Studies on Removal of Heavy Metals from Aqueous Solution Using Melanin Coated Matrix

Abstract

The biopolymer/biopigment Melanin is known for its free radical scavenging property. Melanin obtained from the marine bacteria Pseudomonas stutzeri is used for the removal of Hg (II), As (III), As (V), Pb (II), Cr (VI) and Cu (II) from aqueous medium. The melanin produced by the bacterium is confirmed to be nanoparticle (32 ± 0.98 nm) using TEM and particle size analysis. Different characterisations such as SEM, TGA, DSC, FTIR, BET surface analysis, zeta potential analysis, XRD were conducted to understand the physio-chemical properties of melanin. The kinetic, thermodynamic and equilibrium studies were conducted to achieve the removal of 85%, 87% 92% and 95% Hg (II), Cr (VI), Pb (II) and Cu (II) respectively from water having 10 mg/L adsorbate concentration and 0.2 g/L of adsorbent loading. Trivalent and pentavalent arsenic showed ineffective binding to melanin which was resolved by functionalising melanin using iron and copper. On functionalisation, melanin could remove more than 99 % of As (III) and As (V) from aqueous medium having arsenic concentration of 10 mg/L and loading of 0.8g/L and 2g/L of Fe-melanin and Cu-melanin respectively. For effective heavy metal adsorption and effortless removal of adsorbent from the aqueous medium, melanin nanoparticles were coated on to different matrices and batch removal studies were performed. Adsorption studies using melanin immobilised N, N-diethylacrylamide hydrogel, melanin coated PVDF membrane, melanin impregnated activated carbon were conducted and found that efficient removal of heavy metals was achieved by melanin impregnated activated carbon. Continuous adsorption studies using melanin impregnated activated carbon as a fixed bed column was performed by varying the parameters such as influent flow rate, heavy metal concentration and adsorbent loading in the column. The flow rate of 0.5 mL/min, the heavy metal concentration of 1 mg/L and adsorbent loading of 100 mg were the optimised parameters for efficient heavy metal removal. Thomas model fitted well with the experimental data compared to the Adam-Bohart’s model. Efficient desorption of Hg (II), Pb (II) and Cu (II) were obtained using 3N HCl and Cr (VI) using 1N citric acid. Melanin was re-functionalised after treatment with 5N HCl, and effective reuse of melanin for removal of all heavy metals was achieved until four cycles of study.