Ni-N codoped SnO2/Fe2O3 nanocomposite as advanced bifunctional photocatalyst for simultaneous photocatalytic redox conversion of Cr(VI) and As(III)
Graphical abstract
Introduction
The toxic impacts of heavy metal contamination is a huge challenge, and one of the most critical environmental issues worldwide that affects the health of humans, perturbation of ecological and geological equilibrium [1, 2]. Chromium (Cr) and arsenic (As) as two typical hazardous contaminants of heavy metal ions in industrial wastewater arising from various natural or anthropic sources, such as electroplating, wood industry, leather making, corrosion protection, mining, and metallurgy activities [3, 4]. There are two main forms of chromium in the natural environment: trivalent [Cr(III)] and hexavalent [Cr(VI)]. Compared to Cr(III), Cr(VI) is more mobile, toxic, and carcinogenic to livings due to its weak absorption to inorganic surfaces and high solubility migration in water and soils [5, 6]. Currently, the maximum concentration of Cr(VI) in the drinking water and inland surface water recommended by the World Health Organization (WHO) is 0.05 and 0.1 ppm, respectively [6]. Arsenic can exist in two oxidation states; trivalent [As(III)] and pentavalent [As(V)], that As(III) form is thermodynamically stable, more mobile in groundwater, hard to remove by conventional treatment processes, low adsorption efficiency to various adsorbents and 25–60 times more toxic than As(V) form [7, 8]. According to the WHO standard, the maximum allowable limit for arsenic is 0.01 ppm [4]. Therefore, in order to reduce the toxicity as well as to defer mobility, the effective conversion of Cr(VI) and As(III) to Cr(III) and As(V) is urgently required. To date, the Cr(VI) reduction or As(III) oxidation have been individually investigated through various conventional techniques such as ion exchange [9], biological method [10], carbon activated [11], membrane filtration [12], electrocoagulation [13], adsorption [14], chemical precipitation [15], and reverse osmosis [16]. However, these methods are sometimes rather expensive for large-scale, often inefficient at low concentrations, requiring large exposed liquid surface area and long arrest periods [7, 17, 18]. Besides, how to realize the simultaneous redox conversion of Cr(VI) and Ar(III) co-existing in solution in a facile pathway is still a critical challenge. To overcome these problems, the development of a new, viable, and green technique instead of the mentioned methods becomes of serious practical important. The direct reaction between Cr(VI) and As(III) ions coexist in the solution are thermodynamically impossible because the Gibbs free energy is theoretically positive [19].
It is known that the rate of the direct redox conversion between Cr(VI) and As(III) in the dilute solutions is slow, even though the transformation is possible in viewpoint of their redox potentials (E0(HCrVIO4−/Cr3+)=+1.35 V and E0(H2AsVO4−/HAsIIIO2)=+0.4 V vs NHE at pH=3.0) [20]. To conquer the limitations and accelerate the simultaneous redox conversion of Cr(VI) and As(III), activating agents such as H2O2 can be introduced to the system to change the reaction pathway [19]. In this context, advanced reductive or oxidative processes are regarded as a new alternative that often utilizes heterogeneous photocatalysts for the reduction/oxidation reactions and the transformation of Cr(VI) and As(III). This technology exhibits high efficiency, economic cost, low energy consumption, stable performance, clean and environmental friendliness [5, 6, 21]. In this system, simultaneous redox conversion of Cr(VI) and As(III) can be realized utilizing H2O2 directly or/and photocatalytic materials, potentially providing H2O2 redox agents [22].
Many semiconductor materials such as TiO2 [4, 23], SnO2 [24, 25], ZnO [26], g-C3N4 [22] and MnO2 [27] have been used for Cr(VI) reduction or As(III) oxidation. SnO2 is one of the most useful semiconductors for photocatalytic purposes because of its non-toxicity, chemical stability, relatively low cost and, high photocatalytic activity [28]. However, its wide band gap (⁓ 3.6 eV) and high recombination rate of photogenerated electron/hole pairs intensely confine the ability to maximize the solar energy conversion efficiency [29]. Hence, the development of novel photocatalyst materials with proper optical responses in the visible light region becomes necessary in the practical and technological aspects. Several strategies like plasmonic coupling with noble metals [30], coupling with another semiconductors [31] and doping with transition metals [32] and/or non-metals [33] have been reported in the recent years. As a narrow-band gap (∼ 2.2 eV) n-type semiconductor with a visible light response, α-Fe2O3 can be coupled with SnO2 to achieve the effective separation of photogenerated charge carriers and improve the photocatalytic activity [34]. Heterojunction between these two semiconductors has much interest and has been investigated by many researchers for potential applications in photocatalysis. Pradhan and coworkers [35] reported the mesoporous α-Fe2O3/SnO2 nanoheterostructure for photodegradation of malachite green. Zhang et al. [36] designed hierarchical SnO2/α-Fe2O3 Nanoheterostructures and evaluated visible-light photocatalytic degradation of methylene blue. Devi and Shyamala [34] prepared SnO2-α-Fe2O3 composites by sol-gel method and studied their photocatalytic degradation of phenol under UV/solar light irradiation. All of them have declared that the photogenerated charge carriers were separated at SnO2/Fe2O3 interface, leading to the enhancement of the photocatalytic activity. On the other hand, doping with suitable anions (S, N, F, B, P, and C) or/and cations (Cr, Fe, Ni, Co, Mn, and Mo) can narrow the band gap of SnO2 and extend the photoresponse to visible light range [37, 38]. Among them, N and Ni seem to be the most plausible candidates as dopant to investigate. N has similar characteristic features to oxygen, and modifies the electronic structure of SnO2 lattice due to the introduction of localized states in the band gap at the top of the valence band. This modification reduces the band gap and helps to the photogenerated charges separation, and therefore can be a most probable reason to enhance the photocatalytic activity under visible light. Ni2+ has almost the same ionic radius with Sn4+ (0.69 oA), and is easily doped into the SnO2 lattice, creating impurity energy levels in the forbidden band of SnO2. It can be responsible for the visible light absorption and improving the photocatalytic properties of SnO2 [39]. However, the reports are mostly for the TiO2-based photocatalysts or Cr(VI) reduction and As(III) oxidation have been studied individually in the photocatalytic system. Moreover, as far as we know, there are no reports on the use of SnO2-based heterogeneous semiconductors for simultaneous photocatalytic redox conversion of both Cr(VI) and As(III) pollutants. Herein, simultaneous redox conversion of Cr(VI) and As(III) was investigated over Ni-N codoped SnO2/Fe2O3 nanocomposites under visible light irradiation. The morphology and structure of the synthesized photocatalysts were characterized, and the mechanisms of the redox process at different pH were studied.
Section snippets
Preparation and characterization of photocatalysts
Template-based liquid phase deposition (LPD) approach was applied for preparing the SnO2 nanotube arrays, as shown in previous reports [28, 40]. Alumina membranes used as a template assist in synthesizing highly-order one-dimensional nanostructures and better control of the process [41, 42]. 1.3 g ammonium hexafluorostannate ((NH4)2SnF6, Sigma-Aldrich; 99.99%) was dissolved in 50 mL deionized water (18 μΩ) to produce a 0.1 M treatment solution. Ammonium fluoride (NH4F, Sigma-Aldrich; ≥98%), and
Structural and morphological properties of the photocatalysts
The XRD and Raman measurements were carried out to determine the crystal structure and phases of the resulting products. Fig. 2a shows the XRD patterns of the SnO2 nanotubes, SnO2/Fe2O3, N-doped SnO2/Fe2O3, and 2, 4, 6, 8, 10% Ni/N-codoped SnO2/Fe2O3 nanocomposites calcined at 550 °C for 2 h. The peaks found in the diffraction pattern of SnO2 nanotubes are well indexed to the pure rutile phase of SnO2 in correspondence with the Joint Committee on Powder Diffraction Standard (JCPDS), card no.
Conclusions
Ni/N-codoped SnO2/Fe2O3 nanocomposites were successfully synthesized as efficient photocatalysts for the synergistic photocatalytic redox conversation of Cr(VI) and As(III) under visible light irradiation. It was found that among all samples, N-doped SnO2/Fe2O3 nanocomposite with 8 at.% Ni exhibited the highest photocatalytic removal efficiency of Cr(VI) and As(III) in the co-existing system with reaction rate constant 99% and 96% after 90 min irradiation, respectively. These values were about
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.
References (66)
- et al.
Total concentrations and sources of heavy metal pollution in global river and lake water bodies from 1972 to 2017
Glob Ecol Conserv
(2020) - et al.
Comparision of photocatalysis and photolysis processes for arsenic oxidation in water
Ecotoxicol Environ Saf
(2018) - et al.
Adsorption and reduction of chromium(VI) from aqueous solution using polypyrrole/calcium rectorite composite adsorbent
Water Res
(2019) - et al.
Photocatalytic reduction of Cr(VI) with TiO2 film under visible light
Appl Catal B Environ
(2013) - et al.
Graphitic carbon nitride based photocatalysis for redox conversion of arsenic(III) and chromium(VI) in acid aqueous solution
Appl Catal B: Environ
(2019) - et al.
Polyaniline modified SnO2 nanoparticles for efficient photocatalytic reduction of aqueous Cr(VI) under visible light
Sep Purif Technol
(2018) Boosting the photocatalytic ability of hybrid BiVO4-TiO2 heterostructure nanocomposites for H2 production by reduced graphene oxide (rGO)
J Taiwan Inst Chem Eng
(2020)- et al.
Facile fabrication of mesoporous α-Fe2O3/SnO2 nanoheterostructure for photocatalytic degradation of malachite green
Catal Today
(2014) - et al.
Convenient fabrication of Ni-doped SnO2 quantum dots with improved photodegradation performance for Rhodamine B
J Alloys Compd
(2019) Structural and optical characteristic of single crystal rutile-titania nanowire arrays prepared in alumina membranes
Mater Chem Phys
(2016)