Novelty g-C3N4/HAp composite as highly effective photocatalyst for Cr (VI) photoreduction
Graphical abstract
Introduction
The presence of heavy metal ions in aqueous environment is a major global concern due to its toxicity to animals, plants and humans. Among the various heavy metal contaminants, chromium is widely used in several industrial activities such as electroplating, textiles and plastic manufacturing [[1], [2], [3]]. Chromium can be present in nature in form of two oxidation states, Cr (VI) and Cr (III). Of them, Cr (VI) is toxic to most of the organisms at concentration so low as 0.05 ppm, is carcinogenic in animals and humans and causes irritation; additionally, is very soluble in water [4,5]. In comparison, Cr (III) is considered less toxic and can be recovered by precipitating as Cr(OH)3 in neutral or alkaline solutions [6]. Therefore, the photoreduction of Cr (VI) to Cr (III) by photocatalytic processes seems to be a viable, economic and non-toxic alternative for environmental remediation [[7], [8], [9]]. Photocatalysis is based on the use of semiconductor materials [[10], [11], [12], [13]] which, when are irradiated with light, generate the migration of electrons from the valence band (VB) to the conduction band (CB), thus generating the called electron-hole pair (h+/e−); these generated pairs (h+/e−) promote reactions of oxidation and reduction [14,15]. Among the semiconductor materials used as photocatalyst the most used are metal oxides [10,16,17], metal sulfides [9,18,19] and, in the last years, the formation of heterojunctions of them with metal nanoparticles [20,21] has been reported as a good strategy to improve the photocatalytic behavior of those materials. In the search of green and economical materials applied to photocatalytic reactions, some metal-free materials such as graphitic carbon nitride (g-C3N4) and hydroxyapatite (HAp) have attracted attention due to their easy synthesis, optical properties, low toxicity and feasible application [[22], [23], [24], [25], [26], [27], [28]]. With this in mind, in the present work a novel g-C3N4/HAp composite was synthesized by a combination of simple precipitation and thermal polycondensation processes, in order to developing a metal-free, stable and very active photocatalyst. The photocatalytic behavior of this composite material was studied for the photoreduction of Cr (VI), in presence of UV and visible light irradiation. This research aims to design a novel, stable and economical photocatalyst applicable to the elimination of heavy metal in wastewater.
Section snippets
Hydroxyapatite
Hydroxyapatite (HAp) was synthesized by the precipitation technique. In a typical experiment 1 M solution of calcium nitrate tetrahydrate (Ca5(NO3)2·4H2O) was incorporated by dripping to a 1 M solution of sodium orthophosphate (Na3PO4·12H2O) under magnetic stirring, maintaining the pH of the solution at 10 by adding NaOH solution (1 M). The resulting suspension was kept under magnetic stirring for 24 h at room temperature and finally it was filtered, washed with deionized water and dried at 80
XRD
XRD analysis of g-C3N4/HAp composite photocatalyst (Fig. 1) shows the presence of the characteristic peaks of g-C3N4 at 13.1 and 27.4°, corresponding to the (100) and (002) crystallographic planes, respectively (JCPDS 87-1526) [[35], [36], [37]], and the peaks of the hydroxyapatite phase (Ca5(PO4)3(OH)) at 10.8, 25.8, 31.7, 32.1, 32.9, 34, 39.8, 46.7 and 49.4°, which correspond to the (100), (002), (211), (112), (300), (202), (310), (222) and (213) planes, respectively (JCPDS 09-0432).
Conclusions
g-C3N4/HAp composite material was obtained by a combination of precipitation and thermal condensation techniques. The formation of the phases in the composite was confirmed by the XRD analysis. The morphology of the composite presents the agglomerated laminar structure, common in the materials based on g-C3N4. Although the bandgap of the composite did not show an important change with respect to g-C3N4, the XPS and FTIR studies showed a change in the chemical bonds of the composite. The
CRediT author statement
Y. Jiménez-Flores: Conceptualization, Methodology and Research activities (synthesis, characterization and evaluation), Writing - Review & Editing
K. Jiménez-Rangel: Conceptualization, Methodology and Research activities (synthesis and characterization)
J. E. Samaniego-Benítez: Conceptualization, Methodology and Research activities (characterization and evaluation), Writing - Review & Editing
L. Lartundo-Rojas: Research activities (XPS characterization of the materials)
H. A. Calderón: Research
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors thank the financial support of 20201373, 20200352SIP projects, and CONACyT CB 285711 project. Y. Jiménez thanks to CONACyT for the postdoctoral fellowship. K.Y. Jiménez-Rangel thanks CONACyT and BEIFI (IPN) for doctoral scholarship received. The use of Electron Microscopes at the NCEM (Molecular Foundry–Lawrence Berkeley National Laboratory) for the HRTEM analysis was supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy, under Contract No.
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