Flocculation Behavior and Mechanism of Silver Colloid from Simulated Radioactive Liquid at Nuclear Power Plant

^110mAg is one of the neutron activation products in silver-containing structural materials within the nuclear power plant (NPP) reactor. ^110mAg can exist in either ionic or colloidal forms within the NPP liquid radioactive waste. Conventionally, nuclear-grade filters or ion-exchange resins cannot effectively remove the colloidal ^110mAg. The elimination of ^110mAg colloids from NPP liquid radioactive waste represents a critical challenge for radiation safety management and environmental protection. The stable silver colloids under simulated NPP wastewater conditions were synthesized by chemical reaction method. The regulatory effects of the addition amounts of boric acid H₃BO₃, citric acid C₆H₈O₇, and hydrazine hydrate N₂H₄·H₂O on the silver colloids characteristics were systematically examined. The ultraviolet-visible spectrophotometry (UV-vis), zeta potential measurement, nanoparticle size distribution and transmission electron microscope (TEM) were combined to characterize the sliver colloidal morphology, particle size distribution, and stability. The sliver colloid solutions prepared under optimal synthesis conditions produce the monodisperse Ag colloids with the average particle size of (30.2±2.1) nm and the zeta potential of (−38.9±1.5) mV. The results of flocculation experiments indicate that polymeric aluminum ferric chloride (PAFC) is more suitable for removing silver colloids from solution. Under neutral-alkaline conditions, the total Ag (including the ionic Ag⁺ and colloidal Ag) and colloidal Ag removal rates achieve 96.3% and 98.2%, respectively. Notably, the boric acid inhibits the elimination of Ag colloids by flocculation process. The removal rate of silver colloids reduces to 69.8% when the aqueous boric concentrations achieve to 1 000 mg/L. It can be attributed to the enhanced colloid stabilization caused by boric acid. The coexisting transition metal ions, e.g., Fe³⁺, Co²⁺, Mn²⁺, improve the colloidal removal via hydroxide co-precipitation. When the dosage of PAFC is 5 mg/L, the total Ag removal rate in the solution containing transition metal ions is as high as 98.4%, which is significantly higher than the conditions without transition metal ions. The microscopic morphology of the flocs was characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). The results indicate that the flocs are sheet-like stacked with a dense structure. The elements, including Ag, Al, Fe and Cl are uniformly distributed on the surface of the flocs, with Al (63%) and Ag (24%) being the main components. The work further investigates the floc characters with XPS analysis, which verifies the preservation of metallic Ag⁰ in flocs (with the binding energy 368.1 eV belong to Ag 3d_5/2 and the Auger parameter value of 726.5 eV). In summary, PAFC exhibits excellent performance in removing Ag colloids from solutions. Mechanistically, the removal of Ag colloids is mainly achieved through electrostatic adsorption, sweep flocculation, and bridging effects. This work establishes fundamental guidelines for optimizing Ag colloid removal in complex radioactive wastewater matrices, and highlights the importance of solution chemistry control in practical NPP wastewater treatment applications, and provides the reference for the subsequent process of liquid radioactive waste.