M.M.L. Ribeiro Carrott

Low-Cost Adsorbents: Growing Approach to Wastewater Treatment—a Review

Industrial, agricultural, and domestic activities of humans have affected the environmental system, resulting in drastic problems such as global warming and the generation of wastewater containing high levels of pollutants. As water of good quality is a precious commodity and available in limited amounts, it has become highly imperative to treat wastewater for removal of pollutants. In addition, the rapid modernization of society has also led to the generation of huge amount of materials of little value that have no fruitful use. Such materials are generally considered as waste, and their disposal is a problem. Also, there are some materials that are available in nature that have little or no use. The utilization of all such materials as low-cost adsorbents for the treatment of wastewater may make them of some value. An effort has been made to give a brief idea of an approach to wastewater treatment, particularly discussing and highlighting in brief the low-cost alternative adsorbents with a view to utilizing these waste/low-cost materials.

Photocatalytic decolorization of methylene blue in the presence of TiO2/ZnS nanocomposites.

The synthesis of distinct nanocrystalline TiO2 capped ZnS samples was carried out using a chemical deposition method. The materials characterization showed that the presence of ZnS onto TiO2 surface results in a red shift of the material band edge when compared with the initial semiconductor. The photocatalytic activity of the prepared nanocomposites was tested on the decolorization of methylene blue (MB) aqueous solutions. The dye photodecolorization process was studied considering the influence of experimental parameters such as catalyst concentration, TiO2/ZnS ratio, pH and methylene blue adsorption rate. The material with the best catalytic activity towards the methylene blue photodecolorization was the TiO2 doped with 0.2% of ZnS. The complete photodecolorization of a 20ppm methylene blue solution, at natural pH was achieved in less than 20min, nearly 70min faster than the TiO2 photoassisted process.

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.

Adsorption of volatile organic compounds onto activated carbon cloths derived from a novel regenerated cellulosic precursor

Activated carbon cloths (ACC) were prepared from lyocell, a novel regenerated cellulose nanofibre fabric, by phosphoric acid activation in inert atmosphere at two different final thermal treatment temperatures (864 and 963 degrees C). Benzene, toluene and n-hexane isotherms at 298 and 273K were measured in order to gain insight into the porous structure of the ACC and to evaluate their performance for the removal of volatile organic compounds (VOCs). The Dubinin-Radushkevich equation was employed to evaluate textural parameters of the ACC. The textural characteristics of the ACC were compared with those previously determined from nitrogen (77K) and carbon dioxide (273K) adsorption data. The samples were essentially microporous. The textural parameters calculated from the hydrocarbon isotherms were in good agreement with those evaluated from nitrogen isotherms for the ACC with the wider microporosity. Additionally, the Freundlich model provided a good description of the experimental isotherms for the three volatile organic compounds. The ACC obtained at the higher temperature exhibited a larger adsorption capacity. The ACC were also electrically conductive and showed potential for regeneration by the Joule effect, as determined from macroscopic electrical measurements before and after n-hexane adsorption.

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.

Reference data for the adsorption of methanol on carbon materials

The adsorption of methanol at 298 K on carbon blacks and microporous carbons, including Carbosieve, Takeda molecular sieves, Maxsorb superactivated carbons and an activated charcoal cloth, has been studied. Analysis of the results indicates that the mechanism of adsorption of methanol on carbon blacks shows some similarities to that of water vapour. However, it is still possible to define reference data which can be used to obtain estimates of the surface area of carbon blacks, provided that the surface is reasonably polar, and of external surface area and micropore volume of microporous carbon adsorbents, provided that the pore size is not very large.

Physical adsorption of gases by microporous carbons

A summary of some of the fundamental work, on physical adsorption of gases by microporous carbons, carried out by the authors at Brunel University is presented. The development of the α3, Dubinin—Radushkevich and n-nonane preadsorption methods for the characterisation of micropore structure, and the evidence for the primary and secondary processes of micropore filling are considered. Reference is also made to more recent work on microporous magnesium oxide which provides excellent confirmation of the ideas previously developed in relation to adsorption by carbon adsorbents.

Evaluation of the Stoeckli method for the estimation of micropore size distributions of activated charcoal cloths

Abstract The Stoeckli method for the estimation of micropore size distribution (PSD) is analyzed and quantitative relationships between the mean and width of the PSD and the parameters a and m of the Stoeckli equation, as well as a qualitative relationship between the DA exponent, n, and the PSD mean and width, are presented. The method is applied to nitrogen adsorption data obtained on a series of activated charcoal cloths and quite reasonable and internally consistent semi-quantitative micropore size distributions are obtained. Criteria which should be used to assess the reliability of the results obtained when the Stoeckli method is applied to experimental data are suggested.