Adsorption of Pb2 +, Cd2 +, Cu2 + and Cr3 + onto titanate nanotubes: Competition and effect of inorganic ions Original Research Article

Adsorption of Pb2 +, Cd2 +, Cu2 + and Cr3 + from aqueous solutions onto titanate nanotubes (TNTs) in multiple systems was systematically studied. Particular attention was paid to competitive adsorption and the effect of inorganic ions. TNTs showed large adsorption capacity for the four heavy metals, with the mechanism of ion-exchange between metal ions and H+/Na+ located in the interlayers of TNTs. Binary or quaternary competitive adsorption indicated that the adsorption capacity of the four heavy metals onto TNTs followed the sequence of Pb2 + (2.64 mmol g− 1) ≫ Cd2 + (2.13 mmol g− 1) > Cu2 + (1.92 mmol g− 1) ≫ Cr3 + (1.37 mmol g− 1), which followed the reverse order of their hydration energies. Moreover, inorganic ions including Na+, K+, Mg2 + and Ca2 + inhibited the adsorption of heavy metals on TNTs, because they competed for adsorption sites, decreased the activity of heavy metal ions, and promoted the aggregation of TNTs. However, Al3 + and Fe3 + generally enhanced adsorption because the resulting hydroxyl-Al/Fe intercalated or coated TNTs could also capture metal ions. Furthermore, minor effect of inorganic ions on adsorption of Pb2 + resulted from its strong affinity to TNTs. Difficult desorption and small inhibiting effect by Na+, K+, Mg2 + and Ca2 + on adsorption of Cr3 + was due to the formed stable complex of HOCr(OTi)2triple bond; length of mdash with TNTs. Present study indicated potential applications of TNTs in wastewater treatment for heavy metals.