Photodegradation of strong copper-complexing ligands in organic-rich estuarine waters

Abstract

Exposure to solar radiation substantially decreased the strong copper-complexing capacity of samples collected from the organic-rich Cape Fear estuary, North Carolina. Samples exposed to natural sunlight for 1-2 d experienced a loss of strong Cu ligand (modeled at fixed K‘CuL = 1013.5) ranging from 15-33%, and >90% using long-term exposures under controlled conditions with a Xe arc solar simulator light (14 d summer sunlight). Pseudo firstorder rate constants of strong Cu ligand photodegradation averaged 0.28 d-1 for two separate Cape Fear samples exposed to simulated solar radiation, much higher than corresponding dissolved organic carbon photooxidation rate constants (<0.01 d-1). Degradation rates measured in solar simulator experiments predicted ligand concentrations in separate samples exposed to natural sunlight within 30% after rates were normalized to the ultraviolet (UV) light absorption coefficient at 300 nm. UV light is not solely responsible for photodegradation because exposure to photosynthetically active radiation also decreased strong Cu ligand levels, but with a smaller rate constant (0.06 d-1). Although photodegradation appears to be a relatively minor sink within the Cape Fear estuary because of a shallow light penetration depth (<2% of water column) and short residence times (<1 week), strong Cu ligand levels in optically clearer South Atlantic Bight (SAB) surface waters are likely to be substantially reduced by photodegradation. Ligand degradation may be especially important to Cu-sensitive ecosystems where large increases in bioavailable and potentially toxic levels of free Cu2+ ions are possible.