Immobilised tannin: Efficient trap for nickel and lead ions in aqueous solution

Abstract Quantity of water is a major concern, but the quality is of more urgent concern due to heavy metal pollution. In this study, the focus was to develop adsorbents (tannin resin (TR) and iron-doped tannin resin (Fe-TR)) from invasive Acacia mearnsii for Pb2+ and Ni2+ in solution. The developed TR and Fe-TR were prepared directly by crosslinking and iron-grafting; respectively with thermal stable at ≤ 350 °C. Surface functionality analysis revealed the presence of chelating groups at ≈ 1688 and 3661 cm−1. The crystallite sizes of the adsorbents were smaller compared to tannin; however, the % carbon and nitrogen were significantly higher. Also, the Brunauer–Emmet–Teller study revealed that iron-grafting of tannin after crosslinking increases the surface area, pore volume and pore size. Furthermore, the surface morphology of adsorbents indicated a well-defined structure with the formation of a whitish deposit and rough lump-like after ion uptake. Both adsorbents showed a type II isotherm with adsorption capacities of 40.650 mg/g and 13.763 mg/g for Pb2+and 120.63 mg/g and 110.74 for Ni2+ ion by TR and Fe-TR; respectively. The absorbate-adsorbent interactions were observed to be endothermic with negative and positive Gibb's free energy values for the adsorption of Pb2+ and Ni2+; respectively. The adsorption kinetics of Ni2+ is governed by several models; however, Pb2+ interaction is regulated by Pseudo-second-order model. Furthermore, the data generated offered the possibility of both chemisorption and physisorption processes; highlighting the flexibility and multifunctionality of tannin traps for the erasure of ion impurities in wastewater.