Heterogeneous Cross-Linking of Chitosan Gel Beads: Kinetics, Modeling, and Influence on Cadmium Ion Adsorption Capacity

Removal of hexavalent chromium using a novel cross linked xanthated chitosan

Suitability of a novel cross linked, chemically modified chitosan as highly efficient adsorbent for the recovery of toxic chromium(VI) was studied. After cross linking with glutaraldehyde, xanthate group was grafted onto the back bone of chitosan. Sorption was found to be both pH and concentration dependent, with pH 3 being the optimum value. Both, chemically modified beads (CMCB) and flakes (CMCF) followed a pseudo-second-order kinetics with a rate constant of 2.037 and 4.639 g/mg/min, respectively. The equilibrium data followed the Langmuir isotherm model with maximum capacities of 625 mg/g and 256.4 mg/g and for CMCF and CMCB respectively. Desorption studies revealed the reusability of the sorbent for at least 10 cycles without any significant change in adsorption capacities.

A novel soft and cotton-like chitosan-sugar nanoscaffold

A novel type of chitosan nanoscaffold with a soft and cotton-like appearance is proposed. The key to success is based on two points: (i) the change in morphology of chitin whisker to chitosan nanoscaffold and (ii) the surface modification of the nanoscaffold chitosan with a sugar unit. Simple deacetylation of chitin whisker gives a colloidal solution of chitosan, of which the chitosan is in a nanoscaled scaffold. Surface functionalization of the chitosan nanoscaffold with lactose or maltose via a heterogeneous system in water at room temperature results in a soft and cotton-like chitosan containing mesopores. As all steps are organic solvent free, this chitosan-sugar nanoscaffold might be a promising material for biopolymer-supported tissue engineering.

Kinetics and equilibrium adsorption of Cu(II), Cd(II), and Ni(II) ions by chitosan functionalized with 2[-bis-(pyridylmethyl)aminomethyl]-4-methyl-6-formylphenol

Chitosan biopolymer chemically modified with the complexation agent 2[-bis-(pyridylmethyl)aminomethyl]-4-methyl-6-formylphenol (BPMAMF) was employed to study the kinetics and the equilibrium adsorption of Cu(II), Cd(II), and Ni(II) metal ions as functions of the pH solution. The maximum adsorption of Cu(II) was found at pH 6.0, while the Cd(II) and Ni(II) maximum adsorption occurred in acidic media, at pH 2.0 and 3.0, respectively. The kinetics was evaluated utilizing the pseudo-first-order and pseudo-second-order equation models and the equilibrium data were analyzed by Langmuir and Freundlich isotherms models. The adsorption kinetics follows the mechanism of the pseudo-second-order equation for all studied systems and this mechanism suggests that the adsorption rate of metal ions by CHS-BPMAMF depends on the number of ions on the adsorbent surface, as well as on their number at equilibrium. The best interpretation for the equilibrium data was given by the Langmuir isotherm and the maximum adsorption capacities were 109 mg g-1 for Cu(II), 38.5 mg g-1 for Cd(II), and 9.6 mg g-1 for Ni(II). The obtained results show that chitosan modified with BPMAMF ligand presented higher adsorption capacity for Cu(II) in all studied pH ranges.

Monitoring the self-assembly of chitosan/glutaraldehyde/cysteamine/Au-colloid and the binding of human serum albumin with hesperidin

A new method for monitoring, in real-time, the self-assembly of chitosan/glutaraldehyde/cysteamine (CGC) on the gold surface and the immobilization of Au-colloid on CGC membrane with piezoelectric quartz crystal impedance (PQCI) are firstly proposed. Cyclic voltammogram and electrochemical impedance spectroscopy were also use to investigate the formation of Au-colloid/CGC. The viscosity-average molecular weight of chitosan used was firstly estimated as 16.8 x 10(5) with piezoelectric quartz crystal (PQC) sensor. On the basis of the analysis of the multi-dimensional information provided by PQCI, two stages existed in chitosan drying course: the frequency shift of the first stage was controlled by viscoelastic of the liquid, while the total frequency shift was due to mass change. The cross-link ratio of glutaraldehyde with chitosan was about 0.13, while for glutaraldehyde with cysteamine was about 0.217. PQCI also showed that the Au-colloid immobilization is a first-order reaction, while the HSA immobilization is a sum of two exponential functions, e.g., adsorption and re-arrangement. The association of the immobilized HSA with the purified hesperidin was monitored, and the association constant was estimated as 3.42 ml mg(-1) by Scatchard analysis.

Uptake of mercury by thiol-grafted chitosan gel beads

This study describes the synthesis and characterization of thiol-grafted chitosan beads for use as mercury (Hg) adsorbents. Chitosan flakes were dissolved and formed into spherical beads using a phase inversion technique, then crosslinked to improve their porosity and chemical stability. Cysteine was grafted onto the beads in order to improve the adsorption affinity of Hg to the beads. The beads possessed an average diameter of 3.2 mm, porosity of 0.9, specific surface area of approximately 100 m2/g, average pore size of approximately 120 angstroms, and specific gravity of 2.0. Equilibrium and kinetic uptake experiments were conducted to study the uptake of Hg by the beads. The adsorption capacity was approximately 8.0 mmol-Hg/g-dry beads at pH 7, and decreased with decreasing pH. Hg adsorption kinetics was modeled as radial pore diffusion into a spherical bead with nonlinear adsorption. Use of the nonlinear Freundlich isotherm in the diffusion equation allowed modeling of the uptake kinetics with a single tortuosity factor of 1.5 +/- 0.3 as the fitting parameter for all initial Hg concentrations, chitosan loadings, and agitation rates. At agitation rates of 50 and 75 rpm, where uptake rate was reduced significantly due to the boundary layer effect, the mass transfer coefficient at the outside boundary was also used as a fitting parameter to model the kinetic data. At agitation rates higher than 150 rpm, pore diffusion was the rate-limiting step. The beads exhibited a high initial uptake rate followed by a slower uptake rate suggesting pore diffusion as the rate-determining step especially at high agitation rates. Higher uptake rates observed in this study compared to those in a previous study of chitosan-based crab shells indicate that dissolution and gel formation increase the porosity and pore accessibility of chitosan.

Kinetics of cadmium uptake by chitosan-based crab shells

Crushed crab shells were chemically treated to transform the chitin present into chitosan. Three particle sizes with average diameters of 0.65, 1.43 and 3.38 mm, average pore diameters ranging from approximately 300 to 540 A, and a specific surface area of approximately 30 m2/g were obtained. Batch experiments were performed to study the uptake equilibrium and kinetics of cadmium by chitosan. Adsorption equilibrium followed a Freundlich relationship and was found to be independent of particle size indicating that adsorption takes place largely in the pore space. A high initial rate of cadmium uptake was followed by a slower uptake rate suggesting intraparticle diffusion as the rate-limiting step. The kinetic uptake data were successfully modeled using a pore diffusion model incorporating nonlinear adsorption. The effect of boundary layer resistance was modeled through inclusion of a mass transfer expression at the outside boundary. Two fitting parameters, the tortuosity factor (tau) and the mass transfer coefficient at the outside boundary (k(c)) were used. These parameters were unique for all solute and sorbent concentrations. The tortuosity factors varied from 1.5 for large particles to 5.1 for small particles. The mass transfer coefficient varied from 2 x 10(-7) m/s at 50 rpm to 2 x 10(-3) m/s at 200 rpm. At agitation rates below 100 rpm, boundary layer resistance reduced the uptake rate significantly. Its very high sorption capacity and relatively low production cost make chitosan an attractive sorbent for the removal of heavy metals from waste streams.

Reversible adsorption by a pH- and temperature-sensitive acrylic hydrogel

Thermo- and pH-sensitive hydrogels were synthesized using N-isopropylacrylamide (NIPA) and N-aminopropylmethacrylamide, cross-linked with N,N'-methylenebis(acrylamide). The dependence of the degree of swelling on the cross-linking density was analyzed according to the Flory-Huggins theory and a master curve obtained. To optimize the efficiency of these hydrogels in controlled release, we studied the loading and release of a divalent molecule (naphthalenedisulfonic acid, NS-2) in media of different ionic strengths and pH. The uptake process followed the Langmuir adsorption isotherm model. The highest loading occurred when the amino groups in the gel were protonated (acidic pH) and could come close each other to form a binding site for the two sulfonic groups of NS-2, i.e. low degree of cross-linking and collapsed state. Below the phase transition temperature (33 degrees C), NS-2 loaded hydrogels quickly released a significant amount of adsorbate until a new equilibrium between free NS-2 and adsorbed NS-2 was achieved. Above that temperature, hydrogels not only stopped the release but were even able to take free NS-2 up again from the medium, showing that the loading/release process was reversible and reproducible after several temperature cycles. At 37 degrees C, the release rate was independent of the degree of cross-linking (NIPA caused the hydrogel to collapse), but was strongly affected by the pH and salt concentrations of the medium, which condition the strength of the interaction between the hydrogel amino groups and the NS-2 sulfonic groups. In an acidic medium, the protonated amino groups bind NS-2 strongly and the amount released is small. In contrast, at pH 7.4 or in the presence of a high salt concentration, the hydrogel loses its affinity for NS-2 and the release rate increases, giving pH- or salt-sensitive delivery systems. Additionally, since the hydrogel is collapsed, the release can be prolonged for a long period of time.

Film-pore diffusion model for the fixed-bed sorption of copper and cadmium ions onto bone char

The sorption of copper and cadmium ions onto bone char in single component systems has been studied using fixed-bed column adsorbers. The effects of solution flowrate, initial metal ion concentration and bone char particle size have been studied. A film-pore diffusion model has been developed to predict the fixed-bed breakthrough curves for the two metal ions. A sensitivity analysis has been carried out to investigate the influence of the external mass transfer coefficient (film resistance), the effective diffusion coefficient (pore diffusion) and the solid phase loading capacity. It is found that under the experimental conditions employed in the study, film diffusional resistance was low and the Biot numbers were relatively high. Furthermore, a constant effective pore diffusivity was not sufficient to correlate the breakthrough curves accurately and a variable dependent effective diffusivity was required; suggesting a possible contribution from surface diffusion. Since the metal ion-bone char systems take a long time to reach equilibrium, the solid phase loading capacity, as predicted by the "best-fit" equilibrium isotherm, was not suitable for use in the diffusional mass transport model and the mass balance solid phase loading was utilised instead.

Combined diffusion model for the sorption of cadmium, copper, and zinc ions onto bone char

The ability of bone char to remove three single-component metals (namely, cadmium, copper and zinc) from aqueous solutions has been studied. Equilibrium isotherms have been measured and analyzed using a Langmuir isotherm model. A series of batch contact time experiments were performed to study the effect of sorbent mass and initial metal ion concentration for the three systems. A new film-pore diffusion model has been applied to the batch contact time results, and good correlation is obtained between theoretical predictions and experimental data. A further extension to the new model was made to include a surface diffusion term. The agreement between theoretical results and experimental data improved as the sum of the squares of the errors (SSE) improved by around 20% for cadmium and zinc but by 50% for copper. Therefore, the sorptions of cadmium and zinc ions onto bone char are primarily film-pore diffusion controlled, but for the copper system film-pore-surface diffusion must be considered.