Preparation of activated carbon from a renewable bio-plant of Euphorbia rigida by H2SO4 activation and its adsorption behavior in aqueous solutions

The use of activated carbon obtained from Euphorbia rigida for the removal of a basic textile dye, which is methylene blue, from aqueous solutions at various contact times, pHs and temperatures was investigated. The plant material was chemically modified with H₂SO₄. The surface area of chemically modified activated carbon was 741.2 m² g⁻¹. The surface characterization of both plant- and activated carbon was undertaken using FTIR spectroscopic technique. The adsorption process attains equilibrium within 60 min. The experimental data indicated that the adsorption isotherms are well described by the Langmuir equilibrium isotherm equation and the calculated adsorption capacity of activated carbon was 114.45 mg g⁻¹ at 40° C. The adsorption kinetics of methylene blue obeys the pseudo-second-order kinetic model and also followed by the intraparticle diffusion model up to 60 min. The thermodynamic parameters such as ΔG°, ΔH° and ΔS° were calculated to estimate the nature of adsorption. The activation energy of the system was calculated as 55.51 kJ mol⁻¹. According to these results, prepared activated carbon could be used as a low-cost adsorbent to compare with the commercial activated carbon for the removal textile dyes from textile wastewater processes.     

Preparation and characterization of carbons for the retention of halogens in the condenser vacuum system of a thermonuclear plant

Activated carbons were prepared by air and carbon dioxide activation, from almond tree pruning, with the aim of obtaining carbons that reproduce the textural and mechanical properties of the carbons currently used in the filtering system of the condenser vacuum installation of a Thermonuclear Plant (CNA; Central Nuclear de Almaraz in Caceres, Spain), produced from coconut shell. The variables studied in non-catalytic gasification series with air were the temperature (215-270 °C) and the time (1-16 h) and the influence of the addition of one catalyst (Co) and the time (1-2 h) in catalytic gasification. In the case of activation with CO₂, the influence of the temperature (700-950 °C) and the time (1-8 h) was studied. The resulting carbons were characterized in terms of their BET surface, porosity, and pore size distribution. The N₂ adsorption isotherms at 77 K for both series showed a type I behaviour, typical of microporous materials. The isotherms showed that with both gasificant agents the temperature rise produced an increase in the carbon porosity. With regards to the activation time, a positive effect on the N₂ adsorbed volume on the carbons was observed. The best carbons of each series, as well as the CNA (carbon currently used in the CNA), were characterized by mercury porosimetry and iodine solution adsorption isotherms. The results obtained allowed to state that several of the carbons produced had characteristics similar to the carbon that is target of reproduction (which has S_BET of 741 m² g⁻¹, V_mi of 0.39 cm³ g⁻¹ and a iodine retention capacity of 429.3 mg g⁻¹): carbon C (gasification with CO₂ at 850 °C during 1 h), with S_BET of 523 m² g⁻¹, V_mi of 0.33 cm³ g⁻¹ and a iodine retention capacity of 402.5 mg g⁻¹, and carbon D (gasification with CO₂ at 900 °C during 1 h), whose S_BET is 672 m² g⁻¹, V_mi is 0.28 cm³ g⁻¹ and has a iodine retention capacity of 345.2 mg g⁻¹.     

Preparation of activated carbons from used tyres by gasification with steam and carbon dioxide

Activated carbons were prepared from waste tyres by gasification with steam and carbon dioxide and their characteristics were investigated. A two-stage activation procedure (pyrolysis at 800 °C in N₂ atmosphere, followed by steam or carbon dioxide activation) was used for the production of activated samples. The effect of the activation temperature (750-900 °C) and the activation time (1-3 h) on the surface characteristics of the prepared carbon was investigated. Carbons produced to different degrees of burn-off were characterized by means of their nitrogen adsorption isotherms at 77 K. In both sets of experiments, the mesopore, micropore volume, and BET surface area increased almost linearly with the degree of activation. For burn-off values lower than 53%, the steam activation produced carbons with a narrower and more extensive microporosity and higher BET and external surface area than the carbon dioxide activation. As the activation proceeds (burn-off > 53%), a strong development of the mesoporosity in the carbons was observed and the micropores size distribution revealed broader micropores, that is, a more heterogeneous distribution.     

Characterization of nanoporous carbons by combining CO2 and H2 sorption data with the Monte Carlo simulations

The Monte Carlo method in its grand ensemble variant (GCMC) is used in combination with experimental data in order to characterize microporous carbons and obtain the optimal pore size distribution (PSD). In particular, the method is applied in the case of AX-21 carbon. Adsorption isotherms of CO₂ (253 and 298 K) and H₂ (77 K) up to 20 bar have been measured, while the computed isotherms resulted from the GCMC simulations for several pore widths up to 3.0 nm. For the case of H₂ at 77 K quantum corrections were introduced with the application of the Feynman-Hibbs (FH) effective potential. The adsorption isotherms were used either individually or in a combined manner in order to deduce PSDs and their reliability was examined by the ability to predict the experimental adsorption isotherms. The combined approach was found to be capable of reproducing more accurately all the available experimental isotherms.     

Carbon dioxide-activated carbons from almond tree pruning: Preparation and characterization

Activated carbons were prepared from almond tree pruning by non-catalytic and catalytic gasification with carbon dioxide and their surface characteristics were investigated. In both series a two-stage activation procedure (pyrolysis at 800 °C in nitrogen atmosphere, followed by carbon dioxide activation) was used for the production of activated samples. In non-catalytic gasification, the effect of the temperature (650-800 °C for 1 h) and the reaction time (1-12 h at 650 °C) on the surface characteristics of the prepared samples was investigated. Carbons were characterized by means of nitrogen adsorption isotherms at 77 K. The textural parameters of the carbons present a linear relation with the conversion degree until a value of approximately 40%, when they come independent from both parameters studied. The highest surface area obtained for this series was 840 m² g⁻¹. In the catalytic gasification the effect of the addition of one catalyst (K and Co) and the gasification time (2-4 h) on the surface and porosity development of the carbons was also studied. At the same conditions, Co leads to higher conversion values than K but this last gives a better porosity development.     

Air-activated carbons from almond tree pruning: Preparation and characterization

In this work the results obtained in the preparation and characterization of carbons made from almond tree pruning by non-catalytic and catalytic gasification (using K and Co) with air are analyzed and discussed. The main aim was to obtain high quality activated carbons at the lowest possible cost. The variables studied have been the temperature (190-260 °C) and the time (1-10 h) in non-catalytic gasification and the influence of the catalyst type (K and Co, 1 wt.% referred to cation, at 190 °C and 1 h) and the time (1-4 h) in catalytic gasification with Co at 190 °C. The air flow rate used in all the series was 167 cm³ min⁻¹. In non-catalytic gasification the reaction normalized rate versus the conversion degree was maintained until a conversion value of 10% for the experiment made at 260 °C since, at lower temperatures, this rate drops quickly for low conversion values. The N₂ adsorption isotherms for the carbons of this series resemble type I, although there is an increase of N₂ adsorbed volume at relatively high pressures. A temperature rise produced an increase of the carbon porosity and BET specific surface (116-469 m² g⁻¹). The activation time has a positive effect on the N₂ volume adsorbed by the carbons. The isotherms shapes were similar to those previously commented. A concentration equal to 1 wt.% was used to study the influence of the catalyst type. Under the studied experimental conditions, Co drives to a bigger porosity development than K, although with both catalysts a very similar pore size distribution is obtained. The activation time, in the gasifications catalyzed with Co, gives rise to a very important porosity development in the carbons. This produces a strong increase of the carbon specific surface area with very high values in the 4 h experiment, in which a BET specific surface of 959 m² g⁻¹ was obtained.     

Preparation and textural characterisation of activated carbon from vine shoots (Vitis vinifera) by H3PO4—Chemical activation

An abundant and low-cost agricultural waste as vine shoots (Vitis vinifera) (VS), which is generated by the annual pruning of vineyards, has been used as raw material in the preparation of powder activated carbon (AC) by the method of chemical activation with phosphoric acid. After size reduction, VS were impregnated for 2 h with 60 wt.% H₃PO₄ solution at room temperature, 50 and 85 °C. The three impregnated products were carbonised at 400 °C. The product impregnated at 50 °C was heated either first at 150-250 °C and then at 400 °C or simply at 350-550 °C in N₂ atmosphere. The time of isothermal treatment after each dynamic heating was 2 h. The carbons were texturally characterised by gas adsorption (N₂, −196 °C), mercury porosimetry, and density measurements. FT-IR spectroscopy was also applied. Better developments of surface area and microporosity are obtained when the impregnation of VS with the H₃PO₄ solution is effected at 50 °C and for the products heated isothermally at 200 and 450 °C. The mesopore volume is also usually higher for the products impregnated and heated at intermediate temperatures.     

Wettability modification of pitch-based spherical activated carbon by air oxidation and its effects on phenol adsorption

The surface chemistry of pitch-based spherical activated carbon (PSAC) was modified by air oxidation to enhance its wettability as well as adsorption properties. Changes of PSAC after modification in texture, surface chemistry and wettability were studied by different techniques including N₂ adsorption, X-ray photoelectron spectroscopy (XPS) and dynamic contact angle analyzer (DCA). Phenol adsorption characteristics in different solvents on PSAC were also investigated. When PSAC was modified under an atmosphere with 20 vol.% oxygen at 400 and 450 °C for 5 h, surface acidic groups increased from 0.11 to 1.22 and 1.60 meq/g, while basic groups decreased from 0.52 to 0.03 and 0.02 meq/g, respectively. After PSAC was modified, the increase of the oxygen-containing groups, especially carboxylic and phenolic ones, is responsible for the increasing of the surface acidity and the significant improvement of the wettability of PSAC. PSAC with a relatively high oxygen content provided a low adsorption capacity to phenol in aqueous solution, and the adsorption isotherms change from Langmuir class (L) to the S-shaped curve; while the solvent is changed into cyclohexane, all adsorption isotherms are type L, and the adsorption capacity to phenol increases with increasing oxygen-containing groups. Possible reasons, including the solvent effect, π-π dispersion and donor-acceptor interactions are discussed.     

Preparation of activated carbons from cherry stones by activation with potassium hydroxide

Using cherry stones, the preparation of activated carbon has been undertaken in the present study by chemical activation with potassium hydroxide. A series of KOH-activated products was prepared by varying the carbonisation temperature in the 400-900 °C range. Such products were characterised texturally by gas adsorption (N₂, −196 °C), mercury porosimetry, and helium and mercury density measurements. FT-IR spectroscopy was also applied. The carbons prepared as a rule are microporous and macroporous solids. The degree of development of surface area and porosity increases with increasing carbonisation temperature. For the carbon heated at 900 °C the specific surface area (BET) is 1624 m² g⁻¹, the micropore volume is 0.67 cm³ g⁻¹, the mesopore volume is 0.28 cm³ g⁻¹, and the macropore volume is 1.84 cm³ g⁻¹.     

Removal of chloridazon by natural and ammonium kerolite samples

The adsorption of chloridazon (5-amino-4-chloro-2-phenylpyridazin-3(2H)-one) on natural and ammonium kerolite samples from aqueous solution at 10, 25 and 40 °C has been studied by using batch experiments. The experimental data points were fitted to the Langmuir equation in order to calculate the adsorption capacities (X_m) of the samples; two straight lines were obtained, which indicates that the adsorption process takes place in two different stages. Values for X_m1 (first stage) ranged from 1.1 × 10⁻² mol kg⁻¹ for natural kerolite at 40 °C up to 5.1 × 10⁻² mol kg⁻¹ for ammonium kerolite at 10 °C and the values for X_m2 (second stage) ranged from 9.1 × 10⁻² mol kg⁻¹ for natural kerolite at 40 °C up to 14 × 10⁻² mol kg⁻¹ for natural kerolite at 10 °C. The adsorption experiments showed on the one hand, that the ammonium kerolite is more effective than natural kerolite to adsorb chloridazon in the range of temperature studied and on the other hand, that the lower temperature, the more effective the adsorption of chloridazon on the adsorbents studied.     

Preparation of activated carbon from cherry stones by chemical activation with ZnCl2

Cherry stones (CS), an industrial product generated abundantly in the Valle del Jerte (Cáceres province, Spain), were used as precursor in the preparation of activated carbon by chemical activation with ZnCl₂. The influence of process variables such as the carbonisation temperature and the ZnCl₂:CS ratio (impregnation ratio) on textural and chemical-surface properties of the products obtained was studied. Such products were characterised texturally by adsorption of N₂ at −196 °C, mercury porosimetry and density measurements. Information on the surface functional groups and structures of the carbons was provided by FT-IR spectroscopy. Activated carbon with a high development of surface area and porosity is prepared. When using the 4:1 impregnation ratio, the specific surface area (BET) of the resultant carbon is as high as 1971 m² g⁻¹. The effect of the increase in the impregnation ratio on the porous structure of activated carbon is stronger than that of the rise in the carbonisation temperature, whereas the opposite applies to the effect on the surface functional groups and structures.     

Magnetic relaxation near a fishtail peak of isothermal magnetization in B1 NbCy encapsulated in multiwall carbon nanocages

Isothermal magnetization near a fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 4000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with temperature T and is independent of applied magnetic field H. The results of J(t) and U_eff (T, H) are consistent with the Anderson–Kim flux–creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. U_eff at a fishtail peak field H_fp evolves quickly above a fishtail peak temperature T_fp, but slowly below that temperature. The result suggests that a decrease of flux viscosity coefficient above T_fp at H_fp is the origin of the fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages.     

The influence of nonpolar organics adsorption on the electrochemical behaviour of powdered activated carbon electrodes in aqueous electrolytes

Adsorption and electrochemical studies were carried out on three activated carbon samples first oxidized, then heat-treated under vacuum (at 180, 500 and 900 °C). The investigations were performed with aqueous electrolyte (Na₂HPO₄ and H₃PO₄) solutions containing selected nonpolar organics (benzene and n-hexane). Adsorption measurements were carried out on solution with a wide range of organics concentration (up to saturation point). Cyclovoltammetric curves of powdered electrodes prepared from the activated carbon samples were recorded for the organics in saturated solutions. The electric double layer capacities of the anodic and cathodic parts were estimated, and the surface anodic and cathodic charge was calculated both in absence and presence of organics in the electrochemical systems. The relative surface charge (in relation to systems without organics) was found to decrease with a reduction in the concentration of surface oxygen-containing groups. Other physicochemical parameters characterizing the degree of surface oxidation (total oxygen concentration, primary water adsorption centres) were also taken into consideration. The correlation between adsorption capacity towards the nonpolar organic compounds (obtained from adsorption isotherms) and change of surface charge was analyzed.     

Surface and adsorptive properties of carbons prepared from biomass

A number of activated carbons were prepared from a locally available by-product, corncobs, under currently established activation schemes. Obtained carbons were characterized by N₂ adsorption at 77 K and the isotherms were analyzed by BET and α_s methods. Steam-activation at 900 °C produced a microporous carbon having the highest S^α of 788 m² g⁻¹, whereas activation with air at 350 °C produced a carbon of S^α = 321 m²/g and possess wider pores. KOH impregnation with char in ratio 1:1 (w/w) and impregnated in the same ratio with the raw material prior to pyrolysis at 700 °C for 1 h, gave CK700, K700 respectively. An additional sample was obtained by oxidizing part of K700 with conc. HNO₃. All three KOH carbons show pore structures much close to char itself which may be due to potassium salt left in pores and is not easily leached with repeated water washings. In addition, KOH is more effective on the precursor itself than on its char of already developed porosity. FT-IR spectra show an increase in oxygen functionalties on the carbon surface as a result of activation process and the bands become stronger in the spectra of the acid-treated sample. The oxidized carbon sample showed relatively higher uptake of Pb²⁺ and MB and its surface chemistry plays the key role in their adsorption, while sharp decrease was observed in the uptake of phenol and mono-nitrophenols from aqueous solutions. An SEM study showed that air activation produce obvious voids reflecting its erosive effect on the external carbon surface.     

Influence of activated carbon surface acidity on adsorption of heavy metal ions and aromatics from aqueous solution

Adsorption of toxic heavy metal ions and aromatic compounds onto activated carbons of various amount of surface C-O complexes were examined to study the optimum surface conditions for adsorption in aqueous phase. Cadmium(II) and zinc(II) were used as heavy metal ions, and phenol and nitrobenzene as aromatic compounds, respectively. Activated carbon was de-ashed followed by oxidation with nitric acid, and then it was stepwise out-gassed in helium flow up to 1273 K to gradually remove C-O complexes introduced by the oxidation. The oxidized activated carbon exhibited superior adsorption for heavy metal ions but poor performance for aromatic compounds. Both heavy metal ions and aromatics can be removed to much extent by the out-gassed activated carbon at 1273 K. Removing C-O complexes, the adsorption mechanisms would be switched from ion exchange to Cπ-cation interaction for the heavy metals adsorption, and from some kind of oxygen-aromatics interaction to π-π dispersion for the aromatics.     

Preparation of activated carbon from Enteromorpha prolifera and its use on cationic red X-GRL removal

Enteromorpha prolifera was pyrolyzed to prepare activated carbon using chemical activation by zinc chloride. The effect of activation parameters such as activation temperature, weight ratio (Enteromorpha prolifera to ZnCl₂), and activation time was investigated. The BET results showed that the surface area and pore volume of activated carbons were achieved as high as 1722 m²/g and 1.11 cm³/g, respectively, in the optimal activation conditions. Batch adsorption studies were carried out to study the adsorption properties of cationic red X-GRL onto activated carbon by varying the parameters like initial solution pH, contact time, and temperature. The kinetic studies showed that the adsorption data followed a pseudo second-order model. The isotherm analysis indicated that the adsorption data could be represented by the Langmuir isotherm model. The Langmuir monolayer adsorption capacity of cationic red X-GRL was estimated as 263.16 mg/g at pH 6.0.     

Nano-graphene structures deposited by N-IR pulsed laser ablation of graphite on Si

Thin nano-structured carbon films have been deposited in vacuum by pulsed laser ablation, from a rotating polycrystalline graphite target, on Si 〈1 0 0〉 substrates, kept at temperatures ranging from RT to 800 °C. The laser ablation was performed by a Nd:YAG laser, operating in the near IR (λ = 1064 nm).X-ray diffraction analysis, performed at grazing incidence angle, both in-plane (ip-gid) and out-of-plane (op-gid), has shown the growth of oriented nano-sized graphene particles, characterised by high inter-planar stacking distance (d_? ∼ 0.39 nm), compared to graphite. The film structure and texturing are strongly related both to laser wavelength and substrate temperature: the low energy associated to the IR laser radiation (1.17 eV) generates activated carbon species of large dimensions that, also at low T (∼400 °C), easy evolve toward more stable sp² aromatic bonds, in the plume direction. Increasing temperature the nano-structure formation increases, causing a further aggregation of aromatic planes, voids formation, and a related density (by X-ray reflectivity) drop to very low values. SEM and STM show for these samples a strongly increased macroscopic roughness. The whole process, mainly at higher temperatures, is characterised by a fast kinetic mode, far from equilibrium and without any structural or spatial rearrangement.     

Demineralisation of semi-anthracite char with molten salts/HCl: Effects on the porous texture and reactivity in air

The effects of chemical heat treatments of a semi-anthracite char (AC) on textural properties and reactivity in air of the material are investigated. The starting char was first treated with a mixture of LiCl/KCl or LiCl/KCl/CaO at 743, 873 or 1173 K and the products obtained were then washed thoroughly with distilled water. A small fraction of these samples were treated with 10⁻³ M HCl solution. Valuable information on textural modifications produced in the material was derived from the adsorption isotherms for CO₂ at 273 K. The reactivity tests were carried out at 808, 823 and 823 K. The microporosity developed and the reactivity in air increased in the partially demineralised products. The former effect was stronger for the LiCl/KCl/CaO-treated samples and the latter for a larger number of the LiCl/KCl-treated samples.     

Femtosecond pulsed laser ablation of diamond-like carbon films on silicon

Femtosecond pulsed laser ablation (τ = 120 fs, λ = 800 nm, repetition rate = 1 kHz) of thin diamond-like carbon (DLC) films on silicon was conducted in air using a direct focusing technique for estimating ablation threshold and investigating the influence of ablation parameter on the morphological features of ablated regions. The single-pulse ablation threshold estimated by two different methods were ?_th(1) = 2.43 and 2.51 J/cm². The morphological changes were evaluated by means of scanning electron microscopy. A comparison with picosecond pulsed laser ablation shows lower threshold and reduced collateral thermal damage.     

An ab initio-based approach to adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces

The adsorption-desorption behavior of Si adatoms on GaAs(1 1 1)A-(2 × 2) surfaces is investigated using our ab initio-based approach, in which adsorption and desorption behavior of Si adatoms is described by comparing the calculated desorption energy obtained by total-energy electronic-structure calculations with the chemical potential estimated by quantum statistical mechanics. We find that the Si adsorption at the Ga-vacancy site on the (2 × 2) surfaces with As adatoms occurs less than 1140-1590 K while the adsorption without As adatom does less than 630-900 K. The change in adsorption temperature of Si adatoms by As adatoms is due to self-surfactant effects of As adatoms: the promotion of the Si adsorption triggered by As adatoms is found to be interpreted in terms of the band-energy stabilization. Furthermore, the stable temperature range for Si adsorbed surfaces with As adatoms agrees with the experimental results. The obtained results provide a firm theoretical framework to clarify n-type doping processes during GaAs epitaxial growth.