Mesoporous LaFeO3 as a visible light-driven photocatalyst was prepared by a nanocasting method using mesoporous silica (SBA-15) as a hard template. The as-prepared LaFeO3 photocatalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), and optical absorption spectra. The characterization studies and experimental results showed that LaFeO3 with porous structure caused by the removal of SBA-15 hard template could enhance the specific surface area of the resulting photocatalyst, which improves the phenol adsorption ability of the photocatalyst and in turn enhances its photo-Fenton catalytic activity. The photo-Fenton catalytic activity of the photocatalyst was investigated by photo-Fenton degradation of aqueous phenol under visible light irradiation. The effects of catalyst dosage, H2O2 concentration, and solution pH on the photo-Fenton catalytic degradation of phenol using mesoporous LaFeO3 were studied and optimized. Under the optimal conditions of 20 mg L−1 phenol, 1.0 g L−1 catalyst, and 10 mM H2O2 at pH = 5, the photo-Fenton degradation of phenol (93.47%) was achieved in 180 min under visible light irradiation. Furthermore, our results proved the stability and reusability of mesoporous LaFeO3 and revealed its catalytic mechanism for the photo-Fenton degradation of phenol.
Untreated wastewater released from many industries such as paper making; petrochemical, textile, and flavouring agents; and petroleum industry contains high phenol concentrations. Phenol is considered as a hazardous and lethal pollutant that could be one of the main sources of harm to our ecosystem and human health [
Various technologies have been recently developed for wastewater treatment [
As one of the commonly iron-contained perovskite oxides, LaFeO3 is considered as a promising visible light-driven photocatalyst for the photo-Fenton degradation of organic pollutants [
In this work, the mesoporous LaFeO3 photocatalyst was prepared by a nanocasting method using mesoporous silica (SBA-15) as a hard template. After filling the LaFeO3 precursor into mesoporous channels of SBA-15, the hard template was then leached by NaOH solution. The adsorption capability and photo-Fenton-like catalytic activity for phenol removal over mesoporous LaFeO3 were systematically studied. The effects of different operational parameters were investigated to determine the highest phenol removal efficiency. To the best of our knowledge, no such work has been previously published.
Tetraethyl orthosilicate (Si(OC2H5)4; 99%), pluronic P123 (EO20PO70O20; Mn ∼ 5800), lanthanum nitrate hexahydrate (La(NO3)3.6H2O; 99.9%), iron nitrate nonahydrate (Fe(NO3)3.9H2O; ≥98%), citric acid (C6H8O7.H2O; 99.9%), hydrogen peroxide (H2O2; 30 wt%), and phenol (C6H5OH; 99.5%) were purchased from Sigma-Aldrich. Other chemicals were obtained from our lab in Vietnam. All chemicals were used without additional purifications.
The hard template (HT) was synthesized according to Zhao’s research [
Mesoporous LaFeO3 (LFO-RHT) was prepared by the nanocasting method. Typically, 1.241 g of La(NO3)3.6H2O, 1.158 g of Fe(NO3)3.9H2O, and 1.205 g of citric acid were added to 5 mL of DI water. The mixture was kept on stirring for 3 h at ambient temperature, after which 1 g of SBA-15 was added. This resulting solution was continuously stirred at 70°C for 3 h and then dried at 350°C in an oven for 12 h. The SBA-15 filled with LFO was obtained by calcination at 700°C for 6 h. To remove the SBA-15 hard template, the obtained powder was treated with NaOH 8 M solution at 80°C for 4 h. Finally, the powders were centrifuged, washed with deionized water several times, and then dried in the oven at 60°C overnight. The resulting sample was named LFO-RHT. For comparison, pure LFO was synthesized using the similar method above in the absence of SBA-15 and used to compare with LFO-RHT in the photo-Fenton degradation of phenol.
Powder X-ray diffraction (XRD) patterns of samples were collected on a Bruker D8 diffractometer (Bruker, USA) using CuK
The photo-Fenton experiments were carried out in a 250 ml cylindrical glass reactor with 70 mm in diameter and 115 mm in length. To avoid heating inside the reactor, it was surrounded by a circulating water jacket. A Xenon arc lamp of 300 W (LOT-Quantum Design) was used with a 400 nm cut-off filter as the source of visible light irradiation. 100 ml phenol solution of 20 mg L−1 and LFO-RHT was prepared and added to the reactor. Prior to light irradiation, the suspension was magnetically stirred in the dark for 60 min to reach an adsorption-desorption equilibrium of phenol onto LFO-RHT. To perform the photo-Fenton-like reaction, 1 ml H2O2 aqueous solution was dispersed in the suspension and the lamp was turned on to commence the photo-Fenton reaction. A small amount of sample was extracted from the reactor every 15 min and centrifuged for measurement.
Perkin Elmer Lambda 750 UV–Vis spectrophotometer was used as the analytical technique for confirming the concentration of phenol. The maximum absorbance wavelength of phenol was found at 272 nm. The photo-Fenton degradation efficiency (%) was evaluated as follows:
To understand the degradation kinetics of phenol, the pseudo-first-order model was used (Mahmoodi et al., 2006):
The total removal rate of phenol was calculated as
The reusability of the photocatalyst was performed by repeating the photo-Fenton degradation tests (four times) under the above similar reaction conditions.
The wide-angle XRD patterns of LFO and LFO-RHT are shown in Figure
(a) XRD patterns of LFO and LFO-RHT and (b) XRD pattern of SBA-15 at low angles (the inset: XRD patterns of LFO-RHT at low angles).
The general morphologies of LFO and LFO-RHT before and after alkali leaching are presented in Figures
SEM images of (a) LFO, (b-c) LFO-RHT before and after removing SBA-15 hard template; and (d) TEM images of LFO-RHT.
The nitrogen adsorption-desorption isotherms and BJH pore size distribution curves of LFO-RHT are shown in Figure
(a) N2 adsorption-desorption isotherm and (b) BJH pore size distribution of LFO-RHT.
Characteristics of LFO-RHT and LFO samples.
Sample | Structural property | ||
---|---|---|---|
BET specific surface area (m2 g−1) | Pore volume (cm3 g−1) | Pore size (nm) | |
LFO-RHT | 48.75 | 0.142 | 7.87 |
LFO | 8.06 | 0.028 | 11.43 |
To estimate the surface elemental composition and the valence states of principle elements in LFO-RHT, the XPS spectra of LFO-RHT were investigated. The results presented in Figure
XPS spectra of (a) La 3d, (b) Fe 2p, and (c) O 1s for LFO-RHT.
As is well known that the performances of photocatalysts are highly related to their optical properties [
(a) UV-Vis absorption spectra and (b) corresponding [
It can be seen from Figure
The removal of phenol using LFO and LFO-RHT via adsorption and photo-Fenton degradation under visible light irradiation is presented in Figure
(a) Removal of phenol by adsorption and photo-Fenton reaction using LFO and LFO-RHT catalysts; (b) plots of −ln (
Figure
The effects of catalyst dosage, H2O2 concentration, and initial pH solution on the photo-Fenton degradation of phenol versus irradiation time are presented in Figures
(a) Effect of catalyst dosage (H2O2 concentration = 10 mM); (b) H2O2 concentration; and (c) initial solution pH (H2O2 concentration = 10 mM) on photo-Fenton degradation of phenol (reaction conditions (if not specified): temperature = 25°C, catalyst dosage = 1 g L−1, initial phenol concentration = 20 mg L−1, and initial solution pH = 5).
In the heterogeneous photo-Fenton system, the influence of H2O2 concentration is important for the degradation of phenol because the amount of formed OH radicals depends on the H2O2 concentration [
As can be seen in Figure
Industrial wastewater could be acidic or basic medium due to its discharging from different industrial activities and thus the effect of pH should be studied. The effect of initial solution pH on phenol photo-Fenton degradation is presented in Figure
Based on the above results, a possible photo-Fenton catalytic mechanism using LFO-RHT was proposed, as follows:
As is well known during Fenton-like reaction, the interfacial Fe atoms (denoted as ≡FeIII) of LFO-RHT photocatalyst can react with H2O2 to generate OH radicals (equations (
Photocatalytic stability is considered as one of the important criteria in evaluating photocatalyst performances. In order to explore the reusability of the as-synthesized catalyst, the stability of the LFO-RHT photocatalyst was evaluated by successive experiments, as shown in Figure
(a) The regeneration and reusability of LFO-RHT for the photo-Fenton degradation of phenol over 4 cycles and (b) XRD patterns of LFO-RHT before and after 4 cycles of photo-Fenton degradation test.
Mesoporous LaFeO3 particles with pure perovskite phase have been synthesized via a nanocasting process. Characterization and experimental results showed that LaFeO3 with porous structure could improve the phenol adsorption ability of the photocatalyst and in turn enhance its photo-Fenton catalytic activity, implying synergistic effects of adsorption, and the photo-Fenton process is a good technique for phenol removal. Effects of catalyst dosage, H2O2 concentration, and initial solution pH on the photo-Fenton degradation of phenol were systematically investigated. Furthermore, our results proved that mesoporous LaFeO3 exhibited good photo-Fenton catalytic activity and stability even after four cycles and proposed its photo-Fenton catalytic mechanism for the degradation of phenol under visible light.
The data used to support the findings of this study are available from the corresponding author upon request.
There are no conflicts of interest to declare.
This research was funded by the Hanoi University of Science and Technology (HUST) under Project no. T2020-SAHEP-029.
Figure S1: the pH of point of zero charge of LFO-RHT.