Ecotoxocicological assessment of antifouling biocides and non-biocidal antifouling paints

Summary

The objective of this investigation was to assess the environmental hazards of the active substances, copper, 4,5-dichloro-2-n-octyl-4-isothiazolin-3-on (DCOI) and zinc pyrithione and of substances leaching from non-biocidal antifouling paints.

The bioavailability of copper is the key parameter for the assessment of the toxicity of the metal in the aquatic environment. Sequestration of copper to organic substances normally reduces their bioavailability, however, this sequestration is apparently dependent on the composition of the organic matter. The bioavailability of copper in aquatic sediments depends on the speciation of the metal, on the sediment and on the physiology and food selection of the exposed organisms. It has been demonstrated that metals sequestrated to easily digested food are absorbed more easily by aquatic organisms than metals sequestrated to indigestible food. Bioavailable copper is very toxic to aquatic organisms. A permanent immobilization of copper may occur only by sequestration to undisturbed, anoxic sediments. Harbour sediments are usually anoxic and have a high content of sulfides that sequestrate to copper. Therefore, the bioavailability of copper in harbour sediments is expected to be low. Copper may be released by disposal of sediment and, on the Danish dumping sites, the sediment is usually scattered by water current and waves. As an element, copper is not degradable. The potential toxic effect of copper on the aquatic environment is reduced by sequestration to organic compounds and sediments, which means that the actual bioavailability of copper is low. Disturbances of the sediment, and the consequent changes in the oxygen conditions, may remobilize sequestrated copper, and such changes may cause effects on sensitive organisms in the vicinity of harbour areas and dumping sites.

DCOI is rapidly transformed into metabolites in seawater, where half-lives of 11 and 14 hours were found. The transformation of DCOI is very much quicker in aquatic sediment as half-lives of less than 1 hour have been found. The biodegradation of DCOI was examined in two Danish marine sediments with different textures. The mineralization into CO₂ in a clayey and sandy sediment represented 13% and 24%, respectively, of the added ¹⁴C during an aerobic incubation of 42 days at a temperature of 15°C. The mineralization under anaerobic, sulfate-reducing conditions was examined in the clayey sediment and represented 14% of the added ¹⁴C after an incubation of 56 days at a temperature of 15°C. DCOI is very toxic to aquatic organisms as the lowest effect concentrations (EC/LC50) are lower than 10 µg/L. The aquatic toxicity of the stable metabolite, N-(n-octyl) malonamic acid, is several orders of magnitude lower as the lowest effect concentrations (LC50) are estimated to be between 90 and 160 mg/L. Laboratory tests performed with seawater and sediment containing DCOI showed that degradation and sorption eliminated the acute aquatic toxicity of water samples in less than one day. On the basis of available data regarding effects on aquatic organisms, Predicted No-Effect Concentrations (PNEC) were estimated at 0.06 µg/L for DCOI and 90 µg/L for N-(n-octyl) malonamic acid. PNEC for N-(octyl) malonamic acid is considered to be representative of the other metabolites from the transformation of DCOI. In order to calculate exposure concentrations (Predicted Environmental Concentration, PEC), a model was set up on the basis of principles recommended by the EU "Technical Guidance Document" for risk assessment. The model used was not validated as regards concentrations in harbours and navigation routes. The basis of the calculation of PEC was defined by way of realistic worst-case scenarios, which means that, in practice, the calculated PEC values are seldom exceeded. The highest calculated exposure concentrations for DCOI were PEC (water), which was 0.52 µg/L in a pleasure craft harbour and 0.006 µg/L in a busy navigation route outside the harbour. As for the metabolites, PEC (water) was 2.2 µg/L in the pleasure craft harbour and 0.047 µg/L in the navigation route outside the harbour. On the basis of the values for PNEC and PEC, the risk quotients (PEC/PNEC) for DCOI were calculated at 8.7 for the pleasure craft harbour and at 0.1 for the navigation route outside the harbour. The calculated risk quotients of the total amount of metabolites from the transformation of DCOI were 0.02 in the pleasure craft harbour and 0.0005 in the navigation route outside the harbour. Because of the short half-life in water and sediment, DCOI will most likely be rapidly eliminated as soon as the pleasure craft are taken out of the water at the end of the sailing season.

By photolysis and biodegradation, zinc pyrithione is transformed very rapidly. Analyses of the degradation of zinc pyrithione in two Danish sediments showed that mineralization into CO₂ in a clayey and a sandy sediment represented 2.8 and 5%, respectively, of the added ¹⁴C under aerobic conditions. The mineralization under anaerobic, sulfate-reducing conditions represented 3.5% of the added ¹⁴C in the clayey sediment. Like DCOI, zinc pyrithione is very toxic to aquatic organisms as the lowest effect concentrations (EC/LC50) are less than 10 µg/L. The toxicity of the stable metabolites, omadine sulfonic acid and pyridine sulfonic acid, is several orders of magnitude lower as the lowest effect concentrations (LC50) of these compounds are 36 and 29 mg/L, respectively. Laboratory tests performed with seawater and sediment containing zinc pyrithione showed that degradation and sorption eliminated the acute aquatic toxicity of water samples in less than one day. The available data regarding effects on aquatic organisms form the basis of an estimation of PNEC values at 0.1 µg/L for zinc pyrithione and 30 µg/L for stable metabolites represented by pyridine sulfonic acid. By using the same realistic worst-case scenarios as for DCOI, the highest exposure concentrations (PEC, water) of zinc pyrithione were calculated to be between 0.56 and 1.7 µg/L for the pleasure craft harbour and between 0.0053 and 0.022 µg/L for the navigation route outside the harbour. For the total amount of metabolites, PEC (sediment, pore water) was between 1.6 and 2.7 µg/L in the pleasure craft harbour and between 0.037 and 0.042 µg/L in the navigation route. On the basis of the values for PNEC and PEC, the risk quotients (PEC/PNEC) for zinc pyrithione were calculated to be between 5.6 and 17 for the pleasure craft harbour and between 0.05 and 0.22 for the navigation route. The risk quotients for the total amount of metabolites from the transformation of zinc pyrithione were 0.05-0.09 for the pleasure craft harbour and 0.0012-0.0014 for the navigation route. The lowest risk quotients are based on PEC values, for which transformation of zinc pyrithione by photolysis is included in the calculations. The highest risk quotients are, however, based on PEC values in which transformation by photolysis is not taken into account. Like DCOI, zinc pyrithione will most likely be rapidly eliminated as soon as the pleasure craft are taken out of the water at the end of the sailing season, in consequence of the short half-life in water and sediment.

Effects on aquatic organisms of water samples from leaching tests with non-biocidal paints, the epoxy-based High Protect 35651 and the experimental silicone-containing 86330 paint, were tested on the marine green alga, Skeletonema costatum, and on the marine crustacean, Acartia tonsa. A similar test was performed with an organotin-based antifouling paint, Hempel's Antifouling Nautic 76800. Water samples from the leaching test with High Protect 35651 caused no inhibition of growth of S. costatum, and chronic effects on A. tonsa were observed only in undiluted leachate (No-Effect Concentration, NOEC = 100 mL/L). Water samples from the leaching test with the experimental 86330 paint showed toxicity to S. costatum and in acute and chronic tests with A. tonsa (NOEC, acute <100 mL/L; NOEC, chronic <10 mL/L). However, some factors seem to indicate that variations in production and in application may have an effect on the leaching of substances from this type of paint. These indications should be investigated further before a final assessment of the environmental properties of the paint is made. The leachates of both non-biocidal paints showed a significantly lower effect than water samples from similar tests with the organotin-based paint, Hempel's Antifouling Nautic 76800. Leachates from the paints, High Protect 35651 and the experimental 86330, caused chronic NOEC values for A. tonsa, which were at least 1,000 and 100 times higher, respectively, than the corresponding NOEC values for leachates from the organotin-based paint.