Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy

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Abstract

X-ray photoelectron spectroscopy (XPS) is employed to study chemical interactions between three metal ions — Cu(II), Mo(VI), and Cr(VI) — and chitosan, a natural biopolymer extracted from crab shells. Three forms of chitosan are used — flakes, beads, and modified beads obtained by glutaraldehyde cross-linking. XPS provides identification of the sorption sites involved in the accumulation of metals, as well as the forms of species sorbed on the biopolymer. It is found that sorption occurs on amine functional groups for all the three metals. With copper, the sorption step is not followed by reduction of the metal. More complex phenomena are involved in molybdate removal. A partial reduction (about 20–25% of the total molybdenum content) occurs with chitosan beads and flakes. The distribution of reduced Mo(V) on the surface of the sorbent differs from that in the bulk of the sorbent for raw chitosan beads, while the glutaraldehyde cross-linking allows uniform distribution of reduced Mo(V) throughout the sorbent. The difference between these two forms of chitosan can be related to a complementary photoreduction step occurring on the surface of the biopolymer. For chromium, a similar trend with molybdenum is followed but to a greater extent; with cross-linked sorbents all chromate previously sorbed is reduced to Cr(III), while with raw chitosan beads Cr(VI) reduction does not exceed 60%.

Introduction

The development of industrial activities dealing with metal ions and the strengthening of discharge and health protection regulations have led to an increasing number of studies of alternative wastewater treatment processes. Among the alternative processes suggested in the last 40 years, biosorption has received great attention due to the diversity of the available sorbent materials. Some of the biosorbents used are fungal or bacterial biomass and biopolymers such as alginate or chitosan obtained as by-products of marine industries. Most of the studies on biosorption have been dedicated to the determination of the overall uptake performance; however, there is limited information available on identifying sorption mechanisms.

Volesky and Tsezos [1] have shown that the main mechanisms involved in uranium and thorium biosorption on fungal biomass are due to adsorption, complexation, and precipitation steps. Ion-exchange mechanisms are also frequently cited [2]. For other metals, such as gold, the participation of reductive steps has been suggested by Kuyucak and Volesky [3]. The reduction process can also decrease metal toxicity in solution as cited for uranium uptake by Clostridium sp. [4]. Similar mechanisms are commonly encountered in the sequestration of metals using other sorbents such as biopolymers [5], [6].

Chitosan is a polymer [β-(1→4)-2-amino-2-deoxy-d-glucopyranose] extracted from chitin using an alkaline deacetylation procedure that yields a heteropolymer (polyglucosamine/poly-acetylglucosamine). It is characterized by high nitrogen content and low porosity resulting in kinetic restrictions in the sorption mechanisms, as well as a reduction in the sorption capacities by increasing particle size [6]. To increase sorption performance, it is possible to modify the structure of the biopolymer by chemical modifications and substitutions [6], physical modifications [7], and gel formation [8]. Several metals are preferentially sorbed in acidic media in which chitosan can dissolve. To increase its stability under such experimental conditions, chitosan sorbent is cross-linked using, for example, a Schiff's base reaction with glutaraldehyde [9]. The introduction of aldehyde moieties on the chitosan backbone can reinforce the reductive activity observed on the raw polysaccharide [10]. In addition, light exposure has been shown to increase this reduction step.

Several conventional techniques have been used to identify surface groups on sorbents and sorbed species [11], however, these methods involve extensive preparation of the samples that can affect linkage to the sorbent. Spectroscopic methods are usually non-destructive and supply interesting information. Core electron X-ray photoelectron spectroscopy (XPS) has been found to be a useful tool in characterizing ligand effects in transition-metal complexes; electron-donating ligands will lower the binding energy (BE) of the core level electrons and electron-withdrawing ligands will raise their BE.

XPS techniques have been widely used for the characterization of structure of metal compounds and their interactions with membranes and films constituted by polymers [12], [13], [14], as well their interactions with catalysts [15], algal biomass [3], yeast [16], and synthetic sorbents [17]. XPS has been used by Maruca et al. [18] to study Cr(III) sorption by chitin and chitosan. More recently XPS techniques have been carried out by Ni and Xu [19] for the interpretation of chemical interactions between chitosan-based sorbents (obtained by the grafting of mercapto compounds) and several metal ions such as Ag(I), Au(III), Pd(II), Pt(IV), Cu(II), Hg(II), and Zn(II). They have shown that sorption of these metals occurs mainly on the sulfur moieties of sorbents.

In spite of the interest in identifying sorbent functional groups and their interactions with metals, there are limited studies on the direct interaction of raw and basic chitosan with metal ions. A more focused XPS study is necessary to explain the differences observed in the interactions of chitosan with certain metal ions. For example, studies using various spectroscopic methods have shown that a reduction step can occur during or after metal sorption [20]. This metal reduction does not occur at the same extent or under the same conditions for all metals. To understand this phenomenon better, the present study investigates the interactions of Cu(II), Cr(VI), and Mo(VI) with chitosan (in either flake or bead conditioning) and glutaraldehyde-cross-linked chitosan. The choice of these metals was guided by their chemical redox properties in the optimum experimental conditions for sorption; Cr(VI) is known to be easily reduced in acidic media; under similar conditions Mo(VI) is more stable; while Cu(II) is quite stable in neutral (or mild acidic) media, in the absence of a strong reducing agent. The influence of biopolymer conditioning and that of bulk or surface phenomena is studied and interpreted with reference to light exposure and redox potentials.

Section snippets

Materials

Chitosan was supplied by Aber-Technologies, France (Lot No. A17G28), and the main characteristics of the sorbent are acetylated fraction, FA=0.13; number average molecular mass, MWn=125 000; and weight average molecular mass, MWw=191 000 [8]. The gel bead fabrication protocol has previously been described [8]. After rinsing, the beads were cross-linked using glutaraldehyde (2.5% aqueous solution); a Schiff's reaction occurs between aldehyde groups on glutaraldehyde and amine groups on the

Sorbent characterization

XPS analysis was initially conducted for sorbents prior to sorption in order to characterize the available functional groups. The sorbents were used in three forms — flakes, beads, and cross-linked beads. In some cases, beads were ground in order to provide information on the distribution of functional groups on the surface and in the bulk of the sorbent.

In general, amino-polysaccharides can be characterized by bands representing carbon, nitrogen, and oxygen. The electronic environment of these

Summary and conclusions

XPS methods demonstrate that metal ion interactions with chitosan in several forms involve various changes in the metal oxidation state, as well as in the chemical state of the reactive sites of the biopolymer. These changes and their extent depend on the metal and the conditioning of the biopolymer (flakes, raw or cross-linked beads). Metal sorption occurs on amine functions for all the metals used in this study.

With copper, sorption is not followed by a reduction of the metal. Experiments

Acknowledgements

Acknowledgments are due to Secrétariat d'Etat à l'Industrie, France, for financial support (fellowship application of LD). Support to SY was provided by the U.S. National Science Foundation through a Career award (BES-9702356).

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