Volume 2 Issue 9 - December 28, 2007
Thermodynamics and kinetics of adsorption of Cu(II) onto waste iron oxide
Yao-Hui Huang1,2*, Chan-Li Hsueh1, Hui-Pin Cheng1, Liang-Chih Su1, Chuh-Yung Chen1

1 Department of Chemical Engineering, National Cheng Kung University, Tainan City 701, Taiwan
2 Sustainable Environment Research Center, National Cheng Kung University, Tainan City 701, Taiwan

*E-mail address: yhhuang@ccmail.ncku.edu.tw

Paper published in Journal of Hazardous Materials 144 (2007) 406–411

Heavy metals in water have been a major preoccupation for many years because of their toxicity towards aquatic life, human beings and the environment. As they do not degrade biologically like organic pollutants, their presence in industrial effluents or drinking water is a public health problem due to their absorption and therefore possible accumulation in organisms. Copper (Cu2+) is of particular interest because of its toxicity and its widespread presence in the industrial applications, e.g. electrical, electro-plating, metal-finishing and paint industries.

Several techniques are available for removing heavy metals from aqueous solutions. They include chemical precipitation, conventional coagulation, reverse osmosis, ion exchange and adsorption. One of these techniques, the adsorption method, is simple and cost-effective, and is extensively adopted. Adsorption onto activated carbon has proven to be one of the most effective and reliable physicochemical treatment methodologies. However, commercially available activated carbons are very expensive. Babel and Kurniawan have presented interesting reviews of the potential of a wide range of low-cost sorbents of heavy metals. According to these researchers, a sorbent can be assumed to be low-cost if it requires little prior processing, is naturally abundant, or is either a by-product or a waste material from another industry. These materials may represent alternatives to expensive treatment processes.

Recently, interest in low-cost, high-surface-area materials, especially metal oxides, and their unique applications, such as adsorption and chemical catalysis, has been growing. Iron oxide has a relatively high surface area and charge; numerous researchers have used iron oxide as an adsorbent to treat heavy metals and organic compounds from wastewater.

This study investigates low-cost sorbents as replacements for current costly methods of removing heavy metals from solution. This investigation explores the waste iron oxide material (F1), which is a by-product of the fluidized-bed reactor (FBR)-Fenton reaction, for use in the treatment of the wastewater in Taiwan. In this investigation, F1 are tested as adsorbents for removing copper (Cu2+) from aqueous solutions. The highest Cu2+ adsorption capacity of F1 adsorbent was determined as 0.21 mmol g-1 for 0.8 mmol dm−3 initial Cu2+ concentration at pH 6.0 and 300 K.

Table 1 presents the characteristics of the F1 adsorbent. Figure 1 displays the morphology of the original activated alumina grain support (Fig. 1a) and the F1 adsorbent (Figs. 1b). Comparing Fig. 1a with Fig. 1b reveals that the morphology was smoother after the reaction in the FBR had proceeded for three months. Figure 2 shows the XRD patterns of the F1 adsorbent. The XRD data were analyzed using the F1 adsorbent of iron according to the diffraction files of the Joint Committee on Powder Diffraction Standards (JCPDS). The JCPDS data on oxyhydroxides of iron were compared. The main diffraction peaks of the F1 adsorbent at 2θ = 21.20, 36.60 and 53.20 were carefully compared with the standard for goethite (α-FeOOH—file number 81-0464). Accordingly, the peaks of F1 adsorbent were identified with the phase α-FeOOH. However, the XRD pattern of the F1 adsorbent exhibits very weak diffraction intensities indicating that the F1 adsorbent contained a very small amount of crystalline goethite.

Table 1 Properties of the F1 adsorbent

Fig. 1        Scanning electron micrographs of (a) original support and (b) F1 adsorbent.

Fig. 2        X-ray diffraction pattern of the F1 adsorbent showing the intensities in the region 2θ = 10 — 800.

Fig. 3        Adsorption isotherms of Cu2+ onto F1 adsorbent at different temperatures.

Figure 3 plots the adsorption isotherms of Cu2+ adsorption by F1 adsorbent at various temperatures. All batch experimental data were fitted to the isotherm models of the well-known Langmuir and Freundlich using the method of least squares and an optimization algorithm. Figures 4a and 4b display linear plots of Ce / q e vs. Ce and lnqe vs. lnCe. For each isotherm in Fig. 4a, the values of qm and KL were determined from experimental data by linear regression. According to Fig. 4b, the values of KFand n were obtained similarly.

Fig. 4        Linearized (a) Langmuir and (b) Freundlich isotherm models for Cu2+ adsorption by the F1 adsorbent at different temperatures.

Fig. 5        The standard free energy change versus standard enthalpy change for F1 adsorbent and the other adsorption systems.

Fig. 5 plots the free energy change vs. enthalpy change to yield the thermodynamic parameters evaluated herein and previous literature. A positive standard enthalpy change of 9.2 kJ mol−1 obtained in this study indicates that the adsorption of Cu2+ by the F1 adsorbent is endothermic, which fact is evidenced by the increase in the adsorption of Cu2+ with temperature. A negative change in adsorption standard free energy reveals that the adsorption reaction is a spontaneous process. The positive standard entropy change may be caused the release of water molecules in the ion exchange reaction between the adsorbate and the functional groups on the surfaces of the F1 adsorbent. These results were similar to most of the data shown in Fig. 5. In previous literature, although the standard enthalpy change for adsorption of very different adsorbate onto distinct adsorbent covers a wide range (–85~+160 kJ mol-1), the standard free energy change at 30˚C remain within ±30 kJ mol-1.
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