Removal of antiretroviral drugs stavudine and zidovudine in water under UV254 and UV254/H2O2 processes: Quantum yields, kinetics and ecotoxicology assessment
Graphical abstract
Introduction
In the last decades, as a result of the widespread availability of pharmaceutical drugs, occurrence, identification, quantification, removal and environmental fate of these emerging contaminants have received critical attention [[1], [2], [3]]. Among this new and increasingly growing class of water microcontaminants, the presence of antiretroviral drugs (ARVs) in wastewater and surface water has been the focus of recent research [[4], [5], [6], [7], [8], [9]]. Since their introduction into the market in the early 90s, ARVs have rapidly spread across the world because of their effectiveness in the treatment of the HIV virus [10]. In fact, ARVs inhibit the reverse transcriptase of the HIV virus, repressing viral replication [11]. The most commonly used ARVs include zidovudine (ZDV), stavudine (STV), lamivudine, abacavir and nevirapine which are usually administered as a combination therapy to increase their effectiveness in preventing HIV reproduction [12]. ZDV was the first marketed antiretroviral [12] and is still one of the most widely used. Despite presenting several side effects, STV is still one of the most common ARVs because of its relatively low price [13]. Collectively, ARVs increase the life expectancy of HIV-positive patients, however, significant concerns have been raised about their simultaneous release to the environment [4,5,9]. New concerns are also related to their consumption in the illicit drugs nyaope [14] and whoonga [15]. As a result, STV and ZDV have been often detected in effluents of wastewater treatment plants (WWTPs) and in natural surface water, in Europe and in Africa, at levels of tens of ng L−1 up to hundreds of ng L−1 (Table 1).
In Europe, the main ARVs contamination route of natural waters is through human body excretion and subsequent release in the sewage system [9]. The further presence of ARVs in the effluents of WWTPs and surface water demonstrates the inefficiency of current WWTPs treatment methods. The highest concentrations have been detected in Kenya and South Africa. The levels of ZDV and STV in these African countries have been shown to be higher in surface water compared to WWTPs effluents, which contrast with the general trend in Europe. The level of ZDV in Kenyan surface waters have increased up to three order of magnitude during the period 2012–2016 [16,18]. Recently, ZDV has also been detected in groundwater [16] which can be probably ascribed to the illicit use and direct spillage in water.
It has been reported [7] that ZDV is not completely removed in conventional treatment plants, a conclusion also shown for an aerobic and anaerobic WWTP in Germany [9], although, these authors reported 68% of ZDV removal in different German WWTP with an activated sludge system. Further biological treatment studies performed in synthetic wastewater demonstrated that ZDV is non biodegradable, toxic, and inhibitory to activated sludge bacteria [22]. Higher removals have been reported for STV through activated sludge (>78%) and biological treatment (>89%) [7,9]. Even though the reported LC50 (Daphnid acute 48 h) are 980 mg L−1 and higher than 100 mg L−1 for STV and ZDV respectively [[23], [24], [25]], synergistic and mutagenic effects on the aquatic fauna and humans cannot be ruled out. For example, ZDV has been demonstrated to have carcinogenic potential [26].
Advanced oxidation processes (AOPs) have increasingly been proposed as effective tertiary treatments for the removal of biorecalcitrant emerging contaminants [27,28]. Among these, the UV254/H2O2 is considered one of the most convenient process since it can be simply applied in existing municipal water treatment plants adopting UV254 lamps for water disinfection, such as treatment plants for water reuse and tertiary units in conventional STP [29]. Notably, reclaimed water reuse for irrigation is especially suitable in water stressed areas [30], which often also present the highest incidence of HIV-positive people, such as Central and South Africa. In spite of the apparent effectiveness of AOPs in micropollutants removal, the potential for the formation of highly toxic by-products [31,32] calls for longer treatment times and for the further evaluation of the ecotoxicity of the treated water. In this study the kinetics of ZDV and STV direct photolysis under UV254 radiation and in the presence of hydrogen (UV254/H2O2) was investigated in order to estimate important photo-kinetic parameters, such as the quantum yields and the second-order kinetic constant of reaction between OH radicals and the compounds, which are necessary for design and retrofitting of water treatment plants. The reaction kinetics were investigated by means of a recent developed methodology which uses a microcapillary film (MCF) array photoreactor [33], previously adopted for the investigation of the photolytic kinetics of other micropollutants [[34], [35], [36]]. The use of this new microphotoreactor technology has been shown to be particularly suitable, as an effective experimental tool, for the study on highly priced, hazardous, or poorly available water contaminants since it allows to run the entire experimental campaign using minimal amount of compounds, in this case less than 50 mg of ZDV and STV.
On the other hand, the kinetic constant of hydroxyl radical attack to the organics and the UV254 quantum yield of direct photolysis of an organic substrate are not affected by the type of the reactor adopted. Therefore, both parameters will be applicable for the design of water treatment plants (batch or continuous flow type).
The implementation of water reclamation systems and of the environmental risks posed by the effluents, requires comprehensive ecotoxicological assessment on a set of biological tests on species at different trophic levels [37]. For this purpose, the three most frequently ecotoxicity bioassays in aquatic systems are the assessment on Aliivibrio fischeri and Daphnia magna tests for acute toxicity and the Raphidocelis subcapitata test for chronic toxicity. Although these target organisms have often been used to assess the impact of contaminated water, the main focus of water quality testing should also concern organisms- dependent chemical-physical and biological properties of the target molecules. In particular, several studies have demonstrated that ARVs differ in genotoxic potency, chromosomal damage and aberration types induced in vitro and in perinatally exposed mice and infants [[38], [39], [40]].
In consequence, in the present study we investigated the ecotoxicity of untreated and treated solutions of ZDV and STV using a battery of ecologically relevant testing species to assess the acute and chronic toxicities and genotoxicity and mutagenicity.
Section snippets
Materials
Zidovudine (>99%), stavudine (>98%), NaOH (≥98%), H2SO4 (98%), hydrogen peroxide (30% in H2O), acetonitrile (≥99.9%), methanol (≥99.9%), phosphoric acid (85% in H2O), catalase from Micrococcus lysodeikticus, CaCl2·2H2O(≥ 99.5%), MgSO4·7H2O (≥ 98%), NaHCO3 (≥99.5%), KCl (≥99%), NH4Cl (≥99.9%), MgCl2· 6H2O (≥ 98%), KH2PO4(≥ 99%), FeCl3·6H2O (≥ 98%), Na2EDTA·2H2O (≥ 99.9%), H3BO3 (≥ 99%), MnCl2·4H2O (≥ 98%), ZnCl2 (≥ 99%), CoCl2·6H2O (≥ 98%), Na2MoO4·2H2O (≥ 98%) and CuCl2·2H2O (≥ 98%) were all
Absorbance spectra
The absorbance spectra of ZDV (Fig. 1a) and STV (Fig. 1b) at pH 4.0, 6.5 and 8.0 showed invariance in the pH range from 4.0 to 8.0 for ZDV and from 6.5 to 8.0 for STV. Since pH did not affect ZDV and STV degradation kinetics in the pH ranges 4.0–8.0 and 6.0–8.0, respectively, the reaction kinetics were investigated in the slightly acidic to alkaline pH range from 6.0 to 8.0, which also is more environmentally relevant. The estimated molar absorption coefficients at 254 nm are summarized in
Conclusions
The removal of stavudine and zidovudine by UV254 radiation without and with hydrogen peroxide was investigated in a microcapillary film photoreactor. A key benefit of the microcapillary photoreactor over larger batch or flow reactors is the possibility to carry out the process using very small quantities of reagents in a highly controlled environment. As a result, the adoption of microphotoreactor technology is particularly appropriate for investigation on highly priced or scarcely available
Acknowledgements
The Authors are grateful to ERASMUS-Mobility Student Program, and to Ing. Giulio Di Costanzo for his precious support during the experimental campaign.
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