EXAFS study of the interfacial interaction of nickel(II) on titanate nanotubes: Role of contact time, pH and humic substances

• Outer-sphere surface complexes formed at low pH.
• Inner-sphere surface complexes and surface precipitates formed at high pH.
• The microstructure changed from outer-sphere complexes into inner-sphere complexes over extended time.
• Ligand-bridging and metal-bridging ternary complexes formed in the presence of HA/FA.

In this work, the interaction mechanism and local atomic structures of Ni(II) on titanate nanotubes (TNTs) as a function of contact time, solution pH and addition of humic acid (HA) or fulvic acid (FA) were studied mainly via the extended X-ray absorption fine structure (EXAFS) spectroscope. The results indicate that Ni(II) can be interacted with TNTs by different mechanisms, largely depending on environmental conditions. At pH 4.0 and short contact time, Ni(II) interaction with TNTs is mainly dominated by outer-sphere surface complexation. With medium increasing pH, Ni(II) interaction with TNTs tends to transform from outer-sphere surface complexes to inner-sphere surface complexes, and a mixture of inner-sphere surface complexes and surface precipitates is formed at pH 7.0 or 8.0. Besides, the interaction of Ni(II) with TNTs becomes more favorable over extended time, and EXAFS spectra analysis indicates outer-sphere surface complexes at short contact time is transformed to inner-sphere surface complexes and surface precipitates over long contact time. The addition of HA or FA enhances Ni(II) interaction with TNTs at low pH, which is attributed to the formation of “metal-bridging” (type A) and “ligand-bridging” (type B) ternary surface complexes. The results obtained are very important to understand the interaction of Ni(II) and related heavy metal ion with nano-materials such as TNTs once released into natural environment.