M.V. López-Ramón

Changes in surface chemistry of activated carbons by wet oxidation

Abstract A series of activated carbons with different degrees of activation were oxidized with H2O2, (NH4)2S2O8 and HNO3 in order to introduce different oxygen surface complexes. Changes in the surface chemistry of the activated carbons after their oxidizing treatments were studied by different techniques including temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR), titrations with HCl and NaOH, measurements of the pH of the point of zero charge and catalytic dehydration of methanol. Results showed that treatment with (NH4)2S2O8 fixed the lowest amount of both total oxygen and surface acid groups. However, this treatment yielded the acid groups with the highest acid strength. This could be because it favors fixation of carboxyl groups close to other groups, such as carbonyl and hydroxyl, which enhances their acidity.

On the characterization of acidic and basic surface sites on carbons by various techniques

Abstract Active carbons of different origins have been oxidized with H2O2 and (NH4)2S2O8 and their oxygen surface complexes have been characterized by TPD, classical titration following Boehms method and by neutralization calorimetry. The net enthalpies of neutralization, determined by immersion calorimetry into NaOH and HCl 2 N lead to −41.1±1.8 and −52.3±2.0 kJ eq−1 for the acidic and basic sites on the surface. Experiments with NaHCO3 lead to −39.7±1.7 kJ eq−1 for the carboxylic groups alone. These results suggest that the surface groups of active carbons can also be characterized by immersion calorimetry. Results are also given for the variation of the pH of the point of zero charge with the total oxygen content of the surface.

Adsorption of some substituted phenols on activated carbons from a bituminous coal

Adsorption at 298 K of phenol, p-cresol, m-chlorophenol, m-aminophenol, and p-nitrophenol from aqueous solutions on activated carbons obtained from an original and a demineralized bituminous coal has been studied. The adsorption capacity of the activated carbons depended on the surface area and porosity of the carbon, the solubility of the phenolic compound, and the hydrophobicity of the substituent. The relative affinity of the phenolic compound toward the surface of the carbon was related to the electron donor-acceptor complexes formed between the basic sites on the surface of the carbon and the aromatic ring of the phenol. The adsorption capacity of the carbon depended on the solution pH. As a result, the adsorption capacity began to decrease at a pH value that depended on the difference between the external and internal surface charge density, as measured by electrophoresis and pH measurement of the slurry, respectively.

Thermal regeneration of an activated carbon exhausted with different substituted phenols

The thermal desorption process of phenol, m-aminophenol, p-cresol and p-nitrophenol from an activated carbon has been studied. For this purpose, the activated carbon-phenolic compound system was heated to 1100 K in a He flow following determination of weight loss and the evolution of light gases by a thermobalance and by mass spectrometry, respectively. Results showed that during the heat treatment, part of the phenol evolved from the activated carbon and was deposited at the outlet of the reactor and part underwent degradation reactions to light gases and to a residue that remained on the surface of the activated carbon. Finally, the variation of the adsorption capacity of the activated carbon after several adsorption-regeneration cycles has also been studied.

Kinetics of diuron and amitrole adsorption from aqueous solution on activated carbons

A study was conducted on the adsorption kinetics of diuron and amitrole from aqueous solutions on activated carbons of different particle sizes and on an activated carbon fiber. Different kinetic models were applied to the experimental results obtained. A pseudo-second-order rate equation fitted the adsorption kinetics data better than a pseudo-first-order rate equation. Amitrole showed faster adsorption kinetics compared with diuron because of the smaller size of the former herbicide, despite its lower driving force for adsorption. Both reaction rate constants increased when the particle size decreased. The activated carbon fiber and the activated carbon of smallest particle size (0.03 mm) showed similar adsorption kinetics. The intraparticle diffusion rate constant increased with higher initial concentration of herbicides in solution and with lower particle size of the adsorbent. This is because the rise in initial concentration increased the amount adsorbed at equilibrium, and the reduction in particle size increased the number of collisions between adsorbate and adsorbent particles. Demineralization of the activated carbon with particle size of 0.5mm had practically no effect on the adsorption kinetics.

Adsorption mechanisms of metal cations from water on an oxidized carbon surface.

Adsorption of Cr(III), Mn(II), Cu(II) and Zn(II) on an oxidized activated carbon cloth was studied. Its surface chemistry was characterized by potentiometric titration. This technique revealed the amount of surface oxygen functionalities and their acidity constant distribution. The acidity constant range involved in the metal cation adsorption was obtained from this distribution. Metal cation adsorption increased with higher adsorption temperature due to the increase in the negative surface charge of the oxidized activated carbon. Adsorption was by proton exchange and the number, amount and strength of the surface acid groups involved could be obtained. The proton exchange was by an inner-sphere or outer-sphere surface metal complex formation mechanism. In the case of divalent cation adsorption, the increase in temperature changed the adsorption mechanism from outer-sphere to inner-sphere. However, the adsorption mechanism of Cr(III) was outer-sphere and independent of temperature. Adsorption capacity augmented with the increase in the charge-to-size ratio of the hexa-aquo cations. In addition, the adsorption capacity of divalent cations increased with the rise in stability of the surface metal complex formed.

Batch and column adsorption of herbicide fluroxypyr on different types of activated carbons from water with varied degrees of hardness and alkalinity

There has been little research into the effects of the water hardness and alkalinity of surface waters on the adsorption of herbicides on activated carbons. The aim of this study was to determine the influence of these water characteristics on fluroxypyr adsorption on different activated carbons. At low fluroxypyr surface concentrations, the amount adsorbed from distilled water was related to the surface hydrophobicity. Surface area of carbons covered by fluroxypyr molecules ranged from 60 to 65%. Variations in fluroxypyr solubility with water hardness and alkalinity showed a salting-in effect. Calcium, magnesium and bicarbonate ions were adsorbed to a varied extent on the activated carbons. The presence of fluroxypyr in solution decreased their adsorption due to a competition effect. K(F) from the Freundlich equation linearly increased with water hardness due to salt-screened electrostatic repulsions between charged fluroxypyr molecules. The amount adsorbed from distilled water was largest at high fluroxypyr solution concentrations, because there was no competition between inorganic ions and fluroxypyr molecules. The column breakthrough volume and the amount adsorbed at breakthrough were smaller in tap versus distilled water. Carbon consumption was lower with activated carbon cloth than with the use of granular activated carbon.

Activated carbons from a subbituminous coal: Pore texture and electrokinetic properties

Activated carbons were prepared from an original and demineralized subbituminous coal by a two-stage, pyrolysis and steam activation, treatment. The effect of demineralization, pyrolysis and steam activation processes on the evolution of the surface area, pore texture, and ζ potential has been investigated. The pyrolysis step increased the microporosity of the samples and decreased the negatively charged surface. The steam activation step increased the surface area and porosity as well as the surface basicity of the activated carbons. The electrokinetic behaviour of the carbons was affected by the presence of KNO3, because they specifically adsorbed either K− or NO3− ions depending on the sample. Finally, the demineralization process after the pyrolysis and steam activation was more effective than if the demineralization was carried out prior the two-stage treatment.

Adsorption and thermal desorption of the herbicide fluroxypyr on activated carbon fibers and cloth at different pH values

Adsorption of fluroxypyr was studied at pH values between 2 and 10 and at temperatures of 298 and 313 K. Adsorption capacity decreased when the solution pH increased. This was explained by changes in fluroxypyr solubility and in dispersive and electrostatic adsorbent-adsorbate interactions with the increase in pH. The highest adsorption was found at pH 2, when the solubility was the lowest and only dispersive interactions operated. An increase in temperature produced a decrease in adsorption capacity. Thermal desorption of fluroxypyr up to 1073 K left a residue on the carbon surface, which increased with higher adsorption pH. Differential thermogravimetry (DTG) profiles showed two desorption peaks at pH values of 2 and 4 and only one peak at pH values of 7 and 10. The appearance of one or two peaks may be related to the type of adsorbate-adsorbent interactions established during adsorption. The predominance of electrostatic interactions favours the strongly bound or chemisorbed fluroxypyr. One important conclusion is that the highest amounts of fluroxypyr are adsorbed and thermally desorbed when there are only non-electrostatic interactions between fluroxypyr molecules and carbon surface. Activation desorption energy and pre-exponential factor were obtained from the shift in temperature of desorption peaks with higher heating rate.

Nitroimidazoles adsorption on activated carbon cloth from aqueous solution

The objective of this study was to analyze the equilibrium and adsorption kinetics of nitroimidazoles on activated carbon cloth (ACC), determining the main interactions responsible for the adsorption process and the diffusion mechanism of these compounds on this material. The influence of the different operational variables, such as ionic strength, pH, temperature, and type of water (ultrapure, surface, and waste), was also studied. The results obtained show that the ACC has a high capacity to adsorb nitroimidazoles in aqueous solution. Electrostatic interactions play an important role at pH<3, which favors the repulsive forces between dimetridazole or metronidazole and the ACC surface. The formation of hydrogen bonds and dispersive interactions play the predominant role at higher pH values. Modifications of the ACC with NH3, K2S2O8, and O3 demonstrated that its surface chemistry plays a predominant role in nitroimidazole adsorption on this material. The adsorption capacity of ACC is considerably high in surface waters and reduced in urban wastewater, due to the levels of alkalinity and dissolved organic matter present in the different types of water. Finally, the results of applying kinetic models revealed that the global adsorption rate of dimetridazole and metronidazole is controlled by intraparticle diffusion.