Effective adsorption and removal of Cr(VI) from wastewater using magnetic composites prepared by synergistic effect of polypyrrole and covalent organic frameworks

Cr(VI) as a toxic heavy metal can endanger human physical and mental health. The adsorption method has been widely used for Cr(VI) removal due to its advantages of cost-effectiveness and simplicity of operation. However, the key to Cr(VI) removal by adsorption lies in the construction of adsorption active sites. In this study, Fe3O4@COF-MT@PPy magnetic composites were prepared by the Schiff base method and in situ polymerisation to construct adsorbents with high specific surface area and abundant amino active sites for the removal of Cr(VI) from wastewater. The TEM and BET results showed that PPy was successfully encapsulated on the Fe3O4@COF-MT material, and the COF layer provided sites for the polymerisation of PPy, which resulted in a large specific surface area (400.93 m2/g) for Fe3O4@COF-MT@PPy, thus providing more adsorption sites. The VSM results showed that the prepared composites have good magnetic properties (13.8 emu/g) and are easy to separate from aqueous solutions. Encouragingly, the Fe3O4@COF-MT@PPy composites showed excellent adsorption performance for Cr(VI) in aqueous solution. The maximum adsorption capacity of the material for Cr(VI) was 429.18 mg/g at 298 K, which was much higher than Fe3O4@COF-MT with Fe3O4@PPy. Adsorption kinetics and isotherm analyses showed that the adsorption process was more in line with the pseudo-second-order and Langmuir models. The Fe3O4@COF-MT@PPy composites were able to maintain high adsorption capacity of Cr(VI) after five regenerations, which showed good reusability. In addition, interfering ion experiments demonstrated the selective adsorption of Fe3O4@COF-MT@PPy on Cr(VI). Using XPS for the analysis, it can be speculated that the adsorption mechanism is due to the abundant amino groups on the surface of the adsorbent firstly exerting electrostatic attraction and ion-exchange effect on the anion Cr(VI), and then the protonated amino groups reduce part of the Cr(VI) to Cr(III), and finally adsorb the Cr(III) on the surface of the material by chelation. Therefore, the recyclable Fe3O4@COF-MT@PPy has a good potential for adsorption of Cr-containing wastewater.