Occurrence and persistence of antifouling biocide Irgarol 1051 and its main metabolite in the coastal waters of Southern England

Abstract

The toxicity and persistence of antifouling booster biocides are of major concern. This study reports the occurrence of Irgarol 1051 and its degradation product M1, in coastal waters of Southern England, during 2004–2005. The highest concentrations of Irgarol 1051 were 89 ng/L in water and 45 ng/g dry weight in sediments, with an overall mean (n = 108) of 13 ng/L and 16 ng/g in water and sediments, respectively. As the degradation product of Irgarol 1051, M1 was less widespread, with the highest concentration of 30 ng/L in water and 14 ng/g in sediments, with an overall mean (n = 108) of 5 ng/L and 4 ng/g in water and sediments, respectively. Overall, the concentration of Irgarol 1051 and M1 decreased significantly during the sampling period and in comparison to earlier studies during 2000 to early 2004, indicating that control measures by restricting the use of Irgarol 1051 are effective in reducing its concentrations in coastal waters. The distribution of Irgarol 1051 between sediments and water was significantly related to sediment organic carbon content. In addition, significantly higher concentrations of Irgarol 1051 were detected in paint particles than in sediment. The rate of release of Irgarol 1051 from paint residues is very slow, with a half life of approximately 1 y. Two important findings are emerging, first the importance of organic rich sediments and paint residues as major sites of storage for Irgarol 1051, and secondly Irgarol 1051 may be classified as a persistent organic pollutant due to its long half life.

Introduction

Antifouling paints have long been used in modern shipping industry as they are an effective method of controlling invasive species, from microbes such as algae and bacteria to barnacles and shellfish that may attach themselves to the hull of vessels, thus reducing the speed of the vessels and increasing fuel consumption (Omae, 2003). However, the serious environmental problems caused by the extensive use of tributyltin in antifouling paints, e.g. imposex in dogwhelks, resulted in the introduction of alternative compounds for the protection of ship hulls. Irgarol 1051 (2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine) is one of such substances, which has been used worldwide as an active ingredient for this purpose (Readman et al., 1993, Gough et al., 1994, Tolosa et al., 1996, Zhou et al., 1996, Thomas, 2001). Prior to September 2000, eight organic compounds including Irgarol 1051 were allowed for use in antifouling paints in the UK (Thomas et al., 2002). Since September 2000, as a result of 98/8/ΕΕ directive implementation, restrictions concerning the use of such substances in antifoulants were introduced. According to these restrictions, antifouling paints for use in small vessels are allowed to contain only the substances dichlofluanid, zineb and zinc pyrithione. Irgarol 1051 was approved for use on larger (> 25 m) vessels up to July 2003 (Thomas et al., 2002, Bowman et al., 2003, Cresswell et al., 2006, Gatidou et al., 2007).

Due to the nature and intended purpose of antifouling biocides, they were designed to be highly stable and long lasting in alkaline waters, to ensure they are effective in inhibiting biofouling for months to years. By using a box model, Ranke (2002) estimated that the overall residence time for Irgarol 1051 in the marine systems is over 10 y. If so, Irgarol 1051 can be considered as a persistent organic pollutant (POP). It is therefore not surprising that after Irgarol 1051 has been banned, it can still be detected in marine waters (Bowman et al., 2003, Gardinali et al., 2004, Gatidou et al., 2005, Cresswell et al., 2006, Gatidou et al., 2007) and sediments (Gatidou et al., 2004, Gatidou et al., 2007). Although Irgarol 1051 is not considered to be easily degraded in seawater with a half life of 100–350 d (Thomas et al., 2002, Omae, 2003), several studies (Liu et al., 1997, Okamura, 2002) show that it can be degraded to form its main metabolite M1 (2-methylthio-4-tert-butylamino-s-triazine) through N-dealkylation. Furthermore, M1 has been detected in coastal waters and sediments as a result of natural transformation processes such as photodegradation and biodegradation (Thomas et al., 2000, Thomas et al., 2002, Martinez and Barceló, 2001, Ferrer and Barceló, 2001, Lam et al., 2005, Gatidou et al., 2007). Concentrations of Irgarol 1051 in seawater worldwide vary between non-detectable and low μg/L (Sargent et al., 2000, Biselli et al., 2000, Sakkas et al., 2002, Okamura et al., 2003). Concentrations up to 4.2 μg/L have been detected in coastal areas (Basheer et al., 2002), whereas in the UK the highest concentration observed is 1.4 μg/L (Thomas et al., 2001). In sediment samples concentrations are typically in ng/g range (Albanis et al., 2002, Gatidou et al., 2007) but as high as 1 μg/g have been detected in marinas (Boxall et al., 2000). The levels of M1 are up to 1.9 μg/L (Okamura et al., 2000) and 0.023 μg/g (Gatidou et al., 2007) for seawater and marine sediment respectively, which are generally lower than those of Irgarol 1051 indicating slow degradation rates of the parent compound.

The aim of this study was to determine the occurrence and persistence of Irgarol 1051 and its main degradation product (M1) in coastal waters of Southern England following the restrictions of its use in antifouling paints, by comparing their concentrations against threshold values; assessing their spatial and temporal variations in seawater and sediment; determining the relationship between the physicochemical properties of marine sediment and the concentrations of the target compounds in sediments; and assessing the impacts of paint matrix for affecting the persistence of Irgarol and M1 in marine systems.