Ajna Tóth

Competitive adsorption of phenol and 3-chlorophenol on purified MWCNTs

A commercial multiwall carbon nanotube and its carboxylated derivate (CNTC and COOHC, respectively) was used after purification to study the competitive adsorption of phenol (P) and m-chlorophenol (CP) from 0.1 M aqueous NaCl solutions without external pH control. The adsorption takes place practically exclusively on the external surface of the nanotubes. The uptake of P is suppressed in comparison to its single solute behaviour on both nanotubes, independently of the initial pollutant concentration. The uptake of CP however is more sensitive to the concentration and the surface chemistry of the nanotube. The measured co-adsorption isotherms were compared to the isotherms calculated from the competitive Langmuir model (CLM). Preferential adsorption of CP was observed in about 95% of the relative concentration range. The total adsorption may exceed the corresponding single component sorption capacity.

Interaction of phenol and dopamine with commercial MWCNTs

We report the adsorption of phenol and dopamine probe molecules, from aqueous solution with NaCl, on commercial multiwall carbon nanotubes (MWCNT) and on their carboxylated derivative. The nanotubes were fully characterized by high resolution transmission electron microscopy (HRTEM), small angle X-ray scattering (SAXS), potentiometric titration, electrophoretic mobility, and nitrogen adsorption (77K) measurements. The experimental pollutant isotherms, evaluated using the Langmuir model, showed that only 8-12% and 21-32% of the BET surface area was available for phenol and dopamine, respectively, which is far below the performance of activated carbons. Influence of the pH was more pronounced for the oxidized MWCNT, particularly with dopamine. The strongest interaction and the highest adsorption capacity occurred at pH 3 with both model pollutants on both types of nanotubes. Although the surface area available for adsorption is far lower in MWCNTs than in activated carbons, it is nonetheless substantial. In particular, delayed release of toxic molecules that are either adsorbed on the surface or trapped in the inner bore of such systems could constitute an environmental hazard. The need for further adsorption studies with regard to their environmental aspects is therefore pressing, particularly for MWCNTs in their functionalized state.

Catalytic performance of carbon nanotubes in H2O2 decomposition: Experimental and quantum chemical study

The catalytic performance of multi-walled carbon nanotubes (MWCNTs) with different surface chemistry was studied in the decomposition reaction of H2O2 at various values of pH and temperature. A comparative analysis of experimental and quantum chemical calculation results is given. It has been shown that both the lowest calculated activation energy (∼18.9 kJ/mol) and the highest rate constant correspond to the N-containing CNT. The calculated chemisorption energy values correlate with the operation stability of MWCNTs. Based on the proposed quantum chemical model it was found that the catalytic activity of carbon materials in electron transfer reactions is controlled by their electron donor capability.

Surface-associated metal catalyst enhances the sorption of perfluorooctanoic acid to multi-walled carbon nanotubes

The perfluorooctanoic acid (PFOA) sorption behavior of two commercial multi-walled carbon nanotubes (MWCNTs) (C 150 P from Bayer MaterialScience: BA and C-MWNTs from NanoTechLabs Inc.: CP) was investigated from aqueous solution. The BA nanotubes contained Co/Mn/Mg/Al catalysts both on their outer surface and in the inner bore while CP contained Fe-based catalyst typically within the tubes. The adsorption isotherms of (14)C-radiolabeled PFOA were measured by batch experiments and fitted to the Freundlich model (r(2)>0.92). The adsorption affinity and capacity on BA were significantly higher than on CP. Increasing the pH reduced the adsorption of PFOA due to the electrostatic interaction between the pH-sensitive surface and the adsorbate. Increasing the NaCl concentration led to the aggregation of the MWCNTs reducing the available surface and thus the adsorption capacity. Removal of the catalyst from the outer surface of BA changed the electrophoretic mobility from a positive to a negative value and also decreased the adsorbed amount of PFOA. The surface charge of the surface-associated metal catalyst favors the electrostatic sorption of PFOA. Such surface modifications may be a promising way to improve the sorption capacity of MWCNTs for pollutants such as PFOA and to broaden their potential application in water purification.

Water Adsorption by Carbons. Hydrophobicity and Hydrophilicity

In spite of their intrinsically hydrophobic character, nanoporous carbon materials are able to adsorb a large amount of water, which can compromise their adsorption performance in gas-phase separation applications. The intention of this chapter is to show the importance of surface hydrophobicity and hydrophilicity in the adsorption of water by carbons. A variety of experimental approaches has revealed the key role of polar groups, which act as nucleation centers at the different stages of cluster formation and pore filling, whether in low-resolution experiments such as gas adsorption techniques, in higher-resolution scattering observations such as SAXS or SANS, or again in local methods such as NMR. The conclusions drawn for porous carbons, however, do not necessarily extend to CNTs. Several outstanding questions in water adsorption still remain incompletely understood, or even open. Understanding of these questions may be improved by the synergism between experimental and computational efforts.

The Effect of Ionic Environment on the Adsorption of Phenol

The effect of the ionic environment on the adsorption of phenol from aqueous solutions was investigated in a predominantly microporous carbon and in a commercial carbon designed for wastewater treatment. It was found that not only the pH of the solution but also the method of its setting affects the adsorption capacity. Setting the pH with a buffer solution instead of HCl/NaOH results in a reduced adsorption capacity, owing to the increased number of competing species for adsorption sites, and also to pore blocking. The latter is less critical for the commercial carbon with wider pores. Thermal desorption of phenol exhibits an even stronger dependence on pH setting than adsorption. Upon heating, a mass equivalent to 10–35% of the adsorbed phenol is retained by the surface as a carbon-rich residue, which may modify not only the chemistry but also the pore volume and the pore size distribution of the carbon.