Synthesised phosphine sulphide-type macroporous polymers for the preconcentration and separation of gold (III) and palladium (II) in a column system

Reactive & Functional Polymers 49 (2001) 215–224www.elsevier.com/ locate / react

Synthesised phosphine sulphide-type macroporous polymers forthe preconcentration and separation of gold (III) and palladium

(II) in a column system

*´ ` ´Juan M. Sanchez, Manuela Hidalgo, Victoria SalvadoDepartment of Chemistry, University of Girona, Campus Montilivi, 17071 Girona, Spain

Received 2 October 2000; received in revised form 18 June 2001; accepted 30 June 2001

Abstract

New macroporous polymers with an ionogenic group based on triisobutyl phosphine sulphide, with spacer arms containingO or S atoms, are evaluated in small-scale column adsorption processes. These resins are highly selective towards gold andpalladium in hydrochloric acid media and they do not adsorb other Platinum Group Metals (PGM) such as Pt, Rh and Ir norbase metals such as Fe, Zn, Cu and Ni. The heteroatoms present in the spacer arm enhance the adsorption capacities of thesepolymers by coordinating the metal ions jointly with the functional group. The polymer that contains O atoms permits the

21direct separation of palladium and gold at linear flow rates lower than 80 mm min . The second polymer, which containsone S atom in the spacer arm, shows a higher breakthrough capacity for Pd ions and a decrease in the separation factor ofgold and palladium. A temperature increase from 25 to 608C produces a significant increase in gold adsorption resulting in ahigher separation factor from palladium. Separate metal elution can be achieved by passing, firstly, a sodium nitrite 2 Msolution at pH ¯ 5, which can elute 75% of palladium without gold interference. A second elution with thiourea permits thetotal recovery of the remaining ions. This elution procedure has been applied to the preconcentration of both metal ions.Polymers were evaluated during 15 consecutive cycles without changes taking place in the adsorption and elution profiles ofthe metals. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Phosphine sulphide resins; Column system; Separation; Gold; Palladium; Preconcentration

1. Introduction in trace metal analysis. The main advantage ofchelating polymers over the traditionally used

The synthesis of new polymeric matrix based ion-exchange resins is that both noble metalsorbents, incorporating ligands which are im- separation and concentration can be accom-mobilised by chemical bonds and highly selec- plished in one single step. A great variety oftive towards the noble metals, has great interest chelating polymers with different functionaldue to their potential in the industrial separation groups which are selective towards noble metalsand concentration of these valuable metals and have been synthesised [1–4]. In general, the

good co-ordinating properties of sulphur and/ornitrogen atoms lead to the presence of these

*Corresponding author. Tel.: 134-972-418-271; fax: 134-972-atoms in the ionogenic groups of the polymers.418-150.

´E-mail address: [email protected] (V. Salvado). The most well-known are the commercially

1381-5148/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S1381-5148( 01 )00082-7

´216 J.M. Sanchez et al. / Reactive & Functional Polymers 49 (2001) 215 –224

available poly-isothiourea resins (PITU) [1] The operational life of these resins was alsowhich are employed as either a gel (Srafion determined in order to assess their utility inNMRR) or a macroreticular matrix (Monivex). industrial applications.These resins lack selectivity and the separationof the noble metals is obtained by a later elutingprocess. 2. Experimental

In previous studies [5–7], we synthesised andcharacterised new chelating resins with a tri- 2.1. Reagentsisobutylphosphine sulphide (TIBPS) functionalgroup. Among the polymers synthesised, best Metal solutions were obtained dissolvingperformance characteristics in terms of affinity appropriate amounts of HAuCl , ZnCl (Merck,4 2

and selectivity to gold and palladium was Germany), PdCl ( . 99%, Sigma, Germany),2

observed in those bearing spacer arms con- PtCl , RhCl , IrCl (Johnson Mattey, Ger-4 3 4

taining O (2-[2-(2-ethoxy)ethoxy]ethane) or S many), Fe(NO ) ? 9H O, CuCl ? 9H O (Pan-3 3 2 2 2

and O heteroatoms in the spacer groups (2-[2- reac, Spain), and NiCl ? 6H O (Fluka, Switzer-2 2

(2-thioethane(ethoxy)]ethane). The high rates land) in 1.0 M hydrochloric acid. Stock solu-obtained for gold, up to analytical capacity, tions of palladium and gold were standardisedwere attributed to the joint co-ordination of the gravimetrically and volumetrically, respectivelymetal ions due to the presence of heteroatoms [8]. The concentration of Pt(IV), Rh(III), andsuch as S and O in the spacer arm between the Ir(IV) solutions were determined by Inductivelypolymeric matrix and the functional group. Both Coupled Plasma (ICP) (ARL-3410 ICP-AES,polymers show a great affinity towards gold and Fisons Instruments) using standard solutions of

21palladium in hydrochloric acid media and no 1000 mg l (Aldrich, USA) to prepare theaffinity for other noble metals. Batch adsorption calibration curves. Other metals were analysedstudies [7] have shown that these chelating by Atomic Absorption Spectrometry (AAS)resins perform well in the separation of both (SpectrAA-300, Varian Instruments, Australia).gold and palladium from other noble and base Periodically, we used both ICP and AAS meth-metals. The slow kinetics of the gold adsorption ods to determine the metal content of thein discontinuous studies, more than 70 h in effluent samples in order to ensure the quality ofachieving equilibrium, would seem to suggest the data.that their utility is minimal. However, the Ultra-pure water obtained through a Milli-Qselectivity and kinetic characteristics of the water purification system (Millipore, Bedford,resins need to be tested in dynamic conditions MA, USA) was used in all procedures. Workingto determine their applicability. The results solutions were obtained by dilution with waterobtained in batch conditions, which are equilib- and the appropriate amount of HCl to adjust therium systems, may not always be extrapolated proton concentration. All other reagents were ofto column operations, which are non-equilib- analytical grade (Panreac, Spain).rium systems. Polymers (Table 1) bearing a diisobutylphos-

This study was undertaken in order to char- phine sulphide functional group were synthes-acterise the adsorption and separation of noble ised from chloromethylated polystyrene (cross-metals in continuous small-scale column opera- linked with 4% DVB, 4 meq Cl /g polymer andtions. The influence of different parameters such 50 mesh). Their synthesis and characterisationas flow rate and temperature was investigated by IR spectroscopy, elemental analysis andunder dynamic conditions and, taking into ac- energy dispersive X-ray microanalysis (EDX)

21count the previous results obtained in batch have been described [6]. A band at 590 cm inconditions, we studied the elution efficiency. the IR spectra, characteristic of the P=S group,

´J.M. Sanchez et al. / Reactive & Functional Polymers 49 (2001) 215 –224 217

Table 1aStructure of the polymer employed, and the analytical and experimental capacities obtained for Pd(II) and Au(III) in batch adsorption

studies [6]

Resin Structure Analytical capacity Experimental capacity

mmol S/g polymer mmol Au/g polymer mmol Pd/g polymer

Polymer A 1.76 3.360.1 0.560.1

Polymer B 1.78 6.560.1 0.760.1a Analytical capacities are calculated from the S content determined by Elemental Analysis, divided by the number of S atoms present in the spacer arm and

functional group.

21and sulphur elemental analysis were used to ml min . A fraction collector (FC 203, Gilson,determine the degree of functionalisation of the France) was used to collect effluent samples atpolymers, which always exceeded 75% of the the outlet of the column at different periods ofchloromethyl groups. C–O–C stretching at 1101 time. Metal content was then determined by

21cm shows that O atoms are present in the AAS in the case of Au, Pd and base metals, andspacer arm. The presence of a band at 2727 by ICP for Pt, Rh, and Ir. The values obtained

21cm belonging to the SH group confirms the were used to calculate the breakthrough curvesformation of C–SH before the introduction of for each ion. After complete elution of thethe 2-[2-(2-ethoxy)ethoxy]ethane spacer arm metals adsorbed, the resins were regeneratedand the polymer functionalisation as phosphine with 1.0 M HCl and washed with ultra-puresulphide-type for Polymer B. water before a new cycle was started.

In the breakthrough curves, a non-dimension-al factor C /C , where C is the metal con-2.2. Experimental procedure f 0 f

centration at the outlet of the column and C is0

Glass columns with an internal diameter of the initial metal concentration, is plotted versus0.6 cm were employed for experiments per- the bed volume, BV, which is defined as theformed at room temperature. We used a thermo- effluent volume/ resin volume ratio. This param-statically conditioned glass column with an eter enables comparison of the breakthroughinternal diameter of 1.7 cm to test the effect of curves obtained from the different quantities oftemperature variations on metal ion separations. resin tested. Eq. (1) is used to standardise theEach column was packed with a known amount flow rates as linear rates:of dried resin until a bed volume (BV) of | 2 q

]]cm height was obtained. The resins were con- v 5 (1)linear 0.4Aditioned by passing a volume of ultra-pure

3 21water and hydrochloric acid 1 M through the where q is the flow rate in mm min , A is thecolumns that were equal or higher than three column area and 0.4 is a standard value repre-times the bed volume. senting the percentage of the column area

Feed and eluting solutions were propelled occupied by the polymer.downwards through the column by means of a The reproducibility of the breakthroughperistaltic pump (Minipuls3, Gilson, France). curves and elution profiles was proved byThe flow rate was varied between 0.2 and 2.0 conducting all experiments in triplicate.


3. Results and discussion present different diffusion behaviour. The satu-ration capacity for palladium ions is reached

Table 1 shows the structures of Polymers A after about 60 min at all the flow rates tested.and B used in the study and the capacities for The breakthrough curves for gold ions, how-gold(III) and palladium(II) adsorption estab- ever, are less steep, and are more prone to belished in earlier batch experiments [7]. affected by the flow rate. Moreover, gold ions

do not reach saturation capacity even after 9 hof contact. These results show that palladium3.1. Column loadingions diffuse easily within the resin structure.

The breakthrough curves obtained for Poly- Although it is observed, as can be seen inmer A at different flow rates are represented in Fig. 1 and Table 2, that increasing the flow rateFig. 1. The different profiles of the curves for for gold ions moves the profile towards fastergold and palladium indicate that the two ions breakthrough, the working capacity is not modi-

Fig. 1. Effect of the linear flow rate in the profile of the breakthrough curves for gold and palladium by Polymer A. (Feed solution with 7021mg l of each metal at 1 M HCl).

Table 2aBreakthrough capacities and bed volumes obtained for gold and palladium ions at different linear flow rates

Linear flow Resin Bed volume Breakthrough capacities21(ml min ) (ml) (mmol M/g polymer)

Au(III) Pd(II) Au(III) Pd(II)

176.8 Polymer A 11.2 5.6 0.024 0.024129.1 9 11.2 5.6 0.024 0.02282.2 9 11.2 5.6 0.024 0.02337.1 9 30.7 5.6 0.087 0.02317.7 9 62.6 5.6 0.162 0.02217.7 Polymer B 62.6 12.3 0.159 0.071

a 21 1Experimental conditions: feed solution |70 mg l of individual metal, [H ]51.0 M, |1.6 g of dry resins.


21fied. At flow rates higher than 80 mm min the Three factors would seem to explain thegold ions may move too quickly to be able to behaviour of gold ions. Firstly, the difficultydiffuse within the resin structure effectively and with which gold ions diffuse in these resins.only at lower flow rates do they manage to Secondly, although similar analytical capacities

21reach the internal ligand sites. for Polymers A and B (1.76 and 1.78 mmol g ,The volume of solution passed through the respectively) have been established by a previ-

column before reaching the breakthrough point, ous study [6], the presence of one S atom in thewhich we take as 10% of the feed concentration Polymer B spacer chain has resulted in batchin the effluent, and achieving saturation capaci- experiments giving a greater working capacity.ties, is much higher in the case of gold ions. Thirdly, heteroatoms in the spacer chain mayThis clearly shows the better kinetic perform- participate in the adsorption of gold ions, but aance of palladium ions in column experiments certain quantity of metal must be adsorbedand confirms the results of the earlier batch previously by the functional groups in order toexperiments [7]. enable the participation of heteroatoms [7]. The

Comparison of the two resins tested (Poly- fact that both resins have the same analyticalmers A and B) shows that the loading profiles capacity, indicating the same number of func-(Fig. 2) are the same for gold ions in the first 2 tional groups, and the lack of heteroatom par-h but differ for palladium ions. A similar ticipation in the spacer chain at the beginning ofdifference is observed on comparing the break- the adsorption process, result in the two poly-through capacities of the two resins (Table 2). mers having gold ion loading profiles which areThe change of slope observed for the palladium at first identical. A change in the slope of theions must be due to the different capacities of loading profile occurs when |65% (Polymer A)

21the resins (0.72 mmol g for Polymer B and and |45% (Polymer B) of the initial gold210.51 mmol g for Polymer A in batch experi- concentration is found at the column outlet. The

ments) given that palladium ions diffuse without participation of the spacer heteroatoms in thedifficulty in both resins. gold adsorption mechanism explains the sub-

Fig. 2. Breakthrough curves of gold (m) and palladium (d) for Polymer A (black) and B (white). Experimental conditions: Feed solution21 1 21with 70 mg l of each metal, [H ]51.0 M, linear flow rate 17.7 mm min .


Table 3sequent lower slope. The presence of an S atom,Separation factors of gold and palladium; feed volume of 100 mlwhich has a higher affinity towards gold ions, in 21and linear flow rate52.2 mm min

the spacer arm of Polymer B leads to an earlier AuResin X Initial metal concentration aAu Pdlevelling off of its loading profile.21 21C (mg l ) C (mg l )Au PdPrevious batch experiments [7] showed that

Polymer A 0.36 56.7 102.4 39.6the differences in the adsorption capacity for9 0.34 52.3 100.2 37.9gold and palladium ions are greater in the case9 0.37 59.8 100.4 34.6

of Polymer B than in Polymer A. This would 9 0.48 95.5 102.2 15.09 0.76 310.5 100.3 11.6seem to suggest that Polymer B should offer

more effective separation for both metal ions inPolymer B 0.64 160.2 91.8 10.7

column processes. However, the profiles ob- 9 0.72 265.8 107.1 5.69 0.71 249.0 102.8 6.4tained in column operations indicate that Poly-

mer A separates both metal ions more effective-ly when a continuous system is employed. Thefact that gold ions diffuse with difficulty inthese polymers results in it not being possible to 3.2. Temperature effectextrapolate the results of batch experiments

Processes involving high enthalpy changes,(equilibrium systems) to column operationssuch as those caused by coordinating resins,(non-equilibrium systems). The breakthroughresult in temperature variation having an in-curves obtained show that both resins may befluence on the adsorption process [10–13].used for the selective adsorption of gold andTemperature-based studies of separation pro-palladium from other noble and base metals.cesses using coordinating resins have found thatThe choice of the appropriate resin depends onthe separation factor is temperature dependentthe application selected. Polymer B shows better[12,13].behaviour if it is used for pre-concentration of

The dependence of the equilibrium constantthese two ions. Polymer A adsorbs a lowerof a reaction on the temperature is calculated byamount of Pd and Au than Polymer B, but itthe van’t Hoff equation. In the case of coor-allows the quantitative separation of both ionsdinating resins, an increment in the equilibriumdirectly by the column.constant is expected with higher temperatureThe separation factor of the two ions (a) isalong with a decrease in the selectivity of thethe most useful parameter in practical use [9],resin [14]. The diffusion coefficient within aand is defined as:resin increases with increased temperature[15,16].Y XM M2 1M2 The previously determined coordinating]]]a 5 (2)M1 Y XM M1 2 mechanism of the resins [7] increases the ad-sorption capacity of both metal ions as tempera-ture is increased (Fig. 3). There is little vari-where Y and X are the fraction of the ions to beation in the case of palladium ions and it isseparated in the resin phase and solution, re-slower than for gold. This can be explained byspectively. Table 3 shows the separation factorthe fact that the temperature only affects theobtained by resins A and B in the separation ofequilibrium constant of the adsorption of Pd(II).gold and palladium ion mixtures with varyingThe higher degree of variation obtained for goldcompositions. Both resins are highly selectiveis due to the combination of this change in thetowards gold ions. As would be expected fromequilibrium constant and an increased diffusionthe results we have mentioned, Polymer A is thecoefficient of the gold ions.most selective.


Fig. 3. Evaluation of the amount of metal adsorbed by Polymer A at different temperatures.

Table 43.3. Column elutionPercentages of recovery obtained in the elution of gold andpalladium with different eluting reagents

Different eluting reagents were tested (TableEluting solution % Recovery4). The effect of some physical (flow rate) andReagent Concentration pH Au(III) Pd(II)chemical parameters (concentration and pH)

1Thiourea 0.1 M [H ]51 M 100 100were also tested in order to determine the best10.3 M [H ]51 M 100 100reagents and the best conditions for the elution 10.5 M [H ]51 M 100 1001of gold and palladium ions from the resins. 0.7 M [H ]51 M 100 100

0.7 M 2.0 100 100Thiourea gives the highest rates of elution for0.7 M 4.2 80 25both metal ions at pH,2, but the two metals are0.7 M 5.3 30 8

not eluted separately. The elution volume with NH 5% – ,1 53

HCl 6 M – ,1 ,1thiourea is highly dependent on pH and reagentNaSCN 0.5 M 5.4 ,1 7concentration, and is practically unaffected by

0.5 M 2.5 ,1 6flow rate. The increase in the concentration of Na S O 0.1 M – 10 ,12 2 31thiourea and H reduces the volume required 1.0 M – 40 10

NaNO 1.2 M 7.1 ,1 282for elution (from more than 100 ml at 0.1 M2.0 M 7.4 ,1 33thiourea to |25 ml at 0.7 M). In the extreme 2.0 M 5.6 ,1 49

1conditions tested (0.7 M, 1.0 M H and 17.7 2.0 M 4.9 ,1 7421 2.0 M 4.0 ,1 40mm min ) we observed the formation of a

2.0 M 1.0 ,1 ,1precipitate due to the high concentration ofmetal–thiourea complexes in the eluted solu-tion. we attribute to a redox reaction between Au(III)

Thiosulphate solutions are affected by con- ions and thiosulphate.centration and flow rate, and a precipitate in the The best results in the elution of palladiumcolumn was observed at low flow rates which were obtained using sodium nitrite. With this


reagent it is possible to elute |75% of the having palladium ion impurities, contains anpalladium adsorbed in the resin without gold ion amount of gold which is over 20 times moreinterference. The decrease in the degree of concentrated.elution at low pH values is explained by theprotonation of nitrite ions and its spontaneous 3.4. Useful life of the polymersoxidation into nitric acid [17], which does notelute palladium. The high cost of polymer synthesis makes it

The use of a sequence of eluting reagents, important to establish for how long they may bebased on sodium nitrite 2 M at pH |5 followed usefully employed. After the loading and elution

1by thiourea 0.5 M at 1.0 M H , enables the of the metal samples, a volume of ultra-pureseparation of the ions adsorbed by the resins. water three times higher than the bed volumeFig. 4 shows the elution profiles obtained from was passed through the column to ensure thatthe calculation of the concentration factor (C / the elution solution was totally eliminated fromf

C ) for each metal at the column outlet. The the resin [18]. After the cleaning, the resins0

efficacy of the reagents is demonstrated by the were re-conditioned, as explained in the ex-fact that the maximum value of the concen- perimental section, and reused for further ad-tration factor is obtained with the first fraction sorption experiments. Both resins were em-for both reagents. One hundred percent pure ployed under the conditions described for 15palladium solution, with a concentration over consecutive cycles without changes taking placefour times greater than the initial solution, is in the adsorption and elution profiles of theobtained with nitrite. The posterior application metals.of thiourea leads to the total elution of the restof the metal ions adsorbed in the resin. The fact 3.5. Separation of noble metalsthat nitrite ions only elute 75% of the palladiumimplies that the second elution solution, whilst One of the main applications of the polymers

1Fig. 4. Elution profiles obtained with sodium nitrite (2 M, pH 4.7) and thiourea (0.5 M, [H ]51.0 M) in the recovery of gold (white points)21and palladium ions (black points) adsorbed in Polymer A. (Linear flow rate 37.1 mm min ).


21 21 21Fig. 5. Breakthrough curves obtained in the separation of a synthetic mixture containing 57 mg l Pd(II), 54 mg l Au(III), 88 mg l21 21Pt(IV) and 60 mg l Rh(III) in HCl 1.0 M. (Linear flow rate 17.7 mm min , Polymer A).

studied here is in the separation of gold and Ni, Zn) and the separation factor of gold andpalladium both from one another and from other palladium shows that they are highly selectivenoble and base metals. Fig. 5 shows the break- towards gold ions. The coordinating mechanismthrough curves obtained in the separation of a of the two polymers leads to a greater capacitysynthetic mixture containing different noble for the adsorption of gold and palladium ions atmetals (Pd, Au, Pt, Rh) using Polymer A. As increased temperatures.was to be expected from previous results [6], Sodium nitrite solution can be used to obtainplatinum and rhodium are not adsorbed by the 75% palladium elution without the interferenceresin and appear at the front. The same be- of gold. Thiourea permits the total desorption ofhaviour was observed for Ir and the base metals the gold ions and the palladium that have notusually associated with noble metal liquors (Fe, been previously eluted.Cu, Ni and Zn). The two resins that we have tested can be

used effectively for the preconcentration of goldand palladium solutions. At room temperature, aconcentration factor of over 4 was achieved in4. Conclusionsthe case of palladium and of over 20 for gold,

Polymers A and B with a functional group and at higher temperatures we may expect tobased on triisobutyl phosphine sulphide and obtain much greater concentration factors.spacer arms containing O and S heteroatoms are Whereas the behaviour of conventional com-effective for the selective adsorption of gold and mercial and synthesised resins require variouspalladium ions in hydrochloric acid media using elution steps to separate noble metals, Polymera column system. These resins do not adsorb A has the advantage that it permits directother PGM (Pt, Rh and Ir) and base metals (Cu, separation in one single step.


´ ´[7] J.M. Sanchez, M. Hidalgo, V. Salvado, React. Polym. 46 (3)Acknowledgements(2001) 283–291.

[8] A.I. Vogel, Textbook of Quantitative Inorganic Analysis,This study has been supported by grant Longman, London, 1978.

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[10] A.J. Groszeck, in: M. Streat (Ed.), Ion Exchange for Indus-velopment)try, Ellis Horwood Ltd, Chichester, 1988, p. 286.

[11] J. Noguerol, Ph.D. Thesis, Faculty of Sciences, AutonomousUniversity of Barcelona, 1996.

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Synthesised phosphine sulphide-type macroporous polymers for the preconcentration and separation of gold (III) and palladium (II) in a column system