Melanin-embedded materials effectively remove hexavalent chromium (Cr^VI) from aqueous solution

Currently, it is recognized that water polluted with toxic heavy metal ions may cause serious effects on human health. Therefore, the development of new materials for effective removal of heavy metal ions from water is still a widely important area. Melanin is being considered as a potential material for removal of heavy metal from water.

In this study, we synthesized two melanin-embedded beads from two different melanin powder sources and named IMB (Isolated Melanin Bead originated from squid ink sac) and CMB (Commercial Melanin Bead originated from sesame seeds). These beads were of globular shape and 2–3 mm in diameter. We investigated and compared the sorption abilities of these two bead materials toward hexavalent-chromium (Cr^VI) in water. The isotherm sorption curves were established using Langmuir and Freundlich models in the optimized conditions of pH, sorption time, solid/liquid ratio, and initial concentration of Cr^VI. The FITR analysis was also carried out to show the differences in surface properties of these two beads.

The optimized conditions for isotherm sorption of Cr^VI on IMB/CMB were set at pH values of 2/2, sorption times of 90/300 min, and solid-liquid ratios of 10/20 mg/mL. The maximum sorption capacities calculated based on the Langmuir model were 19.60 and 6.24 for IMB and CMB, respectively. However, the adsorption kinetic of Cr^VI on the beads fitted the Freundlich model with R² values of 0.992 for IMB and 0.989 for CMB. The deduced Freundlich constant, 1/n, in the range of 0.2–0.8 indicated that these beads are good adsorption materials. In addition, structure analysis data revealed great differences in physical and chemical properties between IMB and CMB. Interestingly, FTIR analysis results showed strong signals of –OH (3295.35 cm^− 1) and –C=O (1608.63 cm^− 1) groups harboring on the IMB but not CMB. Moreover, loading of Cr^VI on the IMB caused a shift of broad peaks from 3295.35 cm^− 1 and 1608.63 cm^− 1 to 3354.21 cm^− 1 and 1597.06 cm^− 1, respectively, due to –OH and –C=O stretching.

Taken together, our study suggests that IMB has great potential as a bead material for the elimination of Cr^VI from aqueous solutions and may be highly useful for water treatment applications.