Removal of Congo red dye from aqueous solution with hydroxyapatite/chitosan composite
Highlights
► Hydroxyapatite–chitosan composite for adsorption of dyes is reported. ► The adsorption capacity of HAp-CS composite for Cango red removal is 769 mg g−1. ► The adsorption capacity of composite is much higher than previously reported adsorbents. ► The adsorption mechanism was studied with FT-IR and XPS. ► It can be useful in water purification for its easy shape and high adsorption capacity.
Introduction
Synthetic dyestuffs widely exist in the effluents of industries such as textiles, printing, paper, plastics and leather. Many dyes and pigments contain aromatic rings in their structures, which make them toxic, non-biodegradable, carcinogenic and mutagenic for aquatic systems and human health [1]. Because of their accumulative effects in biota, the presence of dyes even at very low concentrations in water is not allowed. However, dyes cannot be easily removed with the conventional wastewater treatment methods since they are difficult to be biodegraded or photo-degraded. Therefore, removing dyes from aqueous solution has become an important and challenging area in wastewater treatment [2]. A variety of methods have been employed for removing dyes from colored effluents, such as membrane filtration, oxidation, coagulation, biological treatment, electrochemical process and adsorption [3], [4], [5], [6], [7]. Among these techniques, adsorption is characterized as one of the most effective methods due to its simple operation, high treatment efficiency and economy.
Recently, chitosan (CS), a kind of a cationic polymer produced by the N-deacetylation of chitin [8], has attracted great attention for dye removal because of its high affinity for most dyes [9], [10]. However, drawbacks such as poor mechanical strength, low specific gravity, easy agglomeration or gel formation and insufficient solubility in dilute acids largely limit its widespread applications for environmental pollutant removal [11], [12]. To pave the way for using CS for hazardous dyes removal, efforts have been endeavored to improve the mechanical strength and adsorption efficiency by immobilization of CS on rigid inorganic materials [11]. In this regard, composites based on CS–montmorillonite [12], [13], [14], CS–carbon nanotube [15], [16], CS-γ-Fe2O3 [17] and CS-surfactant [18], have been proposed for various environmental pollutants removal.
Hydroxyapatite [Ca10(PO4)6(OH)2, HAp], a main inorganic constituent of the hard tissues (bone and teeth) in human body, has marked potential in adsorption of various ions, organic molecules and polymers [19]. HAp is also capable of establishing bonds with organic molecules of different size. However, HAp is usually provided in powder or calcined pellets form, which limits its industrial applications. Accordingly, HAp-based composite materials have been investigated. Anchoring CS with HAp to obtain HAp-CS biocomposite material for improved biomedical applications has been reported [20], [21], [22]. While the use of HAp-CS composite for some environmental pollutants removal has also emerged [23], [24], [25], [26], [27], no such efforts for adsorption of dyes have been reported.
Therefore, in this work, we attempt to extend the use of HAp-CS biocomposite for hazardous dye removal. Congo red (CR) [1-naphthalene sulfonic acid, 3,3′-(4,4′-biphenylenebis (azo)) bis(4-amino-) disodium salt] (Fig. 1) was chosen as the model dye in this study. CR is seriously hazard to aquatic living organisms and can cause human carcinogen [28]. A series of HAp-CS composites were prepared through a co-precipitation method [29]. The adsorption capacity, adsorption pattern and possible mechanism, as well as operation parameters including weight contents of chitosan in HAp-CS composite, adsorbent dosage, initial pH, adsorption time and initial dye concentration, were investigated in detail. The results showed HAp-CS composite is a promising adsorbent for removing CR from aqueous solutions.
Section snippets
Materials
Chitosan (>85% de-acetylation) and Congo red used in this study were purchased from Kelong Chemical Industry Co., Chengdu, China. Other reagents were analytical grade and all solutions were prepared with high-purity water.
Preparation of HAp and HAp-CS composite
HAp and HAp-CS composite was synthesized via a co-precipitation method [29], [30]. Briefly, (NH4)2HPO4 solution was dropwisely added to the solution of Ca(NO3)2·4H2O with a stoichiometric ratio of Ca/P = 1.67. The pH of the reaction media was maintained above 10 with a 25%
Characterization of HAp-CS composite adsorbent
FT-IR spectra of pure HAp, pure CS and HAp-CS composite with varied CS weight contents were shown in Fig. 2. It can be seen that all characteristic peaks of both HAp and CS appear in the spectra of HAp-CS composites, except for slight band-shifts and peak height decreases. For example, upon anchoring of CS into HAp, the two specific bands of amino groups (3434 and 1628 cm−1) shifted to higher wavenumbers, indicating possible interactions between CS and HAp. Considering the structures of
Conclusion
In summary, a promising adsorbent for CR removal, based on embedding of HAp into CS, was prepared. The HAp-CS composite was synthesized via a co-precipitation approach. HAp-CS composite with CS weight content of 50% exhibited the highest adsorption capacity for CR (769 mg g−1). Kinetic and isotherm studies showed that pseudo-second-order model and Langmuir model could well describe the adsorption behavior. Thermodynamic investigation indicated that the CR adsorption by HAp-CS composite was a
Acknowledgements
This work was financially supported by the Basic Research Program of Sichuan Province, China (2011JY0016) and the National Science Foundation of China (21145002). The authors also thank Professor Xiandeng Hou of Analytical & Testing Center , Sichuan University for his valuable comments and stimulating discussions.
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