Biosorptive removal of heavy metals from water is an eco-friendly green technology. In this study, raw pomegranate (Punicagranatum L.) peel (RPP), without any modification or activation, was used as biosorbent for the removal of chromium (Cr(VI)). The influence of operating parameters such as pH, contact time, temperature and concentration of Cr(VI) were studied in batch mode. A maximum removal of 100% was achieved for a Cr(VI) solution of concentration 20 mg/L at an optimum pH and temperature of 2 and 313 K respectively in 3 min. Among the isotherms tested, Langmuir adsorption isotherm has good correlation with experimental data. A maximum Cr(VI) biosorption capacity of 370.4 mg/g was observed at 313K under equilibrium conditions. RPP is also effective in the removal of Cr(VI) at higher concentrations. Among the kinetic models, pseudo-second order kinetic model fits the data well. The thermodynamic study reveals that the endothermic biosorption taking place on RPP is physico-chemical in nature. The biosorption mechanism of RPP with Cr(VI) indicates that intra-particle diffusion is not the only rate limiting step and film diffusion also plays a major role in biosorption.

During biosorption, the biosorbent releases many organic compounds to the medium. In the present study, extracts of pomegranate peel (PP), green coconut shell (GCS), and karuvelam wood (KW) were prepared at three different conditions, namely 12 h, 3 h, and 3 h at pH 2, for the removal of hexavalent chromium [Cr(VI)]. The amount of organic compounds, mainly the leached organic compounds in the extract before and after treatment with Cr(VI) solution, was determined by chemical oxygen demand analysis. The total phenolic content, antioxidant activity, and total flavonoid content were used to estimate the chromium reduction potential of the extracts. The PP extract is the richest in all the three factors, followed by GCS and KW. The disappearance rate of Cr(VI) in the presence of PP extract reached 99.63% for the 50 mg/l concentration within 3 min, while it was 12% and 10% for GCS and KW, respectively, for the same concentration and time. Reaction mechanisms were formulated with the help of Fourier transform infrared spectroscopy to confirm the role of leached compounds from natural materials for the removal of heavy metal.