J. Roussy

Vanadium (IV) sorption by chitosan: Kinetics and equilibrium

Abstract The adsorption of vanadium (IV) by chitosan, a naturally occurring material, is studied according to equilibrium and kinetics. Sorption isotherms are determined and single mechanisms of diffusion are studied. These are regarded as the main limiting steps. The parameters studied are: pH, the initial metal concentration, the particle size of the polymer and the stirring speed. While the fourth parameter has no effect on overall sorption performances, equilibrium and kinetics are greatly influenced by the other three. The speciation of metal in solution, relative to pH and total metal concentration, plays an important part in the separation factor between the solid and liquid phases, and on the diffusion of solute through the polymer surface. It has been demonstrated that the sorption, in the case of chitosan is mainly located on the surface. The diffusion mechanisms are both external and intraparticle phenomena: but diffusion is restricted to a thin layer of the particle. An increase of the particle size results in a greater time to reach equilibrium. The Langmuir and Freundlich models show relative correlations difficult to estimate considering to the pseudo rectangular isotherm obtained: the equilibrium plateau is quickly reached.

A Review of the Use of Chitosan for the Removal of Particulate and Dissolved Contaminants

Abstract Chitosan has unique properties among biopolymers, especially due to the presence of primary amino groups. Chitosan has been used for the chelation of metal ions in near‐neutral solution, the complexation of anions in acidic solution (cationic properties due to amine protonation), the coagulation of negatively charged contaminants under acidic conditions, and for precipitative flocculation at pH above the pKa of chitosan. The coagulation and flocculation properties can be used to treat particulate suspensions (organic or inorganic) and also to treat dissolved organic materials (including dyes and humic acid). This paper will give an overview of the principal results obtained in the treatment of various suspensions and solutions: (a) bentonite suspensions; (b) organic suspensions; (c) anionic dye solutions; and (d) humic acid solutions. Stoichiometry and charge restabilization were determined for the coagulation of humic acid, kaolin, and organic dyes with chitosan, indicating charge neutralization as the dominant mechanism for removal. Charge patch destabilization and bridging mechanisms were inferred in other cases, based on the effects of the apparent molecular weight of the chitosan preparations and effectiveness of sub‐stoichiometric doses of chitosan. For dye solutions, results showed that color can be removed either by sorption onto solid‐state chitosan or by coagulation‐flocculation using dissolved‐state chitosan; the reactivity of amine groups was significantly increased when dissolved chitosan was used. For humic materials, chitosan can be used as a primary coagulant or as a flocculant after coagulation with alum or other inexpensive coagulants. The influence of the degree of deacetylation and the molecular weight of chitosan on its performance as coagulant/flocculant is illustrated by several examples.

Chitosan for the coagulation and flocculation of mineral colloids

Abstract Chitosan dissolved in acetic or hydrochloric acid was used for the coagulation and the flocculation of bentonite colloids. The influence of pH was investigated and the double effect of the biopolymer was identified. Experiments were performed at different polymer concentrations and at different pHs with simulated suspension (prepared with different amounts of bentonite). At pH 5 (acidic region), the protonation of amine groups led to coagulation effect and very low amounts of polymer were required to achieve a fast and efficient decantation of colloids; at pH close to neutral, a different mechanism was involved in the flocculating effect of the polymer chain (larger amounts of polymer were usually required). The effect of the characteristics of the polymer, i.e., degree of deacetylation, and polymer weight (related to polymer viscosity) was also investigated in order to optimize the selection of the biopolymer for targeted experimental concentrations.

Influence of Hydrolysis Mechanisms on Molybdate Sorption Isotherms Using Chitosan

Molybdate sorption using chitosan sorbents has proved to be strictly controlled by the pH of the solution. Sorption isotherms exhibit a sigmoid trend, which has been correlated to the appearance of polynuclear hydrolyzed species, the most favorable species for sorption on chitosan. Sorption capacity exceeds 7 mmol·g−1, which corresponds to a molar ratio between Mo and the amine group significantly higher than 1. The formation of complexes in a pendant fashion and/or the ion-exchange mechanism of polynuclear metal ions are suspected to occur between polynuclear molybdate species and protonated amine groups, though several amine groups can interact with the same polynuclear molybdate group.

Uptake of uranyl ions by new sorbing polymers: discussion of adsorption isotherms and pH effect

Abstract Chitosan is a natural polymer well known for its efficient uptake of heavy metals. The modification of this polymer by the substitution of functional groups from organic acids allows new sorbents with high sorption capacities to be obtained: NDTC with ascorbic acid and glutamate glucan with oxo-2-glutaric acid. The mechanism of sorption is investigated and several sorption models are applied such as the Freundlich, Langmuir and Temkin models which fit experimental results well. These results are explained by the heterogeneity of the polymer surface, the variability of metal species in solution and possible interactions between the molecules sorbed. The effects of several parameters such as pH and metal ion concentration are examined and demonstrate the relative importance of metal speciation on sorption performances and mechanisms. The grafting of functional groups creates some heterogeneity of sorption energy, which explains the selectivity seen in the best models. Metal ions are firstly bound onto more energetic sites (nitrogen groups) and later onto other substituting functions.

Influence of polymer structural parameters and experimental conditions on metal anion sorption by chitosan

Chitosan is effective at removing molybdate from acidic solutions, especially when it is stabilized by crosslinking treatment. The influence of structural and physicochemical properties of chitosan and the glutaraldehyde crosslinking step on molybdate has been studied. This study shows that the maximum sorption capacity mainly depends on the crystallinity and degree of deacetylation. Hydration and accessibility to internal sites are controlled by these parameters, which may explain the intraparticle diffusion control on mass transfer. Chitosan gel beads can be used instead of flaked particles to decrease sorbent crystallinity. These sorbents were used in column systems, and the breakthrough curves obtained under several experimental conditions (flow velocity and column depth) were modelled using numerical analysis. Molybdate-impregnated chitosan beads (MICB) can be used for As(V) removal: sorption isotherms show high sorption levels even at low arsenic concentrations, and at pH 2-3, arsenic removal is optimal and molybdenum release is minimal. The desorption and regeneration of the sorbent was effective using phosphoric acid solutions.

Molybdate sorption by cross-linked chitosan beads: Dynamic studies

Recent trends in environmental monitoring have induced increasing development of new wastewater treatment techniques. Membrane processes, electrochemical techniques, or ion-exchange systems are widely used, but biosorption has been recognized in the last 30 years as a promising way to reduce the contamination of surface water issued from industrial effluent. Chitosan, a biopolymer extracted from crustacean shells, exhibits high sorption capacities for metal ion recovery. Sorption efficiency and removal rates are controlled by several diffusion mechanisms. Chitosan gel beads have been prepared and have shown enhanced sorption performance in batch systems. This study shows that, in continuous systems, sorption capacities can reach 700 mg/g, a level close to that obtained in batch studies. The effects of metal concentration, flow velocity, and column size are investigated and demonstrate that, because of diffusion mechanisms, the optimum concentration range is approximately 50 to 100 mg/L. In column systems, the Biot number, though greater than 1, is lower than the Biot number obtained in batch systems, indicating that external mass transfer influences mass transfer at the low superficial velocity investigated in this work.

Mercury Recovery by Polymer‐Enhanced Ultrafiltration: Comparison of Chitosan and Poly(Ethylenimine) Used as Macroligand

Abstract: Chitosan is an aminopolysaccharide that has been widely studied for metal ion recovery. In most cases it is used as a sorbent in solid form, but the polymer can also be used in a dissolved form in the so-called Polymer-Enhanced UltraFiltration (PEUF) process. The present work focuses on the use of dissolved chitosan for the removal of mercury from dilute solutions using an Amicon ultrafiltration unit. Recovery performance is compared to that obtained with poly(ethylenimine) (PEI), a synthetic amine-bearing polymer. The pH, metal concentration, and polymer concentration are the principal parameters to be taken into account in evaluating the recovery process. The impact of these parameters was tested with respect to metal and polymer retention and the filtration flow rate. In the case of chitosan, the comparison of molar metal/amine group ratios at saturation of the polymer in its solid state (adsorption process) and dissolved state (PEUF process) shows that dissolving the polymer improves the accessibility of sorption sites and enhances the sorption capacity. Although the addition of chloride strongly decreased mercury retention, it hardly influ-enced PEUF performances when using PEI; this indicates a different binding mechanism or, at least, different contributions on the part of electrostatic attraction and chelating mechanisms at different pHs for these different polymers; linear polymer (chitosan) and branched polymer (PEI).

Mercury sorption on a thiocarbamoyl derivative of chitosan

The grafting of thiourea on chitosan backbone allows synthesizing a thiocarbamoyl derivative that was very efficient for mercury sorption in acidic solutions. Though the sorption capacity is not increased compared to raw chitosan in near neutral solutions, this modification allowed maintaining high sorption capacity (close to 2.3 mmol Hg g(-1)) at pH 2. Mercury sorption in acidic solutions is not affected by the presence of competitor metals (such as Zn(II), Pb(II), Cu(II), Cd(II), Ni(II)) or the presence of nitrate anions (even at concentration as high as 0.8M)). The presence of chloride or sulfate anions (0.8M) decreased Hg(II) sorption capacity to 1 mmol Hg g(-1). Kinetics are controlled by a combination of pseudo second-order reaction rate and resistance to intraparticle diffusion. Mercury desorption reached about 75% using thiourea (in HCl solution).

Adsorption and selectivity of activated carbon fibers application to organics

The adsorption of contaminants from polluted water was performed by Fibrous Activated Carbon (FAC) in batch and in continuous flow reactors. In batch reactors, the adsorption capacity and the adsorption velocity of micropollutants (phenol, atrazine) was higher for fibers than for Granular Activated Carbon (GAC). In continuous flow reactors, the breakthrough curves obtained from the micropollutants were quite steep, suggesting a smaller mass transfer resistance than with GAC. Integration of the breakthrough curves gave a high maximum adsorption capacity for both micropollutants. On the other hand, molecules with high molecular weight (MW), humic substances, were not adsorbed on FAC. The selectivity of FAC was studied in batch and in continuous flow reactors, with mixtures containing micropollutants (phenol) and humic substances. In both reactors, humic substances were not adsorbed on fibers, and the adsorption of phenol was not disturbed by the presence of humic substances.