Sorption hysteresis of Cd(II) and Pb(II) on natural zeolite and bentonite
Introduction
Regarding acute toxicity, Cd(II) and Pb(II) together with Hg(II) form “the big three” of heavy metals with the greatest potential hazard to humans and the environment [1]. Much greater effort will be needed to reduce pollution from these elements, e.g. through immobilization of Cd(II) and Pb(II) in soil and removal of these toxic elements from contaminated water and wastewater. Utilization of low cost and efficient natural sorbents is one of the techniques, which can be used to immobilize or eliminate toxic heavy metals from soil and wastewater [2], [3]. High specific surface areas, high cation exchange capacities, and low cost and ubiquitous presence in most soils, are the reasons to choose natural sorbents such as zeolite and bentonite to sorb and immobilize heavy metals in the environment [3]. Bioavailability and remobilization of heavy metals in soil and water treated with natural sorbents such as zeolite and bentonite is generally controlled by sorption-desorption reactions [3], [4]. Extensive researches have been conducted on heavy metal sorption by natural sorbents. However, desorption behavior of Cd(II) and Pb(II) from clay minerals are still poorly described and understood.
To predict the fate and transport of heavy metals in the environment pollutant transport and bioavailability models commonly rely on distribution coefficients and maximum sorption levels that are obtained from equilibrium batch sorption experiments [4], [5]. However, one of the discrepancies that causes errors in predicting the potential toxicity of a metal contaminant is the often neglect of desorption, which is important in the control of metal bioavailability in the environment [4], [6]. If sorption is irreversible, these models will incorrectly over predict the movement or biological fate of the metal contaminant. Thus, in order to improve remediation strategies, risk assessments, and to make better predictions about the mobility of contaminants, it is critical that the mechanism of sorption-desorption reactions as well as the reversibility or irreversibility of sorbed heavy metal cations on natural sorbents and soil constituents be quantified.
Hysteresis, or non-singularity, is a phenomenon in which the sorption and desorption isotherms do not coincide [7]. Several mechanisms have been suggested to explain hysteresis including chemical precipitation, variation of the binding mechanism with time, migration and incorporation of the solute into the soil matrix, micropore deformation and trapping [8], [9], [10]. Pseudo-hysteresis is related to slow desorption kinetics, non-attainment of equilibrium of the sorption before desorption was started [8] mass loss from vessels and sorption to non-settling colloids [9]. One possible explanation for slow desorption is that chemisorption reactions usually require a much higher activation energy in desorption direction than sorption, to break the energetically very favorable bonds of the sorbate with the surface [8], [11]. The pseudo-hysteresis depends on conditions and can be eliminated, while the true hysteresis is reproducible in repeated sorption-desorption cycles [9].
Strawn et al. [12] found that sorption of Pb(II) by aluminum oxide is completely reversible within 3 days Similar results were found by Ainsworth et al. [13] for Pb(II) sorption on iron oxides, while Shirvani et al. [14] reported that a large fraction of sorbed Cd(II) by palygorskite and sepiolite was not desorbed after 24 h. Morton et al. [15] reported that Cu(II) sorption irreversibility on montmorillonite was due to the formation of kinetically irreversible Cu(II) dimer complexes or coordination of Cu on the high-energy edge sites of the clay.
Several studies on sorption of Cd(II) and Pb(II) by zeolite and bentonite [3], [16], [17] as well as examples of the sorption hysteresis studies with significant irreversibility exist in the literature for various heavy metals-sorbent systems: Cd(II) sorbed to sepiolite and palygorskite [14]; Th(IV) sorbed to attapulgite [18]; Cd(II), Pb(II) and Cu(II) sorbed to soils [19]. However, no information regarding reversibility of heavy metals from zeolite and bentonite minerals is available. The objectives of the study were: (i) to quantify the hysteresis for Cd(II) and Pb(II) sorption to zeolite and bentonite and, (ii) to compare sorption irreversibility of Cd(II) and Pb(II) onto the sorbents.
Section snippets
Characterization of the sorbents
The bentonite and zeolite samples used in this study were obtained from Anarak and Firouzkoh mines in central and northern Iran, respectively. The mineral samples were powdered in a mortar and sieved using a no. 270 mesh (0.05 mm) sieve. The mineralogical composition was investigated by X-ray diffraction (XRD), and the data was analyzed by the Reitveld method (program AutoQuan, GE Seifert)(data not shown). The samples were saturated with Ca2+ using 1 M Ca(NO3)2 solution. Excess salts were then
Sorbent characteristics
According to XRD, the zeolite was composed of 82.6% clinopetilolite, 8.6%. quartz, 5.3% illite and 3.5% feldspar, the bentonite of 86.5% montmorillonite, 9.5% quartz, 2.5% illite and 1.5% calcite. The CEC of bentonite and zeolite were 76 ± 0.5 and 91 ± 0.5 cmol(+) kg−1, respectively. The N2-surface areas of the samples were 28 ± 1 and 32 ± 1 m2 g−1 for bentonite and zeolite, respectively. It is believed that BET-N2 determined specific surface area does not reflect the true surface area of the bentonite as
Conclusion
Sorption and desorption of Cd(II) and Pb(II) results were well described by the Freundlich model. The results revealed that the desorption isotherms of Cd(II) and Pb(II) from zeolite deviated from sorption data indicating irreversible or very slowly reversible sorption, while, for bentonite sorption/desorption isotherms showed little deviation indicating reversible sorption. The amount of Cd(II) or Pb(II) desorbed from bentonite was more than that of zeolite, indicating that zeolite is a more
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