Mechanisms of EDDS Adsorption on Goethite and Hematite Under Aqueous and Dehydrated Conditions

Chelating agents have been widely studied for enhancing the effectiveness of soil remediation. Natural and induced drying process may alter the interactions between chelating agents and iron oxides requiring a better mechanistic understanding. This study employed attenuated total reflectance (ATR) and transmission Fourier-transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) to evaluate the influence of aqueous and dehydrated conditions on the adsorption mechanisms of EDDS ([S,S]-ethylene-diamine-N,N-disuccinic acid) on goethite and hematite, while EDTA (ethylene-diamine-tetraacetic acid) was studied for comparison. In aqueous phase, ATR-FTIR analysis revealed that outer-sphere surface complexation was the dominant adsorption mechanism for both EDDS and EDTA. Chelant adsorption was more significant on goethite than hematite because of higher surface charge and stronger electrostatic attraction at acidic pH. Upon dehydration on the iron oxides, inner-sphere surface complexation was not observed for adsorbed EDDS on goethite and hematite, whereas EDTA formed inner-sphere surface complexes on goethite (bidentate binuclear adsorption) and hematite (monodentate adsorption), as shown by transmission FTIR and XPS analysis. Therefore, EDDS remained weakly bound and mobile compared with EDTA, possibly due to its weaker zwitterionic properties and lower charge density. These results suggest that the environmental fate of EDDS is different from that of EDTA, where wetting-drying process and types of iron oxides also play an important role.