Effect of dimensions of multi-walled carbon nanotubes on its enrichment efficiency of metal ions from environmental waters

The effect of dimensions (length and external diameter) of multi-walled carbon nanotubes (MWCNTs) on its preconcentration efficiency towards some metal ions (Pb²⁺, Cd²⁺, Cu²⁺, Zn²⁺ and MnO₄⁻) from environmental waters prior to their analysis by flame atomic absorption spectroscopy (FAAS) was investigated. MWCNTs (as-received from the manufacturer) of various external diameters and lengths were involved. Other variables optimized included effects of pH of water sample, composition and volume of eluent, mass of the MWCNTs, breakthrough volume and coexisting ions. Maximum recovery of metal ions was obtained at pH 9 where it was thought that precipitation of metals as their hydroxides played the major factor in metals uptake by MWCNT. It was suggested that the use of appropriate dimensions of MWCNTs may support the trapping process of the precipitated metal hydroxides by MWCNTs. It was found that long MWCNT of length 5-15 μm and external diameter 10-30 nm gave the highest enrichment efficiency towards almost all the targeted metal ions. It could be used for preconcentration of MnO₄⁻, Cu²⁺, Zn²⁺ and Pb²⁺ with almost full recovery; but not for Cd²⁺ due to its low recovery. The optimized solid phase extraction (SPE) procedure was capable of determining metal ions in the linear range 20-100 ng mL⁻¹ (except for Zn²⁺ from 20 to 150 ng mL⁻¹). Detection limits were 0.709 ng mL⁻¹ for MnO₄⁻, 0.278 ng mL⁻¹ for Pb²⁺, 0.465 ng mL⁻¹ for Cu²⁺, 0.867 ng mL⁻¹ for Zn²⁺. Application of the optimized SPE procedure to environmental waters (tap water, reservoir water and stream water) gave spike recoveries of the metals in the range of 81-100%.