『Abstract
　Iron (III) oxides are ubiquitous in near-surface soils and sediments and interact strongly with dissolved phosphates via sorption, co-precipitation, mineral transformation and redox-cycling reactions. Iron oxide phases are thus, an important reservoir for dissolved phosphate, and phosphate bound to iron oxides may reflect dissolved phosphate sources as well as carry a history of the biogeochemical cycling of phosphorus (P). It has recently been demonstrated that dissolved inorganic phosphate (DIP) in rivers, lakes, estuaries and the open ocean can be used to distinguish different P sources and biological reaction pathways in the ratio of ¹⁸O/¹⁶O (δ¹⁸OP) in PO4^3-. Here we present results of experimental studies aimed at determining whether non-biological interactions between dissolved inorganic phosphate and solid iron oxides involve fractionation of oxygen isotopes in PO4. Determination of such fractionations is critical to any interpretation of δ¹⁸OP values of modern (e.g., hydrothermal iron oxide deposits, marine sediments, soils, groundwater systems) to ancient and extraterrestrial samples (e.g., BIF's, Martian soils). Batch sorption experiments were performed using varied concentrations of synthetic ferrihydrite and isotopically-labeled dissolved ortho-phosphate at temperatures ranging from 4 to 95℃. Mineral transformations and morphological changes were determined by X-ray, Mossbauer（oの頭に¨） spectroscopy and SEM image analyses.
　Our results show that isotopic fractionation between sorbed and aqueous phosphate occurs during the early phase of sorption with isotopically-light phosphate (P¹⁶O4) preferentially incorporated into sorbed/solid phases. This fractionation showed negligible temperature-dependence and gradually decreased as a result of O-isotope exchange between sorbed and aqueous-phase phosphate, to become insignificant at greater than 〜100 h of reaction. In high-temperature experiments, this exchange was very rapid resulting in negligible fractionation between sorbed and aqueous-phase phosphate at much shorter reaction times. Mineral transformation resulted in initial preferential desorption/loss of light phosphate (P¹⁶O4) to solution. however, the continual exchange between sorbed and aqueous PO4, concomitant with this mineralogical transformation resulted again in negligible fractionation between aqueous and sorbed PO4 at long reaction times (＞2000 h). This finding is consistent with results obtained from natural marine samples. Therefore, ¹⁸O values of dissolved phosphate (DIP) in sea water may be preserved during its sorption to iron-oxide minerals such as hydrothermal plume particles, making marine iron oxides a potential new proxy for dissolved phosphate in the oceans.』

1. Introduction
2. Materials and methods
　2.1. Synthesis and purification of ferrihydrite
　2.2. Phosphate sorption experiments
　2.3. Methods of aqueous and sorbed phosphate O-isotope analysis
　2.4. XRD
　2.5. ⁵⁷Fe-Mossbauer（oの頭に¨） spectroscopy
3. Results
　3.1. Properties of ferrihydrite and its transformation products
　3.2. Kinetics of phosphate sorption and isotopic fractionation
　　3.2.1. 21℃
　　3.2.2. 70℃
　　3.2.3. 95℃
　　3.2.4. 4℃
　　3.2.5. Fractionation of phosphate at low phosphate concentrations
4. Discussion
　4.1. Kinetics of mineral transformation and its effect on partitioning of phosphate
　4.2. Isotopic effects of phosphate sorption
　4.3. Isotopic fractionation during mineral transformation
　4.4. Mechanism of phosphate sorption
5. Conclusions and implications
Acknowledgements
References