In vitro binding of heavy metals by an edible biopolymer poly(gamma-glutamic acid)

An edible biopolymer poly(gamma-glutamic acid) (gamma-PGA) was evaluated for possible use as an chelating/binding agent in the treatment of metal intoxication in humans. In vitro binding of the toxic heavy metals lead and cadmium as affected by pH, contact time, metal concentration, gamma-PGA dose, and essential metals was carried out in a batch mode. A maximum binding occurred in the pH range 5-7, corresponding to the gastrointestinal pH values except for the stomach. Binding isotherms at pH 5.5 were well described by the heterogeneous models (Freundlich and Toth), while the lead isotherm at pH 2.5 showed a S-type curve, which was fitted as multiple curves with the Langmuir model and a shifted-squared Langmuir model. However, no adsorption occurred for cadmium at pH 2.5. The maximum binding capacities of lead and cadmium at pH 5.5 were 213.58 and 41.85 mg/g, respectively. A curvilinear biphasic Scatchard plot signified a multisite interaction of metals. Binding was extremely rapid with 70-100% of total adsorption being attained in 2 min. Kinetics at low and high metal concentrations obeyed pseudo-first-order and pseudo-second-order models, respectively. The gamma-PGA dose-activity relationship revealed a low dose of gamma-PGA to be more efficient in binding a large amount of metals. Incorporation of Cu, Zn, Fe, Mg, Ca, and K showed only a minor influence on lead binding but significantly reduced the binding of cadmium.