Effects of affinity and aggregation on parallel diffusion of tri-sulfonated azo compounds into water-swollen cellulose membranes

Adsorption and diffusion of tri-sulfonated azo dyes, C.I. Acid Red 18 and C.I. Acid Red 27 onto waterswollen cellulose membrane has been studied at 25°C. Affinities of these dyes onto cellulose were evaluated by the coefficients of Freundlich equation. Diffusion behavior of these days was analyzed on the basis of a parallel transport theory by surface and pore diffusion. The results could be described by the parallel diffusion model provided that adsorption was stimulated by addition of NaCl. The surface diffusivities for the parallel diffusion model were correlated by the affinity of the dyes, on the other hand, the pore diffusivities for the model were affected by aggregation of the dye depending on its structure and NaCl concentration.     

Effects of type of adsorption isotherms on parallel diffusion of sulfonated dyes into porous cellulose membrane

Transport phenomenon of three sulfonated azo dyes, C.I. Acid Red 88, C.I. Direct Yellow 12, and C.I. Direct Blue 15 into water-swollen cellulose membranes has been analyzed on the basis of parallel transport theory by surface and pore diffusion. Langmuir equation was applied into the mass balance equation to estimate dye concentration in the pores. The results were compared with the results obtained by applying Freundlich equation in our previous papers. The surface diffusivity (D _s) and the pore diffusivity (D _p) for the parallel diffusion model obtained by applying Langmuir equation agreed with those obtained by applying Freudlich equation. The theoretical concentration profiles for parallel diffusion calculated usingD _s andD _p coincided accurately with the experimental data when we applied either Langmuir or Freundlich equations.     

Mixture diffusion of sulfonated dyes into cellulose membrane. III. Application of parallel diffusion model to binary system of direct dyes with different affinity

Mixture diffusion of two dyes (C.I. Direct Blue 15 (DB15) and C.I. Direct Yellow 12 (DY12)) with different affinity onto the substrate into cellulose membrane from the binary solution was studied at 55°C. Uptake curves and concentration–distance profiles were measured experimentally in the ratios (DB15:DY12) 1:0.5, 1:1 and 1:2. It was examined whether the diffusion of the dyes could be analyzed based on the parallel diffusion theory of surface and pore diffusion. It was revealed that the diffusion of DB15 with higher affinity could be analyzed based on the model in the ratios 1:0.5 and 1:1, although the theoretical value deviated slightly from the data in the concentration–distance profile in the ratio 1:1. On the other hand, the diffusion of DY12 with smaller affinity could not be described by the model, because the diffusivity of the dye changed during the adsorption process against the assumption of the model.     

Adsorption of xanthene food additive dyes to cellulose granules

It was found that three kinds of the synthetic food additive dyes, red nr. 3 (erythrosine), nr. 104 (phloxine), and nr. 105 (rose bengal) were adsorbed to the surface of charred cellulose granules and the maximum amounts of adsorption of these dyes were 3.75, 3.42, and 4.74 mg/g cellulose, respectively. Scanning electron microscopy-electron probe micro analysis (SEM-EPMA) showed a coating of the dyes on the surface of charred cellulose granules. Electron spectroscopy for chemical analysis (ESCA) suggested the presence of NH₃ ⁺ in the surface of charred cellulose granules. Since all three dye compounds have both anionic carboxylate and hydrophobic groups and were released from the surface of charred cellulose granules by 0.1 N NaOH solution, it was surmised that these three food additive dyes were bound to the surface of cellulose granules by both ionic and physical interactions.     

Effects of polymer concentration and coagulation temperature on the properties of regenerated cellulose films prepared from LiOH/urea solution

Aqueous 5 wt% LiOH/12 wt% urea solution pre-cooled to −12 °C has a more powerful ability to dissolve cellulose compared to that of NaOH/urea and NaOH/thiourea solution system. The influences of the cellulose concentration and coagulation temperature on the structure, pore size and mechanical properties of the cellulose films prepared from LiOH/urea system were investigated. The cellulose films exhibited good mechanical properties either at wet or dry state and their pore size and water permeability at wet state can be controlled by changing the cellulose concentration or coagulation temperature. With a decrease of the coagulation temperature, the mechanical properties and optical transmittance of the cellulose films enhanced, as a result of the formation of relative smaller pore size and denser structures. This work provided a promising way to prepare cellulose films with different pore sizes at wet state and good physical properties at dry state.     

Sorption of dyes on cellulose II: effect of alkali treatment of fibre and dye structure

Cellulose is a linear 1,4-β-glucan polymer where the units are able to form highly ordered structures, as a result of extensive interaction through intra- and intermolecular hydrogen bonding of the three hydroxyl groups in each cellulose unit. Alkali has a substantial influence on morphological, molecular and supramolecular properties of cellulose II polymer fibres causing changes in crystallinity. Lyocell fibres pre-treated with 0.0, 2.0, and 4.0 mol dm⁻³ aqueous NaOH solution were dyed with hydrolyzed reactive dyes that had different molecular shapes and sizes. Overall exhaustion (q _e), value of K, and −ΔG increased for lyocell samples pre-treated with aqueous NaOH solution in the following order: 2.0 > 4.0 > 0.0 mol dm⁻³ NaOH. The same trends were observed for colour strength (K/S) values of the dyeings. Pre-treatment of lyocell with 2.0 mol dm⁻³ NaOH creates the substrate that achieves the most thermodynamically favourable system for sorption of hydrolyzed reactive dyes, as at this concentration crystallinity decreases (with respect to 0.0 mol dm⁻³ NaOH treated lyocell) to afford higher sorption; however, at higher alkali concentrations the macro-sorbent forms a compacted unit that limits diffusion within the sorbent interior. Molecular size of the sorbate dye has a significant effect on the sorption process: for the largest dye structure the sorption isotherm is most closely correlated to a Langmuir isotherm; as the size of the dye decreases correlation to a Langmuir isotherm is observed, but with good correlation to the Freundlich isotherm; as the size of the dye is decreased further sorption is more typical of a Freundlich isotherm.     

A model for the coupled transport of ions and electrons in redox conductive microcrystals

Coupled diffusion of ions and electrons in microcrystals of insertion compounds immobilized at an electrode surface is theoretically analysed by a lattice-gas model without interactions. The transport in the direction perpendicular to the electrode surface depends on Wagner's factor for electrons, while the transport parallel to the electrode depends on this factor for ions. The iso-concentration profiles may depend on the orientation of the particle on the electrode surface. Chronoamperometric responses of volume and surface redox reactions are calculated. Received: 5 June 1998 / Accepted: 22 August 1998     

Adsorption of basic dyes onto montmorillonite

Ca-montmorillonite (Ca-Mont) was exchanged with titanium cations and the adsorption equilibrium and kinetics of Basic Green 5 (BG5) and Basic Violet 10 (BV10) on these montmorillonites were measured to examine the ion-exchange effects on the basic dyes adsorption. The relationship between the dye adsorption and the alteration of pore structures of montmorillonite induced by ion-exchange processes was discussed. Moreover, the changes in the surface and pore structure of montmorillonites during adsorption were characterized based on classical and fractal analyses of the nitrogen adsorption isotherms as well as the XRD patterns. The decrease in BET surface area of montmorillonites after adsorption of dyes was interpreted in terms of both the coverage of some surface roughness (surface screening effect) and the inhibition of the movement of nitrogen molecule into some pores (pore blocking effect). The surface fractal dimension D was used to examine whether or not the surface screening effect exists and the pore blocking effect was examined with the changes of mean pore size before and after adsorbing basic dyes.     

Interpretation of relaxation and swelling phenomena in lyocell regenerated cellulosic fibres and textiles associated with the uptake of solutions of sodium hydroxide

The uptake of solutions of sodium hydroxide by lyocell fibre results in a phenomenon in textiles described as swelling–shrinkage. The response of woven fabrics in a tensile stress–relaxation experiment shows two time-dependent processes, corresponding to different mechanisms of pressure development. Rapid diffusion has been assigned to osmotic swelling through the interconnected pore structure of the fibre (D = 6–15 × 10⁻¹² m²/s), which is influenced by the extent of ionization of hydroxyl groups at the pore surfaces. A ratio for the cellulose and water dissociation constants (K_cell/K_w) of 70 provides best agreement with experimental data. A second slower diffusion process (D = 2–10 × 10⁻¹⁴ m²/s) is assigned to transport through the cellulose polymer structure, associated with the Na-cellulose transition. This can be modeled assuming an ion-exchange equilibrium, where the cellulose gel converts reversibly between compact hydrogen and expanded sodium forms, with K = 1.04 × 10¹⁴, in favour of the hydrogen form. The model successfully predicts the concentration dependence of the transition and the movement to higher concentration with external constraint. The slow diffusion process only becomes apparent at high alkali concentrations, as the pores in the fibre collapse due to the expansion of the gel. Continued gel-diffusion is only possible through the polymer phase, which then dominates over fast pore-diffusion.     

A Film-Pore-Surface Diffusion Model for the Adsorption of Acid Dyes on Activated Carbon

The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80, Acid Red 114 and Acid Yellow 117 removal have been investigated. A three-resistance mass transport model based on film, pore and surface diffusion control has been applied to model the concentration decay curves. The model incorporates an effective diffusion coefficient D _eff, which is dependant on the equilibrium solid phase concentration or fractional surface coverage. The results of the film-pore-surface diffusion model are compared with the data obtained from the basic film-pore diffusion model. It has been found that the film-pore-surface diffusion model provides a major improvement over the data correlated by the film-pore diffusion model. Also, the relationship between surface diffusion and fractional surface coverage has been investigated for the adsorption of acid dyes on activated carbon.     

A Film-Pore-Surface Diffusion Model for the Adsorption of Acid Dyes on Activated Carbon

The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80, Acid Red 114 and Acid Yellow 117 removal have been investigated. A three-resistance mass transport model based on film, pore and surface diffusion control has been applied to model the concentration decay curves. The model incorporates an effective diffusion coefficient D _eff, which is dependant on the equilibrium solid phase concentration or fractional surface coverage. The results of the film-pore-surface diffusion model are compared with the data obtained from the basic film-pore diffusion model. It has been found that the film-pore-surface diffusion model provides a major improvement over the data correlated by the film-pore diffusion model. Also, the relationship between surface diffusion and fractional surface coverage has been investigated for the adsorption of acid dyes on activated carbon.     

Direct dyes as molecular sensors to characterize cellulose substrates

Six dyes were selected based on their molecular structure to test theirsuitability as sensors to characterize the fine structure of cellulosesubstrates. Cotton, mercerized cotton and microcrystalline cellulose werechosento represent a variety of pore structures typically encountered in practicalapplications. Internally available surface areas were calculated. It ispostulated that roughly 25% of the Connolly surface areas (CSA) of the sensorsDirect Blue 1, Direct Blue 14, Direct Blue 53, Direct Red 28, and Direct Red 2and 30% of the CSA of Direct Yellow 4 are representative of the space requiredfor the sensor to dock onto cellulose surfaces. Molecular weight of the dyeprobes does not serve as a good indicator of sensor size. Molecular structureisa critical factor to take into account when selecting a probe.     

Microfiltration performance of regenerated cellulose membrane prepared at low temperature for wastewater treatment

A series of regenerated cellulose membranes with pore diameters ranging from 21 to 52 nm have been prepared by dissolving cellulose in 5 wt% LiOH/12 wt% urea aqueous solution re-cooled to −12 °C. The influences of cellulose concentration on the structure, pore size, and the mechanical properties of the membrane were studied by using Wide angle X-ray diffraction, scanning electron micrography and tensile testing. Their pore size, water permeability, equilibrium-swelling ratio and fouling behaviors of the cellulose membranes were characterized. The water-soluble synthetic and natural polymers as organic matter were used to evaluate the microfiltration performance of the regenerated cellulose membrane for wastewater treatment in aqueous system. The results revealed that the organic matter with molecular weight more than 20 kDa effected significantly on the membrane pore density, and reducing factor a _2, whereas that having molecular weight less than 20 kDa exhibited a little influence on the membrane pore size reducing factor a ₁. Furthermore, a simple model to illustrate of microfiltration process of the RC membrane for wastewater treatment was proposed.     

Theoretical investigation of azo dyes adsorbed on cellulose fibers: 2. Spectroscopic study

Electronic absorption spectra and the frontier orbitals of 1-arylazo-2-naphtol dyes are computed and analyzed in four models, namely in the gas phase (model I), in a solvent (model I + CPCM), adsorbed on the cellulose surface (model II), and model II in the presence of solvent (model II + CPCM) via time-dependent density functional theory (TD-DFT) and conductor-like polarizable continuum model (CPCM) at the B3LYP/6-31G** level of theory. A bathochromic shift is observed for the λ_max peak due to both short-range and long-range interactions of the non-ionic dyes with cellulose, while the ionic dyes exhibit hypsochromic shift in their λ_max peak. The results predict that the studied dyes should be nearly yellow after being adsorbed on cellulose with excellent color strength. Furthermore, the ionic dyes are suitable for the dyeing of cellulose fibers. The nuclear magnetic resonance (NMR) chemical shieldings calculated for the azo dyes in the gas phase and adsorbed states and for their tautomeric equilibrium mixtures show that the NMR technique can be used successfully to follow the dyeing process.     

Theoretical investigation of azo dyes adsorbed on cellulose fibers: 1. Electronic and bonding structures

Structural, bonding and electronic characteristics of complexes of anthraquinone and 1-arylazo-2-naphtol dyes and cellulose I _β are studied using B3LYP density functional method with 6-31G** basis set based on the partially and fully optimized structures. Results reveal that for both partially and fully optimized complexes, there is a stabilizing attraction between dyes and cellulose surface. The hydrazone (Hy) tautomer in anionic state (Hy–SO₃ ^?) shows the strongest interaction with the cellulose surface. Natural bond orbital (NBO) and atoms-in-molecules (AIM) analyses have been carried out to study the nature of azo dyes-cellulose bonds in detail. According to NBO analysis, a remarkable charge transfer occurs between the –SO₃ ^? and –SO₃H functional groups of the dye and the cellulose surface which can be regarded as the main source of the large dye–cellulose interaction energy. AIM analysis confirms the existence of hydrogen and van der Waals bonds between the azo dyes and cellulose. Furthermore, a very good agreement is observed between the number of hydrogen bonding sites and dye–cellulose interaction energies.     

Photocatalytic reduction of azo dyes Naphthol Blue Black and Disperse Blue 79

Photocatalytic reduction of two textile azo dyes, Naphthol Blue Black (NBB) and Disperse Blue 79 (DB79) has been carried out in colloidal WO₃ and TiO₂ suspensions. Under bandgap excitation of the semiconductor colloids these dyes undergo irreversible reduction as they react with the trapped electrons. The quantum efficiency for the photocatalytic reduction of these dyes were 5.4% and 4.8% for NBB and DB79 respectively. The kinetics and mechanism of the interfacial charge transfer in these colloidal suspension has been elucidated with transient absorption spectroscopy. The reaction between the dye and trapped electrons is diffusion limited and occurs with rate constants of 1.1×10⁸ M⁻¹s⁻¹ and 4.0×10⁷ M⁻¹s⁻¹ for NBB and DB79 respectively.     

Theoretical modeling of water vapor transport in cellulose-based materials

The theory of mass transport in porous media is of fundamental importance for different applications such as food, paper packaging, textiles, and wood for building materials. In this study, a theoretical water vapor transport model has been developed for cellulose-based materials, such as paper and regenerated cellulose film. Pore diffusivities were determined from the dynamic moisture breakthrough experiments comprising a stack of paper sheets and regenerated cellulose films in a configuration similar to a packed adsorption column. Other mass transfer parameters were determined from transient moisture uptake rate measurements. The model incorporates pore and surface diffusion as a lump parameter into a variable effective diffusion coefficient. The mass transport, involving both pore and surface diffusions, is evaluated independently. The theoretical water vapor transmission rates (WVTRs) obtained from the model were compared with experimentally determined WVTRs measured under steady-state conditions. The theoretical model, based on intrinsic diffusion, stipulates higher WVTR values compared to the experimental results. However, the theoretical water vapor transfer rates agree well with the experimental results when external mass transfer resistance is incorporated in the model.     

A comparative adsorption study with different industrial wastes as adsorbents for the removal of cationic dyes from water

Four adsorbents have been prepared from industrial wastes obtained from the steel and fertilizer industries and investigated for their utility to remove cationic dyes. Studies have shown that the adsorbents prepared from blast furnace sludge, dust, and slag have poor porosity and low surface area, resulting in very low efficiency for the adsorption of dyes. On the other hand, carbonaceous adsorbent prepared from carbon slurry waste obtained from the fertilizer industry was found to show good porosity and appreciable surface area and consequently adsorbs dyes to an appreciable extent. The adsorption of two cationic dyes, viz., rhodamine B and Bismark Brown R on carbonaceous adsorbent conforms to Langmuir equation, is a first-order process and pore diffusion controlled. As the adsorption of dyes investigated was appreciable on carbonaceous adsorbent, its efficiency was evaluated by comparing the results with those obtained on a standard activated charcoal sample. It was found that prepared carbonaceous adsorbent exhibits dye removal efficiency that is about 80-90% of that observed with standard activated charcoal samples. Thus, it can be fruitfully used for the removal of dyes and is a suitable alternative to standard activated charcoal in view of its cheaper cost.     

Longitudinal diffusion behavior of hemicyanine dyes across phospholipid vesicle membranes as studied by second-harmonic generation and fluorescence spectroscopies

The adsorption and longitudinal diffusion behaviors of a series of hemicyanine dyes to phospholipid vesicle membranes were studied by second-harmonic generation (SHG) and fluorescence spectroscopies. It was observed that the longitudinal diffusion of cationic hemicyanine dyes takes place immediately after the initial adsorption of these dyes to the outer surface of the vesicle membrane. In contrast, hardly any amount of a zwitterionic hemicyanine dye with a sulfonate group diffused across the vesicle membrane within the measurement time (<2000 s). Based on the difference in the time-course responses of SHG and fluorescence spectroscopies for all of the hemicyanine dyes tested, we propose that hydration of the sulfonate group is mainly responsible for the low diffusivity of the zwitterionic hemicyanine dye.     

The dissolution of cellulose in NaOH-based aqueous system by two-step process

A new dissolution method, a two-step process, for cellulose in NaOH/urea aqueous system was investigated with ¹³C NMR, wide X-ray diffraction (WXRD), and solubility test. The two steps were as follows: (1) formation and swelling of a cellulose–NaOH complex and (2) dissolution of the cellulose–NaOH complex in aqueous urea solution. The dissolution mechanism could be described as strong interaction between cellulose and NaOH occurring in the aqueous system to disrupt the chain packing of original cellulose through the formation of new hydrogen bonds between cellulose and NaOH hydrates, and surrounding the cellulose–NaOH complex with urea hydrates to reduce the aggregation of the cellulose molecules. This leads to the improvement in solubility of the polymer and stability of the cellulose solutions. By using this two-step process, cellulose can be dissolved at 0–5 °C in contrast to the known process that requires −12 °C. Regenerated cellulose (RC) films with good mechanical properties and excellent optical transmittance were prepared successfully from the cellulose solution.