Photocatalytic aptitude of titanium dioxide impregnated chitosan beads for the reduction of Cr(VI)
Introduction
Industrial effluents enclose various toxic metals, harmful gases, and several organic and inorganic compounds. Due to the discharge of these toxic effluents, there has been a major loss in the ecological, social and economic perception. The pollution of toxic metals is increasing with wide-ranging of industrial developments. Large amounts of chromium are introduced into the environment through various industries like dyeing and printing in textile industries, chemical manufacture, leather tannery, metal plating, and processing industrial effluents affected living cells [1], [2], [3], [4], [5], [6]. Hexavalent chromium is 500 times more toxic than trivalent chromium [7]. Among the two forms of chromium, hexavalent chromium, which forms chromate (CrO42−) is considerably more soluble in water than Cr(III) [8]. The maximum level of Cr(VI) permitted in wastewater is 0.05 mg L−1 [9]. World health organization (WHO) has determined that chromium (VI) is carcinogenic. Hexavalent chromium compounds are considered to be highly toxic, carcinogenic, and mutagenic to living organisms [10]. When chromium enters the gastric system, epigastric pain, nausea, vomiting, severe diarrhoea, corrosion of the skin, respiratory tract and lung carcinoma are noticed. Trivalent chromium is generally toxic only to plants at very high concentrations and is less toxic or non-toxic to human and animals [11].
Currently, disposal of chromium (VI) present in wastewater is the difficult problem faced by various industries. The chromium removal treatment includes precipitation, ion exchange, photocatalysis, reverse osmosis, adsorption and electrochemical coagulation process [12], [13], [14], [15]. The majority of these methods entail high capital and persistent expenses and therefore they are not suitable for small-scale industries [16]. Photocatalysis is a highly effective and cheaper process than the other methods and it is one of the potential methods to either oxidize or reduce risky pollutants. Various heavy metals such as Pb(II), Hg(II), As(III), U(VI) and Cr(VI) have been effectively removed by the application of the photocatalytic reduction process [17], [18], [19], [20], [21].
Chitosan is widely found in the exoskeleton of shellfish and crustaceans. It is actually a heteropolymer containing both glucosamine units and acetyl glucosamine units [22]. It can be formulated into beads and films [23] and found to be an ideal material for simple water treatment processes. It has excellent properties for the adsorption of metal ions [24], [25], principally due to the presence of high content of amino (−NH2) and hydroxyl (−OH) groups in the polymer matrix. Like chitosan, various semiconductor materials were used for the removal of heavy metals by the photocatalytic degradation method. TiO2 is an attractive water treatment material because it is inexpensive, non-toxic, and widely available as it is included in a variety of consumer products. Titanium dioxide has a good capacity to degrade organic compounds by oxidation and also remove heavy metals by reduction of them [17]. In TiO2/UV light process; a titanium dioxide semiconductor absorbs UV light and generates powerful oxidizing hydroxyl radicals, which completely degrade the most of environmental pollutants. The recombination of electron–hole pair limits the efficiency of TiO2 catalyst. Also, the recovery of this photocatalyst from its aqueous suspension is an expensive and time-consuming process [26]. For these reasons and to improve the efficiency of TiO2, the surface of TiO2 is modified by impregnation with chitosan in the present study.
The main aim of this work is to prepare and characterize titanium dioxide impregnated chitosan beads (TCB) for selective reduction of Cr(VI) ion for the first time and have not been reported elsewhere. Hence, TCB was prepared and characterized in this study for photo-reduction of Cr(VI). The performance of photo-reduction of Cr(VI) using TCB was examined in order to recognize the synergistic effect with the adsorption and photocatalytic ability. The suitability of the prepared TCB in the presence of UV irradiation has been tested with the samples collected in a nearby industrial area also.
Section snippets
Materials
Chitosan (85% deacetylated) was supplied by Pelican Biotech and Chemicals Labs, Kerala (India). Titanium dioxide was purchased from Sigma–Aldrich. Potassium dichromate, glacial acetic acid, glutaraldehyde, sodium hydroxide and hydrochloric acid were purchased from Merck chemicals and all other chemicals and reagents were of analytical grade. Double distilled water was used throughout the study.
Preparation of TiO2 impregnated chitosan beads (TCB)
The chitosan beads were prepared as reported by Jeon and Holl [27]. About 14 g of chitosan was
Characterization of the sorbents
FTIR spectra of (a) CS (b)TiO2 (c) TCB and (d) Cr(VI) sorbed TCB are revealed in Fig. 1. The hard-hitting broad band at the wave number region of 3300–3500 cm−1 is the characteristic of –NH2 stretching vibration although there is a possibility of overlapping between –NH2 and –OH stretching vibrations, which is exemplified in Fig. 1a. Several peaks related to TiO2 are observed in Fig. 1b. The broad band centered at 500–600 cm−1 is probably owing to the vibration of the Ti–O bonds in the TiO2
Conclusions
Hexavalent chromium in aqueous solutions can be effectively reduced to the trivalent state using TCB under UV irradiation. The extent of reduction of Cr(VI) was increased with the loading of CS, TiO2 and TCB in the presence of UV irradiation. The highest reduction rate was obtained at the lower pH for all the sorbents studied. The maximum percentage reduction of Cr(VI) using CS/UV, TiO2/UV and TCB/UV were obtained at 60 min. TCB/UV possesses higher percentage of Cr(VI) removal compared to CS/UV
Acknowledgements
The authors are grateful to University Grants Commission (No.F.37-153/2009 (SR)), New Delhi, India for the provision of financial support to carry out this research work. The first author is thankful to UGC-BSR for Research Fellowship in Sciences for Meritorious students.
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