Water vapor adsorption and the microporous structures of carbonaceous adsorbents

The principal role in adsorption of almost all vapors organic and inorganic substances on nonporous and microporous carbonaceous adsorbents is played by dispersion interactions. They are characterized by a considerable increase in adsorption potentials as a result of superposition of the fields of the opposite pore walls. This effect determines the entire specifics of adsorption in micropores and, in particular, the substantial increase in adsorbability. A theoretical estimate of the adsorption potentials of benzene and water in adsorption on graphite, and a comparison of the differential heats of adsorption of water vapors on a non-porous carbon black previously heated in a vacuum at 950°C and on an active carbon show that water adsorption is due to the formation of hydrogen bonds both between the oxygen complexes on the surface of carbonaceous adsorbents and between the adsorbed molecules themselves. Dispersion interactions are weak and can be neglected to a first approximation. It has been shown for microporous structures and the slitlike model that one can calculate, from the parameters W₀ and E₀ of the adsorption equation of the theory of volume filling of micropores (determined from the adsorption isotherm of a standard vapor, benzene) the volume of the micropores, their halfwidth, and the specific surface area of the micropore walls. The latter are in good agreement with the specific surface areas of the micropores, as estimated by the independent method of similarity of the adsorption isotherms of water in micropores and on the surface of a nonporous carbonaceous adsorbent. The application of the BET and t-methods to microporous carbonaceous adsorbents is physically unsubstantiated.     

富微孔炭质吸附剂吸附天然气的热效应

为了确定吸附天然气汽车的储气罐在充放气时的热效应及其对储气量的影响,实验室利用自行设计的天然气吸附评价装置,研究了富微孔炭质吸附剂吸附天然气的热效应。结果表明,在室温下充气时最高温度可超过 75℃,放气时最低温度可低于-5℃。充气时最高温度随充气速度的增大而升高,放气时最低温度随放气速度的增大而降低,但当充放气超过一定时间后,这种因热效应带来的吸附床层的温度变化明显减缓。快速充气时,因热效应的影响,吸附剂的天然气储气量体积分数大约有 16%—23%的下降。     

On the characterization of microporous adsorbents

Two exponential adsorption isotherms are discussed with regard to adsorption on microporous adsorbents at low and moderate surface coverages. One of them may be treated as a certain generalization of Dubinin isotherm. However, the other equation discussed is the exponential virial isotherm. Both isotherms may be used for characterization of the adsorbents showing a very differentiated microporous structure, because they predict universal expressions for the distribution function of adsorption potential. Adsorption data of hydrocarbons on molecular sieving carbon measured at different temperatures are interpreted in terms of the theory presented. Special cases of the exponential virial isotherm are discussed in detail.     

Novel ideas in the theory of the physical adsorption of vapors on micropore adsorbents

A general equation has been derived for the physical adsorption of vapors on adsorbents with nonhomogeneous microporous structure, in which the energy of adsorption is increased as a result of the effect of superposition of dispersion force fields of the opposite micropore walls. The equation is based on experimentally determined reciprocal dependence of the generalized dimensions of the micropores expressed as their radii of inertia and the characteristic adsorption energy. The expression of the inertia radii of micropores takes into account their geometrical sizes. The nonhomogeneity of the microporous structure is described in terms of normal distribution of the micropore volumes with respect to reciprocal values of the characteristic adsorption energies. For a homogeneous elementary micropore volume, the adsorption equation derived from the theory of volume filling of the micropores is used. The adsorption equation obtained contains three parameters: the total micropore volume, the characteristic adsorption energy at the maximum of the distribution curve and the dispersion. The equation can be used for micropores with various geometric shapes. It leads to the following consequences: If the dispersion equals zero, then the Dubinin-Radushkewich equation is obtained for homogeneous microporous structures; the Dubinin-Stoeckli equation is obtained for the model of slitlike micropores in carbonaceous adsorbents. The parameters of the equation were found to be effective quantities for increased mesopore surface areas in microporous adsorbents. We considered methods for determining the specific surface areas of mesopores; these methods are necessary for obtaining realistic parameters for the microporous structure of adsorbent.     

Adsorption of associating and non-associating adsorbates on microporous carbon adsorbents

The adsorption of different chemical substances on microporous carbon adsorbents has been investigated. It has been determined that the chemical character of the molecules adsorbed as well as the adsorbent texture determine the adsorption kinetics. In the case of alcohols, one of the parameters determining the sorption mechanism is the formation of complexes. As was shown earlier, the rates of complex molecule formation for methyl and ethyl alcohols differ significantly.     

改性吸附剂脱除燃油中二苯并噻吩的研究

采用浸渍法将5种金属离子Zn2+,Cu2+,Fe3+,Pb2+,Hg2+负载于活性炭和γ-氧化铝上,制备出一系列改性的吸附剂;用静态吸附法测定了燃料油中二苯并噻吩在不同吸附剂上的吸附等温曲线。结果表明,活性炭对97#汽油中二苯并噻吩吸附能力显著高于氧化铝;活性炭表面负载Zn2+,Cu2+,Fe3+,可以提高其对97#汽油中的二苯并噻吩的吸附量(摩尔质量浓度),其中负载Zn2+的活性炭吸附能力最大,硫饱和吸附量为478.3mmol/kg;γ-氧化铝上Pb2+的最佳负载质量分数为5.0%。     

Adsorption properties and microporous structures of carbonaceous adsorbents

The theory underlying the absorption behavior of carbons is discussed as a basis for quantitative analysis of their adsorption properties and microporous structure. The equations of the theory of volume filling of micropores for homogeneous and inhomogeneous microporous structures and a rational method for determining the specific surface area of micropores are considered. Four principal parameters are proposed which describe the physical vapor adsorption with good accuracy and quantitatively characterize the microporous structure of carbonaceous adsorbents.     

微孔木薯淀粉吸附特性研究

对微孔木薯淀粉吸附次甲基兰的特性进行了研究，发现微孔淀粉对次甲基兰的吸附遵循Langmuir吸附等温方程式，其吸附速率方程为：A=0．4228t^0.243。淀粉颗粒表面ξ电位决定其对次甲基兰的饱和吸附量，ξ负电位增大，吸附量增加；当ξ正电位增大，则吸附量下降。     

Water adsorption equilibria on microporous carbons correlated using a modification to the Sircar isotherm

Water vapor adsorption equilibria are measured for several unimipregnated and impregnated microporous carbons, including BPL, ASC, ASC-TEDA and a polymeric beaded carbon. Both the shape of the isotherms and the presence of hysteresis are characteristics of the capillary condensation mechanism. Sircar's isotherm expression, which incorporates capillary condensation with the Kelvin equation, has been modified to correlate the data for both adsorption and desorption. Two gamma distribution functions are required to represent the pore size distribution for the activated carbons while only one gamma distribution function is required to represent the pore size distribution for the polymeric beaded carbon. The pore distributions that result from correlating the isotherm data are consistent with the known structure of these adsorbents.     

Improved phenol adsorption from aqueous solution using electrically conducting adsorbents

The electrically conducting and partially porous graphite based adsorbent (called NyexTM 2000) was tested for its adsorption capacity and electrochemical regeneration ability for the removal of phenol from aqueous solution. Nyex? 2000 was tested in comparison with Nyex? 1000, which is currently being used for a number of industrial waste water treatment applications. Nyex? 1000 exhibited small adsorption capacity of 0.1 mg g^?1 for phenol because of having small specific surface area of 1 m² g^?1. In contrast, Nyex? 2000 with specific surface area of 17 m² g^?1 delivered an adsorption capacity of 0.8 mg g^?1, which was eight-fold higher than that of Nyex? 1000. Nyex? 2000 was successfully electrochemically regenerated by passing a current of 0.5 A, charge passed of 31 C g^?1 for a treatment time of 45 minutes. These electrochemical parameters were comparable to Nyex? 1000 for which a current of 0.5 A, charge passed of 5 C g^?1 for a treatment time of 20 minutes were applied for complete oxidation of adsorbed phenol. The comparatively high charge density was found to be required for Nyex? 2000, which is justified with its higher adsorption capacity. The FTIR results validated the mineralization of adsorbed phenol into CO₂ and H₂O except the formation of few by-products, which were in traces when compared with the concentration of phenol removed from aqueous solution. The electrical energy as required for electrochemical oxidation of phenol adsorbed onto Nyex? 1000 & 2000 was found to be 214 and 196 J mg^?1, respectively. The comparatively low energy requirement for electrochemical oxidation using Nyex? 2000 is consistent with its higher bed electrical conductivity, which is twice that of Nyex? 1000.     

Affinity adsorption mechanism studies of adsorbents for oligopeptides using model polymer

To understand the principle of designing efficient adsorbents toward specific peptides, adsorption mechanism between oligopeptide VVRGCTWW (VW-8) and alkylamine modified polyacrylamide adsorbents was studied in this article. Linear models of the adsorbents were used to facilitate various measurements except for adsorption experiment. 2D ¹H NMR experiments reveal the main binding site of VW-8 to the adsorbents is C-terminal Trp. When peptide CTWW (CW-4) was introduced as the reference, the same binding site was found. However, in adsorption experiments, a remarkable difference was observed with the adsorption capacity of the two peptides. According to isothermal titration calorimetry (ITC) and surface plasmon resonance (SPR) experiments, which are in good agreement with the adsorption results, VW-8 has much higher affinity to the adsorbents than CW-4 and the main driving force in adsorption process is considered to be electrostatic force. However, the unusual affinity adsorption of two oligopeptides on the adsorbents contrasted to the traditional view of electrostatic interaction. With a comprehensive understanding of above experimental results along with computer-aided analysis, we conclude that the difference in conformation of the two peptides has a great impact on the interaction mode and further affects the affinity between the peptides and the adsorbents. This provides valuable information for the further design of synthetic affinity ligand for VW-8.     

Selection of microporous adsorbents for the clean-up of manufacturing waste water

Conclusions -- A procedure has been developed for selecting microporous sorbents for the clean-up of waste water, which takes into account overall the diverse technological characteristics of adsorbers.-- The procedure has been checked out as applicable to selecting carbon sorbents for cleaning up caprolactam-containing waste water in the manufacture of polyamide fibres.Translated from Khimicheskie Volokna, No. 5, pp. 24–27, September–October, 1990.     

Sorption of polar substances on active carbons

The mechanism of polar substances sorption on active carbons have been discussed using some aliphatic alcohols as an example. An effort has been undertaken to determine the role of both the specic adsorbate-adsorbent sorption and the adsorbate-adsorbate association in the total sorption of studied alcohols. It has been stated that, at small filling degrees, there is no consistence with the theory of volume filling of micropores because of the existence of both the association and the specific adsorbate-adsorbent interactions. An analysis of changes in fundamental thermodynamic functions has shown that, at great filling degrees, the fundamental assumptions of Dubinin's theory can be realized.     

Pre-treatment of adsorbents for waste water treatment using adsorption coupled-with electrochemical regeneration

With the aim to address waste water treatment problems, a novel and economic water treatment technology was introduced at the University of Manchester. It comprised of a unique combination of adsorption and electrochemical regeneration in a single unit. This process successfully eliminated a number of organic pollutants by using an electrically conducting adsorbent material called Nyex? which was a modified form of synthetic graphite. To expand the scope of other graphite types in waste water treatment applications, natural vein and recycled vein graphite materials were selected for electrochemical surface treatment (pre-treatment) in order to evaluate their adsorptive and electrical properties. New graphite based adsorbents were developed and characterized using a laser diffraction particle size analyser, BET surface area, SEM analysis, X-ray (EDS) elemental analysis, X-ray powder diffraction, Boehm surface titration, Zeta potential electrical bed conductivity and bulk density measurements. Boehm surface titration and EDS (X-ray) elemental analysis showed a significant increase in oxygen containing surface functional groups. Although, no significant improvement in bed electrical conductivity was found to occur after electrochemical surface treatment, however, natural vein and recycled vein graphite materials presented highest bed electrical conductivity amongst competing graphite materials. Aqueous solution of acid violet 17 as a standard pollutant was used to evaluate the comparative performance of these adsorbents. The investigations revealed that electrochemical surface treatment contributed to an increase in the adsorption capacity by a factor of two only for natural vein graphite. Un-treated recycled vein graphite adsorbent delivered the same adsorptive capacity (3.0 mg g^?1) to that of electrochemically treated natural vein graphite. The electrochemical regeneration efficiency at around 100% was obtained using a treatment time of 60 and 30 min, current density of 14 mA cm^?2, charge passed of 36 and 18 C g^?1 for synthetic graphite, natural and recycled vein graphite materials respectively. Relatively a small consumption of electrical energy, 24 J g^?1 for regenerating natural vein graphite adsorbent versus 36 J g^?1 for synthetic graphite adsorbent, was found to be required for destruction/oxidation of adsorbed acid violet 17. Multiple adsorption/regeneration cycles presented no loss in adsorptive capacity over 5 adsorption/regeneration cycles. The use of natural and recycled vein graphite adsorbents offered some advantages over graphite intercalation based adsorbents with reduced electrical energy consumption during regeneration and simpler separation of particulate adsorbent.     

Surface potential theory of multilayer adsorption from gas mixtures

Methods of calculating equilibria for adsorption of gas mixtures are compared. It is shown that methods proposed earlier by Myers and Prausnitz [14], by Cook and Basmadjian [16], by Lewis [11] and by Arnold [3] are based upon the assumption of an ideal adsorbed solution but differ in their choice of standard states. All of these methods fail for multilayer adsorption because the standard state is undefined.A new surface-potential theory of multilayer adsorption is proposed; it is based upon the formation of an ideal adsorbed solution at high surface coverage where the vapor is close to condensation. The new theory agrees with published experimental data for adsorption isotherms of Types I and II.