Transition Metal Transfer from Water-Soluble Chelates to a Chelating Polymer: Kinetic and Mechanistic Insights

Noam Dolev, Biochemistry, Hebrew University, Rehovot, Israel
Zvi Ludmer, Biochemistry, Hebrew University, Rehovot, Israel
Avraham Bromberg, Hebrew University, Rehovot, Israel
Roman Goikhman*, Biochemistry, Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University, Rehovot, Israel

Chelating polymers such as Chelex-100 are used nowadays for detection, concentration and separation of metal ions in various media from biological samples to ground water. However, their applicability in environmental chemistry remains poorly studied.

Within our project of developing a new soil remediation process with emphasis on environmental-friendly chelators, we tested simple amino acids, in comparison to commonly used chelators like EDTA. We have found that different chelated metals have different transfer rates and different molar adsorption capacity at Chelex-100. Particularly, copper was readily adsorbed from copper (II) diglycinate complex CuG₂, but not from copper-EDDS complex. Adsorption of nickel from nickel (II) diglycinate NiG₂ onto Chelex-100 proceeds pretty fast, but it is considerably slowed down by 2.5 fold excess of glycine, that clearly indicates expected metal pre-dissociation followed by fast adsorption of Ni²⁺ by Chelex-100 (dissociative mechanism). Surprisingly, in contrast to nickel, adsorption of copper from CuG₂ is not affected by excess of free glycine or sodium glycinate. This finding excludes an option of copper pre-dissociation as a major mechanistic pathway, and allows suggesting an associative (conjoined) mechanism of Cu²⁺ transfer. Thus, different metal complexes react with Chelex-100 by different preferred mechanistic pathways. 

Moreover, metal transfer rate is found to be temperature-independent when viscosity of the chelate solutions is kept constant.  This finding suggests that the metal transfer rate is determined by solution viscosity rather than by chemical reaction.

Kinetic models of the studied processes are discussed. Currently, theoretical calculations aimed to disclose molecular mechanism of the associative metal transfer, are underway, as well as synthesis of a stronger chelating polymer designed for environmental chemistry needs.

The present study is aimed to understand mechanisms of the widely used metal transfer processes, and to take into account metal-specific reactivity when chelating polymers are applied for extracting chelated metals from aqueous solutions.