Environmental Engineering Reference
In-Depth Information
10.3.5.1 Preconcentration Using CNT
Until now, most research has focused on using CNTs as adsorbents in the SPE
process, whereas there is only one report related to employing CNTs for coating fibers
used in a solid-phase microextraction (SPME) process (Wang et al., 2006). Carbon
nanotubes showed impressive results for their preconcentration of various target
compounds such as trace metal ions, trace rare earth elements, volatile organic
compounds, herbicides, insecticides, organochlorine pesticides, phthalate esters,
endocrine disruptors, chlorophenols, dioxins, and related compounds. These results are
going to be presented briefly in this section.
Trace Metal Ions and Rare Earth Elements.
The analytical potential of nitric
acid-treated MWNTs was explored as SPE adsorbents for the preconcentration of trace
Cd, Mn, Ni (Liang et al., 2004), and Cu (Liang et al., 2005a), as well as for rare earth
elements (REEs) such as La, Sm, Eu, Gd, Tb, Yb, and Ho (Liang et al., 2005b). Similar
conditions for preconcentrating trace metal ions were found. A typical procedure for
SPE was proposed as follows: 30 mg of MWNTs were introduced into a PTFE
microcolumn and then washed with 1 M HNO
3
solution and doubly distilled water; a
sample volume was loaded on the cartridge (the maximum volume is 50 ml for Cd, Mn,
and Ni; 150 ml for Cu; and 100 ml for REEs). Thereafter, 1 ml of 0.5 M HNO
3
, 2 ml of
0.5 M HNO
3
, and 2 ml of 1 M HNO
3
were used as an eluent for Cu, three metal cations
(Cd, Mn, and Ni) and REEs, respectively, with the same flow rate (< 2 ml/min). In
addition, the pH of the sample solution should be in the range of 8-9 for Cd, Mn, and Ni,
greater than 5 for Cu, and more than 3 for REEs. A decrease in pH leads to the
neutralization of the CNT surface charge, so the adsorption of cations onto CNTs
decreases quickly. With these conditions, the recoveries were found to be higher than
95% for Cu and REEs, and more than 90% for Cd, Mn and Ni.
The effect of coexisting ions was also investigated. High tolerance limits
(defined as the largest concentration of coexisting ions that reduces the recovery of the
preconcentrated cations to less than 90%) were observed as follows: 5000 mg/L for
alkaline metals (Na
+
and K
+
); 2000 mg/L for earth-alkaline metals (Ca
2+
and Mg
2+
);
1000 mg/L for Al
3+
(200 mg/L in the case of Cu); 5000 mg/L for divalent anions (SiO
4
2-
and SO
4
2-
); 500 mg/L for PO
4
3-
; and 100 mg/L for Fe
3+
(50 mg/L in the case of Cu)
(Liang et al., 2004; Liang et al., 2005a; Liang et al., 2005b). The detection limits of the
method for Cd, Mn, Ni, and Cu were 48, 21, 36, 420 ng/L, respectively, and for REEs
from 3 to 57 ng/L with relatively low standard deviations (< 3.5% for Cd, Mn, Ni and
Cu; < 6% for REEs). In every case, the column can be reused for up to 50 adsorption-
elution cycles by washing with 10 ml of 1 M HNO
3
and 20 ml of distilled water. For
lake water and synthetic seawater, recoveries of 94-104% for all cations were obtained.
In conclusion, MWNTs have great potential as adsorbents for the preconcentration of
metal ions and REEs.
Search WWH ::
Custom Search