The Role And Its Mechanism Of Humic Acid In The Sorption Of Heavy Metals To Carbon Nanotubes

With many extraordinary properties, carbon nanotubes (CNTs) have been used in various fields, which subsequently raises concerns over their environmental behavior and ecological effect. Sorption behavior obtains wide research attention and may control transport, fate, and bioavailability of CNTs and coexisting contaminants. Commercially available CNTs often contain catalytic metal residues and these impurities may release into the sorptive solution and affect the sorption behavior of CNTs. The main mechanism of the sorption of heavy metals by CNTs is attributed to the interaction between heavy metal ions and oxygen-containing functional groups on the CNT surfaces, whereas the potential effect of the catalytic metal residues in CNTs is ignored. What's more, natural organic matter (NOM) widespread in water environment may interact with CNTs and change the physicochemical properties and sorption behavior of CNTs. Up to date, however, only very limited studies are available on the effect of NOM on the sorption behavior of CNTs. Therefore, more specific studies are warranted to reveal the effect and its mechanism of NOM on the sorption of heavy metal ions to CNTs. This thesis is aimed to study the effects of catalytic metal residues and humic acids on the sorption of heavy metal ions to CNTs. Some valuable research results are obtained.(1) The effect of catalytic metal residues in CNTs on the sorption of heavy metal ions by CNTs was firstly studied. The results indicated that the residues of Mo and Co could release into the aqueous phase.The released Mo in form of MoO42-could precipitate Pb(Ⅱ) with the formation of PbMoO4, and the precipitation accounted for 84% of the apparent sorption capacity of CNTs for Pb(Ⅱ).(2) The effect and mechanism of humic acid (HA) on the sorption of Pb(Ⅱ) to CNTs was intensively investigated. It was demonstrated that HA can increase the content of oxygen-containing functional groups and the surface charge negativity of CNTs and thus promote the sorption of Pb(Ⅱ) to CNTs. Sorption capacity of Pb(Ⅱ) by HA-coated CNTs increased with the increase of the content of surface-bound HA. No desorption of CNTs-adsorbed Pb(Ⅱ) was observed, indicating a chemical sorption. However,50% of the CNTs-adsorbed Pb(Ⅱ) could be desorbed from HA-coated CNTs in the presence of 0.1 M Ca2+ions, suggesting half of the sorption could be ascribed to ligand/ion exchange, while the other half may mainly be due to the chemical complexation between Pb(II) and the polar functional groups (mainly-COOH) in the CNTs-bound HA.(3) The effect of humic acid on the sorption of Cd(II) to CNTs was systematically examined. The maximum sorption capacities of Cd(II) by the HAs-coated CNTs were 5.8～12.3 times as that by the HAs-free CNTs, with the ratio increased with the increase in polarities of the CNT-bound HAs and with the decrease in diameters of the CNTs.