Theretical Basis Study On Environmental Remediation By Carbon Nanotubes

Nano-material is known as the most promising materials in the 21 th century, and leading to international research boom. As a new type of nano-materials, carbon nanotubes(CNTs) exhibit unique tubular structure and outstanding physicochemical properties which make them attract special attention in many fields. During the produce and use, CNTs could release to the environment by many ways, the potential harmful impacts on humans and the environment are attracting increasing research and public attention. Meanwhile, CNTs can be carrier of organic pollutants because of their high adsorption affinity and preferential adsorption, and affect the environmental behavior of organic pollutants and then the toxicity, tansformation, fate. At present, most of studies on adsorption of organic compounds by CNTs focus on strong hydrophobic organic compounds and nonionic organic compounds, but for the adsorption mechanisms of ionizable organic compounds(especially for those high hydrophilic organic compounds) on CNTs has attracted little attention. Therefore, this study chose three types of functionalized carbon nanotubes(H-CNTs, C-CNTs and G-CNTs) as the adsorbent, and mainly investigated the adsorption and desorption behavior of two kinds of organic pollutants(ionizable and non-ionic organic compounds) on CNTs, and effect of p H on adsorption mechanism of ionizable compounds on CNTs. Meanwhile, competitive adsorption between an ionizable and a nonionic organic compound, and competitive adsorption between two different ionizable organic compounds were also discussed. This study not only provides theoretical foundation for the transformation and fate of organic pollutants in the environment, but also provides certain theoretical basis for environmental remediation by CNTs. The main research conclusions as following:(1) The physical and chemical properties of CNTs are one of the most important factors affecting the adsorption behavior. FTIR, elemental analysis, XPS, N2 adsorption, Boehm’s titration and Zeta potential methods were used to measure the basic physicochemical properties of CNTs. The results showed that: the carbon contents of three CNTs were all greater than 94%, and ash less than 0.1% of the total weight. The specific surface area of three CNTs was between 117 and 228 m2/g, and the surface oxygen contents of three CNTs were between 0.7 and 4.2%. The major O-groups on three CNTs surface were-OH,-COOH and –COO, and their total contents were between 0.021 and 0.382 mmol/g. The PZC of H-CNTs and C-CNTs were p H 5.5~6.0 and p H 5.0~5.5, respectively, but G-CNTs did not show stable result due to lack of O-groups on its surface. H-CNTs and C-CNTs showed clearly buffer capacity between p H 6.0 and p H 10.0, but G-CNTs also did not show any buffering.(2) Adsorption/desorption processes of organic pollutants play a pivotal role in their mobility, fate and bio-availability in the natural environment. In this study, benzoic acid(BA), phthalic acid(PA) and 2,6-dichloro-4-nitrophenol(DCNP) were chosen as the ionizable organic compounds, and nitrobenzene(NB) was chosen as the nonionic organic compound, to investigate the adsorption/desorption character of organic compounds in their molecular form. The results showed that: adsorption capacity of organic compounds increased with the specific surface area of CNTs increasing. All isotherms of four compounds were highly nonlinear, and their adsorption affinity followed in order PA < BA < NB < DCNP. Both the Freundlich and Langmuir models fitted the isotherms well for BA, PA and NB(all r2 > 0.952), while for DCNP the Freundlich model(r2, 0.939-0.966) gave better than the Langmuir model(0.706-0.883). Adsorption/desorption hysteresis was not observed for all four molecular compounds on CNTs.(3) Through the stoichiometry experiments, effect of p H and ionic strength on adsorption of BA, PA, DCNP and 3,4-dinitrophenol(DNP) by CNTs was studied, and adsorption mechanism of ionizable organic compounds at different p H was also discussed. The results showed that: the effect of increasing p H varied with the acids; adsorption of BA first increased and then decreased with the increasing p H, whereas adsorption generally decreased with p H for PA and DCNP. Negative charge-assisted hydrogen bond,(-)CAHB, played a crucial role in ionizable compounds adsorption, especially for dissociated form; adsorption through(-)CAHB appeared to contribute from ～10% to ～ 100% of total adsorption over the tested p H and concentration range in this experiment. Apparent acidity constant p Ka,ads of the adsorbed ionizable compound was shifted to a more positive value than the corresponding p Ka in solution. The(-)CAHB between DCNP and an oxyl group on CNTs surface was sterically hindered by the bulky Cl atoms at the 2- and 6-positions. Ionic strength had no effect on ionizable compounds adsorption by H-CNTs.(4) Co-occurrence of various organic compounds such as ionizable and non-ionic compounds in the natural environment is common. Therefore, BA and DNP as ionizable compounds, and NB as a non-ionic compound were selected, to investigate competitive adsorption between an ionizable and a nonionic compound, and competitive adsorption between two different ionizable compounds. The results showed that: at p H 7.0, adsorption of both BAˉ and DNPˉ on all three CNTs were suppressed by co-existing NB, as shown by the decrease in both adsorption affinity and capacity of BAˉ and DNPˉ, while different trends were observed for NB adsorption with co-existing BAˉ or DNPˉ. BAˉ hardly affected NB adsorption, while the increased DNPˉ could significantly inhibit NB adsorption. The addition of BAˉ almost did not affect DNPˉ adsorption on CNTs, i.e., both adsorption affinity and capacity, while co-existing DNPˉ showed significant competitive effect on BAˉ adsorption by CNTs.