Synergistic adsorption and photocatalytic properties of AC/TiO₂/CeO₂ composite for phenol and ammonia–nitrogen compound degradations from petroleum refinery wastewater

Highlights:

• AC/TiO₂/CeO₂ composite has a synergistic adsorption and photocatalytic properties.

• The adsorption process followed Langmuir isotherm and pseudo-second order kinetic.

• Photocatalysis improved the pollutant removal with pseudo-first order kinetic.

• Three-cycles test showed that the AC/TiO₂/CeO₂ composite has excellent durability.

Abstract

This study focuses on the utilization of AC/TiO₂/CeO₂ composite on the removals of phenol and ammonia–nitrogen (NH₃-N) compounds from petroleum refinery wastewater (PRW) through a combination of adsorption and photocatalytic process. The composite was prepared by hydrothermal method with the composite formulation of 53.43%-wt of AC, 21.96 %-wt of TiO₂, and 24.61 %-wt of CeO₂. Characterization results revealed that the surface micromorphology, elemental, and oxide contents on synthesized composite significantly changed with the addition of TiO₂ and CeO₂. Optical UV–vis analysis showed that the estimated bandgap energy of the synthesized composite was 2.60 eV, which is lower than pristine TiO₂ or CeO_2, indicating that the photocatalysis is easier to perform that reflecting the successful combination of TiO₂ and CeO₂ as the photocatalyst in the composite. The Langmuir isotherm and pseudo-second order kinetic showed to be the most favorable isotherm and kinetic model for the adsorption process. The photocatalytic activity and kinetic evaluations showed that the photocatalytic process remarkably improved the removal performance and was followed the pseudo-first order kinetic. Moreover, the rate constants of photocatalysis were higher than the adsorption process, indicating that the rate of photocatalysis on the pollutant degradations is faster than the adsorption process. The final pollutant removals by using AC/TiO₂/CeO₂ composite were 50.91% for phenol removal and 65.83% for NH₃-N removal. Three consecutive cycles test showed that the AC/TiO₂/CeO₂ composite has excellent durability and can nicely maintain the phenol and NH₃-N removals from PRW.

Introduction

Petroleum industrial activities have been expanded in the recent decades caused by the continuously high demand for petroleum main products and derivatives. In the process, a petroleum plant, specifically a refinery plant, generates approximately 200–20,000 m³ of wastewater daily [1], [2], [3]. In the refining process, the crude oil is treated through a specific series of processes to separate hydrocarbons from the impurities such as sulfur, carbon dioxide, nitrogen compounds, and heavy metal compounds [4], [5]. During the process, some amounts of hydrocarbons and impurities are released into the wastewater stream and can cause severe damages to the ecosystem and environment. Therefore, petroleum refinery wastewater (PRW) contains numerous types of pollutants with considerably high in content. This wastewater contains heavy metals (Cr, Pb, Hg, etc.), grease content (100–400 ppm), chemical oxygen demand (COD) (310–4700 ppm), phenolic compounds (100–1200 ppm), ammonia–nitrogen (NH₃-N) substances (50–950 ppm), total suspended solids (TSS) (30–400 ppm), total dissolved solids (TDS) (1000–5000 ppm), high electrical conductivity (2000–7000 µS/cm), and most likely appeared at alkaline condition (pH: 8.00–8.80) [4], [5], [6], [7], [1]. These contaminants have been reported to cause plenty of problems, including a reduction in equipment unit efficiency due to corrosion and scaling. It is also poisoning biotic elements in ecosystem caused by the compounds that difficult to be degraded biologically such as heavy metals, phenolic and NH₃-N compounds. Numerous studies have been conducted in order to find a solution for treating PRW efficiently. The methods using coagulation-flocculation [8], conventional filter, adsorption[9], membrane filtration [10], [11], photocatalysis [12], ion exchange [13], electrocoagulation [14], advanced oxidation process (AOP) [15], activated sludge[7], and as organic ligand of Fe-metal organic frameworks (Fe-MOFs) for lithium-ion battery composite [16] have been performed, and the results have been reported nicely. Some results show great potential for better technology for treating PRW. However, mostly those methods require extremely high-cost if they are applied in an actual circumstance. Therefore, it is essential to propose a proper technique to treat this polluted water with efficient, cost-effective, and cleaner technology.

Over the decades, adsorption has been applied for the removal of numerous pollutants, including heavy metals and organic compounds in industrial wastewater. It has also gained thousands of scientists and engineers to develop this particular technique due to its simplicity, low toxicity, cost-effectiveness, and ease of being coupled with other methods. Activated carbon, zeolite, bentonite, and silica are the commonly used adsorbent [17], [18], [19]. Fortunately, scientists are also paying close attention to applying the clean technology principle, particularly in water and wastewater treatment. Therefore, photocatalysis has also gained tremendous scientists’ attention worldwide. Surface adsorption has been recognized to speed up photocatalysis by increasing the number of contacts and the shorter path between the photocatalyst and contaminants [20]. Additionally, adsorbent materials with high electronic transfer properties can improve photoinduced charged carrier transport [21]. Furthermore, pollutants adsorbed on the adsorbent's surface are more easily destroyed by photocatalysis, increasing the speed of pollutant removal. As a result, combining conventional adsorption and photocatalytic processes in a composite could be a potential strategy to develop a PRW treatment method that is both efficient and environmentally beneficial.

The study of pollutant removals using composites of carbon/metal oxide has been commonly used in adsorption and photocatalytic processes. Because of its high capacity of adsorption, chemical resistance, and cost-effectiveness, activated carbon (AC) is the famously utilized carbon-based adsorbent material in a variety of applications [9], [22], [23]. For now, titanium oxide (TiO₂) has been identified as the most promising photocatalyst because of its prominent photocatalytic property [21], [24]. Organics can theoretically be converted to water (H₂O), carbon dioxide (CO₂), and mineral acids via a photocatalytic reaction using TiO₂ [24]. The photoexcited electrons that are emitted have such high reducibility to hazardous organic contaminants, which are difficult to decompose. However, its vast bandgap energy, low photo-quantum availability, and high electron-hole pair coupling probability have limited its use [25]. Because a single use of TiO₂ nanoparticles may struggle to meet the requisite performance, several photocatalysts can be blended via several methods, and their characteristics are merged [26], [27], [28]. The morphology or performance of the synthesized composite will be improved by the new structure. By controlling the surface characteristics to boost surface adsorption and delaying the electron/hole (e^-/h⁺) recombination speed by modifying the structure and particle shape has been proven that improve the composite photocatalytic ability [25], [29]. To improve the photocatalytic characteristic of TiO₂, some strategies have been carried out, including doping, photocatalysts coupling, morphology, and structure modification. Combining with other photocatalysts has been shown to be an effective and practical method for facilitating electron/hole separation by heterojunction scheme and improving TiO₂′s photocatalytic properties.

Integrating photocatalysts with the oxides of rare earth metal element has been investigated, and they have resulted in some positive photocatalyst enhancements. The close contacts between iron oxide (Fe₂O₃) and cerium oxide (CeO₂) have been reported to have a lower energy gap and increase electron-hole e^-/h⁺ separation-transfer in a composite of Fe₂O₃@CeO₂ [30]. The CeO₂/ Co₃O₄ composite substructure also has a high photocatalytic capacity due to the low energy gap and remarkable reusability due to the effective oxygen vacancies at the interconnection surface [31]. The distinctive pair interaction of Ce(IV)/Ce(III) in cerium oxide has also been investigated, which has the potential to enhance the absorption of photon energy capacity under visible light exposure [32]. The combined Pd/CeO₂ photocatalyst has been reported to have an exquisite performance on dye compounds degradation [33]. Due to the excellent activity and improvement from application of CeO₂, it possesses a particular interest by combining TiO₂ and CeO₂ nanoparticles onto the activated carbon to synthesize AC/TiO₂/CeO₂ composite.

This study provides a comprehensive investigation regarding the utilization of AC/TiO₂/CeO₂ composite on the removals of phenol and NH₃-N compounds from PRW through a combination of adsorption and photocatalytic process, which has not been investigated. The composite was prepared by hydrothermal method, whereas the composite formulation has been optimized from our previous study [34]. The surface micromorphology, elemental and oxide compositions, and the mapping distributions of the fabricated composite were characterized using scanning electron microscopy (SEM) and energy dispersive x-ray (EDX). The crystalline structure was analyzed using x-ray diffraction (XRD), and their functional groups were assessed using Fourier-transform infrared (FTIR) analysis. The isotherms and pore features were evaluated by Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) analyses, and their optical features were analyzed using diffuse reflectance spectrometry (DRS) analysis. Point of zero charge (PZC) analysis was applied to evaluate the surface charge of the composite and the electrostatic interaction between composite’s surface and the pollutants. The effect of time, dosage, and temperature, and also characteristics of adsorption isotherms, kinetics, and thermodynamics were studied to deeply understand the adsorption properties of AC/TiO₂/CeO₂ composite. Similarly, the investigations on the photocatalytic activity and degradation kinetics were also carried out to get clear findings regarding the photocatalytic ability of the synthesized composite. The cycle test on the composite was conducted to reveal the durability of the composite degrading phenol and NH₃-N compounds from PRW. Last but not least, the mechanisms of adsorption and photocatalytic process were proposed to clearly describe the removal step on degradations of phenol and NH₃-N compounds from PRW.