Binary adsorption of heavy metals from aqueous solution onto natural clays

• Binary adsorption of metals on clays was interpreted by multicomponent isotherms.
• Bentonite capacity for adsorbing Cd(II) was similar to that for adsorbing Ni(II).
• Zn(II) presented strong antagonism to the adsorption of Cd(II) on sepiolite.
• Adsorption of Zn(II) on sepiolite was slightly affected by presence of Cd(II).
• Vermiculite adsorption capacity toward Pb(II) was higher than that toward Cu(II).

The binary adsorption of Cd(II)–Ni(II) on bentonite, Zn(II)–Cd(II) on sepiolite and Pb(II)–Cu(II) on vermiculite was investigated in this work. The experimental binary adsorption data were interpreted using the following multicomponent adsorption isotherms: non-modified, extended and modified Langmuir; non-modified and modified Redlich–Peterson; extended Freundlich; and Sheindorf–Rebuhn–Sheintuch. The modified Redlich–Peterson multicomponent isotherm best fitted the adsorption data of Cd(II)–Ni(II) on bentonite and Pb(II)–Cu(II) on vermiculite, while the modified Langmuir multicomponent isotherm best fitted the data of Zn(II)–Cd(II) on sepiolite. The competitive adsorption of Cd(II)–Ni(II) on bentonite revealed that both metals presented very similar antagonism toward the adsorption of the other metal. The single metal adsorption on sepiolite showed that the Cd(II) adsorption capacity of sepiolite was 1.3 times greater than that of Zn(II); however, in the binary adsorption, the Zn(II) presented a strong antagonism to the adsorption of Cd(II), but Cd(II) did not affected the adsorption of Zn(II). The single adsorption isotherms of Cu(II) and Pb(II) on vermiculite revealed that the affinity of the vermiculite towards Pb(II) was slightly higher than that of the Cu(II). However, in the competitive adsorption, the affinity of Pb(II) towards the vermiculite was much higher than that of the Cu(II).

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