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The influence of surface chemistry of activated carbons on adsorption and freezing/melting processes
Abstract
In this work, the adsorption and freezing/melting processes on activated carbons of different chemical nature (different concentrations of oxygen surface groups) were investigated. The role of solvent (cyclohexane and water)...
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In the paper the chemical modification of activated carbon was presented. The activated carbon was modified by nitric acid. For the tested activated carbons, the following physical and chemical properties were determined: bulk density, pH of the water extract, and effective diameter of the grains. Elemental and technical analyses were performed. The pore distribution was determined using mercury porosimetry. Low-temperature nitrogen isotherms were used to analyze the microporous structure. The chemical properties of the surfaces of the tested adsorbents were interpreted by means of the Boehm method. The water vapor adsorption isotherms at 303 K were determined and the adsorption capacity towards methanol was tested using the desiccator method. Thermogravimetric tests were used and, at the same time, the released oxidizing gases from the tested samples were analyzed using a mass spectrometer. As a result of the modification, activated carbon enriched with acidic oxygen functional groups was obtained. The resulting modified activated carbon showed worse structural parameters (when compred to umodified activated carbon), however it was characterized by a higher adsorption capacity in relation to polar adsorbates such as water and methanol, especially in the low pressure range.
A coconut shell (AC1230CX) and a bituminous coal based (F400) granular activated carbon (GAC) were ground with mortar and pestle (MP), a blender, and a bench-scale ball milling unit (BMU). Blender was the most time-efficient for particle size reduction. Four size fractions ranging from 20 × 40 to 200 × 325 were characterized along with the bulk GACs. Compared to bulk GACs, F400 blender and BMU 20 × 40 fractions decreased in specific surface area (SSA, -23% and -31%, respectively) while smaller variations (-14% to 5%) occurred randomly for AC1230CX ground fractions. For F400, the blender and BMU size fraction dependencies were attributed to the combination of (i) radial trends in the F400 particle properties and (ii) importance of shear (outer layer removal) versus shock (particle fracturing) size reduction mechanisms. Compared to bulk GACs, surface oxygen content (At%-O1s) increased up to 34% for the F400 blender and BMU 20 × 40 fractions, whereas all AC1230CX ground fractions, except for the blender 100 × 200 and BMU 60 × 100 and 100 × 200 fractions, showed 25-29% consistent increases. The At%-O1s gain was attributed to (i) radial trends in F400 properties and (ii) oxidization during grinding, both of which supported the shear mechanism of mechanical grinding. Relatively small to insignificant changes in point of zero charge (pHPZC) and crystalline structure showed similar trends with the changes in SSA and At%-O1s. The study findings provide guidance for informed selection of grinding methods based on GAC type and target particle sizes to improve the representativeness of adsorption studies conducted with ground GAC, such as rapid small-scale column tests. When GACs have radial trends in their properties and when the target size fraction only includes larger particle sizes, manual grinding is recommended.
The influence of activation methods on the surface characteristics of activated carbons (ACs) and the relationships of these characteristics and the parameters of a medium with the efficiency of adsorption of herbicide molecules on ACs are considered. The main factors affecting the adsorption efficiency of ACs with respect to these pollutants are discussed.
The main purpose of the research was to obtain and study hybrid materials based on three different nano-oxides commonly used in the cosmetic and pharmaceutical industries: Al2O3, TiO2, and ZnO, with the natural bioactive polysaccharide fucoidan. Since the mentioned oxides are largely utilized by industry, there is no doubt that the presented studies are important from an environmental point of view. On the basis of the textural studies (dynamic light scattering DLS, low temperature nitrogen adsorption, X-ray diffraction analysis XRD, scanning electron microscopy SEM) it was proved that the properties of the hybrid materials differ from the pure components of the system. Moreover, the advanced thermal analysis (TG-DTG-DSC) combined with the evolved gas analysis using Fourier transformed infrared spectroscopy (FTIR) and mass spectrometry were applied to describe the thermal decomposition of fucoidan, oxides and hybrid materials. It was found that the interactions between the polymer and the oxides results in the formation of the hybrid materials due to the functionalization of the nanoparticles surface, and that their thermal stability increased when compared to the pure substrates. Such findings definitely fill the literature void regarding the fucoidan based hybrid materials and help the industrial formulators in the preparation of new products.
Effective and low-cost removal of dye and heavy metals from wastewater still is a great challenge for researchers. Adsorption using activated carbon is widely used in removing these toxic pollutants. Physical, chemical, and biological modifications have been studied for improving activated carbon adsorption performance. Literature suggests that chemical modified activated carbon showed maximum adsorption capacity towards dye and heavy from aqueous solution. Chemical modifications, including acid, base, and impregnation, are studied extensively due to reagent availability, easy modification, and tuning facilities of surface functional groups. However, systematic documentation of chemical modifications on activated carbon is required for dye and heavy metals removal efficiency improvement from wastewater. This review focused on the up to date experimental chemically modified activated carbon that showed improved adsorption capacity towards dye and heavy metals from aqueous solution. The available experimental data recommends that an appropriate treatment strategy of a chemical modification process enhanced dye and heavy metals adsorption capacity of the modified activated carbon. Optimum modification process developed textural or surface functional groups properties of modified activated carbon that improved adsorption or binding capacity toward adsorbate or a particular species. In addition, the adsorption capacity of modified and corresponding activated carbon is compared.
The effect of chemical activators on the properties of activated carbon from sago waste was conducted in this study by using ZnCl2, H3PO4, KOH, and KMnO4 chemical solutions. The carbonized sago waste was added to each chemical solution, boiled at 85 °C for 4 h, and heated at 600 °C for 3 h. The porosity, microstructural, proximate, and surface chemistry analyses were carried out using nitrogen adsorption with employing the Brunauer Emmett Teller (BET) method and the Barret-Joyner-Halenda (BJH) calculation, scanning electron microscopy by using energy dispersive spectroscopy, X-ray diffractometer, simultaneous thermogravimetric analysis system, and the Fourier-transform infrared spectroscopy. The results showed that the activated carbon prepared using ZnCl2 acid had the highest specific surface area of 546.61 m²/g, while the KOH activating agent surpassed other chemicals in terms of a refined structure and morphology, with the lowest ash content of 10.90%. The surface chemistry study revealed that ZnCl2 and KOH activated carbon showed phenol and carboxylate groups. Hence, ZnCl2 acid was suggested as activating agents for micropore carbon, while KOH was favorable to producing a mesopore-activated carbon from sago waste.
Solvent plays a key role in biological functions, catalysis, and drug delivery. Metal-organic frameworks (MOFs) due to their tunable functionalities, porosities and surface areas have been recently used as drug delivery vehicles. To investigate the effect of solvent on drug adsorption in MOFs, we have performed integrated computational and experimental studies in selected biocompatible MOFs, specifically, UiO-AZB, HKUST-1 (or CuBTC) and NH2-MIL-53(Al). The adsorption of three drugs, namely, 5-fluorouracil (5-FU), ibuprofen (IBU), and hydroxyurea (HU) were performed in the presence and absence of the ethanol. Our computational predictions, at 1 atmospheric pressure, showed a reasonable agreement with experimental studies performed in the presence of ethanol. We find that in the presence of ethanol the drug molecules were adsorbed at the interface of solvent and MOFs. Moreover, the computationally calculated adsorption isotherms suggested that the drug adsorption was driven by electrostatic interactions at lower pressures (<10-4 Pa). Our computational predictions in the absence of ethanol were higher compared to those in the presence of ethanol. The MOF-adsorbate interaction (U HA) energy decreased with decrease in the size of a drug molecule in all three MOFs at all simulated pressures. At high pressure the interaction energy increases with increase in the MOFs pore size as the number of molecules adsorbed increases. Thus, our research shows the important role played by solvent in drug adsorption and suggests that it is critical to consider solvent while performing computational studies.
In this work, organic-inorganic materials with spherical shape consisting of divinylbenzene (DVB) and triethoxyvinylsilane (TEVS) were synthesized and investigated by different complementary techniques. The obtained microspheres may be applied as sorbent systems for the purification of organic compounds from water. The hybrid microspheres combine the properties of the constituents depending on the morphologies and interfacial bonding. In this work, the influence of the molar ratio composition of crosslinked monomer (DVB) and silane coupling agent (TEVS) (DVB:TEVS molar ratios: 1:2, 1:1 and 2:1) on the morphology and quality of organic-inorganic materials have been examined. The materials were analysed using small angle X-ray scattering (SAXS) analysis, low-temperature nitrogen sorption, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) to provide information on their structural and surface properties. Moreover, thermal analysis was performed to characterize the thermal stability of the studied materials and the adsorbent-adsorbate interactions, while adsorption kinetic studies proved the utility of the synthesized adsorbents for water and wastewater treatment.
Agricultural waste materials (strawberry seeds and pistachio shells) were used for preparation of activated carbons by two various methods. Chemical activation using acetic acid and physical activation with gaseous agents (carbon dioxide and water vapor) were chosen as mild and environmentally friendly methods. The effect of type of raw material, temperature, and activation agent on the porous structure characteristics of the materials was discussed applying various methods of analysis. The best obtained activated carbons were characterized by high values of specific surface area (555–685 m²/g). The Guinier analysis of small-angle X-ray scattering (SAXS) curves showed that a time of activation affects pore size. The samples activated using carbon dioxide were characterized mostly by the spherical morphology of pores. Adsorbents were utilized for removal of the model organic pollutants from the single- and multicomponent systems. The adsorption capacities for the 4-chloro-2-methyphenoxyacetic acid (MCPA) removal were equal to 1.43–1.56 mmol/g; however, for adsorbent from strawberry seeds it was much lower. Slight effect of crystal violet presence on the MCPA adsorption and inversely was noticed as a result of adsorption in different types of pores. For similar herbicides strong competition in capacity and adsorption rate was observed. For analysis of kinetic data various equations were used.
Adsorption is one of the oldest and extensively studied techniques for water and wastewater treatment. As a surface phenomenon, adsorption efficiency relies heavily on the surface chemistry of the natural and modified material. The mode of surface modification and the nature of the precursor have significant impacts on surface functionalities. Chemical method of surface treatment is a technique that alters the structure and the surface properties of the precursor to enhance adsorption process. The present work intends to deliver precise knowledge on the choice and impacts of chemical modification on some known adsorbing materials.
Activated carbon (AC), a porous material with high pore volume, attracts increasing attention owing to its potential applications in several fields. The development of a porous structure in AC marginally relies on both the treatment methods and the type of precursor. Thus far, both renewable and nonrenewable precursor sources have been used to synthesize AC with high surface area and pore volume. This study presents the synthesis of AC via physicochemical treatment of waste oil fly ash (OFA), a waste material produced from power plants. The aim was to produce AC by adding surface pores and surface functional groups to the basal plane of OFA. Toward this objective, OFA was first chemically leached/activated with various combinations of H2 SO4 and H3 PO4 , and then physically activated with CO2 at 900 °C. The chemical activation step, synergistically combined with CO2 activation, resulted in an increase of 24 times the specific surface area of the OFA. The maximum increase in surface area was obtained for the sample physicochemically treated with 100% H2 SO4 . Moreover, the spectroscopic analysis confirmed the presence of acid functional groups after the chemical treatment step. To explore the surface heterogeneity, adsorptive potential distribution in terms of surface energy was also discussed as a function of the surface coverage. Following chemical activation, the OFA surface became heterogeneous. A major portion of the AC showed surface energy in the range of 40–50 erg/K, which was further increased as a result of physical activation at a higher temperature. Thus, the synergism created by physico-chemical activation resulted in a material with high surface area and pore volume, and excellent adsorption characteristics. From the findings of this study, it was concluded that OFA is a cost-effective and environmentally benign precursor for the synthesis of AC.
Solvent selection is a pressing challenge in developing efficient and selective liquid phase catalytic processes, as predictive understanding of the solvent effect remains lacking. In this work, an attenuated total reflection infrared spectroscopy technique is developed to quantitatively measure adsorption isotherms on porous materials in solvent and decouple the thermodynamic contributions of van der Waals interactions within zeolite pore walls from those of pore-phase proton transfer. While both the pore diameter and the solvent identity dramatically impact the confinement (adsorption) step, the solvent identity plays a dominant role in proton-transfer. Combined computational and experimental investigations show increasingly favorable pore-phase proton transfer to pyridine in the order: water < acetonitrile < 1,4 – dioxane. Equilibrium methods unaffected by mass transfer limitations are outlined for quantitatively estimating fundamental thermodynamic values using statistical thermodynamics.
Surface functionalities of activated carbon can be affected by the presence of heteroatoms such as oxygen, sulfur, and nitrogen. In this work, nitrogen-doped activated carbons (NACs) were prepared from shrimp shells, and the effects of the mixing ratio (raw material to an activating agent) on the porous texture and surface functionalities were investigated. It was found that, with increasing the mixing ratio (resulting in increasing N/C), the development of mesoporosity was significantly observed. This led to decreasing microporosity and specific surface areas (SSAs). The obtained NACs exhibited nitrogen functionalities in the forms of pyridinic and pyrrolic groups. It was found that although the pyridinic-N has a detrimental effect on the SSA, it does favor the pseudocapacitance, leading to an enhancement in the ion storage capability regardless of the low SSA.
This study examined change in pore structure and microstructure of nanoporous carbon after surface oxidation and how it affects the adsorption performance of metronidazole antibiotics. The surface oxidation was performed by hydrogen peroxide at 60 °C. The properties of porous carbon were investigated by N2-sorption analysis (pore structure), scanning electron microscopy (surface morphology), the Boehm titration method (quantification of surface functional group), and Fourier transform infrared spectroscopy (type of surface functional group). The results showed that the oxidation of porous carbon by hydrogen peroxide has a minor defect in the carbon pore structure. Only a slight decrease in specific surface area (8%) from its original value (973 m²g⁻¹) was seen but more mesoporosity was introduced. The oxidation of porous carbon with hydrogen peroxide modified the amount of oxide groups i.e., phenol, carboxylic acid and lactone. Moreover, in the application the oxidized carbon exhibited a higher the metronidazole uptake capacity of up to three-times manifold with respect to the pristine carbon.
Carbon-based materials is considered one of the oldest and extensively studied research areas related to gas adsorption, energy storage and wastewater treatment for removing organic and inorganic contaminants. Efficient adsorption on activated carbon relies heavily upon the surface chemistry and textural features of the main framework. The activation techniques and the nature of the precursor have strong impacts on surface functionalities. Consequently, the main emphasis for scientists is to innovate or improve the activation methods in an optimal way by selecting suitable precursors for desired adsorption. Various approaches, including acid treatment, base treatment and impregnation methods, have been used to design activated carbons with chemically modified surfaces. The present review article intends to deliver precise knowledge on efforts devoted by researchers to surface modification of activated carbons. Chemical modification approaches used to design modified activated carbons for gas adsorption, energy storage and water treatment are discussed here.
Ordered mesoporous carbon obtained by the soft template method was subjected to oxidation with the use of ammonium persulfate or nitric acid. The material was modified in different conditions in order to generate on its surface oxygen functional groups. Functionalization of carbon with nitric acid or ammonium persulfate at low temperatures caused a reduction in its surface area and pore volume. At elevated temperatures, the oxidation with (NH4)2S2O8 solution brought about an increase in these parameters, which is related, among others, with increased area of micropores. Transmission electron microscopy images confirm that the application of ammonium persulfate as an oxidizer does not lead to structural changes in carbon. FT-IR spectra and Boehm titration results proved that irrespective of the oxidation conditions, the process leads to generation of oxygen functional groups. Their highest content was observed for the carbon sample oxidized by a 5 M solution of HNO3 at 100 °C.
Adsorption and desorption of benzoic and salicylic acids and phenol from a series of synthesized mesoporous carbons is measured and analyzed. Equilibrium adsorption isotherms are best described by the Langmuir–Freundlich isotherm. Intraparticle diffusion and McKay’s pore diffusion models, as well as mixed 1,2-order (MOE), integrated Langmuir kinetic equation (IKL), Langmuir–Freundlich kinetic equation and recently derived fractal-like MOE (f-MOE) and IKL models were compared and used to analyze adsorption kinetic data. New generalization of Langmuir kinetics (gIKL), MOE and f-MOE were used to describe desorption kinetics. Analysis of adsorption and desorption half-times shows simple relation to the size of carbon pores.
The dependence between the surface chemistry characteristics of thermally modified active carbon samples with previously oxidized surfaces and their ability to adsorb organic solutes from aqueous solutions was investigated. The characteristics of the surface functional groups and the porous structure of the samples obtained were estimated using various experimental methods. The adsorption isotherms of nitrobenzene and 4-nitrophenol from dilute aqueous solutions onto all the carbon samples obtained were measured. The experimental systems were analyzed in terms of the theory of adsorption onto energetically heterogeneous solids. The relationships between the optimized isotherm parameters, i.e. equilibrium constants and heterogeneity parameters, and the characteristics of the active carbons were discussed.
The surface chemistry and pore structure of porous carbons determine its application. The surface chemistry could be modified by various methods, such as, acid treatment, oxidization, ammonization, plasma, microwave treatment, and so on. In this paper, the modification methods were illustrated and compared, some new methods also reviewed. The surface chemical functional groups were determined by the treatment methods, the amminization could increase its basic property while the oxidization commonly improved its acids. In the end, the commonly characterization methods were also mentioned. Some interesting patents are also discussed in this article.
In this work, the mesoporous carbon (MC) of the ordered structure was synthesized using silica (SBA-15) as a template and glucose as a carbon precursor. The received MC was subjected to wet oxidation in nitric acid and thermal treatment at 150, 400, 600 and 800 °C to differ its hydrophobic/hydrophilic character. The obtained unmodified (MC), oxidized (MC-OX) and thermally modified oxidized carbon (MC-OX-150, MC-OX-400, MC-OX-600 and MC-OX-800) were investigated by various techniques: transmission electron microscopy (TEM), scanning electron microscopy (SEM), and low-temperature nitrogen adsorption to characterize material porosity, X-ray photoelectron spectroscopy (XPS) and suspension potentiometric titration (SPT) to provide the statement of quality and quantity of groups presented onto the surface and the surface charge, and dielectric spectroscopy (DS) and differential scanning calorimetry (DSC) to register the changes in the water melting processes in carbon pores. The DS and DSC studies revealed the correlation between the water melting temperature inside the pores of MCs and their chemical characteristics, which were determined mainly by surface functional groups and only slightly by textural properties.
The influence of the geometric modification (GM) of fumed nanoscale silica A300 (NS) on the adsorption capacity of human serum albumin (HSA) as well as the physicochemical and textural properties of the protein/nanosilica system was analyzed. An effective medical enterosorbent based on fumed nanosilica was designed and produced in the Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine. To design an effective nanomaterial for biomedical applications as a wound-healing material, the adsorption, physicochemical and surface properties of the initial nanosilica (NS), nanosilica after geometric modification (GM-nanosilica), and HSA/nanosilica biocomposites were characterized. The differences in sorption capacities, acid-base, textural, and surface properties of the obtained materials were monitored using the diffuse UV–vis reflectance spectroscopy, the potentiometric titration of suspension, the nitrogen adsorption/desorption isotherms, the scanning electron microscopy with the energy dispersive X-ray microanalysis and the digital optical microscope. For a deeper understanding of the nature of immobilized HSA molecules on the nanosilica and GM-nanosilica, the surface functional groups were characterized by the FTIR spectroscopy. It was found that the adsorption properties, physicochemical and textural characteristics of fumed nanoscale silica depend on the mechanical treatment time (tMT) and bulk density (db). In fact, as the tMT increases, the db of initial fumed nanosilica increases, and the protein adsorption capacity decreases, however, it remains at an acceptable level: 0.075 g/g, 0.056 g/g, 0.032 g/g for GM-nanosilica after 1, 4, and 7 h of mechanical treatment, respectively. HSA adsorption significantly changes the surface morphology, acid-base character, and structure of both unmodified and modified nanosilica.
Adsorption of phenol on activated carbon of strongly differentiated surface chemistry from aqueous and cyclohexane solutions was studied. The role of oxygen surface groups and kind of solvent in phenol adsorption was analyzed. The research showed a decrease in phenol adsorption on the oxidized carbon from water in contrast to organic solvent. The KF parameter values estimated from the Freundlich isotherm for unmodified (C1) and oxidized material (OC1) were respectively 0.19 vs. 0.26 (mmol g–1) (L mmol–1)1/n for organic solvent and 1.03 vs. 0.76 (mmol g–1)(L mmol–1)1/n for aqueous solution. Thermal analysis (TG, DTG, DTA) was used to know the kind of interactions between carbon surface and both solvents. Temperature Programmed Desorption technique was applied for characterization of carbon surface and to recognize how sorption of cyclohexane may influence its properties, e.g. changing sorption of phenol. The thermal measurements showed that phenol adsorption process on the carbons is mainly of a physical nature. Additionally, the shift of combustion exothermic peaks on DTA curves to lower temperatures for the samples with phenol adsorbed from aqueous solutions indicated the adsorbate interactions only with graphene layers, whereas, for the samples containing phenol adsorbed from cyclohexane, interactions with oxygen groups of OC1 also occurred.
In this work, granular activated carbon (GAC1240W Norit N.V.) was chemically and thermally modified to diverse its acid-base character. The main aim of this work was to assess the efficiency of surface modifications of activated carbons (ACs) concerning studying the differentiation of their affinity towards aromatic organic compounds of various properties. For this purpose, the comprehensive characteristics of oxidized activated carbon (AC-OX) and oxidized thermally modified carbons at 180, 280, 380, 480, 640, and 800 ◦C (AC-OX, AC-OX-180, 280, 380, 480, 640, and 800) were analyzed using several analytical techniques:low-temperature nitrogen sorption (porosity characteristics), potentiometric titration of suspension (SPT) (acid-base properties), X-ray photoelectron spectroscopy (XPS) (quality and quantity analysis of elements), thermal analysis (TA) coupled with mass spectrometry (MS) (materials thermal stability, identification of gaseous products emitted during thermal decomposition). To verify the applicability of adsorbents for removing selected organic compounds, the equilibrium and kinetic experimental data were measured and analyzed by applying various equations and
models.
Despite extensive literature regarding the effect of solvent on numerous physicochemical processes, the solvent effect on the adsorption kinetics of a surface-active compound is yet to be explored. This study first investigated the adsorption kinetics of dibutyl phthalate (DBP) in 1 wt% aqueous methanol (MeOH) solvent: dynamic and equilibrium surface tension (ST) data were measured using a pendant bubble tensiometer and the resultant ST data were examined via theoretical simulation. The adsorption of DBP onto a clean air-water interface was found to be of mixed-control (Frumkin model with K = –0.89), indicating a non-negligible interaction amongst the adsorbed DBP molecules. Subsequent examination of DBP’s adsorption kinetics in 0, 1, and 5 wt% MeOH(aq) solvents revealed a strong solvent effect on the (i) dynamic and equilibrium ST relaxations, (ii) dependency of surface concentration on DBP concentration, and (iii) intermolecular interaction amongst the adsorbed DBP molecules. Besides, although all the adsorption mechanisms of DBP in these three solvents were of mixed control, it was observed to be more biased towards diffusion-control at increasing MeOH concentration.
The adsorption behavior of activated carbons is determined not only by their porous structures but also by the chemical nature of its surface. The surface chemistry of activated carbons can be selectively modified in order to improve their adsorption capacity. In this study, a NORIT granular activated carbon was treated by oxidant (HNO3) and non-oxidant acid (HCI) at different concentrations and temperatures. The surface chemistries of the materials were characterized by Boehm titration method and by the determination of the point of zero charge (pHPZC).The adsorption properties of the selected samples were studied by adsorption of methylene blue, which is one of the important dyes and found in many textile effluents. In addition, the pore structures of the modified carbons were also studied by argon adsorption at 87.29 K. As results, it was observed that both HN03 and HCI treatments could increase the surface acidity of activated carbons. Activated carbons modified by HCI gave the best performance on the adsorption of methylene blue.Keywords: Activated Carbon, Surface Chemistry, Chemical Treatment, Boehm Titration Method, Adsorption AbstrakKemampuan adsorpsi karbon akti.ftidak hanya ditentukan oleh struktur pori tetapijuga dipengaruhi oleh sifat kimia dari permukaannya. Sifat kimia permukaan karbon aktif dapat secara selektif dimodifikasi dengan tujuan untuk lebih meningkatkan kapasitas adsorpsinya. Pada penelitian ini, karbon aktif NORIT granular ditreatment dengan menggunakan asam oksidator (HNO) dan non-oksidator (HCI) pada berbagai konsentrasi dan suhu. Sifat kimia permukaan karbon aktif dikarakterisasi dengan menggunakan metode titrasi Boehm serta dengan penentuan point of zero charge (pHPZC). Kemampuan adsorpsinya diuji dengan mengadsorp larutan methylene blue, dimana methylene blue merupakan salah satu komponen dalam limbah tekstil. Sedangkan struktur pori karbon aktif dianalisa dengan adsorpsi Ar pada suhu 87,29 K. Penelitian ini menunjukkan bahwa baik treatment dengan HNO3 maupun HCI dapat mengakibatkan terjadinya peningkatan sifat asam pada permukaan karbon aktif. Karbon aktif yang diberi perlalatan dengan HCI memberikan kemampuan adsorpsi yang paling baik dalam adsorpsi larutan methylen biru.Kata Kunci: Karbon Aktif, Sifat Kimia Permukaan, Perlakuan dengan Larutan Kimia, Metode Titrasi Boehm, Adsorpsi
Water confined in nanoscale exhibits unique structural properties compared to the bulk phase. The effects of confinement of water in carbon nanopores were investigated in this study using differential scanning calorimetry (DSC), dielectric spectroscopy (DS), and neutron diffraction (ND). The ice was confined in slit-shaped pores of activated carbon fibers (ACF) and cylindrical-shaped porous system (ordered carbon mesoporous structure - CMK-5) of different inner diameter. Melting behaviors of the confined ice were studied using DSC and DS methods, while ND technique was used to determine the structure of nanoconfined ice. ND measurements for D 2 O in different carbon nanopores were taken in a temperature range of 300 to 100 K. The results of the DS and ND studies showed the existence of hexagonal ice, Ih, and cubic ice, Ic at temperatures below the pores melting point. We have found that the content of confined hexagonal and cubic ice phases depend on the type of porous materials used in the confinement. Our results of the confined ice in the carbon nanopores revealed the features of stacking disordered ice I sd formed by disordered hexagonal and cubic ice layers, which has been recently reported in the literature.
The oxidized mesoporous carbons obtained by hard or soft-template methods have been used for removal of heavy metal ions – Ni²⁺and Co²⁺ from the liquid phase. The materials were functionalized with 0.5 and 5 M solutions of nitric(V) acid. The use of two different temperatures of oxidation 70 °C and 100 °C, permitted achieving functionalization in two different degrees and examination of the effect of temperature on the ordering of carbon material structure. Functionalization of mesoporous carbon materials with HNO3 brought a decrease in the surface area, pore volume and area of micropores, irrespective of temperature and method of synthesis. However, it also resulted in a significant increase in the number of oxygen functional groups, especially on the surface of carbons oxidized at 100 °C, as proved by FT-IR data and Boehm titration. The oxidized mesoporous carbon materials show high sorption capacities towards Ni²⁺and Co²⁺ ions. Adsorption of metal ions is endothermic and spontaneous. The examination of adsorption isotherm revealed that the sorption fitted well the Langmuir isotherm model.
The adsorption of herbicides belonging to the group of halogenated phenoxyacids on the activated carbon was studied. They are differentiated in terms of quantity and type of functional groups (such as chloride, bromide, fluoride) and their position on an aromatic ring. The experimental equilibrium data were analyzed using adsorption isotherm equations taking into account energetic heterogeneity of the adsorption systems. The calculated concentration profiles from the kinetic data were discussed applying two diffusion models, MOE, f-MOE and multi-exponential equations. The dependences between the properties of adsorbates, adsorption uptake and rate were analyzed. The adsorption affinity of pesticides was correlated with adsorbate hydrophobicity, character of functional group, molecular structure. The applicability of kinetic models and equations was investigated; the assumptions of the models were analyzed with regard to consistency with adsorption mechanism. Similarity of adsorption mechanism was found for all adsorbates confirmed by similarity of kinetic curves and corresponding distributions of rate coefficients. The differences in kinetic profiles were attributed to differentiation of herbicide's molecules - number and type of functional groups and their positions on aromatic ring.
The effects of co-influence of a pore size distribution (PSD) and a surface chemistry of activated carbon (AC) on the p-chlorophenol (PCP) adsorption from water, heptane and cyclohexane have been studied. To modify the surface basicity, the commercial activated carbon and the ash free commercial activated carbon were subjected to a heat treatment in a hydrogen atmosphere. The ACs were also oxidized with a hydrogen peroxide to increase the acidity. The applied modifications caused negligible changes in the porous texture and a significant modification on the surface characteristics. The adsorption of PCP was carried out in static conditions at an ambient temperature. The time needed to obtain the adsorption equilibrium from organic solvent was shorter than from water. The boundary layer effect was found to increase in the direction of water<cyclohexane<heptane and was heteroatom dependent. The equilibrium adsorption isotherms showed all spectrum of isotherm types according to the Gilles classification. The strong relationship between the volume of the PCP adsorbed and the volume of ACs micropore with size smaller than 1.6 nm was presented. This work shows that the surface heterogeneity influences the adsorption mechanism in the low adsorbate concentration range, more specifically the final adsorption capacity is pore size dependent regardless of the kind of solvent used.
Porous carbon is used as the electrode material in electrochemical double-layer capacitors (EDLCs) due to its high surface area and electrical conductivity. However, one of the challenges associated with porous carbon is the presence of electrochemically active surface oxygen groups, which undergo rapid charge transfer reactions in alkaline or acidic electrolytes. In this study, oxygen groups have been removed from porous carbon samples in order to quantify their contribution to the double-layer capacitance. The oxygen-free porous carbon samples have similar pore size distributions to the original sample; however, the specific surface area (SSA) is observed to increase slightly with heat treatment. The original porous carbon sample, containing oxygen, has an SSA-normalized capacitance of 12.2 ± 0.2 μF cm⁻², while the oxygen-free samples have SSA-normalized capacitances of 8.8 ± 1.0 μF cm⁻², regardless of total SSA. Based on these results, the relative contributions of the carboxyl and phenol oxygen groups are determined, which are the two electrochemically active oxygen groups in alkaline solutions. From this work, it is shown that the carboxyl groups and phenol groups contribute approximately 1.10 μF cm⁻²at.%⁻¹. These results demonstrate the importance of oxygen groups for accurate characterization of the performance of novel porous carbons for EDLCs.
To examine the influence of pore-wall hydrophobicity on freezing and melting behavior of a confined water, we measured the X-ray diffraction pattern of water in the nearly cylindrical pores of ordered mesoporous carbons during cooling and heating processes. The pore walls of the ordered mesoporous carbons are crystalline and cannot form hydrogen bonds with water molecules. Capillary condensation of water in the hydrophobic mesopores takes place very slowly only close to the saturation pressure of a bulk water. Nevertheless, freezing and melting of water confined in the hydrophobic mesopores occurred almost in the same way as those in mesoporous silicas with similar pore sizes, the pore walls of which are amorphous and can form hydrogen bonds with water molecules. This clearly indicates that the pore-wall hydrophobicity does not affect appreciably the freezing and melting behavior of the confined water. The existence of a thin water layer between core ice and the pore walls is responsible for it.
In this study, the influence of chitosan immobilization method on properties of final hybrids materials was performed. Chitosan immobilized on the surface of mesoporous (ChS2) and fumed silica (ChS3) by physical adsorption and through sol-gel method (ChS1). It was found that physical immobilization of chitosan allow to obtain hybrid composites (ChS) with homogeneous distribution of polymer on the surface, relatively wide pores and specific surface area about 170 m²/g, pHPZC=5.7 for ChS3 and 356 m²/g, pHPZC=6.0 for ChS2. The microporous chitosan-silica material with specific surface area 600 m²/g and more negatively charged surface (pHPZC=4.2) was obtained by sol gel reaction. The mechanisms of azo dyes adsorption were studied and correlation with composite structure was distinguished. The Generalized Langmuir equation, and its special cases, i.e. Langmuir-Freundlich and Langmuir equations, were applied for analysis of adsorption isotherm data. The adsorption study showed that physically adsorbed chitosan (ChS1 and ChS2) on silica surface have higher sorption capacity, eg. 0.48 mmol/g for AR88 dye (ChS2) and 0.23 mmol/g for AO8 dye (ChS1), comparing to the composite obtained by sol gel method (ChS1, 0.05 mmol/g for AO8 dye). For deeper understanding of the behavior of immobilized chitosan in adsorption processes, various kinetic equations were applied: first-order, second-order, mixed 1,2-order (MOE), multi-exponential, fractal-like MOE as well as intraparticle and pore diffusion models equations. In the case of AO8 dye the adsorption rates were differentiated for three composites: for ChS3 50% of dye was removed from the solution after merely 5 min and almost 90% after 80 min. The slowest adsorption process controlled by a diffusion rate of dye molecules into the internal space of pore structure was found for ChS1 (225 min halftime). In the case of ChS2 the rates for various dyes change in the following order: AO7 > OG > AR1 > AR88 > AO8 (half-times: 10.5 < 15.7 < 23.7 < 34.9 < 42.9 min).
The main and new surface modification methods of activated carbon (AC) and their influence on application (adsorption capacity) were reviewed. Adsorption capacity is an important issue, contributing to hazardous substances environment management. According to literature, it is true that surface chemistry strongly affects adsorption capacity. Surface chemistry can be modified by several methods that lead to different activated carbon properties. Furthermore, adsorbate properties, and their relationships with surface structure, can impact adsorption properties. Surface modifications can be conducted by adding some atoms to the surface structure, making the surface more acidic or basic. Introduction of oxygen and ammonia atoms (chemical modification) are the main processes to make the surface more acidic and basic, respectively, although may bring chemical wastes to environment. Surface modification is done by chemical and physical modifications that lead activated carbons to present different properties. The main and new methods of chemical and physical modifications are compared and presented in this paper. Some new physical methods, like corona treatment, plasma discharge and microwave radiation, can be applied to cause surface modifications. Corona treatment can be a practical and new way to cause surface modification on an activated carbon surface.
We report experimental measurements of melting behavior of liquids adsorbed in carbon and silica nanopores. Phase transition of H2O, D2O and C6H5NO2 confinement in pores of different inner diameter in the range of 2.2-7.5nm has been characterized by Dielectric Spectroscopy and Differential Scanning Calorimetry methods. The measurements presented in this paper have been made in a wide temperature range from 150K to 300K.We find that the melting point inside pores strongly depend on pores size and fluid-wall interaction. We show that melting temperatures Tmp of confinement system decrease relative to the bulk for weaker fluid-wall than fluid-fluid interaction (silica glasses) and increase in the case of stronger fluid-wall interaction (carbon pores). © 2018 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
Different potassium permanganate oxidation processes were used to modify activated carbon, and then surface structure and chemical properties of modified activated carbon with different preparation methods as well as their influences on carbon dioxide reforming of methane were studied. BET and SEM surface structure analysis indicated that after modification with potassium permanganate under alkaline condition, specific surface area of activated carbon increased, microspore and mesoporous capacities enlarged, but the pore diameter of activated carbon slightly reduced. XPS and FT-IR characterization indicated that after modification with potassium permanganate under alkaline condition, acid oxygen-containing functional groups dropped and new alkaline active substances like ionic OH⁻ and manganite were generated on the activated carbon surface. Increased specific surface area and pore capacity of activated carbon, reduced acid oxygen-containing groups and generated alkaline active substances like ionic OH⁻ and manganite exerted significant influences on methane activation and carbon dioxide reforming of methane. Under alkaline condition, the activity of the potassium permanganate modified activated carbon and reforming conversion slightly increased because of interaction between alkaline functional groups and acid carbon dioxide; In the meantime, newly generated ionic OH⁻ on the surface and manganite participated in reforming reaction, which also improved activity of potassium permanganate-modified activated carbon for carbon dioxide reforming of methane. Under acid condition, CO2-CH4 reforming conversion and catalyst activity decreased because of interaction between acidic functional groups and acid carbon dioxide.
In this study, the structure of nanoconfined ice and its behavior during the melting process have been investigated. For this purpose, deionized water was inserted into the pores of the ordered carbon structures CMK-3 and CMK-8 having pores of different diameters. The first set of experiments was performed using differential scanning calorimetry (DSC), from which the melting transition temperature of the confined ice was determined. In order to investigate the structure of ice formed inside the mesopores, wide-angle X-ray scattering was used. The measurements were performed at temperatures from 173 K up to and above the pore melting point for each system. The results of the XRD experiments showed features characteristic of both hexagonal, Ih, and cubic, Ic, ice at temperatures below the melting point. The structure of the confined ice corresponds to disordered stacking ice layers, ice Isd, and our results agree well with recent simulations of X-ray diffraction of such ice crystals by Murray and co-workers.
The effects of solvent, particle size and temperature on the adsorption isotherm for phenol on to activated carbon were investigated in this work. The first two effects were studied by determining the adsorption isotherm at 298 K in both aqueous and cyclohexane solutions, and using particle diameters of 0.338, 0.635, 0.940 and 1.494 mm. The last effect was analyzed by measuring the adsorption isotherm at temperatures of 283 K, 298 K and 313 K in both solvents, and using a particle diameter of 0.940 mm. It was found that in the aqueous solution the amount of phenol adsorbed is greater than that in the cyclohexane solution, which was attributed to the fact that phenol has a higher affinity for cyclohexane than for water. Furthermore, the results revealed that in aqueous solution the amount of phenol adsorbed increased when the particle size decreased and was slightly reduced by an increase in the temperature; however, in cyclohexane solution, the amount of phenol adsorbed was independent of particle size and considerably reduced by increasing temperature.
The effects of dry and wet oxidation treatments of activated carbon (AC) on the surface chemistry and porous structure are studied. Using cherry stones (CS), AC was first prepared by carbonization at 900 °C for 2 h in N2 and activation at 850 °C for 2 h in CO2. Then, the resulting AC was oxidized in O2(air) or O3 atmosphere and with HNO3 and H2O2 solutions. The acidic–basic surface sites were analyzed by FT-IR spectroscopy, Boehm method, and pH of the point of zero charge (pHpzc) and the porous structure by N2 adsorption and mercury porosimetry. It has been found that the oxidizing agent, under specific reaction conditions, rather than whether it was a gas or a solute in aqueous solution, is the main factor that controls the changes produced in the surface chemistry and porous structure of AC. O3 and HNO3 are the most effective oxidants to form acidic oxygen surface groups. However, the content of basic groups decreases for the four oxidants, the effect being much stronger for HNO3. A microporosity reduction is also observed, which is more important for O2(air) and especially for HNO3 than for O3 and H2O2. The percentage of microporosity loss is as high as 43.3 for HNO3. Mesoporosity significantly develops, whereas macroporosity usually remains practically unchanged. Dry oxidation of AC at 100 °C in O3 has proved to be the most promising method to increase the content of acidic oxygen surface groups in the material without greatly decreasing the content of basic sites and microporosity and with a significant mesoporosity development.
Two commercial activated carbons (AC) were treated with HNO3 and (NH4)2S2O8. The changes in surface area and pore size distribution resulting from the oxidising treatments were studied by means of nitrogen adsorption isotherms. The modifications in the surface chemistry were evaluated by means of the point of zero charge (PZC) and the oxygen content. Selective removal of oxygen complexes by heating under N2 and CO2 flow at 723 and 1123 K was also studied. The treatment with HNO3 was found to broaden the microporosity and mesoporosity of the carbons when low concentrations of the oxidising agent were employed. Severe oxidation conditions resulted in an almost total destruction of the texture of the carbons. The point of zero charge values changed systematically with the extent of the oxidation; the more oxidised the carbon, the lower its point of zero charge. Thermal treatment at high temperatures leads to an enhancement of the carbon basicity and a slight decrease in the textural properties of the original carbons.
Enthalpies of wetting of several pure liquids onto active carbon were measured at 298.15 K with a quasi-isothermal microcalorimeter Setaram MS 80 II. New measuring cells with three independent cavities of about 15 cm3 were created. The advantage of this new method is the high weight-in of active carbon and a reduced time per measurement for reaching a constant baseline. Experiments were carried out with n-alkanes, 1-alkanols, cyclohexane, 2,2,4-trimethyl-pentane (isooctane) and water. The experimental results for the enthalpy of wetting show a dependence of the geometric heterogeneity of the active carbon and the polarity of liquids.
Ordered mesoporous carbons (OMC) were synthesized using mesoporous silica SBA-15 as templating material. Prepared carbons had a specific area of 570m2/g and a pore volume of 0.37cm3/g. Its texture and surface chemistry were modified by oxidation treatments in liquid phase using nitric acid with different concentrations as oxidizing agent for different times. The effect of liquid phase oxidation on the texture, surface chemistry and structure of ordered mesoporous carbons was studied by means of different analytical techniques as N2-physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed oxidation (TPO), temperature programmed desorption (TPD) and infra-red Fourier transform spectroscopy (IRTF). During these treatments, surface oxygen groups were created and their number was function of the concentration of the oxidizing agent used and the treatment time. Nitration reactions occurred simultaneously to oxidation reactions through HNO3 treatments. On the other hand, the porous structure of OMC was hexagonally ordered and it was maintained after oxidation treatments in nitric acid, as XRD and TEM showed.
The adsorption of four dyestuffs onto bagasse pith, a waste product from the sugar cane industry, has been studied. The equilibrium isotherms have been measured and concentration versus time decay curves have been determined in an agitated batch adsorber. A two-resistance model has been developed based on external film mass transfer and pore diffusion. The model incorporates a concentration-decay curve fitting routine which was optimized during the model development. The experimental batch contact time data have been compared with the theoretically predicted data in the form of Sherwood numbers.
The process of adsorption of selected benzene derivatives from aqueous solution is investigated on two carbonaceous materials of differentiated surface properties – quantity of oxygen functional groups. Carbon samples were prepared by removal of external layers from granules of unmodified and oxidized active carbon. The surface and structure characteristics of carbon samples were estimated by various methods. The experimental isotherms of organics adsorption from liquid phase were measured and interpreted in terms of the theory of adsorption on heterogeneous solid surfaces. The influence of differences in adsorbate and adsorbent properties on adsorption uptake was analyzed. The adsorption effectiveness was regarded as a result of the differences in adsorbate hydrophobicities and the effect of specific interactions of its functional groups with active sites on carbon surface.
Adsorption of benzene derivatives of various chemical properties from dilute aqueous solutions on the Norit activated carbons is investigated. The experimental systems are analyzed in terms of adsorption theory on energetically heterogeneous solids. The relations between the optimization isotherm parameters, i.e. equilibrium constants and heterogeneity parameters, and the solute properties, e.g. number, character and position of functional groups are discussed.
We report both experimental measurements and molecular simulations of the melting and freezing behavior of simple fluids in porous media. The experimental studies are for carbon tetrachloride and nitrobenzene in controlled pore glass (CPG) and Vycor. Differential scanning calorimetry (DSC) was used to determine the melting point in the porous materials for each of the glass samples. In the case of nitrobenzene (which has a nonzero dipole moment), dielectric spectroscopy was also used to determine melting points. Measurements by the two methods were in excellent agreement. The melting point was found to be depressed relative to the bulk value for both fluids. With the exception of smallest pores, the melting point depression was proportional to the reciprocal of the pore diameter, in agreement with the Gibbs−Thomson equation. Structural information about the different confined phases was obtained by measuring the dielectric relaxation times using dielectric spectroscopy. Monte Carlo simulations were used to determine the shift in the melting point, Tm, for a simple fluid in pores having both repulsive and strongly attractive walls. The strength of attraction to the wall was shown to have a large effect on the shift in Tm, with Tm being reduced for weakly attracting walls. For strongly attracting walls, such as graphitic carbon, the melting point increases for slit-shaped pores. For such materials, the adsorbed contact layer is shown to melt at a higher temperature than the inner adsorbed layers. A method for calculating the free energies of solids in pores is presented, and it is shown that the solid−liquid transition is first order in these systems.
Static and kinetic studies on adsorption of methylene blue on four synthesized mesoporous carbons are presented. The carbon properties are analyzed by means of nitrogen adsorption. The static experiments are analyzed by means of Langmuir–Freundlich and Freundlich isotherms. The Lagergren, pseudo-second-order and mixed order as well as the multi-exponent equations are used in analysis of kinetic equilibria. The properties of rate equations are compared and analyzed.