Abstract

Key Words

Cadmium, adsorption, edge, kaolinite, extended constant capacitance model

Introduction

Soil is an important reservoir of metals, and contains a range of biologically essential and non-essential elements. The growing concern about the quality of the natural environment has stimulated an increasing interest in the occurrence and behaviour of heavy metals in soils and natural waters. Adsorption is one of the most important physico-chemical processes responsible for the retention of inorganic and organic substances in the soil environment. Some of the important factors affecting the adsorption of metals are pH (James and Healy 1972), nature and concentration of substrate and adsorbing ion (Spark et al. 1995), ionic strength and the presence of competing and complexing ions (Ikhsan et al. 1999).

Cadmium pollution in soils has rapidly increased in recent decades as a result of rising consumption of Cd by industries (Alloway 1995). Sources of Cd contamination include phosphatic fertilizers; the mining of Cd and smelting; atmospheric pollution from metallurgic industries; the disposal of wastes containing Cd and urban/industrial pollution (Nriagu 1990; Tiller 1989). Contaminated soils often contain elevated concentrations of heavy metals such as Cd, Cu, Pb and Zn (Singh 2001). However, it is important to note that presence of more than one heavy metal can have potential impact on the adsorption behaviour of each metal present in the system. They might experience competition by the presence of other metal(s) depending upon their concentrations and other factors such as pH, type and abundance of clay minerals and organic matter content.

Kaolinite is the most abundant phyllosilicate mineral in highly weathered soils. It is a 1:1 aluminosilicate comprising a tetrahedral silica sheet bonded to an octahedral sheet through the sharing of oxygen atoms between silicon and aluminium atoms in adjacent sheets. Successive 1:1 layers are held together by hydrogen bonding of adjacent silica and alumina layers. The tetrahedral sheet carries a small permanent negative charge due to isomorphous substitution of Si⁴⁺ by Al³⁺, leaving a single- negative charge for each substitution (Schulze 1989). Both the octahedral sheet and the crystal edges have a pH-dependent variable charge caused by protonation and deprotonation of surface hydroxyl (SOH) groups. Thus, two different populations of metal ion adsorption sites are present on the kaolinite surface. The intensive interaction between kaolinite and contaminants is primarily controlled by the surface acid-base properties.

Schindler et al. (1987) proposed that the binding of Cd on kaolinite could be described by a model that assumes two kinds of binding sites. Adsorption is via ion exchange on the first site type, whereas inner-sphere binding to ampholytic SOH groups occurs on the second population of surface sites. This view was supported by comprehensive studies on adsorption of transition metals onto Kaolinite by Angove et al. (1998) and Ikhsan et al. (1999). They applied a surface complexation modelling approach to adsorption results obtained from three different types of experiments to determine possible surface reactions.

The onset of adsorption and the formation of hydrolysis products have been linked in adsorption studies (James and Healy 1972), therefore, one might expect that differences in metal ion hydrolysis may result in different adsorption reactions. Here we have used an extended constant capacitance surface complexation model to describe the sorption process in an effort to further understand the mechanisms involved in metal uptake. Potentiometric titration and a range of sorption data were fitted using the program GRFIT (Ludwig 1992) to determine various reaction constants.

Previous studies show that Cd adsorption onto kaolinite increases with increasing pH (Farrah and Pickering 1977; Lackovic et al. 2003; Spark et al. 1995; Schindler et al. 1987); however, little information is available pertaining to its adsorption onto kaolinite in systems containing other competing metals. The objective of our study was to examine the adsorption of Cd(II) at across a range of pH values in the absence and presence of Cu(II), Pb(II) and Zn(II) – metals, which occur commonly at elevated concentrations in contaminated soils.

Materials and Methods

All solutions were prepared in E-Pure^® (Barnstead) deionized water. Acid washed kaolinite supplied by Ajax Chemicals, Sydney, Australia was used without any further treatment in the present study. The ‘Brunauer-Emmett-Teller’ (BET) surface area determined by Quantasorb Surface Area Analyser (Model Autosorb-1, Quantachrome Corp., NY) was 14.4 m² g^-1. The X-Ray diffractometry (XRD) showed characteristic peaks of kaolinite and no other mineral component was detected.

The adsorption and potentiometric titration experiments were carried out in a borosilicate reaction vessel at controlled room temperature (22 ºC) under a nitrogen atmosphere. The mineral suspension (2 g kaolinite in 300 mL 0.01 M NaNO₃ in single-element system and 4 g kaolinite in 600 mL 0.01 M NaNO₃ in multi-element system to give a mineral concentration of 96.27 m²/l) was stirred overnight for 16 hours at its natural pH (~5.0) to hydrate the mineral surface. The adsorption experiments were conducted with a metal concentration of 133.33 µM in single-element system and 33.33 µM each of Cd, Cu, Pb and Zn (i.e. total concentration of metals 133.33 µM) in multi-element system. The pH was measured using a Radiometer combined pH electrode with Radiometer standard buffers (pH 4 and 7) at 22ºC. The pH was varied from 3.5 to 10.0 with a 0.5-pH unit increment using 0.1 M HCl. One-hour equilibration period was allowed for adsorption and an aliquot (10 mL in single-element system and 20 mL in multi-element system) was collected every hour. The aliquots were centrifuged, filtered through Whatman No. 1 filter paper and the supernatant was analysed for respective metal(s) using Varian SpectrAA – 220 FS flame atomic absorption spectrophotometer. The amount adsorbed was calculated from the difference between the initial and equilibrium concentrations. The adsorption curves were plotted between percent metal adsorbed and pH.

Potentiometric titrations were performed in systems containing kaolinite in the absence and presence of heavy metal (Cd, Cu, Pb and Zn) ions at the same concentrations as the adsorption experiments. After the kaolinite system was equilibrated overnight, the metal ions(s) was/were added and the system was titrated with 0.1 M NaOH to pH 10 and with 0.1 M HCl to pH 3, with a Radiometer TIM800 Autotitrator. After each addition of acid/base, the pH was allowed to stabilize until the pH drift was less than 0.01 pH unit per minute.

Cadmium speciation in solution

The speciation diagram of Cd in solution (Figure 1) shows that Cd²⁺ is the only species dominant upto pH 10.0. The concentration of Cd²⁺ is almost consistent up to pH~8.0, after which, CdOH^- and Cd(OH)₂ start to form, however, the fractions of these species are only ~30 % and ~20 %, respectively. Other hydroxyl and nitrate species e.g. Cd(NO₃)₂ (aq), Cd(OH)₃^-, Cd(OH)₄^2-, Cd₂OH³⁺ and CdNO₃⁺ are also formed, but their concentration in solution was less than 2 percent. While modelling Cd²⁺ adsorption on kaolinite, we considered only Cd²⁺ species participating in the adsorption on permanent charges and variable charges, as it is most dominant species over the entire pH range studied.

Figure 1. Distribution of Cd species in solution as a function of pH.

Modelling the data

An extended constant capacitance surface complexation model was used for modelling Cd adsorption on kaolinite. This model accounts for both inner sphere and outer sphere complexation and requires fewer adjustable parameters than some of the other models. Values for equilibrium constants and site densities were estimated using the computer program GRFIT (Ludwig 1992). Parameters for surface protonation reaction were obtained from the titration data of kaolinite suspensions without metals. The surface acidity constants and site densities were then introduced as fixed values for modelling the systems containing kaolinite and heavy metal ions. Earlier modelling studies by Schindler et al. (1987) and Angove et al. (1998) suggest that transition metal ions adsorb at both the permanent and variable-charge sites. The permanent, negatively charged sites, represented by X^-, can undergo an exchange reaction that may be characterized by:

XNa + H⁺ = XH + Na⁺

In the absence of transition metal ions, the X^- sites are assumed to be occupied by Na⁺ ions from the background electrolyte at higher pH values. The variable-charge sites can undergo both protonation and deprotonation reactions, which may be shown as following:

SO^- + H⁺ = SOH
SOH + H⁺ = SOH₂⁺

Where SOH represents a surface hydroxyl group. No distinction is made between aluminol and silanol surface groups, but it is likely that SOH groups involved in Cd adsorption are mostly AlOH (Schindler et al. 1987). Two additional equilibria were required to model adsorption of Cd onto kaolinite:

Cd²⁺ + 2 XNa = X₂Cd + 2 Na⁺
Cd²⁺ + 2 SOH = (SO)₂Cd + 2 H⁺

Adsorption of other Cd species like CdOH⁺ and Cd(OH)₂ as shown in figure 1 were also tried during the modelling process, but only the reaction scheme outlined above gave the best statistical fit to the experimental data. The equilibrium constants were deemed acceptable only when the data from the titrations and adsorption edges were adequately fitted by a given set of equilibrium constants.

Table 1. Extended constant capacitance surface complexation model parameters for Cd adsorption onto kaolinite using the program GRFIT (Ludwig 1992).

Site density XH (μmol m-2)	1.04
Site density SOH (μmol m-2)	6.93
Specific inner capacitance κ (F m-2)	7.0
Specific outer capacitance κ (F m-2)	3.0
Protonation reactions	Log K
XH + Na+ = XNa + H+	-3.30
SOH + H+ = SOH2+	3.81
SOH = SO- + H+	-6.16
Hydrolysis reactions*
Cd 2+ + H2O = CdOH+ + H+	9.60
Cd 2+ + 2H2O = Cd(OH)2 + 2H+	18.80
Surface reactions	Cd (II)
2XNa + Cd2+ = X2Cd + 2Na+	4.13
2SOH + Cd2+ = (SO)2Cd + 2H+	-8.79

Results

Effect of pH on Cd adsorption

The adsorption of Cd on kaolinite increased with increase in pH both in single- and multi-element systems (Figure 2). The adsorption curve in the single- element system was characterized by two distinct adsorption stages, with each stage separated by a plateau in the pH range ~6.0 to ~6.5. The first stage of adsorption edge commenced with about 5 % Cd adsorption at pH~4.0 and ended at pH~6.0, at which about 50 % of the total Cd had been adsorbed. The second stage started at pH~6.5 and continued up to pH 10.0 where about 95 per cent of the total Cd was adsorbed. The plateau observed in the middle of adsorption curve in single-element system was not as well defined in the multi-element system. Comparing the pH₅₀ (i.e. the pH at which 50 % metal is adsorbed) of both systems, it may be noted that in multi-element system the adsorption edge shifted to a lower pH. The values of pH₅₀ in single- and multi-element systems were 6.40 and 6.10, respectively. The adsorption curve of the two systems crossed at pH~6.0.

Figure 2. Effect of pH on Cd adsorption onto kaolinite in single- (Cd concentration 133.33 µM) and multi-element (Cd, Cu, Pb and Zn concentration 33.33 µM each i.e. total metal concentration 133.33 µM) systems.

Cadmium adsorption onto different sites of kaolinite

The solid and broken lines in the adsorption curve (Figure 3) show predicted adsorption based on the extended constant capacitance surface complexation model, with best fit parameters shown in Table 1. The model fits the experimental data closely, with only slight overestimation above pH 7.5 in the single-element system. The model suggests that Cd adsorbs as a bidentate complex on the permanent charge surface (X₂--Cd²⁺) at lower pH values, while at higher pH another bidentate complex, (SO)₂Cd, forming on hydroxyl surfaces dominates the speciation Interestingly the model predicts that more Cd adsorbs to the permanent charge surface in the multi-element system: up to 50 % X₂--Cd²⁺ in the single-element system compared to about 70 % in the multi-element system.

Figure 3. Cadmium adsorption onto different sites of kaolinite in (a) single- and (b) multi-element systems as derived from extended constant capacitance modelling and using surface complexation parameters described in Table 1.

Discussion

Cadmium adsorption in single- and multi-element systems occurred differently (Figure 1) as the adsorption edge in single-element system had two distinct stages separated by a plateau between pH~6.0 and ~6.5. This plateau is a feature that has been observed previously by several investigators (eg. Schindler et al. 1987; Spark et al. 1995; Angove et al. 1997, 1998). The plateau region was not as obvious in the adsorption curve in the multi-element system. The Cd adsorption curve (Figure 2) reveals that the adsorption in single-element took place predominantly on permanent negatively charge sites in the first stage up to pH~6.0. The Cd adsorption at the variable charge sites on the crystal edges started from pH~6.5 onwards. In the multi-element system, however, Cd adsorption took place predominantly on the permanent charge surface up to pH 7.5. The increased uptake of Cd by the permanent charge surface in the multi-element system can be explained by considering the behaviour of the other metals present. Modelling in the multi-element system (data not shown) reveals that metals that form hydrolysis products more readily (Pb and Cu) adsorb to variable charge surfaces from about pH 5 and above. This tendency for readily hydrolysable metals to interact strongly with oxide surfaces has been noted previously (James and Healy 1972; Spark et al. 1995; Ikhsan et al. 1999). Because of its lower tendency to form hydrolysis products, Cd does not compete effectively for variable charge surfaces, and so is more restricted to permanent charge sites, hence we see an increased uptake of Cd at these sites when other metals are present.

Conclusions

The kinetics and magnitude suggest that in single-element system, Cd adsorption onto kaolinite occurs in two stages. The first stage involves adsorption on permanent charge sites and the second stage involves adsorption on variable charge sites. Adsorption in multi-element system is different from single-element system as Cu, Pb and Zn, having greater affinity for adsorption onto kaolinite, occupy most of the sites and Cd is left with very few sites available for its adsorption onto kaolinite surface. Cadmium adsorption occurred onto permanent and variable charges by forming bidentate complexes. The findings, however, require confirmation by direct methods such as extended x-ray absorption fine structure (EXAFS) spectroscopy etc.

Acknowledgement

Prashant Srivastava acknowledges the financial support by International Postgraduate Research Scholarship (IPRS) funded by the University of Sydney and the Department of Education, Science and Technology (DEST), Government of Australia.

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