Indirect estimation of degradation time for zinc pyrithione and copper pyrithione in seawater

The degradability of two antifouling biocides: zinc pyrithione (ZPT) and copper pyrithione (CPT) in seawater was examined. Reduction in toxicity due to degradation was monitored over two days using a bioassay with natural assemblages of coastal marine bacteria from Roskilde Fjord, Denmark. To investigate photo-degradation of the compounds, bacteria were exposed to sterile ZPT- and CPT-dilution that had either been exposed to sunlight or darkness. Bio-degradation was examined by diluting ZPT and CPT in sterile seawater or natural seawater. Photo-degradation half-life for ZPT was estimated to be 8.3 ± 0.9 min and for CPT to 7.1 ± 0.2 min. Total and microbial degradation in combination with photo-degradation did not further shorten the degradation time, suggesting no bio-degradation. Bio-degradation without the influence of sunlight was also negligible over the time-period investigated. ZPT and CPT are therefore suggested to persist in the marine environment where the influence of the light is limited.