Colloids and Surfaces A: Physicochemical and Engineering Aspects
Lead sorption from aqueous solutions on chitosan nanoparticles
Introduction
Biopolymers, chitosan is the deacetylated form of chitin and composed of glucosamine, known as (1–4)-2-amino-2-deoxy-β-d-glucose. Chitosan has three types of reactive functional groups, an amino group as well as both primary and secondary hydroxyl groups at the C-2, C-3 and C-6 positions, respectively [1]. This special structure makes it exhibit chelation with various metal ions [2]. Muzzarelli [3] pointed out that chitosan combines with metal ions by three forms: ion exchange, sorption and chelation. Chitosan has been broadly used for the sorption of heavy metal ions [4], [5], [6]. Further physical and chemical modifications of chitosan have been made to improve the selectivities and the capacities for metals ions [7], [8], [9], [10]. Chitosan is also characterized by weak diffusion properties: long contact times are required to reach equilibrium. Sorption capacity can be controlled by sorbent particle size [8]. Due to the low porosity of chitosan, sorption performances are frequently controlled by mass transfer resistance. To reduce this resistance to mass transfer, chitosan gel beads have been developed to expand the polymer structure and reduce its crystallinity favouring sorption rate [11], [12], [13]. However, these treatments results in either a decrease of the number of available sorption sites (cross-linking treatment), or the volumetric sorption capacity (the percentage of water in the gel beads can reach 95%) [14]. Controlled drying can increase the volumetric sorption capacity [15]. Another possibility for increasing this volumetric sorption capacity is the grafting of supplementary functional groups [16].
A number of nanoscale inorganic particles (NIPs) offer favourable properties in regard to selective removal of target contaminants. For example, hydrated Fe(III) oxides particles can selectively sorb dissolved heavy metals like zinc, copper or metalloids like arsenic oxyacids or oxyanions [17]. Amorphous and crystalline Fe(III) hydroxides have long been known to possess selective sorption properties toward arsenites and arsenates [18]. Polymer supported nanoparticles have been prepared and used for selective removal of target arsenic compounds and heavy metals [17]. Chitosan nanoparticles had been synthesized based on polymer and applied as drug carriers as reported in previous studies [19], [20], [21].
However, researches about the sorption properties of chitosan nanoparticles are seldom reported now. The unique character of nanoparticles for their small size, great surface area and quantum size effect could make it exhibit higher capacities for metal ions. The present work aims to synthesize chitosan nanoparticles by ionic gelation of chitosan and tripolyphosphate and evaluates their sorption capacity of lead ions. Freeze-drying was applied in preparation of chitosan nanoparticles, and new sorption sites polyphosphoric groups were introduced by slightly crosslinking with tripolyphosphate to increase the volumetric density of sorption sites and the sorption capacity.
Section snippets
Materials
Chitosan, obtained from Chitosan Company of Panan city of Zhejiang province in China, was refined twice by dissolving it in dilute acetic acid solution, filtered, precipitated with aqueous sodium hydroxide, and finally dried in vacuum at room temperature [22]. The degree of deacetylation was about 85%, and weight of chitosan was 220 kDa, determined by viscometric methods [23]. Sodium tripolyphosphate (TPP) was supplied by Sigma Chemical Co. (USA). Lead nitrate was purchased from Shanghai
Size and morphology of chitosan nanoparticles
The preparation of chitosan nanoparticles was based on an ionic gelation interaction between positively-charged chitosan and negatively-charged tripolyphosphate at room temperature [25], [26]. The chitosan nanoparticles prepared in the experiment exhibit white powder shape, and were insoluble in water, dilute acid, and alkalescent solution.
The mean size and size distribution of each batch of nanoparticles suspension was analyzed using the Zetasizer analysis. The size distribution profile, as
Conclusion
Different mean size of chitosan nanoparticles have been prepared by ions gelation of chitosan and tripolyphospahte. The morphology changes of nanoparticles after sorption of lead ions are revealed by AFM observations. FTIR spectra reveal the functional groups of chitosan nanoparticles and the interaction with lead ion, the amine and phosphoric groups of nanoparticles provide sorption sites for lead ions. Chitosan nanoparticles possess lower crystallinity than chitosan illuminated by XRD
Acknowledgements
We would like to thank Zhejiang Provincial Science and Technology Committee of China (No. 021102680) for financial support.
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