Adsorption Science & Technology

The evaluation of agricultural by-products as a means of removing pollutants from contaminated wastewater is attracting increasing interest for sustainable development applications. The present work deals with the removal of phenol and 4-chlorophenol from aqueous solutions by chemically activated olive stones. The olive stones were soaked in 50% phosphoric acid for 3 h at 443 K. After washing, the product was activated in an inert atmosphere for 2 h or 3 h at temperatures within the range 873–1023 K. The carbons thus prepared were characterized by Methylene Blue and iodine adsorption, N₂ adsorption at 77 K in conjunction with the BET equation, scanning electron microscopy (SEM) and Boehm titrations. The sorption capacity towards both pollutants was determined by fitting the Langmuir model to the adsorption isotherms. The optimum activation temperature and time were 1023 K and 3 h, respectively. A maximum adsorption capacity of 189 mg/g was obtained for phenol and 436 mg/g for 4-chlorophenol. In comparison, the Merck and Aldrich commercial-grade powdered activated carbons ranked lower with adsorption capacities of 145 mg/g and 179 mg/g for phenol and 244 mg/g and 316 mg/g for 4-chorophenol, respectively.

In the current studies we are focusing on the utility of the theory of hard and soft acids and bases (HSAB), as proposed by Pearson, for the description of the reactivity of different types of two-radical graphene sheets where two neighbouring carbon atoms are unsaturated in the zigzag position. The closed- and open-shell systems were taken into account. The well-known theoretical reactivity indices (ionization potential, electron affinity and global softness) were calculated for the adsorbents analyzed. We have shown that the values of these parameters are independent of the size of the carbon models. To test the reactivity of adsorption sites as probe molecules, we have used the chloride anion (Cl⁻), cation (Cl⁺) and radical (Cl⁰), respectively. Respective correlations between the enthalpy of reaction, the ionization and/or electronic chemical potential of the carbon structures were found.

The results obtained confirm the original HSAB concept of Pearson which has been improved recently by Nalewajski. A similar relationship was observed for the adsorbent charge-transfer term, i.e. the change in the electron densities of the analyzed graphene sheets before and after the adsorption process. From an analysis of the results presented, it has been deduced that a charge-transfer mechanism is probable for chloride ion/adsorbent complexes. Moreover, the acid–base duality of carbon models during the adsorption process was shown and confirmed.

The peculiarities of template syntheses of ordered porous silicas with spherical particle morphologies were investigated in alkaline and acidic media. The silicas obtained were characterized by nitrogen adsorption/desorption measurements, X-ray diffraction, scanning and transmission electron microscopies. Mesoporous silica matrices with high specific surface areas (450–1200 m²/g), an ordered pore structure and a spherical particle granulation of ca. 0.5 μm size were synthesized in alcohol/ammonia media. In acidic media, silica spheres with an average diameter of ca. 10 μm, a slightly ordered pore structure and a lower specific surface area (600 m²/g) were obtained.

Results of Grand Canonical Monte Carlo (GCMC) simulations of hydrogen storage at 303 K in ordered mesoporous carbons (OMCs) which are inverse replicas of cubic Im3¯m silica are presented. Of the ones investigated here, the highest gravimetric total storage of hydrogen (ca. 2 wt% at P = 22 MPa and 303 K) was observed for the OMC replica of the cubic Im3¯m material which was characterized by the largest lattice parameter and internal pore radius of the carbon sphere (a = 9.98 nm and R_int ≈ 3.0 nm). However, the corresponding volumetric density of hydrogen did not exceed 17 kg H₂/m³. The highest excess storage was observed for a threshold value given approximately by a ≈ 7.98 nm and R_int ≈ 2.4 nm. Above this value, the excess storage and volumetric density of hydrogen did not change.

The adsorption was dominated by the bulk storage, with the adsorbent serving as a tank whose storage properties were governed by the available space for hydrogen molecules (bulk storage) and not by physical adsorption (i.e. densification of the supercritical fluid on the walls of the porous material). The calculated average value for hydrogen adsorption in internal spaces was twice as high as in the internal spaces of the spherical cavities, i.e. in the lattice parameter range of the Im3¯m OMC replicas that were studied experimentally at 303 K.

Our calculations indicate that the calculated maximum gravimetric weight per cent and volumetric density of hydrogen lay well below the target of 2010 for 6.0 wt% and 45 kg H₂/m³ indicated by the United States Department of Energy. The storage of hydrogen at 303 K and P < 22 MPa in the OMC replicas of the cubic Im3¯m silica was effective against compression of the hydrogen fluid. However, it was not sufficiently effective to justify the application of these materials for the development of fuel cells for the automobile industry.

Mesoporous activated carbon fibres (ACFs) were prepared from pitch-based microporous carbon fibres by additional chemical activation using calcium nitrate [Ca(NO₃)₂] and disodium hydrogen phosphate (Na₂HPO₄) as activation agents. The porosity change was examined by performing N₂ adsorption measurements at 77 K. The nitrogen adsorption isotherm varied as a result of re-activation with Na₂HPO₄ or Ca(NO₃)₂ at 1123 K or 1173 K for 1 h or 2 h, with that of the re-activated ACFs exhibiting explicit H4 or H1 hysteresis. The use of Ca(NO₃)₂ for chemical activation produced more mesopores than Na₂HPO₄, the maximum mesopore volume being 0.92 ml/g. The largest volume ratio of mesopores to micropores was greater than three and the surface areas were greater than 1000 m²/g.

After KOH activation, the pyrolysis residues of sewage sludge could be converted to a porous carbon with a pore size situated mainly in the 0.6–1.5 nm range. The adsorption rate and capacity of the porous carbon towards Methylene Blue could be improved by the use of acoustic and magnetic fields. The best adsorption behaviour was displayed when the acoustic field frequency was 50 Hz and the magnetic field strength was 485 mT.

For ecological reasons, there is an increasing demand for recycling polyethylene terephthalate (PET) wastes in developed countries. Although one potential application might be its utilisation for the production of activated carbons, the behaviour of these wastes when subjected to different heat treatments and activation processes is still not very well known.

In the present work, samples with different degrees of burn-off were prepared by pyrolysis in an inert atmosphere and subsequent CO₂ activation at high temperatures. The derived changes in the textural and structural properties of the residual solids were studied by helium picnometry, N₂ and CO₂ adsorption isotherms, powder XRD, Raman spectroscopy and XPS. The study reveals that CO₂ activation of PET wastes develops a carbonaceous matrix with micropores. Helium measurements showed that the mass density of the activated samples increased as the degree of burn-off increased. Characterisation studies revealed that the structural changes derived from pyrolysis and further CO₂ activation mostly involved a progressive decrease in the number of structural units.