Recovery of trace metal isotopes in seawater samples using multifunctional neem (azadirachta indica) biosorbent: A comparison with monofunctional NOBIASChelatePA1 resin

Neem (Azadirachta indica) tree leaves (NL) represents a potential alternative to costly conventional materials for recovery of trace metals from seawater. Metal uptake increased in the following order: hard metals ( ⁴⁴Ca, ²⁴ Mg, and ⁸⁸Sr) < borderline metals (⁵⁵ Mn, ⁵⁹Co, ⁶¹Ni, ⁶³Cu, and ⁶⁶Zn), and soft metals (²⁰⁸Pb and ¹¹¹Cd) based on covalent bond formation with biological ligands. Improved removal of major ions was achieved using NL instead of the monofunctional (N(CH2COOH)2) NOBIASChelatePA1 resin. However, major ion (1.5500.8 mg/kg) and heavy metal concentrations were much higher (1.0 × 10² to 1.0 × 10⁵ μg/kg) in fresh NL compared to the metal concentrations (0.0298.2 ng/kg) in seawater. Therefore, before use, the freeze-dried NL was subjected to a rigorous acid cleaning process using HCl, HNO₃, and buffer (NH₄COOCH₃) to remove heavy metals and major ions. Trace metal recovery levels ascertained using inductively coupled plasma-mass spectrometry (ICP-MS) were in the range of 91115%, with 99.999% of the matrix ions removed. For each trace metal isotope, the procedure blank was <10% of the mean concentration of the seawater, and the precision was 58% RSD. The use of reference seawater (NASS-6) was extremely valuable in validating the metal-free condition of the NL and for data quality control. The current adsorption capacities of NL are relatively low; however, biological ligands have excellent potential for rapid separation of trace metals from the seawater matrix.