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

Phytic acid (myo-inositol hexaphosphate, IHP) is a dominant form of organic phosphate (OP) in organic carbon-rich surface soil. The IHP impact on Fe (oxyhydr)oxide transport is critical for iron and phosphorus (bio)geochemical processes in iron and phosphorus rich soil and subsurface systems. Three typical Fe (oxyhydr)oxides (ferrihydrite, hematite, and goethite) were studied in this research. The effects of IHP and morphology on Fe (oxyhydr)oxide transport and IHP cotransport had been investigated using saturated sand columns. The results showed that IHP significantly enhanced the mobility of Fe (oxyhydr)oxide by 30-90% due to the stronger electrostatic repulsion. At low IHP concentration (< 50 µM IHP), the rod-like goethite and goethite-facilitated IHP showed high mobility due to their orientation and motion along the water flow, which is 70% faster than ferrihydrite and hematite at pH 5 and 90% faster at pH 10. The mobility of amorphous ferrihydrite was slowest among three selected iron oxides (< 37% at pH 5 and < 72% at pH 10). At high IHP concentration (> 50 μM IHP), the surface precipitation might have occurred on ferrihydrite because of its poorly ordered crystallinity, contributing to its less negatively charged surface and weak transport. The new insight provided in this study is essential for evaluating the fate and transport behavior of iron and iron-facilitate OP in soil rich in iron and OP.