Crystalline/Amorphous Blend Identification from Cobalt Adsorption by Layered Double Hydroxides.

Area of Science:

• Environmental Science
• Materials Science
• Chemistry

Background:

• Layered double hydroxides (LDHs) are effective adsorbents for heavy metals.
• Cobalt (Co(II)) removal from aqueous solutions is crucial for environmental remediation.
• The adsorption mechanism of Co(II) on CaAl-Cl-LDH typically involves co-precipitation, yielding crystalline products.

Purpose of the Study:

• To investigate the adsorption behavior and mechanisms of Co(II) ions onto CaAl-Cl-LDH at pH 6.
• To elucidate the nature of the adsorbed products, particularly the formation of crystalline/amorphous blends.
• To clarify the interaction between cobalt and CaAl-Cl-LDH using advanced analytical techniques.

Main Methods:

• Adsorption experiments with controlled pH (6) and Co(II) concentration (8 mM).
• X-ray Diffraction (XRD) for phase analysis.
• Fourier-transform infrared spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), High-resolution Transmission Electron Microscopy (HR-TEM), X-ray Photoelectron Spectroscopy (XPS), and Inductively Coupled Plasma (ICP) for detailed characterization.

Main Results:

• An unexpected crystalline/amorphous blend was observed in the XRD patterns of Co-adsorbed LDH.
• Crystalline phases were identified as Co₆Al₂CO₃(OH)₁₆·4H₂O, consistent with co-precipitation.
• Amorphous phases were attributed to surface complexation on hydrolyzed amorphous Al(OH)₃.

Conclusions:

• The adsorption of Co(II) by CaAl-Cl-LDH involves two distinct mechanisms, leading to a mixture of crystalline and amorphous products.
• Co-precipitation is responsible for the formation of crystalline Co-LDH.
• Surface complexation on amorphous aluminum hydroxide contributes to the amorphous phase formation.