PEG assisted P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst with enhanced elimination of emerging organic pollutants in salinity condition under solar light illumination

Highlights

• PEG modified P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst was successfully synthesized.

• PEG molecular weight strongly affect morphology and structure of the photocatalyst.

• The prepared photocatalyst exhibited superior photoactivity at salinity condition.

• Generation of Cl and free chlorine species further enhanced purification process.

• The photocatalyst is promising to purify the seawater environment practically.

Abstract

In this study, the effect of salinity on degradation of model environmental pollutants (organic dyes and E. coli) was investigated using polyethylene glycol (PEG) assisted P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst. The photocatalyst was firstly synthesized with structure modifier PEG to determine the optimum molecular weight by sol-gel method, and was characterized by XRD, UV–Vis, PL, SEM, TEM, EDS, and photocurrent experiment. The PEG₃₀₀ modified P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst was found to be the optimal molecular weight which exhibited small crystalline size, narrow band gap, better absorption ability, smaller particles size, large photocurrent density, low recombination rate and higher photocatalytic activity among other PEG modified photocatalyst. The PEG₃₀₀-P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalytic degradation of emerging organic pollutants such as rhodamine B, methyl orange, and methylene blue was significantly higher at aqueous NaCl (2.6 and 3.2 wt%) and artificial salinity (25 and 35‰) compared to freshwater under simulated solar light illumination. Furthermore, the elimination of organic dyes under different environmental factors such as initial pH, light intensity, reaction temperature showed substantial degradation in real environmental conditions at salinity. Moreover, PEG₃₀₀-P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst demonstrated high practicability on complete inactivation of E. coli, simultaneous degradation of organic dyes, and high stability in seawater. Superoxide anion (O₂⁻) radicals was found to be the dominant reactive species, and subsequent formation of Cl and free chlorine oxidative agents further improved the photocatalytic purification process. The PEG₃₀₀-P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst showed great potential in treating environmental organic pollutants at salinity condition under solar light illumination.

Introduction

Marine and coastal areas are constantly contaminated by increase in disposal of pollutants from anthropogenic activities. The persistent and recalcitrant property of emerging organic pollutants (EOPs) such as dyes, pharmaceuticals, and endocrine-modulating substances with high potential of bioaccumulation has detrimental effect on marine ecosystem and human health [1]. Another concern is increase in microbial contamination from discharge of untreated sewage wastewater into sea, which also poses a potential health hazard to marine life [2]. The release of highly toxic industrial saline pollutants in freshwater sources has similar consequence on the aquatic life. The loss of rich biodiversity in and around the coastal areas affects fisheries and its socio-economic development of the region [3]. Conventional treatment methods such as biological remediation [4], physical adsorption and chemical flocculation/coagulation [5] are employed for treatment of wastewater. However, they are either expensive, ineffective in complete removal of pollutants or leads to formation of toxic by-products [6].

Photocatalysis, especially TiO₂ photocatalyst, has attracted wide attention as an environmentally friendly, inexpensive and effective technique for wastewater treatment [7]. However, the application of pristine TiO₂ is limited in real wastewater treatment due to its wide band gap (~3.2 eV), which is only activated under UV light irradiation that accounts for 3–5% of solar light spectrum, indicating a poor utilization efficiency of solar light along with high recombination of e⁻/h⁺ rate [8]. Several methods have been employed to extend absorption of TiO₂ into visible light region to enhance photocatalytic efficiency, and incorporation of noble metals (Ag, Au and Pt) [9], [10] and non-metals (C, N, P, F, and S) [11] was considered to be an effective modification strategy for the development of TiO₂ photocatalyst. As per the aforementioned method, previous study in our lab showed a novel sol-gel synthesized P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst with narrow band gap, efficient charge transfer and e^-/h⁺ separation, anti-bacterial and excellent photocatalytic activity on degradation of organic pollutants in freshwater compared to pristine TiO₂ under simulated solar light [12]. On the other hand, significant efforts have been directed towards surfactant assisted sol-gel synthesis of composite photocatalyst by using structure modifiers to improve the physicochemical properties and catalytic performance [13]. The surfactants commonly used in the synthesis of TiO₂ composite are sodium dodecyl sulfate (SDS), polyvinylpyrollidone (PVP), polyethylene glycol (PEG), and cetyltrimethylammoniumbromide (CTAB) [13]. Among them, polyethylene glycol (PEG), a non-ionic surfactant, is considered as a suitable structure directing agent due to its high water solubility, stability, non-toxicity, and thermal decomposition upon calcination [14], [15]. Considering its advantages, PEG has been widely used in morphological tuning for effective synthesis of nanoparticles based biosensors [16]. In recent years, it has been extensively studied in the preparation of dye-sensitized TiO₂ nanoparticles for improving the stability and durability to ensure better charge transfer in production of energy [17], [18], [19], [20], [21]. Armelle et al. showed the addition of PEG during the synthesis significantly improved the morphology and optical properties leading to higher photocatalytic activity [22]. Taicheng et al. [23] and Hu et al. [24] reported the change in morphology, crystalline size and grain growth of PEG-TiO₂ photocatalyst were strongly depended on molecular weight of PEG. Similarly, Meizhou et al. [25] observed the interaction between different molecular chains PEG and photocatalyst which inhibited phase transformation of TiO₂ (anatase to rutile) and controlled the interfacial charge transfer [26]. This illustrated that optimal length of molecular chain could regulate growth and particle size of nanoparticles. Hence, modification of novel P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst with optimum molecular weight PEG may improve the crystalline structure and morphology to further enhance catalytic efficiency under solar light irradiation.

In seawater treatment, the presence of major ions like Na⁺, Cl⁻, Mg²⁺, Ca²⁺, CO₃²⁻, and HCO₃⁻ may influence the photocatalytic degradation efficiency of organic pollutants [27], [28]. Guilliard et al. reported the inhibitory effect of anions (Cl⁻, SO₄²⁻ and CO₃²⁻) on photoreduction of organic dyes in textile wastewater [29]. Dugandzic et al. observed similar inhibition of nicosulfuron degradation in presence of cations (Na⁺, Mg²⁺, and Ca²⁺) by using TiO₂ under UV light [30]. The reduction in activity was caused due to competitive adsorption [29], scavenging activity and active site blocking [31] of inorganic ions on the surface of photocatalyst. However, Ortiz et al. [32] and Wang et al. [33] observed an enhanced photodegradation of Escherichia coli (E. coli) and antibiotics, respectively, due to the formation of reactive oxidizing species in the photocatalytic reaction. Although, the individual effect of inorganic ions showed both inhibitory and promoting effect on degradation in freshwater, the elimination mechanism in seawater is still unclear. Additionally, in real condition, the photocatalytic activity is affected by environmental factors such as initial pH [34], light intensity [35], and reaction temperature [36]. So far, there are very few studies on the photodegradation of organic pollutants in seawater [33], [37], [38], and lack of comprehensive assessment on the effect and influence of different environmental parameters limits the application of photocatalytic treatment under salinity condition. However, to the best of our knowledge, there is no research focused on elimination of emerging organic pollutants in seawater using PEG modified P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst under solar light illumination.

Hence, the objective of this study is to understand the effect of structure directing agent PEG and determine the optimal molecular weight for controlling morphology and structural properties to further improve the P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalytic efficiency. Emerging aquatic pollutants such as rhodamine B (Rh B), methylene blue (MB) and methyl orange (MO) were used to investigate the effect of major salt NaCl and artificial salinity on photocatalytic activity. Further, the influence of environmental factors (pH, light intensity, temperature) in real seawater treatment was evaluated. The practicality of the photocatalyst on bacterial disinfection, simultaneous reduction of organic pollutants and recyclability were examined. A possible mechanism was proposed for PEG-P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photodegradation of organic pollutants in environmental seawater purification process under solar light irradiation.