Binary adsorption of heavy metals from aqueous solution onto natural clays
Graphical abstract
Introduction
The heavy metals have very diverse applications and are commonly found in wastewater from the mining and metal finishing industry, refining and smelting of metals, and manufacture of batteries. Wastewater from these industries normally contains Pb(II), Cu(II), Cd(II), Ni(II) and Zn(II), and the presence of these metals in wastewater is of great concern because some of these metals are very toxic to the human health and the environment [1].
The adsorption is a separation process widely used for the removal of heavy metals from wastewater due to its versatility and easy operation. Among the most commonly used adsorbents are activated carbon, biosorbents, natural zeolites, and natural clays. The individual adsorption of heavy metals from water solution on these adsorbents has been extensively studied; however, industrial wastewater normally contains two or more metals. The presence of another metal in solution affects the adsorption capacity of the single metal. Therefore, it is very important to study the multicomponent adsorption of heavy metals and to find out about the selectivity or affinity of each metal for a given adsorbent and to know if there is competition between the metal cations for the same adsorption sites.
In the last 10 years, the multicomponent or competitive adsorption of metals from water solution has been investigated on various adsorbents. The following competitive adsorption systems have been studied: Pb(II)–Zn(II) on sulfured orange peel [2]; Pb(II)–Cu(II) on Sphaerotilus natans [3], on cow bone char [4], on granular activated carbon [5] and on Rhizopus arrhizus [6]; Pb(II)–Cd(II) on clinoptilolite [7], on green macroalga Caulerpa lentillifera [8] and on calcium alginate [9]; and Pb(II)–Cd(II)–Zn(II) on Eichhornia crassipes [10]. In these works, it was shown that the Pb(II) presented higher affinity towards these adsorbents than the other metals since the presence of Cu(II), Cd(II) or Zn(II) in water solution affected very slightly the mass of Pb(II) adsorbed on these adsorbents.
The single adsorption of heavy metals on natural clays has been extensively investigated since natural clays have a high cation exchange capacity, mechanical and chemical stability and low-cost, and are very abundant [11], [12], [13], [14], [15], [16], [17], [18], [19]. The main disadvantages are that the natural clays have very low or no capacity for adsorbing anionic species [20], and the removal efficiency of clays for heavy metals is normally less than that of natural zeolites. The adsorption mechanisms of metal cations on natural clays have been argued and reviewed in various works [21].
The binary adsorption of metals on bentonite [22], beidellite [23], kaolinite [24], montmorillonite [25] and smectite [26] have been examined very superficially in recent works, but the binary adsorption of metals on sepiolite and vermiculite has not been investigated. Zhi-rong and Shao-qi [22] evaluated the adsorption of Cu(II)–Ni(II) on a bentonite from Gaomiaozi (Mongolia, China) and found that the capacity of the bentonite for adsorbing Cu(II) decreased slightly by the presence of the Ni(II) and the same trend happened in the other way around. The single and multicomponent adsorption of Cd(II), Cu(II), Pb(II), and Zn(II) from aqueous solution onto kaolinite was analyzed by Srivastava et al. [24] and their results showed that the adsorption capacity increased in the following orders: Cu(II) < Zn(II) < Pb(II) < Cd(II) in the single metal systems, and Pb(II) < Cu(II) < Zn(II) < Cd(II) in the multicomponent system. Srivastava et al. [24] did not interpret the adsorption equilibrium data with the typical multicomponent adsorption isotherm models.
In general, the experimental binary adsorption equilibrium data of metals on natural clays has been obtained for few metals and clays systems, but no work has been reported regarding the modeling of the adsorption equilibrium data using the multicomponent adsorption isotherms.
The main objectives of this work were to study the binary adsorption of Cd(II)–Ni(II) on bentonite, Zn(II)–Cd(II) on sepiolite and Pb(II)–Cu(II) on vermiculite, and to model the adsorption equilibrium data with the binary adsorption isotherms. Furthermore, it was investigated the effect of the presence of Cd(II) on the adsorption of Ni(II) on bentonite, Zn(II) on the adsorption of Cd(II) on sepiolite, and Pb(II) on the adsorption of Cu(II) on vermiculite and vice versa.
Section snippets
Single and multicomponent adsorption equilibrium isotherms
The adsorption equilibrium data for a single component can be normally interpreted by the Langmuir, Freundlich and Redlich–Peterson isotherms, which are represented mathematically as follows:
The multicomponent adsorption isotherms are normally obtained by extending the above single component adsorption isotherms and can be classified accordingly to the single adsorption isotherm that originated the multicomponent isotherm. The following multicomponent
Natural clays
The calcium bentonite employed in this work was obtained from a location in Guadalcazar, San Luis Potosi, Mexico. The sepiolite sample was supplied by the company SEPIOLSA, and it was collected from a location near Madrid, Spain. The vermiculite sample was provided by the company Virginia Vermiculite, situated in Virginia, USA. The particle average diameter of the three clays was 0.375 mm.
Characterization of clays
The chemical composition of the clays was determined following the method proposed by Rettig et al. [32] and
Single metal adsorption isotherms on natural clays
The single metal and clay systems investigated in this work were Cd(II) and Ni(II) on bentonite at pH = 7, Cd(II) and Zn(II) on sepiolite at pH = 7, and Cu(II) and Pb(II) on vermiculite at pH = 4. The solution pH was chosen as the pH where the clay presented a reasonable adsorption capacity towards both metals, and no metal precipitation occurred in the solution. In all the systems, the temperature of the solution was 25 °C.
The Langmuir, Freundlich and Redlich–Peterson isotherms were fitted to the
Conclusions
The binary adsorption equilibrium data of heavy metals can be predicted reasonably well by the multicomponent adsorption isotherm models reported in the literature. The binary adsorption data of Cd(II)–Ni(II) on bentonite, Cd(II)–Zn(II) on sepiolite and Cu(II)–Pb(II) on vermiculite were best interpreted with the MRPMI, MLMI and MRPMI models, respectively.
In the competitive adsorption of Cd(II) and Ni(II) on bentonite, both metals presented very similar antagonism against the adsorption of the
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