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Colloids and Surfaces A: Physicochemical and Engineering Aspects (v.265, #1-3)
Visualization of colloid transport through heterogeneous porous media using magnetic resonance imaging by Thomas Baumann; Charles J. Werth (pp. 2-10).
The effects of heterogeneous grain packing on colloid transport were evaluated in flow-through columns using magnetic resonance imaging (MRI). Two columns were packed, each with a core of fine-grained silica gel surrounded by a shell of coarse-grained silica gel. In column 1, 600–850μm silica gel was surrounded by 850–1000μm silica gel. In column 2, 250–600μm silica gel was surrounded by 850–1000μm silica gel. Both columns were continuously purged with water and colloids were introduced as pulses.MRI images of column 1 showed that colloid transport in the core and shell was not distinguishable. However, colloid transport was slightly faster along the bottom of the column. T1-weighted images showed that small variations in the packing density of silica gel caused this effect. MRI images of column 2 showed that colloid transport in the core was much slower than colloid transport in the shell. Colloid exchange between the shell and the core was also observed.Colloid transport velocities and collision efficiencies were calculated from the images. In agreement with the visualization, velocities for column 1 increased from the top to bottom of the column and velocities for column 2 were greater in the shell than in the core. Collision efficiencies were calculated, but trends were not apparent because of the difficulty of applying filtration theory to heterogeneous media. Velocities from images were compared to those from conventional experiments where colloid concentrations were measured at the column effluent. While often comparable, results from the latter mask many of the complexities that control the overall rate of colloid transport. Since these complexities can give rise to very different transport behavior, it is critical to understand their effects in real systems. Hence, MRI is a technique that has the power to elucidate many of the small-scale processes that affect the behavior of colloids in the field.
Keywords: Colloid transport; Magnetic resonance imaging; Attachment
Exploring organic compound interactions with organic matter: The thermodynamic cycle approach by Ellen R. Graber; Mikhail Borisover (pp. 11-22).
Several approaches for studying sorption of organic compounds by natural organic matter (OM) include correlations of compound OM-water distribution coefficients ( KOM) with such properties as n-octanol–water distribution coefficients ( KOW), compound (liquid) aqueous solubilities ( SAq), or multiple parameter linear free energy relationships (LFERs). However, such KOM-based comparisons of sorbate–OM interactions for different organic compounds may be obscured by compound solvation interactions in the bulk aqueous phase. To avoid this complication, we employ two alternative means for simultaneously eliminating the contribution from solute hydration in the aqueous phase, and for providing a single inert reference standard state. Both means utilize a thermodynamic cycle for replacing the aqueous reference state with an inert reference state (either gas phase or inert solvent), while maintaining the same final hydrated sorbed state. In this review, we describe both approaches and demonstrate their utility for elucidating sorption mechanisms, as well as discuss the rationale for choosing one approach (the gas phase reference state) or the other (inert solvent reference state).Specifically we demonstrate that: (i) specifically interacting compounds have an essentially greater ability to undergo interactions with hydrated soil organic matter as compared with non-specifically interacting compounds of the same molar refraction; (ii) in many cases, greater interaction ability is correlated with compound ability to undergo hydrogen bond formation with the hydrated soil organic matter phase; (iii) interactions of solute molecules with the bulk aqueous phase compensate for specific interactions between sorbate molecules and the soil organic matter phase, resulting in ‘apparent’ hydrophobic partitioning behavior of specifically-interacting solutes that masks compound specific interactions with soil organic matter; (iv) the overall trend in increasing strength of compound interactions with OM is as follows: aromatic and Cl-substituted aromatic hydrocarbons≤aliphatic hydrocarbons and Cl-substituted aliphatic hydrocarbons≈anisole≈ o-Cl-nitrobenzenem-nitrophenol; (v) only those compounds that interact most strongly with OM exhibit significant sorption isotherm nonlinearity; (vi) other commonly used standard states such as pure compound (liquid) state and solution in n-octanol are considerably less useful in deciphering sorption mechanisms; (vii) strength of compound interaction with hydrated OM is correlated with the effect of OM hydration on compound sorption. Thus, using a single inert reference state (either gas phase or inert solvent), is shown to be valuable for delineating the differences in organic compound interactions with OM.
Keywords: Sorption; Organic compounds; Organic matter; Classification; Organic matter hydration; Specific interactions; Inert reference state; Thermodynamic cycle; Non-linear sorption isotherm
Preparation and photoactivity of nanostructured TiO2 particles obtained by hydrolysis of TiCl4 by Maurizio Addamo; Vincenzo Augugliaro; Agatino Di Paola; Elisa García-López; Vittorio Loddo; Giuseppe Marcì; Leonardo Palmisano (pp. 23-31).
This work reports the preparation of nanostructured polycrystalline TiO2 photocatalysts obtained by hydrolysis of TiCl4 in very mild conditions. Several samples were prepared as suspensions or colloidal dispersions by using TiCl4/H2O volume ratios ranging between 1:1 and 1:100. The samples were characterised by X-ray diffraction analysis, specific surface area determination, diffuse reflectance spectroscopy and scanning electron microscopy. 4-Nitrophenol photodegradation was used as probe reaction to test the photoactivity of the catalysts. Some samples revealed a photoactivity higher than that of Degussa P25 in similar experimental conditions and the most photoactive one was that prepared by using the TiCl4/H2O volume ratio equal to 1:50. A dialysis process was performed in some cases to reduce the amount of chloride ions present in the system. The presence of Cl− ions appeared to be dramatically detrimental for Degussa P25 while a lower influence was found for the home prepared samples.
Keywords: Photocatalysis; Titanium dioxide; Nanostructured materials; Titanium tetrachloride
Adsorption from aqueous phenol and aniline solutions on activated carbons with different surface chemistry by Krisztina László (pp. 32-39).
Microporous carbons of similar surface area (1200–1500m2/g) and porosity but different surface composition were prepared from poly(ethyleneterephthalate) (PET) based activated carbon by chemical (cc HNO3) and thermal (700°C) post-treatment. pH, pHPZC measurements and Boehm titration proved that the concentration and distribution of the surface functional groups is different.The waste removal capacity was studied by adsorption from buffered aqueous phenol and aniline solutions. Adsorption isotherms were satisfactorily fitted by both the Langmuir and Freundlich model. Since the parameters of the former approach can be converted to a physical model, however, discussion is based on this fit.The adsorption of phenol and aniline from their aqueous solution is a complex process governed by the pH of the medium, due to the acid/base character of both these molecules and the carbon surface. The surface concentration of the aromatic pollutant molecules was found to be always greater than that of functional groups. Even when electrostatic repulsion is present the graphene layer adsorbs 70–80molecules/100nm2. This means that the major contribution to the adsorption comes from the dispersion effect and enhancement of the interactions is obtained only through attractive electrostatic forces. Adsorbates decorating the graphene edge, however, can limit access of further molecules to the interlayer region.Aromatic monolayers are never completed as the surface layer also contains a considerable amount of water.The weak acid and base studied do not behave symmetrically: the uptake of phenol exhibits a maximum, while a monotonic increase with pH was found with aniline. The pH dependence of the adsorption capacity and the interaction parameter is stronger for aniline than for phenol. The lower water solubility of the aniline does not result in higher adsorption capacity. Its maximum uptake and interaction parameter are lower than those of phenol.
Keywords: Polymer-based carbon; Acid/base properties; pH; Langmuir model; Freundlich parameters
Ion binding to natural organic matter: General considerations and the NICA–Donnan model by L.K. Koopal; T. Saito; J.P. Pinheiro; W.H. van Riemsdijk (pp. 40-54).
The general principles of cation binding to humic matter and the various aspects of modeling used in general-purpose speciation programs are discussed. The discussion will focus on (1) the discrimination between chemical and electrostatic interactions, (2) the binding site heterogeneity, (3) the models for the electrostatic interactions, such as the Donnan model and (4) the complications inherent to ion binding in multi-component systems, including a different stoichiometry for different ions. The basic NICA (non ideal competitive adsorption) model addresses issues (1), (2) and (4) and ensures thermodynamic consistency. The electrostatic interactions (3) can be included using the ion activity adjacent to the binding sites. To compute this activity it is necessary to use an electrostatic model that relates the particle charge and the characteristic particle potential. Several simplifications to arrive at such a relation are analyzed. The Donnan model is proposed as the most convenient model. A short derivation of the NICA–Donnan model is presented to illustrate that the derivation is quite straightforward once it is clear which assumptions should be made. The main parameters of the NICA model are the site density, the median affinity constants for the different ions, the ion stoichiometry and the width of the generic affinity distribution. All these parameters are intrinsic properties that are independent of pH, salt concentration or metal concentration. The key parameter of the Donnan model is the Donnan volume, for which an empirical relation with one adjustable parameter is advised. The NICA–Donnan model has been used in several studies. To illustrate the capabilities of the model some results obtained for purified humic acid are presented together with model predictions. Key features of the model are its ability to adjust to different heterogeneities and binding stoichiometries and its predictive abilities with respect to metal ion–proton and metal–metal ion competition. With the application the model should be regarded as semi-empirical because assumptions have been made for its derivation that are not met in practice. Both for this reason and since it is difficult, if not impossible, to find a unique set of parameters for a given system the interpretation of the parameter values may be difficult. A brief discussion is given on the generic NICA–Donnan description of fulvic and humic acids presented by Milne et al. [C.J. Milne, D.G. Kinniburgh, E. Tipping, Environ. Sci. Technol. 35 (2001) 2049; C.J. Milne, D.G. Kinniburgh, W.H. van Riemsdijk, E. Tipping, Environ. Sci. Technol. 37 (2003) 958]. Finally a brief literature review is presented on the use of the NICA–Donnan model for other heterogeneous natural sorbent systems.
Keywords: Non ideal competitive adsorption; Multicomponent ion binding; Site heterogeneity; Electrostatic interactions; Binding stoichiometry; Intrinsic affinity distribution; Electrostatic models; Humic and fulvic acids; NICA–Donnan model; NICA derivation; NICA–Donnan applications
Charge characteristics and related dispersion/flocculation behavior of soil colloids as the cause of turbidity by Katsuhiko Itami; Hideaki Fujitani (pp. 55-63).
Dispersion/flocculation (D/F) behavior of soil colloids is closely related to numerous environmental issues and regulated by the charge characteristics of the colloids. The charge characteristics of five clays commonly distributed in Japan (montmorillonite, sericite, kaolinite, halloysite and allophane) were determined by the modified ion adsorption method using caesium as an index cation and compared with the D/F behavior under the similar conditions. The flocculation effect of Cs on negatively charged clays enabled accurate charge analysis in the high pH and low electrolyte concentration region where measurement had been difficult by conventional methods due to clay dispersion. The dispersion ratio of montmorillonite was evenly high in all pH regions reflecting its large permanent charge. The pH dependence of sericite charge was rather variable than permanent. The dispersion ratio of sericite was increased abruptly above pH 6.5. The pH dependence of kaolinite negative charge was small, indicating the predominance of permanent charge. Both negative and positive charges were detected from halloysite in large pH regions. The dispersibility of kaolinite and halloysite was increased above pH 7. To explain the observed discrepancy between the charge characteristics and the D/F behavior, the significant role of edge surfaces in flocculation was emphasized for sericite, kaolinite and halloysite. In case of halloysite, the reverse imogolite structure curling with the Al-octahedral layers inside was proposed to account for the constant negative charge detected in all pH regions. Allophane showed the charge characteristics typical of variable charge minerals and dispersed in both acid and alkali regions. The difference between the charge characteristics and the dispersibility of allophane was attributed to the heterogeneity of its charge. From the aspect of environmental conservation, keeping the suspension pH below 6–7 so as to prevent edge surfaces from dissociating is likely to be primarily important to control clay dispersion depending on the clay mineralogy. By combining the information derived from this Cs adsorption method with the proper land management, more effective techniques to reduce soil losses and water pollution will be established according to field conditions.
Keywords: Charge characteristics; Dispersion/flocculation behavior; Clay mineralogy; Edge surface; Point of zero charge
ZnAl-layer double hydroxides as photocatalysts for oxidation of phenol in aqueous solution by Ágnes Patzkó; Robert Kun; Viktória Hornok; Imre Dékány; Thomas Engelhardt; Norbert Schall (pp. 64-72).
ZnAl-layer double hydroxides (ZnAl-LDHs) with various Zn/Al ratios from 1:1 to 4:1 were prepared by the sol–gel method. The amount of ZnO in the layer double hydroxide was increased by the addition of more Zn hydroxide sol and Zn-oxide-hydroxide/2:1 ZnAl-LDH composites were prepared by heterocoagulation. The samples were calcinated at different temperatures (80–700°C) and it was shown by XRD and TG measurements that the crystallinity of the samples improves as temperature is increased. The photocatalytic properties of the heat-treated samples were studied in a batch reactor with aqueous phenol solution as substrate. The samples prepared from ZnAl-LDH were shown to have higher photocatalytic efficiencies at 3:1 and 4:1 Zn/Al ratios in the case of mixed phase double hydroxide and ZnO crystals.
Keywords: Zinc oxide; Layer double hydroxide; Photocatalysis; Photooxidation; Phenol
FT-IR–ATR as a tool to probe photocatalytic interfaces by Paula Z. Araujo; Cecilia B. Mendive; Luis A. García Rodenas; Pedro J. Morando; Alberto E. Regazzoni; Miguel A. Blesa; Detlef Bahnemann (pp. 73-80).
The catalytic photo-oxidation on TiO2 (Degussa P-25) of oxalic acid at pH 3.7 and of catechol at pH 6.2 has been studied by in situ ATR–FT-IR. In the case of catechol, both FT-IR and HPLC demonstrate the formation of an intermediate, the accumulation of carbonate, and the depletion of the ligand on the illuminated surface. Ligand depletion is also demonstrated by the spectral evolution of adsorbed oxalate; the data in the case also suggest that different surface complexes may inter-convert directly on the surface. These findings illustrate the potentiality of in situ ATR–FT-IR to follow the evolution of the catalytic surface. It is also clearly demonstrated that under our experimental conditions, the photolytic oxidation of the adsorbed ligands is fast as compared with the rate of exchange with the bulk. In terms of simple mechanistic considerations, the systems behave as expected for low degrees of coverage, determined by fast surface reactions, k2{ h+}≫ k−1, where k−1 is the rate constant for hole trapping by the adsorbed ligand, k−1 the rate constant for ligand desorption, and { h+} is the effective hole concentration, determined by the intensity of light. The rate law then turns out to be R= k1 NS[HL], where R is the reaction rate, k1 the adsorption rate constant, NS the surface site density, and [HL] is the bulk ligand concentration.
Keywords: Photocatalysis; Gallic acid; Catechol; Intermediates; Surface complexes; FT-IR/ATR
Sorption study of 2,4-dichlorophenol on organoclays constructed for soil bioremediation by B. Witthuhn; T. Pernyeszi; P. Klauth; H. Vereecken; E. Klumpp (pp. 81-87).
The role of the type and amount of cationic surfactant in the sorption behaviour of partially modified clays was studied and the applicability of a partially modified organoclay for bioremediation was demonstrated. Partially modified surfactant/clay complexes (“organoclays�) were prepared from Na-montmorillonite by using monoalkyl-(dodecyltrimethylammonium and octadecyltrimethylammonium) and dialkylammonium (didodecyldimethylammonium and dioctadecyldimethylammonium) bromides. The degree of modification was varied between 35 and 89% of the cation exchange capacity of montmorillonite (MM). The sorption of 2,4-dichlorophenol (DCP) on organoclays was studied by using the batch technique. The intercalation of DCP into the interlayers of organo-MM was detected by X-ray diffraction.The adsorption isotherms found were non-linear indicating that the sorption is not only based on a simple partition mechanism. The adsorption coefficients Kd calculated from the initial isotherm slope turned out to be exponential with the organic carbon content of organo-MM ( foc). In particular, dialkylammonium-modified MM revealed outstanding sorption characteristics for DCP. The comparison of partially modified organo-MMs with similar foc and volume fraction of surfactant indicated that the surfactant arrangement in the interlayers significantly influences the DCP adsorption. It could be assumed that both the kind and degree of exchanged surfactant play a decisive role in the adsorption of DCP on the organoclays. By performing biodegradation experiments in the presence of dioctadecyldimethylammonium montmorillonite, the complete biodegradation of DCP was achieved demonstrating the applicability of partially modified organoclays for bioremediation.
Keywords: Adsorption; Biodegradation; Organic pollutant; Organoclay; Soil remediation
Oxidative transformation of contaminants using colloidal zero-valent iron by Andrew J. Feitz; Sung Hee Joo; Jing Guan; Quan Sun; David L. Sedlak; T. David Waite (pp. 88-94).
It has recently been demonstrated that nano scale zero-valent iron (nZVI) oxidizes the herbicide molinate when it is used in the presence of oxygen. Further batch and column experiments were conducted to investigate the potential application of nZVI for the treatment of contaminants. Results of batch studies reveal a rapid initial degradation of molinate followed by slower degradation over more than a day. Additional insight into the nZVI-mediated process has been obtained from studies of the formation of para-hydroxybenzoic acid ( p-HBA) from the oxidative degradation of benzoic acid with slow ongoing production of p-HBA over 24h observed after an initial pulse of byproduct production. Addition of EDTA enhances the initial pulse by 50% but does not appear to reduce surface passivation over a longer time frame. Oxygen availability during the initial pulse appears to be a limiting factor. Results of column studies reveal that the arrangement of the nZVI, sand and gravel within the column strongly affect the degradation performance despite each column having the identical nZVI loading. Under the optimal column configuration, >90% removal of 100ppb molinate was observed over a 3h period. These promising results suggest that nZVI, despite initial rapid oxidation of the particle surface, has sufficient residual oxidizing power to enable it to be incorporated into a continuous treatment process.
Keywords: Molinate; Zero-valent iron; EDTA; Oxidation; Pesticide
Size and shape of lead–organic associations by F. Lang; H. Egger; M. Kaupenjohann (pp. 95-103).
The relevance of mobile organic matter for the transport of Pb through soils has been shown through several studies. Up until this study, no investigation has been published on whether the adsorption of Pb alters the size and/or shape, and consequently the mobility, of organic substances. We hypothesize that the addition of Pb induces the formation of organic precipitates by intermolecular cross-linking. We extracted organic matter from forest floor samples and determined the size and shape of the organic colloids via small angle X-ray scattering (SAXS) and their zeta potential. Measurements were conducted at three Pb concentrations (0, 0.05 and 0.10mmoll−1) and at varying pH values (2.5, 4, 7, and 8). Under neutral and alkaline conditions, we observed geometrically weakly defined colloids possessing a maximal dimension of 33nm. Either decreasing the pH value or increasing the Pb concentration induced the formation of organic matter rods with a corresponding decrease in the maximal dimension to 24nm. Zeta potential measurements indicate that this decrease is due to a reduction of the repulsion between functional groups of the natural polyelectrolytes. SAXS curves indicate the formation of polymolecular Pb–organic agglomerates at pH 4 and 0.1mM Pb, which exhibit a high colloidal stability. We conclude from our results that Pb is not merely passively co-transported by organic matter in soils but that it may also accelerate its mobile sorbents through the compaction of organic molecules or by the formation of highly mobile organic colloids.
Keywords: Lead; Soil organic matter; Colloids; Cross-linking; Small angle X-ray scattering
Electrostatic interaction models for ion binding to humic substances by Takumi Saito; Shinya Nagasaki; Satoru Tanaka; Luuk K. Koopal (pp. 104-113).
Preferably, the description of ion binding to humic substances (HSs) is done with thermodynamic constants that do not depend on the environmental conditions. To solve this problem, models have to be made that describe the electrostatic and specific interactions. With a given electrostatic model the charge/pH curves of HS at different salt levels can be re-plotted as a function of the local pH near the sites (pHloc) of HSs. If the model is appropriate, the charge/pHloc curves will merge into a master curve (MC). In this study five electrostatic models were investigated to obtain pHloc for purified Aldrich humic acid (PAHA): rigid sphere (RS), ion-permeable sphere, Donnan (NICA), Donnan-EV, and Donnan-EDL. The RS model is tested in two versions; one based on the measured hydrodynamic radius (RS- ah) and the other on an optimized radius (RS- aopt). The ion-permeable sphere model uses the hydrodynamic radius and provides the potential distribution; as characteristic potential the radial average inside the sphere is used. In the Donnan (NICA) model the volume of the sphere, VD, in which the charge of PAHA is neutralized, is optimized with a constraint between VD and ionic strength, and in the Donnan-EV model VD is calculated by setting the radius of the gel as the sum of the hydrodynamic radius of PAHA and the Debye length. The Donnan-EDL model uses the hydrodynamic particle radius and is based on a combination of the Donnan model and the diffuse electrical double layer model. Only the RS- aopt, Donnan (NICA), and Donnan-EV models give adequate MCs. The positions of the MCs differ with respect to each other. This means that the discrimination between electrostatic and intrinsic interactions is model-dependent and therefore arbitrary. The Donnan (NICA) model has a practical advantage over the other two models because this model needs no measurements of the size of HS. For the purpose of the routine fitting of ion-binding data to an ion-binding isotherm equation that includes the electrostatics, this advantage is quite important.
Keywords: Humic substances; Ion binding; Electrostatic interaction; NICA-Donnan; Master curve
Enhanced electrokinetic remediation of metals-contaminated clay by B. Kornilovich; N. Mishchuk; K. Abbruzzese; G. Pshinko; R. Klishchenko (pp. 114-123).
Analysis of polarization processes at electrokinetic remediation of clay soils and evaluation of the contribution of various components into the transport of charged contaminants are performed. The peculiarities of soil decontamination at constant and pulse voltages are investigated using kaolinite and mixture of montmorillonite and sand contaminated with radionuclides (137Cs,90Sr, U) and heavy metals (Co) as model systems. It is shown that the use of unipolar pulse voltage changes the distribution of contaminations in soil and allows to decrease power inputs. It is also demonstrated that the introduction of cation-exchangers CU-2-8 into electrode chambers considerably improves the degree of soil decontamination. The most effective method to intensify the decontamination of clay soils from radionuclides and heavy metals is the wetting of soil with enhancement reagents (acetic, nitrilotriacetic and ethylenediaminetetraacetic acid).
Keywords: Decontamination; Electrokinetic remediation; Cation exchangers; Clay soils; Radionuclides; Heavy metals; Polarization processes; Pulse regime
Effects of contaminants on biological model membranes: The advantage of the ASAXS method for the study of the location of copper ions and dihalogenated phenol molecules by Attila Bóta; Erwin Klumpp (pp. 124-130).
The features of the location of copper ions and the dihalogenated phenol molecules were investigated at their low concentration in the dipalmitoylphosphatidylcholine (DPPC)–water liposomes by using small-angle X-ray scattering (SAXS) and anomalous small-angle X-ray scattering (ASAXS) methods. Only the ASAXS method can provide information about the structural features of the domains rich in these ions and organic guest molecules. The copper ions are mainly located in large domains between the lamellae and only partly in the characteristic periodic water shells of the liposomes. The dihalogenated phenol molecules are embedded in the layers and do not destroy the lamellar arrangement. Their inhomogeneous distribution appears laterally and phase separation occurs. The local structures induced by the copper ions and the dihalogenated phenols were also visualised by freeze-fracture method.
Keywords: Anomalous small-angle X-ray scattering (ASAXS); Liposome; Localisation; Phase separation; Copper(II) ion; Dihalogenated phenol
Effects of pH and ionic strength on electrokinetic properties of imogolite by H. Tsuchida; S. Ooi; K. Nakaishi; Y. Adachi (pp. 131-134).
Imogolite, an aluminosilicate with thread-like morphology and a nano-size tubular tunnel, is a unique clay mineral not only in morphology but also in colloidal stability. This clay is normally contained in volcanic ash soil and important in characterizing the physical and chemical properties of volcanic ash soil. In the present study, we performed measurements of acid–base potentiometric titration, electrophoresis, sediment volume and electrical conductivity for suspensions of natural imogolite. From these results, we clarified: (i) point of zero charge (PZC) existed near pH 6, where the imogolite has the same amount of positive and negative charges. An exchangeable cation was able to penetrate into the tube, but the counter-ion inside the tube played a small roll in the electrokinetic phenomena. (ii) The sediment volume increased with an increase in pH. The formed flocs were bulky at a high pH in accordance with reduction in the electrical repulsive force. This behavior was more pronounced with an increase in ionic strength. (iii) Surface conductmetry was very useful for detecting the mobile ion in the electrical double layer near the outer surface of the tube. The electrical surface conductance, the sediment volume and the electrophoretic mobility indicated that only the outer mobile ions contributed to the electrokinetic phenomena.
Keywords: Imogolite; Surface conductmetry; Acid–base titration; PZC; Sediment volume
Photoassisted oxidative degradation of surfactants and simultaneous reduction of metals in titanium dioxide dispersions by Ottó Horváth; Eszter Bodnár; Judit Hegyi (pp. 135-140).
Photoinduced oxidative degradation of both anionic (dodecyl sulfate) and cationic (cetyltrimethylammonium) surfactants can be efficiently realized by simultaneous reduction of chromium(VI) to chromium(III) and mercury(II) to its metallic form in aqueous systems containing colloidal titanium dioxide photocatalyst. The charge of the reactants (surfactant and metal ions) has considerable effect on the efficiency of the photoassisted processes. Similarly charged reactants in general show mutual inhibition, mostly due to the competition for the active sites on the surface of the catalyst, while oppositely charged detergent and metal ions promote the redox change of both components. The cetyltrimethylammonium–chromate system displays significant photoactivity even in the absence of titanium dioxide, due to the strong ion-pair formation resulting in direct electron transfer from the surfactant to the metallate ions. The initial pH in the studied systems may considerably affect both the Coulombic interactions between the reactants and the surface of the catalyst, and the driving forces of some of the redox key reactions.
Keywords: Photocatalysis; Titanium dioxide; Surfactants; Chromate; Decontamination
“Active filtrationâ€? for the elimination and recovery of phosphorus from waste water by U. Berg; D. Donnert; A. Ehbrecht; W. Bumiller; I. Kusche; P.G. Weidler; R. Nüesch (pp. 141-148).
A seed-induced crystallization process was developed to remove and recover phosphorus from waste water, i.e. attention has been especially drawn to gain a phosphorus-rich product, which can be recycled by the phosphorus industry and/or may directly be used as a fertilizer. The necessity of phosphorus recovery from phosphorus sinks is due to the fact that the world reserves of phosphorus rock will run out within the following 80–100 years. On the other hand, phosphorus is an essential nutrient for all forms of life, which cannot be replaced by any other element.For the development of this phosphorus recovery process, continuous-flow fixed-bed column and expanded-bed experiments were carried out in the laboratory scale using calcite (cc) and tobermorite-rich crushed gas concrete (cgc) as seeds. The experiments revealed that cgc was much more efficient in phosphorus removal from organic-enriched waste water compared to calcite seeds. Under suitable conditions, a phosphorus removal efficiency of 80–100% could be achieved for a time period of up to 4 months yielding a product containing about 10% P per weight. Furthermore, the physical and chemical properties of the product obtained proved the ability for phosphorus recovery. The results of FTIR and XRD investigations showed the transformation of cgc into calcite, the formation of hydroxyl apatite-like compounds on the seed material during the reaction with waste water, and finally the residual quartz originating from the cgc. A comparison of the results of laboratory- and half-scale fixed-bed column experiments revealed a shortened life-cycle of the columns with higher phosphorus concentrations of the inflow waste water, i.e. the velocity of Ca depletion within the fixed-bed system is governed by the P concentration.Active filtration using cgc seems a promising development for phosphorus removal and recovery from waste water. Thus, it should be possible in the near future to replace state-of-the-art methods of waste water treatment plants (WWTPs), such as metal precipitation or enhanced biological phosphorus removal (EBPR), which have the only aim of phosphorus removal, by a rather simple technology with the paramount goal of phosphorus recovery.
Keywords: Phosphorus recycling; Phosphorus removal; Crushed gas concrete; Crystallization; Active filtration
Study of colloidal stability of allophane dispersion by dynamic light scattering by Y. Adachi; S. Koga; M. Kobayashi; M. Inada (pp. 149-154).
The aggregation kinetics of the aqueous dispersion of allophane was measured by dynamic light scattering as functions of pH and ionic strength. The sample was purified from natural Kanuma soil (Tochigi, Japan). A notable feature is that the dispersion is composed of aggregated flocs of allophane particles. The experiments were performed adjusting the initial floc diameter by centrifugation. The aggregation was induced by charge neutralization or compression of the electrical double layers, by merely mixing the suspension with an electrolyte solution of controlled pH and ionic strength. After the initial mixing operation, the suspension was placed in a static condition and the temporal variation of the average hydrodynamic diameter was monitored in situ . In the initial stage, the diameter dramatically increased reflecting the aggregation induced by the hydrodynamic mixing. Due to the Brownian aggregation, this initial enhancement was followed by a moderate increase. Under the Brownian aggregation, the aggregation rates were found to take a certain limiting value denoting fast coagulation irrespective of charge neutralization or compression of the electric double layers. When the salt concentration is not sufficiently high, the rate of aggregation against pH gradually increases and approaches the fast-coagulation domain; however, the rate decreases rather rapidly as the pH increases. This behavior was interpreted considering pH-dependent charge distribution generated on the surface of allophane particles with peculiar morphology. This interpretation was supported by electrophoretic mobility data. In contrast with the Brownian aggregation, the rate of orthokinetic aggregation induced by charge neutralization was detected to be slightly faster than that induced by compression of the electric double layers.
Keywords: Allophane; Aggregation kinetics; Dynamic light scattering; Charge neutralization; Electrophoretic mobility
Photooxidation of organic dye molecules on TiO2 and zinc–aluminum layered double hydroxide ultrathin multilayers by Robert Kun; Margit Balázs; Imre Dékány (pp. 155-162).
Multilayered self-assembling films from colloidal particles with constant or variable surface charge were prepared on the surface of glass plates using the “layer by layer� (LBL) technique. As particles with variable charge, TiO2 photocatalyst particles and silica sol particles were used for the preparation of multilayered, up to 20-layer films. The colloid particles with constant charge was a suspension of zinc–aluminum layer double hydroxide (ZnAl-LDH), 20 layers of which were deposited on the surface of glass plates using the LBL technique. Layer buildup was monitored by absorbance measurements at a wavelength of λ=500nm. The buildup of ZnAl-LDH layers was also followed by XRD measurements. Both TiO2 or TiO2/SiO2 multilayered films and ZnAl-LDH multilayered films were shown to disintegrate acridine orange (AO) dye via photooxidation. The photocatalytic efficiency of ZnAl layer double hydroxide, the new photocatalyst we introduce is close to that of the well-known P25 Degussa titanium dioxide.
Keywords: Multilayered films; Layered double hydroxide; TiO; 2; Photocatalysis
Matrix relaxation and change of water state during hydration of peat by Gabriele E. Schaumann (pp. 163-170).
In this study, hydration of a peat sample was investigated with differential scanning calorimetry (DSC) in terms of glass transition behavior and the formation of freezable and unfreezable water. Special attention was drawn to the development of these characteristics in the course of hydration and to plasticizing and antiplasticizing effects of water. Freezable water was formed above a water content threshold of Θcrit=(23±7)% and revealed structured melting peaks indicating freezable bulk-like water and freezable bound water. The freezable bound water revealed a broad, kinetically controlled melting endotherm, and the melting barrier increased with increasing hydration time. Glass transitions were found in between 43°C and 68°C with a change in heat capacity of Δ C=(0.13±0.08)Jg−1K−1. Glass transition behavior does not fully match the theoretical expectations and is linked with water binding. Water reveals a short-term plasticizing function in the range of days as well as a slow antiplasticizing function in the range of weeks or even months. The findings are consistent with the hydrogen bond based cross-linking model (HBCL) suggested in a previous study. Non-equilibrium and matrix relaxation are considered the rule rather than the exception in nature and underline the ecological relevance of hydration, the relevance for sorption and transport phenomena was well as possibly for soil development.
Keywords: Hydration; Peat; Plasticizer; Antiplasticizer; Unfreezable water; Glass transition
Catalytic detoxification of C2-chlorohydrocarbons over iron-containing oxide and zeolite catalysts by János Halász; Mária Hodos; István Hannus; Gyula Tasi; Imre Kiricsi (pp. 171-177).
Catalytic hydrodechlorination is an effective and environmentally friendly alternative for the destruction of chlorine-containing organics. Since the byproducts are waste materials (HCl and Cl2), it is important to find and apply inexpensive or natural materials having catalytic activity in hydrodechlorination. A potential catalyst of this type, the calcined “red mud� originated from the alumina process, was studied in comparison with other iron-containing catalysts, such as single (Fe2O3) and mixed oxides (Fe2O3/Al2O3) and modified zeolite (Fe-ZSM5). The red mud is a dangerous waste formed in large amount, nevertheless, it contains catalytically active oxides. The hydrodechlorination reactions were carried out in a fixed-bed flow reactor in the temperature range of 200–400°C using hydrogen as reducing agent. The structure of the catalysts was characterized by XRD, IR and adsorption methods. The metal and chlorine contents of the used samples were also determined. The efficiency of the reductive destruction of 1,2-dichloroethane (DCE), 1,1,2-trichlorethane (TRCE), 1,1,2,2-tetrachloroethane (TCE), trichloroethene (TRCEE) and tetrachloroethene (TCEE) over mixed oxide originating from red mud is comparable to that obtained for synthetic catalysts. The solids used are not only catalysts, but reactants as well in these reactions. Therefore, these processes could be considered as gas–solid reactions.
Keywords: Hydrodechlorination; Mixed oxide catalyst; Red mud; Volatile organic compounds (VOC); Modified zeolites