Adsorption of Pb(II), Cd(II), Ni(II) and Cu(II) onto natural kaolinite clay
Introduction
Heavy metal is one of important pollutants in water and especially in wastewater, and it has become a public health concern because of its non-biodegradable and persistent nature. The toxicity of these metals is enhanced through accumulation in living tissues and consequent biomagnification in the food chain [1]. There have been studies on the removal of heavy metals from water and wastewater using chemical precipitation, physical treatment such as ion exchange, solvent extraction, reverse osmosis and adsorption [2], [3], [4], [5]. Among them, reverse osmosis, although very effective, is a cost-prohibitive process as the membranes get easily spoiled requiring frequent replacement. Chemical precipitation however is not very suitable when the pollutants are present in trace amounts and also a large amount of sludge is produced. Ion exchange is expensive and sophisticated. Solvent extraction or electrolytic processes are also available but they are considered to be cost-effective only for more concentrated solutions [4]. Natural clays as the adsorbent with a low cost have received much attention on heavy metals sorption from contaminated water [4], [6].
Clay minerals have different adsorption capacities for metal ions, such as mesoporous silica [7] and montmorillonite clay for Hg(II) [8], which depend on the absorption conditions as they have been found to be very effective, economical, versatile and simple [9], [10]. The removal of metal ions using kaolinite clay is based on ion exchange and adsorption mechanisms and kaolinite has a relative low cation-exchange capacity (CEC) [3–15 meq/100 g of clay] and smaller surface area ranged from 10 to 20 m2/g [11]. A number of studies concerning kaolinite clay used to remove heavy metals from aqueous solution have been reported. Recently, kaolinite clay used to remove Pb(II), Cd(II) and Ni(II) in aqueous medium has been investigated through the process of adsorption under a set of variables [12]. Similarly, kaolinite has been used as adsorbents for the removal of Fe(III), Co(II) and Ni(II) in aqueous medium [13] and for removal of Cu(II) [14] and Pb(II) [15]. In addition, modification of clay minerals can be achieved by either impregnation or grafting of organic/inorganic molecules on its surface to improve its properties, for example, kaolinite clays were modified with orthophosphate and tri-polyphosphate [16], [17], [18], and 2-mercaptobenzothiazole impregnated clay surface was used for the removal of some heavy metal ions from water samples [19], [20].
This study investigated the feasibility of kaolinite clay used as a low-cost adsorbent for the removal of Pb(II), Cd (II), Ni(II) and Cu(II) from aqueous solution. To achieve the aim, the conditions for adsorption, including initial metal ion concentration, pH, electrolytes and their ionic strength, and contact time were investigated with respect to the removal efficiency of metal ion. In addition, the evidence for physicochemical characteristics of kaolinite clay obtained from X-ray diffraction (XRD) and scanning electron microscope (SEM) were investigated to understand the adsorption mechanism. Finally, kaolinite clay potentially used to remove metal ions in wastewater was demonstrated.
Section snippets
Clay minerals
Kaolinite clay was supplied by the Kaolin Company of Long Yan, Fujian, China. It was parched at 80 °C for 3 h and sieved to pass 220 μm mesh (220 μm mesh was proved an appropriate size in the pre-experiment) before being used as adsorbent. The cation-exchange capacity (CEC) of kaolinite clay is 95 mmol/kg. The BET surface area of kaolinite clay is 3.7 m2/g. The chemical compositions of the kaolinite clay were 53.7% SiO2, 43.6% Al2O3, 2.0% Fe2O3, 0.2% MnO, 0.5% K2O, and 0.1% TiO2.
Reagents
The reagents used in
Effect of contact time
The effect of contact time on the adsorption of Pb(II), Cd(II), Ni(II) and Cu(II) was investigated at different time intervals in the range of 2–240 min at 12.5, 9.6, 6.4, and 20 mg/L respectively in a NaNO3 solution with an ionic strength of 0.01 M. After they were centrifuged, the supernatant solutions were taken for metal ion analysis using AAS [17]. As shown in Fig. 1, the equilibrium adsorption was established rapidly within 30 min, indicating that the initial adsorption was very fast and
Conclusions
The results showed that kaolinite clay can be used to remove metal ions such as Pb(II), Cd(II), Ni(II) and Cu(II) from aqueous solution in both single and multi-metal ions. The adsorption capacity of Pb(II) onto kaolinite clay is bigger than other ions tested. There are several factors affecting the adsorption of metal ions onto kaolinite clay, however, the solution pH is a significant impact on the adsorption of metal ions onto kaolinite clay. The selectivity sequence of the adsorption of
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