Polypyrrole modified bentonite nanocomposite and its application in high-efficiency removal of Cr(VI)

The disposal of Cr(VI) contamination in water urgently required an optimal material, nanocomposite of chloride doped polypyrrole/bentonite (Cl-PPy/BT) was synthesized successfully through in situ polymerization to achieve synergistic reduction and adsorptive removal of Cr(VI). The adsorbent property was characterized methodically. The Cr(VI) adsorption results indicated that introduction of Cl-PPy significantly favored the performance of bentonite, with maximum adsorption capacities of 314.5-454.9 mg g(-1) at 25-55 degrees C. These capacities are much higher than those of many other organoclays (16.4-185.2 mg g(-1)). Additionally, the removal efficiency of Cr(VI) by Cl-PPy/BT was respectively 3.4-fold and 5.4-fold higher than those of Cl-PPy and bentonite at Cr(VI) concentration of 100 mg L-1, indicating the strong synergistic effect between bentonite and Cl-PPy. This is because Cl-PPy and BT could well resist the aggregation of each other, and therefore resulted in a loose-packed structure and good exposure of active sites. The adsorption kinetics were well described by the pseudo-second-order kinetic model, suggesting the existence of chemical adsorption process; while the thermodynamic fitting results demonstrated the existence of physical adsorption process. Furthermore, the intraparticle diffusion model indicated that the adsorption was controlled by both film and intraparticle diffusion. The combination of fitting results and instrumental analysis results suggested that electrostatic interactions between Cr(VI) and -NH+- of polypyrrole, ion exchange between Cr(VI) and Cl- as well as reduction of Cr(VI) to Cr(III) by -NH- contributed to the Cr(VI) removal. The results maybe give some new insight about the removal of Cr(VI) with negative charge by polypyrrole/bentonite nanocomposites in water.