Degradation of NSAID Naproxen in Wastewater by Electro-Fenton

Abstract

The removal of non-steroidal anti-inflammatory drug naproxen in tap water by hydroxyl radicals (▪OH ) formed by electro-Fenton process was conducted either with Pt or DD anodes and a 3D carbon felt cathode. 0.1 mM ferrous ion was proved to be the optimized dose to reach the best naproxen removal rate in electro-Fenton process. Both degradation and mineralization rate increased with increasing applied current intensity. The degradation of naproxen by ▪OH vs. electrolysis time was well fitted to a pseudo–first–order reaction kinetic. An almost complete mineralization was achieved under optimal catalyst concentration and applied current values. Considering efficiency of degradation and mineralization of naproxen, electro-Fenton process with DD anode exhibited better performance than that of Pt anode. The absolute rate constant of the second order kinetic of the reaction between naproxen and ▪OH was evaluated by competition kinetics method and the value (3.67 ± 0.3) × 10λ M-1·s-1 was obtained. Identification and evolution of the intermediates, as aromatic compounds and carboxylic acids, were deeply investigated, leading to the proposition of oxidation pathway for naproxen. The evolution of the degradation products and solution toxicity were determined by monitoring the luminescence of bacteria Vibrio fischeri (Microtox method).

Keywords: Naproxen, Electro-Fenton, BDD Anode; Degradation Pathways; By-Products, Toxicity

Introduction

The recent public interest regarding the presence of pharmaceutical pollutants in water has raised important concerns due to the unknown environmental impact and possible damages to the botany and fauna present in aquatic system. Pharmaceutical compounds are extensively consumed annually. Most of them are metabolized and excreted by humans and animals, and spilled into wastewaters that are treated in wastewater treatment plants. However, some of these pharmaceuticals are not completely eliminated by the procedures applied in conventional water treatment processes, and consequently are found in effluents exiting the plants, and could even reach drinking water. There are even reports that these contaminants are recently been detected in sewage effluents and sometimes in drinking water, suggesting that their possible environmental impacts is an emerging environmental issue. And due to the increase of its consumed quantity, there is a high possibility that it is present in our waste and surface waters, too. The traditional wastewater treatment plants are mostly not designed to deal with polar micro pollutants such as pharmaceuticals. With the respect of pharmaceutical characteristic being resistant to microbial degradation, low removal percentages are performed in the secondary treatment in traditional water treatments. Such final effluents containing residual pharmaceuticals are discharged into natural surface water bodies (stream, river or lake).

Most of the traditional wastewater treatment plants (WWTPs) are especially not designed with tertiary treatment step to eliminate pharmaceuticals and their metabolites [4]. WWTPs therefore act as main pharmaceuticals released sources into environment. The released pharmaceuticals into the aquatic environment are evidenced by the occurrence of pharmaceuticals up to 􀈝g L-1 level in the effluent from medical care units and sewage treatment plants, as well as surface water, groundwater, and drinking water. It is urgent to supply the adapted technologies to treat the pharmaceuticals in WWTPs before releasing them into natural water system. Nevertheless, increased attention is currently being paid to pharmaceuticals as a class of emerging environmental contaminants. Because of the presence of the pharmaceuticals in the aquatic environment and their low volatility, good solubility and main transformation products dispersed in the food chain, it is very important to investigate their greatest potential risk on the living organisms. Since the pharmaceuticals are present as a mixture with other pollutants in the waste and surface waters, effect as synergistic or antagonistic can occur as well. Therefore, their long-term effects have also being taken into consideration. In the last years, European Union and USA have taken action to establish regulations to limit the pharmaceuticals’ concentrations in effluents to avoid environmental risks.

To date, most of the research has been focused on aquatic environments. There's a lot we don't know yet, but what we do know is that pharmaceuticals are blamed for various negative effects on marine life. Populations of male fish are being feminized when exposed to wastewater containing low concentrations of estrogen from oral contraceptives. They are growing ovaries, suffering from low sperm count, producing egg proteins usually found only in females, and in some cases, trying to lay eggs. Generations of aquatic organisms have lived and continue to live in this chemical “soup”. For them there is no escape. The toxic exposure is constant in the aquatic environment. For this reason, even low concentrations of pharmaceuticals can have very significant impacts on many species. Some impacts are profound and even disturbing, while others are unknown. As with humans health risks, there are many questions and few answers other than the all too familiar response—we do not know.

As a consequence of this problem, it is advisable to reduce their concentrations in wastewater purification plants by using different treatments, such as chemical procedures, which have demonstrated to be efficient to remove non – biodegradable pollutants. Among these techniques, UV radiation is widely used to induce photoreactions of pharmaceuticals. Similarly, ozone is also an efficient oxidant for the purification of surface and drinking waters. At the same time, advanced oxidation processes (AOPs), which are based in the generation of hydroxyl radicals (·OH) in solutions, have attracted great interest for the degradation of bio refractory or hazardous organic compounds in water systems. The AOP procedure is particularly useful for cleaning biologically toxic or non-degradable materials such as aromatics, pesticides, petroleum constituents, and volatile organic compounds in waste water. The contaminant materials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization. The development and application of several Advanced Oxidation Processes (AOPs) to destroy toxic and biologically refractory organic contaminants in aqueous solutions concentrated significant research in the field of environmental engineering during the last decades. Among AOPs, electrochemical advanced oxidation processes (EAOPs) are being regarded as the most perspective treatments for removing persistent organic micropollutants.

Due to the interest of this research field, a study was designed for the removal of pharmaceutical in wastewater AOPs. The pharmaceutical selected, which have been found in different aquatic environments at concentrations in the range ng L-1 toμg L-1 is the Non-Steroidal Anti-Inflammatory Drug compound (NSAID) Naproxen(Nap). The degradation of these substances was conducted by single agents (UV radiation and ozone), as well as by several AOPs: Fenton’s reagent, Fenton-like system, photo-Fenton system, and different combinations of UV radiation and ozone with H2O2, TiO2, Fe(II), and Fe(III).

This research work is only focused in the degradation of NSAIDs which have been detected in many water-bodies around the world and is consider as micropollutants. The removal of Naproxen has been widely study by different means like the AOPs, but some of the showed low removal efficiency or the cost of the electrodes are too expensive. The focus of this research is on the degradation of Naproxen using AOPs, the optimal parameters for the electro-Fenton process and the effect of different catalyst ions in the electro-Fenton process to oxidize Naproxen.

Chapter 2

REVIEW OF LITERATURE

Advanced Oxidation Processes

In a broad sense, refers to a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials inwaterandwaste waterbyoxidationthrough reactions withhydroxyl radicals(·OH).[1]In real-world applications ofwastewater treatment, however, this term usually refers more specifically to a subset of such chemical processes that employozone(O3),hydrogen peroxide(H2O2) and/or UV light.[2]One such type of process is calledin situ chemical oxidation.

Contaminants are oxidized by four different reagents: ozone, hydrogen peroxide, oxygen, and air, in precise, pre-programmed dosages, sequences, and combinations. These procedures may also be combined with Ultra Sound reactors, UV irradiation and specific catalysts. This results in the development of hydroxyl radicals.

The AOP procedure is particularly useful for cleaning biologically toxic or non-degradable materials such as aromatics, pesticides, petroleum constituents, and volatile organic compounds in waste water.

The contaminant materials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization.

A goal of the waste water purification by means of AOP procedures is the reduction of the chemical contaminants and the toxicity to such an extent that the cleaned waste water may be recycled or, at least, dumped into a conventional sewage treatment.

Electro-Fenton Process

Electro-Fenton (EF) process, which can be defined as electrochemically assisted Fenton’s process, is one of the most popular techniques among EAOPs. A suitable cathode applied to be fed with O2 or air reduces dioxygen to superoxide ion (O2−),leading to the formation of H2O2 continuously in an acidic medium (Eq. (2.22)).

Catalysts such as Fe2+, Fe3+, or iron oxides react with H2O2 (Eq. (2.23)), following Fenton’s reaction to yield OH radicals. Fe3+ ions produced by Fenton’s reaction are electrochemically reduced to Fe2+ ions (the Fe3+/Fe2+ electro catalytic system), which catalyze the production of OH from Fenton’s reaction [92, 155]. On the other hand, molecular oxygen can also be produced in the anodic compartment simply by the oxidation of water with Pt or other low O2 overvoltage anodes (Eq. (2.25)).

O2 (g) + 2H+ + 2e-→ H2O2 E0 = 0.695 V/SHE (2.22)

Fe2+ + H2O2 + H+→ Fe3+ + H2O + OH (2.23)

Fe3+ + e-→ Fe2+ E0 = 0.77 V/SHE (2.24)

H2O → 1/2 O2 + 2H+ + 2e- E0 = 1.23 V/SHE (2.25)

Then the generated strong oxidant radical (OH) can either dehydrogenate unsaturated compounds (RH) or hydroxylate aromatic pollutants (Ar) or other compounds having unsaturated bonds until their overall mineralization (conversion intoCO2, H2O and inorganic ions). The oxidation of organic pollutants by EF process can be visualized in the catalytic cycle.

Fenton Reagent

The Fenton Reagent (or reaction) is defined as a mixture of hydrogen peroxide and ferrous iron. The process is based on the formation of reactive oxidizing species, able to efficiently degrade the pollutants of the wastewater stream but the nature of these species is still under discussion and its formulation is subject to controversy in the past and recent Fenton oxidation related literature. Fenton Reagent is one of the most effective methods of the oxidation of organic pollutant. The efficiency of the Fenton reaction depends mainly on H2O2 concentration, Fe2+/ H2O2 ratio, pH and reaction time. Also, initial concentration of the pollutant and its character as well as temperature, have been substantial influence on final efficiency.

The Fenton reagent destroys wide variety of organic compounds without the formation of toxic by-products. Among the different technologies reported in literature for the treatment of highly contaminated effluents, Fenton’s reagent is characterized by its cost effectiveness, simplicity and suitability to treat aqueous wastes showing a variable composition. This method offers a cost-effective source of highly oxidizing species, using easy-to-handle reagents. The important advantage of the Fenton process is that oxidation and coagulation take place simultaneously. The comprehensive investigations showed that Fenton reagent is effective in treating various industrial wastewater components including aromatic amines, a wide variety of dyes, pesticides, surfactants, explosives and many other substances. Therefore, the Fenton reagent has been applied to treat a variety of wastes such as those associated with the textile industry, chemical manufactures, refinery and fuel terminals, engine and metal cleaning, etc.

[...]