Photodegradation of the synthetic fragrance OTNE and the bactericide triclosan adsorbed on dried loamy sand – Results from models and experiments

Abstract

Fragrances such as OTNE (marketed as Iso-E-Super®) and bactericides such as triclosan (marketed as Igrasan) are present in waste water and thus finally sorbed to sewage sludge. With that sludge they can reach agricultural fields where they potentially can undergo photodegradation processes. In this study the photodegradation of OTNE and triclosan on dried loamy sand was measured under artificial sunlight conditions in laboratory experiments. These compounds were artificially added with concentrations of 1 μg g⁻¹ on pre-rinsed dried loamy sand. The decrease in concentration with light irradiation was measured for 32 d in comparison to soil samples without light irradiation. The estimated light source intensity was 27 W m⁻². Within the experiment, the apparent half-life was 7 and 17 d for OTNE and triclosan respectively. The decrease did not simply follow first-order kinetics. The apparent rate constant decreased in the latter stage of reaction, suggesting that part of the chemicals were inaccessible for degradation. Two models, i.e., a diffusion-limited model, and a light penetration-limited model, were used in comparison to the measured data to explain the observed degradation limitations in the latter stages of the experiments. Comparing the hereby obtained model parameters with estimated physico-chemical parameters for the soil and the two chemical compounds, the light penetration-limited model, in which the degradation in the soil surface layer is assumed to be limited due to the shading effect of light in the upper thin soil layer, showed to be the most realistic in describing the photodegradation.

Introduction

OTNE ([1,2,3,4,5,6,7,8-octahydro-2,3,8,8-tetramethylnaphthalene-2yl]ethan-1-one) is the one of the major constituents of fragrance mixtures and is produced as a technical mixture (e.g., Iso-E-Super®) with 2500–3000 t annually (Gautschi et al., 2001). Triclosan is used as an antibacterial agent in toothpaste, mouthwash, functional clothing such as sport shoes and underwear as well as a stabilizing agent in detergents and cosmetics (Adolfsson-Erici et al., 2001). 1500 t of this bactericide are produced annually worldwide (Singer et al., 2002). Since the emissions and impacts to environment of such compounds are becoming an issue, several researchers have measured the concentrations and the fate of them in aquatic environments (Kolpin et al., 2002, Lindström et al., 2002, Simonich et al., 2002, Bester, 2005, Bester et al., 2008a, Bester et al., 2008b). Both compounds are used in consumer products for cleaning and personal care, thus a major fraction of the used amount is disposed of with waste water. Waste water treatment plants can perform a partial cleaning of the water (Lindström et al., 2002, Bester, 2005, Bester et al., 2008b). During this process a considerable fraction of the triclosan and OTNE is partitioning into the sludge during waste water treatment (Singer et al., 2002, Simonich et al., 2002, Bester, 2003, Bester et al., 2008b). In those countries that use sludge as fertilizer, such as Denmark, these xenobiotics are then easily transported to the soil this way. Another way to expose soil is using the treated water that still contains some of the pollutants for irrigation (Oppel et al., 2004). On the soil surfaces, OTNE and triclosan can possibly be either be persistent, be degraded microbially or decompose with solar irradiation (photo-oxidation). In this study we focus on the photo-oxidation. In aquatic environments, the photolytic decomposition of triclosan has been measured extensively (Kanetoshi et al., 1992, Tixier et al., 2002, Ferrer et al., 2004, Latch et al., 2005, Chen et al., 2008) whereas the photolytic decomposition on the soil surface has not been investigated well. Further, for OTNE, the information of photolytic behavior is very limited (Aschmann et al., 2001). Triclosan undergoes degradations and conversions when irradiated with UV- or sunlight (Tixier et al., 2002, Latch et al., 2005, Chen et al., 2008). The half-life varies with pH values, as chinoidic structures are formed, and the half-life in water is estimated to be in the order of minutes to hours in natural sunlight conditions, and the extent is strongly dependent on pH values. Photolysis of triclosan in water leads to hydrolysis, dechlorination, and catechol products (Kanetoshi et al., 1992), or 2,8-dichlorodibenzo-p-dioxin and 2,4-dichlorophenol by high energy UV light (Ferrer et al., 2004) or natural sunlight (Latch et al., 2005).

In this work OTNE and triclosan were used as markers as they are (a) present in sludge in relatively high concentrations, (b) allow some comparison, as triclosan is a phenolic structure with relatively well understood photodegradation pattern, while very little is known about the photodegradation of the aliphatic OTNE. It was studied whether OTNE and triclosan can be decomposed or converted at the soil surface layer (top few millimeters) under light irradiation in similar ways as in the water phase. In this study the photodegradation of OTNE and triclosan on dried loamy sand (as typical for Denmark) is measured by artificial sunlight in the laboratory. From the result, three different models describing the limitation of the reactions of the compounds on the solids were compared with the measured data. Factors affecting the extent of photodegradation on the soil were discussed.