Elsevier

Chemical Engineering Journal

Volume 191, 15 May 2012, Pages 85-94
Chemical Engineering Journal

Novel thiourea-modified magnetic ion-imprinted chitosan/TiO2 composite for simultaneous removal of cadmium and 2,4-dichlorophenol

https://doi.org/10.1016/j.cej.2012.02.071Get rights and content

Abstract

In the present study, a novel type of adsorbent called thiourea-modified magnetic ion-imprinted chitosan/TiO2 (MICT) was prepared with the purpose of improving its feature as a composite adsorbent. The simultaneous cadmium ion adsorption and 2,4-dichlorophenol (2,4-DCP) degradation by this novel composite adsorbent were investigated. The obtained results showed that the optimum pH values for adsorption of cadmium and degradation of 2,4-DCP were approximately 6.0–7.0. The kinetics study demonstrated that the adsorption process proceeded according to the pseudo-second-order model. The maximum adsorption capacity for cadmium was 256.41 mg/g according to the Langmuir model. The intermediate products of the reaction consisted of 4-chlorophenol, 1,4-benzoquinone, phenol, cyclohexanol, and some other trace substances, as identified by gas chromatography/mass spectroscopy (GC/MS) technique. The sequential degradation process was proposed, including reductive dechlorination or reaction with hydroxyl radicals based on the products identified. The used sorbent was reusable after regenerated through desorption, and the adsorption and degradation capacities were barely affected after five cycles.

Highlights

► MICT was developed for effective Cd(II) adsorption and 2,4-DCP degradation. ► Ion-imprinted and photocatalysis technologies were coupled in the synthesis process. ► The properties of MICT were greatly improved compared with those previously reported. ► The sorption and degradation capacities are barely affected after five cycles.

Introduction

The co-contamination of aquatic systems with heavy metal ions and toxic organic pollutants is a problem of global concern [1]. Heavy metals and aromatic compounds from industrial activities, such as plating, metallurgy, and dyeing, are a threat to humans and to the environment due to their toxicity and persistence after they are released into the natural environment [2]. Among these pollutants, cadmium has attracted the attention of environmentalists as one of the most toxic heavy metals [3]. Some aromatic compounds, such as 2,4-DCP can provoke disturbances in the structure of cellular bilayer phospholipids, which may cause carcinogenic and mutagenic effects [4]. Many traditional methods applied to remove the two kinds of pollutants, such as electrochemical precipitation, ion exchange, reverse osmosis and solvent extraction, are mainly based on applications in single systems containing either metal ions or organic solutes [5]. Moreover, these conventional separation techniques have many disadvantages for example; the high cost, possible production of secondary toxic compounds and the generation of sludge leading to high disposal costs [6], [7]. Adsorption has been recognized as one of the most popular and effective methods for the removal of the above-mentioned pollutants from wastewaters due to the flexibility in design and operation offered by the adsorption process [8]. However, this process is expensive, so low-cost biosorbents have been given increasing attention as they can significantly reduce the cost of an adsorption system [9].

Chitosan and chitin are the most important materials examined for removal of toxic metal ions owing to their inexpensive and effective in natures [10]. Chitosan is hydrophilic, biodegradable, harmless to living things, and offers ease of chemical derivatization. Moreover, chitosan has many amino and hydroxyl groups that can chelate heavy metals. Therefore, chitosan is a very promising material for chelating resins [11], [12]. However, its flaws, such as weak mechanical strength, dissolution in acidic solutions, and leaching of organics like carbohydrates, are serious when raw chitosan is used. These drawbacks have hindered the applications for treatment of waste metallic streams [12], [13].

In recent years, various efforts have been focused on the stability of chitosan, which is a potential way to overcome the disadvantages. Chitosan is often crosslinked to confer better microbiological and mechanical resistance [14]. Crosslinking agents like glutaraldehyde, epichlorohydrin, triphosphate, and ethyleneglycol diglycidylether have been used to stabilize chitosan in acid solutions [11], [12], [13], [14], [15]. However, the metal uptake efficiency of crosslinked chitosan is reportedly often much lower than that of raw chitosan. The functional groups on chitosan for metal binding are involved in the crosslinking reaction; thus, the sorption decreases. In the present study, an innovative ion-imprint technology was developed with the objective of achieving higher sorption capacity of resin and stability. The ability of a material to capture metals is controlled in part by the number of available functional groups used for binding metals. Thus, efforts in the current research were directed towards modification with chelating functionalities to improve the adsorption capacity.

Adsorption capacity was improved extensively through the above process. For organic pollutants removal, a novel approach to achieve this purpose is developing photo-oxidation via an advanced oxidation process (AOP). AOPs have been proven to be an effective treatment method for the degradation of toxic organic pollutants from wastewaters [16]. In this application, TiO2 is a benchmark photocatalyst because of its various merits, such as chemical stability, high photocatalytic activity, and non-toxicity. TiO2 has an excellent ability to degrade numerous kinds of organic pollutants in water into harmless end-products of CO2, H2O, and some simple mineral acids [17].

The topic of TiO2 coupled with chitosan was studied by some researches in recent years. For example, Qian et al. [18] prepared a novel bactericidal and mildew-proof fabric through immobilization of TiO2-chitosan composite on cotton fibers, which exhibits high bactericidal ratios for Escherichia coli, Staphylococcus aureus and Aspergillus niger under visible light irradiation. In the wastewater-purification area, TiO2 and chitosan were also taken together to synthesize microporous materials for the arsenic, Ni2+ adsorption and methyl, reactive dyes removal [19], [20], [21]. However, there were many drawbacks existed in these researches, for example; a single treatment object, low removal efficiency, difficult for separation and recycle. In light of the foregoing, the current study presents the combined effect of photodegradation-adsorption using the prepared chitosan/TiO2 composite under the illumination of UV light as a new method for the treatment of heavy metal and organic co-contaminating wastewater. Efforts were directed towards overcome the above drawbacks to improve its feature.

Separating the adsorbents from the reaction solution is a problem after adsorption. Traditional separation methods, such as filtration and sedimentation, are ineffective due to the small size and low density of the adsorbents. Magnetic separation technology is a much better solution. Magnetic carriers are used as support material that can be easily separated from the reaction medium and stabilized in a fluidized bed reactor by applying a magnetic field [11], [22], [23].

In the present work, a novel type of adsorbent, namely, thiourea-modified magnetic ion-imprinted chitosan/TiO2 composite (MICT), was prepared for the purpose of improving separation, adsorption, and degradation capacities. Molecular imprinting technology and photodegradation technology were coupled through the immobilization of nanometer TiO2 and Fe3O4 on ion-imprinted chitosan matrixes during the synthesis. The multiple functions of the composite were investigated.

Section snippets

Chemicals

Chitosan (degree of deacetylation: 92%) was purchased from Haidebei Ocean Biology Co. (Jinan, China). Analytical pure 2,4-DCP purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China) was used in the preparation of solution for the tests. All other inorganic chemicals of analytical grade were purchased from Shanghai First Reagent Co., China.

Preparation of solution

A stock solution containing 2.0 g/L Cd(II) was prepared by dissolving Cd(NO3)2·4H2O in ultrapure water. The exact concentration of the

Effect of pH

Metal ion and organic material adsorption on both non-specific and specific adsorbents is pH-dependent [28], [29]. The pH of the medium affects the solubility of metal ions and the surface characteristics of sorbents [30]. The effects of the initial pH on cadmium removal and 2,4-DCP degradation at pH 2.0–8.0 are shown in Fig. 1. Studies beyond pH 8.0 were not attempted as the precipitation of the ions as hydroxides would have likely occurred [31]. As shown in Fig. 1, the adsorption capacities

Conclusion

A novel type of composite called MICT was developed for effective cadmium adsorption and 2,4-DCP degradation in the current study through the combination of ion-imprinted technology and photodegradation technology. The prepared adsorbent has better adsorption and degradation capacity compared with those previously reported. The maximum adsorption capacity of cadmium was 256.41 mg/g according to the Langmuir model and the 2,4-DCP degradation efficiency was up to 98% at the initial 2,4-DCP

Acknowledgments

This study was financially supported by the National Natural Science Foundation of China (51178171, 50908078, 50978088, 51039001), Program for New Century Excellent Talents in University (NCET-10-0361, NCET-08-0181), Xiangjiang Water Environmental Pollution Control Project subjected to the National Key Science and Technology Project for Water Environmental Pollution Control (2009ZX07212-001-02 and 2009ZX07212-001-06), the Hunan Key Scientific Research Project (2009FJ1010) and the Hunan

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