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%.