V. Gómez-Serrano

FT-IR STUDY OF ROCKROSE AND OF CHAR AND ACTIVATED CARBON

Fourier transform-infrared spectroscopy was used in the study of rockrose (Cistus ladaniferus, L.) and of rockrose chars and activated carbons. Chars were prepared by heating the starting material between 200 and 1000 °C in nitrogen. Also, a sample was obtained at 600 °C in the atmosphere of the products of the thermal decomposition of rockrose. This charred product was used in the preparation of activated carbons. It was heated at 800 °C in air, CO2 or steam to 40% burn-off. The presence of oxygen groups and of olefinic and aromatic structures was detected in rockrose. The heat treatment of rockrose in the range 300–500 °C produced the most significant changes in the chemical structure of the material. Olefinic CC bonds and ether structures increased first in the chars and then the aromatic structure developed. Above 500 °C, the oxygen groups decreased with the temperature increase. The charring method of rockrose also influenced markedly the chemical structure of the chars. The use of a greater mass of sample slowed down the pyrolysis of rockrose. The activated carbons possessed aromatic CC bonds and oxygen groups.

Formation of oxygen structures by air activation. A study by FT-IR spectroscopy

Abstract Using cherry stones (CS) as starting material and commonly air as activating agent, formation of oxygen structures in activated carbon is investigated. In the preparation of samples, CS was first heated at temperatures between 450 and 900°C in N2 atmosphere. Then, in a successive activation stage, the product carbonized at 600°C was maintained in contact with an air stream at 25–325°C for 24 h, 300–600°C for 1 h, and 250°C for 1–96 h. The rest of the carbonization products of CS were also heated at 250°C in air for 24 h. Moreover, the product carbonized at 900°C was activated at 750 or 900°C in CO2 for 1 h. Furthermore, in a second activation stage, the products activated at such temperatures in CO2 and those at 300–600°C in air were heated at 250°C in air for 24 h. The starting material, carbonized products, and activated carbons were examined by FT-IR spectroscopy. A number of carbon–hydrogen atomic groupings and of oxygen groups and structures, i.e., OH, CO, and C–O–C) have been identified in CS. The yield of the activation and carbonization processes and also the chemical structure of the resultant products are strongly dependent on the carbonization temperature. In the products carbonized at 600–900°C, only ether type structures are detected. The activation at 250°C in air results in activated carbons that contain different oxygen structures when CS is carbonized at 450 or 600°C. At 750 or 900, by contrast, oxygen structures are not formed as a result of the activation treatment. This also applies when the carbonization product of CS at 900°C is activated solely in CO2 or first in CO2 and then in air. The heating conditions in air greatly influence the formation of oxygen structures (specifically, of lactonic and ion-radical types) to a large extent. It only occurs when activating at relatively low temperatures for a long time; at 300–600°C for 1 h, however, the oxygen structures are not formed.

Adsorption of mercury, cadmium and lead from aqueous solution on heat-treated and sulphurized activated carbon

The adsorption of mercury, cadmium and lead from aqueous solutions on heat-treated and sulphurized activated carbon has been studied comparatively. The adsorption isotherms for the various metals were measured at 298 K, using adsorptive solutions at two pH values. The adsorption was much higher for mercury than for cadmium and lead for all adsorbents. The adsorption of mercury greatly increased for the samples of sulphurized carbon and also, though less, for the sample prepared in N2. The decrease of pH in the adsorptive solution to pH 2.0 drastically reduced the adsorption of mercury. The effect of pH change on the adsorption of Cd2+ and Pb2+ occurred with less adsorbents than for mercury.

Oxidation of activated carbon by hydrogen peroxide. Study of surface functional groups by FT-i.r.

Abstract Outgassed activated carbon was oxidized using H2O2 aqueous solutions and the surface oxygen groups were identified by FT-i.r. spectroscopy. The influence of solution concentration and pH, outgassing and oxidation conditions of the carbon, and the addition of catalytic or stabilizer agents to the oxidizing solution was investigated. The stability of the surface groups was also studied for a series of samples. The surface groups initially present in the carbon were largely OH and COC. Oxidation of material with H2O2 solution, at unchanged pH, increased the OH groups present in the oxidized products. With solutions of pH 2.5 and 11.5, the OH groups decreased and various oxygen groups such as CO and C O, or CO2−, were formed. At the lowest pH value, formation of COC structures with an increased number of CO bonds occurred. Ether structures were lost when outgassing under more drastic conditions and when contact with the oxidizing solution was established at very different temperatures. The addition of Fe2+ to the oxidizing solution favoured the oxidation of carbon. However, Fe3+, ethanol and ether were not effective agents. Significant changes in the surface chemistry of samples were observed after a long period of time.

Enhanced adsorption of metal ions onto functionalized granular activated carbons prepared from cherry stones

Some granular activated carbons (GACs) were prepared from cherry stones (CS), an agricultural waste, by thermal methods following pyrolysis in nitrogen and subsequent activation (with air, carbon dioxide and steam) or single-step activation in steam. A GAC prepared by activation with carbon dioxide was further treated with several oxidizing agents (air, air-ozone mixture, nitric acid and hydrogen peroxide). The non-oxidized GACs produced have surface areas ranged from 508 to 901m(2)/g and show a predominantly micro- and macropores structure. Oxidation treatments, especially with nitric acid and ozone, led to the fixation of high amounts of acidic surface oxygen complexes (SOCs), thus making the carbon surface more hydrophilic. Contrary to oxidation with the other agents, the ozone treatment, at the conditions applied in this work, does not decrease the GAC surface area. Both, the non-oxidized and the oxidized GACs were used as adsorbent to study the adsorption of Cu(II) from aqueous solution. Adsorption isotherms were obtained at 25 degrees C and data were well fitted to the Langmuir equation. The results show that the Cu(II) uptake of the non-oxidized GACs is not great, but similar to that of the commercial grade Filtrasorb 400. The adsorption capacity for Cu(II) could be greatly enhanced by GAC oxidation, especially with nitric acid and ozone treatments. The larger adsorption capacity of the oxidized GACs has been attributed to oxygen functionalities (mainly carboxylic) fixed on the GAC surface, which can remove Cu(II) species from water by both ion-exchange and surface complexation mechanisms. The ozonated GAC was also evaluated for the adsorption of Co(II) species from single solute and Cu(II)-Co(II) binary mixture solutions. The functionalization of GAC by the ozone treatment improved the adsorption of both Cu(II) and Co(II) regardless of the fact that they were in single solute or binary systems.

Adsorption of Pb2+ by heat-treated and sulfurized activated carbon

Abstract Using activated carbon (AC) and samples obtained by heat and sulfurizing treatments of the material, the adsorption of Pb2+ from aqueous solution was studied at two pH values and at three temperatures in the range 25–45°C. Adsorbents were prepared either by heat treatment of AC from 30 to 900°C in N2 or H2S, or by treatment of the material first at 30°C successively in SO2 and H2S and then at 200°C in N2. Their characterization was in terms of textural properties and surface chemistry. Techniques used were gas adsorption (N2, −196°C), mercury porosimetry, density measurements (He, 30°C) and FT-IR spectroscopy. The treatments effected on AC originated a great increase in the adsorption of Pb2+, but only when it occurred from solutions of metallic ion at unchanged pH and at 25°C. Adsorption decreased strongly by pH and temperature variations, except for AC. The adsorption rate depended on the adsorbent, pH and temperature.

Formation of oxygen complexes by ozonation of carbonaceous materials prepared from cherry stones: I. Thermal effects

Abstract In this study, the feasible use of ozone to form oxygen complexes in chars prepared from cherry stones (CS) is investigated. CS were charred at 450, 600 or 900°C for 2 h in nitrogen. Char samples were ozonated over the 25–250°C temperature range for 1 h. Elemental chemical analysis was effected for a few selected samples. The oxygen complexes were successfully analyzed by Fourier infrared spectroscopy (FT-IR) and by titration methods. Thermal decomposition of ozone in the gas stream was also studied and the mechanism of the ozonation process dealt with. The ozonation treatment of CS chars was found to yield products with a relatively high concentration of a number of oxygen complexes. These include phenolic hydroxyl, quinonic, carboxylic acid, and ether structures. The content of lactonic structures was very low in the ozonated samples. The type and quantity of oxygen complexes depended on the ozonation and charring temperatures. The formation of oxygen complexes was favored when the charring of CS was effected at 450°C and when the ozonation of the char prepared at 600°C was performed at 100°C. The ozone content in the gas stream was very sensitive to the temperature increase in the reactor. Several reaction routes have been proposed for the transformation of ether, aromatic, and olefinic structures present in CS chars into oxygen complexes.

Characterization study of char and activated carbon prepared from raw and extracted rockrose

Abstract Using raw rockrose ( Cistus Ladaniferus, L. ) and rockrose extracted into petroleum ether, the effect of extraction on the thermal behaviour and chemical-physical properties of chars and activated products was studied. Chars were prepared by heat treatment of the starting materials under dynamic and isothermal conditions over the temperature range 200–1000°C. Activations of two carbonized products, which were obtained at 600°C, was accomplished in air, CO 2 , or steam at 850°C until burnoff 40%. Techniques used were thermogravimetry, gas adsorption, mercury porosimetry, density measurements and FT-IR spectroscopy. Extraction delayed the rockrose pyrolysis, as inferred from the thermal behaviour of the starting materials and by the composition and texture of the chars prepared between 200–600°C. At higher temperatures, extraction reduced the ash content of chars and mitigated the loss produced in the surface area and the microporosity. Differences in textural properties of activated carbon depended on the starting material and the activating agent, being especially significant between air and CO 2 or steam. The FT-IR spectra of the carbonized and activated products displayed absorption bands compatible with the presence in the materials of surface olefinic CC double bonds, aromatic rings, and oxygen functional groups.

Degradation kinetics of p-nitrophenol ozonation in water

Abstract The degradation of p -nitrophenol with ozone in water has been studied. The influence of pH and ozone partial pressure on p -nitrophenol conversion has been observed. The kinetic regime of ozone absorption has been shown to be highly dependent on both variables. Thus, at pH 2 the kinetic regime corresponds to a slow reaction regardless of the ozone partial pressure applied. At pH 6.5 the absorption of ozone develops in the fast kinetic regime the reaction being of pseudo first-order with respect to ozone at ozone partial pressures lower than 500 Pa. Finally, at higher pH (8.5 and 12.4) and ozone partial pressures higher than 2000 Pa, the reaction becomes instantaneous. Under conditions of slow and instantaneous kinetic regimes the liquid phase volumetric mass transfer coefficient, k L a , was calculated. The results are in agreement with those obtained from the absorption of ozone in organic-free water at similar pHs. On the other hand, under the situation of fast pseudo first-order reaction the rate constant for the p -nitrophenol-ozone direct reaction was determined to be 4.5 × 10 6 M −1 s −1 . The ozone decomposition reaction develops to some extent only at pH 12.4.

Adsorption of cadmium by sulphur dioxide treated activated carbon

Merck carbon (1.5 mm) was treated in three ways: heating from ambient temperature to 900 degrees C in SO(2); treatment at ambient temperature in SO(2); or successive treatments in SO(2) and H(2)S at ambient temperature. All samples were then characterised and tested as adsorbents of Cd(2+) from aqueous solution. The characterisation was in terms of composition by effecting ultimate and proximate analyses and also of textural properties by N(2) adsorption at -196 degrees C. Kinetics and extent of the adsorption process of Cd(2+) were studied at 25 and 45 degrees C at pH of the Cd(2+) solution (i.e., 6.2) and at 25 degrees C also at pH 2.0. The various treatments of the starting carbon had no significant effect on the kinetics of the adsorption of Cd(2+), but increased its adsorption capacity. The most effective treatment was heating to 900 degrees C, the adsorption in this case being 70.3% more than that of the starting carbon. The adsorption increased at 45 degrees C but decreased at pH 2.0 when compared to adsorption at 25 degrees C and pH 6.2, respectively.