Measurement of copper release rates from antifouling paint under laboratory and in situ conditions: implications for loading estimation to marine water bodies

Abstract

The release of biocides, such as copper (Cu), from antifouling (AF) coatings on vessel hulls represents a significant proportion of overall Cu loading in those harbors and estuaries where substantial numbers of small craft or large vessels are berthed. Copper release rates were measured on several self-polishing, tin-free coatings and an ablative Cu reference coating applied to steel panels using three measurement methods. The panels were exposed in natural seawater in San Diego Bay, and release rates were measured both in the laboratory and field over 2 years. Results with the static (20 cm × 30 cm) panels indicated that Cu release rates were initially high (25–65 μg Cu cm −2 day −1), with a large range of values between paint types. Release rates declined to substantially lower rates (8–22 μg cm −2 day −1) with reduced variability within 2 months. Release rates continued to decrease over time for approximately 6 months when relatively constant release rates were observed for most coatings. Over time, relative differences in Cu release rates measured by three exposure methods decreased, with all coatings exhibiting similar behavior toward the end of the study. Lowest overall Cu release rates were observed with the self-polishing experimental paint no. 7 in static–dynamic and in situ treatments. The highest periodic release rates were measured from panels that experienced periods of both static and dynamic exposure (8.7 m s −1 rotation). The lowest release rates were measured from panels that experienced static, constant depth exposure, and where release rates were evaluated in situ, using a novel diver-deployed measurement system. Results from this in situ technique suggests that it more closely reflects actual Cu release rates on vessel hulls measured with intact natural biofilms under ambient conditions than measurements using standardized laboratory release rate methods. In situ measurements made directly on the AF surface of vessels demonstrated typically lower release rates than from the panel studies, averaging 8.2 μg cm −2 day −1 on pleasure craft, and 3.8 μg cm −2 day −1 on Navy vessels. The data suggest that the presence of an established biofilm likely serves to moderate the release of Cu from field-exposed antifouling coatings both on panels and hull surfaces.