Photochemical fate of a mixture of emerging pollutants in the presence of humic substances

Abstract

The photodegradation of a mixture of the emerging pollutants (EPs) clofibric acid, amoxicillin, acetamiprid, acetaminophen, carbamazepine, and caffeine was studied under irradiation with a xenon lamp. The quantum efficiencies of the EPs were determined when irradiated individually. Experiments with the mixture of the EPs showed that indirect photoprocesses attributable to interaction between EPs can either enhance the photodegradation rate by photosensitization or decrease it by quenching processes. The addition of humic substances (HS) to the solutions resulted in an increase of indirect photoprocesses with higher effects on acetaminophen and carbamazepine; this was more remarkable when a filter was used to cut off radiation in the range 280–295 nm. Experiments carried out with chemical probes indicated that the triplet excited states of HS play a major role in the photosensitization process, although the contribution of other species cannot be completely ruled out. Additionally, V. fischeri toxicity tests showed a synergistic effect produced by the mixture of EPs before irradiation. Photodegradation resulted in an enhanced toxicity of the solution at the initial steps of the process, which was associated both with synergistic effects and with the formation of toxic photodegradation by-products of clofibric acid.

Introduction

Emerging pollutants (EPs) are a group of chemical substances that have been recently detected in fresh waters, as a result of human activities. Perfluorinated compounds, pharmaceuticals, hormones, endocrine disruptors, disinfection byproducts, sunscreens, flame retardants, algal toxins, dioxane, pesticides and their degradation byproducts or nanomaterials are considered within this group (Petrovic and Barceló, 2006; Richardson, 2008). Although they are commonly found at low concentration (typically μg L⁻¹), they constitute an important concern because their potential effects on humans and natural ecosystems still remain unexplored (Murray et al., 2010).

Different natural processes have been described to remove EPs from surface waters, namely, adsorption onto sediments, biodegradation or photolysis with sunlight (Pal et al., 2010). Solar-driven photo-degradation has been identified as the most efficient among these processes for some families of EPs, such as pharmaceuticals (Khetan and Collins, 2007; Tixier et al., 2003). Two different mechanisms have been proposed for sunlight mediated reactions: direct substrate photolysis or indirect reactions promoted by reactive species generated by natural photosensitizers like nitrate or dissolved organic matter (DOM) (Khetan and Collins, 2007; Razavi et al., 2011).

The presence of DOM might be of critical importance for photochemical reactions of EPs in surface waters (Xu et al., 2011). In particular, the study of the effect of humic substances (HS), the main fraction of DOM, on the photochemical removal of EPs seems meaningful because HS can promote the transformation of many organic contaminants under sunlight irradiation, either through the formation of reactive oxygen species (ROS), such as hydroxyl radical (HO), singlet oxygen (O₂(a¹Δ_g)) and superoxide (Goldstone and Voelker, 2000; Paul et al., 2004; Vaughan and Blough, 1998), or mediated by the triplet-excited states of the HS (³HS*) (Canonica, 2007). However, HS may also retard the removal of the EPs by screening sunlight due to HS light absorption in a wide range of wavelengths. Therefore, HS are expected to influence the photochemical fate of the EPs and their persistence in natural water (Guerard et al., 2009b) although the overall effect of the HS on the photodegradation rates of organic compounds depends on the magnitude of these two opposite contributions.

Several papers have been published on the role of HS in the photochemical degradation of pollutants (Guerard et al., 2009a; Peuravuori and Pihlaja, 2009; Sturini et al., 2010; Yu et al., 2010). In addition to humic substances, the use of related organics such as photoactive macromolecules extracted from compost or solid urban wastes in advanced oxidation processes is gaining momentum (Bianco Prevot et al., 2010). In those systems, the concentration of pollutants might be several orders of magnitude above those found in natural waters (Richardson, 2008). Hence, much research is still needed on this issue. Since the EPs show very different chemical structures and properties and they are commonly found in complex mixtures in natural waters, their behavior might differ from that of single compounds. Furthermore, primary removal of the EPs does not guarantee decontamination because toxic photodegradation by-products can be formed (Farré et al., 2008). Thus, the identification of photoproducts and monitoring of the toxicity of the solution is also needed since, despite their low concentration, synergistic or antagonistic effects can be produced in complex mixtures of EPs and their photoproducts (Farré et al., 2008).

With this background, the aim of this paper is to study the photodegradation of a mixture of six emerging pollutants belonging to different families (Scheme 1): acetaminophen (analgesic), caffeine (stimulating agent), acetemiprid (insecticide), clofibric acid (metabolite of clofibrate, also employed as herbicide), carbamazepine (psychiatric drug) and amoxycillin (antibiotic). They were chosen because they are commonly found in the environment and at the effluents of wastewater treatment plants (Khetan and Collins, 2007). Although, some information on the photochemical degradation of these pollutants is available (Chiron et al., 2006; Doll and Frimmel, 2003; Lam et al., 2004; Matamoros et al., 2009; Peuravuori and Pihlaja, 2009), as far as we know no systematic study has been performed on the photolysis of the mixture of the pollutants in the presence of HS. Thus, we here investigate the influence of the mixture of the six EPs on the photodegradation of each EP in the presence and in the absence of HS. In addition, the evolution of the dissolved organic carbon and toxicity of the mixture have been also investigated.