Effect of Wetting and Drying Cycles on the Stability of Cadmium and Zinc Immobilized with Ferrihydrite

 The repetitive wetting and drying cycles, driven by intense rainfall and drought, intricately alter subsurface conditions, affecting redox states and pH levels. Consequently, these alterations may prompt the dissolution or transformation of ferrihydrite (Fh), an amorphous iron oxide used as a stabilizing agent in soil, thereby influencing the leaching behavior of heavy metals. This study investigates the impact of wetting and drying cycles on Fh crystallinity, and the leaching behavior of immobilized heavy metals, specifically focusing on cadmium (Cd) and zinc (Zn) at Fe/heavy metals molar ratios of 1 and 10. Fh synthesis was induced by neutralizing pH to 7.0 with 1 M NaOH, simultaneously fostering the coprecipitation of Fh-heavy metals. The experimental design entailed 12 cycles, each involving 8 hours of wetting at room temperature, followed by 16 hours of drying at 40 °C. The Synthesis Precipitation Leaching Procedure (SPLP) was then used to confirm the variations in leaching quantities observed throughout the cycles, following the guidelines of EPA METHOD 1312. Analyses using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) revealed distinct variations in morphology and crystallinity based on the type and ratio of heavy metals. Pure ferrihydrite remained stable after 12 cycles; however, heightened crystallinity emerged after 3 cycles in high heavy metal concentrations, forming otavite (CdCO₃) in Cd samples and petalline-shaped ferrihydrite in Zn samples. In contrast, low heavy metal concentrations displayed no such changes, indicating variable effects of wetting-drying cycles depending on the iron-to-heavy-metals ratio. The supernatant concentration decreased by 11-37% during wetting in all samples, yet SPLP leaching tests exhibited consistent heavy metal concentrations between wetting and drying, suggesting a minimal impact on heavy metal stability. These findings underscore the environmental factors influencing the stability of iron oxide-based immobilized heavy metals and raise the need for long-term stability assessments to consider variables that can cause more complex changes in the field.