Molecular mechanisms of phosphorus immobilization by nano-clay mediated by dissolved organic matter

Abstract

The adsorption of phosphorus (P) by minerals, particularly clay minerals, plays a crucial role in the biogeochemical cycling of P in geological environments such as soil and groundwater. However, the mechanisms underlying the immobilization and subsequent release of element P by nano-clay are still not well comprehended. This study delves into the mechanism of adsorption of inorganic phosphate and organic phytate onto laponite, serving as a representative nano-clay. This aspect has frequently been disregarded due to research method limitations; nevertheless, the substantial specific surface areas (SSA) are abundant in a multitude of potential adsorption sites. Batch experiments were conducted to evaluate surface reaction and adsorption capacity under varying solution conditions, including pH, ionic strength, and the presence of dissolved organic matter (DOM). The results demonstrate that the adsorption capacity of laponite decreases with increasing pH. Additionally, the adsorption capacity initially increases within the range of 10–50 mM NaCl and then decreases within 50–100 mM NaCl, while the presence of DOM significantly inhibits P adsorption. Complementary techniques such as X-ray photoelectron spectroscopy (XPS), zeta potential analysis, atomic force microscopy (AFM) were employed to gain further insights into the adsorption process. These analyses revealed that DOM obstructs P adsorption by occupying adsorption sites and promoting the aggregation of laponite nanoparticles. Furthermore, AFM-based dynamic force spectroscopy (DFS) provides molecular-scale thermodynamic support for the observed P adsorption phenomena on laponite under different conditions. The findings of this study contribute to a better understanding of the P adsorption mechanism of nano-clay and have valuable implications for the biogeochemical cycling of P in soil and groundwater.