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The use of biodegradable chelating agent,[S,S]-ethylenediamine disuccinic acid(EDDS),for soil remediation(e.g.,phytoextraction and soil washing)has attracted extensive research interests.This study employed attenuated total reflectance-Fourier transform infrared(ATR-FTIR)spectroscopy to investigate the interaction mechanisms between Pb and EDDS at the solid-liquid interface of iron oxides(goethite and hematite),while ethylene-diamine-tetraacetic acid(EDTA)was studied for comparison.The influence of aqueous and dehydrated conditions due to natural/induced drying process was then elucidated by using transmission-FTIR and X-ray photoelectron(XPS)spectroscopy.The ATR-FTIR 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.However,there was a higher amount of re-adsorption of Pb-EDTA than Pb-EDDS,because zwitterionic EDTA resulted in a greater Pb extraction and facilitated iron dissolution which significantly altered the goethite morphology,particle size,and surface area.On the other hand,in aqueous phase,ATR-FTIR analysis revealed that outer-sphere surface complexation was the dominant adsorption mechanism for both EDDS and EDTA.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.These results indicated that,due to difference in zwitterionic properties and charge density,EDDS and EDTA resulted in varying extents of Pb extraction/re-adsorption and their environmental fate was different under wetting-drying process.