Mechanism of Congo red adsorption on new sol-gel-derived hydroxyapatite nano-particle
Graphical abstract
Introduction
The dye and organic content of wastewaters discharged industrial are generally high result in severe problems such as delaying the growth of micro−organisms, increasing the chemical oxygen demand (COD), reducing light penetration, and visibility [1], [2], [3]. Synthetic dyes having stability, carcinogenic, and toxic properties such as health and visibility problems that cause not completely removed from wastewater as a worldwide major environmental problem [4]. Moreover, their metabolic processes in animals and plants make hazardous aromatic amines that cause problems of environmental pollution. Therefore, many removing techniques including chemical oxidation, adsorption, photo degradation, biodegradation, and electrochemical process are used [5], [6], [7], [8], [9]. However, for this purpose the conventional biological treatment processes are not so successful [10].
Congo red (sodium salt of benzidinediazo bis−1−naphthylamine−4−sulfonic acid) (CR) as a most used color in the textile industries, because of its aromatic structure shows high physicochemical, thermal and, optical stability against photo or biodegradation [11]. The good water solubility of CR makes difficult it's discoloring in wastewater and aquatic environment. Among different using methods, adsorption by organic or inorganic adsorbents has been introduced as a most simple, high efficiency, and reusability methods with ease decolorization of textile wastewaters [12]. In recent years, absorbing of different azo dyes including CR using compounds such as the, bentonite, hematite, γ-Fe2O3/SiO2, CuFe2O3, nanoporous silica, mesoporous TiO2, red mud, fly ash, kaolin, leaf powder, marble powder, and activated carbon have been studied [13], [14], [15], [16], [17]. The problem is that most of the applied compounds have not sufficient adsorption capacities (less than 100 mg/g) towards CR molecules, therefore in recent years several novel alternative adsorbents such as calcium hydroxyapatite (CaHA) bioceramic attract growing interest because of the low−cost, non−toxicity, non−immunogenicity, and high adsorptive capabilities to pollutants [18], [19], [20], [21], [22], [23], [24], [25]. CaHA shows high stability in physiological conditions [26], [27] that makes it a good biomaterials employing in tissue engineering [26].
CaHA possessing two different ionic C and P binding sites on the crystal surface, respectively, rich in calcium ions with positive charge and rich in phosphate ions with negative charge, respectively [28], [29], [30]. Therefore, CR as an anionic dye should be adsorbed on C sites through electrostatic interactions. The unsaturated nanoparticles of CaHA can bind with other atoms resulted in remarkable adsorption ability. Therefore it has been identified as a good absorbent for many compounds and pollutants like anionic and cationic proteins, heavy metals, phenol, and azo dyes [31], [32], [33], [34]. Also, these biomaterials widely used through chromatography to separate different proteins and nucleic acid fragments [35]. There are several works on dyes adsorption of CaHA like disperse blue SBL [36], reactive yellow 4 [37], direct yellow 27 [38], and reactive blue 204 and methylen blue (MB) [39], although there are no report, about the adsorption of CR onto sol−gel−derived nanoparticles of HAp. Herein, for the first time the new synthesized sol−gel−derived CaHA introduced as an effective adsorbent of CR.
In this work, sol−gel method because of homogeneity of product, capability to make nanoparticles, bioactivity, and low crystallinity of prepared materials with high absorption surface area chose to synthesize nanoparticles of CaHA. In followings, the synthesized nanoparticles used for removing CR in water. Finally, the sample characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and particle size estimated by XRD and Transmission electron microscopy (TEM). It should be noted that the experimental results of absorption of CR will be discussed in details about kinetics, equilibrium adsorption isotherms, and thermodynamic parameters.
Section snippets
HA sol-gel synthesis & physical measurement
Firstly as the novel part of the applied method, triethyl phosphate ((C2H5O)3PO, TEP, Fluka) was hydrolyzed for 24 h at R.T. along with the vigorous stirring. In following, an aqueous solution of 3 M Ca(NO3)2·4H2O (Merck) was added to 4 M hydrolyzed TEP (Ca/P = 1.67) slowly at a rate of 6 mL/min. Then the respect sol solution was vigorously mixed for 60 min at 80 °C. Finally, a clear solution was obtained that aged at room temperature for 24 h. The progress of sol–gel process was gradually
XRD analysis
According to Fig. 2(a) the XRD pattern of CaHA sample shows the intense peaks of hydroxyapatite (HA) and impurity phase of CaO which match the ICDD standards (JCPDS). The indices of HA ((002), (210), and (211)), tri−calcium phosphate (TCP) ((217) at 30.7°), and calcium pyrophosphate (Ca2P2O7) ((100) at 26.7°) confirm the hexagonal, rhombohedral, and monoclinic systems for them. Overall, synthesized CaHA had a good and stable crystal shape. The control of the heat treatment and aging steps of
Conclusions
In the present study, the nano−particles of a new CaHA through an easy low temperature sol-gel method including new conditions was first synthesized and then the structural evolution and particle size evaluation using XRD, FTIR, DTA, SEM, and TEM analyses investigated. However, sol-gel approach results in particles agglomeration that causes the small particles to be overlooked. The modified Scherrer's equation and TEM images show the determined 25 nm as the average of particle size. The
Acknowledgment
MCH gratefully acknowledges the financial support by the Hakim Sabzevari University, Sabzevar, IRAN.
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