Degradation pathways of emerging contaminants using TiO₂-activated carbon heterostructures in aqueous solution under simulated solar light

Highlights

• TiO₂/AC heterostructures were prepared by different synthesis methods.

• Microwave-assisted heterostructure was the most photoactive.

• Ibuprofen was the most easily removed.

• Degradation pathways for acetaminophen, ibuprofen and antipyrine were proposed.

• In pharmaceuticals mixture, the highest and fastest mineralization occurred at pH 7.

Abstract

This work deals with the degradation of three emerging contaminants (acetaminophen, ibuprofen and antipyrine) in water under simulated solar light using different catalysts of TiO₂/activated carbon heterostructures. The heterostructures, based on anatase phase, were successfully synthesized following three different methods (solvothermal, microwave-assisted and sol-gel), using lignin as carbon precursor. The sol-gel photocatalyst only yielded 50% conversion of acetaminophen and a low mineralization (15%), probably due to the higher crystal and particle size and lower surface area of this heterostructure, as a consequence of the higher temperature reached during the heat-treatment included in this synthesis route to achieve anatase crystallization. In contrast, the heterostructure prepared by the microwave-assisted procedure achieved complete conversion after 6 h of reaction. Regarding the contaminants, ibuprofen was the most easily removed, requiring 3 h for complete disappearance, while antipyrine showed the highest resistance to photodegradation, not being completely removed after 6 h. The photocatalytic performance was also evaluated for a mixture of these three pharmaceuticals at different initial pH. The fastest and highest mineralization (ca. 50%) occurred around neutral pH. The study proposes the oxidation degradation pathways of the three pharmaceuticals under solar-simulated irradiation from the analysis of the reaction intermediates.

Introduction

The awareness about the presence of contaminants of emerging concern (CECs) in water bodies is strongly increasing, especially in the last two decades. This type of compounds, also known as emerging contaminants due to recent detection and quantification in water streams, are considered to be biologically active, altering the metabolism of living beings, despite they are usually detected in very low concentrations [1], [2], [3], [4], [5]. CECs include, among other species, pharmaceuticals and personal care products that are considered endocrine disruptors and can cause negative effects on health and aquatic environments due to their continuous release [6], [7]. They are commonly present in sewage and wastewater treatment plants (WWTPs) allow only partial removal in most cases, so that continuous municipal discharges give rise to accumulation in the receiving water bodies. The average removal of antibiotics in WWTPs has been reported in the range of 40–60% and close to 25–55% in the case of analgesics and anti-inflammatories [8], [9]. These species can be also present in the wastewaters from pharmaceutical plants, in this case at substantially higher concentrations, in the order of mg·L⁻¹. The growing concern about water quality promotes action plans by United Nations [10] and research efforts on the development of efficient and sustainable technologies for the abatement of those pollutants, in general of hazardous character.

Several advanced treatments have been investigated for the removal of CECs [11], [12], including advanced oxidation processes (AOPs), which degrade contaminants via generation of reactive oxygen species (ROS). Among the AOPs, heterogeneous photocatalysis has been widely reported for the removal of many different types of contaminants [13]. In this technology, ROS can be generated upon light absorption in a semiconductor (being TiO₂ the most used so far) giving rise to separation of electron-hole charges. Nowadays, there is a growing trend to use solar light as renewable and sustainable energy source, reducing the operation costs [8]. In spite of the wide use of TiO₂ (due to its well-stated physical and chemical properties), this material has the main drawbacks of low adsorption capacity, limited photocatalytic activity under visible light and difficult recovery from the aqueous medium, especially in the case of commercial TiO₂ [14]. Supporting TiO₂ on porous materials, as activated carbons (ACs), zeolites or clays, can partly overcome these drawbacks [15], [16]. ACs are characterized by their relatively low cost, high surface area and well-developed porosity. Moreover, activated carbons can be prepared from almost any carbonaceous waste, providing a way of valorization of those residues. Lignin, a biopolymer with relatively high carbon content is a main component of lignocellulosic biomass. Different types of modified lignin are produced in huge amounts from cellulose pulp manufacture, being mostly used by its fuel value. The future development of biorefinery is expected to generate high quantities of waste lignin whose valorization becomes mandatory. Regarding valorization possibilities, lignin has been studied as precursor for the synthesis of activated carbons and other carbon-based materials [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. In a recent work [27], our research group investigated the use of different activating agents (FeCl₃, ZnCl₂, H₃PO₄ and KOH) in the preparation of activated carbons from lignin for synthesizing TiO₂/activated carbon heterostructures. It was observed that the photocatalyst obtained with FeCl₃-activated carbon yielded higher removal of acetaminophen (around 29, 42 and 74% higher than those obtained with KOH-, H₃PO₄- and ZnCl₂-derived carbons, respectively, after 3 h under solar light). In that previous research, the photocatalytic performance of bare TiO₂ was already studied, being higher than the synthesized TiO₂/AC heterostructures, which was attributed to a better contact of contaminant with non-supported TiO₂ as corroborated in literature [28]. However, the photocatalyst obtained with FeCl₃-activated carbon showed around 5-fold increment in the initial settling velocity compared to bare TiO₂, and therefore it is much easily recovered from the reaction media.

The preparation of the TiO₂/AC heterostructures can be addressed by different methods [29], [30], [31], [32]. The properties of these heterostructures depend on the synthesis route because of the different conditions used. For instance, sol-gel synthesis requires high temperature post-treatment to obtain the highly-photoactive anatase phase; solvothermal synthesis uses lower temperatures, while microwave-assisted methodology allows a faster heating rate and different distribution of heat involving hot spots [28]. To the best of our knowledge, a detailed comparative study of the performance of TiO₂-carbonaceous heterostructures prepared by these three synthesis routes on the photocatalytic degradation of emerging contaminants has not been reported before. Additionally, in many cases it has not been established a clear difference between the contribution of adsorption and photodegradation, despite this can be a key factor when using highly porous materials as catalyst supports. In the current study, TiO₂/AC heterostructures have been synthesized by three different methods (solvothermal, microwave-assisted and sol-gel), being the activated carbon previously prepared by chemical activation of lignin with FeCl₃. The synthesized heterostructures have been tested in the solar-driven photocatalytic degradation of three target pharmaceuticals (acetaminophen, ibuprofen and antipyrine), paying special attention to the effect of pH and the extent of mineralization. Experiments with three single compounds as well as with mixtures of them have been performed. In addition to this, intermediates of the photocatalytic oxidation of individual contaminants were identified, being proposed the degradation pathways of these pharmaceuticals under solar-simulated light.