João M. Valente Nabais

Preparation of activated carbon fibres from acrylic textile fibres

Abstract Acrylic textile fibres have been used to prepare activated carbon fibres (ACF). Characterisation by means of elemental analysis, XRD, SEM and low temperature nitrogen adsorption show that the properties of the acrylic ACF compare favourably with those of non-textile PAN, Kevlar and Nomex ACF. A particularly interesting, and never previously reported, feature was observed with fibres activated at 900°C. It was found, with one fibre in particular, that over a very limited range of burn-off between 40 and 50% the micropore volume tripled, the mean pore width suddenly increased, the mean stack height, Lc, suddenly decreased and the reactivity decreased by more than a half. The observed changes suggest a change in the mechanism of activation from one involving principally gasification of amorphous or more reactive carbon at low burn-off to one involving principally attack of individual crystallites and their reorganisation at higher burn-off.

Influence of preparation conditions in the textural and chemical properties of activated carbons from a novel biomass precursor: The coffee endocarp

In this work a novel biomass precursor for the production of activated carbons (AC) was studied. The lignocellulosic material used as precursor is the coffee bean endocarp, which constitutes an industrial residue from the Portuguese coffee industry. Activation by carbon dioxide and potassium hydroxide produces activated carbons with small external areas and pore volumes up to 0.22 and 0.43cm3g(-1), respectively, for CO2 and KOH activation. All the ACs produced are very basic in nature with point of zero charge higher than 8. SEM/EDX studies indicate the presence of K, O, Ca and Si. By FTIR it was possible to identify the formation on the ACs surface of several functional groups, namely phenol, alcohol, quinone, lactone, pyrone and ether as well as SiH groups. The tailoring of the porous and chemical structure of the activated carbons produced is possible by selecting the appropriate production conditions.

Thermal treatments of activated carbon fibres using a microwave furnace

Abstract Thermal treatment of activated carbon fibres (ACF) in a flow of N2 gas has been carried out using a microwave device operating at 2450 MHz and with a power input of 1000 W, instead of a conventional furnace, and the samples were analysed by means of low temperature N2 adsorption, elemental analysis and determination of points of zero charge. The results show that microwave treatment for periods between 5 and 30 min affects the porosity of the ACF, causing a reduction in micropore volume and micropore size. More importantly, the results also show that microwave treatment is a very effective method for modifying the surface chemistry of the ACF. During microwave treatment surface groups are completely eliminated, whereas oxygen and nitrogen atoms bonded within the pseudo-graphitic layer planes are retained. On re-exposure to air the surface groups only reform to a very limited extent and as a result very basic carbons, with points of zero charge approximately equal to 11, are readily obtained.

INFLUENCE OF SURFACE IONIZATION ON THE ADSORPTION OF AQUEOUS ZINC SPECIES BY ACTIVATED CARBONS

A surface ionization and adsorption model is used to simulate the adsorption of zinc species from aqueous solutions of varying pH on a number of different activated carbons and the results compared with experimental data. In all cases good agreement is obtained between theory and experiment. It is shown that for most of the carbons the uptake of zinc is due to adsorption of Zn2+ ions on ionized acid sites, the calculated equilibrium constants being of the order of 106–107. On the other hand, one of the carbons used (NORIT AZO) contains virtually no acid sites, but a high concentration of strong basic sites, and the uptake of zinc in this case appears to be due to adsorption of a negatively charged hydroxy complex on protonated basic sites, the estimated equilibrium constant being 108.

Influence of surface ionization on the adsorption of aqueous mercury chlorocomplexes by activated carbons

The adsorption of aqueous mercury species from chloride solutions on a number of activated carbons has been studied. It was found that whereas the adsorption of neutral HgCl2 or positive Hg2+ was very low, significant quantities of the tetrachloromercury(II) complex, HgCl42−, were adsorbed. Adsorption isotherms of this complex were measured at different pH values, and the results analysed by the Langmuir equation and by a simple surface ionization and specific adsorption model in order to obtain estimates of the adsorption stoichiometry and the mean free energy of adsorption.

Scanning electron microscopy of activated carbons prepared from commercial acrylic textile fibres

Scanning electron microscopy has been used to study the variation in surface texture and dimension during carbonisation and activation of three acrylic textile fibres. The results show that both ordered and disorganised forms of carbon exert a direct influence on the macroscopic structure of the carbon materials. In addition, the results also show that the presence of small amounts of an inorganic additive, namely titanium dioxide, can disrupt the graphene structure and lead to modification of the surface texture and reactivity of the carbon materials.

Adsorption of Aqueous Mercury(II) Species by Commercial Activated Carbon Fibres with and without Surface Modification

The adsorption of HgCl2, [HgCl4]2– and Hg2+ onto a series of activated carbon fibres was studied. These included the as-received commercial activated carbon fibre (K), that obtained after modification via by sulphuric acid oxidation (KAC) and that obtained after modification by reaction with pentaethylenehexamine (KBAS). The effects of concentration (10–1500 mg/l), solution pH (1–10) and temperature (25°C, 35°C and 45°C) were studied. The mercury(II) adsorption isotherms followed the Langmuir model with maximum adsorption capacities of 361.0, 142.2 and 300.3 mg/g for HgCl2, [HgCl4]2– and Hg2+, respectively. Fibre K proved to have the highest adsorption capacity towards HgCl2 but the best results for the adsorption of [HgCl4]2– and Hg2+ were obtained with the fibre KAC. The performance of fibre KBAS was always worse than those of the other two fibres tested. The negative values obtained for ΔH0 and ΔG0 indicate that the adsorption was an exothermic and spontaneous process and also demonstrated that the adsorption of Hg(II) is a feasible process.

Removal of amitriptyline from simulated gastric and intestinal fluids using activated carbons

In this work, the adsorption behavior of a tricyclic antidepressant, amitriptyline hydrochloride, onto several activated carbons (ACs) is reported. The adsorption was done using in vitro simulated gastric and intestinal fluid at 37°C to test the performance of the carbons as treatment in overdose cases. The tested materials were one commercial AC (carbomix) and two ACs produced in our laboratory. The highest adsorption capacity was achieved by carbomix, followed by the laboratory-made carbons that still have a very good performance with adsorption capacity up to 120 and 100 mg/g for the gastric and intestinal fluids, respectively.

Simulations of Phenol Adsorption onto Activated Carbon and Carbon Black

Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations have been used to study the adsorption of phenol onto carbon materials. Activated carbon was modelled in terms of its pore-size distribution based on DFT methods, while carbon black was represented by a single carbon slab with varying percentages of surface atoms removed. GCMC results for adsorption from the corresponding gas phase were in reasonable agreement with experimental adsorption results. MD simulations, that studied the influence of the presence of water and surface roughness on the arrangement of the adsorbed phenol molecules, showed that the interaction between the adsorbed molecules was strongly influenced by the presence of water.