Application of enteromorpha polysaccharides as coagulant aid in the simultaneous removal of CuO nanoparticles and Cu2+: Effect of humic acid concentration.

Humic acid (HA) influences the aggregation and stability of nanoparticles (NPs), which determine the removal performance of NPs in coagulation. Consequently, in this study, the impact of HA concentration on the simultaneous removal of CuO NPs, Cu2+, and natural organic matter (NOM) was investigated. Enteromorpha polysaccharides (Ep), as the novel recycling coagulant aid, were integrated with polyaluminum chloride (PAC) to treat composite contaminants in this coagulation process. Removal performance, floc properties, zeta potential, scanning electron microscope (SEM) images, and Fourier Transform Infrared Spectra (FT-IR) were measured and analyzed. Results showed that PAC with Ep (PAC-Ep) was more beneficial for improving removal performances than PAC alone. Further, the coagulation performance became better with the increase in HA concentration. When the HA concentration was 10 mg/L (used PAC-Ep), the removal efficiencies of CuO NPs and Cu2+ were both more than 80%; and in particular, the highest removal efficiency of turbidity was 98%. However, excessive HA (more than 10 mg/L) reduced the removal efficiency of Cu by more than 31%. Smaller and denser flocs with better recoverability were formed as the HA concentration increased. Furthermore, because of affluent functional groups, HA was easily adsorbed on the surface of NPs and combined with dissociative Cu2+, thereby forming a composite contaminant. During the coagulation process, the colloidal system was destabilized preferentially by the charge neutralization between composite contaminants and PAC. Furthermore, a chelated reticular structure was formed by the conjunction of carboxyl and hydroxyl groups (from Ep) and Al (III) species (from PAC). Flocs were further enlarged and precipitated by bridging and sweeping of this structure.