Elsevier

Materials Chemistry and Physics

Volume 202, 1 December 2017, Pages 340-351
Materials Chemistry and Physics

Mechanism of Congo red adsorption on new sol-gel-derived hydroxyapatite nano-particle

https://doi.org/10.1016/j.matchemphys.2017.09.047Get rights and content

Highlights

  • Synthesis the new sol–gel derived compound of nano–particles Hydroxyaptite (CaHA).

  • CaHA shows high qm of Congo red adsorption as 487.80 mg g−1 in comparision with others.

  • Fitting well the adsorption isotherms data with the Langmuir model.

  • Kinetic data well fitted with the pseudo–second order equation.

  • Exothermic and spontaneous adsorption process as ΔG° < 0 and ΔH° < 0.

Abstract

In this study, an effective adsorbent calcium hydroxyapatite (CaHA) having high adsorption capacity about aqueous solution of Congo red dye in ambient temperature is presented. Nano particles of CaHA were prepared through a new easy alkoxide–based sol–gel technique. The structural characterization of synthesized CaHA were performed by X–ray diffraction (XRD), Fourier transform infrared (FTIR) analysis, thermal behavior (DTA), morphology and elemental analysis via scanning electron microscopy (SEM). The nano particle size and micro–strain of CaHA were measured using modified Shcerrer equation as Williamson–Hall method and transmission electron microscopy (TEM). XRD pattern shows hexagonal structure of the nano–particle CaHA with P63/m space group that the size measured through XRD and TEM methods are in good agreement as 25 nm. The equilibrium adsorption capacity (qm) for 100 ppm CR was measured to be 487.80 mg g−1 under optimal conditions. The adsorption isotherms and kinetic data well fitted with the Langmuir model and the pseudo–second order equation with correlation coefficients of 0.9979 and 0.9902, respectively. The calculated structures of CR and its complex including Ca2+ and PO43− ions show the electrostatic interactions of anionic CR dye and Ca2+ site and hydrogen bonding of amine group and phosphate of CaHA surface. Thermodynamic parameters (ΔG° < 0 and ΔH° < 0) imply the exothermic and spontaneous adsorption process. Finally, the experimental results suggest that this new synthesized CaHA can be good candidate for application in the management of environmental.

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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