The thermodynamics and biodegradability of chelating agents upon metal extraction

Abstract

Chelation technology, one of emerging green approaches, has shown its potential to extract metals from industrial waste; however, the stability of metal–chelate complexes and the biodegradability of chelating agents are opposing aspects which need to be balanced carefully in order to develop a sustainable metal extraction process. In present study, stability of transition metal (Ni2+, Co2+, Rh3+, Mo6+) complexes with different aminopolycarboxylic chelating ligands (EDTA, EDDS, NTA) was investigated using density functional theory (DFT) calculations and the results were compared to previous experimental studies (Chauhan et al., 2012, 2013b; Cruywagen et al., 1994; Smith and Sawyer, 1968a;Marcus, 1985). Desolvation and complexation processes were examined to rationalize the thermodynamics of ligand substitution reactions using generalized gradient approximations (GGA) and hybrid DFT methods. A combination of explicit metal–water complexation and implicit solvation treatment was used to calculate the free energies of desolvation and chelation. Standard state corrections were also included for solvation free energies to enable a direct comparison with literature values. Thermodynamic analysis was performed in order to estimate the metal–ligand binding free energies at room and elevated process temperatures. Existence of different Rh3+ species in solution and in complexes was also explored and the most stable complex was identified. Differences in the coordination chemistry associated with Mo6+ were observed which motivated to perform free energy calculations upon MoO3 as a central coordination unit for different metal-to-ligand complexes (1:1, 2:1) in order to characterize the thermodynamically most stable complex at optimum reaction pH. In general, the metal–chelate ligand complexes followed the stability trend i.e. (EDTA)>(EDDS)>(NTA). The high stability of EDTA4− ligand with a favorable minimal strain can be the reason for its inherent resistance to degradation; it is therefore reckoned to consider the environmental impact of chelate assisted metal extraction processes by assessing ligand degradation pathways also. Possible pathways for the biodegradability of the chelating agents were investigated by calculating the bond dissociation energies of the ligands.