Iron-polyphenol nanomaterial removes fluoride and methylene blue dye from water and promotes plant growth

Abstract

Iron-Polyphenols are attractive as adsorbents as polyphenols are abundant in plants and tend to form high-surface-area nanoparticles with metal. However, reports on the large-scale synthesis and adsorption properties are rather limited. We are reporting a multi-gram scale synthesis of a nanostructured material from tannic acid and iron(II) salt. Tannic acid (TA) is a plant polyphenol available in abundant quantities. Analysis and spectroscopic studies of the material confirm the composition as a Fe(III) complex of TA (Fe: TA ~ 9:1) with 25% of iron is in the divalent state. The material has a relatively high surface area (99.6 m2/g). It adsorbs both anionic fluoride and cationic methylene blue (MB) from aqueous solutions. It adsorbs 66% fluoride from contaminated natural water at pH ~7 (C0 4.4 mg/L, dose 3 g/L). It removes 99% methylene blue (MB) dye from an aqueous solution at pH 7 (C0 150 mg/L, dose 4 g/L). The equilibrium study and salt addition experiments suggest that the fluoride removal depends on surface charge and binding with iron. MB can be desorbed using salts of cations with a higher charge. Several reports showed that materials from plant sources could be harmful to plant growth, thereby limiting the use of the adsorbent. We found that tannic acid alone stunts the growth of Mung bean (Vigna radiata) plants, but plants did show significant growth when grown with the material reported here (shoot, 155%, root 200%, twice daily 1 mL of 1 mg/mL).