The effect of wastewater effluent derived ligands on copper and zinc complexation

Abstract

The shift toward bioavailability-based standards for metals such as copper and zinc improves the ecological relevance of the standard but also introduces significant complexity into assessing compliance. This study examined differences in the copper and zinc complexation characteristics of effluents from a range of different sewage treatment works and in relation to so-called ‘natural’ samples. This information is essential to determine whether the inclusion of effluent-specific complexation characteristics within the regulatory framework could enhance the environmental relevance of compliance criteria. The data show that for copper, binding affinity was not greater than that measured for material derived from the receiving water environment with mean log K of between 4.4 and 5.15 and mean complexation capacity ranging from 38 to 120 µg/mg DOC for effluents compared with a log K of 5.6 and complexation capacity of 37 µg/mg DOC for Suwannee River fulvic acid. For zinc however, effluents exhibited much higher complexation capacity with effluent means ranging from 3 to 23 µg/mg DOC compared with Suwannee River fulvic acid, for which complexation capacity could not be determined. Synthetic ligands in sewage effluent such as ethylenediamine tetraacetic acid are implicated as contributing to these observed differences. This suggests that current biotic ligand models for zinc might overstate the risk of harm in effluent impacted waters. The data also show that the copper and zinc complexation characteristics of effluent samples obtained from the same treatment works were less different from one another than to effluents from other treatment works and therefore that sewage source has an important influence on complexation characteristics. The findings from this study support the case that the contribution to complexation from effluent derived ligands could enhance the environmental relevance of bioavailability-based compliance criteria in particular for zinc owing to the additional complexation capacity afforded by effluent-derived ligands.