Abstract
Recent developments in analytics using infrared spectroscopy have enabled us to identify the adsorption mechanism at interfaces, but such methods are applicable only for simple systems. In this study, the preferential adsorption of phosphate on binary goethite and maghaemite was investigated. As a result, monodentate and bidentate complexes were the major complexes on goethite and maghaemite, respectively. A shrinking effect in goethite and a swelling effect in maghaemite were identified, and environmental perturbations caused a significant decrease in the integrated absorbance of phosphate complexes on maghaemite, while no effect was observed on goethite, which implies that different adsorption mechanisms were involved. Based on the results, a bridging complex was proposed, and the swelling effect is explained by the negatively charged maghaemite surface resulting from the bidentate complex. The isolation of phosphate by the shrinking effect explains the low phosphate bioavailability in the soil environment, while the colloidal properties of the bidentate complex on maghaemite are the reason for colloidal mobilization. To the best of our knowledge, this study not only addresses the shrinking and swelling properties of iron (hydr)oxide nanoparticles but also demonstrates preferential adsorption on binary sorbents using in situ ATR-FTIR for the first time.
Highlights
The developed technologies are not satisfactory for interpreting adsorption mechanisms in the real environment because in situ ATR-FTIR with computational chemistry is available only for simple systems, for example, one or two solutes on a single sorbent
Gt and Mh were identified as rod- and spherical-shaped nanoparticles (NPs) with sizes of 50.3 × 10.8 nm and 50.7 nm, respectively, using high-resolution transmission electron microscopy (HR-TEM)
The calculated surface area (SA) was higher than the measured SA for Gt, but the opposite pattern was observed for Mh
Summary
The developed technologies are not satisfactory for interpreting adsorption mechanisms in the real environment because in situ ATR-FTIR with computational chemistry is available only for simple systems, for example, one or two solutes on a single sorbent. The results for phosphate adsorption on a single sorbent showed a distinctive spectrum compared with previous studies[12,28,29,30]: the background increased at 1500-1200 and 930-850 cm−1 for Gt29,30 (Fig. 2a) and decreased at 1500-1300 cm−1 for Mh (Fig. 2b).
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