The rates and equilibria of the adsorption of dissolved. Cd", Pb" and Cu" (at concentration levels of 2 X 10-8 and. 8 X moll-l) from sea-water and from a 0.55 mol ...
Analyst, August 1996, Vol. 121 (1127-1131)
1127
Adsorption of Trace Metals From Sea-water Onto Solid Surfaces: Analysis by Anodic Stripping Voltammetry
Vlado CuculiCa and Marko Branicaath Centre for Marine Research-Zagreb, Ruder BoSkoviC Institute, POB 1016 , 10000 Zagreb, Croatia Institute for Applied Physical Chemistry (IPC),Research Centre Jiilich (KFA), 52425 Jiilich, Germany a
The rates and equilibria of the adsorption of dissolved Cd", Pb" and Cu" (at concentration levels of 2 X 10-8 and moll-l) from sea-water and from a 0.55 mol 1-1 8X NaCl model solution onto electrochemical glass and quartz cells and Nalgene [fluorinated ethylene-poly(propy1ene)l sample bottles with and without added glass beads at pH 6.2 and 8.1 were measured by differential-pulseanodic stripping voltammetry. Lead(I1) shows higher adsorption than Cu", whereas no adsorption of Cd" is observed. Nalgene is the most suitable material for samplers and storage bottles, whereas quartz is the best material for the electroanalytical vessels. The maximum surface covering concentrations of Pb", (r,) (from sea-water at pH 8.1) on the surfaces of a quartz cell, a glass cell, a Nalgene bottle and glass beads were found to be 2.0 X 10-l1, 3.1 X 10-l1, 2.0 X and 2.6 X 10-11 mol cm-2, respectively. The maximum Pb capacities of the glass and quartz cells with the electrode assembly were calculated to be 2.3 XlO-9 and 1.5 X 10-9 mol, respectively. A procedure is proposed for the measurement of the trace metal capacity of the cell and the electrode assembly used in the experiments, for the determination of the metal concentration in natural samples. Keywords: Adsorption; isotherm; cadmium(ii); lead(i1); copper(ir);sea-water; glass; quartz; fluorinated ethylene-poly(propy1ene)
Introduction Various electroanalytical techniques, mainly differential-pulse anodic stripping voltammetry (DPASV), 1-5 have been used for the determination of trace metals at ppb levels in model solutions or in natural water samples. However, an important problem occurs in the actual electrochemical determination of dissolved metals at low natural levels, namely, their adsorption onto sampling, storing and measuring vessel surfaces results in experimental errors that are not negligible.6.7 The adsorption has been studied by several workers in order to explain the adsorption processes of trace metals on electrochemical cell materials. Some workers also included the particulate matter.*-l3 In order to understand these processes under natural sea-water conditions it is necessary to study model solutions with higher metal concentration levels ( > 10-8 mol 1-I). However, in most papers, the experiments were performed in model solutions or, if performed in natural seawater samples, at significantly higher metal concentrations (10-7-10-5 moll-I). The aim of this research is primarily oriented towards natural sea-water samples and a study of the adsorption of dissolved
Cd", Pb" and Cu" ions at low concentration levels onto quartz and glass cells, and Nalgene bottles with and without added glass beads. The adsorption isotherms of Pb" for different adsorbing surfaces are also presented. These experiments are a step towards finding the most convenient vessel material that will minimize loss of the dissolved trace metals from the natural water sample. A procedure for the characterization of the cell and electrode assembly in terms of cell Pb capacity is also given.
Experimental Instrumentation and Reagents All voltammetric measurements were performed with a PAR 174 polarographic analyser, (Princeton Applied Research, Princeton, NJ, USA), connected to a Hewlett-Packard (Avondale, PA, USA) 704514 X-Y recorder. pH was measured with an Orion Research (Cambridge, MA, USA) pH-meter. A constant temperature of 25 "C was maintained during the experiments by a HAAKE D8 thermostat (HAAKE Mess-Technik, Karlsruhe, Germany). The electrochemical vessels were quartz and glass cells (both 60 ml, adsorbing area about 50 cm2) with a corresponding universal cap (Metrohm, Herisau, Switzerland, No. 6.1414.0 10). The adsorption experiments were performed in a 1 1 Nalgene bottle [fluorinated ethylene-poly(propy1ene)l with an adsorbing area of approximately 580 cm2. Glass beads were used as an additional adsorptive surface (area of one bead about 0.24 cm2; diameter 0.3 cm). The working electrode was a hanging mercury drop electrode (HMDE) (Metrohm, No. 6.0335.OOO), the reference electrode was an Ag/AgCl electrode (saturated NaC1) and platinum wire served as the counter electrode. Stirring of the solution was performed with a 'turbo' stirrer constructed in this laboratory, with a constant speed of 2000 rev min-1. The capillary of the working electrode, together with the reference and counter electrodes, the stirrer and the N2 tube comprised the electrode assembly with a surface area of 24 cm2. Pure N2 was saturated with C 0 2 by passing it through wash bottles [the first contained a buffered solution of MgC03(s), Na2B4O7.10H2O(each 10.4 g 1-l) and 0.1 mol 1-1 KH2P04 and the second contained natural sea-water] maintaining the natural pH of sea-water (8.1) in the reaction vessels. The pre-electrolysis was performed for 5 min with stirring and for 15 s without stirring. The pre-electrolysis potentials were -0.8 V for the adsorption experiments and -0.6 V for the isothermal measurements; the pulse amplitude was 50 mV, the scan rate 10 mV s-l and the drop time 0.5 s. All solutions were prepared from analytical-reagent grade chemicals. Stock solutions of Cd", Pb" and Cu" were prepared by dissolution of an appropriate amount of their nitrate salts (all from Merck, Darmstadt, Germany) in doubly distilleG water. The sea-water sample was taken from Jadrija (near Sibenik, Croatia), with a pH of 8.1 and a salinity, S, of 3 8 % ~ ~ .
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Analyst, August 1996, Vol. 121
Procedure
Adsorpion isotherm of the Nalgene bottle
Adsorption measurements
The procedure for adsorption onto the surface of the Nalgene bottle and glass beads excludes adsorption on the cell. This was achieved by keeping the cell supporting electrolyte at pH 2.5 where no adsorption was observed. The construction of the isotherms was performed by using the following equations: 12
Prior to the experiments, the experimental vessels (quartz cell, Nalgene bottle) and glass beads were thoroughly washed. The quartz cell was washed with chromic acid, then with 10% nitric acid and finally with doubly distilled water; the Nalgene bottle was washed with 50% nitric acid and then with doubly distilled water. Glass beads were boiled for 1 h in concentrated nitric acid, then for 1 h in concentrated hydrochloric acid and finally washed with doubly distilled water.'" All measurements were performed at pH 8.1 and 6.2, achieved by the addition of 1 X lo-' moll-' nitric acid, whereas in 0.55 moll-I NaCl solution pH values were regulated by the addition of borate buffer. Adsorption onto the quartz and glass cells and the electrode assembly was studied for natural sea-water and NaCl solution. A mixture of 2 X mol I-' each of Cd", Pb" and Cu" was added to 40 ml of electrolyte in the quartz cell. Measurements in theglass cell were performed with only natural sea-water as the electrolyte. For the first seven values, the anodic stripping voltammetric peak current was recorded every 8 min and subsequently every 30 min until the adsorption equilibrium was achieved. Adsorption onto the surface of the Nalgene bottle was measured for 500 ml of sea-water to which 8 X 10-8 moll-1 of each dissolved metal ion had been added. The bottle was rotated in order to mix the solution. At the same time, in a quartz cell, 30 ml of sea-water (at pH 2.5) were prepared, and a 10 ml aliquot of the solution from the bottle was then added to the quartz cell. The anodic peak current was recorded, representing the concentration of dissolved metal at zero time, t = 0 min. The electrochemical cell was then washed and the whole procedure was repeated with a further aliquot from the adsorption vessel until adsorption equilibrium was achieved. The same experimental procedure was also performed with the addition of glass beads at two levels with approximately (i) 3050 beads (total area 725 cm2) and (ii) 9150 beads (total area 2175 cm2).
c(i)o- c ( i ) m = C(i)ads (2) where r(,) are the surface covering concentrations, C(i)oare the added concentrations of Pb2+, C(i).. are the equilibrium concentrations of Pb2+ in the solution, A is the area of the electroanalytical vessel surface (cm2) and V is the volume of the electrolyte (1). The abscissa represents the equilibrium concentrations achieved after 3 h of equilibration, [C(i,,/mOl l-l] and the ordinate represents the surface covering concentrations ( T h o 1 cm-2). Results and Discussion Adsorption of Cd", Pb" and Cu" Onto the Quartz Cell and Electrode Assembly The adsorption of Cd", Pb" and Cu" onto the quartz cell and electrode assembly from natural sea-water and the model solution of 0.55 rnol 1-1 NaCl at two pH values (8.1 and 6.2) was measured. The time dependence of the concentration of dissolved Pb" in the electrolyte is shown in Fig. I (A). The ratio ii : il is the anodic peak current normalized to the first measured value ( i l ,where t = 0). Adsorption equilibrium at pH 8.1 was reached after approximately 3 h from sea-water and after only 45 min from the NaCl solution. At equilibrium, 48% of Pbrlwas adsorbed from sea-water and about 55% from the NaCl electrolyte, whereas at pH 6.2 41% of Pb" was adsorbed from sea-water and 35% from the NaCl solution. The stronger adsorption of trace metals at higher pH values is in good agreement with the results of previous studies.6-9.13 A longer period is necessary for establishment of the adsorption equilib-
Adsorption isotherms of Pb'I The Langmuir isotherms were constructed from the results obtained in the glass cell, quartz cell, Nalgene bottle and Nalgene bottle with glass beads of adsorptive area 725 cm2 for natural sea-water (at pH 8.1; S = 38%0). The volume of the electrolyte in the cells was 60 ml and in the Nalgene bottle 500 ml. Prior to all experiments, the entire equipment was thoroughly washed in order to remove any residual trace metals. 14
(A) 9
seawater (S=38%0) 0.55 mol I-'NaCl
. =
seawater (S=38%0) 0.55 mol I-'NaCI
0.8 0.6
0.2
Lead(I1) was added to 60 ml of sea-water in the range from 6 X 10-9 to 2 X moll-1. The first measured peak current, I(,)", corresponds to the C(l)o,i.e., 6 X 10-9 moll-' of Pb2+ added. After 3 h (the equilibration time determined from the adsorption procedure described above), adsorption equilibrium occurs, and the peak current i(1 represents the equilibrium concentration of Pb2+ in the solution. Z(l)o- I(,).. is the adsorbed Pb2+ value, C(l)ads. The numerical value of a further added amount of Pb2+ was summarized with the previous numerical vlue of C(lIm measured in solution. The peak current obtained is equivalent to the new concentration of Pb2+ in the solution, i(2)o.The addition of Pb" to the solution up to a concentration of 2 X 10-7 moll-l was repeated every 3 h until sufficient points for the construction of the Langmuir isotherm had been obtained.
(B)
1.o
0.4
Adsorption isotherms of the electroanalytical cells
[Cu2+]=2x1 o-* mol I-'
[Pb*+]=2~10-~ mol 1-l
0.4 -
pH=6.2
0.2 -
.
\ -
a- 0.0
" " " " '
1 5 0 . 0
1.o
pH=6.2
0.8
0.6 0.4
)33,
0.4-
pH=8.1
0.2
0.0
0.2 I
l
pH=8.1
~
l
Fig. 1 current on the time of adsorption in the quartz cell at pH values b.2 and 8.1. Initial concentrations of Cd", Pb" and Cu" were 2 X lo-* mol 1-1. S = 38%0.
Analyst, August 1996, Vol. 121
riurn in natural sea-water, probably because of its complex composition and also its higher ionic strength6 ( = 0.7 moll-1). Competition between ions naturally present in sea-water (such as Mg2+ and Ca2+) and the added metals also affects the equilibration time of the adsorption. The ionic strength of seawater is higher than that of the 0.55 mol 1-1 NaCl solution; hence, the concentrations of Na+ and K+ in sea-water are much greater than those of the trace metals, and therefore these ions are able to compete efficiently for active surface sites.6 For 0.55 moll-' NaCl, the adsorption curve shows a more regular shape than that for natural sea-water which is in the form of a 'broken' line. These observations are also applicable to the complex competition with sea-water constituents. The diffusion coefficient, D , is also a parameter that influences the time at which adsorption equilibrium is achieved, as well as the shape of the adsorption curves. It has been shown that diffusion affects the adsorption phenomena.7 A difference between the diffusion coefficients in the natural sea-water and in the-model solution is evident, and is caused by the trace metal speciation in the two systems.15 The mobility of Pb" species is lower in natural sea-water than in 0.55 moll-' NaCl, where Pb" is mainly present as the Pb2+ ion and PbCl,, whereas in seawater it also occurs as an organic and/or inorganic complex.'6,'7 This fact probably affects the over-all adsorption processes. The amount of adsorbed Pb'I is similar for both electrolytes (natural sea-water and the model solution of 0.55 moll-' NaC1) and at both pH values (Table 1). This result indicates that the cell and electrode assembly adsorbing capacity for Pb" from natural sea-water and 0.55 moll-' NaCl solution is similar and could be the subject of further investigations. Fig. 1(B) shows the rate of the Cu" current decrease. The shape of the curves is similar to that of the corresponding curves for Pb. The conclusions concerning Pb" adsorption can be partially used to explain Cull adsorption. The amount of metal adsorbed is different. The loss of dissolved CuT1 is lower than for Pb". At pH 8.1, the losses of Cull are 25 and 23% for sea-water and 0.55 moll- NaCl solution, respectively. At pH 6.2 the loss of Cu" is 15% for both electrolytes (Table 1). The very similar extent of adsorption indicates that Cu" is present in the same ionic form in both the 0.55 mol 1-1 NaCl solution and natural sea-water. At the lower pH value (6.2), the shape of the curve is the same for the NaCl solution and sea-water. The lower adsorption of Cu" species in comparism with Pb" could be explained by a smaller extent of hydrolysis.",I2,'~20The major inorganic Cu" species in natural sea-water at 25 "C and salinity S = 35%0 are: CuC03 (73.8%), Cu(CO&- (14.2%), &OH* (4.9%) and Cu2+(3.9%).17 The lower amount of Cu adsorbed in
comparison with Pb cannot be ascribed to the competition between Cd, Cu and Pb species for the free sites on the adsorptive surface. This is illustrated in Fig. 2(A) and (B). The amount of Cu adsorbed is virtually the same with or without Cd and Pb species in the measuring system. The same situation was observed for the Pb adsorption measurements in the quartz cell at pH 8.1. It should be noted that after the addition of Cu" to sea-water, Cu" reacts with the organic compounds present21 and/or adsorbs onto the cell walls and electrode assembly. From our results we cannot distinguish between these two cases. The results for Cd" show that there is virtually no adsorption from sea-water and 0.55 mol 1-1 NaCl at both pH values (8.1 and 6.2) in comparison with Pb" and CuT1ions. This can be explained by the fact that the main Cd" species in sea-water and the NaCl model solution are dissolved Cd"-chloro comple~es,2~.23 and, according to other workers, Cd-chloro complexes do not adsorb in contrast to free Cd ions.24,25
Adsorption of Cd", Pb" and Cu" onto the Glass Cell and Electrode Assembly The adsorption of Cd", Pb" and Cull onto the glass cell and electrode assembly from natural sea-water at pH 8.1 was measured. Adsorption equilibrium was reached after 90 and 45 min for Pb and Cu, respectively. At equilibrium, about 52% of Pb" and 50% of Cu" were adsorbed (Fig. 3). No adsorption of Cd2+was observed. A similar situation occurred in the glass cell as in the quartz cell when the metal examined was with the other Co(Pb2') = 2 ~ 1 rnol 0 ~I-'
(A)
It
1.0
Pb"
cu"
Cd"
Adsorbing PH PH PH PH PH vessel 6.2 8.1 6.2 8.1 6.2 Quartz cell + 41.0 48.0 15.0 25.0 assembly 35.0* 5S.O* 15.0* 23.0* Glass cell + assembly 52 so Nalgene bottle 1.1 1.5 1.5 1.6
