A practical review of the performance of organic and inorganic adsorbents for the treatment of contaminated waters

A wide range of materials known for their adsorption properties have the potential for being used for the removal of trace substances from drinking and wastewaters. These include physically and chemically activated carbons, surface modified carbons, non‐porous resins with ion exchange capacities, inorganic microporous solids like zeolites and clays, and mixed organic–inorganic materials like bone chars. The adsorption capacity exhibited by each material relates primarily to its textural and chemical properties. Other factors, however, such as apparent density, regeneration potential and cost, need to be taken into consideration when selecting one adsorbent over another. A comparative investigation of 18 solids and their capacity to remove organics and metals from natural waters and solutions reconstituted to simulate the conditions in natural waters is presented. The experiments were carried out using batch and small‐scale column adsorption tests. In general, zeolites and ion exchange resins exhibited limited capacities to remove organic matter from solution but were highly effective with metallic species like manganese and aluminium. Activated carbons adsorbed organic matter very efficiently, with results showing a correlation between adsorption capacity and surface area (up to 1791 m² g^?1). Metal removal was highly variable and was enhanced in activated carbons subjected to acid washing. Owing to its mixed organic/inorganic nature, and despite its poorly developed micropore structure, bone char exhibited a strong adsorption capacity for both organic and metal species. The high apparent density of this material (0.763 g cm^?3) meant that its performance was greatly improved when tests were conducted on the basis of volume, matching and surpassing the performance of the best carbons. The possibility of using mixtures of complementary adsorbents for the removal of organic and inorganic species from solution was also successfully evaluated in this work. Copyright © 2006 Society of Chemical Industry     

Effects of compression on the textural properties of porous solids

This work deals with the analysis of the effects of compression in the textural properties, the crystallinity and the packing density of porous solids. It has been found that compression produces the decrease of both, the pore volume and the interparticle voids of porous solids. The reduction of the pore volume depends on the mechanical strength of the material. Activated carbons and inorganic porous oxides with a high mechanical strength, show a relatively low reduction of their pore volume, while porous solids in which the pore walls are constituted by organic frameworks, like MOF-5, have a lower mechanical strength and thus, their porous texture is largely affected by compression. In general terms, an increasing compressive pressure produces the removal of pores in the following sequence: mesopores, broad micropores and narrow micropores. Thus, compression of porous materials could be considered as a procedure to tailor the pore volume and the pore size distribution of porous solids. The results obtained in this work allow affirming that, in the preparation of adsorbent materials, together with the development of a high specific pore volume or a specific surface area, the behaviour upon compactatation must, undoubtedly, be taken into account.     

Sorption Properties of Polyacrylonitrile Fibres

As-spun undrawn PAN fibre wet-spun from solutions of the polymer in DMF, DMA, and DMSO are porous with a developed specific surface (up to 80 m²/g for nitrogen), while fibres spun from aqueous solutions of sodium thiocyanate or zinc chloride are nonporous with a specific surface of under 1 m²/g. The finished PAN fibres spun from solutions in different solvents have a closely packed structure, equivalent to highly crystalline, according to the data from sorption of inert gases and water vapors, and according to the results on the kinetics of sorption of vapors of liquids with different molecule sizes, they can be considered ultramicroporous.     

Synthesis of macroporous ZrO2–Al2O3 mixed oxides with mesoporous walls, using polystyrene spheres as template

Macroporous ZrO₂–Al₂O₃ mixed oxides with mesoporous walls were synthesized. The three-dimensional interconnected macroporous structures, of inorganic zirconia–alumina mixed oxides containing different alumina compositions (25, 50, 100 wt%), were prepared by sol–gel method from inorganic precursors and using polystyrene microspheres with diameters of 685 and 1520 nm as templates. The final porous arrays with controllable pore size in the submicrometer range could be obtained by calcination of the organic template. The structural characteristics are discussed. The physicochemical characterization of the samples was carried out by N₂ physisorption (S_BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The shrinkage of pore diameter was approximately 35%, and the wall thickness of inorganic framework varied between 135 and 154 nm. The specific surface areas, of the samples, were between 123 and 287 m²/g, obtaining the largest surface area with the highest alumina contents and the smallest templates.     

The Significance of Surface Charge and Structure on the Accessibility of Cellulose Fibres

The electrokinetic properties of macroscopic solids can describe both the surface modifications as well as the interaction abilities during processing. Normally the zeta potential (ζ) is measured and can be used to develop, optimise and control the manufacturing process. The zeta potential of natural and regenerated cellulose fibres changes noticeably during the pre‐treatment and finishing process and it is important for the quality of the final product (whiteness, colour, hydrophilicity). The pre‐treatment of cellulose fibres improves the accessibility of dissociable groups. Further improvement of hydrophilicity (mercerisation) causes a pronounced increase in the surface charge density and a reduction in the degree of crystallinity and structural modifications. The progress of the chemical as well as the enzymatic treatment correlates with ζ. These data correlate very well with the results obtained using tensiometry, water retention, and water vapour and iodine adsorption. The changes in crystallinity index and the accessibility for low molecular components are well described by the latter methods.     

Mechanism for the anti-redeposition action of sodium carboxy-methyl cellulose with cotton. I. Radiotracer studies

The adsorption of a ¹⁴C-labelled sodium carboxy-methyl cellulose (SCMC) on to textile fibres and soil particles has been investigated. Attempts have been made to relate the amounts of SCMC adsorbed to its anti-redeposition action. The results show that SCMC is only effective as an anti-redeposition agent with fabrics on to which it readily adsorbs from detergent solutions.     

Removal of reactive dyes using organofunctionalized mesoporous silicas

Mesoporous inorganic–organic hybrids with mono- or bifunctional pendant chains anchored onto the silica backbones were investigated for reactive dye removal. The organofunctionalized silicas obtained from silylating agents contain the functional groups: 3-aminopropyl, octadecyldimethyl(silylpropyl)ammonium and 3-mercaptopropyl. The pendant electrophilic chains attached on the new porous silicas interact with the negative charge of industrial textile reactive dyes: yellow GR, red RB and blue RN. The best maximum sorption results were obtained for both mercapto and amino groups, to give 351, 388 and 203 mg g^?1 for GR, RN and RB dyes. These silicas also have the advantage of undergoing sorption without initial pH adjustment and surface saturation in short times. The high sorption capacity is due to the combination of properties associated with higher pore volume, surface area, smaller particle and the presence of two different functional groups, also emphasizing the presence of one mercapto group in the hybrid. These silicas, mainly the structurally bifunctional arrangement, with available disposition of pendant chains, are promising for textile effluent removal.     

Ultralight,high-surface-area,multifunctional graphene-based aerogels from self-assembly of graphene oxide and resol

The self-assembly between graphene oxide sheets and resol-type phenolic prepolymers was investigated as a method to form three-dimensional porous carbon objects with high surface areas and low densities. After freeze-drying and subsequent pyrolysis of the assembled hydrogels, ultralight graphene/carbon composite aerogels with high surface areas and porosity, good conductivity, and well-defined bulk shape were obtained. By adjusting the amount of graphene oxide and resol in the precursor mixture, aerogels with a density as low as 3.2 mg/cm³ or a surface area as high as 1019 m²/g could be prepared. It is proposed that resol molecules are first adsorbed on the surface of graphene oxide sheets, and then the surface-coated sheets are crosslinked by the polymerization of resol prepolymers. The absorption performance was evaluated for the aerogel with the lowest density. Due to the high porosity, the aerogel displayed fast absorption rates for organic solvents as well as high absorption efficiencies. The high conductivity of the aerogels permits good performance as binderless monolithic electrodes for supercapacitors.     

Synthesis,characterization and activity in cyclohexene epoxidation of mesoporous TiO2–SiO2 mixed oxides

A series of porous TiO₂–SiO₂ mixed metal oxides were synthesized via sol–gel route using n-alkyl amine as pore directing templates. The mixed oxides were characterized by nitrogen physisorption for surface area, pore volume and pore size distribution, as well as by FTIR, TGA, SEM, and TEM for the dispersion of titanium. These compounds are good catalysts for liquid phase epoxidation of cyclohexene using hydrogen peroxide as the oxidant. The effects of various factors on catalyst reactivity were also studied. The catalysts performance and selectivity are enhanced in protic solvents when compared to aprotic solvents.     

Hydration of magnesium oxides in the presence of water vapour

The uptake of water vapour on to magnesium oxide has been studied using a silica spring technique. In the experiment reported here, porous magnesium oxides were investigated, and the amount adsorbed at equilibrium at various vapour pressures of water was determined. The magnesium oxides used were characterised using low temperature nitrogen adsorption to give a pore size distribution and a specific surface area, and chemical analysis from X-ray fluorescence data. The nitrogen adsorption isotherms obtained were a mixture of type II and type IV using the BET classification. The oxides sorbed water vapour to give an equilibrium isotherm which was sigmoid in character. Rehydration of the samples after desorption and degassing at 298 K produced samples with an enhanced capacity for water vapour sorption. This is associated with a change in the surface of the oxide from a hydrophobic to a hydrophilic character upon the second adsorption cycle. This is of importance in assessing the water sorption capacity of industrial produced porous magnesium oxides and their subsequent use as desiccants.     

Aqueous synthesis of high surface area metal oxides

By applying high throughput synthesis and characterization technologies, we have been optimizing common dry or aqueous synthetic routes for the preparation of high surface area metals and oxides, such as precipitation and modified Pechini methods. For wet combustion synthesis, we have been screening a variety of organic acids as dispersants and developed proprietary recipes for individual metals. By resorting to easily decomposable organic acids (as opposed to citric acid in the original Pechini combustion method), such as glyoxylic acid, oxalacetic acid and ketoglutaric acid, it is possible to obtain high surface area materials for many metals after careful optimization of acid/metal ratio and calcination conditions. Examples are Sn, In, Co, Ru, Ni, Fe, Mn, Y, Ce and Rare Earth oxides and their mixtures. After calcination in the temperature range of about 300–400 °C, surface areas >150 m²/g could be obtained for Er, Tm, Co, Ru, and Nb; >200 m²/g for Sn, Fe, Mn, and Y; >300 m²/g for Ce; and >400 m²/g for Ni oxide. Noteworthy are also >140 m²/g for La₂O₃, >80 m²/g for CuO, and 75 m²/g for ZnO. For V, around 40 m²/g was possible for the nearly carbon-free V₂O₅, whereas up to 90 m²/g was obtained for a 90% V–10% carbon composite (by incomplete combustion of the organic acid). Residual carbon helps in stabilizing the porous oxide against sintering. Thus, conventional aqueous routes (precipitation, Pechini) can be competitive to more elaborate and costly methods such as those using organic solvents, sol–gel, supercritical drying or template/hydrothermal synthesis. Combustion synthesis is well suited for the preparation of mixed oxides from mixed metal solutions in aqueous organic acids. Bulk porous Co and CoRu mixed oxides have been screened for liquid phase alcohol oxidations and CoRuCe oxides for CO oxidation and VOC destruction, and doped NiO has been reduced to the metal and tested for various hydrogenations.     

Determination of specific surface area of ferric oxide and alumina powders

Methods for measuring the surface areas of powdered solids are considered and the specific surface areas of ferric oxide and alumina powders using adsorption of stearic acid from benzene solution are determined. It has been found that the value of the specific surface area of both powders agree with the BET nitrogen gas adsorption area if it is considered that the area occupied per molecule of stearic acid on the powder surface is of the order of 60–70Ų. This finding shows that orientation of the stearic acid molecule on the solid/liquid interface takes place so that stearic acid molecules lie neither completely perpendicular nor completely parallel to the surface. Possible orientations of stearic acid chains on the surface are envisaged.     

The influence of different factors on the porous structure of the positive electrode in lead-acid batteries

Geometric structure measurements of positive electrodes have been determined by measuring cumulative pore volume, pore size distribution, porosity, pore specific surface area and BET specific surface area. The influence of technological parameters such as concentration of SO₄²⁻ ions in the paste, the temperature of the paste and the quality of oxides on the structure of porous positive electrode have been studied. The relation between BET specific surface area and the electrical capacity of the positive electrode is shown and interpreted.     

Note on stability of p-nitrophenol in aqueous solutions

It has been shown that p-nitrophenol (PNP) solution is stable only at pH ≥ 9 when PNP is in the completely ionised form. In benzene, PNP exists in a neutral molecular form. These considerations are important in the estimation of surface areas of solids by adsorption of PNP from solution.     

Modeling of Adsorptive Filtration of a Leather Dye in a Fixed Bed Column

Abstract

Activated carbons offer an efficient option for the removal of organic and inorganic contaminants from water. However, due to its high costs and difficulty in the regeneration, other low cost adsorbents have been used. In this work, the adsorption capacity of an adsorbent carbon with high iron oxides concentration was compared with that of a commercial activated carbon in the removal of a leather dye from an aqueous solution. The adsorbents were characterized using SEM/EDAX analysis and BET surface area. The capacity of adsorption of the adsorbents was evaluated through the static method at 25°C. The results showed that the color removal was due to the adsorption and precipitation of the dye on the surface of the solids. The adsorption equilibrium was described according to the linear model for the adsorbent carbon and the equilibrium constant was 0.02 L g^?1. The equilibrium of adsorption on activated carbon exhibited a behavior typical of the Langmuir isotherm and the monolayer coverage was 24.33 mg g^?1. A mathematical model was proposed to describe the dynamics of the color removal using a fixed bed considering that the color removal is due to the adsorption and the precipitation of the dye on the adsorbent.     

Pseudo-capacitance properties of porous metal oxide nanoplatelets derived from hydrotalcite-like compounds

Nanoplatelets of metal oxides with interesting porous structure were obtained by thermal treatment of Ni/Al hydrotalcite. Structural and surface properties of the porous oxides were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM and HRTEM), and N₂ adsorption–desorption. The electrochemical performance of the electrodes was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and constant current charge–discharge measurements. Ni/Al hydrotalcite calcined at 450 °C (NA-450) displayed a maximum specific capacitance (419.0 F g⁻¹) due to the porous structure with the highest specific surface area (142.3 m² g⁻¹) and small pore size (4.4 nm). The present study shows the potential of NiO nanoplatelets composite material for electrochemical pseudo-capacitors.     

Sol-gel preparation and thermo-mechanical properties of porous xAl2O3–ySiO2 coatings on SiC Hi-Nicalon fibres

Porous thin films of mixed aluminium silica oxides were elaborated by the sol-gel process from aluminium tri-sec-butoxide and tetraethylorthosilicate on Hi-Nicalon fibres. The porosity was generated by addition of a surfactant, namely cetyltrimethylammonium bromide (CTAB). SiC Hi-Nicalon fibres were coated by the dip-coating technique. After annealing in air (500–1200 °C) crack-free coatings were observed, with a thickness in the range 100–1000 nm. The fibres were tensile tested and results were analysed by the Weibull statistic. They showed good mechanical properties compared to the commercial fibres. The systems were characterized by thermal gravimetry, differential scanning calorimetry, X-ray diffraction, BET and scanning electron microscopy. The powders obtained in the same conditions as the coatings were highly porous with surface areas in the range 540–150 m²/g depending on the annealing temperature (400–1000 °C).     

偏氯乙烯共聚物/纳米水滑石复合材料及多孔炭的制备与表征

采用原位悬浮聚合和熔融加工制备了不同纳米水滑石含量的偏氯乙烯-丙烯酸甲酯(VDC-MA)共聚物/纳米水滑石复合材料,并通过高温炭化和模板消除得到多孔炭材料。采用电镜、X射线衍射、N₂吸脱附法表征了复合材料和多孔炭的结构。结果表明,纳米水滑石含量≤6.25%（质量）时,纳米水滑石基本以初级粒子均匀分散在VDC-MA共聚物基体中,并在炭化过程中转化为金属氧化物;金属氧化物可经酸洗去除,起到模板致孔作用;同时VDC-MA共聚物炭化过程形成大量微孔,因此得到的多孔炭具有微孔和中孔分布。当炭化温度较低时(600～700℃)相似文献   

Modification of the oxygen storage capacity of CeO2–ZrO2 mixed oxides after redox cycling aging

High surface area CeO₂–ZrO₂ mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H₂, TPO, magnetic susceptibility measurements to obtain the Ce³⁺ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides.

All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides.     

The Problem of Cyclic Oligomers in Dyeing and Processing Polyester and Ways of Solving It

Approaches to reducing the content of oligomers on the surface of PET fibres are substantiated. The first one is based on reducing surface sorption of oligomers on the fibre by rational selection of textile auxiliaries for dyeing polyester fabric and creating new preparations based on them; the second consists of chemical decyclization of the oligomers by use of catalysts of ammonolysis and hydrolysis in low concentrations. A method of eliminating cyclic oligomers and unfixed dye from the surface of the dyed textile material was developed based on use of aqueous solutions of ammonia in 0.04-0.06% concentration. Technology is created for giving polyester fibres stable antistatic properties, and this significantly reduces its resistivity from 2.8·10¹⁴ to 2.68·10⁹.