Morphological investigation of chemically treated poly(ethylene terephthalate)-based activated carbons

Enhanced adsorption of diclofenac onto activated carbon derived from PET plastic by one-step pyrolysis with KOH

Plastic pollution is a severe threat to the health of ecosystems, and recycling plastics is recognized as a key control strategy. This study used the one-step pyrolysis assisted with KOH activation to recycle the widely used polyethylene terephthalate (PET) plastic as activated carbon (PET-AC) which was subsequently applied to adsorb diclofenac (DCF), a frequently detected emerging contaminant in water, for the first time. It was found that both the pyrolysis temperature and the addition of KOH can effectively regulate the pore sizes and volumes of PET-AC. PET-AC obtained at 700 °C demonstrated a high adsorption capacity of DCF up to 179.42 mg g-1 at 45 °C. The adsorption kinetics was conducted with both static jar and dynamic column tests and analyzed with various models. Thermodynamic results demonstrated that the adsorption of DCF was spontaneous and endothermic. The material also presented an excellent potential to adsorb other pharmaceuticals and personal care products in water. XPS and FTIR analysis indicated that the adsorption might be mainly driven by the physical forces, especially π-π interaction and hydrogen bonding. This study provided a reference for recycling waste plastic as an efficient adsorbent to eliminate organic contaminants from water.

Water-Ionic Liquid Binary Mixture Tailored Resorcinol-Formaldehyde Carbon Aerogels without Added Catalyst

The potential applications of mesoporous carbon aerogels are wide-ranging. These gels are often obtained from resorcinol-formaldehyde (RF) hydrogel precursors. The sol-gel method in this synthesis provides an efficient and versatile means of product control through systematic variation of process conditions, such as pH, stoichiometry, concentration, catalyst, further additives, etc., in addition to the drying and pyrolytic conditions. Here, a novel means of tuning the texture of carbon aerogels is proposed. Water-1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO₄] mixtures constitutes a polycondensation medium that requires no added catalyst, thus yielding an intrinsically metal-free carbon aerogel after pyrolysis. We also show that the carbon morphology is tailored by the supramolecular structure of the aqueous ionic liquid. The results of scanning electron micrographs, low-temperature nitrogen adsorption/desorption isotherms, and small-angle X-ray scattering (SAXS) confirm that changing the initial water concentration from 9 to 55 wt % gives rise to systematic alteration of the mesopore size and volume, as well as of the bead size. The pore structure becomes consolidated only when the water content exceeds 25 wt %. When the water content reaches 55 wt %, the bead size increases by two orders of magnitude. The electrocatalytic performance, however, is compromised, most probably by structural defects.

Effect of surface acidic oxides of activated carbon on adsorption of ammonia

The influence of surface acidity of activated carbon (AC) was experimentally studied on adsorption of ammonia (NH(3)). Coconut shell-based AC was modified by various acids at different concentrations. There were five different acids employed to modified AC, which included nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, and acetic acid. Acidic functional groups on the surface of ACs were determined by a Fourier transform infrared spectrograph (FTIR) and by the Boehm titration method. Specific surface area and pore volume of the ACs were measured by a nitrogen adsorption apparatus. Adsorption amounts of NH(3) onto the ACs were measured by a dynamic adsorption system at room temperature according to the principle of the ASTM standard test method. The concentration of NH(3) in the effluent stream was monitored by a gas-detecting tube technique. Experimental results showed that adsorption amounts of NH(3) on the modified ACs were all enhanced. The ammonia adsorption amounts on various activated carbons modified by different acids are in the following order: nitric acid>sulfuric acid>acetic acid approximately phosphoric acid>hydrochloric acid. It is worth to note that the breakthrough capacity of NH(3) is linearly proportional to the amount of acidic functional groups of the ACs.

Surface area and microporosity of carbon aerogels from gas adsorption and small- and wide-angle X-ray scattering measurements

A carbon aerogel was obtained by carbonization of an organic aerogel prepared by sol-gel polymerization of resorcinol and formaldehyde in water. The carbon aerogel was then CO(2) activated at 800 degrees C to increase its surface area and widen its microporosity. Evolution of these parameters was followed by gas adsorption and small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) with contrast variation by using dry and wet (immersion in benzene and m-xylene) samples. For the original carbon aerogel, the surface area, S(SAXS), obtained by SAXS, is larger than that obtained by gas adsorption (S(ads)). The values become nearly the same as the degree of activation of the carbon aerogel increases. This feature is due to the widening of the narrow microporosity in the carbon aerogel as the degree of activation is increased. In addition, WAXS results show that the short-range spatial correlations into the assemblies of hydrocarbon molecules confined inside the micropores are different from those existing in the liquid phase.

A microscopic view of physical and chemical activation in the synthesis of porous carbons

The nanostructure and porosity of activated carbon fibers (ACFs) prepared by physical activation with CO2 and by chemical activation with H3PO4 of the highly ordered polymer poly(m-phenylene isophthalamide) have been investigated and compared by means of scanning tunneling microscopy (STM), scanning electron microscopy (SEM), and gas adsorption measurements. In general terms, both types of activation led to porous carbons with similar nanometer-scale structure, which consisted of relatively ordered and homogeneous arrays of platelets below 10-nm wide, the porous structure being mainly comprised by the network of narrow trenches present between neighboring platelets. This similarity was attributed to the influence of the crystalline structure of the polymeric precursor, which should favor a homogeneous, uniform transformation of the polymer into the final carbon material. Such influence was only lost in chemical activation with the use of very large amounts of activating agent. A comparison of samples before and after physical activation allowed a direct identification of the local areas where gasification (activation) took place. For chemical activation, the STM measurements suggested that porosity was developed at a lower temperature than the highly cross-linked nanographitic structure of the final ACF. This result was discussed in terms of the thermal transformation mechanism of the precursor polymer into a carbonaceous solid in the presence of H3PO4.

Carbide-derived carbons: a comparative study of porosity based on small-angle scattering and adsorption isotherms

Porous carbons have received much attention recently for potential applications in energy generation and storage, molecular sieving, and environmental remediation. Property optimization for specific applications rests largely on controlling the volume, size, and shape of the pores at the synthetic level. Direct atom-scale experiments which might accurately and reliably measure these quantities are problematic, so indirect methods such as gas sorption are generally employed. Here we apply a second indirect method, small-angle X-ray scattering (SAXS), to study porosity in carbide-derived carbons (CDC). The results qualitatively confirm and reinforce model-dependent conclusions drawn from gas sorption isotherms. In particular, both techniques indicate the onset of broad polydispersity under the same processing conditions for particular porous carbon materials.

Influence of surface chemistry on the SAXS response of polymer-based activated carbons

Small-angle X-ray scattering (SAXS) measurements using contrast variation are reported for activated carbons prepared from poly(ethyleneterephthalate). The carbon surfaces are functionalized to different degrees by cold and hot nitric acid treatment. The latter treatment reduces the surface area by 75%, but the pore size distribution in the micropore range is hardly affected. Seven liquids, n-hexane, i-octane, i-propanol, cyclohexane, toluene, alpha-pinene, and nitrobenzene, in addition to water vapor, were used as contrast modifiers. Although the values of the specific surface area S(X) deduced from these measurements are relatively insensitive to the solvent, the detailed SAXS spectra reveal interactions occurring on different spatial scales that depend on the surface chemistry of the carbon and on the physicochemical properties of the solvent. In the most heavily oxidized sample, the amphiphilic molecule i-propanol stabilizes the surface structure, whereas nonpolar molecules make the rough surface smoother. In the untreated and the lightly functionalized carbons, water vapor at 50% relative humidity condenses only partially in the micropores at room temperature, whereas in the heavily treated sample condensation in the micropores is practically complete.

Inorganic-organic phase arrangement as a factor affecting gas-phase desulfurization on catalytic carbonaceous adsorbents

Dried sewage sludge was physically mixed with waste paper (paper-to-sludge ratios from 25% to 75%). To increase the catalytic activity, from 1% to 6% calcium hydroxide was added to the mixtures. Then the precursors were carbonized at 950 degrees C. The performance of materials as H2S adsorbents was tested using a home-developed dynamic breakthrough test. The samples, before and after the adsorption process, were characterized by adsorption of nitrogen, potentiometric titration, thermal analysis, XRF, and SEM. Differences in the performance were linked to the surface properties. Itwas found that mixing paper with sludge increases the amount of H2S adsorbed/oxidized in comparison with that adsorbed/oxidized by the adsorbents obtained from pure precursors (sludge or waste paper) and the capacity is comparable to those of the best activated carbons existing on the market. Although both sewage sludge and waste paper provide the catalytic centers for hydrogen sulfide oxidation, the dispersion of the catalyst and its location within accessible pores is an important factor. The presence of cellulose in the precursor mixture leads to the formation of a light macroporous char whose particles physically separate the inorganic catalytic phase of the sewage sludge origin, decreasing the density of the adsorbent and thus providing more space for storage of oxidation products. This, along with calcium, contributes to a significant increase in the capacity of the materials as hydrogen sulfide adsorbents. On their surface about 30 wt % H2S can be adsorbed, mainly as elemental sulfur or sulfates. The results demonstrate the importance of the composition and arrangement of inorganic/ organic phases for the removal of hydrogen sulfide. The interesting finding is that although some microporosity is necessary to increase the storage area for oxidation products, the carbonaceous phase does not need to be highly microporous. It is important that it provides space for deposition of sulfur, which is formed on the inorganic-phase catalyst. That space can be in meso- and macropores as shown in the case of char derived from the waste paper.

Characteristic adsorption functions and the surface structure of solid adsorbents

A thermodynamic model of gas/solid adsorption has been constructed from two elements. The first is the original Gibbs equation. The second consists of functions psi(Theta) or psi(P) that are calculable from measured isotherms. Based on this model the characteristic adsorption functions (CAFs) were defined and calculated. The CAFs, which concentrate into one function all measured isotherms having the same change in relative free energy of the surface, are very sensitive to the structure of the adsorbents. This statement was tested with nitrogen isotherms measured at 77 K on well-characterized chemically/physically treated activated carbons prepared from poly(ethylene terephthalate). Changes in the surface structure were followed by small angle X-ray scattering (SAXS). This experimental approach made it possible to observe the correspondence between structural changes and the CAF.