The effect of dissolved organic matter on copper bioavailability in aquatic ecosystems: A review

Abstract

Copper is a very common substance occurred in the natural environment and was the first metal used by human since very early days, the Bronze and Iron Ages. In the first several thousand years, copper production only contributed little to global and even local pollution because its usage was too little to affect human health or ecosystems. However, things have rapidly changed during the industrial revolution. Copper has begun playing a critical role in the technologies developed and being applied in many aspects, such as electrical industries and agricultures. Hence, copper quantities in the environment have hugely increased and are still rising. Many studies have shown that naturally occurred organic matter in different aquatic environments controls the speciation, bioavailability, and, thus, its toxicology and distribution of soluble copper in aquatic ecosystems. And the binding potential of dissolved organic matter (DOM) is largely depending on its components, functional groups, and origins. In this article, we summarized sources, characteristics, and reactivity of aquatic dissolved organic matter (DOM) and its complexation abilities of soluble copper. Aquatic DOM can be classified into exogenous DOM and endogenous DOM. Exogenous DOM is mainly from overland runoff and terrestrial origins leaching from soils and plants, whereas endogenous DOM is from extracellular release, cell decomposition of phytoplankton and bacteria. DOM composition varies a lot among different sources and thus changes the binding potential of copper. The effects of DOM on the bioavailability of copper are mainly manifested in two aspects: (1) DOM can reduce or increase the biological toxicity of Cu2+; (2) DOM deprives the entry of metals into the food chain by reducing the absorption of copper at the bottom level of the food chain. Aquatic DOM can reduce the biotoxicity of Cu by forming complexation so as to serve as buffer solution in natural environments in most instances. When the environmental conditions change, such as pH or salt, it would favor the formation of free Cu2+, the most toxic form of copper. In an active ecosystem, copper complexes to some lipophilic DOM, such as diethyldithiocarbamate (DDC) and 8-hydroxyquinoline (O x ), may induce cellular toxicity through membrane-facilitated transport. In the marine ecosystem, the class 1 ligands (L1) and the class 2 ligands (L2) balance the copper biotoxicity and bioavailability between surface and deep seawater. Although DOM-Cu complexation may increase the bioavailability of copper through DOM uptake process, Cu-DOM complexes are less bioavailable to organisms at lower trophic level. Thus, DOM can deprive Cu entering the food chain and being bioaccumulated in the higher trophic levels. The effects of aquatic DOM on the bioavailability of copper are mediated by the complexation or chelation potentials, as well as the characteristics of DOM itself and the size of DOM molecules. In the aquatic ecosystem, the extent of Cu binding to DOM as affected by the binding sites depends on its strength. Cu speciation is mostly controlled by DOM with weaker binding sites; whereas the DOM which has stronger binding sites bind Cu at lower Cu concentration environment, such as nitrogen DOM. The chelation of DOM to copper is strongly affected by pH which can both influence the number of the binding sites of DOM and the valence state of copper. In generally, small DOM can increase the uptake of Cu while large DOM can reduce or limit the uptake and adsorption of Cu. With the development of research techniques, research now can more accurately evaluate the effect of DOM on metals from the perspective of molecular structure and its functional groups.