Geochemical factors affecting the solubility of copper in seawater

Abstract

Environmental contextMany trace metals, including copper, are only sparingly soluble in seawater and may exist in both dissolved and particulate forms (e.g. as precipitates). Aquatic organisms may experience different toxic effects from exposure to dissolved and particulate trace metals. This study investigates how concentration, reaction time and changes to precipitate composition/mineral formation affect copper solubility in seawater, thus influencing metal bioavailability and toxicity in the field and laboratory. AbstractA lack of knowledge on the solubility of metals such as copper affects the ability to predict the forms (dissolved and particulate) that organisms are exposed to in field and laboratory waters. Laboratory tests were conducted where copper (total concentrations of 0.5 to 20mg L−1) was added to natural and artificial seawater (pH 8.15, 22°C), equilibrated for 28 days and dissolved copper monitored at periodic intervals. At 0.5mg L−1, dissolved copper concentrations remained stable over 28 days and no precipitates were detected. However, at higher total copper concentrations, an initial rapid precipitation phase was followed by the establishment of a metastable equilibrium that persisted for periods of days to weeks, and whose solubility concentrations and duration were influenced by the total copper concentration and typically in the range 0.6 to 0.9mg L−1. After 5 to 15 days, a step change decrease in dissolved copper concentration followed by a slow decline was observed in the >2mg L−1 total copper treatments. The minimum solubility measured after 28 days was 0.053mg L−1. Elemental and X-ray diffraction analyses indicated that the copper precipitates comprised similar proportions of amorphous copper hydroxycarbonate and amorphous dicopper trihydroxide chloride after 1 day and transformed to predominantly mineralised dicopper trihydroxide chloride in the clinoatacamite polymorph form after 28 days. These observations have particular relevance for toxicity tests of less sensitive organisms and highlight the need to consider metal solubility, exposure to precipitates and changes in precipitate mineral phases.