Metal Impurities Dominate the Sorption of a Commercially Available Carbon Nanotube for Pb(II) from Water

Numerous studies suggested carbon nanotubes (CNTs) as a type of promising sorbent for heavy metals from water and explained the sorption mechanism mainly by oxygen-containing functional groups on CNT surfaces but neglected the potential role of metal catalyst residues in CNTs. This is a first study showing that metal impurities could dominate the sorption of one type of commercially available CNTs (P-CNTs) for Pb(II) from water, which will help to understand and guide environmental applications of CNTs as a sorbent. Sorption capacity of P-CNTs (27.3 mg g(-1)) for Pb(II) was much higher than that of the water-washed P-CNTs (W-CNTs, 4.7 mg g(-1)). SEM-EDS and ICP-MS analyses showed that both P-CNTs and W-CNTs contained metal impurities (mainly Co and Mo) which released into the solutions during the sorption, especially P-CNTs. XAFS examination and precipitation experiments demonstrated that PbMoO4 formation between Pb(II) and CNT-released MoO42- and subsequent precipitation in the sorptive solutions was the dominant mechanism for the apparent sorption of Pb(II) by P-CNTs.