Construction of an in-situ Fenton-like system based on a g-C₃N₄ composite photocatalyst

Highlights

• An in-situ Fenton-like system was successfully constructed with the photocatalyst.

• The main ROS in gCPF system for degradation of PNP was OH and O₂–.

• Degradation efficiency of PNP was promoted dramatically in gCPF/NTA system.

Abstract

In this study, g-C₃N₄/PDI/Fe (gCPF) composite material was prepared by incorporating Fe ion on the composite catalyst of g-C₃N₄/PDI (gCP). X-ray photoelectron spectroscopy (XPS) showed that the Fe was successfully incorporated on the pristine g-C₃N₄/PDI. UV–vis diffuse reflectance spectrometry (UV–vis DRS) and Photoluminescence spectral (PL) analysis confirmed the enhancement of the visible absorption band following a decline in the photoelectron/hole recombination rate with gCPF. A preparatory experiment was performed on photocatalytic degradation of p-nitrophenol (PNP) to examine the activity of gCPF. Results obtained in the radical quenching and the electron paramagnetic resonance (EPR) spectroscopic studies indicated that an in-situ Fenton-like system has been successfully established and the main reactive oxygen species (ROS) changed from O₂– to both O₂– and OH in the gCPF system. However, a competition toward conduction band electrons between Fe³⁺ and O₂ caused an inhibitory effect on PNP degradation. To overcome the effect, nitrilotriacetic acid (NTA) was introduced as a reducing agent for Fe³⁺. Upon adding NTA, the efficiency of PNP degradation greatly enhanced from 33 to 80%. The effect of initial pH, dosage of NTA and content of dissolved O₂ on PNP degradation was also studied. The photocatalytic stability was confirmed by recycling experiments.

Introduction

In recent decades, the advanced oxidation processes (AOPs) have been so attractive in developing state of the art technology for wastewater treatment [1]. The AOP techniques have been successfully applied so far to degrade the persistent organic pollutants, and converted the toxic pollutant into easily degradable less molecular weight metabolite compounds as well [2]. As one of the advanced oxidation processes, the Fenton reaction system can generate hydroxyl radicals (∙OH) via the excitation of hydrogen peroxide (H₂O₂) using iron ions. Based on this, the ∙OH can degrade almost the entire organic pollutants in aqueous solution as a non-selective radicals [3]. However, its application is severely limited by the requirements of iron ions and aggressive reaction environment (pH < 3) [4,5]. To overcome the drawbacks of so-called conventional homogeneous Fenton reaction, the heterogeneous Fenton-like reactions wherein the solid catalysts replaces the aqueous iron ions are developed [[6], [7], [8], [9], [10]]. In addition, the classical Fenton reaction encounters problems of lower rate of H₂O₂ utilization and its migration difficulties along with a high operating cost. The surplus quantity of H₂O₂ will in turn react with the generated OH and yield a less reactive radical (HO₂). Hence, the construction of in-situ Fenton system needs to be considered for both practical applications as well as academic research [[11], [12], [13]].

Graphitic Carbon Nitride (g-C₃N₄), a metal-free polymeric photocatalyst, has drawn increasing attention in recent years due to its visible-light driven photocatalytic activity [14,15]. The g-C₃N₄ has been reported to be producing H₂O₂ with higher than 90% selectivity under sunlight irradiation [16,17] via two-electron reduction of O₂ (Eq. 2) rather than one-electron and four-electron reduction (Eqs. 1 and 3).

$${\text{O}}_2\text{\hspace{0.17em}+\hspace{0.17em}}{\text{H}}^{+}\text{\hspace{0.17em}+\hspace{0.17em}}{\text{e}}^{-}\text{\hspace{0.17em}→\hspace{0.17em}}\text{•}\text{OOH}$$

$${\text{O}}_2\text{\hspace{0.17em}+\hspace{0.17em}2}{\text{H}}^{+}\text{\hspace{0.17em}+\hspace{0.17em}2}{\text{e}}^{-}\text{\hspace{0.17em}→\hspace{0.17em}}{\text{H}}_2{\text{O}}_2$$

$${\text{O}}_2\text{\hspace{0.17em}+\hspace{0.17em}4}{\text{H}}^{+}\text{\hspace{0.17em}+\hspace{0.17em}4}{\text{e}}^{-}\text{\hspace{0.17em}→\hspace{0.17em}2}{\text{H}}_2\text{O}$$

However, the low energy level of valence band [14,18,19] and fast recombination of the photoelectron and hole pairs [20] result in the poor performance of g-C₃N₄ in aqueous solution. To overcome the drawbacks, the attempts on developing a modified g-C₃N₄, such as reduced g-C₃N₄ [21], ZnO/g-C₃N₄ [22], TiO₂/g-C₃N₄ [23], g-C₃N₄-Ti³⁺/TiO₂ [24] and S-doped g-C₃N₄ [25] have been made. By compounding pyromellitic dianhydride (PMDA) unit into the g-C₃N₄ network structure to form g-C₃N₄/PDI, the position of the valence band could largely be pulled down. Thus, the photocatalytic activity of the g-C₃N₄/PDI becomes effective compared to the pristine g-C₃N₄ [20,26], thereby an improved photocatalytic production of H₂O₂. However, it is noteworthy that there are a few reports on application of photocatalytically generated H₂O₂.

In this study, considering the effective production of H₂O₂ by g-C₃N₄/PDI, a composite material g-C₃N₄/PDI/Fe (gCPF) was fabricated by loading iron specie onto the g-C₃N₄/PDI, with an aim of constructing an in-situ Fenton-like system. P-nitrophenol (PNP), a typical persistent organic pollutant with acute toxicity [27], was used as the target pollutant to examine the efficiency of the photocatalytic system. To promote the reduction of Fe³⁺ into Fe²⁺ and then accelerate the Fenton reaction, nitrilotriacetic acid (NTA) was introduced into the system. The NTA widely used as a chelating agent [28] and proved to be very effective in the recent studies in degrading the pollutants with a modified Fenton process [[29], [30], [31]]. Furthermore, the efficiency of modified Fenton reaction was confirmed to be greatly enhanced via the reduction of Fe(III)NTA to Fe(II)NTA under natural sunlight [32]. The biodegradability of NTA and the less toxic behavior of metal-NTA complexes made it a suitable candidate for environmental remediation [33].