Phosphorus adsorption onto ferrihydrite and predicting colloidal phosphorus transport

Losing soil phosphorus to aquatic environments often causes eutrophication; however, the stability and transport of colloidal phosphorus, especially amorphous iron colloid-bound phosphorus in porous soil, is poorly understood. In this study, adsorption experiments were conducted to investigate phosphorus adsorption onto ferrihydrite. The effects of pH, ionic strength, and humic acid (HA) on the dissolved phosphorus distribution and colloidal and solid ferrihydrite adsorbed phosphorus were explored. Ferrihydrite colloid-bound phosphorus stability was calculated using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to predict the transport of colloidal complexes. The results showed that the pseudo-second-order kinetic model (R² = 0.964) best described the phosphorus onto ferrihydrite adsorption process. Adsorption was controlled by liquid film diffusion as well as internal diffusion, and chemisorption. The Freundlich model (R² = 0.970) was a better fit for isothermal adsorption than the Langmuir model (R² = 0.842), indicating that phosphorus onto ferrihydrite adsorption was multi-layer; however, the parameters of Langmuir model revealed that the maximum theoretical phosphorus onto ferrihydrite adsorption capacity was 22.55 mg∙g^-1. Phosphorus adsorption onto ferrihydrite decreased with increasing pH and decreasing ionic strength; low ionic strength and high pH were considered beneficial for releasing ferrihydrite colloids. Approximately 5%–20% phosphorus bound to ferrihydrite colloids in alkaline and low ionic strength conditions (1–10 mmol∙L^-1) regardless of HA, and the electrostatic repulsion between ferrihydrite-phosphorus colloids was notably. According to the DLVO theory, the colloids were stable and easily transported in the soil pores due to their negative surface charge. Negatively charged ferrihydrite colloids can transport long distances in negatively charged water-bearing media, such as soil or aquifer. In agricultural activities, excessive phosphate fertilizer application may cause large amounts of phosphate ion loading onto iron minerals and promote the formation of stable, negatively charged iron mineral colloids. Ferrihydrite-phosphorus colloid transport is likely to become another form of phosphorus leaching. Thus, this study investigated ferrihydrite-phosphorus colloid generation and stability in variable pH, ionic strength, and HA conditions, qualitatively predicted their transport, and assessed colloid-facilitated phosphorus loss risk. However, in complex soil systems, chelation and precipitation of co-existing ions may alter the fate of ferrihydrite-phosphorus colloids, which needs further investigation.