Metallothionein isoforms for selective biosorption and preconcentration of cadmium at ultra-trace levels

Abstract

Metallothioneins (MTs) are low molecular weight, cysteine-rich proteins widely distributed in nature. Two isoforms of MT, e.g., metallothionein isolated from rabbit liver (rMT) and recombined cyanobacteria metallothionein (SmtA), were immobilized on spherical SiO₂ particles to evaluate their biosorption behaviors for cadmium. We found that cadmium binding on both MT isoforms is pH dependent and follows Langmuir adsorption, and their adsorption dynamic fits the pseudo-second-order kinetics model. The affinity of cadmium on rMT is higher than that on SmtA, which is in accordance with the HSAB theory. On the other hand, however, SmtA exhibits a higher cadmium sorption capacity than rMTs both statically and dynamically. The SmtA–SiO₂ composite was thus used to pack a mini-column for the evaluation of cadmium preconcentration. The cadmium retained on the SmtA surface was recovered with a small amount of thiourea in nitric acid and quantified by graphite furnace atomic absorption spectrometry (GFAAS). Within a range of 5–100 ng L⁻¹ and a sample volume of 1 mL, an enrichment factor of 13.8 was achieved along with a detection limit of 1.4 ng L⁻¹, and a precision of 3.2% RSD at 50 ng L⁻¹. The procedure was validated by analyzing cadmium in certified reference materials and a series of environmental water samples.