Insight into the synergistic effect on adsorption for Cr(vi) by a polypyrrole-based composite

Polypyrrole-based (PPy) composite are promising candidates for the treatment of water pollution. Adsorption selectivity as well as a large adsorption capacity are two key factors for treating wastewater containing multiple ions. The structure and morphology of the prepared composites were characterized by the FT-IR, XRD and SEM examinations. The results indicate that the Fe₃O₄ and PPy nanosphere coats attapulgite (ATP) closely and evenly. Herein, a novel Fe₃O₄ and ATP doped three-dimensional network structure PPy/Fe₃O₄/ATP composite was demonstrated as an excellent adsorbent to effectively remove Cr(vi). The as-synthesized PPy/Fe₃O₄/ATP composite is suitable for Cr(vi) adsorption in a wide pH range (pH 2-6). Up to a 96.44% removal rate was found with 400 mg L^-1 Cr(vi) aqueous solution in 30 min for 0.2 g PPy/Fe₃O₄/ATP adsorbent. Adsorption results showed that Cr(vi) removal efficiency by PPy/Fe₃O₄/ATP decreased with an increase in pH. The removal rate of Cr(vi) had already reached 93.63% in 15 min contact time. Co-existing ions studies exhibit inorganic oxyacid anion and transition metal cation showed negative effects on Cr(vi) removal rate. A chemical rather than a physical adsorption occurred for these adsorbents as revealed by a pseudo-second-order kinetic study. The results of the adsorption isotherms showed that the adsorption process was similar to the Langmuir isotherm adsorption. Furthermore, the PPy/Fe₃O₄/ATP composite exhibited a high stability for Cr(vi) adsorption during recycling tests process. This work may provide some useful guidelines for designing adsorbents with selectivity toward specific heavy metal ions.