Separation and Sequential Recovery of Tetracycline and Cu(II) from Water Using Reusable Thermoresponsive Chitosan-Based Flocculant.

Coexistence of antibiotics and heavy metals is typically detected in water containing both organic and inorganic contaminants. In this work, a flocculation method using a reusable thermoresponsive chitosan-based flocculant (CS-g-PNNPAM) was applied for separation and sequential recovery of tetracycline (TC) and Cu(II) from water. High synergistic removal rates of both TC and Cu(II) from water (>90%) were reached. Interactive effects among targeted water temperature (T1), stock solution temperature (T2), and flocculant dosage on flocculation performance were assessed using response surface methodology. To optimize flocculation, operation strategies of adjusting T2 and dosage according to T1 based on the interactive effects were given through mathematical analyses. The flocculation mechanism as well as interfacial interactions among CS-g-PNNPAM, TC, and Cu(II) were studied through experimental investigations (floc size monitoring, X-ray photoelectron spectroscopy, and UV spectra) and theoretical calculations (density functional theory and molecular dynamics simulations). Coordination of Cu(II) with TC and the flocculant promoted flocculation; switchable interactions (H bonds and hydrophobic association) of the TC-flocculant at different temperatures were key factors affecting operation strategies. When these interactions were weakened step by step, TC and Cu(II) were sequentially recovered from flocs using certain solutions. Meanwhile, the flocculant in flocs was regenerated and found reusable with high flocculation efficiency.