Solid phase extraction and uptake properties of multi-walled carbon nanotubes of different dimensions towards some nitro-phenols and chloro-phenols from water

Equilibrium sorption studies and solid phase extraction (SPE) of various phenols (Phenol (Ph), 2-chlorophenol (2-CP), 3-chlorophenol (3-CP), 4-chlorophenol (4-CP), 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP)) on oxidized and raw multi-walled carbon nanotubes (MWCNTs) of various external diameters (10–20, 10–30, 20–40, 40–60 and 60–100 nm) and various lengths (short: 1–2 µm and long: 5–15 µm) were tested. Equilibrium sorption studies showed that 2-NP fits the Langmuir isotherm model (LIM), while the other phenolic compounds fit the Freundlich isotherm model (FIM). There was generally an inverse relation between external diameter of MWCNT and its sorption capacity towards phenolic compounds. Long MWCNT showed higher sorption capacity than short MWCNT. Thus dimensions of MWCNT play a role in retaining the sorbed molecules. Oxidation of MWCNT caused a decrease in sorption capacity of phenolic compounds of lower acidity where hydrophobic interaction is predominant, while it caused an increase in sorption capacity of phenolic compounds of higher acidity where H-bonding is predominant. The dependence of sorption on the acidity of phenols (pKa values) indicates that the basic groups (pyrone-like groups) on the MWCNT surface play a role in the sorption process. In SPE experiments, it was found that hydrogen peroxide-oxidized MWCNT of external diameter 40–60 nm and length 5–15 µm was the best extractant at pH 6.5 using acetonitrile as eluting solvent. The optimized SPE procedure gave detection limits range: 0.027–0.202 ng mL−1 within the studied concentration range (10–100 ng mL−1). Application of the optimum SPE method on spiked tap water, reservoir water and stream water gave recovery range of 84.3–100% for 2-CP, 3-CP and 4-CP (%RSD range 2.8–9.2%); while Ph, 2-NP, 4-NP, and 2,4-DNP gave recoveries <67.6%.