Structured carbon haemoadsorbents for the removal of middle molecular weight toxins

There is continued interest in the development of novel haemoadsorbents for the removal of middle molecular weight molecules from blood in patients suffering with chronic renal failure. Conventional activated carbons have shown poor biocompatibility whereas polymer coated carbons have been applied with limited success. The external biocompatible coating significantly reduces the adsorption capacity for middle molecules due to size exclusion and slows down the adsorption kinetics due to mass transport limitations. In this work, we show that polymer derived mesoporous activated carbons (pore size in the range 2-50 nm) and surface area of between 400 and 800 m²/g are efficient adsorbents for middle molecular weight species such as interleukin IL-1beta.     

The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres

SO₂ removal from flue gases by carbonaceous materials is determined by their behaviour as catalysts for SO₂ oxidation into SO₃ or H₂SO₄ in the presence of O₂ or O₂ and steam, respectively. Previous studies have demonstrated that nitrogen (N) functional groups are active sites for the adsorption and oxidation of SO₂, although the nature of the N groups with the higher activity had not been established yet. For this reason, in the present work a number of activated carbons (AC) and activated carbon fibres (ACF) doped with N atoms have been prepared using different methods. The number and nature of these N groups have been assessed by XPS. The materials prepared have a wide range of nitrogen content, which is distributed into different chemical species. In this way, we were able to determine the effect of the N content and the role of the different N-containing functional groups on the catalytic activity for SO₂ oxidation. The results confirm that, although the pore volume and the pore size distribution strongly influence the catalytic activity, the presence of N species at the surface increases the catalytic activity. They also demonstrate that, among the different N functional groups, pyridinic nitrogen is the most active for this reaction.     

Preparation of activated carbons by utilizing solid wastes from palm oil processing mills

Feasibility of producing activated carbons by utilizing solid wastes (extracted flesh fibre and seed shell) from palm oil processing mills was investigated. The effects of activation conditions (CO₂ flow rate, activation temperature and retention time) on the characteristics of the activated carbons, i.e. density, porosity, BET surface area, pore size distribution and surface chemistry were studied. In this study, the optimum conditions for activation were an activation temperature of 800 °C and a retention time of 30 min for fiber or 50 min for shell, which gave the maximum BET surface area. Pore size distribution revealed that the shell-based activated carbons were predominantly microporous whilst fiber activated carbon had predominant mesopores and macropores, suggesting the application of shell and fiber activated carbon as adsorbents for gas-phase and liquid-phase adsorption, respectively. This was confirmed by further gas- and liquid-phase adsorption tests.     

The characterization of nitrogen-enriched activated carbons by IR, XPS and LSER methods

The aim of this work is to study the efficiency of the nitrogen enrichment by urea of lignites and the induced changes of the adsorptive properties towards volatile organic compounds (VOCs) of the activated carbons derived from these modified precursors. The study is made using infrared and X-ray photoelectrons spectroscopies and the LSER (linear solvation energy relationship) modeling. Four activated lignites derived from the same raw material, original or enriched with nitrogen, are characterized in this way. The effect of the chemical treatment by urea and of the burn-off amount are investigated in term of evolution of the chemistry of the studied materials. The influence of these parameters on the selective behavior of the activated lignites towards two pairs of VOCs is also discussed in terms of molecular interactions using the LSER approach. The results show that the chemical treatment of the raw material is successful, leading to significant enrichment with nitrogen under pyridinic form at the surface of the activated carbons. Moreover, they reveal some selective properties well explained by the LSER analysis and spectroscopic measurements. The selective character of the studied materials is modulated by the duration of the activation step.     

Adsorption of Hg(II) ions from aqueous chloride solutions using powdered activated carbons

Activated carbons were developed from Casurina equisetifolia leaves, by chemically treating with sulfuric acid (1:1) or zinc chloride (25%), at low (425 °C) and high (825 °C) temperatures. The resulting powdered activated carbons were applied for removing mercuric ions from aqueous solution at different agitation times and mercuric ion concentrations. The equilibrium data fitted well the Langmuir adsorption isotherm. The Langmuir adsorption capacities were 12.3 and 20.3 mg g⁻¹ for low temperature carbons and 43.9 and 38.5 mg g⁻¹ for high temperature carbons impregnated with H₂SO₄ and ZnCl₂, respectively. Studies of the effects of carbon dosage, NaCl concentrations and solution pH values were carried out for the more effective, high temperature carbons. Increasing NaCl concentration resulted in a significant decrease in the adsorption efficiency. Adsorption was high from solutions with low and neutral pH values and lower for solutions with alkaline pH values for the high temperature carbons.     

Simultaneous removal of VOC and NO by activated carbon impregnated with transition metal catalysts in combustion flue gas

Activated carbon (AC) was conventionally sprayed into the incineration flue gas for controlling the emissions of volatile organic compounds (VOC) by adsorption at low temperature (200–250 °C). However, the spent carbons required to be further disposed with more energy. In the present study, the inertness and hydrophobic AC was used as support for transition metal catalysts (Cu, Co, Fe and Ni) to remove VOC by deep oxidation into CO₂ and H₂O, and also to converse NO to N₂ with reduction at the temperature of 200–250 °C. The properties of the catalysts were analyzed by ICP-MS, XPS, FTIR, BET and SEM. The chemical state of transition metals supported on AC playing a significant role in improving the ability of VOC adsorption or catalytic oxidation at low temperature. The XPS analysis proved that the oxidized and reduced metals were coexisted on the fresh catalysts, but there was only the oxidized metal on the spent catalysts. These investigations indicated that AC impregnated with transition metal catalysts had the potential to simultaneously remove VOC and NO at 250 °C, with Co/AC being found to have higher activity.     

Competitive adsorption of a benzene-toluene mixture on activated carbons at low concentration

Previous studies of the adsorption of benzene and toluene at low concentration showed that both porosity and surface chemistry of the activated carbon play an important role. This paper analyses the adsorption behaviour of a mixture of VOCs (benzene-toluene) on AC, due to the lack of information regarding the adsorption of mixtures. Thus, the performance of chemically activated carbons, physically activated carbon with steam and commercial samples is studied. This study shows that chemically activated carbons have better performance than the other samples, showing much higher adsorption capacities, breakthrough times and separation times. Porosity is a key factor and those activated carbons with higher volumes of micropores exhibit higher adsorption capacities and breakthrough times. This work also analyses the state of the adsorbed phase resulting from the mixture adsorption and comparison of the composition of the adsorbed hydrocarbons with that predicted by the ideal adsorption solution theory (IAST), shows good agreement.     

The influence of demineralisation and ammoxidation on the adsorption properties of an activated carbon prepared from a Polish lignite

The preparation of cheap nitrogen-enriched materials with large adsorptive capacities and selectivity towards volatile organic compounds remains a challenge. Ammoxidation has been used to prepare nitrogen-enriched activated carbons using a demineralised Polish lignite. The lignite samples were demineralised by two different methods before nitrogen-enrichment by ammoxidation and physical activation in steam. The surface chemistry was investigated by elemental analysis, Boehm titration, infrared and XPS spectroscopies and adsorptive properties by a linear solvation energy relationship approach. Results show a quasi-total demineralisation and a higher reactivity towards nitrogen for the demineralised samples. The BET surface is also higher than for the non-demineralised lignite. Active carbons previously ammoxidated and demineralised are more interesting in terms of selective removal of gaseous pollutants.     

Adsorption of cationic dye from aqueous solutions by activated carbon

Batch sorption experiments were carried out to remove a cationic dye, methylene blue (MB), from its aqueous solutions using a commercial activated carbon as an adsorbent. Operating variables studied were pH, stirring speed, initial methylene blue concentration and temperature. Adsorption process was attained to the equilibrium within 5 min. The adsorbed amount MB dye on activated carbon slightly changed with increasing pH, and temperature, indicating an endothermic process. The adsorption capacity of methylene blue did not significantly change with increasing stirring speed. The experimental data were analyzed by various isotherm models, and found that the isotherm data were reasonably well correlated by Langmuir isotherm. Adsorption measurements showed that the process was very fast and physical in nature. Thermodynamic parameters such as the adsorption entropy (ΔS^o) and adsorption enthalpy (ΔH^o) were also calculated as 0.165 kJ mol⁻¹ K⁻¹ and 49.195 kJ mol⁻¹, respectively. The ΔG^o values varied in range with the mean values showing a gradual increase from −0.256 to −0.780 to −2.764 and −7.914 kJ mol⁻¹ for 293, 313, 323 and 333 K, respectively, in accordance with the positive adsorption entropy value of the adsorption process.     

Tailoring activated carbons for enhanced removal of natural organic matter from natural waters

Several pathways have been employed to systematically modify two granular activated carbons (GACs), F400 (coal-based) and Macro (wood-based), for examining adsorption of dissolved natural organic matter (DOM) from natural waters. A total of 24 activated carbons with different physical and chemical characteristics was produced. The impact of carbon treatment on the DOM adsorption was examined by conducting isotherm experiments at a neutral pH using the modified carbons and a DOM isolated from the influent to Myrtle Beach drinking water treatment plant in South Carolina (USA). Adsorption of the DOM by two activated carbon fibers, with relatively uniform pore size distributions, showed that only pores with widths larger than 1 nm were accessible to the DOM macromolecules. Increases in the carbon supermicropore and mesopore volume (i.e., >1 nm) increased the DOM uptake, if the surface chemistry was favorable. The isotherms normalized on a surface area basis showed the significance of carbon surface chemistry on the DOM uptake. At neutral pH, adsorption of negatively charged DOM molecules was favored by basic and positively charged surfaces, while the DOM uptake was minimized when the surface had acidic characteristics. High temperature ammonia treatment of oxidized carbons considerably enhanced the DOM uptake, mainly due to the increase in accessible surface area and surface basicity. Iron-impregnated carbons indicated an enhanced affinity of iron-laden carbon surface toward the DOM species, if the surface was not negatively charged.     

Adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons

In order to understand the adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons, the carbon yield, specific surface area, micropore area, zeta potential, and the effects of pH value, soaking time and dosage of bamboo activated carbon were investigated in this study. In comparison with once-activated bamboo carbons, lower carbon yields, larger specific surface area and micropore volume were found for the twice-activated bamboo carbons. The optimum pH values for adsorption capacity and removal efficiency of heavy metal ions were 5.81–7.86 and 7.10–9.82 by Moso and Ma bamboo activated carbons, respectively. The optimum soaking time was 2–4 h for Pb²⁺, 4–8 h for Cu²⁺ and Cd²⁺, and 4 h for Cr³⁺ by Moso bamboo activated carbons, and 1 h for the tested heavy metal ions by Ma bamboo activated carbons. The adsorption capacity and removal efficiency of heavy metal ions of the various bamboo activated carbons decreased in the order: twice-activated Ma bamboo carbons > once-activated Ma bamboo carbons > twice-activated Moso bamboo carbons > once-activated Moso bamboo carbons. The Ma bamboo activated carbons had a lower zeta potential and effectively attracted positively charged metal ions. The removal efficiency of heavy metal ions by the various bamboo activated carbons decreased in the order: Pb²⁺ > Cu²⁺ > Cr³⁺ > Cd²⁺.     

Polymorphic phase transition in BaTiO3 by Ni doping

We demonstrate a phase transition between BaTiO₃ polymorphs induced by Ni substitution experimentally. The structural evolution of tetragonal-to-hexagonal polymorph in Ni-doped BaTiO₃ ceramics is observed macroscopically and then, visualized in an atomic scale. Under this structural transition, the long-range ferroelectric ordering in the undoped BaTiO₃ with tetragonal symmetry is suppressed and eventually, vanishes in the heavily Ni-doped BaTiO₃ with hexagonal symmetry and dielectricity. An oxygen vacancy defect can be created at a local Ni doping site with a decrease in the charge valence state from Ti⁴⁺ (3d⁰) to Ti³⁺ (3d¹). With the reduction of the Ti⁴⁺ oxidation state, the polar Ti–O ionic displacement is degenerated in the tetragonal BaTiO₃ and thereby, the short-range Coulomb repulsion may be restored in the hexagonal BaTiO₃ resulting in a polymorphic phase transition. Our work is of potential interest for artificial realization of an exotic phase in complex oxide materials by doping.     

TiO2 photocatalyst deposition by MOCVD on activated carbon

Activated carbon modified by HNO₃ was used as the support during the production of TiO₂ by metal organic chemical vapor deposition (MOCVD). The HNO₃ modification increased mesopores surface area of activated carbon indicating the size of pores increased. The concentration of surface oxygen bearing groups on the HNO₃ modified activated carbon was much higher than that of the original activated carbon. It was found that a modification of activated carbon by 6 mol/L HNO₃ increased the deposition rate of TiO₂ by 4.5 times. The modification of activated carbon by HNO₃ significantly raised the photocatalytic activity of TiO₂ resulting from the formation of smaller-sized TiO₂ particles well dispersed confirmed by the results of XRD patterns and N₂ adsorption-desorption isotherms.     

Application of different equations to adsorption isotherms of phenolic compounds on activated carbons prepared from cork

Activated carbons were prepared from solid cork wastes by physical activation with carbon dioxide or steam, and chemical activation by impregnation with phosphoric acid. In this work we show the possibility of using these activated carbons for the adsorption of phenolic compounds from the aqueous phase. The materials present a different response to the adsorptives used (p-nitrophenol, p-chlorophenol, p-cresol and phenol), depending on the type of activation and the parameters (burn-off, absolute concentration) used in each case. All the samples were capable of retaining the contaminants, with the best result being reached by the sample with higher burn-off and the worst with the carbonised, while intermediate values were reached with the remaining samples. The experimental isotherms were analysed with two and three parameters equations (Freundlich, Langmuir, Dubinin-Radushkevich-Kaganer and Redlich-Peterson). The results obtained from the application of the equations are similar in some aspects, but the degree of confidence is quite different. The best fit was achieved with the Redlich-Peterson equation, which can be explained by the fact that this has three adjustable parameters. However, overall the Freundlich and DRK equations appear to be more useful and provide parameters which can be correlated with the structural characteristics of the solids obtained from N₂ adsorption measurements.     

Preparation of photocatalytic TiO2 coatings of nanosized particles on activated carbon by AP-MOCVD

Anatase TiO₂ coatings on highly porous activated carbon were prepared by a novel method—atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). At a source temperature of 423 K, the TiO₂ particles were mostly coated on the external surface of activated carbon. These particles were well dispersed with their sizes ranging from 10 to 50 nm. The optimum loading of TiO₂ was found to be 12 wt%. The TiO₂ photocatalysts so prepared behave similarly to that of the pure commercial TiO₂ powder. The activated carbon supported TiO₂ catalyst could be easily separated from the treated water with its catalytic performance maintained even after 10 cycles, indicating that the TiO₂ coating was stable. It was observed that TiO₂ supported on activated carbon had a high capacity to mineralize pollutants. Consequently, activated carbon supported TiO₂ by AP-MOCVD is a promising photocatalyst for the photodegradation of pollutants in water.     

Synthesis and characterization of high-quality activated carbons from hard-shelled agricultural wastes mixture by zinc chloride activation

In this study, a mixture apricot and peach stones, and almond shell, which are hard-shelled agricultural wastes, was evaluated as a precursor for activated carbon synthesis. Effects of the impregnation and pyrolysis conditions on ACs synthesis were investigated. The ACs were characterized by surface area, pore size, pore volume, surface functional groups by Boehm’s titration and FTIR analysis, ash content, pH_pzc, and SEM-EDX. It has been found that the impregnation conditions and pyrolysis temperature have important effect on the microspores formation. ACs having specific surface area in the range of 1,125.73–2,073.04?m²g^?1 and total pore volumes in the range of 0.5498–1.0918?cm³ g^?1 can be synthesized from the mixture activated by ZnCl₂ at impregnation ratios of higher than 15/40. These values are higher than those of the most commercial ACs. Therefore, it can be concluded that the mentioned wastes mixture is a promising precursor for producing high-quality ACs.     

Adsorption of toluene onto activated carbons exposed to 100 ppb ozone

The effects of 100 ppb ozone exposure on the adsorption of 1 ppm toluene on activated carbon are presented for dry (less than 5% RH) and humid (55% RH) air. In dry air, the 10% toluene breakthrough times of granular carbon beds exposed to ozone for 208 days are 17% less than those of unexposed carbon beds. At 55% RH, the corresponding reduction in toluene breakthrough time is 78%. For a humid environment with 100 ppb ozone, filter life would be reduced by more than half compared to the expected life based on tests in the absence of ozone. This degradation is attributed to changes in carbon surface chemistry, surface area, and pore volume that occur with relatively brief exposure to the ozone.     

Removal of ammonium ions from aqueous solutions with coal-based activated carbons modified by oxidation

The main purpose of this work is to study the possibilities for removal of ammonium ions from aqueous solutions by the two coal-based activated carbons (one obtained from Bulgarian lignite from the Chukurovo deposit, and the second, available as commercial product) and their oxidized modifications. The porous texture and surface chemistry of the adsorbents were characterized. Adsorption of ions was investigated using solutions with different concentrations in the range 35-280 mg l⁻¹ in a static mode. Equilibrium modeling data were fitted to linear Langmuir’s and Freundlich’s equations and maximum adsorption capacities were calculated.