Cadmium and Lead

Aqueous solution of cadmium (25 g l^-1, 100 ml) was stirred with ion exchanger (1 g) in a beaker, and pH was continuously potentiometrically monitored and adjusted at pH by addition of 0.2 M NaOH. At chosen time intervals, 2 ml samples were withdrawn form the solution and, after diluting, analyzed by means of AAS.

Am 0.1 g sample of the ion exchange pre-dried at room temperature was stirred with cadmium solution (250 mg l^-1, 100 ml) 2 h, and pH of solution was monitored potentiometrically and maintained at the required value by adding either 0.2 M HCl or 0.2 M NaOH solution. Then the sorbent was separated, washed with 10 ml liquid of the given pH value, and shaken with 20 ml 1 M HCl in a 50 ml calibrated flask 1 h. The cadmium concentration of desorbate was determined by means of AAS.

Dependence of exchange capacity of Iontosorb DTTA on ionic strength of solution

An 0.1 g sample of ion exchanger was shaken with lead or cadmium (initial concentration 250 mg l^-1, 100 ml) solution in acetate buffer of pH 4. The ionic strength was adjusted at I = 0.04 - 4.00 by addition of solid sodium perchlorate. Then the sorbent was filtered off, the metals trapped were desorbed by means of 20 ml 1 M HCl, and their concentration in solution was determined with the help of AAS. The same procedure was adopted also in the experiments with the ionic strength adjusted with potassium chloride.

Determination of yields of cadmium and lead in the sorption-desorption processes

In a 1000 ml calibration flask, Iontosorb DTTA (0.1 g) was shaken with cadmium or lead solution (500 ml) in acetate buffer of pH 4, ionic stremgth I = 0.04, and initial concentration of metals p_o (Cd) = 0.6 mg l^-1 and p_o (Pb) = 1 mg l^-1. After two hours, the ion exchanger was filtered off, washed with 10 ml buffer, and extracted with 20 ml 1 M HCl in a 50 ml calibrated flask. The experiment described was repeated five times with solutions containing lead or cadmium or none of the metals.

Property	Value
Density, g . cm^-1	1.19 ^a
Specific weight, g . cm^-1	0.76 ^a 0.84 ^b
Specific volume, cm³ . g^-1	1.31 ^a 1.19 ^b
Mass swelling capacity, g H₂O/g	1.43 ^a
Volume swelling capacity, cm³ . g^-1	3.22 ^a
Water content, %	10.1 ^b
Particle size, µm	80 - 340

The title time course is given in Fig. 1 wherefrom it is obvious that the exchange equilibrium was established approximately 2 h after the beginning of sorption. The contact time of exchanger and metal was used in the rest of experiments.

Time dependence of sorption of cadmium with Iontosorb OXIN at pH 7, p/p_o ratio of actual and initial weight concentration of cadmium

From the data of Table II it is obvious that the most efficient sorption of Pb(II) and Cd(II) ions by the Iontosorb OXIN ion exchanger only occurs in alkaline medium at pH > 8 where the nitrogen atom of the oxine moiety is fully deprotonated already (the corresponding dissociation constants are pK₁ 4.91 and pK₂ 9.81) [2] and can take part in chelate bond formation with the metal ion. The significant difference between the values of exchange capacities Q_a of lead and cadmium is obviously due to different stabilities of hydroxy compounds of lead and cadmium which are formed in solutions of these metals at pH > 7 and thus compete with the exchange equilibrium between the cations and ion exchanger. This interpretation is also supported by the magnitude of stability constants of hydroxo complexes of lead and cadmium whose logarithmus have the following values [2]: log ß₂(Cd) = 7.7, log ß₂(Pb) = 10.9, log ß₄₄(Cd) = 23.2, log ß₄₄(Pb) = 35.1.

pH Dependence of exchange capacity Q_a of Iontosorb OXIN, Iontosorb DETA and Iontosorb DTTA in sorption of Cd(II) and Pb(II) ions

The formation of insoluble hydroxy compounds of the two metals, whose solubility products have the values of pK_s[Pb(OH)₂] 14.9 and pK_s[Cd(OH)₂] 14.35, can unfavourably affect the reliability of determination of exchange capacity in this pH region.

Table III presents the Q_a values of Iontosorb DTTA for the sorptions of Cd(II) and Pb(II) ions from a medium of acetate buffer of pH 4 and varying ionic strength adjusted by addition of sodium perchlorate or potassium chloride. The values of both Q_a(Cd) and Q_a(Pb) estimated for the solutions with NaClO₄ show no systematic dependence on ionic strength, being randomly distributed around the mean value of Q_a(Cd) = 0.94 ± 0.04 mmol g^-1 and Q_a(Pb) = 0.93 ± 0.02 mmol g^-1, respectively. On the other hand, increasing concentration of chloride ions in solution results in a distinct decrease of exchange capacities for both lead and cadmium, the latter being practically not trapped by the ion exchanger from a solution with c(KCl) = 4 mol l^-1 (I = 4). This phenomenon can be explained by the ability of a number of metals, inclusive of Pb and Cd, to for anionic complexes with chloride ions, the different stabilities of these complexes forming a basis of, inter alia, a method of separation of these metals from hydrochloric acid media of different concentrations on strongly basic anion exchangers [3]

Dependence of exchange capacity Q_a (mmol g^-1) of Iontosorb DTTA on ionic strength I in sorption of Cd (II) and Pb (II) ions ^a

^a In acetate buffer of pH 4, the ionic strength was adjusted by adding sodium perchlorate (A) or potassium chloride (B). ^b In a buffer without added salts for ionic strength adjustment.

The above-mentioned experiments were carried out with solutions of high cadmium and lead concentrations but in analytical practice much more dilute samples are encountered. Therefore the following measurements were accomplished with solutions containing the two metals at concentrations as low as the limit of direct detectability by the AAS method, and the overall recovery of the sorption-desorption processes was examined. The statistical treatment of five repeated experiments showed that acceptable recovery values R(Pb) = 93.6 ± 0.8% and R(Cd) = 94.3 ± 0.7% were obtained by twentyfold concentrating 500 ml original lead and cadmium solutions, respectively (pH 4, I = 0.04, initial concentrations p_o(Pb) = 1 mg l^-1, p_o(Cd) = 0.6 mg l^-1). The mean values of estimated amounts of metal were m(Pb) = 0.468 mg and m(Cd) = 0.283 mg, the relative standard deviations of determination of m being s_r(Pb) = 1.63%, s_r(Cd) = 2.43%. The metals were not found in the reference solutions without added lead and cadmium. Provided the detection limits of AAS apparatus are 0.5 mg l^-1 Pb and 0.02 mg l^-1 Cd, the concentration procedure suggested will enable determination of lead and cadmium present in original samples at the concentrations p(Pb) = 25 µg l^-1 and p(Cd) = 1 µg l^-1. When analyzing real samples one will have to verify the recovery of concentration procedure e.g. by the method of standard addition because such solutions often contain a variety of more or less interfering compounds whose presence will result in a decreased efficiency of sorption by the ion exchanger of the ion monitored.

1. Svoboda L., Vorechovsky P.: Collect. Czech. Chem. Commun. 59 (1994) 1311-1318 PDF files

2. Kotrly S., Sucha L.: "Chemicke rovnovahy v analyticke chemii", SNTL, Praha 1988

Sorption of Cadmium and Lead by Chelating Ion Exchangers Iontosorb OXIN, Iontosorb DETA and Iontosorb DTTA [1]

Sorption of Cadmium and Lead by Chelating Ion Exchangers Iontosorb OXIN, Iontosorb DETA and Iontosorb DTTA ^[1]