Photocatalytic oxidation and catalytic wet air oxidation of real pharmaceutical wastewater in the presence of Fe and LaFeO₃ doped activated carbon catalysts

Highlights

• Real pharmaceutical wastewater was treated in photocatalytic and CWAO processes.

• The performances of Fe/AC and LaFeO₃/AC catalysts were compared.

• 72.7% and 83.1% COD removal efficiencies were achieved by photocatalysis and CWAO, respectively.

• CWAO reaction followed a two-step first-order reaction kinetics.

Abstract

Real pharmaceutical wastewater was treated by photocatalytic oxidation and catalytic wet air oxidation (CWAO) processes. The catalytic performance of Fe/AC and LaFeO₃/AC (AC: Commercial activated carbon) was compared in the advanced oxidation processes. The influences of the reaction parameters were determined by using Box–Behnken design. Fe/AC catalyst was more effective in both of the photocatalytic oxidation and catalytic wet air oxidation processes. In the photocatalytic oxidation process, 72.7% COD removal was achieved at pH = 4.5, 2.0 g/L, and [H₂O₂]_o = 0.32 mM. Under the optimum conditions of the CWAO, which were determined as 3 g/L of catalyst loading, pH = 3, and 50℃, 83.1% COD removal efficiency was obtained. CWAO method was determined as more suitable since a higher COD removal efficiency was achieved in a shorter reaction time. The toxicity in terms of L. sativum growth inhibition was evaluated as 4.82% after the CWAO treatment. A two-step first-order reaction kinetics provided the best fit of the CWAO experimental data. The activation energies of the fast and the slow steps were 25,661.16 and 28,327.46 J/mole, respectively.

Introduction

Water pollution has become a serious global problem due to the rapid population growth and urbanization. In recent years, increasing attention has been paid to the detection of pharmaceutical contaminants in water resources. Pharmaceutical industry wastewaters contain organic solvents, additives, intermediates, and active ingredients used in various drugs (Gosu et al., 2014). It is reported that conventional wastewater treatment plants are not specifically designed for the degradation of pharmaceutical residues and the pharmaceutical removal efficiencies obtained in the wastewater treatment facilities are highly variable depending on the content of the wastewater (Angeles et al., 2020). The presence of pharmaceutical residues in the water resources even at low concentrations may result in development of drug-resistant microorganisms in addition to many other health problems such as chronic toxicity and endocrine system disorders (Isari et al., 2020).

There is an urgent demand for the development of effective treatment methods for the pharmaceutical wastewaters characterized by high chemical and biochemical oxygen demand, suspended solids, auxiliary chemicals and presence of complex pharmaceutical compounds and their metabolites. Though several conventional treatment methods such as coagulation and flocculation, adsorption and filtration have been considered for pharmaceutical wastewater treatment, application of conventional methods have been demonstrated as insufficient for the degradation of persistent pharmaceutical compounds generally (Changotra et al., 2020).

Chemical oxidative treatment methods, particularly Advanced Oxidation Processes (AOPs) are promising alternatives to the conventional treatment methods employed for the toxic and nonbiodegradable wastewaters (Sirtori et al., 2009). AOPs have a great potential for treating a wide range of emerging contaminants present in wastewaters (Huang et al., 2020).

Among various AOPs, increasing attention was raised by photocatalytic oxidation, since non-specific reactive species have been generated on the catalyst surface under UV irradiation (Collivignarelli et al., 2020). Heterogeneous photocatalytic oxidation has many advantages including the high mineralization efficiencies, low toxicity, low operation cost, and the effectiveness under ambient conditions (Xiao et al., 2013).

Catalytic wet air oxidation (CWAO) is another environmentally friendly advanced oxidation technology, which uses air as the oxidant at relative low cost. The performance of the CWAO depends on the catalysts used in the treatment process (Wang et al., 2020). CWAO can be operated under mild conditions in the presence of effective catalysts even at atmospheric conditions (Gao et al., 2018; Zhang et al., 2014). Perovskites are proved to be highly efficient for oxidation reactions and they are the potential catalysts in substitution of noble metal catalysts due to their high thermal stability (Cho et al., 2009). Supported metal and perovskite catalysts are the promising candidates to be used in advanced oxidation processes due to the large surface area and improved chemical stability of the composite catalyst structures.

In this context, the main objective of this study is to propose an eco-friendly, sustainable and innovative method for the treatment of pharmaceutical industry wastewater. Activated carbon supported iron and LaFeO₃ perovskite catalysts are good alternatives for the application of green catalytic methods since these materials have many advantages over traditional advanced oxidation catalysts. For instance, it is difficult to recover the homogeneous catalysts (e.g. transition metal salts) from the reaction medium, the noble metals are quite expensive, and the metal oxides suffer from the deactivation due to the leaching of the active sites. CWAO is an efficient method for the treatment of highly polluted industrial wastewaters and toxic organic compounds without the emissions of dioxins, furans, NO_x, SO₂, HCl, fly ash, etc (Jing et al., 2012). Since air is used as the oxidant directly instead of the expensive and harmful oxidants CWAO can be considered as an environmentally advanced oxidation technology. In addition, the photocatalytic oxidation is an effective and economically feasible method due to its easy operation at ambient conditions. Furthermore, in the presence of heterogeneous catalysts no solid wastes and sludge formation is observed by the application of these advanced oxidation methods. For this purpose, two different advanced oxidation processes; photocatalytic oxidation and catalytic wet air oxidation processes were tested, and the efficiencies were compared in the presence of two prepared catalysts which are Fe/AC and LaFeO₃/AC (AC: Activated Carbon).

Recently, various advanced oxidation methods have been applied effectively for the treatment of real pharmaceutical wastewater. Ahmadi et al. (2017) used MWCNT-TiO₂ catalysts under UVC light irradiation in the photocatalytic oxidation process (Ahmadi et al., 2017). Changotra et al. (2019) treated pharmaceutical industry wastewaters by combining the photo-Fenton oxidation and biological treatment methods (Changotra et al., 2019). Collivignarelli et al. (2020) investigated the efficiency of H₂O₂-assisted photoelectrocatalysis on TiO₂ meshes for the treatment of real pharmaceutical wastewater (Collivignarelli et al., 2020). Deng et al. (2017) used CdS/SnIn₄S₈ nanoheterojunctions under visible-light irradiation for the mineralization (total organic carbon removal) of real pharmaceutical wastewater (Deng et al., 2017). Huang et al. (2020) used iron foam combined ozonation as an advanced oxidation process to treat the organic pollutants in real pharmaceutical wastewater (Huang et al., 2020). Isari et al. (2020) improved the biodegradability of real pharmaceutical wastewater by sono-photocatalysis in the presence of WO₃/CNT (carbon nano-tube) catalysts (Isari et al., 2020). Accordingly, several advanced oxidation methods have been effectively applied for the treatment of pharmaceutical wastewaters in the presence of various composite catalysts.

Carbon based materials supported Fe or LaFeO₃ catalysts have also been used effectively in wastewater treatment processes. Esteves et al. (2021) studied on catalytic wet peroxide oxidation of olive oil wastewater in the presence of Fe/AC catalysts (Esteves et al., 2021). Medina et al. (2021) used iron-functionalized activated carbon catalysts for the treatment of two real water matrices including the effluent of a wastewater treatment plant and a landfill leachate (Medina et al., 2021). Qin et al. (2018) performed the degradation of benzoic acid by using Fe/AC catalyst (Qin et al., 2018). Zhang et al. (2017) investigated the catalytic performance of N-Fe/AC for the treatment of pharmaceutical wastewater (Zhang et al., 2017). Additionally, in literature, there are several studies on use of composite LaFeO₃/carbon materials such as LaFeO₃/Carbon sphere (Wang et al., 2017), LaFeO₃/g-C₃N₄ (Xu et al., 2020; Ye et al., 2018) and LaFeO₃/Graphene Oxide (Jing et al., 2021; Mutalib et al., 2018) in advanced oxidation processes for the removal of organic pollutants.

However, the catalytic activity of perovskite catalysts supported on activated carbon has not been tested so far in photocatalytic and CWAO processes for the treatment of pharmaceutical industry wastewaters. In addition, the performances of catalytic wet air oxidation and photocatalytic oxidation processes have not been investigated comparatively so far for the treatment of real pharmaceutical wastewaters. Therefore, the use of Fe/AC and LaFeO₃/AC catalysts for the treatment of real pharmaceutical wastewater and comparison of the efficiencies of the two advanced oxidation processes constitute the main innovative approaches of the study.