Adsorption of Cr(VI) on hexadecylpyridinium bromide (HDPB) modified natural zeolites

Adsorption of Cr(VI) on Pohang zeolite (PZ) and Haruna zeolite (HZ) modified with surfactant hexadecylpyridinium bromide (HDPB) was investigated by batch studies, X-ray diffraction (XRD), FT-IR spectroscopy and scanning electron microscopy (SEM) analyses. The results show that Cr(VI) adsorption on HDPB-zeolites have quick initial speed and adsorption kinetics follow pseudo-second order kinetic model within the initial adsorption stage. HDPB-zeolites have higher affinities toward chromate than that to natural zeolites, and particularly, HDPB-HZ exhibits the highest adsorption capacity in the all samples studied. Additionally, the adsorption of Cr(VI) on HDPB-PZ is almost constant in a wide pH range 3.0–11.0 of solution. In contrast, the Cr(VI)-uptake for HDPB-HZ strongly depends on the solution pH. The highest Cr(VI) adsorption on HDPB-HZ occurs in acidic solutions (pH 3.0–5.0) and the amount of sorbed Cr(VI) decreases rapidly with increasing pH. High solution ionic strength has a significant effect on chromate adsorption. The chromate adsorptions by HDPB-zeolites are not affected by coexisting chloride, nitrate, sulfate, and calcium as well as magnesium ions but are reduced drastically in the presence of bicarbonate and phosphate ions. Moreover, HDPB-zeolites can achieve high regeneration efficiency after using sodium carbonate extraction and hydrochloric acid. The analyses of XRD, FTIR and SEM reveal that HDPB cations are incorporated into the zeolite’s structure via ion-exchange and Van der Waals forces. It also suggests that anion exchange and electrostatic interaction are probably the main mechanisms that govern the Cr(VI) adsorption.