Effect of Fe (oxyhydr)oxide morphology on phytic acid transport under saturated flow condition

Natural iron (oxyhydr)oxides are ubiquitous in subsurface environments. Phytic acid (myo-inositol hexaphosphate, IHP), a dominant form of organic phosphate (OP) in organic carbon-rich surface soils, strongly binds with Fe (oxyhydr)oxide. The cotransport of IHP and Fe (oxyhydr)oxide with different morphology under acid and alkaline conditions in the subsurface is mostly overlooked. These cotransport processes are critical for P (bio)geochemical processes in the subsurface that is rich in Fe (oxyhydr)oxides. Three Fe (oxyhydr)oxides (ferrihydrite, hematite, and goethite) were chosen in this study, and the cotransport of IHP and Fe (oxyhydr)oxide was investigated in saturated columns by injecting Fe (oxyhydr)oxide under different IHP concentrations (0, 10, 25, 50, and 100 µM) at pH of 5 and 10. The presence of IHP significantly enhanced the mobility of Fe (oxyhydr)oxide at both pH 5 and 10 due to the stronger electrostatic repulsion between Fe (oxyhydr)oxide and quartz sand. At low IHP concentrations (< 50 µM IHP), goethite with a rod-like morphology showed strong mobility due to its orientation transport along with the water flow streamline. The mobility of amorphous Fe (oxyhydr)oxide, ferrihydrite, was much slower than the goethite. However, ferrihydrite could facilitate more IHP transport due to its sorption capacity for IHP that is higher than goethite and hematite. At high IHP concentrations (> 50 μM), surface precipitation might have occurred on ferrihydrite because of its poorly ordered crystallinity, which contributed to its less negatively charged surface and weak ferrihydrite facilitated IHP transport. The new insight provided in this study is important for evaluating the transport behavior and impact of IHP in a saturated solum rich in Fe (oxyhydr)oxides.