Interactions of chelating agents with Pb-goethite at the solid-liquid interface: Pb extraction and re-adsorption.
The use of ethylenediaminetetraacetic acid (EDTA) for soil remediation is under concern due to its non-biodegradability and toxicity; thus, its biodegradable structural isomer, [S,S]-ethylenediamine disuccinate ([S,S]-EDDS), has been proposed as an emerging substitute. In this study, batch experiments were performed to quantify the Pb extraction from goethite by EDTA and EDDS, respectively, and attenuated total reflectance-Fourier transform infrared (ATR/FT-IR) spectroscopy was used to investigate the corresponding mechanisms at the solid-liquid interface at different pH and reaction times. The Pb extraction was pH-dependent for both chelating agents; mildly alkaline condition was favorable for EDDS, while mildly acidic condition for EDTA. The discrepancy between two structural isomers might be caused by their pH-dependent zwitterionic structures. The ATR/FT-IR results revealed that under acidic conditions, hydrogen-bonded carboxyl groups were present in both zwitterionic EDDS and EDTA. However, ring structure of zwitterionic EDDS formed with stable intramolecular hydrogen bond might limit the availability for EDDS to extract Pb from goethite. On the other hand, each protonated amine of zwitterionic EDTA could form hydrogen bonds with two neighbouring carboxyl groups, intensifying the negative charge of carboxyl groups and enhancing the Pb extraction efficiency. However, there was a higher amount of re-adsorption of PbEDTA than PbEDDS, because zwitterionic EDTA resulted in a greater Pb extraction and facilitated iron dissolution which significantly altered the goethite morphology, particle size, and surface area. These results suggested that, despite being structural isomers, EDDS and EDTA resulted in varying extents of Pb extraction and re-adsorption due to their different zwitterionic properties.