Effects of copper ions on removal of nutrients from swine wastewater and on release of dissolved organic matter in duckweed systems.

High concentration of Cu2+ in swine wastewater raises concerns about its potential adverse effects on nutrient removal by aquatic plants like duckweed. In this work, the effects of copper ions on nutrient removal and release of dissolved organic matter (DOM) were investigated in duckweed systems. Results showed that the removal performance of ammonia nitrogen (NH3N) and total phosphorus (TP) increased at 0.1-1.0 mg/L of Cu2+, while dropped at 2.0-5.0 mg/L of Cu2+. A novel kinetic model in which Cu2+ was taken into account was then developed which was used to optimize Cu2+ concentration at 0.96 mg/L for nutrient removal in duckweed systems. NADH, detected in DOM by the parallel factor (PARAFAC) analysis, exhibited high capacities of binding copper ions, so it played an important role on the decrease of Cu2+ concentrations in duckweed systems. The principle component analysis (PCA) showed that the dominant DOM were lower molecular weight compounds at 1.0 mg/L of Cu2+ and higher molecular weight compounds at 2.0-5.0 mg/L of Cu2+. The bonds of CH (humic-like), NO (NO3-) and ArH (tyrosine) in DOM were responsible for not only the fastest binding with Cu2+ from the result of the two-dimensional Fourier transform infrared correlation spectroscopy (2D-FTIR-CoS) but also the variations of DOM conformations at a critical concentration of 0.5 mg/L Cu2+ from the perturbation correlation moving window two-dimensional (PCMW2D) analysis. These findings lead to a better understanding on the environmental behaviors and mechanisms of Cu2+ in duckweed systems.