Full Length Article
Highly dispersed FeOOH to enhance photocatalytic activity of TiO2 for complete mineralisation of herbicides

https://doi.org/10.1016/j.apsusc.2020.145479Get rights and content

Highlights

  • Highly dispersed FeOOH instead of Fe2O3 species are the most active reaction sites.

  • The composition of Fe(III) species is dependent on the experimentla conditons.

  • Enhanced generation of hydroxyl radicals by FeOOH on TiO2 plays a major role.

  • Six intermediate products were thoroughly detected during TCP mineralisation.

Abstract

Although there were many new photocatalysts reported recently, TiO2 has still been considered as one of the best candidates for real application of environmental decontamination. Fe-based oxides were synthesised as efficient and equally important non-toxic active species to improve the efficiency of TiO2 photocatalysts. Such nano-architectured FeOx/TiO2 was tested for herbicides mineralisation e.g. 2,4,6-trichlorophenol (2,4,6-TCP) and 2,4-dichlorophenoxyacetic acid (2,4-D) under full arc light irradiation. The consistent results were achieved by HPLC, TOC and UV–vis spectra measurements, which show among three Fe species, Fe4NO3(OH)11, FeOOH and Fe2O3, FeOOH is the best to improve TiO2 activity. This active specie of FeOOH was readily controlled by synthesis temperature and precursor concentration, leading to 250 °C being the optimum temperature for the synthesis of very stable FeOOH/TiO2 nanocomposite with excellent photocatalytic activity, representing nearly two times activity of the benchmark PC50 TiO2 photocatalyst for all herbicides tested. Such high activity was attributed to the enhanced photo-generated electron-hole separation and improved generation of hydroxyl radicals by FeOOH. The multifunction of FeOOH is very crucial for organic pollutants mineralisation. The mechanistic studies also show that degradation of 2,4,6-TCP was mostly dominated by hydroxyl radicals and superoxide radicals. The possible degradation pathway of 2,4,6-TCP was also proposed.

Introduction

The presence of toxic and persistent organic pollutants in wastewater effluents causes serious environmental problems [1], [2]. Chlorophenols (CPs) and derivatives are common and recalcitrant environmental pollutants believed to have high bioaccumulation capability, carcinogenic and mutagenic effects [2], [3], [4]. However, CPs and their derivatives find extensive application in the chemical, forestry, and wood-working industries. They are used as herbicides, insecticides, fungicides, wood preservatives and chemical intermediates [3]. Generally, these organic pollutants are released into the environment because of several man-made activities including water disinfection, waste incineration, uncontrolled used of pesticides and herbicides, and as by-products in the bleaching of paper pulp with chlorine [2]. The Environmental Protection Agency (EPA) recommended maximum allowable concentration for chlorinated phenols is 0.1 µg/L in drinking water and 200 µg/L in wastewater (EPA 2003) [2], [5].

Various strategies have been employed to remove organic and inorganic contaminants from the environment. Conventional methods include coagulation-flocculation [6], reverse osmosis [7], active carbon adsorption [8], biodegradation [9], air stripping [10] and incineration [11]. However, these techniques have some drawbacks and limitations e.g. toxic by-product generation, incomplete mineralisation, low efficiency, high energy and capital cost [3], [12], [13], [14]. Chlorophenols absorb light of wavelength below 290 nm, thus they do not undergo direct sunlight photolysis [15]. Therefore, it is important to find innovative and cost-effective techniques for the safe and complete degradation of chlorinated organic pollutants such as chlorophenols.

TiO2 based materials have been the most studied photocatalysts for the degradation of various chlorophenols e.g. 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP [16]. Several TiO2 based photocatalysts e.g. Ag-doped TiO2 [17], Fe-doped TiO2 [18], [19], La-doped TiO2 [20], V2O5/TiO2 [21] and other photocatalysts like ZnO [22], [23], [24], α-Bi2O3 [25], Ag3PO4 [26], BiVO4 [27] and g-C3N4 [28], [29], [30] have also been reported for the degradation of 2,4,6-TCP in aqueous solution. However, photocatalyst stability and/or mineralisation efficiency (not decomposition efficiency) are the major challenges encountered during degradation of 2,4,6-TCP. Due to this drawback of the doped TiO2 and non-TiO2 photocatalysts for the degradation of chlorinated phenols, extensive research is still required to develop a cheap, non-toxic, photo-chemically stable and highly efficient TiO2-based photocatalyst for scalable wastewater purification.

The surface modification of TiO2 with Fe(III) species as co-catalysts has been reported due to its high efficiency and robust synthesis procedure, unlike doping into TiO2 lattice which requires a very high temperature [31], [32]. It has been reported that the use of low Fe(III) concentration in TiO2 surface modification typically led to the formation of isolated ions or clusters of Fe(III) species [33], [34]. The Fe(III) species that accepted photogenerated electrons got reduced to Fe(II) species, which were unstable and would easily be oxidised back to Fe(III) through oxygen reduction reaction [34]. The photocatalytic enhancement contributed by Fe2O3 clusters has been broadly investigated in the literature and assigned to the interfacial charge transfer (IFCT) from TiO2 to Fe(III) or cross excitation from TiO2 to Fe species [35], [36], [37], [38]. However, the active Fe species to improve the photocatalytic activity of TiO2 was mainly regarded as Fe2O3 or ferric oxide [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. Production of reactive oxygen species and reduction of charge recombination during mineralisation of organic pollutants are important in photocatalytic water treatment.

Herein, we report the synthesis of novel nano-architecture comprising different phases of Fe(III) species on PC50 (commercial anatase TiO2), using a reproducible surface impregnation method. The active species of Fe4NO3(OH)11, FeOOH and Fe2O3 were found to be readily controlled by synthesis temperature. The Fe(III) species were thoroughly characterised in order to clarify their functionality and actual active species. The degradation of 2,4,6-TCP in water was carried out under full arc light irradiation. The effects of co-catalyst concentration, choice of Fe(III) precursor and calcination temperature were investigated. The charge transfer mechanism and the reaction pathway were also discussed. Photocatalytic mineralisation ability of the optimised catalyst was also evaluated with another widely used herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D) to demonstrate its wide feasibility.

Section snippets

Chemicals

PC50 TiO2 (purely anatase) was purchased from Millennium chemicals. 2,4,6-trichlorophenol (98%) was purchased from Alfa Aesar. 2,4-dichlorophenoxyacetic acid was purchased from Cayman Chemical Company. 1,4-benzoquinone (99%) was purchased from Acros Organics. Fe(NO3)3·9H2O, FeCl3·6H2O, Fe2(SO4)3·H2O, DMPO and EDTA (99%) were purchased from Sigma-Aldrich. Isopropanol (HPLC grade), KHP and Acetonitrile (HPLC grade) were purchased from Fischer Scientific. All reagents were used as received without

Characterisation of FeOx/TiO2 nanocomposites

The XRD patterns of unmodified PC50 TiO2 and the selected FeOOH/TiO2 samples with varying Fe concentration are shown in Fig. 1a. Typical diffraction peaks corresponding to anatase (JCPDS 21-1272) are observed in these samples. No characteristic diffraction peaks of Fe-related species (e.g. Fe, FeO, FeOOH, Fe(OH)x, Fe2O3 or Fe3O4) are observed on the surface modified TiO2. This could be due to the highly dispersed and low amount of iron species loaded on TiO2 and the intensive background signal

Conclusions

In summary, facile and robust synthesis procedure was successfully used in decorating PC50 TiO2 nanoparticles with highly dispersed FeOOH, which plays a key role for efficient photocatalytic herbicide decomposition. The Fe loading and properties in the composites were thoroughly controlled by varying the Fe concentrations from 0.07 to 2.8 wt% Fe and calcination temperatures from 120 to 450 °C, respectively. At 120 °C, iron nitrate hydroxyl impurities and FeOOH were impregnated on surface of TiO2

CRediT authorship contribution statement

Ayoola Shoneye: Conceptualization, Methodology, Investigation, Data curation, Writing - original draft. Junwang Tang: Supervision, Writing - review & editing.

Acknowledgments

A. Shoneye acknowledges studentship from the Federal Scholarship Board (FSB), Nigeria. J. Tang acknowledges funding from the Royal Society-Newton Advanced Fellowship award, UK (NA170422) and Leverhulme Trust grant, UK (RPG-2017-122).

Declaration of Competing Interest

The authors declare no competing financial interest.

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