Effects of GLDA, MGDA, and EDTA chelating ligands on Pb sorption by montmorillonite
2020
Parsadoust, Farzad | Shirvani, Mehran | Shariatmadari, Hossein | Dinari, Mohammad
In this study, the effects of the new biodegradable chelating ligands, i.e., methylglycinediacetic acid (MGDA) and L-glutamic acid-N,N-diacetic acid (GLDA), were compared with that of ethylenediaminetetraacetic acid (EDTA) on lead (Pb) sorption by montmorillonite (MMT) at equimolar Pb:ligand ratios. The results of PHREEQC simulations indicated that Pb was predominantly present in the form of negatively-charged chelates with significantly different sorption behaviors compared to that of the Pb²⁺ cations. The time-dependent sorption of the Pb species on MMT was biphasic, with an initial fast phase occurring within 3 h followed by a slower one reaching equilibrium after approximately 12 h. The analysis of equilibrium data revealed that both capacity and affinity of MMT for Pb sorption were decreased in the presence of the chelating ligands. The moderating potential of ligands on Pb sorption decreased in the order of EDTA ≫ MGDA > GLDA in accordance with complex stability constants. The FE-SEM/EDX, ATR-FTIR, and XRD evidence indicated that Pb and Pb complexes were adsorbed, most likely through physical forces, on both external planar and edge surfaces of the MMT. In conclusion, GLDA and MGDA represented lower ability than EDTA to depress Pb²⁺ adsorption on MMT, and hence, lower potential to mobilize Pb in soils and sediments where MMT is an important mineral sorbent.
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