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Sorption of Gold, Platinium, Palladium and Rhodium by Chelating Ion Exchanger Iontosorb DETA ^[1]

Experimental

Preparation of Sorption Solutions and Determination of their Concentration

For measurements of sorption properties of the sorbent Iontosorb DETA there were prepared solutions of CuCl₂ and FeCl₃ with the concentration c = 0,1 mol/l.

To the sorption of precious metals there were used solutions containing 2 mg of Au or Pt or, in case of Pd and Rh, 1 mg in 1 ml 0,2 M HCl. The solutions were prepared by dissolving purest Au, Pt, Pd in aqua regia, in case of Rh by pressure decomposition in HCl + HNO₃ mixture. Before using, Rh (standard, Johnson & Hatley) was annealed under reduction atmosphere of H₂ and N₂. Other metals were delivered by the firm SAFINA, Jesenice. 

Besides sorption of Au, Pt, Pd and Rh, also Fe (III) and Cu sorption was measured. The content of Cu and Fe(III) was determined by chelatometric titration. 

The content of common and precious metals was detemined by photometric method. In case of Au and precious metals there was measured extinction of chloride complexes [2]. Calibration curves for three various concentrations of HCl were measured: c = 0,1 mol/l, 1,0 mol/l and 3 mol/l. For Fe extinction of chloride complex was measured too. The concentration of Cu was measured in ammonium complex.

The Treatment of Sorbents

Iontosorb DETA was delivered in swollen state, particle size was 0,2 - 0,3 mm. About 50 ml of the sorbent were transferred into a column, the sorbent was firstly rinsed by 1000 ml of HCl (c = 1 mol/l), then the acid was rinsed out from the sorbent by 3000 ml of demineralized water. After that, free water was sucked off and the sorbent was transferred into a PE bottle.

The Determination of Dry Matter and Nitrogen Content in Dry Matter 

For dry matter determination, the sorbent was dried out at 60 °C till constant weight. Nitrogen content was determined by Dumas' method. From this nitrogen content theoretical capacity of the sorbent was determined. 

The Sorption in Statical Arrangement 

Sorption Kinetics 

The sorption kinetics was measured from the dependence of metal content (in mol) in the sorbent on time.

Into a closed bottle  of 250 ml volume the quantity of sorbent, adequate to 1,0 g dry matter, was placed. Then metal solution, containing metal in quantity adequate to 1/10 of theoretical sorbent capacity, was added (0,217 mmol). The HCl and water were added to obtain resulting HCl concentration 0,1 mol/l and volume of the whole solution 200 ml. Then the bottle content was shaken, in certain time intervals 5 ml samples were taken for metal concentration determination by a photometric method.

To calculate the quantity of sorbed metal from its decreasing concentration in the solution, it must be assumed that not only concentration, but also solution volume change.

Parameters:
V = sloution volume [ml]
P = total quantity of metal [mg] in the solution and sorbent
k = the concentration of metal in the solution [mg/ml]
x = the volume of taken sample [ml]
n = the weight of dry matter in the sorbent  [g]

index o resp. N means initial, resp. N-th value. 

Before taking out the N-th sample the sorption takes place in the volume V_N-1 ml. In the N-th sample the concentration of metal k_N [mg/ml] is measured. The total quantity of metal in the solution before taking out the N-th sample, P_N-1, is calculated from the following equation:

P_N-1 = P_o - k_ox_o - k₁x₁ - ....... - k_N-1 x_N-1

The quantity of the metal in the sorbent [mmol/g] is calculated from the equation: 

The results of kinetic measurements and calculations are given in the following table:

The Kinetics Au and Pd sorption from 0,1 M HCl on Iontosorbu DETA

Given 0,217 mmol of metal, initial volume 200 ml, volume of individual samples 5 ml
The weight of dry matter in Iontosorb DETA          for Au: 0,9747 g          for Pd:  1,005 g

The Sorption of Metal Ions from HCl solution 

The sorption of Cu, Fe, Au, Pt, pd and Rh was measured by static method from HCl solution of concentration  c = 0,1;   0,5;   1,0;   2,0   and   5 mol/l.

The amount of Iontosorb DETA containing 0,125 g of dry matter was given into a bottle of 25 ml. Then the quantity of metal adequate to 1/10 of theoretical sorbent capacity (0,027 mmol) was added. Then HCl and water were added to obtain 20 ml resulting volume. After shaking 3 hours metal concentration was determined by a photometric method.

From measured values, weight distribution coefficients Dg, giving information on the distribution of a metal between both phases, were calculated. 

n_Me,s = the quantity of metal in sorbent [mg]
m_s = dry matter of the sorbent [g]
n_Me,aq = the quantity of metal in liquid phase [mg]
V_aq = the volume of liquid phase [ml]

 From distribution coefficients of individual ions, a separation factor (alpha) was calculated as follows:

Dg (Me)1 = the weight distribution coefficient of the first metal from a couple 
Dg (Me)2 = the weight distribution coefficient of the second metal from a couple 

Separation factors were calculated to find couples of metal ions proper for their separation from HCl.

The Dependence of Weight Distribution Coefficient on HCl concentration during Sorption of Metal Ions on  Iontosorb DETA

The weight of Iontosorb DETA was adequate to 0,125 mg dry matter, given 0,027 mmol of metal in 20 ml volume, shaken 3 hours.

Separation Factors for Metal Ions Sorbed on Iontosorb DETA from HCl ( c = 0,1 mol/l  a  0,5 mol/l )

Sorption on Dynamic Condition

Sorption and Desorption Curves of Au, Pt, Pd, Rh

For sorption of precious metals the column of diameter 4 mm was used, the volume of filling was 1 ml, which corresponded to 0,160 g dry matter of Iontosorb DETA. Before experiment, the sorbent in the column was rinsed with 3 ml HCl (c = 0,1 mol/l) and water. The influent contained metal in the concentration 5.10^-3 mol/l in 0,1 M HCl. The flow velocity 0,5 ml/min corresponded to the specific loading S = 30 hour^-1. The fraction of 2 ml volume were taken and the metal content in individual fractions was determined by the photometric method. The penetration was defined by the moment when c/c₀ reached 0,01. This value was determined by a UV detector. After the concetration of metal in the efluent reached the same value as in influent (c = c_o), influent was replaced with HCl (c = 0,1 mol/l). Fractions of eluate were taken and analyzed till the whole 100 ml volume of HCl flew through the sorbent.

 The sorption and desorption curves are given in the picture 1. The values of penetration capacity Qp, determined as the quantity of the metal in the sorbent till penetration moment, and the total capacity Qg, are given in the following table and calculated from the equations:

Q_p  =  V_p  .  c_o                                Q_g,total  =  V_0,5  .  c_o

c_o = metal ion concentration in influent
V_p = the volume till penetration
V_0,5 = the volume till the moment in which  the concentration of efluent c/c_o = 0,5

Picture 1:

Sorption and desorption curves of Rh, Pt, Pd a Au on Iontosorb DETA from HCl (c = 0,1 mol/l)

1 = Rh          2 = Pt          3 = Pd          4 = Au

Column:   internal diameter d = 4 mm, the volume of filling V_c = 1 ml (corresponds to 0,160 g dry matter)
c/c_o = the ratio of metal concentration in efluent (c) and metal concentration in influent (c_o)     c_o = 5 . 10^-3 mol/l
V/V_c = the ratio of  efluent volume (V) and filling volume (V_c)

The vertical line in the picture means the beginning of desorption. 

The Weight Capacities of Sorption from HCl of c = 0,1 mol/l

Metal concentration of  influent c_o = 5 . 10^-3 mol/l

The Desorption by HClO₄

Static experiments showed that in HClO₄ of c = 2 mol/l Au and precious metals are sorbed very little. On dynamic conditions the sorption of Au, Pt and Pd in dependence on concentration of ClO₄^-  ions in the solution of constant acidity.

On the column with 1 ml of Iontosorb DETA was sorbed the amount of metal corresponding to 40 % of sorption capacity, found from sorption curve. Then metal ions were desorbed by the solution containing in 1 litter 2 mol HClO₄ and 0,1 mol HCl and, in other experiments, by the solution containing in 1 litter  0,1 mol HCl and 2 mol NaClO₄ For photometric determination modelling solutions were prepared to find how ClO₄^- ions influence extinction.

The results are given in the folowing tables. 

The Desorption of Au sorbed on Iontosorb DETA

Given 10,00 mg Au

The Desorption of Pd sorbed on Iontosorb DETA

Given 7,00 mg Pd

Desorpce platiny sorbované na  Iontosorb DETA roztokem o c_HClO4 = 2 mol/l  a  c_HCl = 0,1 mol/l

The separation from the mixture Pt - Rh

From static experiments and sorption and desorption curves it can be seen that Iontosorb DETA enables to separate Cu and Fe from Au, Pt and Pd during sorption from HCl. Also separation of these three precious metals, which are sorbed from HCl of c = 0,1 mol/l very little, from Rh can be well done. Among these possibilities the couple Pt-Rh was chosen for separations, because the separation factor alpha = 54,5, which is advantageous.

According to the equation for resolution

k = capacity ratio Pt
n = number of plates in the column

for ion exchange chromatography

both metals are sufficiently separated in the column containing 1 ml of iontosorb DETA.

Firstly both metals were given on the column separatelly to find whether HCl does not rinse Pt and Rh.

Onto the column containing 1 ml Iontosorb DETA it was given: 0,01 mmol Rh;  0,10 mmol Rh  and  0,05 mmol Pt. Caught metals were eluted by HCl of c = 0,1 mol/l under specific loading S = 30 hod^-1. 

The result was calculated from three determinations:

Pt and Rh were separated on the column containing 1 ml of Iontosorb DETA, rinsed before experiment by 3 ml of HCl (c = 0,1 mol/l) and water. 10 ml of solution contained 0,05 mmol Pt and 0,05 mmol Rh in HCl (c = 0,1 mol/l). After the solution of Pt and Rh sorption was finished, the sorbent was rinsed with HCl of the same concentration to remove Rh from the column. The caught Pt was desorbed by 25 ml of HClO₄ (c = 2 mol/l) and HCl (c = 0,1 mol/l). The content of both metals was determined by photometric method in three determinations. The results are given in the following table:

The Separation of Pt and Rh on Iontosorb DETA from HCl (c = 0,1 mol/l)

Results and Discussion

In the whole HCl concentration range (c = 0,1 to 5 mol/l) Cu (²⁺) and Fe (³⁺) are not sorbed, their distribution coefficients are zero. All studied precious metals show the highest sorption at HCl concentration c = 0,1 mol/l. Distribution coefficients for Au (³⁺), Pt (⁴⁺) and Pd (²⁺) are at this concentration of HCl much higher then for Rh (³⁺). The mechanism of Rh sorption is probably different from the other precious metals, because Dg for Rh nearly does not depend on HCl concentration. On the contrary, Dg for Au, Pt and Pd decreases with increasing HCl concentration very much, upto 100 times.

From the values of separation factors the couples of metal ions most proper for separation can be determined.The separation factors were calculated for HCl concentration (c = 0,1 mol/l and 0,5 mol/l), because at higher HCl concentration the differences in Dg are small. On Iontosorb DETA it is possible to separate Cu and Fe from Pt and Pd in the solution of HCl (c = 0,1 mol/l and 0,5 mol/l). The separation of these three precious metals from Rh in HCl (c = 0,1 mol/l) can be well done. In HCl with higher concentration (c = 0,5 mol/l) can be separated Pd-Au and Pd-Pt.

 For desorption of precious metals from Iontosorb DETA the mixture of HCl and HClO₄ was used, because it was necessary to keep metals in chloride complexes due to photometric determination. The distribution coefficients for sorption from this media are low for all studied precious metals. On dynamic conditions the desorption of Au, Pt and Pd was studied by using HCl and HClO₄ of given concentrations. Rh can be rinsed from the sorbent with HCl of c = 0,1 mol/l. Pt can be desorbed easily, caught Pt was quantitatively desorbed by 25 ml of HCl and HClO₄ mixture. Photometric determination of Pt was not influenced by present HClO₄ . Au and Pd were desorbed slowly, after rinsing the column with 55 ml of desorption solution the eluted quantity of metal was not higher then 90 % of the given amount. Therefore for desorption of Au and Pd the solution with higher ClO₄^- concentration was used, containing 2 mol of NaClO₄ in 1 litter. By this change no improvement in Au and Pd was reached and both metals were found in the sorbent. It seems that these two metal ions are sorbed by another mechanism then ion exchange. This idea is enforced also by the fact that by complexing agents these precious metals can be eluted, which is visible.

From calculated separation factors the following couples of precious metals could be separated on the column:

Pd²⁺  -  Au³⁺ z prostředí HCl c = 0,5 mol/l

Pd²⁺  -  Pt⁴⁺ z prostředí HCl c = 0,5 mol/l

Literature

1.    Vera Cernajová: Thesis, VŠCHT Praha 1983

2.    Vorlicek J., Dolezal J.:  Fresenius´z Anal. Chem. 260, 369 (1972)