Effects of the pH and anions on the adsorption of tetracycline on iron-montmorillonite

Highlight
• Iron-intercalated montmorillonite (Fe-Mt) forms the peculiar delaminated structure.
• High-valent anions can significantly affect the tetracycline adsorption onto Fe-Mt.
• Multiple mechanisms coexist responsible for the tetracycline adsorption by Fe-Mt.

Tetracycline frequently needs to be detected in the environment; therefore, iron-intercalated montmorillonite (Fe-Mt) was evaluated as a model adsorbent to evaluate tetracycline adsorption behavior. The Fe-Mt sample derived from Na+-montmorillonite (Mt) by the introduction of iron-(hydr)oxides was fully characterized using the techniques of XRD, FTIR, TEM, and SEM as well as N2-adsorption/desorption methods. The delaminated structure of Fe-Mt could cause remarkable increases in both the surface area and pore volume. Fe-Mt also showed remarkably distinct behavior from Mt for TC adsorption. The TC adsorption by Fe-Mt was found to be highly influenced by the pH as well as the types and concentrations of background electrolytes. The positive effect of anions on the TC adsorption by Fe-Mt at acidic pH values follows the following adsorption maximum order: PO43 − > SO42 − » NO3− > Cl− > ClO4−. The solution pH affects the TC adsorption, and the adsorption maximum occurs at a pH around 5.0. The Langmuir and Freundlich equations were shown to fit well to the TC adsorption isotherms on Fe-Mt, and the former equation was a better fit. These adsorption-related performances could be ascribed to Fe-Mt having formed characteristic bulk/surface/pore structures. The multiple potential coexisting efficient mechanisms or pathways responsible for the TC adsorption by Fe-Mt were finally proposed and discussed in comparison with Mt and ferrihydrite.

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