Two-stage synergistic flocculation based on chitosan/sodium hyaluronate polyelectrolyte complexes for rapid underwater visual clarification in turbid environments

CTS-SH complexes were developed for rapid flocculation in turbid water. • The complexes significantly enhanced floc growth and structural integrity. • The synergistic system reduced settling time by 68.01% compared with CTS used alone. • Floc size increased by ∼103-fold in CTS/SH synergistic system than in CTS. • Clear defect contours were identified within 2.5–7.5 min in real water samples. Underwater defect detection is markedly hampered by image degradation from light scattering in turbid water, obscuring critical defect features. Herein, the CTS-SH polyelectrolyte complexes were constructed in a two-stage synergistic flocculation strategy for rapid clarification under turbid conditions. CTS was first added to induce pre-flocculation and form initially small flocs. Then, the addition of SH and CTS constructed complexes for further floc growth. Flocculation performance was evaluated in simulated turbid water, while zeta potential, SEM, FTIR, and XPS were employed to elucidate the structural characteristics and underlying mechanisms. Raw water sampled from the SX Dam was used to verify practical applicability. Under simulated turbid-water conditions, the synergistic mode required only 2.45 ± 0.14 min of settling after stirring to reach the target turbidity for clear defect detection, reducing the required time by 64.80% compared with CTS-only system. The CTS-SH complexes formed a three-dimensional leaf network, which offered numerous attachment sites and enhanced sweep capture, increasing floc projected area by approximately 103 times. In raw water tests, defect contours and clear defects could be identified within 2.5 and 7.5 min, respectively. These findings demonstrate a green and effective strategy for rapid clarification and underwater visual detection in turbid environments.