THE BIOAVAILABILITY OF COPPER IN WASTE-WATER TO LEMNA-MINOR WITH BIOLOGICAL AND ELECTROCHEMICAL MEASURES OF COMPLEXATION

Lemna minor (duckweed) was grown in treated domestic wastewater containing added Cu to study the relationship between complexation and bioavailability. Growth was not inhibited until total Cu exceeded 0.079-0.119 mg/l or internal Cu exceeded 81 to 235 mu g/g. Cu2+ was detectable in the wastewater when total Cu measured greater than or equal to 0.378 mg/l. There was a significant (P < 0.05) relationship between Cu bioconcentration and total Cu in wastewater that was attributed, in part, to heterotrophy of duckweed. Results indicated that, in addition to the bioavailable Cu2+ species, some complexes of Cu formed with ligands in wastewater may also be bioavailable. Measurement of the Cu Complexing Capacity (CC) of the wastewater by Ion-Selective Electrode (ISE) gave values of 0.26 to 0.29 mg/l and indicated the presence of ligands with a continuum of binding strengths which eliminated two of three commonly used methods for calculating CC. The biologically-measured CuCC based on growth was 0.077-0.125 mg/l when calculated by two out of three measures of Effective Concentration (EC) and was two to three times more sensitive than the ISE method for measurement of CC. ECs based on tissue concentration of Cu rather than solution concentration were 81-471 mu g/g and have been proposed as an alternative for work in complex solutions like wastewater.