Adsorption behavior of copper and cyanide ions at TiO2–solution interface
Introduction
Copper-containing wastewaters are waste products of various chemical industries including mining, smelting, extracting and finishing processes. Copper is a highly toxic element; the removal of Cu(II) from wastewater has been the subject of many studies [1], [2], [3], [4], [5], [6]. Studies on the adsorption of heavy metal ions onto the inorganic material surface have received much attention due to the recent development of more advanced skills and sophisticated equipments [7], [8]. The coexistence of ligands and metal ions in aqueous solutions often results in the formation of metal complexes. Complex formation may alter the adsorptive behavior of a metal due to charge, size, and stereo-chemical configuration. It has been suggested that upon complex formation, the adsorption characteristics of the metal ion may be rendered more metal-like, more ligand-like, or unchanged compared to that of the metal-only systems [9]. Elliott and Huang [10], [11], and Bowers and Huang [12] reported that the adsorption characteristics of metals in the presence of complex formation can not be characterized simply. These metal complexes possess unique adsorption characteristics and therefore any change in adsorption characteristics leads to variation in the removal efficiency of heavy metals in water and wastewater treatment.
Hydrogen cyanide has been a common leaching agent for the extraction and refinement of valuable meals. Wastewaters from such operations often contain significant amount of cyanide and metal ions such as copper. Several investigators [13], [14] have stressed the importance of the equilibrium copper–cyanide speciation distribution in determining the adsorption behavior of the metal ions. They reported that the distribution of Cu–CN complexes in solution at equilibrium is highly dependent upon the CN/Cu molar ratio.
Titanium dioxide, TiO2, is among the most studied metal oxides; it exists in two major polymorphic forms, anatase and rutile. Since the surface area of anatase powder is typically greater than that of rutile, anatase is a preferred catalyst support in many applications [15]. Adsorption of copper on TiO2 in solution have been studied in several previous works [16], [17], [18], [19], [20], [21]. Lee and Chung [20] reported that Langmuir adsorption isotherms were suitable in the acidic region, while Freundlich and Sips adsorption isotherms were suitable in the basic region at pH 9. It is well reported that such adsorption processes are strongly pH-dependent [22], [23]. Sol–gel-derived nanostructured TiO2 thin films and powders have also been investigated; it is reported that the adsorption of copper increases with increasing surfactant concentration, attains a maximum, and then decreases [24], [25], [26]. Zhou et al. [27] reported that the particle size of TiO2 nanoparticles has a strong influence on copper growth on the catalyst surface. Kim et al. have recently studied the removal of Cu(II) in CN− solution under UV irradiation [28]. They reported that the Cu(II) adsorption was enhanced in the present of UV irradiation.
Information on the adsorption of Cu(II) in the presence of CN− in dark environments is not available. The main objective of this study was to investigate how and under what conditions Cu(II) ions can be adsorbed onto the TiO2 surface in both the absence and the presence of complex formation species, i.e., CN− ions.
Section snippets
Materials
Titanium dioxide, TiO2, (Degussa-P25), was provided from Degussa Company, NJ. Monthly stock solutions of TiO2 (10 g/L) were prepared, shaken for 24 h, and left for 3 days for hydration before using. Stock solutions of copper chlorate [ Cu(ClO4)2⋅6H2O] and cyanide [ NaCN] were prepared using deionized water. NaOH and HClO4 were used for pH adjustment. NaClO4 was used to adjust the ionic strength. All used chemicals were of ACS grade.
Batch adsorption experiments
Batch adsorption experiments, in three
Copper and cyanide ion speciation
Copper ions can be present in aqueous solution in two major oxidation states, Cu(I) and Cu(II). In the absence of a ligand agent, such as cyanide, the Cu(II) species is predominant. As pH increases, the hydrated Cu(II) ions become hydrolyzed, yielding hydrolysis species such as Cu(OH)+, Cu(OH)2, Cu(OH)−3, and Cu(OH)2−4. In the presence of a ligand agent, complexes formed are obtained by replacement of water molecules by more preferred ligands [13]. In the case of the CN− ligand, there is a
Conclusions
Adsorption is a useful technique for the removal of Cu(II) ions from wastewater using TiO2. Complete separation of Cu(II) with concentration of was achieved at 20 °C in a TiO2 suspension of 1 g/L and pH ⩾ 8. Cu(II) hydroxyl complexes such as Cu(OH)2 and Cu(OH)+ are major Cu(II) species responsible for Cu(II) adsorption in the pH rang from 7–11. Complexation with cyanide ligand affects the adsorption of Cu(II) negatively. This is due to the formation of unstable Cu(II)–CN complex which
Acknowledgement
The author gratefully appreciates Professor C.P. Huang, University of Delaware, for his helpful advice and discussions in this work.
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