Efficient removal of Cr(VI) from aqueous solutions by polypyrrole/natural pyrite composites

Easily agglomeration of polypyrrole (PPy) significantly limits its practical applications. In this study, natural pyrite (NP) was applied to disperse PPy and overcome this agglomeration; a novel composite of PPy/NP was synthesized and applied for the efficient removal of hexavalent chromium [Cr(VI)] from aqueous solutions. Results showed that the PPy/NP composites exhibited better Cr(VI) removal performance than PPy alone. This enhanced removal capacity is attributed to the addition of NP, which prevents PPy agglomeration and reduces Cr(VI) to trivalent chromium[Cr(III)]; Cr(VI) removal was significantly dependent on the solution's pH. The sorption kinetics of Cr(VI) with PPy/NP was described by the pseudo-second-order model and Elovich models, which highlight the chemicalsorption process. The sorption isotherm was well simulated by Langmuir model and the maximum adsorption capacity of Cr(VI) on PPy/NP was 261.78–395.26 mg/g at 303 – 323 K, which is significantly higher than that of PPy alone (124.84–194.17 mg/g). Thermodynamic analyses confirmed that this adsorption process was spontaneous and endothermic. PPy/NP composites could be used as a recyclable absorbents for four Cr(VI) reduction cycles without any evident loss in adsorption performance. The mechanism Cr(VI) removal by PPy/NP composites is attributed to ion exchange (9.47 %), adsorption (31.03 %), and reduction (59.50 %).