Studies on the kinetics of carbamazepine degradation in aqueous matrix in the course of modified Fenton's reactions

Highlights

• Carbamazepine degradation by classical and modified Fenton's reagent.

• The efficiency of Fenton and Fenton-like reagent in ratio to the carbamazepine.

• The influence of Fenton's systems on a persistence of carbamazepine.

Abstract

The present article describes a study into the kinetics of carbamazepine degradation under influence of the standard Fenton's reagent, light-enhanced Fenton's reagent, as well as modified Fenton's systems in which iron(II) ion is replaced by Cu(I), Cu(II), Ni(II), Mn(II), Cr(III) and V(V) ions. In the course of the study it was established that V(V) ion modified Fenton's reagent was equally effective in relation to carbamazepine as the standard reagent. Parameters of both standard and modified Fenton's reagents were optimized. It was observed that an increased concentration of inorganic ions and acidic pH levels precipitated the decomposition of carbamazepine.

Introduction

Dynamic expansion of pharmaceutical markets along with elevated drug consumption are principal reasons behind the increasing pollution of water environments with pharmaceutics and products of their partial decomposition. These substances penetrate into water environments mostly by way of municipal waste, and as biological agents, they pose serious harm to water organisms and to humans [1]. For this reason, one of the overriding goals of environmental chemistry has become to analyze the life cycle of biologically active pharmaceutical substances existent in untreated and treated sewage, as well as in surface water (e.g. rivers, streams, lakes), groundwater, and drinking water [2].

Presence of drugs in surface water is a result of various factors, the most influential among them being: amounts in which the drugs are taken, frequentness and periodicity of their use, forms of their excretion (i.e. as metabolites or in an unchanged form), and effectiveness of their neutralization during sewage treatment. Small as the individual quantities of drugs that enter water environments may be, the sheer constancy of the process leads to their considerable accumulation, followed by increased concentration in water milieus. Pharmaceutical substances prevalent in water ecosystems may be harmful to indigenous water organisms, and – in an indirect way – to humans. Worse still, the negative effects can be non-immediate, cumulating slowly and imperceptibly over a time span of many years to a moment when they become severe threat to human health, especially considering the fact that lifetimes of many drugs in the natural environment reach up to ca. one year, and there are substances which sustain much longer [3]. One of the compounds frequently found in various sections of water ecosystems is carbamazepine (5H-dibenzo[b,f]azepine-5-carboxamide), a drug belonging to the derivatives of dibenzazepine (Fig. 1) which, apart from its common application in the treatment of epilepsy, is administered in neuropathic pains and multiple sclerosis. Carbamazepine exhibits mood-stabilizing properties, and is often used in the therapy of manic episodes and bipolar affective disorders [4], [5]. It is characterized by strong lipophilicity; also, ca. 70–80% of the drug bonds with proteins. In the liver, carbamazepine is oxidized, deaminated, hydroxylated and partly esterificated with glucuronic acid. Its main route of excretion is urine, and the metabolites include pharmacologically inert trans-10,11-dihydro-10,11-dihydroxy carbamazepine and pharmacologically active carbamazepine-10,11 epoxide. The biological half-life of the drug is relatively long, and it depends on the duration of the treatment period. In particular, the elimination half-life from the human organism corresponds to the treatment time, amounting to 24–45 h when a single dose is administered, and 15–24 h in long-term therapies [6]. Due to the fact that carbamazepine is so widely used, it belongs to drugs which are most frequently detected in environmental samples, considering the whole gamut of pharmaceutical substances present in the environment. Its mean concentration in municipal waste reaches 2.1 μg L⁻¹ [7]; moreover, it can be found in surface or even drinking water [7], [8]. Numerous experiments have confirmed that the compound is highly resistant to photodegradation and biodegradation in conventional sewage treatment plants. Specifically, its photodecomposition lasts between 50 and 100 days [9], [10], and the ensuing photoproducts impinge on the natural environment event to a greater extent than the original compound. For that matter, one of the most frequently encountered products of this photodecomposition is acridine, often present in municipal waste [9], [11].

In recent years, much attention has been devoted to research on advanced oxidation processes (AOPs). These processes are characterized by great efficiency in the cases of contaminants with high degree of chemical stability and low susceptibility to biological degradation [12], [13]. Many organic compounds present in water and sewage in the course of AOP undergo complete mineralization to carbon dioxide and water, or, alternatively, in the course of incomplete oxidation processes, are subjected to decomposition into much simpler molecules such as e.g. alcohols, aldehydes, or biodegradable carboxylic acids.

All methods of advanced oxidation are reliant on generation of highly reactive free radicals, especially hydroxyl radicals (HO) [14]. It happens that in AOPs the (HO) radicals are oftentimes provided by the Fenton's reaction. Fenton's reagent exhibits strong oxidative properties in relation to some compounds precisely due to the triggered formation of (HO) radicals [15], [16]. However, in order to boost the performance, efficiency and cost-effectiveness of the reaction, further improvements of Fenton's reagent are sought after. The classic Fenton's reaction, despite its already high efficiency, has certain limitations related to the stability of the solution's pH. Essentially, the acidic character of the solution has to be maintained to ensure that iron(III) hydroxide does not precipitate. Many researchers have been introducing various improvements to the Fenton's reaction, mostly using various sources of iron ions, or other transition metals ions [17], [18], [19], [20] alongside additional sources of energy. Accordingly, one can distinguish UV radiation enhanced Fenton's reaction (photo-Fenton), ultrasound aided Fenton's reaction (sono-Fenton), both UV radiation and ultrasound supported Fenton's reaction (sono-photo-Fenton), Fenton's reaction improved by the presence of photocatalyst (UV/TiO₂ + Fenton's reaction), the sono-photocatalytic Fenton's reaction (US + UV/TiO₂ + Fenton's reaction), as well as the electro-Fenton's reactions. All of these have been used for the purpose of efficient removal of harmful substances [21], [22], [23], [24].

The experiments described within the scope of the present article aimed at investigating the kinetics of carbamazepine decomposition under the influence of transition metal – hydrogen peroxide type Fenton's reagent, and comparing the results to those obtained by means of the classic and photocatalytic Fenton's reactions.