Evaluation of a novel kaolinite loaded sulfidized nZVI aggregates for removing Cr(VI) and Cd(II) in water: New sulfurization strategy, morphological control, performance, and synergistic mechanisms

Nano-zero-valent iron (nZVI) is commonly used in the treatment of heavy metal pollution in water, but its application is limited due to a variety of defects. In this study, a new method was used to successfully synthesize the vulcanized nano-zero-valent iron (S-nZVI@K) supported by kaolin, which has excellent properties compared with nZVI related materials. In the process of vulcanization, both the aggregates and the vulcanized states of zero-valent iron were obtained. The formation of zero-valent iron aggregates and the morphology of aggregates were found in the process of optimization synthesis. Characterization of the morphology and structure revealed that S-nZVI was uniformly distributed on the surface of kaolin, with particle shape being adjustable by varying hydrothermal conditions. Batch experiments demonstrated excellent adsorption performance of S-nZVI@K towards heavy metal ions. Kinetic analysis indicated that the removal process followed a pseudo-second-order model and Langmuir adsorption isotherm model, with maximum adsorption capacities reaching 88.88 mg/g for Cr(Ⅵ) and 174.69 mg/g for Cd(II), respectively. Furthermore, thermodynamic analysis confirmed that the removal of the heavy metals proceeded through spontaneous endothermic chemisorption. The removal mechanism of S-nZVI@K for Cr(VI) primarily involved adsorption, reduction, and co-precipitation processes, while complexation adsorption and precipitation played key roles in removing Cd(II).