Removal of hydrogen sulfide from biogas on sludge-derived adsorbents

Desulfurization adsorbents were prepared from the mixtures of sewage sludge and metal sludge of various compositions and individual sludges by pyrolyses at 650, 800 and 950 °C. The resulting materials were used as adsorbents of hydrogen sulfide from simulated digester gas mixture. The adsorbents before and after H₂S removal were characterized using adsorption of nitrogen, elemental analysis, pH measurements, and thermal analysis. The behavior of materials as desulfurization media does not depend strongly on the humidification pretreatment. The pyrolysis temperature and composition of the mixture play a role in the development of final properties of adsorbents. When the content of sewage sludge is high the strong synergetic effect is noticed after high temperature of pyrolysis. Such factors as development of mesoporosity and new catalytic phases formed as a result of solid-state reactions contribute to this behavior. The removal of hydrogen sulfide on the materials obtained is complex due to the competition between H₂S and CO₂ for adsorption centers and deactivation of those centers by CO₂/H₂CO₃.     

PBI-based polymer electrolyte membranes fuel cells: Temperature effects on cell performance and catalyst stability

In this work, it has been shown that the temperature (ranging from 100 to 175 °C) greatly influences the performance of H₃PO₄-doped polybenzimidazole-based high-temperature polymer electrolyte membrane fuel cells by several and complex processes. The temperature, by itself, increases H₃PO₄-doped PBI conductivity and enhances the electrodic reactions as it rises. Nevertheless, high temperatures reduce the level of hydration of the membrane, above 130-140 °C accelerate the self-dehydration of H₃PO₄, and they may boost the process of catalyst particle agglomeration that takes place in strongly acidic H₃PO₄ medium (as checked by multi-cycling sweep voltammetry), reducing the overall electrochemical active surface. The first process seems to have a rapid response to changes in the temperature and controls the cell performance immediately after them. The second process seems to develop slower, and influences the cell performance in the “long-term”. The predominant processes, at each moment and temperature, determine the effect of the temperature on the cell performance, as potentiostatic curves display. “Long-term” polarization curves grow up to 150 °C and decrease at 175 °C. “Short-term” ones continuously increase as the temperature does after “conditioning” the cell at 125 °C. On the contrary, when compared the polarization curves at 175 °C a continuous decrease is observed with the “conditioning” temperature. A discussion of the observed trends is proposed in this work.     

Hydrophobisation of active carbon surface and effect on the adsorption of water

A technique of surface hydrophobisation has been applied to two microporous carbonaceous adsorbents. A granular active carbon and an activated carbon fibre, both formerly chemically treated in order they preferentially present hydroxyl surface functions, were modified by action of vinyltrimethoxysilane (vtmos) in liquid phase. The resulting samples were characterised using sorption of nitrogen, FTIR, XPS and ²⁹Si MAS-NMR spectroscopy, and elemental analysis. Their stability and heat treatment have also been investigated through thermal analysis.The efficiency of the hydrophobisation treatment was evaluated by static adsorption of water vapour and vapours of chlorinated volatile organic compounds (VOCs): dichloromethane and trichloroethylene. Grafting of the vtmos and development of a “coating” of polysiloxane onto the adsorbent induced a modification of the carbon surface but also a partial filling of the porosity. These modifications accounted for a decrease of both the amounts of water and VOC adsorbed by the hydrophobised materials. However, water uptakes were found to be much lower than those of the VOCs, evidencing an enhanced selectivity of the hydrophobised adsorbents toward VOCs.     

New applications for carbonaceous adsorbents

The most important adsorption properties of carbonaceous adsorbents are based on the microporous structure which can be determined by the mechanism of pore filling at equilibrium and by the diffusion behaviour of adsorbent molecules from the external surface into the grain. New processes of application of granular activated carbon in the gaseous phase besides the solvent recovery have been developed for the adsorption of sulfur containing and radioactive gases and in the aqueous phase for the purification of drinking and wastewater. New carbon molecular sieves are the base for gas separation processes, e.g. O₂- or N₂-recovery from air or H₂-recovery from H₂ containing gases.     

Optimum structured adsorbents for gas separation processes

Recent developments in separation technology by adsorption have included the development of new structured adsorbents which offer some attractive characteristics compared to a typical packed bed. These improved features include lower energy consumption, higher throughput and superior recovery and purity of product. However, the exact combination of structural, geometric parameters which yields optimum performance is unknown. This study formulates a methodology for comparison based on a variety of analytical and numerical models and uses it to examine the performance of different adsorbent configurations. In particular, monolithic, laminate and foam structures are evaluated and compared to a packed bed of pellets. The effects of physical adsorbent parameters which govern the performance of a PSA process are considered during model development. Comparisons are carried out based on mass transfer kinetics, adsorbent loading and pressure drop of a PSA system for CO₂/N₂ separation. The results indicated that structured adsorbents can provide superior throughput to packed beds provided their geometrical parameters exceed certain values. For example, laminate structures can offer superior performance to a packed bed of pellets only if the critical sheet thickness and spacing are less than about 0.2 mm. Each adsorbent structure should be designed to operate at its “optimal” velocity. When operating at velocities higher than the “optimal” value, the increase in pressure drop and length of the mass transfer zone more than offsets gains accrued through reduction in cycle time.     

Reactive adsorption of thiophene on Ni/ZnO: Role of hydrogen pretreatment and nature of the rate determining step

Reactive adsorption of thiophene on reduced and unreduced NiO/ZnO adsorbents was studied by thermal gravimetric analysis and by sulfidation in a fixed bed reactor at 330–375 °C and 10–40 mbar of thiophene in hydrogen. The adsorbents (12 wt% Ni) were prepared by co-precipitation of corresponding nitrates with sodium carbonate followed by calcination at 400 °C. We have found that such solids can react with thiophene without any prior reduction. Metallic Ni, indispensable for thiophene decomposition, is formed in this case in situ upon the contact with thiophene/H₂ reaction mixture. The reduction of NiO/ZnO in H₂ (360 °C, 6 h) results in the formation of Ni–Zn alloyed particles (as attested by XRD data) and leads to a decrease of the sulfidation rate in comparison with the unreduced sample. Concerning the mechanism of the reaction, we found that H₂S is absent in the gas phase during sulfidation in a fixed bed reactor for both reduced and unreduced solids, showing that all produced H₂S is rapidly absorbed by ZnO. This observation points out that catalytic thiophene decomposition on Ni is the rate determining process under used conditions. Existence of two stages in the reactive adsorption, characterized by different rates and activation energies, is explained by the change of the rate determining step of thiophene decomposition from desulfurization reaction itself in the first stage to thiophene transport in the second one. Based on these findings we attributed the lower reaction rate of the reduced sample to a lower activity and/or accessibility of metallic Ni in such adsorbents.     

Development of multiphase bioceramics from a filler-containing preceramic polymer

Multiphase bioceramics based on wollastonite and wollastonite/hydroxylapatite (W/HAp) have been successfully prepared by the heat treatment of a filler-containing preceramic polymer. CaO-bearing precursors (Ca-carbonate, Ca-acetate, and CaO nano-particles) were dispersed in a solution of silicone resin, subsequently dried and pyrolysed in nitrogen. The reaction between silica, deriving from the oxycarbide (SiOC) residue of the silicone resin, and CaO “active filler” led to the formation of several calcium silicates, mainly consisting of wollastonite (CaSiO₃), in both low and high temperature forms. The phase assemblage of the final ceramic varied with the pyrolysis temperature (varying from 1000 to 1200 °C). HAp was additionally inserted, as “passive filler” (i.e. not reacting with SiOC), for the preparation of bioceramics based on W/HAp mixtures.     

Impedance spectroscopy of oxidized polyaniline and poly(o-ethoxyaniline) thin film modified Pt electrodes

Impedance spectra of potentiodynamically formed polyaniline (PANI) and poly(o-ethoxyaniline) (POEA), thin film modified Pt electrodes measured at potentials of conductive, emeraldine form in H₂SO₄ electrolyte solution were reported. Specific features of spectra obtained for films of different voltammetric charge densities were modelled and interpreted according to “homogeneous” and “distributed” impedance models. The values of charging capacitances of different PANI and POEA films were found independent of the model used. Charging of polymer films was in terms of both models controlled by charge transport processes being ion diffusion and migration in the first case, and polymer film and ionic conductivities in the second case. Distortion from Warburg-like response observed for both polymer films was explained by either differences in relaxation times of diffusion and migration processes, or by impedance of ionic channels with “anomalous” frequency dispersion. There were some discrepancies between theories and results suggesting that “homogeneous” model was more appropriate for POEA films while “distributed” model was more appropriate for PANI films, at least if both films were formed and measured in presently described conditions.     

Characterisation of sewage sludge-derived adsorbents for H2S removal. Part 2: Surface and pore structural evolution in chemical activation

This paper presents results of chemical activation of sewage sludge, a waste material generated in sewage treatment processes, to produce an adsorbent for H₂S removal. Dewatered sewage sludge samples were subjected to chemical treatment by sulfuric acid and zinc chloride at various molar concentrations and were then pyrolysed in inert gas atmosphere at various temperatures for different hold times. Resulting adsorbents were characterised in terms of BET surface area, micropore area and pore size/volume distributions. In this study, it was shown that pyrolysis temperature and activation chemicals used significantly affect the surface area development and pore structure evolution. Solution molar concentration of the activating agent is a particularly important factor. H₂S adsorption tests were carried out on the derived adsorbents using a thermogravimetric analyser. Experimental results demonstrate that sewage sludge, a waste material in abundant supply at virtually no cost, is a viable source of activated adsorbents. Its potential use for odour control is reinforced by the need to find environmentally safe disposal alternatives for sewage sludge. From both economics and environmental perspectives, these experimental results warrant further efforts, perhaps in terms of large scale manufacturing and testing.     

Application of two phase model to linear dynamic rheology of filled polymer melts

The linear dynamic rheology for a series of filled polymer melts is investigated to take account for the respective contributions of the bulky polymer phase away from the filler inclusions and the “filler phase” composed of filler particles coated with polymer layer. Time-concentration superposition principles are introduced to the linear viscoelasticities of both the bulky polymer phase and the “filler phase”. The result highlights the importance of the polymer dynamics to both the “filler phase” and the composite melt. Polymer mediated filler jamming towards glass formation is revealed for the “filler phase”, which accounts for origin of the fluid-to-solid transition upon filling.     

Removal of Lead Ions from Aqueous Solutions by Different Types of Industrial Waste Materials: Equilibrium and Kinetic Studies

Abstract

A comparative study of the adsorbents prepared from several industrial wastes for the removal of Pb²⁺ has been carried out. Fertilizer industry waste viz. carbon slurry and steel plant wastes viz. blast furnace (B.F.) slag, dust, and sludge were investigated as low‐cost adsorbents after proper treatment in the present study. The adsorption of Pb²⁺ on different adsorbents has been found in the order: B.F. sludge>B.F. dust>B.F. slag>carbonaceous adsorbent. The least adsorption of Pb²⁺ on carbonaceous adsorbent even having high porosity and consequently greater surface area as compared to other three adsorbents, indicates that surface area and porosity are not important factors for Pb²⁺ removal from aqueous solutions. The adsorption of Pb²⁺ has been studied as a function of contact time, concentration, and temperature. The adsorption has been found to be exothermic, and the data conform to the Langmuir equation. The kinetic results reveal that the present adsorption system follows Lagergren's first order rate equation. Since all three waste products from the steel industry show higher potential to remove lead from water, therefore, it is suggested that these metallurgical wastes can be fruitfully employed as low‐cost adsorbents for effluent treatment containing toxic metal ions.     

Adsorption of H2, CO2, CH4, CO,N2 and H2O in Activated Carbon and Zeolite for Hydrogen Production

Abstract

The design of a layered pressure swing adsorption unit to treat a specified off-gas stream is based on the properties of the adsorbent materials. In this work we provide adsorption equilibrium and kinetics of the pure gases in a SMR off-gas: H₂O, CO₂, CH₄, CO, N₂, and H₂ on two different adsorbents: activated carbon and zeolite. Data were measured gravimetrically at 303–343 K and 0–7 bar. Water adsorption was only measured in the activated carbon at 303 K and kinetics was evaluated by measuring a breakthrough curve with high relative humidity.     

The adsorption of sulphate, hydrogenchromate and dihydrogenphosphate anions on surfactant-modified clinoptilolite

The adsorption of sulphate, hydrogenchromate and dihydrogenphosphate anions on surfactant-modified clinoptilolite (SMC) was investigated. The SMCs were prepared by the adsorption of cis-1-aminoctadecen-9 (oleylamine) on both modified and unmodified natural clinoptilolite tuff. The properties of the modified clinoptilolite samples, such as cation type, structure of the zeolite framework and ECEC value, determined the mechanism of oleylamine adsorption, and consequently anion adsorption on the external clinoptilolite surface. According to the strength of the anion adsorption, two groups of SMCs could be distinguished: strong and weak anion adsorbents. Strong anion adsorbents were obtained by oleylamine adsorption on H⁺-clinoptilolites by protonation of the –NH₂ groups. This mechanism of oleylamine adsorption resulted in the surface precipitation mechanism of anion adsorption being the dominant mechanism. The oleylamine derivatives of Ca- and Na-clinoptilolite were weak anion adsorbents. Oleylamine is adsorbed on Ca- and Na-clinoptilolite by hydrogen bonding, thus yielding insufficient adsorption sites for anions. Hydrogenchromate and dihydrogenphosphate anions were nevertheless adsorbed on these SMCs by interaction with oleylamine. The experiments of anion adsorption on various oleylamine loaded SMCs confirmed the existence of two types of anion adsorption sites and showed that excess oleylamine did not significantly influence the anion adsorption in the investigated concentration range. The kinetic results showed that SO₄²⁻ and H₂PO₄⁻ adsorptions were slow processes while HCrO₄⁻ adsorption was completed in a few minutes.     

Sorption of alkanes on sulfated zirconias—Modeling of TAP response curves

The transient TAP method was applied to investigate sorption phenomena of alkanes on differently prepared sulfated zirconias to provide further explanation of different catalytic performances in the n-butane isomerization. A modified “three-zone” model was developed to describe the experimental results of a single-pulse response experiment. It allowed the extraction of the ratios of adsorption and desorption rate constants. The introduction of a second sorption center in the model is proposed to reproduce the sorption behavior for larger alkanes quantitatively better, especially in the low-temperature region. Moreover, the new model is able to reflect the heterogeneous distribution of active centers, e.g., different acidic sites in combination with surface sulfate species. Temperature dependent measurements have been performed to calculate heats of adsorption for various alkanes. The impact of these values on the catalytic properties is discussed.     

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.     

Characteristics of pellet-type adsorbents prepared from water treatment sludge and their effect on trimethylamine removal

We optimized the preparation method of pellet-type adsorbents based on alum sludge with the aim of developing a high-performance material for the adsorption of gaseous trimethylamine. Effects of calcination temperature on physical and chemical properties of pellet-type adsorbents were investigated. The porous structure and surface characteristics of the adsorbents were studied using N₂ adsorption and desorption isotherms, scanning electron microscope, X-ray diffraction, temperature-programmed desorption of ammonia, and infrared spectroscopy of adsorbed pyridine. The adsorbents obtained from the water treatment sludge are microporous materials with well-developed mesoporosity. The pellet-type adsorbent calcined at 500 °C had the highest percentage of micropore volume and the smallest average pore diameter. The highest adsorption capacity in trimethylamine removal attained over the pellet-type adsorbent calcined at 500 °C can be attributed to the highest number of acid sites as well as the well-developed microporosity.     

The functionalities of Pt/γ-Al2O3 catalysts in simultaneous HDS and HDA reactions

A Pt/γ-Al₂O₃ catalyst was tested in simultaneous hydrodesulfurization (HDS) of dibenzothiophene and hydrodearomatization (HDA) of naphthalene reactions. Samples of it were subjected to different pretreatments: reduction, reduction–sulfidation, sulfidation with pure H₂S and non-activation. The reduced catalyst presented the best performance, even comparable to that of Co(Ni)Mo catalysts. All catalyst samples were selective to the HDS reaction over HDA, and to the direct desulfurization pathway of dibenzothiophene HDS over the hydrogenation reaction pathway of HDS. The effect of H₂S partial pressure on the functionalities of the reduced Pt/γ-Al₂O₃ catalyst was studied. The results showed that an increase in H₂S partial pressure does not cause poisoning, but an inhibition effect, without changing the catalyst selectivity. Accordingly, the activity trends were ascribed to adsorption differences between the different reactive molecules over the same catalytic active site. TPR characterization along with a thermodynamics analysis showed that the active phase of reduced Pt/γ-Al₂O₃ is constituted by Pt⁰ particles. However, presulfidation of the catalyst leads to a mixture of PtS and Pt⁰ which has a negative effect on the catalytic performance without changing catalyst functionalities.     

Adsorption of styrene sulfonate vs. polystyrene sulfonate on layered double hydroxides

The adsorption properties of a water-soluble polymer, polystyrene sulfonate (PSS), on the surface of a layered double hydroxide (LDH), Zn₂Al(OH)₆Cl·nH₂O, an anionic clay of the hydrotalcite type, were investigated and compared to the adsorption of the monomer, styrene sulfonate (vinylbenzene sulfonate (VBS)). PSS adsorption can be described by the Freundlich model and is of the S-type, while the adsorption of VBS is of the Langmuir type. Solid-state ¹H and ¹³C CP MAS NMR experiments confirm the strong interaction between the adsorbed monomer and the LDH inorganic framework. The shielding of the resonance peak of the aromatic carbon attached to the sulfonate group is transmitted through the carbonaceous skeleton to the vinylic carbon, which is more deshielded. This phenomenon is induced by the electrostatic binding between the molecule and the clay surface. This situation is observed to a lesser extent for the polymer. The adsorption of polymer changes the pore size distribution, creating a larger macropore volume compared to the pristine chloride material and to the adsorbed monomer phase. This is explained by the entanglement of the polymer, pushing apart the stacked units of the LDH.     

Gas adsorption on activated carbons from PET mixtures with a metal salt

Activated carbons were prepared from carbonized PET by steam activation via pretreatment by mixing PET with a metal salt [Ca(NO₃)₂·4H₂O, Ca(OH)₂, CaCO₃, ZnO, and AlNH₄(SO₄)₂·12H₂O], and with acid treatment after carbonization. The porous properties of the activated carbons were determined by the nitrogen adsorption method. The adsorption isotherms of CO₂, C₂H₆, nC₄H₁₀ and iC₄H₁₀ at 298 K on the prepared activated carbons were measured to determine practical applications and to obtain a better understanding of the porous structure of the prepared carbons. Steam-activated carbons via pretreatment have a larger mesoporosity than carbons with no pretreatment. The metal salt used in the pretreatment for steam activation has no influence on the microporous structure, but it does influence the mesoporous structure of the prepared carbons. Activated carbons prepared via pretreatment show a large adsorption capacity for nC₄H₁₀ and iC₄H₁₀. These carbons are suitable as adsorbents for canisters, etc. Application of the potential theory to adsorption data for the prepared carbons suggests that the pretreatment contributes to the formation of pores larger than 0.50 nm at high burnoff.