as chloride and thiourea complexes under acidic conditions or cyanide complexes under alkaline conditions. It seems interesting that gold can be recovered from varied waste materials, which is proved by the fact that 40% of overall gold production in the 1980s was obtained through recycling [1]. Gold is one of the rarest elements on earth. Its concentration is about 4 mg/t (ppb) in the upper crust and 0.01 mg/m3 in seawater. Adsorption methods are widely used and are the
* Corresponding author. Tel: +48-71-320-38-26; fax: +48-71320-3678. E-mail address: [email protected] (B.N. Kolarz).
most effective methods of preconcentration and separation of gold from aqueous solution. Many adsorbents have been used in this respect. Selectivity for gold is displayed by the functional resins, ion exchange resins as well as chelating resins bearing ligands with nitrogen groups [2–8]. Fundamental studies on guanidine as a ligand for gold and metals sorption have been undertaken. Researchers incorporated sulfonyl guanidine ligand into a styrene– divinylbenzene copolymer [9,10]. Guanidine and its derivatives are very interesting organic compounds. Their ligands can be incorporated into polymer matrices to form novel functional resins. Aminoguanidine is a strong organic amine [7,11] (with pKa 11) since the resulting positive charge upon protonation can be delocalized throughout the molecule as shown in Fig. 1 [11,12]. The introduction of the aminoguanidine group to a polymer matrix has given resins with high pKa values which, in turn, allowed gold removing from cyanide leach solution at the natural pH (about 9–10) and even at pH 13 [13]. In our previous papers we have found that
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 1 2 9 - 5
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D. Jermakowicz-Bartkowiak, B.N. Kolarz / European Polymer Journal 38 (2002) 2239–2246
Fig. 1. Resonance forms of aminoguanidine monocation tautomer [12].
the introduction of guanidine or aminoguanidyl ligands to the polymer matrices could lead to novel chelating and ion exchange resins. Their synthesis, characteristics and abilities of gold recovery from cyanide and acidic solutions were discussed [14–18]. Few reports concerning guanidine resin are available [17,18]. Some others are patented [19–22]. The aim of our work is to present gold sorption as dicyanoaurate Au(CN) 2 and tetrachloroaurate AuCl4 ions on two selected resins, obtained from various matrices: porous and gel, bearing aminoguanidyl groups and highlight their abilities of multiple sorption after some sorptions and desorptions cycles from HCl solutions.
2. Experimental 2.1. Materials All copolymers were obtained by suspension polymerisation with 0.5 wt.% of benzoyl peroxide as an initiator. The first one is a porous AN/VA/DVB terpolymer obtained from the following monomers: acrylonitrile (AN), vinyl acetate (VA) and divinylbenzene (DVB) as crosslinker (the nominal crosslinking equals 10 wt.%). The content of monomers in their mixture reached 63/27/10 wt.% (15/5/1% mol). Polymer was synthesised in the presence of inert diluents cyclohexanol and dodecanol (9:1 v/v). The second polymer is a gel copolymer of vinylbenzyl chloride/divinylbenzene (2 wt.%) (VBC/DVB) and was prepared in the presence of toluene (50 wt.% in respect to the amount of monomers). More details on the preparation of AN/VA/ DVB and VBC/DVB copolymers can be found in [23,24]. Copolymer 1, AN/VA/DVB, was swollen for 24 h in mixture of: dioxane–butanol–water (10/5/1 v/v per gram of dry polymer) but copolymer 2, VBC/DVB, was swollen in dioxane only. Then the copolymers were reacted with aminoguanidine bicarbonate for 24 h (4 mol excess over the amount of nitrile groups and chlorine atoms in polymers) by refluxing for 24 h resulting in Resin 1 and Resin 2, respectively [23,24]. All reagents were purchased from Aldrich Chemical Co. Vinylbenzyl chloride was purified by distillation under reduced pressure.
2.2. Methods Water regain was measured using the centrifugation technique in which about 1 g of polymer swollen in water was placed in glass vial having fritted glass bottom and centrifuged for 5 min at 3000 rpm. After drying under high vacuum at 60 °C for 24 h polymer was weighted. Water regain expressed as ratio (mw –md ) and md , where mw is the weight of wet polymer and md is the weight of dry polymer [27]. The nitrogen content was determined using Kiejdahl’s method employed to polymers [28]. The chlorine content was measured by burning of ca. 30 mg of dry polymer sample in an oxygen-filled flask. The products of burning were adsorbed in diluted hydrogen peroxide solution. The content of Cl was determined using Volhardt’s method of titration [29]. FTIR spectra in KBr pellets were recorded on a Perkin–Elmer System 2000 spectrophotometer. The porosity of copolymer 1 swollen in water was determined by Inverse Steric Exclusion Chromatography according to Ref. [23] and Gorbunov et al. [30]. The anion exchange capacity was determined according to Hecker’s method [31] and used to calculate ligand concentration. Concentration of carboxyl group was determined by method described with details in Ref. [23]. A batch method was used to examine the sorption of gold [14] in acidic and alkali solutions. Resin, in a swollen form, (equivalent to 0.13 mmol of ligand concentration), was introduced into 100 ml PP bottle containing 50 ml of gold solution and shaken for 48 h in an ambient temperature. Stock solutions of Au(III) were prepared by dissolving appropriate amounts of HAuCl4 3H2 O in HCl solutions. Isotherms studies were conducted by varying initial concentrations of hydrogen tetrachloroaurate solutions containing from 50 to 1000 mg/dm3 Au. Stock solutions of Au(I) were prepared by dissolving appropriate amount of KAu(CN)2 formulated with 0.05 N KCN. pH of this solution has been adjusted to 9.6. The resin in the swollen form was contacted for 48 h with dicyanoaurate potassium solutions containing from 50 to 500 mg/dm3 Au. After 48 h the polymers were separated by filtration and the concentration of Au was determined using Perkin–Elmer Analyst 100 atomic spectrophotometer set at 242.8 nm wavelength. The amount of gold sorption in the resin phase (in mg Au/g resin) was calculated from the mass balance and the initial concentration in the aqueous
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in the presence of inert diluents––cyclohexanol and dodecanol (9:1 v/v). More details on the preparation of AN/VA/DVB can be found in [23]. The second starting copolymer 2 (VBC/DVB), was obtained from monomers mixture of vinylbenzyl chloride and 2% divinylbenzene by method described in our previous paper [24]. The polymerisation was carried out in the presence of toluene (50 wt.% in respect to the monomers mixture) in order to obtain material with an expanded gel structure. The characteristics of copolymers and resins derived from them are presented in Tables 1 and 2. The schemes of modifications are showed in Fig. 2. Resin 1 was obtained by the aminolysis of nitrile groups of copolymer 1 using aminoguanidine bicarbonate in a boiling butanol– dioxane–water mixture. It was found that during that process under strongly alkaline conditions nitrile groups were converted to N-substituted amides (1.9 mmol/g) and to carboxyl groups (3.3 mmol/g) as a result of alkaline hydrolysis of the ester and nitrile groups. Resin 2 was prepared by direct reaction of chlorine atoms in VBC mers (5.5 mmol of Cl/g) of copolymer 2 with aminoguanidine bicarbonate resulting in aminoguanidine ligands attached to the matrix, in which the concentration of ligands reaches 1.5 mmol/g. Elemental analysis of Resin 2 gave 6.6 mmol of N/g. That amount corresponds to about 30% of the theoretical content calculated based on the composition of the monomer mixture. Polymers have various matrices and various structures. Copolymer 1 is porous with the porosity of 0.62 and water regain of 1.52 g/g, which is about five times higher compared with the gel expanded copolymer 2. Resin 1 obtained from AN/VA/DVB copolymer displays smaller water regain than the initial copolymer 1. This can be explained by additional crosslinking during the modification reaction. Its ligand concentration is greater than in Resin 2 and amounts to 1.9 mmol/g while carboxyl group content is 3.3 mmol/g.
solution. The distribution coefficients (K) were calculated as the ratio of the amount of gold adsorbed by 1 g of resin and the amount of metal remaining in 1 ml of solution after sorption. All elution studies were carried out at an ambient temperature in batch reactions from AuCl 4 anions loaded resins. The procedure was as follows: definite amounts of swollen resin beads were contacted with 200 ml of gold solution, ½Au ¼ 50 mg/dm3 in 0.1 and 1 M HCl. After 24 h of shaking the resin was filtrated off and the gold concentration was determined. Then the gold loaded resin was washed with distilled water and shaken with 50 ml of 5% thiourea solution in 0.1 HCl. The filtrates were analysed for the indirect determination of the amount of gold desorbed. Then the resin bed was washed again with water and used again for gold sorption. This sorption–desorption procedure was repeated five times.
3. Results and discussion It has been previously shown [13–16] that chemical compositions and structures of copolymers matrices with guanidyl and aminoguanidyl ligands influence the dicyanoaurate sorption and its selectivity from alkaline solutions. In this paper we report the results of our studies concerning comparison of two resins obtained from different matrices, containing aminoguanidine ligands, and their gold sorption abilities from both cyanide and acidic solutions. Copolymers were obtained by the suspension polymerisation. These copolymers differ in both the crosslinking degree, determined by nominal DVB concentration in the starting monomer mixture, and the structure developed during synthesis. They were synthesised under various solvation conditions, generated by appropriate solvents used to dilute the monomers. The acrylic porous terpolymer (AN/VA/DVB), copolymer 1 containing 10 wt.% of DVB, was obtained
Table 1 Characteristics of starting copolymers Copolymer
Matrix
Porosity
Water regain (g/g)
N (mmol/g)
Cl (mmol/g)
1 2
AN/VA/DVB (10%) VBC/DVB (2%)
0.62 Gel
1.52 0.28
11.9 –
– 5.5
Table 2 Characteristics of anion exchangers Resin
1 2 a
Matrix
AN/VA/DVB VBC/DVB
Water regain (g/g)
1.04 0.44
N (mmol/g)
14.5 6.6
Ligands (mmol/g)
1.9 1.5
AuCl 4
Au(CN) 2
Sorptiona (mg/g)
K
Sorptiona (mg/g)
K
36 68
30000 33300
2.3 23
52 2260
mg of gold metal sorbed by 1g of dry resin, gold concentration 50 mg/dm3 , K distribution coefficient.
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Fig. 2. Scheme of aminoguanidyl ligand introduction into polymer matrices.
Fig. 3. FTIR spectra of copolymer 1 and Resin 1.
Figs. 2 and 3 are the FTIR spectra of resultant polymers. It is reported [32] that there are four characteristic peaks for guanidine compounds as following: mNH at about 3300 cm1 ; mC@N at 1689–1650 cm1 ; dNH at about 1640 cm1 and dCAN at about 1300 cm1 . The introduction of aminoguanidine groups to copolymers 1 and 2 was confirmed by FTIR spectra. As shown in Fig. 3 after the modification of the copolymer 1 peak ascribed to the nitrile groups at 2240 for cm1 is decreased. Degree of substitution calculated from these spectra is ca. 50%. In these spectra the intensity of the bands at 1235 and 1740 cm1 resulting from CAO and C@O stretching vibrations, respectively, clearly decreases compared with starting copolymer 1. The spectra in Fig. 3 exhibit the appearance of new peak at 1684 cm1 for the Resin 1 due to the C@N stretching vibration responsible for existing aminoguanidyl groups. This band in am-
inoguanidine bicarbonate spectra is shifted to 1689 cm1 . The broadening of bands at 3400–2935 cm1 in the spectrum of the Resin 1 may suggest the presence of existing amino and OH groups. The presence of aminoguanidine groups in Resin 2 (see Fig. 4), formed by replacement of chlorine atom in copolymer 2 (VBC/ DVB) is revealed by disappearance of the band at 1265 cm1 responsible for existing CH2 Cl groups and appearance of the band at 1645 cm1 . Moreover, a wide band of strong intensity with peak at 3412 cm1 can be observed that is due to vibrations of the NAH bonds in NH2 groups. IR spectra of investigated Resin 2 shows the appearance of new peak at 3348 cm1 region due to stretching vibration of ANH2 groups. Decrease of CN band (2240 cm1 ) for the Resin 1 and disappearance of the CH2 Cl band (1265 cm1 ) for the Resin 2 in the IR spectra proves conversion of CN and CH2 Cl groups. It
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Fig. 4. FTIR spectra of copolymer 2 and Resin 2.
is obvious that there are clear characteristic peaks for investigated resins, which means that the resultant polymers have aminoguanidine ligands. Figs. 5 and 6 present the isotherms of gold sorption for Resin 1 and Resin 2 in the range 0.1–1 M HCl and at pH 9.6 for the Resin 2. As shown in Fig. 6, Resin 2 displays excellent sorption capacity for gold Au(III) in HCl solution and much better that Resin 1 presented in Fig. 5. Resin 2 shows particularly strong preferences for tetrachloroaurate and dicyanoaurate anions in contrast to the Resin 1 and the highest sorption of Au(III)
Fig. 6. Sorption isotherms of Au(I) dicyanoaurate and Au(III) tetrachloroaurate complexes on Resin 2. Gold sorption, mg of gold sorbed by g of dry resin. [Au]eq mg/g; gold concentration in aqueous phase after 48 hr. Experimental conditions: 0.125 mg of swollen resin and 50 ml of gold solution in 0.1–1 M HCl and at 9.6 pH at room temperature.
Fig. 5. Sorption isotherms of Au(III) tetrachloroaurate complexes on Resin 1. Gold sorption, mg of gold sorbed by g of dry resin. [Au]eq mg/g; gold concentration in aqueous phase after 48 h. Experimental conditions: 0.140 mg of swollen resin and 50 ml of gold solution in 0.1–1 M HCl at room temperature.
reaches 68 mg Au/g in 0.1 HCl solution and 23 mg Au/g from cyanide solution at 9.6 pH. The sorption from 0.1–1 HCI solutions for the Resin 1 is two to three times smaller. Moreover, Resin 1 does not reveal significant gold sorption in cyanide solution (isotherm not shown in Fig. 5). It reaches only 2.3 mg/g from 50 mg/dm3 solution whereas sorption for the Resin 2 is over 23 mg/g. The high concentration of negatively charged carboxylate anions in the Resin 1 probably can adversely influence
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the sorption of dicyanoaurate anions. Repulsive forces between cyanoaurate and negatively charged carboxylates can explain low Au(CN) 2 sorption on this resin. The aurocyanide anion has a relatively low charge density (1 electric charge is carried by five atoms) and according to Diamond and Whitney’s theory ‘‘the ions with low charge density will favour the poorly solvating phase’’ [33]. According to this principle Au(CN) 2 anions are strongly extracted by more hydrophobic Resin 2 which displays lower aminoguanidine ligand content (1.5 mmol/g), despite the fact that Resin 1 contains 1.9 mmol of aminoguanidine ligands per gram (Tables 1 and 2). The effect of HCl concentration on the gold sorption was investigated and is presented in Figs. 5 and 6. The decrease of gold sorption with the increasing concentration of counter-ion (Cl ) can be explained by the competition between the gold chlorocomplexes and the chloride anions, which may interact with the functional aminoguanidyl groups of resins [25,26]. The sorption of gold chlorocomplexes by resins containing nitrogen ligands can take place according to the mechanism of ion exchange when nitrogen atoms are protonated in the reaction:
or can occur through formation of gold complexes when nitrogen acts as the coordinating atom. As shown in Figs. 5 and 6 the sorption at 0.1 M HCl is the best, as could be expected, as there are fewer protons in the solution competing for the sorption sites on the resin. These abilities can be caused by some factors concerning coordinating resins and are connected with hydrophilicity or hydrophobicity of resins, the structures of their matrices and the steric hindrance around ligands [27]. The other factors, which can be relevant, are charge, size and geometry of the complex anions and properties of the central atom [34]. The sorption capacity of a resin bearing amino groups is pH dependent due to the protonation of the nitrogen atom of the functional groups. Gold complexes, tetrachloroaurate and dicyanoaurate, can be extracted from aqueous solution by a variety of sorbents containing N-coordinating atom. When amino resins are used, both the ion exchange and the complex formation mechanisms can take place via the nitrogen atoms of the functional groups. The decrease in sorption of gold at higher HCl concentration was probably caused by the mass action effect of chloride and increased competition from chloride ion. At high HCl concentrations HAuCl4 is largely undissociated and so would not participate in the ion exchange and its extraction declines [25,26,35]. However,
Fig. 7. Logarithm of distribution coefficient and sorption of Au(III) from HCl solutions. Experimental conditions: 0.125 mg of swollen Resin 2; 0.140 mg Resin 1, and 50 ml of gold solution (350 ppm Au(III)) in 0.1–3 M HCl at room temperature.
with increasing HCl concentrations, the distribution coefficient K decreases as a result of the competition of increasing chloride ions in the external solutions [35]. Fig. 7 shows the dependence of log K and sorption from 350 mg/dm3 of gold solution in acidic solutions. The investigated resins were tested in 0.1–3 M hydrochloric solutions. The highest distribution coefficients and sorption values were displayed by Resin 2. The plotting of log K as a function of pH gives lines with a slope of 1.236 and 0.949 for the Resin 1 and 2 respectively, suggesting that the ion exchange plays a great role in the sorption of Au(III) by Resin 2 under these conditions. Resin 1 is strongly hydrophilic resin, containing hydrophilic hydroxyl and carboxyl groups obtained during the hydrolysis of nitrile and acetate groups. Its hydrophilic matrix and higher degree of crosslinking, 10% DVB, and the porous structure can be responsible for smaller tetrachloroaurate sorption from HCl solutions compared with low crosslinked Resin 2 obtained from gel expanded VBC/DVB copolymer. The complete gold desorption from investigated resins was carried out using 5% thiourea solution in 0.1 M HCl. As shown in Fig. 8 their ability of gold sorption in 0.1 HCl was invariable and reached 36 and 68 mg/g for Resin 1 and Resin 2, respectively. Five cycles of the sorption–desorption processes were investigated and the resins’ ability of gold sorption in HCl solution remains unchanged after the first and the subsequent cycles. After the fifth process of gold desorption the nitrogen content was determined again and turned out to be the same as in the initial resin. Based on nitrogen content and on IR spectra we can say that resin does not change in subsequent sorption–desorption processes. Resins 1 and 2 are suitable for multiple processes of gold sorption–desorption from HCl solution in the range from 0.1–1 M HCl.
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Fig. 8. Gold sorption from HCl solution after processes desorption by elution with 5% thiourea.
4. Conclusions 1. Resins bearing aminoguanidyl ligands (Resin 1 and 2) are useful in the sorption of tetrachloroaurate anions from acidic solution. 2. The porous, hydrophilic acrylic Resin 1, containing high amount of carboxylic groups is not suitable for aurocyanide anions sorption at pH 9.6 and its sorption reaches only 2.3 mg/g. 3. The gel Resin 2 obtained from hydrophobic matrix of vinylbenzyl chloride and divinylbenzene copolymer reveals significant sorption of gold from both acidic and alkali solutions, 68 and 23 mg/g, respectively. 4. The investigated resins may be useful in many cycles of sorption–desorption of tetrachlorourate anions in acidic solutions. 5. Gold can be quantitatively eluted from loaded resins with acidic thiourea solution at an ambient temperature. Acknowledgements This work was done with financial support from State Committee for Scientific Research through grant 3 T09B 067 16. References [1] Chmielewski AG, Urbanski TS, Migdal W. Separation technologies for metals recovery from industrial wastes. Hydrometallurgy 1997;45:333–44. [2] Sanchez JM, Hidalgo M, Salvado V. The separation of Au(III) and Pd(II) in hydrochloric acid solutions by strong anion type II exchange resins: the effect of counter ion concentration and temperature. Solv Extr Ion Exch 2000;18(6):1199–217.
[3] Hainey P, Sherrington DC. Oligoamine-functionalised poly(glycidyl methacrylateethyleneglycol dimethacrylate) resins as moderate base extractants for gold from cyanide solutions. React Funct Polym 2000;43:195–210. [4] Lukey GC, Van Deventer JSJ, Chowdhury R, Shallcross DC. The speciation of gold and copper cyanide complexes on ion-exchange resins containing different functional groups. React Funct Polym 2000;44:121–43. [5] Lawson PJ, Dicinoski GW, Rideout JA. Gold and silver anion exchange resins. Proc Randol Gold Forum, Beaver Creek, 1993. p. 301–8. [6] Li W, Coughlin M, Albright R. Polymer pendant ligand chemistry, 4. Recovery of precious metal ions from strongly acidic solution with a polymer-supported o-phenylenediamine hydrochloride ligand. React Funct Polym 1995;28: 89–96. [7] Dicinoski GW. Novel resins for selective extraction of gold from copper rich ores. S Afr J Chem 2000;53(1):33– 43. [8] Warshawsky A, Kahana N, Kampel V, Rogachev I, Meinhardt E, Kautzmann R, Cortina JL, Sampaio C. Ion exchange resins for gold cyanide extraction containing a piperazine functionality 1. Macromol Mater Eng 2000;283:103–14. [9] Gulko A, Feigenbaum G, Schmuckler G. Separation of palladium(II) and platinum(II) chlorides by means of a guanidine resins. Anal Chim Acta 1971;51:397–402. [10] Koster G, Schmuckler G. Separation of noble metals from base metals by means of a new chelating resin. Anal Chim Acta 1967;38:179–84. [11] Perrin DD. Dissociation constants of organic bases in aqueous solution. London: Butterworths; 1965. [12] Koskinen JT, Koskinen M, Mutikainen I, Mannfors B, Elo H. Experimental and computational studies on aminoguanidine free base, monocation and dication, Part I. Zeitschrift fur Naturforschung 1996;52b:1771–8. [13] Kolarz BN, Jermakowicz-Bartkowiak D, Trochimczuk AW, Apostoluk W. Influence of the structure of chelating resins with guanidyl groups on gold sorption. React Funct Polym 1999;42:213–22.
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[14] Kolarz BN, Bartkowiak D, Trochimczuk AW, Apostoluk W, Pawł ow B. New selective resins with guanidyl groups. React Funct Polym 1998;36:185–95. [15] Trochimczuk AW, Kolarz BN, Jermakowicz-Bartkowiak D. Sorption of dicyanoaurate anions by novel resins containing N,N’-substituted diamides of malonic acid. Sep Sci Technol 1999;34:2243–50. [16] Jermakowicz-Bartkowiak D, Kolarz BN, Trochimczuk AW, Apostoluk W. Selectivity of resins with guanidyl groups towards dicyanoaurate anions. Solv Extr Ion Exch 1999;17:613–33. [17] Kordosky GA, Kotze MH, Mackenzie JMW, Virnig MJ. New solid and liquid ion exchange extractantas for gold. XVIII International Mineral Processing Congress, Sydney, Australia, 23–28 May 1993. [18] Virnig MJ, Mackenzie JM, Adamson C. The use of guanidine-based extractants for recovery of gold. In: Hidden wealth, SAIMM, Johannesburg, South Africa, 1996. p. 151–6. [19] Lin WL, Mattison PL, Virnig MJ. Patent USA, Recovery of precious metals,12-02-1991, no 4 992 200. [20] Lin WL, Mattison PL, Virnig MJ. Patent USA, Recovery of precious metals, 02-06-1991, no 5 028 259. [21] Virnig MJ. Patent USA, Recovery of precious metals, 3003-1993, no 5 198 021. [22] Virnig MJ. Patent USA, Recovery of precious metals, 2308-1994, no 5 340 380. [23] Kolarz BN, Jezierska J, Bartkowiak D, Gontarczyk A. Acrylic resins with complexes of guanidyl groups and copper(II). React Funct Polym 1994;23:53–61. [24] Kolarz BN, Trochimczuk AW, Jermakowicz-Bartkowiak D, Jezierska J, Apostoluk W. Anion exchangers with alkyl substituted guanidyl groups. Gold sorption and Cu(II) coordination. React Funct Polym 2001;48:169–79.
[25] Kumar S, Verma R, Venkataramani B. Solv Extr Ion Exch 1995;13(6):1097–121. [26] Hudson MJ, Leung BKO. Synthesis and characterization of a copolymer with pendant weak base groups suitable for the separation of gold (III) from base metals in chloride media. Eur Polym J 1992;28(8):1001–6. [27] Trochimczuk AW. Selective recovery of gold from acidic solutions using resins with methylenediphosphonate and carboxymethylphosphonate ligands. Solv Extr Ion Exch 2001;19(2):301–13. [28] Trochimczuk AW, Kolarz B, Jermakowicz-Bartkowiak D. Metal ion uptake by ion-exchange/chelating resins modified with cyclohexene oxide and cyclohexene sulphide. Eur Polym J 2001;37:559–64. [29] Vogel AI. Handbook of quantitative inorganic analysis. London: Longman; 1978. [30] Gorbunov AA, Solovyova LYa, Pasechnik VA. Fundamentals of the theory and practice of polymer gel-permeation chromatography as a method of chromatographic porosimetry. J Chrom 1988;448:307–32. [31] Hecker H. Beitrag zur basizit€atsprufung von stark basischen anionenaustaschern. J Chrom 1974;102:135–8. [32] Yumei Z, Jianming J, Yanmo C. Synthesis and antimicrobial activity of polymers guanidine and biguanidine salts. Polymer 1999;40:6189–98. [33] Riveros PA. Selectivity aspects of the extraction of gold from cyanide solutions with ion exchange resins. Hydrometallurgy 1993;33:43–58. [34] Iglesias M, Antico E, Salvado V. Characteristic of metalfix–chelamine and its application in precious metal adsorption. Solv Extr Ion Exch 2000;18(5):965–79. [35] Leung BKO, Hudson MJ. A novel base anion exchange resin which is highly selective for the precious metals over base metals. Solv Extr Ion Exch 1992;10(1):173–90.
Gold sorption on weak base anion exchangers with aminoguanidyl groups Dorota Jermakowicz-Bartkowiak, Bo_zena N. Kolarz
*
Institute of Organic and Polymer Technology, Wrocław University of Technology, Wyspianskiego 27, 50-370 Wroclaw, Poland Received 25 October 2001; received in revised form 1 April 2002; accepted 17 April 2002
Abstract Two resins with different matrices, both bearing aminoguanidyl ligands, were obtained and used for gold sorption from hydrochloric acidic and from alkali solutions. Resin 1 was a aminoguanidine derivative of poly(acrylonitril-covinyl acetat-co-divinyl benzene) terpolymer (AN/VA/DVB), (67:27:10 wt.%), and Resin 2 the same derivative of gel expanded poly(vinylbenzyl chloride-co-divinylbenzene) copolymer (VBC/DVB). The resins sorption capacity and sorption isotherms of both resins were determined for AuCl 4 and Au(CN)2 anions. Resin 2 revealed the best sorption of gold from acidic and alkali solutions: 68 and 23 mg/g, respectively. The effect of the HCl concentration on AuCl 4 removal from solution was investigated. Gold was recovered quantitatively from both resins using a 5% thiourea solution in 0.1 HCl. Both resins remained ability of multiple gold sorption and desorption under acidic conditions. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aminoguanine resin; Gold sorption; Dicyanoaurate sorption; Tetrachloroaurate sorption; Gold desorption
1. Introduction Polymeric ion exchangers for gold recovery are subject of research although so far active carbons have been used for the extractions of gold on an industrial scale from the gold-bearing pulps for more than 30 years. There exist three processes of gold recovery in which Au is present either as chloride and thiourea complexes under acidic conditions or cyanide complexes under alkaline conditions. It seems interesting that gold can be recovered from varied waste materials, which is proved by the fact that 40% of overall gold production in the 1980s was obtained through recycling [1]. Gold is one of the rarest elements on earth. Its concentration is about 4 mg/t (ppb) in the upper crust and 0.01 mg/m3 in seawater. Adsorption methods are widely used and are the
* Corresponding author. Tel: +48-71-320-38-26; fax: +48-71320-3678. E-mail address: [email protected] (B.N. Kolarz).
most effective methods of preconcentration and separation of gold from aqueous solution. Many adsorbents have been used in this respect. Selectivity for gold is displayed by the functional resins, ion exchange resins as well as chelating resins bearing ligands with nitrogen groups [2–8]. Fundamental studies on guanidine as a ligand for gold and metals sorption have been undertaken. Researchers incorporated sulfonyl guanidine ligand into a styrene– divinylbenzene copolymer [9,10]. Guanidine and its derivatives are very interesting organic compounds. Their ligands can be incorporated into polymer matrices to form novel functional resins. Aminoguanidine is a strong organic amine [7,11] (with pKa 11) since the resulting positive charge upon protonation can be delocalized throughout the molecule as shown in Fig. 1 [11,12]. The introduction of the aminoguanidine group to a polymer matrix has given resins with high pKa values which, in turn, allowed gold removing from cyanide leach solution at the natural pH (about 9–10) and even at pH 13 [13]. In our previous papers we have found that
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 1 2 9 - 5
2240
D. Jermakowicz-Bartkowiak, B.N. Kolarz / European Polymer Journal 38 (2002) 2239–2246
Fig. 1. Resonance forms of aminoguanidine monocation tautomer [12].
the introduction of guanidine or aminoguanidyl ligands to the polymer matrices could lead to novel chelating and ion exchange resins. Their synthesis, characteristics and abilities of gold recovery from cyanide and acidic solutions were discussed [14–18]. Few reports concerning guanidine resin are available [17,18]. Some others are patented [19–22]. The aim of our work is to present gold sorption as dicyanoaurate Au(CN) 2 and tetrachloroaurate AuCl4 ions on two selected resins, obtained from various matrices: porous and gel, bearing aminoguanidyl groups and highlight their abilities of multiple sorption after some sorptions and desorptions cycles from HCl solutions.
2. Experimental 2.1. Materials All copolymers were obtained by suspension polymerisation with 0.5 wt.% of benzoyl peroxide as an initiator. The first one is a porous AN/VA/DVB terpolymer obtained from the following monomers: acrylonitrile (AN), vinyl acetate (VA) and divinylbenzene (DVB) as crosslinker (the nominal crosslinking equals 10 wt.%). The content of monomers in their mixture reached 63/27/10 wt.% (15/5/1% mol). Polymer was synthesised in the presence of inert diluents cyclohexanol and dodecanol (9:1 v/v). The second polymer is a gel copolymer of vinylbenzyl chloride/divinylbenzene (2 wt.%) (VBC/DVB) and was prepared in the presence of toluene (50 wt.% in respect to the amount of monomers). More details on the preparation of AN/VA/ DVB and VBC/DVB copolymers can be found in [23,24]. Copolymer 1, AN/VA/DVB, was swollen for 24 h in mixture of: dioxane–butanol–water (10/5/1 v/v per gram of dry polymer) but copolymer 2, VBC/DVB, was swollen in dioxane only. Then the copolymers were reacted with aminoguanidine bicarbonate for 24 h (4 mol excess over the amount of nitrile groups and chlorine atoms in polymers) by refluxing for 24 h resulting in Resin 1 and Resin 2, respectively [23,24]. All reagents were purchased from Aldrich Chemical Co. Vinylbenzyl chloride was purified by distillation under reduced pressure.
2.2. Methods Water regain was measured using the centrifugation technique in which about 1 g of polymer swollen in water was placed in glass vial having fritted glass bottom and centrifuged for 5 min at 3000 rpm. After drying under high vacuum at 60 °C for 24 h polymer was weighted. Water regain expressed as ratio (mw –md ) and md , where mw is the weight of wet polymer and md is the weight of dry polymer [27]. The nitrogen content was determined using Kiejdahl’s method employed to polymers [28]. The chlorine content was measured by burning of ca. 30 mg of dry polymer sample in an oxygen-filled flask. The products of burning were adsorbed in diluted hydrogen peroxide solution. The content of Cl was determined using Volhardt’s method of titration [29]. FTIR spectra in KBr pellets were recorded on a Perkin–Elmer System 2000 spectrophotometer. The porosity of copolymer 1 swollen in water was determined by Inverse Steric Exclusion Chromatography according to Ref. [23] and Gorbunov et al. [30]. The anion exchange capacity was determined according to Hecker’s method [31] and used to calculate ligand concentration. Concentration of carboxyl group was determined by method described with details in Ref. [23]. A batch method was used to examine the sorption of gold [14] in acidic and alkali solutions. Resin, in a swollen form, (equivalent to 0.13 mmol of ligand concentration), was introduced into 100 ml PP bottle containing 50 ml of gold solution and shaken for 48 h in an ambient temperature. Stock solutions of Au(III) were prepared by dissolving appropriate amounts of HAuCl4 3H2 O in HCl solutions. Isotherms studies were conducted by varying initial concentrations of hydrogen tetrachloroaurate solutions containing from 50 to 1000 mg/dm3 Au. Stock solutions of Au(I) were prepared by dissolving appropriate amount of KAu(CN)2 formulated with 0.05 N KCN. pH of this solution has been adjusted to 9.6. The resin in the swollen form was contacted for 48 h with dicyanoaurate potassium solutions containing from 50 to 500 mg/dm3 Au. After 48 h the polymers were separated by filtration and the concentration of Au was determined using Perkin–Elmer Analyst 100 atomic spectrophotometer set at 242.8 nm wavelength. The amount of gold sorption in the resin phase (in mg Au/g resin) was calculated from the mass balance and the initial concentration in the aqueous
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in the presence of inert diluents––cyclohexanol and dodecanol (9:1 v/v). More details on the preparation of AN/VA/DVB can be found in [23]. The second starting copolymer 2 (VBC/DVB), was obtained from monomers mixture of vinylbenzyl chloride and 2% divinylbenzene by method described in our previous paper [24]. The polymerisation was carried out in the presence of toluene (50 wt.% in respect to the monomers mixture) in order to obtain material with an expanded gel structure. The characteristics of copolymers and resins derived from them are presented in Tables 1 and 2. The schemes of modifications are showed in Fig. 2. Resin 1 was obtained by the aminolysis of nitrile groups of copolymer 1 using aminoguanidine bicarbonate in a boiling butanol– dioxane–water mixture. It was found that during that process under strongly alkaline conditions nitrile groups were converted to N-substituted amides (1.9 mmol/g) and to carboxyl groups (3.3 mmol/g) as a result of alkaline hydrolysis of the ester and nitrile groups. Resin 2 was prepared by direct reaction of chlorine atoms in VBC mers (5.5 mmol of Cl/g) of copolymer 2 with aminoguanidine bicarbonate resulting in aminoguanidine ligands attached to the matrix, in which the concentration of ligands reaches 1.5 mmol/g. Elemental analysis of Resin 2 gave 6.6 mmol of N/g. That amount corresponds to about 30% of the theoretical content calculated based on the composition of the monomer mixture. Polymers have various matrices and various structures. Copolymer 1 is porous with the porosity of 0.62 and water regain of 1.52 g/g, which is about five times higher compared with the gel expanded copolymer 2. Resin 1 obtained from AN/VA/DVB copolymer displays smaller water regain than the initial copolymer 1. This can be explained by additional crosslinking during the modification reaction. Its ligand concentration is greater than in Resin 2 and amounts to 1.9 mmol/g while carboxyl group content is 3.3 mmol/g.
solution. The distribution coefficients (K) were calculated as the ratio of the amount of gold adsorbed by 1 g of resin and the amount of metal remaining in 1 ml of solution after sorption. All elution studies were carried out at an ambient temperature in batch reactions from AuCl 4 anions loaded resins. The procedure was as follows: definite amounts of swollen resin beads were contacted with 200 ml of gold solution, ½Au ¼ 50 mg/dm3 in 0.1 and 1 M HCl. After 24 h of shaking the resin was filtrated off and the gold concentration was determined. Then the gold loaded resin was washed with distilled water and shaken with 50 ml of 5% thiourea solution in 0.1 HCl. The filtrates were analysed for the indirect determination of the amount of gold desorbed. Then the resin bed was washed again with water and used again for gold sorption. This sorption–desorption procedure was repeated five times.
3. Results and discussion It has been previously shown [13–16] that chemical compositions and structures of copolymers matrices with guanidyl and aminoguanidyl ligands influence the dicyanoaurate sorption and its selectivity from alkaline solutions. In this paper we report the results of our studies concerning comparison of two resins obtained from different matrices, containing aminoguanidine ligands, and their gold sorption abilities from both cyanide and acidic solutions. Copolymers were obtained by the suspension polymerisation. These copolymers differ in both the crosslinking degree, determined by nominal DVB concentration in the starting monomer mixture, and the structure developed during synthesis. They were synthesised under various solvation conditions, generated by appropriate solvents used to dilute the monomers. The acrylic porous terpolymer (AN/VA/DVB), copolymer 1 containing 10 wt.% of DVB, was obtained
Table 1 Characteristics of starting copolymers Copolymer
Matrix
Porosity
Water regain (g/g)
N (mmol/g)
Cl (mmol/g)
1 2
AN/VA/DVB (10%) VBC/DVB (2%)
0.62 Gel
1.52 0.28
11.9 –
– 5.5
Table 2 Characteristics of anion exchangers Resin
1 2 a
Matrix
AN/VA/DVB VBC/DVB
Water regain (g/g)
1.04 0.44
N (mmol/g)
14.5 6.6
Ligands (mmol/g)
1.9 1.5
AuCl 4
Au(CN) 2
Sorptiona (mg/g)
K
Sorptiona (mg/g)
K
36 68
30000 33300
2.3 23
52 2260
mg of gold metal sorbed by 1g of dry resin, gold concentration 50 mg/dm3 , K distribution coefficient.
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Fig. 2. Scheme of aminoguanidyl ligand introduction into polymer matrices.
Fig. 3. FTIR spectra of copolymer 1 and Resin 1.
Figs. 2 and 3 are the FTIR spectra of resultant polymers. It is reported [32] that there are four characteristic peaks for guanidine compounds as following: mNH at about 3300 cm1 ; mC@N at 1689–1650 cm1 ; dNH at about 1640 cm1 and dCAN at about 1300 cm1 . The introduction of aminoguanidine groups to copolymers 1 and 2 was confirmed by FTIR spectra. As shown in Fig. 3 after the modification of the copolymer 1 peak ascribed to the nitrile groups at 2240 for cm1 is decreased. Degree of substitution calculated from these spectra is ca. 50%. In these spectra the intensity of the bands at 1235 and 1740 cm1 resulting from CAO and C@O stretching vibrations, respectively, clearly decreases compared with starting copolymer 1. The spectra in Fig. 3 exhibit the appearance of new peak at 1684 cm1 for the Resin 1 due to the C@N stretching vibration responsible for existing aminoguanidyl groups. This band in am-
inoguanidine bicarbonate spectra is shifted to 1689 cm1 . The broadening of bands at 3400–2935 cm1 in the spectrum of the Resin 1 may suggest the presence of existing amino and OH groups. The presence of aminoguanidine groups in Resin 2 (see Fig. 4), formed by replacement of chlorine atom in copolymer 2 (VBC/ DVB) is revealed by disappearance of the band at 1265 cm1 responsible for existing CH2 Cl groups and appearance of the band at 1645 cm1 . Moreover, a wide band of strong intensity with peak at 3412 cm1 can be observed that is due to vibrations of the NAH bonds in NH2 groups. IR spectra of investigated Resin 2 shows the appearance of new peak at 3348 cm1 region due to stretching vibration of ANH2 groups. Decrease of CN band (2240 cm1 ) for the Resin 1 and disappearance of the CH2 Cl band (1265 cm1 ) for the Resin 2 in the IR spectra proves conversion of CN and CH2 Cl groups. It
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Fig. 4. FTIR spectra of copolymer 2 and Resin 2.
is obvious that there are clear characteristic peaks for investigated resins, which means that the resultant polymers have aminoguanidine ligands. Figs. 5 and 6 present the isotherms of gold sorption for Resin 1 and Resin 2 in the range 0.1–1 M HCl and at pH 9.6 for the Resin 2. As shown in Fig. 6, Resin 2 displays excellent sorption capacity for gold Au(III) in HCl solution and much better that Resin 1 presented in Fig. 5. Resin 2 shows particularly strong preferences for tetrachloroaurate and dicyanoaurate anions in contrast to the Resin 1 and the highest sorption of Au(III)
Fig. 6. Sorption isotherms of Au(I) dicyanoaurate and Au(III) tetrachloroaurate complexes on Resin 2. Gold sorption, mg of gold sorbed by g of dry resin. [Au]eq mg/g; gold concentration in aqueous phase after 48 hr. Experimental conditions: 0.125 mg of swollen resin and 50 ml of gold solution in 0.1–1 M HCl and at 9.6 pH at room temperature.
Fig. 5. Sorption isotherms of Au(III) tetrachloroaurate complexes on Resin 1. Gold sorption, mg of gold sorbed by g of dry resin. [Au]eq mg/g; gold concentration in aqueous phase after 48 h. Experimental conditions: 0.140 mg of swollen resin and 50 ml of gold solution in 0.1–1 M HCl at room temperature.
reaches 68 mg Au/g in 0.1 HCl solution and 23 mg Au/g from cyanide solution at 9.6 pH. The sorption from 0.1–1 HCI solutions for the Resin 1 is two to three times smaller. Moreover, Resin 1 does not reveal significant gold sorption in cyanide solution (isotherm not shown in Fig. 5). It reaches only 2.3 mg/g from 50 mg/dm3 solution whereas sorption for the Resin 2 is over 23 mg/g. The high concentration of negatively charged carboxylate anions in the Resin 1 probably can adversely influence
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the sorption of dicyanoaurate anions. Repulsive forces between cyanoaurate and negatively charged carboxylates can explain low Au(CN) 2 sorption on this resin. The aurocyanide anion has a relatively low charge density (1 electric charge is carried by five atoms) and according to Diamond and Whitney’s theory ‘‘the ions with low charge density will favour the poorly solvating phase’’ [33]. According to this principle Au(CN) 2 anions are strongly extracted by more hydrophobic Resin 2 which displays lower aminoguanidine ligand content (1.5 mmol/g), despite the fact that Resin 1 contains 1.9 mmol of aminoguanidine ligands per gram (Tables 1 and 2). The effect of HCl concentration on the gold sorption was investigated and is presented in Figs. 5 and 6. The decrease of gold sorption with the increasing concentration of counter-ion (Cl ) can be explained by the competition between the gold chlorocomplexes and the chloride anions, which may interact with the functional aminoguanidyl groups of resins [25,26]. The sorption of gold chlorocomplexes by resins containing nitrogen ligands can take place according to the mechanism of ion exchange when nitrogen atoms are protonated in the reaction:
or can occur through formation of gold complexes when nitrogen acts as the coordinating atom. As shown in Figs. 5 and 6 the sorption at 0.1 M HCl is the best, as could be expected, as there are fewer protons in the solution competing for the sorption sites on the resin. These abilities can be caused by some factors concerning coordinating resins and are connected with hydrophilicity or hydrophobicity of resins, the structures of their matrices and the steric hindrance around ligands [27]. The other factors, which can be relevant, are charge, size and geometry of the complex anions and properties of the central atom [34]. The sorption capacity of a resin bearing amino groups is pH dependent due to the protonation of the nitrogen atom of the functional groups. Gold complexes, tetrachloroaurate and dicyanoaurate, can be extracted from aqueous solution by a variety of sorbents containing N-coordinating atom. When amino resins are used, both the ion exchange and the complex formation mechanisms can take place via the nitrogen atoms of the functional groups. The decrease in sorption of gold at higher HCl concentration was probably caused by the mass action effect of chloride and increased competition from chloride ion. At high HCl concentrations HAuCl4 is largely undissociated and so would not participate in the ion exchange and its extraction declines [25,26,35]. However,
Fig. 7. Logarithm of distribution coefficient and sorption of Au(III) from HCl solutions. Experimental conditions: 0.125 mg of swollen Resin 2; 0.140 mg Resin 1, and 50 ml of gold solution (350 ppm Au(III)) in 0.1–3 M HCl at room temperature.
with increasing HCl concentrations, the distribution coefficient K decreases as a result of the competition of increasing chloride ions in the external solutions [35]. Fig. 7 shows the dependence of log K and sorption from 350 mg/dm3 of gold solution in acidic solutions. The investigated resins were tested in 0.1–3 M hydrochloric solutions. The highest distribution coefficients and sorption values were displayed by Resin 2. The plotting of log K as a function of pH gives lines with a slope of 1.236 and 0.949 for the Resin 1 and 2 respectively, suggesting that the ion exchange plays a great role in the sorption of Au(III) by Resin 2 under these conditions. Resin 1 is strongly hydrophilic resin, containing hydrophilic hydroxyl and carboxyl groups obtained during the hydrolysis of nitrile and acetate groups. Its hydrophilic matrix and higher degree of crosslinking, 10% DVB, and the porous structure can be responsible for smaller tetrachloroaurate sorption from HCl solutions compared with low crosslinked Resin 2 obtained from gel expanded VBC/DVB copolymer. The complete gold desorption from investigated resins was carried out using 5% thiourea solution in 0.1 M HCl. As shown in Fig. 8 their ability of gold sorption in 0.1 HCl was invariable and reached 36 and 68 mg/g for Resin 1 and Resin 2, respectively. Five cycles of the sorption–desorption processes were investigated and the resins’ ability of gold sorption in HCl solution remains unchanged after the first and the subsequent cycles. After the fifth process of gold desorption the nitrogen content was determined again and turned out to be the same as in the initial resin. Based on nitrogen content and on IR spectra we can say that resin does not change in subsequent sorption–desorption processes. Resins 1 and 2 are suitable for multiple processes of gold sorption–desorption from HCl solution in the range from 0.1–1 M HCl.
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Fig. 8. Gold sorption from HCl solution after processes desorption by elution with 5% thiourea.
4. Conclusions 1. Resins bearing aminoguanidyl ligands (Resin 1 and 2) are useful in the sorption of tetrachloroaurate anions from acidic solution. 2. The porous, hydrophilic acrylic Resin 1, containing high amount of carboxylic groups is not suitable for aurocyanide anions sorption at pH 9.6 and its sorption reaches only 2.3 mg/g. 3. The gel Resin 2 obtained from hydrophobic matrix of vinylbenzyl chloride and divinylbenzene copolymer reveals significant sorption of gold from both acidic and alkali solutions, 68 and 23 mg/g, respectively. 4. The investigated resins may be useful in many cycles of sorption–desorption of tetrachlorourate anions in acidic solutions. 5. Gold can be quantitatively eluted from loaded resins with acidic thiourea solution at an ambient temperature. Acknowledgements This work was done with financial support from State Committee for Scientific Research through grant 3 T09B 067 16. References [1] Chmielewski AG, Urbanski TS, Migdal W. Separation technologies for metals recovery from industrial wastes. Hydrometallurgy 1997;45:333–44. [2] Sanchez JM, Hidalgo M, Salvado V. The separation of Au(III) and Pd(II) in hydrochloric acid solutions by strong anion type II exchange resins: the effect of counter ion concentration and temperature. Solv Extr Ion Exch 2000;18(6):1199–217.
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